Delayed Fluorescence Lifetime Research Articles

A pyrimidine-5-carbonitrile acceptor combined with an ortho-linked donor for long lifetime through facilitated reverse intersystem crossing in thermally activated delayed fluorescence emitters

Molecular design combining a 4,6-diphenylpyrimidine-5-carbonitrile (PyCN) acceptor and an ortho-linked donor was developed to extend the lifetime of OLEDs with shortened the delayed fluorescence lifetime and promoted the reverse intersystem crossing.

Starburst Type Benzofuroindolocarbazole Donor for High Efficiency and Long Lifetime in Thermally Activated Delayed Fluorescence Emitters

AbstractA series of thermally activated delayed fluorescence (TADF) emitters of 5‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)‐2‐(15‐phenylbenzofuro[3,2‐a]indolo[3,2‐c]carbazol‐10(15H)‐yl)benzonitrile (TrzCNTrx1) and 5‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)‐2‐(10‐phenylbenzofuro[3,2‐a]indolo[3,2‐c]carbazol‐15(10H)‐yl)benzonitrile (TrzCNTrx2) are developed through a molecular design strategy that incorporates a starburst type benzofuro[3,2‐a]indolo[3,2‐c]carbazole as a rigid donor. The benzofuro[3,2‐a]indolo[3,2‐c]carbazole is developed as a strong and rigid donor for strong charge transfer character in the TADF emitter in combination with a diphenyltriazine acceptor and a benzonitrile linker. Two starburst type donors with different active sites for the aromatic linker substitution are prepared. Among the TrzCNTrx1 and TrzCNTrx2 emitters, the TrzCNTrx2 emitter shows a small singlet–triplet energy gap below 0.10 eV, high photoluminescence quantum yields over 90%, and short exciton lifetimes less than 5.0 µs. In the device application, the TrzCNTrx2 emitter shows maximum external quantum efficiency of 20.6% with little efficiency roll‐off less than 2% at 3000 cd m−2 and color coordinate of (0.37, 0.57). Moreover, the TrzCNTrx2 device is much better than the TrzCNTrx1 in terms of device lifetime because of short delayed fluorescence lifetime.

Highly Efficient Aggregation-Induced Red-Emissive Organic Thermally Activated Delayed Fluorescence Materials with Prolonged Fluorescence Lifetime for Time-Resolved Luminescence Bioimaging

Organic thermally activated delayed fluorescence (TADF) materials are emerging as potential candidates for time-resolved fluorescence imaging in biological systems. However, the development of purely organic TADF materials with bright aggregated-state emissions in the red/near-infrared (NIR) region remains challenging. Here, we report three donor-acceptor-type TADF molecules as promising candidates for time-resolved fluorescence imaging, which are engineered by direct connection of electron-donating moieties (phenoxazine or phenothiazine) and an electron-acceptor 1,8-naphthalimide (NI). Theoretically and experimentally, we elucidate that three TADF materials possessed remarkably small ΔEST to promote the occurrence of reverse intersystem crossing (RISC). Moreover, they all exhibit aggregation-induced red emissions and long delayed fluorescence lifetimes without the influence of molecular oxygen. More importantly, these long-lived and biocompatible TADF materials, especially the phenoxazine-substituted NI fluorophores, show great potential for high-contrast fluorescence lifetime imaging in living cells. This study provides further a molecular design strategy for purely organic TADF materials and expands the versatile biological application of long-lived fluorescence research in time-resolved luminescence imaging.

Aromatic-imide-based TADF material as emitter for efficient yellow and white organic light-emitting diodes

Abstract An aromatic-imide based TADF emitter (AI-4Cz) was designed and synthesized. The TADF emitter showed high thermal stability, good electrochemical properties, obvious AIE activity, small ΔEST value of 0.02 eV, and intense yellow emissions with PLQY of 76%. AI-4Cz also exhibited obvious microsecond-scale delayed fluorescence lifetime. Yellow OLED based on AI-4Cz was further fabricated, which achieved EQEmax of 16.7%. Moreover, a white OLED was fabricated through the dual emission layer strategy by using AI-4Cz and DMAC-DPS as emitting dopants, and the device obtained CIE coordinates of (0.30, 0.40), CRI value of 66, and an EQEmax of 13.6%.

Highly Efficient, Red Delayed Fluorescent Emitters with Exothermic Reverse Intersystem Crossing via Hot Excited Triplet States

Three donor–acceptor–donor molecular emitters have been designed by taking triphenylamine or N-phenylcarbazole as the donor and maleimide or phenyl maleimide as the acceptor, in which the highest occupied molecular orbital interaction between two donor units is maximized via the acceptor bridge. This is envisaged to enable both strong fluorescence radiation and fast exoergic reverse intersystem crossing via the second triplet state. Detailed photophysical characterization and theoretical calculations confirm that all the compounds have large oscillator strengths and short delayed fluorescence lifetimes of ∼0.2 μs. The fabricated organic light-emitting diodes (OLEDs) give red emission above 600 nm, luminance exceeding 6000 cd m–2, and external quantum efficiencies (EQEs) of over 6%. In particular, the best device shows an emission at 645 nm and a maximum EQE of 10.4%. Moreover, the EQEs remain above 3% at 1000 cd m–2 for all the emitters. This work provides an effective method to develop organic emitters for highly efficient OLEDs with low-efficiency roll-off.

High-Performance Solution-Processed Red Thermally Activated Delayed Fluorescence OLEDs Employing Aggregation-Induced Emission-Active Triazatruxene-Based Emitters

Two novel red thermally activated delayed fluorescence (TADF) emitters (TAT-DBPZ and TAT-FDBPZ) were developed by incorporating triazatruxene (TAT) as the electron donor (D) and dibenzo[a,c]phenazine (DBPZ) or fluorine-substituted dibenzo[a,c]phenazine (FDBPZ) as the electron acceptor (A). Both compounds showed aggregation-induced emission (AIE) behaviors and bright red emission in neat films. Benefited from the rigid and large planar conjugated structures of TAT and DBPZ, TAT-DBPZ and TAT-FDBPZ realized high photoluminescence quantum yields (PLQYs) in solid states. Meanwhile, the large steric hindrance between TAT and DBPZ segments produced small singlet-triplet energy splitting (EST), leading to short delayed fluorescence lifetimes and high reverse intersystem crossing (RISC) rate (106 s-1) for both compounds. The solution-processable doped organic light-emitting diodes (OLEDs) based on TAT-DBPZ achieved a high external quantum efficiency (EQE) of 15.4% with a red emission peak at 604 nm, which was one of the highly efficient solution-processable red TADF OLEDs. TAT-FDBPZ based doped devices also showed a red emission peak at 611 nm with a maximum EQE of 9.2% and low efficiency roll-off ratios of 1.0% at 100 cd m-2 and 19% at 1000 cd m-2. Furthermore, their solution-processable non-doped devices displayed EQEs of 5.6% and 2.9% with the red-shifted emission peaks at 626 nm and 641 nm, respectively. These results indicate the huge potential of utilization of triazatruxene as donor unit to achieve highly efficient and low efficiency roll-off solution-processable red TADF OLEDs.

A general framework for non-exponential delayed fluorescence and phosphorescence decay analysis, illustrated on Protoporphyrin IX

Delayed fluorescence (DF) is a long-lived luminescence process used in a variety of applications ranging from oxygen sensing in biological tissues to organic Light Emitting Diodes. In common cases, DF results from the de-excitation of the first excited triplet state via the first excited singlet state of the chromophore, which produces a mono-exponential light signal whose amplitude and lifetime give an insight into the probed environment. However, non-linear de-excitation reactions such as triplet-triplet annihilation, which can cause decays to lose their mono-exponential nature, are often neglected. In this work, we derive a global framework to properly interpret decays resulting from a combination of linear and non-linear de-excitation processes. We show why the standard method of using multi-exponential models when decays are not mono-exponential is not always relevant, nor accurate. First, we explain why the triplet de-excitation and light production processes should be analyzed individually: we introduce novel concepts to precisely describe these two processes, namely the deactivation pathway – the reaction which mainly contributes to the triplet state de-excitation – and the measurement pathway – the reaction which is responsible for light production. We derive explicit fitting functions which allow the experimenter to estimate the reaction rates and excited state concentrations in the system. To validate our formalism, we analyze the in vitro Transient Triplet Absorption and DF of Protoporphyrin IX, a well-known biological aromatic molecule used in photodynamic therapy, cancer photodetection and oxygen sensing, which produces DF through various mechanisms depending on concentration and excitation intensity. We also identify the precise assumptions necessary to conclude that triplet-triplet annihilation DF should follow a mono-exponential decay with a lifetime of half the triplet state lifetime. Finally, we describe why the commonly used definitions of triplet / DF lifetime are ill-defined in the case where second-order reactions contribute to the deactivation process, and why the fitting of precise mixed-orders DF kinetics should be preferred in this case. This work could allow the correct interpretation of various long-lived luminescence processes and facilitate their understanding.

Imaging of hypoxia, oxygen consumption and recovery in vivo during ALA-photodynamic therapy using delayed fluorescence of Protoporphyrin IX.

Hypoxic lesions often respond poorly to cancer therapies. Particularly, photodynamic therapy (PDT) consumes oxygen in treated tissues, which in turn lowers its efficacy. Tools for online monitoring of intracellular pO2 are desirable. The pO2 changes were tracked during photodynamic therapy (PDT) with δ-aminolevulinic acid (ALA) in mouse skin, xenograft tumors, and human skin. ALA was applied either topically as Ameluz cream or systemically by injection. Mitochondrial pO2 was quantified by time-gated lifetime-based imaging of delayed fluorescence (DF) of protoporphyrin IX (PpIX). pO2-weighted images were obtained with capture-times of several seconds, radiant exposures near 10 mJ/cm2, spatial resolution of 0.3 mm, and a broad dynamic range 1-50 mmHg, corresponding to DF lifetimes ≈20-2000 μs. The dose-rate effect on oxygen consumption was investigated in mouse skin. A fluence rate of 1.2 mW/cm2 did not cause any appreciable oxygen depletion, whereas 6 mW/cm2 and 12 mW/cm2 caused severe oxygen depletion after radiant exposures of only 0.4-0.8 J/cm2 and <0.2 J/cm2, respectively. Reoxygenation after PDT was studied too. With a 5 J/cm2 radiant exposure, the recovery times were 10-60 min, whereas with 2 J/cm2 they were only 1-6 min. pO2 distribution was spatially non-uniform at (sub)-millimeter scale, which underlines the necessity of tracking pO2 changes by imaging rather than point-detection. Time-gated imaging of PpIX DF seems to be a unique tool for direct online monitoring of pO2 changes during PDT with a promising potential for research purposes as well as for comparatively easy clinical translation to improve efficacy in PDT treatment.

Achieving Submicrosecond Thermally Activated Delayed Fluorescence Lifetime and Highly Efficient Electroluminescence by Fine-Tuning of the Phenoxazine-Pyrimidine Structure.

Thermally activated delayed fluorescence (TADF) materials, combining high fluorescence quantum efficiency and short delayed emission lifetime, are highly desirable for application in organic light-emitting diodes (OLEDs) with negligible external quantum efficiency (EQE) roll-off. Here, we present the pathway for shortening the TADF lifetime of highly emissive 4,6-bis[4-(10-phenoxazinyl)phenyl]pyrimidine derivatives. Tiny manipulation of the molecular structure with methyl groups was applied to tune the singlet–triplet energy-level scheme and the corresponding coupling strengths, enabling the boost of the reverse intersystem crossing (rISC) rate (from 0.7 to 6.5) × 106 s–1 and shorten the TADF lifetime down to only 800 ns in toluene solutions. An almost identical TADF lifetime of roughly 860 ns was attained also in solid films for the compound with the most rapid TADF decay in toluene despite the presence of inevitable conformational disorder. Concomitantly, the boost of fluorescence quantum efficiency to near unity was achieved in solid films due to the weakened nonradiative decay. Exceptional EQE peak values of 26.3–29.1% together with adjustable emission wavelength in the range of 502–536 nm were achieved in TADF OLEDs. Reduction of EQE roll-off was demonstrated by lowering the TADF lifetime.

Electron Transfer in a Naphthalene Diimide System Studied by Single-Molecule Delayed Fluorescence

Electron transfer (ET) is a key chemical reaction in nature and has been extensively studied in bulk systems, but remains challenging to investigate at the single-molecule level. A previously reported naphthalene diimide (NDI)-based system (Higginbotham et al., Chem. Commun. 2013, 49, 5061–5063) displays delayed fluorescence with good quantum yield (~0.5) and long-lived (nanoseconds) prompt and delayed fluorescence lifetimes, providing an opportunity to interrogate the underlying ET processes in single molecules. Time-resolved single-molecule fluorescence measurements enabled forward and reverse ET rate constants to be calculated for 45 individual molecules embedded in poly(methylmethacrylate) (PMMA) film. Interpretation of the results within the framework of Marcus–Hush theory for ET demonstrates that variation in both the electronic coupling and the driving force for ET is occurring from molecule to molecule within the PMMA film and over time for individual molecules.

Silica nanoparticles with thermally activated delayed fluorescence for live cell imaging.

Thermally activated delayed fluorescence (TADF) has revolutionized the field of organic light emitting diodes owing to the possibility of harvesting non-emissive triplet states and converting them in emissive singlet states. This mechanism generates a long-lived delayed fluorescence component which can also be used in sensing oxygen concentration, measuring local temperature, or on imaging. Despite this strong potential, only recently TADF has emerged as a powerful tool to develop metal-free long-lived luminescent probes for imaging and sensing. The application of TADF molecules in aqueous and/or biological media requires specific structural features that allow complexation with biomolecules or enable emission in the aggregated state, in order to retain the delayed fluorescence that is characteristic of these compounds. Herein we demonstrate a facile method that maintains the optical properties of solvated dyes by dispersing TADF molecules in nanoparticles. TADF dye-doped silica nanoparticles are prepared using a modified fluorescein fluorophore. However, the strategy can be used with many other TADF dyes. The covalent grafting of the TADF emitter into the inorganic matrix effectively preserves and transfers the optical properties of the free dye into the luminescent nanomaterials. Importantly, the silica matrix is efficient in shielding the dye from solvent polarity effects and increases delayed fluorescence lifetime. The prepared nanoparticles are effectively internalized by human cells, even at low incubation concentrations, localizing primarily in the cytosol, enabling fluorescence microscopy imaging at low dye concentrations.

Circularly Polarized Electroluminescence of Thermally Activated Delayed Fluorescence-Active Chiral Binaphthyl-Based Luminogens.

Two pairs of thermally activated delayed fluorescence (TADF)-active chiral luminogens (R/S-1 and R/S-2) can be achieved by introducing D-A-type groups to chiral BINOL skeletons. The resulting chiral luminogens can exhibit aggregation-induced emission properties in THF-water mixtures and TADF emission in a doped-film state. The absolute photoluminescence quantum yield (ΦPL) and delayed fluorescence lifetimes (τdelayed) were measured to be 18.5% and 1.03 μs for R-1 and 15.7% and 0.97 μs for R-2. However, only R/S-1 with the fixed conjugation structure can emit circularly polarized luminescence signals, and glum can reach 1.6 × 10-3 in toluene solution and 9.2 × 10-4 in the neat film. Most importantly, R/S-1 was chosen as the emitting layers for orange-red circularly polarized organic light-emitting diodes, which can display low turn-on voltage (Von) of 3.4 V, high maximum brightness (Lmax) up to 40 470 cd m-2, moderate external quantum efficiency of 4.1%, as well as circularly polarized electroluminescence signal with gEL = -0.9 × 10-3/+1.0 × 10-3.

Oxygen-dependent delayed fluorescence of protoporphyrin IX measured in the stomach and duodenum during upper gastrointestinal endoscopy.

Protoporphyrin IX‐triplet state lifetime technique (PpIX‐TSLT) is a method used to measure oxygen (PO2) in human cells. The aim of this study was to assess the technical feasibility and safety of measuring oxygen‐dependent delayed fluorescence of 5‐aminolevulinic acid (ALA)‐induced PpIX during upper gastrointestinal (GI) endoscopy. Endoscopic delayed fluorescence measurements were performed 4 hours after oral administration of ALA in healthy volunteers. The ALA dose administered was 0, 1, 5 or 20 mg/kg. Measurements were performed at three mucosal spots in the gastric antrum, duodenal bulb and descending duodenum with the catheter above the mucosa and while applying pressure to induce local ischemia and monitor mitochondrial respiration. During two endoscopies, measurements were performed both before and after intravenous administration of butylscopolamine. Delayed fluorescence measurements were successfully performed during all 10 upper GI endoscopies. ALA dose of 5 mg/kg showed adequate signal‐to‐noise ratio (SNR) values >20 without side effects. All pressure measurements showed significant prolongation of delayed fluorescence lifetime compared to measurements performed without pressure (P < .001). Measurements before and after administration of butylscopolamine did not differ significantly in the duodenal bulb and descending duodenum. Measurements of oxygen‐dependent delayed fluorescence of ALA‐induced PpIX in the GI tract during upper GI endoscopy are technically feasible and safe.

Isomeric Bright Sky‐Blue TADF Emitters Based on Bisacridine Decorated DBNA: Impact of Donor Locations on Luminescent and Electroluminescent Properties

AbstractThree isomeric boron‐containing thermally activated delayed fluorescent (TADF) emitters, namely m‐AC‐DBNA, p‐AC‐DBNA, and m′‐AC‐DBNA, are constructed by incorporating an electron‐donor acridine (AC) moiety into meta‐, para‐, or meta′‐positions of an electron‐accepting boron‐embedded rigid framework. The substitutional positions are found to dramatically affect thermal, photophysical, and electroluminescent (EL) properties. The experimental results show that the para‐substituted compound (p‐AC‐DBNA) exhibits higher decomposition temperature, higher photoluminescence (PL) quantum efficiencies, smaller singlet–triplet energy splitting, shorter delayed fluorescence lifetimes as well as a fast reverse intersystem crossing rate of over 106 s−1, compared to the meta‐isomers (m‐AC‐DBNA and m′‐AC‐DBNA). Bright and highly efficient organic light‐emitting diodes (OLEDs) with external quantum efficiencies (EQEs) up to 20.5% and 14.1% are achieved by employing p‐AC‐DBNA as doped and nondoped emitters in sky‐blue OLEDs, respectively. Moreover, excellent doping‐concentration independent EL properties and very low efficiency roll‐off at a high luminance are achieved. This isomeric strategy provides a simple method to extend structural diversity of highly efficient TADF emitters, optimize optoelectronic properties, and demonstrate the relationship of delayed fluorescence lifetime and efficiency roll‐off of the TADF devices. The three isomers also display distinct temperature‐dependent emission and mechanochromism.

Theoretical study on photophysical properties of a series of functional pyrimidine-based organic light-emitting diodes emitters presenting thermally activated delayed fluorescence.

Issue concerning accurate prediction of the reverse intersystem crossing rate (kRISC ) is critical for developing novel efficient thermally activated delayed fluorescence (TADF) materials. In this contribution, the kRISC rates from the lowest excited triplet T1 state to the lowest excited singlet S1 state were evaluated for five donor-π-acceptor-type pyrimidine-based TADF emitters using the semiclassical Marcus theory. Both the singlet-triplet energy difference (ΔEST ) and spin-orbit coupling (V) between the S1 and T1 states were investigated by performing the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. In addition, their fluorescence emission wavelengths (λem ) were also calculated at the TD-DFT level. The predicted kRISC and λem values are found to reproduce well the available experimental findings. The present results reveal that the kRISC rates of molecules possessing the unsymmetrical diphenyl pyrimidine acceptor core are calculated to be slightly larger than those of their analogues with the symmetrical diphenyl pyrimidine. In addition, introducing two tert-butyl groups into the 2,7-positions of the donor moiety of the latter is also an effective method for increasing kRISC when designing TADF emitters. Such a difference is related to the nature of the T1 excited state. A more remarkable charge-transfer (CT) contribution to the state can achieve a smaller ΔEST , leading to a more efficient RISC process, and consequently a shorter delayed fluorescence lifetime as observed experimentally. © 2019 Wiley Periodicals, Inc.

Triazatruxene-based thermally activated delayed fluorescence small molecules with aggregation-induced emission properties for solution-processable nondoped OLEDs with low efficiency roll-off

Triazatruxene-based TADF small molecules with AIE properties exhibited a short delayed fluorescence lifetime, resulting in a low efficiency roll-off of 1.0% even at 1000 cd m−2 in solution-processable nondoped OLEDs.

Rational Molecular Design Overcoming the Long Delayed Fluorescence Lifetime and Serious Efficiency Roll-Off in Blue Thermally Activated Delayed Fluorescent Devices.

Blue thermally activated delayed fluorescent (TADF) devices with short excited-state lifetime, high reverse intersystem crossing rate, and low-efficiency roll-off were developed by managing the molecular structure of donor-acceptor-type blue emitters. Three isomers of blue TADF emitters with a diphenyltriazine acceptor and three carbazole donors were synthesized. The position of the donor moieties in the phenyl linker connecting the donor and acceptor moieties was controlled to devise compounds with a short delayed fluorescence lifetime. A blue TADF emitter with three carbazole donors at 2-, 3-, and 4- positions of a phenyl linker shortened the excited state lifetime to 4.1 μs, showed a high external quantum efficiency of 20.4 %, and low efficiency roll-off of less than 10 % at 1000 cd m-2 . Therefore, a molecular design distorting the donors by aligning them in a consecutive way is useful to resolve the issues of long delayed fluorescence lifetime and efficiency roll-off of blue TADF devices.

Red-Emitting Thermally Activated Delayed Fluorescence Polymers with Poly(fluorene-co-3,3′-dimethyl diphenyl ether) as the Backbone

A series of red-emitting thermally activated delayed fluorescence (TADF) polymers have been designed and synthesized based on poly(fluorene-co-3,3′-dimethyl diphenyl ether) (PFDMPE) as the backbone. Compared with polyfluorene (PF, 2.16 eV), the introduction of 3,3′-dimethyl diphenyl ether into the main chain of PFDMPE leads to the increased triplet energy of 2.58 eV, which is higher enough than the tethered red TADF guest (2.13 eV) to prevent the unwanted triplet energy back-transfer. Meanwhile, there is a good overlap between the absorption spectrum of the red guest and the photoluminescence (PL) spectrum of the polymeric host, ensuring the efficient energy transfer from host to guest. Consequently, the resultant polymers PFDMPE-R01 to PFDMPE-R10 in solid states show obvious red TADF properties with delayed fluorescence lifetimes of 126–191 μs and PL quantum yields of 0.18–0.55. Among them, PFDMPE-R05 obtains the best device performance, revealing a bright red electroluminescence peaked at 606 nm and a p...

Effect of donor substituents on thermally activated delayed fluorescence of diphenylsulfone derivatives

Diphenylsulfones substituted by acridan, carbazole, phenothiazine and phenoxazine moieties were synthesized and characterized by thermal analysis, UV-, steady-state and time-resolved luminescent spectrometries, cyclic voltametry. Quantum chemical calculations on the molecular level were performed to interpret photophysical properties of the derivatives. Structural parameters, electronic properties, HOMO-LUMO gaps, molecular orbital densities, ionization potentials, reorganization energies were determined. The lowest excitation energies and the wavelengths of absorption maxima were also estimated using the time-dependent density functional theory. All the compounds were found to be capabale to form glasses with glass transition temperatures ranging from 82° to 91°C. They exhibited high thermal stabilities, with 5% weight loss temperatures exceeding 385 °C. Strong solvatochromism arising from the intramolecular charge transfer in the excited state was evidenced by bathochromic shifts of emission maxima with increasing solvent polarity. The compounds containing acridan and phenoxazine moieties showed relatively high photoluminescence quantum yield (up to 35%) in the non-doped solid state, long delayed fluorescence lifetime (in µs range) and small singlet-triplet energy splitting (ΔEST) that is attributed to thermally activated delayed fluorescence. These compounds were tested as emissive species for the fabrication of OLEDs. The sky-blue and green devices showed maximum brightness of 3200 and 12300 cd/m2 and maximum external quantum efficiency of 6.3% and 6.9%, respectively.

Dihedral Angle Control of Blue Thermally Activated Delayed Fluorescent Emitters through Donor Substitution Position for Efficient Reverse Intersystem Crossing.

This study shows a molecular design strategy for controlling the dihedral angle of two carbazole donors linked to a 2,4-diphenyl-1,3,5-triazine acceptor by a phenyl unit. Using this approach, six thermally activated delayed fluorescence emitters were synthesized with donors placed in various positions around a central phenyl core, and the photophysical relationship between the donor position and its dihedral angle was investigated. We demonstrate that this angle can affect both the strength of the charge transfer state and the conjugation across the entire molecule, effectively changing the singlet-triplet energy gap of the system. We conclude that materials containing two substituted -ortho donors or one -ortho and an adjacent -meta have the smallest energy gaps and the shortest delayed fluorescence lifetimes. On the other hand, emitters with no -ortho substituted donors have larger energy gaps and slow-to-negligible delayed fluorescence. When applying these materials to organic light-emitting diodes, these blue-emitting devices have a range of electrical properties, the best producing efficiencies as high as 21.8% together with high resistance to roll-off that correlate with the reverse intersystem crossing rates obtained.