Developing Photosensitizer-Cobaloxime Hybrids for Solar-Driven H2 Production in Aqueous Aerobic Conditions

Summary

We have directly incorporated a stilbene-based organic dye into a cobaloxime core to generate a photosensitizer-catalyst dyad for photocatalytic H₂ production. We have also developed a simple experimental setup for evaluating the light-driven H₂ production by photocatalytic assemblies.

Abstract

Developing photocatalytic H₂ production devices is the one of the key steps for constructing a global H₂-based renewable energy infrastructure. A number of photoactive assemblies have emerged where a photosensitizer and cobaloxime-based H₂ production catalysts work in tandem to convert light energy into the H-H chemical bonds. However, the long-term instability of these assemblies and the need for hazardous proton sources have limited their usage. Here, in this work, we have integrated a stilbene-based organic dye into the periphery of a cobaloxime core via a distinct axial pyridine linkage. This strategy allowed us to develop a photosensitizer-catalyst hybrid structure with the same molecular framework. In this article, we have explained the detailed procedure of the synthesis of this hybrid molecule in addition to its comprehensive chemical characterization. The structural and optical studies have exhibited an intense electronic interaction between the cobaloxime core and the organic photosensitizer. The cobaloxime was active for H₂ production even in the presence of water as the proton source. Here, we have developed a simple airtight system connected with an online H₂ detector for the investigation of the photocatalytic activity by this hybrid complex. This photosensitizer-catalyst dyad present in the experimental setup continuously produced H₂ once it was exposed in the natural sunlight. This photocatalytic H₂ production by the hybrid complex was observed in aqueous/organic mixture media in the presence of a sacrificial electron donor under complete aerobic conditions. Thus, this photocatalysis measurement system along with the photosensitizer-catalyst dyad provide valuable insight for the development of next generation photocatalytic H₂ production devices.

Introduction

In the modern world, fossil fuels such as coal, oil, and natural gas supply a majority share of the energy. However, they produce copious amount of CO₂ during the energy harvesting to negatively impact the global climate¹. In coming years, a steep rise in energy demand is predicted worldwide following the continuous growth of population and constant improvement in human lifestyle. Thus, there is an active search for a suitable alternative energy resource to match the global energy requirement. Renewable energy resources like solar, wind, and tidal power have emerged as one of the best solutions due to their environment-friendly zero carbon energy transduction process². However, the intermittent nature of these energy resources has so far limited their extensive application. A possible solution of this problem can be found in biology; solar energy is efficiently transformed into chemical energy during photosynthesis³. Following this clue, researchers have developed artificial photosynthetic strategies for storing solar energy into chemical bonds following a number of small molecule activation reactions^4,5. The H₂ molecule has been considered one of the most appealing chemical vectors due to their high energy density and simplicity of their chemical transformation^6,7.

The presence of a photosensitizer and a H₂ production catalyst are essential for an active solar-driven H₂ production setup. Here in this work, we will focus on the cobalt-based molecular complex cobaloxime for the catalytic segment. Typically, a hexa-coordinated cobalt center is bound in a square planar N₄ geometry, derived from the dimethylglyoxime (dmg) ligands, in cobaloximes. The complementary Cl^- ions, solvent molecules (such as water or acetonitrile) or pyridine derivatives ligate in the residual axial positions⁸. Cobaloximes are long known for active H₂ production electrocatalysis and their reactivity can be tuned by appending variable functionalities on the axial pyridine^9,10,11,12. The relatively uncomplicated syntheses, oxygen tolerance under catalytic conditions, and moderate catalytic response of cobaloximes have prompted researchers to explore their photocatalytic H₂ production reactivity. The Hawecker group was the pioneer in demonstrating the light-driven H₂ production activity of cobaloximes by utilizing Ru(polypyridyl)-based photosensitizers¹³. Eisenberg and his coworkers utilized platinum (Pt)-based inorganic photosensitizers to induce photocatalytic H₂ production in tandem with cobaloxime catalysts^14,15. Later, the Che group utilized organo-gold photosensitizer to replicate similar activity¹⁶. Fontecave and Artero expanded the range of photosensitizers by applying iridium (Ir)-based molecules¹⁷. The practical applications of these photocatalytic systems were heading towards a roadblock due to the use of expensive metal-based photosensitizers. The Eisenberg and Sun research groups have countered that by independently devising organic dye-based photo-driven H₂ production systems^18,19. Despite the successful photo-driven H₂ production by all these systems, it was observed that the overall catalytic turnovers were relatively slow²⁰. In all these cases, the photosensitizer and cobaloxime molecules were added as separate moieties in the solution, and the lack of direct communication between them might have hindered the overall efficiency of the system. A number of photosensitizer-cobaloxime dyads were developed to rectify this issue, where a variety of photosensitizers were directly linked with the cobaloxime core via the axial pyridine ligand^{21,22,23,24,25,26}. Sun and co-workers were even successful in developing a noble-metal free device by introducing a Zn-porphyrin motif as a photosensitizer²⁴. Recently, Ott and coworkers have successfully incorporated the cobaloxime catalyst within an metal organic framework (MOF) that displayed photocatalytic H₂ production in the presence of organic dye²⁷. However, the inclusion of the high molecular weight photosensitizers into the cobaloxime framework reduced the water solubility while affecting the long-term stability of the dyads under catalytic conditions. The stability of the active dyads under aqueous conditions during the catalysis is crucial as the omnipresent water is an attractive source of protons during the catalysis. Thus, there is a serious need for developing an aqueous soluble, air-stable photosensitizer-cobaloxime dyad system to establish an efficient and economical photo-driven H₂ production setup.

Here in this work, we have anchored a stilbene-based organic dye²⁸ as photosensitizer to the cobaloxime core via the axial pyridine linker (Figure 1). The light molecular weight of the dye ensured improved water solubility of the dyad. This stilbene-cobaloxime hybrid molecule was characterized in detail via optical and ¹H NMR spectroscopy along with its single crystal structure elucidation. The electrochemical data revealed the active electrocatalytic H₂ production by the cobaloxime motif even with the appended organic dye. This hybrid complex exhibited significant photo-driven H₂ production when exposed to direct sunlight in the presence of an appropriate sacrificial electron donor in a 30:70 water/DMF (N,N′-dimethylformamide) solution without any degradation of the hybrid structure as complemented by optical spectroscopy studies. A simple photocatalytic device, consisting of a H₂ detector, was employed during the photocatalysis of the hybrid complex that demonstrated continuous production of H₂ gas under aqueous aerobic condition without any preliminary lag period. Thus, this hybrid complex has the potential to become the base for developing the next generation of solar-driven H₂ production catalysts for efficient renewable energy utilization.

Protocol

1. Synthesis of the photosensitizer-catalyst hybrid

1. Synthesis of catalyst precursor Co(dmg)₂Cl₂ complex
   NOTE: This complex was synthesized following the modified version of the reported procedure²⁹.
   1. Dissolve 232 mg (1 mmol) of dimethylglyoxime (dmg) ligand (two equivalents in this reaction) in 27 mL of acetone.
   2. Dissolve 118 mg (0.5 mmol) of CoCl₂∙6H₂O (one equivalent in this reaction) in 3 mL of deionized water separately that produces a pink color solution.
   3. Add the aqueous CoCl₂ solution drop wise to the acetone solution containing dmg with continuous stirring at room temperature.
   4. Closely monitor the change in the solution color, which will sequentially turn to bluish green color following the metal addition.
   5. Continue the reaction for 2 h.
   6. Filter the reaction mixture through a Grade 40 filter paper and keep the filtrate at 4 °C overnight.
   7. The next day, obtain the green colored precipitate of Co(dmg)₂Cl₂ complex (cobaloxime) from the solution and filter it through grade 40 filter paper.
   8. Dry the sample under air.
2. Synthesis of the photosensitizer (PS)-cobaloxime hybrid
   NOTE: The stilbene based photosensitizer (PS) was synthesized as per the reported method²⁸. The following steps were followed for the PS-catalyst hybrid complex synthesis.
   1. Add 100 mg (0.277 mmol) of cobaloxime (one equivalent) (synthesized in Step 1) in 5 mL of methanol. It will form a green suspension.
   2. Add 38 µL (0.277 mmol) of triethylamine (TEA) base (one equivalent) to the green suspension with continuous stirring. The solution will turn transparent brown after within 1 min.
   3. Add 65 mg (0.277 mmol) of solid stilbene dye (one equivalent) to the previously mentioned TEA added cobalt solution in methanol.
   4. Continue the stirring for 3 h. Closely monitor the change in the solution, which will sequentially produce the reddish-brown precipitate of the PS-cobaloxime hybrid.
   5. Filter the reddish-brown precipitate with Grade 40 filter paper and wash it with copious amount of cold methanol (20 mL).
   6. Dissolve the precipitate in chloroform (10 mL) and collect the reddish-brown filtrate.
   7. Evaporate the filtrate under reduced pressure using a rotavapor at room temperature.
   8. Collect the solid reddish-brown product [Observed yield: 76 mg (65%)].
   9. Recrystallize the product from chloroform solution at room temperature, where the chloroform evaporates slowly to produce reddish-brown crystals of the complex.

2. Characterization of the photosensitizer-cobaloxime hybrid

1. NMR characterization
   1. Dissolve 5.0 mg of the purified PS-Cobaloxime hybrid complex in 650 μL of d⁶-DMSO.
   2. Record the ¹H NMR in NMR spectrometer at room temperature.
      NOTE: ¹H NMR signals, in δ (ppm) units with the corresponding number of protons, their identity, and the splitting pattern in parentheses (s = singlet, d = doublet, m = multiplet), are as following: ¹H NMR: 2.34 (12H, -dmg-CH₃, s), 2.97 (6H, -dye-N-(CH₃)₂, s), 6.74 (2H, dye-aromatic, d), 6.84 (1H, allylic-H, d), 7.48 (5H, four dye-aromatic, one allylic-H, m), 7.82(2H, dye-aromatic, d), 18.47(2H,dmg-NOH, s).
2. UV-Vis spectroscopy
   1. Prepare a 1.0 mM solution of the PS-cobaloxime complex in N,N′-dimethylformamide (DMF) by adding the appropriately weighed amount of the complex in the solvent.
   2. Dilute the solution 10 times with blank DMF to generate 0.1 mM solution of the hybrid complex in DMF.
   3. Further dilute it 5 times with blank DMF to generate 20 μM solution of the hybrid complex in DMF.
   4. Record the optical spectra of the 20 μM PS-cobaloxime complex solution using a spectrophotometer.
      NOTE: UV-Vis peaks (λ/nm), with the corresponding molar extinction coefficient (ε /M^-1cm^-1) in parentheses, are as follows: 266 (13400) and 425 (14600).
3. Single crystal structure determination
   1. Prepare a concentrated 0.2 M sample of the PS-catalyst hybrid complex in 5 mL of chloroform. Grow reddish-brown (cubic) crystals of the complex from that chloroform solution over 3 days.
   2. Select a suitable crystal of the complex and mount on a cryo-loop using cryoprotectant (e.g., Paratone oil).
   3. Collect the single crystal diffraction data for the hybrid complex at 298 K on the diffractometer.
   4. Apply the empirical absorption correction to the data by employing the multi-scan method in SADABS programming³⁰.
   5. Resolve the structure by direct methods with SHELXS-97 and refine by the full matrix least square methods on F² using the SHELXL-2014³¹.
4. Electrochemical studies
   1. Sample preparation
      1. Prepare a 1 mM solution of the PS-catalyst hybrid complex in HPLC grade DMF containing 0.1 M tetra-N-butyl ammonium fluoride (n-Bu₄N⁺F⁻/TBAF).
      2. Place 2 mL of the sample solution prepared in step 1 in the electrochemical cell (volume 5 mL).
      3. Purge N₂ gas through the solution for 30 min to remove oxygen.
   2. Electrode preparation
      1. Polish the 1 mm diameter glassy carbon-disc working electrode with 0.25 μm alumina paste prepared in water on a polishing pad.
      2. Rinse the polished electrode thoroughly with a copious amount of deionized water.
      3. Place the clean working electrode in the electrochemical cell.
      4. Place the Ag/AgCl (in 1.0 M AgNO₃) reference electrode and the platinum (Pt)-wire counter electrode in the electrochemical cell.
      5. Connect all the electrodes accordingly to the potentiostat.
   3. Collecting data
      1. Stop the N₂ gas purging before the electrochemical experiment.
      2. Keep a continuous flow of N₂ above the sample solution in the electrochemical cell.
      3. Record cyclic voltammograms (CV) of the sample starting from the anodic direction to cathodic direction with appropriate scan rate (0.1 V/s scan rate was used in this experiment).
      4. Repeat the above experiment by adding appropriate amounts of water (30% water in DMF) and trifluoroacetic acid (TFA) (8 μL of 10x diluted neat TFA), respectively.
      5. Add ferrocene to the sample solution and record the corresponding CV. Adjust the potential scale with the ferrocene couple (FeCp₂^+/0 = 0V vs. Ferrocene) for all the collected data. Thus, all the potential values mentioned in this work was internally referenced against Ferrocene couple.

3. Catalytic H₂ production by the photosensitizer-catalyst hybrid in sunlight

1. Photocatalytic H₂ production by the PS-catalyst hybrid complex
   1. Prepare 0.2 mM PS-catalyst hybrid complex in 10 mL of 70:30 DMF water (pH 7, 0.1 MES buffer) in a two-neck test tube.
   2. Add 1 mL of triethanolamine (TEOA) as the sacrificial electron donor to the sample solution.
   3. Close the two openings of the test tube with the air-tight septum.
   4. Connect this setup with the H₂ detector with appropriate tubing connections.
      NOTE: The H₂ detector has two tube connections. One of them acts as the input that goes through an in-built detector to measure the amount of H₂ (in ppm units) present in the sample. The measured gas sample then connects back to the reaction vessel by the output tubing.
   5. Place the set up under sunlight for 30 min and monitor the H₂ production rate via the detector.
2. Monitoring the solar-driven H₂ production via gas chromatography (GC)
   1. Collect 1 mL of headspace gas via gas-tight syringe.
   2. Inject the collected gas in the gas chromatography (GC) instrument.
   3. Monitor the resulted gas chromatograph.
   4. Inject 1 mL of headspace gas collected from a control sample placed under dark.
   5. Inject 1 mL of gas from a calibrated standard gas mixture containing 1% H₂.

Results

In this work, a stilbene photosensitizer-cobaloxime hybrid complex (C1) was synthesized successfully by anchoring the organic dye (L1) derived pyridine motif as the axial ligand to the cobalt core. The ¹H NMR data of the hybrid complex clearly demonstrated the presence of both the cobaloxime and organic dye protons in the same complex. As shown in Figure 2, the up-fielded aliphatic region highlighted the presence of both oxime-bound methyl and sti...

Discussion

The organic photosensitizer stilbene moiety was successfully incorporated into the cobaloxime core via the axial pyridine linkage (Figure 1). This strategy allowed us to devise a photosensitizer-cobaloxime hybrid complex C1. The presence of both the oxime and organic dye in the same molecular framework was evident from the single crystal structure of the C1 (Figure 4). The phenyl and pyridine functionalities of the stilbene moti...

Materials

Name	Company	Catalog Number	Comments
1 mm diameter glassy carbon disc electrode	ALS Co., Limited, Japan	2412	1
Acetone	SD fine chemicals	25214L10	27 mL
Ag/AgCl reference electrode	ALS Co., Limited, Japan	12171	1
Co(dmg)2Cl2	Lab synthesised	NA	100 mg
CoCl2.6H2O	Sigma Aldrich	C2644	118 mg
d6 dmso	Leonid Chemicals	D034EAS	650 µL
Deionized water from water purification system	NA	NA	500 mL
Dimethyl formamide	SRL Chemicals	93186	5 mL
Dimethyl glyoxime	Sigma Aldrich	40390	232 mg
Gas-tight syringe	SGE syringe Leur lock	21964	1
MES Buffer	Sigma	M8250	195 mg
Methanol	Finar	67-56-1	15 mL
Platinum counter electrode	ALS Co., Limited, Japan	2222	1
Stilbene Dye	Lab synthesised	NA	65 mg
TBAF(Tetra-n-butylammonium fluoride)	TCI Chemicals	T1338	20 mg
Triethanolamine	Finar	102-71-6	1 mL
Triethylamine	Sigma Aldrich	T0886	38 µL
Trifluoroacetic acid	Finar	76-05-1	10 µL
Whatman filter paper	GE Healthcare	1001125	2