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 Synthesis of catalyst precursor Co(dmg)2Cl2 complex NOTE: This complex was synthesized following the modified version of the reported procedure29. Dissolve 232 mg (1 mmol) of dimethylglyoxime (dmg) ligand (two equivalents in this reaction) in 27 mL of acetone. Dissolve 118 mg (0.5 mmol) of CoCl2∙6H2O (one equivalent in this reaction) in 3 …

Representative 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

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