Modified Photoreactivity Due to Mixed Crystal Formation. III. Decelerated Photoisomerization upon Topochemically Unfavorable Conformational Change

Abstract

The cobaloxime complexes of (2-cyanoethyl)(aniline)dimethylglyoximatocobalt(III) and (2-cyanopropyl)-(aniline)dimethylglyoximatocobalt(III) formed mixed crystals with (4-cyanobutyl)(aniline)dimethylglyoximatocobalt-(III), Mix-I and Mix-II, respectively, although the component molecules have considerably different structures. The conformations of Co-C-C-CN of the 2-cyanoethyl and 2-cyanopropyl groups in the respective pure crystals are gauche, but they changed to trans in the mixed crystals. Because the conformations of Co-C-C-C and C-C-C-CN of the 4-cyanobutyl group are trans in both the pure crystal and the mixed crystals, structural mimicry or conformational adoption occurred at the Co-C-C-CN group of each minor component in the mixed crystal formation.The 2-cyanoethyl and 2-cyanopropyl groups of the cobaloxime complexes were isomerized to 1-cyanoethyl and 1-cyanopropyl groups on exposure to a xenon lamp. The isomerization rates were estimated from the changes of the FT/IR spectra. The rate constants of the two groups in both pure crystals and mixed crystals were calculated assuming first-order kinetics. Since the conformations of the reactive 2-cyanoethyl and 2-cyanopropyl groups were changed to topochemically unfavorable trans conformations on mixed crystal formation, the rate constants of isomerization of the 2-cyanoethyl and 2-cyanopropyl groups in the Mix-I and Mix-II crystals, were reduced to about one-third and one-fourth of the original values, respectively, compared to the corresponding rates in the pure crystals.

Multicolor Photochromism of Fulgide Mixed Crystals with Enhanced Fatigue Resistance

“…This implies that in MIX-1E formation, 61% of 1E host molecules have been replaced by 2E guest molecules. This is a very strange and rare situation, because generally, in mixed crystals, cell parameters and the space group of the mixed one are similar to those of the major component. ,, Contrastingly, an equimolar mixture of 1E and 3E cocrystallized to give MIX-2E having cell parameters closer to those of the host component 1E , and here only 12% of 3E has been incorporated as the guest component. With respect to Figure , molecular structures of 1E , 2E , and 3E are almost similar having differences only in the pendent groups at the para positions.…”

“…This is a very strange and rare situation, because generally, in mixed crystals, cell parameters and the space group of the mixed one are similar to those of the major component. 18,20,24 Contrastingly, an equimolar mixture of 1E and 3E cocrystallized to give MIX-2E having cell parameters closer to those of the host component 1E, and here only 12% of 3E has been incorporated as the guest component.…”