Exciplex Formation between Indole Derivatives and Polar Solutes

The interaction between 3‐methylindole (3‐MeI) in cyclohexane with the polar molecules ethyl acetate (EA) and dimethylacetamide (DMA) was studied. The kinetic analysis of the fluorescence data shows that 3‐MeI forms an excited state complex with EA in a diffusion controlled process. The temperature dependence of the exciplex formation indicates that its dissociation rate constant too is of appreciable magnitude. UV, IR and fluorescence measurements all indicate that DMA forms complexes with 3‐MeI both in the ground and in the excited states. The latter complex is rather strong i.e. its dissociation rate is negligibly small. The findings imply that the fluorescence properties of tryptophan residues within a protein often must be determined by their interaction with neighboring polar groups rather than by the degree to which they are exposed to the ambient solution.