Vibrational dynamics study of the effect of the substituents on the π-conjugation of different bithiophene molecules

We report on the FT-Raman spectroscopic study, aided by DFT model chemistry calculations, of three different classes of π-conjugated oligomers: (i) a dicyanomethylene end-capped bithiophene with a quinoidal chemical structure, (ii) a symmetrically dimethyl-substituted system bearing a non-polar aromatic structure in its ground electronic state and (iii) a highly polarized push-pull system with an electron-donor dimethylamino and a electron-acceptor cyano groups attached to its end α,ω-positions. We have optimized the molecular geometries of these three bithienyls at the DFT//B3LYP/6-31G** level, and compared the main skeletal bond lengths of the π-conjugated backbone in terms of the so-called bond-length-alternation (BLA) parameter. The overall summations of the B3LYP/6-31G** atomic charges for the thienyl rings and the various types of end α,ω-substituents have also been compared along the bunch of compounds. Finally we make use of the well-known effective conjugation coordinate (ECC) theory to assess useful information about the π-conjugation, computing the B3LYP/6-31G** value of the force constant associated to the collective ECC vibrational normal mode.