| dc.description.abstract | The field of organic, or molecular, electronics is currently dominated by both polymeric
and molecular organic materials, while considerably less research efforts are devoted to
transition metal based complexes. Despite this, such compounds can offer advantages
including additional tuneability of the spatial distribution and energy levels of the
frontier orbitals or stable paramagnetic species by manipulating the metal-ligand interactions
which may be accomplished selectively modifying the ligand framework or
changing the central metal.
A series of Ni(II) and Cu(II) acenaphthenequinone bis(thiosemicarbazonato) complexes
were prepared and characterised using X-ray diffraction, cyclic voltammetry,
UV/Vis and EPR spectroscopy, as well as magnetic susceptibility and field effect transistor
measurements and computational calculations. The observed charge transport
properties are discussed in terms of the structural and electronic trends both within
the series and in the context of the two more established analogue series, namely the
bis(3-thiosemicarbazonato) and the diacetyl bis(3-thiosemicarbazonato) metal complexes.
The Ni(II) analogues of the acenaphthenequinone bis(thiosemicarbazonato)
family were found to exhibit p-type charge transport with mobilities between 10¯9 and
10¯5 cm2V¯1s¯1 depending on the exocyclic substitutent and resulting packing pattern.
The observed results were rationalised in terms of the reorganisation energy and
the charge transfer integrals.
A series of 4,4`-phenyl-substituted nickel dithiolene complexes was synthesised and
characterised. Initially with the aim of investigating the effect of varying the para-substituent
of the phenyl ring on the charge transport properties, these efforts were
undermined by the poor processability of these molecules by both vapour and solution
phase methods. As a result, n-type charge transport could be observed under ambient
conditions only for the phenyl and 4-bromo-phenyl substituted analogues, but the
device performance was extremely poor. Nonetheless, the calculated reorganisation
energies, charge transfer integrals and predicted mobilities were encouraging and may
prompt further work on these materials.
An all-organic analogue series of 4,4`-(4-halogen-phenyl)-substituted tetrathiafulvalenes
was also investigated. The hole transport materials displayed mobilities of
between 10¯3 and 10¯7 cm2V¯1s¯1 for both solution and vapour processed devices,
depending on the nature of the halogen. These results are discussed in terms of their
molecular properties and the calculated charge transport parameters and put in context
of the performance of the 4,4`-bis(phenyl)-substituted benchmark analogue. Interestingly,
the obtained crystal structure of the bromo-substituted analogue showed
the molecule to be in the cis conformation, an observation that is unprecedented for
simple, 4-phenyl,5-hydrogen substituted tetrathiafulvalenes, and indicates that both
conformers are initially formed.
Finally, a series of 4,4`-(2-alkyl)thienyl substituted nickel dithiolene salts and tetrathiafulvalenes
was synthesised and characterised. While the charge transport properties of
the former were not further investigated due to the low solubility of the neutral species,
the tetrathiafulvalenes were incorporated into FET devices via solution processing. All
exhibited comparatively high conductivity at room temperature (1.6x10¯3S m¯1), exceeding
that of their quarterthiophene analogues. This masked the observed gate effects
but indicates potential applications as conducting or charge transfer materials. While
the two resolved analogues displayed trans geometry in the single crystal structures,
powder diffraction and preliminary DSC measurements indicate that the materials displayed
at least one additional phase, which once again likely corresponded to the cis
conformer. | en |
