Theoretical Studies of Relaxation, Energy Transfer and Non-linear Spectroscopy in Molecular Systems

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The Theoretical and Computational Chemistry program is supporting Professor Silbey at MIT. During the next three years theoretical models will be developed for the study of several distinct areas. The first involves low temperature optical properties of molecules in condensed phases, in particular glassy solids. Spectral hole burning, photon echoes, and single molecule spectroscopy have all shown that the so called homogeneous line shape of a molecule is time dependent because of slow fluctuations in the condensed phase. These methods have different but often overlapping time scales, so that a complete picture of the time dependence from picoseconds to weeks can be found. The theoretical interpretations of these experiments based on a quantum statistical mechanical approach is the goal of the first area of research. The second involves optical and electronic properties of molecular aggregates in solutions. Molecular aggregates, such as the light harvesting complexes of the photosynthetic apparatus, have been under intense experimental study over the past decade. The advent of femtosecond lasers in the correct spectral range has led to new experiments studying the time dependence of excitation energy transfer and vibrational relaxation in these aggregates. The theoretical understanding of the electronic and vibronic states and the dynamics of energy flow of these aggregates is the central problem of this phase of the supported research. Because of experimental advances, second harmonic light scattering from molecules in solution has reemerged as an important tool to measure the non-linear optical properties of molecules. However, the interpretation of these experiments in terms of the molecular and solvent properties requires a modern theoretical basis, which is the goal of the third part of this grant. In particular, the description of the fluctuations of the solvent field at the solute molecule is essential to a correct interpretation of the experimental results. The optical spectrum of a molecule is a signature of its electronic structure and the environment in which the molecule finds itself. The effects of the environment can be profound and can make the identification and understanding of the optical properties of the molecule difficult. Thus, a study of these effects provides information not only on the molecular spectrum, but also on the environment. The purpose of this research is to understand theoretically the effects of the environment (solvent) on the optical properties of a molecule. The molecules that will be studied are organic molecules that have a large linear and non-linear optical response; these include: dye molecules used in laser research, chlorophyll and aggregates of chlorophyl (antenna complexes and reaction centers), and conjugated molecules that are candidates for use in optical switching and communication devices. The solvents or environments that the molecules are in include normal liquids, solids and glasses. The theoretical research will be carried out in collaboration with experimentalists who are measuring these optical properties.