1. XUV Observation of High-Valent States in Co4O4 Cubane
Yusef Shari’ati
Vura-Weis Group
June 19th, 2019

2. Metal-Oxo “Cubanes” Oxidize Water
2 H2O  O2 + 4 H+ + 4 e-
Nocera Group Catalyst, “CoPi”
The Oxygen Evolving Complex of Photosystem II
Cobalt Cubane Cluster, “[Co4O4]”
Solar fuels
Water oxidation difficult half-reaction, large overpotential.
Mechanistically complex, multi-step reaction in which … must be marshalled.
Plants do it!
OEC, CoPi, and Cubane have in common are multiple metals.
Monarch Butterfly resting on Honey Locust
Umena, Y; Kawakami, K.; Shen, J.-R.; Kamiya, N. Nature 2011, 473, 55.
Kanan and Nocera, Science 2008, 321, pp
Dismukes, J. Am. Chem. Soc. 2011, 133 (30),
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XUV
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3. Proposed Mechanisms are Indistinguishable
PET
Unstable
Obtain by (electro)chemical oxidation
- e-
Kinetically indistinguishable.
Why Co(V)?
Why Co(IV)?
- e-
Tilley et al. J. Am. Chem. Soc. 2015, 137, 12865−12872
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XUV
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4. High Harmonic Generation  XUV ProbeMn Fe Co Ni
Zhang et al. J. Phys. Chem. Lett. 2016, 7 (17), 3383−3387
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5. Simulated M-Edge XANES Spectra
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6. Simulated M-Edge XANES Spectra
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7. Oxidized Cubane is Distinguishable
CHEMICALLY OXIDIZED CUBANE, ISOLATED obtain singly oxidized cubane – the first step in the catalytic cycle.
1.2 eV shift upon oxidation! This is ~ ¼ that seen by RIXS, and makes sense.
OK, let’s get back to the idea of PHOTOINDUCED ELECTRON TRANSFER.
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8. Simulated M-Edge XANES Spectra
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9. PET via Perylene Diimide Chromophore
1*PDI-[Co4O4]
2.3 eV
PDI-[Co4O4]
0 eV hν
Energy (eV)
1.4 eV
PDI●--[Co4O4]+ * + e- ℎ𝑣
Pyridine ligand is platform. Acetates labile during water oxidation.
To SENSITIZE cubane towards light we need a CHROMOPHORE
Highly absorptive
Highly photostable
Redox Tunable
As a stepping stone to the 2+ cubane which is the REAL GOAL of this endeavour, I want to show you the 1+.
[Co4O4](OAc)4(py)4 + PDIpy  [Co4O4](OAc)4(py)3 (PDIpy) + py
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10. Dyad is 1:1 in chromophore:cubane
Molar excess of cubane.
No untoward coupling – wish to probe UNPERTURBED cubane, and
That is suggested here.
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11. Fluorescence is Quenched in Sensitized Cubane
ENERGY TRANSFER or
ELECTRON TRANSFER?
(DPP)2PDIpy
(DPP)2PDIpy-[Co4O4]
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12. (DPP)2PDIpy OTA Spectrum
𝝉=𝟐.𝟕 𝒏𝒔
(DPP)2PDIpy
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13. Electron Transfer in Sensitized Cubane is Clear
QY here is ~1
In PS it’s slightly decreased: 0.75
(DPP)2PDIpy-[Co4O4]
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14. Electron Transfer in Sensitized Cubane is Clear
τET = 2.45 ps
τCR = 59.9 ps
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15. PDI Aggregates in Thin Films
PDIpy will still aggregate in a PS film. Concentrations needed for XUV are unattainable with this system without aggregation.
Fluorescence is greatly quenched. Normally ~4 ns…will compete with ET.
Excitonic effects will certainly complicate analysis.
PDIpy
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16. Polymer Films Mitigate Aggregation
Si3N4 Substrate
100 nm membrane
$30 each
Image Credit: Silson Ltd.
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17. XUV Spectrum of Dyad is Comparable to Cubane
(no high energy data for PDI cubane)
Slightly more distinct in DPP cubane?
Could be resolution, damage.
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18. XUV Transient Signal is Missing
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19. XUV Radiation Alters Cubane Spectrum
This is a PVC film of (DPP)2PDIphpyCubane.
Surprising Discovery
(Each minute ~ 0.6 Mgy)
Controls: tried in PVC, PS, and Neat.
It’s not the pump laser. It’s XUV.
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20. XUV Radiation Reduces Cobalt Cubane
Co(III) LS  Co(II) HS
This is cubane in PS film. Compare to difference spectrum.
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XUV

21. XUV Photoreduction is Manageable
PVC Matrix
0.3 MGy in 27 seconds.
1MGy in ~1.5 minutes of continuous exposure.
Kubin et al, Phys. Chem. Chem. Phys., 2018, 20, 16817–16827.
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22. XUV Photoreduction is Manageable
PVC Matrix
PS or No Matrix
0.3 MGy in 27 seconds.
1MGy in ~1.5 minutes of continuous exposure.
Kubin et al, Phys. Chem. Chem. Phys., 2018, 20, 16817–16827.
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XUV

23. Summary Cobalt cubanes:
molecular model for the CoPi catalyst
structural relative of the OEC
itself a controversial water oxidation catalyst
Synthesized PDI-cubane dyads and verified electron transfer.
Obtained GS XUV spectra.
XUV irradiation photoreduces cobalt cubane.
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24. Acknowledgements Vura-Weis Group Prof. Josh Vura-Weis
Kris Benke, Suds Dwaraknath, Aastha Sharma, Clare Leahy, Dr. Liz Ryland, Dr. Michaela Carlson, Ryan Ash, Kori Sye, Max Verkamp, Kaili Zhang, Hannah Toru
Family Siddiqua & Adam
Funding from NSF GRFP