Intramolecular Excimer Formation between 3,6-Di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione Chromophoric Groups Linked by a Flexible Alkyl Spacer

Abstract

Bichromophoric molecules containing two 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) moieties linked via aliphatic spacers of different length are synthesized. Optical absorption spectroscopy indicates that the molecules adopt an extended conformation in solution. Fluorescence spectroscopy shows that photons are emitted from the locally excited singlet state in an extended conformation. In sufficiently polar solvents, quenching of fluorescence occurs and fluorescence quantum yield (Φ_F) and fluorescence lifetime (τ_F) measurements indicate formation of an intramolecular excimer as the quenching mechanism. The redox potentials of the molecules and the solvent polarity dependence of the quenching are consistent with significant charge-transfer character of the excimer state. Photoinduced absorption measurements show enhanced intersystem crossing to the triplet state in polar solvents. Results indicate that in donor–acceptor π-conjugated materials involving the DPP moiety, excimer-like interchain polaron pair excited states could play an important role in the photophysics because of their close proximity in energy to the lowest singlet excited state.