The effect of phenyl substitution on the fluorescence characteristics of fluorescein derivatives via intramolecular photoinduced electron transfer

Abstract

UV-vis absorption, steady state fluorescence emission, time-correlated single photon counting and laser flash photolysis methods were employed to examine the excited state properties of fluorescein derivatives to understand the mechanism that controls their fluorescence efficiency. The fluorescein derivatives contain amino, t-butyl, carboxyl or nitro on their phenyl moieties, respectively. These substituents are not directly connected to the fluorophore but still showed a very remarkable effect on the fluorescence properties. Compared to fluorescein, the introduction of nitro, a strong electron withdrawing group, or amino, a strong electron donating group, caused a substantial quenching of both the fluorescence quantum yield and lifetime. The presence of a t-butyl or carboxyl, on the other hand, caused a smaller decrease. The mechanism for the substituent effect is due to the involvement of an additional de-excitation process, i.e. intramolecular photoinduced electron transfer (PET). The thermodynamics and kinetics of PET were analyzed. Depending on the nature of the substituent, the xanthenic ring acts as an electron acceptor (or donor), while the phenyl moiety is the corresponding electron donor (or acceptor) in PET. The rate constant of PET for the amino case is larger than 4.79 × 10⁹ s⁻¹, while for nitro substitution it is 0.67 × 10⁹ s⁻¹. Both values are much larger than the radiation rate constant of 0.20 × 10⁹ s⁻¹, meaning that PET plays important roles in the deactivation of S₁ for the two dyes. The charge transfer state generated by PET was observed by laser flash photolysis.