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

Here, we are developing a photocatalytic system that can produce hydrogen from water, using water as a proton source. Secondly, we would like to stabilize the system under fully aerobic condition. The blueprint of our system is based on biology.

Biology uses photosystems too, where it harvests solar energy to produce chemical energy. We are going to mimic that system in a synthetic background. Over here, our system will consist of three important parts.

First, there will be a photosensitizer, which will nothing but a simple organic dye. Secondly, there will be a cobalt-based catalyst that will produce hydrogen from proton. And the third important part of this whole molecule hybrid will be a linker, which will be connecting the photosystem and the catalyst.

This linker will be an extension of the organic molecule, which will be nothing but a pyridine motif which will be anchoring with the cobaloxime linker. The whole experimental segment is divided into three parts. First one is synthetic procedure of the complex.

Second one is the characterization of the complex. The third one is the application of this complex for hydrogen production under sunlight in aerobic conditions. First we start the synthesis of cobaloxime.

At first we take the acetone. Then we add the dimethylglyoxime to it, and dissolve it, through stirring. On the other hand, we take an aqueous solution of cobalt chlorate.

The aqueous solution of the cobalt chlorate was added dropwise to the acetone solution of dimethylglyoxime. At first the solution will be blue, which will slowly turn to bluish-green. Then the solution was filtered after two hours of the reaction, and the filtrate was kept at four degrees centigrade over night.

Then we obtained a green colored precipitate. In the second step of the synthesis we are going to use the cobaloxime we synthesized in the first step. Here, we are going to use our photosensitizer dye, which actually has a pyridine linker that is going to add to the cobaloxime in an axial position.

In the second step of the synthesis, first we take a metallic solution of our cobaloxime, which is green in color. Then we add one equivalent of triethylamine base to it, which will slowly change the color of the solution to brown, and it will be a transparent brown color solution. Then, we add one equivalent of the photosensitizer dye into it, and then stir the solution for three hours.

After half an hour, we started seeing a precipitate coming out of the solution. We take this precipitate out after three hours of experiment through filtration of Whatman 40 filter paper. Then, the precipitate was washed with cold methanol.

Then, the precipitate was collected in a solution form by adding chloroform to it. The brown colored solution we obtained in this step was slowly evaporated at room temperature to obtain the brown colored product of the photosensitizer-cobaloxime hybrid complex. So, in the characterization part, we first dissolved the complex in deuterated DMSO, and got the proton spectra of the complex.

So, here is a proton spectra. The aliphatic region consists of two types of protons:12 protons from dimethylglyoxime, and six protons from photosensitizer. The aromatic region mainly accounts for PS dye, and the two protons from dimethylglyoxime.

Here, we are highlighting aromatic region, where all protons are assigned accordingly. Next, we did the optical spectroscopy of our complex. We serially diluted the complex up to 20 micromolar, and then recorded its optical spectrum in this UV spectrophotometer.

If we look into the UV spectrum, we can see there are two important bands coming into the complex. First, in the UV region we see a band that is possibly from the pi to pi-star transition. And then secondly, one band in the visible region which is for the LMC transition.

If we compare that with the starting material of the photosensitizer and the cobaloxime complex we can see these peaks we observe for our hybrid complex is clearly different from the starting materials. Here, we have gone the single crystal of our photocatalyst-cobaloxime hybrid complex, from chloroform solution. Then, we diffract it, this brown colored crystal, so a single crystal exited the diffractometer.

The structure we have obtained from the single crystal diffractometer, we can clearly see the cobaloxime complex bind the photosensitizer through the axial pyridine linker. The cobalt and nitrogen pyridine linker bond distance over here is 1.965 angstrom, which is very similar to the analogous complex of similar jar. We have performed the electrochemistry of the cobalt-photosensitizer hybrid system in a standard three electrode mechanism.

The first electrode is the working electrode, which is nothing but a one millimeter radius glassy carbon disc electrode. This glassy carbon disc electrode was thoroughly polished with 125 micron aluminum powder, and washed with deionized water before it's used. Then, it was assembled in a three electrode system along with the silver silver chloride reference electrode, and a platinum, or counter electrode.

Then, the solution was degassed with nitrogen, before the actual experiment. Then, we recorded the cycling voltammogram of our complex. First, we started in the DMF solution.

We started the scan from the anodic region, and slowly moved to the cathodic region. And we see, a few stoichiometric peaks. Then, when we add water to the solution, one of the cathodic peak increases in intensity.

That is possibly due to the hydrogen production on that center. Which was confirmed later, when we added acid in the same solution, and that current is going to increase further. So that clearly shows that in presence of water, the cobalt complex, in presence of photosensitizer, can be active for hydrogen production.

This complex was studied for photocatalytic hydrogen production under ideal sunlight and in aerobic conditions. In a closed system we have photosensitizer-cobalt hybrid complex dissolved in 7H23 DMF water, with added sacrificial electron donor. This closed system was connected to hydrogen detector, so we can monitor hydrogen from solution continuously.

We have observed a continuous growth in hydrogen in this setup, when the PS catalyst hybrid was exposed to sunlight. The accumulation of hydrogen was continuous, and no lag period was noticed. We have further confirmed the formation of hydrogen via gas chromatrography, or GC, experiment.

We have collected the headspace gas via syringe, and injected that in GC to observe a signal for hydrogen. The hydrogen identity in GC was confirmed, by complementary experiment with control and blank injection. Here, we have synthesized a successful model, where we have included the photosystem and the catalyst together in the same molecule.

The overall mechanism of the reaction possibly start from the excitation of the photosensitizer. Where it takes the sunlight, and get to the excited state. In the excited state, it loses the electron.

After it loses the electron, it becomes a positively charged ion, which takes an electron from the Sacrificial Electron Donor to come to the ground state again. On the other hand, the released electron probably traveled through the linker to the catalyst. On the catalyst system, once it gets the electron, it goes to the reduced state.

And in the reduced state, it reacts with proton to produce the hydrogen, to complete the cycle of the catalysis. In this project, we have successfully developed a photosensitizer-cobalt catalyst hybrid system, that can produce hydrogen directly from water. And this hydrogen production happens continuously for one hour, without any lag period.

And this full system is stable and active under aerobic conditions.