The forbidden emission of protonated proton sponge

The forbidden and strongly redshifted emission of monoprotonated proton sponge (PS·H+) in water and in acidified acetonitrile is reported. An explanation consistent with both the strongly forbidden nature of the emission as well as its highly redshifted character is proposed to involve an overlap forbidden transition connected with two approximately orthogonal orbitals localized in different parts of the molecule. The corresponding relaxation coordinate leading to the fluorescing charge transfer state is linked with pyramidal (p) to nonpyramidal (np) interconversion of the dimethylamino group of PS·H+ and results in a significant energetic lowering of the CT state.