Fluorescence properties of electropolymerised 5-substituted indoles in solution

Electropolymerisation of the 5-substituted indole monomers, 5-cyanoindole, indole-5-carboxylic acid, 5-chloroindole, 5-bromoindole and 5-methoxyindole results in a redox-active film consisting of a cyclic trimer and chains of linked cyclic trimer (polymer). The monomer, trimer and polymer species ar e fluorescent and have been studied using steady state fluorescence spectro scopy in solution at room temperature. The excitation and emission spectra of the trimer species show a significant shift to longer wavelength compare d to the monomer, consistent with the greater extent of electron delocalisa tion. The emission properties of the 5-substituted indole monomers are very dependent upon solvent polarity and the nature of the 5-substituent; in co ntrast, the trimer species show little dependence. Controlling the electroc hemical conditions allows variation of the relative proportions of trimer a nd polymer species. The excitation and emission spectra of the polymer spec ies are shifted to longer wavelength, are broader and are of lower intensit y than those of the trimer.