Control of the photochromic behavior of cobaloxime complexes with salicylidene-3-aminopyridine and 2-cyanoethyl groups by dual photoisomerization

Abstract

New dual photoisomeric cobaloxime complexes composed of salicylidene-3-aminopyridine derivatives and a 2-cyanoethyl group as axial ligands were synthesized. Photoisomerization of the 2-cyanoethyl group in the crystalline state was performed to control the photochromic properties of the salicylidene-3-aminopyridine derivatives. The colors of the three cobaloxime crystals changed from pale yellow to orange or dark red when the crystals were irradiated with UV light. The red crystals returned to their original color when they were irradiated with visible light or kept in the dark. When the crystals were exposed to visible light before UV irradiation, the 2-cyanoethyl group bonded to the cobalt atom isomerized to the 1-cyanoethyl group with retention of the single crystal form. The crystals containing the 1-cyanoethyl group after photoirradiation also showed photochromism on exposure to UV light. However, the lifetime of the photochromic red species after 2-1 isomerization of the cyanoethyl group was significantly different from that before isomerization. The difference is well explained by the cavity volume around the central C-N bond of the salicylidene moiety in the crystal of the red species.