Micellar Effects on Photoinduced Electron Transfer in Aqueous Solutions Revisited: Dramatic Enhancement of Cage Escape Yields in Surfactant Ru(II) Diimine Complex/[Ru(NH₃)₆]²⁺ Systems

Abstract

The effect of cationic micelle incorporation on light induced electron transfer, charge separation and back electron transfer between an aqueous electron donor, [Ru(NH3)6](2+), and a series of Ru(II) diimine complex chromophores/acceptors, is presented. The chromophores have the general formula [(bpy)2Ru(LL)](2+) (LL = bpy; 4-R-4'-methyl-2,2'-bpy, R = pentyl (MC5), terdecyl (MC13), heptadecyl (MC17); 4,4'-di(heptadecyl)-2,2'-bpy (DC17)). Of the five chromophores, the MC13, MC17, and DC17 complexes associate with the added micelle forming surfactant, cetyltrimethylammonium bromide (CTAB). Quenching of the luminescence of the bpy and MC5 complexes by [Ru(NH3)6](2+) is unaffected by addition of surfactant, while rate constants for quenching of the MC13 and MC17 complexes are decreased. Cage escape yields following photoinduced electron transfer to generate [(bpy)2Ru(LL)](+) and [Ru(NH3)6](3+) are approximately 0.1 for all the water-soluble chromophores (excluding DC17) in the absence of added CTAB. In the presence of surfactant, the cage escape yields dramatically increase for the MC13 (0.4) and MC17 (0.6) complexes, while remaining unchanged for [Ru(bpy)3](2+) and the MC5 complex. Back electron transfer of the solvent separated ions is also strongly influenced by the presence of surfactant. For the MC13 and MC17 complexes, back electron transfer rate constants decrease by factors of 270 and 190, respectively. The MC5 complex exhibits two component back electron transfer, with the fast component having a rate constant close to that in the absence of surfactant and a slow component nearly 200 times smaller. Results are interpreted in terms of the partitioning of the 2+ and 1+ forms of the chromophores between aqueous and micellar phases. The extended lifetimes of the radical ions may prove useful in coupling the strong reductants formed to kinetically facile catalysts for reduction of water to hydrogen.

Accessing Photoredox Transformations with an Iron(III) Photosensitizer and Green Light

“…These spin states are markedly different from that of the prototypical [Ru(bpy) 3 ] 2+ , which is rigorously diamagnetic in the ground state and exhibits a 3 MLCT excited state with a distorted octahedral geometry that possesses a single reduced 2,2′-bipyridine ligand. 67,68,71−73 Charge recombination for this prototypical 3 MLCT 60,74,75 thus involves a spin-forbidden 3 CS → 1 GS electron transfer, very different from the spin-allowed 2 CS → 2 GS charge recombination of the 2 LMCT iron(III) photosensitizer 1 (Figure 3). These spin states might be expected to result in distinct solvent effects on cage-escape yields.…”

Controlling Spin‐Correlated Radical Pairs with Donor–Acceptor Dyads: A New Concept to Generate Reduced Metal Complexes for More Efficient Photocatalysis

“…Quantitative studies on cage escapey ields or inherent efficiencies for PET reactions requiret he determination of molar absorptionc oefficients of the quenching products.R eductive quenching of triplet-excited ruthenium(tris)bipyridine 3 Ru(bpy) 3 2 + ( : 3 M) in water to yield M À is probably the best-investigated photoreduction of ah eavy-atom containing chromophore. [22,[27][28][29][30][31] Amongt he readily availables acrificial donors, [32] the ascorbate dianion Asc 2À is the most promising candidate for the alternative photoreduction mechanism presented herein,b ecause of the very low potential for its oxidation. The direct photoreduction of 3 Ru(bpy) 3 2 + by Asc 2À (Scheme 2a)p roceeds with an h of about 0.4; [29,30] this means that some 60 %o fa ll quenching eventsd on ot produce the desired reduced complex Ru(bpy) 3…”

“…Despite practically quantitative excited-state quenching, the actual yield of free radicals or radicali ons might be close to zero, [20,21] but the determinationo fc age-escape efficiencies h for ag iven PET event requires more sophisticated experimental techniques such as quantitative transienta bsorption spectroscopy.O wing to the lack of systematic quantitative studies in photochemistry,t he exact factorst hat govern the overall efficiencies of PET processes are stillp oorly understood and areliable predictiono fthe h values cannot be made in advance.F or instance, the inherent photoreduction efficiencies for an anionic ruthenium(II) complex by as eries of dianionic electron donors differ by as much as af actor of 30. [20] Most h values for PET reactions with triplet-excited Ru complexes are in the range from 0.05 (5 %) to 0.6 (60 %), [21][22][23][24][25][26][27] clearlyi ndicating that unproductive in-cage recombination is ag eneral energywasting problem.…”

Bathophenanthroline Disulfonate Ligand-Induced Self-Assembly of Ir(III) Complexes in Water: An Intriguing Class of Photoluminescent Soft Materials

“…Red-shifted emission spectra have been previously observed in the self-assembly of other Ru(II) chromophores. 42−46 Further, sodium chloride was added to a 100 μM solution of 7 to test the effect of salt on the system and produced enhancements in excited-state lifetime consistent with aggregate formation, Figure S24. The DLS spectrum for a 2 mM sample of 7 was similar to that observed in 1–6 , further confirming the presence of an aggregate, Figure S25.…”