Syntheses, structures, and one- and two-photon excited fluorescence of dimesitylboryl-ended quadrupolar hybrid oligothiophenes

Abstract

Four dimesitylboryl-ended quasi-oligothiophene chromophores composed of central 2,1,3-benzothiadiazole (1), thieno[3,2-b]thiophene (2), and benzo[1,2-b:4,5-b′]dithiophene (3, 4), respectively, have been synthesized via concise Pd-catalyzed cross-coupling reactions. The crystal structures of 1–3 and a mono dimesitylboryl-ended derivative (5) were determined. The extensive examination and comparison of their photo-physical properties indicate that the electron-donating or -accepting features of the central moiety may greatly tune their photon absorption and emission properties. Among these quasi-oligothiophene chromophores, 2,1,3-benzothiadiazole derivative 1 has the highest photoluminescence quantum yield (0.70) and longest fluorescence lifetime (12.89 ns). In particular, the excitation spectra of their two-photon excited fluorescence (TPEF) in the range of 740–880 nm demonstrate that the maximum of the two-photon absorption cross-sections of the A–π–D–π–A type compound 4 is nearly 8 times that of A–π–A–π–A type compound 1, which clearly proves that the length of the conjugated bridge and the number of efficient delocalized electrons may play important roles in enhancing the two-photon absorption.