THE PHOTOCHEMICAL PROPERTIES OF THE MONOTHIOIMIDE AND DITHIOIMIDE DERIVATIVES OF (E,E)-DIBENZYLIDENE-N-PHENYLSUCCINIMIDE

Novel (E,E)-dibenzylidene-N-phenyl succinimide thioimide lb and dithio imide Ic derivatives of the photochromic fulgimide 1a were synthesized . The thioimide lb and dithioimide Ic derivatives were found to underg o 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 la. The X-ray c rystal structures of lb and 1c were determined, and the influence of t he thiocarbonyl groups on the structural parameters were investigated in comparison to the crystal structure of la. The H-1 and C-13 NMR spe ctra were recorded for all three compounds la, lb, and ic and are disc ussed in terms of their structural and electronic variations, as is th e effect on the photochemical behavior. The twisting of the N-phenyl r ing was found to be a consequence of intramolecular steric interaction s and not of intermolecular packing forces. This was substantiated by a series of semiempirical (AM1) 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 wa s evident throughout the whole thiocinnamic moiety. Crystal data for m onothioimide lb: spacegroup P2(1)/c with a = 8.794(2) angstrom, b = 9. 623(1) angstrom, c = 22.182(5) angstrom, beta = 91.45(2)-degrees, Rw = 0.038, and R = 0.067. Dithioimide 1c: spacegroup Pnna with a = 9.411( 2) angstrom, b = 17.304(2) angstrom, c = 13.574(2) angstrom, Rw = 0.05 5, and R = 0.077.