Singlet adiabatic states of solvated PRODAN: A semiempirical molecular orbital study

X-ray structural analysis depicts PRODAN (6-propionyl-2-(dimethylamino)naphthalene) as a planar system with four molecules stacked in a unit cell of a monoclinic crystal. AM1 semiempirical calculations predict planar conformation for isolated PRODAN with rotational barriers 0.03 kcal/mol for the propionyl and 0.33 kcal/mol for the amino group, and ΔH_f = 6.74 kcal/mol. Adiabatic INDO/S-CI calculations suggest antiquinoidal distortion as the likely ring structural change in the lowest excited singlet state. Twisting of the dimethylamino group at position 2 induces, in comparison with either 1- or 3-substituted analogues, strong molecular orbital (MO) localization in the former derivative and occurrence of a highly polar excited singlet state. Decrease of the charge density on nitrogen atom, induced by specific electrostatic interactions with the surrounding medium, promotes the MO with strong N(2p_y) character into the HOMO level. The transition to that state, determined within the {π*,π} ← n(N) singlet manifold, is shifted strongly to the red, and the electric dipole moment of the lowest excited state is quadrupled. These predictions are corroborated by the strong emission shifts PRODAN and chromophores of similar structure and topology of substitution exhibit in media of different polarity and viscosity.