Structural study of the solid-state photoaddition reaction of arylidenoxindoles

The photochemistry of isomeric 2-furyliden- and benzylidenoxindoles (2H-indol-2-ones) is examined. In solution E-Z isomerization is the only process via the excited singlet state (which fluoresces in glassy solution at 77 K and not at room temperature). In the crystalline state, the two (Z) derivatives are photostable, in accordance with the prediction based on the structural determination of the furylidene derivative, which adopts the unreactive Schmidt's gamma type arrangement. The (E) furylidene derivative (1a) gives efficiently (Phi = 0.3) the head-to-tail dimer, as indicated by the crystal structure, which is of the reactive alpha type, in full accord with the topochemical principles. In contrast, the corresponding benzylidene (1b) derivative reacts sluggishly (Phi < 0. 01) and mainly gives polymers, despite the fact that crystal structure determination shows that it likewise pertains to the alpha type and complies with the topochemical rules. The difference in reactivity is explained on the basis of (i) the twist of the phenyl ring with respect to the indole plane, and (ii) the higher overall cohesion energy and the lower interaction energy between facing molecules, as found from the charge density analysis for the crystals of 1b in comparison to those of 1a. This evidences a further stringent requirement for the occurrence of topochemical photodimerizations.