Classifying the Photophysical Dynamics of Single- and Multiple-Chromophoric Molecules by Single Molecule Spectroscopy

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Abstract

The single molecule fluorescence spectroscopy of various isolated single-chromophoric dye molecules and multiple-chromophoric conjugated polymer molecules has been investigated. For each system the transient fluorescence “intensity”, ICW(t) (i.e., detected photons/dwell time), has been recorded with continuous wave (CW) irradiation. ICW(t) has been analyzed to yield an occurrence histogram for the different “intensities”, H(I), and an intensity time-autocorrelation function CI(t). The histograms H(I) for the various examples show highly diverse behavior with one, two, or even three peaks as well as “flat regions”. The different features in the histograms are shown to arise from distinct photophysical processes. From the study of model systems, characteristic features in the intensity histograms and autocorrelation functions are shown to result from photon shot noise, “blinking” due to triplet bottlenecks, spectral diffusion due to environmental fluctuations, and interchromophoric energy transfer. Classification of the relevant photophysical processes is aided by single molecule spectroscopic data on these systems, including wavelength-resolved emission spectroscopy and “two-color excitation spectroscopy”, as well as stochastic simulations. The results indicate that a combined analysis of H(I) and CI(t) is a valuable approach in sorting out single molecule behavior involving multiple photophysical processes in complex systems. For single molecule systems that exhibit “on−off blinking” involving the formation of dark states, the paper also explores the practical advantages of studying the duration histograms (H(ton) and H(toff)) versus the intensity autocorrelation function CI(t), for quantifying the underlying photophysical dynamics.

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