XCHEM

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

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'The interaction of light and matter gives rise to a multitude of important and fascinating phenomena. Computational studies of excited states are vital to further our basic understanding of these processes, and design and optimise new processes for particular applications. However, the computational chemistry of excited states gives rise to many challenging features, including differential static and dynamic correlation effects, which can often be difficult to separate. Furthermore, regions of non-adiabatic coupling between various potential energy surfaces are ubiquitous in photochemistry. Such regions where the Born- Oppenheimer approximation breaks down are among the most difficult to treat. The computational chemist must use a wide variety of methods to study photochemistry. However, one important ‘tool’ in the computational arsenal is currently missing for general photochemical problems: namely the ability to undertake systematically converging computations over all of the relevant regions of the various (multi-state) potential energy surfaces. The Monte-Carlo Configuration Interaction (MC-CI) method is ideal for this purpose, and has many desirable features, including automatic inclusion of strong static correlation effects, and a balanced treatment of all states. Development of MC-CI methods, including gradients and non-adiabatic couplings is proposed. This will give rise to the unprecedented ability to benchmark a large variety of photochemical problems, across the entire potential energy surfaces, with systematic accuracy. The method will be further extended by coupling within molecular mechanics in a quantum mechanics / molecular mechanics (QM/MM) framework to study general excited state / open-shell problems in complex environments. The work will lead onto the applications research which spans the length scales of chemistry from small molecules to large supramolecular systems. The above MC-CI method and other state-of-the-art techniques will be applied to photochemical problems of enormous scientific interest. These include high accuracy studies of inorganic photochemistry where the computational demands can be greatest, but also where high-level electronic structure and dynamics simulation offers exceptional possibility to understand complex molecular photochemistry. A practical area of photochemical research with a huge potential is photodynamic therapy. Here light is used to destroy cancer tissue via the creation of the highly reactive singlet molecular oxygen species. A deeper understanding of the many processes involved in this is required. These include, single- vs multi-photon absorption, sensitizer internal conversion and intersystem crossing, energy transfer processes with molecular oxygen, solvent effects, and aggregation effects. Detailed and systematic studies of these fundamental aspects are proposed. The final applied area of study follows naturally from this and is the supramolecular photochemistry of host-guest molecular sensors. Here advances are required to allow a detailed understanding. These include the use of molecular dynamics simulation in conjunction with QM/MM and statistical sampling.'