Understanding the evolving geometric and electronic structure in the course of a chemical reaction or biological process has been, and still is, one of the main goals of modern science. The goal of our research group is to achieve a sophisticated understanding leading to the design of molecules and molecular properties on the atomic level. To achieve this goal we use and develop a variety of theoretical and computational approaches. During this research a particular emphasis is placed upon dynamics occurring in electronically excited states. Wherever possible we try and combine our simulations with experiments, especially the new and exciting experiments made possible from the development of X-ray free-electron lasers.
Thermally Activated Delay Fluorescence
The issue currently holding back Organic Light Emitting Diodes (OLEDs) from dominating the display and lighting markets is the reliance upon Iridium. Indeed, as it is the 4th most scarce naturally occurring element on earth, it is unsustainable to base large-scale high volume industries on this resource. Thermally Activated Delayed Fluorescence (TADF) is an attractive new approach. Here the triplet states are thermally activated such that they can undergo crossing (so called reserve intersystem crossing, rISC) to the singlet manifold and emit light. This opens the potential for devices reaching 100% efficiency and importantly the possibility for truly organic only OLEDs.
We are interested in probing the fundamental photophysics and factors influencing the efficiency of the TADF process. In collbaration with Prof. Andy Monkman we have recently demonstrated that nonadiabatic coupling between the lowest local excitation triplet and lowest charge transfer triplet is crucial and can increase the rate of reserver intersystem crossing by 4 orders of magnitude!
1. J. Phys. Chem. C 119: 13535-13544 (2015).
2. ChemPhysChem DOI: 10.1002/cphc.201600662 (2016).
Metal organic complexes are excellent model systems to study fundamental photophysical processes. After photoexitation, these molecular complexes may undergo any of a plethora of important phenomena including radiative (fluorescence and phosphorescence) decay, non-radiative intramolecular relaxation processes, such as internal conversion (IC) intersystem crossings (ISC, i.e. a spin change) and intramolecular vibrational redistribution (IVR). Most of these processes are accompanied or driven by structural changes occurring as the nuclei adapt to the new electronic structure. Besides this, transition metal complexes have also gathered significant attention owing to potential applications, such as photosensitisers in photovoltaics or photocatalysts. At the heart of such applications are the photoactive metal-to-ligand charge transfer (MLCT) states, which has been the subject of numerous of our recent studies.
Proc. Natl. Acad. Sci. USA 112:12922–12927 (2015).
Chem. Comm. 51: 16629-16632 (2015).
J. Phys. Chem. A. 119: 7026-7037 (2015).
J. Phys. Chem. A 118: 9861-9869 (2014).
Angew. Chem. Int. Ed. 53:5858 (2014)
Structural Dynamics. 1, 024901 (2014)
J. Phys. Chem. A 118:411-9418 (2014).
Probing ultrafast non-equilibrium dynamics became possible with the advent of ultrafast time-resolved linear and non- linear optical spectroscopies. However, because optical spectroscopy consists of transitions between delocalised valence states, the link between the spectroscopic observable and structure is ambiguous for systems of more than one nuclear degree of freedom, i.e. >2 atoms. To overcome this, the last decades have witnessed a significant research effort aimed at exploiting short wavelength probe pulses to achieve direct structural sensitivity in time-resolved pump–probe experiments. The focus of this research effort is to develop the rigorous theoretical framework for simulating the experimental observables of time-resolved core-hole spectroscopy and diffraction experiments.
J. Phys. B 48:214001 (2015).
Phys. Chem. Chem. Phys. 17:23298-23302 (2015).
Structural Dynamics, 2:024302 (2015).
Coord. Chem. Rev. 277-278, 44-68 (2014).
Phys. Chem. Chem. Phys. 16:23157- 23163 (2014).
Within the Born-Oppenheimer approximation the coupling between nuclear and electronic motion is neglected. This approximation is behind most ground state electronic structure methods for molecules and led to the development of efficient mixed quantum/classical schemes for per- forming ab initio molecular dynamics (AIMD), gives rise to considerable computational savings in comparison to the full quantum dynamics. However, as soon as more than one electronic state plays a role in the dynamics (for example for a photochemical reaction), the Born- Oppenheimer approximation will break down whenever the coupling between electronic states due to nuclear motion becomes important. Our group is interestingly in developing new theoretical and computational approaches for improving the rigorous description of coupled electronic and nuclear dynamics, while restricting the computational expense as much as possible! Developments are usually performed within the Quantics Quantum Dynamics Package, largely based around the Muti-Configurational Time-Dependent Hartree (MCTDH) method.
J. Chem. Phys. 140, 144103 (2014).
Chemphyschem, 16:2026 (2015).
Cent. Eur. J. Phys. 11,1059-1065 (2013).