Single-Molecule Optical Switching: A Mechanistic Study of Nonphotochemical Hole-Burning

Abstract

Persistent spectral hole-burning of dopant chromophores embedded in solid matrices has proven to be a sensitive high-resolution spectroscopic tool to investigate structural and dynamic properties of amorphous and crystalline hosts at low temperature [1]. A commonly encountered mechanism of holeformation is the nonphotochemical process, for which it is assumed that the frequency selective laser excitation and the subsequent relaxation of guest and host eventually leads to a change of configurational degrees of freedom in the nearby environment of the photo-excited centers or in the impurities themselves (or both) [2]. However, detailed knowledge about the microscopic mechanism of the nonphotochemical process is rare. Methyl group spin conversion [3] and rearrangement of hydrogen bond networks [1] belong to the few mechanisms known in the literature. In the present work we want to introduce a model system which allows the reproducible observation of nonphotochemical hole-burning at the single molecule level, a phenomenon which amounts to the controlled optical manipulation of an isolated chromophore. We will illustrate how a number of experimental techniques available in single molecule spectroscopy can be combined to obtain ample information about the underlying hole-burning mechanism. Then we will introduce a theoretical approach [4] to elucidate the microscopic nature of the configurational degrees of freedom responsible for the formation of the photoproduct: the results of recent molecular dynamics simulations do indeed admit a detailed mechanistic scenario for the hole-burning process in our model system. We thus hope to demonstrate how studies at the single molecule level can serve to improve our understanding of the structural dynamics of solids at low temperatures. To do so, we will start by giving a brief overview of the basic concepts of low-temperature single-molecule spectroscopy.