The photochemical reaction of (E,E)-dibenzylidenesuccinic thioanhydride: A spectroscopic and structural study

The first photochromic thioanhydride, (E,E)‐2,3‐dibenzylidenesuccinic thioanhydride (1b), was synthesized. Irradiation of (1b) in benzene, dichloromethane, or chloroform gives isomerization and cyclization products similar to those obtained for the anhydride analogue (1a). The crystal structure of (1b) was determined (X‐rays) and compared to the structures of the anhydridge (1a) and N‐phenylimide (1c). 1H and 13C NMR spectra were recorded, and the electron charge densities of the carbon atoms were calculated with AM1. These results are discussed and compared with those for the compounds 1a–1c. The thioanhydride (1b) ring is much more twisted than either the anhydridge (1a) or the imide (1c) rings. The linear relationship between the 13C NMR chemical shifts and the electron charge density on the given atom has been investigated. The C(carbonyl) and C(α) carbon atoms of 1b were found to be greatly deshielded relative to the corresponding atoms in 1a and 1b. The increase in corresponding atoms in 1a and 1b. The increase in sensitivity toward cyclization of the thioanhydridge (1b) is attributed to the marked change in the charge distribution. Crystal data. Thioanhydride (1b): space group P21/c with a = 12.956(3) Å, b = 14.843(6) Å, c = 7.512(6) Å, β = 98.05(2)°, Rw = 0.085 and R = 0.089.