Excited-State Intramolecular Charge Transfer Materials Based On "Hot Exciton" Mechanism And Their Application In OLEDs

With the establishment of the first low-voltage OLED device in 1987 by Tang et al.,a new era in OLED technology was ushered in.Since then,extensive research and development have been carried out in both academic and industrial spheres.For OLED devices,the proportion of singlet excitons and triplet excitons are 25%and 75%,respectively.Aiming to optimize the device performance and achieve high external quantum efficiency（EQEs）,effective harnessing of singlet and triplet excitons in OLED emissive materials is highly demanded.To achieve this goal,various approaches have been explored,including the development of phosphorescent and thermally delayed fluorescence materials.However,the commercial utilization of phosphorescent materials has been impeded by factors such as high costs of noble metals and the accumulation of triplet excitons which cause triplet-triplet annihilation.The subsequent development of purely organic materials,such as thermally activated delayed fluorescence materials,hot exciton materials,and multiple resonance delayed fluorescence materials,has allowed for theoretical exciton utilization rates of up to 100%.In recent years,these materials have been continuously improved,achieving high efficiency levels comparable to those of phosphorescent devices,and thus have attracted significant attention.In particular,hot excitons materials based on intramolecular proton transfer have emerged as a new focus of research in the scientific community due to their large Stoke shift and multiple efficient reverse intersystem crossing pathways.In this study,we have synthesized two series of excited-state intramolecular proton transfer（ESIPT）materials,which were based on the ESIPT core skeleton of 2-（2’-hydroxyphenyl）benzothiazole（HBT）andβ-diketone,respectively.The two types of materials incorporated hot excitons mechanisms and exhibited hybridized local excited and charge-transfer（HLCT）features.By rapidly undergoing reverse intersystem crossing from the high-energy triplet states to the lowest singlet state,these materials achieved high exciton utilization efficiency of ESIPT materials.A utilization of 57.4%of excitons was achieved with the S2-based ESIPT material,which displayed a blue-white OLED color coordinate of（0.18,0.25）and a maximum EQE of 2.84%.To further improve the device performance,we have designed another three compounds of S3,S4,and S5 that incorporated a D-π-A-π-D structure.Hereβ-Diketones were used as the ESIPT core electron acceptor,and carbazole and diphenylamine groups were employed as the electron donor,thereby forming rigid and relatively planar molecular configurations.Since the energy level of the enol form is slightly lower than that of the keto form,only a small proportion of the enol form can undergo rapid ESIPT and transform into the keto structure,while the keto structure can quickly revert to the enol form through the reverse ESIPT process.As a result,the keto structure has a short emission lifetimeand the efficient utilization of excitons mainly takes place on the enol structure.The emission from the enol form dominates in both PL and EL spectra,as its realatively longer lifetime is a result of the fast reverse intersystem crossing from T₃ to S₁.This leads to slower emission enhancement and slower fluorescence decay of the enol emission than that of the keto form.The excited-state intramolecular proton transfer is suppressed,which effectively reduces energy loss.Among them,the S3 doped blue OLED device exhibits a CIE of（0.14,0.16）,a maximum EQE of 9.3%,and a narrow FWHM of only50 nm.This indicated a broader prospect of these ESIPT molecules for wider applications in the OLED field.