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Structural and Optical Properties of Inherently Chiral Polythiophenes: A Combined CD-Electrochemistry, Circularly Polarized Luminescence, and TD-DFT Investigation Giovanna Longhi,*,†,‡ Sergio Abbate,†,‡ Giuseppe Mazzeo,† Ettore Castiglioni,†,§ Patrizia Mussini,∥ Tiziana Benincori,⊥ Rocco Martinazzo,∥ and Francesco Sannicolò∥ †

Dipartimento di Medicina Molecolare e Traslazionale, Università di Brescia, Viale Europa 11, 25123 Brescia, Italy CNISM Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, Via della Vasca Navale, 84, 00146 Roma, Italy § JASCO Europe, via Cadorna 1, 23894 Cremella, Lecco, Italy ∥ Dipartimento di Chimica, Università di Milano, Via Venezian 21, 20133 Milano, Italy ⊥ Dipartimento di Scienze e Alta Tecnologia, Università dell’Insubria, via Valleggio 11, 22000 Como, Italy ‡

S Supporting Information *

ABSTRACT: Circular dichroism (CD) and ultraviolet absorption (UV) spectra of films obtained by electrochemical polymerization of inherently chiral 2,2′-bis(2,2′-bithiophene-5-yl)-3,3′-bithianaphthene (T4-BT2) are taken during electrochemical polarization cycles. Besides the bisignate CD features in the near UV range recorded at zero potential, new features in the visible−near-infrared range are observed under increased applied potential. Results are interpreted with the help of static and time-dependent (TD) density functional theory (DFT) calculations, which shed light on the structural and electronic properties of neutral and charged oligomers (from monomers to tetramers) and reproduce UV and CD spectra satisfactorily. Furthermore, properties of the excited state of T4-BT2 monomers in solution are enlightened by combining circularly polarized luminescence (CPL) measurements with TD-DFT calculations.

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INTRODUCTION Conducting polymers with a helical twist have attracted the attention of many scientists and technologists because of the possibility of preferred interactions with biological molecules or of potentially large nonlinear optical susceptibility terms. They present an intriguing combination of two apparently conflicting characteristics, namely, highly extended conjugation, favoring conduction, on one hand, and a twisted conformation, providing noncentrosymmetric constraint, on the other. Among the most studied molecular systems are polyalkylthiophenes (PAT), where the alkyl substituent is a chiral pendant and regioregularly connected to the highly conductive polythiophene backbone.1,2 Their configurational and conformational/aggregation properties in solution and film samples have been probed with different techiques, including circular dichroism (CD), both in the ultraviolet (UV) and in the infrared (IR) range, and X-ray diffraction.2−5 Circularly polarized luminescence techniques have also been applied to solutions containing PAT systems with chiral pendant groups.6,7 The difficulty in studying such systems lies in the fact that chirality emerges only under some circumstances, namely, in solution of mixed solvents or in specially prepared films, and is not inherent to the molecular core. Recently, we succeeded in synthesizing and characterizing an inherently chiral thiophene-based molecule, the 2,2′-bis(2, © 2014 American Chemical Society

Scheme 1. Chemical Structures for (R)-2,2′-Bis(2,2′bithiophene-5-yl)-3,3′-bithianaphthene [(R)-T4-BT2] (left) and (S)- 2,2′-Bis(2,2′-bithiophene-5-yl)-3,3′bithianaphthene [(S)-T4-BT2] (right)

2′-bithiophene-5-yl)-3,3′-bithianaphthene (T4-BT2), Scheme 1,8 which, upon chiral high-performance liquid chromatography (HPLC) separation of the enantiomers, displayed a large optical rotation of about 1000 (c = 0.1%, CHCl3),9,10 thereby showing the potentiality of introducing chirality inherently in the repeat units. A further asset is the full coincidence of the stereogenic Received: May 2, 2014 Revised: June 21, 2014 Published: June 23, 2014 16019

dx.doi.org/10.1021/jp504307v | J. Phys. Chem. C 2014, 118, 16019−16027

The Journal of Physical Chemistry C

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element with the electroactive backbone, which can afford fine tuning of the chiroptical properties by modulating the electrical potential, i.e., the film charge. Here we report on a combined experimental and theoretical study of the chiroptical properties of T4-BT2before and after electrochemical polymerization which connects to and extends our previous work.9,10 In particular, we provide here a detailed description of the combined voltammetry-CD experiments, on the basis of static and time-dependent (TD) density functional theory (DFT) calculations on some of its oligomers, consider in detail the structural and electronic properties of such systems, assign the observed UV and CD features, and enlighten their helicization. TD-DFT will also be employed to interpret the circularly polarized luminescence (CPL) spectra of the T4-BT2 monomers in solution.

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For wavelength range up to 1100 nm a larger size photomultiplier tube, namely, Hamamatsu R-316 with S1 response, had to be used so that sampling was performed in the normal sample compartment but once more keeping the cell as close as possible to the PM tube sensitive surface; only the data taken in this extended range are shown in Figure 1.

MATERIALS AND METHODS

a. Materials Employed in CD Measurements. Synthesis of the T4-BT2 monomer was described in previous publications,8−10 along with separation of enantiomers (R)-T4-BT2 and (S)-T4-BT2 by means of chiral HPLC.9,10 Enantiopure films were prepared by electrodeposition from 5 × 10−4 M solutions of enantiopure (R)-T4-BT2 or (S)-T4-BT2 in CH3CN + 0.1 M TBAPF6 (Fluka, electrochemical grade) on ITO (indium tin oxide) coated glass slides (8−12 W/sq, Sigma-Aldrich) in a 3-electrode minicell (4 cm3); the counter electrode was a platinum one, while the reference electrode was an aqueous saturated calomel electrode (SCE) inserted in a double bridge, filled with the working medium, to avoid water and KCl leakage into the working solution. The conducting oligomer films were electrodeposited by repeated oxidative potential cycling at 0.2 V s−1 around the first oxidation peaks, followed by repeated stability cycles in a monomer-free solution. The enantiopure oligomer films thus electrodeposited on ITO were then used as working electrodes for the in situ CD spectroelectrochemistry experiments, fitting them into a 1 cm path length quartz cuvette also including a mini SCE reference electrode and a platinum wire counter electrode. The potential of the Fc+|Fc reference redox couple versus the SCE operating electrode is 0.39 V. Both the film electrodepositions and the electrochemical polarization cycles in the CD-spectroelectrochemistry experiments were carried out using Autolab PGSTAT potentiostat facilities of EcoChemie (Utrecht, The Netherlands), run by a PC with the GPES or NOVA software of the same manufacturer. b. CD Experimental Spectra. ECD spectra were obtained on a J-815SE spectrometer at 1000 nm/min scanning speed; band pass was 8 nm, and integration time was 0.25 s. The large band pass was selected to reduce artifacts in the absorption spectra, which were simultaneously collected; the latter artifacts are caused by the sharp emission lines of the Xe light source of the spectrometer above 800 nm. For the wavelength range from the near UV up to 950 nm a dedicated, small size, sample compartment had been designed to fit the 10 mm path rectangular quartz cell containing the ITO transparent plate and the various necessary electrodes immersed in solution. A miniature Hamamatsu R-7400U-20 extended red multialkali photomultiplier tube (PM) was directly placed in close contact with the cell to reduce possible light loss due to scattering. Black light masks were placed in front of the cuvette to keep the size of the sampling beam in the spectrometer smaller than the size of the various employed ITO electrodes, whose dimensions are somewhat irregular.

Figure 1. Spectroelectrochemistry measurements: (a) Cyclic voltammetry (stability upon oxidative−redox cycles is shown), (b) absorption, and (c) CD spectra of films of oligo-(R)-(−)- T4-BT2 and oligo-(S)(+)- T4-BT2 on ITO electrodes in MeCN + 0.1 M TBAPF6 at potentials indicated by the arrows, referred to the saturated calomel electrode (SCE). For the sake of clarity, only a subset of tested potentials is shown.

The three-electrode setup is described above. A blank spectrum was first recorded on a new and clean ITO glass in the cuvette filled with the working solution; then an ITO glass with the enantiopure sample film was inserted in the cuvette, paying attention to alignment, so that the film is perpendicular to the incoming light ray. Few voltammetric anodic cycles were recorded to check its electroactivity, and excellent reversibility was ascertained (see Figure 1A). Different potentials were applied, each one for about 60 s, starting from the neutral state to the heavily p-doped state, corresponding to the maximum of the voltammetric peak, and the CD spectrum was recorded at each potential. Before applying a new potential, a voltammetric scan restored the polymer film to the neutral state, and we checked that electroactivity had not been lost during the preceding CD measurement. After the first increasing ramp a decreasing potential ramp was applied by setting the same potential values 16020



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Figure 2. (A) Optimized structures (CAM-B3LYP/6-31G*) of neutral and charged (S)-(+)-T4-BT2 dimer (front and side view); (first row) neutral dimer, (second row) charged dimer. CC bonds marked in bold are examined in Figure 3 and discussed in the text. (B) Optimized structures (CAMB3LYP/6-31G*) of the neutral, cation, and dication trimer of (S)-(+)-T4-BT2. CC bonds marked in bold are examined in Figure 3. Also, the structure (B3LYP/6-31G) of calculated neutral tetramer structure is shown (see text).

c. CPL Experiments. CPL is not so common a technique but is unique in providing chiral information on the excited state;11−13 lately it has received some attention in the field of

used in the increasing ramp. A couple of potential values were chosen also in the cathodic region, but generally the CD spectra were found to be the same as in the neutral state. 16021



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Figure 3. CC bond lengths of the dimer and trimer of (S)-(+)-T4-BT2 as a function of the thiophene backbone position, as evidenced in bold in Figure 2A and 2B as obtained at CAM-B3LYP/6-31G* level. First plot for trimer, second plot for dimer, third plot for a model built with the longer flat part of dimer (figureSI-2, Supporting Information).

chiral polyalkylthiophenes and helicenes.6,7,14 For solutions of (R)-(−)- and (S)-(+)-T4-BT2 in toluene we first measured the ECD spectra and then employed a homemade apparatus12−14 for running CPL experiments. Irradiation was brought to the samples through an optical fiber from a commercial FP8200 Jasco fluorimeter at λ = 375 nm. d. TD-DFT Calculations. Linear-response, time-dependent density functional theory is an established and economical tool for interpreting UV and CD spectra of midsized molecules.15 When applied to investigate the spectral properties of electrochemically prepared films, careful selection of the target oligomer species which predominantly form the film is required. In our case, MALDI experiments10 showed that the films mainly contain dimers and a nonnegligible amount of trimers, and we thus restricted our analysis to such species, including the tetramer for completeness. Dimer, trimer, and tetramer were built from the lowest energy structure of the monomer10 favoring the anti conformation.16 Geometry optimization was conducted for either the neutral or the charged state using both B3LYP and CAM-B3LYP functionals, though for the tetramer convergence was obtained for the neutral state only with B3LYP. The Pople 6-31G basis set was used to treat all cases; optimizations were also performed at the 6-31G* level with the CAM-B3LYP functional for all systems but the tetramer. Analysis of the obtained structures will be

presented and comparison among the different charged states discussed. After geometry optimization, TD-DFT calculations were performed at the 6-31G and 6-31G* levels, including a large number of states in order to simulate the recorded spectra for the charged species (the size of the systems and the high number of states needed to reproduce the spectra are the reason for the choice of the low-level basis sets). Different functionals and basis sets tested herein gave similar results except for some shift in wavelength. Calculation of CPL spectra for the monomer was carried out as described in refs 13, 14, 17, and 18; excited state optimization and spectra calculations were performed at the CAM-B3LYP/631G* and CAM-B3LYP/TZVP levels with similar results. Calculations were all performed with the help of the Gaussian09 package,19 and computed frequencies (wavelength), dipole, and rotational strengths for the considered electronic transitions were Gaussian broadened to obtain ECD, UV absorption spectra, and CPL and fluorescence spectra. A broadening factor of 0.2 eV was applied throughout if not stated otherwise. All calculations were carried out on isolated molecules: this is justified since the monomer itself gives chiroptical signals with no need of bad solvents favoring aggregation. As for the ensuing discussion, the molecular stereogenic torsion is preserved in polymers and the observed spectroscopic data are well accounted 16022



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for; however, further contributions to the optical response from intermolecular interactions cannot be excluded.

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RESULTS AND DISCUSSION The superimposed experimental CD and absorption spectra for poly(R)-T4-BT2 and poly(S)-T4-BT2 for a few values during a voltammetric cycle are presented in Figure 1, the corresponding electrode potential values being indicated along the cycle. CD data obtained in the backward polarization half-cycle coincide with the ones obtained in the forward one, within experimental error, showing that the process is fully reversible and repeatable even though some loss of electroactivity was observed after several repeated slow voltammetric scans. UV−vis spectra demonstrate that progressively shifting the potential in the positive direction produces a strong reduction in intensity of the absorption band at 450 nm and a parallel growth of a new band at 780 nm (absorption at wavelengths higher than 1100 nm has been also observed10). Parallel ESR experiments conducted in ref 10 show the presence of a polaronic state. Coming now to CD, one may notice that the CD spectrum of the neutral species consists of a bisignate feature with a negative band at ca. 400 nm and a positive band at ca. 500 nm for the (S) enantiomer; the bisignate spectra of the polymers are similar to those shown by the original monomers,10 but they are strongly shifted toward higher wavelength values, as expected for longer conjugation length. Furthermore, we notice that the two features are separated by about 50 nm in the monomer case, by 100 nm for the film. In the spectra recorded for the largest applied positive potentials one may notice the appearance of a positive broad feature at ca. 830 nm; concurrently, the higher energy component of the 400−500 nm couplet practically does not vary, while the lower energy component at 500 nm markedly decreases in intensity. The decrease in intensity of the positive 500 nm feature proceeds along the polarization cycle and is accompanied by the increase in intensity of the positive feature at 830 nm; another very small negative feature appears at 700 nm. Throughout the polarization cycle the negative band at the lowest wavelengths does not change significantly in intensity. This means that the two CD bands at 400 and 500 nm are not simply the two components of an excitonic type couplet,20,21 as one could assume at first sight. The interpretation is more complex, and some hints as to what is really going on come from higher level calculations, as provided by the present TD-DFT approach. In Figure 2A and 2B we provide the optimized geometries for the dimer and trimer of (S)-T4-BT2 in the neutral and charged state. One may notice that the structure consists of one or two tetrathienyl long ribbons, respectively, terminating with two shorter arms, each one with two thiophene rings; the “nodes” of the structures are the bithianaphthenes (BT); they are the chemical constraints controlling the absolute configuration. The two thienyl terminal short arms are not parallel to one another and are slightly bent. The tetrathienyl ribbons are gently twisted in the neutral case; on the contrary, they are flat in the charged case. From the structures of Figure 2 we can derive useful parameters, which permit not only deeper insight into the stereochemistry of the system but permits us also to get a first picture of the electroactive sites of poly-T4-BT2. This analysis brings us to conclude that the structure is that of a right-handed square staircase, which proceeds with addition of other “arms” and “hinges”. Let us analyze the aromatic/quinoid structure and compare neutral and charged systems, similarly to what has been done in the literature on similar systems:22,23 the CC bond lengths of the backbone marked in bold in Figure 2A and 2B are reported in

Figure 4. Comparison of CD (top panel) and UV (lower panel) experimental spectra of poly(S)-T4-BT2 (neutral and charged) with corresponding calculated spectra (B3LYP/6-31G/CAM-B3LYP/6-31G), for dimer (neutral and cation), trimer (neutral and cation and dication) scaled in intensity by 0.66, and tetramer (neutral) scaled in intensity by 0.5 (bdw 0.2 eV, 85 nm above 600 nm). Experimental intensities are in arbitrary units.

Figure 3 as a function of backbone atom positions for both the trimer and the dimer. These plots provide a rationale for the charged species being flat, a condition imposed by the quinoid structure. This analysis allows us also to establish the localization of the polaron defect (dihedral angles along the same backbone are reported in Figure SI-1, Supporting Information). In this instance, it is interesting to compare results for the actual molecules studied here with those for a model molecule containing just the linear part of the dimer and missing the two nearly perpendicular “arms” (Figure SI-2, Supporting Information). The expected change in the pattern of CC bond lengths from neutral molecule to cation is the same in the true dimer and in the model. We can expect that longer oligomers can sustain higher oxidation states; for this reason we considered both the cation and the dication for the case of the trimer. In the latter case we notice that two positive charges, symmetrically placed on the two long strands, are necessary to have the same CC bond length pattern of the dimer. The neutral molecules present slight twists of about 20° from planarity around the interthiophene CC bonds (a−a′), which we report in Figure SI-1, Supporting Information, for the dimer and trimer: they get completely flattened in the charged case. This is expected since the conformation is the result of a balance of two competing effects: delocalization of the π electrons force the molecule to be planar, while the steric repulsions favor a twisted conformation.24 The dihedral angles at the hinges are 105° and 98.6° in the neutral and charged dimer, respectively; for the trimer we obtain 107.5°−106.5°−107.5° for the neutral case, 101.6°−103.8°− 101.6° for the cation, and 104°−108.2°−104° for the dication. 16023



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Figure 5. Calculated spectra at the CAM-B3LYP/6-31G* level for the neutral dimer of (S)-(+)-T4-BT2 (60 nm shift, bdw 0.2 eV; bars proportional to calculated dipole, and rotational strengths superimposed to the calculated spectra). Electronic states involved in the three principal transition are indicated and illustrated in the Supporting Information.

Figure 6. Calculated spectra at the CAM-B3LYP/6-31G* level for the charged dimer of (S)-(+)-T4-BT2 (60 nm shift, bdw 0.2 eV; 85 nm above 600 nm; bars proportional to calculated dipole and rotational strengths superimposed to the calculated spectra).

In Figure 4A and 4B, respectively, we report the experimental CD and UV absorption spectra for the neutral and charged film, respectively, and compare them with the calculated corresponding spectra for the neutral and charged species of three oligomers. Spectra reported in Figure 4 were calculated at the 6-31G level of theory; this low-level basis set permits a comparison among the different species: neutral and charged dimer, trimer in the three forms, i.e., neutral cation and dication, and finally neutral tetramer. As a check a comparison of calculations conducted on the neutral and charged dimer at different levels of approximation is given in Figure SI-3, Supporting Information: qualitatively results are similar, differences concern just transition wavelengths. In order to identify the transition involved in observed spectra we refer in the following to the CAM-B3LYP/6-31G* calculations. Before discussing the details, however, let us notice from Figure 4 that all species, except the calculated doubly charged trimer, show a negative CD feature at about 380 nm. The band around 500 nm is more influenced by the extent of oligomerization and, most importantly, exhibits the largest variation from neutral to charged species, resulting in a quite weak positive feature at about 420 nm; a positive feature appears at high wavelength values, which depend on the degree of oligomerization; correspondingly, also an absorption feature is calculated. Simulated spectra are in good agreement with observation. A detailed examination of the calculated results permits us to assign the observed features. In Figures 5 and 6 we report in more detail the comparison of experimental and calculated CD and UV absorption spectra for the dimer in the neutral and charged case, respectively. Taking into account the results of Figure SI-4, Supporting Information, illustrating the principal transitions, we conclude that (i) the negative CD band at 400 nm is due to a nearly degenerate couplet, which is predicted for both the neutral and the charged dimer; the negative component prevails in both cases, and the algebraic sum of the two features is

Figure 7. Calculated spectra at the CAM-B3LYP/6-31G* level for the neutral trimer of (S)-(+)-T4-BT2 (60 nm shift, bdw 0.2 eV; bars proportional to calculated dipole and rotational strengths superimposed to the calculated spectra).

similar in the two cases; from Figure SI-4, Supporting Information, we know that these transitions involve the two identical short arms of the dimer; (ii) calculations allow us to predict, for both the neutral and the charged case, a positive feature in correspondence with the observed positive experimental feature; it is reassuring to notice that the intensity of this band is considerably smaller in the charged case (with a shift at lower wavenlength), 16024



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Figure 8. Comparison of experimental spectra of (R)-2,2′-bis(2,2′-bithiophene-5-yl)-3,3′-bithianaphthene [(R)-T4-BT2] in toluene solution with spectra calculated at the CAM-B3LYP/TZVP level: (A, B) experimental and calculated CD spectra, (C, D) CPL spectra, (E, F) absorption spectra, and (G, H) fluorescence spectra. Bars are proportional to calculated D01 and R01 for absorption and to E3, D01, and E3 R01 for emission (E is the excited state energy), see ref 13. Gaussian bandwidth 0.22 eV.

The first couplet (at high wavelength) is due to transitions involving the two central longer strands; the second couplet mixes a larger number of orbitals principally but not completely localized on the two short arms. These results confirm that the features observed on the films cannot be explained with the same picture appropriate for the monomer. For the sake of completeness we report in Figure 8 the comparison of experimental and calculated ECD spectra for the monomer case (see also ref 10): a bisignate Cotton effect is generated by two transitions delocalized on the whole molecule comprising two short arms. This is the case of a real “exciton couplet” involving HOMO−1, HOMO, LUMO, and LUMO+1 orbitals; the dipole strengths for the two transitions are comparable, and the excitonic interaction results in about 10 nm separation, the observed doublet showing a higher wavelength splitting (about 50 nm) as the result of partial cancellation of the two Gaussian bands.

allowing us to provide meaning to the observed decrease of that feature with voltage: considering the neutral case this feature is due to the HOMO−LUMO transition; (iii) at the same time calculations allow us to predict, for the charged case, a positive feature at ca. 850 nm in correspondence with the observed positive experimental feature, while no feature is calculated for the neutral case, in accordance with observation of no CD band for low voltages; (iv) Similar observations, as in i, ii, and iii above, can be made, looking at the comparison of UV calculated and experimental spectra. In Figure 7 we present calculated spectra and transition probabilities (represented by bars in Figure 7) for the case of the neutral trimer; reported in Figure SI-5, Supporting Information, are the results of CI calculations. The two CD bands of opposite sign are due to two couplets, with opposite sign as a net result. 16025



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In Figure 8 we also provide a comparison of experimental and calculated fluorescence and CPL spectra of the monomer in solution. One may appreciate very good correspondence of theory and experiment: the sign of the CPL band is the same as that for the ECD band at the highest wavelength. Since for the monomer the ECD bands are helical sensitive,14 also the observed CPL band is so. We also observe that in the present case not only is CD a property embedded in the axial molecular chirality and easily observed for both the monomer solution and the polymer films (differently from the case of polyalkylthiophenes with regioregular chiral alkyl substituents6,7) but also CPL is easily and inequivocally observed: this is further proof of the importance of inherent chirality for endowing thiophene systems with large chiroptical characteristics. Finally, it is instructive to take a look in Figure 9 at the superposition of

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ASSOCIATED CONTENT

S Supporting Information *

Dihedral angles along the CC backbone of dimer and trimer; optimized structures (CAM-B3LYP/6-31G*) of the monomer (S)-T4-BT2 and a linear model compound; comparison of spectra calculated with different functional/basis sets for the case of neutral and charged dimer; scheme of the principal molecular orbitals involved in the observed electronic transitions of the neutral dimer; scheme of the principal molecular orbitals involved in the observed electronic transitions of the neutral trimer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the CARIPLO foundation for funding the research program 2011-0417 under the auspices of which this work was carried out. We thank also CINECA-Bologna, Italy, for computational facilities and Regione Lombardia for the LISA grant “ChirConj: Chirooptical properties of conjugated systems”.

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Figure 9. Superimposed calculated structures for the monomer of (S)T4-BT2 in the ground state (light blue + yellow colors) and in the first excited electronic state (gray + orange colors).

REFERENCES

(1) Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W. On the Origin of Optical Activity in Polythiophenes. J. Mol. Struct. 2000, 521, 285−301. (2) Catellani, M.; Luzzati, S.; Bertini, D.; Bolognesi, A.; Lebon, F.; Longhi, G.; Abbate, S.; Famulari, A.; Meille, S. V. Solid-State Optical and Structural Modifications Induced by Temperature in a Chiral Poly-3alkylthiophene. Chem. Mater. 2002, 14, 4819−4826. (3) Lebon, F.; Longhi, G.; Abbate, S.; Catellani, M.; Wang, F.; Polavarapu, P. L. Circular Dichroism Spectra of Regioregular Poly {3 [(S)-2-methylbutyl]-thiophene} and of Poly {3, 4-di [(S)-2-methylbutyl]-thiophene}. Enantiomer 2002, 7, 207−212. (4) Wang, F.; Polavarapu, P. L.; Lebon, F.; Longhi, G.; Abbate, S.; Catellani, M. Absolute Configuration and Conformational Stability of (S)-(+)-3-(2-methyl butyl)thiophene and (+)-3,4-di[(S)-2methylbytyl)]thiophene and Their Polymers. J. Phys. Chem. A 2002, 106, 5918−5923. (5) Arosio, P.; Famulari, A.; Catellani, M.; Luzzati, S.; Torsi, L.; Meille, S. V. First Detailed Determination of the Molecular Conformation and the Crystalline Packing of a Chiral Poly(3-alkylthiophene): Poly-3-(S)2-methylbutylthiophene. Macromolecules 2007, 40, 3−5. (6) Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Christiaans, M. P. T.; Meskers, S. C. J.; Dekkers, H. P. J. M.; Meijer, E. W. Circular Dichroism and Circular Polarization of Photoluminescence of Highly Ordered Poly{3,4-di[(S)-2-methylbutoxy]thiophene}. J. Am. Chem. Soc. 1996, 118, 4908−4909. (7) Castiglioni, E.; Abbate, S.; Lebon, F.; Longhi, G. UV, CD, Fluorescence and CPL Spectra of Regioregular Poly-[3-((S)-2methylbutyl)-thiophene] in Solution. Chirality 2012, 24, 725−730. (8) Sannicolò, F.; Rizzo, Si.; Benincori, T.; Kutner, W.; Noworyta, K.; Sobczak, J. W.; Bonometti, V.; Mussini, P. R.; Pierini, M. An Effective Multipurpose Building Block for 3D Electropolymerisation: 2,2′Bis(2,2′-bithiophene-5-yl)-3,3′-bithianaphthene. Electrochim. Acta 2010, 55, 8352−8364. (9) Bonometti, V. Electrochemistry for the Development of Innovative Three-Dimensional and Chiral Thiophene-based Organic Semiconductors, Ph.D. Thesis, University of Milano, Italy, 2012.

the calculated three-dimensional structures of T4-BT2 in the ground and first excited state: the latter one, from which emission and CPL are originated, exhibits the two BT2 moieties tilted with respect to the ground state. This is a further aspect of the propensity of thiophenes to structurally respond to perturbations from various sources (charge injection, light excitation, etc.).

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CONCLUSIONS In this work we evidenced the great difference existing between these nonconventional, inherently chiral polythiophene polymers and the traditional ones, which exhibit CD spectra only in particular chain aggregation states. Simple DFT structure optimization and TD-DFT calculations permit a good reproduction of the observed signals and characterization of the polaronic defect. The investigated molecular models appear appropriate in this case, since evidence from MALDI measurements confirms the presence of dimers, trimers in lower amount, and longer oligomer in trace amounts. Also, concerning charge, ESR measurements reported in ref 10 suggest the prevalence of a single charge defect with spin. The high chiral response of the single molecules accounts for the observed spectroscopic data; however, further contributions to the optical response from intermolecular interactions cannot be excluded. Good correspondence between theory and experiments is also found for CPL spectra obtained for the monomer in solution, indirectly confirming the three-dimensional structure found by TD-DFT for T4-BT2 monomer in the excited state. The fluorescence and CPL spectra reported here evidence why these systems can be materials with interesting optoelectronic potentialities. 16026



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