Rate constants for intramolecular excimer formation and dissociation in 1,3hyphen;bis(Nhyphen;carbozolyl)propane have been measured in a number of different solvents using time resolved fluorescence decay techniques. It is found that within homologous series of solvents both rate constants decrease progressively with increasing viscosity and obey a relationship originating with the concept of free volume requirements for intramolecular conformational change. Among a group of chemically and structurally different solvents both rate constants are observed to vary randomly with the macroscopic solvent viscosity indicating that other solvent properties are also important. The results are discussed in terms of a qualitative model in which the microstructure of the solvent shell surround the interacting chromophores, as determined by the degree of solvation, ultimately determines the ease with which this intramolecular photophysical process can occur.
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