Photochemistry of α‐oxo oximes. Part 11: Photochemical E‐Z isomerization of two 2‐(methoxyimino)‐1‐indanones. Evidence for excimer formation upon direct excitation at λ 300 nm

Abstract

AbstractPhotochemical E‐Z isomerization of 2‐(methoxyimino)‐1‐indanone (1) and its 7‐methyl derivative (2) was studied under conditions of direct irradiation (λ 350 and 300 nm) and triplet photosensitization. The decay ratios k5/k6 for triplet photosensitization and direct irradiation at λ 350 nm are the same for a given compound, indicating that isomerization of the studied α‐oxo oxime ethers upon direct irradiation at λ 350 nm also proceeds from the (lowest) excited triplet state. Upon direct irradiation at λ 300 nm, the photostationary state (pss) was found to depend on the concentration of the 2‐(methoxyimino)‐1‐indanone: at high concentrations, the (E)‐isomer strongly dominates (>90%), whereas, at low α‐oxo oxime ether concentrations, a substantial amount of the (Z)‐isomer is present (up to 35%). The concentration dependence of the pss is explained in terms of excimer formation of the S2(ππ*) state, the excimer being capable of inducing efficient E‐Z isomerization with a different decay ratio than that of the excited triplet monomer.