| Abstract: The fluorescence quantum yield (Φf), fluorescence lifetime (τf) and singlet oxygen formation quantum yield (ΦΔ) of fluorescein and 4,5-dihalogenated fluoresceins in their dianion forms (FLX2, X=H, Cl, Br, I) were characterized by using UV-Vis spectroscopy, steady-state/time-resolved fluorescence and singlet oxygen traping technique. The Φf value is 0.92, 0.96, 0.69, and 0.16; the τf value is 4.16, 4.72, 3.50 and 0.80 ns; the ΦΔ value is 0.003 4, 0.049, 0.30, and 0.68 for FLX2 (X=H, Cl, Br, I), respectively. In comparison with that of fluorescein, the increase of atom size from Cl, Br to I does not cause the decrease of Φf and τf monotonically, so that heavy atom effect alone cannot explain the experimental results. The observations can be rationalized by the presence of intramolecular photo-induced electron transfer, in which the benzonate acts as the electron donor while the xanthene moiety is the acceptor. |
