Photochromic Transitional Metal Complexes for Photosensitization

Abstract

IntroductionPhotochemical reactions of most organic compounds (except those with carbonyl substituent) occur predominantly at the lowest-lying singlet excited state (S 1 ). This is attributed to the very inefficient intersystem crossing (ISC) from the singlet to the triplet state due to its spin-forbidden nature. In contrast, triplet excited-state photochemistry of carbonyl-containing organic compounds are well reported because the singlet-triplet energy splitting of these types of compounds are much smaller, leading to the effective population of their triplet excited state on light absorption [1]. To tackle the inefficient ISC, the photoreactivity of an organic compound derived from the triplet state can be achieved by using a triplet photosensitizer, through which light is absorbed by the photosensitizer and energy is transferred from its triplet excited state to the acceptor organic molecule to populate the triplet excited state (Figure 2.1). To achieve photosensitization, which bypasses the reactivity from singlet excited states, the photosensitizer must possess long-lived and higher-lying triplet (T 1 ) energy than the acceptor organic molecule for efficient energy transfer but lower-lying singlet state (S 1 ) for selective photoexcitation of the photosensitizer (Figure 2.1).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 [2]. Studies on the photochromic reactivity of azo compounds [3], stilbenes [3], spiropyrans [4], and spirooxazines [5] via the triplet excited-state pathway achieved by intermolecular photosensitization with organic photosensitizers have also been reported in the literature [3][4][5]. 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) [6] and tricarbonyl rhenium(I) bipyridine [8] complexes has been demonstrated. The

Photoswitchable Photochromic Chelating Spironaphthoxazines: Synthesis, Photophysical Properties, Quantum-Chemical Calculations, and Complexation Ability

“…One way to achieve this is to try to trap the photomerocyanine in a metal-chelated form. This approach has been reported for spiropyranes , and in single cases for spirooxazines. , Metal ion complexation has also been used to exploit this class of compounds for the development of photochromic chemosensors that allow colorimetric or fluorimetric detection of certain analytes. ,,,,− Combining a photochromic moiety with an organometallic or coordination compound provides new properties arising from the combination of redox, optical, and magnetic properties of the metal complexes with the photochromic reaction. Metal complexes of photosensitive ligands are an interesting alternative to all-organic photochromic compounds.…”