Light-Driven Hydrogen Generation from Microemulsions Using Metallosurfactant Catalysts and Oxalic Acid

Article abstract of DOI:10.1021/acs.inorgchem.7b00463

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 = 2-phenylpyridine 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.

Full text of DOI:10.1021/acs.inorgchem.7b00463

Article
pubs.acs.org/IC
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
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 = 2-
phenylpyridine and dhpdbpy = 4,4′-diheptadecyl-2,2′-bipyr-
idine), 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.
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
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 conven-
tional 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 haloa-
cids,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 homoge-
neous 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 H2
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
form micelles or microemulsions. Seminal work by Gratzel 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 photo-
physical 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 cetyltrimethy-
lammonium bromide (CTAB) as the micelle forming
surfactant. Their study further supports the idea of
amphiphile-based photocatalytic systems.
Received: February 20, 2017
© XXXX American Chemical Society
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Scheme 1. Mechanism of (A) Photocatalytic “Total Water Splitting” and (B) Proton/Water Reduction in the Presence of a
a
Sacrificial Electron Donor
a
For simplicity, both schemes depict oxidative quenching mechanisms. WOC = water oxidation catalyst.
tetrabutylammonium hexafluorophosphate (N(Bu)4PF6) were pro-
cured from Sigma-Aldrich. All deuterated solvents for NMR analysis
were obtained from Cambridge Isotopes. All chemicals were used as
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
1
received. A Bruker Avance spectrometer was used to record H NMR
and 13C NMR at 300 and 75 MHz, respectively, at room temperature
(RT). Positive ion electrospray ionization mass spectrometry (ESI-
MS) 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, GasRaman-
NOCH-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,
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-to-
hydrogen schemes. Before the onset of catalysis, the metal-
losurfactants 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 metal-
losurfactants (present in the organic layer) more likely.
Optimization studies afforded 240 iridium and over 400
rhodium turnover numbers (TONs). Moreover, this photo-
catalytic 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, electro-
chemistry, gas-phase Raman analysis, and luminescence
quenching.
1
and then dried overnight to give the complex in 75−80% yield: H
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,
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. HPLC-
grade xylene and toluene were obtained from Fischer Scientific.
Anisole and 1-octanol were purchased from TCI Chemicals. 1-
Octylamine, oxalic acid dihydrate (C2H2O4·2H2O), ferrocene, and
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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
3
recorded in toluene due to intense solvent absorptions in the UV region. The inset shows the low intensity MLCT 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 CH-
Instruments 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
cycles of vacuum degassing and backfilling with N2 to ensure complete
deaeration. The samples were then illuminated in a 12-well, home-
built 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.
RESULTS AND DISCUSSION
■
Ir Metallosurfactant: Photophysical and Computa-
tional Analysis. To determine its effectiveness as a photo-
sensitizer, [Ir(ppy)2(dhpdbpy)]Cl was first subject to a
photophysical evaluation. The observed photophysical charac-
teristics 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
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
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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.
could tentatively be attributed to excitation to the spin-
forbidden 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
(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-octyl-
amine 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 micro-
emulsion 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).
3
suggest the emission to be taking place from a mixed MLCT
62
3
and LC excited state.
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
1
(region I), mixed MLCT-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 chromo-
phore.
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 1-
octylamine (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
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 light-
absorption 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.
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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.
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
excess 1-octylamine would negatively affect this ideal pH (∼4−
7).
To explore further, the effect of metallosurfactant concen-
tration 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 H2 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 photo-
catalytic H2 generation systems, where higher-order degrada-
tion pathways prevail at high PS and WRC concentrations.69,70
Aggregation of the metallosurfactants at increased concen-
trations 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.
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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 photo-
catalytic system was further inspected by several techniques to
gain mechanistic insight. First, isotope studies were undertaken
using deuterated water-containing solvent mixtures (exper-
imental 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,
F
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Article
a
Table 1. Electrochemical Properties of Metallosurfactants Used in This Study
El/2ox (ΔE)
El/2red (ΔE)
c
c
compound
I
II
1
II
E*ox
−1.1
E*red
b
b
[lr(ppy)2(dhpdbpy)]CI
[Rh(dhpdbpy)2CI2]CI
+1.10
+1.14
+1.31(80)
−1.48 (90)
+0.72
b
b
−1.03
−1.34
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,
b
c
using ferrocene as an internal standard. Potentials in volts and ΔE in millivolts. Irreversible, depicts Epa or Epc. Calculated 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
a
Redox Potentials 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 1-
octylamine 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 photo-
reaction 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
G
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Article
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 lumines-
cence 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 metallosurfac-
tants was determined via cyclic voltammetry, using CH2Cl2
solutions. The corresponding voltammograms are displayed in
Figure S11. Unlike analogous cyclometalated iridium com-
plexes, [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 metal-
losurfactants, 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, provid-
ing further support to the proposed reaction pathway.
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.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00463.
1
Additional experimental methods, H NMR and ESI-MS
data, 13C NMR study of photoreaction mixtures, H2
evolution traces, additional photoreaction results, and
cyclic voltammograms (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: bern@cmu.edu.
■
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.
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 instru-
mentation at CMU was partially supported by NSF (CHE-
0130903 and CHE-1039870).
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