Photochromic Transitional Metal Complexes for Photosensitization

Photochemical reactions of most organic compounds occur predominantly at the lowest-lying singlet excited state. This is attributed to the very inefficient intersystem crossing (ISC) from the singlet to the triplet state due to its spin-forbidden nature. In the organic photochromic systems, even though the photochromism is originated from different reaction mechanisms such as reversible trans-cis photoisomerizations as well as the reversible photoinduced ring-opening and ring-closing reactions, most of these reactions are derived from the excited singlet state. Studies on the photochromic reactivity of azo compounds, stilbenes, spiropyrans, and spirooxazines via the triplet excited-state pathway achieved by intermolecular photosensitization with organic photosen-sitizers have also been reported in the literature. Along with the rapid development of phosphorescent metal polypyridine complexes in the 1970s, diffusion-controlled intermolecular sensitization of photochromic reactions of stilbenes with the triplet excited state of tris (bipyridyl) ruthenium(II) and tricarbonyl rhenium(I) bipyridine complexes has been demonstrate