| Upconversion luminescence can convert low-energy,long wavelength photon into high-energy,short wavelength photon,so it has a wide range of applications in photocatalysis,solar cells and so on.Among all kinds of upconversion luminescence systems,triplet-triplet annihilation upconversion（TTA-UC）system can achieve high quantum yield upconversion luminescence at low excitation power density,with low energy demand and high efficiency.TTA-UC system is usually composed of sensitizer,acceptor and solvent.At present,the most widely used sensitizers are transition metal complexes,for example ruthenium,palladium and platinum.However,its large-scale application is limited because of the use of rare and precious metals.Recently,some studies have reported that pure organic thermally activated delayed fluorescence（TADF）materials are used as sensitizers.The smallerΔEST let TTA process can easily use the singlet and triplet exciton of sensitizer to achieve efficient upconversion.In addition,the good sensitizer needs to have good absorption in visible light even up to near infrared region,which can efficiently get use of low-energy and long wavelength photon.And the lower triplet energy level is necessary to reduce the energy gap between sensitizer and acceptor,which is benefit to the triplet-triplet energy transfer（TTET）.Therefore,it is of great significance to develop a kind of TTA sensitizer,which should have the TADF properties,appropriate triplet energy level,red emission and high quantum yield.These characteristics should help the sensitizer have great performance in upconversion.The benzothiadiazole（BTZ）as acceptor has good planarity and electron absorption,and it is the acceptor of red emission materials in common use.Benzothiadiazole has multiple reaction sites,which can connect donor to form"donor-acceptor"（D-A）type luminescent materials,and it is a hot research topic at present.In this paper,based on the above requirements of TTA sensitizer and the design criteria of red TADF materials,three D-A type materials（BTZ-DMAc,Br-BTZ-DMAc,CN-BTZ-DMAc）were designed and synthesized,which use the BTZ acceptor unit as core.Due to the steric hindrance effect between the benzothiadiazole acceptor unit and the acridine donor unit,a large distorted structure is formed.The introduction of heavy atom Br further enhances the spin orbit coupling effect of the molecule,making its TADF characteristics more obvious.And the introduction of cyano enhanced the electron absorption property,increased the intramolecular charge transfer state,decreased the triplet energy level of the molecule,and its fluorescence emission spectrum was significantly red shifted.The rationality of molecular design was verified by theoretical calculation,and the photophysical properties and upconversion luminescence properties were studied in detail.As a result,the delayed fluorescence lifetimes of BTZ series compounds are up to micro second level.In addition,BTZ series compounds can achieve the upconversion from green-excitation to blue-emission in TTA-UC system,and the two molecules with TADF properties can obtain significantly lower excitation power threshold,and the highest upconversion quantum yields up to 1.06%.The excitation power threshold is an important parameter.In order to get further study on reducing the excitation power threshold and improving upconversion quantum yield.Starting from the significant TTET process,we designed a novel sensitizer,DPA-BTZ-DMAc,in which the DPA unit used as the acceptor was directly covalently linked to the red emission material unit.The introduction of DPA unit does not greatly change the photophysical properties of the molecule.It can make full use of its structural similarity to enhance the TTET process between DPA structure in molecule and DPA in the solvent system,and reduce the excitation threshold significantly.Theoretical calculation,photophysical properties test and upconversion luminescence performance show that the excitation threshold of DPA-BTZ-DMAc is 2.63×104 m W/cm2,which is reduced by one order of magnitude compared with BTZ-DMAc as sensitizer.And the maximum upconversion efficiency is greatly improved,reaching 1.39%.In this paper,the molecular design,synthesis,photophysical properties and upconversion performance of a series of red emission materials based on benzothiadiazole as acceptor.From the aspects of heavy atom effect,charge transfer state enhancement and similarity,the effects of TADF properties,PLQY of sensitizer,the excitation threshold and up conversion efficiency of TTA-UC were investigated.It provides useful molecular design ideas and methods for the development and utilization of high-performance TTA sensitizer. |
