CLASSIFYING THE PHOTOPHYSICAL DYNAMICS OF SINGLE-CHROMOPHORIC AND MULTIPLE-CHROMOPHORIC MOLECULES BY SINGLE-MOLECULE SPECTROSCOPY

The single molecule fluorescence spectroscopy of various isolated sing le-chromophoric dye molecules and multiple-chromophoric conjugated pol ymer molecules has been investigated. For each system the transient fl uorescence ''intensity'', I-cw(t) (i.e., detected photons/dwell time), has been recorded with continuous wave (CW) irradiation. I-cw(t) has been analyzed to yield an occurrence histogram for the different ''int ensities'', H(I), and an intensity time-autocorrelation function C-l(t ). The histograms H(I) for the various examples show highly diverse be havior with one, two, or even three peaks as well as ''flat regions''. The different features in the histograms are shown to arise from dist inct photophysical processes. From the study of model systems, charact eristic features in the intensity histograms and autocorrelation funct ions are shown to result from photon shot noise, ''blinking'' due to t riplet bottlenecks, spectral diffusion due to environmental fluctuatio ns, and interchromophoric energy transfer. Classification of the relev ant photophysical processes is aided by single molecule spectroscopic data on these systems, including wavelength-resolved emission spectros copy and ''two color excitation spectroscopy'', as well as stochastic simulations. The results indicate that a combined analysis of H(I) and C-l(t) is a valuable approach in sorting out single molecule behavior involving multiple photophysical processes in complex systems. For si ngle molecule systems that exhibit ''on-off blinking'' involving the f ormation of dark states, the paper also explores the practical advanta ges of studying the duration histograms (H(t(on)) and H(t(off))) versu s the intensity autocorrelation function C-l(t), for quantifying the u nderlying photophysical dynamics.