Commercial carbazole has been widely used to synthesize organic functional materials that have led to recent breakthroughs in ultralong organic phosphorescence
, thermally activated delayed ...fluorescence
, organic luminescent radicals
and organic semiconductor lasers
. However, the impact of low-concentration isomeric impurities present within commercial batches on the properties of the synthesized molecules requires further analysis. Here, we have synthesized highly pure carbazole and observed that its fluorescence is blueshifted by 54 nm with respect to commercial samples and its room-temperature ultralong phosphorescence almost disappears
. We discover that such differences are due to the presence of a carbazole isomeric impurity in commercial carbazole sources, with concentrations <0.5 mol%. Ten representative carbazole derivatives synthesized from the highly pure carbazole failed to show the ultralong phosphorescence reported in the literature
. However, the phosphorescence was recovered by adding 0.1 mol% isomers, which act as charge traps. Investigating the role of the isomers may therefore provide alternative insights into the mechanisms behind ultralong organic phosphorescence
.
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GEOZS, IJS, IMTLJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK, ZAGLJ
Chemical modification of phenothiazine‐benzophenone derivatives tunes the emission behavior from triplet states by selecting the geometry of the intramolecular charge transfer (ICT) state. A ...fundamental principle of planar ICT (PICT) and twisted ICT (TICT) is demonstrated to obtain selectively either room temperature phosphorescence (RTP) or thermally activated delayed fluorescence (TADF), respectively. Time‐resolved spectroscopy and time‐dependent density functional theory (TD‐DFT) investigations on polymorphic single crystals demonstrate the roles of PICT and TICT states in the underlying photophysics. This has resulted in a RTP molecule OPM, where the triplet states contribute with 89 % of the luminescence, and an isomeric TADF molecule OMP, where the triplet states contribute with 95 % of the luminescence.
Rapid and efficient utilization of triplet states to generate room temperature phosphorescence (RTP) or highly efficient thermally activated delayed fluorescence (TADF) is achieved by structural modification to give a planar or twisted intramolecular charge transfer (PICT or TICT) geometry, respectively.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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.
Regio- and conformational isomerization are fundamental in chemistry, with profound effects upon physical properties, however their role in excited state properties is less developed. Here two ...regioisomers of bis(10H-phenothiazin-10-yl)dibenzob,dthiophene-S,S-dioxide, a donor-acceptor-donor (D-A-D) thermally-activated delayed fluorescence (TADF) emitter, are studied. 2,8-bis(10H-phenothiazin-10-yl)dibenzob,dthiophene-S,S-dioxide exhibits only one quasi-equatorial conformer on both donor sites, with charge-transfer (CT) emission close to the local triplet state leading to efficient TADF via spin-vibronic coupling. However, 3,7-bis(10H-phenothiazin-10-yl)dibenzob,dthiophene-S,S-dioxide displays both a quasi-equatorial CT state and a higher-energy quasi-axial CT state. No TADF is observed in the quasi-axial CT emission. These two CT states link directly to the two folded conformers of phenothiazine. The presence of the low-lying local triplet state of the axial conformer also means that this quasi-axial CT is an effective loss pathway both photophysically and in devices. Importantly, donors or acceptors with more than one conformer have negative repercussions for TADF in organic light-emitting diodes.
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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IJS, KILJ, NUK, PNG, UL, UM
The key to engineering an efficient TADF emitter is to achieve a small energy splitting between a pair of molecular singlet and triplet states. This work makes important contributions towards ...achieving this goal. By studying the new TADF emitter 2,7-bis(phenoxazin-10-yl)-9,9-dimethylthioxanthene- S , S -dioxide (DPO-TXO2) and the donor and acceptor units separately, the available radiative and non-radiative pathways of DPO-TXO2 have been identified. The energy splitting between singlet and triplet states was clearly identified in four different environments, in solutions and solid state. The results show that DPO-TXO2 is a promising TADF emitter, having Δ E ST = 0.01 eV in zeonex matrix. We further show how the environment plays a key role in the fine tuning of the energy levels of the 1 CT state with respect to the donor 3 LE D triplet state, which can then be used to control the Δ E ST energy value. We elucidate the TADF mechanism dynamics when the 1 CT state is located below the 3 LE triplet state which it spin orbit couples to, and we also discuss the OLED device performance with this new emitter, which shows maximum external quantum efficiency (E.Q.E.) of 13.5% at 166 cd m −2 .
We expose significant changes in the emission color of carbazole-based thermally activated delayed fluorescence (TADF) emitters that arise from the presence of persistent dimer states in thin films ...and organic light-emitting diodes (OLEDs). Direct photoexcitation of this dimer state in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) reveals the significant influence of dimer species on the color purity of its photoluminescence and electroluminescence. The dimer species is sensitive to the sample preparation method, and its enduring presence contributes to the widely reported concentration-mediated red shift in the photoluminescence and electroluminescence of evaporated thin films. This discovery has implications on the usability of these, and similar, molecules for OLEDs and explains disparate electroluminescence spectra presented in the literature for these compounds. The dimerization-controlled changes observed in the TADF process and photoluminescence efficiency mean that careful consideration of dimer states is imperative in the design of future TADF emitters and the interpretation of previously reported studies of carbazole-based TADF materials.
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IJS, KILJ, NUK, PNG, UL, UM
Four new symmetrical donor–acceptor–donor (D–A–D)-type molecules are reported with diphenylamine (DPA) or 10,11-dihydro-5H-dibenzb,fazepine (Az) as electron donors and ...9,9-dimethylthioxanthene-S,S-dioxide (TXO2) as the electron acceptor. The donors are attached at different positions on the acceptor core: either para or meta to the sulfone unit. This series provides new insights into the effects of chromophore rigidity/flexibility on the efficiency of thermally activated delayed fluorescence (TADF). The molecules have been characterized by X-ray crystallography, by in-depth photophysical studies, and by theoretical calculations. The clear differences observed in the photophysical properties when using DPA or Az as a donor are shown to originate from different geometries of the donor unit which, in turn, influence the geometry of the nitrogen lone pair and the donating strength of the corresponding fragment. Thus, a para-substituted Az derivative demonstrated blue TADF in polar media, while the compounds with more flexible DPA units did not show delayed fluorescence. To obtain deep-blue emitters, weaker donating units are needed. A more flexible donor unit leads to increased local excited state (donor) LE emission and reduced TADF. However, a certain amount of flexibility has to be present to ensure deep-blue TADF.
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IJS, KILJ, NUK, PNG, UL, UM
An efficient synthetic route to 2‐ and 2,7‐substituted pyrenes is described. The regiospecific direct CH borylation of pyrene with an iridium‐based catalyst, prepared in situ by the reaction of ...{Ir(μ‐OMe)cod}2 (cod=1,5‐cyclooctadiene) with 4,4′‐di‐tert‐butyl‐2,2′‐bipyridine, gives 2,7‐bis(Bpin)pyrene (1) and 2‐(Bpin)pyrene (2, pin=OCMe2CMe2O). From 1, by simple derivatization strategies, we synthesized 2,7‐bis(R)‐pyrenes with R=BF3K (3), Br (4), OH (5), B(OH)2 (6), and OTf (7). Using these nominally nucleophilic and electrophilic derivatives as coupling partners in Suzuki–Miyaura, Sonogashira, and Buchwald–Hartwig cross‐coupling reactions, we obtained 2,7‐bis(R)‐pyrenes with R=(4‐CO2C8H17)C6H4 (8), Ph (9), C≡CPh (10), C≡C{4‐B(Mes)2}C6H4 (11), C≡CTMS (12), C≡C(4‐NMe2)C6H4 (14), C≡CH (15), N(Ph)(4‐OMe)C6H4 (16), and R=OTf, R′=C≡CTMS (13). Lithiation of 4, followed by reaction with CO2, yielded pyrene‐2,7‐dicarboxylic acid (17), whilst borylation of 2‐tBu‐pyrene gave 2‐tBu‐7‐Bpin‐pyrene (18) selectively. By similar routes (including Negishi cross‐coupling reactions), monosubstituted 2‐R‐pyrenes with R=BF3K (19), Br (20), OH (21), B(OH)2 (22), 4‐B(Mes)2C6H4 (23), B(Mes)2 (24), OTf (25), C≡CPh (26), C≡CTMS (27), (4‐CO2Me)C6H4 (28), C≡CH (29), C3H6CO2Me (30), OC3H6CO2Me (31), C3H6CO2H (32), OC3H6CO2H (33), and O(CH2)12Br (34) were obtained from 2. These derivatives are of synthetic and photophysical interest because they contain donor, acceptor, and conjugated substituents. The crystal structures of compounds 4, 5, 7, 12, 18, 19, 21, 23, 26, and 28–31 have also been obtained from single‐crystal X‐ray diffraction data, revealing a diversity of packing modes, which are described in the Supporting Information. A detailed discussion of the structures of 1 and 2, their polymorphs, solvates, and co‐crystals is reported separately.
The point of catalytic CH borylation! Regioselective iridium‐catalyzed borylation of pyrene takes place at the 2‐ and 2,7‐positions. The resulting mono‐ and bisboronate esters can be readily converted into both nucleophilic and electrophilic cross‐coupling partners and serve as useful precursors to a wide range of pyrene derivatives of significant photophysical and structural interest, which are otherwise difficult to prepare (see scheme).
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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