My research is the theoretical study of quantum processes in macromolecular systems, in particular in π-conjugated molecules. Conjugated molecules (e.g., polymers, nanotubes, porphyrins and DNA) occur widely in many biological and synthetic systems; for example, in polymer optoelectronic devices and light harvesting complexes.
These systems are characterised by both strong electron-electron interactions and electron-nuclear coupling, and are subject to spatial and temporal disorder. Moreover, they are strongly coupled to their 'noisy' environment. Part of my research is focussed on understanding the effect of these interactions on the electronic and optical properties of conjugated macromolecules. Another goal is to understand excited state dynamics, from ultrafast decoherence and localization processes to post-ps exciton migration and diffusion, and to relate these predictions to experimental observables.
These goals are being pursued using a variety of theoretical methods and computational techniques (e.g., DMRG (including time-dependent DMRG), MPS methods and CI-S) on a wide variety of models (e.g., Pariser-Parr-Pople, Hubbard-Peierls and Frenkel-Holstein models).
Singlet fission in carotenoids
Singlet fission in polyacenes and carotenoids has the potential to enhance the efficiency of photovoltaic devices. It is also a fascinating process in its own right, because it requires an understanding of the roles of electronic correlation, electron-phonon coupling, and the coupling of a quantum system to its environment. My group, in collaboration with experimentalists at the University of Sheffield, is applying the t-DMRG and TEBD methods to model Hamiltonians in order to understand this mechanism in polyenes (especially carotenoids).
There are two complementary strands to this work. First, we are investigating state interconversion from the optically excited singlet state (S2) to triplet-pair states (see Phys Rev B 102, 125107 (2020)). Second, we are investigating decoherence and disentanglement of triplet-pair states to become spin-uncorrelated (non-geminate) triplet pairs (see Phys Rev B 102, 035134 (2020)).
Modelling exciton and charge dynamics
Exciton dynamics in conjugated polymers is a fascinating and complex topic, as it encompasses multiple time and length scales, and is determined by many factors, including electron-nuclear coupling, disorder and system-bath interactions. In recent years the development of realistic coarse-grained exciton-phonon models coupled to sophisticated numerical techniques and theoretical insights have led to a wide-range of theoretical predictions of exciton dynamics, from ultrafast intrachain relaxation and decoherence to sub-ns Forster-type interchain transfer and diffusion. These predictions can now be used to interpret a wide-range of time-resolved spectroscopic experiments. See here for a review.
Developing the DMRG method for quantum chemistry and condensed matter physics
Density matrix renormalization group (DMRG), matrix products states (MPS), and their associated time-dependent methods, are extremely powerful computational tools to solve one-dimensional quantum systems. As such, they are particularly suited to study conjugated polymers. My group is pioneering these methods to study one-dimensional linear conjugated systems. We are also developing computational methods to simulate open-quantum systems.