Investigation in linker functionality of main-chain thermally activated delayed fluorescence polymers and through-space charge transfer in supramolecular polymers
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Abstract
In recent years, organic light-emitting diodes (OLED) using thermally activated delayed fluorescence (TADF) emitters were shown to be one of the most promising technologies for display and lighting applications. The TADF mechanism enables the utilization of both singlet and triplet excitons for light generation based on a thermal upconversion resulting from a small energy difference (Δ𝐸𝑆𝑇) between the lowest excited triplet and singlet states. While excellent performances have been reported for small molecule TADF emitters, the necessary vacuum processing makes large-area device fabrication difficult and expensive, explaining the high research interest in the development of polymeric TADF emitters enabling solution processing.
The focus of this dissertation was on the systematic investigation of the influence of the linker moiety on the photophysical properties and performance in optoelectronic devices in main-chain TADF polymers. Two sets of yellowish-green emitting polymers with alkyl spacers of different lengths as well as a conjugated biphenyl linker were synthesized and compared to structurally equivalent small molecules. While the charge transfer (CT) character of the excited state and the Δ𝐸𝑆𝑇 showed no linker dependent change, all polymeric materials were identified to suffer from a pronounced concentration quenching effect in a film, reducing their photoluminescence quantum yield (PLQY) and performance in OLEDs. This adverse effect was successfully suppressed by diluting the materials with polystyrene in blended films, resulting in the TADF polymers with nonconjugated linkers showing similarly high PLQY values as well as efficiencies in OLEDs compared to the small molecule emitter. Furthermore, with regard to device performance they were observed to be superior to their conjugated polymer counterpart.
In the second part of this dissertation, the field of use of TADF emitters was expanded by incorporating donor and acceptor moieties into peptide-polymer conjugates capable of pH-responsive self-assembly. Upon increasing pH in aqueous solution, a supramolecular polymer is formed leading
to donor and acceptor coming in close proximity to one another exhibiting a through-space charge transfer (TSCT) indicated by a significant red-shift in photoluminescence (PL).