Sensitizing Ru(II) polyimine redox center with strong light-harvesting coumarin antennas to mimic energy flow of biological model for efficient hydrogen evolution

In natural photosynthetic system, sunlight is trapped by strongly absorbing chromophores, followed by excitation energy transfer to redox centers to initiate redox reactions. This inspired chemists to decorate redox centers with antenna molecules to mimick energy flow of biological model. Herein, three strong light-harvesting coumarin antennas were firstly decorated to a Ru(II) complex, resulting in a tetrads (Ru-3) with high molar extinction coefficient of 144000 M-1 cm(-1) at 481 nm, 13-fold higher than that of typical Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine, Ru-1). Photocatalytic hydrogen evolution activity of Ru-3 is over 27 times higher than that of Ru-1 under weak visible-light condition, while achieving a turnover number (TON) of 5510 under 175 W Xenon lamp irradiation. Steady and transient spectra confirm that the strong absorbing coumarins are responsible for capturing visible light and subsequently funneling the excitation energy to Ru redox center, which can efficiently promote the electron transfer from N, N-dimethyl-p-toluidine (DMT) to excited redox center, as well as the subsequent photocatalytic hydrogen evolution.