Photodriven water oxidation initiated by a surface bound chromophore-donor-catalyst assembly

In photosynthesis, solar energy is used to produce solar fuels in the form of new chemical bonds. A critical step to mimic photosystem II (PS II), a key protein in nature's photosynthesis, for artificial photosynthesis is designing devices for efficient light-driven water oxidation. Here, we describe a single molecular assembly electrode that duplicates the key components of PSII. It consists of a polypyridyl light absorber, chemically linked to an intermediate electron donor, with a molecular-based water oxidation catalyst on a SnO2/TiO2 core/shell electrode. The synthetic device mimics PSII in achieving sustained, light-driven water oxidation catalysis. It highlights the value of the tyrosine–histidine pair in PSII in achieving efficient water oxidation catalysis in artificial photosynthetic devices.

Preparation of Electrodes. The FTO|SnO 2 /TiO 2 slides were prepared by immersing the slides into molecular solutions (4 mM) with soaking for 12 h. The molecular-loaded slides were 5mM Zr(OCl) 2 methanol solution for 2 hours to link Zr IV . After depositing Zr IV , the slides were immersed in 2 mM CH 2 Cl 2 solutions of TPA for an additional 1 h followed by rinsing with CH 2 Cl 2 .The as prepared slides was stored in Glovebox for further use. For other slides preparation, using different substrate but similar procedure.

UV-Visible spectroscopy.
Agilent Technologies Cary 8454 UV-visible spectrometer was used to perform UV-visible absorbance measurements.

Photoelectrochemical Measurements. Electrochemical and photoelectrochemical experiments
were performed by using either a CH Instruments 660D potentiostat or a CH Instruments 760E bipotentiostat. A Thor Labs HPLS 30-04light source was used to provide white-light illumination.
For all indicated experiments using 100-mW cm −2 white-light illumination, the electrochemical cell was positioned an appropriate distance from the light source to receive the indicated light intensity as measured with a photodiode (Newport), and a 400-nm cutoff filter (Newport) was used to prevent direct bandgap excitation of the semiconductor layer. In the water oxidation experiments, a two-compartment cell with a Nafion membrane was used in a three-electrode configuration, with Ag/AgCl as the reference electrode and a Pt mesh counter electrode for H 2 evolution. The experiments were carried out under N 2 at pH = 7.0 in a 0.1 M aqueous sodium phosphate buffers in 0.4 M NaClO 4 with a 100-mW/cm 2 white-light source (400-nm cutoff filter) at a bias of 0.4 V versus Ag/AgCl. For the electrode, we have used 3 samples to test FTO|SnO 2 /TiO 2 |-Ru II P (TPA)(Cat) 2+ water oxidation stability.

IPCE Measurement.
It is well-known that the potential of the Ag/AgCl reference (saturated KCl) is constant with respect to absolute potentials such as NHE (+0.1976 V at 298 K). Incident photon-to-current conversion efficiencies (IPCEs) were measured in a similar manner using a 300 W xenon lamp with a band-pass filter. In all cases, we confirmed that the Ag/AgCl reference electrode correctly worked during the photoelectrochemical experiments, by measuring the potential difference between the used Ag/ AgCl reference electrode and another fresh one. For the electrode, we have used 3 samples to test FTO|SnO 2 /TiO 2 |-Ru II P (TPA)(Cat) 2+ IPCE value.
Quantifying O 2 Evolution. To quantify the amount of evolved O 2 , generation, collector-generator (C-G) experiments were carried out with a dual-electrode design described elsewhere (31, 32). The results of a 1-h illumination period with an ∼1-sun intensity light source (100-mW cm−2 and 400nm long-pass filter). At the end of a photolysis cycle, the generator current decayed instantaneously with slower decay at generator electrode as diffusion of O 2 to the electrode occurs.
Faradaic efficiencies (FE) for O 2 production were calculated with Qcollector and Qgenerator the total charge passed at the collector and generator electrodes, respectively. The constant, 0.7, is the experimentally derived, collection efficiency for the cell, as described previously.
XPS spectra were acquired by using a Kratos Axis Ultra DLD X-ray photoelectron spectrometer with a base pressure of 6 × 10−9 Torr, a monochromatic Al Kα X-ray source, and an analyzer pass energy of either 80 eV or 20 eV for survey and high resolution scans respectively.

Photostability Measurement.
Derivatized electrodes were exposed to constant irradiation at 455 nm (fwhm      Table S1. Table S1. Fitting result for Ru II P 2+ on ZrO 2 in Figure S12. In order to determine the electron injection rate k 1 and recombination rate k 4 , a Ru II P 2+ sample was prepared on CaF 2 substrate and excited by 400 nm excitation light. The probe was selected to be at 5000 nm to probe the electron intraband transition in TiO 2 's conduction band. As shown in Figure S13, the electron injection in TiO 2 results in the growth of signal at a sub picosecond time scale. This signal decays at tens of picosecond. The growth and decay of this signal could be fitted with one exponential function for growth and three exponential functions for decay convoluted with instrument response function. The fitting results are shown table below. Therefore,  1 =124±13 fs and  5 =56.0±9.1 ps. The recombination rate by Ru II P  4 is determined by replacing the electron accepting TiO 2 with insulating ZrO 2 . The associated time constant is shown in Figure   S12 and the Table S1 below. It is worth noticing that the recombinateion discussed here is biphasic, with the fast recombination rate constant determined here. The slower recombination rate constant is determined in Figure S12. The origin of biphasic recombination is attributed to different location of electron in TiO 2 /SnO 2 . Table S2. Fitting result for CaF 2 |SnO2/TiO 2 |Ru II P 2+ in Figure S13.  Comparing the GSB kinetics of -Ru II P 2+ and -Ru II P(TPA) 2+ on FTO|SnO 2 /TiO 2 , the hole transfer with the rate constant  2 could be determined. The GSB of Ru II P 2+ could be described by e -(k4+k5)t where -Ru II P(TPA) 2+ could be described by e -(k2+k4+k5)t . In order to simplify the fitting procedure, we divide these two GSBs and fit the result with a single exponential function to obtain  2 .
Subsequently, the -Ru II P(TPA +. ) 3+ lifetime  6 could be determined as well by fitting the decay of the TPA +. signal at 680 nm as shown in Figure S15 with KWW function as shown in the right panel. The obtained TPA radical kinetics growth agrees well with the initial decay of the GSB of -Ru II P(TPA) 2+ after scaling as shown in the right panel. This proves the TPA radical is a product of the hole transfer from Ru II P 2+ . Figure S16. Determination of  8 . Comparison of GSB kinetics of -Ru II P 2+ and -Ru II P(Cat) 2+ could determine the hole transfer to catalyst rate  8 .
Similarly, comparing the GSB kinetics of -Ru II P 2+ and -Ru II P(Cat) 2+ on FTO|SnO 2 /TiO 2 , the hole transfer with the rate constant k 8 could be determined. The GSB of Ru II P 2+ could be described by e -(k4+k5)t where -Ru II P(TPA) 2+ could be described by e -(k8+k4+k5)t . In order to simplify the fitting procedure, similar to the treatment in Figure S15, we divide these two GSBs and fit the result with a single exponential function to obtain  8 . The recombination rate for hole in catalyst  7 is also fitted by a KWW function and the result is 64.2±8.5 s. The TPA +. signal at 680 nm is used to determine the hole transfer from TPA +. to catalyst as shown in Figure S17. The contribution from Ru II P 2+ at this wavelength is subtracted as shown. The TPA +. signal in both -Ru II P(TPA) 2+ (blue) and Ru II P(TPA)(Cat) 2+ (purple) should follow the GSB kinetics at 470 nm as shown previously in Figure S16. As shown in the right panel, the kinetics of TPA +. in Ru II P(TPA)(Cat) 2+ could be described by: [TPA](t)=-a 1 e -(k2+k8)t +a 2 e -k3t where k 2 , k 8 are determined by previous fitting in Figure S16 and S17. The k 3 that corresponds to the TPA +. decay due to the hole transfer to the catalyst can be determined without interfering with the charge recombination at a later time. This is done by selectively investigating the kinetics between 1 ns and 50 ns shown in the right panel. Because without the catalyst (blue curve) the TPA +. shows no decay yet, it is safe to assume the decay at this time range is due to the transfer.
This kinetics is fitted with an exponential to determine k 3 , which is 3.6±1.6 ns. Table S3: The rate constants for electron transport processes shown in molecular assemblies were tabulated from the TA trace above.