Journal of Molecular Structure 1047 (2013) 80–86

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Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

The electronic characterization of conjugated aryl-substituted2,5-bis(2-thien-2-ylethenyl) thiophene-based oligomers

0022-2860/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.molstruc.2013.04.054

⇑ Corresponding author.E-mail address: kgordon@chemistry.otago.ac.nz (K.C. Gordon).

John C. Earles a, Holly van der Salm a, Pawel Wagner b, David L. Officer b, Keith C. Gordon a,⇑a Department of Chemistry, MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Otago, PO Box 56, Dunedin 9054, New Zealandb ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, University of Wollongong, Australia

h i g h l i g h t s

� A series of thiophene oligomers were synthesised and their electrochemistry characterized.� Polaron and bipolaron species are formed and their optical spectra measured.� DFT methods are used to model the optical spectra which are consistent with the Furukawa model showing intra-band gap states.� Oligomers without methyl end-caps show dimerization with oxidation; methyl end-capped systems do not.

a r t i c l e i n f o

Article history:Received 10 February 2013Received in revised form 18 April 2013Accepted 18 April 2013Available online 28 April 2013

Keywords:Thiophene oligomersDFTElectrochemistrySpectroelectrochemistry

a b s t r a c t

A series of 2,5-bis(2-thien-2-ylethenyl) thiophene-based oligomers with a para-R-arylethenyl substituenthave been subjected to electrochemical (cyclic voltammetry and electronic absorption spectroscopy) anddensity functional theory characterization. The primary aim of this investigation is to characterize thebehavior of these oligomers in the oxidized state. Oligomers without methyl ‘end-caps’ undergo faciler-dimerization; however there is no evidence for the formation of higher oligomers. The oxidizedr-dimers exist in both cationic and dicationic form. Oligomers with methyl ‘end-caps’ do not showany evidence of r-dimerization. The inductive capacity of the para-R substituent has a significant bearingon the electronic properties of the oligomer, in particular, oligomers with more electron-withdrawingsubstituents have charge transfer character associated with the dominant electronic excitations.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Conducting polymers are a unique class of materials with thepotential to incorporate interesting electronic properties intocheap and readily processable organic molecules. The future devel-opment of organic based electronic devices is contingent upon thecharacterization of conjugated materials in both the neutral andactive (charged) states. However, owing to complications associ-ated with long chain polymers (polydispersity, lack of solubility),it is necessary to look at smaller systems, namely the oligomerunits from which they are constructed. These smaller structuralunits have well-defined chemical and spectroscopic characteristicsthat have proven in the past to be reliable models for better under-standing the corresponding polymers [1,2]. Furthermore, oligo-mers are interesting materials in their own right, having beenincorporated into a variety of electronic devices, including: photo-voltaic cells [3]; organic light emitting diodes [4] and thin filmtransistor devices [5–7]. Nonhomogeneous co-oligomers built up

from different conjugated units have been shown to exhibit im-proved electroluminescent properties compared with their homo-geneous analog [8–10]. The electronic properties of oligomers arelargely controlled by the delocalization of p-electrons [11,12]. Forsystems involving aromatic units, the extent of delocalization ismoderated by the resonance energy of the aromatic rings and theinter-ring rotations [12]. Oligomers such as 2,5-bis(2-thien-2-ylethenyl) thiophene that are based on alternating thiophene andvinylene moieties are often termed ‘nTVs’. The ethylene linkagesreduce steric interactions between neighboring thiophene rings,thus encouraging a more planar structure [12–15]. As a result ofthese phenomena nTVs have the longest effective conjugationlength and lowest band-gap of any p-conjugated oligomer per unitlength [14,16–18] and exhibit excellent charge transmission alongthe conjugation path; in fact, nTVs are the most efficient conduct-ing polymer molecular wires [16,19]. A number of recent studiesdescribe these properties. Li et al. [20] describe how the incorpora-tion of a thienylenevinylene units appended to a polythiophenebackbone can lead to enhanced performance in polymer solar cells.A similar type of strategy using cross-linked polythiopheneswith vinylene–terthiophene–vinylene bridges to improve

J.C. Earles et al. / Journal of Molecular Structure 1047 (2013) 80–86 81

charge-transport has been utilized by Gong et al. [21] The use ofthienylene–vinylene as molecular wires is exemplified by Casadoet al. [22] in which they show that ferrocene units terminally ap-pended to thienylene–vinylene units of varying chain lengths exhi-bit differing degrees of charge localization between the terminalferrocence upon formal oxidation of one of these units. For shorterchains, comprising of two or four thienylene–vinylene strong inter-ferrocene coupling is observed. As the chain is extended to six unitsweaker coupling is observed and at eight units no ferrrocene-to-ferrocene interaction can be detected with the electrochemicallyformed radical anion residing entirely on the bridging chain. Thestructural nature of the radical anion species was probed using Ra-man spectroscopy in concert with DFT calculations. Casado et al.show that the interferrocene coupling may be transmitted across40 Å; highlighting the ability of thienylene–vinylene to act as amolecular wire [22].

Ten molecules have been investigated and are divided into twoseparate groups (see Fig. 1). Group I is comprised of 2,5-bis-(2-thien-2-ylethenyl) thiophenes in which the central thiophene issubstituted with phenylethenyl containing the para-groups (in or-der of strongest electron donor to strongest electron acceptor) –OMe > –H > –CN > –NO2. These molecules have been investigatedboth with and without methyl ‘end-caps’ (at the 5-position of theterminal thiophenes). Group II is comprised of the two

(a) I-R1/R2 (b) II-R2

(c) (I-R1/H)2

(d) (II-H)2

Fig. 1. The structures and naming scheme of the oligomers under consideration.There are eight monomeric oligomers with the phenylethenyl substituent (a): R1 =–OMe, H, CN and NO2; R2 = H and Me. There are two non-substituted monomericoligomers (b): R2 = H and Me. Oligomers without methyl end-caps (R2 = H) canundergo r-dimerization to yield (I-R1/H)2 (c) and (II-H)2 (d).

(end-capped and non-end-capped) 2,5-bis(2-thien-2-ylethenyl)thiophenes without the phenylethenyl substituent.

We have previously reported a spectroscopic and computa-tional characterization of this series of oligomers, with a particularemphasis on the confinement/delocalization of the charged defectthat is formed upon oxidation [13]. We present here a follow upstudy in which the nature of the charged defect (polaron or bipola-ron) is assigned and the electronic characteristics of the oligomersin both the neutral and oxidized state are discussed. The nature ofthe thienylenevinylene unit lends itself to a variety of conforma-tions and a discussion of those that exist in the compounds ofinterest has already been given [13]. In brief, the structures showE isomers only which are consistent with the NMR data [13]. E,Zisomers have been observed in the synthesis of aldehydes to ulti-mately form porphyrin–oligothienylenevinylene–C60 systems[23]. In addition the conformation of the thiophene ring with re-spect to the ethylene link may be anti or syn. In our previous stud-ies [13] the anti conformation (as depicted in Fig. 1) is the lowestenergy form; this is consistent with other researchers, such asCasado et al. [22].

The polymerization of thiophene-based oligomers has beenproposed to proceed via a radical–radical coupling mechanisminvolving the a positions of the terminal thiophene rings [24]. Thisis a facile process: the terthiophene radical cation exists on themicrosecond time scale [25–28] before undergoing r-dimerizationto form the sexithiophene. However, the reaction does not proceedwhen methyl end caps are on the a-positions of the terminal thio-phene rings. The end-capped and open-ended molecules provide ameans of examining the oxidized monomeric and dimeric speciesrespectively.

All non-degenerate conjugated polymers have an energeticpreference for the benzenoid bond alternation arrangement overthe quinoid one. Upon oxidation the positive charge is coupledvia a quinoidal conjugation path to either an unpaired electronor another positive charge to give the polaron and bipolaronrespectively. These are collectively termed ‘charged-defects’ andact as the mobile charge carriers. The polaron domain extends overthree to four [29,30] thiophene units in a polythiophene, while the

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Fig. 2. The cyclic voltammograms of (a) I-CN/H and (b) the end-capped analogI-CN/Me. Each voltammogram was recorded from a 1 � 10�4 mol L�1 solution in0.1 mol L�1 TBAP/acetonitrile with a scan rate of 100 V s�1. The potentials werecalibrated using Me10Fc+/0.

82 J.C. Earles et al. / Journal of Molecular Structure 1047 (2013) 80–86

bipolaron extends over six to nine [29,31–33]. The structural defor-mations (that is, changes in the bond length alternation arrange-ment) associated with the charged defects serve to raise theenergy of the HOMO and lower the energy of the LUMO, thus pro-ducing two localized intra-band gap states.

Table 1Cyclic voltammetry data recorded from 10�4 mol L�1 solutions (except those of I-NO2/Me, which were recorded from a saturated solution) in 0.1 mol L�1 TBAP/acetonitrile with a scan rate of 100 V s�1. The potentials were calibrated usingMe10Fc+/0. – Indicates that the peak is broad and ill-defined.

Oligomer First scan; ox/red Subsequent scans; ox/redV vs Ag/AgCl V vs Ag/AgCl

I-OMe/H 0.89/0.67 0.76/0.52 0.87/0.68I-H/H 0.95/– 0.79/– 0.93/0.75I-CN/H 0.97/0.83 0.75/0.56 0.95/0.83I-NO2/H 0.95/– 0.75/– 0.93/0.80II-H 0.98/– 0.78/– 0.97/–I-OMe/Me 0.82/0.77 1.07/0.96I-H/Me 0.87/0.81 1.10/0.99I-CN/Me 0.86/0.81I-NO2/Me 0.89/0.84II-Me 0.88/0.83

2. Results and discussion

2.1. Cyclic voltammetry

The cyclic voltammograms of I-CN/H and I-CN/Me are given inFig. 2 as representative examples of all non-end-capped and end-capped molecules, respectively.

The formation of higher oligomers is evident in the CVs of allmolecules without end-caps. The initial scan of I-CN/H (Fig. 2a) re-veals one cathodic peak at 0.97 V (vs Ag/AgCl), corresponding tothe formation of the radical cation. The cationic species is able toform the r-dimer. On subsequent forward scans there are twocathodic peaks observed at 0.75 V and 0.95 V; these are attributedto the oxidation of the r-dimer to form the cation and dication,respectively. The first oxidation potential for the r-dimer is signif-icantly lower than that of the monomer because of the increasedconjugation length.

It is anticipated that as the chain length increases beyond the r-dimer and the coulombic repulsion between like charges dimin-ishes, the difference in the potential of the two oxidation processesdecreases, until they begin to coalesce when the conjugation pathconsists of six to eight thienylenevinylene units [17]. Furthermore,as the conjugation path lengthens beyond eight TV units, addi-tional peaks assigned to the trication radical and tetracation areobserved [16,14,34]. However, there is no such evidence of poly-merization in the cyclic voltammograms of these systems: two dis-tinct redox processes remain at constant potentials throughoutmultiple scans.

Dimerization or higher oligomer formation that falls short ofcomplete polymerization has previously been observed for bothnTVs [14,16] and sexithiophenes [35]. Explanations profferedfor this behavior include polaron trapping [35], steric hindranceto planarity during the coupling process brought about by sub-stituents appended to the thiophene [16,14] and the stabilizationof the cation radical by the substituent [14]. We have previouslyshown that the phenylethylene substituent interacts with thenTV conjugation path to stabilize the cationic and dicationicstates of these oligomers and their r-dimers [13]. Furthermore,the phenylethylene substituent affords an alternative conjugationpath to the charged defects that does not extend to the a-termini [13]. As such, the spin density at the termini is markedlydiminished relative to the unsubstituted analog, thus hinderingthe terminal spin-coupling necessary for polymerization. All ofthese explanations involve the influence of substituents; how-ever, it should be noted that the CV scans of II-H do not showany clear evidence of polymerization (see supplementaryinformation).

The progressive increase in current with each additional sweepis also indicative of the formation of higher oligomers that are lesssoluble in solution and form a visible solid film on the workingelectrode, thus increasing the effective concentration of moleculesat the electrode.

The CV of I-CN/Me (Fig. 2b) exhibits one cathodic peak at 0.86 Vcoupled to a reduction at 0.81 V. The decreased oxidation potentialrelative to the first cathodic peak of I-CN/H can be explained as aresult of the electron donating effects of the methyl end-capswhich stabilize the positively charged species. Significantly, theCV is reproducible over multiple cycles, indicating that no higheroligomer formation or degradation processes take place.

The oxidation potentials for all species studied are presented inTable 1. Electron-donating groups give rise to a more stable radicalcation and result in a lower oxidation potential. AccordinglyI-OMe/H and I-OMe/Me (which have the most electron-donatingsubstituent) give the lowest oxidation potentials of 0.89 and0.82 V, respectively.

2.2. Electronic absorption spectroscopy

Electronic absorption spectroscopy and TDDFT calculated dataare given in Table 2. The correlation between the experimentallyobserved and the calculated electronic transitions of I-CN/H,I-NO2/H and II-H are shown in Fig. 3. TDDFT calculations areknown to exhibit deviations from experimentally observed transi-tion energies of up to 0.7 eV for both neutral [36,37] and charged[38,39] species. Accordingly, only a qualitative comparison of the-oretical and experimental data can be performed. The B3LYP func-tional does not describe charge-transfer interactions well, and forthis reason we have also conducted TDDFT using the modifiedCAM-B3LYP functional which has been successfully used oncharge-transfer systems [40].

The data (experimental and TDDFT) shown for I-CN/H (Fig. 3a)are typical of all I-OMe/R, I-H/R and I-CN/R type oligomers. Thereare two strong absorption bands both of which exhibit vibrationalfine structure. The high energy band has peaks at 342 and 357 nmand has been assigned to a TDDFT calculated excitation at 374 nmthat is primarily (73%) HOMO � 1 ? LUMO in nature. The low en-ergy band is comprised of a prominent peak at 420 nm that isflanked by vibrational shoulders at 398 and 444 nm. This band isassigned to a TDDFT calculated excitation at 472 nm that is primar-ily HOMO ? LUMO in nature for B3LYP; for CAM-B3LYP the tran-sition is predicted at 424 nm, with similar character. The vibronicstructure on the lower energy transition is confirmed for I-NO2/Husing resonance Raman spectroscopy see Fig. 4.

The resonance Raman effect causes an enhancement of Ramanbands. The enhancement pattern is not random; rather vibrationalmodes that mimic the structural changes attendant with the elec-tronic transition are preferentially enhanced [41–43]. Thus if thenature of the vibrational modes is known then the pattern ofenhancement reveals the sections of the molecule that are the ac-tive chromophore. These data show identical enhancement pat-terns for the compound at 406, 413 and 448 nm excitation. Thisis consistent with a single electronic transition, which supportsthe TDDFT data. Furthermore the pattern of enhancements sug-gests that the transition involves perturbation of the thiophene,phenylethene and NO2, which is consistent with a charge-transferfrom thiophene to phenylethene and NO2, as predicted.

The correlation between the electronic absorption spectrum ofII-H and the corresponding TDDFT data are depicted in Fig. 3c.

Table 2Experimental electronic absorption spectroscopy data and calculated TDDFT and population analyzes associated with the transition. The experimental spectra were recordedfrom 1 � 10�5 mol L�1 solutions in acetonitrile except that of I-NO2/Me which was recorded in dichloromethane (I-NO2/Me is sparingly soluble in acetonitrile).

Molecule Experimental B3LYP CAM-B3LYP Mulliken population analysis (% change)

k (nm) � (mM�1 cm�1) k (nm) f CI (% contribution) k (nm) f CI (% contribution) Thiophene Phenylethenyl R1

I-OMe/H 332 56.5347 55.8 389 1.04 H,L + 1 (50) 334 1.56 H,L + 1 (61) 69 ? 74 27 ? 25 4 ? 1399 32.5421 41.7 484 0.83 H,L (75) 423 1.07 H,L (94) 79 ? 92 19 ? 7 2 ? 0443 33.4

I-CN/H 342 59.0357 58.7 374 0.90 H-1,L (73) 348 1.39 H,L + 1 (80) 86 ? 52 13 ? 42 1 ? 6398 34.5420 40.0 472 0.74 H,L (75) 424 1.00 H,L (93) 89 ? 75 11 ? 23 1 ? 2

I-NO2/H 375 49.8414 48.8 453 0.99 H,L + 1 (78) 362 1.35 H,L + 1 (69) 88 ? 58 11 ? 20 0 ? 21439 40.7 544 0.33 H,L (81) 429 0.92 H,L (83) 90 ? 49 10 ? 33 0 ? 18

II-H 387 37.1406 50.1 433 1.46 H,L (75) 403 1.40 H,L (95)429 38.0

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Fig. 3. Experimental electronic absorption spectra and corresponding TDDFT datafor (a) I-CN/H, (b) I-NO2/H, and (c) II-H. The spectra were recorded from1 � 10�5 mol L�1 solutions in acetonitrile. The TDDFT data were calculated usingthe B3LYP/6-31G(d) method. These data and corresponding data for all oligomersare given in Table 2.

1309

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448

Fig. 4. Resonance Raman spectra for I-NO2/H, with various excitation wavelengths.Spectra were recorded on 1 � 10�3 mol L�1 solutions.

J.C. Earles et al. / Journal of Molecular Structure 1047 (2013) 80–86 83

The spectrum exhibits a low energy band with vibronic structure,similar to I-CN/H. The band is centered at 429 nm and assignedto a calculated transition at 433 nm (for B3LYP) and 403 nm (forCAM-B3LYP); in both cases these are assigned as (75%) HOMO ?LUMO in nature.

The correlation between the electronic absorption spectrum ofI-NO2/H and the corresponding TDDFT data are depicted inFig. 3b. The electronic absorption spectrum is different to thoseof other I-R1/R2-type oligomers insofar as the two bands describedabove merge to form a single, broad and convoluted band.

The electronic absorption spectra of oxidized oligomer solutionshave been recorded in order to determine whether polarons orbipolarons predominate. Solutions were doped with Cu(ClO4)2 un-til additional oxidant no longer affected the spectrum and subse-quently reduced using a 2% hydrazine aqueous solution.

The formation of both polarons and bipolarons results in intra-band-gap states that are manifested in the electronic absorptionspectrum. The electronic absorption spectra of the oxidized specieshave been interpreted within the framework described by

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Fig. 5. Electronic absorption spectra measured during the chemical oxidation of (a)I-H/H, (b) I-CN/H, (c) II-H, and (d) I-OMe/Me. The spectra were recorded from 2 mLof 1 � 10�5 mol L�1 neutral solutions in acetonitrile (heavy, solid line). 100 lLaliquots of 8 � 10�4 mol L�1 Cu(ClO4)2 in acetonitrile were added stepwise, withspectra recorded at each step (dashed lines). Each solution was subsequentlyreduced by a 2% hydrazine aqueous solution (red line). (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version ofthis article.)

84 J.C. Earles et al. / Journal of Molecular Structure 1047 (2013) 80–86

Furukawa [44]. Within this framework, a polaron gives rise to twodominant electronic transitions labeled x1 and x2. Both of thesetransitions are lower in energy than the band gap of the corre-sponding neutral species. A bipolaron gives rise to a single transi-tion (labeled x01

� �) that is intermediate in energy to the transitions

associated with the corresponding polaron.The experimental electronic absorption spectra recorded during

the oxidation of I-CN/H are depicted in Fig. 5b and will be dis-cussed below. Initially, however, let us consider the absorptionspectrum following the reduction of the oxidized solution. Reduc-tion of the oxidized solution does not regenerate the original spec-trum; specifically a new and prominent band emerges at 515 nm.The band is red shifted relative to the HOMO ? LUMO transitionof the neutral monomer (444 nm), indicative of a longer conjuga-tion length, suggesting that r-dimerisation has taken place to formhigher oligomers. The new feature at 515 nm has been assigned tothe HOMO ? LUMO transition for the neutral r-dimer (I-CN/H)2.Therefore analysis of the spectra recorded throughout the oxida-tion of I-CN/H must be carried out with regards to the oxidizedr-dimer (as opposed to the oxidized monomer). These spectrashow the growth of two distinct bands at approximately 780 and1350 nm. The two band feature is indicative of polaron formationon the basis of Furukawa’s assignment and the bands are attribut-able to the x1 and x2 transitions, respectively [44]. The two bandsare significantly red-shifted relative to the neutral dimer (515 nm),indicative of the formation of mid-band-gap states. As more oxi-dant is added, less of the neutral monomer remains and the prom-inent bands of the neutral species gradually diminish in intensity.The electronic absorption spectra of the oxidation of most non-end-capped molecules exhibit similar evidence of r-dimer polaronformation.

There are two exceptions however, the first being the spectrarecorded throughout the oxidation of I-H/H, depicted in Fig. 5a. Po-laron formation is evident initially with two distinct peaks emerg-ing at approximately 800 and 1360 nm. However, an intense peak

at 1270 nm with a shoulder at 1100 begins to appear after one mo-lar equivalent of Cu(ClO4)2 oxidant is added; this feature is as-signed to the bipolaron. Following the addition of two molarequivalents of oxidant, the bipolaron features dominate the spec-trum. The greater intensity of the bipolaron transition and theassociated high energy shoulder have been observed in the spec-trum of oligothiophenes [45]. Clarke et al. [26] have reported theabsorbance spectrum of a structurally similar terthiophene basedoligomer and observed the intense bipolaron band at 984 nm.The red shift down to 1270 nm observed for ðI-H=HÞ2þ2 can be ex-plained in terms of the increased conjugation length impartedupon the molecule by the ethylene moieties.

The other exception is observed during the oxidation of II-H(Fig. 5c). Bipolaron formation is evident from the initial stages ofoxidation with one peak observed at approximately 900 nm, whichdoes not shift nor become resolved into separate peaks upon theaddition of excess oxidant.

A summary of the experimental and theoretical TDDFT data forthe oxidation products of the non-end-capped molecules is shownin Table 3. In general there is a close correlation between experi-mental and theoretical data. For example, in the case ofðI� CN=HÞþ�2 the lowest energy transition is from MO220b to221b. In this case the SOMO is 221a, so this corresponds to thex1 band in Furakawa’s model [44]. The next highest energy transi-tion, corresponding to x2 is 221a ? 222a. Furthermore, the elec-tronic transitions calculated for the neutral r-dimer correlate wellwith the experimental spectrum of the reduced species. The bandobserved experimentally at 515 nm corresponds to a calculatedband at 552 nm that is primarily (83%) HOMO ? LUMO in nature.The bipolarons observed experimentally in the spectra recordedduring the oxidation of I-H/H and II-H are likewise in good agree-ment with calculated results. Consider II-H, the intense bipolaronband is observed experimentally at 1270 nm, predicted at974 nm and assigned to the HOMO ? LUMO transition x01

� �.

The electronic absorption spectra recorded throughout the oxi-dation of I-OMe/Me are depicted in Fig. 5d and are typical of thecorresponding data for I-H/Me and II-Me (see Table 3). Throughoutthe oxidation a single band emerges at approximately 660 nm. Thisis calculated at 618 nm and is associated with 119b? 121b (wherethe SOMO is 121a). Experimentally these transitions are observedbetween 0.35 and 0.76 eV higher in energy than those of the corre-sponding r-dimers, indicative of their shortened conjugation path.

On the basis of Furukawa’s assessment a single peak is indica-tive of a bipolaron formation. However, Furukawa states that theabsence of a two band feature does not necessarily rule out theexistence of polarons [44]. In addition, the cyclic voltammetry datareported here clearly indicate that no higher oligomer formationtakes place upon the oxidation of the end-capped oligomers. TheTDDFT data calculated for the cationic species are an accurate rep-resentation of the experimental spectra insofar as they predict anintense transition in the 590–650 nm region.

It should be noted that the spectra recorded throughout the oxi-dation of I-CN/Me do not show any low energy peaks, while thoserecorded throughout the oxidation of I-NO2/Me exhibit a distincttwo band feature. This latter anomaly may be related to the factthat the spectra were recorded from solutions in CH2Cl2 (all otherexperiments were carried out using acetonitrile).

2.3. Molecular orbital and Mulliken population analyzes

A light-harvesting material with intramolecular charge transfercharacteristics is able to effect the necessary charge separation todecouple the excited electron and hole pair in an exciton, withouthaving to first travel to an interface. The calculated molecular orbi-tals for oligomers I-OMe/H, I-NO2/H and II-H are depicted inTable 4. The molecule with the most electron-donating substituent,

Table 3Experimental electronic absorption spectroscopy data and calculated TDDFT (CAM-B3LYP/6-31G(d)) for the oxidation products of the non-end-capped oligomers. Thespectra were recorded from 2 mL of 1 � 10�5 mol L�1 solutions in acetonitrile (exceptthose of I-NO2/Me, which were recorded in dichloromethane) that were oxidized byapproximately 1 mL of 8 � 10�4 mol L�1 Cu(ClO4)2. The data pertaining to the neutraldimers are those resulting from the reduction by a 2% hydrazine aqueous solution.Square brackets indicate the HOMO, or in the case of radical cations, the SOMO.

Oligomer Experimental Calculated

k (nm) � (mM�1 cm�1) k (nm) f CI (% contribution)

ðI-CN=HÞ�þ2 1350 20 1581 1.64 220b, 221b (90)780 24 971 1.05 221a, 222a (58)

[221a](I-CN/H)2 515 22 552 2.59 221, 222 (83)[221]ðI-H=HÞ�þ2 1360 40 1570 1.62 208b, 209b (91)

880 28 977 1.06 209a, 210a (60)[209a]

ðI-H=HÞ2þ21270 80 974 3.35 208, 209 (93)

1100 42 603 1.22 206, 209 (75)[208](I-H/H)2 500 14 546 2.70 209, 210 (84)[209]

ðII-HÞ2þ2900 16 975 4.21 154, 155 (95)

[154](II-H)2 490 24 506 3.21 155, 156 (82)[155](I-OMe/Me)�+ 660 15 618 1.04 119b, 121b (44)[121a]

J.C. Earles et al. / Journal of Molecular Structure 1047 (2013) 80–86 85

I-OMe/H, has large HOMO amplitude over the functionalized phen-ylethenyl moiety. Conversely the molecule with the most electron-withdrawing substituent, I-NO2/H, has negligible HOMO densityon the phenylethenyl moiety. The LUMOs exhibit the oppositetrends: the phenylethenyl substituent harbors negligible LUMOdensity in I-OMe/H yet has significant LUMO amplitude forI-NO2/H.

The extent of charge transfer in the optical transitions of thesesystems may be parameterised by factorising the molecule intoconstituent groups and examining the distribution of charge thatoccurs with excitation. As such this provides a qualitative pictureof the reorganisation of the electron density. These data are shownin Table 2 in which each molecule is grouped into: the thiophene–vinylene chain; phenylethyleyl group and the R1 substituent (seeFig. 1). The pattern of behavior shows that for electron donatinggroups, such as I-OMe/H, the lowest energy transition slightly in-creases charge on the thiophene–ethenylene backbone with reduc-tion at the phenylethenyl group. The MOs show this, as the lowestenergy transition is HOMO ? LUMO in which the HOMO has sig-nificant phenyl group character and the LUMO does not (Table 4).In the case of I-NO2/H in which the R1 group is electron withdraw-ing, the change in charge with the lowest energy excitation showsa marked drop in electron density at the thiophene (90 ? 49) witha concomitant increase in density at the phenylethyleyl and R1

groups. The MO diagrams reflect this as the transition is HOMO ?LUMO, the HOMO is thiophene based and the LUMO phenylethy-leyl R1 in nature (Table 4). These calculations are also supportedby the resonance Raman spectra which show enhancement ofmodes associated with the phenylethenyl group (Fig. 4).

Table 4The B3LYP/6-31G(d) calculated frontier molecular orbitals of some representativenon-end-capped oligomers.

Molecule HOMO � 1 HOMO LUMO LUMO + 1

I-OMe/H

I-NO2/H

3. Experimental

3.1. Synthesis

The synthesis of the oligomers under consideration is describedin detail elsewhere [13].

II-H

3.2. Physical measurements

Cyclic voltammograms were measured at room temperatureusing 10�4 mol L�1 solutions, except for that of I-NO2/Me, whichwas measured from a saturated solution of concentration less than10�4 mol L�1(I-NO2/Me is sparingly soluble in acetonitrile). Allsolutions were prepared in 0.1 mol L�1 tetrabutylammonium per-chlorate (TBAP) in spectrophotometric grade acetonitrile. The datawere acquired at a scan speed of 100 mV s�1 using an AdInstru-ments Powerlab 4sp potentiostat controlled by a PC running Pow-erlab EChem software. The electrochemical cell consisted of a1.0 mm diameter platinum working electrode embedded in aKel-F cylinder with a platinum auxiliary electrode and a saturatedpotassium chloride calomel reference electrode (SCE). The decam-ethylferrocenium/decamethylferrocene (Me10Fc0/+) redox couplewas used as an internal standard [46,47].

Electronic absorption spectra were recorded from neutral solu-tions of approximately 1 � 10�5 mol L�1 in acetonitrile, except thatof I-NO2/Me which was recorded from a 1 � 10�5 mol L�1 solutionin dichloromethane (I-NO2/Me is sparingly soluble in acetonitrile).Chemical oxidations were performed by adding aliquots of 100 lLof 8 � 10�4 mol L�1 Cu(ClO4)2 in acetonitrile to 2 mL of the neutral1 � 10�5 mol L�1 solutions. Spectra were recorded immediatelyfollowing the addition of each aliquot, until additional oxidant nolonger affected the spectra. The neutral solutions are yellow/or-ange in color. Upon oxidation all solutions turned faint blue.Reduction of each final oxidation product was achieved by addingone drop of 2% hydrazine aqueous solution. No obvious color

change was observed upon reduction. Spectra were recorded atroom temperature from 200 to 2000 nm on a Varian Cary 500 ScanUV–vis–NIR Spectrophotometer using a quartz cell of 1 mm andCary Win UV Scan Application software.

3.3. Computational methods

A detailed discussion of the DFT calculated geometry and vibra-tional spectra of the oligomers is given elsewhere [13]. Time-dependent DFT and the B3LYP/6-31G(d) method were used to pre-dict the 12 lowest energy electronic vertical excitation energieswithout symmetry or spin restrictions. For the radical speciesunrestricted B3LYP and CAM-B3LYP were used. All calculationswere performed using the Gaussian 03 program [48]. Mullikenpopulation analyzes were carried out using GaussSum, version2.1.6 [49].

4. Conclusion

A series of substituted R-arylethenyl 2-thien-2-ylethenyl withand without a thiophene methyl end caps have been studied usingelectrochemistry, optical spectroscopy, spectroelectrochemistryand DFT calculations. Electrochemical studies show that all

86 J.C. Earles et al. / Journal of Molecular Structure 1047 (2013) 80–86

end-capped molecules form polarons in oxidized solution and un-dergo no significant polymerization or degradation processes. Evi-dence of polaron formation, bipolaron formation or both wasobserved in the electrochemical and electronic absorption datafor the various non-end-capped molecules. Furthermore, all non-end-capped molecules exhibit evidence of r-dimer formationwhen oxidized in solution.

The optical properties for this series of 2-thien-2-ylethenylcompounds show transitions that may be p, p� in nature or havecharge-transfer character if suitable R-arylethenyl groups are used.Lower energy bands emerge in the electronic absorption spectra ofthe oxidized species that are not present in the neutral spectra.These are a result of intra-band-gap states that are associated withcharged-defects. The specific nature of the charged defect (that is,polaron or bipolaron) formed for each oligomer has been assignedon the basis of the Furukawa model [44].

The electronic transitions for each neutral molecule and the cor-responding oxidation products were calculated using TDDFT. Theresults were used to interpret the experimental electronic absorp-tion spectra, specifically to assign which MOs are involved in a par-ticular electronic transition. Visualizations of the frontier MOsreveal interesting charge transfer characteristics. The charge trans-fer character associated with calculated electronic transitions hasbeen quantified using Mulliken population analysis. The resultssuggest that there is significant electron transfer character fromthe thiophene conjugation path to the phenylethenyl substituentassociated with the prominent transitions of oligomers bearingan electron withdrawing substituent (most notably I-NO2/H andI-NO2/Me). Such oligomers are of particular interest because theypromote the tendency for the conjugated ‘backbone’ to preferen-tially carry holes; a finding with potential implications to solar celltechnology.

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