Proton phototransfers in doubly hydrogen bonded dimers: the photophysics of 6,7,8,9-tetrahydro-5H-pyrido[2,3-b]indole dimers.

A photophysical study of 6,7,8,9-tetrahydro-5H-pyrido[2,3-b]indole dissolved in 2-methylbutane at decreasing temperatures from 293 to 93 K revealed the presence of two different fluorescence emissions due to doubly hydrogen-bonded dimers of the monomer species. One of the emissions is assigned to the normal tautomer form of the dimer of C(2) symmetry, and the other is mostly assigned to the most stable dimer, which is nonsymmetric (C(1)) and exhibits single-proton transfer upon electronic excitation. As shown here, when the doubly hydrogen-bonded 7-azaindole dimer loses its centrosymmetry, by fusing a six-membered ring to the molecular structure, its first excited electronic state does not undergo a double-proton transfer but rather a single proton transfer, which, in a fluid medium, causes the dimer to dissociate; if the medium can trap the resulting excited structure, then the compound exhibits fluorescence in the 410 nm region, which is typical of its protonated or deprotonated forms.