Hydrogen bonding and charge transfer interactions in exciplexes formed by excited indole and monosubstituted benzenes in cyclohexane

The interaction between the excited singlet state of indole derivatives with simple monosubstituted benzenes has been investigated in cyclohexane by steady state and time resolved fluorescence spectroscopy. The fluorescence quenching efficiency of indole increases in the order benzene, toluene, chlorobenzene and benzonitrile. For indole derivatives substituted on the N atom of the heterocyclic ring the quenching efficiency is much lower. At high concentrations of the benzenes derivatives the emission spectrum is broader and red shifted and the decay of the emission becomes biexponential. The results are analysed in terms of an exciplex forming mechanism. The binding forces in the exciplex are predominantly of hydrogen bonding for the case of benzene and toluene, while a charge transfer interaction is more likely in the case of chlorobenzene and benzonitrile.