1. Synthesis of the photosensitizer-catalyst hybrid
2. Characterization of the photosensitizer-cobaloxime hybrid
3. Catalytic H2 production by the photosensitizer-catalyst hybrid in sunlight
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 stilbene N-dimethyl proton signals in appropriate proportions at δ (ppm) 2.34 and 2.97, respectively. The aromatic and unique allylic proton signals from the stilbene skeleton were sighted in the 6.74-7.82 δ (ppm) region, which was highlighted in detail in the inset of Figure 2. The stability of the cobaloxime core was exemplified by the presence of the intra-molecular hydrogen bonding in the oxime moiety in the far down field region (~12.47 δ (ppm))11. The optical spectra of the hybrid complex C1 exhibited two major signals (Figure 3). In the UV region, a distinct signal was observed at 266 nm. This signal resembled the characteristic π‒π* transition originated from the oxime scaffold. Another optical transition was noticed for C1 in the visible region at 425 nm. This signal is significantly red-shifted compared to the typical π‒π* transition observed for the stilbene compound (λmax 385 nm) (Figure 3)32. This transition observed in C1 possibly has significant contribution from the Npyrdine‒Co(III) ligand to metal charge transfer (LMCT) transition, analogous to similar axial pyrine bound cobaloximes29,33. The ligation between stilbene-derived pyridine motif and cobaloxime was definitively verified with the single crystal structure data of C1. As shown in Figure 4, the critical Npyrdine‒Co bond distance was measured at 1.965 Å, similar to typical axial Npyrdine‒Co bonds9. The aromatic rings along with the allylic group remained in the same plane in the hybrid complex C1 that ensure an elongated conjugation in the stilbene moiety. The details of the crystal data collections and data refinement parameters are given in Table 1. The complete crystallographic information file (CIF) of the PS-catalyst hybrid complex was deposited in the Cambridge crystallographic data centre (CCDC No: 1883987)34.
A cyclic voltammetry (CV) experiment was performed with the PS-catalyst-hybrid complex C1 staring with a cathodic scan in the range of 0.5 V to -1.8 V in DMF (Figure 5). An irreversible reduction signal was observed at -1.0 V (vs. Fc+/0) followed by two successive reversible signals at -1.3 and -1.5 V. The first reductive signal can be assigned as the metal based Co(III/II) reduction while the reversible signals were attributed to the stoichiometric redox processes at aromatic organic dye framework32. C1 demonstrated a distinct catalytic signal at -1.25 V when water was added to the solution. Electrocatalytic H2 production was possibly responsible for this cathodic catalytic behavior. This hypothesis was corroborated by a gradual increase in that catalytic response following the addition of TFA in the same solution (Figure 5). The turnover frequency (TOF) for these catalytic responses was tabulated using the following equation:
where icat = catalytic current, ip = stoichiometric current, n = number of electrons involved in this process, R = universal gas constant, T = temperature in K, F = 1 Faraday, and ν = scan rate. The TOF for H2 production in the presence of water and aqueous TFA were 30 s-1 and 172 s-1, respectively. The complementary chronocoulometric (bulk electrolysis) experiment was used along with the complementary gas chromatography (GC) to provide further evidence of H2 production during the catalytic step with 70% Faradaic efficiency (details in supplementary section, Figure S1).
The H2 production activity of the cobaloxime core in C1 was further investigated during the photo-catalytic studies. In this experiment, C1 was loaded in an airtight container containing 30:70 water/DMF solvent along with TEOA sacrificial electron donor. This system was connected to the H2 sensor and exposed to natural sunlight (power density ~ 100 mW/cm2) (Figure 6). As shown in Figure 7, the PS-catalyst hybrid complex C1 displayed catalytic H2 production immediately following the sunlight exposure. In this case, an almost linear increase in photocatalytic H2 production was observed over time. The identity and purity of the photo-generated gas accumulated in the headspace of the set-up was validated by gas chromatography (GC). As illustrated in Figure 8, solar-driven, H2 production was confirmed by the GC results. The minimal change in the comparative optical spectra demonstrated the stability of C1 during this experiment (Figure S2).
Figure 1: Reaction scheme. The scheme represents the synthetic route for the PS-catalyst hybrid complex. Please click here to view a larger version of this figure.
Figure 2: 1H NMR spectra of PS-catalyst hybrid complex C1. This figure displays the 1H NMR of PS-catalyst hybrid complex recorded in d6-DMSO at room temperature. The aliphatic region consists of oxime-methyl groups (12 H, a), and PS-bound N-methyl groups (6 H, b) (black trace). The aromatic region consists of 10 H, containing both aromatic (c, d, e, f) and allylic (g and h) protoms. The oxime (-NOH) protons are the most down-shielded protons (i) (red trace). The inset highlights the detailed splitting pattern of the aromatic (blue trace) and allylic protons (green trace). Please click here to view a larger version of this figure.
Figure 3: Comparative optical spectra. The comparative Uv-vis spectra of PS (black trace), cobaloxime precursor (red trace), and PS-catalyst dyad C1 (blue trace) recorded in DMF at room temperature. The formation of the hybrid complex distinctly red-shifted the LMCT band, while the π‒π* transition remained same. Please click here to view a larger version of this figure.
Figure 4: Single crystal structure of photosensitizer-Cobaloxime hybrid C1. ORTEP representation for C1 with 50% thermal ellipsoids probability. The carbon (grey), hydrogen (white), oxygen (red), nitrogen (sky-blue), chlorine (green), and cobalt (deep blue) atoms are shown in the figures accordingly. One chloroform molecule was found inside the crystal lattice, but it is omitted here for clarity. Please click here to view a larger version of this figure.
Figure 5: Comparative cyclic voltammograms. The comparative cyclic voltammograms (CVs) of 1 mM C1 in only DMF (black trace), in the presence of 30:70 water/DMF (blue trace), and in the presence of 16 equivalent TFA in 30:70 water/DMF(red trace) were shown in the figure. The scans were performed in the presence of 0.1 M tetra-N-butyl ammonium fluoride (n-Bu4N+F−/TBAF) as supporting electrolyte utilizing 1mm glassy carbon disc working electrode, Ag/AgCl (in 1.0 M AgNO3) reference electrode and platinum (Pt)-wire counter electrode at room temperature with 0.1 V/s scan rate. The initial scan direction is shown with the horizontal black arrow. Please click here to view a larger version of this figure.
Figure 6: The photocatalytic H2 production monitoring system. The schematic representation of the experimental set-up, consisting of an online H2 detector, used for continuous monitoring H2 production by photosensitizer-cobaloxime dyad C1 under natural sunlight and complete aerobic condition. Please click here to view a larger version of this figure.
Figure 7: Photocatalytic H2 production by C1 over time. The accumulation of H2 over time during the natural sunlight-driven photocatalysis by photosensitizer-cobaloxime hybrid complex C1 as detected by the online H2 detector. Please click here to view a larger version of this figure.
Figure 8: Comparative gas chromatography data. Comparative gas chromatography (GC) data recorded for the head space gas collected from the photosensitizer-cobaloxime dyad C1 placed under dark (black trace) and natural sunlight (blue trace). The red trace signified the signal from the 1% H2 calibration gas mixture sample. Please click here to view a larger version of this figure.
Figure 9: Photocatalytic scheme for H2 production by C1. Possible photo-catalytic H2 production cycle for the PS-catalyst hybrid complex C1. This mechanism presumably follows the sequence of excitation of the photosensitizer, transfer the excited electron to the catalyst via linker, and H2 production catalysis at the reduced catalytic centre. The cationic photosensitizer returns to the ground state by accepting electron from the sacrificial electron donor. Please click here to view a larger version of this figure.
Supplementary Materials. Please click here to download this file.
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 |
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.
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.
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.