Photoinduced Electron Transfer (PET) as Key to Accelerate the Cycloreversion Reaction of Arylquadricyclanes.

The development of methods for the utilization of sustainable energy resources is an important scientific challenge. In one approach, the conversion and storage of solar light energy as chemical energy in a photoreaction has been established, namely as molecular solar thermal energy storage (MOST). In particular, the norbornadiene-quadricyclane system offers favorable photochemical and physicochemical properties for this purpose. However, the chemical reaction to release the stored energy, i.e., the cycloreversion of the quadricyclane, still requires improvement. In this context, we demonstrate that a fast and controlled cycloreversion of 2-(5-(1-methoxynaphthyl))quadricyclane can be accomplished in a photoinduced electron transfer (PET) reaction. Specifically, we identified 9-mesityl-10-methylacridinium, 9,10-dicyanoanthracene, and 9-nitrobenzo[b]quinolizinium as suitable catalysts that initiate the fast cycloreversion of the quadricyclane to the norbornadiene with visible light (420 nm) and with low catalyst loading. At the same time, there is no significant loss of favorable MOST properties, namely half-life and energy storage density. Thus, this method provides an efficient, complementary tool for the targeted cycloreversion of quadricyclane with high temporal and local control, as required for MOST applications.