Exploring the Design and Synthesis of Conjugated Materials for Applications in Organic Electronics

Organic electronics offer a variety of advantages over traditional electronics, but their study is hindered by their difficult syntheses. Herein, I tackle several difficult targets of interest to the molecular electronics and organic photovoltaics disciplines, and in doing so I try to illuminate what is and isn’t possible with this frontier of molecular design. I explore the viability of highly sterically-hindered systems enforce intramolecular π-π stacking and tune the 3-dimensional packing of otherwise flat conjugated molecules. In general, these molecules face many of the solubility issues that face large conjugated systems, while now also facing difficult bond formation because of steric bulk. In the case of building small molecule electron acceptors for organic photovoltaics, the electron deficient nature of these compounds also appears to inhibit the formation of these constrained systems. Then, I explore the limits of divergent synthesis by attempting to incorporate two distinct divergent steps into the synthesis of an acceptor unit for photovoltaic polymers. This proved to be a challenging goal, and many issues with orthogonality and reactivity are addressed. Ultimately, the development of this synthesis remains in progress because of Then, I discuss the development of oligophenyl dithiols used for studying tunneling and other electron transport through self-assembled monolayers. These monolayers are a promising frontier in designing molecular electronics. First, our syntheses produce terphenyl and quaterphenyl dithiols for a length-dependent study of electron transport. Then, we design a synthesis of similar oligophenyl dithiols that have sterically hindered rotations so that we can study the effect of inter-ring conjugation on electron transport through the monolayer. I conclude by discussing the themes shared by each of these diverse projects in organic electronics and by touching on issues that remain in the field of organic electronics. Finally, I touch briefly on the culture of scientific publishing against which these projects have strived to be high impact and publishable.Doctor of Philosoph

Charge transport through molecular ensembles: Recent progress in molecular electronics featured

This review focuses on molecular ensemble junctions in which the individual molecules of a monolayer each span two electrodes. This geometry favors quantum mechanical tunneling as the dominant mechanism of charge transport, which translates perturbances on the scale of bond lengths into nonlinear electrical responses. The ability to affect these responses at low voltages and with a variety of inputs, such as de/protonation, photon absorption, isomerization, oxidation/reduction, etc., creates the possibility to fabricate molecule-scale electronic devices that augment; extend; and, in some cases, outperform conventional semiconductor-based electronics. Moreover, these molecular devices, in part, fabricate themselves by defining single-nanometer features with atomic precision via self-assembly. Although these junctions share many properties with single-molecule junctions, they also possess unique properties that present a different set of problems and exhibit unique properties. The primary trade-off of ensemble junctions is complexity for functionality; disordered molecular ensembles are significantly more difficult to model, particularly atomistically, but they are static and can be incorporated into integrated circuits. Progress toward useful functionality has accelerated in recent years, concomitant with deeper scientific insight into the mediation of charge transport by ensembles of molecules and experimental platforms that enable empirical studies to control for defects and artifacts. This review separates junctions by the trade-offs, complexity, and sensitivity of their constituents; the bottom electrode to which the ensembles are anchored and the nature of the anchoring chemistry both chemically and with respect to electronic coupling; the molecular layer and the relationship among electronic structure, mechanism of charge transport, and electrical output; and the top electrode that realizes an individual junction by defining its geometry and a second molecule–electrode interface. Due to growing interest in and accessibility of this interdisciplinary field, there is now sufficient variety in each of these parts to be able to treat them separately. When viewed this way, clear structure–function relationships emerge that can serve as design rules for extracting useful functionality

Understanding and Controlling Chemical Modifications of Rubicene for Their Envisioned Use as Molecular Organic Semiconductors

We discuss here the relationship between the structure of a set of halogenated and cyanated molecules containing the rubicene moiety and a set of relevant electronic properties related to the optoelectronic and semiconductor character of these systems, namely, frontier molecular orbital shape and energy levels, electron affinity, ionization potential, reorganization energy, and electronic coupling between neighboring dimers, calculated over experimental (X-ray) or simulated crystal structures. To do it, we always employ accurate and validated density functional theory methods. The obtained results will be compared with some reference organic semiconductor systems, in order to determine the potential use of the studied compounds in the fabrication of optoelectronic devices.This work is supported by the “Ministerio de Economía y Competitividad” of Spain and the “European Regional Development Fund” through project CTQ2014-55073-P. M.M. thanks the E2TP-CYTEMA-SANTANDER Program for their financial support

Electron structure and charge transport properties of thiols and dithiocarbamates in self-assembled monolayers

It is accepted that the potential of molecular electronics for future device applications critically depends on the formation of a stable and defined link between the organic layer and the metallic contacts. Here we present investigations on dithiocarbamates used as an alternative connecting group for metals, from which their promising properties with respect to thermal stability and electrical conductance emerge. The structure and the electronic properties of dithiocarbamate- and comparable thiolate-based self-assembled monolayers is elucidated using photoelectron spectroscopy, scanning tunneling microscopy (STM), contact angle measurements and interlinked nanoparticle arrays. The experimental results are interpreted in the light of density functional theory calculations, that show the principal difference between the two anchor groups. In the first chapter of the thesis, we show that dithiocarbamates improve the electrical coupling to the metal due to the presence of delocalized electronic states at 0.5±0.1 eV below the Fermi level of Au. The significantly increased density of states at the interface, as revealed by photoelectron spectroscopy and density functional theory calculations, proves that these states are related to the hybridization of the metal d band with delocalized orbitals on the dithiocarbamate anchor group. A low charge injection barrier between the monolayer and the metal is the consequence. Finally, the improved stability of dithiocarbamates on gold is shown by thermal desorption experiments. In the second chapter, the overlayer structure of alkanethiol, benzyl-mercaptan and highly conjugated methyl-phenyl-dithiocarbamate self-assembled monolayers on Au(111) is studied by STM and the conductance of those monolayers measured by current-distance spectroscopy. Whereas alkanethiol monolayers exhibit the known c(4 x 2) overlayer structure, benzyl-mercaptan monolayers show a novel reconstruction, consisting of extended, striped phase domains having a commensurate, p(4½√3 x 2) overlayer structure with an oblique unit cell. In contrast, methyl-phenyl-dithiocarbamate monolayers are found to be disordered. The tunnelling decay constant β for the molecular medium, as well as the molecular conductance at the STM tip-monolayer contact point, are determined. A decay constant of β = 1/Å for alkanethiols and β = 0.5/Å for the phenyl ring is found, in line with reported values, whereas the methyl-phenyl-dithiocarbamate is roughly one order of magnitude more conductive than benzyl-mercaptan. In the last chapter, the structure and the electrical properties of self-assembled monolayers of cyclic aromatic and aliphatic dithioacetamides and of mixed dithioacetamide/alkanethiol monolayers are characterized. The co-assembly and the insertion method are compared for the formation of mixed dithioacetamide/alkanethiol monolayers, and it is found that small and well defined dithioacetamide domains are realized by insertion of dithioacetamides into defect sites of closely packed octanethiol monolayers. These domains are used to determine the molecular conductance by means of STM height profiles, using molecular lengths resulting from density functional theory calculations. The difference in the tunneling decay constant β measured for aromatic dithioacetamides (β = 0.74-0.76/Å) and for aliphatic dithioacetamides (β = 0.84-0.91/Å) highlights the influence of the conjugation within the cyclic core on molecular conductance. In conclusion, different alternative approaches have been used to determine the conductance of molecular junctions, and it is shown that molecules coupled to metals via the dithiocarbamate anchor group could overcome some of the fundamental limitations currently encountered in molecular electronics

New Concepts in Interfacial Dipole Engineering by Self-Assembled Monolayers

Self-assembled monolayers (SAMs) are frequently used for interfacial engineering in organic electronics and photovoltaics. The manipulation of injection barriers by introduction of a specific dipole moment at the interfaces between the electrodes and adjacent organic layers (e.g., an organic semiconductor (OSC)) is of a particular interest. This manipulation is usually achieved by selection of a suitable dipolar terminal tail group comprising the SAM-ambient interface, which however has several essential drawbacks. This approach has been recently complemented by embedding dipolar groups into the molecular backbone of the SAMs, with both aliphatic and aromatic SAMs being engineered and mixed aromatic SAMs comprised of the molecules with the oppositely oriented dipolar groups being studied. The major goal of this work is extension and optimization of the embedding dipole approach, along with several other concepts in general context of interfacial dipole engineering. At first, I studied the mixed aliphatic SAMs comprised of molecules which were modified by a dipolar ester group embedded into the alkyl backbone at two different orientations, viz. with the dipole directed upwards and downwards from the substrate. Applying X-ray photoelectron spectroscopy (XPS) as a morphology tool, I could estimate that the mixed SAMs represent homogeneous intermolecular mixtures of both components, down to the molecular level, excluding existence of "hot spots" for charge injection. The composition of the mixed SAMs was found to mimic fully the mixing ratio of both components in solutions from which these SAMs were prepared, which suggests a minor role of the dipole-dipole interaction in the overall balance of the structure-building forces. Varying this composition, work function of the gold substrate could be tuned linearly and in controlled fashion within a ~1.1 eV range, between the ultimate values for the single-component monolayers. As the next task, I studied the applicability of the embedded dipole concept to the different substrates, taking Ag(111) as a representative example. The aromatic SAMs with the embedded pyrimidine group were found to be much more robust in this context as compared to the aliphatic ones (with the embedded ester group), which makes the former systems especially useful in context of the electrostatic interface engineering. In view of these favorable properties, the next task was optimization of the aromatic SAMs with the embedded pyrimidine group. This was achieved by shortening the molecular backbone and excluding aliphatic building blocks. The resulting, optimized monolayers preserved all useful properties of their prototypes in context of dipole engineering but exhibited much better electrical transport properties, which allowed our partners to fabricate organic thin film transistors with high performance and extremely low contact resistance. Another promising tool for tuning the dipole attributes and the respective work function was found to be electron irradiation. This was demonstrated by the example of aromatic SAMs with the embedded pyrimidine group and terminal pyridine group. The observed behavior is presumably related to specific chemical transformations involving the nitrogen atom in these moieties. It leads to several practical implications, including work function lithography, which could be demonstrated by representative patterns. Alternatively, to the embedding of a dipolar group, the selection of a specific anchoring motif was tried in context of interfacial dipole engineering, taking dithiocarbamate-based SAMs as a representative example. The combination of the spectroscopic and work function data with the results of theoretical simulations performed by our partners allowed understanding the structure and electrostatic properties of these monolayers in very detail, paving the way for their applications

Solution Processable Nanostructures for Molecular Electronics

PhDIn molecular electronics, the building material (traditionally elemental semiconductor) is replaced by single molecules or a nanoscale collection of molecules. Key to molecular electronics is the ability to precisely embed molecules into a nano device/structure and to manipulate large numbers of functional devices so they can be built in parallel, with each nano-device precisely located on the electrodes. In this work, the assembly of organic and inorganic nanostructures dispersed in aqueous solutions has been controlled via chemical functionalisation. By combining this bottom-up assembly strategy with traditional top-down lithographic apporaches, the properties of these nanostructures have been investigated via a range of different techniques. The high degree of control on the molecular design through chemical synthesis and the scalability by self-assembly make this approach of interest in the field of molecular electronics. In this regard, this dissertation presents a solution-based assembly method for producing molecular transport junctions employing metallic single-walled carbon nanotubes as nanoelectrodes. On solid substrates, electrical and electronic properties have been investigated by Conducting Atomic Force Microscopy (C-AFM). Furthermore, different strategies for asymmetric junction formation have been explored towards the development of a potential nanoscale Schottky diode. Moreover, various patterning techniques based on shadow evaporation and AFM probe scratching have been investigated for the assembly of 1-D nanostructures. Nanostructures dispersed in solution were organised onto surfaces by means of dielectrophoretic assembly, and their electronic properties was then measured by the means of a probing station. In addition to the aforementioned organic nanostructures, we also report on the dispersion of boron nitride nanotubes (BNNT) by DNA wrapping, followed by the formation of nano-hybrids of boron nitride nanotubes and carbon nanotubes. Previously, researchers have adopted BNNT as a 2D dielectric layer. The work inspires me to adopt boron nitride nanotubes as 1D dielectric materials. The techniques developed in this thesis are of interest for fundamental studies of electron transport in molecules and nanostructures. Addtionally, the approaches developed in this work may facilitate the advancement of new technologies for electronics, including, but not limited to, future circuits based on single-wall carbon/boron nitride nanotubes with specific functionality

Work Function Tuning at Interfaces by Monomolecular Films

The control over the work function of surfaces and interfaces is one of the most important issues of modern surface science and nanotechnology, e.g. in context of organic electronics and photovoltaics. The goal of this work was to look for new ways to control the work function of metal substrates by using molecular self-assembly. Two different strategies were used. The first strategy was to use aliphatic and aromatic molecules which contain an embedded dipolar group (midchain functionalization). Such self-assembled monolayers (SAMs) allow for tuning the substrate work function in a controlled manner, independent of the docking chemistry and, most importantly, without modifying the SAM-ambient interface. In the case of aliphatic films, we used alkanethiols functionalized with an embedded ester dipole, with the length of both top and bottom segments as well as the direction of the embedded dipole being varied. In the case of aromatic systems, we used terphenyl based thiols functionalized with an embedded pyrimidine dipolar group, with the direction of the dipole being varied. The electronic and structural properties of these embedded-dipole SAMs were thoroughly analyzed using a number of complementary characterization techniques combined with quantummechanical modeling. It is shown that such mid-chain-substituted monolayers are highly interesting from both fundamental and application viewpoints, as the dipolar groups are found to induce a potential discontinuity inside the monolayer, electrostatically shifting the core-level energies in the regions above and below the dipoles relative to one another. Particularly imptortant, in context of the present work, is the fact that the mid-chain functionalized films are indeed well suited to adjust the work function of metal substrates. This could be e.g. done by varying the orientation of the dipolar group but also by mixing the molecules with differently oriented dipoles as was demonstrated in the present work. Within the second strategy, we used photoresponsive systems, viz. azobenzene substituted alkanethiols, having a specially designed architecture to control the packing density and carrying an additional dipolar tail group. These novel SAMs were studied in detail by using spectroscopic and microscopic techniques. Performing photoisomerization experiments we obtained a reproducible, stimuli-responsive change in the work function which was, however, limited to some extent due to the strong steric hindrance effects. In order to reduce these effects, we diluted the azobenzene molecules with short spacer molecules, which resulted in an improvement in the photoswitching behavior

CONJUGATED POLYMERS AND INTER-CHROMOPHORE INTERACTIONS: SYNTHESIS, PHOTOPHYSICAL CHARACTERIZATION AND APPLICATION

Ever since the discovery of conducting polymers (CPs)in the late 1970s, organic conjugated materials and polymers is one of the most popular and fascinating research area among the scientists due to the remarkably unique properties and broad range of applications of (CPs), notably organic photovoltaic (OPVs), organic or polymer based light emitting devices (OLEDs / PLEDs), field effect transistors (FETs), nonlinear optical (NLO) devices and sensor for biologically relevant analytes, metal ions and anions. Optical properties of these materials depend on intra / interchromophoric interaction, geometry and relative orientation in space. Detailed study of such materials can also help us to understand various intriguing properties of conducting polymers such as charge carrier mobility, quantum yield, and how the excited states transfer energy. The primary focus of my research is to develop novel materials which can be employed as sensors for biological relevant molecules and ions and the synthesis of novel scaffolded chromophores to investigate the interchromophoric interactions and to tune their photophysical properties. The basic introduction of conducting polymers and its applications are described in chapter one followed by the detailed study of m-xylene and m-terphenyl based phosphate and pyrophosphate anions sensors in chapter two and three. Also polymer supported m-xylene based materials were studied for catalytic decomposition of phosphoesters in chapter two. Synthesis and photophysical characterization of π-conjugated cruciforms is reported in chapters four and five. Chapter four accounts for the synthesis of 1,3-bis(dimethylaminomethyl)phenyl based cruciform receptors as metal ion sensor whereas chapter five describes the synthesis and photophysical characterization of five phosphorous-containing cruciform derivatives. Synthesis of novel m-terphenyl based halogenated oxacyclophane scaffolds for covalent attachment of chromophores using C-C coupling reaction is reported in chapter six to study the controlled interchain interaction for both monomer and polymers

The development of electron deficient materials for organic electronics applications

This thesis reports on the development of electron deficient (n-type) small molecule and polymer semiconductors. Firstly, the synthesis of a new electron deficient 4,5,6-trifluoro-2,1,3-benzothiadiazole (TFBT) end group is presented. Coupling of TFBT to an electron rich indacenodithiophene (IDT) core, through direct arylation conditions, affords a TFBT IDT material which performs as a poorly ambipolar semiconductor in organic field effect transistor (OFET) devices. A range of related TFBT-based materials with expanded IDT or cyclopentadithiophene (CDT) cores were prepared and characterised. A six-fold nucleophilic aromatic substitution reaction with cyanide was developed and applied to all fluorinated materials. This one step modification resulted in the formation of 2,1,3-benzothiadiazole-4,5,6-tricarbonitrile (TCNBT) end group. This modification dramatically changed structural, optoelectronic and semiconducting properties compared to their fluorinated counterparts. This highlights the importance of strong π-acceptors, like cyano groups, in influencing electron accepting properties, compared to inductively withdrawing fluorine atoms. TCNBT-based semiconductors were utilised in a range of applications such as organic field effect transistors (OFETs) and organic photodetectors (OPDs), demonstrating good stability and high electron mobility. The strong electron accepting properties of the TCNBT end group resulted in low band gap materials that strongly absorb in the NIR range and were utilised to afford semi transparent electronic devices. A range of electron deficient donor-acceptor (D-A) type polymers were also synthesised, based on benzothiadiazole, functionalised with a mixture of cyano, nitro and fluoro groups. This study highlights the importance of molecular engineering and how small structural modifications have a great impact on the nature of the resulting semiconductor. More specifically, the effect of fluorine atoms and their influence on backbone planarity is shown to affect charge transport properties. On the other hand, cyano groups cause backbone twisting that is not easily overcome in the solid state and in turn disrupts charge transport in the polymer backbone. When applied to OFET and organic photovoltaic (OPV) devices, these polymers performed differently to the small molecules presented in this thesis, with fully cyanated polymers showing lower electron mobilities compared to the ones containing fluorine atoms. This demonstrates that the impact of cyano groups have a different effect in polymeric compared to small molecule systems.Open Acces