Metal coordination-assisted near-infrared photochromic behavior: A large perturbation on absorption wavelength properties of N,N-donor ligands containing diarylethene derivatives by coordination to the rhenium(I) metal center

A new series of diarylethene-containing 1-aryl-substituted 2-(2-pyridyl)imidazole (L) ligands have been prepared and characterized. Subsequent reactions of the 2-(2-pyridyl)imidazole ligands with Re(CO)(5)Cl under reflux conditions afforded [Re(CO)(3)(L)Cl]; one of the complexes has also been structurally determined. The open forms of the ligands showed an intense absorption band at ca. 320 nm corresponding to pi -> pi* and n -> pi* transitions of the 1-aryl-2-(2-pyridyl)imidazole moiety, with mixing of pi -> pi* and n -> pi* transitions of the thiophene moieties. The corresponding rhenium(I) complexes showed IL absorption bands at ca. 352 nm, with additional MLCT [d pi(Re) -> pi*(L)] transitions at ca. 425 nm. Upon photocyclization of the diarylethene moieties by UV irradiation, two new absorption bands emerged at ca. 410-425 and 576-586 nm for the ligands, while the rhenium(I) complexes showed three new absorption bands at ca. 288-290, 475-480, and 712-713 nm upon excitation into either the IL or MLCT bands. Such a large shift of the absorption bands of the closed forms of the rhenium(I) complexes to the NIR region could be attributed to the planarization of the four heterocyclic rings relative to the open forms. On the contrary, the organic ligand systems alone did not display such a shift of the absorption maxima to the NIR region upon photocyclization.