[Rubpy

Rubpy Latimer Diagram

Fig. 5. Latimer diagram of the excited- and ground-state redox processes for [Ru(bpy)3]2+.

Potentials given in V vs. NHE reference electrode.

Fig. 5. Latimer diagram of the excited- and ground-state redox processes for [Ru(bpy)3]2+.

Potentials given in V vs. NHE reference electrode.

given step in the cycle. If we analyze the energetics of the water-splitting reaction within these two manifolds, as shown in reactions 9 to 17, we can see that by designing a system properly we can deliver more driving force where it is most needed. All the reactions are normalized to a 4-photon, 4-electron process as presumably (though not necessarily) would be required and a pH of 7.0 is assumed. The first three photo-reactions, 9-11, summarize the capture of light energy (at ~ 450 nm or 2.75 eV) to form the excited singlet molecule [Ru(bpy)3]2+**. This molecule rapidly loses some energy upon intersystem-crossing to the triplet state and relaxation to vibronically cooled E00 triplet state, [Ru(bpy)3]2+*.17 The useable energy of this species is ~ 2.1 eV

4Ru2+ (X ~ 450 nm) ^ 4 Ru2+** AE = +1060 kJ/mol (9)

4 Ru2+ + 4 hv ^ 4 Ru2+* AE = +810 kJ/mol (11) Reductive quenching water-splitting path:

4 Ru2+* + 2H2O ^ 4 Ru+ + O2 + 4 H+ AG = -76 kJ/mol (12)

2 H2O + 4 Ru2+* ^ O2 + 2 H2 + 4 Ru2+ AG = -336 kJ/mol (14) Oxidative quenching water-splitting path:

4 Ru2+* + 4 H+ ^ 4 Ru3+ + 2 H2 AG = -62 kJ/mol (15)

4 Ru3+ + 2 H2O ^ 4 Ru2+ + O2 + 4 H+ AG = -274 kJ/mol (16)

2 H2O + 4 Ru2+* ^ O2 + 2 H2 + 4 Ru2+ AG = -336 kJ/mol (17)

Anti Bacterial Bpy
Fig. 6. Photoreactions based on reductive quenching of the photoexcited [Ru(bpy)3]2+*.

As can be seen both pathways are exothermic by -336 kJ/mol, however the exo-thermicity of the HER, Eq. (13), can be favored in the reductive quenching path at the expense of the OER, Eq. (12). The reverse is true for the oxidative quenching pathway as may be expected.

Of course, the solution pH is a major factor that can be tuned to favor hydrogen reduction or water oxidation. Application of the Nernst equation for the two manifolds yields expressions 18-21:

reductive quenching pathway:

AGHER =-420 + 22.85(pH) (kJ/mol) (19) oxidative quenching pathway:

From these expressions, we find the OER and HER are isoenergetic at pH 11 and 2.3 for the reductive and oxidative quenching pathways, respectively. More importantly, the pH range in which water splitting can occur can be determined, assuming the two half-reactions are not energetically coupled, i.e., the free energy of HER is not used to help drive the OER (or vice versa). Following the reductive quenching pathway, both the OER and HER are only spontaneous above pH 3.7. Conversely, the oxida-tive quenching pathway requires a pH below 9.6 for both to be spontaneous. Thus the quenching pathway plays an important role in determining optimum pH conditions. These energies and pH ranges are specific for [Ru(bpy)3]2+assuming a 4 pho-

Table 1. Room temperature electrochemical and photophysical data for homoleptic ru-

polypyridyl complexes.

Table 1. Room temperature electrochemical and photophysical data for homoleptic ru-

polypyridyl complexes.

Complex

£(Ru3+/2+)°

£(Ru2+/+)

£(Ru3+/2+*)

£(Ru2+*/ +)

Xem, nm (t, ns)'

^em

[Ru(bpz)3]2+

2.10

-0.56

-0.06

1.60

627 (720)

0.024

[Ru(4,4'-Ckbpy)]2+

1.64

-0.83

-0.42

1.23

632 (480)

-

[Ru(bpy)3]2+

1.53

-1.09

-0.57

1.02

610 (890)

0.073

[Ru(phen)3]2+

1.51

-1.11

-0.68

1.08

604 (460)

0.028

[Ru(terpy)2]2+

1.50

-1.05

-0.58

1.03

(<250 ps)

[Ru(4,4'- Me2bpy)3]2+ d

1.10

-1.37

-0.94

0.69

631 (335)

0.014

[Ru(4,4'-(Et2N)2bpy)3]2+

0.84

-1.07

700 (130)c

0.010c

aAs 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 +0.247 V for SCE and +0.225 for Ag/Ag+ where required.

bdegassed.

cin MeOH/EtOH.

din H2O.

bpz = 2,2'-bipyrazine. Data obtained from Ref. 54.

aAs 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 +0.247 V for SCE and +0.225 for Ag/Ag+ where required.

bdegassed.

cin MeOH/EtOH.

din H2O.

bpz = 2,2'-bipyrazine. Data obtained from Ref. 54.

ton process but are tunable by modifying the complex structure. This is most commonly achieved by modification of the diimine ligand with electron withdrawing or donating groups. As seen in Table 1, ligand substitution can shift the relevant redox potentials by nearly a volt in either direction, however any gain in the oxidizing potential is offset by a loss in reducing potential or vice-versa. An extensive listing of the ground and excited state redox data for Ru(II) and Os(II) diimine complexes can be found in a review by Balzani and coworkers.54 Also, Vlcek et. al. have reported that in many cases the excited state energies can be predicted simply from ligand redox parameters.55

This above analysis assumes that the free-energies of the HER and OER cannot be coupled which need not be true. As we observe in the natural photosystems, the free energy of one reaction may be used to change local pH by the translocation of protons across a membrane. The resulting pH change could easily be applied to increase the exothermicity of both O2 and H2 evolution in a straightforward manner and would be highly desirable. However, such a system requires an additional layer of sophistication in the design of the photosynthetic machinery. That said, artificial photodriven proton pumps are a reality as Gust, Moore and coworkers have shown that a membrane-bound carotenoid-porphyrin-quinone triad can actively transport protons against a gradient upon visible irradiation.56

There are additional aspects that must be considered for artificial or man-made photosystems. Nature's photosynthetic machinery is easily repaired or regenerated by the living organism. Artificial systems are likely to be harder to repair and therefore should be constructed of the most robust components available. The various components are likely to be subject to relatively harsh environmental conditions including high photon flux, thermal stress, aqueous solutions with variable pH and ionic strengths and oxygen. Only a few molecular species can meet all, or even most, of these conditions and while it is feasible to encapsulate or otherwise protect less stable molecular components, it is preferable that they be as robust as possible. No doubt the popularity of metalloporhyrins and ruthenium polypyridyl complexes as sensitizers is due, in large part, to their exceptional stability and favorable photo-physical properties. Below we review the properties of ruthenium polypyridyl complexes that warrant all this attention, the current state of the art in 'wiring-up' these sensitizer to efficiently undergo energy or electron-transfer over large distances, the current ability to store multiple redox equivalents, and our ability to generate long-lived charge-separated states. Finally the development of efficient co-catalysts for water oxidation (OER) and hydrogen evolution (HER) are reviewed as are aspects of linking these catalysts with the sensitizer as indicated in Fig. 4.

Was this article helpful?

0 0
Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment