Studies On The Synthesis And Properties Of 4H-pyrimido [2,1-b]benzothiazole Derivatives

The aggregation-induced emission（AIE）is the phenomenon of the fluorescence enhancement of organic molecules upon aggregation.Since the concept of AIE was proposed,a large number of AIEgens have been reported and have been widely used in many fields such as cell imaging and organic optoelectronic devices.Currently,most known AIEgens have a typical propeller structure and a stator withπ-conjugate.The AIE mechanism can be attributed to the restriction of intramolecular motion（RIM）mechanism.In recent years,some AIEgens with atypical propeller structures or without aπ-conjugate stator have been reported.These molecules exhibit AIE properties through excited-state intramolecular proton transfer（ESIPT）or through-space/through-bond conjugation.The discovery of these new AIEgens not only gives us a deeper understanding of AIE mechanism,but also provides more effective guidance for the rational design of AIE molecules.In our research on multi-component reactions,we found 4H-pyrimido[2,1-b]benzothiazole（PBT）as a novel AIE molecular skeleton with an atypical propeller shape（unpublished results）.Because of the high molecular diversity of starting materials of each component,and convenient synthetic procedure,the PBT molecular skeleton has excellent structural derivatization capability.In chapters 2 to 4 of this thesis,the synthesis and properties of three series of 19 novel PBT derivatives were studied.The detailed studies are described as follows.In chapter 2,the synthesis and properties of 9 2-methyl modified PBT derivatives were studied.Firstly,3 PBT derivatives substituted at the 2-position with CH₂Cl-,CHF₂-,and Et OOCCH₂-were directly synthesized via a one-pot reaction catalyzed by Hf（OTf）₄.Then,the 2-CH₂Cl PBT derivative was used to synthesize 4PBT derivatives with CH₃COOCH₂-、Ph OCH₂-、（CH₃）₃N⁺CH₂-、and p-CH₃OPh SCH₂-groups via nucleophilic substitution reactions.Finally,the2-p-CH₃OPh SCH₂-PBT derivative was used to synthesize 2 PBT derivatives with p-CH₃OPh-SOCH₂-and p-CH₃OPh-SO₂CH₂-goups via oxidation reactions.Among these new PBT derivatives,the single crystals of 6 compounds were successfully obtained.Subsequently,their photophysical properties,such as UV absorption,fluorescenc spectra,fluorescence quantum yields,and fluorescence lifetime were determined.The experimental results show that the new PBT derivatives generally exhibit no fluorescence in solution,but emit fluorescence when excited in a solid state.Therefore,these compounds are typical AIEgens.The structure-reactivity relationship studies show that the different functional groups at the 2-position have limited effects on their UV absorption and excitation/emission wavelengths,but have significant effects on their fluorescence quantum yields and fluorescence lifetime.The analysis of crystal structures shows many weak intermolecular effects in the crystals of PBT derivatives.In chapter 3,the synthesis and properties of 4 2-and 3-position cyclized PBT derivatives were studied.In this study,the 2-CH₂Cl PBT derivative was used as starting material,and 4 new cyclized PBT derivatives of lactone,lactam,and cyclohydrazide were synthesized via tandem substitution and intramolecular cyclization reactions under basic conditions,and single crystals of 3 compounds were obtained.Subsequently,their photophysical properties,such as UV absorption,fluorescenc spectra,fluorescence quantum yields,and fluorescence lifetime were determined.The experimental results show that these derivatives have typical solid fluorescence characteristics,but exhibit notable fluorescence in solution.Due to the fact that the fluorescence is enhanced upon aggregation,these derivatives have AIEE properties.These results indiate that the rotation of ester group at 3-position of PBT molecules plays a crucial role in energy dissipation of excited state in solution,which has been rarely reported in previous AIE research.The photophysical properties of solid lactone,hydrazide,and phenyl lactam derivatives are similar,in terms of their UV absorption,excitation/emission wavelength,fluorescence lifetime,and fluorescence quantum yields.Methyl lactam PBT derivative has two emission wavelengths,and higher fluorescence quantum yield.The analysis of the crystal structures shows their pattern of stacking and various intermolecular forces.In chapter 4,the synthesis and properties of 6 4-heteroaryl PBT derivatives were studied.The PBT derivatives with pyridin-2-yl,pyridin-3-yl,pyridin-4-yl,pyrimidin-5-yl,thiazol-2-yl and thiophen-2-yl were synthesized via the one-pot method catalyzed by Hf（OTf）₄with a series of heteroaryl aldehydes as substrates.Single crystals of 3 compounds were obtained.Their properties such as UV absorption,fluorescenc spectra,fluorescence quantum yields,and fluorescence lifetime were determined.The experimental results show that the 4-heteroaryl PBT derivatives have typical AIE properties.The fluorescence emission wavelength of the pyridyl and pyrimidinyl derivatives is red-shifted,while the fluorescence emission wavelength of the thiazole derivative is blue-shifted.The decrease in the aromaticity of the 4-heteroaryl group significantly increases the solid fluorescent quantum yield of the corresponding PBT molecules,and also affects their fluorescence lifetime.The analysis of crystal structures shows the pattern of stacking and various intermolecular forces of this series of PBT derivatives.