Controllable electron transfer
Electron transfer (ET) is a ubiquitous chemical process and the simplest chemical reaction. There are a number of ways of influencing the electronic properties of a given molecular system, ranging from static changes, for example by changing the solvent environment, as well as dynamic changes, for example by external perturbation. Recently, the Weinstein and Meijer groups have shown that ET pathways in the excited states of molecules can be influenced by excitation of specific vibrational modes in photoexcited molecules.[3,4]
This effect, dubbed "vibrational control", is born of the interactions of excited state Potential Energy Surfaces (PESs). My interests are in identifying the features of these PESs that are necessary for the effect to be seen. To achieve this, we use Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) to explore the electronic structure of molecules that show the effect.
Currently, we are extend our investigations to identify target molecules for synthesis that we expect may show the behaviour, as well as incorporate higher level theoretical methods to better understand this phenomenon.
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The fundamental approximation of a great majority of contemporary computational chemistry is the Born-Oppenheimer (BO) approximation, first described in the early 1920s by Max Born and Robert Oppenheimer. A translation of their original paper into English is available here. The BO approximation separates the nuclear and electronic components of the total wavefunction, on the basis that the changes in kinetic energy associated with changing the eigenstates of the nuclei in a molecule is far smaller than the change in potential energy when changing the eigenstates of their associated electrons. This approximation, the central dogma of quantum chemistry, drastically reduces the complexity of electronic structure problems by effectively removing electronic and nuclear coupling, thus rendering the associated equations of motion far simpler to solve. The success of the BO approximation cannot be denied and for a large number of chemical problems it remains entirely valid. However, the key requirement of differing energy scales separating nuclear and electronic wavefunctions is not always met.
Conical intersections are a prominent example of where the BO approximation breaks down.
Commonly, excited state electron transfer processes are facilitated by the breakdown of the BO regime. In such processes, the electronic eigenstates become close in energy, thus allowing significant coupling between nuclear and electronic components of the wavefunction. A recent series of papers from Hush and coworkers eloquently lays out the conditions required for the breakdown of the BO approximation, its consequences and the requisite corrections.[3,4]
In addition to the problems of separation of nuclear and electronic degrees of freedom, another key approximation in contemporary quantum chemistry is the use of non-relativistic Hamiltonians. As the atomic number of nuclei increases, the core electrons begin to move faster and faster. This results increasing relativistic character in molecules featuring heavy elements and starts to become prominent as the d orbitals become available. These relativistic effects contribute alongside breakdown of the BO approximation to the exceptionally interesting photochemistry of transition metal complexes.
The combination of these two effects - breakdown of the BO approximation and relativistic behaviour - contribute to the so called non-adiabatic character of the photochemistry of transition metal complexes, but are seldom included in contemporary quantum chemical investigations of photochemical processes due to their complexity. I am interested in applying recently developed techniques to include non-adiabaticity to photochemical electronic structure problems, such as the ones described above.
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