Exploring the Photophysics and Photocatalytic Activity of Heteroleptic Rh(III) Transition-Metal Complexes Using High-Throughput Experimentation.

High-throughput synthesis and screening (HTSS) methods were used to investigate the photophysical properties of 576 heteroleptic Rh(III) transition-metal complexes through measurement of the UV-visible absorption spectra, deaerated excited-state lifetime, and phosphorescent emission spectra. While 4d transition-metal photophysics are often highly influenced by deleterious metal-centered deactivation channels, the HTSS of structurally diverse cyclometalating and ancillary ligands attached to the metal center facilitated the discovery of photoactive complexes exhibiting long-lived charge-transfer phosphorescence (0.15-0.95 µs) spanning a substantial portion of the visible region (546-620 nm) at room temperature. Further photophysical and electrochemical investigations were then carried out on select complexes with favorable photophysics to understand the underlying features controlling these superior properties. Heteroleptic Ir(III) complexes with identical ligand morphology were also synthesized to compare these features to this family of well understood chromophores. A number of these Rh(III) complexes contained the requisite properties for photocatalytic activity and were consequently tested as photocatalysts (PCs) in a water reduction system using a Pd water reduction cocatalyst. Under certain conditions, the activity of the Rh(III) PC actually surpassed that of the Ir(III) PC, uncovering the potential of this often-overlooked class of transition metals as both efficient photoactive chromophores and PCs.

Continuous nucleation of metallic nanoparticles via photocatalytic reduction.

Whether in organic synthesis or solar energy conversion, light can be a powerful reagent in chemical reactions and introduce new opportunities for synthetic control including duration, intensity, interval, and energy of irradiation. Here, we report the use of a molecular photosensitizer as a reducing agent in metallic nanoparticle syntheses. Using this approach, we report three key findings. (1) Nanoparticles produced by photocatalytic reduction form via a continuous nucleation mechanism, as opposed to burst and burst-like nucleation processes typically observed in metal nanoparticle syntheses. (2) Because nucleation is continuous, as long as the solution is irradiated (and there remains excess reagents in solution), nanoparticle nucleation can be turned on and off by controlling the timing and duration of irradiation, with no observable particle growth. (3) This synthetic method extends to the formation of bimetallic nanoparticles, which we show also form via a continuous nucleation pathway, and follow predicted patterns of metal incorporation as a function of the magnitude of the difference between the reduction potentials of the two metals. Taken together, these results establish a versatile synthetic method for the formation of multimetallic nanoparticles using visible light.

Nickel(II) complexes based on dithiolate-polyamine binary ligand systems: crystal structures, hirshfeld surface analysis, theoretical study, and catalytic activity study on photocatalytic hydrogen generation.

To ascertain the influence of binary ligand systems [1,1-dicyanoethylene-2,2-dithiolate (i-mnt-2) and polyamine {tetraen = tris(2-aminoethyl)amine, tren = diethylene triamine and opda = o-phenylenediamine}] on the coordination modes of the Ni(ii) metal center and resulting supramolecular architectures, a series of nickel(ii) thiolate complexes [Ni(tetraen)(i-mnt)](DMSO) (1), [Ni2(tren)2(i-mnt)2] (2), and [Ni2(i-mnt)2(opda)2]n (3) have been synthesized in high yield in one step in water and structurally characterized by single crystal X-ray crystallography and spectroscopic techniques. X-ray diffraction studies disclose the diverse i-mnt-2 coordination to the Ni+2 center in the presence of active polyamine ligands, forming a slightly distorted octahedral geometry (NiN4S2) in 1, square planar (NiS4) and distorted octahedral geometries (NiN6) in the bimetallic co-crystallized aggregate of cationic [Ni(tren)2]+2 and anionic [Ni(i-mnt)2]-2 in 2, and a one dimensional (1D) polymeric chain along the [100] axis in 3, having consecutive square planar (NiS4) and octahedral (NiN6) coordination kernels. The N-HO, N-HS, N-HN, N-HS, N-HN, and N-HO type hydrogen bonds stabilize the supramolecular assemblies in 1, 2, and 3 respectively imparting interesting graph-set-motifs. The molecular Hirshfeld surface analyses (HS) and 2D fingerprint plots were utilized for decoding all types of non-covalent contacts in the crystal networks. Atomic HS analysis of the Ni+2 centers reveals significant Ni-N metal-ligand interactions compared to Ni-S interactions. We have also studied the unorthodox interactions observed in the solid state structures of 1-3 by QTAIM and NBO analyses. Moreover, all the complexes proved to be highly active water reduction co-catalysts (WRC) in a photo-catalytic hydrogen evolution process involving iridium photosensitizers, wherein 2 and 3 having a square planar arrangement around the nickel center(s) - were found to be the most active ones, achieving 1000 and 1119 turnover numbers (TON), respectively.

High-Throughput Screening of Earth-Abundant Water Reduction Catalysts toward Photocatalytic Hydrogen Evolution.

Noble-metal photosensitizers and water reduction co-catalysts (WRCs) still present the highest activity in homogeneous photocatalytic hydrogen production. The search for earth-abundant alternatives is usually limited by the time required to screen new catalyst combinations; however, here, we utilize newly designed and developed high-throughput photoreactors for the parallel synthesis of novel WRCs and colorimetric screening of hydrogen evolution. This unique approach allowed rapid optimization of photocatalytic water reduction using the organic photosensitizer Eosin Y and the archetypal cobaloxime WRC [Co(GL1)2pyCl], where GL1 is dimethylglyoxime and py is pyridine. Subsequent combinatorial synthesis generated 646 unique cobalt complexes of the type [Co(LL)2pyCl], where LL is a bidentate ligand, that identified promising new WRC candidates for hydrogen production. Density functional theory (DFT) calculations performed on such cobaloxime derivative complexes demonstrated that reactivity depends on hydride affinity. Alkyl-substituted glyoximes were necessary for hydrogen production and showed increased activity when paired with ligands containing strong hydrogen-bond donors.