Lowest Excited Triplet State Research Articles

Role of Carbonyl Distortions Facilitating Persistent Room-Temperature Phosphorescence

A decrease in the room-temperature phosphorescence (RTP) yield and RTP lifetime with increasing temperature is often explained by the increased nonradiative transition from the lowest triplet excited state (T1) caused by molecular vibrations of the chromophores. Here, we report a positive contribution of molecular vibrations and the distortion of vanillic acid (VA) dispersed in amorphous insulating polymer hosts to phosphorescence characteristics. We investigated triplet generation as well as photophysical processes from T1 of VA dispersed in poly(methyl methacrylate) (PMMA) and poly(vinyl alcohol) (PVA). Comparisons between optically measured data and calculation-based data, regarding the phosphoresce rate (kp) and the rate constant of the nonradiative transition (knr) from T1, reveal that kp and knr of the dispersed VA negligibly changed in PMMA or PVA, indicating that intermolecular processes between VA and PMMA are related to a large RTP quenching of VA in PMMA. Vibrational out-of-plane distortion of the carbonyl moiety of VA induced ππ*–nπ* mixing between the high-order singlet excited state and the ground state, mainly enhancing kp compared with knr of VA. Although vibrations are often reported to quench RTP, this report suggests that some distortions induced by vibrations of other chromophores contribute to RTP enhancement of molecular solid materials.

Light Alters the NH3 vs N2 H4 Product Profile in Iron-catalyzed Nitrogen Reduction via Dual Reactivity from an Iron Hydrazido (Fe=NNH2 ) Intermediate.

Whereas synthetically catalyzed nitrogen reduction (N2 R) to produce ammonia is widely studied, catalysis to instead produce hydrazine (N2 H4 ) has received less attention despite its considerable mechanistic interest. Herein, we disclose that irradiation of a tris(phosphine)borane (P3 B ) Fe catalyst, P3 B Fe+ , significantly alters its product profile to increase N2 H4 versus NH3 ; P3 B Fe+ is otherwise known to be highly selective for NH3 . We posit a key terminal hydrazido intermediate, P3 B Fe=NNH2 , as selectivity-determining. Whereas its singlet ground state undergoes protonation to liberate NH3 , a low-lying triplet excited state leads to reactivity at Nα and formation of N2 H4 . Associated electrochemical and spectroscopic studies establish that N2 H4 lies along a unique product pathway; NH3 is not produced from N2 H4 . Our findings are distinct from the canonical mechanism for hydrazine formation, which proceeds via a diazene (HN=NH) intermediate and showcase light as a tool to tailor selectivity.

Photophysical Rate Constants and Oxygen Dependence for Si and Ge Rhodamine Zwitterions.

The family of group XIV rhodamine zwitterions are fluorescence probes with carbon, silicon, germanium, or tin substituted in the 10-position of the xanthene ring. Because of their inherent near-infrared fluorescence, photostability and high quantum yields in aqueous solutions, the Si and Ge containing fluorophores in this class have become increasingly important for fluorescent labeling of proteins and biological molecules. This study fully characterizes photophysical rates derived from a model consisting of a singlet ground state, the lowest singlet excited state, and the lowest triplet excited state for two exemplar group XIV rhodamine zwitterions, one containing Si and the other Ge. Within a simple Jablonski diagram, all radiative and non-radiative rates, including intersystem crossing and triplet depopulation rates, were measured as a function of oxygen concentration. It was shown that the triplet depopulation rates are intrinsically fast in comparison with traditional xanthene containing fluorophores, probably due to the increased spin-obit coupling from the Si and Ge substitution in the xanthene ring. Dissolved oxygen increases both the intersystem crossing and triplet depopulation rates. Stern-Volmer analysis was conducted to estimate rates of quenching by oxygen. The experimental data was used to estimate the initial rates for reactive oxygen production by Si and Ge containing fluorophores in aqueous solutions containing different concentrations of dissolved O2. These estimates showed a significantly slower initial rate of reactive oxygen production in comparison with rhodamine 6G. This goes a long way to explaining their inherent photostability. Spectroscopic experiments were also conducted in 77 K viscous aqueous glasses where it was observed that the fluorescence spectra remained unchanged, and the quantum yields increased from 0.53 to 0.84 and from 0.52 to 0.89 for the Si and Ge containing fluorophores respectively; no phosphorescence was observed. All intersystem crossing and triplet depopulation rates were measured using fluorescence correlation spectroscopy (FCS) and analyzed using a new method that extrapolated the power dependence of the FCS curves to optical saturation. This method was verified using published data.

Selective Triplet–Singlet Förster‐Resonance Energy Transfer for Bright Red Afterglow Emission

AbstractFörster‐resonance energy transfer (FRETT‐S) from the lowest excited triplet state (T1) of a donating sensitizer to a fluorescence acceptor can be used to obtain bright room‐temperature afterglow emission at long wavelengths. However, the energy transfer from the lowest excited singlet state of the donor to the acceptor is an undesirable deactivation pathway that prevents FRETT‐S. Herein, heteroatoms in chromophores are shown to allow selective and efficient FRETT‐S for enhanced triplet emission for bright room‐temperature afterglow emission at long wavelengths. Different transition characteristics between the lowest singlet excited state and triplet states in heteroatom‐containing chromophores accelerate triplet generation, enabling near‐zero fluorescence yields. Out‐of‐plane vibrations of the heteroatoms in aromatic fused rings greatly enhance the radiative rate from T1 by a factor of 88 relative to non‐heteroatom‐containing fused chromophore. The compatibility of the near‐zero fluorescence and the enhanced triplet emission in a heteroatom‐containing fused chromophore enable selective and efficient FRETT‐S pathways, resulting in room‐temperature red afterglow emission with a yield of 17%. The bright emission at long‐wavelengths allows distinguishable, multiple spectral signals in ambient white light.

Symmetry Breaking with the SCAN Density Functional Describes Strong Correlation in the Singlet Carbon Dimer.

The SCAN (strongly constrained and appropriately normed) meta-generalized gradient approximation (meta-GGA), which satisfies all 17 exact constraints that a meta-GGA can satisfy, accurately describes equilibrium bonds that are normally correlated. With symmetry breaking, it also accurately describes some sd equilibrium bonds that are strongly correlated. While sp equilibrium bonds are nearly always normally correlated, the C2 singlet ground state is known from correlated wave function theory to be a rare case of strong correlation in an sp equilibrium bond. Earlier work that calculated atomization energies of the molecular sequence B2, C2, O2, and F2 in the local spin density approximation (LSDA), the Perdew-Burke-Ernzerhof (PBE) GGA, and the SCAN meta-GGA, without symmetry breaking in the molecule, found that only SCAN was accurate enough to reveal an anomalous under-binding for C2. This work shows that spin symmetry breaking in singlet C2, which involves the appearance of net up- and down-spin densities on opposite sides (not ends) of the bond, corrects that underbinding, with a small SCAN atomization-energy error more like that of the other three molecules, suggesting that symmetry breaking with an advanced density functional might reliably describe strong correlation. This article also discusses some general aspects of symmetry breaking and the insights into strong correlation that symmetry breaking can bring. The normally correlated low-lying triplet excited state has the right vertical excitation energy in SCAN but not in LSDA or PBE, where the triplet is a false ground state. Fractional occupation numbers are found only for the symmetry-unbroken singlet and only in LSDA and PBE GGA.

Dual emissive cyclometallated iridium complexes: Synthesis, structure and photophysical properties

Herein, we present a new set of dual emissive heteroleptic cationic cyclometallated iridium complexeDepartment of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal By-pass Road, Bhauri, Bhopal, 462 066, Madhya Pradesh, Indias [(L)2Ir(N^N)]+ ([1]+ - [3]+) using extended π-conjugated 1-isopentyl-2-phenyl-1H-naphtho [2,3-d] imidazole (L) as C^N chelating ligand. The complexes were characterized using various analytical techniques including single crystal X-ray crystallography. The complexes exhibit dual emission behavior (560 nm, 610 and 670 nm) at ambient condition (298 K). The photophysical properties along with DFT results indicate that the origin of emission bands is not only from the lowest energy T1 state but also there is involvement of other higher energy excited triplet states. The DFT study further reveals that the lowest excited triplet state (T1) has 3MLLCT character, whereas the closely lying other higher excited triplet states are predominately of 3LLCT/3ILCT character which could be responsible for such dual emission.

Structural stability and the low-lying singlet and triplet states of BN-n-acenes, n=1-7.

The chemical stability and the low-lying singlet and triplet excited states of BN-n-acenes (n=1-7) were studied using single reference and multireference methodologies. From the calculations, descriptors such as the singlet-triplet splitting, the natural orbital (NO) occupations and aromaticity indexes are used to provide structural and energetic analysis. The boron and nitrogen atoms form an isoelectronic pair of two carbon atoms, which was used for the complete substitution of these units in the acene series. The structural analysis confirms the effects originated from the insertion of a uniform pattern of electronegativity difference within the molecular systems. The covalent bonds tend to be strongly polarized which does not happen in the case of a carbon-only framework. This effect leads to a charge transfer between neighbor atoms resulting in a more strengthened structure, keeping the aromaticity roughly constant along the chain. The singlet-triplet splitting also agrees with this stability trend, maintaining a consistent gap value for all molecules. The BN-n-acenes molecules possess a ground state with monoconfigurational character indicating their electronic stability. The low-lying singlet excited states have charge transfer character, which proceeds from nitrogen to boron.

Distinguishing the Quantum Yield and Lifetime of Carbazole‐Based Room‐Temperature Phosphorescence Materials: QM/MM Study

AbstractCommercial carbazole derivatives are widely used as typical electron donors in room‐temperature phosphorescence materials. The high phosphorescence quantum yield (Φp) and short lifetime (τp) of the carbazole‐based (9H‐carbazol‐9‐yl)isonicotinonitrile (P‐34N) crystal are distinguished by quantum mechanics and molecular mechanics method. Two inducements are accounted for the high Φp of P‐34N: i) large spin–orbit coupling (0.63 cm−1) induces fast intersystem crossing (ISC) of the lowest singlet excited state (S1) to the lowest triplet excited state (T1), ii) the intermolecular π–π interactions stabilize triplet excitons. Meanwhile, the 3(n, π*) →1(π, π*) of P‐34N favors the nonradiative decay of T1to the ground state (S0), and thus, causes short τp. In contrast to P‐34N, the low Φp of 6‐(9H‐carbazol‐9‐yl)nicotinonitrile (P‐24N) crystal results from the effective reverse ISC process induced by small ΔEST (0.3 eV) between S1 state and the higher triplet state (T2), and the long τp is due to the unfavorable orbital reversals of T1 (π, π*)→S0 (π, π*) transition and the internal conversion process of T2 → T1. This work presents a reasonable interpretation of the Φp and τp of two carbazole‐based crystals.

Yellow-Emitting, Pseudo-Octahedral Zinc Complexes of Benzannulated N^N^O Pincer-Type Ligands.

A series of yellow-emitting, pseudo-octahedral Zn(II) complexes supported by monoanionic, tridentate acetylacetone-derived N^N-^O ligands incorporating phenanthridine (benzo[c]quinoline) units is presented. These species emit weakly in solution but exhibit extended millisecond luminescence lifetimes in the solid state at room temperature, and in a frozen glass at 77 K, indicative of phosphorescence from low-lying triplet excited states. Excitation spectra indicate a role for aggregation in enhancing emission in the solid state. In contrast to four-coordinate phenanthridinyl amide-supported tetradentate Zn(II) complexes which are nonemissive in fluid solution, solid-state X-ray crystallographic structures, solution IR spectroscopy, and computational analysis all indicate a delocalized character for the central deprotonated NH which tempers the amido character of the ligand. This design provides a mechanism for "turning on" long-lived luminescence from N-heterocycle/amido-supported Zn(II) coordination compounds.

Theoretical perspective for substitution effect on luminescent properties of through space charge transfer-based thermally activated delayed fluorescence molecules

Recently, through space charge transfer (TSCT)-based thermally activated delayed fluorescence (TADF) molecules have shown advantages in achieving high efficiencies and tunable emissions. However, the relationships between basic molecular structures and luminescent properties are unclear. Theoretical investigations to reveal the substitution effects with different numbers and positions on excited-state properties are highly desired. Herein, by taking TSCT-based TADF molecules S-CNDF-S-tCz, S-CNDF-D-tCz and T-CNDF-T-tCz as skeletons, a series of promising TADF molecules are designed by adopting ortho, meta and para substitutions with different numbers and positions. Photophysical properties of total 16 molecules are theoretically studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods in chloroform combined with polarizable continuum model. Results indicate that molecules with ortho-substitution possess small geometric changes and short Donor-Acceptor distances which are induced by the intramolecular van der Waals interactions. Decreased non-radiative consumption and increased TSCT ratio and therefore excellent performance for them can be expected. For molecules with large substitution numbers, twist structures facilitate them to realize small adiabatic energy gaps between the lowest singlet excited state (S1) and the lowest triplet excited state (T1), this designing strategy is consistent with the TADF dendrimers. Thus, the relationships between molecular structures and luminescent properties are revealed and promising TSCT-based TADF molecules with high efficiencies are theoretically proposed. Our investigations provide theoretical perspectives for inner mechanisms of substitution effect, which could further afford meaningful guidance to design new efficient TSCT-based TADF molecules.

Multiple resonance type thermally activated delayed fluorescence by dibenzo [1,4] azaborine derivatives.

We studied the photophysical and electroluminescent (EL) characteristics of a series of azaborine derivatives having a pair of boron and nitrogen aimed at the multi-resonance (MR) effect. The computational study with the STEOM-DLPNO-CCSD method clarified that the combination of a BN ring-fusion and a terminal carbazole enhanced the MR effect and spin-orbit coupling matrix element (SOCME), simultaneously. Also, we clarified that the second triplet excited state (T2) plays an important role in efficient MR-based thermally activated delayed fluorescence (TADF). Furthermore, we obtained a blue–violet OLED with an external EL quantum efficiency (EQE) of 9.1%, implying the presence of a pronounced nonradiative decay path from the lowest triplet excited state (T1).

Impact of the Anthryl Linking Mode on the Photophysics and Excited-State Dynamics of Re(I) Complexes [ReCl(CO)3(4'-An-terpy-κ2N)

Rhenium(I) complexes with 2,2′:6′,2″-terpyridines (terpy) substituted with 9-anthryl (1) and 2-anthryl (2) were synthesized, and the impact of the anthryl linking mode on the ground- and excited-state properties of resulting complexes [ReCl(CO)3(4′-An-terpy-κ2N)] (An—anthryl) was investigated using a combination of steady-state and time-resolved optical techniques accompanied by theoretical calculations. Different attachment positions of anthracene modify the overlap between the molecular orbitals and thus the electronic coupling of the anthracene and {ReCl(CO)3(terpy-κ2N)} chromophores. Following the femtosecond transient absorption, the lowest triplet excited state of both complexes was found to be localized on the anthracene chromophore. The striking difference between 1 and 2 concerns the triplet-state formation dynamics. A more planar geometry of 2-anthryl-terpy (2), and thus better electronic communication between the anthracene and {ReCl(CO)3(terpy-κ2N)} chromophores, facilitates the formation of the 3An triplet state. In steady-state photoluminescence spectra, the population ratio of 3MLCT and 3An was found to be dependent not only on the anthryl linking mode but also on solvent polarity and excitation wavelengths. In dimethyl sulfoxide (DMSO), compounds 1 and 2 excited with λexc > 410 nm show both 3MLCT and 3An emissions, which are rarely observed. Additionally, the abilities of the designed complexes for 1O2 generation and light emission under the external voltage were preliminary examined.

Thermally activated delayed fluorescence materials based on acridin-9(10H)-one acceptor for organic light-emitting diodes

It is essential to manage the alignment of the locally excited (3LE) and charge-transfer triplet (3CT) and singlet (1CT) states of thermally activated delayed fluorescence (TADF) materials to accelerate reverse intersystem crossing (RISC) process. A series of donor-acceptor (D-A) type TADF emitters were developed by attaching acridine (DMAC) or phenoxazine (PXZ) donor at 3-site of acridin-9(10H)-one (i.e. acridone, AD) acceptor. Regardless of the donor, the lowest triplet excited states (T1) are always dominated by the 3LEA state with certain contribution from the 3CT state. By introducing methyl (Me) or trifluoromethyl (CF3) at 6-site of AD acceptor, the energy levels of 1CT, 3CT and 3LE states were tuned in different ways. When the donor is DMAC, incorporating Me on AD ring (3-DMAC-6-Me-AD) destructed the emission performance, while CF3 (3-DMAC-6-CF3-AD) strongly stimulated the TADF by pulling down the 1CT state level and reducing the energy splitting ΔEST to almost zero. In the case of strong donor PXZ, decorating acceptor with Me (3-PXZ-6-Me-AD) did not bring extinct effect on the TADF property. In particular, 3-DMAC-6-CF3-AD and 3-PXZ-6-Me-AD exhibited high rate constants of RISC (kRISC) of 10−6 and of radiation (kR) of 107 S−1, respectively. They achieved the maximum external quantum efficiency (EQEmax) of 21.6% and 23.3% in the doped OLEDs, and realized EQEmax of 14.7% and 17.6% with low efficiency roll-offs in non-doped devices.

Enhanced Red Persistent Room-Temperature Phosphorescence Induced by Orthogonal Structure Disruption during Electronic Relaxation.

Bright, persistent, room-temperature phosphorescence (RTP) at long wavelengths is crucial for high-resolution imaging in the absence of in vivo autofluorescence. However, efficient long-wavelength RTP is difficult. Here, enhanced red RTP based on a unique mechanism was observed from deuterated dibenzo[g.p]chrysenes substituted with a phenoxazine. The yield was 16%, with an average lifetime of 1.8 s. An orthogonal dihedral angle between the dibenzo[g.p]chrysene and the phenoxazine in the lowest excited singlet state allowed a forbidden fluorescence to increase triplet generation. When the dihedral angle changed, disengagement of the forbidden fluorescence from the excited singlet state occurred, and the lowest triplet excited state had a facilitated phosphorescence rate without increasing its nonradiative transition rate. The facilitated phosphorescence rate as well as the large triplet yield led to the enhanced red RTP.

A Computational-Experimental Approach to Unravel the Excited State Landscape in Heavy-Atom Free BODIPY-Related Dyes.

We performed a time-gated laser-spectroscopy study in a set of heavy-atom free single BODIPY fluorophores, supported by accurate, excited-state computational simulations of the key low-lying excited states in these chromophores. Despite the strong fluorescence of these emitters, we observed a significant fraction of time-delayed (microseconds scale) emission associated with processes that involved passage through the triplet manifold. The accuracy of the predictions of the energy arrangement and electronic nature of the low-lying singlet and triplet excited states meant that an unambiguous assignment of the main deactivation pathways, including thermally activated delayed fluorescence and/or room temperature phosphorescence, was possible. The observation of triplet state formation indicates a breakthrough in the “classic” interpretation of the photophysical properties of the renowned BODIPY and its derivatives.

Multiple-Resonance Extension and Spin-Vibronic-Coupling-Based Narrowband Blue Organic Fluorescence Emitters with Over 30% Quantum Efficiency.

Achieving narrow-bandwidth emission and high external quantum efficiency (EQE) simultaneously is a challenge for next-generation blue-emitting organic light-emitting diodes (OLEDs). In this study, novel multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters are developed by fusing an indolocarbazole unit with two carbazole skeletons using para-oriented nitrogen atoms. The resulting rigid and planar π-system without electron-accepting atoms exhibits pure blue photoluminescence at 470nm, reaching a 100% quantum yield with a full-width-at-half-maximum (FWHM) of 25nm. Higher-level quantum chemistry calculations confirm an MR effect within the extended π-conjugation and an enhanced triplet-to-singlet crossover (104 s-1 ) through a reduced energy gap (ΔEST ) coupled with large spin-vibronic coupling mediated by low-lying triplet excited states. An OLED fabricated using the MR-TADF emitter with CIE color coordinates of (0.12, 0.16) exhibits a record high EQE of 30.9% and a small FWHM of 23nm. With further optimization of the device structure, a high EQE of 33.8% is achieved without additional outcoupling enhancements owing to the near-perfect horizontal alignment of the emitting dipoles.

Advances in Blue Exciplex-Based Organic Light-Emitting Materials and Devices.

Exciplexes possessing thermally activated delayed fluorescence (TADF) characteristics have received much attention in the fields of organic light-emitting materials and devices over the past decade. In general, an exciplex is a physical mixture between a donor (D) with hole transport properties and an acceptor (A) with electron transport characteristics, and the energy difference between the lowest excited singlet state and the lowest excited triplet state is usually fairly small in terms of the long-range charge-transfer process from D to A. In the processes of photoluminescence and electroluminescence, triplet excitons can be converted to singlet excitons through reverse intersystem crossing and then radiate photons to achieve TADF. As a consequence, triplet excitons can be effectively harvested, and the exciton utilization can be significantly enhanced. Up to now, a large number of exciplexes have been developed and applied to organic light-emitting devices. Notably most of them showed green or red emission, while blue exciplexes are relatively few owing to the spectrum characteristics of the large red-shift and broadened emission. In this study, the latest progress of blue exciplex–based organic light-emitting materials and devices is briefly reviewed, and future research is prospected.

Theoretical insights into room temperature phosphorescence emission with anti-Kasha behavior in aggregate

Organic room temperature phosphorescence (RTP) materials with long lifetimes have shown promising applications in organic light emitting diodes and bioimaging fields. However, the inner luminescent mechanisms for RTP especially for high lying triplet state emission, have not been unveiled. Herein, based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT), the photophysical properties of four RTP systems with triphenylethylene derivatives as skeletons are studied. Excited state dynamic process in aggregate is systematically studied by combining quantum mechanics with molecular mechanics (QM/MM) method coupled with thermal vibration correlation function (TVCF) method. Moreover, the intermolecular interactions are measured by the independent gradient model based on Hirshfeld partition (IGMH) method, the Huang-Rhys factor, reorganization energy and spin-orbit coupling (SOC) constant are calculated, radiative and non-radiative decay as well as intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes are all investigated. Results indicate that the radiative decay process from the lowest triplet excited state (T1) to ground state (S0) is low and the non-radiative decay process is high, various calculations are also performed to exclude the emission process from T1 to S0, thus non-luminescence is determined for T1. Furthermore, a large energy gap between T2 and T1 is observed, and high radiative decay and low non-radiative decay processes from T2 to S0 are confirmed. Thus, the mechanisms of RTP from high lying triplet state T2 are revealed. Furthermore, through wise molecular design and detailed calculations, the mechanical insights are detected that the RTP emissions with anti-Kasha behavior are largely related to the unique molecular configurations with triphenylethylene derivatives as skeletons. Our studies give reasonable explanations for the previous experimental measurements and provide underlying perspectives for RTP mechanisms from high lying triplet state T2, which could promote the development of new efficient RTP emitters.

Dearomatization of Benzenoid Arenes Triggered by Triplet Excited State Intramolecular Proton Transfer.

The detailed mechanism of photoinduced dearomatization of benzenoid arenes is investigated using both the high-level ab initio method and density functional theory. The results suggest that the optically allowed singlet excited state (S2) can quickly decay to the lowest triplet excited state (T1) through a barrierless internal conversion and intersystem crossing. Importantly, we find a triplet excited state intramolecular proton transfer (T-ESIPT) pathway to produce a diradical triplet intermediate (3MO-H), which can trigger the subsequent [4 + 2] dearomatization reaction. Furthermore, the diastereoselectivity of the reaction was illustrated by the rotation of the O-H group of 3MO-H, which could be effectively modulated by the solvent effect (arising from the strength of the intermolecular hydrogen bond) and the substituted effect (arising from the strength of the electron-donation group). This photochemical mechanism can explain well the experimental observations, and the novel T-ESIPT process can open a new door in studying the photoinduced proton transfer reactions.

Down-conversion-induced delayed fluorescence via an inverted singlet-triplet channel

Inverted singlet-triplet molecules possessing a down-conversion process from the lowest excited triplet state (T1) to the lowest excited singlet state (S1) have recently roused much attention on account of potential merits in organic optoelectronics and photocatalysis. Nevertheless, investigations on inverted singlet-triplet molecules predominantly depend on the sophisticated ab initio calculations, while experimental attempts are extremely rare. In this work, we experimentally demonstrate the inverted singlet-triplet character of a heptazine derivative (HAP-3MF) which exhibits negative S1-T1 energy gaps (ΔEST) of −0.22 eV in toluene and −0.19 eV in acetonitrile based on meticulous photophysical investigations. To the best of our knowledge, this is the first time to directly obtain negative ΔEST based on fluorescence and phosphorescence spectra. Moreover, to better evaluate and reveal the photophysical features of extremely weak delayed emission in HAP-3MF, a mixed solution system of HAP-3MF:1,3-di(9H-carbazol-9-yl)benzene (mCP) in toluene at various molar ratios was subtly designed, and the energy transition mechanism involving a down-converted triplet-to-singlet channel was carefully elucidated.