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Title: Fluorination as a tool for controlling properties, molecular order and functionality of polythiophene derivatives
Author: Boufflet, Pierre
ISNI:       0000 0004 6495 7856
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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In this work, we explore the uses of fluorination in the context of π-conjugated polymers. Using extensively studied model systems such as poly(3-hexylthiophene) (P3HT) and poly(2,5-bis(3- alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), we are able to probe the influence of thiophene fluorination on the electronic structure and thin-film morphology, two defining aspects of organic semiconductor performance in electronic devices such as organic field effect transistors (OFET). Experimental data obtained from Ultraviolet photoelectron, UV-Visible and Raman spectroscopy along with differential scanning calorimetry, atomic force microscopy and X-ray diffraction suggest that fluorination leads to a stabilisation of the frontier molecular orbital levels and increased backbone planarity. The latter manifest itself as an enhanced propensity to aggregate and much higher melting point. Hole carrier mobilities in OFET devices are considerably enhanced in the backbone-fluorinated polymers, an observation attributed to the increased planarity of the polymer backbone. Computational calculations also predict an increased backbone rigidity and a strong preference for the coplanar arrangement of fluorinated thiophene rings. The origins of such planarisation are thought to be intramolecular in nature, likely from sulfur-fluorine inter-ring interactions as evidenced by S-F short contacts observed in the single crystal X-ray structure of a bithiophene monomer precursor. By synthesising well-defined diblock and alternating copolymers, backbone fluorination is then explored as a potential tool to control phase separation and dielectric constants, two key factors involved in the context of organic photovoltaic (OPV) devices. The utility of fluorine atoms as a good leaving-group in nucleophilic aromatic displacements on electron-poor aromatic systems is then used to produce a P3HT derivative end-capped with pentafluorobenzene. This polymer can then be reacted with good nucleophiles in mild conditions. As a proof of concept, three functional moieties are tethered to the P3HT derivative in this manner, opening the door towards sensing applications and hybrid organic/inorganic semiconductor interface engineering.
Supervisor: Heeney, Martin Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral