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 ....

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 so....

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

List of materials used in this article

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