Article pubs.acs.org/JPCB
Probing Co-Assembly of Supramolecular Photocatalysts and Polyelectrolytes Using Isothermal Titration Calorimetry Theodore R. Canterbury, Shamindri M. Arachchige, Karen J. Brewer, and Robert B. Moore* Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061-0212, United States S Supporting Information *
ABSTRACT: The creation of renewable fuels to replace dwindling fossil energy resources is one of the greatest challenges facing the scientific community. Generating H2 fuel from water is a carbon-neutral strategy that demonstrates great promise. Photocatalysts of the molecular architecture [{(TL)2Ru(BL)}2RhX2]5+ (BL = bridging ligand, TL = terminal ligand, X = halide) catalyze the formation of H2 in deoxygenated organic solvents but are limited by poor performance in air-saturated aqueous solutions. Addition of the water-soluble polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS) was recently shown as being a promising new strategy to increase efficiency and stability of H2 evolving photocatalysts in air-saturated aqueous solutions. Herein we investigate intermolecular interactions between Ru,Rh,Ru photocatalysts and water-soluble polyelectrolytes using isothermal titration calorimetry (ITC). ITC studies provide insight into the thermodynamic forces that drive assembly of PSS−photocatalyst aggregates and give new evidence for the intermolecular forces that lead to increased photocatalytic efficiency.
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INTRODUCTION Harnessing solar energy for the production of H2 fuel is a promising carbon-neutral strategy to decrease dependence upon nonrenewable energy sources. 1 Photocatalytic H 2 production from water is an energetically uphill reaction that involves the collection of multiple reducing equivalents, bond breakage, and bond formation.2,3 Initially, photocatalytic water reduction efforts employed Ru complexes as light absorbers (LA) and heterogeneous colloidal Pt as a reactive metal (RM). 3,4 Homogeneous multicomponent photocatalytic schemes have now been developed that use Rh, Co, or Ni RMs combined with Ru, Ir, or fluorescein-type LAs.5−7 Photocatalysts that couple the LA to the RM, creating a single molecule having individual components that carry out a complex function, are termed supramolecular complexes.8−11 Supramolecular complexes that couple Ru LAs to Rh, Pd, or Pt RMs through bridging ligands are H2 evolving photocatalysts. 12,13 Photocatalysts having the architecture [{(TL)2Ru(BL)}2RhX2]5+ (Figure 1, TL = bpy (2,2′-bipyridine), BL = dpp (2,3-bis(2-pyridyl)pyrazine), X = Cl) couple two Ru(II) LAs to a Rh(III) RM through dpp BLs carry out multielectron transfer to a Rh metal center to photochemically produce H2 from water.14,15 While supramolecular photocatalysts have demonstrated high turnovers (TON = 1300) in organic solvents, a dramatic decrease in photocatalytic efficiency is observed in aqueous solutions (TON = 15).16,17 Water, compared to polar organic solvents, dramatically quenches the metal-to-ligand charge transfer (3MLCT) excited state (Figure 2) of Ru polypyridyl complexes.18−20 Increased quenching in aqueous solutions is a result of the larger vibrational trapping energy of water owing to coupling of the high-frequency O−H vibration (3500 cm−1) through electro© 2017 American Chemical Society
Figure 1. Structures of [{(bpy)2Ru(dpp)}2RhCl2]Cl5 (Ru,Rh,Ru), poly(sodium 4-styrenesulfonate) (PSS), p-toluenesulfonate (TS), and poly(sodium vinylsulfonate) (PVS).
static interactions with the polar excited state of the chromophore.21,22 Early reports on the photophysics of the LA [Ru(bpy)3]Cl2 in the presence of water-soluble polymers demonstrated very different results depending upon polyelectrolyte structure. 19,23,24 While a dramatic increase in luminescence from the 3MLCT excited-state (Figure 2) of [Ru(bpy)3]Cl2 was observed for aqueous solutions containing poly(styrenesulfonate) (PSS, Figure 1), little to no emission enhancement was detected for solutions containing nonReceived: March 15, 2017 Revised: May 27, 2017 Published: May 30, 2017 6238
DOI: 10.1021/acs.jpcb.7b02462 J. Phys. Chem. B 2017, 121, 6238−6244
Article
The Journal of Physical Chemistry B
prepared using the previously reported building block approach.37 Steady-state luminescence spectroscopy was measured in screw-top 1 cm quartz cuvettes (Starnes Cells Inc.) at room temperature in ultrapure water following deoxygenation with argon. Spectra were measured using a Quanta Master QM-20045E from Photon Technologies International Inc. The samples were excited using a water-cooled 150 W xenon arc lamp, and emission was measured at a 90° angle using a thermoelectrically cooled Hamamatsu R2658 photomultiplier tube operating in photon counting mode. The emission quantum yields were referenced to [Os(bpy)3]2+ (Φ = 4.62 × 10−3) after correcting for PMT response.38,39 Time-resolved emission spectroscopy was measured using a PL-2300 nitrogen laser equipped with a PL-201 dye laser. The dye used was Coumarin 500, and excitation monochromator was set to 520 nm. Emission was detected at 90° after passing through a monochromator set to 790 nm and measured using a Hamamatsu R928 PMT operating in direct output mode. The emission decay was recorded on a Techtronix TDS 3052C oscilloscope. Excitedstate lifetime was calculated from the average of 300 sweeps. Data were fitted to mono- and biexponential decay functions in the absence and presence of PSS, respectively. Isothermal titration calorimetry was carried out on a TA Instruments-Waters LLC NanoITC titration calorimeter at 25 °C. Solutions of complex were titrated into the cell containing polyelectrolyte using an automated buret equipped with a 0.050 mL Hamilton syringe. Injections of 1.43 × 10−3 mL were performed having 300 s between injections to allow for heat signals to return to baseline intensity. Control titrations of the complex titrated into ultrapure water and titration of ultrapure water into polyelectrolyte solutions were subtracted from the heats obtained. Experiments were performed in triplicate and fitted with TA NanoAnalyze software using an independent site model, and the values were averaged to obtain binding parameters reported herein. Fitting the two-stage thermogram obtained for the titration of Ru,Rh,Ru into PSS was performed using the method of Kim and is detailed in the Supporting Information.40 Static light scattering studies were measured in 1 cm quartz cuvettes. Scattering was measured at room temperature in ultrapure water using a Quanta Master QM-200-45E from Photon Technologies International Inc. Samples were irradiated using a water-cooled 150 W xenon arc lamp, and the scattering intensity was measured at a 90° angle using a thermoelectrically cooled Hamamatsu R2658 photomultiplier tube operating in photon counting mode.
Figure 2. Jablonski-type state diagram for [Ru(bpy)3]Cl2.
aromatic polyelectrolytes such as poly(vinylsulfonate) (PVS, Figure 1).23,25,26 Increased emission intensity of [Ru(bpy)3]Cl2 in aqueous solutions containing PSS was attributed to strong hydrophobic interactions between the chromophore and the aromatic polymer. Sequestration of the complex within the hydrophobic domains of the polymer backbone decreases the concentration of water molecules surrounding the complex, thus reducing excited-state quenching.19,27 Similar to previous reports on [Ru(bpy)3]Cl2 and analogous Ru polypyridyl complexes, excited-state properties of photocatalysts bearing Ru LAs were also significantly impacted by the presence of water-soluble polymers. Addition of PSS to aqueous solutions of [{(bpy)2Ru(dpp)}2RhCl2]Cl5 (Ru,Rh,Ru) afforded a dramatic increase in emission quantum yield and excited-state lifetime of the supramolecular complex. Recently, we detailed the use of the polyelectrolyte PSS to improve H2 production in water, including H2 evolution under air saturation.28 Aggregate formation between the photocatalyst and the sulfonated polymer was found to increase the 3MLCT excited-state lifetime and protect the photocatalyst from oxygen quenching owing to increased ionic strength near the charged polymer. A fundamental understanding of the thermodynamic forces driving assembly of PSS−photocatalyst aggregates and the impact this binding has on excited-state decay processes could lead to further increases in catalyst function and stability. Isothermal titration calorimetry (ITC) is a widely used method to investigate intermolecular interactions between ligands or drugs and macromolecules such as proteins or DNA.29−31 Recent work investigated the binding of surfactants with various synthetic macromolecules such as polyelectrolytes.32−35 Herein, we report the novel application of ITC to investigate intermolecular interactions between the supramolecular photocatalyst Ru,Rh,Ru and polyelectrolytes in aqueous solutions. This is the first report on the thermodynamics of binding between well-defined synthetic polyanions and Ru-based photocatalysts, including the model light absorber [Ru(bpy)3]Cl2.
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RESULTS AND DISCUSSION Luminescence titrations allow for the study of polyelectrolyte concentration on emission quantum yield of Ru,Rh,Ru (Figure 3). Titration of PSS into solutions containing the photocatalyst indicates an increase in the Ru,Rh,Ru emission intensity with increased PSS concentration (Figure 3a). A plot of I/I0 versus the number of styrenesulfonate units per Ru,Rh,Ru (mole ratio) indicates that the emission intensity reaches a plateau at a mole ratio of 3 styrenesulfonate units per Ru,Rh,Ru (Figure 3b). Optimization of emission intensity prior to charge balance (5:1) is the result of hydrophobic interactions between Ru,Rh,Ru and PSS, which leads to aggregate formation observed in cryogenic transmission electron microscopy (Cryo-TEM) (Figure 4) and dynamic light scattering studies (Figure 5).28,41 Comparison of emission intensity for Ru,Rh,Ru
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EXPERIMENTAL SECTION All chemicals were used as received unless noted otherwise. Poly(sodium 4-styrenesulfonate) (PSS) [Mn = 1.44 kDa (PDI = 1.17); Mn = 57.8 kDa (PDI = 1.18); Mn = 1.01 MDa (PDI = 1.18)] was purchased from Scientific Polymer Products. PSS having a Mn = 57.8 kDa (PDI = 1.18) was used for all experiments unless noted otherwise. Poly(sodium vinylsulfonate) (PVS) and p-toluenesulfonate (TS) were purchased from Aldrich. Ultrapure water was obtained from a Millipore Milli-Q water purification system. The starting material [(bpy)2Ru(dpp)][PF6]2 was synthesized using previously reported procedures.36 The complex [{(bpy)2Ru(dpp)}2RhCl2]Cl5 was 6239
DOI: 10.1021/acs.jpcb.7b02462 J. Phys. Chem. B 2017, 121, 6238−6244
Article
The Journal of Physical Chemistry B
Table 1. Photophysical Properties of Ru,Rh,Ru (0.12 mM) in Aqueous Solutions solution
λmaxabs [nm]a
λmaxem [nm]a
Φem × 104 a,b
τ [ns]a
H2O H2O (25 mM PSS) H2O (0.5 mM PSS)
518 518 518
840 794 784
0.91 ± 0.05 2.5 ± 0.1 3.4 ± 0.1
17 ± 1 23 ± 2c 40 ± 1d
a
Measured in deoxygenated ultrapure H2O at RT in a 0.2 cm quartz cuvette. bReferenced to [Os(bpy)3]2+. cAdditional decay component of 52 ± 3 ns (α = 0.24 ± 0.01) observed. dAdditional decay component of 26 ± 1 ns (α = 0.04 ± 0.02) observed. Figure 3. Relative emission spectra (a) and normalized emission intensity (b) of Ru,Rh,Ru with increasing poly(4-styrenesulfonate) (PSS) concentration. Mole ratio given as the number of styrenesulfonate units per Ru,Rh,Ru. Measured in deoxygenated ultrapure water, corrected for PMT response.
aqueous solutions has been associated with significantly enhanced H2 production.28 Steady-state emission spectroscopy allows for the study of PSS molecular weight on the excited state properties of the Ru,Rh,Ru photocatalyst (Figure 6). Luminescence studies
Figure 4. Cryogenic transmission electron micrograph of Ru,Rh,Ru (0.12 mM) in the presence of 0.50 mM poly(sodium 4-styrenesulfonate) in ultrapure water.
Figure 6. Relative emission intensity of Ru,Rh,Ru (10 μM) with increasing molecular weight of poly(4-styrenesulfonate) (PSS) (2.5 mM). Measured in deoxygenated ultrapure water, corrected for PMT response. Mn = number-average molecular weight.
indicate that PSS oligomers with as few as 7 monomer repeat units (M̅ n = 1.44 kDa) significantly impact emission intensity of the complex, reaching nearly 90% of the emission quantum yield (Φ = (2.3 ± 0.1) × 10−4) for polymers having over 300 repeat units (M̅ n = 57.8 kDa) (Φ = (2.6 ± 0.1) × 10−4). Increasing M̅ n of the polymer to 1.01 MDa had only a slight increase in emission quantum yield (Φ = (2.8 ± 0.1) × 10−4) compared to M̅ n = 57.8 kDa. Luminescence in the presence of p-toluenesulfonate (TS), the monomeric analogue, was nearly identical (Φ = (0.80 ± 0.06) × 10−4) to the photocatalyst in water alone (Φ = (0.91 ± 0.05) × 10−4), demonstrating the importance of the polymeric form on emission enhancement. Luminescence of Ru,Rh,Ru was not increased in the presence of poly(vinylsulfonate) PVS, which lacks the aromatic group found in PSS (Figure S2), similar to previous reports for [Ru(bpy)3]Cl2 (Table 2).42 To probe the thermodynamic forces driving assembly of PSS−photocatalyst aggregates and understand the forces responsible for emission enhancement, intermolecular interactions were investigated using ITC. During isothermal titration measurements, the photocatalyst was titrated into a solution of the water-soluble polymer and the heat change
Figure 5. Relative emission intensity of Ru,Rh,Ru (0.12 mM) in ultrapure water (blue) and in the presence of 25 mM (orange) or 0.50 mM (red) poly(4-styrenesulfonate) (PSS). Measured in deoxygenated aqueous solutions, corrected for PMT response.
(0.12 mM) within the aggregates (0.5 mM PSS) to the photocatalyst in the presence of excess (25 mM) PSS (Figure 5), where aggregation is not observed, indicates emission quantum yield and excited-state lifetime of Ru,Rh,Ru is further increased within the aggregated form (Table 1). We attribute this observation to the photocatalyst being less accessible to H2O within the aggregates compared to the photocatalyst in the presence of excess PSS (25 mM). Increased excited-state lifetime of Ru,Rh,Ru upon aggregate formation is an important discovery as increased excited-state lifetime of Ru,Rh,Ru in 6240
DOI: 10.1021/acs.jpcb.7b02462 J. Phys. Chem. B 2017, 121, 6238−6244
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natural polyelectrolytes such as DNA (Ka = 0.7 × 103 M−1).43 The increased binding affinity is attributed to the greater hydrophobicity and increased chain mobility of PSS compared to DNA leading strong aromatic−aromatic interactions between the chromophore and the sulfonated polymer. NMR spectroscopy (1H) of [Ru(bpy)3]Cl2 in the presence and absence of polyelectrolytes provide further insight into intermolecular forces between Ru complexes and PSS. Because of the atactic polymer having broad 1H resonances throughout the aromatic region, deuteration of PSS was necessary to distinguish 1H resonances associated with [Ru(bpy)3]Cl2.44 The H4−H6 resonances of the Ru chromophore in D2O are significantly shifted in the presence of the deuterated polymer (PSS-d4) (Figure 8)a stark contrast to solutions containing
Table 2. Photophysical Properties of Ru,Rh,Ru in Aqueous Solutions solution
λmaxabs [nm]a
λmaxem [nm]a
H2O H2O (2.5 mM TS) H2O (2.5 mM PVS) H2O (2.5 mM PSS)
518 518 518 518
845 845 843 798
Φem × 104 a,b 0.91 0.80 0.63 2.6
± ± ± ±
0.05 0.06 0.06 0.1
τ [ns]a 16 16 17 44
± ± ± ±
1 2 1 2c
a
Measured in deoxygenated ultrapure H2O at RT in a 1 cm quartz cuvette. bReferenced to [Os(bpy)3]2+. cAdditional decay component of 2 ± 0.5 ns (α = 0.30 ± 0.01) observed. dAdditional decay component of 7 ± 0.5 ns (α = 0.27 ± 0.02) observed.
monitored. Each injection of the photocatalyst was observed as a spike on the thermogram that corresponded to the heat absorbed or released upon binding of the photocatalyst to the polyanion (Figure 7a). Integration of the heat change over time
Figure 8. 1H NMR (400 MHz) spectrum of [Ru(bpy)3]Cl2 (5 mM) in the absence (black) and presence of d4-poly(styrenesulfonate) (50 mM) (red) in D2O.
TS-d7 (Figure S8) or PVS (Figure S9) in which the resonances of the Ru complex are only slightly perturbed. The large upfield shifts demonstrate the close proximity of the chromophore and polyanion, which greatly shields the aromatic protons of the Ru complex. The upfield shifts observed for H5 and H6 resonances are significantly larger than the H3 and H4 resonances, suggesting the ligands on the complex are interacting with the polyanion through edge-to-face type π−π interactions. Similar upfield shifts of 1H NMR resonances have been reported for polyazine dyes in the presence of PSS, attributed to aromatic− aromatic interactions between the cationic dye and polymer.45 Consistent with previous reports, the 1H resonances of the LA are significantly broadened in the presence of PSS due to decreased tumbling of the chromophore residing in a variety of microenvironments upon solvation by the atactic polymer.46 As expected, thermograms for the isothermal titration of Ru,Rh,Ru into solutions containing PSS are similar to [Ru(bpy)3]Cl2 with an additional binding event observed at high concentration of complex (Figure 9). At the start of the titration, when the concentration of PSS was much greater than that of the photocatalyst, large exothermic heats were observed. The large heat release is the result of combined electrostatic and hydrophobic interactions, which drive the aromatic groups to within close proximity and allows for strong aromatic− aromatic interactions.33,47 Beyond the equilibrium point of the titration, conformation changes of the polyanion due to charge neutralization results in aggregation of PSS-photocatalyst assemblies and a switch from exothermic to endothermic heats, not observed with [Ru(bpy)3]Cl2.33 After all binding sites were occupied, heats returned to the baseline. Fitting of the thermogram (Figure 9b) was performed using a two-stage model developed for the study of [Co(NH3)6]Cl3 binding to DNA, which also displayed two sequential binding events and aggregate formation.40 The initial binding event (NDH1) is
Figure 7. Thermogram of raw heat (a) and enthalpy with independent model fit (b) for the isothermal titration of 1.04 mM [Ru(bpy)3]Cl2 into 2.12 mM poly(4-styrenesulfonate) (PSS). Measured in ultrapure water at 25 °C, corrected for dilution response. Mole ratio = number of [Ru(bpy)3]2+ per PSS repeat unit.
allowed for the determination of the enthalpy of binding (ΔH) (Figure 7b), while the slope and position of the inflection point allowed for the calculation of binding stoichiometry (n) and association constant (Ka). Change in entropy (ΔS) and free energy of binding (ΔG) were calculated using eqs 1 and 2. ΔG = −RT ln K a
(1)
ΔG = ΔH − T ΔS
(2)
Titration of the model [Ru(bpy)3]Cl2 into TS (Figure S4) or the nonaromatic polymer PVS (Figure S5) yielded only low exothermic heats, which coincides with the negligible impact of TS or PVS on the excited-state properties of the chromophore. However, titration of [Ru(bpy)3]Cl2 into solutions containing PSS resulted in large exothermic heats (−21 ± 1 kJ/mol), indicating strong specific interactions between PSS and the Ru complex (Figure 6b). Fitting the thermogram using an independent site model confirmed a high affinity between the chromophore and the sulfonated polymer (Ka = (4.3 ± 0.6) × 105 M−1) having a stoichiometric ratio of 3.8 ± 0.2 monomer repeat units per light absorber (Table 3). Binding affinity for PSS was greater than previously reported for interactions with 6241
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as previous reports of decreased H2 production efficiency.28 1H NMR studies for Ru,Rh,Ru in the presence and absence of polyanions were not investigated due to the large number of equivalent 1H resonances throughout the aromatic region owing to the complex having many geometric isomers.48 To further clarify the nature of the endothermic heats observed during isothermal titration of Ru,Rh,Ru into PSS, static light scattering studies were investigated (Figure 10).
Figure 10. Scattering spectra (a) and normalized scatter intensity (b) measured at 90° angle from source (λ = 600 nm) for the titration of 1.18 mM Ru,Rh,Ru into 1.31 mM poly(4-styrenesulfonate) (PSS). Measured in ultrapure water at 25 °C. Mole ratio = number of Ru,Rh,Ru per PSS repeat unit.
Figure 9. Thermogram of raw heat (a) and enthalpy (b) with first stage independent fit (green), second stage independent fit (red), and sum of fits (black) for the isothermal titration of 1.18 mM Ru,Rh,Ru into 1.31 mM poly(4-styrenesulfonate) (PSS). Measured in ultrapure water at 25 °C, corrected for dilution response. Mole ratio = number of Ru,Rh,Ru per PSS repeat unit.
Table 3. Thermodynamic Binding Parameters for Independent Site Modela
attributed to binding of Ru,Rh,Ru to the elongated polyelectrolyte, while the second binding event (NDH2) is the result of binding to the collapsed form of PSS. Binding of Ru,Rh,Ru to PSS was found to be largely enthalpic in nature, having a binding enthalpy of −29 ± 1 kJ/mol and stoichiometry of 6.5 ± 0.3 polymer repeat units per photocatalyst, slightly greater than charge balance. Binding affinity for PSS-photocatalyst interactions (Ka = (1.3 ± 0.4) × 106 M−1) is increased compared to [Ru(bpy)3]Cl2, a result of greater Coulombic and hydrophobic interactions between the supramolecular complex and the sulfonated polymer.27 Thermograms for the isothermal titration of Ru,Rh,Ru into solutions containing PVS, a polymer that lacks the aromatic side chain, are quite different from that of PSS (Figure S6). While ΔH for PSS−photocatalyst interactions is −29 kJ/mol, interactions with PVS yield only low exotherms (−8.4 ± 0.6 kJ/ mol) and a binding affinity of Ka = (8.6 ± 0.1) × 104 M−1. Therefore, binding of Ru,Rh,Ru to PVS is largely driven by entropic effects (TΔS = 20 ± 1 kJ/mol), similar to that observed for [Ru(bpy)3]Cl2 (TΔS = 22 ± 1 kJ/mol). The large difference in binding enthalpy between PVS and PSS is the result of combined electrostatic and hydrophobic interactions between the aromatic groups on the photocatalyst and PSS, not observed in PVS due to the hydrophilic nature of the polymer backbone, similar to previous reports on surfactant/polyelectrolyte studies.33 Titration of Ru,Rh,Ru into solutions containing the monomeric analogue TS showed only negligible heat release (Figure S7). The lack of exothermic events for the titration of the photocatalyst into TS suggests that specific interactions between Ru,Rh,Ru and TS are minimal despite the presence of the aromatic moiety, consistent with ITC and NMR studies for [Ru(bpy)3]Cl2. These results demonstrate the importance of the polymer chain and coincide with the negligible impact of TS on the excited-state properties of the photocatalyst as well
complex/anion [(bpy)3Ru]Cl2/ PSS [(bpy)3Ru]Cl2/ PVS [(bpy)3Ru]Cl2/ TS Ru,Rh,Ru/PSSe Ru,Rh,Ru/PVS Ru,Rh,Ru/TS
Ka× 10−5 b (M−1)
ΔH (kJ mol−1)
nd
TΔS (kJ mol−1)
4.3 ± 0.6
−21 ± 1
3.8 ± 0.2
11 ± 1
0.30 ± 0.07
−2.9 ± 0.6
5±2
22 ± 1
c 13 ± 4 0.86 ± 0.01 c
c
c
−29 ± 1 −8.4 ± 0.6 c
6.5 ± 0.3 9±1 c
c 6±1 20 ± 1 c
Measured in H2O at 25 °C, corrected for dilution response. bThe Ka is reported as the affinity of each polymer repeat unit for each photocatalyst site. cNegligible heat response detected. dNumber of polymer repeat units per photocatalyst. eAn additional binding event having Ka = 3.1 × 106 M−1, ΔH = 12.5 kJ mol−1, n = 0.055, and TΔS = 46 kJ mol−1 was detected. a
Light scattering was previously reported as a useful tool in the study of aggregate formation between cationic surfactants and PSS.33 Scattering intensity was found to correlate well with the endothermic events detected in ITC thermograms during aggregate formation. At the start of the titration when large exothermic heats are observed in the thermogram, scattering intensity for the sample is negligible and indicates aggregation does not occur at low concentration of the photocatalyst. However, at approximately the same molar ratio at which the transition from exothermic to endothermic heats is detected in ITC studies, a sharp increase in scattering intensity is observed (Figure 10b). The large increase in scattering at nearly the same molar ratio as the endothermic heats in the ITC thermogram indicates the heats are the result of aggregation between photocatalyst−PSS assemblies. Increased scattering was not observed for titrations of Ru,Rh,Ru into aqueous solutions in the absence of the polymer (Figure S10). Additionally, titration 6242
DOI: 10.1021/acs.jpcb.7b02462 J. Phys. Chem. B 2017, 121, 6238−6244
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of [Ru(bpy)3]Cl2 into PSS does not show an increase in scattering throughout the titration (Figure S11). The lack of aggregate formation between [Ru(bpy)3]Cl2 and PSS even at high concentrations accounts for the lack of endothermic heats detected in [Ru(bpy)3]Cl2/PSS calorimetric experiments.
CONCLUSIONS ITC and NMR studies of the model complex [Ru(bpy)3]Cl2 reveal intermolecular forces between Ru photocatalysts and PSS are the result of combined electrostatic and hydrophobic interactions. Hydrophobic interactions drive the molecules into close proximity and allows for strong aromatic−aromatic interactions between the chromophore and sulfonated polymer, resulting in large exothermic heats. Titration of the photocatalyst Ru,Rh,Ru into PSS was found to display a sizable increase in exothermic heats compared to [Ru(bpy)3]Cl2 owing to increased Coulombic and hydrophobic interactions, not detected for the monomeric model TS or the nonaromatic polyelectrolyte PVS. Moreover, PVS and TS were not found to greatly impact the excited-state properties of Ru,Rh,Ru while PSS was found to significantly decrease vibrational quenching of the 3MLCT excited state, previously reported to increase photocatalytic efficiency of the H2 evolving complex. Therefore, a link between the thermodynamics of coassembly, excited-state properties, and H2 production efficiency can be obtained by this novel application of ITC in the study of photocatalyst− polyelectrolyte assemblies. Future work will probe increasing hydrophobicity of the photocatalyst through terminal ligand variation in order to further increase binding affinity. The modified photocatalysts are expected to display even greater increases in quantum yield and excited-state lifetime, previously found to increase H2 production efficiency. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b02462. Experimental details, fitting procedures, ITC thermograms, and supporting photophysical data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*(R.M.) E-mail:
[email protected]. ORCID
Theodore R. Canterbury: 0000-0003-4100-9696 Robert B. Moore: 0000-0001-9057-7695 Notes
The authors declare no competing financial interest. Prof. Karen J. Brewer is deceased (November 2014).
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ACKNOWLEDGMENTS Acknowledgement is made to the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Sciences, U.S. Department of Energy DE FG02-05ER15751, the National Science Foundation (1301131), and the Virginia Tech Institute for Critical Technology and Applied Science (ICTAS) for the generous financial support for our research. Acknowledgement is also made to Hannah M. Rogers and Gregory Fahs for assistance with manuscript preparation. 6243
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