High Radiative Rate Research Articles

Interplay of kernel shape and surface structure for NIR luminescence in atomically precise gold nanorods

It is challenging to attain strong near-infrared (NIR) emissive gold nanoclusters. Here we show a rod-shaped cluster with the composition of [Au28(p-MBT)14(Hdppa)3](SO3CF3)2 (1 for short, Hdppa is N,N-bis(diphenylphosphino)amine, p-MBT is 4-methylbenzenethiolate) has been synthesized. Single crystal X-ray structural analysis reveals that it has a rod-like face-centered cubic (fcc) Au22 kernel built from two interpenetrating bicapped cuboctahedral Au15 units. 1 features NIR luminescence with an emission maximum at 920 nm, and the photoluminescence quantum yield (PLQY) is 12%, which is 30-fold of [Au21(m-MBT)12(Hdppa)2]SO3CF3 (2, m-MBT is 3-methylbenzenethiolate) with a similar composition and 60-fold of Au30S(S‑t‑Bu)18 with a similar structure. time-dependent DFT(TDDFT)calculations reveal that the luminescence of 1 is associated with the Au22 kernel. The small Stokes shift of 1 indicates that it has a very small excited state structural distortion, leading to high radiative decay rate (kr) probability. The emission of cluster 1 is a mixture of phosphorescence and thermally activated delayed fluorescence(TADF), and the enhancement of the NIR emission is mainly due to the promotion of kr rather than the inhibition of knr. This work demonstrates that the metal kernel and the surface structure are both very important for cluster-based NIR luminescence materials.

End Cap Effect on Solution-Processable Deep Blue Lasing Materials with Low-Amplified Spontaneous Emission Thresholds.

Organic lasers have attracted increasing attention owing to their superior characteristics such as lightweight, low-cost manufacturing, high mechanical flexibility, and high emission-wavelength tunability. Recent breakthroughs include electrically pumped organic laser diodes and an electrically driven organic laser, integrated with an organic light-emitting diode pumping. However, the availability of efficient deep blue organic laser chromophores remains limited. In this study, we develop two novel rigid oligophenylenes, end-capped with carbazole and phenylcarbazole groups, to demonstrate exceptional optical and amplified spontaneous emission (ASE) properties. These oligophenylenes are not only solution processable but also exhibit remarkably high solution photoluminescence quantum yields (PLQYs) of 90% and high radiative rates of 1.35 × 109 s-1 in the deep blue range. Our theoretical calculations confirm that the carbazole and phenylcarbazole end groups play a pivotal role in enhancing the optical transitions of the oligophenylene laser chromophores, thereby elevating their emission oscillator strengths. Remarkably, these materials demonstrate low solid-state ASE threshold values of 1.0 and 1.5 μJ/cm2 (at 431 and 418 nm, respectively). To the best of our knowledge, these ASE thresholds represent the lowest reported at these specific ASE wavelengths in the literature, regardless of whether they are solution-processed or thermally evaporated films. Furthermore, they exhibit excellent thermal and photostability, low triplet quantum yields, as well as negligible overlap of excited-state absorption within the ASE emission region, making them excellent candidates for a new class of deep blue materials for organic lasers. By integrating insights from theoretical calculations and experimental validation, our study provides a comprehensive understanding of the design principles behind these high-performing organic laser chromophores, paving the way for the development of advanced organic lasers with enhanced performance characteristics.

Low Amplified Spontaneous Emission Threshold from Solution Processable Excited‐State Intramolecular Proton Transfer Chromophores

AbstractChromophores undergoing excited‐state intramolecular proton transfer (ESIPT) feature a distinct four‐level photocycle and significant Stokes shift, which make them ideal for achieving population inversion upon photoexcitation, a fundamental prerequisite for lasing, by mitigating the concerns of self‐absorption owing to spectral overlap. This study focuses on functionalizing the hydroxyphenyl‐benzothiazole (HBT) ESIPT laser dye through the introduction of fluorene and phenyl‐carbazole moieties at the para‐position to the hydroxyl group, to demonstrate amplified spontaneous emission (ASE) activities. The synthesized HBT derivatives not only exhibit high thermal and photostability, but also high photoluminescence quantum yields, exceeding 40%, which is noteworthy for ESIPT materials emitting in the yellow–orange to red color ranges. Computational analyses show that these new HBT materials possess substantially lower ESIPT energy barriers, compared to their parent HBT, facilitating their efficient ESIPT processes. Moreover, these derivatives demonstrate a high radiative rate of 1.47 × 108 s−1. Thin films of a fluorene‐substituted HBT (HBT‐pFl) show a low solid‐state amplified spontaneous emission threshold value of 3.8 µJ cm−2 at 582 nm, representing the lowest reported solid‐state ASE threshold for laser dyes in the orange emission region (575–600 nm) to date.

Oxidation Resistance of Ir/HfO2 Composite Coating Prepared by Chemical Vapor Deposition: Microstructure and Elemental Migration

In this study, a HfO2 coating was developed on an Ir matrix using a customized open-tube airflow, cold-wall chemical vapor deposition instrument. The preparation process and structure of the as-prepared coating were investigated to gain insights into its characteristics. The HfO2 coating effectively prevents direct contact between Ir and O, leading to a reduction in the oxidation rate of Ir. Furthermore, defects such as micropores and cracks generated during sealed oxidation erosion contribute to Ir’s decelerated oxidation failure. The as-prepared HfO2 coating exhibits low thermal conductivity and a high heat radiation rate, reducing the coating’s surface temperature. These characteristics significantly enhance adversity tolerance and increase the working temperature of the coating. Moreover, the as-prepared HfO2 coating can serve as a diffusion barrier, blocking both the direct contact of O with the Ir coating and the diffusion of other elements to the Ir coating. As a result, the rates of diffusion of other elements to the Ir coating are reduced.

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 %.

Deep-Blue and Fast Delayed Fluorescence from Carbene-Metal-Amides for Highly Efficient and Stable Organic Light-Emitting Diodes.

Linear gold complexes of the "carbene-metal-amide" (CMA) type are prepared with a rigid benzoguanidine amide donor and various carbene ligands. These complexes emit in the deep-blue range at 424 and 466nm with 100% quantum yields in all media. The deep-blue thermally activates delayed fluorescence originates from a charge transfer state with an excited state lifetime as low as 213ns, resulting in fast radiative rates of 4.7×106s-1. The high thermal and photo-stability of these carbene-metal-amide (CMA) materials enabled the authors to fabricate highly energy-efficient organic light-emitting diodes (OLED) in host-guest architectures. Deep-blue OLED devices with electroluminescence at 416 and 457nm with practical external quantum efficiencies of up to 23% at 100cdm-2 with excellent color coordinates CIE (x; y)=0.16; 0.07 and 0.17; 0.18 are reported. The operating stability of these OLEDs is the longest reported to date (LT50=1h) for deep-blue CMA emitters, indicating a high promise for further development of blue OLED devices. These findings inform the molecular design strategy and correlation between delayed luminescence with high radiative rates and CMA OLED device operating stability.

Basic Performance Evaluation of a Radiation Survey Meter That Uses a Plastic-Scintillation Sensor.

After the Fukushima nuclear power plant accident in 2011, many types of survey meters were used, including Geiger-Müller (GM) survey meters, which have long been used to measure β-rays. Recently, however, a novel radiation survey meter that uses a plastic-scintillation sensor has been developed. Although manufacturers' catalog data are available for these survey meters, there have been no user reports on performance. In addition, the performance of commercial plastic-scintillation survey meters has not been evaluated. In this study, we experimentally compared the performance of a plastic-scintillation survey meter with that of a GM survey meter. The results show that the two instruments performed very similarly in most respects. The GM survey meter exhibited count losses when the radiation count rate was high, whereas the plastic-scintillation survey meter remained accurate under such circumstances, with almost no count loss at high radiation rates. For measurements at background rates (i.e., low counting rates), the counting rates of the plastic-scintillation and GM survey meters were similar. Therefore, an advantage of plastic-scintillation survey meters is that they are less affected by count loss than GM survey meters. We conclude that the plastic-scintillation survey meter is a useful β-ray measuring/monitoring instrument.

Effective donor and acceptor modifications of carbazole-benzonitrile thermally activated delayed fluorescence molecules enabling high-performance OLEDs

Carbazole-benzonitrile derivatives (CzBNs) have great potential to enable thermally activated delayed fluorescence (TADF) molecules with both high efficiency and good stability simultaneously. However, the device performances of most CzBN-based organic light-emitting diodes (OLEDs) still lag behind other types of TADF molecules. In this work, we proposed a design strategy to construct efficient CzBN-TADF molecules through acceptor and donor modifications. The cyano-modified acceptor enables the extension of the lowest unoccupied molecular orbital while mostly preserving the overlapped area with the highest occupied molecular orbital. The optimized distributions of frontier molecular orbitals contribute to tiny singlet-triplet energy gaps while maintaining a high radiative decay rate. The incorporation of tert-butyl units in the donor groups further enhances the TADF property of resulted tPCNBN molecule due to suppressed non-radiative decay and reduced exciton self-quenching. Accordingly, OLEDs based on tPCNBN present the best device performances, such as a maximum external quantum efficiency of 33.1 % at 400 cd m−2, which is still as high as 26.3 % at a high luminance of 4000 cd m−2, demonstrating relatively low efficiency roll-off. These results confirm the effectiveness of the proposed design strategy in developing efficient and stable CzBN-TADF materials and devices.

Highly Luminescent Chiral Carbon Nanohoops via Symmetry Breaking with a Triptycene Unit: Bright Circularly Polarized Luminescence and Size-Dependent Properties.

Chiral carbon nanohoops with both high fluorescence quantum yield and large luminescence dissymmetry factor are essential to the development of circularly polarized luminescence (CPL) materials. Herein, the rational design and synthesis of a series of highly fluorescent chiral carbon nanohoops TP-[8-13]CPPs via symmetry breaking with a chiral triptycene motif is reported. Theoretical calculations revealed that breaking the symmetry of nanohoops causes a unique size-dependent localization in the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular obtitals (LUMOs) as the increasing of sizes, which is sharply different from those of [n]cycloparaphenylenes. Photophysical investigations demonstrated that TP-[n]CPPs display size-dependent emissions with high fluorescence quantum yields up to 92.9% for TP-[13]CPP, which is the highest value among the reported chiral conjugated carbon nanohoops. The high fluorescence quantum yields are presumably attributed to both the unique acyclic, and radial conjugations and high radiative transition rates, which are further supported by theoretical investigations. Chiroptical studies revealed that chiral TP-[n]CPPs exhibit bright CPL with CPL brightness up to 100.5M-1cm-1 for TP-[11]CPP due to the high fluorescence quantum yield. Importantly, the investigations revealed the intrigued size-dependent properties of TP-[n]CPPs with regards to (chir)optical properties, which follow a nice linear relationship versus 1/n. Such a nice linear relationship is not observed in other reported conjugated nanohoops including CPPs.

Predictions of photophysical properties of phosphorescent platinum(II) complexes based on ensemble machine learning approach.

Cyclometalated Pt(II) complexes are popular phosphorescent emitters with color-tunable emissions. To render their practical applications as organic light-emitting diodes emitters, it is required to develop Pt(II) complexes with high radiative decay rate constant and photoluminescence (PL) quantum yield. Here, a general protocol is developed for accurate predictions of emission wavelength, radiative decay rate constant, and PL quantum yield based on the combination of first-principles quantum mechanical method, machine learning, and experimental calibration. A new dataset concerning phosphorescent Pt(II) emitters is constructed, with more than 200 samples collected from the literature. Features containing pertinent electronic properties of the complexes are chosen and ensemble learning models combined with stacking-based approaches exhibit the best performance, where the values of squared correlation coefficients are 0.96, 0.81, and 0.67 for the predictions of emission wavelength, PL quantum yield and radiative decay rate constant, respectively. The accuracy of the protocol is further confirmed using 24 recently reported Pt(II) complexes, which demonstrates its reliability for a broad palette of Pt(II) emitters.

A Rising Star: Luminescent Carbene-Metal-Amide Complexes.

Coinage metal (gold, silver, and copper) complexes are attractive candidates to substitute the widely studied noble metal complexes, such as, iridium(III) and platinum(II), as luminescent materials in organic light-emitting diodes (OLEDs). However, the development of coinage metal complexes exhibiting high emission quantum yields and short exciton lifetimes is still a formidable challenge. In the past few years, coinage metal complexes featuring a carbene-metal-amide (CMA) motif have emerged as a new class of luminescent materials in OLEDs. Thanks to the coinage metal-bridged linear geometry, coplanar conformation, and the formation of excited states with dominant ligand-to-ligand charge transfer character and reduced metal d-orbital participation, most CMA complexes have high radiative rates via thermally activated delayed fluorescence. Currently, the family of CMA complexes have rapidly evolved and remarkable progresses in CMA-based OLEDs have been made. Here, a Concept article on CMA complexes is presented, with a focus on molecular design principles, the correlation between molecular structure/conformation and optoelectronic properties, as well as OLED performance. The future prospects of CMA complexes are also discussed.

Luminescent Bimetallic Two-Coordinate Gold(I) Complexes Utilizing Janus Carbenes.

A series of bimetallic carbene-metal-amide (cMa) complexes have been prepared with bridging biscarbene ligands to serve as a model for the design of luminescent materials with large oscillator strengths and small energy differences between the singlet and triplet states (ΔEST). The complexes have a general structure (R2N)Au(:carbene─carbene:)Au(NR2). The bimetallic complexes show solvation-dependent absorption and emission that is analyzed in detail. It is found that the molar absorptivity of the bimetallic complexes is correlated with the energy barrier to rotation of the metal-ligand bond. The bimetallic cMa complexes also exhibit short emission lifetimes (τ = 200-300 ns) with high photoluminescence efficiencies (ΦPL > 95%). The radiative rates of bimetallic cMa complexes are 3-4 times faster than that of the corresponding monometallic complexes. Analysis of temperature-dependent luminescence data indicates that the lifetime for the singlet state (τS1) of bimetallic cMa complexes is near 12 ns with a ΔEST of 40-50 meV. The presented compounds provide a general design for cMa complexes to achieve small values for ΔEST while retaining high radiative rates. Solution-processed organic light-emitting devices (OLEDs) made using two of the complexes as luminescent dopants show high efficiency and low roll-off at high luminance.

Towards Fast Circularly Polarized Luminescence in 2-Coordinate Chiral Mechanochromic Copper(I) Carbene Complexes.

A series of chiral mechanochromic copper(I) cAAC (cAAC = cyclic (alkyl)(amino)carbene) complexes with a variety of amide ligands have been studied with regard to their photophysical and chiroptical properties to elucidate structure-property relationships for the design of efficient triplet exciton emitters exhibiting circularly polarized luminescence. Depending on the environment, which determines the excited state energies, either thermally activated delayed fluorescence (TADF) from 1/3LLCT states or phosphorescence from 3LLCT/LC states occurs. However, neither chiral moieties at the carbene nor at the carbazolate ligands provide detectable luminescence dissymmetries glum. An exception is [Cu(phenoxazinyl)(cAAC)], showing orange to deep red TADF with λmax = 601-715 nm in solution, powders and in PMMA. In this case, the amide ligand can undergo distortions in the excited state. This design motif leads to the first linear, non-aggregated CPL-active copper(I) complex with glum of -3.4·10-3 combined with a high radiative rate constant of 6.7·105 s-1.

Carbon Dots with Blue-to-Near-Infrared Lasing for Colorful Speckle-Free Laser Imaging and Dynamical Holographic Display.

As a new solution-processable laser material, carbon dots (CDs) offer advantages of non-toxicity, low cost, and high stability, which are conducive to the sustainable development of miniaturized lasers. We prepared full-color CDs (FC-CDs) with bright-blue, green, yellow, red, deep-red, and near-infrared (NIR) fluorescence. Their photoluminescence emission ranged from 431 to 714nm. The FC-CDs showed narrow full widths at half maximum in the range of 44-76nm, with concurrent high radiative transition rates (KR ) of 0.54-1.74 × 108 s-1 ; their performance is comparable to that of organic laser dyes, indicating their good gain potential for lasers. Laser pumping of the FC-CDs gave laser outputs at 467.3, 533.5, 577.4, 616.3, 653.5, and 705.1nm, spanning from blue to NIR region, and covering 140% of the NTSC color gamut. The FC-CDs showed high Q-factors (2000-5500), appreciable gain coefficients (9-21.5 cm-1 ), and better stability (∼100%@4-7h) than commercial laser dyes. These excellent properties make them suitable for high-quality, colorful, speckle-free laser imaging and dynamic holographic display. Our findings will be helpful in promoting the practical applications and development of solution-processable CD-based lasers. This article is protected by copyright. All rights reserved.

Tailoring Carbene-Metal-Amides for Thermally Activated Delayed Fluorescence: A Computationally Guided Study on the Effect of Cyclic (Alkyl)(amino)carbenes.

Gold-centered carbene-metal-amides (CMAs) containing cyclic (alkyl)(amino)carbenes (CAACs) are promising emitters for thermally activated delayed fluorescence (TADF). Aiming at the design and optimization of new TADF emitters, we report a density functional theory study of over 60 CMAs with various CAAC ligands, systematically evaluating computed parameters in relation to photoluminescence properties. The CMA structures were primarily selected based on experimental synthesis prospects. We demonstrate that TADF efficiency of the CMA materials originates from a compromise between oscillator strength coefficients and exchange energy (ΔEST). The latter is governed by the overlap of HOMO and LUMO orbitals, where HOMO is localized on the amide and LUMO over the Au-carbene bond. The S0 ground and excited T1 states of the CMAs adopt approximately coplanar geometry of carbene and amide ligands, but rotate perpendicular in the excited S1 states, resulting in degeneracy or near-degeneracy of S1 and T1, accompanied by a decrease in the S1-S0 oscillator strength from its maximum at coplanar geometries to near zero at rotated geometries. Based on the computations, promising new TADF emitters are proposed and synthesized. Bright CMA complex (Et2CAAC)Au(carbazolide) is obtained and fully characterized in order to demonstrate that excellent stability and high radiative rates up to 106 s-1 can be obtained for the gold-CMA complexes with small CAAC-carbene ligands.

Theoretical studies on spiro[acridine-fluorene]-based emitters with efficient thermally activated delayed fluorescence

Thermally activated delayed fluorescence (TADF) emitters with through space charge transfer (TSCT) have been attracting great attention owing to their unique characteristics and potential applications in organic light-emitting diodes. In this study, five TSCT TADF emitters based on spiro[acridine-fluorene] donor have been investigated in detail using density functional theory (DFT) and time-dependent DFT calculations. The results indicate that five emitters exhibit cofacial orientations between acridine donor and acceptor units, leading to the well-separated HOMO and LUMO and small singlet-triplet energy gaps (ΔEST) (less than 0.20 eV). The lowest singlet excited state (S1) and lowest triplet excited state (T1) are characterized to be the mixed charge transfer (CT) and local excitation (LE) natures, which ensures strong oscillator strength of S1 state and triggers high radiative rates over 106s-1. Importantly, the obvious differences in the CT component between S1 and T1 states lead to significantly strong spin-orbit coupling, especially for SAF-XT, SAF-Q and SAF-Qx, reaching 3.93, 2.17, and 4.26 cm−1, respectively, resulting in high reverse intersystem crossing (RISC) rates up to 4.4 × 106s-1, which endow them with great potential to be efficient TSCT-TADF emitters. Our calculation results would be beneficial to enriching the TSCT-TADF emitters.

An Air- and Moisture-stable Zinc(II) Carbene Dithiolate Dimer Showing Fast Thermally Activated Delayed Fluorescence and Dexter Energy Transfer Catalysis.

A dimeric ZnII carbene complex featuring bridging and chelating benzene-1,2-dithiolate ligands is highly stable towards air and water. The donor-Zn-acceptor structure leads to visible light emission in the solid state, solution and polymer matrices with λmax between 577-657 nm and, for zinc(II) complexes, unusually high radiative rate constants for triplet exciton decay of up to kr =1.5×105 s-1 at room temperature. Variable temperature and DFT/MRCI studies show that a small energy gap between the 1/3 LL/LMCT states of only 79 meV is responsible for efficient thermally activated delayed fluorescence (TADF). Time-resolved luminescence and transient absorption studies confirm the occurrence of long-lived, dominantly ligand-to-ligand charge transfer excited states in solution, allowing for application in Dexter energy transfer photocatalysis.

Selective Triplet–Singlet Förster‐Resonance Energy Transfer for Bright Red Afterglow Emission (Adv. Funct. Mater. 11/2023)

Selective Förster-Resonance Energy Transfer In article number 2211604, Shuzo Hirata and co-workers demonstrate the efficient triplet-singlet Förster-resonance energy transfer from the N-fused chromophore to an acceptor leads to the bright red afterglow emission. The selective triplet-singlet energy transfer is due to the high radiation rate from the lowest triplet state facilitated by the out-of-plane molecular vibrations in the N-fused chromophore and near-zero fluorescence suppressing the singlet-singlet energy transfer pathways.

Mechano-Stimulus and Environment-Dependent Circularly Polarized TADF in Chiral Copper(I) Complexes and Their Application in OLEDs

Molecular emitters that combine circularly polarized luminescence (CPL) and high radiative rate constants of the triplet exciton decay are highly attractive for electroluminescent devices (OLEDs) or next-generation photonic applications, such as spintronics, quantum computing, cryptography, or sensors. However, the design of such emitters is a major challenge because the criteria for enhancing these two properties are mutually exclusive. In this contribution, we show that enantiomerically pure {Cu(CbzR)[(S/R)-BINAP]} [R = H (1), 3,6-tBu (2)] are efficient thermally activated delayed fluorescence (TADF) emitters with high radiative rate constants of kTADF up to 3.1 × 105 s-1 from 1/3LLCT states according to our temperature-dependent time-resolved luminescence studies. The efficiency of the TADF process and emission wavelengths are highly sensitive to environmental hydrogen bonding of the ligands, which can be disrupted by grinding of the crystalline materials. The origin of this pronounced mechano-stimulus photophysical behavior is a thermal equilibrium between the 1/3LLCT states and a 3LC state of the BINAP ligand, which depends on the relative energetic order of the excited states and is prone to inter-ligand C-H···π interactions. The copper(I) complexes are also efficient CPL emitters displaying exceptional dissymmetry values glum of up to ±0.6 × 10-2 in THF solution and ±2.1 × 10-2 in the solid state. Importantly for application in electroluminescence devices, the C-H···π interactions can also be disrupted by employing sterically bulky matrices. Accordingly, we have investigated various matrix materials for successful implementation of the chiral copper(I) TADF emitters in proof-of-concept CP-OLEDs.

Tailoring D-π-A architectures with hybridized local and charge transfer fluorophores exhibiting high electroluminescence exciton utilization and low threshold amplified spontaneous emission.

Novel amorphous compounds which could simultaneously use 25% singlet excitons and 75% triplet excitons as the energy source for light amplification enable the reduction of the threshold current density for electrically pumped organic semiconductor laser diodes (OSLDs); however, there is always a trade-off between the high radiative decay rate of the local excited (LE) state that is required for amplified spontaneous emission (ASE) and high exciton utilization benefiting from the charge-transfer (CT) state during electroluminescence (EL). Herein, we have explored a delicate balance to achieve both low ASE threshold and high EL exciton utilization by adopting a carefully tailored hybridized local and charge-transfer (HLCT) molecular design. A series of donor-π-acceptor (D-π-A) molecules (SBz-1, SBz-2 and SBz-3) are synthesized, and the structural change mainly refers to the spatial distance between D and A which could regulate the excited-state character via adjusting the CT length. Notably, the ASE phenomenon with a low threshold (2.97 μJ cm-2) and a high exciton utilization of 57.6% are achieved at the same time for SBz-2 with an appropriate CT length. The results provide guidance for molecular design toward harvesting triplet excitons in organic laser materials.