Light-Driven Hydrogen Generation from Microemulsions Using

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Light-Driven Hydrogen Generation from Microemulsions Using Metallosurfactant Catalysts and Oxalic Acid Husain N. Kagalwala, Danielle N. Chirdon, Isaac N. Mills, Nikita Budwal, and Stefan Bernhard* Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States

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S Supporting Information *

ABSTRACT: A unique microemulsion-based photocatalytic water reduction system is demonstrated. Iridium- and rhodium-based metallosurfactants, namely, [Ir(ppy) 2 (dhpdbpy)]Cl and [Rh(dhpdbpy)2Cl2]Cl (where ppy = 2phenylpyridine and dhpdbpy = 4,4′-diheptadecyl-2,2′-bipyridine), were employed as photosensitizer and proton reducing catalyst, respectively, along with oxalic acid as a sacrificial reductant in a toluene/water biphasic mixture. The addition of 1-octylamine is proposed to initiate the reaction, by coupling with oxalic acid to form an ion pair, which acts as an additional surfactant. Concentration optimizations yielded high activity for both the photosensitizer (240 turnovers, turnover frequency (TOF) = 200 h−1) and catalyst (400 turnovers, TOF = 230 h−1), with the system generating hydrogen even after 95 h. Mechanistic insights were provided by gas-phase Raman, electrochemical, and luminescence quenching analysis, suggesting oxidative quenching to be the principle reaction pathway.

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INTRODUCTION The notion of using solar fuels for fulfilling day-to-day electricity and transportation needs has gained immense attention, considering the rapidly declining stocks of conventional fuels and their negative environmental effects.1,2 This “artificial photosynthesis” encompasses many fuel production schemes, including CO2 reduction3−6 and dihydrogen (H2) evolution from various substrates like alcohols,7−9 haloacids,10−12 and water.13 Among all solar fuel schemes, photocatalytic splitting of water is highly popular due to its utilization of an abundant substrate to produce high specific energy hydrogen fuel. Conducting water splitting in homogeneous solution is attractive (Scheme 1A) but involves such a complex mechanism and high kinetic barriers that current work commonly breaks the process into two separate half reactions.14 Traditionally, the H2-evolving half-reaction, or water reduction, has been light-driven by means of a photosensitizer (PS) and a water (or proton) reducing catalyst (WRC), with a sacrificial reductant (SR) substituting the oxidative half (Scheme 1B). Efforts to maximize the output of photocatalytic H 2 generation have led to an exhaustive exploration of PSs,15−19 WRCs,20−23 and to a smaller extent SRs.24−26 Examination of new constituents for H2 production also requires a similar scrutiny of solvents, since solvents play a crucial role in determining the interactions between the constituents and their overall stability.27−29 Both aqueous and organic solvent-based catalytic systems have been reported so far, with the latter being used more prominently. Using a biphasic solvent system, however, would be of particular interest. Concurrently utilizing solvents of opposing polarity can tremendously broaden the © 2017 American Chemical Society

scope of applicable catalysts, electron donors, and substrates. Additionally, such a setup could potentially allow the integration of the H2 evolution reaction with the oxidation half-reaction, which is typically pursued in aqueous media. However, biphasic H2 generation has rarely been studied, and very few examples exist in literature.30,31 Ideally, biphasic catalysis could become possible via the utilization of amphiphilic molecules like surfactants and metallosurfactants. These molecules can encapsulate a wide range of polar and nonpolar molecules through their ability to form micelles or microemulsions. Seminal work by Grätzel and co-workers has previously established photoinduced electron transfer and charge separation in such microscopic entities, justifying their use in photochemical conversion schemes.32−36 Additionally, the presence of a metal center as the polar headgroup in metallosurfactants imparts fascinating photophysical and electrochemical properties, and as a result, such amphiphiles are beginning to find use in an extensive range of applications.37−42 Very recently, the Schmehl group reported the effects of micellar incorporation on photoinduced electron transfer between metallosurfactant chromophores and a [Ru(NH3)6]2+ donor.43 A drastic decrease in back-electron transfer rates was observed in the presence of cetyltrimethylammonium bromide (CTAB) as the micelle forming surfactant. Their study further supports the idea of amphiphile-based photocatalytic systems. Received: February 20, 2017 Published: May 10, 2017 10162

DOI: 10.1021/acs.inorgchem.7b00463 Inorg. Chem. 2017, 56, 10162−10171

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Inorganic Chemistry

Scheme 1. Mechanism of (A) Photocatalytic “Total Water Splitting” and (B) Proton/Water Reduction in the Presence of a Sacrificial Electron Donora

a

For simplicity, both schemes depict oxidative quenching mechanisms. WOC = water oxidation catalyst. tetrabutylammonium hexafluorophosphate (N(Bu)4PF6) were procured from Sigma-Aldrich. All deuterated solvents for NMR analysis were obtained from Cambridge Isotopes. All chemicals were used as received. A Bruker Avance spectrometer was used to record 1H NMR and 13C NMR at 300 and 75 MHz, respectively, at room temperature (RT). Positive ion electrospray ionization mass spectrometry (ESIMS) was performed using a Thermo-Fisher LCQ instrument equipped with an ESI ion source. Gas-phase Raman spectroscopy was performed employing a Raman spectrometer (Enwave Optronics, GasRamanNOCH-1, maximum output power = 500 mW, excitation wavelength = 532 nm). ProRaman XC1 V79XNT software program was used to record the Raman spectra. Synthesis. The ligand 4,4′-diheptadecyl-2,2′-bipyridine (dhpdbpy) and the rhodium metallosurfactant [Rh(dhpdbpy)2Cl2]Cl were synthesized using a modified literature procedure.48 1H NMR of [Rh(dhpdbpy)2Cl2]Cl (500 MHz, chloroform-d): δ 9.78 (d, 2H, J = 6.1 Hz), 8.60 (s, 2H), 8.49 (s, 2H), 7.68 (d, 2H, J = 5.8 Hz), 7.53 (d, 2H, J = 4.9 Hz), 7.33 (d, 2H, J = 6.1 Hz), 3.05 (m, 4H), 2.84 (m, 4H), 1.88 (m, 4H), 1.68 (m, 4H), 1.50 (m, 4H), 1.42 (m, 4H), 1.27 (m, 104H), 0.90 (t, 12H, J = 6.3 Hz). MS (ESI, MeOH): m/z calculated: 1438.04 (M+), found: 1438.0 (M+, 88%) (lowered abundance is believed to be due to protonated forms (see Figures S1 and S2). [Ir(ppy)2(dhpdbpy)]Cl: Performed by first synthesizing the dimer, [Ir(ppy)2-μ-Cl]2 using reported procedures,49,50 and then cleaving the dimer with the ancillary ligand using a modified literature procedure.42 Briefly, a three-necked round-bottom flask was charged with the dimer (200 mg, 0.187 mmol), to which was added an EtOH/CH2Cl2 mixture (1:3, 200 mL). The resulting solution was magnetically stirred, and the ancillary ligand 4,4′-diheptadecyl-2,2′-bipyridine (260 mg, 2.2 equiv) was added. The solution was then refluxed under Ar and in the dark for 12 h. After the reaction was brought to room temperature, the solvent was removed via rotary evaporation. The solids were dissolved in hot EtOH (∼70 mL), heated for a while, and then cooled to 0 °C. The solution was filtered through diatomaceous earth, which was then rinsed with additional EtOH. The filtrate was concentrated to dryness; the solids were redissolved in minimum quantity of hot EtOH and recrystallized. The bright yellow crystals were isolated via vacuum filtration, washed with a minimum amount of cold EtOH and water, and then dried overnight to give the complex in 75−80% yield: 1H NMR (300 MHz, acetonitrile-d3): δ 8.42 (s, 2H), 8.08 (d, 2H, J = 6.0 Hz), 7.83 (m, 6H), 7.60 (d, 2H, J = 6 Hz), 7.34 (dd, 2H, J = 5.5, 0.9 Hz), 7.05 (m, 4H), 6.93 (td, 2H, J = 7.2, 1 Hz), 6.30 (dd, 2H, J = 7.5, 1 Hz), 2.82 (t, 4H, J = 7.6 Hz), 1.72 (m, 4H), 1.23−1.39 (br, m, 56H), 0.9 (t, 6H, J = 7.0 Hz). MS (ESI): m/z calculated: 1133.71 (M+), found: 1133.8 (M+,100%) (see Figures S3 and S4). Photophysical Characterization. Photophysical experiments were performed using acetonitrile (CH3CN) solutions (10 μM, 2 mL) in a septa-equipped quartz cuvette, thoroughly purged with Ar for 10 min. UV−visible spectroscopy was performed using a Shimadzu UV-1800 double beam spectrophotometer. Emission spectra were obtained using a Fluorolog-3 spectrophotometer associated with dual monochromators and a photomultiplier tube at right-angle geometry. Samples were excited at 400 nm. To obtain the excited-state lifetimes,

In this paper we demonstrate an amalgamation of these concepts to achieve photocatalytic H2 evolution using a novel microemulsion system generated in a toluene/water mixture. The reaction was photosensitized by an amphiphilic iridium metallosurfactant, [Ir(ppy)2(dhpdbpy)]Cl (Figure 1A), while a

Figure 1. Molecular structures of (A) Ir and (B) Rh metallosurfactants used in this study.

rhodium-based amphiphile, [Rh(dhpdbpy)2Cl2]Cl (Figure 1B) was employed as the WRC (where ppy = 2-phenylpyridine and dhpdbpy = 4,4′-diheptadecyl-2,2′-bipyridine). Oxalic acid, which can act as a two-electron donor was chosen to be the sacrificial reductant.44−47 Occurring naturally, this molecule is attractive as a model substrate for inexpensive organics-tohydrogen schemes. Before the onset of catalysis, the metallosurfactants and oxalic acid were present in the organic and aqueous phases, respectively. The microemulsion is proposed to be initiated by the addition of 1-octylamine, which forms an in situ ion pair with oxalic acid. This ion pair acts as an additional surfactant, making interactions with the metallosurfactants (present in the organic layer) more likely. Optimization studies afforded 240 iridium and over 400 rhodium turnover numbers (TONs). Moreover, this photocatalytic system was found to produce H2 for ∼95 h, highlighting its stability. Further proof and insight into the mechanistic aspects were obtained through 13C NMR, electrochemistry, gas-phase Raman analysis, and luminescence quenching.

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EXPERIMENTAL SECTION

Materials and Methods. Iridium(III) chloride (IrCl3·4H2O) and rhodium(III) chloride (RhCl3·3H2O) were purchased from Pressure Chemicals. 2-Phenylpyridine was obtained from Alfa Aesar. HPLCgrade xylene and toluene were obtained from Fischer Scientific. Anisole and 1-octanol were purchased from TCI Chemicals. 1Octylamine, oxalic acid dihydrate (C2H2O4·2H2O), ferrocene, and 10163


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Figure 2. (A) Absorption (blue line) and emission (red dotted line) profile for [Ir(ppy)2(dhpdbpy)]Cl in CH3CN (10 μM). Spectra were not recorded in toluene due to intense solvent absorptions in the UV region. The inset shows the low intensity 3MLCT transition at 470 nm. (B) Comparison of calculated and experimental UV−visible spectra for [Ir(ppy)2(dhpdbpy)]Cl, using CH3CN as a solvent model. The oscillator strengths are denoted by the black lines. (C) Singlet energy-level diagram for [Ir(ppy)2(dhpdbpy)]Cl, depicting the frontier orbitals, along with some of the predicted transitions from regions I−III of the electronic absorption spectrum. the samples were pulsed using the fourth harmonic (266 nm) of a Nd:YAG (Continuum Minilite) laser and a digital oscilloscope (Tektronix TDS 3032B) was employed to measure the luminescence decay. The decay plots were converted to a linear regression via a LabView interface to get the lifetimes. Quantum yield was calculated using the equation φs = φref (Is/Iref) (Aref/As) (ηs/ηref) using a CH3CN solution (10 μM, 2 mL) of [Ru(bpy)3](PF6)2 as the reference (where φs = quantum yield of sample, φref = quantum yield of reference (6.2%), Is and Iref are the maximum emission intensities of sample and reference, respectively, Aref and As are the respective reference and sample absorbances at excitation wavelength, and ηs and ηref represent the refractive indices of the solvents used for the measurements). Computational Methods. Density functional theory (DFT) calculations were performed using a Gaussian 09 suite,51 with the B3LYP functional52−54 and LANL2DZ basis sets55−57 to predict ground-state geometries. No solvent or geometry constraints were applied. Time-dependent DFT calculations were performed on the optimized ground-state geometry, obtaining energies and oscillator strengths for the 150 lowest possible transitions. Simulations were performed with CH3CN as the solvent constraint. The prediction of the UV−visible spectra was done using GaussSum 2.0.58,59 Electrochemistry. Cyclic voltammetry was conducted via a CHInstruments electrochemical analyzer (model 600C) potentiostat, employing a 3 mm glassy carbon working electrode, silver wire pseudoreference electrode, and a platinum coiled wire counter electrode. All measurements were performed using dichloromethane (CH2Cl2) solutions, containing 1 mM analyte and 0.1 M N(Bu)4PF6 as supporting electrolyte. The potentials were referenced to the standard calomel electrode (SCE), using ferrocene as the internal standard.60 Photolysis Protocol. A typical photocatalytic experiment was performed as follows: stock solutions of [Ir(ppy)2(dhpdbpy)]Cl and [Rh(dhpdbpy)2Cl2]Cl were prepared using the respective organic solvent. Oxalic acid solutions were prepared in DI water. Aliquots (2 mL each) of the PS and catalyst solutions along with 4 mL of the oxalic acid solution were added to 40 mL screw-top EPA vials (Fischer Scientific) to give 1:1 organic/aqueous biphasic solutions. The specified amount of 1-octylamine was added to each of the reaction vials under dark conditions along with a magnetic stir bar. Pressure transducers were screwed on, and the solutions were subject to seven

cycles of vacuum degassing and backfilling with N2 to ensure complete deaeration. The samples were then illuminated in a 12-well, homebuilt cylindrical photoreactor with side-illumination (24 W 460 nm LED strip with 300 diodes, Solid Apollo SA-LS-BL-3528−300−24 V) under constant magnetic stirring. Gas evolution traces were obtained via a LabView PC interface. Postillumination, small quantities of the headspace were injected into a gas chromatograph (GOW-MAC, series 400 G/C, thermal conductivity detector, Ar carrier gas) to quantify H2. Photosensitizer Luminescence Quenching Studies. Following the protocol used for photophysical characterization, increasing concentrations of respective quencher were added to an Ar-purged solution (10 μM, 2 mL, either CH3CN or toluene) of the iridium PS, and the corresponding excited-state emission spectra (at 587 nm) or lifetimes were recorded. Excitation was done at 400 nm, and the solutions were degassed with Ar for 15 min before every measurement. Stern−Volmer plots were generated by plotting relative lifetimes/ emission intensities versus the quencher concentrations. Quenching of the PS excited state by the octylamine/oxalate ion pair was performed by extracting increasing concentrations of oxalic acid from water into an organic mixture containing the PS, octanol, octylamine, and toluene. Detailed procedures are provided in the Supporting Information.

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RESULTS AND DISCUSSION Ir Metallosurfactant: Photophysical and Computational Analysis. To determine its effectiveness as a photosensitizer, [Ir(ppy)2(dhpdbpy)]Cl was first subject to a photophysical evaluation. The observed photophysical characteristics are strongly comparable to iridium complexes having the general architecture [Ir(C^N)2(N^N)]+.61 Figure 2A displays its UV−visible absorption profile (blue line) measured in CH3CN. Strong absorption peaks in the 200−290 nm range can be assigned to ligand-centered (LC) π−π* transitions, while moderate features around 300−450 nm are due to mixed metal-to-ligand charge transfer (1MLCT) and interligand charge transfer (1ILCT) excitations. A low-intensity transition can also be seen around 470 nm (Figure 2A, inset), which 10164


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Figure 3. Solvent study. (A) Variation of organic solvent, with the final solution containing 4 mL of the respective organic solvent, 4 mL of water, and 1 mL of 1-octylamine. Toluene with 5% 1-octanol (v/v) was found to be the most optimal solvent. Oxalic acid (red bars) was observed to be a better SR than sodium oxalate (blue bars). (B) Variation of water content and its effect on performance. All vials contained 5% 1-octanol in toluene as the organic solvent and 1 mL of 1-octylamine, with the final solvent volume remaining constant (9 mL). (C) H2 evolution traces depicting comparison of an optimal reaction with controls. All studies were performed in the presence of 0.25 mM Ir, 0.25 mM Rh, and 75 mM oxalic acid, unless stated otherwise. Photoreaction conditions: 460 nm, RT, 17 h.

(Figure 3A, blue bars) instead of oxalic acid significantly reduced the quantity of H2 produced. These results lend further support to the possibility of the ion pair formation, which would be favorable under initial acidic conditions. To probe further, the biphasic layers were monitored in situ by 13C NMR (see Supporting Information for experimental details), using toluene-d8 and D2O. As illustrated in Figure S5, the oxalic acid signal (162 ppm) is no longer seen in the aqueous layer upon addition of 1-octylamine, suggesting its extraction into the organic layer. Better solubility of ion pairs in organic solvents supports this observation.65 The oxalic acid peak also could not be detected in the organic phase, but this may result from the comparatively higher concentrations of octanol and amine, making the oxalate signal difficult to visualize. New signals, possibly belonging to the amine, could be observed in the 10−50 ppm range for the aqueous layer. This further suggests that some extent of partitioning between the two phases occurs, pointing toward the likelihood of a “water-in-oil” microemulsion formation. Addition of 1-octylamine is suspected to promote this formation, bringing the aqueous oxalic acid in close proximity to the PS and WRC, which in turn, assists in the electron transfer and proton reduction. Using toluene-miscible amines like 4-heptylaniline or 2-(p-tolyl)pyridine instead led to negligible H2 generation. This result could be attributed to their inability to contact oxalic acid in the aqueous phase, certifying the importance of microemulsion formation and the presence of active species at the phase interface. Within the microemulsion system, 1-octanol is not an active component but plays the key role of a stabilizing cosurfactant/cosolvent,33,66,67 thus slightly improving the catalytic performance. Using excess cosolvent probably destabilizes the aggregates, explaining why 1-octanol on its own was ineffective as a solvent (see Figure 3A). Using the optimal organic solvent, the amount of water was then varied to understand its role in the photocatalytic reaction. As seen in Figure 3B, the reaction becomes progressively less productive upon decreasing the amount of water. It was observed that such conditions resulted in precipitation of the oxalic acid and excessive turbidity, thus hampering the lightabsorption process and leading to the decrease in performance. Control experiments performed in the absence of Ir, Rh, or illumination (Figure 3C) did not generate H2. However, the absence of water (Figure 3B) or the SR (Figure 3C, red line) led to the evolution of small, yet measurable quantities of H2.

could tentatively be attributed to excitation to the spinforbidden 3MLCT and 3LC state. The metallosurfactant displays a broad and featureless emission at 565 nm (Figure 2A, red dotted line), exhibiting a quantum yield of ∼18% and an excited-state lifetime of 646 ns. All these characteristics suggest the emission to be taking place from a mixed 3MLCT and 3LC excited state.62 TD-DFT simulations were performed using CH3CN as the solvent model. As seen in Figure 2B, the predicted UV−visible spectrum closely matches the experimentally procured spectrum, with the features corresponding to the spin forbidden (region I), mixed 1MLCT-1ILCT (region II), and LC (region III) transitions. Static DFT calculations helped determine the frontier orbitals and energy gap of the [Ir(ppy)2(dhpdbpy)]Cl singlet state. The resulting molecular orbital diagram is shown in Figure 2C. As with traditional iridium cyclometalated PSs,63 the highest occupied molecular orbital (HOMO) has contributions from both the cyclometalating ppy ligand and the metal center, while the lowest unoccupied molecular orbital (LUMO) primarily resides on the ancillary bpy ligand. The HOMO−LUMO gap was calculated to be 2.82 eV. Both computational techniques aided in confirming the identity of the electronic transitions occurring in the iridium chromophore. Microemulsion Photocatalytic Hydrogen Generation: Solvent Studies. Encouraged by its photophysical properties, [Ir(ppy)2(dhpdbpy)]Cl was employed to pursue photocatalytic hydrogen generation, along with oxalic acid as the SR and a rhodium-based metallosurfactant, [Rh(dhpdbpy)2Cl2]Cl, as the WRC. Initial investigation involved a solvent study using 1:1 organic/aqueous biphasic solutions, in the presence of 1octylamine (see Experimental Section for details). As depicted in Figure 3A (red bars), toluene was found to be the optimal solvent, and addition of small quantities of 1-octanol (5% v/v) led to a moderate improvement in performance, producing up to 62 μmol H2. 1-Octylamine was found to be crucial for the reaction, since no H2 was generated in its absence. Moreover, considerable mixing of the two layers was observed only upon addition of the amine. On the basis of previous reports demonstrating in situ ion-pair formation between acids and long-chain alkyl amines,64,65 a similar association is proposed to occur between 1-octylamine and oxalic acid. This pair would behave as a surfactant, consequently interacting with the metallosurfactants more effectually. Using sodium oxalate 10165


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Figure 4. Concentration optimization of the metallosurfactants, using 75 mM oxalic acid in 4 mL of 5% 1-octanol in toluene, 4 mL of water, and 1 mL of 1-octylamine. Catalytic performance was evaluated in terms of (A) μmol H2, (B) maximum H2 evolution rate, (C) Ir TONs, (D) maximum Ir TOF, (E) Rh TONs, and (F) maximum Rh TOF. Conditions: 460 nm, RT, 17 h.

excess 1-octylamine would negatively affect this ideal pH (∼4− 7). To explore further, the effect of metallosurfactant concentration on catalytic activity was studied (see Figure S7 for all H2 evolution and maximum rate traces). As illustrated in Figure 4A, larger quantities of H2 were generated at higher PS concentrations. These studies also helped determine an optimal concentration for Rh (0.25 mM), producing ∼120 μmol H2. Plotting maximum H 2 evolution rates against the PS concentration (Figure 4B) revealed a nonlinear growth in rate, with the highest (65 μmol/h) being achieved in the presence of larger quantities of PS and WRC. In terms of iridium TONs (Figure 4C) and TOFs (Figure 4D), the photocatalytic system performs ideally at lower iridium and the optimal rhodium concentration (0.25 mM), yielding up to 240 TONs, with a TOF of 200 h−1. The Rh catalyst too performs better at lower concentrations, exhibiting over 400 TONs, in the presence of excess PS (Figure 4E). The corresponding maximum TOF (Figure 4F) was calculated to be 230 h−1. These results are consistent with previously reported photocatalytic H2 generation systems, where higher-order degradation pathways prevail at high PS and WRC concentrations.69,70 Aggregation of the metallosurfactants at increased concentrations would further accelerate such deactivation routes. This also explains the optimal concentration for the Rh WRC, beyond which H2 evolution is negatively affected. Using different PS concentrations at the optimal Rh concentration, the reactions were subject to illumination for an extended period of time. After 24 h, the vials were removed from illumination briefly (3 min) for gas-phase Raman analysis (Figure S8). Upon reilluminating, H2 generation resumed, especially for the vials with higher PS concentrations. These were found to produce H2 even after 95 h generating over 280 μmol H2 (70 PS TONs), highlighting the robustness of the photocatalytic system.

The former result implies that although not as effective, electron donation by oxalic acid to the PS still occurs without water, promoted by the presence of 1-octylamine as well as the vigorous stirring. Protons released during the oxidation of oxalic acid are then available for reduction by the WRC. Of course though, the low efficiency of this process suggests that water acts as the main proton source, along with being a dispersion medium for oxalic acid. H2 generation from the control vial without oxalic acid indicates oxidative quenching of the PS being the principle mechanistic route. Replacing toluene with water-miscible solvents like CH3CN or CH3OH did not yield H2. The use of highly polar, water-miscible solvents would certainly not support the formation of the microemulsions, further establishing the importance of the biphasic medium. The microemulsion system seems to be desirable, since it would help to create a high surface area for component interactions, as well as improve the photosensitization process due to accumulation of chromophores.33 Overall, these studies necessitate the presence of all components for catalysis to take place optimally. Concentration Optimization and Time-Dependent Analysis. Further optimizations were performed by varying the concentrations of oxalic acid, 1-octylamine, and 1-octanol, at fixed PS and catalyst concentrations. The final results after 17 h of illumination are displayed in Figure S6. It can be observed that the photocatalytic system performs optimally under conditions used initially (i.e., 75 mM oxalic acid, 1:1 organic solvent/water, and 1 mL of 1-octylamine). With few exceptions, most of the trends can be explained as follows: reactions containing oxalic acid concentrations greater than 75 mM were prone to precipitation upon amine addition. This, as observed earlier, adversely affects the photosensitization process. Also, as established by solvent studies, 1-octanol on its own was not a good medium, and thus, the presence of additional 1-octanol leads to diminished H2 production. Lastly, the effect of pH on oxalic acid oxidation has been previously documented,45,68 and 10166


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Figure 5. Results from kinetic isotope studies performed using D2O-containing solvent mixtures. (A) Raman signals (black ○) for H2, HD, and D2 produced during photoreactions in pure D2O. The corresponding Gaussian fits are represented by the red lines. (B) Gas evolution traces and (C) maximum gas evolution rate traces for the vials containing H2O, D2O, or a 1:1 H2O/D2O mixture. Photocatalysis conditions: 460 nm, RT, 24 h.

Figure 6. Results from Stern−Volmer analysis. (A) Nonlinear quenching of the photosensitizer luminescence by the Rh metallosurfactant (blue ◇). The absence of quenching observed for both 1-octanol (green triangles) and 1-octylamine (red squares). (B) The inability of the octylamine/oxalate ion pair to quench the photosensitizer in a biphasic environment which suggests oxidative quenching to be the dominant pathway.

Mechanistic Evaluation: Raman, Luminescence Quenching and Electrochemical Analysis. The photocatalytic system was further inspected by several techniques to gain mechanistic insight. First, isotope studies were undertaken using deuterated water-containing solvent mixtures (experimental details in Supporting Information) to confirm the earlier speculation that water is the proton source. The headspaces of the corresponding reactions were monitored by gas-phase Raman spectrometry. Figure 5A depicts the Raman signals for the plausible gas products (black hollow circles) and their Gaussian fits (red line), obtained for the experiment conducted with pure D2O. It can be clearly seen that D2 is the major contributor (∼73%), designating water to be the primary proton donor. The detection of HD (∼27%) ascertains the availability of protons from the oxidation/electron donation of oxalic acid.

Additionally, a kinetic isotope effect was observed, where both the amount (Figure 5B, red line) and rate (Figure 5C, red line) of gas production diminishes upon using a heavier isotope. Upon using an H2O/D2O (1:1) mixture, almost identical quantities of gas were evolved at similar rates (Figure 5B,C, yellow lines), when compared to pure H2O (Figure 5B,C, purple lines). Moreover, the gas evolved from the mixture was found to contain mostly H2 (Figure S9), emphasizing proton reduction to be one of the rate-determining steps. Photosensitizer luminescence quenching studies were also performed to elucidate the occurrence of oxidative quenching during photolysis. Because of solubility differences, quenching with oxalic acid was performed in CH3CN, while the remaining studies were performed using toluene as solvent. The corresponding Stern−Volmer plots are displayed above and in the Supporting Information. As depicted by Figure S10A, 10167


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Inorganic Chemistry Table 1. Electrochemical Propertiesa of Metallosurfactants Used in This Study El/2ox (ΔE)

El/2red (ΔE)

compound

I

II

1

II

[lr(ppy)2(dhpdbpy)]CI [Rh(dhpdbpy)2CI2]CI

+1.10b +1.14b

+1.31(80)

−1.48 (90) −1.03b

−1.34b

E*oxc

E*redc

−1.1

+0.72

a

Recorded using CH2CI2 solutions containing 1 mM analyte and 0.1 M N(Bu)4PF6 in a three-electrode setup. All potentials referenced to SCE, using ferrocene as an internal standard. Potentials in volts and ΔE in millivolts. bIrreversible, depicts Epa or Epc. cCalculated using the equations E*ox = Eox − Eλem and E*red = Ered + Eλem.

Scheme 2. (A) Proposed Mechanistic Pathway for Photocatalytic H2 Generation in the Present System. (B) Comparison of Redox Potentialsa of the Different Components Involved, Suggesting Oxidative Quenching As the Thermodynamically Favored Mechanism

a

All potentials are in volts, referenced to SCE.

oxalic acid was found to be an unsuitable quencher. Similar results were obtained upon using 1-octanol and 1-octylamine (Figure 6A). The Rh metallosurfactant, on the other hand, was found to quench the iridium excited state, linearly at low concentrations and in a nonlinear fashion at higher Rh concentrations (Figure 6A, blue diamonds). Such behavior is commonly attributed to “static” quenching, where the quencher molecule is proposed to be associated with the chromophore via aggregation at the time of excitation.71

As proposed earlier, formation of an ion pair between 1octylamine and oxalic acid would allow better interactions with the metallosurfactants. To test whether this may be enabling reductive quenching by the ion pair, quenching studies were performed in an environment closely mimicking the photoreaction solutions (see Supporting Information for details). Briefly, aqueous solutions of oxalic acid were extracted with an organic mixture containing the PS, 1-octylamine, 1-octanol, and toluene. An aliquot of the organic layer was then subjected to excitation, and the resulting emission was monitored. Through 10168


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Inorganic Chemistry

components were found to be essential for the proton reduction to take place effectively. Solvent and concentration studies were performed to maximize catalytic performance. The robustness of this photocatalytic system was unveiled through time-dependent analysis, with some combinations producing H2 even after 95 h. Gas-phase Raman analysis confirms water to be the chief proton source, while electrochemical evaluation and luminescence quenching studies indicate oxidative quenching to be the dominant pathway to H2 evolution.

these experiments, it was clear that ion-pair formation does take place, as evidenced by the presence of oxalate in the toluene layer (determined via precipitation method; see Supporting Information). This corroborates well with the 13C NMR study. However, the corresponding Stern−Volmer analysis (Figure 6B) suggests no quenching. A parallel experiment was performed in a homogeneous MeOH/octylamine mixture to view the effect of the ion pair without interference from water or PS extraction, which might affect photosensitizer luminescence in the extracted solutions. As seen in Figure S10B, even in homogeneous solution, no quenching was observed. While providing proof of oxalic acid extraction, these results clearly indicate the dominance of oxidative quenching by the Rh metallosurfactant in this photocatalytic system. Further, the electrochemical behavior of the metallosurfactants was determined via cyclic voltammetry, using CH2Cl2 solutions. The corresponding voltammograms are displayed in Figure S11. Unlike analogous cyclometalated iridium complexes, [Ir(ppy)2(dhpdbpy)]Cl shows an irreversible oxidation at +1.10 V, followed by a reversible feature centered at +1.31 V. On the basis of DFT calculations, these anodic events can be assigned to the metal center as well as the ppy ligand, although the specific order could not be ascertained. A single reversible peak at −1.48 V could be ascribed to the bpy/bpy− reduction. On the basis of these recorded potentials, the emission energy, and using Hess’s law, the excited-state potentials were calculated to be E*ox = −1.1 V and E*red = +0.72 V. The cyclic voltammogram of [Rh(dhpdbpy)2Cl2]Cl depicts an irreversible feature at +1.14 V, which could be designated as the metal center oxidation. The metallosurfactant also exhibits irreversible reductions at −1.03 and −1.34 V. The first reduction process is characteristic of RhIII polypyridine complexes and can be attributed to an ECEC mechanism involving two successive reductions, each followed by the loss of a −Cl ligand.72 Further reduction of [Rh]I to [Rh]0 gives rise to the second cathodic process. The properties for both complexes are summarized in Table 1. On the basis of these results, a mechanism is proposed where addition of 1-octylamine leads to the formation of an ion pair with oxalic acid. As noted in earlier sections, this ion pair can act as a surfactant and thus associates with the metallosurfactants, resulting in the formation of a microemulsion (Scheme 2A). Upon illumination, the excited state of the Ir PS is oxidatively quenched by the Rh catalyst, which reduces protons to evolve H2. Given the close match between the oxidation potential of oxalic acid (onset = +1.25 V vs SCE, Figure S12) and the [Ir]2+/[Ir]1+ redox event (+1.27 V vs SCE), it is likely that the oxidized PS is brought back to its resting state via electron donation by the extracted oxalic acid, completing the cycle. An overall comparison of the redox potentials of all active components (Scheme 2B) suggests that reductive quenching is thermodynamically unfavorable, providing further support to the proposed reaction pathway.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00463. Additional experimental methods, 1H NMR and ESI-MS data, 13C NMR study of photoreaction mixtures, H2 evolution traces, additional photoreaction results, and cyclic voltammograms (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Husain N. Kagalwala: 0000-0002-5628-5901 Stefan Bernhard: 0000-0002-8033-1453 Author Contributions

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 The authors thank V. Van Benschoten for her assistance in the quenching studies. S.B. gratefully acknowledges support by the National Science Foundation (CHE-1362629). NMR instrumentation at CMU was partially supported by NSF (CHE0130903 and CHE-1039870).

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REFERENCES

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CONCLUSIONS To summarize, we report our investigations of a microemulsion-based photocatalytic hydrogen evolution system. Iridium and rhodium-based metallosurfactants served as the photosensitizer and proton-reducing catalyst, respectively, while oxalic acid acts as an electron donor in a toluene/water biphasic solvent mixture. The addition of 1-octylamine results in an ionpair formation with oxalic acid, which interacts with the metallosurfactants and commences the catalytic process. All 10169


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