STUDIES OF PHOTOPHYSICS OVERARCHING THE SPIN MANIFOLDS USING CORRELATED QUANTUM CHEMISTRY

Abstract

The thesis shows various applications of quantum chemistry in uncovering the photophysics of processes involving transitions between different spin manifolds. From the simple intersystem crossing between two spin manifolds enabled by relativistic effects to electronically coupled transitions such as singlet fission and triplet-triplet annihilation, the crossover of different spin manifolds of molecular excited states offers numerous applications in catalysis and optoelectronic devices. The experiments have accumulated rich history of suggesting new solutions to energetically efficient execution of chemical reactions and harvesting of light energy. However, the experiments often do not suffice to shed light on the mechanism behind the phenomena that will give insights to reinforce the further development of the field. The questions that were addressed with the quantum chemical methods include the mechanism of efficient intersystem crossing of organic photocatalyst, the spectroscopic features encountered during singlet fission, and the effect of substituents on oscillator strength of molecules with inverted energy levels of singlet and triplet excited states. The physical insights obtained from the calculations show that the electronic configurations that mix into the dominant configuration through electronic correlation play cardinal roles in explaining the experimental results or suggesting a new strategy of molecular design. The reorganization of catalyst upon photoexcitation changes the originally single configuration nature of electronic states to highly mixed configuration that enables rapid intersystem crossing, the charge transfer states between monomers in singlet fission system bring new features to the excited state absorption spectrum, and geometries modified by steric hindrance leads to orbital overlaps that satisfies both inverted singlet triplet energy and appreciable transition dipole moment.