Enhanced FRET Efficiency and Kinetics Mediated by ESIPT Molecules: Insights from Solvent Regulation and Quantum Dot Comparison.

High-performance fluorescence resonance energy transfer (FRET) systems are of critical importance for advancing applications in biosensing, optical imaging, and photonic devices. Using an excited-state intramolecular proton transfer (ESIPT)-active molecule as an FRET donor, the solvent effect facilitates the optimization of FRET efficiency and kinetics. Herein, two FRET systems PPC-Rhodamine 101 (Rh101) (incorporating ESIPT) and CdSe/ZnS quantum dot (QD)-Rh101 using steady-state spectroscopy and femtosecond transient absorption measurements were explored. A central finding is that the introduction of ESIPT into the FRET framework allows the solvent effect to dualistically optimize the FRET performance. It not only enables precise tuning of the spectral overlap integral between donor emission and acceptor absorption but also capitalizes on the inherent short donor-acceptor separation advantage of PPC. Relative to the CdSe/ZnS-Rh101 system, this ESIPT-integrated strategy boosts efficiency by nearly 3-fold. These results demonstrate that the ESIPT-active molecule exhibits distinct advantages over conventional QDs as an FRET donor, particularly in organic solvent environments, which can be attributed to its intrinsic spectral tunability and size-related benefits for enhancing FRET kinetics. Beyond establishing a clear link between ESIPT-modulated donor properties and FRET efficiency, this study offers a versatile approach for designing high-performance FRET systems with potential utility in biosensing, optical imaging, and photonic devices.