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We present a joint experimental and theoretical study of the through-space charge transfer (CT) TADF molecule TpAT-tFFO. The measured fluorescence has a singular Gaussian line shape but two decay components, coming from two distinct molecular CT conformers, energetically only 20 meV apart. We determined the intersystem crossing rate (1 × 107 s–1) to be 1 order of magnitude faster than radiative decay, and prompt emission (PF) is therefore quenched within 30 ns, leaving delayed fluorescence (DF) observable from 30 ns onward as the measured reverse intersystem crossing (rISC) rate is >1 × 106 s–1, yielding a DF/PF ratio >98%. Time-resolved emission spectra measured between 30 ns and 900 ms in films show no change in the spectral band shape, but between 50 and 400 ms, we observe a ca. 65 meV red shift of the emission, ascribed to the DF to phosphorescence transition, with the phosphorescence (lifetime >1 s) emanating from the lowest 3CT state. A host-independent thermal activation energy of 16 meV is found, indicating that small-amplitude vibrational motions (∼140 cm–1) of the donor with respect to the acceptor dominate rISC. TpAT-tFFO photophysics is dynamic, and these vibrational motions drive the molecule between maximal rISC rate and high radiative decay configurations so that the molecule can be thought to be “self-optimizing” for the best TADF performance.

The balance between strain relief and aromatic stabilization dictates the form and function of non-planar π-aromatics. Overcrowded systems are known to undergo geometric deformations, but the energetically favourable π-electron delocalization of their aromatic ring(s) is typically preserved. In this study we incremented the strain energy of an aromatic system beyond its aromatic stabilization energy, causing it to rearrange and its aromaticity to be ruptured. We noted that increasing the steric bulk around the periphery of π-extended tropylium rings leads them to deviate from planarity to form contorted conformations in which aromatic stabilization and strain are close in energy. Under increasing strain, the aromatic π-electron delocalization of the system is broken, leading to the formation of a non-aromatic, bicyclic analogue referred to as ‘Dewar tropylium’. The aromatic and non-aromatic isomers have been found to exist in rapid equilibrium with one another. This investigation demarcates the extent of steric deformation tolerated by an aromatic carbocycle and thus provides direct experimental insights into the fundamental nature of aromaticity.

Considering the relevance of room temperature phosphorescent (RTP) materials, we discuss the influence of donor and acceptor groups substituted on to a twisted three-fold symmetric hydrocarbon homotruxene, which presents a persistent RTP, even in the absence of donor or acceptor moieties, under ambient conditions as a result of the twisted π-system. Compared to a fluorine acceptor, a donor methoxy group increases the phosphorescence decay rate in solution, while in the solid-state, molecular aggregation and packing yield a very persistent phosphorescence visible by the eye. The RTP of the intrinsically apolar homotruxene is found to be modulated by polar substituents, whose main impact on the solid-state emission is due to altered packing in the crystal.

Heterocyclic dimers consisting of combinations of butterfly-shaped phenothiazine (PTZ) and its chemically oxidized form phenothiazine-5,5-dioxide (PTZ(SO2)) have been synthesized. A twist is imposed across the dimers by ortho-substituents including methyl ethers, sulfides and sulfones. X-ray crystallography, cyclic voltammetry and optical spectroscopy, underpinned by computational studies, have been employed to study the interplay between the oxidation state, conformational restriction, and emission mechanisms including thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP). While the PTZ(SO2) dimers are simple fluorophores, the presence of PTZ induces triplet-mediated emission with a mixed PTZ-PTZ(SO2) dimer displaying concentration dependent hallmarks of both TADF and RTP.

Here two D–A3 regioisomers, comprising three dibenzothiophene-S,S-dioxide acceptor units attached to a central triazatruxene core, are studied. Both molecules show thermally activated delayed fluorescence (TADF), however, the efficiency of the TADF mechanism is strongly affected by the D–A substitution position. The meta- substituted emitter (1 b) shows a slightly higher-lying singlet charge transfer state and a lower-lying triplet state than that observed in the para- substituted emitter (1 a), resulting in a larger singlet–triplet splitting (ΔEST) of 0.28 eV compared to only 0.01 eV found in 1 a. As expected, this ΔEST difference strongly impacts the reverse intersystem crossing (rISC) rates and the para- isomer 1 a exhibits a much faster delayed fluorescence emission. Calculations show that the triplet energy difference between the two isomers is due to steric hindrance variances along the donor–acceptor rotation axis in these molecules: as 1 b is less restricted, rotation of its acceptor unit leads to a lower T1 energy, further away from the region of high density of states (the region where larger vibronic coupling is found, favouring rISC). Therefore, our results show how the substitution pattern has a marked effect on triplet state energies and character, verifying the key structural designs for highly efficient TADF materials.

Control of photophysical properties is crucial for the continued development of electroluminescent devices and luminescent materials. Preparation and study of original molecules uncovers design rules towards efficient materials and devices. Here we have prepared 7 new compounds based on the popular donor–acceptor design used in thermally activated delayed fluorescence emitters. We introduce for the first time benzofuro[3,2-e]-1,2,4-triazine and benzothieno[3,2-e]-1,2,4-triazine acceptors which were connected to several common donors: phenoxazine, phenothiazine, carbazole and 3,6-di-tert-butylcarbazole. DFT calculations, and steady-state and time-resolved photophysical studies were conducted in solution and in solid states. While derivatives with azine moieties are non-emissive in any form, the compounds comprising 3,6-di-tert-butylcarbazole display TADF in all cases. More interestingly, the two derivatives substituted with a carbazole donor are TADF active when dispersed in a polymer matrix and phosphorescent at room temperature in neat films (microcrystalline form).

A pair of thermally activated delayed fluorescence (TADF) emitters with symmetric and asymmetric D-A-D structure are investigated. Despite displaying near-identical photoluminescence spectrum and quantum yields, the symmetric material possesses significantly better delayed fluorescence characteristics and OLED performance. Building on a previous study of analogous D-A materials we are able to explain these differences in terms of different strengths of electronic interactions between the two donor units. This interaction lowers the energy of the TADF-active triplet state in the asymmetric molecule, increasing its singlet–triplet energy gap and leading to worse performance. This result therefore demonstrates a new strategy to selectively control the triplet states of TADF molecules, in contrast to established control of singlet states using host environment. These results also show that multi-donor TADF emitters cannot be understood simply as the sum of their isolated parts; these parts have different electronic interactions depending on their relative positions, even when there is no scope for steric interaction.

Quantum chemical studies employing combined density functional and multireference configuration interaction methods suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO. Three of them, a pair of singlet and triplet charge transfer (CT) states (S1 and T1) and a locally excited (LE) triplet state (T3), can be associated with the (Me → N) conformer, the other two CT-type states (S2 and T2) form the lowest excited singlet and triplet states of the (Me → Ph) conformer. The two conformers, which differ in essence by the shearing angle of the face-to-face aligned donor and acceptor moieties, are easily interconverted in the electronic ground state whereas the reorganization energy is substantial in the excited singlet state, thus explaining the two experimentally observed time constants of prompt fluorescence emission. Forward and reverse intersystem crossing between the singlet and triplet CT states is mediated by vibronic spin–orbit interactions involving the LE T3 state. Low-frequency vibrational modes altering the distance and alignment of the donor and acceptor π-systems tune the S1 and T3 states (likewise S2 and T3) into and out of resonance. The enhancement of intersystem crossing due to the interplay of vibronic and spin–orbit coupling is considered a general feature of organic through-space charge-transfer thermally activated delayed fluorescence emitters.

The bridging phenyl group in a fluorescent phenoxazine-benzothiazole donor-acceptor dyad is replaced by either a naphthalene or a thiophene moiety to probe the influence of a more extended conjugated system or the presence of a sulfur-containing heteroaromatic spacer on the emissive properties. These seemingly small structural alterations strongly affect the relative positions of the excited states, the fluorescence intensity, and the emission mechanism. Consequently, thermally activated delayed fluorescence (TADF) is observed at longer timescales for the materials with phenyl and naphthalene linkers, whereas the thiophene group promotes room temperature phosphorescence (RTP), both in the solid state and in solution, and enhances singlet oxygen generation. Phosphorescence in solution at ambient temperature from a purely organic molecule without heavy halogen functionalisation is quite rare, and this unique property calls for further specific attention. The authors thank the Research Foundation – Flanders (FWO Vlaanderen) for financial support (projects G087718N, G0D1521N, I006320N, GOH3816NAUHL, the Scientific Research Community ‘Supramolecular Chemistry and Materials’ (W000620N), and PhD scholarship S. Paredis). The calculations were performed on the computers of the ‘Consortium des´equipements de Calcul Intensif (CE´CI)’ (http://www.ceci-hpc.be), including those of the ‘UNamur Technological Platform of High-Performance Computing (PTCI)’ (http://www.ptci.unamur.be), for which we gratefully acknowledge the financial support from the FNRS-FRFC, the Walloon Region, and the University of Namur (Conventions No. 2.5020.11, GEQ U.G006.15, U.G018.19, 1610468, and RW/GEQ2016). A. Danos and A. P. Monkman are supported by EU Horizon 2020 Grant Agreement No. 732013 (HyperOLED) and EPSRC grant EP/T02240X/1.

10H-Dibenzo[b,e][1,4]thiaborinine 5,5-dioxide (SO2B) - a high triplet (T1 = 3.05 eV) strongly electron-accepting boracycle was successfully utilised in thermally activated delayed fluorescence (TADF) emitters PXZ-Dipp-SO2B and CZ-Dipp-SO2B. We demonstrate the near-complete separation of highest occupied and lowest unoccupied molecular orbitals leading to a low oscillator strength of the S1→S0 CT transition, resulting in very long ca. 83 ns and 400 ns prompt fluorescence lifetimes for CZ-Dipp-SO2B and PXZ-Dipp-SO2B, respectively, but retaining near unity photoluminescence quantum yield. OLEDs using CZ-Dipp-SO2B as the luminescent dopant display high external quantum efficiency (EQE) of 23.3% and maximum luminance of 18600 cd m-2 with low efficiency roll off at high brightness. For CZ-Dipp-SO2B, reverse intersystem crossing (rISC) is mediated through the vibronic coupling of two charge transfer (CT) states, without involving the triplet local excited state (3LE), resulting in remarkable rISC rate invariance to environmental polarity and polarisability whilst giving high organic light-emitting diode (OLED) efficiency. This new form of rISC allows stable OLED performance to be achieved in different host environments.

Hidden photophysics is elucidated in the very well-known thermally activated delayed fluorescence (TADF) emitter, DMAC-TRZ. A molecule that, based on its structure, is considered not to have more than one structural conformation. However, based on experimental and computational studies, two conformers, a quasi-axial (QA) and a quasi-equatorial (QE) are found, and the effect of their co-existence on both optical and electrical excitation isexplored. The relative small population of the QA conformer has a disproportionate effect because of its strong local excited state character. The energy transfer efficiency from the QA to the QE conformer is high, even at low concentrations, dependent on the host environment. The current accepted triplet energy of DMAC-TRZ is shown to originate from the QA conformer, completely changing the understanding of DMAC-TRZ. The contribution of the QA conformer in devices helps to explain the good performance of the material in non-doped organic light-emitting diodes (OLEDs). Moreover, hyperfluorescence (HF) devices, using v-DABNA emitter show direct energy transfer from the QA conformer to v-DABNA, explaining the relatively improved Förster resonance energy transfer efficiency compared to similar HF systems. Highly efficient OLEDs where green light (TADF-only devices) is converted to blue light (HF devices) with the maximum external quantum efficiency remaining close to 30% are demonstrated.

Intramolecular hydrogen bonding between donor and acceptor segments in thermally activated delayed fluorescence (TADF) materials is now frequently employed to─purportedly─rigidify the structure and improve the emission performance of these materials. However, direct evidence for these intramolecular interactions is often lacking or ambiguous, leading to assertions that are largely speculative. Here we investigate a series of TADF-active materials incorporating pyridine, which bestows the potential ability to form intramolecular H-bonding interactions. Despite possible indications of H-bonding from an X-ray analysis, an array of other experimental investigations proved largely inconclusive. Instead, after examining computational potential energy surfaces of the donor–acceptor torsion angle we conclude that the pyridine group primarily alleviates steric congestion in our case, rather than enabling an H-bond interaction as elsewhere assumed. We suggest that many previously reported “H-bonding” TADF materials featuring similar chemical motifs may instead operate similarly and that investigation of potential energy surfaces should become a key feature of future studies.

Herein, we expose how the antagonistic relationship between solid-state luminescence and photocyclization of oligoaryl alkene chromophores is modulated by the conjugation length of their alkenyl backbones. Heptaaryl cycloheptatriene molecular rotors exhibit aggregation-induced emission characteristics. We show that their emission is turned off upon breaking the conjugation of the cycloheptatriene by epoxide formation. While this modification is deleterious to photoluminescence, it enables formation of extended polycyclic frameworks by Mallory reactions. We exploit this dichotomy (i) to manipulate emission properties in a controlled manner and (ii) as a synthetic tool to link together pairs of phenyl rings in a specific sequence. This method to alter the tendency of oligoaryl alkenes to undergo photocyclization can inform the design of solid-state emitters that avoid this quenching mechanism, while also allowing selective cyclization in syntheses of polycyclic aromatic hydrocarbons.

We have designed and synthesized a pair of highly asymmetric D-aA-D′ type pyridyl-sulfonyl based isomers comprising phenothiazine (PTZ) and carbazole (Cz) donor units, which are able to emit thermally activated delayed fluorescence. PTZ-pS4-Py-2Cz and PTZ-mS4-Py-2Cz both possess spatial separation of HOMO/LUMO on the donor and acceptor moieties, resulting in small calculated singlet–triplet energy gaps (~0.25 eV). Both isomers exhibit dual emission, which is attributed to charge transfer states associated with the Cz and PTZ moieties at higher and lower energies, respectively. Photoluminescence quantum yields and time-resolved emission decays show significant differences for the two isomers, with the para- isomer exhibiting more efficient emission and stronger delayed fluorescence than the meta- isomer – in strong contrast to recently reported analogous Cz-Cz D-aA-D isomers. The findings clearly show that the interconversion of triplets via the rISC mechanism is promoted when parallel Cz and PTZ charge transfer states are allowed to interact, explaining the improved performance of the Cz-PTZ materials compared to the previous Cz-Cz ones. Finally, moderate device performance was achieved in warm-yellowish (CIE; 0.41; 0.53 & 0.49; 0.48) non-doped OLEDs, which exhibited 0.5% & 1.9% maximum external quantum efficiencies for the meta- and para- isomers, respectively.

Colour purity and stability in multi-donor thermally activated delayed fluorescence (TADF) emitters has significant implications for commercial organic light-emitting diode (OLED) design. The formation of emissive dimer states in the well-known 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) chromophore at elevated dopant concentrations has recently been confirmed both experimentally and via theoretical calculations, indicating that multi-donor emitters such as 4CzIPN might suffer from a lack of colour stability due to the presence of multiple emissive states. This poses a serious issue for OLED manufacturers. In this work, dithieno[3,2-b:2',3'-d]pyrrole (DTP) is applied as an alternative donor unit in a TADF emitter for the first time. In combination with isophthalonitrile (IPN), the 4CzIPN analogue termed 4DTPIPN is obtained. The strong electron donating nature of the DTP moiety gives rise to a red shift of the emission with respect to that of 4CzIPN. We identify that 4DTPIPN has a very stable emission spectrum throughout all solid-state thin film concentrations and host materials. Rather interestingly, this colour stability is obtained via the formation of dimer/aggregate species that are present even at 0.01 wt% concentration. Unfortunately, the higher colour stability is paired with a low photoluminescence quantum yield, making 4DTPIPN unviable for device applications. Nonetheless, this work shows the importance of dimer contributions, even at dilute doping concentrations. This molecule and study provide important understanding of the aggregation behaviour of small-molecule emitters necessary for the successful application of doped and, especially, non-doped OLED architectures. Colour purity and stability in multi-donor thermally activated delayed fluorescence (TADF) emitters has significant implications for commercial organic light-emitting diode (OLED) design. The formation of emissive dimer states in the well-known 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) chromophore at elevated dopant concentrations has recently been confirmed both experimentally and via theoretical calculations, indicating that multi-donor emitters such as 4CzIPN might suffer from a lack of colour stability due to the presence of multiple emissive states. This poses a serious issue for OLED manufacturers. In this work, dithieno[3,2-b:2',3'-d]pyrrole (DTP) is applied as an alternative donor unit in a TADF emitter for the first time. In combination with isophthalonitrile (IPN), the 4CzIPN analogue termed 4DTPIPN is obtained. The strong electron donating nature of the DTP moiety gives rise to a red shift of the emission with respect to that of 4CzIPN. We identify that 4DTPIPN has a very stable emission spectrum throughout all solid-state thin film concentrations and host materials. Rather interestingly, this colour stability is obtained via the formation of dimer/aggregate species that are present even at 0.01 wt% concentration. Unfortunately, the higher colour stability is paired with a low photoluminescence quantum yield, making 4DTPIPN unviable for device applications. Nonetheless, this work shows the importance of dimer contributions, even at dilute doping concentrations. This molecule and study provide important understanding of the aggregation behaviour of small-molecule emitters necessary for the successful application of doped and, especially, non-doped OLED architectures. This work is supported by the University of Namur and Hasselt University [PhD BILA scholarship T. Cardeynaels]. The authors also thank the Research Foundation-Flanders (FWO Vlaanderen) for financial support [project G087718N, G0D1521N, I006320N, GOH3816NAUHL and PhD scholarship S. Paredis]. The calculations were performed on the computers of the `Consortium des e ' quipements de Calcul Intensif (CE ' CI)' (http://www.ceci-hpc.be), including those of the `UNamur Technological Platform of High-Performance Computing (PTCI)' (http://www.ptci.unamur.be), for which we gratefully acknowledge the financial support from the FNRS-FRFC, the Walloon Region and the University of Namur [Conventions No. 2.5020.11, GEQ U.G006.15, U.G018.19, 1610468 and RW/GEQ2016]. M. K. Etherington and A. P. Monkman are supported by EU Horizon 2020 Grant Agreement No. 732013 (HyperOLED). K. Stavrou and A. P. Monkman acknowledge the TADFlife project funded by the European Union's Horizon 2020-MCSA-ITN Research and Innovation Programme under grant agreement no 812872.

The St Andrews team would like to thank the Leverhulme Trust (RPG-2016-047) for financial support. Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F. R. S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n1117545. We acknowledge support from the European Union's Horizon 2020 research and innovation programme under the ITN TADFlife (GA 812872). Y.O. acknowledges funding by the Fonds de la Recherche Scientifique-FNRS under Grant no. F.4534.21 (MIS-IMAGINE). D. B. is a FNRS Research Director. EZ-C is a Royal Society Leverhulme Trust Senior Research fellow (SRF\R1\201089). In this work we present a new multi-resonance thermally activated delayed fluorescence (MR-TADF) emitter paradigm, demonstrating that the structure need not require the presence of acceptor atoms. Based on an in silico design, the compound DiICzMes4 possesses a red-shifted emission, enhanced photoluminescence quantum yield, and smaller singlet-triplet energy gap, ΔEST, than the parent indolocarbazole that induces MR-TADF properties. Coupled cluster calculations accurately predict the magnitude of the ΔEST when the optimized singlet and triplet geometries are used. Slow yet optically detectable reverse intersystem crossing contributes to low efficiency in organic light-emitting diodes using DiICzMes4 as the emitter. However, when used as a terminal emitter in combination with a TADF assistant dopant within a hyperfluorescence device architecture, maximum external quantum efficiencies of up to 16.5% were achieved at CIE (0.15, 0.11). This represents one of the bluest hyperfluorescent devices reported to date. Simultaneously, recognising that MR-TADF emitters do not require acceptor atoms reveals an unexplored frontier in materials design, where yet greater performance may yet be discovered. Publisher PDF

Thermally-activated delayed fluorescence (TADF) compounds are promising materials used in emissive layers of organic light-emitting diodes (OLEDs). Their main benefit is that they allow the internal quantum efficiency of the OLED to reach up to 100% by converting non-radiative triplet states into radiative singlets. Besides the importance of having a high reverse intersystem-crossing rate, which governs triplet conversion, minimizing the non-radiative decay processes is also extremely important to reach high efficiency. In this study we provide a new method to quantify not only the most important decay rates involved in the TADF process, but also the non-radiative decay rates of both singlet and triplet states individually from transient and steady state experimental optical data. In addition, the different contribution that the two non-radiative decay pathways have on the internal quantum efficiency is investigated. Finally, the method is applied to experimental data from two TADF materials.

Three pyrene-containing boronated dyes have been examined in order to evaluate the effect of pyrene moiety localization on the photophysical properties. (3-Fluorophenyl)-1-pyreneborinic 8-oxyquinolinate was obtained by reacting 1-lithiopyrene with 3-fluorophenylboronic diethyl ester followed by complexation with 8-hydroxyquinoline. This compound consists of a pyrene unit (Py) directly bonded to a boron-quinolinate moiety (BQ). Two diarylborinic complexes (boranils) with salicylaldimine ligands (SA) bearing a pyrene moiety were obtained by reacting bis(2,6-difluorophenyl)borinic ethyl ester with salicylaldehyde and 1-aminopyrene or N1-(pyren-1-ylmethyl)benzene-1,4-diamine. The first compound consists of pyrene unit directly bonded to the boron-containing boranil moiety (BA), while the other consists of a pyrene unit connected to the BA moiety via a 1-amino-4-methyleneaminophenyl linker. The obtained compounds were characterized by 1H and 13C NMR spectroscopy and X-ray crystallography. UV–Vis fluorescence measurements showed the strong influence of solvent on the observed emission maximum (λmax) and quantum yield of emission. For (3-fluorophenyl)-1-pyreneborinic 8-oxyquinolinate the excited pyrene unit lead to dual fluorescence (from Py and BQ) which was modulated by solvent polarity. The possible formation of the open form of this complex via B–N bond breakage influences strongly the photoinduced processes which was supported by the fluorescence lifetimes measurements and quantum mechanical calculations. For two boranils the BA emission was observed upon excitation of pyrene regardless on solvent polarity. In the case of boranil complex containing 1-amino-4-methyleneaminophenyl linker a significant increase in quantum yield in the solid state was observed which may be attributed to aggregation-induced emission.