Photoinduced intramolecular electron transfer between fluorescein and carbazole

Several dyads consisting of a fluorescein covalently linked with a carbawle at site 2′ or site 6 have been synthesized and characterized. Studies of absorption spectra, emission spectra and fluorescence lifetime quenching indicate that the ground-state interaction between fluorescein and carbazole in dyads is negligible and the intramolecular electron transfer (ET) reactions are mainly of dynamic process. Moreover, the efficiency and rate constant of electron transfer reactions in ZF03 (carbazole linked at site 2′ of fluorescein) (ΦET-D= 0.74;KET-D = 2.9 X l09 s-1) are larger than those in 4FOZ (carbazole linked at site 6 of fluorescein) (ΦET-D = 0.74;KET-D = 1.1 X 109 s-1), because the mutual orientation of donor and acceptor in ZF04 is nearly face-to-face, which is more favorable to the ET process than the shoulder-to-shoulder mutual orientation in 4FOZ. Estimations are also formed of the free energy change of the photoinduced electron transfer and the back reactions in the dyads.