Spectroscopic and theoretical study of the molecular and electronic structures of a terthiophene-based quinodimethane

The UV/Vis, infrared absorption, and Raman scattering spectra of 3′,4′-dibutyl-5,5″-bis(dicyanom ethylene)-5,5″ -dihydro-2,2′:5′,2″-terthiophene have been analyzed with the aid of density functional theory calculations. The compound exhibits a quinoid structure in its ground electronic state and presents an intramolecular charge transfer from the terthiophene moiety to the C(CN)₂ groups. The molecular system therefore consists of an electron-deficient terthiophene backbone end-capped with electron-rich C(CN)₂ groups. The molecule is characterized by a strong absorption in the red, due to the HOMO ↑ LUMO π-π electronic transition of the terthiophene backbone that shifts hypsochromically on passing from the solid state to solution and with the polarity of the solvent. The analysis of the vibrational spectra confirms the structural conclusions and supports the existence of an intramolecular charge transfer. Vibrational spectra in several solvents and as a function of temperature have also been studied. Significant frequency upshifts of the vibrations involved in the π-electron-conjugated pathway have been noticed upon solution in polar solvents and with the lowering of the temperature. Finally, we propose a quinoid molecule as a reliable structural and electronic model for dication species in doped oligothiophenes or for bipolaron charged defects in doped polythiophene.