Electronic properties of phenanthrimidazoles as hole transport materials in organic light emitting...

Electronic properties of phenanthrimidazolesas hole transport materials in organic lightemitting devices and in photoelectron transferto ZnO nanoparticlesChockalingam Karunakarana, Jayaraman Jayabharathia*,Marimuthu Venkatesh Perumala, Venugopal Thanikachalamb

and Prasoon Kumar Thakurc

Phenanthrimidazoles as hole transport materials have been synthesized, characterized, and applied as nondopingemitters in organic light emitting devices. The synthesized molecules possess high fluorescent quantum yield andthermal properties and display film forming abilities. The highest occupied molecular orbital (HOMO) energies of thesematerials are shallower than the reported tris(8-hydroxyquinoline)aluminum (Alq3), which enables the hole transportability of these phenanthrimidazoles. Taking advantage of the thermal stability and hole transporting ability, thesecompounds can be used as a functional layer between NPB [4,4-bis(N-(1-naphthyl)-N-phenylamino)biphenyl] andAlq3 layers and show that these phenanthrimidazoles can be alternatively used as novel hole transport materials andto improve the device performances. Geometrical, optical, electrical, and electroluminescent properties of thesemolecules have been probed. Further, natural bond orbital, nonlinear optical materials (NLO), molecular electrostaticpotential, and HOMO–lowest unoccupied molecular orbital (LMO) energy analysis have been made by densityfunctional theory (DFT) method to support the experimental results. Hyperpolarizability analysis reveals that thesynthesized phenanthrimidazoles possess NLO behavior. The chemical potential, hardness, and electrophilicity indexof phenanthrimidazoles have also been computed by DFT method. Photoinduced electron transfer explains theenhancement of fluorescence by nanoparticulate ZnO, and the apparent binding constant has been obtained. Adsorptionof the fluorophore on ZnO nanoparticle lowers the HOMO and LUMO energy levels of the fluorophore. The strongadsorption of the phenanthrimidazoles on the surface of ZnO nanocrystals is likely due to the chemical affinity ofthe nitrogen atomof the organicmolecule to Zn(II) on the surface of nanocrystal. Copyright © 2013 JohnWiley & Sons, Ltd.

Keywords: phenanthrimidazole; hole transport materials; optical properties; nanosemiconductor; lifetime

INTRODUCTION

A great deal of effort has been made on the studies of organicelectroluminescent (EL) materials and devices.[1–4] Because ofthe optical and conductive properties, conjugated materialscontaining thiophene, imidazole, and phenanthroline heterocy-cles have found many applications.[5–12] These aryl-imidazo-phenanthrolines play an important role in materials scienceand medicinal chemistry due to their optoelectronic propertiesand possess high thermal stabilities. Intensive research has beencarried out to find materials with high light emitting efficiencies,high thermal stability, and good amorphous film formationproperty.[13–16] The optoelectronic properties for organic lightemitting devices (OLEDs) depend on appropriate highest occupiedmolecular orbital (HOMO) and lowest unoccupied molecularorbital (LUMO) energy levels and suitable electron and hole mobil-ities. Although the guidelines for designing small molecules withthe desirable photo and thermal properties[17–19] are well-known,analogous guidelines on the mobilities of charge carriers inorganic materials are limited because of the scarce of experimen-tal data. Nevertheless, the mobilities are important in optimizingthe performance of OLED; high mobilities reduce the resistance

of the device leading to high power efficiency. In addition, therelative mobilities of electron and hole in the same material canalso affect the power efficiency.Oxadiazoles, triphenylamine, and their derivatives are known

electron and hole transporting materials and are used for theconstruction of hetero-junction multi-layer OLED.[20,21] A newclass of materials for EL devices having both the processability

* Correspondence to: J. Jayabharathi, Professor of Chemistry, Department ofChemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India.E-mail: [email protected]

a C. Karunakaran, J. Jayabharathi, M. Venkatesh PerumalDepartment of Chemistry, Annamalai University, Annamalainagar, Tamilnadu,India

b V. ThanikachalamDepartment of Chemistry Wing (DDE), Annamalai University, Annamalainagar,Tamilnadu, India

c P. Kumar ThakurDivision of Nematology, Indian Agricultural Research Institute, New Delhi-12,India

Research Article

Received: 7 December 2012, Revised: 18 January 2013, Accepted: 27 January 2013, Published online in Wiley Online Library: 2 April 2013

(wileyonlinelibrary.com) DOI: 10.1002/poc.3100

J. Phys. Org. Chem. 2013, 26 386–406 Copyright © 2013 John Wiley & Sons, Ltd.

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of polymers and the defined optical and electrical properties oflowmolecular weight materials are of interest; spiro and branchedderivatives containing oxadiazoles as well as dendrimers are beingused as electron-transporting and hole-blocking materials.[22–24]

Shirota[25,26] has recently suggested a new method for themolecular design of low molecular weight amorphous materials,which involve increasing the number of conformers by loweringthe symmetry of nonplanar molecules. Bazan and co-workers[27]

demonstrated that tetrahedral oligophenylenevinylene tetrameracted as an efficient OLED. Quite recently, amorphous blue light-emitting materials based on tetraphenylsilane compounds or bis(spirobifluorenyl) anthracenes and morphologically stable carba-zole with peripheral aryl amines possessing dual functions, lightemitting, and hole transporting have been reported.[28–31] Thedevices, however, have been prepared by sequential vacuumdeposition method. As of today, the development of organicmolecules with hole transport property is less than those withelectron transport properties. Thus, development of better emis-sive materials with hole transport properties is an importantapproach for performance enhancement. Our experimental find-ings prompted us to develop new hole-transporting materialswith high glass transition temperature (Tg) using a new coremolecule for OLEDs.Study of interfacial electron transfer dynamics between

molecular adsorbates and nanocrystalline semiconductors is ofcurrent interest[32–37] because of its essential role in dye-sensitizedsolar cells,[38,39] photocatalysis,[40] and molecular electronics.[41]

The kinetics of charge injection from dye excited state to semicon-ductor nanoparticles and recombination are found to be, ingeneral, nonsingle exponential, suggesting a heterogeneousdistribution of interfacial electron transfer rates. The nonexponentialkinetics could result from static heterogeneities in energetics ofthe adsorbate and semiconductor and their electronic coupling aswell as dynamic fluctuation of these quantities. Nanocrystalline zincoxide (nano-ZnO) is a wide band gap semiconductor possessing alarge excitonic binding energy and a unique photoluminescence(PL) spectrum. These properties make nano-ZnO a promisingmaterial for use in the areas of photonics, electronics, and sensors.For example, the ability of nano-ZnO to act as a gas sensorhas been demonstrated by utilizing changes in its electricalresistivity. To our knowledge, most researchers have studiedthe ligand effects on the optical properties of nanocrystals,[42–52]

and comparison of the photoelectric properties of different ligandswith ZnO nanocrystals is rare. In this paper, we examine electrontransfer from photoexcited phenanthrimidazole to ZnO nanocrys-tal. We have analyzed these systems with PL and time-resolvedPL decay spectra. Our results show that, unlike the widely reportedquenching of fluorescence of different ligands by variousnanoparticulate semiconductors[53–63] including ZnO,[64] thefluorescence of synthesized phenanthrimidazoles is enhancedby nano-ZnO.

EXPERIMENTAL

Chemicals

4-Fluorobenzaldehyde, 2,4-difluorobenzaldehyde, and 4-trifluoromethyl-benzaldehyde were supplied by Sigma Aldrich (St. Louis, USA). Benzaldehydeand 3,5-dimethoxyaniline used were of analytical grade and receivedfrom S.D.Fine (Mumbai, India). The solvents used for cyclic voltammetric,UV-visible spectroscopic, PL studies and lifetime measurements were ofspectroscopic grade supplied by Himedia (Chennai, India).

Synthesis of phenanthrimidazoles

1,10-Phenanthroline-5,6-dione was synthesized and purified accordingto the reported literature procedure.[65] The various substitutedphenanthrimidazoles[66–68] were prepared by four components conden-sation of 1,10-phenanthroline-5,6-dione (40mmol), ammonium acetate(30mmol), arylaldehyde (30mmol), and arylamine (30mmol). These fourcomponents were refluxed in ethanol (20mL) at 80 �C. The completion ofthe reaction was monitored by thin layer chromatography. The reactionmixture was extracted with dichloromethane and purified by columnchromatography using benzene : ethyl acetate (9:1) as the eluent.

1-(3,5-Dimethoxyphenyl)-2-phenyl-1H-imidazo[4,5-f][1,10]phenan-throline (1)

Yield: 55%. Anal. Calcd for C27H20N4O2: C, 74.98; H, 4.66; N, 12.95. Found:C, 74.68; H, 4.53; N, 12.57. 1H NMR (500MHz, CDCl3): d 4.01 (s, 6H), 6.50(d, 2H), 7.59–7.81 (m, 3H), 8.36 (s, 1H), 8.69 (t, 2H), 8.96 (d, H-aryl), 9.26(d, 2H), 9.38 (d, 2H). 13C (100MHz, CDCl3): d 55.58, 92.33, 96.46, 102.88,116.19, 118.01, 119.17, 122.97, 123.44, 124.07, 124.46, 124.99, 126.98,127.32, 128.58, 129.08, 130.78, 131.53, 132.41, 134.88, 137.82, 138.31,139.33, 140.70, 143.32, 144.84, 147.40, 148.31, 149.64, 156.72, 158.91,163.72, 164.55. MS: m/z. 431.17 [M� 1].

2-(4-Fluorophenyl)-1-(3, 5-dimethoxyphenyl)-1H-imidazo [4, 5-f][1,10]phenanthroline (2)

Yield: 55%. Anal. Calcd for C27H19N4FO2: C, 71.99; H, 4.25; N, 12.44.Found: C, 71.78; H, 4.21; N, 12.18. 1H NMR (500MHz, CDCl3): d 3.81 (s,6H), 7.80–7.87 (m, 4H), 8.46 dd, 2H), 8.72 (s, 1H), 9.06 (dd, 2H, J=8.4 Hz),9.33 (dd, 2H, J=4.4 Hz), 9.38 (dd, 2H, J=4.4 Hz). 13C (100MHz, CDCl3):d 52.16, 121.07, 124.39, 124.48, 125.48, 125.57, 132.55, 137.91, 144.33,146.27, 147.72, 151.56, 153.65, 158.87, 162.56, 163.95. MS: m/z. 451.17[M+ 1], 453.23 [M+ 2].

2-(2, 4-Difluorophenyl)-1-(3,5-dimethoxyphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (3)

Yield: 60%. Anal. Calcd for C24H18F2N4O2: C, 69.23; H, 3.87; N, 11.96.Found: C, 69.97; H, 3.63; N, 11.69. 1H NMR (400MHz, CDCl3): d 3.95 (s,3H), 7.81–7.87 (m, 3H), 8.46 (d, 2H), 8.73 (s, 1H), 9.06 (dd, 2H, J=8.8 Hz),9.33 (dd, 2H, J=4.4 Hz), 9.38 (dd, 2H, J=4.4 Hz). 13C (400MHz, CDCl3):d 55.60, 121.08, 124.38, 124.47, 125.49, 125.56, 132.54, 137.90, 144.34,146.29, 147.73, 151.56, 153.64, 159.63, 160.32. MS: m/z. 469.42 [M+ 1].

2-(4-(Trifluoromethyl)phenyl)-1-(3,5-dimethoxyphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (4)

Yield: 60%. Anal. Calcd for C28H19F3N4O2: C, 67.20; H, 3.83; N, 11.19.Found: C, 67.03; H, 3.75; N, 11.04. 1H NMR (400MHz, CDCl3): d 3.65 (s,3H), 7.78–7.84 (m, 4H), 8.44 (d, 2H), 8.68 (s, 1H), 9.02 (dd, 2H, J=8.4 Hz),9.30 (dd, 2H, J=3.2 Hz), 9.35 (dd, 2H, J=3.2 Hz). 13C (400MHz, CDCl3):d 55.62, 90.90, 95.07, 97.72, 121.02, 123.01, 124.64, 125.39, 126.28,128.32, 128.60, 129.54, 130.33, 131.07, 132.50, 135.83, 137.23, 137.89,144.26, 145.62, 146.21, 147.66, 150.15, 151.48, 152.57, 153.60, 159.49,161.72. MS: m/z. 501.58 [M+ 1].

Synthesis of nanocrystalline ZnO by sol–gel method

To zinc acetate (0.1 g) solution under continuous stirring, aqueousammonia (1:1) was added dropwise to reach a pH of 7, and thestirring was continued for another 30min. The formed glassy-like whitegel was allowed to age overnight. It was filtered, washed with waterand ethanol, dried at 100 �C for 12 h, calcinated at 500 �C for 2 h[69] at aheating rate 10 �Cmin�1.

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Thermal analysis

Thermal analysis of the phenanthrimidazoles 1–4 was made withNETZSCH-Geratebau Gmbh thermal analysis STA 409 PCO. The differen-tial scanning calorimetric (DSC) and thermogravimetric (TG) analyseswere made under nitrogen atmosphere (100mLmin�1). The sensitivityof the instrument was set at 0.01mg, and the sample (10mg) was heatedfrom 30 to 700 �C at the rate of 10 or 15 or 20 Kmin�1.

Spectral measurements

The infrared spectra were recorded with an Avatar 330-Thermo NicoletFourier transform infrared (FT-IR) spectrometer (Thermo, America). Theproton spectra at 400MHz were obtained at room temperature using aBruker 400MHz NMR spectrometer (Bruker biospin, California, USA).Proton decoupled 13C NMR spectra were also recorded at room temper-ature employing a Bruker 400MHz NMR spectrometer operating at100MHz. The mass spectra of the samples were obtained using a ThermoFischer LC-Mass spectrometer in fast atom bombardment (FAB) mode(Thermo, France). The cyclic voltammetry (CV) analyses were performedwith a CH Instrument electrochemical analyzer 604C (CH Instrument elec-trochemical analyzer, USA) at scan rate of 100mV s�1 using 0.1Mtetra-(n-butyl)-ammonium hexafluorophosphate as supporting electrolytewith Ag/Ag+ (0.01M AgNO3) as the reference electrode and Pt electrodeas the working electrode under nitrogen atmosphere at room tempera-ture. The UV–Vis absorption and fluorescence spectra were recorded witha PerkinElmer Lambda 35 spectrophotometer (Lambda 35, PerkinElmer,Singapore) and a PerkinElmer LS55 spectrofluorimeter (LS55, PerkinElmer,Singapore), respectively. Solid state emission spectra were recorded using aFluoromax 2 ISA SPEX fluorimeter with Xenon-arc lamp as light source. Fluo-rescence lifetime measurements were carried out with a nanosecond timecorrelated single photon counting spectrometer Horiba Fluorocube-01-NL life-time system (Horiba, UK) with NanoLED (pulsed diode excitation source) asthe excitation source and TBX-PS as detector. The slit width was 8nm, andthe excitation wavelength was 282nm. The fluorescence decay was analyzedusing DAS6 software. The PL quantum yield for all phenanthrimidazoleswere measured in dichloromethane using coumarin 47 in ethanol as thestandard.[70,71] The radiative and nonradiative rate constants, kr and knr,were deduced from the phosphorescence yield (Φp) and phosphorescencelifetime (t) using the equation,Φp =Φisc{kr/(kr + knr)} where Φisc is theintersystem crossing yield,[72] kr =Φp/t, knr = 1/t�Φp/t; t= (kr + knr)

�1.

Device fabrication

The EL devices based on the phenanthrimidazoles were fabricated byvacuum deposition of the materials at 5� 10�6 torr onto a clean glassprecoated with a layer of indium tin oxide (ITO) as the substrate. The glasswas cleaned by sonication successively in a detergent solution, acetone,methanol, and deionized water before use. Organic layers were depositedonto the substrate at a rate of 0.1nms�1. LiF and Alq3 were thermally evap-orated onto the surface of organic layer. The thickness of the organic mate-rials and the cathode layers were controlled using a quartz crystal thicknessmonitor. A series of devices (I, II, III, and IV) of configuration [ITO/NPB(10 nm)/buffer layer (30 nm)/Alq3 (60nm)/LiF (1nm)/Al] with compounds1–4 as buffer layer were fabricated, where NPB is the hole-transportinglayer and Alq3 was the electron-transporting layer as well as the emittinglayer (EML). Compounds 1–4 as film was the buffer layer in devices I, II, III,and IV, respectively. For comparison, the reference device (device V) withthe configuration of [ITO/NPB (40nm)/Alq3 (60nm)/LiF/Al] was fabricated.Measurements of current, voltage, and light intensity of a series of deviceswere made simultaneously using a Keithley 2400 sourcemeter (Keithley,Cleveland, Ohio). The EL spectra and luminance of the devices were carriedout in ambient atmosphere without further encapsulations.

Computational details

All the calculations were performed with Gaussian 03[73] package. Thegeometry of all involved structures was fully optimized with the DFT

method, using B3LYP/6-31G (d,p) basis set. The density functional theorywas used to calculate the dipole moment (m), mean polarizability (a),and the total first static hyperpolarizability (b)[74,72,75–82] of thesephenanthrimidazole derivatives in terms of x, y, and z components usingthe following equations:

m ¼ m2x þ m2y þ m2z� �1

2; a ¼ 1=3 axxþayyþazz� �

;btot

¼ b2x þ b2y þ b2z� �1=2

(1)

btot ¼h

bxxx þ bxyy þ bxzz� �2

þ byyy þ byzz þ byxx� �2

þ bzzz þ bzxx þ bzyy� �2i1=2

(2)

The reported b components of Gaussian output are in atomic units, andthe calculated values are in e.s.u. units (1 a.u. = 8.3693� 10�33 e.s.u.). Thesecond order Fock matrix was carried out to evaluate the donor–acceptorinteractions in the natural bond orbital (NBO) analysis.[83–92] For eachdonor (i) and acceptor (j), the stabilization energy E(2) associated with the

delocalization i! j is estimated as, E(2) = qiF i;jð Þ2ej�ei , where qi is the donor

orbital occupancy, ei and ej are diagonal elements, and F(i, j) is the offdiagonal NBO Fock matrix element. The larger the E(2) value, the moreintensive is the interaction between electron donors and electron accep-tors. To find out the heat of formation of phenanthrimidazole withZnO nanoparticle, structures of the molecules were generated usingChemSketch software and converted into 3Dmodels.[93] Molecular network(MN) format converter was used to convert .mol2 file to .pdb file formatavailable at http://www.molecular-networks.com. The quantummechanical(QM) calculations were performed by semi-empirical molecular orbital cal-culations MOPAC 7.01 with AM1(Hamiltonian) implementation of Vega zz3.0.0.[94] All calculations were carried out on an Intel pentium4, 2.4GHz-basedmachine runningMSWindows XP SP2 as operating system. Visualiza-tion of the optimized structure was performed on PyMol moleculargraphics program, a comprehensive software package for rendering andanimating three-dimensional structures that produces high-quality three-dimensional images of small molecules.[95]

RESULTS AND DISCUSSION

Photophysical studies of the phenanthrimidazoles

UV–Vis absorption and fluorescence spectral studies ofphenanthrimidazoles are presented in Figs 1 and 2, respectively.All the four phenanthrimidazoles show the main absorptionband around 275 nm; compounds 1 and 2 at 275 nm, 3 at270 nm, and 4 at 265 nm. In addition, some weaker bands arealso observed around 325 nm. All the observed absorptionbands do not show remarkable difference between them andsuggest very similar optical energy gap. However, the fluores-cence emission differs significantly; compounds 1 and 2 emitaround 400 nm, 3 at 410 nm, and 4 at 450 nm. It is interestingto note that the fluorescence emission of compounds 1 and 2are at the same wavelength, whereas the emissions of 3 and 4are red shifted with respect to the parent molecule 1. The factthat compounds 1 and 2 absorb at the same wavelength(275 nm) and also emit at identical frequencies indicates thattheir optical energy gaps are similar. The emission of compound4 is red shifted in comparison with compounds 1–3, and this redshifted emission indicates that the polarity of the excited state ishigher than the ground state. These results suggest an importantgeometrical rearrangement in the S1 excited state.All the four phenanthrimidazoles absorb at 270� 5 nm but

emit at longer wavelength (400–450 nm). This suggests theexistence of a-twist in the molecule as given in Fig. 3. The

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wavelength (nm)280275270265

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n-Hexane Dichloromethane Ethyl acetate

1,4-dioxane 1-Butanol DMSO

Benzene Ethanol Acetonitrile

Chloroform Methanol 1-Propanol


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Figure 1. Absorption spectra of phenanthrimidazoles 1–4 (1� 10�8M) in different solvents

n-Hexane Dichloromethane Ethyl acetate

1,4-dioxane 1-Butanol DMSO

Benzene Ethanol Acetonitrile

Chloroform Methanol 1-Propanol

420410400390380

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1.00

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440420400380360

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480460440420400

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(4)

Figure 2. Emission spectra of phenanthrimidazoles 1–4 (1� 10�8M) in different solvents

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correlation of the emission wavelength with the a-twist indicatesthe importance of noncoplanarity between the imidazole andthe aryl ring. This may be due to the variation of dihedral angles.The measured fluorescence quantum yields in dioxane solutionare between 0.25 and 0.36; 0.25 for 1, 0.31 for 2, 0.36 for 3,and 0.33 for 4. That is, a smaller dihedral angle will give a moreextended conjugation system and a higher fluorescent quantumyield in solution. Stronger intermolecular interaction in thesolid state increases the quenching. Therefore, it is importantto design a molecule with a suitable dihedral angle to achieve

an ideal compromise between the fluorescent quantum yieldin solution and the aggregation induced quenching in solid.As shown in Fig. 4, the lifetime of these derivatives liesbetween 2.6 and 5.8 ns. The fluorescence decays for thephenanthrimidazoles 1–4 fit satisfactorily to a biexponentialkinetics, S(t) = a1e

�t/t1 + a2e�t/t2; where ai and ti are amplitudes

and time constants of the ith (= 1, 2) exponential component.From these fitting parameters, the amplitude weighted averagetime constant, tave, can be calculated, tave = (a1t1 + a2 t2)/(a1 + a2).These fitting parameters, the average time constants, radiative(kr), and nonradiative rate (knr) constants are listed in Table 1. Fromthe table, it could be easily seen that the radiative rate constantincreases with the number of fluoro substituent.The distortion of the geometry in the excited state implies a

decrease in the resonance energy so that the fluorescence bandis bathochromically shifted to a larger extent than the absorptionband. Moreover, the loss of planarity in the excited state of thephenanthrimidazole derivatives could explain the lower fluores-cence quantum yield in apolar solvents owing to an increase inthe nonradiative processes. The nonradiative rate constants ofthe four phenanthrimidazoles in different solvents have beendeduced. Figure 5 shows a fair correlation between the knr valuesand the fluorescence wavenumbers of the phenanthrimidazoles1–4 in different solvents. This linear relationship indicates theinfluence of solvent parameters on the photophysical character-istics of phenanthrimidazoles, as have been observed forother aromatic systems.[96–99] The distorted geometry of thephenanthrimidazoles is also confirmed by potential energysurface (PES) diagram; during the calculation, all the geometricalparameters were simultaneously relaxed, whereas the C21-C22-O22-C25 torsional angles were varied in steps of 10� from0� to 360�. The PES diagrams of 1–4 as displayed in Fig. 6 show

Figure 3. Molecular modeling of phenanthrimidazoles 1–4 usingGaussian-03

Figure 4. Time resolved fluorescence decay spectra of phenanthrimidazoles 1–4



390

that the minimum energy during the rotation about C21-C22-O22-C25 torsional angles are 117.51 kcal mol�1 (1) or139.23 kcal mol�1 (2) or 43.93 kcalmol�1 (3) or �22.30 kcalmol�1 (4). In these minimum energy conformation, the tiltingangle of 2,5-dimethoxyaniline ring attached to the nitrogenatom (N1) about C5-N1-C18-C19 bond is 87.66� (1 and 2) or103.04� (3) or 98.35� (4), whereas tilting angle of aldehydic phe-nyl ring attached to the carbon atom (C2) about N3-C2-C26-C31is found to be 0.23� (1 and 2) or �49.16� (3) or �25.67� (4).

Taft and Catalan solvatochromism in thephenanthrimidazoles

The multi-parameter correlation analysis employed is linearcorrelation of a physicochemical property with several solventparameters. Figure 7a and b shows the obtained correlationbetween the absorption or fluorescence wavenumbers calcu-lated using the multi-component linear regression by employingTaft[100]and Catalan[101] solvent parameters and the experimen-tal results. Table 2 presents the coefficients of multi-parametercorrelation. The absorption and fluorescence bands of thephenanthrimidazoles 1 and 2 show that the solvatochromicshifts are associated with the solvent polarity. The H-donorcapacity or acidity of the solvent coefficient (Ca or CSA) is only

moderate and does not play any important role in absorptionas well as fluorescence. The electron releasing ability or basicityof the solvent (Cb or CSB) has a negative value. With the increaseof electron-donating ability of the solvent, the absorption as wellas fluorescence bands are shifted to lower energies. For 3 and 4,the basicity of solvent (Cb or CSB) is the lowest, and hence, thesolvent basicity does not affect the absorption and fluores-cence. The coefficient of the correlation with the H-donorcapacity or acidity of the solvent (Ca or CSA) is of negativevalue. This suggests that with increasing acidity character ofthe solvent, the absorption and fluorescence bands are shiftedto lower energies. This effect can also be interpreted in termsof the resonance stabilization of the chromophore that isshown in Fig. 8. For 1 and 2, the resonance structure (b) haspositive charge located at the nitrogen atom, and it will bestabilized in basic solvents. This resonance structure is pre-dominant in the S1 state and the stabilization of the S1 statewith the solvent basicity is more important than that of theS0 state. Consequently, the energy gap between S1 and S0states decreases. With increasing solvent basicity, the absorp-tion and fluorescence wavelengths are shifted to longer wave-lengths. Thus, the polar solvents stabilize the S0 state moreextensively than the S1 state and thereby increase the energygap between both the states and explain the solvatochromic

Table 1. Bi-exponential fitting parameter for fluorescence decay of 1–4

Compound Фf t1 (ns) a1 (%) t2 (ns) a2 (%) w2 kr knr ket� 108 s�1

1 0.25 2.6 1.46 5.2 98.54 1.03 0.05 0.14 —1+ZnO 0.22 0.12 67.84 7.6 32.16 1.23 0.03 0.12 6.842 0.31 3.3 2.74 5.3 97.26 1.05 0.06 0.13 —3 0.36 2.3 17.26 5.1 82.74 1.23 0.08 0.15 —4 0.33 2.8 68.40 5.8 41.60 1.96 0.10 0.21 —

1 2

43

Figure 5. Correlation of nonradiative deactivation rate with the fluorescence wavenumbers of phenanthrimidazoles 1–4 in different solvents

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shifts.[69] In the case of 3 and 4, because of the electrostaticinteraction between the lone pair of electrons and acidicsolvents, the resonance structure (a) will be stabilized in acidic

solvents. This resonance structure is predominant in the S1state, and the stabilization of the S1 state with the solventacidity is more important than that of the S0 state.

21

43

Figure 6. Potential energy surface of phenanthrimidazoles 1–4



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Optical properties of the phenanthrimidazoles

The absorption and PL spectra of the phenanthrimidazole mate-rials 1–4 have been measured in solid state and in solutions.Intensive emission peak ranging from 401 to 428 nm (for solids)and 390 to 453 nm (for solutions) are observed. It is apparentthat the phenanthrimidazoles have similar absorption andemission in solution, whereas they differ in their bathochromicshifts in the solid state. In CH2Cl2, all the compounds show simi-lar absorption peaks around 275 nm, which may originate fromthe aryl rings,[102] and the absorption band that appearedbetween 325 and 330 nm is assigned to the p–p* electronic tran-sition in the phenanthrimidazoles. The observation that theabsorption and the emission spectra of the phenanthrimidazolematerials 1–4 shows relatively small differences in solutions, andsolid state suggests that the bulky and noncoplanar parts ofthese compounds have successfully restrained intermolecularaggregation. It can also be correlated to the dihedral parameters

of the molecules as shown in Table 3 and optimized geometriesof 1–4 are given in Fig. 3. Their solid state PL spectra are redshifted by about 30 to 40 nm with respect to those in solution.The larger red shifts can be explained by their stronger p–pstacking interactions of the aryl ring.

The electrochemical properties of the phenanthrimidazoles(1–4) have been examined by cyclic voltammetry, and the redoxpotentials have been measured from the plot of potential versuscurrent that is shown in Fig. 9. The energies of the HOMO andLUMO have been calculated using the relation, EHOMO = 4.4 +E½oxi; ELUMO = EHOMO� 1239/lonset, and the calculated values are

given in Table 4. The LUMO energies have been deduced fromthe HOMO energies and the lowest-energy absorption edges ofthe UV–Vis absorption spectra.[103,104] The calculated energygap (Eg = EHOMO� ELUMO) of the phenanthrimidazoles 1–4 are3.33, 3.36, 3.38, and 3.40 eV. Therefore, the HOMO stability andthe emission energy gap are controlled by the nature andsubstituent present in the phenanthrimidazole moiety. The

1 1

2 2

a

a

b

b

3 3

4 4

a

a

b

b

Figure 7. a Taft correlation of the observed absorption (a) and fluorescence wavenumber (b) with the predicted values of phenanthrimidazoles 1 and2. b. Taft correlation of the observed absorption (a) and fluorescence wavenumber (b) with the predicted values of phenanthrimidazoles 3 and 4

MATERIALS SCIENCE


393

Table

2.Adjustedcoefficien

ts((υ

x)0,p

*,c a

andc b)for

themultilinearregression

analysisof

theab

sorptio

nυ a

bsan

dfluo

rescen

ceυ e

miwaven

umbe

rsan

dstok

esshift

(Δυ s

s)of

1–4

with

thesolven

tpo

larity/po

larizab

ility

andtheacid

andba

secapa

city

usingtheTaft(p*,a,

andb)

andtheCatalan

(SPP

N,SAan

dSB

)scales

Com

poud

(υx)

(υx)0cm

�1

(p*)

c ac b

1l a

bs

(3.63�0.01

3)�10

4(0.72�2.47

3)�10

3(1.28�7.96

2)�10

3�(

2.45

6�6.23

5)�10

3

l emi

(2.56�0.02

6)�10

4�(

1.14

�4.74

3)�10

3�(

10.71�5.26

8)�10

3(9.76�11

.957

)�10

3

Δυ s

s=υ a

bs�υ e

mi

(1.06�0.03

0)�10

4(0.51�5.41

6)�10

3(8.28�17

.434

)�10

3�(

8.86

�13

.654

)�10

3

(υx)

(υx)0cm

�1

c SPP

Nc S

Ac S

B

l abs

(3.64�0.03

6)�10

4�(

1.47

�4.14

7)�10

3(6.67�10

.987

)�10

3�(

5.99

�8.22

9)�10

3

l emi

(2.58�0.07

3)�10

4�(

0.25

�8.25

4)�10

3�(

7.78

�21

.868

)�10

3(8.78�16

.379

)�10

3

Δυ s

s=υ a

bs�υ e

mi

(1.06�0.08

2)�10

4�(

1.23

�9.33

9)�10

3(14.47

�24

.740

)�10

3�(

14.77�18

.530

�10

3

2(υ

x)(υ

x)0cm

�1

(p*)

ca

cb

l abs

(3.63�0.01

3)�10

4(0.83�2.41

9)�10

3(1.92�7.78

6)�10

3�(

3.26

�6.09

7)�10

3

l emi

(2.58�0.02

5)�10

4�(

0.12

�4.68

0)�10

3�(

7.32

�15

.064

)�10

3(7.39�11

.797

)�10

3

Δυ s

s=υ a

bs�υ e

mi

(1.04�0.03

2)�10

4(0.85�5.87

8)�10

3(9.25�18

.921

)�10

3�(

10.65�14

.818

)�10

3

(υx)

(υx)0cm

�1

c SPP

Nc S

Ac S

B

l abs

(3.64�0.03

6)�10

4�(

1.13

�4.13

4)�10

3(7.84�10

.951

)�10

3�(

8.24

�8.20

2)�10

3

l emi

(2.61�0.07

1)�10

4�(

2.16

�7.95

5)�10

3�(

3.06

�4.07

5)�10

3(5.59�15

.785

)�10

3

Δυ s

s=υ a

bs�υ e

mi

(1.03�0.08

8)�10

4(1.04�9.96

8)�10

3(10.91

�26

.408

)�10

3�(

13.83�19

.779

)�10

3

3(υ

x)(υ

x)0cm

�1

(p*)

ca

cb

l abs

(3.72�0.03

4)�10

4(5.54�6.20

1)�10

3�(

9.84

�19

.958

)�10

3(2.75�15

.630

)�10

3

l emi

(2.52�0.04

7)�10

4�(

3.74

�8.59

1)�10

3(3.48�37

.653

)�10

3(0.72�21

.656

)�10

3

Δυ s

s=υ a

bs�υ e

mi

(1.20�0.06

9)�10

4(9.26�12

.542

)�10

3�(

13.29�40

.369

)�10

3(1.83�31

.615

)�10

3

(υx)

(υx)0cm

�1

c SPP

Nc S

Ac S

B

l abs

(3.68�0.01

1)�10

4(4.18�11

.392

)�10

3�(

3.02

�30

.179

)�10

3�(

4.34

�22

.604

)�10

3

l emi

(2.62�0.01

2)�10

4�(

12.13�13

.084

)�10

3(20.77

�34

.661

)�10

3�(

9.10

�25

.961

)�10

3

Δυ s

s=υ a

bs�υ e

mi

(1.06�0.01

8)�10

4(16.29

�20

.446

)�10

3�(

23.76�54

.163

)�10

3(4.74�40

.568

)�10

3

4(υ

x)(υ

x)0cm

�1

(p*)

ca

cb

l abs

(3.74�0.01

4)�10

4(2.63�2.47

2)�10

3�(

4.77

�7.95

7)�10

3(2.35�6.23

2)�10

3

l emi

(2.23�0.04

2)�10

4�(

6.58

�7.73

9)�10

3(19.3�24

.908

)�10

3�(

13.31�19

.507

)�10

3

Δυ s

s=υ a

bs�υ e

mi

(1.51�0.03

2)�10

4(9.22�5.91

4)�10

3�(

24.06�19

.036

)�10

3(15.65

�14

.908

)�10

3

(υx)

(υx)0cm

�1

c SPP

Nc S

Ac S

B

l abs

(3.73�0.03

7)�10

4(1.54�4.23

5)�10

3�(

91.75�11

.220

)�10

3�(

1.99

�8.40

4)�10

3

l emi

(2.34�0.10

9)�10

4�(

15.67�12

.338

)x103

(40.86

�32

.685

)x103

�(28

.81�24

.481

)x103

Δυ s

s=υ a

bs�υ e

mi

(1.38�0.08

7)�10

4(17.21

�9.83

9)�10

3�(

40.74�26

.066

)�10

3(26.79

�19

.523

)�10

3



394

decreased HOMO–LUMO energy gap is attributed to the moreplanar configuration between the aryl ring and the imidazolering in 1–4. On the other hand, the lone electron pair in thenitrogen atom may undergo an n–p* transition with lowerenergy absorption with the more coplanar structure.[105,106] Thisindicates that conjugation of the aryl ring to the imidazole ringplays a key role in controlling the HOMO energy of the molecule.With the shallow HOMO, it can effectively lower the hole-injection barrier from the 4,4-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB) layer (i.e., HOMO=�5.4 eV). Furthermore, theshallow HOMOs of the phenanthrimidazoles also imply that it ismore difficult for the holes to leak into the EML. All thesephenanthrimidazoles have HOMO levels similar or shallower thanthe classical hole transport/injection materials[107–109] facilitatingthe hole injection/transport from the hole transport layer.Furthermore, the much shallower HOMO level than those of theclassical electron transport materials[110,111] effectively preventsthe leakage of holes into the EML. With these advantages, thedevices based on these materials are expected to show loweronset and operation voltages compared with devices with recentlyreported emitters.The electron density distribution has been studied using the

frontier molecular orbitals (FMOs) by density function theory(DFT) analysis. The 3D plots of the frontier orbitals HOMO andLUMO for the phenanthrimidazoles 1–4 are shown in Fig. 10.The HOMO orbital acts as an electron donor, and the LUMOorbital acts as an acceptor. In compound 1, the HOMO is locatedon the imidazole ring and partly on the phenanthroline andaldehydic phenyl rings, whereas the LUMO is located partly on

the phenanthroline ring and on the carbon atoms (C2, C4, C25,C26, C28, and C30) of the imidazole and aldehydic phenyl rings.In compound 2, whereas the HOMO is on the imidazole ringand partly on the phenanthroline and aldehydic phenyl rings,the LUMO is located partly on the phenanthroline ring and onthe carbon atoms (C2 and C4) of the imidazole ring. In com-pound 3, the HOMO is on the imidazole ring, partly on thephenanthroline and aldehydic phenyl rings, and the LUMO ispartly on the phenanthroline ring and on the carbon atoms(C2, C4 and C5) of the imidazole ring. In compound 4, whereasthe HOMO is on the imidazole ring, partly on the phenanthrolineand aldehydic phenyl rings, the LUMO is on the phenanthrolinering and on the nitrogen, carbon atoms (N1, C2, N3, C4, C25, C26,C28, and C30) of the imidazole and aldehydic phenyl ring. TheHOMO! LUMO transition implies that intramolecular chargetransfer takes place within the molecule.[112] The energy gap(Eg) of these phenanthrimidazoles has been calculated fromthe HOMO and LUMO energy levels. The energy gap explainsthe probable charge transfer (CD) inside the chromophores.The potential barrier (PB) and potential drop (PD) computedusing the theoretically deduced HOMO–LUMO energies havebeen discussed in detail in the later section.

Table 3. Dihedral angles, electric dipole moment (m in Debye), polarizability (a in 10�24 e.s.u) and hyperpolarisability (btot in10�32 e.s.u) of 1–4

Compound Anglea (�) Angleb (�) Anglec (�) mtotal atot btot

1 99.38 151.93 �2.64 7.04 26.28 237.422 99.45 152.67 �2.89 6.03 27.58 212.553 103.04 131.98 �3.17 6.98 28.16 230.984 99.05 153.85 �3.15 5.48 30.38 281.16aDihedral angle between the imidazole ring and 3,4-dimethoxyphenyl ring; bdihedral angle between the imidazole ring and arylring; cdihedral angle between the phenyl ring and aryl ring.

N N

N N

N N

N N

R1

a b

4, R1 =CF3; R2=H3, R1 = F; R2=F2, R1 = F; R2=H1, R1 = R2 =H

H3CO

H3CO

H3CO

H3CO

R2

R1

R2

Figure 8. Resonance structures of the phenanthrimidazole chromo-phores 1–4

Potential / V-0.4 -0.6 -0.8 -1.0 -1.2

Cur

rent

/ e-6

A

1

2

3

4

Figure 9. Cyclic voltammogram of phenanthrimidazoles 1–4

MATERIALS SCIENCE


395

Thermal analysis

The thermal properties of the phenanthrimidazoles 1–4 havebeen investigated by DSC and thermogravimetric analyses undernitrogen atmosphere. All the phenanthrimidazoles exhibit goodthermal stability as displayed by Fig. 11. Decomposition temper-atures (Td), defined as the temperature at which the materialshowed a 5% weight loss, have been measured to be 310, 335,320, and 315 �C for compounds 1, 2, 3, and 4, respectively

(Table 4). There appears to be a glass transition temperature (Tg)in the range of 98 to 107 �C. The melting points (Tm) of com-pounds 1, 2, 3, and 4 measured by DSC examinations are 248,242, 237, and 228 �C, respectively. The high Tm and Td valuesindicate that the compounds 1–4 are thermally stable and areable to undergo the vacuum thermal sublimation process. There-fore, these derivatives could be used in EL devices because thehigh Tm and Tg values improve the lifetime of the devices.[113]

Electroluminescence and device studies

The electroluminescence properties have been investigated byusing these materials as an organic buffer layer. A series ofdevices (I, II, III, and IV) with NPB as the hole-transporting layer,Alq3 as the electron-transporting layer, and the EML have beenfabricated. Compounds 1–4 as films were the buffer layers in

Table 4. Photophysical, thermal, and electrochemical data of 1–4

t (ns)

Compound labsa (nm) lemi

a (nm) Фfb t1 t2 labs

c (nm) lemi (nm) Tg/Tm/Td (�C) HOMOd (eV) LUMOe (eV)

1 275, 322 392 0.28 2.6 5.2 272, 328, 384 428 98/248/ 310 �5.39 �2.062 275, 322 390 0.31 3.3 5.3 278, 330, 385 421 103/242/335 �5.41 �2.053 267, 327 418 0.36 2.3 5.1 272, 334, 391 417 106/237/320 �5.45 �2.074 266, 331 453 0.33 2.8 5.8 271, 337, 396 445 110/228/315 �5.48 �2.08aMeasured in CH2Cl2 solvent at room temperature. bMeasured in acetonitrile using coumarin 47 in ethanol as a reference. cSolidstate thin-film samples on quartz substrate. dBy measuring ionization potential (IP) with cyclic voltammetry studies.eLUMO=HOMO+Egap.

Compound HOMO LUMO

1

2

3

4

Figure 10. Frontier molecular orbitals (highest occupied molecular orbitaland lowest unoccupied molecular orbital) of phenanthrimidazoles 1–4

50 100 150 200 250 300 350 400 450 500

1234

75

80

85

90

95

100

Temperature/ °C

Temperature/ °C

TG% DTA / (mW/mg)

Exo

50 100 150 200 250

2

DSCmW%

300

1

3

4

End

othe

rmic

Figure 11. Thermogravimetric-differential thermal analysis (TG-DTA)(upper) and differential scanning calorimetric (bottom) curves ofphenanthrimidazoles 1–4 at a heating rate of 20 �Cmin�1



396

devices I, II, III, and IV, respectively. For comparison, the refer-ence device (device V) is also fabricated. The current density–brightness–voltage and the luminous efficiency are listed inTable 5. The devices exhibit emission between 400–430 nm,and Fig. 12 displays the same. The emission is independentof the applied voltages in the range of 3 to 16 V. From thedevice study, it is observed that all the compounds showhigher brightness and current efficiency at low voltage. From theelectrochemical data, the HOMO energy level of compounds 1–4is obtained as ca. �5.6 eV, and this is in between that of NPB(�5.4 eV) and Alq3 (�6.0 eV) that could regulate the hole injection.Thus, the synthesized series of phenanthrimidazole derivatives canprevent excessive holes to enter the EML increasing the efficiency.Comparison of HOMO values of already reported emitters[114–117]

with phenanthrimidazole derivatives 1–4 shows that thesecompounds have HOMO levels similar or shallower than the classi-cal hole transport/injection materials. Therefore, the EL studiesshow an alternative idea to design novel hole transport materialsand also provided a convenient way to improve the performanceof devices by inserting a suitable buffer layer.

DFT analysis

The molecular geometry of 1–4 has been further examined byDFT calculation [DFT/B3LYP/6-31G(d,p)]. Figure 3 shows theoptimized structures of the phenanthrimidazoles 1–4. NBOanalysis has been performed for the phenanthrimidazoles at theDFT/B3LYP/6-31G (d,p) level in order to elucidate the intramolecu-lar charge transfer, within the molecule. Several donor–acceptorinteractions are observed in these phenanthrimidazoles (1–4),and among the strongly occupied NBOs, the most important delo-calization sites are in the p system and in the lone pairs (n) of theoxygen, fluorine, and nitrogen atoms as displayed in Table 6. Thes system shows some contribution to the delocalization, and theimportant contributions to the delocalization that correspond tothe donor–acceptor interactions in the phenanthrimidazoles 1–4are C6–C11!C12–C17, N13–C14!C12–C17, N13–C14!C15–C16, N3–C2!C4–C5, N3–C2!C26–C27, LpN1!N3–C2, C28–C29!C30–C31.The charge distribution has been calculated fromtheMulliken atomic charges by NBO that reveal that among the ni-trogen atoms (N1 and N3), oxygen and fluorine atom, N1 is consid-ered as more basic site. Further, compared with all the nitrogenatoms (N1, N3, N10, and N13), oxygen atoms are less electronega-tive.[118] This shows that more negative charge is concentrated onN1 atom in all these phenanthrimidazoles. The percentage of s andp-character in each NBO natural atomic hybrid orbital for thephenanthrimidazoles 1–4 are displayed in Table 7. Analysis ofHOMO–LUMO and NBO explains the charge transfer within themolecule. Therefore, it is of interest to test their hyperpolarizabilityand to correlate with absorption. The microscopic nonlinearresponses in 1–4 have been obtained by DFT calculation. FromTable 8, it is suggested that the phenanthrimidazoles (1–4) are polarhaving nonzero dipole moment and nonzero hyperpolarizabilitiesand hence have good microscopic NLO behavior.[119] The largermbo values are attributed to the positive contribution of theirconjugation. The hyperpolarizability is a strong function of theabsorption maximum. Because even a small absorption at theoperating wavelength of optic devices can be detrimental, it isimportant to make NLO chromophores as transparent as possi-ble without compromising the molecule’s nonlinearity. Thecomputed b values may be correlated with UV-visible spectro-scopic data in order to understand the molecular structureand NLO relationship in view of a future optimization of themicroscopic NLO properties. The phenanthrimidazoles 1–4possess more appropriate ratio of off-diagonal versus diagonalb tensorial component (r = bxyy/bxxx) that reflects the inplanenonlinearity anisotropy (r= bxyy/bxxx) [r= 0.357 (1), 0.047 (2),0.125 (3), and 0.159 (4)] and the largest mbo values. The b-tensorcan be decomposed in to a sum of dipolar 2D

J¼1b� �

and octupolar2DJ¼3b� �

tensorial components, and the ratio of these two compo-nents strongly depends on their ‘r’ ratios. The zone for r> r2 andr< r1 corresponds to a molecule of octupolar and dipolar,respectively. The critical values for r1 and r2 are (1� √3)/√3√3 + 1) =�0.16 and (√3 + 1)/√3(√3� 1) = 2.15, respectively.

The parameter r2D, [r2D] =2DJ¼3bk k2DJ¼1bk k

� �is convenient to compare

the relative magnitudes of the octupolar and dipolar compo-nents of b. The observed positive small r2D values [11.63 (1),3.54 (2), 3.98 (3), 6.72 (4)] reveal that the biii component cannotbe zero, and these are dipolar component. Because most of thepractical applications for second-order NLO chromophores arebased on their dipolar components, this strategy is more appro-priate for designing highly efficient NLO chromophores.

Table 5. Devices I, II, III, and IV based on phenanthrimidazoles1, 2, 3, and 4 as buffer layer with comparison device V (NPBas HTL)

Device EL (nm) L (cd A�1)a V (V)b �c (cd A�1) CIE (x,y)@ 9 V

I 435 4.7 34870 5.87 (0.35, 0.54)II 431 4.1 35630 4.35 (0.33, 0.52)III 423 3.7 32450 4.07 (0.32, 0.53)IV 452 3.2 30780 4.45 (0.33, 0.52)V 520 2.7 37360 3.22 (0.32, 0.52)

HTL, hole-transporting layer. aThe brightness (L), currentefficiency (�c), bvoltage (V) required for 1 cdm-2; I, II, III, andIV-[ITO/NPB (10 nm)/buffer layer (30 nm)/Alq3 (60 nm)/LiF(1 nm)/Al]; V-[ITO/NPB (40 nm)/Alq3 (60 nm)/LiF/Al].

1.00

1234

0

0.25

0.50

0.75

300 350 400 450 500 550

wavelength (nm)

Nor

mal

ized

Int

ensi

ty (

a.u)

NPB

Figure 12. Electroluminescence spectra of devices I–IV (buffer layer 1–4)with comparison device V (NPB as hole-transporting layer)

MATERIALS SCIENCE


397

Table

6.Sign

ificant

dono

r–acceptor

interactions

of1–

4an

dtheirsecond

-order

perturba

tionen

ergies

(kcal/m

ol)

12

Don

or(i)

ED/e

Accep

tor(j)

ED/e

E(2)

a.u

E(j)–

E(i)

F(i,j)

Don

or(i)

ED/e

Accep

tor(j)

ED/e

E(2)

a.u

E(j)–

E(i)

F(i,j)

C7–

C8

1.69

83C9–

N10

0.33

0624

.08

0.28

0.07

4C7–

C8

1.69

83C9–

N10

0.33

0124

.06

0.28

0.07

4C6–

C11

1.56

27C7–

C8

0.27

7520

.01

0.28

0.06

9C6–

C11

1.56

28C7–

C8

0.27

7319

.99

0.28

0.06

9C9–

N10

1.74

37C6–

C11

0.43

3622

.82

0.32

0.07

9C9–

N10

1.74

34C6–

C11

0.43

3622

.85

0.32

0.08

0C15

–C16

1.69

19C14

–N13

0.32

3724

.63

0.28

0.07

4C12

–C17

1.57

09C4–

C5

0.40

8918

.04

0.28

0.06

3C14

–N13

1.75

22C12

–C17

0.41

1021

.77

0.33

0.07

8C12

–C17

1.57

09C15

–C16

0.26

2418

.96

0.28

0.06

8N3–

C2

1.83

18C4–

C5

0.40

9622

.12

0.34

0.08

2C15

–C16

1.69

18C14

–N13

0.32

3424

.62

0.28

0.07

4C18

–C19

1.71

51C20

–C21

0.41

9322

.94

0.28

0.07

4C14

–N13

1.75

19C12

–C17

0.41

1021

.80

0.33

0.07

8C22

–C23

1.64

47C18

–C19

0.38

9124

.93

0.28

0.07

6N3–

C2

1.83

32C4–

C5

0.40

8922

.01

0.34

0.08

2C20

–C21

1.68

45C22

–C23

0.40

3324

.86

0.29

0.07

7C18

–C19

1.71

61C20

–C21

0.41

8822

.84

0.28

0.07

4C26

–C27

1.64

48C30

–C31

0.30

2219

.24

0.29

0.06

7C22

–C23

1.64

45C18

–C19

0.38

9824

.98

0.28

0.07

6C26

–C27

1.64

48C28

–C29

0.33

2819

.86

0.28

0.06

7C20

–C21

1.68

40C22

–C23

0.40

3824

.93

0.29

0.07

7C26

–C27

1.64

48C30

–C31

0.30

2219

.24

0.29

0.06

7C26

–C27

1.64

81C30

–C31

0.31

2421

.08

0.28

0.07

0C26

–C27

1.64

48C28

–C29

0.33

2819

.86

0.28

0.06

7C26

–C27

1.64

81C28

–C29

0.37

7418

.98

0.27

0.06

5C30

–C31

1.66

48C26

–C27

0.37

7419

.20

0.28

0.06

6C30

–C31

1.68

42C28

–C29

0.37

7423

.56

0.28

0.07

3C30

–C31

1.66

48C28

–C29

0.33

2820

.86

0.28

0.06

8C28

–C29

1.65

44C26

–C27

0.38

3121

.32

0.30

0.07

2C28

–C29

1.66

02C26

–C27

0.37

7420

.76

0.28

0.06

9Lp

N1

1.59

07C4–

C5

0.40

8932

.50

0.30

0.08

8C28

–C29

1.66

02C30

–C31

0.30

2218

.96

0.29

0.06

6Lp

N1

1.59

07N3–

C2

0.39

1847

.04

0.28

0.10

3Lp

N1

1.58

99C4–

C5

0.40

9632

.63

0.30

0.08

9Lp

O22

1.83

42C22

–C23

0.40

3829

.77

0.33

0.09

5Lp

N1

1.58

99N3–

C2

0.38

9146

.77

0.28

0.10

2Lp

O20

1.83

24C20

–C21

0.41

8833

.44

0.33

0.10

0Lp

O22

1.83

46C22

–C23

0.40

3329

.64

0.34

0.09

5Lp

F32

1.91

52C28

–C29

0.37

7420

.38

0.42

0.09

0Lp

O20

1.83

30C20

–C21

0.41

9333

.30

0.33

0.10

0C6–

C11

0.43

36C12

–C17

0.41

1023

1.98

0.01

0.07

4C6–

C11

0.43

36C12

–C17

0.41

1023

0.76

0.01

0.07

4C14

–N13

0.32

34C12

–C17

0.41

1021

1.85

0.01

0.07

9C14

–N13

0.32

37C12

–C17

0.41

1020

8.96

0.01

0.07

9C14

–N13

0.32

34C15

–C16

0.26

2418

9.80

0.01

0.08

2C14

–N13

0.32

37C15

–C16

0.26

2618

9.36

0.01

0.08

2N3–

C2

0.39

18C4–

C5

0.40

8978

.59

0.02

0.06

1N3–

C2

0.38

91C4–

C5

0.40

9682

.23

0.02

0.06

2N3–

C2

0.39

18C26

–C27

0.38

3113

3.98

0.01

0.06

4N3–

C2

0.38

91C26

–C27

0.37

7410

0.49

0.02

0.06

4C28

–C29

0.37

74C30

–C31

0.31

2428

2.83

0.01

0.08

23

4Don

or(i)

ED/e

Accep

tor(j)

ED/e

E(2)

a.u

E(j)–

E(i)

F(i,j)

Don

or(i)

ED/e

Accep

tor(j)

ED/e

E(2)

a.u

E(j)–

E(i)

F(i,j)

C7–

C8

1.69

78C6–

C11

0.43

1916

.18

0.29

0.06

3C7–

C8

1.69

66C9–

N10

0.32

8924

.18

0.28

0.07

4C7–

C8

1.69

78C9–

N10

0.32

9424

.09

0.28

0.07

4C6–

C11

1.56

36C7–

C8

0.27

6420

.01

0.28

0.06

9C6–

C11

1.56

36C7–

C8

0.27

7620

.05

0.28

0.06

9C9–

N10

1.74

18C6–

C11

0.43

2223

.04

0.32

0.08

0C6–

C11

1.56

36C9–

N10

0.32

9416

.32

0.27

0.06

0C12

–C17

1.57

20C4–

C5

0.40

9518

.09

0.27

0.06

3C6–

C11

1.56

36C4–

C5

0.40

8417

.86

0.28

0.06

3C12

–C17

1.57

20C15

–C16

0.26

2018

.94

0.28

0.06

8C9–

N10

1.74

34C6–

C11

0.43

1922

.86

0.32

0.08

0C15

–C16

1.69

04C14

–N13

0.32

2724

.67

0.28

0.07

4C12

–C17

1.57

12C4–

C5

0.40

8417

.98

0.27

0.06

3C14

–N13

1.75

05C12

–C17

0.41

0131

.95

0.32

0.07

9C12

–C17

1.57

12C15

–C16

0.26

1418

.87

0.28

0.06

8N3–

C2

1.82

98C4–

C5

0.40

9521

.85

0.34

0.08

2C15

–C16

1.69

06C12

–C17

0.41

0417

.08

0.29

0.06

5C18

–C19

1.71

81C20

–C21

0.41

7822

.66

0.28

0.07

4C15

–C16

1.69

06C14

–N13

0.32

3624

.71

0.28

0.07

4C22

–C23

1.64

35C18

–C19

0.39

1425

.08

0.28

0.07

6

(Con

tinues)



398

From the absorption analysis, it can be seen that thephenanthrimidazoles absorb around 280 nm (ΔE ~ 1.99 eV), andat the same time, oscillator strength for absorption is very strong.From the analysis of m and Δm, it can be seen that both the valuesare high, and this indicates that the phenanthrimidazoles arepolar and also confirmed charge transfer during excitation. Thenature of charge transfer of the phenanthrimidazoles is analyzedfrom the FMOs participating in the process. Analysis shows thatthe excitation of HOMO to LUMO is associated with the intramo-lecular charge transfer within the molecule. Dipole momentanalysis of the b0 indicates its dipolar nature, and the dipole isalmost unidirectional with little contribution from my and mzbecause of the asymmetric nature of the molecule. Similar typesof situation can also be visualized from the vectorial componentsof b, which show that the maximum contribution to the totalb0 of the molecule is from the x-direction. In view of this, direc-tional hyperpolarizability is calculated from the individualtensorial components of b using Eqns 1 and 2. Reasonably goodvalue of b0 is then explained in the light of two-state model,b0a fΔm/ΔE3, where, f is the oscillator strength of ground toexcited state transition, Δm= me�mg is the state dipole differencebetween the excited and ground state, and ΔE is the excitationenergy for ground to first excited state. Fitting the spectroscopicparameters for phenanthrimidazoles f, Δm, and ΔE in the afore-mentioned equation, large value of b0 can be well explained.Analyzing all the values, it can be concluded that the large Δmand strong oscillator strength for the phenanthrimidazoles aremainly responsible for its large hyperpolarizability (Table 8).

The electron affinity for an N-electron system is equal to thenegative of the LUMO energy, and it is a measure of susceptibilityof molecule towards attack by nucleophiles.[120] The globalhardness is defined as � =½ (d2E/dN2) V(r) or ½ (EHOMO� ELUMO).Electronegativity of the phenanthrimidazoles is calculated usingthe equation: m=�w=�(dE/dN) V(r), where E is the energy, N isthe number of electrons, and V(r) is the constant external potential.By combining the aforementioned equation with the work ofIckowski and Margrave,[121] assuming a quadrate relationshipbetween E and N and in a finite difference approximation,the equation can be rewritten as, w=�m= (I + A)/2; wKoopaman’s =(EHOMO+ ELUMO)/2. The electrophilicity index of a molecule canbe calculated by the equation, o=m2/2�, which can measure thecapacity of electrophile to accept themaximal number of electronsin a neighboring reservoir of Electron Sea. By using the aforemen-tioned equations, the calculated chemical potential (m) are�0.2780 (1), �0.2590 (2), �0.2760 (3), and �0.2775 (4). The hard-ness (�) are �0.0510 (1), �0.0510 (2), �0.0500 (3), and �0.0510(4), and the electrophilicity index (o) are �0.7576 (1), �0.6576(2), �0.7618 (3), and �0.7550 (4).

Phenanthrimidazoles versus electron transport properties

Delocalized nature of the FMOs can be attributed to the planarstructure of the molecule that probably facilitates the easierdelocalization of the orbitals. According to hypothesis, in anelectronic circuit Fermi level of the electrode contact lies approx-imately in the middle of the HLG (HOMO–LUMO Gap).[122,123]

Now in a sufficient applied voltage (the applied voltage mustbe sufficient enough to raise the energy of electron in the Fermilevel of contact to that of the LUMO) one electron will beinjected from the cathode contact to the LUMO of the molecule.Because of the delocalized nature of the vacant LUMO, theinjected electron in the LUMO can travel through the molecule

Table

6.(Con

tinued)

12

Don

or(i)

ED/e

Accep

tor(j)

ED/e

E(2)

a.u

E(j)–

E(i)

F(i,j)

Don

or(i)

ED/e

Accep

tor(j)

ED/e

E(2)

a.u

E(j)–

E(i)

F(i,j)

C14

–N13

1.75

19C12

–C17

0.41

0421

.81

0.32

0.07

8C20

–C21

1.68

30C22

–C23

0.40

4225

.00

0.28

0.07

7N3–

C2

1.84

33C4–

C5

0.40

8421

.29

0.34

0.08

1C26

–C27

1.63

06C30

–C31

0.28

7018

.39

0.29

0.06

6C18

–N19

1.71

56C20

–C21

0.41

9022

.88

0.28

0.07

4C26

–C27

1.63

06C28

–C29

0.36

8621

.91

0.28

0.07

0C22

–C23

1.64

51C18

–C19

0.39

0025

.03

0.28

0.07

6C30

–C31

1.65

62C26

–C27

0.36

9819

.74

0.28

0.06

7C20

–C21

1.68

37C22

–C23

0.40

3924

.97

0.29

0.07

7C30

–C31

1.65

62C28

–C29

0.36

8620

.68

0.28

0.06

8C26

–C31

1.64

08C27

–C28

0.33

8823

.16

0.29

0.07

3C28

–C29

1.66

20C26

–C27

0.36

9819

.12

0.29

0.06

7C27

–C28

1.69

15C29

–C30

0.39

0524

.77

0.27

0.07

5C28

–C29

1.66

20C30

–C31

0.28

7019

.45

0.29

0.06

8C29

–C30

1.67

14C26

–C31

0.41

6923

.90

0.39

0.07

6Lp

N1

1.58

64C4–

C5

0.40

9532

.83

0.30

0.08

9Lp

O22

1.83

48C22

–C23

0.40

3929

.70

0.34

0.09

5Lp

N1

1.58

64N3–

C2

0.39

4746

.91

0.28

0.10

2Lp

O20

1.83

27C20

–C21

0.41

9033

.37

0.33

0.10

0Lp

O22

1.83

36C22

–C23

0.40

4229

.93

0.33

0.09

5Lp

F32

1.91

36C29

–C30

0.39

0521

.11

0.42

0.09

1Lp

O20

1.83

16C20

–C21

0.41

7833

.63

0.33

0.10

0Lp

F33

1.90

96C26

–C31

0.41

6921

.76

0.42

0.09

3C6–

C11

0.43

22C12

–C17

0.41

0122

8.12

0.01

0.07

4C6–

C11

0.43

19C12

–C17

0.41

0422

6.38

0.01

0.07

4C14

–N13

0.32

27C12

–C17

0.41

0121

8.21

0.01

0.07

9C14

–N13

0.32

36C12

–C17

0.41

0421

3.52

0.01

0.07

9C14

–N13

0.32

27C15

–C16

0.26

2019

0.80

0.01

0.08

2C14

–N13

0.32

36C15

–C16

0.26

1418

3.03

0.01

0.08

2N3–

C2

0.39

47C4–

C5

0.40

9577

.84

0.02

0.06

2N3–

C2

0.37

48C4–

C5

0.40

8483

.46

0.02

0.06

2N3–

C2

0.39

47C26

–C27

0.36

9813

2.53

0.02

0.06

5

MATERIALS SCIENCE


399

Table 7. Percentage of s and p-character on each natural atomic hybrid of the natural bond orbital of 1–4

1 2

Bond (A–B) ED/energy(a.u.)

EDA% EDB% s% p% Bond (A–B) ED/energy(a.u.)

EDA% EDB% s% p%

N1–C2 0.7986 63.77 36.23 33.01 66.96 N1–C2 0.7982 63.72 36.28 32.97 29.370.6019 — — 29.31 70.58 0.6023 — — 29.37 70.51

C2–N3 .07645 58.44 41.56 35.33 64.45 C2–N3 0.7643 58.41 41.59 35.30 64.480.6447 — — 32.81 67.15 0.6449 — — 32.84 67.11

N3–C4 0.6473 41.90 58.10 29.59 70.36 N3–C4 0.6471 41.87 58.13 29.57 70.380.7622 — — 32.84 66.96 0.7624 — — 32.87 66.93

C4–C5 0.7139 50.96 49.04 34.18 65.78 C4–C5 0.7139 50.96 49.04 34.20 65.760.7003 — — 34.10 65.86 0.7003 — — 34.10 65.86

N1–C5 0.6105 37.28 62.72 27.33 72.57 N1–C5 0.6103 37.25 62.75 27.30 72.600.7920 — — 32.74 67.23 0.7921 — — 32.77 67.20

N1–C18 0.7911 62.58 37.42 34.16 65.81 N1–C18 0.7911 62.58 37.42 34.16 65.81.6117 — — 26.26 73.63 0.6117 - — 26.25 73.64

C2–C26 0.7063 49.89 50.11 37.84 62.12 C2–C26 0.7053 49.74 50.26 37.74 62.220.7079 — — 31.07 68.89 0.7090 — — 31.38 68.58

N10–C11 0.6459 41.71 58.29 30.67 69.28 N10–C11 0.6459 41.72 58.28 30.68 69.270.7635 — — 35.46 64.37 0.7634 — — 35.45 64.37

N13–C12 0.6467 41.83 58.17 30.72 69.23 C29–F32 0.5230 27.35 72.65 22.15 77.540.7627 — — 35.21 64.62 0.8523 — — 30.68 69.24

C22–O22 0.5725 32.78 67.22 25.06 74.73 C22–O22 0.5726 32.79 67.21 25.07 74.720.8199 — — 33.35 66.58 0.8198 — — 33.36 66.57

O22–C25 0.8256 68.16 31.84 28.42 71.52 O22–C25 0.8257 68.18 31.82 28.41 71.520.5643 — — 20.88 78.85 0.5641 — — 20.86 78.87

C26–C31 0.7186 51.64 48.36 33.86 66.11 C26–C31 0.7175 51.48 48.52 33.67 66.300.6954 — — 34.11 65.81 0.6966 — — 34.44 65.52

3 4Bond (A–B) ED/energy

(a.u.)EDA% EDB% s% p% Bond (A–B) ED/energy

(a.u.)EDA% EDB% s% p%

N1–C2 0.7985 63.76 36.24 32.67 67.30 N1–C2 0.7980 63.68 36.32 32.90 67.070.6020 — — 29.35 70.53 0.6026 — — 29.47 70.41

C2–N3 0.7634 58.27 41.73 35.49 64.29 C2–N3 0.7643 58.41 41.59 35.23 64.550.6460 — — 33.57 66.38 0.6449 — — 32.82 67.13

N3–C4 0.6478 41.97 58.03 29.54 70.41 N3–C4 0.6467 41.82 58.18 29.56 70.390.7618 — — 32.37 67.43 0.7628 — — 32.97 66.83

C4–C5 0.7142 51.01 48.99 34.23 65.73 C4–C5 0.7139 50.97 49.03 34.16 65.800.6999 — — 34.09 65.87 0.7002 — — 34.07 65.88

N1–C5 0.6100 37.21 62.79 27.29 72.61 N1–C5 0.6102 37.24 62.76 27.35 72.550.7924 — — 33.02 66.95 0.7922 — — 32.82 67.15

N1–C18 0.7915 62.65 37.35 34.25 65.72 N1–C18 0.7916 62.66 37.34 34.17 65.800.6111 — — 26.23 73.66 0.6111 — — 26.16 73.74

C2–C26 0.7011 49.16 50.84 37.02 62.94 C2–C26 0.7049 49.69 50.31 37.65 62.310.7130 — — 32.04 67.92 0.7093 — — 31.28 68.68

N10–C11 0.6459 41.72 58.28 30.66 69.28 N10–C11 0.6460 41.73 58.27 30.68 69.270.7634 — — 35.44 64.38 0.7633 — — 35.44 64.38

N13–C12 0.6468 41.84 58.16 30.73 69.22 N13–C12 0.6469 41.84 58.16 30.74 69.210.7627 — — 35.20 64.62 0.7626 — — 35.20 64.62

C22–O22 0.5725 32.77 67.23 25.05 74.74 C22–O22 0.5727 32.80 67.20 25.10 74.690.8199 — — 33.34 66.59 0.8197 — — 33.38 66.55

O22–C25 0.8257 68.18 31.82 28.42 71.52 O22–C25 0.8259 68.21 31.79 28.42 71.510.5641 — — 20.87 78.86 0.5638 — — 20.83 78.90

C26–C31 0.7182 51.58 48.42 32.95 67.01 C29–C32 0.7197 51.80 48.20 28.95 70.990.6958 — — 38.78 61.18 0.6943 — — 36.08 63.85

C29–F32 0.5240 27.45 72.55 22.30 77.39 C32–F33 0.5140 26.42 73.58 21.34 78.220.8517 — — 30.67 69.24 0.8578 — — 29.10 70.81



400

from one end to the other, thus showing the conductance abilityof the molecule. This applied voltage, which is required to bringthemolecule to behave as a conductor, is called the PB for conduc-tion. So to gage the electron transport property in a moleculethrough FMO analysis, PB for the molecules is calculated usingequation, PB=½ HLG+q, when LUMO is the conduction channelq= 0 and q=ΔLUMO for the conduction channel other than LUMO.Lowest unoccupied molecular orbital is the delocalized low

lying vacant orbital for all the molecules and can serve thechannel for conduction process. So using the extreme conditionq= 0, for all the molecules, we have calculated the PB requiredfor the conduction. Along with the HOMO and LUMO orbitalenergies, PB values for all the molecules are shown in Table 9.In Table 9, it is clear that increasing the number of electronwithdrawing substituent increase the PB value. Because of suchtrend of PB required to achieve the conductance at molecularlevel, poly-phenylene and poly-thiophene-like systems are studied

as potential molecular wires.[124,125] In phenanthrimidazole deriva-tives, although the decrease in PB is not large, when such typemolecules tailored into polymeric type of system, an overall large

a

b

Figure 13. (a) Enhancement of absorption spectra of phenanthrimidazoles by nano-ZnO. (b) Enhancement of emission spectra ofphenanthrimidazoles by nano-ZnO

Figure 14. Highest occupied molecular orbital–lowest unoccupiedmolecular orbital energy levels of isolated phenanthrimidazoles,conduction, and valence bands of nano-ZnO

Table 8. Excitation energy (ΔE) in eV, absorption maximum (lmax) in nm, oscillator strength (f), and ground to excited state dipoledifference (Δm) in Debye, Ci coefficient for the transition

Compound btot lmax ΔE f Δm Major transition Ci m

1 237.4 280 1.92 1.07 5.03 HOMO! LUMO 0.52 7.052 212.2 282 1.91 1.11 4.92 HOMO! LUMO 0.49 6.043 231.0 278 1.99 1.09 4.99 HOMO! LUMO 0.55 6.984 281.2 280 1.92 1.07 4.25 HOMO! LUMO 0.53 5.48

Table 9. Frontier molecular orbital energies and PB values

Compound HOMO LUMO LUMO+1 Eg PB PD

1 �8.32 �5.63 �4.38 2.69 1.34 1.252 �8.40 �5.58 �4.03 2.82 1.41 1.553 �8.46 �5.50 �3.76 2.96 1.48 1.744 �8.50 �5.45 �3.62 3.05 1.52 1.83

MATERIALS SCIENCE


401

decrease in PB value can be expected and in this regard the afore-mentioned structure–property analysis will be helpful in designingnew polymeric molecules for molecular electronics.

The PD is one of the parameters to gage the ability of themolecule to show rectification, can be calculated from the orbitalenergy difference between the LUMO and the LUMO+1. As theLUMO and LUMO+1 are localized on different parts of themolecule, the tunneling of electron is possible and the rectifying

ability of the molecule can be expected. Rectification on electrontransport property can be achieved where there is differentialpopulation localization of the FMOs.[126] It has been suggestedthat when such a molecule is placed in between the twoelectrodes and voltage is applied, there will be an inelastictunneling of electron from the acceptor side to the donorside.[126] In considering the FMOs, the resonant transport ofelectron from the LUMO to the next unoccupied level localizedon the donor part could be explained. The rectifying propertyof the molecular systems, PD across the molecular rectifier canbe calculated as, PD (ΔLUMO) = ELUMO� ELUMO+1. This increase inPD and PB is important because the rectifier molecule can beused for a higher range of applied voltage.

Electron transfer from phenanthrimidazoles tonanocrystalline ZnO

ZnO nanoparticles enhance the absorbance of thephenanthrimidazoles remarkably without shifting its absorptionmaximum at 275nm and it is displayed in Fig. 13a. This indicatesthat the nanocrystals do not modify the excitation process of thephenanthrimidazoles. The enhanced absorption at 275 nmobserved with the dispersed semiconductor nanoparticles is dueto adsorption of the phenanthrimidazoles on semiconductorsurface. This is because of effective transfer of electron from theexcited state of the phenanthrimidazole to the conduction bandof the semiconductor nanoparticle as visualized in photoinducedelectron transfer mechanism.The emission spectra of the phenanthrimidazole in the pres-

ence of ZnO nanoparticles dispersed at different loading andalso in their absence are displayed in Fig. 13b. The nanoparticle

Figure 16. Molecular electrostatic potential surface of phenanthrimidazoles 1–4

Figure 15. Mechanismoffluorescenceenhancementof phenanthrimidazolesby nano-ZnO



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enhances the emission of the phenanthrimidazole remarkablywithout shifting its emission maximum at 403 nm. This indicatesthat the nanocrystals do not modify the excitation process ofthe phenanthrimidazole. The enhanced emission at 403 nmobserved with the dispersed semiconductor nanoparticle is dueto the adsorption of the phenanthrimidazole on semiconductor

surface. This is because of effective transfer of electron fromthe excited state of the phenanthrimidazole molecule to theconduction band of the semiconductor nanoparticle. The fluo-rescence enhancement arises because of the formation of thefluorophore–nanoparticulate ZnO complex and the bindingconstant (K) has been evaluated as 5.03� 103. Figure 14 presentsthe HOMO and LUMO energy levels of an isolated imidazolemolecule alongwith the conduction band and valence band edgesof ZnO nanoparticle. The energy levels presented in Fig. 15suggests enhancement of fluorescence of the phenanthrimidazolemolecule by ZnO nanocrystal. On illumination at 275 nm, both theligand and nanosemiconductor are excited. Duel emission isexpected because of LUMO!HOMO and CB!VB electrontransfer. Also possible is the electron jump from the excitedorganic molecule to the nanocrystal; the electron in the LUMO ofthe excited ligand is of higher energy compared with that in theCB of ZnO nanocrystals. This should lead to quenching offluorescence in the phenanthrimidazole molecule. However,contrary to the expectations, enhancement of fluorescence isobserved in the presence of ZnO nanocrystal. This may be becauseof the lowering of the HOMO and LUMO energy levels of thephenanthrimidazole molecule due to adsorption on ZnO nanopar-ticle. The polar ZnO surface enhances the delocalization of the

Figure 17. Visualized mode of attachment of phenanthrimidazolewith ZnO

Figure 18. Time resolved fluorescence decay spectra of phenanthrimidazole with nano-ZnO

Table 10. Selected parameters obtained from MOPAC calculations

Parameter Ligand ZnO (Wurzite) Conjugate

Heat of formation (kcal) 276.204720 536.623765 780.006160Electronic energy (eV) �45436.98896 �3051.723501 �57946.333560Core–core repulsion (eV) 40247.149971 1673.737891 51377.085678Dipole(debye) 4.90349 0.30906 19.81556No. of filled levels 80 16 96Ionization potential (eV) 8.659489 7.699572 6.905978Molecular weight 432.481 325.518 757.998SCF calculations 1 1 1

Ref: MOPAC 93: J.J.P. Stewart, Fujitsu, 1993. MOPAC 7: I. Cserny, Linux Public Domain: ftp://esca.atomki.hu, or ftp://infomeister.osc.edu

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p electrons and lowers theHOMOand LUMOenergy levels due to ad-sorption.[127] The azomethine nitrogen atomof phenanthrimidazole ismore basic, and the molecular electrostatic potential shown inFig. 16 supports the same; the nitrogen, fluorine, and oxygenatoms represent the most negative potential region. The chemicalaffinity between the zinc ion on the surface of the nano-oxide andthe azomethine nitrogen atom of phenanthrimidazole may be areason for strong adsorption of phenanthrimidazole molecule onZnO nanoparticle that causes the enhancement. The visualizedmode of attachment is displayed in Fig. 17.

The thermodynamic feasibility of excited state electrontransfer reaction has been confirmed by the calculation of freeenergy change by employing the well known Rehm-Wellerexpression,[128] ΔGet= E1/2 (ox)� E1/2(red)� Es + C, where, E1/2 (ox) isthe oxidation potential of phenanthrimidazole (0.19 V), E1/2(red) isthe reduction potential of ZnO nanoparticle, i.e., the conductionband potential of nano-ZnO, Es is the excited state energy ofphenanthrimidazole molecule and C is the coulombic term.Because the phenanthrimidazole is neutral and the solvent usedis polar in nature, the coulombic term in the aforementionedexpression can be neglected.[129] The values of ΔGet is calculatedas �2.80 eV. The high negative value indicates the thermody-namic feasibility of the electron transfer process.[130,131]

The phenanthrimidazoles adsorbed on the semiconductorparticle surface had significantly shorter fluorescence lifetimethan the unadsorbed molecules; this decrease in lifetime canbe correlated with the electron transfer process.[132,133] Thefluorescence decay of phenanthrimidazole–ZnO nanoparticle isshown in Fig. 18. This emission fits into a bi-exponential decaywith short-lived and a long-lived components of life time 0.12and 7.6 ns, respectively. The long-lived state may correspond tothe free phenanthrimidazoles molecule and the short-lived onemay be adsorbed on ZnO nanoparticles. The populations of thetwo states are 67.8% and 32.2%, respectively. In the absence ofZnO nanocrystals also, phenanthrimidazoles decay bi-exponentially with lifetime of 2.6 and 5.2 ns. The respectivepopulations are 1.46% and 98.54%. This clearly shows that themol-ecule in the excited state exists in two forms. Themajor one is likelyto be involved in the adsorption process. The shorter lifetime of thefluorophore bound to the nanoparticle is likely due to the electrontransfer process from HOMO of phenanthrimidazole molecule toconduction band of ZnO by way of emission of light. The rateconstant for electron transfer[134] (ket) from the excited statesensitizer into semiconductor can be calculated by using the rela-tion ket = 1/(tads� 1/t). The calculated value of ket is found to be6.8� 108 s�1. The distance between the phenanthrimidazoleand the ZnO nanoparticle can be estimated by Forster’snonradiative energy transfer theory. According to Forster’s

nonradiative energy transfer theory, the energy transfer effi-ciency (E) can be defined by the following equations:E=1� F

Fo=R0

6/(R06+ r 6

0 ); R06 = 8.8� 1023[қ2n-4ΦD J(l)] in Å6; (l) =Z 1

0FD (l) eA (l) l4 dl, where E is the efficiency of transfer

between the donor and the acceptor and R0 is the critical dis-tance when the efficiency of transfer is 50%. FD (l) is thecorrected fluorescence intensity of the donor at wavelength lto (l+ Δl), with the total intensity normalized to unity andeA(l) is the molar extinction coefficient of the acceptor at wave-length l. The Forster distance (R0) has been calculated assumingrandom orientation of the donor and acceptor molecules. In thepresent case, қ2 = 2/3, n = 1.334, ΦD=0.25, and from the availabledata, it results that J(l) = 7.75� 10�12 cm3 L mol�1, E = 0.63,R0 = 8.13� 10�7 cm and r = 0.741 nm. The donor-to-acceptordistance is less than 8nm that indicates that the energycould transfer from phenanthrimidazole to ZnO nanoparticle[135]

with high probability and the distance obtained by FRET withhigher accuracy.

Computational support

In order to further confirm the adsorption of phenanthrimidazoleon semiconductor ZnO nanoparticle surface, the semiempiricalAM1 MO calculations were carried out with the MOPAC 93[136]

program package using a single self-consistent-field method(1SCF).The structure was subjected to geometry optimization intwo stages, whereby first just the hydrogen atoms were relaxedand second all the atoms were relaxed. The parametrization ofthis technique has been developed to best reproduce thegeometry, heat of formation, and ionization potential of theirground state. The results are as shown in Table 10. It revealsthe increase of heat of formation which indicates that thephenanthrimidazole molecule adsorbs strongly on ZnO nanopar-ticle which was shown in Fig. 19, in a shorter lifetime (tads).Theelectronic energy also supports the proposition. The low ioniza-tion potential of the adsorbed species is in agreement with theadsorption of phenanthrimidazole on ZnO.

CONCLUSION

In summary, we have synthesized a series of phenanthrimidazoles(hole transport materials) with increasing number of fluoro-substituents. The photophysical, electrical, optical, thermal, anddevice properties of the phenanthrimidazole molecules have beenstudied and explained. From the physicochemical studies on thephenanthrimidazoles, it is concluded that molecules of higher

Figure 19. AM1 optimized molecular structure of (a) phenanthrimidazole, (b) nano-ZnO, and (c) adsorption of phenanthrimidazole on ZnO nanopar-ticle. Color code: C, green; N, blue; O, red; Zn, gray. Hydrogen atoms were omitted for clarity and black dotted line showing surface interaction



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hyperpolarizability have larger dipole moments and could be usedas potential NLOmolecules. On the basis of the PES scan study, theminimumenergy conformation of themolecule is drawn. From theelectrochemical data of the phenanthrimidazoles, the HOMOenergy level of 1–4 is deduced as�5.6 eV, which is in between thatof NPB (�5.4 eV) and Alq3 (�6.0 eV) and could regulate the holeinjection. These phenanthrimidazoles can prevent excessive holesto enter the EML and increase the efficiency. The high Tm and Tdvalues indicate that these compounds are thermally stable andcan be used to fabricate efficient EL devices. EL data stronglysupport that these phenanthrimidazoles are better hole transportmaterials. Photoinduced electron transfer explains the enhance-ment of fluorescence by nanoparticulate ZnO and the apparentbinding constant has been obtained. Adsorption of the fluorophoreon ZnO nanoparticle lowers the HOMO and LUMO energy levels ofthe fluorophore. The strong adsorption of the phenanthrimidazoleson the surface of ZnO nanocrystals is likely due to the chem-ical affinity of the nitrogen atom of the organic molecule toZn(II) on the surface of nanocrystal. Theoretical calculationsalso support the experimental results.

REFERENCES[1] D. E. Loy, B. E. Koene, M. E. Thompson, Adv. Funct. Mater. 2002, 12,

245–249.[2] C. W. Tang, S. A. Vanslyke, Appl. Phys. Lett. 1987, 51, 913–915.[3] C. Adachi, K. Nagai, N. Tamoto Appl. Phys. Lett. 1995, 66, 2679–2681.[4] D. F. O’Brien, P. E. Burrows, S. R. Forrest, B. E. Koene, D. E. Loy, M. E.

Thompson, Adv. Mater. 1998, 10, 1108–1112.[5] W. S. Hung, J. T. Lin, C. H. Chien, Y. T. Tao, S. S. Sun, Y. S. Wen, Chem.

Mater. 2004, 16, 2480–2488.[6] X. R. Bu, H. Li, D. V. Derveer, E. A. Mintz, Tetrahedron Lett. 1996, 37,

7331–7334.[7] S. Wang, L. Zhao, Z. Xu, C. Wu, S. Cheng, Mater. Lett. 2002, 56,

1035–1038.[8] C. R. Moylan, R. D. Miller, R. J. Twieg, K. M. Betterton, V. Y. Lee, T. J.

Matray, C. Nguyen, Chem. Mater. 1993, 5, 1499–1508.[9] J. Santos, E. A. Mintz, O. Zehnder, C. Bosshard, X. R. Bu, P. Günter,

Tetrahedron Lett. 2001, 42, 805–808.[10] S. Wang, L. Zhao, Z. Xu, C. Wu, S. Cheng, Mater. Lett. 2002, 56,

1035–1038.[11] F. Effenberger, F. Wuerthner, F. Steybe, J. Org. Chem. 1995, 60,

2082–2091.[12] F. Mariani, S. Pina, A. Santarelli, A. Tamayo, B. Valtancolia, Dalton

Trans. 2006, 33, 4000–4010.[13] Y. Shirota, K. Okumoto, H. Inada, Synth. Met. 2000, 111, 387–391.[14] I. Y. Wu, J. T. Lin, Y. T. Tao, E. Balasubramaniam, Y. Z. Su, C.-W. Ko,

Chem. Mater. 2001, 13, 2626–2631.[15] J. Salbeck, N. Yu, J. Bauer, F. Weissotel, H. Bestgen, Synth. Met.

1997, 91, 209.[16] B. C. Maiti, S. Z. Wang, C. P. Cheng, D. J. Huang, C.-I. Chao, J. Chin.

Chem. Soc. 2001, 48, 1059–1062.[17] E. L. Wehry, Effects of Molecular Structure on Fluorescence and

Phosphorescence, in: Practical Fluorescence, 2nd ed. (Ed.: G. C.Guilbault), Marcel Dekker, New York, 1990.

[18] B. Valeur, Molecular Fluorescence Principles and Applications, Wiley-VCH, Weinheim, Germany, 2002.

[19] T. Yasuda, I. Yamaguchi, T. Yamamoto, Adv. Mater. 2003, 15, 293–296.[20] L. Tao, J. Li, Y. Yu, Y. Jiang, C. S. Zhou, S. T. Lee, X. Zhang, O. Kwon,

Chem. Mater. 2009, 21, 1284–1287.[21] P. I. Shih, C. H. Chien, F. I. Wu, C. F. Shu, Adv. Funct. Mater. 17, 2007,

3514–3520.[22] K. H. Weinfurtner, F. Weissortel, G. Harmgarth, J. Salbeck, Proc. SPIE

Int. Soc. Opt. Eng. 1998, 3476, 40–48.[23] S. Berleb, W. Brutting, M. Schwoerer, R. Wehrmann, A. Elschner, J.

Appl. Phys. 1998, 83, 4403–4409.[24] J. Bettenhausen, M. Greczmiel, M. Jandke, P. Strohriegl, Synth. Met.

1997, 91, 223–228.[25] Y. Shirota, J. Mater. Chem. 2000, 10, 1–25.

[26] Y. Shirota, M. Kinoshita, T. Noda, K. Okumoto, T. Ohara, J. Am.Chem. Soc. 2000, 122, 11021–11022.

[27] M. R. Robinson, S. Wang, G. C. Bazan, Y. Cao, Adv. Mater. 2000,12, 1701–1704.

[28] L. H. Chan, H. C. Yeh, C. T. Chen, Adv. Mater. 2001, 13, 1637–1641.[29] Y. H. Kim, D. C. Shin, S. H. Kim, C. H. Ko, H. S. Yu, Y. S. Chae,

S. K. Kwon, Adv. Mater. 2001, 13, 1690–1693.[30] K. R. J. Thomas, J. T. Lin, Y. T. Tao, C. W. Ko, J. Am. Chem. Soc. 2001,

123, 9404–9411.[31] S. Grigalevicius, G. Blazys, J. Ostrauskaite, J. V. Grazulevicius, V.

Gaidelis, V. Jankauskas, E. Montrimas, Synth. Met. 2002, 128, 127–131.[32] F. Willig, Surface electron transfer processes, VCH Publishers, Inc.,

New York, 1995.[33] A. Hagfeldt, M. Gratzel, Chem. Rev. 1995, 95, 49–68.[34] P. V. Kamat, Chem. Rev. 1993, 93, 267–300.[35] P. V. Kamat, Prog. React. Kinet. 1994, 19, 277–316.[36] P. V. Kamat, D. Meisel, Semiconductor Nanoclusters - Physical,

Chemical, and Catalytic Aspects, Elsevier, Amsterdam, 1997, 103.[37] N. A. Anderson, T. Lian, Annu. Rev. Phys. Chem. 2005, 56, 491–519.[38] B. O’Regan, M. Gratzel, Nature 1991, 353, 737–740.[39] W. U. Huynh, J. J. Dittmer, A. P. Alivisatos, Science 2002, 295,

2425–2427.[40] N. Serpone, E. Pelizzetti, Photocatalysis, Fundamentals and Applica-

tions, John Wiley & Sons, New York 1989.[41] A. Nitzan, M. A. Ratner, Science 2003, 300, 1384–1389.[42] O. Schmelz, A. Mews, T. Basche, A. Herrmann, K. Mullen, Langmuir

2001, 17, 2861–2865.[43] M. Tomasulo, I. Yildiz, S. L. Kaanumalle, F. M. Raymo, Langmuir

2006, 2, 10284–10290.[44] C. Bullen, P. Mulvaney, Langmuir 2006, 22, 30073013.[45] J. Locklin, D. Patton, S. Deng, A. Baba, M. Millan, R. C. Advincula,

Chem. Mater. 2004, 16, 5187–5193.[46] Q. Shang, H. Wang, H. Yu, G. Shan, R. Yan, Colloids Surf., A 2007,

294, 86–91.[47] M. Y. Odoi, N. I. Hammer, K. Sill, T. Emrick, M. D. Barnes, J. Am.

Chem. Soc. 2006, 128, 3506–3507.[48] M. Anni, L. Manna, R. Cingolani, D. Valerini, A. Creti, M. Lomascolo,

Appl. Phys. Lett. 2004, 85, 4169–4171.[49] N. I. Hammer, K. T. Early, K. Sill, M. Y. Odoi, T. Emrick, M. D. Barnes, J.

Phys. Chem. B 2006, 110, 14167–14171.[50] C. Landes, C. Burda, M. Braun, M. A. El-Sayed, J. Phys. Chem. B 2001,

105, 2981–2986.[51] B. R. Fisher, H. J. Eisler, N. E. Stott, M. G. Bawendi, J. Phys. Chem. B

2004, 108, 143–148.[52] S. F. Wuister, C. M. Donega, A. Meijerink, J. Phys. Chem. B 2004,

108, 17393–17397.[53] A. Kathiravan, M. Chandramohan, R. Renganathan, S. Sekar,

Spectrochim. Acta A 2009, 72, 496–501.[54] A. Kathiravan, R. Renganathan, Spectrochim. Acta A 2008, 71,

1106–1109.[55] A. Kathiravan, V. Anbazhagan, M. Asha Jhonsi, R. Renganathan,

Spectrochim. Acta A 2008, 70, 615–618.[56] A. Kathiravan, R. Renganathan, Polyhedron 2009, 28¸ 1374–1378.[57] A. Kathiravan, R. Renganathan, J. Colloid Interface Sci. 2009,

331, 401–407.[58] M. Asha Jhonsi, A. Kathiravan, R. Renganathan, J. Lumin. 2009,

129, 854–860.[59] A. Kathiravan, R. Renganathan, J. Colloid Interface Sci. 2009,

335, 196–202.[60] M. Asha Jhonsi, A. Kathiravan, R. Renganathan, Colloids Surf., B

2009, 72, 167–172.[61] A. Kathiravan, R. Renganathan, Colloid Surf., A 2008, 324, 176–180.[62] M. Asha Jhonsi, A. Kathiravan, R. Renganathan, J. Mol. Struct. 2009,

921, 279–284.[63] J. M. Chen, J. H. Hou, Y. F. Li, X. W. Zhou, J. B. Zhang, X. P. Li,

X. R. Xiao, Y. Lin, Chinese Sci. Bull. 2009, 54, 1669–1676.[64] A. Kathiravan, G. Paramaguru, R. Renganathan, J. Mol. Struct. 2009,

921, 279–284.[65] M. Yamada, Y. Tanaka, Y. Yoshimoto, S. Kuroda, I. Shimao, Bull.

Chem. Soc. Jpn. 1992, 65, 1006–1011.[66] I. Sjoholm, B. Ekman, A. Kober, I. Ljungstedt-Pahlman, B. Seiving,

T. Sjodin, Mol. Pharmacol. 1979, 16, 767–767.[67] Y. Marcus, Chem. Soc. Rev. 1993, 22, 409–416.[68] P. Wang, P. Zhu, W. Wu, H. Kang, C. Ye, Phys. Chem. Chem. Phys.

1999, 1, 3519–3525.

MATERIALS SCIENCE


405

[69] C. Karunakaran, V. Rajeswari, P. Gomathisankar, Mater. Sci.Semicond. Process. 2011, 14, 133–138.

[70] S. Okada, K. Okinaka, H. Iwawaki, M. Furugori, M. Hashimoto,T. Mukaide, J. Kamatani, S. Igawa, A. Tsuboyama, T. Takiguchi,K. Ueno, Dalton Trans. 2005, 9, 1583–1590.

[71] H. Y. Chen, Y. Chi, C. S. Liu, J. K. Yu, Y. M. Cheng, K. S. Chen,P. T. Chou, S. M. Peng, G. H. Lee, A. J. Carty, S. J. Yeh, C. T. Chen,Adv. Funct. Mater. 2005, 15, 567–574.

[72] C. Anbuselvan, J. Jayabharathi, V. Thanikachalam, K. Jayamoorthy,Spectrochim. Acta Part A 2012, 96, 848–853.

[73] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,J. R. Cheeseman, J. A. Montgomery, T. Vreven, Jr., K. N. Kudin,J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone,B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson,H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda,J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,M. Klene, X. Li, J. E. Knox, P. Hratchian, J. B. Cross, C. Adamo,J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma,G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck,K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul,S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian 03, Revision C.02,Gaussian, Inc., Wallingford, CT, 2004.

[74] M. Wagener, J. Sadowsky, J. Gasteiger, J. Am. Chem. Soc. 1995,117, 7769–7775.

[75] N. Vijayan, N. Rani, G. Bhagavannarayana, D. Haranath,J. Jayabharathi, M. A. Wahab, S. Das, Spectrochim. Acta A 2012,93, 75–80.

[76] J. Jayabharathi, V. Thanikachalam, M. Vennila, K. Jayamoorthy,Spectrochim. Acta A 2012, 95, 446–451.

[77] J. Jayabharathi, V. Thanikachalam, M. Vennila, K. Jayamoorthy,Spectrochim. Acta A 2012, 95, 589–595.

[78] J. Jayabharathi, V. Thanikachalam, N. Srinivasan, K. Jayamoorthy,Spectrochim. Acta A 2011, 83, 329–336.

[79] J. Jayabharathi, V. Thanikachalam, N. Vijayan, N. Srinivasan,M. Venkatesh Perumal, Struct. Chem. Commun. 2010, 1, 46–52.

[80] J. Jayabharathi, V. Thanikachalam, K. Jayamoorthy, M. VenkateshPerumal, Spectrochim. Acta A 2011, 79, 6–16.

[81] J. Jayabharathi, A. Manimekalai, M. Padmavathy, Med. Chem. Res.2011, 20, 981–995.

[82] N. Subramanian, N. Sundaraganesan, J. Jayabharathi, Spectrochim.Acta A 2010, 76, 259–269.

[83] Y. Yang, W. J. Zhang, X. M. Gao, Int. J. Quantum Chem. 2006,106, 1199–1207.

[84] J. Jayabharathi, V. Thanikachalam, R. Sathishkumar, Spectrochim.Acta Part A 2012, 97, 582–588.

[85] J. Jayabharathi, V. Thanikachalam, K. Jayamoorthy, Spectrochim.Acta Part A 2012, 97, 131–136.

[86] J. Jayabharathi, V. Thanikachalam, M. Venkatesh Perumal,Spectrochim. Acta A 2011, 92, 113–121.

[87] J. Jayabharathi, V. Thanikachalam, K. Jayamoorthy, Spectrochim.Acta Part A 2012, 89, 301–307.

[88] J. Jayabharathi, V. Thanikachalam, K. Brindha Devi, M. VenkateshPerumal, Spectrochim. Acta A 2012, 86, 69–75.

[89] J. Jayabharathi, V. Thanikachalam, N. Srinivasan, M. VenkateshPerumal, K. Jayamoorthy, Spectrochim. Acta A 2011, 79, 137–147.

[90] J. Jayabharathi, M. Venkatesh Perumal, V. Thanikachalam,Spectrochim. Acta Part A 2012, 95, 497–504.

[91] J. Jayabharathi, V. Thanikachalam, M. Venkatesh Perumal,Spectrochim. Acta Part A 2012, 95, 614–621.

[92] J. Jayabharathi, V. Thanikachalam, M. Venkatesh Perumal,Spectrochim. Acta A 2012, 85, 31–37.

[93] Z. Li, H. Wan, Y. Shi, P. Ouyang, J. Chem. Inf. Comput. Sci. 2004,44, 1886–1890.

[94] A. Pedretti, L. Villa, G. Vistoli, J. Comput. Aided Mol. Des. 2004, 18,167–173.

[95] M. A. Lill, M. L. Danielson, J. Comput. Aided Mol. Des. 2011, 25, 13–19.

[96] R. Dunsbach, R. Schmidt, J. Photochem. Photobiol., A 1994, 83,7–13.

[97] L. Biczok, T. Berces, F. Marta, J. Phys. Chem. 1993, 97, 8895–8899.[98] P. O. Andersson, S. M. Bachilo, R. L. Chen, T. Gillbro, J. Phys. Chem.

1995, 99, 16199–16209.[99] Y. Porter, K. M. Ok, N. S. P. Bhuvanesh, P. S. Halasyamani, Chem.

Mater. 2001, 13, 1910–1915.[100] M. J. Kamlet, R. W. Taft, J. Am. Chem. Soc. 1976, 98, 377–383.[101] J. Catalan, V. Lopez, P. Perez, J. Fluoresc. 1996, 6, 15–22.[102] Y. Zhang, S. L. Lai, Q. X. Tong, M. Y. Chan, T. W. Ng, Z. C. Wen,

G. Q. Zhang, S. T. Lee, H. L. Kwong, C. S. Lee, J. Mater. Chem.2011, 21, 8206–8214.

[103] S. Ranjan, S. Y. Lin, K. C. Hwang, Y. Chi, W. L. Ching, C. S. Liu,Y. T. Tao, C. H. Chien, S. M. Peng, G. H. Lee, Inorg. Chem.2003, 42, 1248–1255.

[104] S. L. Lin, L. H. Chan, R. H. Lee, M. Y. Yen, W. J. Kuo, C. T. Chen,R. J. Jeng, Adv. Mater. 2008, 20, 3947–3952.

[105] A. P. Kulkarni, A. P. Gifford, C. J. Tonzola, S. A. Jenekhe, Appl. Phys.Lett. 2005, 86, 061106.

[106] W. L. Yu, J. Pei, Y. Cao, W. Huang, J. Appl. Phys. 2001, 89, 2343–2350.[107] J. Staudigel, M. Stossel, F. Steuber, J. Simmerer, J. Appl. Phys. 1999,

86, 3895–3910.[108] Y. Kawabe, J. Abe, Appl. Phys. Lett. 2002, 81, 493–495.[109] M. T. Lee, H. H. Chen, C. H. Liao, C. H. Tsai, C. H. Chen, Appl. Phys.

Lett. 2004, 85, 3301–3303.[110] Q. X. Tong, S. L. Lai, M. Y. Chan, Y. C. Zhou, H. L. Kwong, C. S. Lee,

S. T. Lee, J. Phys. Chem. C 2009, 113, 6227–6230.[111] S. Tokito, H. Tanaka, K. Noda, A. Okada, Y. Taga, Appl. Phys. Lett.

1997, 70, 1929–1931.[112] V. Crasta, V. Ravindrachary, R. F. Bharantri, R. Gonsalves, J. Cryst.

Growth 2004, 267, 129–133.[113] Y. Zhang, S. L. Lai, Q. X. Tong, M. Y. Chan, T. W. Ng, Z. C. Wen, G. Q.

Zhang, S. T. Lee, H. L. Kwonge, C. S. Lee, J. Mater. Chem. 2011, 21,8206–8214.

[114] Y. Yang, W. J. Zhang, X. M. Gao, Int. J. Quant. Chem. 2006,106, 1199–1207.

[115] M. Szafran, A. Komasa, E. B. Adamska, J. Mol. Struct. 2007, 827, 101–107.[116] G. Wang, F. Lian, Z. Xie, G. Su, L. Wang, X. Jing, F. Wang, Synth. Met.

2002, 131, 1–5.[117] K. Fukui, T. Yonezawa, H. Shingu, J. Chem. Phys. 1952, 20, 722–725.[118] F. Bergstrom, I. Mikhalyov, P. Hagglof, R. Wortmann, T. Ny,

L. B. Johansson, J. Am. Chem. Soc. 2002, 124, 196–204.[119] P. Wang, P. Zhu, W. Wu, H. Kang, C. Ye, Phys. Chem. Chem. Phys.

1999, 1, 3519–3525.[120] R. G. Parr, R. A. Donnelly, M. Levy, W. E. Palke, J. Chem. Phys.

1978, 68, 3801–3807.[121] R. P. Iczkowski, J. L. Margrave, J. Am. Chem. Soc. 1961, 83, 3547–3551.[122] K. Sohlberg, N. Vedova Brook, J. Comput. Theo. Nanosci. 2004, 1,

256–260.[123] H. Chen, J. Q. Lu, J. Wu, R. Note, H. Mizuseki, Y. Kawazoe, Phys. Rev.

B 2003, 67, 113408.[124] J. M. Tour, Polym. News 2000, 25, 329–336.[125] X. Lu, M. Grobis, K. H. Khoo, S. G. Louie, M. F. Crommie, Phys. Rev.

Lett. 2003, 90, 096802–1.[126] C. Majumder, H. Mizuseki, Y. Kawazoe, J. Phys. Chem. A 2001,

105, 9454–9459.[127] H. M. Cheng, W. F. Hsich, Nanotech. 2010, 21, 485202–485209.[128] G. J. Kavarnos, N. J. Turro, Chem. Rev. 1986, 86, 401–449.[129] S. Parret, F. M. Savary, J. P. Fouassier, P. Ramamurthy, J. Photochem.

Photobiol. A 1994, 83, 205–209.[130] K. Kikuchi, T. Niwa, Y. Takahashi, H. Ikeda, T. Miyashi, J. Phys. Chem.

1993, 97, 5070–5073.[131] S. Nath, H. Pal, D. K. Palit, A. V. Sapre, J. P. Mittal, J. Phys. Chem.

A 1998, 102, 5822–5830.[132] P. V. Kamat, J. Phys. Chem. 1989, 93, 859.[133] Y. N. Yan, W. L. Pan, H. C. Song, Dyes Pigm. 2010, 86, 249–258.[134] H. A. Benesi, J. H. Hildebrand, J. Am. Chem. Soc. 1949, 71,

2703–2707.[135] M. M. Yang, X. L. Xi, P. Yang, Chin. J. Chem. 2006, 24, 642–648.[136] MOPAC 93: J.J.P. Stewart, Fujitsu, 1993. MOPAC 7: I. Cserny, Linux

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