Muons as a tool for studying molecular motion
05 Feb 2026 - Rosie de Laune
Using muon spectroscopy, Francis Pratt from ISIS, Iain McKenzie from TRIUMF and Iain Wright from the University of Edinburgh have studied the molecular motions of the propellor-shaped molecule triptycene.
Understanding the movement of molecules in their solid and liquid phases is critical for informing the design of new molecular materials. Molecules based on triptycene are one such example, as they could be promising candidates for new functional dyes as components in organic solar cells.
Inspired by the rotational dynamics observed in previous muon spin resonance (μSR) studies of 3D molecular electronic materials such as fullerenes, the group used μSR to study triptycene in its solid phase. They wanted to understand how the molecular movement varies with respect to temperature to better inform its applications, and that of its derivatives.
To do this, they used μSR on both the HiFi instrument at ISIS and the M15 beamline at TRIUMF in Canada. μSR provides unique information as it is sensitive to motions in between those that can be studied using NMR and neutron scattering; typically, those on the nano- to microsecond time scale. In this respect, the technique is an underutilised tool for organic materials chemists to study (macro)molecular motion and changes in morphology in real-time.
μSR may provide a powerful new method to examine through-space interactions between aromatic rings in such molecules.
Francis Pratt, ISIS
They identified that there is only limited motion of triptycene in its crystalline form over a wide range of temperatures, and it acts more like a two-dimensional molecule than a three-dimensional spheroidal molecule such as a fullerene. “We also made the intriguing observation that the muonium adds to the more crowded barrelene core of the molecules as well as adding to the aromatic rings,” explains Francis. “Therefore, μSR may provide a powerful new method to examine through-space interactions between aromatic rings in such molecules.”
Their work confirmed that μSR can be an insightful tool for studying molecular motion and interactions of organic solar cell components and could potentially be extended to analysis of blends themselves. Their future work will involve demonstrating the capability of using muon spectroscopy to study a wider range of functionalised triptycenes and related materials such as propellanes and naphthopleiadenes.
The full paper can be found at DOI: 10.1021/acs.jpclett.5c03482
