Proton-transfer mechanism for dispersed decay kinetics of single molecules isolated in potassium hydrogen phthalate

The excited-state decay kinetics of single 2',7'-dichlorofluorescein (DCF) molecules oriented and overgrown within crystals of potassium acid phthalate (KAP) are reported. Time-correlated single-photon counting measurements (TCSPC) of 56 DCF molecules in KAP reveal that single-exponential decay is exhibited by roughly half of the molecules. The remainder demonstrates complex excited-state decay kinetics that are well fit by a stretched exponential function consistent with dispersed kinetics. Histograms of single-molecule luminescence energies revealed environmental fluctuations and distinct chemical species. The TCSPC results are compared to Monte Carlo simulations employing a first-passage model for excited-state decay. Agreement between experiment and theory, on both bulk and single-molecule levels, suggests that a subset of the DCF molecules in KAP experience fluctuations in the surrounding environment that modify the energy barrier to proton transfer leading to dispersed kinetics.