1-(Arylalkenyl)pyrenes – Synthetic, Structural, Photophysical, Theoretical, and Electrochemical...

FULL PAPER

DOI: 10.1002/ejoc.201100620

1-(Arylalkenyl)pyrenes – Synthetic, Structural, Photophysical, Theoretical, andElectrochemical Investigations

Muhammad Sharif,[a,b][‡] Sebastian Reimann,[a,b][‡] Kai Wittler,[c] Leif R. Knöpke,[b]

Annette-E. Surkus,[b] Christian Roth,[c] Alexander Villinger,[a] Ralf Ludwig,*[b,c] andPeter Langer*[a,b]

Keywords: Arenes / Polycycles / Pyrene / Density functional calculations / Electrochemistry / UV/Vis spectroscopy

As a new approach for tuning the electronic properties ofpyrene derivatives, we converted 1-bromopyrene into dif-ferent substituted styrenes using the Mizoroki–Heck reac-tion. Several 1-(arylalkenyl)pyrenes have been characterizedand their electronic properties studied by absorption andemission spectroscopy. The effect of the electronic ambienceon the emission spectra of these compounds is discussed.Amongst the intramolecular influences, such as electron do-nating or withdrawing groups, other influences in the formof solvatochromatism are considered. Electrochemical oxi-

Introduction

Pyrene is an important and thoroughly investigated or-ganic chromophore. Its electronic[1,2] and electrochemicalproperties[3,4] and its supramolecular chemistry[5] have beenwidely studied. Structural modifications[6] allow for manyapplications, which include the development of fluorescentdyes, optical sensors,[7] molecular electronics,[2,8] photovol-taic cells, and field-effect transistors.[8] This wide range offabrication techniques coupled with our synthetic ability al-lows the realization of smart materials for future demandand application. For the synthesis of a wide variety of alk-enyl conjugated organic materials the Heck coupling ap-proach has been proved to be one of the best establishedtechniques.

The synthesis and the photophysical properties of substi-tuted pyrenes have been studied previously. For example,1-arylpyrenes and 1-ethynylpyrenes have been prepared bySuzuki–Miyaura and Sonogashira cross-coupling reactionsof halogenated pyrenes and characterized with regard totheir absorption and emission spectral properties.[2,9,10] In

[a] Leibniz-Institut für Katalyse e.V. an der Universität Rostock,Albert-Einstein-Str. 29a, 18059 Rostock, Germany

[b] Institut für Chemie, Universität Rostock,Albert-Einstein-Str. 3a, 18059 Rostock, GermanyFax: +49-381-498-6410E-mail: [email protected]

[c] Institut für Chemie, Universität Rostock,Dr. Lorenz Weg 1, 18059 Rostock, Germany

[‡] These authors contributed equally to this paper.Supporting information for this article is available on theWWW under http://dx.doi.org/10.1002/ejoc.201100620.

Eur. J. Org. Chem. 2011, 5261–5271 © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 5261

dation potentials determined by DPV (differential pulse vol-tammetry) are discussed with regard to substituent effects.The fine structure of the absorbance spectra obtained fromphotophysical measurements is compared with theoreticalcalculations performed by time dependent B3LYP DFT (TD-DFT) methods using the 6-31G* basis set. In this context, wediscuss the calculated potential energy surfaces and geomet-ric structures with regard to the substitution pattern of thepyrenes.

this aspect, the fluorescent properties of the parent pyrenesare well known and characterized by long excited-state life-times and distinct solvatochromic shifts. In addition, pyr-enes exhibit a characteristic excimer formation in concen-trated solutions and in the solid state, due to self-associa-tion by π–π stacking.[11,12,13a] These effects are disadvan-tageous because they lead to a dramatic decrease of thefluorescence intensity and to less defined, broadened fluo-rescence spectra. However, excimer formation of pyrenescan be used to study the phenomenon of aggregation.Furthermore, the sensitive solvatochromic shifts of pyreneshave been used to explore the inner structure and polarityof dendrimers by introducing the pyrene moiety into theouter and inner areas of the dendritic structure.[13b]

As a further extension of our research and investigationon the design and development of new and interesting ma-terials, we have designed a set of molecules based on 1-(arylalkenyl)pyrenes (Scheme 1). We selected four substi-tuted alkenylpyrenes containing electron donor or electronacceptor substituents and investigated the general trends of

Scheme 1. Synthesis of alkenylpyrenes bearing a styrene moiety.

R. Ludwig, P. Langer et al.FULL PAPERtheir absorption and emission spectral properties and theirelectrochemical properties.[14] Moreover, the fine structureof the absorption spectra were compared with the electronicstates derived from theoretical calculations performed bythe B3LYP TD-DFT method using the 6-31G* basis set.In this context, we discuss the calculated potential energysurfaces and geometric structures with regard to the substi-tution pattern of the pyrenes.

Results and Discussion

Synthesis and Structure

The reaction of pyrene with hydrobromic acid in thepresence of hydrogen peroxide afforded 1-bromopyrene (1,Scheme 2).[2,15]

Scheme 2. Synthesis of 1, i) HBr (1.1 equiv.), H2O2 (1.0 equiv.),CH3OH/Et2O (1:1), room temp., 12 h.

The Mizoroki–Heck reaction of 1 with styrenes 3a–g(1.2 equiv.) afforded the 1-pyrenylstyrenes 4a–g in 72–94%yield (Scheme 3, Table 1). The reaction conditions were op-timized for compound 4b (Table 2).

Scheme 3. Synthesis of 4a–g, i) 1 (1.0 equiv.), 3a–g (1.2 equiv.),K2CO3 (2.0 equiv.), Pd(OAc)2 (5 mol-%), XPhos (10 mol-%),DMF, 60 °C, 6 h.

Table 1. Synthesis of 4a–g.

3, 4 Ar 4 (% yield)[a]

a C6H5 79b 4-MeC6H4 92c 4-tBuC6H4 94d 4-(MeO)C6H4 89e 4-(tBuO)C6H4 86f 4-(AcO)C6H4 77g 4-ClC6H4 72

[a] Yields of isolated products.

Time, temperature, and choice of base played an impor-tant role. The best yield was obtained when the reactionwas carried out using Pd(OAc)2 (5 mol-%), the biarylmono-phosphane ligand XPhos (10 mol-%), and when the reac-tion was carried out in DMF at 60 °C for 6 hours. The

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Table 2. Optimization of the synthesis of 4b.

Entry Cat. (mol-%) L (mol-%) T [°C] Base t [h] % Yield[a]

1 Pd(OAc)2 (10) XPhos (10) 120 TEA[b] 12 02 Pd(PPh3)4 (10) – 120 K2CO3 12 03 Pd(OAc)2 (5) XPhos (10) 110 NEt3 10 224 Pd(OAc)2 (5) XPhos (10) 100 K2CO3 10 605 Pd(OAc)2 (5) XPhos (10) 80 K2CO3 8 726 Pd(OAc)2 (5) XPhos (10) 60 K2CO3 6 92

[a] Yields of isolated products. [b] Triethanolamine.

yields significantly decreased when the temperature was in-creased. It was evident that the use of potassium carbonatewas also important. However, the application of Pd-(PPh3)4, which is a widely used catalyst for palladium-cata-lyzed coupling reactions in the presence of K2CO3, provedto be unsuccessful.

The structures of all products were confirmed by spectro-scopic techniques (NMR and IR spectroscopy and massspectrometry). The structures of products 4f and 4g wereconfirmed by X-ray crystal structure analysis (see Figures 1and 2).[16]

Figure 1. Ortep plot of 4f.

Figure 2. Ortep plot of 4g.

The molecules in the unit cell of compound 4g are ar-ranged in parallel sheets opposed to each other and exhibitan average separation of 4.2390(35) Å between the molecu-lar planes (Figure 3). In this case, π stacking interactionsare parallel superimposed.[17] The packing behavior of thesemolecules differs from the herringbone motif observed incrystalline pyrene [3.487(4) Å].[18]

1-(Arylalkenyl)pyrenes

Figure 3. Crystal packing of 4g showing molecules in the unit cellwith parallel superimposed order (above) and molecules of threeparallel sheets with an interplanar distance of 4.2390(35) Å (be-low).

Photophysical Properties

Absorption Spectra

The electronic absorption spectra of 4d, 4e, 4f, and 4g(Figure 4) contain characteristic patterns similar to those inthe spectra of arylethynylpyrenes.[2] Yang et al. describethese patterns as typical of pyrene spectra. Hence, the as-signments of pyrene were the base for assigning the bandsof the pyrenylstyrenes.

The spectrum of pyrene contains three strong bands ofslightly decreasing intensity with increasing wavelength inthe 215–245, 245–280, and 285–350 nm regions. A fourthlow intensity band appears between 350 and 375 nm.[19] All

Eur. J. Org. Chem. 2011, 5261–5271 © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org 5263

Figure 4. Overview of the absorption spectra of 4d, 4e, 4f, and 4g(c = 1�10–6 mol/L in n-hexane).

four bands can be characterized as π�π* transitions. Acomprehensive spectroscopic investigation of pyrene hasbeen carried out by Becker et al.[20] who assigned the statesof pyrene as well as the main transitions more preciselyusing Platt’s nomenclature for polycondensed hydro-carbons. Generally, all the bands of the pyrenylstyrenes 4d,4e, 4f, and 4g are shifted to higher wavelengths. This mightbe an effect of the extension of the π-electron system. Thesame conclusion was drawn by Yang et al. for the spectraof arylethynylpyrenes.[2]

On closer examination the absorption spectra of the pyr-enylstyrenes 4d, 4e, 4f, and 4g contain fine structure. Thisfine structure is less distinct than that of the parent pyrene.The root of the fine structure may be vibronic bands whichappear in nonpolar solvents such as n-hexane. This effecthas been described in detail for trans-stilbene by Suzuki[21]

and for pyrene by Kalyanasundaram and Thomas.[22] Su-zuki points out that planarity of the molecule is essentialfor the appearance of the fine structure. Therefore, the spec-trum of the nonplanar cis-stilbene shows no fine structure.

The absorption spectra of the pyrenylstyrenes can be sep-arated into two groups: 4d and 4e as one group and 4f and4g as the second group. Generally the spectra of 4d and 4eshow a broader 1La transition compared with those of 4fand 4g. The broadening is not symmetrical; the transitionat 372/374 nm remains equal and the transitions between330 and 360 nm become more intense for 4d and 4e. Analy-sis of the 1Bb transitions confirms that 4d and 4e are similarto each other as well as 4f and 4g. For the first group, thetransition shows one band at 278 nm with shoulders at269 and 295 nm. Conversely, the second group has only oneband at 288/289 nm and one shoulder at 279/281 nm. Fi-nally the 1Ba transition appears at 236 and 238 nm for 4dand 4e and at 233 nm for 4f and 4g. The positions of the 1Ba

transitions in the spectra are contrary, which is expected.

R. Ludwig, P. Langer et al.FULL PAPERConsidering the electronic properties of the substituents

it is obvious that the broader 1La transitions refer to 4d and4e, which possess electron donating groups. The electronwithdrawing groups of 4f and 4g do not cause such a broad-ening. This shift to longer wavelengths due to electron with-drawing groups is already known from trans-stilbene,[21] styr-ylpyrenes,[23] and arylethynylpyrenes.[2]

The 1Bb and 1La transitions of the substances with elec-tron withdrawing groups are shifted to longer wavelengthscompared with the spectra of those with electron donatingsubstituents. Conversely, the 1Ba transition shows the oppo-site behavior to the 1Bb and 1La transitions. Interestingly,the forbidden 1Lb band does not appear in the spectra ofthe pyrenylstyrenes. We measured the substituted pyrenesunder the same conditions and could not observe the 1Lb

band either. So it is likely that our measurement conditionsare not sufficient for observing this forbidden transitionwhich appears generally at very low intensity.

Emission Spectra

Figure 5 shows the emission spectra of the pyrenylstyr-enes 4d, 4e, 4f, and 4g. Surprisingly the emission spectra donot bear a mirror image relationship to the absorption spec-tra, which was pointed out by Nakajima when he studiedthe absorption and emission spectra of pyrenes.[24] This lossof the mirror image relationship is analogous to the spectrapublished by Kikutchi et al.[23] These spectra, as well as ourspectra of 4d, 4e, 4f, and 4g, contain two bands at shorterwavelengths and two shoulders at longer wavelengths.

Figure 5. Normalized emission spectra of selected compounds (c =1�10–6 mol/L in n-hexane).

Compared to the fluorescence spectra of pyrene, thespectra of the pyrenylstyrenes contain no fine structure. Aloss of fine structure by photochemical induced isomeri-sation from trans � cis or ionic photoisomerisation can bediscounted.[2] Hence, other effects have to take precidence.Yang et al. gave an explanation based on intramolecularcharge transfer due to the effect of the electron donating oraccepting substituent to the molecule.[23] In other words, theelectronic ambience is responsible for the loss of the finestructure. Strong electron donating or withdrawing groupsshould cause a greater loss of fine structure than weakergroups. This could explain why the spectra of the pyrenyl-


styrenes 4d, 4e, 4f, 4g, and similar published compoundshave less fine structure than pyrene.[2,24] Moreover, this ef-fect should not be restricted to substituent groups on themolecule and should also be valid for solute–solvent inter-actions. Tran-Thi et al. point out the effect of solvent–soluteinteractions on fluorescence spectra in their work on thesolvatochromatism of pyrenol and pyranine. They ex-plained the loss of fine structure by the formation of hydro-gen bonds.[25] In order to elucidate the loss of fine structureand to prove the previous conclusions, solvatochromaticstudies of the solute–solvent interactions have been carriedout.

Solvatochromatic Studies

Solvatochromatism is the ability of a compound to shiftcertain spectroscopic signals or bands with a change of sol-vent. The first systematic solvatochromic studies were car-ried out by Scheibe[26] and Burawoy.[27] Solvatochromatismfor aromatic hydrocarbons was firstly described by Ham[28]

for benzene and is known as the Ham effect. This effect isnot restricted to benzene and can be observed with manybenzene derivatives such as pyrene.[24] Jiama points out thatthe solvatochromatism of pyrene is rooted in changes in thevibronic band structures. Due to effects of solvent polarity,the forbidden vibronic bands (assigned 0–0 and ag vibrationband) become more intense. The intensity enhancementscan be accounted for by solute–solvent interactions, whichmix the allowed and forbidden solute transitions.[29] Wewere unable to observe the forbidden 0–0 or 1Lb transitionin the absorption spectra. Therefore, we applied fluores-cence spectroscopy to study the solvatochromatism of 4d,4e, 4f, and 4g.

The results of the solvatochromic investigations areshown in Figure 6 and Table 3. The solvatochromatism hasbeen studied with respect to the shift of the β-transition.All of the compounds show the same trends so they arediscussed as one. The degree of solvatochromic shift corre-lates in all cases with the type of solvent. In solvents such asn-hexane or ethyl acetate the β-transition occurs at around440 nm and in solvents such as dimethylformamide or di-methyl sulfoxide at around 450–455 nm. Furthermore,emission spectra of 4d, 4e, 4f, and 4g have a distinct struc-ture with a typical pattern of two transitions, followed bytwo shoulders, which is best observed in n-hexane. A lossof this pattern caused by the solvent can be observed. Insolvents as dichloromethane where the β-transition isshifted to the longest wavelengths, the loss is stronger thanin other, less shifting solvents. Finally in dimethyl sulfoxideno pattern is observed. Additionally, the spectral intensitydecreases with the loss of pattern. This loss of intensity isnot shown, and, for reasons of clarity, the normalized spec-tra are shown only. The loss is caused by the effect of thepolar solvents, which decrease the quantum yield and,therefore, the spectral intensity indicated by the loss ofpattern in the spectra caused by dipole–dipole interactionsbetween solute and solvent, which is in line with our pre-vious conclusions.


Table 3. Solvatochromatic shifts of selected compounds and selected solvent properties.

Solvent Wavelength λb(max) [nm] Polarity HBD HBA Dielectric Dipoleconstant moment

4d 4e 4f 4g π* α β ε D

n-Hexane 443 443 441 443 –0.08 0.00 0.00 1.89 –Ethyl acetate 444 444 442 446 0.55 0.19 0.45 6.02 1.78Dioxane 446 447 444 448 0.55 0.00 0.37 2.21 0.45Tetrahydrofuran 445 447 444 448 0.58 0.00 0.55 7.50 1.75Acetonitrile 445 448 443 446 0.75 0.00 0.35 37.50 3.92Dichloromethane 448 449 446 449 0.82 (0.30) 0.00 8.93 1.60Dimethylformamide 449 451 446 450 0.88 0.00 0.76 36.70 3.82Dimethyl sulfoxide 455 459 448 439 1.00 0.00 0.76 46.68 3.96

Figure 6. Solvatochromatic shifts of 4d in selected solvents (c =1�10–6 mol/L).

Regrettably, the solvents were not classified on their sol-vatochromatism in order to assign solvent properties to thesolvatochromic shift. This is because the dielectric constantε and the dipole moment D as were chosen as the influenc-ing variables.[22] Table 3 shows that these variables do notfit with the degree of solvatochromatic shift. Finally theclassification of Kamlet and Taft seems to be adequate toclassify the solvents[30] in order to describe our system. Thedescription can be made based on the Kamlet–Taft linearsolvation energy Equation (1).

XYZ = XYZ0 + aα + bβ + s(π* + dδ) (1)

The polarisability correction term d is 1.0 for aromatic,0.5 for polyhalogenated, and zero for nonchlorinated ali-phatic solvents. So except for dichloromethane, δ is zero,and the coefficient d is mostly zero or finite negative. Asimplified equation can be used [Equation (2)].

XYZ = XYZ0 + aα + bβ + sπ (2)

Furthermore the equation contains a, b, and s as solvent-independent coefficients that reflect the susceptibility of thepolarity terms upon XYZ. XYZ0 is a solvent free referencespectrum and XYZ the spectrum of the system where solva-tochromatism takes place. Moreover, there are three param-eters: polarity/polarisability π*, hydrogen-bond donor(HBD) ability (α), and hydrogen-bond acceptor (HBA) abil-ity (β). This gives us the opportunity to distinguish betweensolvent-to-solute and solute-to-solvent hydrogen bonds.


Additionally the Kamlet–Taft parameters are obtained em-pirically due to solvatochromatism.

Table 3 shows that the shift to longer wavelengths due tothe influence of the solvent correlates with the polarity (π*)of the solvent. Only the value of 4g in dimethyl sulfoxidewith λb(max) = 439 nm does not fit into the order, and unfor-tunately there is no explanation for this deviation. However,referring to the other values of 4g the Kamlet–Taft classifi-cation fits here as well as for the other compounds. Gen-erally the π* values of ethyl acetate, dioxane, and tetrahy-drofurane are very similar (0.55, 0.55, and 0.58, respec-tively), which may cause interchange of their order with re-spect to the experimental error.

Considering that a fluorophore is a dipole because of itsbipolar structure, the fluorescence spectra should be affec-ted by dipole–dipole interactions. These dipole–dipole in-teractions could be solute–solvent interactions. The Kam-let–Taft parameter π*, which is the influencing variable forsolvatochromatism in this system, is the polarity or polaris-ability of the solvent. The solvatochromic shift is evidencethat the deactivation of the excited state does not take placeexclusively by fluorescence. The deactivation process partlytakes place as nonradiative deactivation in the form of vi-brations. This process is more likely in solvents with highpolarity, polarisability or π*. In summary, solvatochromaticstudies show that the electronic ambience, whether intramo-lecular (e.g. substituent group) or intermolecular (e.g. sol-vent), has a huge influence on the nature of the emissionspectra. The polarity π* of the solvent is sufficient to intro-duce solvent–solute interactions, which affect the electronicstate of the solute.

Electrochemistry

DPV measurements were carried out to determine theelectrochemical activity and the potential values of the 1-alkenylpyrenes. Figure 7 shows the influence of differentsubstituents on the oxidation potential. Most of the sub-strates exhibit two oxidation peaks. One of which is alwayslocated at 1.4 V. This peak is only weakly developed andcan also be found in the DPV of the starting material. Thesecond oxidation peak is mostly well developed and locatedbetween 1.0 and 1.2 V. The exact position of this peak de-pends on the substituent. For 4d and 4e, containing elec-tron-donating substituents, a slight shift to negative poten-

R. Ludwig, P. Langer et al.FULL PAPERtial (1.05 V) is observed. For 4f and 4g, containing electron-withdrawing substituents, a slight shift to positive potential(1.15 V) is observed.

Figure 7. Oxidative DPV measurements of the substituted pyrenesin DMF (0.1 mol/L TBABF4); working electrode: platinum.

Computational Studies

For a better understanding of the geometric and elec-tronic structure of the molecules, we carried out DFT calcu-lations using Becke’s three parameter set with Lee–Yang–Parr modification (B3LYP) and the 6-31G* basis set for thegeometry optimization. Furthermore, for the calculation of

Table 4. DFT calculated ground state geometry, electrostatic potential surface, HOMO and LUMO orbitals and dipole momentum.


the excitation energies we used the TD-DFT method withthe B3LYP functional. The 6-31g* basis set is small butyields a better fit of excitation energies to the experimentaldata than larger basis sets, which result in a stronger red-shift of the bands in the absorbance spectra. As shown inTable 4, the geometry optimizations reveal that the prod-ucts and pyrenylstyrene are not planar. The phenyl andpyrene groups are twisted along the ethylene group primar-ily as a result of steric barriers. However, the X-ray crystalstructures of the compounds show that the pyrene moiety,the double bond, and the phenyl ring (for products 4d–g)are planar (vide supra). This result suggests that the planarstructures seen in the solid state might be a result of thecrystal packing (π-stacking).

The nature of the substituent does not have a significanteffect on the distances between the carbon atoms in thepyrene group (1.36 Å � rCC � 1.44 Å in each case, Sup-porting Information) nor on the phenyl side (1.39 Å � rCC

� 1.41 Å). Actually, the angular relationship between thepyrene and ethylene group is not affected by the substitu-ents, the bond angle (α234) is 127.5° throughout, and thecorresponding dihedral angle ϑ1234 is about 37°. A slighteffect can be observed at the phenyl side, depending on thefunctional group R at the phenyl the dihedral angle (ϑ3456)between the phenyl and ethylene group vary from 15.71° (R= H) to 11.79°(R = OMe) (Table 5). The substituents areterminal, so this variation is not caused by steric effects butrather their electronic nature. Donors such as the alkoxygroups (4d, 4e) lead to a more contracting effect than theacceptors such as acetate or chloride (4f, 4g).


Table 5. Dihedral angles ϑ (°) between the ethenyl group and pyr-enyl plane (ϑ1234) and the phenyl plane (#3456), respectively. Thecorresponding atom indices are depicted on the right.

Molecule ϑ3456

4d 11.794e 13.374f 14.254g 14.72Styryl 15.71

Table 4 shows the HOMO and LUMO orbitals, as wellas the electrostatic potential, which show that twisting doesnot significantly disturb the conjugation. Both the HOMOand LUMO orbitals are allocated over almost the entiremolecule in 4d–g and the styrylpyrene. This finding is con-trary to the allocation of HOMO and LUMO orbitals ofthe ethynylpyrenes discussed by Yang et al. TD-DFT calcu-lations give us two significant magnitudes for each exci-tation (I): the excitation frequency (ωI) and the correspond-ing oscillator strength (fI). To determine the shape of thespectrum we calculated the sum of the Gaussian functionsdepending on fI, ωI, and the half band width Δ1/2,I as shownin Equations (3) and (4).

(3)

(4)

with the normalization factor cI.A wave number of 3000 cm–1 is assumed for Δ1/2,I. The

visualization of the single Gaussian functions facilitates thefinding of the contributions of the bands concerned in theabsorbance spectrum.

The wavelengths of the lowest excitation state, denotedas band 1La (according to the notation in Figure 4) areabout 402–409 nm for 4d–g and 396 nm for the unsubsti-tuted styrylpyrene (Figure 8, a–e, and Table 6). Consideringthat the TD-DFT excitation energies are typically slightlyredshifted with respect to the experimental data, the lowestexcitation is the one electron HOMO–LUMO or π–π* tran-sition, which refers to the 1La band at about 370 nm in theabsorbance spectra for 4d–g. There is only one contributionthat shapes the 1La band (red dashed lines in Figure 8, firstrow in Table 6). The second contribution is too weak tohave an effect on the shape in general. Comparing the first


excitation energies of the products 4d–g (Table 6) and thecorresponding HOMOs and LUMOs (Table 4) with that ofthe unsubstituted styrylpyrene, we find that each substitu-ent at the phenyl group enhances the spatial distribution ofthe π system and leads to a redshift of the transition ener-gies. The redshift of 4f and 4g is smaller than that of thealkoxy products. The electrostatic potential surfaces �which imply the electronic density � of 4d and 4e in Table 4point out that the corresponding electron donating substit-uents enhance the electron density of the conjugated ethynylmoiety. Obviously, the higher density of the π systems, be-longing to alkoxy derivatives, leads to a slight redshift ofabout 2–3 nm of the 1La band compared to those of 4f and4g. In addition, alkoxy groups with extended side chainssuch as that in 4e lead to a stronger donor effect and largerredshift.

The root of the broadening of the 1La band in the experi-mental absorbance spectrum for product 4d may be vi-brational transitions.

A further maximum, denoted as 1Bb, is determined atabout 300 nm for all products, caused primarily by the exci-tation state 4 (distinct blue dashed lines in Figure 8). How-ever, for the alkoxy products the excitation state 4 has asmaller oscillator strength (f ≈ 0.3) than that of 4e, 4g, andstyrylpyrene (f ≈ 0.45). However, the corresponding exci-tation state 5 (f ≈ 0.15) of 4d and 4e at about 295 nm inter-feres with state 4 significantly. Both of these contributionslead to 1Bb being quite similar to the corresponding bandof 4e and 4g. This interference may be due to the electrondonating effects of the alkoxy groups.

In the range from 200 to 270 nm a third distinct maxi-mum at about 225 nm can be observed in the calculatedspectra. Additionally, at about 260 nm in the spectra for 4dand 4e, a shoulder and a weak maximum appear, respec-tively. These noticeable patterns can be assigned to 1Ba andthey are the result of a set of single contributions (greendashed lines in Figure 8).

In the spectra of 4f, 4g, and styrylpyrene, these contri-butions lead to a weak shoulder at 240 and 235 nm, respec-tively. Considering styrylpyrene, the intensive single contri-butions in the spectra of the pyrenylstyrenes, 4f, 4g, and thealkoxy products are redshifted from 5 to 25 nm, which maybe caused by the extension of the π system on one side. Onthe other hand, the redshift of the contributions dependson the nature of the particular substituent. The electrondonating alkoxy groups are responsible for larger redshifts,and the electron withdrawing substituents cause a smallerredshift.

Considering the intensities of the experimental ab-sorbance spectra, the intensities predicted by TD-DFT cal-culations are weak and the redshift trends cannot be ob-served well by UV/Vis spectroscopy (see Figures 9 and 10).The determination of pure electronic excitations is compli-cated by the coherencies of vibrational effects and interac-tions with the solvent. This kind of effect is neglected byTD-DFT calculations. However, DVP studies confirm thatthe electron-donating substituents decrease the electrontransition energy (Figure 10).

R. Ludwig, P. Langer et al.FULL PAPER

Figure 8. Sum of Gaussian functions (black solid lines), single Gaussian functions (short dashed lines, colored according to the bands1La, 1Bb, 1Ba and II), and the experimental absorbance spectra (solid grey lines) for the products 4d–g and styrylpyrene. The right axiscorresponds to experimental data.

Table 6. Excitation states, excitation energies, and oscillator strengths f � 0.04, a 5% percentage of strongest band 1La, which are mainlyresponsible for the distinct maxima in the absorption spectra of the styrenyl pyrenes.

Molecule 4d 4e 4f 4g StyrylpyreneBand State λ/nm f State λ/nm f State λ/nm f State λ/nm f State λ/nm f

1La 1 404.6 0.85 1 408.5 0.90 1 402.9 0.82 1 401.8 0.86 1 396.6 0.751Bb 4 303.1 0.32 4 305.1 0.30 4 301.1 0.45 4 302.5 0.46 4 297.2 0.43

5 293.1 0.15 5 294.5 0.14 6 289.2 0.05 5 290.0 0.041Ba 11 258.1 0.14 10 261.5 0.10 13 240.0 0.26 11 258.7 0.06 14 235.8 0.33

13 242.2 0.10 11 259.7 0.09 14 240.8 0.2415 238.2 0.08 14 240.1 0.09 15 238.3 0.05

II 18 225.1 0.34 18 225.7 0.31 20 223.8 0.43 20 224.1 0.36 16 226.2 0.0619 223.2 0.14 19 223.9 0.18 22 223.1 0.14 18 223.1 0.4020 219.5 0.08 20 219.6 0.09

I 30 200.5 0.05 28 205.0 0.11 27 207.5 0.06 28 202.9 0.08



Figure 9. Bottom: Trends of maxima of the bands determined fromthe absorbance spectra and DFT calculations. Top: oxidation po-tential, from DPV measurements.

Figure 10. Trends of maxima of the 1Ba bands determined from theDFT calculations and absorbance spectra including the oxidationpotential from DPV measurements.

The maxima at 225 nm in the calculated spectra men-tioned above and the shoulder at 200 nm (according toBecker’s studies on pyrenes)[20] are assigned to II and I,respectively. There are no considerable differences in exci-tation wavelengths of the maxima and they are independentof the nature of the substituents. These shoulders indicatethe existence of a further maximum.

Conclusions

1-Alkenylpyrenes were prepared by the Mizokoki–Heckreaction of 1-bromopyrene with styrenes. The absorptionand emission properties of the products were studied. Sol-vatochromatic studies show that the electronic ambience af-fects the nature of the emission spectra. The polarity of thesolvent is sufficient to introduce solvent–solute interactions,


which affect the electronic state of the solute. These effectscan be linked with the Kamlet–Taft equation; with increas-ing polarity of the solvent the effect on the solute wasgreater. This was observed in the emission spectra in formof a redshift as well as the loss of fine structure. The electro-chemical characteristics were also studied by DPV measure-ments. The oxidation potential of the products depends onthe nature of the substituents. For a better understandingof the geometric and electronic structure of the molecules,calculations were carried out using the B3LYP TD-DFTmethod with 6-31G* basis set. The properties of the excitedstates depend on both the extent of the π system and theelectron withdrawing/donating nature of the substituents.The results suggest that the alkenyl moiety acts as a conju-gation bridge in all the molecules.

Experimental SectionChemicals and Solvents: Tetrakis(triphenylphosphane)palladium(0)was prepared according to the literature procedure. Pyrene, palladi-um(II) acetate, 2-dicyclohexylphosphanyl-2�,4�,6�-triisopropylbi-phenyl ligand (XPhos) and the corresponding styrenes were pur-chased from a commercial source (Aldrich, Acros). Solvents (DMF,DCM) were distilled and purged with argon before use. TEA, Et3N,and K2CO3 were purchased from a commercial source (Aldrich)and purged with argon before use. Potassium carbonate was driedand ground before use. TLC was performed with Merck precoatedaluminium plates (Si 60 F254). Column chromatography was per-formed using Merck Silica gel 60 (0.043–0.06 mm). 1-Bromopyrenewas synthesized according to a literature procedure.[16,17]

Synthesis of 1-Styrylpyrenes 4. General Procedure: Palladium(II)acetate (5 mol-%) and XPhos ligand (10 mol-%) were placed underan argon atmosphere in a pressure tube and DMF (5 mL) wasadded. After stirring for 15 min, 1, 3, and K2CO3 were added. Sub-sequently, the mixture was heated at 60 °C for 6 h. To the mixturewere added water and CH2Cl2 (20 mL) and the organic and theaqueous layers were separated. The aqueous layer was washed withCH2Cl2 (2�20 mL). The combined organic layers were dried(Na2SO4), filtered, and the filtrate was concentrated in vacuo. Theresidue was purified by a column chromatography (hexane/ethylacetate).

Data Collection: NMR spectroscopic data were recorded withBruker ARX 300 and Bruker ARX 400 spectrometers. 13C and 1HNMR spectra were referenced to signals of deuterated solvents andresidual protonated solvents, respectively. GC–MS was carried outwith an Agilent HP-5890 instrument with an Agilent HP-5973Mass Selective Detector (EI) and HP-5 capillary column using he-lium carrier gas. ESI HRMS measurements were performed withan Agilent 1969A TOF mass spectrometer.

All UV/Vis spectra were recorded with a Lambda 5 (Perkin–Elmer)spectrophotometer with a solution concentration of 10–6 mol/L. Allfluorescence spectra were recorded with a Hitachi F-4010 fluores-cence spectrophotometer using similar solution concentrations invarious solvents. The solvents used (ethyl acetate, hexane, DCM,CH3CN, THF, DMF, DMSO, dioxane) were distilled before use.

DPV studies were performed at room temperature in dry DMFunder an argon atmosphere in the presence of tetrabutylammoniumtetrafluoroborate (0.1 mol/L) as a conducting salt using an Autolab

R. Ludwig, P. Langer et al.FULL PAPER(PGSTAT 302N, Eco Chemie). The working electrode was a plati-num disk electrode (Eco Chemie, d = 2 mm), and the counter elec-trode was homemade Pt plate electrode. The reference electrode Ag/AgCl/ 3 m KCl (Sensortechnik Meinsberg GmbH) was connectedthrough a 3 m KCl salt bridge. All potentials were measured withregard to this reference system and were checked by using a ferro-cene/ferrocenium internal reference. The CV scans were repeatedthree times at a scan rate of 25 mVs–1. The differential pulse vol-tammograms were performed in oxidative and reductive directionswith a scan rate of 5 mVs–1 (step potential 2.5 mV, modulationamplitude 25 mV, modulation time 0.05 s, interval time 0.5 s). Con-centrations of 1 mmol/L analyte were used for the measurements.

Theoretical Calculations: The optimization of the structures andelectrostatic potential surface were calculated with the 6-31g* basissets and the Becke3LYP density functional method using theGAUSSIAN-03-Package. For the B3LYP method the Becke-3 pa-rameter gradient corrected exchange functional is combined withthe gradient-corrected correlation LYP functional by Lee, Yang,and Parr. Calculation of the UV/Vis excitation energies requires thesolutions of the time dependent Schrödinger equations and wascarried out by using the TD-DFT method with the B3LYP func-tional. Orbitals and energies, atomic charges, vibrational modes,and thermodynamic properties were chosen as output parameters.HOMO and LUMO orbital surfaces and electrostatic potentialdensity maps were then obtained from the output.

Supporting Information (see footnote on the first page of this arti-cle): Materials and methods, experimental procedures and charac-terizations, 1H and 13C NMR spectra, solvatochromatic spectra,theoretical calculations, crystal data and structure refinement.

Acknowledgments

The authors would like to thank Prof. Dr. O. Kühn and Dr. Schu-mann for scientific discussions. Financial support by the Universityof Rostock (interdisciplinary faculty of the University of Rostock/Dept. LLM, scholarship to S. R. and C. R.) and the State ofMecklenburg-Vorpommern is gratefully acknowledged. L. K.would like to thank the Leibniz Society for financial support.

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Received: May 3, 2011Published Online: July 28, 2011

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