Dr Tim Hele

Theoretical Chemist

Proleptic Lecturer and Royal Society University Research Fellow,
University College London, UK

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My research examines the quantum mechanical motion and structure of small and light particles such as the atoms and electrons in molecules and how physical properties such as reaction rates and spectra can be calculated for them. The research ranges from mathematical methodological development to applications in large multidimensional systems. Themes include:

Organic Light-Emitting diodes

In 2018 I was the theoretician on a team who discovered the world's most efficient deep-red LED and published this in Nature. Surprisingly this was not based on a conventional, 'closed shell' molecule but used a molecule with an unpaired electron known as a radical. Radicals were previously considered too unstable, dangerous or weakly emissive for use OLEDs and this discovery opened a new field to exploration.

I then used theoretical arguments to explain why the majority of organic radicals do not emit efficiently and provided a simply design rule to creat those that do. I then used theoretical arguments to determine how to blue-shift the emission of radicals into the visible, and my experimental collaborators made pure-red radical OLEDs - the most efficient radical OLEDs in the visible ever made. These results, published in Nature Materials, pave the way for the future design and synthesis of highly efficient radical OLEDs throughout the visible and infra-red.

I have also investigated molecules for highly efficient organic light-emitting diodes (OLEDs) known as carbene metal amides (CMAs). There has been substantial debate concerning why they are so efficient and with Dan Credgington and advice from Richard Friend I discovered a theoretical description of carbene-metal-amides which clarifies may of the seemingly conflicting results and suggests how to make further improvements to these molecules. Further research of mine on CMAs has appeared in Advanced Functional Materials.

In addition I have used molecular orbital and group theory to explain the orbital structure and function of thermally-activated delayed fluorescence (TADF) OLEDs, with results published in JPCL.

Non-adiabatic dynamics

While in the Ananth group at Cornell University, I derived the exact nonadiabatic propagator in the mapping variable representation, writing the result in the Liouvillian formalism. Approximations to this exact result lead to pre-existing methods and their error terms from the exact quantum evolution.

As a master's student with Prof David Manolopoulos FRS, I investigated the computation of thermal reaction rates with a mean field form of non-adiabatic ring polymer molecular dynamics, showing that it was a considerable improvement on rate calculation only on the lower adiabatic surface in one-dimensional models.

More recently, with Matt Church and others I have invented the MInt algorithm, a symplectic propagator for the Meyer-Miller Hamiltonian, which we have then implemented in the mixed quantum-classical initial value representation (MQC-IVR) form of semiclassical dynamics. I have also contributed to the first application of mapping variable ring polymer molecular dynamics to proton-coupled electron transfer, where we showed that MV-RPMD could correctly distinguish between concerted PCET (where the electron and proton move together) and sequential PCET (where either the proton or electron move first, followed by the other one).

Chromophore Design and Singlet Fission

Research includes the design of intensely absorbent organic chromophores for photovoltaic applications (solar cells). There is a particular focus on singlet fission candidates which have the potential to substantially increase solar cell efficiency, with research on pi-bridge-pi chromophores published in the Journal of the American Chemical Society.

Further research on Singlet Fission, this time looking at 'hot fission', where the molecule has to be excited with light that is more energetic than its lowest-energy absorption, has also been published in JACS.

I discovered a general design rule for highly absorbent dimers of acenes, published in JPCA, and currenty research looks at design of efficient organic light-emitting diodes.

This involves high-level electronic structure computation for individual molecules and examining theoretical models to provide general rules for a wide variety of systems.

Quantum Dynamics

Collaborating with the Althorpe group, Michael Willatt, Andrea Muolo, Stuart Althorpe and I derived a form of linearized dynamics which conserves the quantum Boltzmann distribution: Matsubara Dynamics.

We then showed that two popular and successful approximate quantum dynamics methods, Centroid Molecular Dynamics (CMD) and Ring Polymer Molecular Dynamics (RPMD) could be related to Matsubara dynamics, deriving for the first time the explicit error terms between these methods and the exact quantum result.

Researching independently, I then related Thermostatted Ring Polymer Molecular Dynamics (TRPMD) to exact quantum dynamics via Matsubara dynamics, giving the explicit error in the propagator and a dynamical justification for the friction parameter. This research won the 2016 Longuet-Higgins Prize from Molecular Physics.

This research has been summarized in a recent New View article in Molecular Physics which reviews thermal quantum time-correlation functions from classical-like dynamics.

Quantum Transition-State Theory and Rate Theory

Stuart Althorpe and I have derived Quantum Transition-State Theory, the quantum analogue of transition-state theory, which had been claimed not to exist since 1939.

In an international collaboration with Prof Yury Suleimanov (MIT/Cyprus) I applied TRPMD to reaction rate theory, linked it to Kramers theory, and showed that, despite its promise for the computation of spectra, it is generally no more accurate than (unthermostatted) RPMD for the computation of reaction rates.

In a collaboration with Prof Mark Tuckerman and his group at New York University, we applied QTST and RPMD rate theory to diffusion in clathrate hydrates, showing how hydrogen diffuses more slowly that deuterium at intermediate temperates (50K) as the hydrogen molecule "swells" as it passes through the bottleneck between the clathrate cages more than the deuterium molecule. However, at low temperatures (around 8K) the effect is reversed and hydrogen tunnels through the barrier faster than deuterium.

With Stuart Althorpe I have also published an alternative derivation of quantum transition-state theory and, as part of a review article on thermal quantum time-correlation functions, showed how it could be derived in the Matsubara formalism.

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