Local Triplet Research Articles

Molecular engineering charge transfer and triplet exciton formation in donor-acceptor cocrystals.

Organic donor-acceptor (D-A) cocrystals are gaining attention for their potential applications in optoelectronic devices. This study explores the dynamics of charge transfer (CT) and triplet exciton formation in various D-A cocrystals. By examining a series of D-A cocrystals composed of coronene (COR), peri-xanthenoxanthene (PXX), and perylene (PER) donors paired with N,N-bis(3'-pentyl)perylene-3,4:9,10-bis(dicarboximide) (PDI), naphthalene-1,4:5,8-tetracarboxy-dianhydride (NDA), or pyrene-4,5,9,10-tetraone (PTO) acceptors, using transient absorption microscopy and time-resolved electron paramagnetic resonance spectroscopy, we find that the strength of the CT interaction influences the nature and yield of triplet excitons produced by CT state recombination. In particular, in the PER-PDI, COR-PTO, and PER-PTO cocrystals, localized triplet excitons are lower in energy than the CT state. By contrast, no localized triplet excitons are available to the CT states of the PXX-NDA, PER-NDA, and PXX-PTO cocrystals, and as a result, the CT states rapidly decay to ground state with no triplet formation. Moreover, density functional theory calculations show that the transition between delocalized CT states to a triplet state localized to a single donor or acceptor unit provides the source of spin-orbit coupling necessary when the triplet states are energetically accessible. These findings provide insights into the design of molecular materials with tailored exciton properties for optoelectronic applications.

Localized and Excimer Triplet Electronic States of Naphthalene Dimers: A Computational Study

We perform DFT calculations with different hybrid (ωB97X-D and M05-2X) and double hybrid (B2PLYP-D3 and ωB2PLYP) functionals to characterize the lowest energy triplet excited states of naphthalene monomer and dimers in different stacking arrangements and to simulate their absorption spectra. We show that both excimer and localized triplet minima exist. In the former, the spin density is delocalized over the two monomers, adopting a face-to-face arrangement with a short inter-molecular distance. In the latter, the spin density is localized on a single naphthalene molecule, and different minima or pseudo-minima are possible, the most stable one corresponding to a slipped parallel arrangement. According to B2PLYP-D3 calculations, excimer minima are the most stable, in line with the indications of ADC(2) studies. However, the relative stability of the minima is reverted when including thermal and vibrational effects. Excimer minima exhibit a very intense absorption spectrum, peaking above 500 nm. The computed absorption spectra of localized minima significantly depend on the stacking geometry and do not coincide with that of isolated naphthalene. Hybrid functionals provide very accurate vibronic absorption spectra for naphthalene monomer, both in the singlet and in the triplet state, but underestimate the stability of the excimer triplet.

MP-GIEN: Vehicle Re-Identification Method Based on Multi-View Progressive Graph Interactive Embedding Network

Vehicle re-identification primarily faces two challenges: significant intra-class variations caused by different camera views and subtle inter-class differences between vehicles of the same model. In this paper, we propose a Multi-View Progressive Graph Interaction Embedding Network (MP-GIEN) for vehicle re-identification. First, we design a global semantic extractor. By employing Graph Interaction Units (GI Units) and semantic context, we combine semantic information to facilitate contextual reasoning of image regions, thereby extracting global semantic features of the target. The global loss is then calculated using ID loss and triplet loss. Second, we devise a local view-aware extractor. The U-Net network is utilized to parse the image into four views (front, rear, top, and side). Subsequently, features are aligned through mask average pooling, and a progressive training strategy is adopted to address the learning of fine-grained information. Lastly, we design a feature fusion enhancer. We revise the typical triplet loss to avoid mismatches in local features. By optimizing the local triplet loss and global loss, we learn the embedding of visual features, which not only reduces the intra-instance distance but also enlarges the inter-instance differences. MP-GIEN facilitates the capture of stable discriminative information on vehicles under different views. Experiments conducted on the VeRi-Wild dataset demonstrate that our model significantly outperforms state-of-the-art methods.

Solvent Effects on Spin-Orbit Charge-Transfer Intersystem Crossing in Aryl-Substituted Boron-dipyrromethene Donor-Acceptor Dyads.

In heavy-atom-free organic molecules, the rate of triplet generation through charge recombination, as dictated by the El-Sayed rule, can be enhanced by 101-102 times compared with the rate of spontaneous spin flipping between π-π* orbitals. This mechanism is known as the spin-orbit charge-transfer intersystem crossing (SOCT-ISC). Within the framework of the SOCT-ISC mechanism, facilitating the generation of charge-separated (CS) states and suppressing the spin-allowed direct charge recombination to the ground state are pivotal for maximizing the efficiency of generating localized triplet states. Herein, a series of orthogonal aryl-substituted boron-dipyrromethene dyads were studied by time-resolved spectroscopy to unravel the multichannel competitive relationships in the SOCT-ISC mechanism. The energy level of the electron donor and the stabilization of the solvent effect to the charge-transfer state are reflected in the Gibbs free energy changes of the electron transfer and recombination reactions, leading to significantly different triplet quantum yields. Additionally, solvation-induced electronic coupling changes in excited states lead to the fact that the spin-allowed charge recombination rate cannot be well simply predicted by the Marcus inverted region but has to consider the specific excited-state dynamics in optimizing the proportion of triplet generation channels based on charge recombination.

Influence of Multiple rISC Channels on the Maximum Efficiency and Roll-Off of TADF OLEDs.

In this work, we look into the detailed photophysical characterization of a multidonor-acceptor (D-A) family of thermally activated delayed fluorescent (TADF) emitters to find correlations with their device performance. Increasing the number of closely packed Ds around the A core leads to changes in dihedral angles between Ds and A, affecting the highest occupied molecular orbital (HOMO)/lowest unpccupied molecualar orbital (LUMO) separation and impacting the singlet-triplet energy gaps. Moreover, D-A dihedral angles change molecular conjugation affecting the spread of charge-transfer state energies as well as the energy of D local triplet states. The coupling between these triplet states and the dispersion in CT states lead to the appearance of multiple rISC channels, a phenomenon that is host-dependent, i.e., hosts with different rigidities twist the dihedral angles differently. We show that different subsets of rISC rates directly impact device performance, where faster rISC leads to external quantum efficiencies above 20% while slower rISC rates act as parasitic traps, severely affecting device roll-off. This explains why emitters with excellent peak external quantum efficiencies can also present very poor roll-off.

Revelation of room temperature liquid crystallinity and yellow-orange electroluminescence (EQE>7%) in a columnar self-assembled N-annulated perylene bisimide

The synthesis of highly fluorescent compounds is a critical necessity to facilitate cost-effective and metal-independent production of organic light-emitting diodes (OLEDs). This investigation introduces a readily soluble, electron-deficient N-annulated perylene bisimide (PBI-NST) equipped with a swallow tail, exhibiting room temperature columnar oblique mesophase with a low clearing point and high photoluminescence quantum yield, positioning it as a promising candidate for efficient OLEDs. Leveraging the luminescent properties of PBI-NST, a series of OLED devices were fabricated with PBI-NST as the sole emitter and at various concentrations in CBP host material. Impressively, doped devices employing the compound at 0.5 wt% emitter within the host material CBP displayed an exceptional external quantum efficiency (EQE) of 7.2 %, accompanied by a luminance of 2689 cd/m2. Further, a voltage dependent emission tuning was also noticed. The notable increase in EQE is mainly attributed to the contributions of hybrid local and charge transfer (HLCT) and triplet–triplet annihilation (TTA) process. The substantial enhancement in EQE marks a significant advancement in the application of multifunctional columnar self-assembled materials for OLED development.

Efficient Blue Photo- and Electroluminescence from CF3-Decorated Cu(I) Complexes.

Luminescent organometallic complexes of earth-abundant copper(I) have long been studied in organic light-emitting diodes (OLED). Particularly, Cu(I)-based carbene-metal-amide (CMA) complexes have recently emerged as promising organometallic emitters. However, blue-emitting Cu(I) CMA complexes have been rarely reported. Here we constructed two blue-emitting Cu(I) CMA emitters, MAC*-Cu-CF3Cz and MAC*-Cu-2CF3Cz, by introducing one or two CF3 substitutes into carbazole ligands. Both complexes exhibited high thermal stability and blue emission colors. Moreover, two complexes exhibited different emission origins rooting from different donor ligands: a distinct thermally activated delayed fluorescence (TADF) from ligand-to-ligand charge transfer excited states for MAC*-Cu-CF3Cz or a dominant phosphorescence nature from local triplet excited state of the carbazole ligand for MAC*-Cu-2CF3Cz. Inspiringly, MAC*-Cu-CF3Cz had high photoluminescence quantum yields of up to 94 % and short emission lifetimes of down to 1.2 μs in doped films, accompanied by relatively high radiative rates in the 105 s-1 order. The resultant vacuum-deposited OLEDs based on MAC*-Cu-CF3Cz delivered pure-blue electroluminescence at 462 nm together with a high external quantum efficiency of 13.0 %.

Long-lived photoinduced charge separation in adamantane-bridged donor-acceptor dyads: A case of adamantane bridge conferring orthogonal donor-acceptor orientation and restricted rotation

Design of compact donor-acceptor dyads that can exhibit charge separated (CS) state lifetimes extending to the microsecond time domain remains a major challenge even today. A simple but effective design strategy that can lead to long CS state lifetimes, in at least a few compact dyads, is still elusive. Herein we propose that use of adamantane (AD) as bridge can enhance CS state lifetimes in small molecule donor-acceptor dyads. This paper reports AD-bridged dyads, AN-AD-NB and PY-AD-NB, wherein the anthracene (or pyrene) donor and nitrobenzene acceptor are attached to bridgehead positions of AD. The singlet CS states generated upon irradiation were long-lived (lifetimes >1 μs) and decayed to the ground and local triplet states, simultaneously. Electron transfer reactions of the CS state with secondary donor and acceptor established that the CS state is sufficiently long-lived to participate in intermolecular reactions. X-ray crystal structure of the AN-AD-NB dyad reveals that the donor and acceptor are orthogonally oriented, which reduces the donor-acceptor interaction, and thereby the charge recombination rate, significantly. By analysing the X-ray structure, we predict that all adamantane-bridged compact dyads will have orthogonal donor-acceptor orientation and exhibit long-lived CS states. We also report studies with bisadamantyl-substituted dyads and the results confirmed that the local triplets are formed from the CS state through hyperfine interaction. Based on our work we propose that the hyperfine interaction, which is not expected to occur in compact dyads because of the short donor-acceptor separation, can indeed occur if the CS state is sufficiently long-lived.

Impact of Diimine Ligand on Singlet-to-Triplet Charge Transfer Electroabsorption in Iron and Ruthenium Complexes.

The electroabsorption and absorption spectra of eight homoleptic complexes of the general form [M(LL)3]2+ where M = Ru, Fe, and LL = 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), and 4,4',-(R)2-bpy where R = -OCH3, -CF3, were quantified at 77 K in a butyronitrile glass. Intense metal-to-ligand charge transfer (MLCT) absorption bands were evident in the visible region. Electroabsorption spectra measured with applied electric fields >0.2 MV/cm were analyzed by the two-state Liptay model. Significant light-induced dipole moment changes of = 4-13 D were found consistent with a metal-to-ligand charge transfer (MLCT) excited state comprised an electron localized on a single diimine ligand, [MIII(LL-)(LL)2]*2+, in the initially formed Franck-Condon excited state. A low energy feature evident in the electroabsorption spectra was assigned to a direct singlet-to-triplet MLCT excited state. The identity of the diimine ligand had an unexpected and large impact on these transitions. Analysis relative to the higher energy absorption provides a comparison of spin-allowed and disallowed transitions for first- and second-row transition metal complexes. With the notable exception of [Fe(CF3bpy)3]2+, the change in dipole moment for the 3MLCT excited states was less than or equal to that of the 1MLCT excited states. The charge transfer distances for the iron complexes were generally larger than those for the Ru complexes, a behavior attributed to a smaller degree of iron-diimine coupling in the ground state. A striking result was the sensitivity of the extinction coefficient and spectral profile of the low energy electroabsorption assigned to the identity of the diimine ligand; data that suggests electronic coupling with ligand localized triplet states and high spin metal centered states must be considered when modeling the Franck-Condon excited state.

Forward layer-wise learning of convolutional neural networks through separation index maximizing

This paper proposes a forward layer-wise learning algorithm for CNNs in classification problems. The algorithm utilizes the Separation Index (SI) as a supervised complexity measure to evaluate and train each layer in a forward manner. The proposed method explains that gradually increasing the SI through layers reduces the input data’s uncertainties and disturbances, achieving a better feature space representation. Hence, by approximating the SI with a variant of local triplet loss at each layer, a gradient-based learning algorithm is suggested to maximize it. Inspired by the NGRAD (Neural Gradient Representation by Activity Differences) hypothesis, the proposed algorithm operates in a forward manner without explicit error information from the last layer. The algorithm’s performance is evaluated on image classification tasks using VGG16, VGG19, AlexNet, and LeNet architectures with CIFAR-10, CIFAR-100, Raabin-WBC, and Fashion-MNIST datasets. Additionally, the experiments are applied to text classification tasks using the DBPedia and AG’s News datasets. The results demonstrate that the proposed layer-wise learning algorithm outperforms state-of-the-art methods in accuracy and time complexity.

Ultrafast spectroscopy study of DNA photophysics after proflavine intercalation.

Proflavine (PF), an acridine DNA intercalating agent, has been widespread applied as an anti-microbial and topical antiseptic agent due to its ability to suppress DNA replication. On the other hand, various studies show that PF intercalation to DNA can increase photogenotoxicity and has potential chances to induce carcinomas of skin appendages. However, the effects of PF intercalation on the photophysical and photochemical properties of DNA have not been sufficiently explored. In this study, the excited state dynamics of the PF intercalated d(GC)9 • d(GC)9 and d(AT)9 • d(AT)9 DNA duplex are investigated in an aqueous buffer solution. Under 267nm excitation, we observed ultrafast charge transfer (CT) between PF and d(GC)9 • d(GC)9 duplex, generating a CT state with an order of magnitude longer lifetime compared to that of the intrinsic excited state reported for the d(GC)9 • d(GC)9 duplex. In contrast, no excited state interaction was detected between PF and d(AT)9 • d(AT)9. Nevertheless, a localized triplet state with a lifetime over 5 µs was identified in the PF-d(AT)9 • d(AT)9 duplex.

Ultrafast and Multichannel Generation of the Triplet State in DNA-Intercalated Gilvocarcin V for an Enhanced Photoaddition Reaction.

Gilvocarcin V (GV) is a natural antibiotic exhibiting excellent antitumor activities and remarkably low toxicity in near-ultraviolet or visible light-dependent treatment. Notwithstanding, the [2 + 2] cycloaddition reaction between GV and thymine has been proven to be the key for its function in photodynamic therapy, and crucial mechanistic details about such a reaction are poorly understood. In this study, the electronic relaxation pathways and photoaddition reaction are characterized by femto- to nanosecond time-resolved spectroscopy combined with quantum chemical calculation. Our results reveal that ultrafast intersystem crossing (<3 ps) leads to the population of a local triplet excited state in DNA-intercalated GV. Such a state can further induce the formation of a biradical state, which is identified as the important reactive precursor for photoaddition between GV and thymine. The overall photoaddition quantum efficiency is determined to be 11.57 ± 1.0%. These results are essential to the elucidation of the DNA photoaddition mechanism of C-aryl glycoside-based artificial photocytotoxic agents and could help further development of those medicines.

Exploring the Early Time Behavior of the Excited States of an Archetype Thermally Activated Delayed Fluorescence Molecule.

Optical pump-probe techniques allow for an in-depth study of dark excited states. Here, we utilize them to map and gain insights into the excited states involved in the thermally activated delayed fluorescence (TADF) mechanism of a benchmark TADF emitter DMAC-TRZ. The results identify different electronic excited states involved in the key TADF transitions and their nature by combining pump-probe and photoluminescence measurements. The photoinduced absorption signals are highly dependent on polarity, affecting the transition oscillator strength but not their relative energy positions. In methylcyclohexane, a strong and vibronically structured local triplet excited state absorption (3LE → 3LEn) is observed, which is quenched in higher polarity solvents as 3CT becomes the lowest triplet state. Furthermore, ultrafast transient absorption (fsTA) confirms the presence of two stable conformers of DMAC-TRZ: (1) quasi-axial (QA) interconverting within 20 ps into (2) quasi-equatorial (QE) in the excited state. Moreover, fsTA highlights how sensitive excited state couplings are to the environment and the molecular conformation.

Energetic inversion of singlet/triplet interfacial charge-transfer states for reduced energy loss in organic solar cells

Significant CT inversion can be achieved via hybridization of the triplet CT state with high-lying local triplet states, which benefits to concurrently reduce the triplet recombination and driving force for higher-efficiency organic photovoltaics.

The influence of hindered rotation on electron transfer and exchange interaction in triarylamine-triptycene-perylene diimide triads.

Stretched electron-donor-bridge-acceptor triads that exhibit intramolecular twisting degrees of freedom are capable of modulating exchange interaction (J) as well as electronic couplings through variable π-overlap at the linear bond links, affecting the rate constants of photoinduced charge separation and recombination. Here we present an in-depth investigation of such effects induced by methyl substituents leading to controlled steric hindrance of intramolecular twisting around biaryl axes. Starting from the parent structure, consisting of a triphenyl amine donor, a triptycene (TTC) bridge and a phenylene-perylene diimide acceptor (Me0), one of the two phenylene linkers attached to the TTC was ortho-substituted by two methyl groups (Me2, Me3), or both such phenylene linkers by two pairs of methyl groups (Me23). Photoinduced charge separation (kCS) leading to a charge-separated (CS) state was studied by fs-laser spectroscopy, charge recombination to either singlet ground state (kS) or to the first excited local triplet state of the acceptor (kT) by ns-laser spectroscopy, whereby kinetic magnetic field effects in an external magnetic field were recorded and analysed using quantum dynamic simulations of the spin dependent kinetics of the CS state. Kinetic spectra of the initial first order rate constants of charge recombination (k(B)) exhibited characteristic J-resonances progressing to lower fields in the series Me0, Me2, Me3, Me23. From the quantum simulations, the values of the parameters J, kS, kT and kSTD, the singlet/triplet dephasing constant, were obtained. They were analysed in terms of molecular dynamics simulations of the intramolecular twisting dynamics based on potentials calculated by density functional theory. Apart from kT, all of the parameters exhibit a clear correlation with the averaged cosine square products of the biaryl angles.

High-efficiency color-tunable ultralong room-temperature phosphorescence from organic-inorganic metal halides via synergistic inter/intramolecular interactions.

Materials exhibiting highly efficient, ultralong and multicolor-tunable room-temperature phosphorescence (RTP) are of practical importance for emerging applications. However, these are still very scarce and remain a formidable challenge. Herein, using precise structure design, several novel organic-inorganic metal-halide hybrids with efficient and ultralong RTP have been developed based on an identical organic cation (A). The original organic salt (ACl) exhibits red RTP properties with low phosphorescence efficiency. However, after embedding metals into the organic salt, the changed crystal structure endows the resultant metal-halide hybrids with excellent RTP properties. In particular, A2ZnCl4·H2O exhibits the highest RTP efficiency of up to 56.56% with a long lifetime of up to 159 ms. It is found that multiple inter/intramolecular interactions and the strong heavy-atom effect of the rigid metal-halide hybrids can suppress molecular motion and promote the ISC process, resulting in highly stable and localized triplet excitons followed by highly efficient RTP. More crucially, multicolor-tunable fluorescence and RTP achieved by tuning the metal and halogen endow these materials with wide application prospects in the fields of multilevel information encryption and dynamic optical data storage. The findings promote the development of phosphorescent metal-halide hybrids for potential high-tech applications.

Charge Separation from Triplet Excited States in Nonfullerene Acceptor‐Based Organic Solar Cells

Suppressing charge recombination is pivotal for further improving the power conversion efficiency of organic solar cells (OSCs). The bimolecular recombination of free charge carriers leads to the formation of both singlet and triplet charge transfer (CT) states at a ratio of 1:3 when considering simple spin statistics, meaning that charge recombination via the triplet excited state is the main deactivation channel for OSCs. Although the formation of local triplet excitons through back charge transfer from triplet CT states is thought to be a terminal loss process, it is shown that charge separation from triplet excitons can occur in nonfullerene acceptor (NFA)‐based OSCs. To reveal the triplet exciton dynamics, a triplet sensitizer PtTPBP is doped into an OSC consisting of PM6 and Y6 as an electron donor and acceptor, respectively. Upon photoexcitation of PtTPBP, triplet excitons are formed, which subsequently transfer their energy to Y6, resulting in Y6 triplet excitons formation. Based on transient absorption and external quantum efficiency measurements, clear experimental evidence of charge photogeneration from Y6 triplet excitons at the PM6:Y6 interface is provided. This study highlights the importance of minimizing the energy difference between the singlet and triplet excited states of NFAs to suppress charge recombination.

Impact of π-Expanded Boron-Carbonyl Hybrid Acceptors on TADF Properties: Controlling Local Triplet Excited States and Unusual Emission Tuning

Three donor-acceptor-type thermally activated delayed fluorescence (TADF) emitters (PXZBAO (1), PXZBTO (2), and PXZBPO (3)) comprising a phenoxazine (PXZ) donor and differently π-expanded boron-carbonyl (BCO) hybrid acceptor units are proposed. The emitters exhibit red (1) to orange (3) emissions with an increase in the π-expansion in the BCO acceptors. The control of the strength of local aromaticity for the BCO unit and the corresponding LUMO level is attributed to inducing the unusual emission color shifts. The photoluminescence quantum yield and delayed fluorescence lifetime of the emitters are also adjusted by the π-expansion. Notably, although 1 possesses a 3nπ* state in the acceptor unit as a local triplet excited state (3LE, T2), the T2 states of 2 and 3 mainly comprise a 3ππ* state in the acceptor. Consequently, all of the emitters exhibit strong spin-orbit coupling between their T2 and excited singlet (S1) states, leading to a fast reverse intersystem crossing with rate constants of ∼106 s-1. By employing the emitters as dopants, we realize efficient red-to-orange TADF-OLEDs. Maximum external quantum efficiencies of 17.7% for the yellowish-orange (3), 15.5% for the orange (2), and 13.9% for the red (1) devices are achieved, and the values are very close to the theoretical limit predicted from the optical simulation.

Triple- Q partial magnetic orders induced by quadrupolar interactions: Triforce order scenario for UNi4B

We theoretically investigate possible symmetry-broken states in ${\rm UNi_4B}$, constructing a localized pseudo triplet crystalline-electric field model. For a long time, its low-temperature symmetry-broken phase in ${\rm UNi_4B}$ has been considered to be a magnetic toroidal order forming atomic-scale vortices lattice with disordered sites at each center of the vortices. However, recent observation of current-induced magnetizations offers a reinvestigation about the validity of this order parameter because of the contradiction in their anisotropy. Our model takes into account the quadrupole degrees of freedom, whose importance is recently evidenced by the sound-velocity softening. We find that the quadrupole moments play an important role in determining the magnetic structure in the ordered states. For a wide range of parameter space, we obtain two triple-$\mathcal{Q}$ magnetic orders in our 36-site mean-field analysis: toroidal order and another one with the same number of disordered sites as in the toroidal order. We name the latter ``triforce'' order after its magnetic structure. Importantly, the triforce order possesses exactly the same spin structure factor as the toroidal order does, while the phase factors in the superposition of the triple-$\mathcal{Q}$ structure are different. We show that the triforce order is consistent with the observed current-induced magnetization when the realistic crystal structure of ${\rm UNi_4B}$ is taken into account. We compare the predictions of the triforce order with the experimental data available at present in detail and also discuss possible applications of the present mechanism of triple-$\mathcal{Q}$ orders to anisotropic correlated systems.

Donor, Acceptor, and Molecular Charge Transfer Emission All in One Molecule.

The molecular photophysics in the thermally activated delayed fluorescence (TADF) spiro-acridine-anthracenone compound, ACRSA, is dominated by the rigid orthogonal spirocarbon bridging bond between the donor and acceptor. This critically decouples the donor and acceptor units, yielding photophysics, which includes (dual) phosphorescence and the molecular charge transfer (CT) states giving rise to TADF, that are dependent upon the excitation wavelength. The molecular singlet CT state can be directly excited, and we propose that supposed "spiro-conjugation" between acridine and anthracenone is more accurately an example of intramolecular through-space charge transfer. In addition, we show that the lowest local and CT triplet states are highly dependent upon spontaneous polarization of the environment, leading to energy reorganization of the triplet states, with the CT triplet becoming lowest in energy, profoundly affecting phosphorescence and TADF, as evident by a (thermally controlled) competition between reverse intersystem crossing and reverse internal conversion, i.e., dual delayed fluorescence (DF) mechanisms.