Tracking photophysical relaxation in spiropyran with simulated time-resolved X-ray absorption spectroscopy

Spiropyran is a prototype photoswitch that undergoes photoinduced ring-opening via C-O bond fission. The quantum yield for this photochemical reaction is low, with photophysical relaxation being the dominant process. Previous theoretical studies have suggested that photophysical relaxation proceeds via cleavage and subsequent reformation of a C-N bond. However, experimental evidence for this mechanism is lacking; most time-resolved studies have probed in the ultraviolet-visible domain, where C-N bond fission is unlikely to leave clear signatures. Here, we use non-adiabatic dynamics simulations in conjunction with X-ray absorption spectroscopic calculations to investigate the possibility of tracking the photophysical relaxation in spiropyran with time-resolved X-ray absorption spectroscopy at the nitrogen K-edge. The simulations predict that cleavage of the C-N bond results in a transient red-shift of the X-ray absorption. These results indicate a potential experimental route to gain a mechanistic understanding of the efficient photophysical relaxation that limits the efficiency of spiropyran-based photochromic systems.