Multi‐Layer π‐Stacked Molecules as Efficient Thermally Activated Delayed Fluorescence Emitters

Redox Active cAAC-Fluorene/Indene Systems Displaying Solvatochromism, Green Luminescence and pH Sensing: Functionalization of Fluorenyl/Indenyl Rings with Radical Carbene

Two new series of air stable compounds of cAAC^X = fluorene/indene (X = Me₂ , Et₂ , Cy) [cAAC = cyclic (alkyl) amino carbene] have been isolated and well characterized by X-ray single crystal diffraction, photoluminescence, cyclic voltammogram (CV) and electron paramagnetic resonance (EPR) studies. Fluorescence studies reveals green light emission of cAAC bonded fluorene, whereas free fluorene generally displays a violet emission. Interestingly, the sterically crowded cAAC-fluorene analogue display solvatochromism and CF₃ CO₂ H sensing in solution. CV of the these compounds show a quasi-reversible electron transfer process, indicating the functionalization of fluorene/indene with radical anionic form of carbene, confirmed by CV/EPR measurements. DFT/TDDFT calculations and energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) have been carried out to study different aspects of bonding and electronic transitions. Such a class of redox active and thermally stable organic molecules may be suitable for molecule based spin memory devices in future.

Accelerating Radiative Decay in Blue Through-Space Charge Transfer Emitters by Minimizing the Face-to-Face Donor-Acceptor Distances

Thermally activated delayed fluorophors (TADF) featuring through-space charge transfers (TSCT) suffer from low radiative decay rates (k_r s), especially for blue emitters. Here, a xanthene bridge is adopted to construct space-confined face-to-face donor-acceptor alignment and minimize their distances down to 2.7-2.8 Å, even shorter than the interlayer distance of graphite and thus strengthening the electronic interactions. The resulting blue TSCT-TADF emitters exhibit peaks around ≈460 nm, photoluminescence quantum yields of >90 %, and k_r s of nearly 10⁷  s^-1 , almost 2-10 times higher than previously observed values with comparable reverse intersystem crossing rates. The corresponding blue organic light-emitting diodes show maximum external quantum efficiencies of 27.8 % and 34.7 % with Commission Internationale de L'Eclairage y coordinates of 0.29 and 0.15 using those molecules as emitters and sensitizers, respectively.

Lifting Triplet Energy and Bipolar Characteristics by Limiting the Rotation of the Peripheral Groups in Host Materials to Achieve High-Efficiency Blue OLED

Bipolar host materials with high triplet energy are of great significance for highly efficient blue organic light-emitting diodes (OLEDs). In this work, three donor-acceptor-donor (D-A-D) type host materials with identical non-rigid diphenylsulfone center but differing in rotation degree of peripheral amino substituted derivatives from rotating freely diphenylamine (SODP) to rotating partially iminodibenzyl (SOId) and rotating restricted carbazole (SOCz) were designed and synthesized. It was demonstrated that the triplet energy (E_T ) level of the materials promoted by limiting the rotation degree of the peripheral groups, which was 2.72 eV for SODP, 2.73 eV for SOId and 2.78 eV for SOCz, respectively. Besides, the results of the single-carrier devices indicate SOCz possess better bipolar characteristic. Using FIrpic as guest emitter, the blue OLED with SOCz as host material exhibited superior device performance with a low turn-on voltage of 3.3 V, a maximum current efficiency (CE) of 30.1 cd A^-1 , a maximum power efficiency (PE) of 32.2 lm W^-1 , and a maximum external quantum efficiency (EQE) of 14.0%. This work provides a beneficial guideline for realizing promising host materials in efficient blue OLEDs.

S, Stachelek P, Minakata S, de Silva P, Data P, Takeda Y. The Impact of C2 Insertion into a Carbazole Donor on the Physicochemical Properties of Dibenzo[a,j]phenazine-Cored Donor-Acceptor-Donor Triads

Novel electron donor-acceptor-donor (D-A-D) compounds comprising dibenzo[a,j]phenazine as the central acceptor core and two 7-membered diarylamines (iminodibenzyl and iminostilbene) as the donors have been designed and synthesized. Investigation of their physicochemical properties revealed the impact of C2 insertion into well-known carbazole electron donors on the properties of previously reported twisted dibenzo[a,j]phenazine-core D-A-D triads. Slight structural modification caused a drastic change in conformational preference, allowing unique photophysical behavior of dual emission derived from room-temperature phosphorescence and triplet-triplet annihilation. Furthermore, electrochemical analysis suggested sigma-dimer formation and electrochemical polymerization on the electrode. Quantum chemical calculations also rationalized the experimental results.

π-Stacked Donor-Acceptor Dendrimers for Highly Efficient White Electroluminescence

π-Stacked dendrimers consisting of cofacially aligned donors and acceptors are developed by introducing three dendritic teracridan donors with orthogonal configuration and three triazine acceptors in periphery of hexaphenylbenzene skeleton. The dendritic structure and orthogonal configuration of teracridan not only make their outer acridan segments approaching to acceptor in close distance, but also fix donor and acceptor in face-to-face alignment, leading to through-space charge transfer emission with thermally activated delayed fluorescence (TADF) effect. By regulating charge transfer strength via substituent effect of acceptor, emission color of the dendrimers can be tuned from blue to yellow/red region. Solution-processed two-color white organic light-emitting diodes (OLEDs) based on blue and yellow π-stacked donor-acceptor dendrimers exhibit the maximum external quantum efficiency of 20.6 % and maximum power efficiency of 58.9 lm W^-1 , representing the state-of-the-art efficiency for all-TADF white OLEDs by solution process.