Abstract
The excited state properties of photoactive molecules govern their efficiency for a range of applications. In this thesis the excited states for a variety of compounds bearing different charge-transfer moieties were investigated. An assortment of spectroscopic techniques supported by density functional theory calculations were used to analyse these states.
The first series of complexes examined featured a 2,2'-bipyridine ligand with bulky triphenylamine substituents in the 6 and 6' positions (diTPAbpy) and demonstrated the importance of geometry on excited state photophysics. The ligand features a strong ILCT absorption which has a decreased intensity when complexed owing to an increased dihedral angle between the TPA and bpy moieties. Emission from a 1ILCT state is observed for the ligand and does not shift for the copper, silver, and rhenium complexes, with an additional 3MLCT emission observed for [Re(diTPAbpy)(CO3Cl]. Transient absorption measurements revealed the population of a 3ILCT for the homoleptic copper and silver complexes (τ = 80 ns). This is particularly notable for the copper complex as it shows that excited state reorganisation is minimised which is a common issue resulting in excited state quenching in other copper complexes.
Copper complexes were further investigated with either 2-pyridyl-1,2,3-triazole (pytri) or the triphenylamine (TPA) substituted (TPA-pytri) ligands alongside varied ancillary ligands. The TPA-pytri ligand introduced a strong ILCT absorption which was shown to be electronically disconnected from the MLCT state. The MLCT state was tuned to above or below the ILCT band (347 - 433 nm) with choice of ancillary ligand however no changes in the 1ILCT emission were observed.
A series of β-ferrocene-modified zinc porphyrins, with indandione and malononitrile based electron-withdrawing units appended to the ferrocene, were studied to investigate the interactions between π to π* and potential charge transfer states from the porphyrin to indandione. The ferrocene limited communication between the porphyrin and acceptor groups although its oxidation potential was able to be tuned by up to 0.3 V. The porphyrin singlet state was unaffected by ferrocene substitution, although the triplet state lifetime was lowered by up to 10.6 µs from that of the unsubstituted ferrocene porphyrin (18.1 µs). A loss in degeneracy of the porphyrin eg orbitals was found and accounted for the emission varying slightly in energy depending on the excitation wavelength.
Finally, a series of ruthenium and iridium complexes bearing ligands with a unique hydrogen-bonding moiety were investigated. A range of charge transfer states were identified which varied considerably in nature. Anionic ligands introduced LLCT transitions to the ancillary bipyridine or phenylpyridine ligands for the ruthenium and iridium complexes respectively which occurred at lower energy than the associated MLCT transitions. High energy ILCT transitions were introduced through pendant diaminotriazine units. Population of a longer lived 3ILCT state was shown for the iridium complexes while for the ruthenium complexes the lower energy MLCT states were shown to be rapidly populated. For all complexes no changes in their photophysical properties were observed in the presence of complimentary hydrogen-bonding species.
Throughout this thesis resonance Raman spectroscopy was used to support the assignments made for the excited states of the studied compounds These findings were further corroborated through DFT calculations. The species studied here demonstrated a range of excited state properties providing insights into the future design of photoactive materials.