THE 3‐METHYLINDOLE/n‐BUTANOL EXCIPLEXES: EVIDENCE FOR TWO EXCIPLEX SITES IN INDOLE COMPOUNDS

The 1‐butanol concentration dependence of fluorescence emission intensities and spectra from 3‐methylindole/1‐butanol exciplexes in 1‐heptane reveals a 1:1 stoichimetry near the isoemissive point increasing to 1:2 at higher concentrations. The large increase in stability of the 1:1 complex relative to indole itself is attributed to its relatively low dissociation rate with an activation energy of 36.8 kJ/mol. Each step of exciplex formation shifts the emission maximum about 15 nm to the red. The stronger combining site is the negative C‐3. The weaker site is the positive N‐1. Stabilization due to charge transfer between excited indole and electrophilic or nucleophilic partners is small relative to dipole‐dipole interactions. There is no indication of significant orbital‐overlap. The detection of both positive and negative centers for exciplex formation on indole, 1‐methyl indole and electronically similar derivatives resolves some long‐standing problems and extends the basis of knowledge necessary to use tryptophanyl‐residue fluorescence as a quantitative probe for protein conformational character and its changes.