Syntheses and photophysical properties of amphiphilic dyads of fluorescein and carbazole linked with a flexible or semi-rigid bridge

Abstract Four amphiphilic dyads consisting of a fluorescein moiety covalently linked to a carbazole moiety at the 2′ or the 6-position with a flexible (-(CH 2 ) 4 -) or semi-rigid (-CH 2 phCH 2 -) bridge were synthesized and investigated by comparison of photophysical properties with those of model compounds. Absorption and emission spectra and fluorescence quenching results indicate that the ground state interaction between fluorescein and carbazole is negligible. When fluorescein was excited, photoinduced electron transfer from carbazole to fluorescein occurred ( K ElT =1.2×10- 10 s −1 mol −1 l in the intermolecular case, Φ ElT =0.92 and K ElT =1.47×10 9 s −1 in the dyad linked with a flexible bridge at site 2′). The face-to-face conformation and a polar solvent were favourable for the photoinduced intramolecular electron transfer. When carbazole was excited, its singlet energy could easily transfer to fluorescein in the intermolecular ( K EnT = 2.4×10 13 s −1 mol −1 l ) or intramolecular ( Φ EnT ≈1) case; in addition, intramolecular electron transfer could also take place, especially in polar solvents.