Comparison of rhenium–porphyrin dyads for CO₂ photoreduction: photocatalytic studies and charge separation dynamics studied by time-resolved IR spectroscopy

Windle, Christopher D.; George, Michael W.; Perutz, Robin N.; Summers, Peter A.; Sun, Xue Zhong; Whitwood, Adrian C.

Abstract

We report a study of the photocatalytic reduction of CO₂ to CO by zinc porphyrins covalently linked to [ReI(2,2′-bipyridine)(CO)₃L]⁺/⁰ moieties with visible light of wavelength >520 nm. Dyad 1 contains an amide C₆H₄NHC(O) link from porphyrin to bipyridine (Bpy), Dyad 2 contains an additional methoxybenzamide within the bridge C₆H₄NHC(O)C₆H₃(OMe)NHC(O), while Dyad 3 has a saturated bridge C₆H₄NHC(O)CH₂; each dyad is studied with either L = Br or 3-picoline. The syntheses, spectroscopic characterisation and cyclic voltammetry of Dyad 3 Br and [Dyad 3 pic]OTf are described. The photocatalytic performance of [Dyad 3 pic]OTf in DMF/triethanolamine (5 : 1) is approximately an order of magnitude better than [Dyad 1 pic]PF₆ or [Dyad 2 pic]OTf in turnover frequency and turnover number, reaching a turnover number of 360. The performance of the dyads with Re–Br units is very similar to that of the dyads with [Re–pic]⁺ units in spite of the adverse free energy of electron transfer. The dyads undergo reactions during photocatalysis: hydrogenation of the porphyrin to form chlorin and isobacteriochlorin units is detected by visible absorption spectroscopy, while IR spectroscopy reveals replacement of the axial ligand by a triethanolaminato group and insertion of CO₂into the latter to form a carbonate. Time-resolved IR spectra of [Dyad 2 pic]OTf and [Dyad 3 pic]OTf (560 nm excitation in CH₂Cl₂) demonstrated electron transfer from porphyrin to Re(Bpy) units resulting in a shift of ν(CO) bands to low wavenumbers. The rise time of the charge-separated species for [Dyad 3 pic]OTf is longest at 8 (±1) ps and its lifetime is also the longest at 320 (±15) ps. The TRIR spectra of Dyad 1 Br and Dyad 2 Br are quite different showing a mixture of 3MLCT, IL and charge-separated excited states. In the case of Dyad 3 Br, the charge-separated state is absent altogether. The TRIR spectra emphasize the very different excited states of the bromide complexes and the picoline complexes. Thus, the similarity of the photocatalytic data for bromide and picoline dyads suggests that they share common intermediates. Most likely, these involve hydrogenation of the porphyrin and substitution of the axial ligand at rhenium.

Citation

Windle, C. D., George, M. W., Perutz, R. N., Summers, P. A., Sun, X. Z., & Whitwood, A. C. (2015). Comparison of rhenium–porphyrin dyads for CO₂ photoreduction: photocatalytic studies and charge separation dynamics studied by time-resolved IR spectroscopy. Chemical Science, 6(12), 6847-6864. https://doi.org/10.1039/c5sc02099a

• Journal Article Type: Article
• Acceptance Date: Aug 18, 2015
• Publication Date: Aug 20, 2015
• Deposit Date: Oct 12, 2017
• Publicly Available Date: Oct 12, 2017
• Journal: Chemical Science
• Print ISSN: 2041-6520
• Electronic ISSN: 2041-6539
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 6
• Issue: 12
• Pages: 6847-6864
• DOI: https://doi.org/10.1039/c5sc02099a
• Public URL: https://nottingham-repository.worktribe.com/output/758681
• Publisher URL: http://pubs.rsc.org/en/Content/ArticleLanding/2015/SC/C5SC02099A#!divAbstract
• Additional Information: : This document is Similarity Check deposited; : Supplementary Information; : Crystal Structure Data; : The Royal Society of Chemistry has an exclusive publication licence for this journal; OPEN ACCESS: This article is freely available. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence (CC BY 3.0); : Single-blind; : Received 10 June 2015; Accepted 18 August 2015; Accepted Manuscript published 20 August 2015; Advance Article published 2 September 2015; Version of Record published 9 November 2015