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The low-lying electronic states of the p-benzosemiquinone radical anion are studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. Vertical excitation energies at the optimized geometries of the anion and the neutral molecule are computed. In light of the present theoretical results, the information provided by different spectroscopic techniques, such as UV/vis absorption, excitation, fluorescence, electron photodetachment, and electron scattering, is rationalized. CASSCF force fields are employed to compute vibronic intensities of the two lowestenergy π-π* transitions in order to solve controversial assignments and to give an interpretation of the available resonance Raman data.

A novel series of terthiophenes bearing electron-donor and electron-acceptor groups at the end alpha-positions has been prepared. The analysis of the UV-vis, infrared, and Raman spectra, performed with the aid of density functional theory calculations, shows that the asymmetrically substituted nitro compounds PhT(3)NO(2) and BrT(3)NO(2) behave as push-pull systems and present an intense photoinduced charge transfer in the visible spectrum. The symmetrically substituted dinitro compound NO(2)T(3)NO(2) displays a highly delocalized structure with a low single-double bond length alternation and also displays a low-energy absorption band in the visible region. The novel nitroterthiophenes possess attractive electrochemical properties since they generate stable species both upon oxidation and reduction. Oxidation mainly involves changes in the oligothiophene backbone and leads to the formation of stable cations even for NO(2)T(3)NO(2). Reduction is mainly nitro-centered but also affects the conjugated structure. Radical anions and dianions are formed for PhT(3)NO(2) and BrT(3)NO(2). Dianions, not radical anions, and trianions are obtained for NO(2)T(3)NO(2). Nitro-functionalized terthiophenes are shown to be very promising as electroactive molecular materials since they behave as push-pull systems, present a very intense photoinduced charge transfer in the visible region, and could act as both n- and p-channel conductors in organic electronic transistors.

Two quinonoid bis(dicyanomethylene) oligothiophenes, terthiophene and quaterthiophene analogues of TCNQ, have been investigated by spectroelectrochemical experiments and density functional theory calculations. Electrochemical data show that the molecules can be both reduced and oxidized at relatively low potentials, and that the quaterthiophene derivative forms four stable redox species, the dianion, neutral, cation radical, and dication. The neutral oligomers are characterized by a strong electronic absorption in the red or near-infrared region and can be viewed as structural and electronic analogues of aromatic oligothiophenes in the dication or bipolaron state. Upon reduction, dianions, not anion radicals, are formed which absorb in the visible region. The theoretical calculations show that the dianions have aromatic oligothiophene moieties with two anionic dicyanomethylene groups. The transition from a quinonoid to an aromatic structure is fully supported by UV-vis-near-IR and vibrational spectroscopic data. Oxidation, generating cation radicals and dications, occurs at rather low potentials similar to those reported for oligothiophenes. The electronic spectra of these cations are understood from the calculations, which suggest that the oxidized species are stabilized by the partial aromatization of the oligothiophene backbone. IR spectra of the species, especially the CN stretching frequencies, confirm the structural conclusions and allow comparison with TCNQ and the TCNQ dianion.