HIGH-RESOLUTION SPECTROSCOPY OF ORGANIC-MOLECULES IN SOLIDS - FROM FLUORESCENCE LINE NARROWING AND HOLE-BURNING TO SINGLE-MOLECULE SPECTROSCOPY

We give a short overview of the selective spectroscopy of organic molecules in solid solutions, starting from Shpol'skii matrices up to single molecule spectroscopy. We discuss the general principles of selectives and different applications of this technique to molecular and solid-state studies. We examine in more detail two new fields to which we have contributed: persistent spectral hole burning in Langmuir-Blodgett (LB) films and the study of individual molecules. We show how persistent spectral hole burning provides information about structure and dynamics of LB films and how energy transfer can be studied in concentrated films. We probed the dynamics of the LB matrix as a function of the depth of the dye in a multilayer. We show that the surface monolayer presents specific dynamics, which we attribute to the long hydrophobic chains. The shift and broadening of a spectral hole under an applied electric field allows us to determine the orientation and direction of the chromophore axes. We then present the new field of single molecule spectroscopy, including the latest results. So far, the observations were made in a molecular crystal and in a polymer. We first consider the general appearance of fluorescence excitation lines and the sudden jumps of their resonance frequencies. The external electric field effects are then discussed. The correlation properties of the light emitted by single molecules give new insight about intramolecular dynamics and spectral diffusion, which would be impossible to obtain in experiments with ensembles of molecules. We demonstrate how single molecule spectroscopy gives truly local information, eliminates averages and populations, and gives access to distributions of molecular parameters in solids.