The intrinsic fluorescence properties of molecules in condensed phases are complicated by intermolecular interactions. In crystalline solids the fluorescence depends on the photophysics of individual molecules as well as the interactions between molecules in the solid state Even a chemically pure crystal has some lattice disorder, which alters the local environment and changes the intermolecular interactions. Sample preparation and experimental technique may influence the fluorescence results.
The aromatic amino acid tryptophan is widely used as an intrinsic fluorescent probe of the solution conformation and dynamics of peptides and proteins. However, its complex photophysics makes it difficult to interpret the fluorescence results. The biexponential fluorescence decay of the tryptophan zwitterion is presumed to be due to ground-state rotamers. Intramolecular proton and electron transfer reactions involving the excited indole ring and amino acid functional groups have been proposed to account for the lifetime differences among rotamers. Excited-state H-D exchange occurs at the C-4 position of indole. In the proposed mechanism for the photosubstitution reaction, the ammonium group loops back over the aromatic ring and assists the proton exchange.