A cyclometalated Ir(III) diimine complex [Ir(NBI)2(phen)]PF6, NBI = 1,8-naphthalenebenzimidizole and phen = 1,10-phenanthroline, exhibits excellent photostability as a sensitizer in a three-component homogeneous aqueous photocatalytic hydrogen producing system. Here, [Co(dmgH)2(py)Cl] serves as the hydrogen evolving catalyst, and N,N-dimethyl-p-toluidine (DMT) serves as a sacrificial single electron donor. The [Ir(NBI)2(phen)]PF6 photosensitizer remained active over the course of 90 h of visible-light-induced (λex = 452 nm, 540 mW) H2 production photocatalysis, yielding a turnover number (H2/photosensitizer) of 575 occurring near neutral pH conditions. The proton source was verified as being derived from aqueous protons by detecting the headspace gas composition through mass spectrometry in experiments using deuterated water. Mercury metal was used as a selective poisoning test, intended to sequester any metal-based nanomaterial(s) produced in side reactions and provide evidence that the current photocatalytic system was indeed operating in a purely homogeneous manner. Stern–Volmer analyses revealed that reductive quenching by DMT was the dominant (reductive) quenching pathway immediately following light activation in the photocatalytic cycle. The electron-transfer cage escape efficiency for this reaction was measured to be 78%, likely attributable to the significant triplet ligand-centered (3LC) character of the lowest excited-state manifold. This enables the majority of the charged-separated species to enter the subsequent catalytic cycle, thereby enhancing H2 production. In the presence of the [Co(dmgH)2(py)Cl] hydrogen evolution catalyst, the one electron reduced species of the ground-state Ir(III) complex transfers an electron to this catalyst with a rate constant of 2.5 × 109 M–1 s–1, producing a Co(II) intermediate while regenerating the resting state of the photosensitizer. The combined experimental results suggest numerous benefits in homogeneous photocatalysis become realized when 3LC excited states become involved in excited-state electron-transfer chemistry and can likely be leveraged in future photocatalytic schemes.
https://www.opotek.com/wp-content/uploads/2018/08/opotek-logo-v4-4-325x100-300x93.png 0 0 Katy https://www.opotek.com/wp-content/uploads/2018/08/opotek-logo-v4-4-325x100-300x93.png Katy2021-11-18 23:00:162021-11-18 23:00:17A Robust Visible-Light-Harvesting Cyclometalated Ir (III) Diimine Sensitizer for Homogeneous Photocatalytic Hydrogen Production