Electronic Properties and Langmuir-Blodgett Films of Discotic Organic Semiconductors

Columnar mesophases of discotic liquid crystals (DLCs) are an emerging class of organic semiconductors that have several advantages over widely applied organic semiconductors based on conductive polymers and glasses of small molecules, such as high charge carrier mobility of \u3e1 cm2V-1s-1 and charge carrier diffusion length of 70 nm. Two important deficiencies that hamper the application of DLCs in organic electronic devices are an insufficient control over their frontier orbital energies and the alignment of their columnar stacks. This dissertation reports a first systematic study on the control of frontier orbital energies, along with other electronic properties, by alterations of molecular structure and two new approaches towards monomolecular alignment layers for columnar discotic mesophases. Solution cyclic voltammetry and UV-Vis absorption spectroscopy as well as computational studies at the DFT level were employed to measure and predict electronic properties of DLCs based on triphenylene and phthalocyanine derivatives (Chapters 2-4). These are the first reported studies that systematically compare changes of the molecular structures of DLCs with changes of their frontier orbital energies and mesomorphism. Our comparative studies on electron acceptor DLCs provide the first ranking of electron withdrawing groups based on their potential of lowering the energy of the lowest unoccupied molecular orbital (LUMO) of the discotic core. Unexpected was the large dependence of the frontier orbital energies on the symmetry of the substitution patterns. Symmetric patterns give higher LUMO energies mainly because of degenerated frontier orbitals. Objective of the investigation of octa-carboxylic acid and octa-alcohol substituted tetraazaporphyrin (TAP) dyes in Langmuir-Blodgett (LB) monolayers (Chapter 5) is the generation of self-assembled monolayers with the elusive face-on orientation of the TAP macrocycles. Monolayers are formed only by the TAP derivatives with the longest tested aliphatic spacer between the macrocycle and the carboxylic acids or alcohol groups and only the octa-carboxylic acid TAP remains in a face-on orientation at surface pressures sufficiently high for the transfer of good quality LB monolayers. However, the molecular conformation is spider-like with the TAP macrocycle being physically and electronically removed from the surface by the length of the orthogonal decyl chains, which is probably unfavourable for charge injection. In the second approach, TAPs containing azide groups were mixed with TAPs containing acetylene groups and cross-linked by interfacial Cu(I) catalyzed click-chemistry in a Langmuir film (Chapter 6). The cross-linking process reached astonishing conversions of 70% but the employed TAPs were insufficiently amphiphilic to produce stable monolayers. Consequently, 3-dimensional cross-linked TAP structures were obtained instead of monolayers