Optical and morphology properties of supramolecularly controlled conjugated polymers

Resumen

Conjugated polymers (CPs), a central part of the organic semiconductor family, have been subject of intensive research on account of their efficient light emission properties, high optical gain and notable charge carrier mobilities which make them suitable for the development of organic light- emitting diodes (OLEDs), organic lasers (OLs) and sensors. However, the complex arrangement of CPs in solid state leads to a diversity of film morphologies and rich photophysics difficult to pin- point, being the subject of current material research. In general, the largely disorder nature of CPs leads to inhomogeneous nanostructures with a wide variety of chain arrangements in the -atom effects. Thus, their light emission properties can experience a tradeoff due to the presence of weakly emissive inter- chain states, (aggregates, excimers, charge transfer states). Therefore, more understanding is required to learn how to control chain conformations and structure in CP films in order to promote their photoluminescence quantum efficiencies (PLQEs), optical and photonic properties. Tuning of the polymer chain conformation in CP films enables programming the delocalization of the electronic wave functions on the chain or across the chains, which makes big differences in their optical properties. Well defined CP self-assemblies with strong supramolecular interactions such as ¿-¿ interactions, hydrogen bonding or metal coordination, provide substantial intermolecular electronic coupling which favors efficient energy transfer/transport. In this thesis, we shed light on the relation between morphology and optical properties on novel supramolecularly controlled CPs. For this purpose, a series of blue, yellow-green and red emitting CPs, all derivatives of the polyfluorene and polythiophene families, are utilized as building blocks to better understand the intra- or/and interchain conformation influence on their photophysical properties. The major research findings are the following: (1) Insulated polythiophenes (IPTs) bearing conjugated backbones with self-sheathing side-chains were investigated to figure out the relation between structure and photophysical properties in the solid state. Efficient solid state photoluminescence was found in the IPTs which manifested with highly vibronic spectra resembling a mirror-image of the absorption spectra. This effect was attributed to their stiff structure hindering geometric rearrangement in the excited-state. Concomitantly, efficient amplified stimulated emission (ASE) in films was achieved. (2) The relation between photophysical properties and solid state morphology was investigated in a polyfluorene prototype, namely Poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-co-[5- (octyloxy)-9,9-diphenyl-fluoren-2,7-diyl] (PODPF). A gradual increase in energy transfer efficiency from the ¿- to the ß-phase was obtained upon enhancing the ß-phase fraction in films. Films with moderate ß-phase contents exhibited interesting properties, such as the presence of dual ASE peaks ascribed to ¿- and ß-phases. (3) Conjugation shortening effects by inserting different numbers of phenyl groups into PODPF backbone, NPhs (NPhs, N=1, 2, 3), were investigated. The consequences of conjugation length shortening on the photophysics of pristine films as well as NPhs:F8BT blends coupled by energy transfer were assessed. We demonstrate how phenyl intercalation introduced less backbone planarization and shorter effective conjugation length. Moreover, phenyl intercalation was found to be beneficial to promote the stimulated emission properties of conjugated polymer blends.