Does PRODAN possess a planar or twisted charge-transfer excited state? Photophysical properties of two PRODAN derivatives

The synthesis and photochemical properties of 1-(4-methyl-1,2,3,4-tetrahydrobenzo[f]quinolin-8-yl)propan-1-one (7) and 2,2-dimethyl-1-(4-methyl-1,2,3,4-tetrahydrobenzo[f]quinolin-8-yl)propan-1-one (8) are reported. These compounds are models for PRODAN, 6-propionyl-2-(dimethylamino)naphthalene, where the dialkyl-amino group is forced to remain coplanar with the naphthalene ring. The Stokes shifts of 7 and 8 in various solvents are compared with those of PRODAN. The absorption and emission transitions are calculated by the AM1 semiempirical method employing a conductor-like screening model for solvent effects. The experimental and calculated solvatochroism of both model compounds are of the same magnitude as PRODAN, suggesting that all fluoresce from a planar ICT state.