Hydrogen-Bond Promoted Intramolecular Electron Transfer to Photogenerated Ru(III):  A Functional Mimic of Tyrosine_Z and Histidine 190 in Photosystem II

Abstract

As a model for redox components on the donor side of photosystem II (PS II) in green plants, a supramolecular complex 4 has been prepared. In this, a ruthenium(II) tris-bipyridyl complex which mimics the function of P680 in PS II, has been covalently linked to a tyrosine unit which bears two hydrogen-bonding substituents, dipicolylamine (dpa) ligands. Our aim is to mimic the interaction between tyrosineZ and a basic histidine residue, namely His190 in PSII, and also to use the dpa ligands for coordination of manganese. Two different routes for the synthesis of the compound 4 are presented. Its structure was fully characterized by 1H NMR, COSY, NOESY, 13C NMR, IR, and mass spectrometry. 1H NMR and NOESY gave evidence for the existence of intramolecular hydrogen bonding in 4. The interaction between the ruthenium and the substituted tyrosine unit was probed by steady-state and time-resolved emission measurements as well as by chemical oxidation. Flash photolysis and EPR measurements on 4 in the presence of an electron acceptor (methylviologen, MV2+, or cobalt pentaminechloride, Co3+) showed that an intermolecular electron transfer from the excited state of Ru(II) in 4 to the electron acceptor took place, forming Ru(III) and the methylviologen radical MV+• or Co2+. This was followed by intramolecular electron transfer from the substituted tyrosine moiety to the photogenerated Ru(III), regenerating Ru(II) and forming a tyrosyl radical. In water, the radical has a g value of 2.0044, indicative of a deprotonated tyrosyl radical. In acetonitrile, a radical with a g value of 2.0029 was formed, which can be assigned to the tyrosine radical cation. In both solvents the electron transfer is intramolecular with a rate constant k ET > 1 × 107 s-1. This is 2 orders of magnitude greater than the one for a similar compound 3, in which no dpa arm is attached to the tyrosine unit. Therefore the hydrogen bonding between the substituted tyrosine and the dpa arms in 4 is proposed to be responsible for the fast electron transfer. This interaction mimics the proposed His190 and tyrosineZ interaction in the donor side of PS II.

Synthesis and Characterization of Dinuclear Ruthenium Complexes Covalently Linked to RuII Tris‐bipyridine: An Approach to Mimics of the Donor Side of Photosystem II

“…The two shorter components dominated, and their lifetimes and relative amplitudes are given in Table 2. These lifetimes are on the timescale of a few nanoseconds to less than 100 ps, which is much shorter than that of about 1200 ns observed for 3 [21] and shows the strong quenching of the [Ru(bpy) 3 ]…”

“…[31] Ligand complexes 3 and 3 a were prepared by the published procedures. [21,32] By refluxing a mixture of 3 or 3 a with two equivalents of cis-[RuCl 2 (DMSO) 4 ] in MeOH in the presence of NaOAc, followed by addition of a saturated aqueous solution of NH 4 PF 6 , the trinuclear ruthenium complex 2 or 2 a was obtained. Complexes 2 and 2 a are ruthenium(ii,ii,iii) complexes with one m-oxo and two m-acetato bridges, as shown by elemental analysis and ESI-MS. One of the Ru II ions in the phenolic dimer was thus oxidized by air to Ru III during preparation of the complex.…”

“…[30] With the aim of improving the performance of the ruthenium complexes made by, for example, Meyer et al, [27][28][29] and as an alternative route towards artificial photosynthesis, we previously prepared the dinuclear complex 1, [31] and here we present a new trinuclear ruthenium complex 2, which was synthesized by reaction of ligand complex 3 [21] with cis-[RuCl 2 (DMSO) 4 ]. We also prepared the complexes 2 a and 2 b, which, in accordance with an earlier study on the dinuclear complexes 5 and 5 a, [24] would be expected to be less sensitive to excited-state quenching by the appended dinuclear complex.…”

Improving the efficiency of water splitting in dye-sensitized solar cells by using a biomimetic electron transfer mediator

“…In this study, the electron transfer rate between the BIP mediator and the 3P-Ru dye was not well controlled because the molecules were separately adsorbed onto the electrode surface. By modifying the molecular structure of the mediator and covalently linking it to the dye, as was done in previous studies (21)(22)(23)(24)(25), it should be possible to control the electron transfer distance in the dye-mediator dyad. This should, in turn, enable tuning of the electron transfer rate to improve the watersplitting efficiency of the system further.…”

“…Synthetic mimics of this biological redox relay have been studied in a number of molecular dyads and triads (21)(22)(23)(24)(25). We incorporated a redox mediator into a water-splitting dye-sensitized solar cell (see Scheme 1).…”