Dispersive Kinetics from Single Molecules Oriented in Single Crystals of Potassium Acid Phthalate

The intermittent emission or “blinking” of single violamine R (1) and 2‘,7‘-dichlorofluorescein (2) molecules incorporated into single crystals of potassium acid phthalate (KAP) is studied using confocal fluorescence microscopy. Blinking dynamics are quantified in terms of switching rates and on- and off-length probability distributions. Mixed crystals of KAP/1 and KAP/2 consist of photophysical subpopulations with ∼40% and ∼20% exhibiting persistent emission, respectively, and the remainder demonstrating a broad range of blinking behavior that is well described by a power-law distribution. The dependence of the power-law exponent on chromophore, experimental bin time, intensity threshold, and excitation power is examined. The blinking dynamics are also modeled using Monte Carlo simulations on the basis of a three-level electronic system with the rate constants for population and depopulation of the “dark” state being distributed. No correlation between molecular orientation and blinking dynamics is observed, suggesting that intermolecular electron transfer is not the origin of power-law behavior. Alternative origins for this behavior (e.g., conformational flexibility and spectral diffusion) are explored using a combination of experimental and computational techniques. Of these possibilities, the distributed kinetics exhibited by KAP/1 and KAP/2 are attributed to spectral diffusion.