Reductive and Oxidative Quenching Dyads and Triads with Donors and Acceptors

Once light energy has been harvested in the form of a localized excited state, spatial separation of the electron-hole pair becomes important to prevent wasteful recombination reactions and to spatially direct the oxidizing and reducing equivalents to the appropriate co-catalysts. Dyads are usually composed of a sensitizer (S) and a donor (D) or acceptor (A) moiety whereas and triads are typically composed of S with both a D and A covalently attached. As opposed to antenna assemblies, the mode of action here is light-driven endogonic electron transfer from the donor to the acceptor as mediated by the Ru sensitizer. The linked assembly favors fast intramolecular reductive or oxidative quenching of the excited state and in the case of the D-S-A triads, subsequent ET to regenerate the ground state sensitizer (e.g., [Ru(bpy)3]2+). This area has been the subject of several excellent reviews over the past 10 years.98461-164

Some of the more donors that have incorporated into dyads and triads are ferrocene , phenathiazine (PTZ),166-169 phenols170 and, more recently some tethered Mn

Molecular Approaches to Photochemical Water Splitting 143 Table 2. Commonly used donor and acceptor moieties in dyads and triads.

Compound

Couple

Ered (vs NHE)

Ref.

Donors

Phenothiazine (PTZ)

0/+

1.08

294

Ferrocene (Fc)

0/+

0.55

Phenol

0/1-

0.65

Acceptors

Methylviologen (MV2+)

2+/1+

-0.11

166

Naphthalene diimide (NDI)

-0.42

170

C60

0/1-

-0.16

175,295

-0.22°

-0.38'

p-benzoquinone (BQ)

0/1-

-0.36

173

Anthroquinone (AQ)

0/1-

-0.52

294

Nitrobenzene

0/1-

53

As measured in acetonitrile unless otherwise noted. Electrochemical measurements were done in the presence of 0.1 M Bun4NPF6 or 0.1 M Bun4NClO4. All potentials are quoted relative to NHE using the correction factor of +0.548 Fc, +0.236 SSCE, +0.247 V for SCE and +0.225 for Ag/Ag+ where required.296 "in THF. 'in CH2Q2.

As measured in acetonitrile unless otherwise noted. Electrochemical measurements were done in the presence of 0.1 M Bun4NPF6 or 0.1 M Bun4NClO4. All potentials are quoted relative to NHE using the correction factor of +0.548 Fc, +0.236 SSCE, +0.247 V for SCE and +0.225 for Ag/Ag+ where required.296 "in THF. 'in CH2Q2.

Dyads And Triads

complexes. 171-174 Common acceptors include C60,175"177 methylviologen (MV2+),166" 169naphthalenediimide (NDI),170 178, quinones179-181, nitroaromatics53and other transition metals complexes.126 Although most dyads and triads are covalently linked an increasing number of assemblies based on supramolecular interactions (e.g., H-bonding, host-guest, salt-bridge) or are appearing.182-186 Many of these donors and acceptors are listed in Table 2 along with the relevant redox couple and potential.

The function of a typical triad is seen in complex 13. This D-S-A triad yields charge separated (CS) state characterized as [RuII(dmb)(bpyCH2PTZ+) (bpyCH2MV+)]4+ upon photoexcitation of the Ru chromophore.166 The half-life of the CS state is only ~160 ns, however it transiently stores 1.1 eV of energy. Complex 14 is similar in function however a benzoquinone function, is used in place of the Mv2+.173 Like complex 13, a CS state (t ~ 90 ns) is formed in 14 upon photoirradiation. In this complex both the quantum yield (90% vs. 22%) and amount of energy stored (1.32 eV) in the CS state are greater. In both 13 and 14, it appears that oxida-tive quenching is favored and subsequent electron transfer from the PTZ group reduces the RuIII site.

In an important step to mimic the natural photosystem, tyrosine residues tethered to a Rubpy sensitizer as in 15 have been shown to reduce the Rum center obtained after oxidative quenching with methylviologen or [Co(NH3)5Cl]2+.187 Formation of the resulting tyrosyl radical is a proton-coupled process and it has been shown to be a concerted process in which the reorganization energy associated with deprotonation can be tuned by H-bonding and pH.188-191 Similar results are observed for tyrosyl residues tethered to Re(I)diimine based chromophores.192

The Uppsala group has shown that a photogenerated tyrosyl radical can oxidize a MnmMnm dimer to the MnIIIMnIV state in an intermolecular reaction.193 This same group has gone on to incorporated the tyrosyl group into a manganese chelating ligand as in 16 (see Fig. 9).194 These results are particularly significant given that in PSII, tyrosine Z plays a pivotal role in reducing the oxidized P680 cofactor and then oxidizing the nearby tetranuclear Mn-cofactor.195 Importantly, complex 16 has been shown to undergo multiple light-induced oxidations as indicated in Fig. 9 to form the MnIIIMnIV complex 17 196 197 which is just one 'hole' short of the 4 electron stoichi-ometry needed for water oxidation. Moreover, this manganese centered oxidation requires water as the acetate groups are replaced by bridging oxo groups suggesting that the development of dyads which are functional in light driven oxidation may not be long in coming. In general, electron transfer rate constants for these Ru/Mn dyads vary between 1x105 to 2x107 s-1 and are related to the internuclear distance and believed to be limited primarily by the large inner reorganization energy of the Mn complex.198,199 Wiegahardt and coworkers have also explored dyads which couple Rubpy sensitizers to a mononuclear MnIV complex containing phenolate ligands and with a Mnn trimer assembled within the same ligand system.171,172 Tethered [Ru(bpy)3]3+ centers are generated by photoexcitation and oxidative quenching with CoIII. Subsequently, electron transfer from the phenolate group in the MnIV complex or from the MnII ions in the trimer is observed to reduce the Ru. Like the Uppsala groups results, ET rates were on the order of 5 x 107 s-1. This result is particularly notable in that the phenolate radical is directly coordinated to the MnIV ion and this unusual complex survives up to 1 ms.

Phenolate Oxidation

Fig. 9. Photodriven multi-electron oxidation of a manganese dimer using a covalently attached

Rubpy sensitizer (see Ref. 193).

Fig. 9. Photodriven multi-electron oxidation of a manganese dimer using a covalently attached

Rubpy sensitizer (see Ref. 193).

While numerous donor-Rubpy-acceptor triads have been studied,98 161,200 triad 18 is notable for several reasons.201 First, the complex shows a remarkably long-lived charge separated state of ~ 600 |S in solution at room temperature. This is at least two orders of magnitude longer than previous triads based on Rubpy sensitizers. Second, the charge separated state localizes the electrons on the NDI acceptors (as NDI-) and the holes on the Mn dimer as a MnnMnm complex. The authors believe the unusually long lifetime is due, in part, to the large inner sphere reorganization that occurs in the Mn dimer, which makes the back reaction strongly activated. Third, the donor 'cofactor' is has the potential to directly act as both a multi-electron donor and ultimately as a water oxidation catalyst. It is also interesting to note that photoexcitation in this triad is first followed by oxidative quenching to give [MnIIM-nII-RuIII-NDI-]3+ which then undergoes intramolecular ET to yield the long-lived charge separated product [MnIIMnIII-RuII-NDI.-] 3+.

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