Memory and spectral diffusion in single-molecule emission

The blinking dynamics of single violamine R (VR) molecules embedded in crystals of potassium acid phthalate (KAP) are analyzed using threshold and change-point detection (CPD) methods (Watkins, L. P.; Yang, H. J. Phys. Chem. B 2005, 109, 617). Analysis employing thresholding resulted in power-law distributions of on and off times corresponding to a power-law exponent of similar to 2, consistent with a distributed kinetics model for population and depopulation of the nonemissive state. When the same emission time traces were analyzed using the CPD method, a power-law exponent of similar to 1.5 is obtained. In addition, multiple emission states are observed using CPD, inconsistent with a simple two-state blinking model, and indicative of spectral diffusion. The role of spectral diffusion in the distributed blinking kinetics of KAP/VR is investigated by spectrally decomposing the emission using a dichroic mirror. Combining the CPD method with this experiment yielded the emission energy, intensity, and temporal duration of blinking events. A wide distribution of emission energies is observed, consistent with molecules experiencing a variety of dielectric environments within the crystal host. Time-dependent fluctuations in the spectral decomposition are observed, corresponding to spectral diffusion. Blinking events exhibited by single VR molecules in KAP are correlated, an effect referred to as memory. To our knowledge, this is the first reported observation of memory for a molecular system. Positive correlations are observed for consecutive on times and consecutive off times. In addition, adjacent on and off times demonstrate anticorrelation. These observations support the observation of spectral diffusion in this crystal environment, with this diffusion contributing to population and depopulation of the nonemissive state.