The Molecular Design, Synthetic Chemistry And Optoelectronic Properties Of Blue-violet Emitting Materials Based On A Tetraphenylsilane Bridge

Wide band gap blue-violet emitting materials are of great significance for full color displaysand solid state white lightings. Not only can they provide blue color which is one of the threeprimary colors but also act as host to illuminate full visible light emission. In addition, comparedto normal blue materials, violet materials with much deeper emission and smller CIEy value aremore economic in terms of power consumption in full color application. Up to date, green and redmaterials have reached very high efficiency and some of them have already been commercialized.In contrast, the development of blue-violet materials falls far behind. One primary reason is thatthe wide band gap of blue-violet materials is not favorable for radiative decay which usually leadsto a low photoluminescence quantum efficiency (PLQY). Hence, chromophores with high rigidityare often adopted to reduce nonradiative vibration processes and ultimately obtain high PLQY. Inaddition, the wide band gap would cause high charge injection barrier and result in poor deviceperformance. A normal strategy is to introduce both electron donor and acceptor to increase theHOMO level and lower the LUMO level and thus lower charge injection barrier. However, thepossible charge transfer (CT) excited state between donor and acceptor will give rise toundesirable emission at longer wavelength. Therefore, a judicious molecular design which cancompromise above contradictions is imperative.It was noticed that the Si atom in tetraphenylsilane adopt sp3hybridization and hence thetetraphenylsilane present a tetrahedron structure with highly twisted configuration. Thus,tetraphenylsilane can effectively interrupt the conjugation between aromatic groups connected to it.If an electron donor with high HOMO level, an electron acceptor with low LUMO level and ahighly emissive blue-violet emitter with narrower optical gap than those of both the donor andacceptor, are simultaneously incorporated into tetraphenylsilane, the expectation of a low chargeinjection barrier and a balanced carrier transport may come true. When the molecule is excited, thecharge transfer emission between donor and acceptor is inhibited as a result of δ-Si interruption.According to Kasha’s rule, when a molecule reaches a higher excited state, it will relax to itslowest excited state before emit a photon. Since the optical gap of selected emitter is narrowerthan those of both the donor and acceptor, the lowest molecular excited state will locate in theemitter, and as a result, the whole molecular will exhibit the same emission behavior as that of theemitter. Moreover, tetraphenylsilane derivatives usually possess highly thermal stability andstrong tendency to form stable amorphous morphology. In conclusion, by connecting electrondonor, electron acceptor and emitter with narrower optical gap than those of both the donor andacceptor through a tetraphenylsilane bridge, it is expected that each of the group can fully exerttheir functions and achieve efficient blue-violet emitting.By summarizing a large amount of related literature, carbazole and pyridine are selected aselectron donor and acceptor, respectively. Spirofluorene and Tetraphenylethene are selected asblue-violet emitter. It is expected that the adduct of carbazole, pyridine,spirofluorene/tetraphenylethene and tetraphenylsilane melting at molecular level can have some integrated properties such as good thermal stability, stable amorphous morphology, low chargeinjection barrier and so on to ultimately carry out high blue-violet device efficiency.In Chapter2, we mainly discuss the synthesis and optoelectronic properties of the bipolarviolet-ultraviolet emitting material CzPySiSF. To the best of our knowledge, the synthesis oftetraphenylsilane derivatives with three different kinds of substituents has not been reported so far.So, we will firstly describe the synthesis details and the stepwise chemical reaction strategy weadopted. Then we will discuss the thermal and optoelectronic properties of the synthesizedmaterials. The bipolar molecule CzPySiSF possesses excellent thermal stability and tends to formuniform amorphous thin film. The emission peak of EL spectrum of CzPySiSF was408nm andeffectively realized violet-ultraviolet emission. Meanwhile, the maximum EQE of the device was0.59%.In Chapter3, we choose Tetraphenylethene (TPE) with high solid state fluorescenceefficiency as the blue emitter, carbazole and pyridine as electron donor and acceptor, respectively.The bipolar molecular CzPySiTPE exhibited high thermal stability and remarkable AIE activity.The maximum EQE of CzPySiTPE was1.12%and the EL spctum located in the blue region witha emission peak of465nm. Not only did the CzPySiTPE maintain the blue emission of single TPEmoiety, but also improved the maximum EQE of TPE from0.4%to1.12%. Herein, by usingtetraphenylsilane as linking bridge, we successfully improved the EQE of TPE as well asmaintained its blue emission. The inspiring results pave a new way for further molecular design ofhighly efficient solid-state bipolar blue materials, in which the modification by bipolar groups canimprove the efficiency of materials without creating long wavelength emission.