Aggregation of oligomeric Ir photosensitizers promotes efficient and long-lifetime photocatalytic hydrogen evolution

In this paper, the effect of aggregation of oligomeric Ir photosensitizers on photocatalytic hydrogen evolution is reported for the first time. Four oligomeric Ir photosensitizers were synthesized via the copolymerization of [Ir(ppy)2(dabpy)](PF6) (H1) with different anhydride linkers. Density functional theory calculations, spectroscopic characterization, and photoelectrochemical results show that the oligomeric photosensitizer molecules aggregate with each other. The aggregation significantly affects intramolecular and intermolecular electron transfer paths, leading to lower fluorescence emission intensity, broader and stronger light absorption, and stronger photocurrent response of the flexible chain structure. All oligomers showed higher photocatalytic activity and longer photocatalytic lifetime than Ir photosensitizer H1. Moreover, compared with oligomeric photosensitizers containing rigid linkers (BPDA and NPTA), oligomeric photosensitizers with flexible linkers, namely 4,4 '-oxydiphthalic anhydride (ODPA) and hydroquinone diphthalic anhydride (HQDA) (denoted as H4 and H5, respectively), showed higher activity and more than two times longer lifetimes in the photocatalytic process. The photocatalytic efficiencies of H4 and H5 were 181 828.6 mu mol g-1 and 144 149.6 mu mol g-1, respectively. Their photocatalytic lifetimes were over 500 h. This is because the oligomeric photosensitizers with flexible linkers show better intramolecular and intermolecular electron transfer due to the possible distortion of the oligomeric structure with the structural rotation of the flexible anhydride chains. The appropriate steric hindrance enhances the stability of the photosensitizers without significantly compromising their activity. This work not only discloses the interaction mechanism between oligomeric Ir photosensitizers during photocatalysis, but also offers new insights into the structural design of high-performance photosensitizers in the future. This work first reports the aggregation effect of Ir oligomeric photosensitizers, which provides extra intermolecular electron transfer path and leads to a high photocatalytic hydrogen evolution activity and a long lifetime of the photosensitizers.