Sn^IVMetalloporphyrin/Co^IIIComplex: An All-Abundant-Element System for the Photocatalytic Production of H₂in Aqueous Solution

Abstract

A new, molecular system for the light-driven production of hydrogen in aqueous solution was developed by combining a water-soluble tin porphyrin ([Sn(IV)Cl2TPPC], A) acting as photosensitizer with a cobalt-based proton-reduction catalyst ([Co(III)Cl(dmgH)2(py)], C). Under visible light illumination and with triethanolamine (TEOA) as electron source, the system evolves H2 for hours and is clearly catalytic in both dye and catalyst. A detailed analysis of the relevant redox potentials in combination with time-resolved spectroscopy resulted in the development of a Z-scheme type model for the flow of electrons in this system. Key intermediates of the proposed mechanism for the pathway leading to H2 are the porphyrin dye's highly oxidizing singlet excited state (1)A* (E ∼ +1.3 V vs NHE), its strongly reducing isobacteriochlorin analogue (E ∼ +0.95 V), and the Co(I) form of C (E ∼ -0.8 V), acting as catalyst for H2 formation. Among other results, the suggested reaction sequence is supported by the detection of a shortened excited-state lifetime for singlet (1)A* (τ ∼ 1.75 ns) in the presence of TEOA and the ultraviolet-visible detection of the Sn(IV) isobacteriochlorin intermediate at λ = 610 nm. Thus, a molecular, conceptually biomimetic, and precious-metal-free reaction chain was found which photocatalytically generates H2 in a 100% aqueous system from an electron donor with a high oxidation potential (E(TEOA) ∼ +1.1 V). On the other hand, at identical conditions, this photoreaction chain yields H2 markedly slower than a system using the photosensitizer [Re(I)(CO)3(bpy) (py)](+), probably due to the much longer excited-state lifetime (τ ∼ 120 ns) of the rhenium dye and better electron-transfer rates caused by its simple single-electron photoreduction chemistry.

Tuning the Electron Storage Potential of a Charge‐Photoaccumulating RuII Complex by a DFT‐Guided Approach

“…Most of these transformations involvet he transfer of multiple electrons, and significant progress has been made in developing molecular catalysts able to perform such kinetically arduous multi-proton multi-electronp rocesses. [5][6][7][8] Widely used photosensitizers (PS) for solarf uelp roduction include Ru II [9][10][11] and Ir III [12,13] polypyridyl coordination complexes, porphyrins, [14][15][16] and organic push-pull dyes. [17][18][19] While these chromophoresf eature broad absorption bands with relatively high extinction coefficients in the visible region, they fundamentally generate one photoexcited electron at at ime.…”

Structure–Activity and Stability Relationships for Cobalt Polypyridyl‐Based Hydrogen‐Evolving Catalysts in Water

“…For instance, quantities are strongly influenced even by subtle variations of the different components (e.g., SED, PS, pH, concentrations/ratios, and light source) . There is a great deal of reported information on the initial steps of photocatalysis in homogeneous solutions, that is, the study of the processes that follow excitation of the photosensitizer (reductive/oxidative quenching, first electron transfer to cobalt, and follow‐up reactions of oxidized electron donors), both in organic and aqueous media, as well as in mixtures . There are, from a spectroscopic point of view, only few studies focusing on the follow‐up reactions of the reduced cobalt species in water or organic solvents .…”

Evidence for photosensitised hydrogen production from water in the absence of precious metals, redox-mediators and co-catalysts

“…Nevertheless it still yields more H 2 than similar molecular systems shown to operate in pure water. 12 It should also be pointed out that the reaction conditions reported here because of the initially unexpected findings were not at all optimized for a possible scenario of H 2 -production with the chlorin complex ZnC instead of ZnP as the actual photocatalyst ( Fig. 1 and 7), and the light-source applied (HBO 100 W/2 mercury lamp equipped with longpass filters) offers only a minor spectral output in the 600-650 nm region.…”

“…More recently, a molecular all-abundant element system has been described by Kurz and Probst et al that utilized a watersoluble tin porphyrin and the same type of cobalt catalyst to produce hydrogen in aqueous solution with a turnover number (TON) barely exceeding unity, just enough to indicate some catalytic activity. 12 Here, we wish to report our own results with a further simplified homogeneous photocatalytic system based on earth-abundant components producing hydrogen gas from neat aqueous solution. The system is operating in the presence of a readily available molecular photosensitizer and different sacrificial electron donors including simple inorganic salts, but notably without the requirement of further mediators or catalysts.…”