Role of Solvent Polarity and Hydrogen-Bonding on Excited-State Fluorescence of 3-[(E)-{4-[Dimethylamino]benzylidene}amino]-2-naphthoic Acid (DMAMN): Isomerization vs Rotomerization.

The present experimental and theoretical study on a new chromophore DMAMN of the type push-π-pull (push = dimethylaniline, π = imine, pull = 2-naphthoic acid), allows understanding of the mechanism by which the molecular conformational undergoes isomerization/rotomerization following electronic excitation. The steady-state fluorescence spectra of this compound, carried out in solvents of different polarities and proticities, showed significant changes in both the shape and peak positions. The wavelength and intensity change depend on the polarity and hydrogen-bonding environment. In highly polar solvents, the emission is weak and red-shifted compared to that for cyclohexane, but it is more red-shifted in moderate aprotic polar solvents. In hydroxyl solvents, a new weak low-energy emission band appears at ∼525 nm, attributed to the intermolecularly H-bonded open conformer. On the basis of the generated potential energy landscapes of the ground state and low-lying excited state in the gas phase and solution, we found that selective photon absorption, brings this molecule to a "bright" state, from which N═C isomerization Z → E, takes place. This isomerization in gas-phase and low-polarity solvents leads to two minima with a barrier, whereas in highly polar-protic media, it gives one minimum on the S surface with low ΔE (0.17 eV), facilitating deactivation via ISC.