Highly Efficient Singlet Oxygen Generation and High Oxidation Resistance Enhanced by Arsole-Polymer-Based Photosensitizer: Application as a Recyclable Photooxidation Catalyst
Susumu Tanaka, Toshiaki Enoki, Hiroaki Imoto, Yousuke Ooyama, Joji Ohshita, Takuji Kato, Kensuke Naka
Abstract
Photosensitizers have attracted considerable attention in various fields such as organic synthesis and medical care. For the development of high-performance photosensitizers, highly efficient and persistent singlet oxygen generators (1O2) having a high oxidation tolerance are strongly required. This study presents a detailed investigation of dithieno[3,2-b:2′,3′-d]arsole-fluorene copolymer for its 1O2 generation ability and application as a photooxidation catalyst in vital organic reactions. Photoirradiation of an air-saturated solution of the polymer generates 1O2, which was detected by 1O2 scavengers such as dihydronaphthoquinone and diphenylisobenzofuran. The polymer photosensitizer was completely stable in the presence of the strong oxidant 1O2. The photosensitizer showed the highest quantum yield of 1O2 generation (Φ = 0.54) in single-component main-chain type π-conjugated polymers. The quantum yield of the arsenic-free analogue of the polymer—bithiophene-fluorene copolymer—was significantly lower (Φ = 0.14), suggesting that the heavy-atom effect of arsenic can improve the efficiency of intersystem crossing (ISC) from the singlet excited state to the triplet excited state of the photosensitizer. In addition, when utilized as a recyclable photocatalyst for the oxidation reaction, the photosensitizer exhibited excellent oxidation resistance without losing its recognizable catalytic activity. Finally, we demonstrated the release of 1O2 into the air by a film of the present polymer. Persistent 1O2 generation was observed on film irradiation without polymer decomposition. These results suggested that the polymer exhibited excellent oxidation resistance in solution as well as in the solid state. The present molecular design concept of the photosensitizer using the main group element can facilitate the development of further functional optical materials.