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 mole cules in solid solutions, starting from Shpol'skii matrices up to sing le molecule spectroscopy. We discuss the general principles of selecti ves and different applications of this technique to molecular and soli d-state studies. We examine in more detail two new fields to which we have contributed: persistent spectral hole burning in Langmuir-Blodget t (LB) films and the study of individual molecules. We show how persis tent spectral hole burning provides information about structure and dy namics of LB films and how energy transfer can be studied in concentra ted films. We probed the dynamics of the LB matrix as a function of th e depth of the dye in a multilayer. We show that the surface monolayer presents specific dynamics, which we attribute to the long hydrophobi c chains. The shift and broadening of a spectral hole under an applied electric field allows us to determine the orientation and direction o f the chromophore axes. We then present the new field of single molecu le spectroscopy, including the latest results. So far, the observation s were made in a molecular crystal and in a polymer. We first consider the general appearance of fluorescence excitation lines and the sudde n jumps of their resonance frequencies. The external electric field ef fects are then discussed. The correlation properties of the light emit ted by single molecules give new insight about intramolecular dynamics and spectral diffusion, which would be impossible to obtain in experi ments with ensembles of molecules. We demonstrate how single molecule spectroscopy gives truly local information, eliminates averages and po pulations, and gives access to distributions of molecular parameters i n solids.