Conjugated, semiconducting polymers are widely used as active compounds of optoelectronic devices. Usually, they are applied as thin films or layers. However, most “conventional” luminogens show an aggregation-caused quenching (ACQ), which is detrimental for their application. In 2001, Tang et al. reported a novel aggregation-induced emission (AIE) phenomenon: Such luminogens weakly emit in dilute solution but intensely in the aggregated state. Restriction of intramolecular motions has been demonstrated to be the main mechanism for occurrence of the AIE effect by theoretical and experimental studies. Till now, plenty of AIE-active molecules have been synthesized, for the applications in organic light emitting devices (OLEDs), chemo/biosensors, cell imaging, and so on. However, most of them are low molecular weight compounds. Based on superior film forming and mechanical properties, also the design of high molecular weight, polymeric luminogens is promising, e.g. for the use in large-area or flexible devices.
In this thesis, we introduce a new class of AIE polymers with AIE- or crystallization-induced emission (CIE)-active tri-/tetraphenylethylene and 2,3,3-triphenylacrylonitrile (TPAN) side chains. This strategy guarantees the occurrence of AIE properties without greatly affecting the electronic properties of the polymers’ backbones. The obtained AIE-active polymers were applied in the detection of nitroaromatic explosives with two main benefits: a) their high fluorescence quantum yields in the aggregated state; b) their twisted and loose structure, which accelerates the diffusion of analyte molecules, thus enhancing the quenching efficiency. This thesis is comprised of four parts.
1. Carbazole is a typical electron-rich building block, which is beneficial for the interaction with electron-deficient nitroaromatic explosives based on the fluorescence changes. Hence, a series of polycarbazoles (PCz3PEs, PCzTPEs) with AIE-active tri-/tetraphenylethylene side groups have been designed and synthesized. Among them, PCzTPEs showed distinct AIE effect and possess high fluorescence quantum yields in solid film. For the application in the detection of 1,3,5-trinitrobenzene (TNB) as prototypical nitroaromatic analyte, a sensitive and amplified quenching effect was observed with a maximum quenching constant of 1.26×10⁶ M-1 based on PCzTPE0.5 aggregates in 1/9 THF/water. Solid-state paper strips experiments based on PCzTPE and PCzTPE0.5 also showed sensitive PL quenching.
2. Similar to carbazole, triphenylamine (TPA) buiding blocks also exhibit electron-rich characteristics and can serve as electron-donor moiety. Polytriphenylamines (P1 and P2) with tetraphenylethylene (TPE) side chains have been synthesized. P1 and P2 showed distinct AIE behavior and have been used as films for the detection of TNB vapour, with excellent fluorescence response. 89% PL quenching within 10 min and outstanding repeatability have been observed. One disadvantage of P1 and P2 is their poor solubility. To solve this problem, bis(tert-butyl)-TPE groups were introduced to the backbone of the copolymers. The obtained copolymers showed improved solubility in common organic solvents and reasonable AIE properties leading to high solid state fluorescence quantum yields.
3. Two polymers PCzTPAN and PTPATPAN have been developed with electron-rich carbazole- or TPA-based backbones and 2,3,3-triphenylacrylonitrile (TPAN) side chains. As a congener of TPE, TPAN is crystallization-induced emission (CIE)-active. Our polymers containing TPAN side groups showed distinct AIE properties. Moreover, the introduction of the electron-deficient TPAN units led to the occurrence of intramolecular charge transfer (ICT) effects. For TNB detection, both polymers showed sensitive PL responses with a maximum PL quenching constant of 5.5×10⁵ M-1. Furthermore, PTPATPAN was used as dopant for detecting the glass transition temperature of polymers, with polystyrene as the example. The method is a straightforward, simple and sensitive way for the detection of the glass transition temperature, especially of thin films.
4. In the last part, we designed conjugated polymers with electron-rich phenothiazine (PTz)- and thiophene-based backbones and TPE or TPAN as side chains. The obtained PTz polymers showed the expected AIE phenomenon. The polythiophenes, however, displayed ACQ effects, also in the presence of the AIE-active side chains, thus demonstrating a competition between intermolecular π-stacking and restriction of intramolecular motion. The photophysical properties of the polymers were studied in detail.