Local Excited State Research Articles

Excited-state dynamics of C3-symmetric heptazine-based thermally activated delayed-fluorescence emitters.

Heptazine-based materials have recently emerged as a promising motif for thermally activated delayed fluorescence, as their near-zero or negative singlet-triplet energy gaps enable extremely fast reverse intersystem crossing (rISC) rates. Another method for achieving a high rate of rISC is through the use of highly symmetric emitters, which benefit from energy-level degeneracies and a high density of states. Here, we investigate the effect of combining these two design strategies on the excited-state dynamics of C3-symmetric emitters containing heptazine cores. We find that in two of the four emitters studied, the S1 state has a high degree of locally excited (LE) character with density on the heptazine moiety, preventing excited-state localization and a loss of symmetry in the energy-minimized S1 geometry. Surprisingly, these symmetric molecules still suffer from a loss of density of triplet states below the S1 state. Overall, we find that maintaining C3 symmetry will not necessarily maintain density of states, but that heptazine-based materials with LE S1 states still benefit from maximized rISC rates via increased spin-orbit coupling with low-lying charge-transfer triplet states and exhibit advantageous photophysical properties, such as near-unity photoluminescence quantum yields and high colour purity.

A computational investigation of the effects of changes of donor–acceptor linker length and acceptor on electronic spectra of small-molecule single-component organic solar cells

Small-molecule single-component organic solar cells (SM-SCOSCs) have great potential for commercial application. However, the effects of changes of donor–acceptor linker length and acceptor unit of small molecules of active layers on their photoelectric properties are still not well understood. In this work, four state-of-the-art dyads 1–4 were selected to investigate these effects since dyad 4 created the highest power conversion efficiency (PCE) of 5.34% of SM-SCOSC; and their electronic structures were investigated. The results indicate that increasing the length of the donor–acceptor connecting bridge leads to a decrease in exciton binding energy, and so does replacing the acceptor component PC61BM with PC71BM. Furthermore, the simulated electronic absorption spectrum of dyad 4 is enhanced with respect to others. We also find that the introduction of PC71BM creates more channels for transitions from local excited states to charge transfer states. This work gains a deep insight into the relationship between molecular structure and performance of SM-SCOSC.

Photophysical Analysis of Aggregation-Induced Emission (AIE) Luminogens Based on Triphenylamine and Thiophene: Insights into Emission Behavior in Solution and PMMA Films.

Aggregation-Induced Emission (AIE) luminogens have garnered significant interest due to their distinctive applications in different applications. Among the diverse molecular architectures, those based on triphenylamine and thiophene hold prominence. However, a comprehensive understanding of the deactivation mechanism both in solution and films remains lacking. In this study, we synthesized and characterized spectroscopically two AIE luminogens: 5-(4-(bis(4-methoxyphenyl)amino)phenyl)thiophene-2-carbaldehyde (TTY) and 5'-(4-(bis(4-methoxyphenyl)amino)phenyl)-[2,2'-bithiophene]-5-carbaldehyde (TTO). Photophysical and theoretical analyses were conducted in both solution and PMMA films to understand the deactivation mechanism of TTY and TTO. In diluted solutions, the emission behavior of TTY and TTO is influenced by the solvent, and the deactivation of the excited state can occur via locally excited (LE) or twisted intramolecular charge transfer (TICT) state. In PMMA films, rotational and translational movements are constrained, necessitating emission solely from the LE state. Nevertheless, in the PMMA film, excimers-like structures form, resulting in the emergence of a longer wavelength band and a reduction in emission intensity. The zenith of emission intensity occurs when molecules are dispersed at higher concentrations within PMMA, effectively diminishing the likelihood of excimer-like formations. Luminescent Solar Concentrators (LSC) were fabricated to validate these findings, and the optical efficiency was studied at varying concentrations of luminogen and PMMA.

Electrochemiluminescence of hot exciton nanomaterial with boosted efficiency for visual bioanalysis

The electrochemiluminescence (ECL) efficiency (ΦECL) of organic ECL emitters is greatly influenced by the excited state property and component. Here we design a hot exciton molecule (BCzP-BT) with hybridized local and charge-transfer (HLCT) excited state property to prepare the first hot exciton organic nanorods (BB NRs). The BB NRs exhibit both sensitive annihilation and intense coreactant ECL emissions with a band gap emission model. Due to the reverse intersystem crossing among high-lying excited states (hRISC), the HLCT excited state property leads to highly efficient utilization of triplet excitons and thus high photoluminescence quantum yields (ΦPL) of more than 80% at 554 nm, which is demonstrated by solvatochromic experiments and quantum chemical calculations. The high ΦPL endows BB NRs with superior ΦECL over other nanoemitters, even the thermally activated delayed fluorescence materials, and thus offers highly-efficient nanoemitters for the design of ECL imaging strategy. As a proof of concept, an ECL probe is constructed by assembling BHQ2-ssDNA on BB NRs to integrate CRISPR/Cas12a system for target recognition, which produces a rapid and high-throughput ECL imaging platform. The proposed imaging method for HPV16 DNA detection shows excellent performance with a detection limit of 0.6 fM. This work broadens the application of hot exciton materials in ECL field, and opens a new avenue for developing next-generation ECL devices.

Simulating the Fluorescence of the Locally Excited State of DMABN in Solvents of Different Polarities.

4-(N,N-Dimethylamino)benzonitrile (DMABN) is a luminescent probe that can be used for tracking changes in the surrounding solvent due to the large change in polarity between its ground and excited states. An important characteristic of DMABN is that it exhibits dual fluorescence with two different emission energies that can be monitored, allowing for better characterization of the surrounding system. The first excited state is called the locally excited (LE) state and is characterized by the movement of charge over the conjugated ring structure. In nonpolar solvents and in the gas phase, the fluorescence of DMABN is entirely attributed to the transition from the near-planar LE state. In more polar environments, emission occurs from both the LE and a second excited state, corresponding to a twisted intramolecular charge-transfer (ICT) structure. For the sake of simplicity, this work considers transitions between only the ground and LE state. Molecular mechanical force field models of DMABN in its ground and LE states have been developed to investigate the sensitivity of the LE state to the polarity of the solvent. Both nonpolarizable and polarizable force fields were developed to simulate the molecule in a series of 10 different solvents of different polarities. The calculated Stokes shift of DMABN increases with increasing orientation polarizability of the surrounding solvent, which is the expected trend, as seen in experimental studies.

Highly efficient deep blue electroluminescent material with high and towards balanced carrier transmission performance based on hybrid local and charge-transfer (HLCT) excited state

Deep blue electroluminescent materials play a decisive role in organic light-emitting diodes (OLEDs). Herein, three novel deep blue molecules based on hybridized local and charge-transfer (HLCT) excited state were reported, namely CZFPY, CZFPI and TPAFPI. Single-crystal analysis of TPAFPI shows abundant intermolecular interactions, which restricts molecular rotation and suppresses structural vibrations, promotes TPAFPI achieved a high photoluminescence quantum yield (PLQY) up to 72 % in solid state. More importantly, these abundant intermolecular interactions significantly increase its carrier mobilities, with hole and electron mobilities as high as 1.2 × 10−5 cm2 V−1 s−1 and 5.4 × 10−6 cm2 V−1 s−1, respectively. Consequently, the non-doped OLED based on TPAFPI achieved excellent device performance, the maximum current efficiency (CEmax) is 12.4 cd/A, the maximum power efficiency (PEmax) is 14.0 lm W−1, the maximum external quantum efficiency (EQEmax) is 7.2 % with high device stability, the efficiency roll-off is only 4.7 % at 1000 cd m−2. What is more, the doped device of TPAFPI exhibits a higher EQEmax of 10.1 % with a higher color purity, the Commission International de L’Eclairage (CIE) coordinate is (0.15, 0.06), matching the National Television Standards Committee (NTSC) standard. The comprehensive performance of TPAFPI-based doped device is at the highest level in the field of deep blue OLEDs (0.06 ≤ CIEy ≤ 0.09).

Unique structural effect on the fluorosolvatochromism and dual fluorescence emission of D-π-A+ cationic chromophores with furyl bridge. An approach to white light emitters

Photophysical behavior of two D − π − A+ cationic compounds with the same furyl bridge and nicotinamidine group as an electron acceptor moiety and two electron donating groups, namely methoxy (I) and N,N-dimethylamino (II) groups was examined using steady-state and time-resolved techniques in variety of solvents. Time-dependent density functional theory (TDDFT) calculations were performed in some representative solvents and compared with the experimental results. Steady state and time-resolved studies in different solvents reveal that fluorescence emission of (I) is ascribed to an emission from an excited state (ICT) with higher dipole moment than the ground state while the emission of (II) is a dual emission from a state with high charge transfer nature (ICT) in addition to the locally excited state (LE). The fluorescence emission spectra of (II) were found to depend on the excitation wavelength and an increase in the excitation wavelength led to the formation of a longer wavelength emission band with lower quantum yield. It has also been found that the fluorescence excitation spectra were dependent on the emission wavelength. The effect of solvent on the nature of dual emission was examined. Correlation of the photophysical properties of the excited states of (I) and (II) with the solvent polarity, ε, reveals the charge transfer nature of (I) and the long wavelength emission band of (II), while their correlation with the solvent polarity parameter (ETN) shows two different trends when the solvents are divided to aprotic and protic solvents. For precise investigation of the impact of each solvent parameter on each photophysical property, Catalán’s and Laurence’s four parametric linear solvation energy relationships were studied. We have found that the non-specific interactions of the solvent are primarily responsible for controlling the photophysical properties, as demonstrated by Catalán's and Laurence's treatments. DFT and TDDFT calculations were used to anticipate the dipole moments in the ground and excited states and geometry of both states.

Unraveling the dual sensing mechanism of hydrogen peroxide probe: Photoinduced electron transfer and unusual local excited state intramolecular proton transfer

An understanding of hydrogen peroxide's (H2O2) sensing process is crucial for the development of effective fluorescent probes. In this investigation, we unraveled the sensing mechanism of the H2O2 fluorescent probe (BBD) derived from phthalimide-borate. The fluorescence of probe BBD is quenching and our findings establish this phenomenon can be attributed to a nonclassical photoinduced electron transfer (PET) process, wherein a crossing of energy curves between the local excited and charge transfer states is observed. The reaction between BBD and H2O2 causes the phenylboronic acid ester to oxidize and release a hydroxyl group, which turns off the PET process and causes the product HMD to enhance the fluorescence significantly. In addition, the results show that the generated HMD-N likewise undergoes fluorescence quenching, and the fluorescence band seen in the experiment originates from the HMD-T created by the unusual local excited state intramolecular proton transfer (ESIPT) process. In conclusion, our theoretical findings provide a comprehensive explanation for the fluorescence characteristics exhibited by BBD and HMD with an unusual local ESIPT combined PET process.

Regulating excited states by varying different acceptors of D-π-A emitters for efficient non-doped blue electroluminescence with high luminance and low efficiency roll-off.

Chromophores with hybridized local and charge-transfer (HLCT) excited state are promising for the realization of high performance blue organic light-emitting diodes (OLEDs). The rational manipulation of HLCT excited state for efficient emitters remains challenging. Herein, we present three donor-π-acceptor (D-π-A) molecules (mPAN, mPANPH, and mPNAPH) with phenanthro[9,10-d]imidazole (PI) and pyridinyl as donor and π-bridge respectively. Changes in various kinds of polycyclic aromatic derivative acceptors (anthracene, 9-phenylanthracene, and 1-phenylnaphthalene) could manipulate the excited states and optoelectronic properties. Theoretical calculations reveal that the S1 state of mPNAPH exhibits HLCT nature while the other two molecules show local excited (LE) state dominated feature. The photophysical properties also demonstrate this characteristic. Therefore, compared with mPAN and mPANPH, mPNAPH has higher photoluminescence quantum yield (PLQY) whether in solutions or neat films.Ultimately, the non-doped devices based on these emitters show high luminance larger than 35000 cd m-2, and high maximum external quantum efficiencies (EQEmaxs) larger than 5% with low efficiency roll-off. In particular, the mPNAPH-based device displays an excellent performance of pure blue emission at 456 nm with Commission Internationale de L'Eclairage coordinate of (0.15, 0.16) and EQEmax of 6.13% that benefited from the HLCT state and high-lying reverse intersystem crossing process.

Study of excited-state dynamics of D-π-A thiophene derivatives: Enhanced excited-state absorption and refraction

Two D-π-A-type thiophene derivatives Br-TH and 3-TH with different donor units are designed and synthesized to investigate the effect of intra-molecular charge transfer (ICT) on their excited state absorption (ESA) and excited state refraction (ESR). The photodynamic evolution of these compounds in different polar solvents is analyzed by transient absorption spectra (TAS), and the electronic relaxation process from local excited-state (LES) to charge transfer state (CTS) is observed. Z-scan experiments indicate that ESA and ESR of these molecules can be significantly enhanced by extending the π-conjugated length. Phase object pump–probe (POPP) experiments further describe the photophysical mechanisms of CTS-based ESA and ESR of 3-TH. ESA cross section and ESR volume are obtained by establishing the equivalent five-energy level with corresponding rate equations. Results indicate that ICT performance can be effectively modulated by extending the π-conjugated length of these thiophene derivatives, and further improve their ESA and ESR properties. Furthermore, nanosecond optical limiting (OL) experiments indicate that compound 3-TH has excellent OL performance, which has potential application prospects in protecting sensitive devices or human eyes.

Acquiring Charge‐Transfer‐Featured Single‐Molecule Ultralong Organic Room Temperature Phosphorescence via Through‐Space Electronic Coupling

AbstractAlthough long‐lived triplet charge‐transfer (3CT) state with high energy level has gained significant attention, the development of organic small molecules capable of achieving such states remains a major challenge. Herein, by using the through‐space electronic coupling effect, we have developed a compound, namely NIC‐DMAC, which has a long‐lived 3CT state at the single‐molecule level with a lifetime of 210 ms and a high energy level of up to 2.50 eV. Through a combination of experimental and computational approaches, we have elucidated the photophysical processes of NIC‐DMAC, which involve sequential transitions from the first singlet excited state (S1) that shows a 1CT character to the first triplet excited state (T1) that exhibits a local excited state feature (3LE), and then to the second triplet excited state (T2) that shows a 3CT character (i.e., S1 (1CT)→T1 (3LE)→T2 (3CT)). The long lifetime and high energy level of its 3CT state have enabled NIC‐DMAC as an initiator for photocuring in double patterning applications.

Acquiring Charge-Transfer-Featured Single-Molecule Ultralong Organic Room Temperature Phosphorescence via Through-Space Electronic Coupling.

Although long-lived triplet charge-transfer (3 CT) state with high energy level has gained significant attention, the development of organic small molecules capable of achieving such states remains a major challenge. Herein, by using the through-space electronic coupling effect, we have developed a compound, namely NIC-DMAC, which has a long-lived 3 CT state at the single-molecule level with a lifetime of 210 ms and a high energy level of up to 2.50 eV. Through a combination of experimental and computational approaches, we have elucidated the photophysical processes of NIC-DMAC, which involve sequential transitions from the first singlet excited state (S1 ) that shows a 1 CT character to the first triplet excited state (T1 ) that exhibits a local excited state feature (3 LE), and then to the second triplet excited state (T2 ) that shows a 3 CT character (i.e., S1 (1 CT)→T1 (3 LE)→T2 (3 CT)). The long lifetime and high energy level of its 3 CT state have enabled NIC-DMAC as an initiator for photocuring in double patterning applications.

Sensing Mechanism and Excited-State Dynamics of a Widely Used Intracellular Fluorescent pH Probe: pHrodo.

The pHrodo with an "off-on" response to the changes of pH has been widely used as a fluorescent pH probe for bioimaging. The fluorescence off-on mechanism is fundamentally important for its application and further development. Herein, the sensing mechanism, especially the relevant excited-state dynamics, of pHrodo is investigated by steady-state and time-resolved spectroscopy as well as quantum chemical calculations, showing that pHrodo is best understood using the bichromophore model. Its first excited state (S1) is a charge transfer state between two chromophores. From S1, pHrodo relaxes to its ground state (S0) via an ultrafast nonradiative process (∼0.5 ps), which causes its fluorescence to be "off". After protonation, S1 becomes a localized excited state, which accounts for the fluorescence being turned "on". Our work provides photophysical insight into the sensing mechanism of pHrodo and indicates the bichromophore model might be relevant to a wide range of fluorescent probes.

Large enhancement of nonlinear optical absorption and refraction in planar chalcone structure with terminal substitution

The ultrafast nonlinear optical (NLO) properties of novel planar π-conjugated chalcone derivatives, (E)-3-(4-nitrophenyl)-1-(3-bromo-4-fluorophenyl) prop-2-en-1-one (1503) and (E)-3-(4-nitrophenyl)-1-(phenothiazin-2-yl) prop-2-en-1-one (1526) are investigated and compared. The effect of intramolecular charge transfer on NLO absorption and refraction is discussed without changing the planar structural properties of the chalcone molecules. Transient absorption experiments are used to observe the evolution of 1526 from local excited (LE) state to the intramolecular charge transfer (ICT) state. In the Z-scan experiments, 1526 exhibits respectively different nonlinear absorption (NLA) mechanisms (reverse saturable absorption and two-photon induced excited state absorption) at 532 nm (4 ns, 22ps, 190 fs) and 700–900 nm, 190 fs. The NLA and nonlinear refraction (NLR) phenomena of 1526 are observed and discussed in the POPP experiments. The experimental results indicate that the chalcone derivative can become the potential materials for laser protection.

A Breakthrough in Solution-Processed Ultra-Deep-Blue HLCT OLEDs: A Record External Quantum Efficiency Exceeding 10% Based on Novel V-Shaped Emitters.

It is always a great challenge to achieve high-efficiency solution-processed ultra-deep-blue organic light-emitting diodes (OLEDs) with the Commission Internationale de l'Eclairage (CIE) 1931 chromaticity coordinates matching the blue primary of Rec. International Telecommunication Union-Radiocommunication BT.2100, which specifies high dynamic range television image parameters. Inspired by hybrid local and charge transfer (HLCT) excited state emitters improving exciton utilization through high-lying reverse intersystem crossing, here, a series of high-performance blue emitters by a V-shaped symmetric donor (D)-π-acceptor (A)-π-D design strategy are developed. Here, the large torsions and unstable bonds of D-A structures can be improved through π bridges, and also the conjugation length and donor groups can be easily adjusted. The obtained emitters merit excellent photophysical and electrochemical properties, thermal stability, solution processibility, and HLCT excited state excellence. Results suggest that the OLEDs based on the obtained blue emitters all achieve high maximum external quantum efficiency (EQEmax ) of more than 8% with very low efficiency roll-off. In particular, the device based on 4',5'-bis(4-(9H-carbazol-9-yl)phenyl)spiro[fluorene-9,2'-imidazole] exhibits a satisfactory ultra-deep-blue emission (CIEx , y = 0.1579, 0.0387) and a record-high EQEmax (10.40%) among solution-processed HLCT OLEDs, which is very close to the record EQEmax of devices by vacuum vapor deposition technology.

Meta-generalized gradient approximation time-dependent density functional theory study of electron trapping in Hf- and Zr-doped lutetium oxide: influencing the oxygen vacancy.

This work analyzes the effects of a nearby Hf or Zr dopant on the electron density trapped at an oxygen vacancy site. The two metals are among the dopants used to achieve thermoluminescence and energy storage in phosphors based on cubic lutetium oxide (c-Lu2O3). The presence of oxygen vacancies is anticipated in those phosphors. If the dopant is located outside the immediate surroundings of the vacancy site, the resulting optical trap depth is similar to that of the isolated oxygen vacancies (1.6-1.7 eV versus 1.7 eV). If the dopant is one of the four metal cations surrounding the vacancy site, the corresponding trap depth is 2.0-2.1 eV. Using time-dependent density-functional theory calculations, it was found that the excitation of the vacancy-trapped electrons can take two forms: a local excited state at the vacancy site can be formed, or an electron transfer to Hf might occur. With charge compensation in mind, several structures with three defects were analyzed: the dopant cation, the vacancy and an interstitial oxygen (Hf/Zr plus a Frenkel pair). These last two systems with the dopant in a +4 oxidation state and a single electron trapped at the vacancy site correspond to zero total charge, while another electron can be trapped. The vacancy site is expected to trap the electron, not the dopant. The composite defects of the dopant and Frenkel pair are thus considered the most likely electron traps in cubic Lu2O3:Hf and cubic Lu2O3:Zr.

Triphenylene-Based Emitters with Hybridized Local and Charge-Transfer Characteristics for Efficient Nondoped Blue OLEDs with a Narrowband Emission and a Small Efficiency Roll-Off.

Developing high-efficiency nondoped blue organic light-emitting diodes (OLEDs) with high color purity and low-efficiency roll-off is vital for display and lighting applications. Herein, we developed two asymmetric D-π-A blue emitters, PIAnTP and PyIAnTP, in which triphenylene is first utilized as a functional acceptor. The relatively weak charge transfer (CT) properties, rigid molecular structures, and multiple supramolecular interactions in PIAnTP and PyIAnTP can significantly enhance the fluorescence efficiency and suppress the structural relaxations to obtain a narrowband blue emission. The photophysical experiments and theoretical simulations reveal that they both exhibit a typical hybridized local and charge-transfer (HLCT) excited state and achieve high external quantum efficiency (EQE) via a "hot exciton" channel. As a result, PIAnTP- and PyIAnTP-based nondoped devices realize blue emission at 456 and 464 nm, corresponding to CIE coordinates of (0.16, 0.14) and (0.16, 0.19), narrow full width at half-maximums of 52 and 60 nm, and the high EQEs of 8.36 and 8.69%, respectively. More importantly, the PIAnTP- and PyIAnTP-based nondoped devices show small EQE roll-offs of only 5.9 and 2.4% at 1000 cd m-2, respectively. These results signify an advance in designing a highly efficient blue emitter for nondoped OLEDs.

Realignment of Local and Charge-Transfer Excited States in Promoting Room-Temperature Phosphorescence of Organic Aggregates

Realignment of Local and Charge-Transfer Excited States in Promoting Room-Temperature Phosphorescence of Organic Aggregates

One-Pot Synthesis and Excited-State Dynamics of Null Exciton-Coupled Diketopyrrolopyrroles Oligo-Grids.

Exciton coupling in molecular aggregates plays a vital role in impacting and fine-tuning optoelectronic materials and their efficiencies in devices. A versatile platform to decipher aggregation-property relationships is built around multichromophoric architectures. Here, a series of cyclic diketopyrrolopyrrole (DPP) oligomers featuring nanoscale gridarene structures and rigid bifluorenyl spacers are designed and synthesized via one-pot Friedel-Crafts reaction. DPP dimer [2]Grid and trimer [3]Grid, which are cyclic rigid nanoarchitectures of rather different sizes, are further characterized via steady-state and time-resolved absorption and fluorescence spectroscopies. They exhibit monomer-like spectroscopic signatures in the steady-state measurements, from which null exciton couplings are derived. Moreover, in an apolar solvent, high fluorescence quantum yields and excited-state dynamics that resembled DPP monomer are gathered. In a polar solvent, the localized singlet excited state on a single DPP dissociates into the adjacent null coupling DPP with charge transfer characteristics. This pathway facilitates the evolution ofthe symmetry-broken charge-separated state (SB-CS). Notable is the fact that the SB-CS of [2]Grid is, on one hand, in equilibrium with the singlet excited state and promotes, on the other hand, the formation of the triplet excited state with a yield of 32% via charge recombination.

Simple A–D–A pure blue fluorescent emitters based on hybridized local and charge-transfer excited state for non-doped OLEDs with narrow full width at half-maximum of 0.20 eV and CIEy< 0.06

Narrowband emitters possess great potential in organic light emitting diodes (OLEDs) to achieve high color purity. However, developing highly efficient pure blue emitters with a simultaneously narrow full width at half-maximum (FWHM) value is a major challenge due to the energy-gap law and molecular stacking effect. Few efforts have focused on this challenging pursuit. Herein, an ingenious synthetic method was devised to achieve pure blue emitters incorporating bulky rigid planes as donors and highly twisted steric units as acceptors. Four emitters with aggregation-induced emission (AIE) characteristics, namely BCzPPM, BC-zPSP, IDCz-DBPM, and IDCz-BPSP, have been developed and used to fabricate non-doped devices. IDCz-BPSP achieves narrowed photoluminescence (PL) and electroluminescence (EL) with the benefit of suppressing vibronic coupling. Non-doped OLEDs based on IDCz-BPSP have achieved maximum external quantum efficiency (EQEmax) of 4.61% with a FWHM of 25 nm/0.20 eV. The Commission Internationale de L'Eclairage (CIE) coordinates was (0.154, 0.059), with CIEy being superior to the standard of European Broadcasting Union (EBU) 0.06. The hybridized local and charge-transfer (HLCT) states of IDCz-BPSP have simultaneously boosted the PL efficiency and exciton utilization. The EL process has even achieved exciton utilization at exceeding the theoretical limit of 25%. These results indicate that the strategy of combining bulky rigid plane donor units and steric hindrance effect is a favorable strategy for constructing emitters with narrow FWHM.