Unraveling the Dispersed Kinetics of Dichlorofluorescein in Potassium Hydrogen Phthalate Crystals

Abstract

The connection between photoluminescence (PL) intermittency and excited-state kinetics is explored for 2',7'-dichlorofluorescein (DCF) isolated in crystals of potassium acid phthalate (KAP) using time-tagged, time-resolved, time-correlated single-photon counting (T3R-TCSPC). In this technique, PL intermittency or "blinking" is measured in conjunction with the time of photon arrival relative to photoexcitation, allowing for the correlation of emissive intensities and excited-state decay kinetics of single molecules. The blinking trace is parsed into emissive and nonemissive segments using change-point-detection analysis, and the duration of these segments are used to quantify PL intermittency. The results presented here demonstrate that two populations of DCF exist in KAP, with one population demonstrating single-exponential excited state decay over the course of the blinking trace, and the other demonstrating stretched-exponential decay. Molecules demonstrating single-exponential decay also demonstrate modest intensity variation in the blinking trace. Correlation of the emission intensity and excited-state lifetimes demonstrates that for these molecules spectral diffusion is largely responsible for the evolution in emission intensity. In contrast, molecules demonstrating nonexponential excited-state decay vary in emission intensity. Correlation of the emissive intensities with the excited-state lifetimes demonstrates that these molecules undergo changes in both radiative and nonradiative decay rate constants as well as spectral diffusion. These observations suggest that DCF exists in two environments in KAP differentiated by the propensity for proton-transfer with the surrounding KAP matrix. The results presented here provide further insight into the origin of PL intermittency demonstrated by DCF in KAP and related systems.

Temperature-dependent fluorescence intermittency for single molecules of violamine R in poly(vinyl alcohol)

“…It should be noted that using time-tagged, time-correlated single photon counting we have established that the variation in on-state intensities is due to both discrete changes in the non-radiative decay rate and changes in the absorption cross-section due to spectral diffusion. 50 Applying this definition to the data presented in Fig. 1, the molecule undergoes 16 switches, and resides in five different intensity states within the first 50 s of this trace.…”

Photoluminescence Intermittency from Single Quantum Dots to Organic Molecules: Emerging Themes

“…Our group has been investigating the role of proton transfer in the PI exhibited by rhodamine-type chromophores isolated in single crystals of potassium acid phthalate (KAP) [102,109–113]. Comparative studies of violamine R (VR) emission in PVOH and KAP found that the temperature dependence of VR PI was markedly different in these two environments.…”

Two mechanisms for fluorescence intermittency of single violamine R molecules

“…A second (and less appreciated) aspect of SM emission is the presence of emissive states of variable intensity. Fluctuations in emissive intensity have been found to coincide with changes in the excited-state lifetime, [36][37][38] and spectral jumps. 39 Discrete emissive states were first statistically identified by Watkins and Yang, and quantified by the change point detection (CPD) algorithm.…”

“…Early PL intermittency studies were limited to systems that demonstrated a single emissive state. Recently, more complex distributions of on-state intensities have been observed 38,40,43,44 that frequently follow power-law distributed emissive and non-emissive state durations. The observation of a power-law is consistent with dispersed kinetics for dark-state production and decay.…”