Energetics of photoconversion of norbornadiene to quadricyclane: Effects of directly attached substituents via ab initio and DFT

An attempt is made to maximize the photochemical and/or solar energy storage in norbornadiene (1)/quadricyclane (2) system, through direct attachment of substituents at C1, C2 or C7 atoms of 1 and 2. Assessments of the corresponding energies are made at B3LYP/6-311++G* level of theory. Electron donating substituents, D (D=-NMe2, -NH2, -OMe, -OH, and Me), directly attached at C2, increase the energy gap between 1 and 2, inducing higher storage of energy in the system. Electron withdrawing subsituents, W (W=-NO2, -F, -Cl, -Br, -CF3 and -COOH), directly attached at C1, moderately induce the energy storage. Attachment of either D or W groups at C7 show no significant difference in the photochemical and/or solar energy storages. A Hammet ρ value of −3.69 is encountered for substituents directly attached at C2 atoms of 1 and 2. This is in clear contrast to the Hammet ρ value of −0.81 that we recently reported for the subsituents indirectly placed at the C2 of 1 and/or 2 (through attachment via the para carbon of phenyl rings).