Crystalline state photochromism of 3-furylfulgides: impact of size and bond flexibility of the non-aromatic alkylidene group.

3-Furylfulgides are photochromic compounds showing high thermal stability in their closed forms. However, their photochromic properties in the solid state should be improved further to fabricate molecular devices. Understanding how the size and the flexibility of the non-aromatic alkylidene moiety alter the crystalline state photochromic properties is also important here, as the alkylidene group is directly involved in the photochromic ring closing and opening reactions. The synthesis of four 3-furylfulgides composed of different alkylidene groups (rigid isopropyl, flexible 2-butyl, rigid cyclopentyl and flexible cyclohexyl), their crystal structures and structure-photochromic property correlation in the crystalline state are reported here. Crystallographic data along with reaction cavity volumes calculated using the program CAVITY [Ohashi et al. (1981), J. Am. Chem. Soc. 103, 5805-5812] disclosed that fulgides with flexible groups at the ring closing site have more free volume around the reactive area in the crystal lattice, which can provide more space for the atomic movements in the reaction and flexibility can reduce the strain built up in the closed C-isomers by making conformations. According to UV-vis spectroscopic data, a higher yield of C-isomers and a better fatigue resistance were obtained for the 3-furylfulgide with the largest and flexible cyclohexyl group showing greater photochromic properties in the crystalline state than the fulgide containing the smallest and rigid isopropyl group.