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Photoinduced Chemistry: Development and Application of Computational Methods for New Understanding

Final Report Summary - XCHEM (Photoinduced Chemistry: Development and Application of Computational Methods for New Understanding)

An exact solution of the electronic Schrödinger equation will give all desired information about a molecule. Unfortunately such solutions are computationally very, very expensive and can only be considered for quite small molecules. We have developed and adapted a new method that allows us to obtain almost all the important information contained the exact solution but at a very small fraction of the cost. We achieve this by randomly exploring the very large space that the electronic wavefunction inhabits and only keeping those parts (known as configurations) that are important in describing a property of interest, for example, energies, dipole moments, quadrupole moments, ionization energies, electron affinities etc. We have shown that this approach can give highly accurate results for all such properties and that crucially the methodology works equally well far from equilibrium and in excited states, often very problematic for standard methods.

Two-photon absorption (TPA) results from the simultaneous absorption of two lower energy photons to populate an excited state that in standard one-photon absorption (OPA) would require a single high-energy photon of twice the energy. This is of much potential in the photodynamic therapy of cancers as it allows us to exploit the tissue transparency window and gives very focused treatment zones. We have shown that very subtle chemical modification of large macrocyclic molecules can result in massively enhanced TPA, where the standard OPA is almost unchanged. This is very important in the rational design of TPA PDT drugs. Other work in this area has focused on chemical modifications that give rise to new electronic states in a desired energy range, and again this work opens up promising synthetic avenues.

The interaction between light and transition metal containing systems is very important across a wide range of modern science from semi-conductors to solar cells to medical applications. These are very difficult to model theoretically, although such treatments are required to understand the complex chemistry occurring. We have looked at some prototype inorganic systems using high levels of theory to fully understand some of the mechanistic aspects involved. For the important class of open-shell Cr(III) systems we have looked at two paradigm mechanisms: (1) photoinduced racemization, where light is used to transfer different chiral isomers of the complex, and (2) photoaquation, where light is used to substitute ligands with solvent water molecules. In both of these we find that molecular geometries where the complex is unsaturated, but adopts its highest possible symmetry at the metal center, leads to very fast conversion between different electronic states that is able to explain the observed photochemistry. We have also looked at (TiO)2 nanoclusters in both their neutral and charged (radical) forms. We see very different behavior caused by the interaction of vibrational and electronic degrees of freedom. Finally, we have looked at TPA (discussed above) in metal complexes, theory jointly with state of the art spectroscopic techniques, as possible route to the photoactivation of anti-cancer drugs. Again, the importance of the reduced coordination leading to ultrafast decay channels has been found. This general feature will be exploited in joint work designing new metal pro-drugs in the future.

Together with experimental colleagues we have mapped out the relaxation pathways that biological chromophores (ie., hetereocycles) have after light absorption. Nature has fine tuned these molecules such that they are able to rapidly get rid of harmful UV radiation energy via a manifold of pathways that turn this excess energy into heat. As above, for the metal complexes the interaction between the nuclear and electronic degrees of freedom in the chromophores is the vital mechanism by which this happens. Our work has highlighted the intricate chemistry that can take place via such quantum features as this and tunneling in these paradigm biological building blocks.