Abstract
Donor acceptor materials have found application in a wide variety of applications, such as dye-sensitised solar cells (DSSCs), organic photovoltaics, organic light emitting diodes, nonlinear optical devices, biological probes and as therapeutic agents. Despite their use in such a wide range of applications, many fundamental properties of donor acceptor materials are still poorly understood, with relatively simple structural modifications imbuing unexpected electronic and photophysical properties. A wider understanding of the interaction between donor and acceptor is therefore required.
This thesis investigates several previously unexplored aspects of the donor acceptor interaction and their influence on the electronic and photophysical properties of the materials, utilising polypyridyl acceptors with increasing levels of spectroscopic complexity: benzo[c][1,2,5]thiadiazole (btd), dipyrido[3,2 a:2ʹ,3ʹ c]phenazine (dppz) and 5,6,11,12,17,18-hexaazatripnaphthalene (hatn). The aspects of the donor acceptor interaction investigated in this thesis are energetics, connectivity and additivity. Re(I) tricarbonyl complexes of the dppz ligands provide competing MLCT processes, the influence of which can be spectroscopically determined and used to characterise the strength of the interaction between donor and acceptor.
Chapter One provides an overview of donor acceptor systems as components of DSSCs and the fundamental processes that make them of interest in such applications. A critical review of the three investigated polypyridyl acceptor systems is also provided, along with an introduction to the characterisation techniques utilised in this thesis.
Chapter Two investigates the use of palladium catalysed cross couplings to incorporate functionality into dppz systems. The electronic and photophysical properties of these ligands and complexes are reported so as to provide context for later chapters. The complexes are shown to exhibit dual 1π,π* and 3MLCT absorption and emission processes, as well as long lived 3π,π* non emissive states.
Chapter Three investigates btd and dppz-based donor acceptor systems utilising 4 dimethylaminobenzene (dmab) and 4 diphenylaminobenzene (dpab) donors. [ReCl(CO)3(L)], [Re(CO)3(L)(dmap)]PF6 and [Re(CO)3(L)(py)]PF6 complexes of each dppz system were further investigated so as to tune the relative energetics of the donor and Re orbitals. ILCT processes are observed for all complexes, with no evidence for MLCT states. All compounds are highly sensitive to the chemical environment, exhibiting strong solvatochromism and a previously unreported thermochromic behaviour.
The dpab systems were chosen to act as the starting point from which to make structural modifications in a systematic manner, so as to investigate the aforementioned aspects of the donor acceptor interaction.
Chapter Four investigates the energetics aspect of the donor acceptor interaction through modulation of donor energy, using substitution of the dpab donor with electron donating and withdrawing groups, and relates the electronic and photophysical properties of the systems to the electronic influence of the substituent, quantified through the use of Hammett constants.
Chapters Five, Six and Seven investigate the connectivity aspect of the donor acceptor interaction. Chapter Five achieves this through modulation of the donor acceptor distance, utilising linear, conjugated bridging units with minimal and consistent electronic influence. Electronic and photophysical properties exhibit dependencies on the donor acceptor distance, consistent with longer distances lowering connectivity between donor and acceptor. Chapter Six probes connectivity through use of steric factors to restrict donor acceptor rotation so as to disfavour conformations that facilitate charge transfer. Larger donor acceptor angles appear to lower connectivity between donor and acceptor, and electronic and photophysical properties correlate with the donor acceptor angle. Chapter Seven describes the effect of bridging units with different electronic influences. The 2,5 thienyl bridge is shown to act as a conductive bridge, whilst the 1,4 disubstituted 1,2,3 triazolyl bridge has been identified for the first time as an insulating unit. Whilst absorption processes show dependence on bridge conductivity, photophysical properties are not influenced by bridge conductivity, but rather donor bridge conformation,
Chapter Eight investigates the additivity aspect of the donor acceptor interaction through incorporation of 1 to 6 donor units around a hatn acceptor. Electronic and photophysical properties are shown to exhibit dependence on the number of donor units incorporated around the acceptor.