Since the first report of the pyrene excimer by Forster,1 intermolecular excited-state interactions have become a key subject in photochemistry and photophysics. If the two chromophores are identical, the stabilizing interaction between a molecule in the ground state and a molecule in the excited state can be described by the configuration interaction model.2 An alternative way to estimate the stabilization energy takes into account properties such as electrostatic interactions, exciton resonance, intermolecular repulsion, charge-transfer interactions, dispersion forces, and overlap repulsions.3 If the two chromophores are different, an exciplex or an heteroexcimer can be formed.4 The stabilization of an excimer depends strongly on the overlap between the two chromophores.Calculations suggest a picture of the excited-state complex in which the two chromophores, if they are planar, are at an optimum distance of 3.5 Á in a planparallel orientation.
SynopsisDipeptides of the aromatic fluorescent amino acid, pyrenylalanine, are studied using both stationary and transient fluorescence techniques. Since the conformational transitions of the peptide chain are slow compared to the decay of the pyrene excited state, both ground state conformations, adopted by the peptide, i.e., C, and C,, can be monitored separately. Kinetic models are proposed to describe the molecular dynamics of the peptide chain as probed by the intramolecular excimer formation between both pyrene chromophores. These kinetic schemes explain the influence of solvent, chain chirality, main chain-side chain interactions, and nature of the protecting groups on the emission spectrum and the fluorescence decay profile of these model peptides. These schemes also provide a tool to calculate rate constants of conformational transitions and excimer formation. By comparing the kinetic and thermodynamic parameters of the various compounds, the influence of a structural modification on the molecular dynamics of the peptide chain is determined.
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