Unraveling Dispersed Kinetic Behavior of Single Photoacid Molecules in Transparent Crystal Hosts

Abstract

This dissertation represents construction and application of powerful tools for single molecule (SM) photophysical analysis. SM fluorescence intermittency studies are presented for violamine R (VR) and 2',7'-dichlorofluorescein (DCF) isolated in potassium acid phthalate (KAP) crystals. Both dyes were shown to follow a broad range of blinking behaviors, ultimately ending up analyzing with power-law statistics for on and off events. The blinking dynamics were modeled with Monte Carlo (MC) simulations, and show that distributed rate constants for dark-state population and depopulation were able to reproduce the same power-law blinking statistics. The molecular-level kinetics of DCF molecule isolated in KAP crystals were investigated with time-correlated single-photon counting (TCSPC). These results gave distributions of SM lifetime decays, which were best represented by stretched exponentials giving evidence for kinetic dispersion at the molecular level, as the previous MC models predicted. A MC simulation for DFF fluorescence was constructed and demonstrated that the observed kinetic dispersion by TCPSC was reproducible by employing the measured SM energy histograms. The TCSCP results were expanded upon with time tagged, time resolved, time-correlated single-photon counting (T3R-TCSPC) measurements. These experiments simultaneously quantified molecular blinking and lifetime data, and showed that the single DCF molecules underwent both radiative and nonradiative lifetime dispersion within the KAP crystal. These findings further reinforced the proton transfer hypothesis as the etiology for DCF blinking in KAP, but were hampered by DCF's lack of spectral separation between its individual protonation states. This was overcome through measurement of 2',7'-difluorofluorescein (DFF) in KAP, which has extremely similar proton chemistries as DCF, but with spectrally resolvable protonation states. Subsequent T3R-TCSPC measurements revealed that the local environment surrounding the individual DFF molecules is highly acidic, spectrally equivalent to ~6 M strong acid, and were dominated by the cationic form of DFF.