Stimulus-Responsive Organic Room Temperature Phosphorescence Cocrystal

Phosphorescent materials have promising application prospects in optical sensors,advanced security and anti-counterfeiting,biological imaging and organic light emitting diode etc.due to their long lifetimes and large Stokes shifts.Nonetheless,achieving room temperature phosphorescence（RTP）emission in pure organic systems under ambient conditions is a commonly difficult project,essentially because of the weak spin orbit coupling（SOC）of pure organic materials and the inevitable non-radiative transition and quenching of triplet excitons at room temperature.Therefore,the two main ways to achieve efficient pure organic RTP are enhancing SOC and intersystem crossing between the lowest singlet excited state（S₁）and triplet excited state（T_n）and inhibiting non-radiative transition.The former can be realized by aromatic carbonyl,heavy atom effect and heteroatom while the latter is mainly achieved by crystal engineering,polymers and host-guest doping.Among them,crystal engineering has attracted widespread attention due to its advantages of low cost,simple preparation and effective restriction of molecular motion and non-radiative transition processes.The cocrystallization strategy is being extensively established as an effective technique to improve the photoluminescence properties of the organic materials,along with to discover the structure–property associations at the molecular level.Stimulus-responsive luminescence materials respond to external stimulus,such as light,force,pH etc.,which applications have been extended to the fields of sensing,data encryption and security and anti-counterfeiting.In comparison with stimulus-responsive fluorescence materials which have made great progress,pure organic RTP materials with stimulus response are really scarce now.So far,stimulus-responsive RTP materials have been constructed mainly by means of single crystals,polymers and host-guest doping,no stimulus-responsive RTP materials have been reported to be constructed by cocrystallization strategy.The development of novel stimulus-responsive organic RTP systems is becoming the absolute key in the field of RTP.In addition,their RTP colors are predominated by blue,green and yellow,construction of RTP materials with both high efficiency and long wavelength are hardly achieved caused by the inherent low band gaps between the lowest triplet excited state（T₁）and the ground states（S₀）.In this thesis,bipyridine are taken as the research object,and by introducing coformer,the aggregation structures can be effectively regulated.It not only realizes the stimulus-responsive RTP,but also has important significance for future fundamental research and practical applications.The main contents are as follows:1.Two cocrystals named 44BD-DITF and 44BD-TFBA with 4,4’-bipyridine（44BD）as the donor were designed and cultured.Both cocrystal 44BD-DITF and44BD-TFBA show RTP properties,and the lifetime of 44BD-TFBA is 86.67 ms.More interestingly,both cocrystals exhibit a reversible acid-base stimulus response,with phosphorescence disappearing after acid treatment and recovering after alkali treatment.In order to better explore the reason of RTP and the principle of stimulus response properties,44BD crystal and protonated single crystal were further cultured.Comparative analysis of crystal structure shows that there are halogen bonds and abundant hydrogen bonds in 44BD-DITF and 44BD-TFBA cocrystals.Combined with theoretical calculation and analysis,this is conducive to enhancing the SOC and restraining the triplet non-radiative transition.The pyridine nitrogen atom on the 44BD molecule has lone pair electrons,which can bind protons through the coordination bond.Therefore,after acid treatment of 44BD-DITF and 44BD-TFBA cocrystals,the protonation breaks the halogen bond leading to the disappearance of phosphorescence,and deprotonation after alkali treatment leads to the restoration of the halogen bond so that phosphorescence appears.This part of work lays a foundation for understanding the relationship between the aggregation structure and optical properties,and provides new ideas for further design of RTP materials with acid stimulus response properties.Moreover,44BD-DITF and 44BD-TFBA cocrystals have potential application value in acid-base sensing and information encryption.2.Two cocrystals 24BD-DITF and 24BD-TITF with 2,4’-bipyridine as the donor were designed and cultured.Both cocrystals emit orange RTP,and the phosphorescence efficiency of 24BD-DITF was as high as 9.63%.The two cocrystals have different acid stimulus response properties.24BD-DITF shows reversible acid-base stimulus response properties,which phosphorescence disappears after acid treatment and recovers after alkali treatment,while 24BD-TITF phosphorescence disappears after acid treatment and emits orange fluorescence after alkali treatment.Further analysis of the crystal structure and intermolecular interactions show that there are halogen bonds and hydrogen bonds in cocrystals,as well asπ-πinteraction,which is conducive to the redder emission of RTP.The halogen bond of 24BD-TITF cocrystal is weak,so the bond between pyridine nitrogen and hydrogen proton is stronger.Alkali treatment is not easy to deprotonation,halogen bond is difficult to recover so that phosphorescence disappears.The study of the orange RTP cocrystals with different acid stimulus response properties not only provides a deeper understanding of stimulus-responsive RTP materials,but also provides new ideas for the further design of organic photoelectric materials with longer wavelength.