Organic molecules for new devices

Voltage sensing is important in many applications and is typically done with conventional electrical devices such as electrodes. Optical sensing of voltage opens the door to a new era of devices. Scientists manipulated molecules involved in electron transfer to develop voltage-sensitive fluorescence within the context of the project 'Optical probes for membrane potential via photo induced electron transfer' (VOLTAGE-PROBE).

Researchers focused on porphyrins, naturally occurring molecules made of four linked heterocyclic groups often with a metal atom at their centre. Porphyrins form many important substances in the body, including haemoglobin — a protein in red blood cells with iron in the centre that carries oxygen. They also make up chlorophyll, the green pigment in plants responsible for absorption of light. These structures also demonstrate unusual magnetic behaviours based on electron spin and could lead to novel organic-based magnetic data storage.

Photo induced electron transfer in monomeric porphyrins has been studied extensively. Very little is known about porphyrin oligomers, molecules made of two or more porphyrin molecules joined together. VOLTAGE-PROBE synthesised porphyrin macromolecules consisting of between one and six porphyrin units covalently linked together. Scientists then used spectroscopic techniques to study electronic properties and electron transfer.

VOLTAGE-PROBE researchers demonstrated that curved p-systems (conjugation formed by the overlap of p-orbitals) have unique electronic properties (spin states and electronic structures) differing from those of their linear analogues. Rigid porphyrin nanorings of six porphyrin units were extensively characterised, providing important insight into electronic structure.

The team then went on to develop novel synthesis methods for extended porphyrin tubes. Although much attention has been given to other carbon-based nanostructures such as fullerenes and carbon nanotubes, very little is known about porphyrin nanotubes. Scientists investigated their optoelectronic properties, which were found to be closely related to those of nanorings. One small difference was the onset of florescence, which was shifted to the red in nanotubes.

Porphyrin-based organic macromolecules have interesting optical, electronic and magnetic properties, rendering them potentially useful in a plethora of applications. The VOLTAGE-PROBE project synthesised several architectures and characterised their electronic properties, bringing the technology one step closer to rational design of innovative devices.