| dc.description.abstract | Ureasils are Class II organic-inorganic hybrids consisting of poly(oxyalkylene) chains covalently linked to a siliceous network via urea bridges. Ureasil monoliths are photoluminescent, waveguiding and photo- and thermally stable and have been used as hosts for emissive species such as lanthanides, organic dyes and conjugated polymers (CPs). CPs and conjugated organic dyes in particular, are promising materials for flexible lightweight devices such as organic light-emitting diodes and luminescent solar concentrators (LSCs). However, their solid-state morphology can significantly influence their optoelectronic properties, leading to the need for sophisticated design methodologies when trying and incorporate them into devices.
To meet this challenge, this work begins with an investigation of different incorporation strategies for π-conjugated fluorophores into ureasils. Firstly, a siloxane-functionalised poly(fluorene) (PF) (Chapter 3) and a perylene dicarboxdiimide (PDI) (Chapter 4) were covalently grafted via co-condensation to the ureasil siliceous backbone, to achieve their selective localisation within the ureasil matrix. The degree of branching and the molecular weight of the poly(oxyalkylene) backbone were also probed. In both cases, covalent grafting influenced the optical properties of the resultant material; in PF-ureasils, it results in controlled packing of the PF chains, which promotes the formation of the π-stacked β-phase, typical for PFs, which has been linked to enhanced optoelectronic properties. For PDI dyes, covalent-grafting inhibits aggregation and minimises re-absorption losses in PDI-ureasils. Moreover, the ureasil behaves as a donor for energy transfer (ET) to the PDI, enabling tuning of the emission colour.
In Chapter 5, a poly(fluorene-alt-phenylene) (PBS-PFP) copolymer containing on-chain PDI units was physically dispersed in ureasil matrices. The possibility of ET between the ureasil and/or the PBS-PFP donors to the PDI acceptor was investigated. Lifetime measurements showed that good spectral overlap, combined with efficient electronic coupling results in excitation ET from the ureasil to the PBS-PFP units. This process however, inhibits subsequent ET to the PDI chromophore, but leads to high photoluminescence quantum yields (>50%). Due to the low on chain PDI/PBS-PFP ratio, the performance of the system as an LSC is mediocre, but can be boosted by further doping with PDI using a model system. These results demonstrate that the use of an active waveguide host is a promising step towards design of next generation LSCs.
Finally, in Chapter 6, a new ureasil architecture is presented, through the development of hybrid nanoparticles (NPs) consisting of a ureasil core and a silica shell. Upon optimisation of the synthesis, NPs with size of ~200 nm and a polydispersity index of ~0.2, were obtained and remained stable for over 50 days. Incorporation of organic fluorophores within the NPs was investigated by: (i) a non-covalent approach, where dyes are encapsulated in the NPs and (ii) a covalent approach, where the dye is covalently grafted the NPs siliceous backbone.
These examples demonstrate that the simplicity and the versatility of the sol-gel process offer a wide range of possibilities for targeted design of fluorophore-integrated ureasil hybrids. This platform enables us to obtain a variety of hybrid architectures capable of incorporating both CPs and organic dyes, with the possibility of targeting some optoelectronic properties and/or to improve their photo- and their thermal stability, for application in both solid-state emitting devices and dye-doped NPs for imaging. | en |
