Perspective for aggregation-induced delayed fluorescence mechanism:A QM/MM study

To enhance the potential application of thermally activated delayed fluorescence(TADF)molecular materials,new functions are gradually cooperated to the TADF molecules.Aggregation induced emission can effectively solve the fluorescence quenching problem for TADF molecules in solid phase,thus aggregation-induced delayed fluorescence(AIDF)molecules were recently focused.Nevertheless,their luminescent mechanisms are not clear enough.In this work,excited state properties of an AIDF molecule DMF-BP-DMAC[reported in Chemistry-An Asian Journal 14828(2019)]are theoretically studied in tetrahydrofuran(THF)and solid phase.For consideration of surrounding environment,the polarizable continuum method(PCM)and the combined quantum mechanics and molecular mechanics(QM/MM)method were applied for solvent and solid phase,respectively.Due to the increase of the transition dipole moment and decrease of the energy difference between the first single excited state(S1)and the ground state(S0),the radiative rate is increased by about 2 orders of magnitude in solid phase.The energy dissipation of the non-radiative process from S1 to S0 is mainly contributed by low-frequency vibrational modes in solvent,and they can be effectively suppressed in aggregation,which may lead to a slow non-radiation process in solid phase.Both factors would induce enhanced luminescence efficiency of DMF-BP-DMAC in solid phase.Meanwhile,the small energy gap between S1 and triplet excited states results in high reverse intersystem crossing(RISC)rates in both solvent and solid phase.Therefore,TADF is confirmed in both phases.Aggregation significantly influences both the ISC and RISC processes and more RISC channels are involved in solid state.The enhanced delayed fluorescence should be induced by both the enhanced fluorescent efficiency and ISC efficiency.Our calculation provides a reasonable explanation for experimental measurements and helps one to better understand the luminescence mechanism of AIDF molecules.

Sky-Blue Aggregation-Induced Delayed Fluorescence Luminogens with High Horizontal Dipole Orientation for Efficient Organic Light-Emitting Diodes

Efficient and stable blue luminescent organic materials are highly demanded in the field of organic light-emitting diodes(OLEDs)but still remain challenging.In this work,two new sky-blue luminescent molecules comprised of electron acceptor of benzophenone and electron donors of spiro[acridine-9,9'-fluorene]and carbazole are designed and synthesized,and their thermal stability,electrochemical behaviors,photophysical properties,carrier transport ability and electroluminescence performance are investigated.

Aggregation-induced delayed fluorescence (AIDF) materials： a new break-through for nondoped OLEDs

Nowadays, purely organic materials with delayed fluorescence （DF） are being enthusiastically developed and extensively utilized as light-emitting materials to fabricate OLEDs because of their high exciton utilization and metal-flee nature. These DF materials usually have a small single-triplet splitting （AEsT）, and the nonradiative triplet excitons （75%） can be converted into radiative singlet excitons via thermally promoted reverse intersystem crossing （RISC） processes,

Three-dimensional organic cage with aggregation-induced delayed fluorescence

A new kind of emissive small-molecular organic cage has been developed via the combination of coupling and condensation reactions,which shows outstanding solubility,structural stability and potential spatial isomeric chirality.Interestingly,through the introduction of proper donor and acceptor units,this emissive organic cage is the first among organic cages to exhibit red aggregation-induced delayed fluorescence with photoluminescence emission at 603 nm.The finding not only expands the types of emissive small-molecular organic cages,but also represents an important step for further development of red delayed fluorescence materials with good solubility and aggregation-induced emission feature.

Quinoline-based aggregation-induced delayed fluorescence materials for highly efficient non-doped organic light-emitting diodes

Three new emitters,namely 10,10’-(quinoline-2,8-diyl)bis(10 H-phenoxazine)(Fene),10,10’-(quinoline-2,8-diyl)bis(10 H-phenothiazine)(Fens) and 10,10’-(quinoline-2,8-diyl)bis(9,9-dimethyl-9,10-dihydroacridine)(Yad),featuring quinoline as a new electron acceptor have been designed and conveniently synthesized.These emitters possessed small singlet-triplet splitting energy(ΔEst) and twisted structures,which not only endowed them show thermally activated delayed fluorescence(TADF)properties but also afforded a remarkable aggregation-induced emission(AIE) feature.Moreover,they also showed aggregation-induced delayed fluorescence(AIDF) property and good photoluminescence(PL) property,which are the ideal emitters for non-doped organic light-emitting diodes(OLEDs).Furthermore,high-performance non-doped OLEDs based on Fene,Fens and Yad were achieved,and excelle nt maximum external quantum efficiencies(EQEmax) of 14,9%,13.1% and 17,4%,respectively,were obtained.It was also found that all devices exhibited relatively low turn-on voltages ranging from 3.0 V to3.2 V probably due to their twisted conformation and the AIDF properties.These results demonstrated the quinoline-based emitters could have a promising application in non-doped OLEDs.

Effect of aggregation on thermally activated delayed fluorescence and ultralong organic phosphorescence:QM/MM study

Aggregation-induced thermally activated delayed fluorescence(TADF)phenomena have attracted extensive attention recently.In this paper,several theoretical models including monomer,dimer,and complex are used for the explanation of the luminescent properties of(R)-5-(9H-carbazol-9-yl)-2-(1,2,3,4-tetrahydronaphthalen-1-yl)isoindoline-1,3-dione((R)-ImNCz),which was recently reported[Chemical Engineering Journal 418129167(2021)].The polarizable continuum model(PCM)and the combined quantum mechanics and molecular mechanics(QM/MM)method are adopted in simulation of the property of the molecule in the gas phase,solvated in acetonitrile and in aggregation states.It is found that large spin–orbit coupling(SOC)constants and a smaller energy gap between the first singlet excited state and the first triplet excited state(△E_(st))in prism-like single crystals(SC_(p)-form)are responsible for the TADF of(R)-lmNCz,while no TADF is found in block-like single crystals(SC_(b)-form)with a larger △E_(st).The multiple ultralong phosphorescence(UOP)peaks in the spectrum are of complex origins,and they are related not only to ImNCz but also to a minor amount of impurities(ImNBd)in the crystal prepared in the laboratory.The dimer has similar phosphorescence emission wavelengths to the(R)-lmNCz-SC_(p) monomers.The complex composed of(R)-lmNCz and(R)-lmNBd contributes to the phosphorescent emission peak at about 600 nm,and the phosphorescent emission peak at about 650 nm is generated by(R)-lmNBd.This indicates that the impurity could also contribute to emission in molecular crystals.The present calculations clarify the relationship between the molecular aggregation and the light-emitting properties of the TADF emitters and will therefore be helpful for the design of potentially more useful TADF emitters.

First-Principles Investigation on Triazine Based Thermally Activated Delayed Fluorescence Emitters

Three kinds of triazine based organic molecules designed for thermally activated delayed fluorescence （TADF） emitters are investigated by first-principles calculations. An optimal Hartree-Fork （HF） method is adopted for the calculation of energy gap between the first singlet state （S1） and the first triplet state （T1）. The natural transition orbital, the electron- hole （e-h） distribution and the e-h overlap diagram indicate that the S1 states for the three systems include both charge-transfer and some localized excitation component. Further quantitative analysis of the excitation property is performed by introducing the index Ar and the integral of e-h overlap S. It is found that symmetric geometry is a necessary condition for TADF emitters, which can provide more delocalized transition orbitals and consequently a small S1-T1 energy gap. Artful inserting aromatic groups between donors and acceptors can significantly enhance the oscillator strength. Finally, the energy state structures calculated with the optimal HF method is presented, which can provide basis for the study of the dynamics of excited states.

Correlations between delayed fluorescence of chlorophyll, metabolism and yield of plants. II. Influence of moisture of leaf and temperature condition on delayed fluorescence of leaves

During various temperatures of incubation the dehydration of leaves up to 3.2-3.8% mainly in-duced increase maximum amplitude of delayed fluorescence of chlorophyll. It was shown that moisture loss with this range could be deter-mined for the most part by the growth of the electrochemical potential of thylakoid mem-branes. The further incubation of detached leaves at 36℃ temperature, with more notable moisture loss, resulted in specific its decline as opposed to cases of 22 and 6℃ of thermal in-cubation. It was confirmed that the increased temperatures and moisture loss damage of the cells of plants occurred together induce a greater influence on plants than in case of oc-curring apart. The results allow to suppose that this can be mostly caused weakly associated polypeptides fallen out from the chloroplast membrane, which may be stipulated by high temperature combined with change ionic and osmotic stresses due moisture loss. Simulta-neously, the results showed that the exposure of the critical lowered air temperature led to considerable typical changes of leaves delayed fluorescence parameters of field plants. There-fore, their use can constitute new approaches to elucidate the molecular basis of plant freezing tolerance in a timely manner, based on concen-tration-related changes and the efficiency of coupling between light and dark processes of plants.

Inuence of Electron Donating Ability on Reverse Intersystem Crossing Rate for One Kind of Thermally Activated Delayed Fluorescence Molecules

First-principles calculations are applied for investigating influence of electron donating ability of donor groups in eight thermally activated delayed fluorescence（TADF） molecules on their geometrical structures and transition properties as well as reverse intersystem crossing（RISC） processes. Results show that the diphenylamine substitution in the donor part can slightly change the bond angle but decrease bond length between donor and acceptor unit except for the lowest triplet state（Ti） of carbazole-xanthone molecule. As the electron donating ability of donor groups is increased, the overlap between the highest occupied molecular orbital（HOMO） and the lowest unoccupied molecular orbital（LUMO） is decreased. As the diphenylamine groups are added in donor part, the delocalization of HOMO is enlarged,which brings a decreased energy gap(△ES1-T1) between the lowest singlet excited state（S1）and T1 state. Furthermore, with the calculated spin-orbit coupling coefficient(HSO), one finds that the larger value of ■ is, the faster the RISC is. The results show that all investigated molecules are promising candidates as TADF molecules. Overall, a wise molecular design strategy for TADF molecules,in which a small △ES1-T1 can be achieved by enlarging the delocalization of frontier molecular orbitals with large separation between HOMO and LUMO, is proposed.

High Efficiency and Stable Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence Emitter

High efficiency, stable organic light-emitting diodes （OLEDs） based on 2-pheyl-4＇-carbazole-9-H-Thioxanthen-9- one-10, 10-dioxide （TXO-PhCz） with different doping concentration are constructed. The stability of the encap- sulated devices are investigated in detail. The devices with the 10 wt% doped TXO-PhCz emitter layer （EML） show the best performance with a current efficiency of 52.1 cd/A, a power efficiency of 32.71re^W, and an external quantum efficiency （EQE） of 17.7%. The devices based on the lOwt%-doped TXO-PhCz EML show the best operational stability with a half-life time （LTSO） of 8Oh, which is 8 h longer than that of the reference devices based on fac-tris（2-phenylpyridinato）iridium（ Ⅲ） （Ir（ppy）a）. These indicate excellent stability of TXO-PhCz for redox and oxidation processes under electrical excitation and TXO-PhCz can be potentially used as the emitters for OLEDs with high efficiency and excellent stability. The high-performance device based on TXO-PhCz with high stability can be further improved by the optimization of the encapsulation technology and the development of a new host for TXO-PhCz.

Theoretical verification of intermolecular hydrogen bond induced thermally activated delayed fluorescence in SOBF-OMe

Thermally activated delayed fluorescence(TADF)molecules have attracted great attention as high efficient luminescent materials.Most of TADF molecules possess small energy gap between the first singlet excited state(S_(1))and the first triplet excited state(T_(1))to favor the up-conversion from T_(1)to S_(1).In this paper,a new TADF generation mechanism is revealed based on theoretical simulation.By systematic study of the light-emitting properties of SOBF-OMe in both toluene and in aggregation state,we find that the single SOBF-OMe could not realize TADF emission due to large energy gap as well as small up-conversion rates between S_(1)and T_(1).Through analysis of dimers,we find that dimers with intermolecular hydrogen bond(H-bond)are responsible for the generation of TADF,since smaller energy gap between S_(1)and T_(1)is found and the emission wavelength is in good agreement with experimental counterpart.The emission properties of SOBF-H are also studied for comparison,which reflect the important role of H-bond.Our theoretical results agree ith experimental results well and confirm the mechanism of H-bond induced TADF.

Correlations between delayed fluorescence of chlorophyll, metabolism and yield of plants. I. Influence of fertilizers on correlations

The increase of the potash fertilizer dose in-duced a raise in efficiency influence of the ni-trogen fertilizer, optimisation of phosphorous fertilizer effect, enhancement of leaf protein production, expansion of assimilating surface and yield growth. In the period of yield forma-tion, the parameters of delayed fluorescence of chlorophyll (DF) of leaf wholly corresponded with key factors that had a dramatic influence on the effectiveness of yield formation. The maximum level of DF amplitude mostly de-pended on the activity of nitrogen metabolism and presumably on active PSII concentration changes per square unit. Half-decay time of this amplitude was predominantly identified by the level of carbohydrate metabolism in the overall plant system, including the quantity of its products and, therefore, mostly by correspon-dence with yield. This is a biological base trig-gering the use of DF parameters for system analyses of plant production process.

Correlations between delayed fluorescence of chlorophyll, metabolism and yield of plants. III. Influence of viral infection on field plants and new technology of clone selection of virus-free planting potato

At the end of potato plants vegetation virus in-fection induced both decrease in maximum amplitude of delayed fluorescence maximal amplitude and increase half time of its decrease, as well as reduction in the amount of stems, plants’ height and assimilation area surface, yield, acceleration of plants development and their early die-off. The differences of DF pa-rameters and yields between strongly and weakly infected plants increase in case of a combined virus infection. In industrial test of the selection of virus-free planting potato by the use of DF parameter, a rise in the yield and de-crease degree of viral infection of crops was obtained.

Luminescent properties of thermally activated delayed fluorescence molecule with intramolecular π-π interaction between donor and acceptor

Influence of intramolecular π-π interaction on the luminescent properties of thermally activated delayed fluorescence（TADF） molecule（3, 5-bis（3,6-di-tert-butyl-9 H-carbazol-9-yl）-phenyl）（pyridin-4-yl） methanone（DTCBPY） is theoretically studied by using the density functional theory（DFT） and time-dependent density functional theory（TD-DFT）.Four conformations（named as A, B, C, and D） of the DTCBPY can be found by relax scanning, and the configuration C corresponds to the luminescent molecule detected experimentally. Besides, we calculate the proportion of each conformation by Boltzmann distribution, high configuration ratios（44% and 52%） can be found for C and D. Moreover, C and D are found to exist with an intramolecular π-π interaction between one donor and the acceptor; the intramolecular interaction brings a smaller Huang-Rhys factor and reduced reorganization energy. Our work presents a rational explanation for the experimental results and demonstrates the importance of the intramolecular π-π interaction to the photophysical properties of TADF molecules.

Molecular Conformational Isomerization:An Efficient Way to Design Novel Emitters with Dual-Thermally Activated Delayed Fluorescence Characteristics and Their Optoelectronic and Bioimaging Applications

Single-molecule luminophores with dual-thermally activated delayed fluorescence(TADF)properties are receiving increasing attention.However,how to achieve these goals requires more in-depth studies.Herein,we demonstrate a novel example emitter,10-(5-(2-(pyridin-3-yl)-[4,5′-bipyrimidin]-6-yl)pyridin-2-yl)-10Hphenoxazine(PmPy-PXZ),enabling dual-TADF properties due to its key feature of conformational isomerization.Introducing a pyridine bridge can greatly reduce the steric hindrance and facilitate dual-stable conformations in the ground state,where the quasi-axial(QA)forms predominate.Moreover,unlike previously reported TADF molecules with dual confirmations,both theoretical and experimental measurements show that not only the quasi-equatorial(QE)forms but also the QA forms exhibit distinct TADF characteristics,which can be attributed to an additional higher reverse intersystem crossing pathway.This is the first time that dual-TADF properties of single molecules have been achieved based on conformational isomerism.Its applications in“self-doping”organic light-emitting diode and biomedical imaging have further been investigated.All these results show the good potential of such dual-band TADF emitters based on molecular conformational isomerization.

Aggregation-induced delayed electrochemiluminescence of organic dots in aqueous media

Full utilization of the excited species at both singlet states(1R*)and triplet states(3R*)is crucial to improving electrochemiluminescence(ECL)efficiency but is challenging for organic luminescent materials.Here,an aggregation-induced delayed ECL(AIDECL)active organic dot(OD)containing a benzophenone acceptor and dimethylacridine donor is reported,which shows high ECL efficiency via reverse intersystem crossing(RISC)of non-emissive 3R*to emissive 1R*,overcoming the spin-forbidden radiative decay from 3R*.By introducing dual donor-acceptor pairs into luminophores,it is found that nonradiative pathway could be further suppressed via enhanced intermolecular weak interactions,and multiple spin-up conversion channels could be activated.As a consequence,the obtained OD enjoys a 6.8-fold higher ECL efficiency relative to the control AIDECL-active OD.Single-crystal studies and theoretical calculations reveal that the enhanced AIDECL behaviors come from the acceleration of both radiative transition and RISC.This work represents a major step towards purely organic,high-efficiency ECL dyes and a direction for the design of next-generation ECL dyes at the molecular level.

Furan-modified thiadiazolo quinoxaline as an electron acceptor for constructing second near-infrared aggregation-induced emission fluorophores for beyond 1300 nm fluorescence/photoacoustic imaging and photothermal therapy

Creation of new fluorophores is important for understanding the structure-property relationship,by which the required optical properties are likely to be attained.Herein,through theory calculation,it is found that furan-modified thiadiazolo quinoxaline acting as an electron acceptor can endow donor-acceptor-donor(D-A-D)type second near-infrared(NIR-Ⅱ)fluorophores with longer emission wavelength than the other thiadiazolo quinoxaline-based acceptors containing pyridine,pyrrole,thiophene,and phenyl groups,respectively.On the basis of this theoretical prediction,a D-A-D type NIR-Ⅱ fluorophore with 6,7-di(furan-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline(DFTQ)as the acceptor and dithieno[3,2-b:2′,3′-d]pyrrole(DTP)as the donor is designed and synthesized,and the aggregation-induced emission(AIE)function is further achieved by introducing the AIE units of tetraphenylethylene(TPE)and triphenylamine(TPA),respectively,totally forming three NIR-Ⅱ fluorophores DFTQ-DTP,DFTQ-DTPE,and DFTQ-DTPA.For biological applications,the fluorophores are encapsulated by amphiphilic DSPE-PEG2000 to generate water-dispersible nanoparticles(NPs).Almost the whole emission of each of the NPs falls into the NIR-Ⅱ spectral range,with part emission beyond 1300 nm.By using DFTQ-DTPA NPs as the contrast and photothermal therapy(PTT)agent,high-resolution in vivo fluorescence imaging is achieved in the greater than 1300 nm window,and their good performance in photoacoustic imaging and high tumor PTT efficacy in tumor-bearing mice are also demonstrated.Taken together,this work mainly provides a strong electron acceptor for constructing longemitting fluorophores,and by using the electron acceptor,a AIE fluorophore with desirable quantum yield(QY)and photothermal conversion efficienciy(PCE)is synthesized and demonstrated to be promising in fluorescence/photoacoustic imaging and PTT.

Heptagonal intramolecular-lock strategy enables high-performance thermally activated delayed fluorescence emitters

The development of highly efficient thermally activated delayed fluorescence(TADF)emitters is persistently pursued for the application of organic light-emitting diodes(OLED)in full-colour display and solid-state lighting.Herein,we present a heptagonal intramolecular-lock strategy to design high-performance TADF emitters.As a proof-of-concept,a new type of tribenzotropone(TBP)acceptor has been designed and synthesized by a cascade decarboxylative cyclization of aryl oxoacetic acid derivative with biphenyl boronic acid.Compared with the unlocked benzophenone(BP)acceptor,the TBP acceptor has a highly twisted heptagonal geometry with moderate rigidity and flexibility,which enables a high-performance TADF emitter with a small single-triplet energy gap(ΔE_(ST))of 0.04 e V,a high photoluminescence quantum yield(Φ_(PL))of 99% and a large horizontal orientation factor(Θ_(//))of 84.0%.Consequently,highly efficient OLEDs with an external quantum efficiency as high as 33.8% are assembled,which is significantly higher than those of DPAC-BP with a highly rotatable BP acceptor(23.8%)as well as DPACFO with a rigid fluorenone(FO)acceptor(6.9%).

Supramolecular Thermally Activated Delayed Fluorescence Materials via Through-Space Charge Transfer

Thermally activated delayed fluorescence(TADF)polymeric materials based on through-space charge transfer(TSCT)have emerged as a highly studied topic in recent years.However,the construction of TSCT TADF materials via a supramolecular approach is still a big challenge.In this work,we report the noncovalent synthesis of TSCT TADF materials using a cyclic peptide-based bottle-brushed supramolecular polymer as a scaffold.By bringing the TSCT donor and acceptor in close proximity in space using the supramolecular scaffold,distinctive TADF emission in both solution and solid states could be achieved.Furthermore,the TADF system could be utilized as a sensitizer to coassemble with fluorescence acceptors to build thermally assisted fluorescence systems,resulting in color-tunable delayed fluorescence with high efficiency and color purity.Our findings provide a facile yet effective approach to designing and fabricating TSCT TADF materials,which might hold great potential for applications in the fields of organic light-emitting diode,bioimaging,and sensing.

A modular approach to efficient thermally activated delayed fluorescence from metal-perturbed intraligand charge-transfer excited state of Au(Ⅰ)complexes

The efficient harvesting of triplet excitons is crucial to the realization of high-performance organic light-emitting diodes(OLEDs).Herein,we show that coordination of donor-acceptor(D-A)type molecules to a metal atom in a monodentate fashion can lead to thermally activated delayed fluorescence(TADF)emissions with wide color tunability only through varying the noncoordinating acceptor moiety.A panel of TADF gold(Ⅰ)complexes with emission maxima(λmax)of 545–645 nm from metal perturbed intraligand charge-transfer(MPICT)excited states have been developed.Synergetic effects of heavy atom-induced spin-orbit coupling(SOC),steric-induced donor-acceptor twisting and suppressed intramolecular motions lead to high emission efficiencies of 65%-85%in doped films with delayed fluorescence lifetime of as short as 2.0μs.Transient absorption spectroscopic studies on selected complexes determined the kISCto be 6.5×10^(9)s^(-1).Theoretical calculations confirmed the participation of minor d orbital into the lowest excited state,which led to an SOC value of 5.19 cm^(-1)between the lowest-lying singlet and triplet excited states.The yellow to deep red solution-processed OLEDs based on the new gold(Ⅰ)complexes incorporated with various D-A ligands demonstrated promising performances.This study validates a modular design for TADF metal complexes,which will broaden the choices of metal centers and allow for facile color tuning via simple ligand synthesis.