Preparation And Properties Of Pyrazine-based AIEgens

Aggregation-induced emission (AIE) is a novel photo-physical phenomenon which was termed by Tang, et al. in 2001. It refers to a unique phenomenon that a series of propeller-shaped molecules are weakly or non-luminescent in their diluted solution, but greatly enhance their emissions in the aggregate state. Compared with traditional luminophores with aggregation-caused quenching (ACQ) effect, AIE luminogens (AIEgens) could enable aggregation to play positive roles in enhance of emission, thus possessing a huge application potential in the fields of organic light-emitting diodes (OLED), biological and chemical sensors, biological imaging and so on. Up to now, lots of AIEgens have been generated, but most of them are originated from archetypal phenyls substituted silole and tetraphenylethene (TPE). Nevertheless, the intrinsic disadvantages of these AIE cores should not be ignored:for example, phenyls substituted silole and its derivatives are troublesome in preparation, especially in their purification, and unstable under basic conditions; the double bonds in TPE and its derivatives worsen their stabilities under photo and heating. Thus, keeping above questions in mind, we developed a series of pyrazine-based AIE gens, and the relationship of structure-property of them was also investigated in detail.Firstly, we developed a new type of AIEgen of tetraphenylpyrazine (TPP) and its derivatives. It is worthy noting that the preparation of TPP-based AIEgens is feasible, making them applicable in diverse areas. The propeller structure of these luminogens makes them easy to disspitate the excited state energy via intramolecular rotation in solution, but greatly open up the radiative transition channel due to the intermolecular C-H…π hydrogen bond to stiffen molecular conformation in aggregate state, which is featured as AIE or aggregation-enhanced emission (AEE). Because no double bonds and Si-C bonds are involved, TPP has good thermo-, photo- and chemostabilities. Moreover, the electron-withdrawing property of TPP provides a facile platform to design derivative with colorful emissions from deep blue to pure blue.Secondly, taking advantage of the electron-withdrawing ability of TPP, we designed a series of molecules containing TPP and TPA units. The photophysical property investigation showed that the number of acceptor-donor pairs contributed more to the molar absorptivity but less to the maximum absorption wavelength. Also, these molecules exhibit both effects of twisted intramolecular charge transfer (TICT) and AEE in emission. Furthermore, they also display the unique properties of solvatochromism, and selective excellent performance of two-photon absorption, self-assembly and electroluminescence, etc.Thirdly, based on our previous results, polytriazoles PI and PII containing the AIEgens of 2,3-dicyano-5,6-diphenyl pyrazine (DCDPP) and TPP were facilely prepared by Cu(I)-catalyzed azide-alkyne click polymerization, respectively. Interestingly, PI and PII display a contrary photo-physical behaviors:the former suffers from the ACQ effect, whereas, the latter is AIE-active. As informed from the crystal packing of parent molecules and deduced:The intermolecular or intramolecular interaction of π-π stacking in PI could be formed based on the plane of dicyanopyrazine groups in repeating units, from which the light quenching species like excimers will probably be generated. Whereas, in PII, no such species could be generated, making it AEE-active according to the restriction of intramolecular rotation mechanism. Furthermore, these polytriazoles could be used to detect silver ions and more sensitive and selective effect was observed for PII.Finally,1,4- and 1,5-regioregular triazole derivatives containing DCDPP moieties were facilely prepared by Cu(I)-catalyzed azide-alkyne cycloaddition and Ru(II)-catalyzed azide-alkyne cycloaddition, respectively. The photophysical property investigation revealed that both isomers possess TICT and AEE effect. In addition, thanks to their electron-withdrawing ability, they could form charge transfer complexes with strong-electron donating molecule of triphenylamine in aggregate state and emit red light, which provides a new strategy to generate red-light emitting materials.