Mechanistic Approach to the Sensitization Process of Aromatic Ketones in the Isomerization between Norbornadiene and Quadricyclane

Article abstract of DOI:10.1246/bcsj.60.2937

The quantum yields for norbornadiene(N)-->quadricyclane(Q) and Q-->N isomerization were determined in acetonitrile using several substituted benzophenones (BPs) as triplet sensitizers.For the N-->Q isomerization, BPs with electron-donating substituents exhibited higher quantum efficiency (0.4-0.6) than BPs with electron-accepting substituents.The quantum yields for Q-->N isomerization increased with electron-accepting ability of the substituents on BPs, but were lower than 0.1.The rate constants for quenching of triplet BPs by N and Q were also determined in acetonitrile by means of laser flash photolysis.The rate constants for N were not dependent on the triplet energies (E_T) of BPs but dependent on electronic properties of the substituents on BPs.On the contrary, those for Q increased with decreasing E_T of BPs and with increasing Hammet's ? constants of the substituents of BPs.Based on these observations an addition-elimination process is proposed for the N-->Q isomerization, and electron transfer process for the Q-->N isomerization.