Incorporating Thermally Activated Delayed Fluorescence into Mechanochromic Luminescent Emitters: High‐Performance Solution‐Processed Yellow Organic Light Emitting Diodes

Abstract

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...

Recent Advancements in and the Future of Organic Emitters: TADF‐ and RTP‐Active Multifunctional Organic Materials

“…Most recently,Y ang andc o-workers developed as eries of TADF-active multicolor-changing MCL materials based on the D-A scaffold by using diphenyl [1,2,4]triazolo[1,5-a]pyrimidine (TzPm) as an acceptora nd phenoxazine as ad onor (Figure 35 a). [111] On doping in CBP matrix (0.7 wt %d oping), all of the compounds showedt ypical TADF properties, with yellow emissions (l em = 527-543 nm, t d = 2-3 ms), narrow DE ST values (60-100 meV), and good F PL values (0.49-0.66, under an argon atmosphere). OLED devices that were fabricatedb yu sing solution processes based on the configuration ITO/PEDOT:PSS/ emitter in CBP (0.7 wt %)/PmPyPB/Liq/Al showedy ellow EL, with peaks within the range l = 542-560 nm and good maxi-mum EQEs of 9.3-14.3%.I nterestingly,D -A compound 7TzPmPXZ exhibited bicolor (yellow-orange) MCL behavior, whereas compounds that contained more donor units (5TzPmPXZ and 5,7TzPmPXZ)s howed tricolor-changing MCL behavior in response to av ariety of external stimuli,s uch as grinding, heating, recrystallization, and solvent fuming (yellow-yellowish-orange-orangeand yellow-red-deep-red, respectively; Figure 35 b-d).…”

“…Most recently, Yang and co‐workers developed a series of TADF‐active multicolor‐changing MCL materials based on the D–A scaffold by using diphenyl [1,2,4]triazolo[1,5‐ a ]pyrimidine (TzPm) as an acceptor and phenoxazine as a donor (Figure a) . On doping in CBP matrix (0.7 wt % doping), all of the compounds showed typical TADF properties, with yellow emissions ( λ em =527–543 nm, τ d =2–3 μs), narrow Δ E ST values (60–100 meV), and good Φ PL values (0.49–0.66, under an argon atmosphere).…”

Recent Developments on Multi-Functional Metal-Free Mechanochromic Luminescence and Thermally Activated Delayed Fluorescence Organic Materials

“…N.B . All photophysical studies in toluene Rajamalli et al, 2016 ; Ganesan et al, 2017 ; all photophysical studied on neat film Chen et al, 2018 ; Zhan et al, 2019 ; all photophysical studies in DCM Takeda et al, 2018 ; all photophysical studies in their respective solid states/crystalline states (Zhang et al, 2017 ; Huang et al, 2018 ; Data et al, 2019 ; Zheng et al, 2019a ); only UV/PL in toluene (Okazaki et al, 2017 ; Pashazadeh et al, 2018b ; Zeng et al, 2018 ; Zheng et al, 2019b ); only UV/PL in DCM Ishimatsu et al, 2014 ; Pashazadeh et al, 2018a ; life time study on thin film (CBP host) Zeng et al, 2018 ; Zheng et al, 2019b ; life time study on thin film (zeonex host) (Pashazadeh et al, 2018a , b ) .…”

“…Lateron Zeng et al ( 2018 ) reported three multifunctional organic emitters (5TzPmPXZ, 7TzPmPXZ, and 5,7TzPmPXZ) comprising PXZ-moiety integrated with electron-acceptor [1,2,4]triazolo[1,5-a]pyrimidine (TzPm) units, that exhibited TADF with excellent photoluminescence quantum yields (PLQYs) of 49–66%, along with MCL properties caused by transitions between microcrystalline and amorphous phases, in response to external mechanical stimuli. These D-A functional molecules exhibited small ΔE ST (0.06–0.10 eV) including good spatial separation between the HOMO and the LUMO, which is greatly encouraged by the large torsion angle between the donor and acceptor moieties (74.68 to 88.17°) resulting from the steric hindrance between the donor units and the neighboring phenyl ring, susceptible to RISC as well as TADF.…”

“…(c) J-V-L curves for devices A, B, and C (inset: the normalized EL spectra of devices). (d) EQE and LE vs. luminance curves for devices A, B, and C. Reproduced with permission from Zeng et al ( 2018 ). Copyright © WILEY-VCH Verlag GmbH andamp; Co. KGaA, Weinheim.…”