Theoretical Investigation of Substituent Effects on the Dihydroazulene/Vinylheptafulvene Photoswitch: Increasing the Energy Storage Capacity.

We have investigated the effects of substituents on the properties of the dihydroazulene/vinylheptafulvene photoswitch. The focus is on the changes of the thermochemical properties by placing electron withdrawing and donating groups on the monocyano and dicyano structures of the parent dihydroazulene and vinylheptafulvene compounds. We wish to increase the energy storage capacity, that is, the energy difference between the dihydroazulene and vinylheptafulvene isomers, of the photoswitch by computational molecular design and have performed over 9000 electronic structure calculations using density functional theory. Based on these calculations, we obtain design rules for how to increase the energy storage capacity of the photoswitch. Furthermore, we have investigated how the activation energy for the thermally induced vinylheptafulvene to dihydroazulene conversion depends on the substitution pattern, and based on these results, we have outlined molecular design considerations for obtaining new desired target structures exhibiting long energy storage times. Selected candidate systems have also been investigated in terms of optical properties to elucidate how sensitive the absorption maxima are to the functionalizations.