Electronic and Optical Processes in Macromolecular Systems
π-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. Part of my research is focussed on understanding the effect of these interactions on the electronic and optical properties of conjugated macromolecules. In addition, the roles of disorder and dissipative processes on the loss of electronic phase coherence in macromolecular systems is also being investigated.
Further aims are to predict how the electronic and optical behaviour of condensed phase systems are determined by the multi-scale structures of the component molecules, as well as the inverse problem: how experimental observables coupled with theoretical modelling can help determine multi-scale structures.
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).
Recent work is listed below under "Selected Publications".
I am a member of the Oxford Theoretical Chemistry Group and the Centre for Doctoral Training in Theory and Modelling in Chemical Sciences (TMCS).
Students interested in Part II projects are welcome to contact me about potential projects. Prospective DPhil/PhD students should apply here or to the CDT in TMCS.
My current projects include:
Modelling exciton and charge dynamics in conformationally disordered polymers in a dissipative environment. The images shown below are surface plots of exciton wavefunctions, Φ, in the light emitting polymer poly(para-phenylene) (shown above). r is the electron-hole separation and R is the electron-hole centre-of-mass position (in monomer units). The 11B1u exciton is also known as the (singlet) 'Frenkel' exciton, while the 21Ag exciton is also known as the (singlet) 'charge-transfer' exciton; see J. Chem. Phys. 129, 164716 (2008) or my book for further details.
The image shown below represents the formation of an exciton-polaron quasi-particle after photoexcitation of a conjugated polymer, caused by the coupling of the exciton to C-C bond vibrations. Ultra-fast exciton-polaron formation causes ultra-fast exciton decoherence; see J. Chem. Phys. 148, 034901 (2018).
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 the UV-Peierls model in order to understand this mechanism in carotenoids.
Developing theories of optical transitions in π-conjugated systems. The figure below shows the theoretical emission spectra of PPV as a function of torsonal disorder. The ratio of the 0-0 to 0-1 vibronic transitions is a measure of the average chromophore size, which decreases with increasing disorder; see J. Chem. Phys. 141, 164102 (2014).
Developing the density matrix renormalization group (DMRG) method for quantum chemistry and condensed matter physics. 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.
- Jonathan Mannouch (4th year DPhil student): Exciton dynamics in π-conjugated polymers
- Darren Valentine (3th year DPhil student): Singlet fission in carotenoids
- Laszlo Berencei (2nd year DPhil student): Modelling charge transfer and dynamics in π-conjugated polymers
- Isabel Gonzalvez (1st year DPhil student): Exciton dynamics and spectroscopy of π-conjugated polymers
- John Gardner (Part II student): Computing 2d-spectroscopy of π-conjugated polymers
- Dr Max Marcus (Postdoctoral research associate (from April 2019)): Singlet fission in carotenoids