Triplet Photosensitizers Showing Strong Absorption of Visible Light and Long-Lived Triplet Excited States and Application in Photocatalysis: A Mini Review

Abstract

Triplet photosensitizers (PSs) have been studied as photocatalysts in photocatalytic H2 evolution by water splitting and photoredox catalytic synthetic organic reactions, etc. The applications share common features that the photocatalysts (triplet PSs) harvest the photoexcitation energy, then undergo intersystem crossing (ISC), and finally initiate the electron transfer or triplet energy transfer, which are intermolecular processes in most cases. Thus, triplet PSs showing a strong visible-light-harvesting ability and long triplet excited state lifetimes are desired because they can enhance the above intermolecular processes. The conventional transition metal complexes (i.e., Ru­(bpy)3X2 or Ir­(ppy)3) have been widely used; however, these complexes are not satisfactory due to their weak absorption of visible light and short triplet excited state lifetimes. To a large extent, the photophysical property of these complexes is due to the low-lying metal-to-ligand charge transfer (MLCT) state, the S0 → 1MLCT transition is weakly allowed, and a strong heavy atom effect exists for the 3MLCT state, which results in weak visible light absorption and a short triplet excited state lifetime, respectively. This mini review introduces the recent progress of the development of the transition metal complexes as well as the organic triplet PSs that show a strong visible-light-harvesting ability and long triplet excited state lifetimes. The application of these new triplet PSs in photocatalytic H2 evolution and photoredox catalytic synthetic organic reactions is summarized.

2021 Pioneers in Energy Research: Vivian Yam

“…Five papers are on the topic of photocatalysis. The mini review by Zhao and co-workers summarizes the recent development of the transition metal complexes and the new organic triplet photosensitizers (PSs) that show a strong visible-light-harvesting ability and long triplet excited state lifetimes and also their applications, such as photocatalytic H 2 evolution. The research team of Bernhard reviews the recent advances in state-of-the-art parallelized reactors for light-driven H 2 evolution, highlighting the benefits of parallel and high-throughput experimentation via robotic automation.…”

Long-lived excited states of platinum(ii)-porphyrins for highly efficient photocatalytic hydrogen evolution

“…So far, such Pt(II)-porphyrins have been widely applied for organic light-emitting diodes (OLEDs), DNA cleavage, water oxidation, sensing applications, and self-assembly studies. [40][41][42][43][44][45] To the best of our knowledge, there is no study of the long-lived excited triplet states of Pt(II)-porphyrins for PHE.…”

Strong visible light-absorbing BODIPY-based Cu(i) cyclic trinuclear sensitizer for photocatalysis

“…[1][2][3] Light-harvesting molecules, also known as photosensitizers (PSs), play a crucial role in absorbing solar energy and achieving highly efficient photocatalytic systems in both nature and artificial systems. 4,5 Therefore, tremendous efforts have been devoted to developing PSs including pure organic chromophores and transition-metal complexes with strong light absorption in the visible range, long-lived excited states, and high quantum efficiency. 6,7 Among the transitionmetal-based PSs, noble metal complexes, especially iridium (Ir) polypyridyl complexes, are widely explored for photocatalytic organic synthesis due to their structural versatility and fascinating photophysical and chemical properties.…”