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

Abstract

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.

Single molecule power-law behavior on a crystalline surface

“…19 More recently, this has also been confirmed for on distributions of molecules embedded in a crystalline environment. 28,29 Note that exclusion of the multilevel trajectories does not affect the shape of the distributions. Defining the lowest intensity as off and all others as on following Reid and co-workers, 28 the same power-law coefficients are recovered ͑m off = 1.17, m on = 1.25͒.…”

“…We would like to mention that in contrast to off distributions, 19 the existence of power-law behavior in on distributions from a single molecule is still debated, since such distributions could possibly result from a static distribution of molecular states. Wustholz et al 28,29 report, however, essentially identical power-law coefficients from small ensembles and single molecules for violamine doped into potassium acid phthalate. For the data presented here, the close agreement and low values of both on and off power-law coefficients observed for C 4 -PTCDI on Al 2 O 3 ͑0001͒ suggest a relationship between both distributions and might shed light on the charge transfer process.…”

Memory and Spectral Diffusion in Single-Molecule Emission

“…13 It was proposed that distributed kinetics resulted from spectral diffusion and/or intermolecular charge transfer. Here, we examined the role of spectral diffusion in the distributed blinking kinetics of KAP/VR by separating two spectral components of the emission using a dichroic mirror.…”

“…13,25 For example, individual violamine R (VR) fluorophores embedded in crystals of potassium acid phthalate (KAP) were found to exhibit blinking dynamics consistent with power-law behavior (m ∼ 2 for both the on-time and off-time distributions). 13 This behavior was reproduced using a distributed kinetics model for population and depopulation of the nonemissive state.…”

“…13 This behavior was reproduced using a distributed kinetics model for population and depopulation of the nonemissive state. 8,13 Two possible mechanisms for distributed kinetics in KAP/VR were proposed: (1) time-dependent geometrical changes in the firstcoordination sphere of a fixed molecule (i.e., dynamical spectral diffusion) and (2) dynamic environmental and/or conformational heterogeneity in combination with charge separation and recombination. In the first hypothesis, configurational changes modify the arrangement of the molecule relative to the surrounding dielectric, thereby altering the overlap between the molecular absorption band and the exciting field.…”