Spectroscopic and theoretical study of push-pull chromophores containing thiophene-based quinonoid structures as electron spacers

Donor-acceptor chromophores containing three different types of thiophene-based electron spacers and the same donor (1,3-dithiol-2-ylidene) and acceptor (dicyanomethylene) end groups have been investigated by infrared and vis-near-IR absorption spectroscopies with the aim of elucidating the ability of the heteroquinonoid spacers as electron transmitters. Density functional theory calculations have been carried out, both within the standard and the time-dependent formalisms, to assign the most relevant electronic and infrared features of these chromophores and to assess useful information about their molecular structures. Both theoretical calculations and vibrational spectra demonstrate the occurrence of a sizable intramolecular charge transfer from the electron-donor unit to the electron-acceptor group in the ground state. The optical properties of the chromophores are strongly influenced by the electron spacer. The intense optical absorptions recorded in the visible mainly correspond to the pi-pi* excitation of the oligothienoquinonoid bridge. As an additional merit of these molecular materials, their infrared spectra recorded at various temperatures between - 170 and + 180 degreesC reveal, at the molecular level, their high thermal stability, what has importance for their potential use in molecular electronics and optoelectronic devices.