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 (k_RISC) is critical for developing novel efficient thermally activated delayed fluorescence (TADF) materials. In this contribution, the k_RISC 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 (ΔE_ST) 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 (λ_em) were also calculated at the TD-DFT level. The predicted k_RISC and λ_em values are found to reproduce well the available experimental findings. The present results reveal that the k_RISC 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 k_RISC 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 ΔE_ST, leading to a more efficient RISC process, and consequently a shorter delayed fluorescence lifetime as observed experimentally. © 2019 Wiley Periodicals, Inc.     

The Importance of Vibronic Coupling for Efficient Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence Molecules

Factors influencing the rate of reverse intersystem crossing (k_rISC) in thermally activated delayed fluorescence (TADF) emitters are critical for improving the efficiency and performance of third‐generation heavy‐metal‐free organic light‐emitting diodes (OLEDs). However, present understanding of the TADF mechanism does not extend far beyond a thermal equilibrium between the lowest singlet and triplet states and consequently research has focused almost exclusively on the energy gap between these two states. Herein, we use a model spin‐vibronic Hamiltonian to reveal the crucial role of non‐Born‐Oppenheimer effects in determining k_rISC. We demonstrate that vibronic (nonadiabatic) coupling between the lowest local excitation triplet (³LE) and lowest charge transfer triplet (³CT) opens the possibility for significant second‐order coupling effects and increases k_rISC by about four orders of magnitude. Crucially, these simulations reveal the dynamical mechanism for highly efficient TADF and opens design routes that go beyond the Born‐Oppenheimer approximation for the future development of high‐performing systems.     

Modulation of Thermally Activated Delayed Fluorescence in Waterborne Polyurethanes via Charge‐Transfer Effect

Here, we designed several waterborne polyurethanes (WPUs) with efficient thermally activated delayed fluorescence (TADF) via serving charge‐transfer (CT) states as a mediate bridge between singlet and triplet states to boost reverse intersystem crossing (RISC). By tuning substituents of diphenyl sulfone (DS), we found that O,O′‐ and S,S′‐substituted DS covalently incorporated in WPUs solely show typical fluorescence emission with lifetimes in the nanosecond range. Interestingly, TADF appears by replacing the substituent with the nitrogen atom, of which lifetimes are up to ≈10 microseconds and ≈1 millisecond in air and vacuum, respectively, even though the energy gap between singlet and triplet states (ΔE_ST) is still large for generating TADF. To explain this phenomenon, an energy level mode based on CT states and an ³(n‐π*) receiver state was proposed. By the rational modulation of CT states, it is possible to tune the ΔE_ST to render TADF‐based materials suitable for versatile applications.     

Red Light-Emitting Thermally-Activated Delayed Fluorescence of Naphthalimide-Phenoxazine Electron Donor-Acceptor Dyad: Time-Resolved Optical and Magnetic Spectroscopic Studies

We prepared an orthogonal compact electron-donor (phenoxazine, PXZ)-acceptor (naphthalimide, NI) dyad ( NI-PXZ ), to study the photophysics of the thermally-activated delayed fluorescence (TADF), which has a luminescence lifetime of 16.4 ns (99.2 %)/17.0 μs (0.80 %). A weak charge transfer (CT) absorption band was observed for the dyad, indicating non-negligible electronic coupling between the donor and acceptor at the ground state. Femtosecond transient absorption spectroscopy shows a fast charge separation (CS) (ca. 2.02∼2.72 ps), the majority of the singlet CS state is short-lived, especially in polar solvents (τ_CR = 10.3 ps in acetonitrile, vs. 1.83 ns in toluene, 7.81 ns in n-hexane). Nanosecond transient absorption spectroscopy detects a long-lived transient species in n-hexane, which is with a mixed triplet local excited state (³LE) and charge separated state (³CS), the lifetime is 15.4 μs. In polar solvents, such as tetrahydrofuran and acetonitrile, a neat ³CS state was observed, whose lifetimes are 226 ns and 142 ns, respectively. Time-resolved electron paramagnetic resonance (TREPR) spectra indicate the existence of strongly spin exchanged ³LE/³CT states, with the effective zero field splitting (ZFS) |D| and |E| parameters of 1484 MHz and 109 MHz, respectively, much smaller than that of the native ³NI state (2475 and 135 MHz). It is rare but solid experimental evidence that a closely-lying ³LE state is crucial for occurrence of TADF and this ³LE state is an essential intermediate state to facilitate reverse intersystem crossing in TADF systems.     

Theoretical investigation on reverse intersystem crossing from upper triplet to lowest singlet: A “hot exciton” path for blue fluorescent OLEDs

Nowadays, blue fluorescent organic light-emitting diodes (FOLEDs) have attracted considerable attention from both academia and industry. According to spin statistics, electrical excitation results in the formation of ∼25% singlet excitons and ∼75% triplet excitons (signifying ~75% energy loss), which triggered wide-ranging efforts to harvest as many triplet excitons as possible. The materials that can convert triplet excitons into singlet excitons from the high-lying excited triplet states (referred as “hot exciton” channel) to realize high efficiency were reported, which can also efficaciously avoid the accumulation of triplet excitons in T₁ state. In this study, by means of density functional theory (DFT) and time-dependent DFT, we have theoretically investigated the electronic and photophysical properties of 16 newly designed molecules with donor-bridge-acceptor framework to search for the blue FOLED materials exploiting the “hot exciton” path. Important properties, such as singlet-triplet energy gaps, absorption and emission parameters, and reverse intersystem crossing rates (k_RISC), of five target molecules were studied. The calculated results demonstrate that thiophene-diphenylamine (k_RISC up to 1.03 × 10⁸ seconds⁻¹) may have promising potential as blue FOLED materials by virtue of the “hot exciton” effect.     

Efficient Direct Reverse Intersystem Crossing between Charge Transfer-Type Singlet and Triplet States in a Purely Organic Molecule

In the field of organic light-emitting diodes, thermally activated delayed fluorescence (TADF) materials have achieved great performance. The key factor for this performance is the small energy gap (ΔE_ST) between the lowest triplet (T₁) and singlet excited (S₁) states, which can be realized in a well-separated donor-acceptor system. Such systems are likely to possess similar charge transfer (CT)-type T₁ and S₁ states. Recent investigations have suggested that the intervention of other type-states, such as locally excited triplet state(s), is necessary for efficient reverse intersystem crossing (RISC). Here, we theoretically and experimentally demonstrate that our blue TADF material exhibits efficient RISC even between singlet CT and triplet CT states without any additional states. The key factor is dynamic flexibility of the torsion angle between the donor and acceptor, which enhances spin-orbit coupling even between the charge transfer-type T₁ and S₁ states, without sacrificing the small ΔE_ST. This results in excellent photoluminescence and electroluminescence performances in all the host materials we investigate, with sky-blue to deep-blue emissions. Among the hosts investigated, the deepest blue emission with CIE coordinates of (0.15, 0.16) and the highest EQE_MAX of 23.9 % are achieved simultaneously.     

Triggering Thermally Activated Delayed Fluorescence by Managing the Heteroatom in Donor Scaffolds: Intriguing Photophysical and Electroluminescence Properties

Establishment of the structure–property relationships of thermally activated delayed fluorescence (TADF) materials has become a significant quest for the scientific community. Herein, two new donors, 10H‐benzofuro[3,2‐b]indole (BFI) and 10H‐benzo[4,5]thieno[3,2‐b]indole (BTI), have been developed and integrated with a aryltriazine acceptor to design the green TADF emitters benzofuro[3,2‐b]indol‐10‐yl)‐5‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)benzonitrile ( BFICNTrz ) and 2‐(10H‐benzo[4,5]thieno[3,2‐b]indol‐10‐yl)‐5‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)benzonitrile ( BTICNTrz ), respectively. The physicochemical and electroluminescence properties of the compounds were tuned by exchanging the heteroatom in the donor scaffold. Intriguingly, the electronegativity of the heteroatom and the ionization potential of the donor unit played vital roles in control of the singlet–triplet energy splitting and TADF mechanism of the compounds. Both compounds showed similar singlet excited states that originated from the charge transfer (CT) states (¹CT), whereas the triplet excited states were tuned by the heteroatom in the donor unit. The origin of phosphorescence in the BTICNTrz emitter was CT emission from the triplet state (³CT), whereas that in the BFICNTrz emitter stemmed from the local triplet excited state (³LE). Consequently, BTICNTrz showed a small singlet–triplet energy splitting of 0.08 eV, compared with 0.26 eV for BFICNTrz . Thus, BTICNTrz showed efficient delayed fluorescence with a high quantum yield and a short delayed exciton lifetime, whereas BFICNTrz displayed weak delayed fluorescence with a relatively long lifetime. Furthermore, a BTICNTrz ‐based device exhibited a maximum external quantum efficiency (EQE) of 15.2 % and reduced efficiency roll‐off (12 %) compared with its BFICNTrz ‐based counterpart, which showed a maximum EQE of 6.4 % and severe efficiency roll‐off (55 %) at a practical brightness range of 1000 cd m^?2. These results demonstrate that the choice of subunit plays a vital role in the design of efficient TADF emitters.     

Dithia[3.3]paracyclophane Core: A Versatile Platform for Triplet State Fine‐Tuning and Through‐Space TADF Emission

Thermally activated delayed fluorescence (TADF) based on through‐space donor and acceptor interactions constitute a recent and promising approach to develop efficient TADF emitters. Novel TADF isomers using a dithia[3.3]‐paracyclophane building block as a versatile 3D platform to promote through‐space interactions are presented. Such a 3D platform allows to bring together the D and A units into close proximity and to probe the effect of their orientation, contact site and distance on their TADF emission properties. This study provides evidence that the dithia[3.3]paracyclophane core is a promising platform to control intramolecular through‐space interactions and obtain an efficient TADF emission with short reverse‐intersystem crossing (RISC) lifetimes. In addition, this study demonstrates that this design can tune the energy levels of the triplet states and leads to an upconversion from ³CT to ³LE that promotes faster and more efficient RISC to the ¹CT singlet state.     

Donor–σ–Acceptor Motifs: Thermally Activated Delayed Fluorescence Emitters with Dual Upconversion

A family of organic emitters with a donor–σ–acceptor (D‐σ‐A) motif is presented. Owing to the weakly coupled D‐σ‐A intramolecular charge‐transfer state, a transition from the localized excited triplet state (³LE) and charge‐transfer triplet state (³CT) to the charge‐transfer singlet state (¹CT) occurred with a small activation energy and high photoluminescence quantum efficiency. Two thermally activated delayed fluorescence (TADF) components were identified, one of which has a very short lifetime of 200–400 ns and the other a longer TADF lifetime of the order of microseconds. In particular, the two D‐σ‐A materials presented strong blue emission with TADF properties in toluene. These results will shed light on the molecular design of new TADF emitters with short delayed lifetimes.     

Converting molecular luminescence to ultralong room-temperature phosphorescence via the excited state modulation of sulfone-containing heteroaromatics

Manipulating the molecular orbital properties of excited states and the subsequent relaxation processes can greatly alter the emission behaviors of luminophores. Herein we report a vivid example of this, with luminescence conversion from thermally activated delayed fluorescence (TADF) to ultralong room-temperature phosphorescence (URTP) via a facile substituent effect on a rigid benzothiazino phenothiazine tetraoxide (BTPO) core. Pristine BTPO with multiple heteroatoms shows obvious intramolecular charge transfer (ICT) excited states with small exchange energy, featuring TADF. Via delicately functionalizing the BTPO core with peripheral moieties, the excited states of the BTPO derivatives become a hybridized local and charge transfer (HLCT) state in the S₁ state and a local excitation (LE) dominated HLCT state in the T₁ state, with enlarged energy bandgaps. Upon dispersion in a polymer matrix, the BTPO derivatives exhibit a persistent bright green afterglow with long lifetimes of up to 822 ms and decent quantum yields of up to 11.6%.

The decoration of a BTPO core results in a change in the luminescence nature from TADF to URTP. The phosphors in an amorphous PMMA matrix showed monomeric URTP with phosphorescence lifetimes of up to 822 ms and quantum yields of up to 11.6%.     

Tailoring Extremely Narrow FWHM in Hypsochromic and Bathochromic Shift of Polycyclo-Heteraborin MR-TADF Materials for High-Performance OLEDs

Developing double boron-based emitters with extremely narrow band spectrum and high efficiency in organic light-emitting diodes (OLEDs) is crucial and challenging. Herein, we report two materials, NO-DBMR and Cz-DBMR , hinge on polycyclic heteraborin skeletons based on role-play of the highest occupied molecular orbital (HOMO) energy levels. The NO-DBMR contains an oxygen atom, whereas the Cz-DBMR has a carbazole core in the double boron-embedded ν-DABNA structure. The synthesized materials resulted in an unsymmetrical pattern for NO-DBMR and surprisingly a symmetrical pattern for Cz-DBMR . Consequently, both materials showed extremely narrow full width at half maximum (FWHM) of 14 nm in hypsochromic (pure blue) and bathochromic (Bluish green) shifted emission without losing their high color fidelity. Furthermore, both materials show high photoluminescence quantum yield (PLQY) of over 82 %, and an extremely small singlet-triplet energy gap (ΔE_ST) of 0.04 eV, resulting in high reverse intersystem crossing process (k_RISC) of 10⁵ s⁻¹. Due to the efficient thermally activated delayed fluorescence (TADF) characteristics, the fabricated OLEDs based on these heteraborins manifested maximum external quantum efficiency (EQE_max) of 33.7 and 29.8 % for NO-DBMR and Cz-DBMR , respectively. This is the first work reported with this type of strategy for achieving an extremely narrow emission spectrum in hypsochromic and bathochromic shifted emissions with a similar molecular skeleton.     

Understanding the differences in photochemical properties of substituted aminopyrimidines

The luminescent patterns of several members of the aminopyrimidine family are very different, showing not fluorescence at all, only a fluorescence band, normal or anomalous, or dual fluorescence, depending on the substituents and on the environment (gas phase vs. polar solvents). In this work, we study the lowest excited states of several members of this family that exhibit different fluorescence patterns to try to explain their photochemistry and to understand the effect of the substituents and the environment. We have found that several excited states (local excited (LE), charge transfer (CT) and n _N?C??* states) have minima on the lowest excited potential energy surface (S₁), being their relative energy the determinant factor of the luminescent behavior. If the more stable S₁ minima are of n _N?C??* character, a non-radiative deexcitation channel is the most efficient and the system shows no fluorescence. If the CT and/or LE states are the most stable, the non-radiative deactivation channel is not accessible and the system fluoresces. The relative energies of the CT and LE minima (affected by substituents and by the presence of a polar solvent) and the different magnitude of the oscillator strength for the radiative transition to the ground state determine which emission is more efficient, giving place to normal, anomalous or dual fluorescence. The study has been carried out by CASSCF/CASPT2 computations, including the solvent effect by means of the PCM model.     

Alignment of Heptagonal Diimide and Triazine Enables Narrowband Pure-Blue Organic Light-Emitting Diodes with Low Efficiency Roll-Off

The endeavor to develop high-performance narrowband blue organic light-emitting diodes (OLEDs) with low efficiency roll-off represents an attractive challenge. Herein, we introduce a hetero-acceptor design strategy centered around the heptagonal diimide (BPI) building block to create an efficient thermally activated delayed fluorescence (TADF) sensitizer. The alignment of a twisted BPI unit and a planar diphenyltriazine (TRZ) fragment imparts remarkable exciton dynamic properties to 26tCz-TRZBPI, including a fast radiative decay rate (k_R) of 1.0×10⁷ s⁻¹ and a swift reverse intersystem crossing rate (k_RISC) of 1.8×10⁶ s⁻¹, complemented by a slow non-radiative decay rate (k_NR) of 6.0×10³ s⁻¹. Consequently, 26tCz-TRZBPI facilitates the fabrication of high-performance narrowband pure-blue TADF-sensitized fluorescence OLEDs (TSF-OLEDs) with a maximum external quantum efficiency (EQE_max) of 24.3 % and low efficiency roll-off even at a high brightness level of 10000 cd m⁻² (EQE₁₀₀₀₀: 16.8 %). This showcases a record-breaking external quantum efficiency at a high luminance level of 10000 cd m⁻² for narrowband blue TSF-OLEDs.     

TADF Invariant of Host Polarity and Ultralong Fluorescence Lifetimes in a Donor-Acceptor Emitter Featuring a Hybrid Sulfone-Triarylboron Acceptor**

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.     

Thermodynamic equilibrium between locally excited and charge-transfer states through thermally activated charge transfer in 1-(pyren-2′-yl)-o-carborane

Reversible conversion between excited-states plays an important role in many photophysical phenomena. Using 1-(pyren-2′-yl)-o-carborane as a model, we studied the photoinduced reversible charge-transfer (CT) process and the thermodynamic equilibrium between the locally-excited (LE) state and CT state, by combining steady state, time-resolved, and temperature-dependent fluorescence spectroscopy, fs- and ns-transient absorption, and DFT and LR-TDDFT calculations. Our results show that the energy gaps and energy barriers between the LE, CT, and a non-emissive ‘mixed’ state of 1-(pyren-2′-yl)-o-carborane are very small, and all three excited states are accessible at room temperature. The internal-conversion and reverse internal-conversion between LE and CT states are significantly faster than the radiative decay, and the two states have the same lifetimes and are in thermodynamic equilibrium.

Reversible conversion between excited-states is key to many photophysical phenomena. We studied the equilibrium between LE and CT states by time-resolved and temperature-dependent fluorescence, fs- and ns-transient absorption, and LR-TDDFT calculations.     

Purely Organic Fluorescence Afterglow: Visible-Light-Excitation,Inherent Mechanism,Tunable Color,and Practical Applications with Very Low Cost

Purely organic materials with visible light excitable fluorescence afterglow are promising for applications. Herein, fluorescence afterglow with various intensity and duration was observed on fluorescent dyes once being dispersed in polymer matrix, thanks to the slow reverse intersystem crossing rate (k_RISC) and long delayed fluorescence lifetime (τ_DF) derived from the coplanar and rigid chemical structure of the dyes. To verify the mechanism, different polymers were used to tune singlet-triplet splitting energy based on solvent effect. And commercial acriflavine (Acf) film showed blue shifted fluorescence compared to purified one, with slower k_RISC (≈10⁰ s⁻¹) and longer τ_DF (0.6 s). Via energy transfer from Acf to rhodamine B, the afterglow color was further regulated, with the largest fluorescence quantum yield of 42.4 %. It was demonstrated that the materials worked on color tunable light sources, and low-cost ($2 for 50 000 labels) anti-counterfeit labels recognized by white light.     

Chemical Bonding as a New Avenue for Controlling Excited-State Properties and Excitation Energy-Transfer Processes in Zinc Phthalocyanine–Fullerene Dyads

Whether chemical bonding can regulate the excited-state and optoelectronic properties of donor–acceptor dyads has been largely elusive. In this work, we used electronic structure and nonadiabatic dynamics methods to explore the excited-state properties of covalently bonded zinc phthalocyanine (ZnPc)-fullerene (C₆₀) dyads with a 6–6 (or 5–6) bonding configuration in which ZnPc is bonded to two carbon atoms shared by the two hexagonal rings (or a pentagonal and a hexagonal ring) in C₆₀. In both cases, the locally excited (LE) states on ZnPc are spectroscopically bright. However, their different chemical bonding differentiates the electronic interactions between ZnPc and C₆₀. In the 5–6 bonding configuration, the LE states on ZnPc are much higher in energy than the LE states on C₆₀. Thus, the excitation energy transfer from ZnPc to C₆₀ is thermodynamically favorable. On the other hand, in the 6–6 bonding configuration, such a process is inhibited because the LE states on ZnPc are the lowest ones. More detailed mechanisms are elucidated from nonadiabatic dynamics simulations. In the 6–6 bonding configuration, no excitation energy transfer was observed. In contrast, in the 5–6 bonding configuration, several LE and charge-transfer (CT) excitons were shown to participate in the energy-transfer process. Further analysis reveals that the photoinduced energy transfer is mediated by a CT exciton, such that electron- and hole-transfer processes take place in a concerted but asynchronous manner in the excitation energy transfer. It is also found that high-level electronic structure methods including exciton effects are indispensable to accurately describe photoinduced energy- and electron-transfer processes. Furthermore, this work opens up new avenues for regulating the excited-state properties of molecular donor–acceptor dyads by means of chemical bonding.     

Luminescence Properties of C‐Diazaborolyl‐ortho‐Carboranes as Donor–Acceptor Systems

Seven derivatives of 1,2‐dicarbadodecaborane (ortho‐carborane, 1,2‐C₂B₁₀H₁₂) with a 1,3‐diethyl‐ or 1,3‐diphenyl‐1,3,2‐benzodiazaborolyl group on one cage carbon atom were synthesized and structurally characterized. Six of these compounds showed remarkable low‐energy fluorescence emissions with large Stokes shifts of 15100–20260 cm^?1 and quantum yields (Φ_F) of up to 65 % in the solid state. The low‐energy fluorescence emission, which was assigned to a charge‐transfer (CT) transition between the cage and the heterocyclic unit, depended on the orientation (torsion angle, ψ) of the diazaborolyl group with respect to the cage C? C bond. In cyclohexane, two compounds exhibited very weak dual fluorescence emissions with Stokes shifts of 15660–18090 cm^?1 for the CT bands and 1960–5540 cm^?1 for the high‐energy bands, which were assigned to local transitions within the benzodiazaborole units (local excitation, LE), whereas four compounds showed only CT bands with Φ_F values between 8–32 %. Two distinct excited singlet‐state (S₁) geometries, denoted S₁(LE) and S₁(CT), were observed computationally for the benzodiazaborolyl‐ortho‐carboranes, the population of which depended on their orientation (ψ). TD‐DFT calculations on these excited state geometries were in accord with their CT and LE emissions. These C‐diazaborolyl‐ortho‐carboranes were viewed as donor–acceptor systems with the diazaborolyl group as the donor and the ortho‐carboranyl group as the acceptor.     

Thermally Activated Delayed Fluorescence of a Dinuclear Platinum(II) Compound: Mechanism and Roles of an Upper Triplet State

A dinuclear Pt(II) compound was reported to exhibit thermally activated delayed fluorescence (TADF); however, the luminescence mechanism remains elusive. To reveal relevant excited-state properties and luminescence mechanism of this Pt(II) compound, both density function theory (DFT) and time-dependent DFT (TD-DFT) calculations were carried out in this work. In terms of the results, the S₁ and T₂ states show mixed intraligand charge transfer (ILCT)/metal-to-ligand CT (MLCT) characters while the T₁ state exhibits mixed ILCT/ligand-to-metal CT (LMCT) characters. Mechanistically, a four-state (S₀, S₁, T₁, and T₂) model is proposed to rationalize the TADF behavior. The reverse intersystem crossing (rISC) process from the initial T₁ to final S₁ states involves two up-conversion channels (direct T₁→S₁ and T₂-mediated T₁→T₂→S₁ pathways) and both play crucial roles in TADF. At 300 K, these two channels are much faster than the T₁ phosphorescence emission enabling TADF. However, at 80 K, these rISC rates are reduced by several orders of magnitude and become very small, which blocks the TADF emission; instead, only the phosphorescence is observed. These findings rationalize the experimental observation and could provide useful guidance to rational design of organometallic materials with superior TADF performances.     

Nido‐Carboranes: Donors for Thermally Activated Delayed Fluorescence

An approach to the design of nido‐carborane‐based luminescent compounds that can exhibit thermally activated delayed fluorescence (TADF) is proposed. 7,8‐Dicarba‐nido‐undecaboranes (nido‐carboranes) having various 8‐R groups (R=H, Me, i‐Pr, Ph) are appended to the meta or para position of the phenyl ring of the dimesitylphenylborane (PhBMes₂) acceptor, forming donor–acceptor compounds (nido‐ m1 – m4 and nido‐ p1 – p4 ). The bulky 8‐R group and meta substitution of the nido‐carborane are essential to attain a highly twisted arrangement between the donor and acceptor moieties, leading to a very small energy splitting between the singlet and triplet excited states (ΔE_ST <0.05 eV for nido‐ m2 , ‐ m3 , and ‐ p3 ). These compounds exhibit efficient TADF with microsecond‐range lifetimes. In particular, nido‐ m2 and ‐ m3 display aggregation‐induced emission (AIE) with TADF properties.