Electronic absorption and Raman studies of BF4−-doped polythiophene based on the spectra of the radical cation and dication of α-sexithiophene

Conducting Electrospun Nanofibres: Monitoring of Iodine Doping of P3HT through Infrared (IRAV) and Raman (RaAV) Polaron Spectroscopic Features

Aligned polymer nanofibres are prepared by means of the electrospinning of a chlorobenzene solution containing regioregular poly(3-hexyltiophene-2,5-diyl), P3HT, and poly(ethylene oxide), PEO. The PEO scaffold is easily dissolved with acetonitrile, leaving pure P3HT fibres, which do not show structural modification. Polymer fibres, either with or without the PEO supporting polymer, are effectively doped by exposure to iodine vapours. Doping is monitored following the changes in the doping-induced vibrational bands (IRAVs) observed in the infrared spectra and by means of Raman spectroscopy. Molecular orientation inside the fibres has been assessed by means of IR experiments in polarised light, clearly demonstrating that electrospinning induces the orientation of the polymer chains along the fibre axis as well as of the defects introduced by doping. This work illustrates a case study that contributes to the fundamental knowledge of the vibrational properties of the doping-induced defects-charged polarons-of P3HT. Moreover, it provides experimental protocols for a thorough spectroscopic characterisation of the P3HT nanofibres, and of doped conjugated polymers in general, opening the way for the control of the material structure when the doped polymer is confined in a one-dimensional architecture.

Understanding the evolution of the Raman spectra of molecularly p-doped poly(3-hexylthiophene-2,5-diyl): signatures of polarons and bipolarons

Molecular doping is a key process to increase the density of charge carriers in organic semiconductors. Doping-induced charges in polymer semiconductors result in the formation of polarons and/or bipolarons due to the strong electron-vibron coupling in conjugated organic materials. Identifying the nature of charge carriers in doped polymers is essential to optimize the doping process for applications. In this work, we use Raman spectroscopy to investigate the formation of charge carriers in molecularly doped poly(3-hexylthiophene-2,5-diyl) (P3HT) for increasing dopant concentration, with the organic salt dimesityl borinium tetrakis(penta-fluorophenyl)borate (Mes₂B⁺ [B(C₆F₅)₄]^-) and the Lewis acid tris(pentafluorophenyl)borane [B(C₆F₅)₃]. While the Raman signatures of neutral P3HT and singly charged P3HT segments (polarons) are known, the Raman spectra of doubly charged P3HT segments (bipolarons) are not yet sufficiently understood. Combining Raman spectroscopy measurements on doped P3HT thin films with first-principles calculations on oligomer models, we explain the evolution of the Raman spectra from neutral P3HT to increasingly doped P3HT featuring polarons and eventually bipolarons at high doping levels. We identify and explain the origin of the spectral features related to bipolarons by tracing the Raman signature of the symmetric collective vibrations along the polymer backbone, which - compared to neutral P3HT - redshifts for polarons and blueshifts for bipolarons. This is explained by a planarization of the singly charged P3HT segments with polarons and rather high order in thin films, while the doubly charged segments with bipolarons are located in comparably disordered regions of the P3HT film due to the high dopant concentration. Furthermore, we identify additional Raman peaks associated with vibrations in the quinoid doubly charged segments of the polymer. Our results offer the opportunity for readily identifying the nature of charge carriers in molecularly doped P3HT while taking advantage of the simplicity, versatility, and non-destructive nature of Raman spectroscopy.

Operando Characterization of Organic Mixed Ionic/Electronic Conducting Materials

Operando characterization plays an important role in revealing the structure-property relationships of organic mixed ionic/electronic conductors (OMIECs), enabling the direct observation of dynamic changes during device operation and thus guiding the development of new materials. This review focuses on the application of different operando characterization techniques in the study of OMIECs, highlighting the time-dependent and bias-dependent structure, composition, and morphology information extracted from these techniques. We first illustrate the needs, requirements, and challenges of operando characterization then provide an overview of relevant experimental techniques, including spectroscopy, scattering, microbalance, microprobe, and electron microscopy. We also compare different in silico methods and discuss the interplay of these computational methods with experimental techniques. Finally, we provide an outlook on the future development of operando for OMIEC-based devices and look toward multimodal operando techniques for more comprehensive and accurate description of OMIECs.

Synthesis, characterization and electro-conducting study of isomeric polythiophene

Herein we have reported for the first time a one-pot, one step methodology to synthesize isomeric polythiophene (IPTh) possessing 2,2, 2,4 and 5,4 linkages. The method of polymerization of thiophene to IPTh involved reacting thiophene with DDQ in the presence of concentrated H₂SO₄ at 40 °C and the polymerization is completed in 10 minutes. The synthesized IPTh was characterized by various spectroscopic and microscopic techniques. The formation of polaron and bipolaron in an iodine doped sample (IPTh-I₂) has been confirmed by IR, Raman and UV-Vis spectra. The electrical conductivity of the synthesized IPTh and IPTh-I₂ have been studied by impedance spectroscopy and found to be ∼10^-5 and 10^-3 S cm^-1 respectively. IPTh exhibits an excellent thermal stability up to 150 °C, and low optical band gap of 3.49 eV suitable for photovoltaic applications. The weight average molecular weight of IPTh has been found to be 18.636 kDa, and it has a better post functionalization capability and hence wider scope than polythiophene (PTh).

Facile Synthetic Route to Atomically Thin Conductive Wires from Single-Species Molecules in One-Dimensionally Confined Space: Doped Conjugated Polymers inside Single-Walled Carbon Nanotubes

A facile synthetic method for doped conjugated molecules by a heating process is demonstrated. Br-terminated terthiophene precursors are encapsulated in single-walled carbon nanotubes by a vapor-phase reaction, and additional heat treatment promotes the thermal condensation of the precursors. Transmission electron microscopy observations and optical measurements show the successful synthesis of sexithiophenes and their doping (oxidation) by Br dopants generated by the condensation reaction. This study provides a new strategy for the synthesis of the doped conjugated polymers from single-species molecules by only a heating process.

Plasmonic enhancement of SERS measured on molecules in carbon nanotubes

We isolated the plasmonic contribution to surface-enhanced Raman scattering (SERS) and found it to be much stronger than expected. Organic dyes encapsulated in single-walled carbon nanotubes are ideal probes for quantifying plasmonic enhancement in a Raman experiment. The molecules are chemically protected through the nanotube wall and spatially isolated from the metal, which prevents enhancement by chemical means and through surface roughness. The tubes carry molecules into SERS hotspots, thereby defining molecular position and making it accessible for structural characterization with atomic-force and electron microscopy. We measured a SERS enhancement factor of 10⁶ on α-sexithiophene (6T) molecules in the gap of a plasmonic nanodimer. This is two orders of magnitude stronger than predicted by the electromagnetic enhancement theory (10⁴). We discuss various phenomena that may explain the discrepancy (including hybridization, static and dynamic charge transfer, surface roughness, uncertainties in molecular position and orientation), but found all of them lacking in enhancement for our probe system. We suggest that plasmonic enhancement in SERS is, in fact, much stronger than currently anticipated. We discuss novel approaches for treating SERS quantum mechanically that appear promising for predicting correct enhancement factors. Our findings have important consequences on the understanding of SERS as well as for designing and optimizing plasmonic substrates.

Resonance Raman Spectroscopy of the T1 Triplet Excited State of Oligothiophenes

The characterization of triplet excited states is essential for research on organic photovoltaics and singlet fission. We report resonance Raman spectra of two triplet oligothiophenes with n-alkyl substituents, a tetramer and hexamer. The spectra of the triplets are more complex than the ground state, and we find that density functional theory calculations are a useful starting point for characterizing the bands. The spectra of triplet tetrathiophene and hexathiophene differ significantly from one another. This observation is consistent with a T1 excitation that is delocalized over at least five rings in long oligomers. Bands in the 500-800 cm(-1) region are greatly diminished for an aggregated sample of hexathiophene, likely caused by fast electronic dephasing. These experiments highlight the potential of resonance Raman spectroscopy to unequivocally detect and characterize triplets in thiophene materials. The vibrational spectra can also serve as rigorous standards for evaluating computational methods for excited-state molecules.

Poly(3-decylthiophene) radical anions and cations in solution: single and multiple polarons and their delocalization lengths in conjugated polymers

Optical absorption spectra of anions and cations of poly(3-decylthiophene) (P3DT) in solution were identified as single polarons. Pulse radiolysis of P3DT in THF determined the spatial extent of one negative polaron to be ~11.5 thiophene units by observing transient absorption of P3DT(-•) radical ions, which are prinicpally free ions, at 850 nm with ε = (7.25 ± 0.47) × 10(4) M(-1) cm(-1) and bleaching of the neutral absorption band at 450 nm. P3DT(-•) was formed in a combination of diffusive reactions and fast "step" processes. Similarly, a positive polaron of P3DT was estimated to delocalize over ~8.7 thiophene units by pulse radiolysis in chloroform. Chemical reduction by sodium and oxidation by FeCl(3) injected multiple charges into a single P3DT chain while showing absorption spectra in the early stages of reaction resembling those observed by pulse radiolysis. The results indicated that multiple polarons exist in a single chain of P3DT before coalescing into bipolarons or transforming into other forms of polaron.

Electronic interactions between "pea" and "pod": the case of oligothiophenes encapsulated in carbon nanotubes

One of the most challenging strategies to achieve tunable nanophotonic devices is to build robust nanohybrids with variable emission in the visible spectral range, while keeping the merits of pristine single-walled carbon nanotubes (SWNTs). This goal is realized by filling SWNTs ("pods") with a series of oligothiophene molecules ("peas"). The physical properties of these peapods are depicted by using aberration-corrected high-resolution transmission electron microscopy, Raman spectroscopy, and other optical methods including steady-state and time-resolved measurements. Visible photoluminescence with quantum yields up to 30% is observed for all the hybrids. The underlying electronic structure is investigated by density functional theory calculations for a series of peapods with different molecular lengths and tube diameters, which demonstrate that van der Waals interactions are the bonding mechanism between the encapsulated molecule and the tube.

Direct evidence for delocalization of charge carriers at the Fermi level in a doped conducting polymer

The infrared absorption spectrum of the polaron charges at the Fermi level EF in a heavily p-doped conducting poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) film has been measured using interferogram-modulated Fourier-transform charge-modulation spectroscopy. The spectrum indicates softer phonons and weaker electron-phonon coupling riding on a strongly redshifted Drude-like electronic transition, different from the population-averaged "bulk" spectrum. This provides direct evidence that the EF holes are sufficiently delocalized even in such disordered materials to reside in an energy continuum (band states) while the rest of the hole population resides in self-localized gap states.

Raman and theoretical study of the solvent effects on the sizable intramolecular charge transfer in the push-pull 5-(dimethylamino)-5'-nitro-2,2'-bithiophene

In this paper, we analyze the degree of intramolecular charge transfer in a push-pull pi-conjugated system, 5-(dimethylamino)-5'-nitro-2,2'-bithiophene, from changes in frequencies and relative intensities of its strongest Raman scatterings in a bunch of solvents with different polarities. Density functional theory (DFT) was used as a support of the experimental study. Solvent effects on the molecular and electronic structures and on the vibrational properties were estimated by performing B3LYP/6-31G calculations within the framework of the polarized continuum model (PCM) developed by Tomasi. Calculations reveal that the molecule is highly polarized in the ground state and behaves as a very efficient photoinduced push-pull system. The polarization of the molecule strongly increases with solvent polarity and determines that the profile of the Raman spectra greatly changes from one solvent to another and in going to the solid. The strongest Raman scattering associated with the nu(sym)(NO(2)) stretching undergoes a downshift of 48 cm(-1) in passing from CCl(4) to the solid. DFT calculations provide a comprehensive interpretation of the evolution of the Raman spectra with solvent polarity.

Thiophene- and Selenophene-Based Heteroacenes: Combined Quantum Chemical DFT and Spectroscopic Raman and UV−Vis−NIR Study

In this article, we report the characterization of a series of thiophene- and selenophene-based heteroacenes, materials with potential applications in organic electronics. In contrast to the usual alpha-oligothiophenes, these annelated oligomers have a larger band gap than most semiconductors currently used in the fabrication of organic field-effect transistors (OFETs) and therefore they are expected to be more stable in air. The synthesis of these fused-ring molecular materials was motivated by the notion that a more rigid and planar structure should reduce defects (such as torsion about single bonds between alpha-linked units or S-syn defects) and thus improve pi-conjugation for better charge-carrier mobility. The conjugational properties of these heteroacenes have been investigated by means of FT-Raman spectroscopy, revealing that pi-conjugation increases with the increasing number of annelated rings. DFT and TDDFT quantum chemical calculations have been performed, at the B3LYP/6-31G** level, to assess information regarding the minimum-energy molecular structure, topologies, and absolute energies of the frontier molecular orbitals around the gap, vibrational normal modes related to the main Raman features, and vertical one-electron excitations giving rise to the main optical absorptions.

Fourier Transform Raman and DFT Study of Three Annulated Oligothiophenes with Different Molecular Shapes

Herein, we study the conjugation properties of three different thienoacenes, each of which has three or four fused thiophene rings, by means of Fourier transform Raman spectroscopy. The B3LYP/6-31G** vibrational analysis of all of the collected spectroscopic data evidences that the selective enhancement of a limited number of Raman scatterings is related to the occurrence in the three thienoacenes of a vibronic coupling between the lowest unoccupied frontier molecular orbital (LUMO) and some Raman-active skeletal nu(C==C) stretching modes of 1600-1300 cm(-1).

Modulation of electronic properties in neutral and oxidized oligothiophenes substituted with conjugated polyaromatic hydrocarbons

The structural and electronic properties of neutral and oxidized terthiophenes substituted with polyaromatic systems have been investigated using a combination of both Raman and electronic absorption spectroscopy in conjunction with density functional theory calculations. Naphthylethenyl terthiophene exhibits structural and electronic properties, in both the neutral and oxidized species, that are dominated by the terthiophene backbone, in a manner similar to that previously reported for phenylethenylterthiophene. Anthracenylethenyl terthiophene, on the other hand, displays properties that are dominated by the anthracene group. Unlike both phenylethenyl and naphthylethenyl terthiophene, which have electronic absorption spectra dominated by transitions between molecular orbitals that are delocalized throughout the molecules, the absorption spectrum of anthracenylethenyl terthiophene consists of a simple addition of the absorption bands of the separate terthiophene and anthracenylethene chromophores. This is the result of a spatial partitioning of its molecular orbitals that effectively electronically decouples the anthracene and terthiophene moieties. Upon oxidation, the naphthylethenylterthiophene sigma-dimerizes to form sexithiophene charged species and spectral signatures of the sexithiophene backbone are evident in both the electronic absorption and resonance Raman spectra. In contrast, these signatures are absent in the corresponding spectra of the oxidized anthracenylethenylterthiophene, suggesting that the anthracene group is the primary site of the structural changes induced by oxidation.

Nature and energies of electrons and holes in a conjugated polymer, polyfluorene

Electrons and holes were injected selectively into poly-2,7-(9,9-dihexylfluorene) (pF) dissolved in a tetrahydrofuran (THF) and a 1,2-dichloroethane (DCE) solution, respectively, using pulse radiolysis. Transient absorption spectra of monoions of both signs revealed two bands attributable to formation of polarons, one in the visible region (pF+* at 580 nm, pF-* at 600 nm) and another in the near-IR region. Additional confirmation for the identification of pF+* and pF-* comes from bimolecular charge-transfer reactions, such as bithiophene-* + pF --> pF-* or pF+* + TTA --> +TTA+* (TTA = tri-p-tolylamine), in which known radical ions transfer charge to pF or from pF. Difference absorption spectra of pF chemically reduced by sodium in THF provided a ratio of absorbance of anions formed to bleaching of the neutral band at 380 nm. In conjunction with pulse-radiolysis results, the data show that each polaron occupies 4.5 +/- 0.5 fluorene units, most probably contiguous units. Extensive reduction of pF by sodium also revealed resistance to formation of bipolarons: excess electrons reside as separate polarons when two or more electrons were injected. Redox equilibria with pyrene and terthiophene by pulse radiolysis established reversible one-electron redox potentials of E0(pF+/0) = +0.66 V and E0(pF0/-) = -2.65 V vs Fc+/0. Together with the excited-state energy, these results predict a singlet exciton binding energy of 0.2 eV for pF in the presence of 0.1 M tetrabutylammonium tetrafluoroborate. This binding energy would increase substantially without an electrolyte.

Theoretical Study of Long Oligothiophene Dications: Bipolaron vs Polaron Pair vs Triplet State

A series of oligothiophene dications (from the sexithiophene dication to the 50-mer oligothiophene dication, nT2+, n = 6-50) were studied. Density functional theory (DFT) at the B3LYP/6-31G(d) level and, in some cases, also at BLYP/6-31Gd, was applied to study the singlet and triplet states of the whole series. We found that the singlet state is the ground state for all oligothiophene dications up to the 20-mer, and that the singlet and triplet states are degenerate for longer oligomers. Thus, the triplet state is never a pure ground state for these dications. We found that, for short oligothiophenes dication (e.g., 6T2+), the bipolaron state is the more important state, with only a small contribution made by the polaron pair state. For medium size oligothiophene dications (e.g., 14T2+), both the bipolaron state and the polaron-pair state contribute to the electronic structure. Finally, in long oligothiophene dications, such as 30T2+ and 50T2+, the contribution from the polaron pair state becomes dominant, and these molecules can be considered as consisting of two independent cation radicals or a polaron pair. Results from isodesmic reactions show that the stability of oligothiophene cation radicals over dications is inversely proportional to chain length. Small oligothiophene dications (n = 6-12) were studied at the CASSCF(m,m)/6-31G(d) (m = 4, 6, and 10) level. The major conclusions of this paper regarding the relative energy of the singlet state versus the triplet state and regarding the relative stability of the bipolaron versus the polaron pair were also supported by CASSCF calculations.

The effect of oxidation on the structure of styryl-substituted sexithiophenes: A resonance Raman spectroscopy and density functional theory study

The structures and vibrational properties of a series of styryl-substituted sexithiophenes and their charged species have been examined using resonance Raman spectroscopy in conjunction with density functional theory calculations. The calculated geometries of the radical cations and dications indicate that the quinoidal charged defects are more strongly localized in the center of the thiophene backbone than is observed in other sexithiophenes. This defect confinement, induced by the positions of the styryl substituents, is particularly evident in the dication species. However, the defect confinement weakens when alkoxy groups are added onto the phenyl rings by causing the extension of the charged defect into the styryl groups. The Raman spectra of the neutral styryl sexithiophenes are dominated by intense thiophene symmetrical stretching modes in both the measured and predicted spectra. Oxidation generates radical cations and dications, both of which can be observed in the solution state resonance Raman spectra. Unlike other sexithiophenes, which generally show a downshift of the intense thiophene stretching mode from the radical cation to the dication, a small upshift is observed for the styryl-substituted sexithiophenes. The theoretical spectra predict an insignificant change during this transition and the eigenvector for this mode reveals that it is localized over the same area occupied by the confined defect. In contrast, the solid state resonance Raman spectra of electrochemically oxidized films reveal evidence of solely radical cations and there is an appreciable downshift of the intense thiophene stretching mode compared with the corresponding mode in the solution spectra. This implies that the increase in the effective conjugation length from the solution to the solid state is greater for the radical cations than for the neutral species. It therefore appears that the radical cations form pi stacks in the solid film and the resulting intermolecular interactions effectively allow a further extension of the electron delocalization.

Combined Quantum Chemical Density Functional Theory and Spectroscopic Raman and UV−vis−NIR Study of Oligothienoacenes with Five and Seven Rings

In this article, we report the characterization of novel oligothienoacenes with five and seven fused thiophene rings, materials with potential applications in organic electronics. In contrast to usual alpha-linked oligothiophenes, these fused oligothiophenes have a larger band gap than most semiconductors currently used in the fabrication of organic field-effect transistors (OFETs) and therefore they are expected to be more stable in air. The synthesis of these fused-ring oligomers was motivated by the notion that a more rigid and planar structure should reduce defects (such as torsion about single bonds between alpha-linked units or S-syn defects) and thus improve conjugation for better charge-carrier mobility. The conjugational properties of these two molecular materials have been investigated by means of FT-Raman spectroscopy, revealing that conjugation still increases in passing from the five-ring oligomer to that with seven-rings. DFT and TDDFT quantum chemical calculations have been performed, at the B3LYP/6-31G level, to assess information regarding the minimum-energy molecular structure, topologies, and absolute energies of the frontier molecular orbitals (MOs.) around the gap, vibrational normal modes related to the main Raman features, and vertical one-electron excitations giving rise to the main optical absorptions.

Synthesis and Characterization of Three Novel Perfluoro-oligothiophenes Ranging in Length from the Trimer to the Pentamer

In this Article, we report on the synthesis and full characterization of three perfluorinated oligothiophenes, ranging in length from the trimer to the pentamer (PF-nT, with n = 3, 4, or 5). The differential pulse voltammetry (DPV) analysis of the compounds showed that they can be both oxidized and reduced (i.e., they display a dual or amphoteric electrochemical behavior), with the reduction peaks positively shifted relative to those of the corresponding unsubstituted oligothiophenes. The electrochemically determined energy gaps are in agreement with those measured from the UV-vis-NIR absorption spectra in solution. The conjugational properties have been investigated by means of FT-Raman spectroscopy, both as pure solids and as dilute solutes in CH(2)Cl(2), revealing that: (i) pi-conjugation does not still reach saturation with chain length for the longest oligomer, and (ii) conformational distortions from a nearly coplanar arrangement of the successive thiophene units upon solution are not too large. DFT and TDDFT quantum chemical calculations have been performed, at the B3LYP/6-31G level, to assess information about the optimized molecular structure, equilibrium atomic charges distribution, energies and topologies of the frontier molecular orbitals (MO) around the gap, vibrational normal modes associated with the most outstanding Raman scatterings, and vertical one-electron excitations that give rise to the main optical absorptions.

Alternated Quinoid/Aromatic Units in Terthiophenes Building Blocks for Electroactive Narrow Band Gap Polymers. Extended Spectroscopic, Solid State, Electrochemical, and Theoretical Study

We report here the synthesis of three novel pi-conjugated heterocyclic mixed trimers that contain two electron-donating 3,4-ethylenedioxy-2-thienyl (EDOT) units covalently attached to a central proquinoid electron-accepting thienopyrazine moiety (two of these narrow-HOMO-LUMO gap D-A-D compounds also bear hexyl side chains attached either to the outermost alpha positions of the EDOT end rings or to the beta positions of the pyrazine fused ring). The modification of the terthiophene structure upon EDO, pyrazine, and hexyl substitutions has been treated in detail with spectroscopic and theoretical arguments. Solid-state properties reveal the occurrence of short intramolecular contacts between heteroatoms of adjacent rings. The analysis of the structure of the pi-conjugated backbone of each molecule is consistent with a partial quinoid-like pattern which partially reverts to be subtly more aromatic depending on the topology of the positive inductive effect of the hexyl chains. This quinoidization is a consequence of the appearance of a D(EDOT)-->A(PyT)<--D(EDOT) intramolecular charge transfer which further polarizes the structure. The same chemical concepts have been applied to address their electrochemical behavior. The three mixed trimers exhibit amphoteric properties due to the combination of electron acceptor and donor groups. Given their relative low HOMO-LUMO energy gap, these trimers promise to be good candidates for obtaining polymers with significant low energy gap combining electroactivity.

Multidisciplinary Physicochemical Analysis of Oligothiophenes End-Capped by Nitriles: Electrochemistry, UV−Vis−Near-IR, IR, and Raman Spectroscopies and Quantum Chemistry

In this article, we investigate a series of alpha,omega-dicyano end-capped oligothiophenes NC(C(4)H(2)S)(n)()CN ranging in length from the dimer to the hexamer (n = 2-6), in the neutral state as pure solids, by means of Fourier transform IR and Fourier transform Raman (FT-Raman) spectroscopies. The cyclic voltammetry analysis of the compounds in dichloromethane reveals that most of them show two oxidation and two reduction waves (i.e., a dual or amphoteric electrochemical behavior), associated with the injection of either positive or negative charges into the pi-conjugated system. The doped species are characterized by in situ vis-near-IR and FT-Raman spectrochemistries. Density functional theory calculations have been also performed, at the B3LYP/6-31G level, to assess information about the molecular geometries and vibrational features of the neutral and doped species and about the topologies of the molecular orbitals involved in the main electronic transitions that appear for the neutral forms in the visible spectral region and for the doped species in the near-IR region.

Electronic modulation of dithienothiophene (DTT) as pi-center of D-pi-D chromophores on optical and redox properties: analysis by UV-Vis-NIR and Raman spectroscopies combined with electrochemistry and quantum chemical DFT calculations

In this paper, we study three symmetrical D-pi-D chromophores containing dithieno[3,2-b:2',3'-d]thiophene (DTT) as the pi-center and various donor end moieties, by means of UV-vis-NIR and FT-Raman spectroscopy and in situ spectroelectrochemistry. The compounds display dual redox properties: all exhibited two oxidations and single stable reduction peaks contrarily to the one or two oxidations and none reduction which could be anticipated in view of their chemical structures. We analyze the possible electronic modulation by the pi-conjugated DTT relay in the redox process and electronic coupling between the two electron donor (D) units attached through conjugation to opposite sides of the pi-linker. To this end, the UV-vis-NIR and FT-Raman spectra of the neutral compounds and of the charged species generated upon in situ p- or n-doping have been recorded and interpreted with the help of Density Functional Theory (DFT) calculations. The analysis of the peculiar Raman features of these pi-conjugated chromophores is guided by the formalism of the Effective Conjugation Coordinate (ECC) theory. This research shows that the Raman spectroscopic characterization of this type of D-pi-D structures is a powerful tool to derive information about their pi-conjugational properties in the pristine and doped states.

Characterization of the Oxidation Products of Styryl-Substituted Terthiophenes and Sexithiophenes Using Electronic Absorption Spectroscopy and Time-Dependent DFT

The electronic absorption spectra of a series of alkoxy-styryl substituted terthiophenes, their corresponding sexithiophenes, and the oxidation products of both have been measured. The terthiophenes studied sigma-dimerize to sexithiophenes during the oxidation process and there is clear evidence of sexithiophene radical cations, dications, and pi-dimers in the electronic absorption spectra. The oxidation of concentrated solutions produces predominantly pi-dimer bands, as expected. The absorption spectrum of the styryl-functionalized sexithiophene dication without alkoxy substitution closely resembles that of unsubstituted sexithiophene, while alkoxy substitution induces changes in the wavelength of the dication band maximum and the overall band shape. Time-dependent density functional theory (TDDFT) calculations have shown that styryl-based molecular orbitals are important in the transitions of the neutral molecules as well as the charged species, the dication in particular. Kinetics analyses confirm the stabilization effect induced by the alkoxy substituents. The presence of a reversible pi-dimer equilibrium was verified by cyclic voltammetry. It is clear from the experimental observations and the theoretical calculations that both the styryl and alkoxy groups are influencing the electronic properties of this class of molecules.