User:Vickili1101/sandbox/Fluorescence Intermittency

Fluorescence Intermittency is a quantum-mechanics event that commonly observed as blinking; it is the shifting of nanoscale emitters' ON/OFF stages. Nanoscale emitters are molecular  fluorophores and colloidal quantum dots. Photoblinking and photobleaching are important characteristics of single molecules. Photoblinking refers to the temporary disappearance of emitted light when molecules undergo reversible transitions between “on” and “off” states, whereas photobleaching is an irreversible process where bleached products that do not absorb or emit at the excitation and emission wavelengths, respectively, are formed from the molecule. In many applications of single molecule spectroscopy there is a pertinent need to increase the photostability of fluorescent probes and to avoid interruptions during light emission which may otherwise interfere with the actual processes under investigation (e.g., protein folding,1 enzyme catalysis,2 electron transfer3). To increase the efficacy of fluorescent probes, it is important to first understand the dynamics behind photobleaching and photoblinking. Even though many studies have been dedicated to unraveling the mechanisms responsible for these two processes, there still remain controversies surrounding the nature of the dark states, and the pathways leading to their formation.4-11 Fluorescence intermittency of single molecules has previously been explained in terms of triplet state dynamics,12,13 molecular reorientation,11 spectral diffusion,15 conformational changes,8,9 and intramolecular electron transfer.3,14 More recently, on/off events following power law distributions for an ensemble of single molecules have been reported. Haase et al. observed longlived dark states that obey power law distributions for perylenemonoimide chromophores embedded in poly(methyl methacrylate) film,16 and Schuster et al. reported similar power law blinking behavior for various dye molecules (e.g., rhodamine 6G, terrylene) dispersed on top of glass surfaces and polymer films.17,18 Hoogenboom et al. reported a power law distributed dark state for a trimer molecule consisting of three rigidly linked tetraphenoxyperylene diimide chromophores.19 A model based on the formation of (dark) radical cations arising from electron transfer between chromophores and self-trapped states found in the host matrix has been used to understand the sensitivity of power law kinetics to environmental polarity. The assignment of long-lived dark states responsible for photoblinking to radicals was further supported by Zondervan and co-workers’ study in which radical anions of rhodamine 6G (Rh6G), suggested to be formed via electron transfer from poly(vinyl alcohol) to Rh6G upon light irradiation, were observed in an electron-spinresonance experiment.