The Influence of Energy Transfer on the Color Temperature Change in Color-Tunable Organic Light Emitting Diodes with Interface Exciplex

The color-tunable white organic light-emitting diode (CT-WOLED) with wide correlation color temperature (CCT) has many advantages in optimizing the artificial light source to adapt to the human physiological cycle. The research on the change trend of CCT and the law of extending the change range of CCT will help to further improve the performance of this kind of device. The present work fabricated a series of CT-WOLED devices with a simple structure, which are all composed of two ultra-thin phosphor layers (PO-01 and Flrpic) and a spacer interlayer. The yellow interface exciplex (TCTA/PO-T2T) formed between the spacer layer (PO-T2T) and transmission material (TCTA) in EML will decrease the CCT value at low voltage. The relationship between the energy transfer in EML and CCT change trend is investigated by adjusting the interface exciplexes and the thickness of the interlayer or the phosphor layer in devices A, B and C, respectively. The results demonstrate that a simple OLED device with an interlayer inserted between two ultra-thin phosphor layers can achieve a wider CCT span from 3359 K to 6451 K at voltage increases from 2.75 V to 8.25 V. .

Exciplex formation and electroluminescent absorption in ultraviolet organic light-emitting diodes

We investigated the formation of exciplex and electroluminescent absorption in ultraviolet organic light-emitting diodes（UV OLEDs） using different heterojunction structures.It is found that an energy barrier of over 0.3 eV between the emissive layer（EML） and adjacent transport layer facilitates exciplex formation.The electron blocking layer effectively confines electrons in the EML,which contributes to pure UV emission and enhances efficiency.The change in EML thickness generates tunable UV emission from 376 nm to 406 nm.In addition,the UV emission excites low-energy organic function layers and produces photoluminescent emission.In UV OLED,avoiding the exciplex formation and averting light absorption can effectively improve the purity and efficiency.A maximum external quantum efficiency of 1.2%with a UV emission peak of 376 nm is realized.

Exciplex elimination in an organic light-emitting diode based on a fluorene derivative by inserting 4,4'-N,N'-dicarbazole-biphenyl into donor/acceptor interface

Organic light-emitting diodes （OLEDs） composed of a novel fluorene derivative of 2,3-bis（9,9-dihexyl-9H-fluoren- 2-yl）-6,7-difluoroquinoxaline （F2Py） were fabricated, and exciplex emission was observed in the device. To depress the exciplex in an OLED for pure colour light emission, 4, 4＇-N,N＇-dicarbazole-biphenyl （CBP） was inserted as a separator at the donor/acceptor interface. It was found that the device without the CBP layer emitted a green light peaking at 542 nm from the exciplex and a shoulder peak about 430 nm from F2Py. In contrast, the OLED with CBP layer emitted only a blue light peak at about 432 nm from F2Py. Device efficiencies were calculated by a simulative mode in an injection controlled type mechanism, and the results showed that exciplexes yield much lower quantum efficiency than excitons. The device with CBP has a higher power efficiency as no exciplex was present.

Blue Emission from Exciplex in Electroluminescent Devices Made from Vinyl Polymer and Oxadiazole Derivative

Two types of molecular and polymer devices employing (2-(4′-biphenyl)-5-(4″-tert-butylphenyl)-1,3,4-oxadiazole) (PBD) and poly(N-vinylcarbazole)(PVK) as electron transport and hole transport materials, respectively, have been fabricated. The structures of these two devices are ITO/PVK(70 nm)/PBD(60 nm)/Al and ITO/PVK∶PBD(1∶1 by mass, 70 nm)/Al. The formation of exciplex is evident by comparing the electroluminescence (EL) of a bi-layer device and the photoluminescence (PL) of a PVK and PBD mixed film. The maximum emission energy of exciplex evaluated is consistent with the peak emission of EL. Blue emissions from both devices are obtained. The type of exicplex is interpreted based on the energy level diagram of the bi-layer device.

EXCIPLEX FORMATION OF A POLYESTER WITH A TEREPHTHALATE MAIN CHAIN HAVING A PENDANT N, N-DIMETHYLAMINO GROUP

The fluorescence behaviours of a new polymer poly [oxy-2 (4-N, N-dimethylaminobenzyl) propane-1 , 3-diyloxy-terephthaloyl] (Ⅰ) in solution were studied. Inter-and intra -molecular exciplex is formed between the singlet excited state of N, N- dimethylanilino group (abbreviated to DMA) and the ground state of terephthalate group (abbreviated to TP). The intensity ratio of the long wavelength exciplex fluorescence to the short wavelength emission of DMA group is used as an index for the inter-and intra-molecular interaction of chromophores. The results are compared with model polymer polyoxy-2-(4-N ,N-dimethylaminobenzyl) propanc-1 , 3-diyloxyadipoyl (Ⅱ), model monomer compound 4-N, N-dimethylaminobenzylmethyl terephthalate(Ⅲ) and diethyl 2-( 4-N, N-dimethylaminobenzyl) malonate (Ⅳ). Polymer association derived by electron donor and acceptor interaction (EDA) is further verified as a key role in the interpolymer exciplex formation . The fluorescence decay time of (Ⅰ) and monomer (Ⅲ) are measured in solutions.

Exciplex formation and properties at organic solid interface in organic light emitting devices

The bilayer organic light emitting devices （OLEDs） using two common aromatic diamines as hole transporting materials （HTMs） and BBOT （2,5-bis（5-tert-butyl-2-benzoxazolyl）thiophene） as electron transporting material have been prepared, in which the electroluminescent spectra are different from the fluorescent spectra of each of the constituent materials. The electroluminescence is mainly attributed to exciplex confirmed by photoluminescence and electroluminescence measurements, and the type of exicplex is deternfined in terms of the energy level diagram of the bilayer devices, By comparing the molecular structures and energy levels of TPD and NPB, it is demonstrated that the structure of a molecule as well as its energy level has an effect on the exciplex formation.

LASER FLASH SPECTROSCOPY STUDIES OF THE ANTHRACENEN,N-DIETHYLANILINE EXCIPLEX FORMATION IN γ-CYCLODEXTRIN

Gamma-cyclodextrin (γ-CD) was used to induce the formation of N,N-diethylaniline (DEA) exciplex in 15:85{φ=0.15)v/v EtOH-H_2O binary mixture.Measurements of the fluorescence spectra of anthracene in the presence or absence of DEA and γ-CD show that γ-CD can form 1:1:1 host/guest-Ⅰ/guest-Ⅱ inclusion complex with anthracene and DEA.

Achieving Seconds-to-Hours Duration-Tunable Organic Long Persistent Luminescence from Carbon Dots-Based Exciplex Systems by Energy Gaps Regulation

Comprehensive Summary Duration-tunable afterglow materials have garnered considerable attention in various applications.Herein,carbon dots(CDs)-based long persistent luminescence(LPL)composites with a tunable duration in an ultrawide range of seconds-to-hours levels were designed and prepared for the first time.In contrast to the established CD-based afterglow materials,we reported that CD-based composites exhibit LPL in the form of exciplexes and long-lived charge-separated states,enabling the LPL to be prolonged from several seconds to over one hour,exceeding the typical regulation range(limited to 1 min).Further studies revealed that the relationship between the excited and charge-transfer states of CDs plays a pivotal role in activating the LPL and regulating its duration.Furthermore,these composites exhibited high photoluminescence(PL)quantum yields of up to 60.63%,and their LPL was robust under ambient conditions,even in aqueous media.Their robust and superior LPL performance endows these composites with a strong competitive advantage in dynamic display systems,such as tags for time-resolved data encryption and displays of the remaining time of takeaways.This study offers an approach to preparing CDs-based LPL composites with tunable durations and may provide new insights for the development of rare-earth-free LPL materials.

Synthesis and Exciplex Emission of a Tricyano-substituted 9-Phenylfluorene Compound

Exciplex-emitters are attractive due to their distinct feature of easily achieving small singlet-triplet energy splitting for thermally activated delayed fluorescence(TADF),but face with challenges in convenient design of electron acceptors.New 9-phenylfluorene-based acceptor 9-[6-(2-cyano-7-isocyano-9-phenyl-9H-fluoren-9-yl)-9-ethyl-9H-carbazol-3-yl]-9-phenyl-9H-fluorene-2,7-dicarbonitrile(TriCNDPFCz)was designed and synthesized through tricyano-substitution.By mixing TriCNDPFCz with conventional donor 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane(TAPC),bright exciplex-TADF was obtained with a maximum current efficiency of 46.7 cd·A^(-1),power efficiency of 52.3 lm·W^(-1)and external quantum efficiency of 14.5%.The experimental characterizations indicate that tricyano-substitution in 9-phenylfluorene skeleton can form strong electron-accepting ability,which is useful and convenient for constructing electron acceptor in exciplex emission.

Blue emitting exciplex for yellow and white organic light‑emitting diodes

White organic light-emitting diodes(WOLEDs)have several desirable features,but their commercialization is hindered by the poor stability of blue light emitters and high production costs due to complicated device structures.Herein,we investigate a standard blue emitting hole transporting material(HTM)N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)benzidine(NPB)and its exciplex emission upon combining with a suitable electron transporting material(ETM),3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole(TAZ).Blue and yellow OLEDs with simple device structures are developed by using a blend layer,NPB:TAZ,as a blue emitter as well as a host for yellow phosphorescent dopant iridium(III)bis(4-phenylthieno[3,2-c]pyridinato-N,C2')acetylacetonate(PO-01).Strategic device design then exploits the ambipolar charge transport properties of tetracene as a spacer layer to connect these blue and yellow emitting units.The tetracene-linked device demonstrates more promising results compared to those using a conventional charge generation layer(CGL).Judicious choice of the spacer prevents exciton difusion from the blue emitter unit,yet facilitates charge carrier transport to the yellow emitter unit to enable additional exciplex formation.This complementary behavior of the spacer improves the blue emission properties concomitantly yielding reasonable yellow emission.The overall white light emission properties are enhanced,achieving CIE coordinates(0.36,0.39)and color temperature(4643 K)similar to daylight.Employing intermolecular exciplex emission in OLEDs simplifes the device architecture via its dual functionality as a host and as an emitter.

Luminescent gold(Ⅲ)exciplexes enable efficient multicolor electroluminescence

The control of excited states and related emissive properties of gold(Ⅲ)complexes mainly depends on the modulation of intramolecular electronic interactions among gold(Ⅲ)metal center,chelating ligands and/or peripheral groups.However,luminescent gold(Ⅲ)systems based on intermolecular electronic interactions have never been explored.Here we report a series of proof-of-concept gold(Ⅲ)exciplexes using a simple gold(Ⅲ)complex,AuDPPy,as an electron acceptor.The emissive properties of gold(Ⅲ)exciplexes can be regulated by combining AuDPPy with different donors.Inspiringly,these gold(Ⅲ)exciplexes have donor-dependent emission mechanisms:dominant phosphorescence or dual radiative channels of thermally activated delayed fluorescence(TADF)and phosphorescence.Consequently,these gold(Ⅲ)exciplexes deliver green-to-red electroluminescence with external quantum efficiencies(EQEs)of up to 10.1%.More importantly,using these gold(Ⅲ)exciplexes to host multi-resonance TADF emitters results in narrowband yellow,orange,and deep-red electroluminescence with high EQEs of 23.5%,24.4%,and 27.4%,respectively,competitive to the highest values for gold(Ⅲ)OLEDs in similar color gamut.

High efficiency yellow fluorescent organic light emitting diodes based on m-MTDATA/BPhen exciplex

High efficient yellow organic light emitting diodes （OLEDs） based on exciplex were reported. The exciplex was formed by 4, 4＇, 4″-tris β-methylphenyl （phenyi） amino]-triphenylamine （m-MTDATA） and 4, 7- diphenyl-1, 10-phenanthroline （BPhen）. The resulting yellow OLEDs exhibited an external quantum efficiency of over 7%, which is attributed to the effective energy back transfer from exciplex triplet state to exciplex singlet state. The maximum power efficiency of 25 lm/W was achieved. Doping a yellow phosphor Ir（bt）2（acac） into m-MTDATA： BPhen blend, a high efficiency device was achieved with a turn-on voltage of 2.1 V, maximum power efficiency and external quantum efficiency of 86.1 lm/W and 20.7%, respectively.

Efficient red phosphorescent OLEDs based on the energy transfer from interface exciplex: the critical role of constituting molecules

A novel acceptor material,9-(4′-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1′-biphenyl]-3-yl)-9H-carbazole(o-DTPPC)was developed to form interface exciplex with commonly used donors,to maximize the performances of red phosphorescent organic light emitting diodes(PHOLEDs).It is found that the exciplex involving 4,4′-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline)(TAPC)exhibits the most significant thermally activated delayed fluorescence(TADF)property,derived from the high triplet energy level as well as strong hole-transporting ability of TAPC.Intriguingly,it is the same donor-acceptor combination which achieved the highest device efficiency when adopted as the host for red PHOLEDs.Maximum efficiencies as high as31.36 cd A^(-1),17.95 lm W^(-1),and 21.01%for the current efficiency,power efficiency and external quantum efficiency,respectively with low efficiency roll-off were realized.The improved performance can be attributed to the efficient TADF properties of the interface exciplex-forming host constituting TAPC,benefiting the F?rster energy transfer.The article first underlines the importance of the constituting molecules in the interface exciplex-forming hosts,shedding new insight about the choice of interface exciplex as the host for PHOLEDs,which may lead to even higher performances,paving their ways towards practical applications.

Novel donor-spacer-acceptor compound as the multifunctional component of exciplexes for efficient organic light-emitting diodes

By connecting tercarbazole(3 Cz)and triphenyltriazine(TRz)units with a diphenyl ether group,we designed and synthesized a new donor-spacer-acceptor structure molecule9’-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenoxy)phenyl)-9’H-9,3’:6’,9"-tercarbazole(3 Cz-o-TRz)as the multifunctional component of exciplex emitters.As expected,the intermolecular charge-transfer transition is dominant for3 Cz-o-TRz because of the space-enough and conjugation-forbidden linkage of the diphenyl ether group,increasing the intrinsic characteristics of the 3 Cz and TRz moieties in its single-molecule state.Accordingly,three common electron acceptors(4,6-bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine(B3 PyMPm),4,6-bis(3,5-di(pyridin-4-yl)phenyl)-2-methylpyrimidine(B4 PyMPm),and(1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl)tris(diphenylphosphineoxide)(POT2 T))and three common electron donors(4,4’-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline)(TAPC),tris(4-(10 H-phenoxazin-10-yl)phenyl)amine(TPA-PXZ),and N1-phenyl-N4,N4-bis(4-(phenyl(m-tolyl)amino)phenyl)-N1-(m-tolyl)benzene-1,4-diamine(m-MTDATA))were chosen to construct six exciplex emitters with 3 Cz-o-TRz.In the organic light-emitting diodes(OLEDs),the emission spectra of these exciplex emitters could be gradually modulated from 510 to 590 nm.In addition,TAPC:3 Cz-o-TRz-and 3 Cz-o-TRz:PO-T2 T-based OLEDs achieved desirable performance with maximum external quantum efficiencies(EQEs)around 12%.Moreover,a simple tandem OLED containing TAPC:3 Cz-o-TRz and 3 Czo-TRz:PO-T2 T emitters realized optimal performance with an ultralow turn-on voltage of 2.4 V and a maximum EQE of14.1%.These results indicate the great potential of the donorspacer-acceptor structure compounds in developing exciplex emitters.

基于激基复合物主体的高效杂化白光有机发光二极管

在有机发光二极管(OLEDs)中,发光层中载流子的平衡以及激子分布区域的拓宽对于提高器件效率是至关重要的。本研究采用SiCzCz∶SiTrzCz2激基复合物(Exciplex)主体设计制备了基于热激活延迟荧光(TADF)材料DMAC-DPS和TDBA-SAF的低效率滚降蓝光发光器件,并以相同Exciplex为主体材料,通过引入红光磷光材料RD071和绿光磷光材料Ir(ppy)2(acac)构建了同一主体的三层发光结构,实现了高显色指数杂化白光OLEDs的设计。SiCzCz∶SiTrzCz2激基复合物作为主体材料不仅拓宽了激子分布区域,平衡了发光层中载流子传输,并通过与TADF/磷光发光材料三线态能级匹配,构建了级联式激子能量传递途径,从而有效地提高了激子利用率、降低了器件的效率滚降。通过优化器件结构,白光OLEDs实现了最高外量子效率(EQE)、电流效率(CE)、功率效率(PE)分别为23.0%、45.9 cd·A^(-1)和33.9 lm·W^(-1),器件具有较高的显色指数(CRI=87)、良好的光谱稳定性和低的效率滚降。本研究为高显色指数、低效率滚降的杂化TADF/磷光白光器件提供了一种新的设计方案。

TIME-RESOLVED FLUORESCENCE STUDY OF TRIPLE EXCIPLEX FORMED IN THE SYSTEM OF POLY(2-VINYL) NAPHTHALENE AND TEREPHTHALIC DIMETHYLESTER

An intramolecular excimer of poly(2- vinyl) naphthalene was formed by non-adjacentchromophores interaction, then a triple exciplex was formed by interaction with the acceptormolecule further in dilute solution. The lifetime of the intramolecular excimer of poly(2-vinyl) naphthalene, τ_2 = 18.83 ns and the rate constant for the triple exciplex formation,k_7 = 4. 18× 10~9 (mol/L)^(-1)s^(-1), under diffusion-control were measured. An excimer or anexciplex could be an intermediate of the triple exciplex formation. A theoretical model ofthe triple exciplex formation is proposed.

STEADY-STATE AND TIME-RESOLVED FLUORESCENCE STUDY OF TRIPLE EXCIPLEX IN THE SYSTEM OF POLYACENAPHTHALENE AND TEREPHTHALIC DIMETHYLESTER

Triple exciplex formed between polyacenaphthalene and terephthalic dimethylester (TDE) has been studied by means of the steady-state and time-resolved fluorescence spectra. The theoretical model of the triple exciplex formation for a flexible polymer chain in a dilute solution has been proposed. The fluorescence decays of the monomer, the exciplex and the triple exciplex obey a double exponential rule in the pbotophysical processes of the triple exciplex formation. The association rate constant of the exciplex formation is k_3=6.0×10~9s^(-1)(mol/L)^(-1); the association rate constant of the triple exciplex formation is k_6=5.2×10~7s^(-1). A mechanism for the triple exciplex formation from the exciplex to the triple exciplex has been proved through the noncrystal films corresponding to the same concentrations of the solution systems and the fluorescence lifetimes of the intramolecular excimer.

EXCIPLEX FORMATION INFLUENCED BY ISOMERS——EXCIPLEX FORMED FROM DIMETHYL TEREPHTHALATE AND PHTHALATE WITH 9-ETHYL CARBAZOLE

Both dimethyl terephthalate (DMTP) and dimethyl phthalate are able to form exciplex with 9-ethyl carbazole (EtCz) but the fluorescence quenching constant of DMTP to EtCz is larger than that of DMP to EtCz. The peak positions of the fluorescence bands of the exciplex formed from DMTP and DMP with EtCz are also different. The former gives a peak at shorter wavelength region. Either the exciplex formation rate constant k_3 or the exciplex dissociation rate constant k_4 is not alike. Both of these constants of DMTP are larger than those of DMP. All these results are difficult to explain with the established theory, so a new model which could be used to explain the results satisfactorily is supposed.

Effects of Self-coiling of Organic Molecules on Intramolecular Exciplex Formation and Fluorescence Quenching in DX-H_2O Solvent System

Effects of self-coiling of organic molecules on intramolecular exciplexformation of compound Ⅰ, in which the carbazole chromophore and terephthalic acid methylesteracceptor group are linked by one (CH_2)_(10) chain, and the decrease of the fluorescence intensitiesof compounds Ⅱ, Ⅲ, and Ⅳ, in which the carbazole chromophore and 3, 5-dinitrobenzoate areconnected by one aliphatic chain of (CH_2)_(10) (Ⅱ), (CH_2)_(12) (Ⅲ), or (CH_2)_4 (Ⅳ), have beenstudied in the dioxane(DX)-H_2O binary system. The results show that self-coiling of organicmolecules in DX-H_2O facilitates intramolecular exciplex formation of Ⅰ and induces the decrease offluorescence intensities of Ⅱ, because of the proximity effect brought about by self-coiling oforganic molecules under hydrophobic-lipophilic interaction (HLI) between the excited carbazolechromophore and the acceptor. Since the similar effects are observed even when the concentrations ofthe probes are less than their CAgCs (critical aggregate concentrations) in the DX-H_2O mixturewith the same φ values, formation of the intermolecular exciplex has been excluded. The effects arefound to be strongly depended on φ values, indicating that they are mainly driven by HLI. Theproperties of the acceptors can also affect the intramolecular exciplex formation. With terephthalicacid methylester moiety as the acceptor, the carbazole chromophore exhibits the fluorescencespectra of the exciplex, while with 3, 5-dinitrobenzoate moiety as the acceptor, only thefluorescence spectra of excited carbazolyl chromophore are observed.

Photoaddition of chloroform to ketones via a singlet exciplex: A photo-CIDNP study

Photolysis of 4-acetyl-4-ethoxyformylcyclohexanone, 4-acetyl-4-phenylcyclohexanone and 1-acetyl-1-phenylcyclohexane in deuteriochloroform using a high pressure Hg-Xe lamp showed unusual polarizations. The results are explained by the formation of a singlet exciplex of the ketone with deuteriochloroform and the photoaddition of deuteriochloroform to the ketone.