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-<i>b</i>]­thiophene and thieno­[2,3-<i>b</i>]­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