Title:
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Molecular-level studies of oligo (aniline) thin films
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The performance of devices based on small molecules depends on their molecular
architecture and their two- and three-dimensional organisation into thin films. In
this thesis scanning probe microscopy and density functional theory calculations
are used to elucidate packing of poly(aniline) oligomers, oligo(aniline)s, into films
formed by vacuum thermal evaporation and solution-processing.
Thermal deposition of oligo(aniline)s under ultra-high vacuum conditions
and subsequent study by scanning tunneling microscopy at various temperatures
highlights two key issues with the preparation of oligo(aniline) thin films by this
method. Firstly, the underlying copper substrate was shown to catalyse the
fragmentation of oligo( aniline)s even at temperatures as low as 30 K. Secondly,
upon an interfacial layer of fragmented molecules further disordered layers
form, these layers are disordered due to presence of multiple isomeric forms of
oligo(anilines). These two issues will prevent the preparation of high-quality
oligo(aniline) thin films, and therefore limit the successful application of these
molecules into devices.
Computational studies of the isomerism of oligo(aniline)s undertaken by ab
initio quantum calculations revealed a wealth of information about the gas-phase
behaviour of these molecules. The energy barriers to cis/trans, syn/anti and ring
rotation are extracted, as well as the kinetics of all these processes. Relatively
low energetic barriers coupled with the small energy differences between isomers
suggest that thin films of these molecules will be subject to conformational disorder
than will inhibit their performance in applications. This led to the calculation of
the molecular structures of ladder oligo( aniline )s. These were shown to be planar
and rigid, and therefore offer attractive potential synthetic targets if the unique
properties of oligo( aniline)s are to be exploited.
Self-assembly of an amphiphilic oligo( aniline) into thin films by solution processing
was investigated by tapping-mode atomic force microscopy and
scanning tunneling microscopy. The driving forces for the formation of
self-assembled thin films of oligo ( aniline)s are a delicate balance of non-covalent
interactions. By taking a methodical approach, thin films of vertically or
horizontally oriented oligo( aniline) molecules in their semi-conducting or
conducting state could be formed by simple drop-casting. Important thin film
properties are highly anisotropic, and the ability to control the orientation of
molecules within a film is crucial for maximising device performances. Scanning
tunneling microscopy of self-assembled monolayers of oligo(aniline)s reveals
their surface structures, and offers the potential to manipulate them on a single
molecule basis.
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