Robust and Long-Lived Excited State Ru(II) Polyimine Photosensitizers Boost Hydrogen Production

摘要

The prototypical [Ru(bpy)(3)](2+) (bpy = 2,2'-bipyridine, Ru-1) with (MLCT)-M-3 state (metal-to-ligand charge-transfer, pi(M) - pi(L)*) is one of the most widely used photosensitizers (PSs) for photocatalytic hydrogen production. However, its photostability and excited state lifetime (1 mu s) 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 (IL)-I-3/(MLCT)-M-3 equilibrated state and [Ru(bpy)(2)(3-pyrenyl ethynylene-1,10-phenanthroline)](2+)(Ru-4) with (IL)-I-3 state (intraligand charge transfer, pi(L) - pi(L)*) as lowest excited state were first introduced into the photocatalytic hydrogen evolution system. Photophysical and photocatalytic characteristics manifest that the (IL)-I-3 state complex (Ru-4) shows a long-lived excited state (up to 120 mu s) 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)(2)pyCl (C-1) as a catalyst, and N, N-dimethyl-p-toluidine (DMT) as an electron donor indicate that performance of Ru-4 and Ru-3 is dramatically enhanced compared to that of Ru-2 and Ru-1, and the TON and TOF towards 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 x 10(4) M-1, which is 4.5-, 42-, and 44-fold higher than those of Ru-3 (6.2 x 10(3) M-1), Ru-2 (6.7 x 10(2) M-1), and Ru-1 (6.3 x 10(2) 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 (similar to 10(5) 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 (IL)-I-3 state for efficient hydrogen production.