Efficient Light-Driven Oxidation of Alcohols Using an Organic

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Efficient Light-Driven Oxidation of Alcohols Using an Organic Chromophore−Catalyst Assembly Anchored to TiO2 Toan V. Pho,†,∥ Matthew V. Sheridan,‡,∥ Zachary A. Morseth,‡ Benjamin D. Sherman,‡ Thomas J. Meyer,‡ John M. Papanikolas,‡ Kirk S. Schanze,*,§ and John R. Reynolds*,† †

School of Chemistry & Biochemistry, School of Materials Science & Engineering, Center for Organic Photonics and Electronics, Georgia Tech Polymer Network, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡ Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States § Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida 32611, United States S Supporting Information *

ABSTRACT: The ligand 5-PO3H2-2,2′:5′,2″-terthiophene-5-trpy, T3 (trpy = 2,2′:6′,2″terpyridine), was prepared and studied in aqueous solutions along with its metal complex assembly [Ru(T3)(bpy)(OH2)]2+ (T3-Ru-OH2, bpy = 2,2′-bipyridine). T3 contains a phosphonic acid group for anchoring to a TiO2 photoanode under aqueous conditions, a terthiophene fragment for light absorption and electron injection into TiO2, and a terminal trpy ligand for the construction of assemblies comprising a molecular oxidation catalyst. At a TiO2 photoanode, T3 displays efficient injection at pH 4.35 as evidenced by the high photocurrents (∼350 uA/cm2) arising from hydroquinone oxidation. Addition of [Ru(bpy)(OTf)][OTf]2 (bpy = 2,2′-bipyridine, OTf− = triflate) to T3 at the free trpy ligand forms the molecular assembly, T3-Ru-OH2, with the oxidative catalyst fragment: [Ru(trpy)(bpy)(OH2)]2+. The new assembly, T3-Ru-OH2, was used to perform efficient light-driven oxidation of phenol (230 μA/cm2) and benzyl alcohol (25 μA/cm2) in a dyesensitized photoelectrosynthesis cell. KEYWORDS: C−H activation, organic alcohol oxidation, dye-sensitized solar cell, dye-sensitized photoelectrosynthesis cell, solar fuels

1. INTRODUCTION Donor−π−acceptor (D-π-A) organic molecules are one of the promising classes of light-harvesting moieties used in dyesensitized solar cells (DSSCs).1,2 In addition to their use in prototypical DSSC applications in nonaqueous solvents such as acetonitrile (MeCN), they have been used with success in the emerging field of aqueous-based DSSCs.3−5 For example, the organic dye, V35, with hydrophilic glycolic side chains has achieved a 3% power conversion efficiency in a 100% waterbased electrolyte DSSC with the I−/I3− couple.5 D-π-A dyes have also been used with success in aqueous media to perform water-splitting reactions producing H2 from H+ in dye/TiO2/Pt hybrid systems with ethylenediamine tetraacetic acid (EDTA) as a sacrificial electron donor.6−8 Another promising application of these D-π-A molecules is in dye-sensitized photoelectrosynthesis cells (DSPECs), where assemblies for water oxidation and CO2 reduction act in concert to generate molecules with stored chemical potential such as methanol or hydrocarbons.9−13 An important difference in a DSPEC relative to a DSSC is the necessity for a catalyst to perform the more difficult oxidation reactions such as water oxidation or C−H activation.14 Furthermore, because these catalysts tend to be poor light absorbers, they are often paired with ancillary chromophores, which act as effective photosensitizers as well as redox mediators (Scheme 1).11,15 Here, © XXXX American Chemical Society

the donor portion of the D-π-A organic dye is replaced with a ruthenium oxidation catalyst to form the chromophore− catalyst assembly. Scheme 1. Chromophore−Catalyst Assembly for Water Oxidation in DSPECs

Received: January 23, 2016 Accepted: March 16, 2016

A

DOI: 10.1021/acsami.6b00932 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

chromophores. First, UV−visible and DFT characterization of the assemblies demonstrated that the new chromophore− catalyst assemblies (“catalyst-π-A”) mimic spectral properties present in D-π-A organic dyes: namely, the presence of an intense charge-transfer (CT) band from the catalyst to the terthiophene ligand across the phenyl linker. Compared to metal-based chromophores which are typically limited to pHs below 7,22 using an organic dye as the chromophore allowed the characterization of T3 and the assemblies at significantly higher pHs up to 14, an effect attributed to the intermolecular hydrophobicity of the organic dye. Lastly, photocurrent measurements for T3 in aqueous solutions with a sacrificial reductant present demonstrated efficient electron injection, and the T3-Ru-OH2 assembly was shown to perform oxidation reactions of benzyl alcohol and phenol when integrated into a DSPEC device.23−26 In light-driven phenol and benzyl alcohol oxidation experiments, sustained photocurrents of 230 and 25 μA/cm2 were achieved, respectively, under 1 sun illumination (AM1.5 100 mW·cm−2) at pH 4.35 at a TiO2-T3-Ru-OH2 photoanode.

In DSPECs, transition-metal complexes have been the predominant chromophore class of choice.9,14 While Ru polypyridyl complexes are ubiquitous sensitizers, as they have reversible anodic oxidations at their RuIII/II couples in aqueous media, they suffer from the inherently low absorption coefficients of chromophores with metal-to-ligand chargetransfer (MLCT) transitions. In this respect, the utilization of organic chromophores with higher extinction coefficients and broader spectral coverage represents an attractive alternative.12,16 Furthermore, through synthetic modification, the energy levels of these chromophores can be appropriately aligned with those of the catalyst. A few recent examples of this approach come from the use of porphyrin and perylene dyes driving water oxidation interfaced with iridium oxide as the water oxidation catalyst (WOC),17,18 and a phosphonic acidderivatized D-π-A organic dye with a ruthenium molecular WOC.19 Herein, we present the development of a molecular oxidation catalyst assembly containing an organic terthiophene chromophore, where the chromophore is directly attached to the catalyst via a covalent bridge for use in a DSPEC.10,11,15 While previous works have studied the coadsorption of the chromophore and catalyst onto TiO2,11 our approach couples the two components via a covalent bridge.10,15 The covalent attachment allows for more intimate contact between the chromophore and catalyst, with the goal of facilitating electron transfer between the two units. In the current design, the donor moiety in the D-π-A dye was replaced with a molecular catalyst center. The T3 ligand (Scheme 2) consists of a terthiophene-π

2. RESULTS AND DISCUSSION Synthesis of T3 and T3-Ru-L. The synthesis of the terthiophene chromophore−catalyst assembly is outlined in Scheme 3. An asymmetric Stille coupling of diethyl (5(trimethylstannyl)thiophen-2-yl)phosphonate27 with a stoichiometric excess of 5,5′-dibromo-2,2′-bithiophene afforded the terthiophene 2. A subsequent Stille coupling of 2 with 4′-(4(tributylstannyl)phenyl)-2,2′:6′,2″-terpyridine28 then provided the terpyridine-terthiophene 3. Treatment with Me3SiBr afforded the T3 chromophore with a phosphonic acid anchoring group. Ligand exchange of 3 with [Ru(bpy)(η6-Bz)(Cl)]Cl29 under microwave heating gave the Ru complex 4. Deprotection of the phosphonate ester groups was achieved via treatment with Me3SiBr. Finally, deprotection of the chloride ligand on the ruthenium center was done via reaction with HOTf in DCM. The chromophore/catalyst assembly T3-Ru-L was isolated from the reaction as the triflate complex T3-Ru-OTf, and dissolution in acetonitrile (MeCN) provided the acetonitrile complex T3-Ru-MeCN. Electrochemistry and Optical Properties. The electrochemistry of the complexes was pursued in order to establish the proper alignment of redox potentials between the chromophore and catalyst. A summary of the electrochemical properties of the complexes in acetonitrile and pH 1 aqueous solutions is reported in Table 1 (Figure S1). In the assembly T3-Ru-OH2 at pH 1, the RuIII/II and T3+/0 couples can be clearly distinguished by differential pulse voltammetry (DPV) matching closely to their respective model complexes (Table 1). The E1/2 potential for T3+/0, 1.08 V vs Ag/AgCl, is correctly positioned and more positive than the RuIII/II couple of the catalyst at 0.73 V vs Ag/AgCl. The T3+ potential is, likely, sufficiently positive to oxidize the closely spaced RuIV/III couple as well. The kinetically slow RuIV/III wave of the assembly was, tentatively, observed at 0.87 V vs Ag/AgCl (Figure S2).30 The ground-state absorption spectra for T3 and T3-RuMeCN are shown in Figure 1. For T3 dissolved in dimethyl sulfoxide (DMSO), the spectrum consists of several weak transitions in the UV region along with an intense visible absorption centered at 410 nm. From density functional theory (DFT) calculations at the B3LYP level (Supporting Information), this transition can be ascribed to a π → π* transition

Scheme 2. Formation of a Molecular Chromophore− Catalyst Assembly with the T3 Chromophore Support on TiO2

system for its strong light absorption and high oxidation potential to drive reactions at the catalyst,20,21 a phosphonic acid anchoring group that also serves as an acceptor, and a terpyridine (trpy) unit to incorporate the catalyst. The trpy ligand is a powerful unit for incorporating a wide variety of ruthenium catalysts to perform either water oxidation or C−H activation. In this report, we describe the preparation and characterization of T3 and the chromophore−catalyst assemblies formed between T3 with [Ru(bpy)(L)]2+ (T3-Ru-L, L = triflate, MeCN, or OH2). Several unique properties associated with the covalent organic chromophore−catalyst assemblies arise when compared to coloaded assemblies or the use of metal-based B


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ACS Applied Materials & Interfaces Scheme 3. Synthetic Strategy for T3 and T3-Ru-L Assembly

transition on the catalyst centered at 476 nm is enhanced and may follow from overlapping metal-to-ligand charge-transfer (MLCT) and intraligand charge-transfer (ILCT) transitions. This observation has been noted previously31,32 for similar Ru(II) complexes with electron-donating substituted trpy ligands and has been attributed to the larger dipole moment for the MLCT transition as the electron density is shifted from the metal center to the trpy ligand substituted with the T3 unit. DFT calculations at the B3LYP level for the T3-Ru-MeCN complex reveal that the HOMO resides primarily on the T3 unit while the LUMO is localized on the trpy moiety while the HOMO-1 and HOMO-2 reside on the Ru(II) center. TD-DFT calculations at the PBE0 level were performed to assess the degree of electronic communication between the chromophore and catalyst within the assembly and assign the electronic transitions within the assembly. The transition at 383 nm is predicted to be primarily the π → π* transition on the terthiophene unit (HOMO → LUMO+3), while the low energy transition at 490 nm contains multiple configurations. The calculations predict that this transition is HOMO to

Table 1. Summary of the Electrochemical Properties in Solution

a

E1/2 (V vs Fc/Fc+)a

E1/2 (V vs Ag/AgCl)b

complex

RuIII/II

RuIII/II

T3 Ru-OH2 Ru-MeCN T3-Ru-OH2

0.41 0.91 0.43

T3+/0 0.69

T3+/0 1.15

0.77 0.70

0.73

1.08

In MeCN; 0.1 M (n-Bu)4NPF6. bAt pH 1; 0.1 M HClO4.

dominated by the HOMO → LUMO configuration, where the HOMO and LUMO are delocalized across the T3 unit. The absorption spectrum of the T3-Ru-MeCN assembly dissolved in acetonitrile (MeCN) is qualitatively different from that of the model chromophore (T3) and “catalyst” [Ru(trpy)(bpy)(MeCN)]2+ units. In the electronic absorption spectrum, the π → π* transition of the terthiophene chromophore is blueshifted to 383 nm and the oscillator strength of the MLCT C


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exchange at the ruthenium center. Shifts in the MLCT or CT absorption are expected depending on the protonation state of the aquo ligand: [Ru(trpy)(bpy)(OH2/OH)]2+/1+ (note the pKa of 1 is ca. 9.7).33 In the model complex, 1, the MLCT band shifts from 481 nm at pH 1 to 494 nm at pH 14 (Figure 2).34 The red shift is associated with the formation of the stronger σ electron-donating ligand, hydroxide, by deprotonation of the aquo ligand at pH 14.

Figure 2. UV−vis absorption spectra of 0.05 mM [Ru(trpy)(bpy)(OH2)]2+ (solid lines) and TiO2-T3-Ru-OH2 (dashed lines) at pH 1 (gray) and 14 (black).

Figure 1. (Top) Electronic absorption spectra for T3 in DMSO (left) and T3-Ru-MeCN in MeCN (right). The black bars correspond to the electronic transitions predicted from TD-DFT calculations. (Bottom) Frontier orbital energies for T3 (blue) and T3-Ru-MeCN (green) obtained from DFT calculations. The predominant orbitals involved in the computed absorbance spectrum are shown. Solvent effects were included for all calculations with a polarizable continuum model, as implemented in Gaussian 09.

At pH 14, the UV−visible spectrum of a TiO2-T3-Ru-OH2 film shows a similar shift in MLCT/CT absorption compared to the absorption of a film treated with a pH 1 solution (dashed lines, Figure 2). For the assembly, a shift of 514 to 534 nm is observed as the pH is changed from 1 to 14. These results confirm that exposure of the complex to the aqueous solution gives rise to displacement of the OTf− ligand to form the aquo (OH2) complex. Interestingly, the opposite spectral shift is observed for TiO2-T3 films treated with pH 1 and 14 solutions (Figure S6). This effect is likely due to protonation of the terpyridine unit at the low pH. T3-Ru-MeCN films on TiO2 show a negligible shift in the CT band at pH 1 and 14 (Figure S7). Interestingly, the T3 ligand and assemblies were found to be stable indefinitely at pH 14 (e.g., monitoring the TiO2-T3-RuOH CT peak showed no loss in absorbance after prolonged exposure to a pH 14 solution, > 48 h). This allowed for the observation of the CT spectrum of the deprotonated aquo ligand: [Ru(trpy)(bpy)(OH]1+. It is suspected that intermolecular forces between neighboring terthiophene units create a substantial hydrophobic barrier to base attack at the phosphonic acid anchoring groups.5 Light-Driven Hydroquinone Oxidation by TiO2-T3. To assess the T3 chromophore as a competent photosensitizer dye in aqueous solution, light-driven hydroquinone oxidation experiments were carried out. T3 was adsorbed onto a nanoTiO2 semiconductor electrode (0.5 mM T3 in DMSO for 24 h) and the photoelectrochemistry was conducted in pH 4.35, 0.2 M acetate buffer, 0.5 M NaClO4 electrolyte solutions containing the sacrificial electron donor, hydroquinone, H2Q (eq 1). The E1/2 for the oxidation of hydroquinone is E1/2 = 0.20 V vs SCE at pH 4.35.35 This is well below the anodic peak potential determined for T3 absorbed on planar indium-tin oxide (ITO) in the same media, Ep = 1.03 V vs SCE (Figure S8), and the resulting couple formed upon continued anodic scanning, E1/2 = 0.91 V vs SCE (Figure S9).36,37 At the TiO2-T3 photoanode, photocurrent arises from the thermodynamically favorable oxidation of H2Q to semiquinone,

LUMO+3 in character, corresponding to a π → π* transition along the T3 unit, but also contains several MLCT transitions and an intraligand transition in which electron density shifts from the T3 unit to the trpy ligand (i.e., T3+-trpy•−), consistent with an ILCT transition. Although the electronic coupling between the T3 chromophore and Ru-MeCN catalyst serves to enhance the light absorption properties of the assembly, the presence of the ILCT transition may serve to direct excitation energy toward the catalyst, introducing competition between charge separation processes at the metal−oxide interface. In this way, the absorbance at 490 nm may mimic the chargetransfer (CT) band associated with many D-π-A dyes.21 In fact, incident photon-to-current efficiency plots (IPCE) for mesoporous TiO2 films loaded with T3-Ru-MeCN or T3-RuOH2 reveal injection at wavelengths ca. 500 nm related to this latter process in the assemblies (Figure S3).

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XPS AND PH TITRATION OF T3-RU-OTF The T3-Ru-OTf assembly was characterized on nano-TiO2 substrates (the complex was loaded by immersion of substrates for 24 h in a 0.5 mM methanol solution of the assembly). X-ray photoelectron spectroscopy (XPS) was performed on TiO2 samples of T3 and T3-Ru-OTf to confirm the surface structure of the T3-Ru-OTf complex (Figures S4 and S5). To assess formation of the aqua complex, TiO2-T3-Ru-OH2, after loss of the triflate ligand in aqueous solutions, a TiO2-T3-Ru-OTf slide was exposed to pH 1 and 14 aqueous solutions. Monitoring the CT peak by UV−visible absorption spectroscopy provides evidence for the OTf → OH2 ligand substitution reaction. Similar to MLCT transitions, shifts in the assembly’s CT band are expected to be influenced by ligand D


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ACS Applied Materials & Interfaces HQ●+, then to quinone, Q, by two sequential T3●+ radical cation reductions. T3●+ is formed after electron injection by the chromophore’s excited state, T3*, into the conduction band of TiO2.34 This process is summarized in eqs 2−5. A surfacebound thiophene derivative, poly(3-methylthiophene), has been previously shown to be an effective electrochemical mediator of the H2Q/Q couple.38 H 2Q → Q + 2H+ + 2e−

(1)

hv

2x[Ti(IV)O2 ‐T3 → Ti(III)O2 ‐T3•+] e− injection

(2)

Ti(III)O2 ‐T3•+ + H 2Q → Ti(III)O2 ‐T3 + HQ• + H+ (3) •+

Ti(III)O2 ‐T3

•

+ HQ → Ti(III)O2 ‐T3 + Q + H

+

Figure 4. IPCE of TiO2-T3 obtained in pH 4.35, 0.2 M acetate buffer, 0.5 M NaClO4 with 20 mM H2Q present, Eappl = 0.2 V (black), and UV−vis spectrum of TiO2-T3 (gray).

(4)

2x[Ti(III)O2 ‐T3 → Ti(IV)O2 ‐T3 + e−] current

(5)

The result of illuminating TiO2-T3 on nano-TiO2 in the absence and presence of hydroquinone with an applied bias, Eapp, of 0.2 V vs SCE is shown in Figure 3. The effect of adding

for the shape of the IPCE and the observed spectrum for TiO2T3. Nano-TiO2 slides loaded with T3 or [Ru(4,4′-PO3H2bpy)(bpy)2]2+ (RuP2+), a common aqueous metal−chromophore sensitizer,24,25,39 were tested against each other for lightdriven H2Q oxidation to compare the new T3 dye to the wellestablished aqueous RuP2+ chromophore (Figure S10). The photocurrents from the T3 modified electrodes were ca. 50% of the photocurrents measured using RuP2+. The higher sustained photocurrents at RuP2+ are likely due to the broader absorbance of RuP2+ over the visible light spectrum. A large portion of T3’s maximum absorption is cutoff by the optical long-pass filter ( 400 nm.

H2Q to the solution leads to a substantial increase in the photocurrent from ca. 24 to 350 μA/cm2 (dotted gray and black traces in Figure 3, respectively). Along with the increase in photocurrent, the photocurrent is sustained in the case of H2Q present, whereas the current drops continually in its absence. These results are consistent with photocurrent generated from the oxidation of H2Q to Q by T3●+. The role of T3 as the sensitizer is confirmed by the contrast of photocurrents obtained at a bare TiO2 slide in the presence of H2Q (gray line, Figure 3). Further confirmation of T3 as the sensitizer was obtained from the incident photocurrent experiment (IPCE) for the same electrode (black line, Figure 4) shown alongside the corresponding UV−visible absorbance spectra of the TiO2-T3 film (gray line, Figure 4). The results are in excellent agreement E


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ACS Applied Materials & Interfaces the pendant catalyst. Complex 1 is well-known for oxidation reactions at a variety of organic substrates and continues to be one of the most widely used catalysts for organic oxidation reactions.43,47,48 Recent work has shown that photochemical oxidations of organic substrates by 1 and other ruthenium oxidation catalysts are possible with an appropriate photosensitizer present such as [Ru(bpy) 3 ]2+,58−64 and the construction of a DSPEC device has been accomplished with a Ru catalyst and chromophore adsorbed onto a TiO2 photoanodeachieving light-driven dehydrogenation of benzyl alcohol and H2 production at the cathode with an applied bias.65 The T3-Ru-OTf assembly was adsorbed onto thin films of TiO2 on FTO slides, followed by solvent−ligand exchange in the aqueous solution to produce the [Ru(trpy)(bpy)(OH2)]2+ catalyst species. Light-driven organic oxidation reactions of phenol and benzyl alcohol driven by this new assembly, T3-RuOH2, were performed on the first target substrate phenol (PhOH) as it has been previously shown to be highly reactive with catalyst 1. The oxidation of PhOH proceeded through the RuIVO state of 1, leading to the formation of substituted quinones.52 Increasing the PhOH concentration results in an increase in photocurrent generated at a TiO2-T3-Ru-OH2 modified electrode in a pH 4.35 solution under 1 sun illumination, as shown in Figure 5. This is consistent with

Figure 6. Linear sweep voltammograms (LSVs) of TiO2-T3 (dotted gray), TiO2-T3-Ru-MeCN (gray), and TiO2-T3-Ru-OH2 (black) in the presence of 50 mM phenol in pH 4.35, 0.2 M acetate buffer, and 0.5 M NaClO4; ν = 50 mV/s. Illumination with 100 mW·cm−2 visible light, λ > 400 nm.

respectively, in acetonitrile) and prevents bond-forming reactions at the Ru−aquo complex. The T3-Ru-OH2 assembly containing the molecular oxidation catalyst moiety has enhanced photocurrent, establishing the fact that catalytic oxidation of PhOH is occurring at the ruthenium center assembly, likely: TiO2-T3-Ru2+O. The decreased photocurrent at T3-Ru-MeCN versus T3-Ru-OH2 is observed in similar experiments with H2Q oxidation (Figure S3). Longterm photolysis (1 h) of a 10 mM phenol solution at pH 4.35 was performed at TiO2-T3-Ru-OH2. Of the two 4e− products possible for the oxidation of phenol, the principal product observed in solution was 1,2-benzoquinone as evidenced by CVs of the solution by a BDD electrode following photolysis (Figures S12 and S13). As complex 1 is a well-studied molecular catalyst for BnOH oxidation, with reaction via the catalyst occurring via the RuIIIOH or Ru IV O oxidation states, this substrate was subsequently investigated.23,66 The effect of adding BnOH to a pH 4.35 solution in contact with TiO2-T3-Ru-OH2 is shown in Figure 7, where the increased photocurrent is consistent with reactivity at the oxidized metal center. The lower photocurrent with BnOH, compared to PhOH, is a reflection of the slower

Figure 5. I−t curves at TiO2-T3-Ru-OH2 in pH 4.35, 0.2 M acetate buffer, and 0.5 M NaClO4 with increasing phenol concentration, 0−16 mM phenol (4 mM increments); Eappl = 0.2 V. Illumination with 100 mW·cm−2 visible light, λ > 400 nm.

oxidation of PhOH from the oxidized catalyst formed from T3+ according to the reactions described in eqs 6−8. hv

Ti(IV)O2 ‐T3‐Ru‐OH 2 → Ti(III)O2 ‐T3•+‐Ru‐OH 2

(6)

Ti(III)O2 ‐T3•+‐Ru‐OH 2 → Ti(III)O2 ‐T3‐Ru+‐OH + H+ (7)

Ti(III)O2 ‐T3 ‐Ru ‐OH → Ti(III)O2 ‐T3‐Ru O + H •+

+

2+

+

(8)

Comparison of the T3-Ru-OH2 assembly to the organic chromophore, T3, and T3-Ru-MeCN assembly on TiO2 is shown in Figure 6. The T3-Ru-MeCN assembly (prepared separately) has an acetonitrile ligand at the metal center blocking electron transfer to the ruthenium center (the E1/2’s of T3 and T3-Ru-MeCN are 0.69 and 0.81 V vs Fc/Fc+,

Figure 7. (Top) I−t curves of TiO2-T3-Ru-OH2 in pH 4.35, 0.2 M acetate buffer, and 0.5 M NaClO4 with (black) and without (gray) 0.1 M BnOH present; Eappl = 0.2 V. Illumination with 100 mW·cm−2 visible light, λ > 400 nm. F



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kinetics for BnOH oxidation by the Ru catalyst center.23,66 The rate constants for BnOH and PhOH oxidations in water by the related Ru(IV)−oxo complex, [Ru(bpy)2(py)(O)]2+ (py = pyridine), vary by several orders of magnitude from 2.43 to 5.6 × 102 M−1 s−1, respectively. In the BnOH solutions, the effect of adding 4-tertbutylpyridine again showed a positive effect, leading to more stable photocurrents over extended light illumination (Figure S14). The numerous beneficial effects of added pyridine highlight an important crossover in the additives often used in nonaqueous DSSCs and their potential importance in improving aqueous light-driven oxidation reactions in DSPEC devices.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.R.R.). *E-mail: [email protected]fl.edu (K.S.S.). Author Contributions ∥

T.V.P. and M.V.S. contributed equally. 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.

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ACKNOWLEDGMENTS This research was wholly 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-SC0001011. Also, we express our thanks to the CHANL staff, Dr. Amar Kumbhar and Dr. Carrie Donley, for their assistance in collecting XPS data.

3. CONCLUSION A novel chromophore−catalyst molecular assembly featuring a terthiophene-terpyridine chromophore ligand (T3) coordinated to a Ru(trpy)(bpy)OH22+ oxidation catalyst center (T3-Ru-OH2) has been synthesized and characterized in solution at a TiO2 interface. The complex features a broad and intense visible absorption with an onset at ∼600 nm that is assigned to a combination of intraligand and charge-transfer transitions. Electrochemistry of T3-Ru-OH2 reveals two distinct oxidation processes, one at low potential (∼0.4 V) arising from the Ru(II/III) couple, and a second at higher potential (∼0.77 V) that is attributed to terthiophene localized oxidation. Photoelectrochemical studies have explored the properties of the terthiophene-terpyridine ligand (T3) and the complex T3Ru-OH2 adsorbed on mesoporous TiO2 films in aqueous buffer solutions. The free ligand T3 displays moderately efficient photocurrent, with peak IPCE ∼ 15%, and 0.35 mA·cm−2 when illuminated with 100 mW·cm−2 white light in the presence of hydroquinone reductant. Metal complex T3-Ru-OH2 displays moderate photocurrent when illuminated with white light in the presence of the organic reductants phenol and benzyl alcohol. The latter result demonstrates the capability of the Ru(trpy)(bpy)OH2 center to catalyze oxidation of organic reductants where hydrogen atom abstraction may be involved in the activation process. The mechanism of the photoelectrochemical processes displayed by T3-Ru-OH2 at the TiO2 interface are currently being explored by using time-resolved laser spectroscopy, and these results will be reported in an upcoming paper. This work provides a new approach to the development of catalytically active chromophores that can be used at metal− oxide interfaces to effect light-driven photochemical oxidation processes that are central to the development of the dyesensitized photoelectrosynthesis cell. Although the Ru(trpy)(bpy)OH2 center is useful as a model to demonstrate the capability of catalyzing complex multielectron oxidations, its turnover rate is modest compared to single site catalysts that have been recently developed.67−69 Future work will explore using a similar approach of using a covalently linked πconjugated chromophore to enhance the light-harvesting properties of high-efficiency water oxidation catalyst centers.70

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00932. Details of the analytical techniques, preparations, and characterization data (PDF) G


Research Article

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Efficient Light-Driven Oxidation of Alcohols Using an Organic

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