Photophysics of Halogenated Fluoresceins: Involvement of Both Intramolecular Electron Transfer and Heavy Atom Effect in the Deactivation of Excited States

The fluorescence quantum yield (Φf), fluorescence lifetime (τf), intersystem crossing quantum yield (Φisc) and redox potentials of seven halogenated fluoresceins in their dianion forms were measured and compared in methanol to get a deep insight into the effect of halogeno atoms on their photophysics. It is found that the heavy atom effect alone cannot explain the experimental results, as (1) Φf for chlorinated dyes exceeds that of fluorescein and close to unity, (2) the sum of Φf and Φisc for brominated and iodinated xanthene dyes is remarkably less than unity. The observations can be rationalized by the involvement of intramolecular photoinduced electron transfer, in which the benzoate acts as the electron donor while the xanthene moiety is the acceptor. The more negative reduction potential of excited singlet state for chlorinated fluoresceins results in their much smaller ket, and hence higher Φf.