DOI: 10.1002/chem.201002981

Solvent-Polarity-Tunable Dimeric Association of a Fullerene (C60)–N,N-Dimethylaminoazobenzene Dyad: Modulated Electronic Coupling of the Azo

Chromophore with a Substituted 3D Fullerene

K. Senthil Kumar and Archita Patnaik*[a]

Introduction

Realization of dye–dye aggregate formation in artificiallysynthesized dyes, as observed in naturally occurring dyes,has been of continued interest and investigation in view ofnew avenues for electronic and photonic materials.[1–5] H-and J-type aggregation[6,7] of dyes enable rational control ofdye–dye interactions in supramolecular assemblies[8–12] thathave been studied through electronic absorption of the J-and H-aggregates and explained by exciton coupling

theory,[13,14] wherein the excited state of the dye aggregatesplits into allowed and forbidden levels (see Figure 1). The

Abstract: The tunable self-assembly ofa fullerene (C60)–N,N-dimethylamino-ACHTUNGTRENNUNGazobenzene (DPNME) molecularsystem as a function of solvent polarityin THF/water binary solvent is report-ed. Gradual increase of the volumefraction of the nonsolvent water in a1 � 10�5

m THF solution of DPNME ata mixed dielectric constant emix�42 re-sulted in initial redshifting of the 1ACHTUNGTRENNUNG(p!p*) absorption band, which signifiedthe 1D head-to-tail or J-type arrange-ment of the DPNME molecularsystem. Further increase in the solventpolarity to emix�66 evidenced forma-tion of an antiparallel head-to-tail orH-type molecular arrangement in con-junction with the J-aggregates, therebyestablishing a solvent-polarity-depen-dent dynamic equilibrium between themonomer $ J-aggregate $ H-aggre-gate. The controlled aggregation wasgoverned by the synergetic effect of in-

termolecular donor–acceptor interac-tion between the electron-deficientfullerene ring and the electron-richN,N-dimethylamino-substituted aro-matic ring; typically, van der Waals andp–p interactions between the mole-cules constituting a pair of dimers wereenvisaged. An agreement between thesemiempirically calculated drasticallyreduced oscillator strength of theDPNME H-dimer in the antiparallelconfiguration (0.69 vs. 1.29 in the mon-omeric DPNME) and the experimentalelectronic absorption spectra beyondemix = 66 further strengthened this as-signment to the hitherto forbidden an-tiparallel H-dimer. Complementing theabove, the periodicity of molecular

self-assembly dictated a monoclinicunit cell in the single-crystal XRDpacking pattern with a C2/c spacegroup; the molecules packed laterallywith mutual interdigitation with thedonor (E)-N,N-dimethyl-4-(p-tolyldi-ACHTUNGTRENNUNGazenyl)aniline (AZNME) parts in anantiparallel fashion (contrary to theusual expectation for H-aggregates)with strong inter- and intrapair van derWaals and p–p interactions betweenthe constituent fullerene moieties.Unlike those of porphyrin/phthalocya-nine bowl-like donor-initiated architec-tures, a rare class of DPNME dyadicsupramolecular self-assemblies was re-alized with p-extended 2D fullerenenetworks, in which the linear geometryof the AZNME donor and the confor-mational rigidity of the fullerene ac-ceptor played crucial roles.

Keywords: azo compounds · dime-rization · electronic structure · ful-lerenes · supramolecular chemistry

[a] K. S. Kumar, Prof. A. Patnaik+

Department of ChemistryIndian Institute of Technology MadrasChennai 600 036 (India)Fax: (+91) 44-2257-4202E-mail : archita59@yahoo.com

[+] Currently Fulbright–Nehru Visiting Research Professor at the Law-rence Berkeley National Laboratory, Berkeley, CA (USA).

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201002981.

Figure 1. Electronic energy level representation showing the splitting ofenergy levels of H- and J-dimers according to exciton theory.[11, 12]

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head-to-tail (linear) or J-type aggregation was characterizedby a symmetry-allowed transition with double the oscillatorstrength of the monomeric unit to the lower excited state.The bathochromically shifted absorption peak with a lack ofvibrational fine structure was observed when the molecularentities were arranged with a slippage angle of 0 to 588 be-tween the lines joining their centers of mass and the parallelplane of the molecule. However, the face-to-face (parallel)or H-type aggregation was characterized by a symmetry-al-lowed transition with a vibrational fine structure to thehigher excited state with a hypsochromic shifting of the ab-sorption maximum.

Various well-known molecular systems that form J- andH-type aggregates include cyanine,[15–21] perylene bisi-mide,[22–25] merocyanine,[26, 27] and azobenzene families ofdyes.[28–30] Among the molecular entities capable of formingordered aggregates, azobenzenes are unique because oftheir intrinsic photoresponsive property.[31–38] Azobenzenesof donor–acceptor type have been found to form character-istic aggregates depending on the solvent composition, po-larity, and the solute concentration. Studies directed towardsaggregation of azobenzenes have mainly concentrated on:1) simpler azobenzene molecular entities of donor–acceptorand electronically symmetric type;[39,40] 2) surfactant-likeazobenzene derivatives functionalized with long alkyl chainsand apexed with a polar head group;[41,42] and 3) azo moiet-ies with polar or nonpolar head groups, appended as sidechains on polymeric backbones.[43,44] With the aim of devel-oping photomodulated phthalocyanine J-aggregates, a-aryl/alkoxy-substituted zinc phthalocyanine dyads (3-azo-ZnPcand 4-azo-ZnPc) have been synthesized by LiHong Niuet al.[45] In noncoordinating solvents, the 3-azo-ZnPc couldbe effectively photocontrolled as a result of changes in thegeometry and dipole moment of azobenzene on the dyadframework. The positions of the oxygen atoms to which thearyl/alkoxy substitution was attached essentially controlledthe extent of aggregation. Supramolecular aggregates ofazobenzene phospholipids and related compounds in bilayerassemblies have been studied for structure, properties, andphotoreactivity by Song et al.[46] Strong evidence of H-aggre-gate formation in the pure and mixed dispersions has beenindicated. On the basis of simulations and studies with simi-lar stilbene phospholipids and the induced circular dichro-ism signals from the aggregate, a chiral “pinwheel” unit ag-gregate structure, similar to that for several aromatics, wasproposed.

Fullerene (C60) is a fascinating 3D molecular entity withinteresting optical and electronic properties.[47–50] By combin-ing the photoactive azobenzene chromophore with the mo-lecular entity of C60, molecular hybrids with novel photo-chemical, electrochemical, and electrical properties can beconstituted. Moreover, the covalent linkage of azobenzenemoieties (with a strong absorption coefficient in the visibleregion), which are prone to change according to the envi-ronment and molecular association type, could lead to un-derstanding of molecular processes involved in the ordered2D and 3D molecular and supramolecular assembly forma-tion. In addition, the solvent-polarity-controlled molecularassociation of azobenzenes can be used to direct the con-trolled self-assembly of fullerenes with discrete optical andelectronic properties.

The site-specific accommodation of a fullerene C60 guestin the Kagome open networks of specific size and symmetry,formed from a tetra-acidic azobenzene molecule at theliquid/solid interface by Li et al.,[51] is the only report thatdealt with the self-assembly of fullerene and azobenzenesthrough noncovalent means. In spite of recent reports onthe covalently linked fullerene–azobenzene hybrids,[52–57]

tunable and discrete molecular assembly of specific typesupon controlled experimental conditions is clearly missing,when viewed on a par with a range of aggregate-formingsystems.[58–65] This investigation is a maiden attempt to fillthe gap by studying the characteristics of fullerene (C60)–N,N-dimethylaminoazobenzene (DPNME; see Figure 2 c)aggregation in a binary THF/water solvent medium alongwith the characteristic self-assembly of its donor and accept-or constituents (E)-N,N-dimethyl-4-(p-tolyldiazenyl)aniline(AZNME) and N-methylfulleropyrrolidine (NMFP), respec-tively (see Figure 2 a and b). The resultant spectroscopic sig-natures and the single-crystal packing pattern are related tothe structure of the aggregates with the association type in-volved. Detailed semiempirical computations provided fur-ther insight into the electronic structure aspects associatedwith the specific aggregate type, electronic absorption spec-tral co-relationships, and the differences invoked on themonomer�s electronic structure upon molecular association.In the following sections, the intertwined experimental andcomputational results, similar in nature, are presented insupport of the experimental findings, along with site-selec-tive fabrication of two-dimensional fullerene arrays fromcontrolled interfacial transfer of the DPNME dyad on suita-ble substrates.

Figure 2. Molecular structures of the systems a) AZNME, b) NMFP, and c) DPNME.

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Results and Discussion

Molecular self-assembly and electronic absorption spectralcharacteristics of AZNME in THF/water binary solvent ofvarying polarity : UV/Vis spectroscopy as a powerful toolwas used to unravel aggregate formation from the monomo-lecular components under investigation. The nature of mo-lecular association in AZNME was studied in a THF/waterbinary solvent. Gradual increase in the volume fraction ofwater, in which AZNME was insoluble, increased the dielec-tric constant (emix) of the binary solvent and facilitated inter-molecular association between AZNME moieties. The emix

was calculated according to Equation (1)[65]

emix ¼ fTHFeTHF þ fH2OeH2O ð1Þ

with e as 7.52 and 79.52, respectively, for pure THF andwater solvents and f their volume fraction. In Figures 3 aand 4 a, the AZNME moiety in THF showed an absorptionmaximum at 407 nm corresponding to its 1 ACHTUNGTRENNUNG(p!p*) electron-ic transition with the transition moment parallel to the longaxis of the azo chromophore, whereas the transitionmoment parallel to the short axis of the azo chromophorewas observed at a lower wavelength of 252 nm. The 440 nmfeature signified the monomeric 1ACHTUNGTRENNUNG(n!p*) absorption. Withgradual increase in solvent polarity, the absorption maxi-mum was bathochromically shifted without any distinct split-ting of the absorption band until emix = 58.26, as shown inFigure 3 a. At emix = 72.32, the Gaussian deconvoluted ab-sorption spectrum in Figure 4 b showed splitting of the ab-sorption band accompanied by growth of a new feature at449 nm, attributed to head-to-tail associated AZNME J-dimer, in accordance with the exciton coupling theory. The% weights in Figure 4 established a solvent-polarity-con-trolled dynamic equilibrium of the monomer $ J-aggregatein the system.

Steady-state fluorescence studies upon excitation ofAZNME in pure THF at its absorption maximum of 407 nmled to no appreciable emission, whereas in more polar THF/water binary solvents, AZNME excited at the correspondingabsorption maxima yielded orders-of-magnitude enhancedemission, as shown in Figure 4 c. This emission can be attrib-uted to the densely packed arrangement of azobenzenechromophores in the bilayer structure, as discussed in thefollowing section. Although the phenomenon of “no fluores-cence” in azobenzene solutions (quantum yield �10�7–10�5)[66] has been observed, a few exceptional self-assembledbilayer aggregates of azobenzene-containing amphi-philes[67,68] and azobenzene-functionalized dendrimers[69] inaqueous solution have exhibited fluorescence emission at�600 nm.

J-aggregate structure elucidation from single-crystal XRD:unit cell packing of AZNME : To validate the above attribu-tion and to gain insight into the electronic structure involvedin the dimeric association of AZNME, single crystals of

AZNME were successfully grown from hexane/ethyl acetatesolution. This methodology renders a reasonable selectionof a starting dimeric structure from possible varying orienta-tions of two interacting monomers for electronic absorptionspectral calculations. The packing attained in Figure 5 abears a direct relevance to the intermolecular interactionsexisting through the specific head-to-tail arrangement of themolecular entities. Partial interdigitation along the b axis isnoted for AZNME in the packing diagram (Figure 5 a).Across the c axis, molecules are stacked with an intermolec-ular distance of 6.153 � as depicted in Figure 5 b. A closeranalysis of the packing diagrams shown in Figure 5 a and breveals a layered pattern stacked along the crystallographica axis, with each layer comprising a directionally distinct yetsimilar head-to-tail molecular orientation in a parallel fash-ion. In each layer, intermolecular interactions were facilitat-ed through two CH–p-type short contacts with a distance of2.883 �, as depicted in Figure 5 a and b. The aforementionedsolid-state molecular orientation of AZNME, amassedthrough single-crystal XRD data, corroborates well with theexciton coupling theory predicted redshifted peaking of the

Figure 3. a) UV/Vis absorption spectra of 1� 10�5m AZNME in varying

volume percentages of THF/water binary solvent mixture, starting frompure THF (e =7.52) to 10:90 THF/water (emix =72.32). The inset showsthe expanded spectra in the 350 to 600 nm range. b) Comparative absorp-tion spectral changes in AZNME recorded in between two extreme sol-vent polarities.

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FULL PAPERDimeric Association of a Fullerene–N,N-Dimethylaminoazobenzene Dyad

new absorption band, upon increased intermolecular inter-action in a head-to-tail manner.

Molecular mechanics and semiempirical/ZINDO validationof AZNME J-aggregates : In an effort to detail the electron-ic structure of the AZNME J-dimer that provides bestagreement with the solution-phase UV/Vis absorption spec-

trum, molecular mechanics (MM) and semiempiricalZINDO (Zerner�s intermediate neglect of differential over-lap) calculations were carried out. First, the electronic ab-sorption spectrum of the monomeric AZNME was obtainedusing the ZINDO semiempirical method (as implemented inthe Gaussian 03 suite of programs[70]), in which the crystalcoordinates obtained from the single-crystal XRD data wereused. The calculated absorption maximum of the monomericAZNME at 404.91 nm in Figure 6 a was in excellent agree-ment with the experimentally observed absorption maxi-mum of 407 nm. The predicted 1ACHTUNGTRENNUNG(p!p*) transition involvedelectronic excitation from HOMO to LUMO, as shown inFigure 6 b. For the AZNME dimer, ZINDO calculation wasperformed with crystallographic dimeric data for short mo-

Figure 4. a,b) Baseline-corrected and Gaussian deconvoluted UV/Vis ab-sorption spectra of AZNME at varied emix values depicting the individualweights of the characteristic monomeric and J-aggregate 1 ACHTUNGTRENNUNG(p!p*) and 1-ACHTUNGTRENNUNG(n!p*) transitions. c) Steady-state fluorescence emission spectra of 1�10�5

m AZNME at varied emix values for the corresponding excitationwavelengths.

Figure 5. a) Packing picture of AZNME[70] viewed through the c axis.Crystal data for AZNME: orthorhombic; space group Pna21; a=

15.3708(4), b =14.0139(04), c =6.1563(02) �; V= 1326.1(7) �3; cell for-mula units =4; T= 273(2) K (standard deviation in parentheses); MoKa

radiation (l=0.71073 nm). The full-matrix least-squares refinement with2928 reflections and 166 parameters converged to R1 factor = 0.044, wR2factor =0.1337, and goodness of fit= 1.081. b) Layered brickwork-type ar-rangement of AZNME viewed through the c axis (slightly tilted). Thenearest intermolecular distance along the a axis is 2.883 � and in thec axis is 6.153 � (not shown).

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A. Patnaik and K. S. Kumar

nomer contacts in the region of 2.8 �. The dominant absorp-tion feature of this dimeric arrangement at �409 nm in Fig-ure 6 a was associated with an oscillator strength of �2.11(see Figure 6 c), which is about double that of the predomi-nant monomeric transition calculated at �405 nm with anoscillator strength �1.07. The HOMOs and LUMOs contri-buting dominantly to this electronic transition wereHOMO�1!LUMO and HOMO!LUMO +1, which ac-counted for 44 and 49 % oscillator strengths, respectively.Although the calculation rightly predicts a bathochromicshift in the absorption wavelength for the dimer versus theAZNME monomer, the calculated difference in the magni-tude is only �4 nm against the experimentally observed42 nm. A close examination of the frontier molecular orbi-tals involved in the electronic absorption of the dimer re-veals a lack of coupling or localized nature of the excitationprocess involving the HOMO and LUMO of the corre-sponding monomers (see Figure 6 c). Thus the excitationenergy for the monomer and the dimer is similar with a verysmall shift in the calculated absorption maximum.

An analysis of the single-crystal XRD dimeric arrange-ment in Figure 7 a reveals a lesser facial overlap (than thestructure in Figure 7 b) between the two constitutingAZNME moieties along the short axis direction. Unlike theedge-to-face arrangement observed in smaller aromatics,this dimer conformation does not seem to have been facili-tated through the quadrupolar electrostatic interaction be-tween the hydrogen atoms and the aromatic benzene�s

p cloud. Edge-to-face interactions between aromatic rings indetermining the tertiary and quaternary crystalline structureof peptides and proteins have been reported.[74] Recent X-ray crystallographic and NMR evidence indicated the rela-tively weak intramolecular edge-to-face interactions be-tween aromatic rings to determine the conformation of or-ganic molecules in the solid state and in solution.[75] A T-shaped structure as the sole energy minimum for the ben-zene dimer was established with a simple electrostaticmodel, which involved a somewhat electropositive ortho hy-drogen atom interacting favorably with the face of the adja-cent p cloud. The AZNME dimer conformation in Figure 7 athus yielded frontier orbitals that showed a lack of electron-ic coupling, therefore demanding the necessity to look foranother conformation with strong facial interactions, facili-tated through dispersive forces as observed in the case oflinear aromatics.[76–78] The absence of a large dipole momentand a weak donor–acceptor character of AZNME furthersignified the above notion.

For a best possible AZNME J-dimer structure to complywith the experimental electronic spectrum at a higher binarysolvent polarity, a new J-dimer was built by using the crystalcoordinates and was full geometry optimized by performingMM calculations using the AMBER force field. TheAZNME minimum-energy J-dimer depicted in Figure 7 bshowed a shorter angle of 38.618 between the lines passing

Figure 6. a) ZINDO[72]-calculated UV/Vis absorption spectra of mono-meric AZNME and J-aggregate of AZNME as packed in the unit cellXRD diagram. Inset: top view of the J-aggregate structure. b, c) Molec-ular orbitals involved in the ZINDO-calculated electronic transitions ofmonomeric AZNME and J-dimer of AZNME, respectively, showingcharacteristic transitions.

Figure 7. Molecular electronic structures of AZNME dimers. a) J-dimerarranged as in the unit cell XRD packing diagram, b) MM fully opti-mized J-dimer, and c) MM fully optimized face-to-face arranged H-dimerof AZNME. The AMBER[73, 74] force field was employed in all MM cal-culations.

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FULL PAPERDimeric Association of a Fullerene–N,N-Dimethylaminoazobenzene Dyad

through the long axis of AZNME and the stacking planewith pronounced intermolecular facial interaction betweenthe electron-rich N,N-dimethylaniline ring and the relativelyelectron deficient methyl-substituted aromatic ring. For thisdimeric arrangement, ZINDO predicted a dominant absorp-tion feature at �428 nm with an oscillator strength of�2.32, which is double that of the predominant monomerictransition calculated at �405 nm with an oscillator strengthof 1.07, as depicted in Figure 8 a and b.

The 1 ACHTUNGTRENNUNG(p!p*) transition of the J-dimer involves promotionof an electron from the HOMO to LUMO+ 1 orbitals thathave large amplitudes of the N=N group, and are delocal-ized throughout the molecule and have a finite contributionfrom both the AZNME moieties, rather than the predomi-nantly contributing single AZNME as depicted in Figure 6 c.A reasonable extent of exciton coupling therefore explainsthe bathochromic shifting of the absorption maximum, asexpected for a head-to-tail dimeric arrangement. The calcu-lation rightly predicts the lowering of energy for the 1ACHTUNGTRENNUNG(p!p*) transition of the J-dimer with a comparable magnitude,as observed experimentally, and its close proximity to the 1-ACHTUNGTRENNUNG(p!p*) transition of the monomeric AZNME. Furtherproof of the reliability of the above calculations was un-earthed by performing ZINDO calculations on the H-dimer

of AZNME, constructed by a similar methodology to thatfor the J-dimer. For this dimeric arrangement, ZINDO pre-dicted a dominant absorption feature at �399 nm with anoscillator strength of �1.84 (see Supporting Information,Figure S.1 a and b). This hypsochromic shifting of the H-dimer�s absorption maximum versus the 405 nm monomericvalue reveals the predictive utility of the computationalmethodology adopted in this study.

The above discussions thus established the importance ofthe right choice of starting geometry for the absorptionspectral calculations of dimeric molecular arrangements andof the intradimer electronic-coupling-induced shifting of theabsorption maxima, upon monomer to specific dimer con-version. The doubling of the oscillator strength and the con-siderable redshifting of the absorption maximum as in Fig-ure 8 b corroborate the experimentally observed absorptioncharacteristics of the AZNME J-dimer. Azobenzenes are aunique class of photoactive molecules, their self-assembly in-dicates potential for nanoscale optical applications.[79]

Self-organization of DPNME in THF/water binary solvent:electronic absorption spectral characteristics : Fullerene C60

derivatives with new functionalities are widely studied am-phiphilic systems in view of their intrinsic hydrophobicityand unique electronic and optical properties.[80–82] Prepara-tion of aqueous solutions of fullerenes[83–91] and fullerene de-rivatives[92–97, 99–110] gained importance due to their interestingbiological applications. Asanuma et al.[111] studied an atypi-cal C60-based surfactant bearing hydrophobic head and tailgroups. “Supramolecular polymorphism” was coined bythese authors in obtaining exotic molecular assemblies uponchanging the chemical structure design and conditions of as-sembly. C60 functionalized with hydrophobic groups hasbeen seen to self-organize into structures that are soluble inpolar solvents.[112–114] The cationic C60–N,N-dimethylamino-pyrrolidinium iodide dissolved in DMSO/H2O has beenshown to form nanorods upon benzene addition whereas,upon ultrasonication, emergence of vesicles was noted.[115]

The nonpolar C60 in these supramolecular structures was re-ported to associate in the interior polar environment. Newand novel architectures of fullerene C60 were attempted innonpolar media by eliminating hydrophilic units from thederivatives for improved solubility, while maintaining suffi-cient amphiphilicity.[116] A fulleropyrrolidine with 3,4,5-(hex-adecyloxy)phenyl assembled into various superstructures de-pending on the nature of the solvent; vesicles originated ina 2-propanol/toluene mixture and 1D fibrous structures of afew micrometers resulted[113] in 1-propanol. In equimolarTHF/H2O, cone-shaped structures with hole diameter�60 nm were predominant.

In a sequel to the investigation of self-assembly of thedonor component AZNME, study of the DPNME dyadicmolecular structure was undertaken towards unraveling itscharacteristic molecular association in varied solvent polari-ty. This maiden attempt on the aggregation of a fullerene–azobenzene hybrid signified the effect of substitution of a

Figure 8. a) ZINDO-calculated UV/Vis absorption spectra of monomericAZNME and J-aggregate of AZNME. Inset: the MM fully optimized J-aggregate structure. b) Molecular orbitals involved in the ZINDO-calcu-lated electronic transitions of the J-dimer of AZNME showing character-istic transitions.

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A. Patnaik and K. S. Kumar

3D fullerene moiety on a 2D azobenzene chromophore, andyielded some fascinating results in terms of physicochemicaland structural governing of dimer formation. Exciton theoryproposed for explanation of molecular dimer formation wasinvoked as in the case of AZNME, and the experimentallyobserved results were interpreted with the help of MM andZINDO calculations in relevance to exciton theory, as ex-plained below.

In Figure 9 a, the absorption spectrum of DPNME in pureTHF shows the 1ACHTUNGTRENNUNG(p!p*) electronic transition of the azomoiety at 417 nm along with the minor, but characteristic,433 nm N-methylfulleropyrrolidine (NMFP)-based peak. Asin AZNME, an increase in the dielectric constant of themedium led to gradual redshifting of the DPNME 1ACHTUNGTRENNUNG(p!p*)transition up to an emix value of 43.79. Further increase insolvent polarity led to the appearance of a new 469 nm fea-ture (see Figure 9 b) at the expense of the pristine one.More importantly, the disappearance of the characteristicNMFP absorption maximum at 433 nm implied aggregationof DPNME. Experiments performed with the NMFP accept-or constituent of DPNME (see Supporting Information, Fig-ure S.2 a and b) resulted in broadening of the absorptionprofiles in the visible region; at emix�43.79, complete disap-pearance of the 433 nm spikelike absorption feature of

NMFP with enhanced absorbance indicated 100 % mono-mer-to-aggregate conversion.

Figure 10 summarizes the characteristic molecular self-as-sembly in the DPNME dyad system as a function of solventpolarity. The deconvoluted absorption spectral profile inpure THF solvent showed all characteristic transitions ofAZNME and NMFP moieties (see Figure 10 a). At a dielec-tric constant of 43.79, broadening of the 433 nm NMFP-cen-tered peak along with a more intense 443 nm band, previ-ously assigned to the 1ACHTUNGTRENNUNG(n!p*) electronic transition of theazo chromophore, were predominant. The enhanced intensi-ty of the latter indicated mixing of the 1ACHTUNGTRENNUNG(n!p*) feature withthe J-dimer-centered electronic transition, that is, the true 1-ACHTUNGTRENNUNG(n!p*) character of the monomer was lost at the expenseof the system tending to aggregate. However, at this polari-ty, a finite DPNME monomeric weight was retained withthe characteristic 433 nm band (see Figure 10 b). The gradu-al bathochromic shifting of the 1ACHTUNGTRENNUNG(p!p*) absorption featurewas attributed to positive solvatochromic behavior of theDPNME moiety, as reported for azobenzene-based push–pull donor–acceptor systems.[117] At a solvent dielectric con-stant of 51.01, a skewed absorption profile was obtainedwith the loss of the 433 nm NMFP feature (see Figure 10 c)indicating the onset of DPNME aggregation. Complete dis-appearance of the 433 nm peak with emergence of two clearabsorption maxima located at 434.12 and 471.53 nm in Fig-ure 10 c were noted; in line with the conclusions reached forAZNME, the larger-intensity 471 nm feature was attributedto the J-type DPNME dimer. The 22 nm bathochromic shift-ing of the J-dimer of DPNME in comparison with the449 nm absorption maximum of the J-type AZNME dimerrevealed the donor–acceptor-type intermolecular interac-tions in the DPNME J-type dimer. At increased emix valuesof �58 and �65, an interconversion between the J- and H-dimer was imminent with the equilibrium shifting more to-wards the H-dimer, as is evident from the relative weightsof the blueshifted 434 nm and redshifted 471 nm features(see Figure 10 d and e). At emix = 72.32, the peak maximumat 435 nm was 18 nm bathochromically shifted in relation tothat of the DPNME monomer in pure THF at e=7.52 THF,which implies the predominant existence of H-type dimerwith its respective 1ACHTUNGTRENNUNG(p!p*) and 1ACHTUNGTRENNUNG(n!p*) transitions cou-pled with fullerene-centered transitions.

The overall structural changes involved in DPNME aggre-gation are depicted in the absorption versus emix plot inFigure 11. A critical dielectric constant for the monomer-to-aggregate conversion was found at 41.9. In a dielectric con-stant range between 58.26 and 65.95, coexistence of J- andH-type dimers of DPNME was established with an in-creased probability of H-type dimer formation at increasedsolvent polarity. Beyond a dielectric constant of 65.95 and inconjunction with the nature of the Gaussian deconvolutedabsorption features at emix =72.32 (see Figure 10 f), the soleexistence of DPNME H-dimers was established. The Gaussi-an components at 432.71 and 466.81 nm were attributed totransitions originating from the DPNME H-type dimer.Thus, solvent polarity dictated the dynamic equilibrium be-

Figure 9. a) UV/Vis absorption spectra of DPNME in varying volumepercentages of THF/water binary solvent, starting from pure THF to50:50 vol % corresponding to emix =43.79. b) From emix = 43.79 to 79.52for a 10:90 volume fraction of THF/water. The insets in (a) and (b) showthe expanded spectra in the 350 to 600 nm range.

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FULL PAPERDimeric Association of a Fullerene–N,N-Dimethylaminoazobenzene Dyad

tween the aggregates due to different intermolecular struc-tural ordering in solutions of varying dielectric constant.

In compliance with the exciton theory and the results ob-tained for AZNME, the 471 nm peak could be attributed tothe DPNME J-dimer. However, attribution of the 435 nmfeature in Figure 10 c (redshifted in relation to the monomer1ACHTUNGTRENNUNG(p!p*) band at 417 nm) to the H-dimer of DPNMEseemed unacceptable under the ambit of exciton theory,since only electronic transitions for the face-to-face, paral-lel-arranged molecular dipoles constituting an H-dimer are

allowed with a hypsochromically shifted absorption maxi-mum versus the monomer band.[13,14] The observed results inretrospect could be due to electronic transitions of the H-dimer for its antiparallel orientation of the dipoles, thoughcited forbidden. The validation of this attribution is followedthrough the single-crystal structure and semiempirical calcu-lations in the next two sections.

Single-crystal XRD from DPNME: solid-state evidence forthe formation of the antiparallel H-dimer : Single crystals of

Figure 10. a–f) Baseline-corrected and Gaussian deconvoluted electronic absorption spectra of DPNME as a function of solvent polarity, which depictthe sequential monomer to J-aggregate to H-aggregate conversion.

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A. Patnaik and K. S. Kumar

DPNME were grown from a saturated solution of chloro-form by allowing cyclohexane to slowly diffuse into the solu-tion. The crystal structure of DPNME in Figure 12 a repre-sents the first ever crystal structure of a fullerene–azoben-zene family member, which will foster interesting develop-

ments in this growing area of research. Single-crystal analy-sis showed DPNME to belong to the monoclinic crystalsystem with the space group C2/c. The unit cell comprisesfour pairs of dimers with each pair arranged in an interdigi-tated manner as shown in Figure 12 b and c, which depictpart of the unit cell viewed through the b and c axes.

The molecules are packed laterally to form a monolayer-like structure in which the donor AZNME parts are ar-ranged in an antiparallel fashion, contrary to the expectationfor H-aggregates, and are mutually interdigitated (see Fig-ure 12 b). A view through the b axis of the unit cell yieldsone-dimensional arrays of stacked molecules which are ex-tended along the crystallographic c axis through: 1) two sim-ilar CH–p interactions between the aromatic hydrogenatoms of the AZNME part of DPNME and the electron-rich 6,6-ring carbon atoms of fullerene with a distance of2.696 �; and 2) the interpair interactions between two fuller-ene moieties through the 6,5-ring carbon junctions with adistance of 3.227 � (see Figure 12 c) The most interestingfeature of the structure is the intradimer packing betweenfullerene and the AZNME moieties. As depicted in Fig-ure 12 b, the 6,6-ring carbon atoms of the fullerenes maketwo intermolecular contacts, one with the azo nitrogen andthe other with the aromatic carbon of the closely interdigi-tated AZNME part of DPNME. The intradimer interactionsin this ball-on-hand-like structure are further strengthenedby the two CH–p interactions, as illustrated in Figure 12 b.

On a larger perspective, the 3D arrangements of DPNMEmoieties were indeed interesting. Figure 13 a and b show a

Figure 11. Variation of absorbance as a function of emix of the THF/waterbinary solvent establishing a dynamic equilibrium between the aggregatesuntil emix = 65.9, beyond which only H-dimers were formed.

Figure 12. a) ORTEP diagram of DPNME[119] crystallized from chloro-form/cyclohexane solvent with 50% thermal ellipsoids along with impor-tant crystal parameters; the solvent molecules are omitted for clarity andonly a single layer of molecules is shown. b) Crystal packing in theDPNME monoclinic unit cell with a view from the b axis, and c) twopairs of dimers visualized through the c axis. Crystal data for DPNME:monoclinic; space group C2/c ; a=34.4177(15), b=14.2934(05), c=

20.8206(08) �; b=91.064(4)8 ; V=10240.8(7) �3; cell formula units =4;T= 293(2) K (standard deviation in parentheses); MoKa radiation (l=

0.71073 nm). The full-matrix least-squares refinement with 8994 reflec-tions and 822 parameters converged to R1 factor = 0.1164, wR2 factor =

0.3324, and goodness of fit =1.368.

Figure 13. Extended unit cell diagrams showing inter- and intramoleculardistances involved in the crystal packing of DPNME. Regular arrange-ment of interdigitated dimeric arrays of DPNME moieties are seen alonga) the crystallographic a axis and b) the c axis with voids filled by cyclo-hexane and chloroform solvent. c) Interdimer (green) and intradimer (iceblue) short contacts.

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FULL PAPERDimeric Association of a Fullerene–N,N-Dimethylaminoazobenzene Dyad

very regular arrangement of interdigitated dimeric arrays ofDPNME moieties along the crystallographic a and c axeswith voids filled by cyclohexane and chloroform solvent en-tities. Perpendicular to the crystallographic c axis, theDPNME moieties interacted through similar short contactsto those mentioned above for the intradimer interactionsand formed a one-dimensional slablike structure, as depictedin Figure 13 c. These unique structural features could be en-visaged to have interesting solid-state properties.

In the present investigation, the obtained packing patternof DPNME belongs to a rare class; unlike the porphy-rin[118–121] or phthalocyanine[122] bowl-shaped electron donors,the linear geometry of the AZNME donor part of DPNMEalong with the dominant fullerene–fullerene intermolecularp–p interactions have brought about structural transforma-tions. The geometrical constraints imposed by rigid C60 onthe dyad framework could contribute substantially towardsthe resultant dyad arrays.

Molecular mechanics and semiempirical calculation-assistedvalidation of DPNME H-aggregates : The ZINDO-calculat-ed absorption maximum of the monomeric DPNME locatedat 413.29 nm was found to be in excellent agreement withthe experimentally observed absorption maximum of417 nm. The predicted 1ACHTUNGTRENNUNG(p!p*) transition essentially in-volved electronic transition from the HOMO�5 toLUMO + 2 orbital, as shown in Figure 14 a. For the dimer,the XRD single-crystal unit cell structure, as shown in Fig-ure 12 b, was taken as the input for ZINDO calculation. In-terestingly, the calculated 70 transitions extending to 390 nmyielded no electronic transition with considerable oscillatorstrength. The few observable transitions associated withvery low oscillator strengths are collected in Figure 14 b;these transitions involve molecular orbital coefficients con-centrated only on fullerenes constituting the DPNMEdimer, which is contradictory to the experimental spectraobtained in Figure 10 with predominant AZNME-based ab-sorptions.

To achieve the electronic absorption characteristics of theexperimental equilibrium weights and the ultimate H-dimer,MM and ZINDO calculations were carried out for a variedrange of DPNME dimer conformations. First, the DPNMEdimer was arranged in a head-to-tail fashion or as a J-dimer(see Figure 15 a). The energy-optimized dimer showed fuller-ene–fullerene and fullerene–arene distances of 3.14 and3.29 �, respectively, in very good agreement with literaturevalues and the distances obtained from the X-ray crystalstructure analysis presented in the previous section.

The ZINDO-calculated predominant absorption maximafor the J-dimer of DPNME could be located at 450 and424 nm with the latter having larger oscillator strength, asdepicted in Figure 15 b. The molecular orbitals in Figure 15 brevealed a predominantly azo-centered nature of the elec-tronic transition at 450 nm with a reasonable electronic cou-pling in the electronic excited state. However, the higher-in-tensity transition at 424 nm involved contributions from

both azo and fullerene moieties with a remarkable couplingin the excited state. These transition characteristics mappedwell the experimental absorption profiles obtained in theemix range between 43 and 51, and the observed transitionscould therefore be attributed to the characteristic absorptionof the DPNME J-dimer. The discrepancies noted betweenthe ZINDO-predicted oscillator strengths and the experi-mentally observed intensities could be due to:

1) Coupling of the J-dimer�s absorption characteristics withthe 1ACHTUNGTRENNUNG(n ! p*) transition of the azo moiety

2) Broadening of the absorption feature upon increasingfullerene–fullerene interactions leading to clusterization(see the Supporting Information, Figure S.2 b)

3) Use of a semiempirical ZINDO method with its in-builtapproximations

The usage of ZINDO was unavoidable considering thelarge molecular framework of DPNME and its dimer; in ad-dition, to the best of our knowledge no such literaturereport is available on the electronic spectral predictions ofdimeric fullerene entities. This investigation with reasonableaccuracy can be considered as a starting point in terms ofadopted methodology and structure–electronic spectrum

Figure 14. Molecular orbitals involved in the ZINDO-calculated electron-ic transitions of a) DPNME monomer and b) its H-dimer (structure as insingle-crystal XRD unit cell) depicting transition types and oscillatorstrengths.

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A. Patnaik and K. S. Kumar

correlations in large dimeric systems based on fullerene(C60).

To establish the dimeric arrangement associated with thehypsochromic shifting of the absorption maximum with in-creasing binary solvent polarity, DPNME moieties were ar-ranged in a dimeric conformation with the phenyl ring car-rying the electron-donating N,N-dimethylamino groups in p-stacking interaction with the fullerene rings of the otherDPNME moiety. Figure 16 a illustrates this arrangement inan interdigitated manner. In view of the 3D spatial orienta-tion of fullerene C60, an antiparallel arrangement of transi-tion dipoles was observed, unlike that in classical H-aggre-gate-forming materials. The 3D spatial orientation alongwith the bulk of the electronic cloud placed above the alter-nate double and single bonds, and an electron-deficient corehave been the main reasons for fullerene aggregation insingle and binary solvents.[123–125] This characteristic electrondensity distribution has prevailed even for functionalizedfullerene derivatives towards aggregate formation and struc-tures of varying size and dimensions, thereby emphasizingthe electronic and van der Waals interactions between theneighboring molecules.

The ZINDO calculations in Figure 16 b predicted an elec-tronic transition of maximum oscillator strength at 427 nm,which predominantly involves contributions from the azoparts of the DPNME units along with substantial contribu-tions from the fullerene moieties with excellent electroniccoupling. The predicted absorption maximum at 427 nm isin excellent agreement with the experimentally observed

435 nm at a emix value of 72.32. Two new transitions predict-ed at �436 nm showcased the intermolecular interactionsbetween the fullerene moieties constituting the DPNMEdimer.

For a DPNME molecule with a donor–acceptor structureand with an aggregation-philic fullerene moiety, the inter-molecular interactions can be modeled based on: 1) thestrong intermolecular electronic interaction between theelectron-rich part of the molecular skeleton of one moietywith the electron-deficient part of the other, as observed inthe case of fullerene–porphyrin co-crystals[126–131] and encap-sulation complexes of fullerene–calixarenes;[132–135] and2) the strong intermolecular interaction between the fuller-ene moieties in close proximity to each other. The molecularorientation in solution or in the solid state therefore wouldhave been governed by such interactions with each typetrying to maximize its influence in the assembly process.The structure of the DPNME aggregate can be exactly as-signed by taking into consideration the above points. Apartfrom the strong p–p and van der Waals interactions betweenthe fullerene moieties, the donor–acceptor-type interactionsbetween the fullerene moiety and the N,N-dimethylaniline-substituted aromatic ring of the other DPNME moiety gainimportance here. In the case of a J-type aggregate with alinear head-to-tail arrangement (see Figure 17 a), only onesuch interaction is possible for a pair of dimers that shows a

Figure 15. a) Molecular structure of the J-dimer of DPNME. b) Molec-ular orbitals involved in the ZINDO-calculated electronic transitions ofthe J-dimer of DPNME depicting transition types and oscillatorstrengths.

Figure 16. a) Molecular structure of the antiparallel H-dimer of DPNME.b) Molecular orbitals involved in the ZINDO-calculated electronic transi-tions of the antiparallel H-dimer of DPNME depicting transition typesand oscillator strengths.

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FULL PAPERDimeric Association of a Fullerene–N,N-Dimethylaminoazobenzene Dyad

much lesser degree of stabilization than the H-dimer in Fig-ure 17 b. When the two DPNME moieties were arranged inan interdigitated manner with their dipole moments pointingdiametrically opposite to each other, the donor and acceptorfragments of each DPNME moiety could interact with theiropposite numbers without compromising the fullerene–full-erene interaction, as shown in Figure 17 b. This doubled pos-sibility of intermolecular donor–acceptor interaction is thesole origin of formation of the interdigitated H-dimer withincreasing solvent polarity, which enhanced the hydropho-bic/van der Waals fullerene–fullerene interaction.

The semiempirical ZINDO calculations on the monomerand the H-dimer of DPNME were found to be in excellentagreement with the experimental observations and the re-sults are collected in Figure 18. More interestingly, theZINDO-calculated absorption maximum of the DPNME H-dimer at 427 nm in Figure 18 b was in agreement with theexperimental absorption maximum at 436 nm, but appearedwith a much reduced oscillator strength (f= 0.69) relative tothe monomeric DPNME (f=1.29), against an ideal strengthdouble that of the monomer. For H-aggregates, excitontheory predicted parallel-oriented dipoles constituting thehigher-energy state, for which transition from the singletground state was allowed; the forbidden transition was thelowest-energy split state with antiparallel alignment of di-poles.

In view of the meager oscillator strength for a dimer, thiselectronic transition attributed to the H-dimer of DPNME(see Figure 19), although it could be forbidden, is experi-mentally observable. This special property of the H-dimer ofDPNME makes it a conceptually important molecularsystem. The close agreement between the experimental andZINDO-calculated UV/Vis absorption spectral featuresprompted us to assign the calculated dimeric structure tothe dimeric H-aggregate existing in the THF/water binarysolvent medium. The driving force to the close proximity inthe binary solvent medium could be the strong intermolecu-lar van der Waals and p–p electronic interactions betweenthe two hydrophobic fullerene moieties augmented by thepresence of water molecules of large dielectric constant.

More concrete proof for the aforementioned assignmentwas deduced by arranging the DPNME moieties in a paral-lel H-dimer conformation, as depicted in Figure S.3 in theSupporting Information, and the absorption characteristicswere calculated semiempirically. This arrangement placed

Figure 17. MM geometry-optimized structures of a) J-type dimer andb) antiparallel H-type dimer of DPNME, showing relative stabilities.

Figure 18. Semiempirically calculated absorption spectra of a) DPNMEand b) antiparallel H-dimer of DPNME, showing a comparison with ex-periment.

Figure 19. Electronic energy level representation showing the splitting ofenergy levels of the H-dimer. The tick indicates observation of the for-bidden transition in the H-dimer of DPNME, facilitated by the antiparal-lel dipole arrangement.

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A. Patnaik and K. S. Kumar

the two AZNME moieties 7.89 � apart and prevented elec-tronic coupling due to bulky fullerene moieties lying paral-lel, also inhibiting the donor–acceptor-type interaction be-tween AZNME and fullerene moieties. This was evidentfrom the ZINDO-calculated electronic transition located at413 nm (see the Supporting Information, Figure S.3 b), closeto the absorption maximum predicted for the monomericDPNME. Further, a lack of chromophoric azo-group cou-pling between the DPNME moieties clearly reveals the lo-calized nature of the electronic transition. In summary, theexperimentally observed 435 nm transition could be justifia-bly assigned to the antiparallel H-dimer of DPNME.

2D fabrication of DPNME H-aggregates: orientation ofDPNME at air/water and air/solid interfaces : Langmuirmonolayer experiments for 1 mm DPNME on a Milliporewater subphase of pH 5.9 indicated formation of highlystable DPNME bilayers of condensed phase area �50 �2,which sustains a large surface pressure of �70 mN m�1 (seeFigure 20). For a flat-lying monolayer dyad orientation withthe hydrophobic fullerene at the interface (see Figure 20,inset), the computed right-angled triangular area of 102 �2

sufficed for the 2D bilayer formation. The DPNME bilayerLangmuir films were transferred onto hydrophilic SiACHTUNGTRENNUNG(100)substrates by a single upstroke as a Langmuir–Blodgett(LB) film at a surface pressure of 10 mN m�1. Atomic forcemicroscopy (AFM) images of the LB films depicted inFigure 21 reveal vesicular structures upon continuous bilayerbending, dictated by thermodynamic requirements fromstrong intermolecular van der Waals and hydrophobic inter-actions. The UV/Vis absorption characteristics of the trans-ferred film on a quartz substrate in Figure 22 upon Gaussiandeconvolution revealed 428 and 468 nm features, whichmimicked those of the THF/water binary solvent media andthereby implied the presence of both the J- and H-typedimer-based aggregates, but with slight predominance of H-aggregates. The obtained results are in excellent agreement

with the conclusions reached in the previous sections and in-dicate the facile formation of characteristic DPNME aggre-gates through judicious molecular association type. The 2Dvesicular architectures in Figure 21 could be visualized fromthe molecular modeling of a hydrophilic (AZNME)–hydro-phobic (C60)/head-to-tail-type bilayer arrangement of theDPNME H-dimers (see Figure 23). Upon full structure opti-mization with MM, the schemes have been obtained. Vesiclegrowth upon bilayer bending would depend upon a numberof internal factors such as the structure of the molecule, thecompeting intramolecular forces, the donor–acceptor inter-action, and the intermolecular forces. The relative magni-tudes of the attractive and repulsive forces associated withthe molecular organization could have resulted in an ulti-mate equilibrium structure, as depicted in the AFM images.J-aggregate formation of dyes in the LB films was examinedby Tachibana et al.[98] who reported large morphologicalchanges accompanying merocyanine J-aggregate LB films,formed from photomodulated single-layer LB films of am-phiphilic spiropyran. The photoisomerization of azobenzenewas used here as a trigger to control the structure and func-tion of the dye LB films.

Conclusion

The DPNME donor–acceptor dyad in polarity-controlledbinary solvent media behaved as a system of sequential mul-tiple equilibria. The monomer-to-J-aggregate conversionwas essentially complete when the critical dielectric constantof the solvent was �42. Beyond this, the J-aggregatesbecame unstable; a dynamic equilibrium between aggregateswas set up for 58�emix�66. The DPNME H-dimer was ob-tained at emix�66, at which electrostatic interaction betweenthe DPNME molecules played a dominant role along withthe dipolar interaction arising from the DPNME moleculararrangement. In view of 1) the large ground-state dipolemoment in DPNME arising from its electron-donating N,N-dimethylaniline and electron-accepting NMFP moieties con-nected by the azobenzene bridge, and 2) the DPNME titlecompound being optoelectronically important, its electronicstructure upon self-assembly was investigated from thestandpoints of crystal structure and exciton coupling.DPNME crystallized in a layered fashion with four pairs ofdimers, interwoven through dominant C60–C60 hydrophobicinteractions. Interdigitation/p–p stacking on the molecularplane built the individual DPNME moieties and aligned thetransition dipoles for the observed shift in the electronicspectra. The diagonal C60 pairs of the brick-wall structureand its exciton coupling showed a bathochromic shift. Ingoing from solution to the solid state, the hydrophobic p–p

stacking on the molecular plane as well as the brick-wall-type molecular stacking displaced the absorption band to-wards a shorter wavelength. A highly directional, p-extend-ed and layered DPNME donor–acceptor supramolecular ar-chitecture, the first of its kind, was thus realized with a char-acteristic linear AZNME donor geometry.

Figure 20. Langmuir isotherm of 1 mm DPNME spread over subphasewater at pH 5.9. Inset: flat-lying molecular orientation of DPNME at theair/water interface along with structural parameters.

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FULL PAPERDimeric Association of a Fullerene–N,N-Dimethylaminoazobenzene Dyad

Experimental Section

Synthesis : DPNME, AZNME,[136] and NMFP[137] were synthesized ac-cording to the reported procedures.

UV/Vis spectroscopy : Electronic absorption spectra in spectroscopy-grade solvents were measured with a Schimadzu double-beam UV/Visspectrophotometer.

Computational methodology : Density functional calculations were car-ried out with the Gaussian 03 set of programs.[70] The ground-state geom-etry of each molecule was fully optimized by using the hybrid B3LYPfunctional with 6-31g ACHTUNGTRENNUNG(d,p) basis set, until the root mean square residualforce became smaller than 2.533 � 10�6 a.u. In B3LYP,[138] the exchange isa combination of 20% Hartree–Fock exchange, Slater functional, andBecke�s generalized gradient approximation correction,[139] whereas thecorrelation part combines VWN[140] and LYP[141] functionals. B3LYP/6-31g ACHTUNGTRENNUNG(d,p) optimized geometries of the AZNME and DPNME moleculeswere used to construct the dimeric structures of desired orientation alongwith the semiempirical ZINDO[71] methodology to compute the excita-tion energies, oscillator strengths, and the composition of electronic tran-sitions involved with the monomers and dimers of AZNME andDPNME. Oscillator strengths obtained from the ZINDO calculations

were convoluted by using an appropriate line width towards a realisticUV/Vis absorption spectrum.

X-ray structural determination : The crystal data for AZNME andDPNME were collected and integrated by using a Bruker AXS KappaApex2 charge-coupled device diffractometer with graphite-monochro-mated MoKa (l=0.71073 �) radiation at 273 and 293 K. The structureswere solved by heavy-atom methods with SHELXS-97 or SIR92[142] andrefined using SHELXL-97.[143] CCDC-792036 and CCDC-761898 containthe supplementary crystallographic data for this paper. These data can beobtained free of charge from The Cambridge Crystallographic DataCentre via www.ccdc.cam.ac.uk/data_request/cif.

Pressure–area experiments : The pressure–area (p–A) isotherms were ac-quired from a computer-controlled double-barrier Langmuir trough(KSV 5000 Finland). A trough of total area 772.5 cm2 was fabricatedfrom a single Teflon block with a dipping well at the center for transferof films, whereas the barriers were made of hydrophilic Delrin. Ultrapurewater was used as the subphase for all studies using a Millipore-Academ-ic system. The temperature of the subphase was controlled with a JulaboF-36 temperature controller with an accuracy of 0.1 8C. Typically, thetrough was cleaned with chloroform, followed by methanol (extrapureAR grade, SRL Fine Chemicals, India) several times, and finally rinsedwith ultrapure water. The 1 mm DPNME was spread from chloroform

Figure 21. a–c) AFM images of LB films of DPNME transferred onto a pretreated Si ACHTUNGTRENNUNG(100) substrate at a surface pressure of 10 mN m�1, illustrating char-acteristic vesicular architecture. d) Cross section of a �250 nm vesicular architecture.

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A. Patnaik and K. S. Kumar

(100 mL; Uvasol, Merck) solution onto the surface of ultrapure water(Millipore-Academic). Surface pressure was measured by the Wilhelmymethod with a platinum sensor of accuracy 0.1 mN m�1. A delay of30 min was allowed for the solvent to evaporate before acquiring the iso-therms; monolayers were compressed at an optimized speed of5 mm min�1. LB films were prepared by transferring DPNME Langmuirfilms at desired pressures by the single withdrawal method on hydrophi-lized Si ACHTUNGTRENNUNG(100) and quartz plates. Hydrophilization was done by etching thesubstrates with hot piranha solution (3:1 concd H2SO4/H2O2) at 70 8C fol-lowed by rinsing with Millipore water. The substrates were freshly etchedprior to use and stored in Millipore water.

Atomic force microscopy : AFM images of the DPNME LB films wereacquired with an XE-100 instrument, Park Systems, Korea.

Acknowledgements

The authors are grateful to the Department of Science and Technology,New Delhi, India, for the financial support (Grant No. SR/S2/CMP-57/2006). K.S.K. thanks IIT Madras for the SRF grant.

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Received: October 15, 2010Published online: March 14, 2011

Chem. Eur. J. 2011, 17, 5327 – 5343 � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org 5343

FULL PAPERDimeric Association of a Fullerene–N,N-Dimethylaminoazobenzene Dyad