Near-Infrared Light-Driven Condensation Using Branched Two-Photon-Absorbing Organic Photocatalysts with Viscosity-Dependent Properties

Most reported photocatalytic systems that primarily rely on the absorption of ultraviolet (UV) and short visible light irradiation face significant limitations, including shallow penetration in reaction media, competing absorption with substrates and catalysts, and incompatibility with light-sensitive molecules. These drawbacks can be largely avoided if the same reaction can be operated under near-infrared (NIR) light irradiation. Herein, we report a novel family of branched pyridinium-based photocatalysts designed with elegant donor-π-acceptor (D-π-A) architectures and exceptional two-photon absorption (TPA) capabilities in the deep-red and NIR regions. Among the three designed complexes, MPP-3arm demonstrates a remarkable TPA cross section and hence efficient NIR-driven photocatalytic condensation of aromatic diamines and aldehydes. Notably, it also exhibits a unique viscosity-dependent photocatalytic performance, attributed to restricted rotational mobility in a highly viscous environment, a feature rarely explored in organic photocatalysis. Overall, this study presents the design of multibranched organic TPA photocatalysts and their potential of overcoming the limitations encountered in conventional photocatalysis, thus unlocking new opportunities in NIR light-driven chemical synthesis.