The photochemical properties of the mono-and dithioimide derivatives of (E,E)-dibenzylidene-N-phenylsuccinimide

Novel (E,E)‐dibenzylidene‐N‐phenyl succinimide thioimide 1b and dithioimide 1c derivatives of the photochromic fulgimide 1a were synthesized. The thioimide 1b and dithioimide 1c derivatives were found to undergo isomerization as the main photoreaction when irradiated in a variety of organic solvents, as opposed to the reversible electrocyclic ring closure/opening reactions in the case of the fulgimide 1a. The X‐ray crystal structures of 1b and 1c were determined, and the influence of the thiocarbonyl groups on the structural parameters were investigated in comparison to the crystal structure of 1a. The 1H and 13C NMR spectra were recorded for all three compounds 1a, 1b, and 1c and are discussed in terms of their structural and electronic variations, as is the effect on the photochemical behavior. The twisting of the N‐phenyl ring was found to be a consequence of intramolecular steric interactions and not of intermolecular packing forces. This was substantiated by a series of semiempirical (AMI) calculations. The C(thiocarbonyl) was found to be highly deshielded compared to the corresponding C(carbonyl) carbon atom, and the deshielding effect of the thiocarbonyl group was evident throughout the whole thiocinnamic moiety. Crystal data for monothioimide 1b: spacegroup P21/c with a = 8.794(2) Å, b = 9.623(1) Å, c = 22.182(5) Å, β = 91.45(2)°, Rw = 0.038, and R = 0.067. Dithioimide 1c: spacegroup Pnna with a = 9.411(2) Å, b = 17.304(2) Å, c = 13.574(2) Å, Rw = 0.055, and R = 0.077.