The effects of cross conjugation on the optical absorption and frontier orbital levels of donor-acceptor polymers

The influence of cross-conjugation on the optical and electrochemical properties of donor–acceptor copolymers is investigated. Isoindigo, substituted at the 6,6' or the 5,5' positions, and thieno[3,2-b]thiophene and thieno[2,3-b]thiophene are taken as conjugated and cross-conjugated electron-deficient and electron-rich building blocks from which four isomeric donor–acceptor polymers were synthesized. Introducing cross-conjugation in isoindigo has only a small effect on the electrochemical band gap and on the onset of the absorption, which remains in the near-infrared. The optical absorption spectra, however, differ strongly because cross-conjugation strongly reduces the absorption coefficient. DFT calculations confirm that the transition to the lowest excited singlet state has a small oscillator strength in cross-conjugated isoindigo model compounds. Cross-conjugation in thienothiophene exerts a different effect. It causes a moderate but distinct blue-shift of the optical absorption and a deeper HOMO energy level.