טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentFirstenberg Michal
SubjectSequence Independent Synthesis of Organic Pi-Conjugated
Oligomers and Polymers
DepartmentDepartment of Chemistry
Supervisor Professor Yoav Eichen
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

The research focused on the design and preparation of monomers for sequence independent synthesis (SIS), which, when combined, form π-conjugated oligomers and polymers. For the purpose of optimization, different kinds of π-conjugated backbones have been explored, namely phenyl, thiophene, fluorene, and ethylene-dioxy-thiophene rings. The SIS principle is based on phosphonat and aldehyde groups present on each of the monomers, making them able to interconnect with one another at any desired sequence. For an efficient SIS, a step-by-step procedure was chosen, utilizing a protection-deprotection protocol (aldehyde protected by an acetal group), for connecting a single monomer to the chain in each synthesis stage. The monomers consisted of single ring systems to three ring systems, enabling the formation of either very short or relatively long oligomers. The side groups, alkyl and alkoxy chains, are crucial for attaining solubility of the forming oligomers and monomers, a must for device making. With these monomers in hand, a library of various oligomers was constructed. The library was characterized by means of absorption and emission spectroscopy, single crystal X-ray diffraction and electrochemistry. The electronic band gap and the work functions of both the valance and the conduction bands were obtained. We have demonstrated a clear and direct relation between the band gap and the oligomers’ conjugation and have found that the thiophene rings contribute more to the conjugation than, e.g., simple backbone structure and phenyl rings. Very low band gaps of ~2 eV were measured for the longest oligomers, which are close to the lowest possible band gap in this family, determined by us to be 1.7 eV. Subsequently, two low-band-gap oligomers were used to construct electronic devices: organic double-layer light-emitting diodes (OLED) and organic field-effect transistors (OFET). For the OLED, our oligomers were sandwiched between an ITO anode covered by a hole conducting layer (PEDOT) and a Ca/Al cathode. The OLEDs had a green color or a red-orange color, with relatively good brightness (1000-3500 cd/m2) and efficiency. For the OFET, our material was suitable for a p-type device (conductance by holes). Due to the low band gap, the required source-drain voltages were low (4 V), with negligible gate threshold (<1 V). In conclusion, SIS was demonstrated for a large variety of monomers and was found to be an efficient way to synthesize short and long conjugated oligomers in a controllable manner, allowing for optimization of large libraries and for the investigation of structure-properties relations.