Design and Modification of Polyazine-Bridged Ru(II),Rh(III) Bimetallic and Trimetallic Supramolecular Complexes Applicable in Solar Energy Harvesting for the Photocatalytic Reduction of Water to Hydrogen

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Date
2012-09-17
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Virginia Tech
Abstract

The goal of this research was to develop a series of mixed-metal supramolecular complexes through systematic component variation to better understand the role of structural modification on basic chemical and photochemical properties including photocatalysis of H₂O to H₂. Varying bidentate polypyridyl terminal ligands (TL), non-chromophoric halides (X), or number of Ru(II) light absorbers (LA) tunes the electrochemical, spectroscopic, photophysical, and photochemical properties within the supramolecular architecture. Ru(II),Rh(III),Ru(II) trimetallics of the design {(TL)₂Ru(dpp)}₂RhX₂₅ (TL = phen = 1,10-phenanthroline or Ph₂phen = 4,7-diphenyl-1,10-phenanthroline; dpp = 2,3-bis(2-pyridyl)pyrazine; X = Cl⁻ or Br⁻) covalently couple two Ru(II) LAs to a central Rh(III) electron collector (EC) through dpp polyazine bridging ligands (BL). Ru(II),Rh(III) bimetallics of the design (TL)₂Ru(dpp)RhCl₂(TL′)₃ (TL = Ph₂phen or bpy = 2,2′-bipyridine; TL′ = Ph₂phen or tBu2bpy = 4,4′-Di-tert-butyl-2,2′-bipyridine) couple only one Ru(II) LA to a Rh(III) metal center through the dpp BL.

The Ru(II),Rh(III),Ru(II) trimetallic and Ru(II),Rh(III) bimetallic complexes are synthesized using a building block approach, permitting facile modification of the supramolecular architecture throughout molecular assembly. Electrochemical analysis of both architectures displays a Ru-based HOMO tuned by TL identity (RuII/III = +1.62 V and +1.58 V vs. Ag/AgCl for TL = phen and Ph₂phen, respectively) and a Rh-based LUMO tuned by X identity (RhIII/II/I = -0.35 V and -0.32 V vs. Ag/AgCl for X = Cl⁻ and Br⁻, respectively). Modification of TL′ at Rh(III) within the bimetallics provided varying LUMO identity. The trimetallics and bimetallics are efficient light absorbers throughout the UV and visible with π⟶ π* intraligand (IL) transitions in the UV and Ru(dπ)⟶ligand(π*) metal-to-ligand charge transfer (MLCT) transitions in the visible. While X identity does not vary the light absorbing properties within Ru(II),Rh(III),Ru(II) trimetallics, TL identity and the number of Ru(II) LAs strongly impacts spectral coverage and the extinction coefficient. Photoexcitation of the Ru(dπ)⟶dpp(π*) ¹MLCT results in near unity population of the weakly emissive, short-lived Ru(dπ)⟶dpp(π*) ³MLCT excited state, which is efficiently quenched by intramolecular electron transfer to populate a non-emissive Ru(dπ)⟶Rh(dσ*) metal-to-metal charge transfer (³MMCT) excited state. Photolysis of the complexes in the presence of the sacrificial electron donor N,N-dimethylaniline (DMA) results in multi-electron collection at Rh, thereby converting Rh(III) to Rh(II) to Rh(I) accompanied by halide loss at each step. This establishes the Ru(II),Rh(III),Ru(II) and Ru(II),Rh(III) complexes as photochemical molecule devices (PMD) for photoinitiated electron collection (PEC).

The ability of these systems to undergo multiple redox cycles, absorb light efficiently, populate photoreactive excited states, and collect electrons at a reactive Rh metal center fulfills the requirements for H₂O reduction photocatalysts. Photolysis of trimetallic or bimetallic complexes at 470 nm in the presence of DMA and H₂O substrate yields photocatalytic H2 production. Within [{(TL)₂Ru(dpp)}₂RhX₂]⁵⁺ trimetallics (TL = phen or Ph₂phen; X = Cl⁻ or Br⁻), varying the TL from phen to Ph₂phen and X from Cl⁻ to Br⁻ yielded the most active and robust photocatalyst with [{(Ph₂phen)₂Ru(dpp)}₂RhBr₂]⁵⁺ producing 44 ± 6 mL H₂, 610 ± 90 mol H₂/mol Rh catalyst, and 7.3% maximum quantum efficiency (max. ΦH₂) in a DMF solvent system after 20 h photolysis.

The proposed mechanism of PEC suggests bimetallic systems might be prepared that are active photocatalysts. Ru(II),Rh(III) bimetallics are synthetically more challenging and the energetic proximity of dpp(π*) and Rh(dσ*) orbitals make electronic tuning with steric protection of the photogenerated Rh(I) difficult. Within [(TL)₂Ru(dpp)RhCl₂(TL′)]³⁺ bimetallics (TL = Ph₂phen or bpy; TL′ = Ph₂phen or tBu₂bpy), a careful balance of steric and electronic effects was required to produce active photocatalysts. The bimetallic [(Ph₂phen)₂Ru(dpp)RhCl₂(Ph₂phen)]³⁺ produces 1.1 ± 0.07 mL H₂, 81 ± 5 TON, and 0.88% max. ΦH₂ in a DMF solvent system after 20 h photolysis. This establishes the [(Ph₂phen)₂Ru(dpp)RhCl₂(Ph₂phen)]³⁺ complex as the first Ru(II),Rh(III) bimetallic to function as a homogeneous single-component H₂O reduction photocatalyst.

This dissertation reports the detailed analysis of the electrochemical, spectroscopic, photophysical, and photocatalytic properties of [{(TL)₂Ru(dpp)}₂RhX₂]⁵⁺ trimetallic (TL = phen or Ph₂phen; X = Cl or Br) and [(TL)₂Ru(dpp)RhCl₂(TL′)]³⁺ bimetallic (TL = Ph₂phen or bpy; TL′ = Ph₂phen or tBu₂bpy) supramolecular complexes. The design of the molecular architecture and the intrinsic properties of each component contribute to the overall function and efficiency of these systems. The careful design, meticulous synthesis and purification, detailed characterizations, and methodical experimentation have led to an in-depth understanding of the properties and factors needed for more efficient photocatalytic reduction of H₂O to H₂.

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supramolecular complexes, electrochemistry, photop
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