Theoretical approach to photochromism of aromatic Schiff bases: A minimal chromophore salicylidene methylamine

The ground and excited electronic states conformations of various tautomers of salicylidene methylamine (SMA) are calculated by ab initio methods. It is demonstrated that the cis-keto form, more stable in the pi pi* S-1 state , can rapidly decay to a strongly nonplanar configuration of the n pi* stat e, which subsequently undergoes very fast internal conversion to one of the trans-keto tautomers. This we identify with the photochromic species of SM A. The calculated photochromic absorption spectrum reproduces very well the observed spectrum. The rates of the proton transfer processes in the groun d and excited states of SMA are calculated with the help of the ab initio g eometries, force fields, and the instanton method. It is shown that the exc ited state proton transfer occurs faster than 50 fs for the CIS barrier of 1.6 kcal/mol. The ground state backproton transfer between the cis-keto and enol tautomers is predicted to take similar to 30 ps and to exhibit a larg e deuterium effect. (C) 2000 American Institute of Physics. [S0021-9606(00) 31242-9].