Photoinduced intramolecular charge-transfer state in thiophene-π-conjugated donor–acceptor molecules

Novel thiophene-π-conjugated donor–acceptor molecules, 5-[2-(1,2,2,4-tetramethyl-1,2,3,4-tetrahydroquinolin-6-yl)-vinyl]-thiophene-2-carbaldehyde (QTC) and (1-cyano-2-{5-[2-(1,2,2,4-tetramethyl-1,2,3,4-tetrahydroquinolin-6-yl)-vinyl]-thiophen-2-yl}-vinyl)-phosphonic acid diethyl ester (QTCP), were designed and synthesized. Combined experimental and theoretical methods were performed to investigate the photoinduced intramolecular charge-transfer (ICT) processes of these compounds. Steady-state absorption and fluorescence measurements in different solvents indicate the photoinduced ICT characters of QTC and QTCP. Solvent dependency of the large Stokes shifts and high dipole moment of the excited state also support the charge-transfer character of the excited state. Theoretical calculations based on time-dependent density functional theory (TDDFT) method were performed to investigate ICT states of these compounds. The results reveal that the excited states have adopted a distortion of the CC double bond between the donor moiety and the thiophene-π-bridge.