Aromaticity-Controlled Energy Storage Capacity of the Dihydroazulene-Vinylheptafulvene Photochromic System

Photochemical conversion of molecules to high-energy isomers that after a stimulus return to the original ones presents a closed-cycle of light-harvesting, energy storage and release. One challenge is to achieve a sufficiently high energy storage capacity. Here we present efforts to tune the dihydroazulene/vinylheptafulvene (DHA/VHF) couple via loss/gain of aromaticity. Two derivatives were prepared, one with aromatic stabilization of DHA and the other of VHF. The consequences for the switching properties were elucidated. For the first type, sigmatropic rearrangements of DHA occurred upon irradiation. Formation of a VHF complex could be induced by a Lewis acid, but addition of H2O resulted in immediate regeneration of DHA. For the second type, the VHF was too stable to convert into DHA. Calculations support the results and provide new targets. We predict that by removing one of the two CN groups at C-1 of the aromatic DHA, the heat storage capacity is further increased as is the life-time of the VHF. Calculations also reveal that a CN group at the fulvene ring retards the back-reaction, and synthetically we show that it can be introduced regioselectively.