Donor–acceptor–donor thienyl/bithienyl-benzothiadiazole/quinoxaline model oligomers: experimental and theoretical studies

A comprehensive spectral and photophysical investigation of four donor–acceptor–donor (DAD) oligomers consisting of electron-deficient 2,1,3-benzothiadiazole or quinoxaline moieties linked to electron-rich thienyl or bithienyl units has been undertaken. Additionally, a bis(dithienyl) substituted naphthalene was also investigated. The D–A–D nature of these oligomers resulted in the presence of an intramolecular charge transfer (ICT) state, which was further substantiated by solvatochromism studies (analysis with the Lippert–Mataga formalism). Hereby, significant differences have been obtained for the fluorescence quantum yields of the oligomers in the non-polar solvent methylcyclohexane vs. the polar ethanol. The study was further complemented with the determination of the optimized ground-state molecular geometries for the oligomers together with the prediction of the lowest vertical one-electron excitation energy and the relevant molecular orbital contours using DFT calculations. The electronic transitions show a clear HOMO to LUMO charge-transfer character. In contrast to the thiophene oligomers (the oligothiophenes with n = 1–7), where the intersystem crossing (ISC) yield decreases with n, the studied DAD oligomers were found to show an increase in the ISC efficiency with the number of (donor) thienyl units.