Acid-sensitive photoswitches: towards catalytic on-demand release of stored light energy

Photochromic compounds are promising for a variety of applications, including molecular solar thermal (MOST) energy storage. The energy release step and cyclability are critical issues to be addressed for the development of this technology. We report herein the synthesis and characterization of two diarylethene molecules featuring one (1) or two (2) pyridine groups as protonatable moieties. Upon UV irradiation, both molecules undergo a cyclization reaction from the open form (OF) to the closed form (CF). Both CF are stable for a few days in acetonitrile, and the addition of acid leads to a 600 (1) or 1500-fold (2) acceleration of the ring-opening reaction, even in catalytic amounts. A kinetic model is proposed to simulate the reaction, elucidating the contribution of each step to the kinetics and evidencing the importance of the kinetic control over the protonation thermodynamic equilibrium. Data fitting leads to the rates of elementary steps and turnover numbers (TON). Following a complete reaction cycle, neutralization of the acid by an equivalent amount of base allowed further cycles. This study represents a significant advancement in the cyclability and the control of the on-demand triggering of the energy-releasing ring-opening reaction of diarylethenes for future MOST applications.