Stepwise Electron Relay Across Stacked Perylene Bisimide (PBI) Dyes

Electrons have been shown to travel in a rapid, stepwise manner across a stacked set of dye molecules in just trillionths of a second, creating a swift, relay-style route through the assembly.

This behaviour means a precisely ordered molecular stack can act as a practical blueprint for steering charge over longer ranges in light-activated materials.

Inside the stack

Within a stack made up of four molecules, red light drove charge from a donor towards a terminal acceptor, passing through a linked bridge in between.

At Julius-Maximilians-Universität Würzburg (JMU), doctoral researcher Leander Ernst demonstrated that the electron progressed through each molecular unit one after another, rather than jumping straight across the whole structure.

When the stack was extended, the charge still followed the same route, instead of cutting out early or dropping back.

That sustained transport relied on the precise molecular packing, which determines when the relay completes successfully and when it can stall.

Perylene bisimide (PBI) molecules steer charge

The chemists selected perylene bisimide (PBI), a robust dye class used in electronics. One end unit absorbed slightly redder light than the rest, allowing the researchers to initiate the relay from a chosen side.

At the other end, a unit with a stronger pull on electrons created an energetically downhill pathway, avoiding a trapped, dead-end state.

“We can specifically trigger the charge transport in this structure with light and have analysed it in detail,” said Ernst.

Early evidence of relay

When red light excited the starting portion of the molecule, the usual emission dimmed very quickly.

In the simple solvent toluene, the brightness fell from roughly one in five flashes to almost nothing in the shortest construct.

Longer constructs still produced only a weak glow, indicating the energy was being carried away rather than re-emitted.

This loss of brightness suggested the system had accessed a quicker route, moving energy through the structure instead of releasing it as light.

Solvent changes everything

In toluene, the travelling charge could reach the central part of the structure, but it frequently failed to complete the final stages.

In a different solvent type-one that stabilises charges more readily-the last step became easier to achieve.

With that support, the charge could continue onwards, reaching the end rather than becoming stuck part-way.

Without such assistance from the surrounding liquid, the charge often slipped back and remained close to where it started.

Distance reveals route

One key metric tracked how effectively charge transport persisted as the distance increased.

In this case, the signal declined only modestly as the pathway lengthened-much less than would be expected if the process were a single long jump.

This trend is consistent with a hop-by-hop mechanism, where charge moves between neighbouring units in small steps.

Thanks to this relatively consistent motion, the stacked dyes acted more like a tiny wire than a delicate, one-off transfer route.

Inspired by plant energy

In photosynthesis, plants protect useful energy by separating charge before it has a chance to recombine.

Artificial systems require the same principle, because once charges are kept apart they can later power fuel-forming chemistry rather than being lost as heat.

These new PBI arrays reproduced that early handover with unusually good control, while still being far simpler than a living cell.

That simplicity is valuable, since chemists can alter one component at a time and directly observe what each redesign changes.

Why speed counts

Rapid forward transfer is important because electrons that linger are more likely to fall back, undoing the separation devices rely on.

In the longest stack, the charge reached its final state in about 1,100 trillionths of a second in the polar solvent.

Even though that later stage was slower, it still followed an ultrafast initial step that occurred in about one trillionth of a second.

As recombination became slower in longer arrays, the design effectively bought more time for useful processes to occur afterwards.

Current limits of design

Even so, the system did not operate as a flawless molecular wire in every setting.

In non-polar environments the final step was blocked, and the longest transfer still slowed as the distance grew.

Using stronger acceptor units, or adjusting how neighbouring molecules overlap, could help drive charge further and faster in future versions.

For that reason, the JMU team is now pursuing longer stacks, working towards a wire built from interconnected molecules.

Applications in electronics

Improved molecular charge routing could enable thinner solar materials that lose less absorbed light before it can be converted into useful work.

The same level of control could also be important for sensors, photodetectors and organic circuits that depend on directed charge movement.

Because this PBI stack relies on close contact rather than a long, rigid bond, designers gain another lever for tuning performance.

That added flexibility could make future materials easier to adapt, particularly when a given formulation performs poorly under real operating conditions.

Why this matters

A four-dye stack demonstrates that carefully spaced molecules can hand charge along via fast, stepwise transfer rather than a single long-range leap.

This shifts the chemistry beyond a laboratory curiosity, while leaving clear engineering challenges around building longer, more robust and more selective systems.