Hybrid QM/QM simulations of photochemical reactions in the molecular crystal N-salicylidene-2-chloroaniline

Abstract

In this paper, we report the application of the QM/QM hybrid simulation technique to the photoisomerisation reactions of anils (i.e., Schiff bases of salicylaldehyde with aniline derivatives) in the solid state, on the example of the photochromic polymorph of N-salicylidene-2-chloroaniline. By propagating molecular dynamics on a potential energy surface constructed using a combination of time-dependent DFT and ground-state DFT calculations, two reaction pathways of the cis-enol isomer were observed, which occur with approximately equal probability. In the first pathway, the photoexcited molecule undergoes an intramolecular proton transfer reaction on average 25 fs after photoexcitation. It then persists in the cis-keto form for a few hundred femtoseconds before undergoing a pedal motion through which it reaches an S₁/S₀ conical intersection. This pathway, whose existence has previously been proposed in the literature to rationalize the feasibility of the photoisomerisation reaction in the confined environment of the crystal lattice, is predicted to lead to the formation of the trans-keto form. The second pathway is nonreactive and is analogous to a previously characterised radiationless de-excitation pathway of the isolated molecule. The cis-enol to trans-keto photoisomerisation is reversible. Following the photoexcitation of a trans-keto molecule, it persists in a largely unchanged geometry for a period of time ranging from a few hundred femtoseconds to over a picosecond, and subsequently undergoes a pedal motion in the same direction as the one involved in the cis-enol to trans-keto photoisomerisation, leading to the cis-keto isomer through another S₁/S₀ conical intersection.