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 are fluorescent and have been studied using steady state fluorescence spectroscopy in solution at room temperature. The excitation and emission spectra of the trimer species show a significant shift to longer wavelength compared to the monomer, consistent with the greater extent of electron delocalisation. The emission properties of the 5-substituted indole monomers are very dependent upon solvent polarity and the nature of the 5-substituent; in contrast, the trimer species show little dependence. Controlling the electrochemical conditions allows variation of the relative proportions of trimer and polymer species. The excitation and emission spectra of the polymer species are shifted to longer wavelength, are broader and are of lower intensity than those of the trimer.