A series of phenothiazine-dibenzothiophene-S,S-dioxide charge-transfer molecules have been synthesized. Increasing steric restriction around the donor-acceptor bond significantly alters contributions ...from TADF and phosphorescence. Bulky substituents on the 1-(and 9) position(s) of the phenothiazine result in no TADF in the solid state; instead strong phosphorescence is observed at ambient temperature.
Materials that display thermally activated delayed fluorescence (TADF) have recently been identified as the third generation emitters for organic light-emitting diodes (OLEDs). However, there are ...only a few reported examples of polymeric TADF materials. This study reports a series of polymers with an insulating backbone and varying ratios of 2-(10H-phenothiazin-10-yl)dibenzothiophene-S,S-dioxide as a pendant TADF unit. Steady-state and time-resolved fluorescence spectroscopic data confirm the efficient TADF properties of the polymers. Styrene, as a comonomer, is shown to be a good dispersing unit for the TADF groups, by greatly suppressing the internal conversion and triplet–triplet annihilation. Increasing the styrene content within the copolymers results in relatively high triplet energy, small energy splitting between the singlet and triplet states (ΔE ST), and a strong contribution from delayed fluorescence to the overall emission. Green emitting OLED devices employing these polymers as spin-coated emitting layers give high performance, which is dramatically enhanced in the copolymers compared to the homopolymer. Within the series, Copo1 with a regiorandom ratio of 37% TADF units:63% styrene units displays the best performance with a maximum external quantum efficiency (EQE) of 20.1% and EQE at 100 cd m–2 of 5.3%.
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The synthesis of 1-methylphenoxazine via CO2-directed lithiation chemistry is reported. This electron donor was coupled with 2,8-dibromodibenzothiophene-S,S-dioxide with Buchwald–Hartwig chemistry to ...give a new donor–acceptor–donor charge-transfer fluorescent molecule 1b. X-ray crystal structures and calculations show that the phenoxazinyl groups are coplanar and equatorial (eq) to the acceptor plane in nonmethylated 1a but are pyramidal and axial (ax) in 1b. The bond rotation energy barriers between donor and acceptor groups for 1a and 1b are only 0.13 and 0.19 eV, respectively, from hybrid-DFT computations at the CAM-B3LYP/6-31G(d) level. Many possible conformers are present in solutions and in zeonex. In zeonex, the methyl groups in 1b shift the emission band 0.13 eV higher in energy compared to 1a. Excited state eq–eq and ax–ax geometries were identified with DFT calculations with charge transfer (CT) emission assigned as 1CT(eq) and 1CT(ax) dominating. The lower energy 1CT(eq) contributes to thermally activated delayed fluorescence, whereas the higher energy 1CT(ax) does not. Phenothiazine analogues 2a and 2b also have major fluorescence emissions assigned as 1CT(eq) and 1CT(ax), respectively. 2a and 2b have substantial room temperature phosphorescence (RTP), whereas 1a and 1b do not, highlighting the importance of the sulfur atom in 2a and 2b to obtain RTP emission.
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Here, a comprehensive photophysical investigation of a the emitter molecule DPTZ‐DBTO2, showing thermally activated delayed fluorescence (TADF), with near‐orthogonal electron donor (D) and acceptor ...(A) units is reported. It is shown that DPTZ‐DBTO2 has minimal singlet–triplet energy splitting due to its near‐rigid molecular geometry. However, the electronic coupling between the local triplet (3LE) and the charge transfer states, singlet and triplet, (1CT, 3CT), and the effect of dynamic rocking of the D–A units about the orthogonal geometry are crucial for efficient TADF to be achieved. In solvents with low polarity, the guest emissive singlet 1CT state couples directly to the near‐degenerate 3LE, efficiently harvesting the triplet states by a spin orbit coupling charge transfer mechanism (SOCT). However, in solvents with higher polarity the emissive CT state in DPTZ‐DBTO2 shifts below (the static) 3LE, leading to decreased TADF efficiencies. The relatively large energy difference between the 1CT and 3LE states and the extremely low efficiency of the 1CT to 3CT hyperfine coupling is responsible for the reduction in TADF efficiency. Both the electronic coupling between 1CT and 3LE, and the (dynamic) orientation of the D–A units are thus critical elements that dictate reverse intersystem crossing processes and thus high efficiency in TADF.
The crucial step in the thermally activated delayed fluorescence (TADF) mechanism is the reverse intersystem crossing that converts triplet to singlet states. Here we show that this is mediated by a spin orbit charge transfer (CT) mechanism between the CT manifold and a local triplet state, and requires dynamic rocking about the D‐A bond. This explains how molecular geometry and environment influences TADF and the photophysics of D‐A‐D molecules.
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Thermally activated delayed fluorescence (TADF) white organic light‐emitting diodes (WOLEDs) have drawn tremendous interest and have been extensively studied because of harvesting both triplet and ...singlet excitons without heavy metals. However, single white‐light‐emitting polymers are currently limited and few strategies exist to design these materials. Herein, a new strategy is proposed to develop the polymers with tunable emission colors by combining fluorescence and TADF based on aggregation‐enhanced emission (AEE) characteristics. The polymers containing different ratios of pendant 2‐(10H‐phenothiazin‐10‐yl)dibenzothiophene‐S,S‐dioxide units with yellow TADF emission and dibenzothiophene units with blue fluorophor emission, which display both TADF and AEE characteristics, are synthesized successfully. Among them, the emission color of P3 in different tetrahydrofuran/water mixtures changes from greenish‐blue to white or yellow. Moreover, P3 displays white emission in the solid state dispersing by poly(methyl methacrylate). In addition, electroluminescent device of P3 achieve white light emission with high color rending indexes (CRI) and low turn‐on voltages (V
on ). P3 OLEDs show two‐color warm‐white emission with high CRI of 77, low V
on of 2.9 V, CEmax of 23.0 cd A−1, PEmax of 32.8 lm W−1, external quantum efficiency (EQE)max of 10.4% with Commission Internationale de l'Eclairage coordinates of (0.37, 0.38) at 5 V. Moreover, EQE of 2.6%, CE of 5.7 cd/A, PE of 4.7 lm W−1 at 100 cd m−2 are obtained. To the best of our knowledge, this work reports the first example of warm‐white TADF polymer OLEDs.
The first thermally activated delayed fluorescence (TADF) polymer giving white emission and aggregation‐enhanced emission is reported. Solution‐processable single‐polymer warm‐white organic light‐emitting diodes (WOLEDs) exhibit a low turn‐on voltage of 2.9 V and high color rending indexes of 77 with Commission Internationale de l'Eclairage coordinates of (0.37, 0.38). This work provides a guide to prepare high efficiency polymeric TADF‐based WOLEDs.
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In this work we successfully developed a strategy for positively influencing the conformation of thermally activated delayed fluorescence (TADF) molecules containing phenothiazine as the electron ...donor (D) unit, and dibenzothiophene-
S
,
S
-dioxide as the acceptor (A), linked in D-A and D-A-D structures. In this strategy the effect of restricted molecular geometry is explored to maximize TADF emission. The presence of bulky substituents in different positions on the donor unit forces the molecules to adopt an axial conformer where the singlet charge transfer state is shifted to higher energy, resulting in the oscillator strength and luminescence efficiency decreasing. With bulky substituents on the acceptor unit, the molecules adopt an equatorial geometry, where the donor and acceptor units are locked in relative near-orthogonal geometry. In this case the individual signatures of the donor and acceptor units are evident in the absorption spectra, demonstrating that the substituent in the acceptor uncouples the electronic linkage between the donor and acceptor more effectively than with donor substitution. In contrast with the axial conformers that show very weak TADF, even with a small singlet triplet gap, molecules with equatorial geometry show stronger oscillator strength and luminescence efficiency and are excellent TADF emitters. Acceptor-substituted molecules
6
and
7
in particular show extremely high TADF efficiency in solution and solid film, even with a singlet-triplet energy gap around 0.2 eV. This extensive study provides important criteria for the design of novel TADF and room temperature phosphorescence (RTP) emitters with optimized geometry.
We report a strategy for positively influencing the conformation of thermally activated delayed fluorescence (TADF) molecules containing phenothiazine and dibenzothiophene-
S
,
S
-dioxide units.
The photophysics of thermally activated delayed fluorescence (TADF) in phenothiazine‐dibenzothiophene‐S,S‐dioxide (PTZ‐DBTO2) molecule is investigated in detail. First, it is shown that the proximity ...of local triplet excited states (3LE), e.g., 3D or 3A, above or below the DA charge transfer states (CT) is crucial for the efficiency of the TADF mechanism in PTZ‐DBTO2. This TADF emitter is then used as a monomer unit to design polymer materials with efficient TADF. The reverse intersystem crossing mechanism (RISC) that supports TADF is able to compete with internal conversion and triplet–triplet annihilation (TTA) in the polymer chains and generates efficient TADF emission in the polymer pristine films. Prototype devices with PTZ‐DBTO2 dispersed in 4,4′‐bis(N‐carbazolyl)‐2,2′‐biphenyl (CBP) host give excellent performance with EQE of ≈22% at low luminance (<100 cd m−2), for 100 cd m−2 the EQE is 19.4%. In the case of solution processed devices, using the novel TADF polymers, the performance is much lower, EQE ≈3.5% at 100 cd m−2, which is still the highest value for a polymer TADF system at useful brightness, yet reported. This results from a combination of weak charge transport properties in these materials and device fabrication methods that require further improvement. Nevertheless, these results pave the way to explore TADF in polymer light emitting diodes (PLEDs), using less costly deposition methods, such as spin‐coating and inkjet printing, which are more appropriate for large area deposition.
The photophysics of an electron donor–acceptor (DA) molecule showing efficient thermally activated delayed fluorescence (TADF) is studied in detail. Evidence shows that the energy ordering of the charge transfer and local triplet excited states is crucial to maximize the TADF efficiency. This DA TADF emitter is used to design two polymers showing TADF in their pristine films. These results open an avenue for the design of new TADF emitters and exploring TADF in solution‐processed polymer light emitting diodes.
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The mechanism of thermally activated delayed fluorescence (TADF) in molecules in aggregated or condensed solid states has been rarely studied and is not well understood. Nevertheless, many ...applications of TADF emitters are strongly affected by their luminescence properties in the aggregated state. In this study, two new isomeric tetradentate Cu
complexes which simultaneously show aggregation induced emission (AIE) and TADF characteristics are reported for the first time. We provide direct evidence that effectively restricting the vibrations of individual molecules is a key requisite for TADF in these two Cu
complexes through in-depth photophysical measurements combined with kinetic methods, single crystal analysis and theoretical calculations. These findings should stimulate new molecular engineering endeavours in the design of AIE-TADF active materials with highly emissive aggregated states.
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Thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) are known to occur in organic D–A–D and D–A systems where the donor group contains the phenothiazine unit ...and the acceptor is dibenzothiophene-S,S-dioxide. This study reports the synthesis and characterization of one new D–A and four new D–A–D systems with methoxy groups on the phenothiazine to examine their effect on emission properties in the zeonex matrix. X-ray analysis and highly specialized NMR techniques were used to characterize asymmetric methoxy-substituted derivative 3b, which is chiral at N because of an extremely high flipping barrier at the phenothiazine N atom. Based on hybrid-density functional theory computations, the methoxy substituents tune the relative stabilities of the axial conformers with respect to equatorial conformers of the phenothiazine units, depending on their substitution position. This conformational effect significantly influences both TADF and RTP contributions compared to the parent D–A–D system. It is also demonstrated that the equatorial forms of D–A–D and D–A systems in zeonex exhibit TADF. Additionally, the methoxy groups promote luminescence in D–A–D systems where only axial conformers exist. This work reveals further design opportunities for more efficient TADF and RTP molecules.
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The West Nile Virus (WNV) NS2B-NS3 protease is an attractive target for the development of therapeutics against this arboviral pathogen. In the present investigation, the screening of a small library ...of fifty-eight synthetic compounds against the NS2-NB3 protease of WNV is described. The following groups of compounds were evaluated: 3-(2-aryl-2-oxoethyl)isobenzofuran-1(3H)-ones; eugenol derivatives bearing 1,2,3-triazolic functionalities; and indan-1,3-diones with 1,2,3-triazolic functionalities. The most promising of these was a eugenol derivative, namely 4-(3-(4-allyl-2-methoxyphenoxy)-propyl)-1-(2-bromobenzyl)-1H-1,2,3-triazole (35), which inhibited the protease with IC50 of 6.86 μmol L-1. Enzyme kinetic assays showed that this derivative of eugenol presents competitive inhibition behaviour. Molecular docking calculations predicted a recognition pattern involving the residues His51 and Ser135, which are members of the catalytic triad of the WNV NS2B-NS3 protease.
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