Luminescence Properties Of Pyrazine Derivatives With Aggregation-induced Emission

Nowadays,with the rapid development of electronic and information technology,the exploration of luminescent materials has become one of the popular research topics.Comparing to the inorganic luminescent materials,the organic ones have the advantages,such as facile preparation,easy modification,flexibility of structure.At present,organic luminophores have been widely used in new generation of flat organic light-emitting diode(OLED)displays,chemical sensors and biological imaging.Most traditional luminescent materials show weak emission in solid states due to the aggregation-caused quenching effect.However,a type of organic molecules emits hardly in solutions but generate strong fluorescence in the aggregate states,which is called “Aggregation-induced emission(AIE)”.The unique charm of AIE creates a lot of innovative scientific research.In recent years,AIE materials have shown the explosive growth,and the new AIE systems have been developed to meet the needs of different scientific fields.In 2015,tetraphenylpyrazine(TPP)was reported as a new star of AIE luminogen.Subsequently,a series of TPP derivatives with various fluorescence colors were obtained through the different structure modifications.Nonetheless,there are few studies on the AIE mechanism and luminescent properties of this structure,whilst the specific design strategy of this system is also absent.Based on this background,the studies presented in this thesis focuses on the theoretical research on the structure-property relationship of pyrazine compounds and the subsequent preliminary explorations in the field of electroluminescence(EL).In Chapter 2,we design three(o-/m-/p-)monosubstituted pyrazine-carbazole derivatives and a full-substituted compound.Combined with the experimental results and theoretical analysis,the functions of three(o-/m-/p-)substituting position are clarified.The meta and para phenyl groups can slightly and outstandingly adjust their emission properties,respectively.The ortho phenyl group can distort the plane of pyrazine owing to the large steric hinderance of two moieties,which triggers many molecular motions and leads to the wasting of excited state energy,so it motivates the AIE characteristic of TPPs.Meanwhile,the ortho-disubstituted pyrazine-fluorene structures are prepared to explore the relation between the planarity of pyrazine and AIE behaviors.These results can serve as a design strategy of the AIE molecules.In Chapter 3,the influence of the structural modification on the luminescent properties is further explored based on a series of polyphenylpyrazine compounds.From the analysis of the structure variety,hole-electron distribution and reorganization energy,we discover that the luminescent quantum yield shows a negative correlation with the degree of the hole-electron overlaping on polyphenylpyrazine.According to the relation among the molecular structure,hole-electron distribution and luminescent properties of these compounds,we propose the corresponding molecular design scheme,which is expectedly an accurate theoretical guidance for the preparation of high on-off ratio AIE fluorescent probes and high efficiency OLED devices.In Chapter 4,we speculate that the solvent effect on the photophysical properties of the pyrazine compounds is similar to the effect of structural modification and expect that the luminescent materials with the characteristics of polarity enhanced emission can be obtained based on this type structure.Hence,we gradually explore the solvatofluorochromic phenomenon of these compounds.First,from the study on TPP-imidazole derivatives,we confirm that electron donating-accepting(D-A)structure is the basic structural condition for achieving solvatofluorochromic behavior with polarity enhanced emission.Second,we find that bonding the strong D groups can show the obvious response to polarity through the research on the solvation process of different TPP-aromatic amine compounds.Third,the electrondeficiency can be increased via pre-twisting the TPP group,and PL intensity of t-TPP-TPA increases first and then decreases with the rise of solvent polarity,which reveals the general rule of intramolecular charge transfer that the molecule chooses the planarized intramolecular charge transfer(PLICT)and twisted intramolecular charge transfer(TICT)process to balance the increasing polarity environment,successively.In Chapter 5,based on the understanding of above studies,triphenylpyrazine(Tr PP)group is chosen as the AIE unit and three Tr PP derivatives(Tr PP-Ph C,Tr PP-3C and Tr PP-TPA)are obtained via combining with the common donors phenylcarbazole and triphenylamine.In virtue of restricted intramolecular motion(RIM)mechanism of Tr PP,Tr PP-Ph C and Tr PP-3C exhibit obvious AIE phenomena,while Tr PP-TPA shows aggregation-enhanced emission(AEE)phenomenon.The non-doped OLED devices fabricated with these emitters display good deepblue to blue EL performance.These preliminary attempts imply the Tr PP compounds have the potential application in deep blue OLED,but these type emitters cannot fix the deep-blue emission “contradiction” between the efficiency and color purity.In Chapter 6,a linear D-A-D structure is designed to solve the contradiction of deep-blue emission that is mentioned in Chapter 5,so two deep blue(DPP-DPh C and DPP-D3C)and a blue(DPP-DTPA)fluorophores were synthesized.In this strategy,the introduction of two D and the linear pyrazine structure is expected to maximize the para-substitution effect,elongate the hole-electron distribution and achieve the high efficiency emission in the short-wavelength region.Consequently,the OLED devices based on DPP compounds show the excellent deep blue to blue EL performance.Among them,the deep blue non-doped OLED device based on DPP-DPh C exhibits the best EL performance with the maximum EQE of 5.73% with the Commission International de L'Eclairage(CIE)coordinate of(0.151,0.078)among them.These results demonstrate the feasibility of our strategy and provide the simple method to achieve the high efficiency deep blue emitters.In summary,this thesis focuses on the luminescence properties of pyrazine compounds.Through the modification and derivatization of the surrounding structure on pyrazine,the aggregation and solvation effects on the luminescence are studied,and the relation among the structural regulation,hole-electron distribution and luminescence are obtained.The development of AIE/AEE pyrazine compounds and theoretical understanding guide their preliminary explorations on the OLED application and have the important reference for the design and prediction of the subsequent functional materials.