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 ππ^* $S_1$ state, can rapidly decay to a strongly nonplanar configuration of the ${\mathrm{n\pi }}^{*}$ state, which subsequently undergoes very fast internal conversion to one of the trans-keto tautomers. This we identify with the photochromic species of SMA. The calculated photochromic absorption spectrum reproduces very well the observed spectrum. The rates of the proton transfer processes in the ground and excited states of SMA are calculated with the help of the ab initio geometries, force fields, and the instanton method. It is shown that the excited 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 ∼30 ps and to exhibit a large deuterium effect.