Spectroscopy and photophysics of bifunctional proton donor-acceptor indole derivatives.

We report spectroscopic and photophysical studies of a series of selected indole derivatives in solutions and under supersonic jet isolation conditions. All the compounds can assume two rotameric forms, syn and anti. The bifunctional molecules containing both the hydrogen bond donor (indole NH group) and acceptor centers (oxygen, nitrogen, or sulfur atoms) located in separate moieties covalently linked by a single bond are compared with the compound that does not have any acceptor center, 2-(1H-pyrrol-2'-yl)-1H-indole. The former compounds (containing furan, thiazole, or thiophene moieties) were anticipated to show solvent-dependent photophysics. Contrary to the expectations, all the compounds reveal very efficient fluorescence, independent of solvent polarity and hydrogen bond donor and acceptor abilities. Laser spectroscopic studies combined with supersonic jet techniques and quantum chemical computations have been performed in order to identify the rotameric forms and to gain insight into the changes in the molecular structure accompanying electronic excitation.