Publications

2024

• Spin-optical design of organic radicals for photochemical upconversion
  Kieran D. Richards, Wenzhao Wang, Philipp Thielert, James D. Green, John M. Hudson, Claire Tonnelé, David Casanova, Yoann Olivier, Timothy J. H. Hele, Sabine Richert, Feng Li and Emrys W. Evans
  Submitted (2024)
  ChemRxiv DOI: 10.26434/chemrxiv-2024-2gf9b
  ChemRxiv
  The first demonstration of photon upconversion where a radical is directly bonded to an annihilator in one molecule. The theory which my PhD student James Green and I derived showed how energy transfer from the sensitizer to the annihilator is quantum mechanically allowed, in contrast to conventional upconversion which often requires a spin forbidden singlet to triplet transfer.
• ExROPPP: Fast, Accurate and Spin-Pure Calculation of the Electronically Excited States of Organic Hydrocarbon Radicals
  James D. Green and Timothy J. H. Hele
  The Journal of Chemical Physics 160 164110 (2024), DOI 10.1063/5.0191373
  JCP arxiv PDF
  Here we formulate what we believe is the fastest known algorithm for radical excited state calculation, which quickly and accurately computes the electronic structure of organic hydrocarbon radicals with spin-purity. To do this we combine two pre-existing methods, Extended Configuration Interation Singles (XCIS) and Pariser-Parr-Pople theory (PPP) to create a method we call Extended Restricted Open-shell Pariser-Parr-Pople theory (ExROPPP). We find ExROPPP computes the excited states of a series of alternant hydrocarbon radicals with comparable accuracy to high-level GMC-QDPT but at approximately two orders of magnitude lower computational cost.
  We also use the underlying algebra from ExROPPP to derive 'alternacy' or 'pseudoparity' rules for alternant hydrcarbon radicals - alternacy rules for closed-shell excited states have been known since the 1950s but to our knowledge this is the first general derivation of them for radicals.
  This article is part of the Emerging Investigators Special Collection for 2024 and has also been selected as an Editor's Pick.
  Typo: There is a small error in the direction of some of the arrows in Figure 1 of the JCP version, which is corrected in the arxiv and pdf versions.

2023

• Which Algorithm Best Propagates the Meyer–Miller–Stock–Thoss Mapping Hamiltonian for Non-Adiabatic Dynamics?
  Lauren E. Cook, Johan E. Runeson, Jeremy O. Richardson, and Timothy J. H. Hele
  Journal of Chemical Theory and Computation 19 (18), 6109-6125 (2023)
  DOI: 10.1021/acs.jctc.3c00709
  JCTC arxiv
  Here we analyse in detail three algorithms for propagating the Meyer-Miller-Stock-Thoss Hamiltonian including the MInt algorithm, the Split Liouvillian (SL), and one we call the Degenerate Eigenvalue (DE). We find that the MInt algorithm is the only symplectic algorithm but that (surprisingly) the SL can give comparable energy conservation at somewhat lower computational cost.

2022

• Inverse molecular design from first principles: Tailoring organic chromophore spectra for optoelectronic applications
  James D Green, Eric G Fuemmeler and Timothy JH Hele
  The Journal of Chemical Physics 156, 180901 (2022)
  JCP arxiv PDF
  In this invited perspective article we show how absorption and emission properties of molecules can be designed from first principles by combining electronic structure theory and intensity borrowing perturbation theory, thereby providing one solution to the inverse design problem. We hope that these results can be used to inform machine learning and artifical intelligence algorithms for faster molecular design.
• Singlet and triplet to doublet energy transfer: improving organic light-emitting diodes with radicals
  Feng Li, Alexander J Gillett, Qinying Gu, Junshuai Ding, Zhangwu Chen, Timothy JH Hele, Richard H Friend and Emrys W Evans
  Nature Communications 13 2744 (2022)
  Nature Comms arxiv PDF
  In a joint experimental and theoretical collaboration, we show how triplet states, which are usually don't emit light, can give their energy to radicals, which can then emit. OLEDs are made using this principle which turn out to have better properties than either the singlet/triplet OLED or radical-only OLED.
• Electronic energies from coupled fermionic 'Zombie' states imaginary time evolution
  Oliver A Bramley, Timothy JH Hele and Dmitrii V Shalashilin
  The Journal of Chemical Physics 156 174116 (2022)
  JCP arxiv PDF
  Here we show hom coupled coherent fermionic 'zombie' states can be used to compute the ground and low-lying electronic states of small molecules using imaginary time evolution. We also propose efficient algorithms for computation of spin and energy, amongst other properties.

2021

• On the electronic structure of alternant conjugated organic radicals for light-emitting diode applications
  Timothy J. H. Hele
  Proceedings Volume 11799, Physical Chemistry of Semiconductor Materials and Interfaces XX; 117991A (2021), https://doi.org/10.1117/12.2593712
  SPIE PDF
  Copyright notice: Copyright 2021 Society of Photo‑Optical Instrumentation Engineers (SPIE). One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this publication for a fee or for commercial purposes, and modification of the contents of the publication are prohibited.
  In this article I derive equations for the electronic structure of radical molecules which are alternant hydrocarbons (no odd-membered rings or heteroatoms), showing how they are not usually suited to applications in OLEDs. However, as I have shown previously (see the Nature Materials paper below), non-alternant radicals can make highly efficient OLEDs.
• Systematic improvement of molecular excited state calculations by inclusion of nuclear quantum motion: A mode-resolved picture and the effect of molecular size
  Timothy J. H. Hele, Bartomeu Monserrat and Antonios M Alvertis
  The Journal of Chemical Physics, 154, 244109 (2021), DOI: 10.1063/5.0052247
  JCP arxiv PDF
  Here we show how the computation of excited-state energies can be improved by accounting for the nuclear quantum effects (zero-point energy) of the molecule. We present a method for ZPE correction which does not require the optimized excited state stucture nor the excited state hessian and show that this substantially improves upon the uncorrected energies from a DFT calculation.
• Intrinsic photogeneration of long-lived charges in a donor-orthogonal acceptor conjugated polymer
  Jordan Shaikh, Daniel G Congrave, Alex Forster, Alessandro Minotto, Franco Cacialli, Timothy JH Hele, Thomas J Penfold, Hugo Bronstein and Tracey M Clarke
  Chemical Science 12 8165-8177 (2021)
  Chemical Science PDF
  Here we show long-lived charges are generated in an orthogonal donor-acceptor copolymer, and using group theory I show how fast intersystem crossing can happen in this system.
• Efficient light-emitting diodes from organic radicals with doublet emission
  John M. Hudson, Timothy J. H. Hele and Emrys W. Evans
  Journal of Applied Physics 129 180901 (2021), DOI 10.1063/5.0047636
  J. Appl. Phys. PDF (accepted manuscript)
  In this article Emrys Evans, John Hudson and I review recent progress in organic radical light-emitting diodes, including the breakthroughs in our Nature and Nature Materials papers, and identify future challenges.

2020

• Understanding the luminescent nature of organic radicals for efficient doublet emitters and pure-red light-emitting diodes
  Alim Abdurahman, Timothy J. H. Hele, Qinying Gu, Jiangbin Zhang, Qiming Peng, Ming Zhang, Richard H. Friend, Feng Li & Emrys W. Evans
  Nature Materials 19, 1224-1229 (2020), https://doi.org/10.1038/s41563-020-0705-9
  Nature Materials Cambridge Repository
  This article explains why the vast majority of organic radicals do not emit, and gives a simple design rule for creating those that do. My theory also explains how to blue-shift emission from the infra red (in the Nature article below) to the visible using aza-substitution. Using this we make the most efficient known radical OLEDs with emission in the visible, and this article paves the way for the design of many other radical OLEDs in the future.
• Environmental Control of Triplet Emission in Donor-Bridge-Acceptor Organometallics
  J. Feng, L. Yang, A. S. Romanov, J. Ratanapreechachai, S.T. E. Jones, A. M. Reponen, M. Linnolahti, T.J.H. Hele, A. Köhler, H. Bässler, M. Bochmann and D. Credgington
  Advanced Functional Materials (2020), 30, 1908715
  Adv. Funct. Mater. arxiv preprint PDF
  This article shows (somewhat surprisingly) that the colour of light-emitting diodes (LEDs) made from carbene-metal-amides can be changed substantially without actually changing the carbene-metal-amide itself, but by changing the host material around it.

2019

• Switching between coherent and incoherent singlet fission via solvent-induced symmetry-breaking
  Antonios M Alvertis, Steven Lukman, Timothy J. H. Hele, Eric G Fuemmeler, Jiaqi Feng, Jishan Wu, Neil C. Greenham, Alex W Chin and Andrew J. Musser
  The Journal of the American Chemical Society (2019) 141 17558-17570 DOI: 10.1021/jacs.9b05561
  JACS
• Anticipating acene-based chromophore spectra with molecular orbital arguments
  T. J. H. Hele, E. G. Fuemmeler, S. N. Sanders, E. Kumarasamy, M. Y. Sfeir, L. M. Campos and N. Ananth
  The Journal of Physical Chemistry A, 123 (2019) 2527-2536
  Arxiv preprint JPCA
  This article solves problem in the spectra (colour) of molecules known as acene dimers which puzzled scientists since 1948. It also develops a design rule for highly absorbent acene dimers which can be used in photovoltaic applications due to their ability to undergo singlet fission.

2018

• Efficient radical-based light-emitting diodes with doublet emission
  X. Ai, E. W. Evans, S. Dong, A. J. Gillett, H. Guo, Y. Chen, T. J. H. Hele, R. H. Friend & F. Li
  Nature 563, (2018) 536–540
  Nature
  The discovery of the world's most efficient deep-red organic light-emitting diode (OLED), with 27% external quantum efficiency. My theory explained the unusual orbital structure which was relevant to the intense, and efficient, emission.
• Vibrationally Assisted Intersystem Crossing in Benchmark Thermally Activated Delayed Fluorescence Molecules
  E. W. Evans, Y. Olivier, Y. Puttisong, W. K. Myers, T. J. H. Hele, S. M. Menke, T. H. Thomas, D. Credgington, D. Beljonne, R. H. Friend and N. C. Greenham
  J. Phys. Chem. Lett. 9 (2018) 4053-4058
  PDF JPCL
• Theoretical description of Carbene-Metal-Amides
  T. J. H. Hele and D. Credgington
  arxiv preprint
  The theoretical model presented in this article were subsequently confirmed experimentally by Hamze et al and led to an eletter in Science.
• Nonadiabatic semiclassical dynamics in the mixed quantum-classical initial value representation
  M. S. Church, T. J. H. Hele, G. S. Ezra and N. Ananth
  J. Chem. Phys., 148 (2018), 102326
  JCP arxiv PDF
  Typos (Compiled by Lauren Cook and Tim Hele)
  This article presents the MInt algorithm, the only known symplectic (energy-conserving) algorithm for computing non-adiabatic dynamics with the commonly-used Meyer-Miller Hamiltonian

2017

• A Mapping Variable Ring Polymer Molecular Dynamics Study of Condensed Phase Proton-Coupled Electron Transfer
  S. Pierre, J. R. Duke, T. J. H. Hele and N. Ananth
  J. Chem. Phys., 147 (2017), 234103, DOI: 10.1063/1.4986517
  JCP PDF
• Tuning Singlet Fission in Pi-Bridge-Pi Chromophores
  E. Kumarasamy, S. N. Sanders, M. J. Y. Tayebjee, A. Asadpoordarvish, T. J. H. Hele, E. G. Fuemmeler, A. B. Pun, L. M. Yablon, J. Z. Low, D. W. Paley, J. C. Dean, B. Choi, G. D. Scholes, M. L. Steigerwald, N. Ananth, D. R. McCamey, M. Y. Sfeir, and L. M. Campos
  J. Am. Chem. Soc., 139 (2017), 12488–12494, DOI: 10.1021/jacs.7b05204
  JACS
• Thermal quantum time-correlation functions from classical-like dynamics
  T. J. H. Hele
  Molecular Physics 13 (2017) 1435-1462, DOI 10.1080/00268976.2017.1303548
  Invited New View article
  Mol. Phys. arxiv PDF
  Published alongside an Author Profile

2016

• Competing quantum effects in the free energy profiles and diffusion rates of hydrogen and deuterium molecules through clathrate hydrates
  J. R. Cendagorta, A. Powers, T. J. H. Hele, O. Marsalek, Z. Bačić and M. E. Tuckerman
  Phys. Chem. Chem. Phys. 18 (2016) 32169-32177
  PCCP arxiv PDF
• Fundamentals: general discussion
  S. C. Althorpe, V. Beniwal, P. G. Bolhuis, J. Brandao, D. C. Clary, J. Ellis, W. Fang, D. R. Glowacki, T. J. H. Hele, H. Jonsson, J. Kastner, N. Makri, D. E. Manolopoulos, L. K. McKemmish, G. Menzl, T. F. Miller III, W. H. Miller, E. Pollak, S. Rampino, J. O. Richardson, M. Richter, P. R. Chowdhury, D. Shalashilin, J. Tennyson and R. Welsch
  Faraday Discussions 195 (2016) 139-169.
  Faraday Discuss.
• Deriving the exact nonadiabatic quantum propagator in the mapping variable representation
  T. J. H. Hele and N. Ananth
  Faraday Discussions 195 (2016) 269-289, DOI: 10.1039/C6FD00106H
  Selected for presentation at the Reaction Rate Theory Faraday discussion held in Cambridge, UK, September 2016.
  Faraday Discuss. arxiv PDF
• Non-adiabatic reactions: general discussion
  S. C. Althorpe, N. Ananth, G. Angulo, R. D. Astumian, V. Beniwal, J. Blumberger, P. G. Bolhuis, B. Ensing, D. R. Glowacki, S. Habershon, S. Hammes-Schiffer, T. J. H. Hele, N. Makri, D. E. Manolopoulos, L. K. McKemmish, T. F. Miller III, W. H. Miller, A. J. Mulholland, T. Nekipelova, E. Pollak, J. O. Richardson, M. Richter, P. R. Chowdhury, D. Shalashilin and R. Szabla
  Faraday Discussions 195 (2016) 311-344.
  Faraday Discuss.
• Application to large systems: general discussion
  S. Althorpe, G. Angulo, R. D. Astumian, V. Beniwal, P. G. Bolhuis, J. Brandao, J. Ellis, W. Fang, D. R. Glowacki, S. Hammes-Schiffer, T. J. H. Hele, H. Jonsson, T. Lelievre, N. Makri, D. Manolopoulos, A. M. Mebel, G. Menzl, T. F. Miller III, M. Parrinello, P. M. Piaggi, E. Pollak, P. Roy, Chowdhury, E. Sanz, D. Shalashilin, E. Skulason, R. Spezia and S. Taraphder
  Faraday Discussions 195 (2016) 671-698.
  Faraday Discuss.
• An alternative derivation of ring-polymer molecular dynamics transition-state theory
  T. J. H. Hele and S. C. Althorpe
  J. Chem. Phys. 144 (2016) 174107
  JCP arxiv PDF
  This article featured as an Editor's Pick on the JCP homepage, and as a 2016 Editors' Choice article
• On the relation between thermostatted ring polymer molecular dynamics and exact quantum dynamics
  T. J. H. Hele
  Mol. Phys. 114 9 (2016) 1461-1471, DOI: 10.1080/00268976.2015.1136003
  Mol. Phys. (published version) arxiv (submitted version) PDF (accepted version)
  This article won the Longuet-Higgins prize from Molecular Physics,who published an accompanying Winner Profile.
  

2015

• Should Thermostatted Ring Polymer Molecular Dynamics be used to calculate thermal reaction rates?
  T. J. H. Hele and Y. V. Suleimanov
  J. Chem. Phys. 143 (2015) 074107.
  JCP arxiv PDF
• Communication: Relation of centroid molecular dynamics and ring-polymer molecular dynamics to exact quantum dynamics
  T. J. H. Hele, M. J. Willatt, A. Muolo and S. C. Althorpe
  J. Chem. Phys 142 (2015) 191101.
  JCP arxiv PDF
  This article was showcased by J. Chem. Phys. as one of the most read papers of 2015
• Boltzmann-conserving classical dynamics in quantum time-correlation functions: ‘Matsubara dynamics’
  T. J. H. Hele, M. J. Willatt, A. Muolo and S. C. Althorpe
  J. Chem. Phys 142 (2015) 134103.
  JCP arxiv PDF

2014

• Quantum Transition-State Theory
  T. J. H. Hele
  Dissertation submitted for the degree of Doctor of Philosophy, University of Cambridge
  arxiv PDF

2013

• On the uniqueness of t → 0 + quantum transition-state theory
  T. J. H. Hele and S. C. Althorpe
  J. Chem. Phys 139 (2013) 084116.
  JCP arxiv PDF
• Derivation of a true (t → 0 + ) quantum transition-state theory. II. Recovery of the exact quantum rate in the absence of recrossing
  S. C. Althorpe and T. J. H. Hele
  J. Chem. Phys 139 (2013) 084115.
  JCP arxiv PDF
• Derivation of a true (t → 0 + ) quantum transition-state theory. I. Uniqueness and equivalence to ring-polymer molecular dynamics transition-state theory.
  T. J. H. Hele and S. C. Althorpe
  J. Chem. Phys 138 (2013) 084108.
  JCP arxiv PDF

2011

• An Electronically Non-Adiabatic Generalization of Ring Polymer Molecular Dynamics
  T. J. H. Hele
  Dissertation submitted for the degree of Master of Chemistry, University of Oxford
  arxiv PDF