Picosecond dynamics of benzophenone anion solvation

Abstract

The dynamics of benzophenone anion solvation in alcohols are studied by pulse-radiolysis techniques. The solvation process is characterized by the blue shift of the transient absorption spectrum of the anion and is faster for the smaller alcohols. The anion is solvated more slowly than the electron in the same solvent, but the solvation times of both are similar to 7 2 , the solvent dielectric relaxation time. The familiar phenomenological "two-state" model of solvation was found to be inappropriate for describing the anion solvation process. A multistate process appears to be a more appropriate description. We modeled the kinetics of the spectral relaxation. In most cases, nearly quantitative agreement between the calculated and observed spectra is achieved.The characteristic relaxation times for the alcohol solvents around the anions were also reproduced. I, IatroductionThe solvent plays multiple roles in determining the course of condensed-phase chemical reactions. The solvent can remove energy from the reactants; it can confine reactants into regions in space (cage effects); and it can stabilize or destabilize the creation of charges. Solventsharge interactions are explored in this paper. A framework for this exploration can be created by posing two important questions. How do solvent molecules react to the sudden creation of a charged species, and what microscopic factors of the charged molecule structure and solvent molecule struture control this response? In the past decade, a number of researchers have employed picosecond time-resolved absorption and emission spectroscopy to probe the dynamics of the solvation of an electron or the solvation of a large molecular dipole created in a polar liquid.'-* Both the size of the spectral shifts and the time scale of these shifts have been probed theoretically.+' * Our understanding of the nature of the solvation has been deepened by these studied. In a recent publication, Jortner reviewed the status of the solvation experiments and divided the work into three classes: (1) solvation of the electron, (2) solvation of a molecular dipole, and (3) extraction of information about the solvation process from the rates of electron-transfer reactions." However, the solvation dynamics of ions in polar solution have not beeen experimentally measured. While structural data exist for small ions in water, neither the structure nor the dynamics of the solvation of larger ions have been studied. This behavior is of prime interest to physical chemists and is important for understanding many chemical and biological processes.There have been few studies of the time-dependent solvent reorganization processes around an ion that is created quickly compared to the relaxation time of the solvent. In this work, we present a pulse-radiolytic study of the solvation of a large aromatic anion in a seriesof primary alcohols. The system selected for this study was benzophenone anion, because its absorption spectrum isstronglyshifted by theso1vent;it issolubleat highconcentration; and its absorpti...

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