Nanotubes incorporated into a simple biomimetic light-harvesting system on a microfluidic platform could be used to develop molecular wires for energy transport thanks to a spectroscopic study undertaken by researchers at the universities of Groningen and Würzburg in Germany. [Kriete, B. et al., Nature Commun. (2019); DOI: 10.1038/s41467-019-12345-9]

Natural photosynthetic complexes can harness photons in a way that no synthetic systems yet can. They are even able to extract energy from their surroundings even when it is dark. Understanding and perhaps emulating their functionality on the nanoscale could revolutionize solar power and even optoelectronics. Unfortunately, natural photosynthetic systems are rather too complex to construct from simple bench-top components, so scientists must first focus on simplified versions of these hierarchical systems in order to approach the problem.

The team has used a new spectroscopic lab-on-a-chip approach based on advanced time-resolved multidimensional spectroscopy, microfluidics and extensive theoretical modeling to investigate their artificial light-harvesting device. The device is based on the multi-walled tubular antenna network of photosynthetic bacteria found in nature and consists of nanotubes made from light-harvesting molecules, self-assembled into double-walled nanotubes.

At low light intensity, the system absorbs photons in both walls, creating excitations or excitons. 'Due to the different sizes of the walls, they absorb photons of different wavelengths,' explains team leader Maxim Pshenichnikov. 'This increases the efficiency.' At high light intensity, a large number of photons is absorbed, creating a huge number of excitons. The team found that when two excitons meet in this system, one of them ceases to exist and this acts as a kind of safety valve for the system as excitons present in too high a number would damage the nanotubes themselves.

The team has thus demonstrated that a double-walled molecular nanotube can adapt to changing illumination conditions. They have emulated the essential functional elements of nature's design toolbox for low light conditions by acting as highly sensitive antennas. At the same time, the system sheds any excess energy when the photon count is much higher as might commonly occur in nature. Both these properties pave the way to better control of the transport of energy through complex molecular materials, the team suggests.

"We envision that the versatility of the microfluidic approach paired with higher order 2D spectroscopy opens the door to further expedite a better fundamental understanding of the excitonic properties of supramolecular assemblies and, thereby, will encompass rational design principles for future applications of such materials in optoelectronic devices," the team concludes.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is now available.