emitting diodes (OLEDs). [6] Very recently, thermally activated delayed fluorescence (TADF) emitters have emerged as a potential alternative to phosphorescent emitters for OLEDs due to their abilities to reach unity internal electroluminescence quantum efficiency without using noble metals. [7] Thanks to the continuous endeavor of investigators, lots of highefficient TADF emitters have sprung up in the past years. [8] However, the multicolor TADF emitter of which the solid state emission could be reversibly tuned is till rarely reported, although it is of great significance for scientific research and practical applications. [9] Taking these into account, integrating the MCL behavior into TADF materials would be an effective strategy for creating multicolor TADF emitters. Moreover, the marriage of MCL and TADF functions will endow these emitters with more applications in advanced fields, including displays and lighting. In this context, Chi and co-workers first reported a dual-emissive emitter which shows TADF and MCL behavior in 2015. [10] Soon after, Minakata and co-workers developed two TADF molecules that exhibit multicolor-changing MCL by introducing phenothiazine as donors. [11] Recently, Grazulevicius and co-workers and co-workers successfully synthesized four multifunctional MCL-TADF luminogens, the external quantum efficiencies (EQEs) of resulting OLEDs reached 10.9%. [12] Despite of these excellent works, the multicolor-changing (not less than three colors) TADF materials, which look more promising in applications of security inks, information storage and sensors, are still limited due to the challenges in molecular design. [11,13] Hence, the exploitation of emitters which endowed with TADF and multicolor-changing behavior is imbued with significance for both scientific research and practical application.In this work, we developed three multifunctional materials which show both TADF and MCL features by first introducing [1,2,4]triazolo[1,5-a]pyrimidine (TzPm) units as an electron acceptor. According to the positions of donor of phenoxazine, they are named 5TzPmPXZ, 7TzPmPXZ, and 5,7TzPmPXZ (Scheme 1). Notably, all the target compounds show high photoluminescence quantum yields (PLQYs) of 0.49-0.66, small ΔE ST of 0.06-0.10 eV, and distinct TADF features. 7TzPmPXZ exhibits reversibly bicolor-switching behavior upon Multifunctional materials with stimuli-responsive features are demonstrated to be promising smart materials. In this work, three multifunctional materials, namely 5TzPmPXZ, 7TzPmPXZ, and 5,7TzPmPXZ, are constructed by integrating phenoxazine with electron-acceptor core [1,2,4]triazolo[1,5-a] pyrimidine (TzPm). Intriguingly, these materials exhibit not only thermally activated delayed fluorescence but also multicolor luminescence switching which is caused by the phase transitions between microcrystalline and amorphous states upon external stimuli. Moreover, the solution-processed organic light emitting diodes (OLEDs) fabricated with these materials furnish a high external quantum efficienc...
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