Unravelling the electronic structure properties of functional molecules

Novel light sources and devices such as solar cells rely on the development of new, functional molecules with enhanced photophysical properties. The purpose of this thesis is to demonstrate how the electronic structures of functional molecules can be understood beyond simple but commonly employed approaches, such as the textbook picture of promoting an electron from an occupied to virtual molecular orbital: the frontier molecular orbital approach. Whilst the frontier molecular orbital picture can provide qualitative insight into the nature of electronic excited states, it is an approximate theory and often fails to adequately describe the underlying wavefunctions. In particular, for chromophoric systems with optoelectronic applications, early design principles have focused on an inspection of frontier molecular orbitals with varying degrees of success. This is due to the highly complex underlying photophysics which cannot be completely described by an analysis of frontier orbitals. As research strives towards developing more efficient chromophores, the need for robust design principles has never been greater. This thesis presents the means to understand electronic excitation energies and characters beyond the frontier molecular orbital approach, and details the development of computational protocols with the objective of informing quantitative design principles.

Modern approaches for designing new optoelectronic devices often consider the energy gap between the first singlet and triplet excited states. Tuning the relative energies of singlet and triplet excited states to optimise the gap between them is an ongoing challenge. This thesis showcases a novel analysis of excited state energy components with physically meaningful interpretations, which can be used to better understand electronic excitation energies. Using two illustrative molecules, it is highlighted that locally excited triplet states can lie below the HOMO-LUMO gap whereas the analogous singlet states are pushed up in energy. Charge transfer excited states are found to be void of any significant stabilising or destabilising effects. Additionally, it is shown how the state orderings can be rationalised with respect to the individual energy components. The shortcomings of the frontier molecular orbital approach are explained by considering the underlying physics of the excited states with respect to their spin multiplicity and character.

An analysis of excited state character is then used to rationalise the photophysical behaviour of two closely related donor-π-acceptor-π-donor chromophores which have been synthesised previously. Using wavefunction analysis to systematically decompose the excited state character into various locally excited and charge transfer contributions, differences between the two molecules are uncovered which the frontier orbitals are unable to explain. The molecules are analysed in the context of their ground and excited state minima, considering the effects of structural relaxation, symmetry breaking and the use of different chemical solvent models. It was found that symmetry broken charge transfer excited state minima, accompanied by a small adiabatic singlet triplet gap, support enhanced emission behaviour. Using the computational protocol developed within this work, the photophysical properties of two closely related, not yet synthesised, molecules are evaluated.

Finally, this thesis includes details of computational method development performed in pursuit of reimplementing the NMR chemical shielding code according to modern standards within the Q-Chem software package. The existing legacy code is limited in its functionality and therefore it was desirable to lay the foundations for a more flexible, extensible code which is compatible with modern post-processing tools. It is detailed in this thesis that the key quantity required for solving the CPSCF equation system has been reproduced. Additionally, the modernised code has a one-to-one mapping of code to equations found in the literature, which makes the code more readable. Suggestions for further development of the code are presented.