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Direct observation of sequential oxidations of a titania-bound molecular proxy catalyst generated through illumination of molecular sensitizers


Natural photosynthesis uses the energy in sunlight to oxidize or reduce reaction centres multiple times, therefore preparing each reaction centre for a multiple-electron-transfer reaction that will ultimately generate stable reaction products. This process relies on multiple chromophores per reaction centre to quickly generate the active state of the reaction centre and to outcompete deleterious charge recombination. Using a similar design principle, we report spectroscopic evidence for the generation of a twice-oxidized TiO2-bound molecular proxy catalyst after low-intensity visible-light excitation of co-anchored molecular Ru(II)–polypyridyl dyes. Electron transfer from an excited dye to TiO2 generated a Ru(III) state that subsequently and repeatedly reacted with neighbouring Ru(II) dyes via self-exchange electron transfer to ultimately oxidize a distant co-anchored proxy catalyst before charge recombination. The largest yield for twice-oxidized proxy catalysts occurred when they were present at low coverage, suggesting that large dye/electrocatalyst ratios are also desired in dye-sensitized photoelectrochemical cells.

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Figure 1: Chemical structures, relative energetics, and processes proposed to occur at TiO2 nanoparticles.
Figure 2: Spectroelectrochemical absorption spectra of functionalized TiO2 thin-film electrodes.
Figure 3: Transient absorption difference spectra of co-functionalized (RuII+RC-11)/TiO2 thin-film electrodes.
Figure 4: Transient absorption data as a function of time for (RuII + RC-11)/TiO2 thin films.
Figure 5: Steady-state difference spectra for a (RuII + RC-11 (97:3))/TiO2 thin film.


  1. 1

    Bard, A. J. & Fox, M. A. Artificial photosynthesis: solar splitting of water to hydrogen and oxygen. Acc. Chem. Res. 28, 141–145 (1995).

    CAS  Google Scholar 

  2. 2

    Vagnini, M. T. et al. Ultrafast photodriven intramolecular electron transfer from an iridium-based water-oxidation catalyst to perylene diimide derivatives. Proc. Natl Acad. Sci. USA 109, 15651–15656 (2012).

    CAS  PubMed  Google Scholar 

  3. 3

    Du, P. & Eisenberg, R. Catalysts made of earth-abundant elements (Co, Ni, Fe) for water splitting: recent progress and future challenges. Energy Environ. Sci. 5, 6012–6021 (2012).

    CAS  Google Scholar 

  4. 4

    Hammarström, L. Accumulative charge separation for solar fuels production: coupling light-induced single electron transfer to multielectron catalysis. Acc. Chem. Res. 48, 840–850 (2015).

    PubMed  Google Scholar 

  5. 5

    Han, Z., Qiu, F., Eisenberg, R., Holland, P. L. & Krauss, T. D. Robust photogeneration of H2 in water using semiconductor nanocrystals and a nickel catalyst. Science 338, 1321–1325 (2012).

    CAS  PubMed  Google Scholar 

  6. 6

    Han, Z. & Eisenberg, R. Fuel from water: the photochemical generation of hydrogen from water. Acc. Chem. Res. 47, 2537–2544 (2014).

    CAS  PubMed  Google Scholar 

  7. 7

    Das, A., Han, Z., Haghighi, M. G. & Eisenberg, R. Photogeneration of hydrogen from water using CdSe nanocrystals demonstrating the importance of surface exchange. Proc. Natl. Acad. Sci. USA 110, 16716–16723 (2013).

    CAS  PubMed  Google Scholar 

  8. 8

    Ardo, S., Achey, D., Morris, A. J., Abrahamsson, M. & Meyer, G. J. Non-Nernstian two-electron transfer photocatalysis at metalloporphyrin–TiO2 interfaces. J. Am. Chem. Soc. 133, 16572–16580 (2011).

    CAS  PubMed  Google Scholar 

  9. 9

    Youngblood, W. J. et al. Photoassisted overall water splitting in a visible light-absorbing dye-sensitized photoelectrochemical cell. J. Am. Chem. Soc. 131, 926–927 (2009).

    CAS  PubMed  Google Scholar 

  10. 10

    McNamara, W. R. et al. Acetylacetonate anchors for robust functionalization of TiO2 nanoparticles with Mn(II)–terpyridine complexes. J. Am. Chem. Soc. 130, 14329–14338 (2008).

    CAS  PubMed  Google Scholar 

  11. 11

    Xu, Y. et al. Synthesis and characterization of dinuclear ruthenium complexes covalently linked to RuII tris-bipyridine: an approach to mimics of the donor side of photosystem II. Chem. Eur. J. 11, 7305–7314 (2005).

    CAS  PubMed  Google Scholar 

  12. 12

    Brimblecombe, R., Koo, A., Dismukes, G. C., Swiegers, G. F. & Spiccia, L. Solar driven water oxidation by a bioinspired manganese molecular catalyst. J. Am. Chem. Soc. 132, 2892–2894 (2010).

    CAS  PubMed  Google Scholar 

  13. 13

    Song, W. et al. Photoinduced stepwise oxidative activation of a chromophore–catalyst assembly on TiO2 . J. Phys. Chem. Lett. 2, 1808–1813 (2011).

    CAS  Google Scholar 

  14. 14

    Moore, G. F. et al. A visible light water-splitting cell with a photoanode formed by codeposition of a high-potential porphyrin and an iridium water-oxidation catalyst. Energy Environ. Sci. 4, 2389–2392 (2011).

    CAS  Google Scholar 

  15. 15

    Klepser, B. M. & Bartlett, B. M. Anchoring a molecular iron catalyst to solar-responsive WO3 improves the rate and selectivity of photoelectrochemical water oxidation. J. Am. Chem. Soc. 136, 1694–1697 (2014).

    CAS  PubMed  Google Scholar 

  16. 16

    Duan, L. et al. A molecular ruthenium catalyst with water-oxidation activity comparable to that of photosystem II. Nat. Chem. 4, 418–423 (2012).

    CAS  PubMed  Google Scholar 

  17. 17

    Heyduk, A. F. & Nocera, D. G. Hydrogen produced from hydrohalic acid solutions by a two-electron mixed-valence photocatalyst. Science 293, 1639–1641 (2001).

    CAS  PubMed  Google Scholar 

  18. 18

    Mann, K. R. et al. Solar energy storage. Production of hydrogen by 546-nm irradiation of a dinuclear rhodium(I) complex in acidic aqueous solution. J. Am. Chem. Soc. 99, 5525–5526 (1977).

    CAS  Google Scholar 

  19. 19

    Teets, T. S. & Nocera, D. G. Halogen photoreductive elimination from gold(III) centers. J. Am. Chem. Soc. 131, 7411–7420 (2009).

    CAS  PubMed  Google Scholar 

  20. 20

    Karlsson, S. et al. Accumulative charge separation inspired by photosynthesis. J. Am. Chem. Soc. 132, 17977–17979 (2010).

    CAS  PubMed  Google Scholar 

  21. 21

    Karlsson, S. et al. Accumulative electron transfer: multiple charge separation in artificial photosynthesis. Faraday Discuss. 155, 233–252 (2012).

    CAS  PubMed  Google Scholar 

  22. 22

    Song, W. et al. Accumulation of multiple oxidative equivalents at a single site by cross-surface electron transfer on TiO2 . J. Am. Chem. Soc. 135, 11587–11594 (2013).

    CAS  PubMed  Google Scholar 

  23. 23

    Song, W. et al. Visible light driven benzyl alcohol dehydrogenation in a dye-sensitized photoelectrosynthesis cell. J. Am. Chem. Soc. 136, 9773–9779 (2014).

    CAS  PubMed  Google Scholar 

  24. 24

    Meyer, G. J. Antenna molecule drives solar hydrogen generation. Proc. Natl Acad. Sci. USA 112, 9146–9147 (2015).

    CAS  PubMed  Google Scholar 

  25. 25

    Coggins, M. K., Zhang, M.-T., Chen, Z., Song, N. & Meyer, T. J. Single-site copper(II) water oxidation electrocatalysis: rate enhancements with HPO42− as a proton acceptor at pH 8. Angew. Chem. Int. Ed. 53, 12226–12230 (2014).

    CAS  Google Scholar 

  26. 26

    Alibabaei, L., Sherman, B. D., Norris, M. R., Brennaman, M. K. & Meyer, T. J. Visible photoelectrochemical water splitting into H2 and O2 in a dye-sensitized photoelectrosynthesis cell. Proc. Natl Acad. Sci. USA 112, 5899–5902 (2015).

    CAS  PubMed  Google Scholar 

  27. 27

    Kanan, M. W. & Nocera, D. G. In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co2+. Science 321, 1072–1075 (2008).

    CAS  PubMed  Google Scholar 

  28. 28

    Gao, Y. et al. Visible light driven water splitting in a molecular device with unprecedentedly high photocurrent density. J. Am. Chem. Soc. 135, 4219–4222 (2013).

    CAS  PubMed  Google Scholar 

  29. 29

    Chang, D. W. et al. Bistriphenylamine-based organic sensitizers with high molar extinction coefficients for dye-sensitized solar cells. RSC Adv. 2, 6209–6215 (2012).

    CAS  Google Scholar 

  30. 30

    Mishra, A., Fischer, M. K. R. & Bäuerle, P. Metal-free organic dyes for dye-sensitized solar cells: from structure: property relationships to design rules. Angew. Chem. Int. Ed. 48, 2474–2499 (2009).

    CAS  Google Scholar 

  31. 31

    Ardo, S. & Meyer, G. J. Photodriven heterogeneous charge transfer with transition-metal compounds anchored to TiO2 semiconductor surfaces. Chem. Soc. Rev. 38, 115–164 (2009).

    CAS  PubMed  Google Scholar 

  32. 32

    Göransson, E. et al. Charge transfer through cross-hyperconjugated versus cross-π-conjugated bridges: an intervalence charge transfer study. Chem. Sci. 4, 3522 (2013).

    Google Scholar 

  33. 33

    Staniszewski, A., Ardo, S., Sun, Y., Castellano, F. N. & Meyer, G. J. Slow cation transfer follows sensitizer regeneration at anatase TiO2 interfaces. J. Am. Chem. Soc. 130, 11586–11587 (2008).

    CAS  PubMed  Google Scholar 

  34. 34

    Cappel, U. B., Gibson, E. A., Hagfeldt, A. & Boschloo, G. Dye regeneration by spiro-MeOTAD in solid state dye-sensitized solar cells studied by photoinduced absorption spectroscopy and spectroelectrochemistry. J. Phys. Chem. C 113, 6275–6281 (2009).

    CAS  Google Scholar 

  35. 35

    Ardo, S., Sun, Y., Staniszewski, A., Castellano, F. N. & Meyer, G. J. Stark effects after excited-state interfacial electron transfer at sensitized TiO2 nanocrystallites. J. Am. Chem. Soc. 132, 6696–6709 (2010).

    CAS  PubMed  Google Scholar 

  36. 36

    Ardo, S., Sun, Y., Castellano, F. N. & Meyer, G. J. Excited-state electron transfer from ruthenium-polypyridyl compounds to anatase TiO2 nanocrystallites: evidence for a Stark effect. J. Phys. Chem. B 114, 14596–14604 (2010).

    CAS  PubMed  Google Scholar 

  37. 37

    Cappel, U. B., Feldt, S. M., Schöneboom, J., Hagfeldt, A. & Boschloo, G. The influence of local electric fields on photoinduced absorption in dye-sensitized solar cells. J. Am. Chem. Soc. 132, 9096–9101 (2010).

    CAS  PubMed  Google Scholar 

  38. 38

    O'Regan, B. C. & Durrant, J. R. Kinetic and energetic paradigms for dye-sensitized solar cells: moving from the ideal to the real. Acc. Chem. Res. 42, 1799–1808 (2009).

    CAS  PubMed  Google Scholar 

  39. 39

    Schmidt-Mende, L., Kroeze, J. E., Durrant, J. R., Nazeeruddin, M. K. & Grätzel, M. Effect of hydrocarbon chain length of amphiphilic ruthenium dyes on solid-state dye-sensitized photovoltaics. Nano Lett. 5, 1315–1320 (2005).

    CAS  PubMed  Google Scholar 

  40. 40

    Chandiran, A. K., Nazeeruddin, M. K. & Grätzel, M. The role of insulating oxides in blocking the charge carrier recombination in dye-sensitized solar cells. Adv. Funct. Mater. 24, 1615–1623 (2014).

    CAS  Google Scholar 

  41. 41

    Clifford, J. N., Yahioglu, G., Milgrom, L. R. & Durrant, J. R. Molecular control of recombination dynamics in dye sensitised nanocrystalline TiO2 films. Chem. Commun. 1260–1261 (2002).

  42. 42

    Farnum, B. H., Wee, K.-R. & Meyer, T. J. Self-assembled molecular p/n junctions for applications in dye-sensitized solar energy conversion. Nat. Chem. 8, 845–852 (2016).

    CAS  PubMed  Google Scholar 

  43. 43

    Walter, M. G. et al. Solar water splitting cells. Chem. Rev. 110, 6446–6473 (2010).

    CAS  PubMed  Google Scholar 

  44. 44

    Sun, K. et al. Enabling silicon for solar-fuel production. Chem. Rev. 114, 8662–8719 (2014).

    CAS  PubMed  Google Scholar 

  45. 45

    Song, W. et al. Visualization of cation diffusion at the TiO2 interface in dye sensitized photoelectrosynthesis cells (DSPEC). Energy Environ. Sci. 6, 1240–1248 (2013).

    CAS  Google Scholar 

  46. 46

    Sullivan, B. P., Salmon, D. J. & Meyer, T. J. Mixed phosphine 2,2′-bipyridine complexes of ruthenium. Inorg. Chem. 17, 3334–3341 (1978).

    CAS  Google Scholar 

  47. 47

    Liu, F. & Meyer, G. J. Remote and adjacent excited-state electron transfer at TiO2 interfaces sensitized to visible light with Ru(II) compounds. Inorg. Chem. 44, 9305–9313 (2005).

    CAS  PubMed  Google Scholar 

  48. 48

    Behl, M., Hattemer, E., Brehmer, M. & Zentel, R. Tailored semiconducting polymers: living radical polymerization and NLO-functionalization of triphenylamines. Macromol. Chem. Phys. 203, 503–510 (2002).

    CAS  Google Scholar 

  49. 49

    Seok, W. K., Jo, M., Kim, N. & Yun, H. Comparative study of ruthenium (II) and ruthenium (III) complexes with the ligand dmbpy (dmbpy=4,4′-dimethyl-2,2′-bipyridine). Z. Anorg. Allg. Chem. 638, 754–757 (2012).

    CAS  Google Scholar 

  50. 50

    Connelly, N. G. & Geiger, W. E. Chemical redox agents for organometallic chemistry. Chem. Rev. 96, 877–910 (1996).

    CAS  PubMed  Google Scholar 

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This work was supported by the School of Physical Sciences at the University of California Irvine and the National Science Foundation under CHE – 1566160. The authors acknowledge the UCI Laser Spectroscopy Facility (LSF) for transient absorption and photoluminescence spectroscopy instrumentations, the NMR Facility for NMR measurements, the Laboratory for Electron and X-ray Instrumentation (LEXI) for SEM measurements and the Mass Spectrometry Facility for ESI–MS measurements. The authors thank J. Cardon, who is supported by a National Science Foundation Graduate Research Fellowship, for performing additional control experiments during the review process. The authors also thank V. Nair for SEM measurements, D. Fishman, W. Van der Veer and A. Alshawa for assistance and guidance with the LSF instrumentation, J. Winkler for general laser and electronics guidance and H. Gray and A. Borovik for use of their laboratory space and their group members for support and guidance.

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S.A. conceived the research, wrote the Monte Carlo code and performed the Monte Carlo simulations. H.-Y.C. synthesized molecules and materials, prepared samples, performed measurements and analysed the data, with advice from S.A. S.A. and H.-Y.C. discussed the results and prepared the manuscript.

Corresponding author

Correspondence to Shane Ardo.

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Chen, HY., Ardo, S. Direct observation of sequential oxidations of a titania-bound molecular proxy catalyst generated through illumination of molecular sensitizers. Nature Chem 10, 17–23 (2018).

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