Light-Harvesting Cross-Linked Polymers for Efficient Heterogeneous Photocatalysis

Abstract

Nonporous, phosphorescent cross-linked polymers (<b>Ru-CP</b> and <b>Ir-CP</b>) were synthesized via Pd-catalyzed Sonogashira cross-coupling reactions between tetra­(<i>p</i>-ethynylphenyl)­methane and dibrominated Ru­(bpy)₃²⁺ or Ir­(ppy)­₂­(bpy)⁺, respectively. The resultant particulate cross-linked polymer (CP) materials have very high catalyst loadings (76.3 wt % for <b>Ru-CP</b> and 71.6 wt % for <b>Ir-CP</b>), and are nonporous with negligibly small surface areas (2.9 m²/g for <b>Ru-CP</b> and 2.7 m²/g for <b>Ir-CP</b>). Despite their nonporous nature, the insoluble CP materials serve as highly active and recyclable heterogeneous photocatalysts for a range of organic transformations such as aza-Henry reaction, aerobic amine coupling, and dehalogenation of benzyl bromoacetate. An efficient light-harvesting mechanism, which involves collection of photons by exciting the ³MLCT states of the phosphors and migration of the excited states to the particle surface, is proposed to account for the very high catalytic activities of these nonporous CPs. Steady-state and time-resolved emission data, as well as the reduced catalytic activity of Os­(bpy)₃²⁺-doped <b>Ru-CP</b>s supports efficient excited state migration for the CP frameworks. This work uncovers a new strategy in designing highly efficient photocatalysts based on light-harvesting cross-linked polymers