The prototypical [Ru­(bpy)3]2+ (bpy = 2,2′-bipyridine, Ru-1) with 3MLCT state (metal-to-ligand charge-transfer, πM → πL*) is one of the most widely used photosensitizers (PSs) for photocatalytic hydrogen production. However, its photostability and excited state lifetime (<1 μs) are eagerly to be improved to further enhance the performance of hydrogen production. Herein, [Ru­(bpy)2(3-pyrenyl-1,10-phenanthroline)]2+ (Ru-3) with 3IL/3MLCT equilibrated state and [Ru­(bpy)2(3-pyrenyl ethynylene-1,10-phenanthroline)]2+(Ru-4) with 3IL state (intraligand charge transfer, πL → πL*) as lowest excited state were first introduced into the photocatalytic hydrogen evolution system. Photophysical and photocatalytic characteristics manifest that the 3IL state complex (Ru-4) shows a long-lived excited state (up to 120 μs) and much enhanced photostability with no photobleaching over 13 h in stark contrast to Ru-1 and [Ru­(bpy)2(1,10-phenanthroline)]2+ (Ru-2). Photocatalytic reactions with these Ru­(II) complexes as PSs, Co­(dmgH)2pyCl (C-1) as a catalyst, and N, N-dimethyl-p-toluidine (DMT) as an electron donor indicate that the catalytic performance of Ru-4 and Ru-3 is dramatically enhanced compared to that of Ru-2 and Ru-1, and the TON and TOF toward Ru-4 can reach up to 9140 and 6.3 min–1 under the optimized condition. Photoluminescence studies reveal that the Stern–Volmer quenching constant of excited state Ru-4 by DMT is determined as 2.8 × 104 M–1, which is 4.5-, 42-, and 44-fold higher than those of Ru-3 (6.2 × 103 M–1), Ru-2 (6.7 × 102 M–1), and Ru-1 (6.3 × 102 M–1), respectively. Transient absorption spectra confirmed that the reductive quenching mechanism is the dominated process, and the quenching constant of electron transfer from reduced PSs of Ru-1–Ru-4 to C-1 catalyst has the same order of magnitude (∼105 M–1). The increased photocatalytic activity of Ru-3 and Ru-4 is due to their prominent photostability and efficient electron transfer from DMT to PSs. This work not only contributes to a deep understanding in the photocatalytic process with the PSs of three different excited state types but also opens up an avenue to explore robust and long-lived PSs with 3IL state for efficient hydrogen production.

Inspired by nature, improving photosensitization represents a vital direction for the development of artificial photosynthesis. The sensitization ability of photosensitizers (PSs) reflects in their electron transfer ability, which highly depends on their excited state lifetime and redox potential. Herein, for the first time, we put forward a facile strategy to improve sensitizing ability via finely tuning the excited state of Ru(II)-PSs (Ru-1–Ru-4) for efficient CO2 reduction. Remarkably, [Ru(Phen)2(3-pyrenylPhen)]2+ (Ru-3) exhibits the best sensitizing ability among Ru-1 – Ru-4, over 17 times higher than that of typical Ru(phen)32+. It can efficiently sensitize dinuclear cobalt catalyst for CO2-to-CO conversion with a maximum turnover number of 66480. Systematic investigations demonstrate that its long-lived excited state and suitable redox driving force greatly contributed to this superior sensitizing ability. This work provide a new insight for dramatically boosting photocatalytic CO2 reduction via improving photosensitization.

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