Mechanistic insights into azo compound back-isomerization from spin-flip time-dependent DFT combined with Marcus theory

Interest in photosensitive molecules has increased significantly over the past decade, with particular attention given to photoswitchable systems. Among these, azobenzene stands out as a reference compound due to its broad range of applications, in particular for solar energy storage. While the trans-to-cis photoisomerization has been relatively well characterized, the reverse cis-to-trans isomerization remains a complex process potentially involving multi-state physics. In this study, we compile recent theoretical advances aimed at modeling this process and introduce, through the spin-flip time-dependent density functional theory (SF-TDDFT) approach combined with the semi-classical Marcus equation, a fast and efficient method to investigate the mechanisms of thermal back-isomerization of azo derivatives. By comparing various exchange-correlation functionals with CASPT2 reference data, we demonstrate that the PBE0(D3BJ) functional provides an accurate description for the non-adiabatic rotational pathway. We successfully reproduce the experimental values (88.6 vs. 88.3 kJ mol(-1) for the experimental enthalpy of activation, and -53.0 vs. -50.2 J mol(-1) K-1 for the experimental entropy of activation) for azobenzene, thus motivating the extension of this methodology to other azo derivatives. This approach can be further generalized to a broader class of azo-based photo-switches in future studies.