The mechanism behind the photochromism and photomagnetism of type II biindenylidenediones: multiconfigurational, perturbative and density functional theory studies.

Abstract

Biindenylidenediones (BIDs) are a family of compounds that have been studied for a relatively short time. The crystals of these compounds are yellowish, and become purplish when they are irradiated and return back to their original color slowly in the dark or quickly when they are heated up. BIDs can be classified into different subfamilies depending on the nature of their substituents. BID-II crystals show a thermally dependent electron paramagnetic resonance (EPR) signal that is a characteristic of chemical species with unpaired electrons. These properties make BIDs very attractive for industrial applications but the mechanisms responsible for their photochromism and photomagnetism are still under debate. In this article, a computational study focused on the BID-II subfamily is presented. A variety of multiconfigurational methods (CASSCF, CASPT2 and IDDCI) have been used to study exhaustively the topography of the potential energy surfaces of the lower electronic states of a single BID molecule. Methods based on density functional theory (DFT) were then used to model the most important structures in a periodic crystal system. Our results suggest that δ-hydrogen abstraction could explain the observed experimental phenomena. After the initial excitation to the 1ππ* state, non-symmetric nπ* minima are populated, which are adiabatically connected to the photoproduct zone through a barrier along the reaction coordinate. Based on our set of results, we propose that an epoxide constitutes the most stable and accessible photoproduct preceded by the population of a triplet biradical of πOπ* nature which has only small geometrical differences in comparison with the reactant. The spin-orbit coupling indicates that the EPR signal arises due to the population of a low energy triplet through a thermally accessible intersystem crossing in the photoproduct zone.