Visible-Light-Driven Photosystems Using Heteroleptic Cu(I) Photosensitizers and Rh(III) Catalysts To Produce H2

Bradley J McCullough; Bertrand J Neyhouse; Briana R Schrage; Demi T Reed; Allen J Osinski; Christopher J Ziegler; Travis A White

doi:10.1021/acs.inorgchem.7b03273

Visible-Light-Driven Photosystems Using Heteroleptic Cu(I) Photosensitizers and Rh(III) Catalysts To Produce H₂

Inorg Chem. 2018 Mar 5;57(5):2865-2875. doi: 10.1021/acs.inorgchem.7b03273. Epub 2018 Feb 15.

Authors

Bradley J McCullough¹, Bertrand J Neyhouse¹, Briana R Schrage², Demi T Reed¹, Allen J Osinski², Christopher J Ziegler², Travis A White¹

Affiliations

¹ Department of Chemistry and Biochemistry, Clippinger Laboratories , Ohio University , Athens , Ohio 45701 , United States.
² Department of Chemistry, Knight Chemical Laboratory , University of Akron , Akron , Ohio 44325 , United States.

PMID: 29446925
DOI: 10.1021/acs.inorgchem.7b03273

Abstract

The synthesis of two new heteroleptic Cu(I) photosensitizers (PS), [Cu(Xantphos)(NN)]PF₆ (NN = biq = 2,2'-biquinoline, dmebiq = 2,2'-biquinoline-4,4'-dimethyl ester; Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene), along with the associated structural, photophysical, and electrochemical properties, are described. The biquinoline diimine ligand extends the PS light absorbing properties into the visible with a maximum absorption at 455 and 505 nm for NN = biq and dmebiq, respectively, in CH₂Cl₂ solvent. Following photoexcitation, both Cu(I) PS are emissive at low energy, albeit displaying stark differences in their excited state lifetimes (τ_MLCT = 410 ± 5 (biq) and 44 ± 4 ns (dmebiq)). Cyclic voltammetry indicates a Cu-based HOMO and NN-based LUMO for both complexes, whereby the methyl ester substituents stabilize the LUMO within [Cu(Xantphos)(dmebiq)]⁺ by ∼0.37 V compared to the unsubstituted analogue. When combined with H₂O, N,N-dimethylaniline (DMA) electron donor, and cis-[Rh(NN)₂Cl₂]PF₆ (NN = Me₂bpy = 4,4'-dimethyl-2,2'-bipyridine, bpy = 2,2'-bipyridine, dmebpy = 2,2'-bipyridine-4,4'-dimethyl ester) water reduction catalysts (WRC), photocatalytic H₂ evolution is only observed using the [Cu(Xantphos)(biq)]⁺ PS. Furthermore, the choice of cis-[Rh(NN)₂Cl₂]⁺ WRC strongly affects the catalytic activity with turnover numbers (TON_Rh = mol H₂ per mol Rh catalyst) of 25 ± 3, 22 ± 1, and 43 ± 3 for NN = Me₂bpy, bpy, and dmebpy, respectively. This work illustrates how ligand modification to carefully tune the PS light absorbing, excited state, and redox-active properties, along with the WRC redox potentials, can have a profound impact on the photoinduced intermolecular electron transfer between components and the subsequent catalytic activity.