On the phosphorescence of 7-azaindole C2h dimer: The case of 3-bromo-7-azaindole

A 10−5 M solution of 7-azaindole in an inert solvent such as 2-methylbutane at a low temperature (77 K) consists of a dimer of C2h symmetry of the compound. Electronic excitation of such a dimer causes a rapid double proton transfer via a concerted mechanism and produces fluorescent emission with an onset at 413 nm and a maximum at 480 nm, but the resulting tautomeric C2h dimer does not emit phosphorescence. In this Letter, the phosphorescence of the tautomeric C2h structure is reported for the first time, which is induced by the intramolecular heavy atom effect present in 3-bromo-7-azaindole dimer.