Tailor made organic molecules for electronic applications

Abstract

Within this thesis, the synthesis of functional molecules with specific characteristics and a wide application spectrum was of major interest. Due to the nature of their applications, it is rather difficult to categorize all the designed molecules within one general topic. For the purpose of a clearly represented structure, this work is divided in three individual chapters, each treating one independent topic by its own introduction, synthetic strategy, results and conclusion. Chapter 1 - Surface Functionalization Protection groups for alkynes, representing structural building blocks in various chemical applications, are sensitive to a fluoride source or basic conditions. The synthetic accessibility and efficient functionalization by click chemistry of the alkyne moiety makes them interesting target structures in surface functionalization approaches. A main challenge is the site-selective functionalization of a surface. To address this challenge, an electrochemically cleavable protecting group for alkynes was developed. The immobilization of the designed molecules on TiO2 surfaces was achieved with an EDTA based anchoring group, linked by a phenyl ethylene moiety to the protected alkyne. Upon reduction of the naphthoquinone based protecting group, an intramolecular ring formation forces the release of the alkyne moiety, which remains immobilized on the TiO2 surface. The site-selectivity was achieved by dividing a wafer into electronically separated sections. By applying a potential of -0.9 V vs. SCE on individual areas of the wafer, selective deprotection was achieved. The received free alkyne moieties were further functionalized with naphthalene diimide based azide dyes by copper catalyzed 1,3-dipolar Huisgen cycloaddition reactions. To confirm the individual steps of the procedure, solid-state UVVis measurements were performed. Chapter 2 - Molecular Wires The scientific challenge to build electronic circuits by bottom-up approaches in a nanometer scale appealed generation of chemists. With organic synthetic techniques, an atomic precision in the structural design is achievable and enables the creation of identical molecules in large quantities. The understanding of structural and electronical characteristics and their influences on the behavior of molecules in electronic circuits has risen during the last decade. As consequence, molecules acting as molecular wires, rectifiers, switches or memory elements have been developed. Most of the conformation effects contributing to the conductivity of molecular wires were investigated with rather short molecules. By elongate the molecular wires, a change in the transport mechanism was reported in literature from a tunneling to a hopping process. Therefore, new molecules were designed with a length within the determined mechanism changing range. The molecular design was based on already investigated biphenyl structures with defined inter-plane angles, to determine if the same conformation effects were observable in the elongated analogues. The second part of this chapter is based on an earlier study with a bipyridyl derivative[1], which has shown a switch between an “on” state of higher conductance and “off” state with lower conductance in break junction measurements. The switching was achieved by applying a positive or negative voltage in the range of ± 0.9 V. The state of the molecule was readable by the conductance difference of both states at an applied potential, lower than the required switching potential. To further investigate this phenomenon, biphenyl derivatives with electron withdrawing and electron donating moieties were synthesized. The resulting intrinsic dipole moment of the molecules is considered crucial for the switching behavior. Chapter 3 - High-Triplet State Energy Materials Used as building blocks for organic light emitting diodes (OLED), 4,4’-dicarbazole-1,1’-biphenyl (CBP) derivatives are of major interest in the field of electrophosphorescence.[2] Carbazole moieties in CBP provide a high triplet energy state and thereby enable the use as matrix materials with an efficient energy transfer to a phosphorescence light emitting dye.[3] For an efficient triplet energy transfer, especially for deep blue light emitting dyes, the triplet energy state of the matrix material should be higher than 2.7 eV. Five new CBP derivatives were synthesized by introducing sterically demanding substituents to the carbazole subunit in 1,8-position or to the biphenyl backbone. With these modifications, a perpendicular alignment between the carbazole subunit and the biphenyl backbone was achieved, resulting in a decreased π-conjugation through the molecule and therefore an increased triplet state energy. Furthermore, by introducing electron withdrawing and electron donating groups at the carbazole subunit, a shift of the HOMO / LUMO level was achieved.[4]