Synthesis and two-photon-excited fluorescence of benzothiazole-based compounds with various π-electron donors

We have synthesized a series of new D-pi-A compounds that feature various electron donors and a fixed benzothiazolyl. unit as an electron acceptor. The crystal structure of compound 3 [trans, trans-2-{4-[ (4-N-carbazolyl)styryl]styryl}-1,3-benzothiazole, CSSB] was determined. All these compounds show high fluorescence quantum yields and 3 in toluene gives the most intense blue emission around 450 nm with a quantum yield of Phi = 0.69. When excited at 800 nm by a Ti:sapphire femtosecond laser, these compounds exhibit strong two-photon-excited fluorescence (TPEF) in the blue-to-orange region. The measured TPEF cross-section of compound 2 [trans,trans-2-{4-[4-(N, N-diphenylamino)styryl]styryl}-1,3-benzothiazole, DPSSB], for example, is about 6.1 times that of Coumarin 307. Photophysical data indicate that these compounds are polar in the ground state and have an enhanced polarity in the excited state, and that the electron donating ability of a dialkylamino group is much stronger than that of a diarylamino group. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.