Experimental and Theoretical Studies of Quadrupolar Oligothiophene-Cored Chromophores Containing Dimesitylboryl Moieties as π-Accepting nEnd-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 (4V-5V), and five thiophene rings with two terminal aryldimesitylboryl groups (5B), as well as an analogue of 5V with a central EDOT ring (5VE), have been synthesized via Pd-catalyzed cross-coupling reactions in high yields (66-89%). Crystal structures of 4V, 5B, bithiophene 2V, and five thiophene-derived intermediates are reported. Chromophores 4V, 5V, 5B and 5VE have photoluminescence quantum yields of 0.26-0.29, which are higher than those of the shorter analogues 1V-3V (0.01-0.20), and short fluorescence lifetimes (0.50-1.05ns). Two-photon absorption (TPA) spectra have been measured for 2V-5V, 5B and 5VE in the range 750-920nm. The measured TPA cross-sections for the series 2V-5V increase steadily with length up to a maximum of 1930GM. We compare the TPA properties of 2V-5V with the related compounds 5B and 5VE, 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.