Experimental and theoretical studies of quadrupolar oligothiophene-cored chromophores containing dimesitylboryl moieties as π-accepting end-groups: syntheses, structures, fluorescence, and one- and two-photon absorption

Quadrupolar oligothiophene chromophores composed of four to five thiophene rings with two terminal (E)-dimesitylborylvinyl groups (4 V-5 V), and five thiophene rings with two terminal aryldimesitylboryl groups (5 B), as well as an analogue of 5 V with a central EDOT ring (5 VE), have been synthesized via Pd-catalyzed cross-coupling reactions in high yields (66-89 %). Crystal structures of 4 V, 5 B, bithiophene 2 V, and five thiophene-derived intermediates are reported. Chromophores 4 V, 5 V, 5 B and 5 VE have photoluminescence quantum yields of 0.26-0.29, which are higher than those of the shorter analogues 1 V-3 V (0.01-0.20), and short fluorescence lifetimes (0.50-1.05 ns). Two-photon absorption (TPA) spectra have been measured for 2 V-5 V, 5 B and 5 VE in the range 750-920 nm. The measured TPA cross-sections for the series 2 V-5 V increase steadily with length up to a maximum of 1930 GM. We compare the TPA properties of 2 V-5 V with the related compounds 5 B and 5 VE, giving insight into the structure-property relationship for this class of chromophore. DFT and TD-DFT results, including calculated TPA spectra, complement the experimental findings and contribute to their interpretation. A comparison to other related thiophene and dimesitylboryl compounds indicates that our design strategy is promising for the synthesis of efficient dyes for two-photon-excited fluorescence applications.