Absorption spectra of substituted indoles in stretched polyethylene films

The stretched film technique has been used to resolve the two overlapping ¹L _a and ¹L _b transitions in the absorption spectra of some substituted indoles. The substituent effects on the absorption spectrum of indole, N-ethyl indole, 3-methyl indole and 7-methyl indole are discussed. The directions of the transition moments of these indoles with respect to the long molecular axis have been determined and the differences were attributed to the changes in the molecular geometry on excitation. The vibronic structure in the absorption spectra of these indoles is also considered.