ABSTRACT
The design of multifunctional materials is a challenging and important objective for a wide array of multidisciplinary applications. However, a multifunctional organic emitter exhibiting simultaneous aggregation-induced emission (AIE), multi-responsive polymorphs, mechanoluminescence and electroluminescence have been scarce. In this study, two anthracene based compounds, namely 10-(4-(9H-carbazol-9-yl)phenyl)anthracene-9-carbonitrile (CzPACN) and 10-(4-(di-p-tolylamino)phenyl)anthracene-9-carbonitrile (DTPACN) was designed and synthesized with rigid and flexible donors, respectively. The CzPACN shows the bright blue emission and DTPACN shows the bright green emission in solution. We have demonstrated an effective strategy to achieve three polymorphic phases such as DTPACN-α, DTPACN-ß and DTPACN-γ from DTPACN by controlling the temperature. Under mechanical stimuli, highly restricted and non-planar crystals of the structurally tuned polymorphs DTPACN-α, and DTPACN-ß exhibited red shifted emission and DTPACN-γ showed blue shifted emission. Conversely, CzPACN is not showing polymorphism and is not sensitive to external stimuli. In addition, blue and green OLEDs were fabricated using CzPACN and DTPACN, respectively, as an emitter and achieved a maximum external quantum efficiency (EQEmax ) of 5.5% and 5.7%, respectively, for blue and green OLEDs. Further, this study suggests designing multi-responsive smart materials via a simple modification by introducing a non-planar unit with a large twist.
ABSTRACT
Metal-free organic emitters with thermally activated delayed fluorescence (TADF) characteristics are emerging due to the potential applications in optoelectronic devices, time-resolved luminescence imaging, and solid-phase sensing. Herein, we synthesized two (4-bromobenzoyl)pyridine (BPy)-based donor-acceptor (D-A) compounds with varying donor size and strength: the emitter BPy-pTC with tert-butylcarbazole (TC) as the donor and BPy-p3C with bulky tricarbazole (3C) as the donor unit. Both BPy-pTC and BPy-p3C exhibited prominent emission with TADF properties in solution and in the solid phase. The stronger excited-state charge transfer was obtained for BPy-p3C due to the bulkier donor, leading to a more twisted D-A geometry than that of BPy-pTC. Hence, BPy-p3C exhibited aggregation-induced enhanced emission (AIEE) in a THF/water mixture. Interestingly, the singlet-triplet energy gap (ΔE ST) was reduced for both compounds in the aggregated state as compared to toluene solution. Consequently, a faster reverse intersystem crossing rate (k RISC) was obtained in the aggregated state, facilitating photon upconversion, leading to enhanced delayed fluorescence. Further, the lone-pair electrons of the pyridinyl nitrogen atom were found to be sensitive to acidic protons. Hence, the exposure to acid and base vapors using trifluoroacetic acid (TFA) and triethylamine (TEA) led to solid-phase fluorescence switching with fatigue resistance. The current study demonstrates the role of the donor strength and size in tuning ΔE ST in the aggregated state as well as the relevance for fluorescence-based acid-base sensing.
ABSTRACT
We report highly efficient, ultrathin non-doped green and bluish-green organic light-emitting diodes (OLEDs) using a thermally activated delayed fluorescence (TADF) emitter. The green OLED with an ultrathin (â¼1 nm) EML showed a 2.6-fold higher external quantum efficiency (EQEmax) of 13.5% with a luminance of 17 250 cd m-2 than the conventional (30 nm) non-doped device.
