Spectral Diffusion of Single Molecules in a Hierarchical Energy Landscape

Spectral diffusion as a result of both the transitions between different molecular conformers and the ′′molecular softness′′ of quasi‐free perylene diimides on a SiO2 surface is investigated by means of single‐molecule spectroscopy, which reveals the time dependence of both the fluorescence spectra and the three‐dimensional orientation. Spectral wavelengths of all single emitters cover a wide energy range of about 0.27 eV, which is due to different types of conformers with large differences in optical transition energy. Time‐dependent spectral trajectories of single emitters within this wavelength manifold are evaluated with a model transcribed from the analysis of spatial diffusion. Spectral diffusion processes are closely correlated with fluorescence emission and excitation power. The overall analysis of spectral diffusion reveals, similar to proteins, a hierarchy of energy barriers in a broad energy landscape. A hierarchy of energy barriers in a broad energy landscape is revealed by a single‐molecule fluorescence spectroscopic study on spectral diffusion of a dipyridyl perylene diimide on a SiO2 surface (see picture). The diffusive processes are described by a model transcribed from spatial diffusion analytics and found to be closely correlated to fluorescence and excitation intensities.