Every green leaf on the planet is running the same extraordinary trick, one that chemists have been trying to copy for decades. Inside the leaf, tucked within the membranes of microscopic structures called chloroplasts, a series of protein complexes absorbs sunlight and uses the energy to pull electrons away from water molecules on one side of a biological membrane and push them toward carbon dioxide chemistry on the other. The oxidation and the reduction happen in separate compartments, held apart by the membrane itself. That physical separation is the secret. It stops the two reactions from interfering with each other, prevents the energised electrons from collapsing uselessly back where they came from, and generates the proton gradient that ultimately powers the synthesis of ATP, the cell's universal energy currency.

Artificial photosynthesis has been trying to replicate this compartmentalised trick since the field began. A research team based across universities in Germany and Austria has now reported a significant step toward that goal in Angewandte Chemie International Edition. They have synthesised a rigid molecular wire small enough to thread completely through the membrane of an artificial lipid vesicle, absorbs visible light, and uses that light energy to ferry electrons from one side of the membrane to the other under conditions, including in the presence of ordinary air, that would cripple most previous approaches.

The molecule is, by the standards of this field, gratifyingly simple to make.

The Problem with Doing It All in One Place

To understand why the membrane matters, it helps to think about what happens when you try to run the two halves of artificial photosynthesis in the same flask. Reduction reactions, the kind that might produce hydrogen fuel or reduce carbon dioxide to useful chemicals, tend to work best under acidic conditions where proton availability favours the chemistry. Oxidation reactions, such as the splitting of water to release oxygen, tend to prefer alkaline conditions. Running both in the same container means compromising on conditions that are optimal for neither. Worse, the highly reactive intermediates generated by one half of the reaction tend to sabotage the other half before useful products can accumulate.

Nature solved this problem by doing what any good factory does: putting incompatible processes in separate rooms and building a controlled channel between them. The membrane is the wall. The photosynthetic protein complexes spanning it are the channel. Electrons flow one way, protons are pumped to create a gradient, and the two reaction spaces remain chemically distinct.

Artificial systems have attempted various approximations of this architecture. Some use freely diffusing molecules that carry electrons across the membrane by drifting back and forth through it, a process known as diffusion based electron transfer. These systems work under carefully controlled, oxygen free conditions, but oxygen present in air rapidly quenches the process, shutting down the chemistry entirely. Others have used more structurally elegant rigid molecular wires spanning the membrane, but these have been asymmetric in design, meaning the molecule can insert into the membrane from either face with no preferred orientation, which complicates reproducible function. Some have required synthetically demanding multi step routes to construct.

The new molecular wire addresses all three limitations simultaneously.

A Wire Designed to Fit

The molecule the team built is called [1]2+ and it is, at its heart, a rigid rod of connected aromatic rings. At the centre sits a unit called benzothiadiazole, a light absorbing component that captures visible light efficiently and whose electronic properties make it particularly suited to participating in redox chemistry. Flanking it on either side are fluorene units that keep the central region hydrophobic, meaning water repelling, which encourages the molecule to bury itself in the fatty interior of a membrane rather than floating freely in water. At the two ends of the rod sit charged trimethylammonium groups that are water loving and anchor the molecule at the polar surfaces of the membrane on either side.

The distance between those two end groups is approximately 3 nanometres. This is not a coincidence. Lipid bilayer membranes, the kind that form the walls of biological cells and artificial vesicles alike, are typically between 3 and 5 nanometres thick. The molecule was designed to match this dimension precisely, threading through the membrane like a bolt through a wall, with its charged ends embedded in the polar head group layers on each face and its hydrophobic core buried in the fatty interior. The symmetric design means the molecule inserts with a defined and reproducible orientation.

The synthesis required only five steps and achieved an overall yield of 18 percent. For a molecule performing this function, this is straightforward.

The team confirmed the membrane orientation using multiple independent methods. Confocal microscopy of giant vesicles tagged with the molecule showed a characteristic pattern in the fluorescence image: a double half moon shape around the perimeter of each vesicle, which arises when a fluorescent molecule adopts a specific orientation in the curved membrane rather than sitting randomly. Molecular dynamics simulations, in which the molecule was placed randomly in a computational model of the membrane and allowed to find its preferred position, showed it consistently adopting the transmembrane configuration within the simulation timescale.

"The system is active in both aerobic and anaerobic atmospheres, rendering it ideal for aerobic conditions or reactions that produce oxygen such as solar-driven water splitting and artificial photosynthesis applications."

KEY FACTS

What is a lipid bilayer and why does it matter for artificial photosynthesis? A lipid bilayer is a two layer sheet of fatty molecules that forms the basic structure of biological membranes. Each molecule has a water loving head and a water repelling tail; when many of these molecules are placed in water they spontaneously arrange themselves into a double layer with tails pointing inward and heads facing outward. This structure is impermeable to most water soluble molecules, making it an ideal barrier for separating two different chemical environments. In artificial photosynthesis, the bilayer plays the same role it plays in leaves: physically separating the oxidation and reduction reactions so each can proceed under conditions optimal for its own chemistry without interfering with the other.

What are liposomes? Liposomes are tiny spherical vesicles made from lipid bilayers, essentially hollow bubbles with walls made of the same membrane material as biological cells. In this experiment, they serve as the reaction vessel: one chemical reagent is trapped inside the liposome, another is placed in the surrounding solution, and the molecular wire threaded through the membrane connects the two compartments electronically when illuminated by light.

Why does ordinary air normally prevent these reactions from working? Most artificial photosynthesis systems use freely diffusing electron carrying molecules that shuttle back and forth through or across membranes. Oxygen in air reacts rapidly with these molecules and with the reactive intermediates they generate, quenching the charge transfer process before useful chemistry can occur. This is why most previous systems had to be run under inert gas, typically argon or nitrogen. The rigid molecular wire in this study does not rely on diffusion: it transfers electrons directly across the membrane through its fixed structure, and this makes it far less vulnerable to oxygen interference.

What is the significance of the symmetric design? Most previous rigid transmembrane molecular wires reported in the literature were asymmetric, meaning they had different chemical groups at each end. An asymmetric molecule can insert into a membrane from either face with equal likelihood, resulting in a mixture of orientations that is difficult to control and characterise. The symmetric design of the new molecule, with identical charged groups at both ends, means it inserts with a single defined orientation regardless of which face it encounters first. This gives the system a predictability and reproducibility that asymmetric designs lack.

Light In, Electrons Across

To test whether the system actually worked, the team prepared liposomes with the molecular wire embedded in their membranes and loaded a molecule called NADH into the interior aqueous compartment. NADH is the biological electron carrier used in cellular metabolism, a molecule that readily donates electrons and whose oxidation is easy to monitor spectroscopically. On the outside of the liposomes, in the surrounding solution, they placed a molecule called XTT, a yellow compound that turns an intense orange colour when it accepts electrons.

The experiment was simple to read. If electrons were crossing the membrane, driven by light absorbed by the molecular wire, XTT on the outside would turn orange as it accepted those electrons. The NADH inside would simultaneously lose its electrons and be oxidised. If no transmembrane electron transfer occurred, nothing would happen.

Under illumination with blue light at 470 nanometres, the orange colour steadily grew. Spectroscopic measurements confirmed that NADH was being consumed inside the vesicles while formazan, the orange reduced form of XTT, was accumulating outside. Multiple control experiments confirmed that the colour change was not due to leakage of the reagents through the membrane, to direct reaction between them in the absence of light, or to any process not involving the molecular wire.

The reaction worked under both inert argon atmosphere and in open air containing oxygen. The rate and quantum yield under air were approximately half those measured under argon, a modest reduction compared to the near complete suppression that oxygen causes in diffusion based systems. The researchers also tested what happened when the two reagents were mixed together in bulk solution with the wire present but without the liposome membrane separating them. Under those non compartmentalised conditions, the efficiency was much lower and oxygen sensitivity much higher, confirming that the physical separation provided by the membrane was itself a significant contributor to the system's robustness.

Adding a Proton Channel

One inherent challenge with transmembrane electron transfer is that moving electrons across a membrane creates a charge imbalance. As electrons move from NADH inside to XTT outside, positive charge accumulates on the inner face and negative charge on the outer face, building up an electrical potential that increasingly opposes further electron transfer. Unless something compensates for this charge imbalance by moving ions in the opposite direction, the process will stall.

In natural photosynthesis, the proton gradient built up by this charge separation is not a problem to be overcome but the very point of the exercise: it drives ATP synthesis. In the artificial system, the team tested whether adding a proton conducting channel could improve performance by dissipating the charge buildup. They incorporated gramicidin A, a naturally occurring peptide that forms proton selective channels in lipid membranes, alongside the molecular wire. Under ambient air conditions, gramicidin A increased the electron transfer rate by 27 percent and improved the initial quantum yield by 50 percent, confirming that charge compensation is a meaningful factor in the system's performance and one that can be tuned independently of the electron transfer mechanism itself.

A second ion carrier, 18 crown 6 ether, which selects specifically for potassium rather than protons, did not produce the same improvement, consistent with the view that proton movement specifically is what needs to be balanced in this system.

What Comes Next

The work described in this paper is explicitly a proof of concept. The model reactions used, the oxidation of NADH on one side and the reduction of a coloured dye on the other, are chosen for their ease of measurement rather than for their practical utility. The ultimate goal is systems capable of driving real solar fuel chemistry: splitting water to produce hydrogen, or reducing carbon dioxide to produce carbon based fuels and feedstocks.

Several challenges remain before that goal is reached. The inner compartment of a liposome is a fixed and finite volume; eventually the NADH supply is exhausted and cannot easily be replenished without disrupting the vesicle. The quantum yield of 0.045 percent under inert conditions, while clearly demonstrating the principle, is far below what would be needed for practical solar energy conversion. And while the system's tolerance of oxygen is a significant advance over diffusion based predecessors, further improvement would be desirable for reactions that actively produce oxygen, as water splitting does.

The researchers note several directions for scaling and adapting the approach. The same transmembrane wire design could in principle be incorporated into polymer based membranes, which are more robust and durable than lipid bilayers, potentially enabling larger scale photoreactor devices. The compatibility of the lipid bilayer with biological molecules opens the possibility of incorporating enzymes into the system, with the membrane serving as an organised platform for a multi step catalytic sequence. And the rigid molecular architecture, which makes the wire work without diffusion, suggests it could function in solid state or gel phase membranes where molecular mobility is restricted.

For now, a tiny yellow bubble smaller than a human cell, threaded with a glowing orange molecule and illuminated by blue light, is turning orange on its outside walls while the electrons that colour it travel in a single directed step across a wall three nanometres thick. It is a very small version of what every leaf has been doing for hundreds of millions of years. But it works in air.

Publication Details: Year of publication: 2025 Journal: Angewandte Chemie International Edition Publisher: Wiley-VCH GmbH Volume / Article: e202423393 DOI: https://doi.org/10.1002/anie.202423393

Credit & Disclaimer: This article is based on the peer reviewed research paper. All scientific facts, findings, and conclusions presented here are drawn directly from the original study and remain unchanged. This popular science article is intended purely for general educational purposes. Readers are strongly encouraged to consult the full research article for complete spectroscopic data, quantum yield measurements, molecular dynamics simulations and mechanistic analysis.