Increased Water Reduction Efficiency of Polyelectrolyte‐Bound Trimetallic [Ru,Rh,Ru] Photocatalysts in Air‐Saturated Aqueous Solutions

Abstract

The groundbreaking use of polyelectrolytes to increase the efficiency of supramolecular photocatalysts in solar H2 production schemes under aqueous aerobic conditions is reported. Supramolecular photocatalysts of the architecture [{(TL)2 Ru(BL)}2 RhX2 ](5+) (BL=bridging ligand, TL=terminal ligand, X=halide) demonstrate high efficiencies in deoxygenated organic solvents but do not function in air-saturated aqueous solution because of the quenching of the metal-to-ligand charge-transfer (MLCT) excited state under these conditions. The new photocatalytic system incorporates poly(4-styrenesulfonate) (PSS) into aqueous solutions containing [{(bpy)2 Ru(dpp)}2 RhCl2 ](5+) (bpy=2,2'-bipyridine, dpp=2,3-bis(2-pyridyl)pyrazine). PSS has a profound impact on the photocatalyst efficiency, increasing H2 production over three times that of deoxygenated aqueous solutions alone. H2 photocatalysis proceeds even under aerobic conditions for PSS-containing solutions, an exciting consequence for solar hydrogen-production research.

Probing Co-Assembly of Supramolecular Photocatalysts and Polyelectrolytes Using Isothermal Titration Calorimetry

“…Optimization of emission intensity prior to charge balance (5:1) is the result of hydrophobic interactions between Ru,Rh,Ru and PSS, which leads to aggregate formation observed in cryogenic transmission electron microscopy (Cryo-TEM) (Figure 4) and dynamic light scattering studies (Figure 5). 28,41 Comparison of emission intensity for Ru,Rh,Ru (0.12 mM) within the aggregates (0.5 mM PSS) to the photocatalyst in the presence of excess (25 mM) PSS (Figure 5), where aggregation is not observed, indicates emission quantum yield and excited-state lifetime of Ru,Rh,Ru is further increased within the aggregated form (Table 1). We attribute this observation to the photocatalyst being less accessible to H 2 O within the aggregates compared to the photocatalyst in the presence of excess PSS (25 mM).…”

“…Increased excited-state lifetime of Ru,Rh,Ru upon aggregate formation is an important discovery as increased excited-state lifetime of Ru,Rh,Ru in aqueous solutions has been associated with significantly enhanced H 2 production. 28 Steady-state emission spectroscopy allows for the study of PSS molecular weight on the excited state properties of the Ru,Rh,Ru photocatalyst (Figure 6). Luminescence studies indicate that PSS oligomers with as few as 7 monomer repeat units (M̅ n = 1.44 kDa) significantly impact emission intensity of the complex, reaching nearly 90% of the emission quantum yield (Φ = (2.3 ± 0.1) × 10 −4 ) for polymers having over 300 repeat units (M̅ n = 57.8 kDa) (Φ = (2.6 ± 0.1) × 10 −4 ).…”

“…Recently, we detailed the use of the polyelectrolyte PSS to improve H 2 production in water, including H 2 evolution under air saturation. 28 Aggregate formation between the photocatalyst and the sulfonated polymer was found to increase the 3 MLCT excited-state lifetime and protect the photocatalyst from oxygen quenching owing to increased ionic strength near the charged polymer. A fundamental understanding of the thermodynamic forces driving assembly of PSS−photocatalyst aggregates and the impact this binding has on excited-state decay processes could lead to further increases in catalyst function and stability.…”

Main group metal chalcogenidometalates with transition metal complexes of 1,10-phenanthroline and 2,2′-bipyridine

“…Taking the feature of their empty * orbital, some of their transition metal complexes, such as Ru(II), Co(II) and Fe(II), show an intense metal-to-ligand charge-transfer (MLCT) characteristic in visible spectra. These complexes have been used as photoelectronic [31][32][33][34] and photoactive catalysis materials [35][36][37][38][39], because they have narrow gap between ground and excited states, and the energy of the excited state matches well with that of the conductive band of the semiconductors. Many recent efforts have focused on the design and synthesis of novel photoactive M-phen/bpy derivatives, especially, the Ru(II)-phen/bpy system.…”

Ultrafast kinetics of supramolecules with a Ru(II)- or Os(II)-polypyridyl light absorber, cis-Rh(III)Cl2-polypyridyl electron collector, and 2,3-bis(2-pyridyl)pyrazine bridge

“…Strong electronic absorption bands in the visible region of the spectrum are assigned to metal (Ru(d)-to-ligand(dpp(*)) charge transfer transitions (MLCT). From electrochemistry, the lowest unoccupied orbital (LUMO) is Rh III (d*), suggesting a low-lying metal(Ru(d)-tometal(Rh(d*)) charge transfer state (MMCT) and thus favorable conditions for photo-initiated electron transfer (k ET ) [9][10][11][12][13][14][15][16][17][18][19][20]. The magnitude of k ET was inferred from a static quenching model that compares the mixed-metal complexes to the Rh-free analogs, e.g.…”