Abstract
Spectroscopic techniques, namely electronic and vibrational spectroscopy, have been shown to be very powerful tools in providing insight into the excited state nature and dynamics of compounds and complexes. A mixture of steady state, time-resolved and temperature dependent vibrational and electronic spectroscopies were used to study and characterise a range of donor-acceptor (DA) compounds.
This work explores a number of DA systems, ranging from conducting polymers, to small molecule organic systems, to inorganic systems. From this breadth of samples insight can be gained into the wide range of behaviours and potential applications DA systems can have. A number of different excited states, ranging from π to π ∗ to metal-to-ligand charge-transfer (MLCT) and their interplay can be explored. In addition to looking at the electronic nature and behaviour of DA systems a, limited, exploration into aggregation, inter- and intra- molecular forces and their influence on behaviour is carried out. The primary aim is to determine what information can be extracted from new systems using the pre-establish techniques, as well as explore the use of less common techniques, such as low-frequency Raman and variable temperature resonance Raman spectroscopies, in different situations to access their suitability.
In the first section a series of three conducting polymers, with various degrees of linearity in their backbone are studied. From this it can be seen that as the backbone rigidity increases the emission become less sensitive to temperature, which was interpreted as being linked to increased stability of order in solution. This conclusion was linked to the changes in emission as a function of temperature; primarily being linked to increased emission for a localised emissive state formed when bending in the polymer prevented full conjugation being achieved. Computational modelling suggested that the polymer with intermediate linearity alternated between a disordered and order configuration, while the two extremes sat more in one configuration or the other. The experimental data supports this conclusion.
In Chapter 4 a pair of regioisomers, structural analogues to monomors used in the polymer construction, were studied to try understand the relationship between fluorination position and previously reported variations in physical and electronic properties. The combination of spectroscopic and computational techniques lead to the conclusion that this is result of F···S through space interactions. These interactions alter the structure and electronic properties of the molecule, by changing the electronic density distribution of the HOMO and LUMO. One example of the impact of the position of the fluorine, is that for one isomer the Stokes shift between absorbance and fluorescence was consistently 400 cm−1 greater than the other.
The systems in the first two section had a complex arrangement of multiple donor and acceptor units, with even the regioisomers being D-A-D-π-D-A-D in nature. In Chapters 5 and 6 simpler DA systems, with a tetraphenylbenzene (TPB) donor attached to a fluorene based acceptor were studied. In Chapter 5 fluorenone was used as the acceptor (FTP series) and in Chapter 6 the more electron withdrawing fluoren-9-ylidene dicyano was used as the acceptor (CNTP series). A large dihedral angle between the TPB and fluorene units disrupted conjugation and lead to the systems showing minimal DA nature. The FTP series showed strong solvent and temperature dependent fluorescence, with the response found to correlate with electronic nature and excited state dynamics shown by fluorenone. The emission showed a Stokes shift of over 9000 cm−1 . For the CNTP series excited state rotation through linker between the fluorene to dicyano units resulted is near complete quenching of the emission. However, this allowed a large amount of resonance Raman data to be collected. From this an atypical and strong progression of overtone and combination bands were observed. This indicates at a large ∆Q in the excited state and allowed for more in detailed modelling of the potential energy surface, with the anharmonicity constant found to be around ∼2 x 10−3.
In the final section the work moves from organic to inorganic systems, with a pair of rhenium-bipyridine based D-A-D and D-A-A based complexes. While changing between the D-A-D and D-A-A was only of limited interest, with variations in excited state lifetimes correlating with those expected from the literature and only minimal variation in the electronic absorbance intensity and Stokes shift, the more interesting part was the solvent sensitivity of the emission. Between low and high polarity solvent the lowest energy emission could be completely quenched. The combination of spectroscopic techniques and computational modelling was used to show that this was due to the interplay between an emissive 3MLCT and dark 3ILCT state. In all solvents a 1MLCT was initially populated, which decayed to a 3 MLCT state. In low polarity solvent the system got trapped here and relaxed via phosphorescence, while in high polarity solvents it could rapidly cross from the 3MLCT to a 3ILCT and undergo non-radiative decay to the ground state. Data suggests the 3MLCT to a 3ILCT crossing required structural reorganisation.