Incisive structure-spectroscopic correlation in oligothiophenes functionalized with (+/-) inductive/mesomeric...

This paper presents a combined experimental Raman and density functional theory (DFT) study of a series of oligothiophenes with variable pi-core lengths and substituted at the alpha,omega- and beta,beta'-terminal positions with perfluorohexyl, perfluorohexylcarbonyl, perfluoroarene, and cyano groups. The study covers the neutral and the electrochemically oxidized samples. The spectra have been assigned with the help of B3LYP/6-31G** calculations and interpreted by the predictions of effective conjugation coordinate (ECC) theory. Direct relationships between the bond length alternation (BLA) parameters and the Raman wavenumbers of the two most important bands of the spectra have been outlined showing the collective character of their electronic and vibrational properties. It is found that the topology of the thienyl electronic structure is not uniform along the conjugated core with the external rings more aromatic, whereas for the innermost part the pi-electron delocalization is greater and extends with the increment of chain length. Perfluorohexyl substitution finely tunes the electronic properties via negative inductive effects. The beta,beta' isomers exhibit larger conformational distortion, which strongly modifies the mean conjugation length. Oxidation provokes electronic structure quinoidization spreading over the whole system but more marked in the central part of the pi-core. The Raman features associated to quinoidization in the oxidized species have been interpreted in relation to the oligomer core length and the substitution regiochemistry.