Stable Molecular Surface Modification of Nanostructured, Mesoporous

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Surfaces, Interfaces, and Applications

Stable Molecular Surface Modification of Nanostructured, Mesoporous Metal Oxide Photoanodes by Silane and Click Chemistry Lei Wu, Michael Eberhart, Bing Shan, Animesh Nayak, M. Kyle Brennaman, Alexander J. M. Miller, Jing Shao, and Thomas J. Meyer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17824 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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ACS Applied Materials & Interfaces

Stable Molecular Surface Modification of Nanostructured, Mesoporous Metal Oxide Photoanodes by Silane and Click Chemistry Lei Wu,†‡ Michael Eberhart,‡ Bing Shan,‡ Animesh Nayak,‡ M. Kyle Brennaman,‡ Alexander J. M. Miller,‡* Jing Shao,†* Thomas J. Meyer.‡* †College of Chemistry and Environment Engineering, Shenzhen University, Shenzhen, 518000, China ‡Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States KEYWORDS: Dye-sensitized, Ru(II) polypyridyl complexes, silane chemistry, click chemistry, stability, photostability, electrostability, photoanode, DSPECs

ABSTRACT: Binding functional molecules to nanostructured mesoporous metal oxide surfaces provides a way to derivatize metal oxide semiconductors for applications in dye-sensitized photoelectrosynthesis cells (DSPECs). The commonly used anchoring groups, phosphonates and carboxylates, are unstable as surface links to oxide surfaces at neutral and high pH, leading to rapid desorption of appended molecules. A synthetically versatile molecular attachment strategy based on initial surface-modification with a silyl azide followed by click chemistry is described here. It

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has been used for the stable installation of surface-bound metal complexes. The resulting surfaces are highly stabilized toward complex loss with excellent thermal, photochemical, and electrochemical stabilities. The procedure involves binding 3-azidopropyltrimethoxysilane (APTMS) to nanostructured mesoporous TiO2 or tin-doped indium oxide (ITO) electrodes by silane attachment followed by azide-terminated, Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reactions with an alkyne-derivatized ruthenium(II) polypyridyl complex. The chromophore-modified electrodes display enhanced photochemical and electrochemical stabilities compared to phosphonate surface binding with extended photoelectrochemical oxidation of hydroquinone for more than ~6 h with no significant decay.

Introduction

Combining the reactivity of transition metal complexes and metal oxide semiconductors in device applications remains a fundamental challenge. The controlled modification of nanostructured mesoporous metal oxide surfaces with reactive molecules has applications in selfassembly,1-4 semiconductor coatings,1,5 nanoelectronics,2,3,5 and solar energy conversion.6-12 Surface-modified semiconducting metal oxides have emerged as a promising approach in artificial photosynthetic systems that mimic natural photosynthesis by converting water and/or carbon dioxide into “solar fuels” such as hydrogen or carbon monoxide with sunlight.6,9,12-19 A variety of photoelectrochemical approaches have been explored in this area, including dyesensitized

photoelectrosynthesis

cells

(DSPECs)11,14,15

that

combine

molecular-level

chromophore/catalyst assemblies for light absorption and catalysis with high band gap metal oxide semiconductors.11,15,20 In a DSPEC photoanode, photoexcitation of a surface-bound chromophore is followed by electron injection into a n-type semiconductor (e.g. TiO2, SnO2, ZnO), producing


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oxidizing equivalents that can drive catalytic water oxidation. The instability of water oxidizing photoanodes is a major challenge in the development of DSPECs. While stability depends on many factors including degradation pathways of the transition metal complexes especially in high oxidation or photoexcited states, surface binding in aqueous conditions under solar flux presents a major limitation. In the literature, surface binding of chromophores, catalysts, and chromophore/catalyst assemblies to electrode surfaces has been dominated by phosphonic acid and carboxylic acid linkers.6,16,21-23 Carboxylic acids offer stable surface binding in non-aqueous solvents and are used in dye-sensitized solar cells (DSSCs) but they undergo rapid loss from surfaces in aqueous solutions.24-29 Phosphonic acid surface binding offers enhanced aqueous stability but is subject to hydrolysis at pH >5.24-29 Since water oxidation is more thermodynamically favorable under neutral or basic conditions, and can be kinetically accelerated with added buffer bases by atom-proton transfer (APT) pathways,6,10,19,30 stable binding plays an essentially important role as the pH is increased. Various approaches have been developed for the stabilization surface-bound molecular assemblies under photoelectrocatalytic conditions.6,10 In atomic layer deposition (ALD), after electrode derivatization with molecular components, metal oxide layers are added by dose-purge cycles of precursor such as Al(CH3)3 (for Al2O3) or TiCl4 (for TiO2) with H2O.27,28,31,32 While this approach provides significant stabilization even at high pH conditions, ALD is expensive, utilizes relatively high temperature conditions with reactive precursors, and deposits an insulating overlayer that hinders the electron transfer.31,33,34 Surface phosphonates have been stabilized by added polymer film overlayers33,35-38 including poly(methyl

methacrylate)

(PMMA),35

fluorinated

polymers,33

and

Fe(II)-containing



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metallopolymer.36 There are limitations in the polymer overlayer method that the diffusion of high molecular-weight polymers is uncontrollable through the mesoporous film, the reactive C-H bonds in the polymers can be unstable toward oxidation by external oxidants, and the electron transfer is inhibited within the polymer overlayers.33,35,39,40 A less explored approach for surface stabilization is the application of new covalent surface linkages for molecular assemblies. Schmuttenmaer, Batista, and Crabtree have reported that hydroxamate surface-binding exhibited improved stability compared to carboxylates and phosphonates.41 An obvious extension is to silanes, where a range of trimethoxysilyl, triethoxysilyl, or trichlorosilyl functional groups have been used in other areas.39 Application to solar fuel assemblies has been limited because of incompatibility in the synthetic conditions required.1 Instead, Brudvig and Moore reported silatrane anchoring groups for attaching porphyrin chromophores42 and water oxidation catalysts, [Ru(Mebimpy)(bpy)(OH2)]2+-type (Mebimpy = 2,6-bis(1-methylbenzimidazole-2-yl)pyridine; bpy = 2,2’-bipyridine),43 to surfaces. The silatrane approach utilizes protected silyl groups but requires a multi-step inorganic synthesis or a multicomponent approach. On the contrary, functionalized silanes can be attached to surfaces via Si-O bond followed by surface link at the functional group. In recent examples, an azide-functionalized silane was combined with click chemistry for a modular molecular attachment on planar or nanoparticle surfaces.44-46 We report here a versatile synthetic strategy that enables preparation of surface-modified photoanodes with long-term stabilization of a Ru(II) polypyridyl chromophore. To avoid complications from silane-derivatization of the complex, the electrode surface was initially functionalized with a silyl azide followed by a Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) (Scheme 1) enabling modular synthesis directly on the electrode surface.47-51 Compared


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to phosphonic or carboxylic acid binding strategies, the silane surface attachment procedure greatly improves stability over a broad pH range.

Scheme 1. Synthetic strategy for immobilization of a Ru(II) polypyridyl chromophore, 12+, on nanoITO or nanoTiO2 electrodes based on (i) silane surface functionalization and (ii) CuAAC click chemistry. Results and Discussion Surface Assembly. A two-step procedure was used in the preparation of the surface-modified electrodes. Surface modification with functional silanes is the most commonly utilized method for preparing closely-packed self-assembled monolayers (SAMs) on metal oxide planar or nanoparticle surfaces in aprotic solvents such as toluene.1 The strategy used here enables stable surface binding of an alkyl azide for additional surface reactions. In the first step, 3azidopropyltrimethoxysilane (APTMS) was synthesized50,51 and used to derivatize mesoporous nano-titanium dioxide (nanoTiO2, ~20-30 nm particle diameter and ~4 μm film thickness) or nanotin-doped indium oxide (nanoITO, ~10-15 nm particle diameter and ~4 μm film thickness) films on fluorine-doped tin oxide (FTO) glass (Step i in Scheme 1) in toluene.



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Evidence from X-ray photoelectron spectroscopy (XPS) confirms the success of surface functionalization. The high resolution N 1s XPS spectrum has resonances at 400 and 404 eV with an intensity ratio of 2:1 from the azide groups on the surface (Figure S1a in Supporting Information).49 Surface functionalization was further verified by Fourier transform infrared (FTIR) spectroscopy based on the appearance of alkyl azide vibrational band at 2100 cm−1 (Figure S2).52 For the coupling strategy, the alkyne-derivatized Ru(II) polypyridyl chromophore 12+ ([Ru(bpy)2(4,4’-(HC≡C)2bpy)]2+) was synthesized as shown in Scheme 2. Sonogashira coupling between 4,4'-dibromo-2,2'-bipyridine and trimethylsilylacetylene, followed by desilylation with potassium carbonate, afforded the ligand 4,4'-(HC≡C)2bpy (i and ii in Scheme 2). The ruthenium polypyridyl complex 12+ was synthesized by a reaction between cis-[Ru(bpy)2Cl2] and 4,4'(HC≡C)2bpy) in a refluxing 1:1 mixture of ethanol and water under argon (iii in Scheme 2). The crude complex product was purified by size exclusion chromatography (Sephadex LH-20) in 40% yield.

Scheme 2. Synthesis of 12+: (i) Sonogashira coupling, PdCl2(PPh3)2, CuI, triethylamine, and (trimethylsilyl)acetylene; (ii) K2CO3, dichloromethane/methanol; (iii) 1 equiv cis-[Ru(bpy)2Cl2], ethanol/water. (Details in Experimental Section)


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Chromophore 12+ was attached to azide-modified nanoTiO2 or nanoITO electrode surfaces by CuAAC click reactions in methanol/water (1/1, v/v) using a Cu(I) catalyst formed in situ from cupric sulfate and sodium ascorbate to give nanoTiO2|-12+ or nanoITO|-12+ (Scheme 1). After allowing the reaction to proceed for 60 hours, the electrodes were immersed in methanol/water overnight to remove any physisorbed chromophore. The appearance of 12+ bound on the surface was confirmed by ultraviolet-visible (UV-vis) spectroscopy based on the presence of a broad metal-to-ligand charge transfer (MLCT) absorption at λmax ~460 nm.6,53 Surface-bound ruthenium was also confirmed by XPS measurements (Figure S1b). The Ru 3d5/2 peak appeared at 280.5 eV with the Ru 3d3/2 peak partly overlapped with the adjacent C 1s peak.54 FT-IR spectra were collected before and after the CuAAC reaction. Reduction of the peak intensity for the surface azide vibration at 2100 cm-1 was observed (Figure S2). Residual azide remains even after prolonged reaction times, consistent with some azide sites blocked by the complexes which inhibit the click reaction. A band was observed at 2165 cm-1 in Raman spectra arising from the unreacted C≡C bonds in the attached chromophores (Figure S3)55 bound through only one triazole in the final structure. Electrochemical Characterization. Cyclic voltammetry (CV) measurements on nanoITO|-12+ and nanoTiO2|-12+ were conducted in 0.1 M HClO4 aqueous solutions from 0.5 to 1.5 V at varying scan rates with Pt-wire counter and Ag/AgCl reference electrodes. For nanoITO|-12+, at slow scan rates e.g. 5 mV/s, a chemically reversible wave appeared for the RuIII/II couple at E1/2 = 1.32 V vs NHE (Figure 1a). The peak current (ip) varied linearly with scan rate (Figure 1b), confirming that the chromophore was bound to the surface.56,57 Increasing the scan rate from 5 to 100 mV/s resulted in an increase in the peak to peak separation, presumably due to slow electron transfer kinetics at the electrode and mass transport of the electrolyte ions within the mesoporous nanostructures for



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a charge equilibration.57,58 The E1/2 value for the RuIII/II redox couple falls within the band gap for TiO2, but electrochemical characterization was possible at slow scan rates (Figure S4), by electron transfer across the FTO interface with site-to-site, cross-surface electron transfer hopping.6,36

Figure 1. (a) CV scans for nanoTiO2|-12+ in 0.1 M HClO4 as function of scan rate; (b) Peak current (ip) vs scan rate. Chromophore loading, Γ in nmol/cm2, was quantified by integrating RuIII/II wave. Analysis of the data gave Γ ~39 nmol/cm2 on nanoITO and ~30 nmol/cm2 on nanoTiO2 with both values comparable to those for phosphonate or carboxylate bound analogs on mesoporous nanoTiO2 or nanoITO electrodes.6,35,36 The surface loading of chromophores was also evaluated from UV−Vis


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measurements using the measured absorbance at λmax with the corresponding molar absorptivity,6 which gave Γ ~36 nmol/cm2 on nanoITO and ~56 nmol/cm2 on nanoTiO2. The surface coverage on nanoTiO2 obtained by UV-vis characterizations is larger than electrochemical analysis since not all chromophores were electro-activated during CV scans.6 Aqueous Stability in the Dark. The stabilities of the nanoTiO2|-12+ or nanoITO|-12+ electrodes were explored in aqueous solutions as a function of pH. UV-visible spectra of 12+ on nanoTiO2 immersed in pH ~1.0 aqueous HClO4, pure water, pH ~7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, 0.5 M NaClO4), and pH ~12.5 phosphate buffer (0.1 M Na2HPO4/Na3PO4, 0.5 M KNO3) under N2 protection were monitored over 22 h periods at 1 h intervals (Figure 2a and S5-7). Due to light absorption and scatter by TiO2, spectra were monitored from 350−700 nm for MLCT bands.25 Desorption from the surface was investigated by recording absorption-time curves at 460 nm (Figure 2b).



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Figure 2. (a) Changes in the absorption spectrum of nanoTiO2|-12+ in the dark at pH ~7.5 in a phosphate buffer (0.1 M NaH2PO4/Na2HPO4, 0.5 M NaClO4); 0 h black to 22 h green at 1h intervals. (b) Relative absorbance with time as a ratio of the initial absorbance (At/A0) for nanoTiO2|-12+ at 460 nm as a function of pH. Prior reports have demonstrated that phosphonic and carboxylic acid linkages are unstable toward surface hydrolysis at pH > 5.25,35 Desorption of [Ru(bpy)2((4,4’-(HO)2PO)2bpy)]2+ (RuP2+) is apparent by visual inspection within 30 min in a phosphate buffer at pH ~12.5, with the electrode undergoing complete bleaching. The silane anchoring strategy led to dramatic reduction in desorption, especially in pure water, at pH ~7.5 and ~12.5 0.1 M phosphate buffers (Figures 2 and


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S5-7). Based on these results, more than 85% of the initial Ru(II) chromophore remained on the electrode surface after a 22 h soaking period in all pH conditions (Figure 2 and S5-7). Photostability. The photostability of 12+ on nanoTiO2 electrodes at different pH values was investigated according to a previously established protocol.28 UV-vis spectra of nanoTiO2-12+ in pH ~1.0 aqueous HClO4, pure water, pH ~7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, 0.5 M NaClO4), and pH ~12.5 phosphate buffer (0.1 M Na2HPO4/Na3PO4, 0.5 M KNO3) under N2 protection were recorded in situ every 15 min with continuous illumination at 455 nm (30 nm fwhm, 475 mW/cm2). Results are shown in Figure 3a and S8-10 with photo-desorption kinetics monitored by absorption-time changes at 480 nm (Figure 3b). Under illumination, the surfacebound chromophore was highly stable under these conditions. More than 80% of the initial Ru(II) chromophore remained on the surface after 16 h of high intensity illumination at pH ~7.5 in a phosphate buffer.



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Figure 3. (a) Changes in UV-vis absorption spectra for nanoTiO2|-12+ in a pH ~7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, 0.5 M NaClO4) under constant illumination at 455 nm (30 nm fwhm, 475 mW/cm2, 0 h black to 16 h green at 15 min intervals). (b) Relative absorbance over time as a ratio of the initial absorbance (At/A0) for nanoTiO2|-12+ monitored at 480 nm under different pH conditions. Photo-desorption kinetics could be fit to a single exponential decay function with the time dependence used to evaluate photo-desorption kinetics, kphoto-des, in Equation 1. Rate constants as a function of pH are listed in Table 1.

y  Ae(kdes)t  y0 (Equation 1)


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Table 1. Rate constants for photochemical surface desorption, kphoto-des, of 12+ on nanoTiO2 electrodes compared to literature values for RuP2+, ALD and polymer overlayer stabilized RuP2+. Constant illumination at 455 nm (30 nm fwhm, 475 mW/cm2) in pH ~1.0 aqueous HClO4, pure water, pH ~7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, 0.5 M NaClO4), and pH ~12.5 phosphate buffer (0.1 M Na2HPO4/Na3PO4, 0.5 M KNO3). kphoto-des (× 10-5 s-1) ALD 3 cycles Al2O3 overlayer28

solvent

RuP2+ 28

pH 1.0

5.0

pure water

> 30

3.2

pH 7.5

*

9.5

pH 12.5

*

*

PMMA (1.0 wt %) overlayer35

12+

0.44

3.9

3.7

5.5 3.1

*

10.5

* Unstable: molecules desorbed before measurements could be made.

Based on the data, at pH ~1.0 and in pure water, kphoto-des is notably lower than for RuP2+. Photodesorption from nanoTiO2|-RuP2+ films was too rapid to be quantified in basic buffer solutions (pH ~7.5 and ~12.5) but can be significantly reduced with stable surfaces for nanoTiO2|-12+ (Table 1). Variations in kphoto-des for nanoTiO2|-12+ show no obvious dependence on pH or the nature of the buffer. The photostability of the surface-bound silane complex was comparable to phosphonate anchored dyes stabilized by ALD and polymer overlayer protocols under all pH conditions. The results obtained here show that the silane immobilization strategy results in greatly enhanced surface stabilization under high-intensity illumination in a variety of aqueous conditions. The mechanistic details for surface destabilization have been discussed elsewhere.25,26,36



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Electrochemical Stability as Ru(III). The oxidative stability of the surface-bound 12+ for Ru(III) form was also investigated (Figure S11a) by successive CV scans for nanoITO|-12+ in a pH ~7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, 0.5M NaClO4) between 0 and 1.5 V vs Ag/AgCl at scan rate of 20 mV/s. CV scans clearly showed the distinct RuIII/II couples from the initial cycle without electron inhibitions in ALD method34 and electrochemically distorted waves for an ion equilibration in polymer overlayers.33 Over a 2 h period, cycling through the RuIII/II wave led to a decrease of 70% in peak current for the RuIII/II couple at E1/2 = 1.32 V vs NHE. The electrochemical kinetics were characterized by loss of the cathodic peak current for the RuIII/II couple on nanoITO over time (Figure S11b).33 Kinetic traces were fit to a biexponential function with the electro-decay rate constant (kelectro-d) calculated as the inverse of the weighted average lifetime with, kelectro-d = 1.4 × 10-4 s-1. Based on the data, there was an increase in electrostability as RuIII by a factor of 10 compared to nanoITO|-RuP3+, with kelectro-d 1.3 × 10-3 s-1 under the same conditions in our previous work.33 Several possible pathways for electro-decay are shown in Figure S11c. Detachment of the surface-bound chromophore is boosted when pH >5 for phosphonate or carboxylate linkages, which is diminished by using the silane immobilization method (pathway 1 in Figure S11c). The other factor originates from the inherent redox properties of the Ru(II) polypyridyl chromophores that oxidation of the complexes results in decomposition through bpy ligand loss and aquation or anation (pathways 2 and 3 in Figure S11c). Sustained Photo-Oxidation of Hydroquinone. To investigate the stability of nanoTiO2|-12+ electrodes in photoanode applications, the electrode was applied to the prolonged photo-oxidation of hydroquinone (H2Q, 50 mM) to quinone (Q). The experiments were conducted in a twocompartment cell with a Nafion membrane separator in a three-electrode configuration with a


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Ag/AgCl reference electrode and a Pt wire counter electrode. The measurements were conducted under nitrogen in a pH ~7.5 in a phosphate buffer (0.1 M NaH2PO4/Na2HPO4, 0.5 M NaClO4) with a 100 mW/cm2 white light source (400 nm long-pass filter). An applied bias of 0.2 V vs Ag/AgCl was used. The photoelectrochemical response of nanoTiO2|-12+ with H2Q as the sacrificial electron transfer donor was compared to nanoTiO2|-12+ without H2Q as the control. With added H2Q, excitation of the Ru(II) chromophore and electron injection into TiO2 would give nanoTiO2(e-)|-13+ which could be rapidly reduced to nanoTiO2(e-)|-12+ by H2Q (Scheme 3).6

Scheme 3. Electron transfer scheme for photolysis with H2Q as the sacrificial electron donor. Current density-time traces under dark/light cycling were explored (Figure 4). Under illumination, the nanoTiO2|-12+ electrode with added H2Q showed significantly higher photocurrents than the control, consistent with light-driven photo-oxidation of H2Q by nanoTiO2|12+.



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Figure 4. Off-and-on photocurrent-time traces for nanoTiO2|-12+ with H2Q (black line) and without H2Q at 50 mM (red line) in pH ~7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, 0.5 M NaClO4) with a 100 mW/cm2 white light source (400 nm long-pass filter). During a 3 h photolysis period for nanoTiO2|-12+ (Figure 5), the photocurrent gradually decreased by ~25% with the cell passing ~200 μA/cm-2 at the end of the photolysis. The initial photocurrent spike at early times arises from surface oxidation of the chromophore and local capacitance effects.34 The silane-attached chromophore exhibits a striking improvement in stability compared to phosphonate or carboxylate linkages, in which ~30% and ~15% photocurrent decay occurred in less than 20 min using RuP2+ and RuP2+ with PMMA stabilizations.35 After prolonged photolysis, the nanoTiO2|-12+ electrode was washed with water and added to a fresh buffer solution for a second 3 h period photolysis with ~75% of the starting photocurrent response retained (Figure S12).

Figure 5. Long-term photolysis of nanoTiO2|-12+ with H2Q added as a sacrificial electron transfer donor at pH ~7.5 (0.1 M NaH2PO4/Na2HPO4, 0.5 M NaClO4) showing a current-time trace during ~3 h at an applied bias of 0.2 V vs Ag/AgCl; illumination at 100 mW/cm-2 with a 380 nm longpass filter.


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Conclusions A facile procedure for immobilizing transition metal complexes on mesoporous metal oxide electrodes based on silane surface functionalization and click chemistry has been developed. The strategy has been used for the preparation of photoanodes with a surface-attached Ru(II) polypyridyl complex. The stabilized complex exhibits exceptional photochemical and electrochemical stability as a chromophore in photoelectrochemical cycles for DSPEC applications. Compared to phosphonic or carboxylic acid surface binding, silane functionalization provides a greatly enhanced surface binding ability with significantly improved stability toward pH changes, electrolysis, and photo-electrochemical conditions. The stabilization method, which is implemented in a simple and straightforward chemical procedure, is comparable to other chromophore stabilization methods such as ALD and polymer dip-coating. The strategy described here provides a basis for a systematic approach to the formation and stabilization of a broad sweep of chromophores, catalysts, and chromophore/catalyst assemblies on metal oxide electrode surfaces for photoelectrochemical applications. Experimental Section Reagents.

4,4’-dibromo-2,2’-bipyridine,

bis(triphenylphosphine)palladium(II)

dichloride

(PdCl2(PPh3)2), copper(I) iodide (CuI), triethylamine, (trimethylsilyl)acetylene, potassium carbonate (K2CO3), CuSO4·5H2O, and (+)-sodium L-ascorbate were commercially available and were used as received. cis-[Ru(bpy)2Cl2]53 and APTMS50,51 were synthesized by literature procedures. Mesoporous films of either nanoTiO2 (~20-30 nm particle diameter and ~4 μm film thickness) or nanoITO (~10-15 nm particle diameter and ~4 μm film thickness) particles on FTO



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glass were prepared according a previous procedure.33,35 Solvents were used without further purification. Synthesis of 4,4'-bis((trimethylsilyl)ethynyl)-2,2'-bipyridine. In a glovebox, a vessel was charged with 4,4’-dibromo-2,2’-bipyridine (1 g, 3.2 mmol), PdCl2(PPh3)2 (115 mg, 0.16 mmol), CuI (65 mg, 0.32 mmol), triethylamine (14 mL, 96 mmol), (trimethylsilyl)acetylene (1.8 mL, 12.8 mmol), and a magnetic stir bar. The vessel was tightly sealed, heated to 90 oC, and stirred overnight. The reaction mixture was then cooled to room temperature and the volatiles removed by rotary evaporation. The crude product was passed through a silica gel column (hexane:ethyl acetate = 10:1) to afford light gray solid (600 mg, 65% yield). 1H NMR (600 MHz, CDCl3): δ 8.62 (dd, 2H), 8.43 (q, 2H), 7.33 (dd, 2H), 0.27 (s, 18H). 13C NMR (600 MHz, CDCl3): δ 0.15, 100.19, 102.33, 123.73, 125.90, 132.45, 149.28, 155.69. Synthesis of 4,4’-(HC≡C)2-2,2'-bipyridine. 4,4'-bis((trimethylsilyl)ethynyl)-2,2'-bipyridine (200 mg, 0.58 mmol) was dissolved in dichloromethane/methanol (60 mL, 5:7) to which K2CO3 (805 mg, 5.8 mmol) was added. The mixture was stirred overnight at room temperature under argon followed by addition of water. Solvent was removed by rotary evaporation resulting in precipitation of the crude product which was washed with water (3 × 50 mL) and extracted with dichloromethane. The organic phase was isolated, washed with water (3 × 50 mL), and dried over MgSO4. Evaporation of the solvent produced a light gray solid (80 mg, 67% yield). 1H NMR (600 MHz, CDCl3): δ 8.66 (dd, 2H), 8.48 (q, 2H), 7.38 (dd, 2H), 3.32 (s, 2H). 13C NMR (600 MHz, CDCl3): δ 81.27, 82.03, 123.90, 126.35, 131.57, 149.40, 155.75. Synthesis of Chromophore 12+. cis-[Ru(bpy)2Cl2] (65 mg, 0.134 mmol) and 4,4’-(HC≡C)2bpy (30.6 mg, 0.15 mmol) were dissolved in 30 mL of ethanol and water (v/v = 1:1) under an argon atmosphere. The solution was heated at reflux in the dark for 24 h. The solvent was removed under


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vaccum and the dark orange-red solid was isolated. The solid was further purified by size exclusion chromatography (Sephadex LH-20) with methanol as eluent (40% yield). 1H NMR (500 MHz, CD3CN): δ 8.65 (d, 2H), 8.56 (dd, 4H), 8.06 (m, 4H), 7.71 (d, 4H), 7.68 (d, 2H), 7.40 (m, 6H), 4.05 (s, 2H). 13C NMR (600 MHz, CD3OD): δ 158.41, 158.37, 152.79, 152.55, 139.53, 133.60, 131.00, 129.10, 128.25, 125.73, 88.75, 80.50. HR-ESI-MS (MeOH): m/z calcd for [M]2+ = 309.05 found at 309.05519, [M + Cl]+ = 653.08 found at 653.07908. Preparation of Azide Self-Assembled Monolayers on nanoITO or nanoTiO2. Clean nanoITO or nanoTiO2 substrates (~10 slides in 1 cm × 1 cm each) were immersed in toluene (55 mL) with 550 μL of APTMS added. After standing for 3.5 d, the substrates were rinsed with methanol and immersed in toluene overnight to remove any physisorbed silane molecules. The slides were dried by a compressed air jet and stored in dark for further use. Caution: Organic azides are potentially explosive substances and any manipulation should utilize appropriate safety protections. Note: A free standing in the concentrated silane stock solution for several hours is enough to form a robust silane SAM on the oxide surface. Anchoring 12+ on Azide-Functionalized Electrode Surfaces. Azide functionalized nanoITO or nanoTiO2 electrodes were added to 60 mL of methanol/water mixtures (1/1, v/v) and 12+ (10 mg, 0.0145 mmol) was added. CuSO4·5H2O (0.32g, 1.29 mmol) and (+)-sodium L-ascorbate (1.28g, 6.45mmol) were added. After 2.5 d, the electrode substrates were removed, washed with methanol/water, and immersed in methanol/water overnight to remove any physically absorbed molecules. The samples were dried by a compressed air jet and stored in the dark for further characterization. Characterization and Measurements. NMR analysis was conducted on Bruker 400, 500, or 600 MHz NMR spectrometer. XPS was performed on a Kratos Analytical Axis Ultra-DLD



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spectrometer with a monochromatized X-ray Al Kα radiation (1486.6 eV) of an analysis area of 300 × 700 μm2. A survey scan was first performed with a step size of 1 eV, a pass energy of 80 eV, and a dwell time of 200 ms. High resolution scans were collected with a step size of 0.1 eV and a pass energy of 20 eV. The binding energy for all peaks were referenced to the C 1s peak at 284.6 eV. FT-IR spectroscopy was carried out on a Bruker Alpha FT-IR spectrometer. Raman spectra were recorded by Raman measurements (Renishaw) with a 514 nm laser. UV-vis absorption spectra were recorded on an Agilent 8453 diode array spectrophotometer, or a Varian Cary 50 UV/Vis spectrophotometer in photostability studies. Photostability measurements were performed in air by a previously reported procedure.25 The light from a Royal Blue (455 nm, FWHM ~30 nm, 475 mW/cm2) Mounted High Power LED (Thorlabs, Inc., M455L2) powered by a T-Cube LED Driver (Thorlabs, Inc., LEDD1B) was focused to a 2.5 mm diameter spot size by a focusing beam probe (Newport Corp. 77646) outfitted with a second lens (Newport Corp. 41230). The light output was directed onto the derivatized thin film placed at 45º in a standard 10 mm path length cuvette containing 3 mL of the solution. The illumination spot was adjusted to coincide both with the thin films and the perpendicular beam path of a Varian Cary 50 UV/Vis spectrophotometer. The absorption spectrum (350 - 700 nm) of the film was taken every 15 minutes over 16 hours of illumination. The incident light intensity was measured with a thermopile detector (Newport Corp. 1918-C meter and 818P-020-12 detector). The solution temperature, 22 ± 2 °C, was consistent throughout the duration of the experiment. CV experiments were conducted with a CH Instruments 600A potentiostat using a Pt wire counter electrode and a Ag/AgCl reference electrode (0.198 V vs NHE). Electrostability studies utilized repeated CV scans over fixed potential ranges in buffer solutions for several hours.


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Photoelectrochemical experiments for photo-oxidizing hydroquinone were conducted on a bipotentiostat (CHI 760E). A two-compartment cell was used in which nanoTiO2|-12+ working electrode (~1 cm2) was separated with Ag/AgCl reference and Pt-wire counter electrodes by a Nafion membrane. The buffer solution containing hydroquinone and working electrode was degassed with nitrogen for half an hour. Illumination was provided by a Thor Labs HPLS-30-04 light source.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XPS spectra, FT-IR spectra, Raman spectra, CVs, UV-vis absorption spectra, CVs of nanoITO|12+ for electrostability study, current-time trace for long-term photolysis, and NMR spectra.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected] *Email: [email protected] Author Contributions



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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the UNC EFRC Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0015739.

REFERENCES (1) Pujari, S. P.; Scheres, L.; Marcelis, A. T. M.; Zuilhof, H., Covalent Surface Modification of Oxide Surfaces. Angewandte Chemie International Edition 2014, 53, 6322-6356. (2) Barbey, R.; Lavanant, L.; Paripovic, D.; Schüwer, N.; Sugnaux, C.; Tugulu, S.; Klok, H.-A., Polymer Brushes via Surface-Initiated Controlled Radical Polymerization: Synthesis, Characterization, Properties, and Applications. Chemical Reviews 2009, 109, 5437-5527. (3) Zoppe, J. O.; Ataman, N. C.; Mocny, P.; Wang, J.; Moraes, J.; Klok, H.-A., Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes. Chemical Reviews 2017, 117, 1105-1318. (4) Russell, T. P., Surface-Responsive Materials. Science 2002, 297, 964-967. (5) Zou, H.; Wu, S.; Shen, J., Polymer/Silica Nanocomposites: Preparation, Characterization, Properties, and Applications. Chemical Reviews 2008, 108, 3893-3957. (6) Ashford, D. L.; Gish, M. K.; Vannucci, A. K.; Brennaman, M. K.; Templeton, J. L.; Papanikolas, J. M.; Meyer, T. J., Molecular Chromophore–Catalyst Assemblies for Solar Fuel Applications. Chemical Reviews 2015, 115, 13006-13049. (7) Armaroli, N.; Balzani, V., Solar Electricity and Solar Fuels: Status and Perspectives in the Context of the Energy Transition. Chemistry – A European Journal 2016, 22, 32-57. (8) Duan, L.; Tong, L.; Xu, Y.; Sun, L., Visible light-driven water oxidation-from molecular catalysts to photoelectrochemical cells. Energy & Environmental Science 2011, 4, 3296-3313.


Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Young, K. J.; Martini, L. A.; Milot, R. L.; Snoeberger, R. C.; Batista, V. S.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W., Light-driven water oxidation for solar fuels. Coordination Chemistry Reviews 2012, 256, 2503-2520. (10) Meyer, T. J.; Sheridan, M. V.; Sherman, B. D., Mechanisms of molecular water oxidation in solution and on oxide surfaces. Chemical Society Reviews 2017, 46, 6148-6169. (11) Swierk, J. R.; Mallouk, T. E., Design and development of photoanodes for water-splitting dye-sensitized photoelectrochemical cells. Chemical Society Reviews 2013, 42, 2357-2387. (12) Xu, P.; McCool, N. S.; Mallouk, T. E., Water splitting dye-sensitized solar cells. Nano Today 2017, 14, 42-58. (13) Lewis, N. S.; Nocera, D. G., Powering the planet: Chemical challenges in solar energy utilization. Proceedings of the National Academy of Sciences 2006, 103, 15729-15735. (14) Grätzel, M., Photoelectrochemical cells. Nature 2001, 414, 338. (15) Concepcion, J. J.; House, R. L.; Papanikolas, J. M.; Meyer, T. J., Chemical approaches to artificial photosynthesis. Proceedings of the National Academy of Sciences 2012, 109, 1556015564. (16) Brennaman, M. K.; Dillon, R. J.; Alibabaei, L.; Gish, M. K.; Dares, C. J.; Ashford, D. L.; House, R. L.; Meyer, G. J.; Papanikolas, J. M.; Meyer, T. J., Finding the Way to Solar Fuels with Dye-Sensitized Photoelectrosynthesis Cells. Journal of the American Chemical Society 2016, 138, 13085-13102. (17) Joya, K. S.; Joya, Y. F.; Ocakoglu, K.; van de Krol, R., Water-Splitting Catalysis and Solar Fuel Devices: Artificial Leaves on the Move. Angewandte Chemie International Edition 2013, 52, 10426-10437. (18) Kärkäs, M. D.; Laine, T. M.; Johnston, E. V.; Åkermark, B., Visible Light-Driven Water Oxidation Catalyzed by Ruthenium Complexes. In Applied Photosynthesis - New Progress, Najafpour, M. M., Ed. InTech: Rijeka, 2016; p Ch. 10. (19) Kärkäs, M. D.; Verho, O.; Johnston, E. V.; Åkermark, B., Artificial Photosynthesis: Molecular Systems for Catalytic Water Oxidation. Chemical Reviews 2014, 114, 11863-12001. (20) Alibabaei, L.; Luo, H.; House, R. L.; Hoertz, P. G.; Lopez, R.; Meyer, T. J., Applications of metal oxide materials in dye sensitized photoelectrosynthesis cells for making solar fuels: let the molecules do the work. Journal of Materials Chemistry A 2013, 1, 4133-4145. (21) Materna, K. L.; Crabtree, R. H.; Brudvig, G. W., Anchoring groups for photocatalytic water oxidation on metal oxide surfaces. Chemical Society Reviews 2017, 46, 6099-6110. (22) Zhang, L.; Cole, J. M., Anchoring Groups for Dye-Sensitized Solar Cells. ACS Applied Materials & Interfaces 2015, 7, 3427-3455. (23) Queffélec, C.; Petit, M.; Janvier, P.; Knight, D. A.; Bujoli, B., Surface Modification Using Phosphonic Acids and Esters. Chemical Reviews 2012, 112, 3777-3807. (24) Hanson, K.; Brennaman, M. K.; Ito, A.; Luo, H.; Song, W.; Parker, K. A.; Ghosh, R.; Norris, M. R.; Glasson, C. R. K.; Concepcion, J. J.; Lopez, R.; Meyer, T. J., Structure–Property Relationships in Phosphonate-Derivatized, RuII Polypyridyl Dyes on Metal Oxide Surfaces in an Aqueous Environment. The Journal of Physical Chemistry C 2012, 116, 14837-14847. (25) Hanson, K.; Brennaman, M. K.; Luo, H.; Glasson, C. R. K.; Concepcion, J. J.; Song, W.; Meyer, T. J., Photostability of Phosphonate-Derivatized, RuII Polypyridyl Complexes on Metal Oxide Surfaces. ACS Applied Materials & Interfaces 2012, 4, 1462-1469. (26) Hyde, J. T.; Hanson, K.; Vannucci, A. K.; Lapides, A. M.; Alibabaei, L.; Norris, M. R.; Meyer, T. J.; Harrison, D. P., Electrochemical Instability of Phosphonate-Derivatized,



Page 24 of 27

Ruthenium(III) Polypyridyl Complexes on Metal Oxide Surfaces. ACS Applied Materials & Interfaces 2015, 7, 9554-9562. (27) Hanson, K.; Losego, M. D.; Kalanyan, B.; Ashford, D. L.; Parsons, G. N.; Meyer, T. J., Stabilization of [Ru(bpy)2(4,4′-(PO3H2)bpy)]2+ on Mesoporous TiO2 with Atomic Layer Deposition of Al2O3. Chemistry of Materials 2013, 25, 3-5. (28) Hanson, K.; Losego, M. D.; Kalanyan, B.; Parsons, G. N.; Meyer, T. J., Stabilizing Small Molecules on Metal Oxide Surfaces Using Atomic Layer Deposition. Nano Letters 2013, 13, 4802-4809. (29) Brown, D. G.; Schauer, P. A.; Borau-Garcia, J.; Fancy, B. R.; Berlinguette, C. P., Stabilization of Ruthenium Sensitizers to TiO2 Surfaces through Cooperative Anchoring Groups. Journal of the American Chemical Society 2013, 135, 1692-1695. (30) Chen, Z.; Concepcion, J. J.; Hu, X.; Yang, W.; Hoertz, P. G.; Meyer, T. J., Concerted O atom–proton transfer in the O—O bond forming step in water oxidation. Proceedings of the National Academy of Sciences 2010, 107, 7225-7229. (31) Vannucci, A. K.; Alibabaei, L.; Losego, M. D.; Concepcion, J. J.; Kalanyan, B.; Parsons, G. N.; Meyer, T. J., Crossing the divide between homogeneous and heterogeneous catalysis in water oxidation. Proceedings of the National Academy of Sciences 2013, 110, 20918-20922. (32) Son, H.-J.; Prasittichai, C.; Mondloch, J. E.; Luo, L.; Wu, J.; Kim, D. W.; Farha, O. K.; Hupp, J. T., Dye Stabilization and Enhanced Photoelectrode Wettability in Water-Based DyeSensitized Solar Cells through Post-assembly Atomic Layer Deposition of TiO2. Journal of the American Chemical Society 2013, 135, 11529-11532. (33) Eberhart, M. S.; Wee, K.-R.; Marquard, S.; Skinner, K.; Nayak, A.; Meyer, T. J., Fluoropolymer-Stabilized Chromophore–Catalyst Assemblies in Aqueous Buffer Solutions for Water-Oxidation Catalysis. ChemSusChem 2017, 10, 2380-2384. (34) Lapides, A. M.; Sherman, B. D.; Brennaman, M. K.; Dares, C. J.; Skinner, K. R.; Templeton, J. L.; Meyer, T. J., Synthesis, characterization, and water oxidation by a molecular chromophore-catalyst assembly prepared by atomic layer deposition. The "mummy" strategy. Chemical Science 2015, 6, 6398-6406. (35) Wee, K.-R.; Brennaman, M. K.; Alibabaei, L.; Farnum, B. H.; Sherman, B.; Lapides, A. M.; Meyer, T. J., Stabilization of Ruthenium(II) Polypyridyl Chromophores on Nanoparticle Metal-Oxide Electrodes in Water by Hydrophobic PMMA Overlayers. Journal of the American Chemical Society 2014, 136, 13514-13517. (36) Lapides, A. M.; Ashford, D. L.; Hanson, K.; Torelli, D. A.; Templeton, J. L.; Meyer, T. J., Stabilization of a Ruthenium(II) Polypyridyl Dye on Nanocrystalline TiO2 by an Electropolymerized Overlayer. Journal of the American Chemical Society 2013, 135, 1545015458. (37) Ashford, D. L.; Lapides, A. M.; Vannucci, A. K.; Hanson, K.; Torelli, D. A.; Harrison, D. P.; Templeton, J. L.; Meyer, T. J., Water Oxidation by an Electropolymerized Catalyst on Derivatized Mesoporous Metal Oxide Electrodes. Journal of the American Chemical Society 2014, 136, 6578-6581. (38) Ashford, D. L.; Sherman, B. D.; Binstead, R. A.; Templeton, J. L.; Meyer, T. J., Electroassembly of a Chromophore–Catalyst Bilayer for Water Oxidation and Photocatalytic Water Splitting. Angewandte Chemie International Edition 2015, 54, 4778-4781. (39) Wu, L.; Glebe, U.; Böker, A., Surface-initiated controlled radical polymerizations from silica nanoparticles, gold nanocrystals, and bionanoparticles. Polymer Chemistry 2015, 6, 51435184.


Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(40) Hirata, T.; Kashiwagi, T.; Brown, J. E., Thermal and oxidative degradation of poly(methyl methacrylate): weight loss. Macromolecules 1985, 18, 1410-1418. (41) Brewster, T. P.; Konezny, S. J.; Sheehan, S. W.; Martini, L. A.; Schmuttenmaer, C. A.; Batista, V. S.; Crabtree, R. H., Hydroxamate Anchors for Improved Photoconversion in DyeSensitized Solar Cells. Inorganic Chemistry 2013, 52, 6752-6764. (42) Brennan, B. J.; Llansola Portoles, M. J.; Liddell, P. A.; Moore, T. A.; Moore, A. L.; Gust, D., Comparison of silatrane, phosphonic acid, and carboxylic acid functional groups for attachment of porphyrin sensitizers to TiO2 in photoelectrochemical cells. Physical Chemistry Chemical Physics 2013, 15, 16605-16614. (43) Materna, K. L.; Brennan, B. J.; Brudvig, G. W., Silatranes for binding inorganic complexes to metal oxide surfaces. Dalton Transactions 2015, 44, 20312-20315. (44) Haensch, C.; Hoeppener, S.; Schubert, U. S., Chemical modification of self-assembled silane based monolayers by surface reactions. Chemical Society Reviews 2010, 39, 2323-2334. (45) Prakash, S.; Long, T. M.; Selby, J. C.; Moore, J. S.; Shannon, M. A., “Click” Modification of Silica Surfaces and Glass Microfluidic Channels. Analytical Chemistry 2007, 79, 1661-1667. (46) Haensch, C.; Chiper, M.; Ulbricht, C.; Winter, A.; Hoeppener, S.; Schubert, U. S., Reversible Supramolecular Functionalization of Surfaces: Terpyridine Ligands as Versatile Building Blocks for Noncovalent Architectures. Langmuir 2008, 24, 12981-12985. (47) Kolb, H. C.; Finn, M. G.; Sharpless, K. B., Click Chemistry: Diverse Chemical Function from a Few Good Reactions. Angewandte Chemie International Edition 2001, 40, 2004-2021. (48) Wang, Q.; Chan, T. R.; Hilgraf, R.; Fokin, V. V.; Sharpless, K. B.; Finn, M. G., Bioconjugation by Copper(I)-Catalyzed Azide-Alkyne [3 + 2] Cycloaddition. Journal of the American Chemical Society 2003, 125, 3192-3193. (49) Iha, R. K.; Wooley, K. L.; Nyström, A. M.; Burke, D. J.; Kade, M. J.; Hawker, C. J., Applications of Orthogonal “Click” Chemistries in the Synthesis of Functional Soft Materials. Chemical Reviews 2009, 109, 5620-5686. (50) Paoprasert, P.; Spalenka, J. W.; Peterson, D. L.; Ruther, R. E.; Hamers, R. J.; Evans, P. G.; Gopalan, P., Grafting of poly(3-hexylthiophene) brushes on oxides using click chemistry. Journal of Materials Chemistry 2010, 20, 2651-2658. (51) Wu, S.; Tan, S. Y.; Ang, C. Y.; Nguyen, K. T.; Li, M.; Zhao, Y., An imine-based approach to prepare amine-functionalized Janus gold nanoparticles. Chemical Communications 2015, 51, 11622-11625. (52) Wu, L.; Glebe, U.; Böker, A., Synthesis of Polystyrene and Poly(4-vinylpyridine) Mixed Grafted Silica Nanoparticles via a Combination of ATRP and CuI-Catalyzed Azide-Alkyne Click Chemistry. Macromolecular Rapid Communications 2017, 38, 1600475-n/a. (53) Norris, M. R.; Concepcion, J. J.; Glasson, C. R. K.; Fang, Z.; Lapides, A. M.; Ashford, D. L.; Templeton, J. L.; Meyer, T. J., Synthesis of Phosphonic Acid Derivatized Bipyridine Ligands and Their Ruthenium Complexes. Inorganic Chemistry 2013, 52, 12492-12501. (54) Ruther, R. E.; Rigsby, M. L.; Gerken, J. B.; Hogendoorn, S. R.; Landis, E. C.; Stahl, S. S.; Hamers, R. J., Highly Stable Redox-Active Molecular Layers by Covalent Grafting to Conductive Diamond. Journal of the American Chemical Society 2011, 133, 5692-5694. (55) Wu, L.; Glebe, U.; Böker, A., Fabrication of Thermoresponsive Plasmonic Core–Satellite Nanoassemblies with a Tunable Stoichiometry via Surface-Initiated Reversible Addition– Fragmentation Chain Transfer Polymerization from Silica Nanoparticles. Advanced Materials Interfaces 2017, 4, 1700092-1700102.



Page 26 of 27

(56) Zhong, Y.-W.; Yao, C.-J.; Nie, H.-J., Electropolymerized films of vinyl-substituted polypyridine complexes: Synthesis, characterization, and applications. Coordination Chemistry Reviews 2013, 257, 1357-1372. (57) Bard, A. J.; Faulkner, L. R., Electrochemical methods :fundamentals and applications. 2nd ed. New York, xxi, 833 s, 2001. ed.; John Wiley & Sons: 2001. (58) Eberhart, M. S.; Sampaio, R. N.; Marquard, S. L.; Shan, B.; Brennaman, M. K.; Meyer, G. J.; Dares, C.; Meyer, T. J., Water Photo-oxidation Initiated by Surface-Bound Organic Chromophores. Journal of the American Chemical Society 2017, 139, 16248-16255.


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Stable Molecular Surface Modification of Nanostructured, Mesoporous

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