Photochromism of Fulgide Crystals: From Lattice-Controlled Product Accumulation to Phase Separation

Abstract

Fulgides are photochromic in solution and in the solid state. This study revealed that the photochromism of fulgide crystals proceeds in two stages. On the basis of a previously developed two-photon excitation method, a single-crystal X-ray diffraction study on (2E,3Z)-2-[1-(2,5-dimethyl-3-furyl)-2-methylpropylidene]-3-sec-butylidenesuccinic anhydride (3) allowed monitoring the structural changes in the solid state and confirmed that the photocyclization proceeded in a conrotatory fashion. The lattice-controlled photoreaction was, however, limited to the early photoconversion stages, where only a small amount of photogenerated colored species was present in the lattice of the reactant crystals. Upon further accumulation of the photoproduct, the reaction proceeded to a second stage, where the crystals of the photoproduct separated from the reactant-like crystals to form a different solid phase. The in situ observation of the solid-state photochromism was thus also limited to the early stages of the photoch...