Mechanistic studies of photoinduced intramolecular and intermolecular electron transfer processes in RuPt-centred photo-hydrogen-evolving molecular devices

Abstract

The photoinduced electron transfer properties of two photo-hydrogen-evolving molecular devices (PHEMDs) [(bpy)2Ru(II)(phen-NHCO-bpy-R)Pt(II)Cl2](2+) (i.e., condensation products of [Ru(bpy)2(5-amino-phen)](2+) and (4-carboxy-4′-R-bpy)PtCl2; bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline; RuPt-COOH for R = COOH and RuPt-CN for R = CN) were investigated. RuPt-CN demonstrates higher photocatalytic performance relative to RuPt-COOH arising from a larger driving force for the intramolecular photoinduced electron transfer (PET) associated with a stronger electron-withdrawing effect of R (ΔGPET = −0.43 eV for RuPt-CN and −0.16 eV for RuPt-COOH). This is the first study on PET events using ultrafast spectroscopy. Dramatic enhancement is achieved in the rate of PET in RuPt-CN (1.78 × 1010 s(−1)) relative to RuPt-COOH (3.1 × 109 s(−1)). For each system, the presence of three different conformers giving rise to three different PET rates is evidenced, which are also discussed with the DFT results. Formation of a charge-separated (CS) state [(bpy)2Ru(III)(phen-NHCO-bpy(−˙)-R)Pt(II)Cl2](2+) in the sub-picosecond time regime and recombination in the picosecond time regime are characterized spectrophotometrically. The CS-state formation was found to compete with reductive quenching of the triplet excited state by EDTA whose dianionic form ion-pairs with dicationic RuPt-COOH. Thus, a key intermediate [(bpy)2Ru(II)(phen-NHCO-bpy(−˙)-R)Pt(II)Cl2](+) (i.e., the one-electron-reduced species) prior to the H2 formation was found to be formed either via reduction of the CS state by EDTA or via formation of [(bpy)2Ru(II)(phen(−˙)-NHCO-bpy-R)Pt(II)Cl2](+) by reductive quenching of the triplet excited state. More importantly, it is also shown that some of the conformers in solution possess a CS lifetime sufficiently long to drive hydrogen evolution from water.

Optimization of Hydrogen‐Evolving Photochemical Molecular Devices

“…To date the nature of the bridging ligand [19,24,25] and/or the peripheral ligands at the photocenter [17] have been considered as the main components of molecular photocatalysts that can be modified to achieve optimized catalytic behavior and this will always involve changes in the photophysical properties of the systems studied. Therefore,t he outcomes of the present study are striking,a su pon av ery simple change in the composition of the catalytic center the catalytic efficiency increases by af actor of almost 40, while the photoinduced intramolecular processes observed in the PtI 2 containing compound 3 are not different from those found for compound 2 based on PtCl 2 .T his shows for the first time that significant improvements in the photocatalytic behavior of intramolecular assemblies can be obtained by targeted co-ligand optimization of the catalytic metal center and without the need for amodification of the photophysical properties.T his is acompletely new starting point in the continuing efforts to optimize the photocatalytic properties of such supramolecular assemblies and suggests that as observed for intermolecular systems the rate determining step could be based on the catalytic center.…”

“…[18] Therefore,a part from continuing studies to improve the electron transfer between the chromophore and the catalytic center,m ore attention needs to be paid to the optimization of the hydrogenproducing catalytic center itself.I na ddition Pd-based catalytic centers tend to be instable and they may create colloidal particles,w hich although they may be involved in the photocatalytic process [19][20][21] are essentially the result of decomposition of the photocatalytic assembly and limit the efficiency of the catalytic process.F or the assembly shown in Figure 1and in other examples in the literature,PdCl 2 is used as ap hotocatalytic center since it tends to yield higher turnover numbers (TONs), defined as the number of hydrogen molecules generated per molecule of catalyst, than PtCl 2 even though the PtCl 2 metal center is more stable. [20,[22][23][24] To date the difference in TONs obtained by Pd-and Ptbased catalytic centers is considerable.W hile compound 1 ([Ru(tbbpy) 2 (tpphz)PdCl 2 ](PF 6 ) 2 ;tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine and tpphz = tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine,F igure 1), containing aP dCl 2 unit as catalytic center, yields am aximum TON of 238, [25] for compound 2 containing aP tCl 2 unit, ac onsiderably lower catalytic activity with maximum TONs of less than 10 is obtained. [20] However,the catalytic activity of 2 compares well with other molecular photocatalysts for hydrogen production containing Pt as catalytic center.…”

Photo-induced redox catalysis for proton reduction to hydrogen with homogeneous molecular systems using rhodium-based catalysts

“…The performance of these latter species remains however modest. The [Ru(bpy) 2 (phen-R(bpy))PtCl 2 ] 2+ (R = COOH) photocatalyst designed by Sakai and co-workers [40,62,135] combining a platinum(II) catalyst with a Ru(II) as the PS, exhibited a maximum turnover number of 12.6 in the presence of EDTA as electron donor and the dyad described by Eisenberg [71] which links a cobaloxime catalyst to a fluorescein organic dye, associated with TEOA only reached a TON value of 11.…”

Tuning the Photophysical Properties of Ru(II) Monometallic and Ru(II),Rh(III) Bimetallic Supramolecular Complexes by Selective Ligand Deuteration

“…55,14 The Rh(III)-based reactive metal (RM) subunit was synthesized by coordinating one bpy TL to the Rh(III) metal center. 56 Deuterated bpy TL (d 8 -bpy) and dpp BL (d 10 -dpp) were selectively substituted for the protiated analogues to develop the desired Ru (II) or Rh(III) monometallic precursors. The Ru(II),Ru (II) and Ru(II),Rh-(III) bimetallic complexes were synthesized in moderate yield by covalently coupling one LA and one RM subunit or another Ru(II) unit using careful control of reaction stoichiometry, Scheme 1.…”