Theoretical study on the photochemical behavior of 4-(dimethylamino)benzonitrile

Ab initio calculations have been performed to examine the photochemical behavior of 4-(dimethylamino)benzenzonitrile (DMABN). The conical intersection between S-2 and S, (S-2/S-1-CIX), where the internal conversion takes place after the main transition Of S-0-S-2 at the equilibrium geometry in So, is characterized by a dimethylamino-twisted quinoid structure where aromaticity of the benzene ring is lost. The optimized geometry of the charge transfer (CT) state in S-1 has a feature similar to that of S-2/S-1-CIX but is not energetically stabilized so much. Consequently, electronically excited DMABN with CT character relaxes into the most stable locally excited (LE) state in S, through a recrossing at S-2/S-1-CIX in gas phase or nonpolar solvent. In polar solvent, in contrast, the equilibration between LE and CT takes place in S, so that the CT state is more stable because of electrostatic interaction. The excited states of DMABN derivatives have been also examined. On the basis of the present computational results, a new and simple guiding principle of the emission properties is proposed, where conventional twisted intramolecular CT (TICT) and planar intramolecular CT (PICT) models are properly incorporated.