Modified photoreactivity due to mixed crystal formation. I. Three mixed crystals between isostructural cobaloxime complexes

Abstract

Three crystals of 2-cyanoethyl cobaloxime complexes with 3-chloropyridine, 3-bromopyridine and 3-methylpyridine as axial base ligands are isostructural to one another. Three mixed crystals were formed between the pairs: (3-bromopyridine)(2-cyanoethyl)cobaloxime-(2-cyanoethyl)(3-methylpyridine)cobaloxime(0.45/0.55);(3-chloropyridine)(2-cyanoethyl)cobaloxime-(2-cyanoethyl)(3bromopyridine)cobaloxime (0.61/0.39); (3-chloropyridine)(2-cyanoethyl)cobaloxime-(2-cyanoethyl)(3-methylpyridine)cobaloxime (0.44/0.56). The X-ray crystal structure analysis revealed that the mixed compounds are also isostructural to the component crystals. It was found from the change in IR spectra that the 2-cyanoethyl groups in the three mixed crystals isomerized to 1-cyanoethyl groups on exposure to a xenon lamp, as observed for the 2-cyanoethyl groups in the component crystals. Rate constants for the three mixed and three component crystals, measured under the same conditions, are approximately the same, as the reaction cavities for the 2-cyanoethyl groups in the six crystals have almost the same size. For the mixed crystal between 3-chloropyridine and 3-methylpyridine complexes, the isomerization proceeded with retention of the single-crystal form. The conformation and configuration of the 1-cyanoethyl group that was produced were well explained by the shape of the reaction cavity before irradiation.

Engineering Crystal Properties through Solid Solutions

“…Then they modify the cavity size by making solid solutions, which confirmed their theory and proved that the rate of reaction could be tuned. 84,85 A similar mechanism is behind the improved water and light stability of Pigment Red 170. Here denser, and hence more stable structures, are obtained through mixed crystals with bromo, chloro, fluoro, and methyl substituted analogues.…”

“…Oashi and co-workers measured the photoreactivity of 3-chloro-, 3-bromo-, and 3-methylpyridine complexes and showed that the rate of reaction depends on the size of the cavity around the reactive group. Then they modify the cavity size by making solid solutions, which confirmed their theory and proved that the rate of reaction could be tuned. , A similar mechanism is behind the improved water and light stability of Pigment Red 170. Here denser, and hence more stable structures, are obtained through mixed crystals with bromo, chloro, fluoro, and methyl substituted analogues …”

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.…”

“…Even though a mixed fulgide composite had been prepared by blending two fulgides in the Poly­(methyl methacrylate) (PMMA) polymer matrix exhibiting two colors, still no fulgide multicolor photochromic systems in the single crystal form have been formulated. On the other hand, successful formation of substitutional solid solutions between isostructural molecular pairs containing methyl, chloro, and bromo groups were reported several times. − Hence, it would be of a great interest if a mixed crystal is formed between fulgides by exchanging methyl, chloro, and bromo groups, which can show multicolor photochromism.…”

Modified photoreactivity due to mixed crystal formation. II. Enhanced reactivity upon conformational mimicry and cavity enlargement

“…The isomerization rate of the (2-cyanoethyl)cobaloxime complex crystal with the cis conformation was signi®cantly greater than that with the trans conformation (Ohashi et al, 1990). Recently, a quantitative relationship between the isomerization rates and the structures of the 2-cyanoethyl groups in three mixed cobaloxime crystals with different axial base ligands was investigated (Vithana et al, 2002). However, striking changes in the reactivity were not observed, because the mixed crystals were composed of isostructural compound pairs and had structures similar to their parent crystals.…”