Theoretical study of photochromism of N-salicylidene-alpha-methylbenzylamine

It is shown that the direct precursor for the formation of a photochrome in Schiff bases is a "twisted" geometry S1 state of a keto tautomer in which the O[centered ellipsis]N\ufffdH hydrogen bond is broken. In photochromic Schiff bases this state is close in energy to the pipi* state of the cis-ketone obtained from the initially optically excited enol species by an ultrafast excited-state proton transfer, in particular, in N-salicylidene-alpha-methylbenzylamine (SMB), it lies within 1 kcal/mol from the pipi* state of cis-ketone and ~16 kcal/mol below the initial pipi* state of enol. The excited-state proton transfer in SMB is predicted to take ~15 fs and to be nearly barrierless. It is also shown that the initial pipi* state of enol can decay via competing route to a twisted geometry nNpi* state of enol with the energy ~20 kcal/mol below that state.Peer reviewed: YesNRC publication: Ye