Quinoidal Oligothiophenes: Towards Biradical Ground‐State Species

A family of quinoidal oligothiophenes, from the dimer to the hexamer, with fused bis(butoxymethyl)cyclopentane groups has been extensively investigated by means of electronic and vibrational spectroscopy, electrochemical measurements, and density functional calculations. The latter predict that the electronic ground state always corresponds to a singlet state and that, for the longest oligomers, this state has biradical character that increases with increasing oligomer length. The shortest oligomers display closed‐shell quinoidal structures. Calculations also predict the existence of very low energy excited triplet states that can be populated at room temperature. Aromatization of the conjugated carbon backbone is the driving force that determines the increasing biradical character of the ground state and the appearance of low‐lying triplet states. UV/Vis, Raman, IR, and electrochemical experiments support the aromatic biradical structures predicted for the ground state of the longest oligomers and reveal that population of the low‐lying triplet state accounts for the magnetic activity displayed by these compounds.     

The frontiers of quinoidal stability in long oligothiophenes: Raman spectra of dicationic polaron pairs

The transformation of bipolarons into polaron pairs in long oligothiophene dications has been reported by Raman spectroscopy. These polaron-pair dicationic species possess singlet open-shell biradicaloid ground electronic states. The formation of biradical polaron pairs marks the end of the quinoidal stability promoted by the intrinsic proaromatic character. The quinoidal stability in TCNQ oligothiophenes in comparison with dicationic oligothiophenes has been addressed.     

Extensive quinoidal oligothiophenes with dicyanomethylene groups at terminal positions as highly amphoteric redox molecules

A series of highly extensive quinoidal oligothiophenes carrying a dicyanomethylene group at both terminal positions is synthesized. As the quinoidal structures extend, they have highly amphoteric abilities and show strong electronic absorptions in the visible to near-infrared region. The higher homologues, quinquethiophene and sexithiophene, exist as equilibrium mixtures with the biradical species.     

Mesitylthio-oligothiophenes in various redox states. Molecular and electronic views as offered by spectroscopy and theory

A series of alpha,omega-bis(mesitylthio)oligothiophenes of various chain lengths and with different side substitution patterns have been studied in their oxidized states by means of electron absorption and Raman spectroscopies in combination with theory in the framework of the density functional theory. Upon chemical oxidation, stable radical cations, dications, and even radical trications are generated. Longer chain lengths better stabilize higher oxidation states. The tetramer can be easily converted to the dication, and a trication can be obtained for the ethylenedioxy derivative. The alpha,omega-sulfur atoms are actively involved in the formation of the charged species and exert a favorable tuning of their electronic structure. Raman spectra provide experimental evidence of the attainment of quinoidal structures within the conjugated path, initially heteroaromatic, with different extension as a function of the p-doping level.     

Quinoidal/Aromatic Transformations in π‐Conjugated Oligomers: Vibrational Raman studies on the Limits of Rupture for π‐Bonds

The vibrational Raman spectra of several series of aromatic and quinoidal compounds have been analyzed considering the downshifts and upshifts of the frequencies of the relevant Raman bands as a function of the number of repeating units. Oligothiophenes, oligophenylene‐vinylenes, and oligoperylenes (oligophenyls) derivatives are studied in a common context. These shifts are taken as spectroscopic fingerprints of the changes in π‐conjugation. For a given family, aromatic and quinoidal oligomers have been studied together, and according to their Raman frequency shifts located in the two‐well BLA–energy curve of their ground electronic state as a function of the bond‐length‐alternation pattern (BLA). The connection among BLA values, π‐conjugation, and Raman frequencies is taken here as the basis of the study. These Raman shifts/BLA changes have been related to important electronic properties of these one‐dimensional linear π‐electron delocalized systems such as quinoidal (polyene) and aromatic characters.     

Raman dispersion and intermolecular interactions in unsubstituted thiophene oligomers

We present a critical analysis of the Raman spectra of unsubstituted oligothiophenes and rediscuss the well-known Raman dispersion of conjugated systems explicitly considering intermolecular interactions. Temperature-dependent Raman spectra and DFT calculations for dimers of different chain lengths show that the effect of intermolecular interactions on the frequency and intensity of carbon-carbon (CC) stretching modes is non-negligible. This effect has not been considered in previous works and might explain many spectral features of this class of compounds which are not completely interpreted by the usual models. Both intensities and frequencies are significantly affected by intermolecular interactions showing that molecular self-organization should be taken into account in future spectroscopic studies of conjugated molecules. In particular, the interactions among molecules cause an upward shift of the frequency of the R mode (amplitude mode) which can explain the lack of frequency dispersion with conjugation length of oligothiophenes, as experimentally observed for solid-state samples at room temperature.     

醌式杂环化合物的设计、合成及应用性能研究进展

醌式杂环(噻吩、吡咯、呋喃等)分子具有结构刚性、最高占据分子轨道(HOMO)/最低未占据分子轨道(LUMO)能级低、能级带隙窄和摩尔消光系数高等特点.醌式分子因其结构平面性特点,分子间作用力较强,因而分子间电荷传输能力强.目前,醌式杂环分子已成为有机半导体材料领域特别是有机场效应晶体管领域的研究热点.根据醌式杂环分子的结构特点,以端基为分类依据,综述了近年来醌式杂环化合物在分子设计、合成及应用性能等方面的研究进展,并展望了醌式杂环分子的发展前景.     

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.     

Effect of ring fusion on the electronic absorption and emission properties of oligothiophenes

A series of fused-ring oligothiophenes were synthesized by a combination of Stille and oxidative coupling reactions. Compounds with the same number of double bonds, but varying in extent of planarization, display a similar longest wavelength absorption maximum in solution. However, the introduction of sulfur linkages into these oligothiophenes leads to a blue shift of the maximum emission wavelength and a correspondingly smaller Stokes shift.     

Enhanced functionality for donor-acceptor oligothiophenes by means of inclusion of BODIPY: synthesis, electrochemistry, photophysics, and model chemistry

We have synthesized several new push-pull oligothiophenes based on the boron dipyrromethene (BODIPY) moiety as the electron acceptor and the more well-known oligothiophenes substituted with N,N-dialkylamino functions to enhance their electron-donor ability. A complete characterization of the electronic properties has been carried out; it consists of their photophysical, electrochemical, and vibrational properties. The compounds have been studied after chemical treatment with acids and after oxidation. In this regard, they can be termed as NIR dyes and amphoteric redox electroactive molecules. We have described the presence of dual fluorescence in these molecules and fluorescence quenching either by energy transfer or, in the push-pull molecules, by electron exchange. The combination of electrochemical and proton reversibility along with the interesting optical properties of the new species offer an interesting platform for sensor and material applications.     

Non-destructive Raman analyses--polyacetylenes in plants

Ferdinand Bohlmann has described the isolation, the identification and the structure elucidation of acetylene compounds in many plants, and confirmed it by its synthesis. We have recorded the Raman spectra of most of these plants non-destructively by FT-Raman spectroscopy using radiation at 1064 nm. We could not observe any interfering fluorescence. We found acetylene compounds in some plants, even distinct compounds with different concentration in various parts of it. The distribution of the different compounds over the plant can be observed and their changes during the ontogenesis can be followed by a FT-Raman mapping technique. Of special help is a library of Raman and IR spectra and the structure of the compounds, synthesized by Bohlmann. Thus, the Raman technique allows analyses in a very short time replacing the usual time-consuming separation procedures and avoiding artefacts during clean-up procedures.     

Tuning the Self‐Assembly of Oligothiophenes on Chemical Vapor Deposition Graphene: Effect of Functional Group,Solvent, and Substrate

Tuning and characterizing the interfacial structure of organic semiconductors on graphene is essential for graphene‐based devices. Regulation of the supramolecular assembling structure of oligothiophenes on graphene by changing functional groups attached to the backbone of oligothiophenes is described and the assembling behavior is compared with that on the basal plane of highly oriented pyrolytic graphite. It reveals that terminal functional groups attached to the conjugated backbone of oligothiophene can entirely change the assembling structures. Significant solvent and substrate effects have also been confirmed by comparing the assembling structures of oligothiophenes deposited from tetrahydrofuran, 1,2,4‐trichlorobenzene, and octanoic acid onto graphene and graphite.     

Transferable semiempirical quadratic force fields: The case of polythiophene and shorter oligomers

The vibrational spectra of oligomers of thiophene are treated theoretically with the main purpose of deriving information for the interpretation of the infrared and Raman spectra of the polymer and isotopic derivatives. We report the results of a series of semiempirical MNDO calculations on the structure and vibrational properties of oligothiophenes, and we compare the calculated MNDO Pulay scaled force field of the monomer with an empirical harmonic force field that we have obtained by least squares refinement on nine isotopic derivatives. The scaling factors obtained were transferred from thiophene for the computation of the vibrational spectrum and the phonon dispersion curves of the polymer. © 1994 by John Wiley & Sons, Inc.     

Raman scattering in crystalline oligothiophenes: a comparison between density functional theory and bond polarizability model

Raman intensity of intramolecular and lattice modes of crystalline alpha-bithiophene (alpha-2T) are investigated within density functional theory using a nonlinear response formalism. First, comparison between the calculated Raman spectrum and the experimental data allows the assignment of the main Raman lines over the whole frequency range. Then, a bond polarizability (BP) model, limited to first neighbors, is built. We show that, although the BP model cannot reproduce the changes of dielectric susceptibility under individual atomic displacements, it is accurate enough to reproduce the profile of the unpolarized nonresonant Raman spectrum of alpha-2T powder. Finally, the BP model, fitted on our first-principles results on alpha-2T, is applied with success to the alpha-quaterthiophene polymorph phases and alpha-sexithiophene, demonstrating on practical examples that first-principles and BP approaches are powerful complementary tools to calculate the nonresonant Raman spectrum of alpha-2T and make reasonable predictions on larger oligothiophenes.     

Towards Molecular Design Rationalization in Branched Multi‐Thiophene Semiconductors: The 2‐Thienyl‐Persubstituted α‐Oligothiophenes

The introduction of branching in multi‐thiophene semiconductors, although granting the required solubility for processing, results in an increased molecular fluxionality and a higher level of distortion, thus hampering π conjugation. Accordingly, branched oligothiophenes require rationalization of their structure–reactivity relationships for target‐oriented design and optimization of the synthetic effort. Our current research on spiderlike oligothiophenes affords deep insight into the subject, and introduces new, easily accessible molecules with attractive functional properties. In particular, a regular series, T′X _Y , of five new multi‐thiophene systems, T′5₃ , T′8₄ , T′11₅ , T′14₆ , and T′17₇ , constituted by five, eight, 11, 14, and 17 thiophene units, respectively, their longest α‐conjugated chain consisting of tri‐, tetra‐, penta‐, hexa‐, and heptathiophene moieties, respectively, has been synthesized and fully characterized from the structural, spectroscopic, and electrochemical point of view. The electronic properties of the monomers and their electropolymerization ability are discussed and rationalized as a function of their molecular structure, particularly in comparison with the series of 5‐(2,2′‐dithiophene)yl‐persubstituted α‐oligothiophenes ( TX _Y ) previously reported by us. These oligothiophenes are easily accessible materials, with promising properties for applications as active layers in multifunctional organic devices including solar cells.     

Cyclic oligothiophenes: novel organic materials and models for polythiophene. A theoretical study

Cyclic oligothiophenes (CnT, n = 6-30, even only) in syn- and anti-conformations are studied theoretically at the B3LYP/6-31G(d) level of theory. Strain energies, ionization potentials, HOMO-LUMO gaps, bond length alternations, NICS values, and IR and Raman spectra have been studied. The properties of anti-conformers change systematically with increasing ring size and were studied in detail in neutral, radical cation, and dication forms. In syn-conformation, the oligomers lose their nearly planar ring shape and bend significantly for n > 14, and thus properties cannot be related to ring size. The HOMO-LUMO gap in C14T-syn is even lower than polythiopehene. IR and Raman spectra calculated at the B3LYP/6-31G(d) level are used to differentiate syn- from anti-conformations. The properties of cyclic oligomers are compared to those of the linear system, and cyclic oligothiophenes are revealed as good models for polythiophene. To assist the experimental study of known cyclic oligomers having dibutyl substituents on alternate thiophene rings, the corresponding dimethyl-substituted oligomers are also studied. The experimentally evaluated HOMO-LUMO gaps for alternately dibutyl-substituted cyclic oligomers match the calculated values; however, they are significantly higher than those of the unsubstituted analogues.     

Quinoidal Oligo(9,10‐anthryl)s with Chain‐Length‐Dependent Ground States: A Balance between Aromatic Stabilization and Steric Strain Release

Quinoidal π‐conjugated polycyclic hydrocarbons have attracted intensive research interest due to their unique optical/electronic properties and possible magnetic activity, which arises from a thermally excited triplet state. However, there is still lack of fundamental understanding on the factors that determine the electronic ground states. Herein, by using quinoidal oligo(9,10‐anthryl)s, it is demonstrated that both aromatic stabilisation and steric strain release play balanced roles in determining the ground states. Oligomers with up to four anthryl units were synthesised and their ground states were investigated by electronic absorption and electron spin resonance (ESR) spectroscopy, assisted by density functional theory (DFT) calculations. The quinoidal 9,10‐anthryl dimer 1 has a closed‐shell ground state, whereas the tri‐ ( 2 ) and tetramers ( 3 ) both have an open‐shell diradical ground state with a small singlet–triplet gap. Such a difference results from competition between two driving forces: the large steric repulsion between the anthryl/phenyl units in the closed‐shell quinoidal form that drives the molecule to a flexible open‐shell diradical structure, and aromatic stabilisation due to the gain of more aromatic sextet rings in the closed‐shell form, which drives the molecule towards a contorted quinoidal structure. The ground states of these oligomers thus depend on the overall balance between these two driving forces and show chain‐length dependence.     

Indandione-Terminated Quinoidal Compounds for Low-Bandgap Small Molecules with Strong Near-Infrared Absorption: Effect of Conjugation Length on the Properties

Low-bandgap organic semiconductors have attracted much attention for their multiple applications in optoelectronics. However, the realization of narrow bandgap is challenging particularly for small molecules. Herein, we have synthesized four quinoidal compounds, i. e., QSN3 , QSN4 , QSN5 and QSN6 , with electron rich S,N-heteroacene as the quinoidal core and indandione as the end-groups. The optical bandgap of the quinoidal compounds is systematically decreased with the extension of quinoidal skeleton, while maintaining stable closed-shell ground state. QSN6 absorbs an intense absorption in the first and second near-infrared region in the solid state, and has extremely low optical bandgap of 0.74 eV. Cyclic voltammetry analyses reveal that the lowest unoccupied molecular orbital (LUMO) energy levels of the four quinoidal compounds all lie below −4.1 eV, resulting in good electron-transporting characteristics in organic thin-film transistors. These results demonstrated that the combination of π-extended quinoidal core and end-groups in quinoidal compounds is an effective strategy for the synthesis of low-bandgap small molecules with good stability.     

Surface enhanced Raman spectroscopic study of α-bithiophene and α-quaterthiophene molecules adsorbed on silver sols

Surface enhanced Raman scattering (SERS) has been observed from α-bithiophene (2T) and α-quaterthiophene (4T) adsorbed on Ag sols. Experimental results suggest that the adsorbed oligothiophene molecules are anti but twisted and the twist angle is smaller in the surface adsorbed state than in solution. It has been observed that the enhancement factor decreases with the increase of the chain length and maximum enhancement is obtained at a higher concentration for the longer oligomer. The Raman excitation frequency dependence of the enhancement suggests a significant classical electromagnetic contribution to the SERS of oligothiophenes in Ag sols