Distinguishing Förster resonance energy transfer and solvent-mediated charge-transfer relaxation dynamics in a zinc(ii) indicator: A femtosecond time-resolved transient absorption spectroscopic study

A bifluorophoric molecule (1) capable of intramolecular Förster Resonance Energy Transfer (FRET) is reported. The emission intensity of the FRET acceptor in 1 depends on the molar absorptivity of the donor, which is a function of zinc(ii) complexation. The FRET dynamics of [Zn(1)](ClO ₄)₂ is characterized by femtosecond time-resolved transient absorption spectroscopy. The solvent-mediated relaxation of the charge-transfer (CT) state of the isolated donor and the FRET process of the donor-acceptor conjugate are on similar time scales (40-50 ps in CH ₃CN), but distinguishable by the opposite solvent polarity dependency. As the solvent polarity increases, the efficiency of Columbic-based FRET is reduced, whereas CT relaxation is accelerated. In addition to revealing a method to distinguish CT and FRET dynamics, this work provides a photophysical foundation for developing indicators based on the FRET strategy.