HIGHLIGHTS
• Multi-electron transfer was achieved through the synthesis of a five-component, molecular complex that reversibly accumulates two positive and two negative charges through multi-electron transfer reactions.
• The stability of the molecular complex is due to the placement of the positive and negative charges at opposite ends.
• The multi-electron transfer can be carried out in dimmer light that is closer to the intensity of sunlight used by plants in photosynthesis. 

Research is continuing in an effort to find cost effective processes for converting carbon dioxide into useful intermediates and products. Plants that contain the pigment chlorophyll have been effectively using light to convert carbon dioxide to the sugar glucose through natural photosynthesis. 

The first phase of natural photosynthesis involves the formation of a charge-separated state due to the interaction of light with chlorophyll creating the transfer of one electron to form a charge-separated state that consists of an oxidized chlorophyll and reduced quinone. While this process can be emulated artificially through the preparation of artificial donor-photosensitizer-acceptor compounds, challenges still exist in developing a feasible artificial photosynthesis pathway.

In a previous TLT article,¹ researchers found that the bacterium, Azotobacter vinelandii, can enzymatically convert carbon dioxide to carbon monoxide. This reduction occurs more effectively under anerobic conditions. Further reaction of carbon monoxide with the bacterium’s enzyme converts carbon monoxide to ethylene, ethane and propane over an eight-hour period at ambient temperature. This hydrocarbon formation was determined to be a secondary metabolic pathway that is not essential for the growth of the bacterium cell. 

The challenge facing researchers in preparing an artificial pathway is to develop multi-electron transfer reactions to efficiently reduce carbon dioxide so that solar energy can be converted into hydrocarbons that can be used to prepare energy-rich chemical fuels and lubricants. 

Oliver Wenger, professor of chemistry at the University of Basel in Basel, Switzerland, says, “There are quite a number of challenges that need to be overcome to enable artificial photosynthesis to become commercially viable. Simple electron, hole combinations can be prepared, but there are many follow-up steps that need to be conducted to produce a viable product such as ethanol or hydrogen that can be used as a fuel.”

As explained by Mathis Brändlin, graduate student at the University of Basel, “Electron donor-photosensitizer-electron acceptor compounds can form charge-separated states with components that have been oxidized (positively charged) and reduced (negatively charged). These species formed from single electron transfer reactions are not stable and will typically spontaneously revert back to their original state in a charge recombination reaction that will occur within only a few hundred nanoseconds.”

The problem faced by this system is that after moving to an initially charged separated state by a single light photon, there is an introduction of a second photon charge separation. 

Wenger, Brändlin and their colleagues have now overcome this issue by preparing a new molecular donor-photosensitizer-acceptor molecule that can produce two positive and two negative charges when exposed to light. 

Molecular complex
The researchers synthesized a five-component, molecular complex that is able to reversibly accumulate two positive and two negative charges through multi-electron transfer reactions. This molecular complex combines a triarylamine, phenothiazine, a ruthenium complex, an anthraquinone and a naphthalene diimide. Figure 3 shows a schematic of the molecular complex which is designated as a pentad. The two blue spheres on the left are electron donors which is why they have “+” charges, while the two purple spheres on the right are electron acceptors which explains the presence of “-” charges. In the middle is the photosensitizer.


Figure 3. A five-component molecular complex, known as a pentamer, has been found to accumulate two positive (blue spheres on the left) and two negative (purple spheres on the right) and may be suitable for use in artificial photosynthesis. Figure courtesy of the University of Basel.

Brändlin says, “All of the components in the pentad are known as electron acceptors, electron donors and the photosensitizer. We were able to synthesize this molecular complex using known methods under ambient conditions.”

A two-step process was used by the researchers to generate the multi-electron transfer. In Step 1, a 20 micromolar solution of the molecular complex in acetonitrile was subjected to laser pulses of 10 nanoseconds in duration at 460 nanometers. The excitation by one photon of the ruthenium component in the molecular complex led to a single electron transfer from the triarylamine to the naphthalene diimide component. 

This produced a charge-separated state designated as CSS-2. Brändlin says, “We found that CSS-2 formed within 10 nanoseconds and stores 1.3 electron volts of energy. A promising characteristic is this charged state is stable for 120 microseconds which is a large lifetime.”

The reason for the stability is that the positive and negative charges of the pentad are at the opposite ends of the molecular complex and are separated by 44 angstroms. Brändlin says, “The large separation of the two charges increased the stability of the charge-separated state by reducing the chance of recombination.”

Further light absorption by CSS-2 produces a second electron transfer leading to the formation of the charge-separated state, CSS-3. By using a continuous-wave-pump-pump probe experiment, each of the electron donors (triarylamine and phenothiazine) contributed one electron to each of the electron acceptors (anthraquinone and naphthalene diimide). The product of the multi-electron transfer, CSS-3, was found to have a lifetime between 100 and 1,100 nanoseconds and to store approximately 3 electron volts.

Wenger says, “An important aspect of this finding is the multi-electron transfer can be accomplished using dimmer light which enables us to move close to the intensity of sunlight instead of relying on extremely strong laser light.”

The synthesis of a molecular complex that can undergo multi-electron transfer is a step forward in creating artificial photosynthesis. A molecular complex now has been able to use two photons to create two-electron transfers that can potentially be used in artificial photosynthesis.

Additional information can be found in a recent article² or by contacting Wenger at oliver.wenger@unibas.ch. 

REFERENCES
1. Canter, N. (2017), “Enzymatic conversion of carbon monoxide to hydrocarbon,” TLT, 73 (4), pp. 14-15. Available at www.stle.org/files/TLTArchives/2017/04_April/Tech_Beat_III.aspx.
2. Brändlin, M., Pfund, B. and Wenger, O. (2025), “Photoinduced double charge accumulation in a molecular compound,” Nature Chemistry, https://doi.org/10.1038/s41557-025-01912-x.