Triplet Exciplex Research Articles

The Importance of Excited‐State Energy Alignment for Efficient Exciplex Systems Based on a Study of Phenylpyridinato Boron Derivatives

AbstractUnderstanding excited‐state dynamics is critical for improving the photoluminescence (PL) efficiency of exciplexes. A series of exciplexes based on conventional hole‐transporting materials as donor and newly developed phenylpyridinato boron derivatives as acceptor were investigated. High PL efficiencies were achieved in only some combinations, and a large difference in performance among combinations provided insight into nonradiative processes in exciplex systems. Furthermore, the triplet local excited states (3LE) of each donor and acceptor were found play an important role in triplet exciplex harvesting. Significant contributions from triplets were clearly observed when the charge‐transfer excited states (1CT and 3CT) and 3LE were ideally aligned. We also demonstrated fine control of relative energy alignment via the concentration to improve the PL efficiency.

Combined Inter- and Intramolecular Charge-Transfer Processes for Highly Efficient Fluorescent Organic Light-Emitting Diodes with Reduced Triplet Exciton Quenching.

Inter- and intramolecular charge-transfer processes are combined using an exciplex-forming host and a thermally activated delayed fluorescent dopant, for fabricating efficient fluorescent organic light-emitting diodes along with the reduced efficiency roll-off at high current densities. Extra conversion on the host from triplet exciplexes to singlet exciplexes followed by energy transfer to the dopant reduces population of triplet excitons on dopant molecules, thereby reducing the triplet exciton annihilations at high current densities.

Interaction of Triplet Excited States of Ketones with Nucleophilic Groups: (π,π*) and (n,π*) versus (σ*,π*) States. Substituent-Induced State Switching in Triplet Ketones

The intramolecular interaction of ketone triplet excited states with nucleophilic substituents is investigated by studying the electronic properties of phenalenone and a range of phenalenones functionalized in position 9 as a model system. In accordance with the literature, a (π,π*) triplet excited state is predicted for phenalenone. Similarly, 9-fluoro-, 9-chloro-, and 9-methoxyphenalenone are calculated to have (π,π*) lowest triplet excited states, whereas the lowest triplet states of 9-bromo-, 9-iodo, 9-methylthio, and 9-dimethylaminophenalenone are predicted to have (σ*,π*) character. As a result of the interaction between halogen and oxygen lone pairs increasing with increasing orbital size, the antibonding linear combination of substituent lone pairs with oxygen lone pairs sufficiently rises in energy to change the character of the lowest triplet excited state of the 9-substituted phenalenones from (π,π*) to (σ*,π*). These unusual triplet excited states or exciplexes should essentially behave like (n,π*) triplets states, but will differ from pure (n,π*) states by showing significant spin densities at the substituent heteroatoms, predicted to reach values of 0.25 for 9-iodophenalenone, and ~0.5 for 9-dimethylaminophenalenone. Vertical T1–T2 excitation energies calculated indicate that the stabilization of the (σ*,π*) relative to the (π,π*) state can reach 1 eV. Preliminary calculations on the triplet excited states of 2-iodobenzophenone, 4-iodo-2-butanone, and iodoacetone indicate that intramolecular triplet exciplex formation should be a general phenomenon, as long as the ring being formed is at least a five-membered ring.

Triplet exciplex electroluminescence from two columnar liquid crystal perylene derivatives

Double layers and mixed single layers of two electronically complementary columnar liquid crystals show enhanced performances in light emitting diodes compared to devices made from single layers of only one of the materials. A shift from green to red electroluminescence is observed between single-material and two-material devices, assigned as resulting from triplet exciplexes in the electroluminescence of the two-material films, whereas singlet exciplex states contribute to the photoluminescence of the mixture and double layer films.

Boosting Triplet Harvest by Reducing Nonradiative Transition of Exciplex toward Fluorescent Organic Light-Emitting Diodes with 100% Internal Quantum Efficiency

The external quantum efficiencies (EQEs) of the fluorescent organic light-emitting diodes (OLEDs) using exciplex as emitters are much lower than fluorescent OLEDs based on single-molecular thermally activated delayed fluorescent (TADF) emitters, even though exciplex can, in principle, realize 100% internal quantum efficiency (IQE). In addition, there are no clear guidelines to improve the efficiency in exciplex-based fluorescent OLEDs. Here, we quantitatively analyze the exciton dynamics influencing IQE of exciplex and demonstrate a fluorescent OLED with IQE of 100% and EQE of 25.2% at 150 K, which are improved from IQE of 48.3% and EQE of 11.0% at room temperature by reducing nonradiative transition of exciplex. This study shows that a high reverse intersystem crossing rate does not necessarily lead to efficient harvesting of triplet exciplex if it occurs with a high nonradiative transition rate (knr). We harvested more triplet exciplex by adopting exciplex with low knr and further suppressed the knr by freezing the exciplex to achieve IQE of 100%.

Immense Magnetic Response of Exciplex Light Emission due to Correlated Spin-Charge Dynamics

As carriers slowly move through a disordered energy landscape in organic semiconductors, tiny spatial variations in spin dynamics relieve spin blocking at transport bottlenecks or in the electron-hole recombination process that produces light. Large room-temperature magnetic-field effects (MFE) ensue in the conductivity and luminescence. Sources of variable spin dynamics generate much larger MFE if their spatial structure is correlated on the nanoscale with the energetic sites governing conductivity or luminescence such as in co-evaporated organic blends within which the electron resides on one molecule and the hole on the other (an exciplex). Here we show that exciplex recombination in blends exhibiting thermally-activated delayed fluorescence (TADF) produces MFE in excess of 60% at room temperature. In addition, effects greater than 4000% can be achieved by tuning the device's current-voltage response curve by device conditioning. These immense MFEs are both the largest reported values for their device type at room temperature. Our theory traces this MFE and its unusual temperature dependence to changes in spin mixing between triplet exciplexes and light-emitting singlet exciplexes. In contrast, spin mixing of excitons is energetically suppressed, and thus spin mixing produces comparatively weaker MFE in materials emitting light from excitons by affecting the precursor pairs. Demonstration of immense MFE in common organic blends provides a flexible and inexpensive pathway towards magnetic functionality and field sensitivity in current organic devices without patterning the constituent materials on the nanoscale. Magnetic fields increase the power efficiency of unconditioned devices by 30% at room temperature, also showing that magnetic fields may increase the efficiency of the TADF process.

Triplet energy management between two signaling units through cooperative rigid scaffolds.

Through-bond triplet exciplex formation in donor-acceptor systems linked through a rigid bile acid scaffold has been demonstrated on the basis of kinetic evidence upon population of the triplet acceptors (naphthalene, or biphenyl) by through-bond triplet-triplet energy transfer from benzophenone.

Large magneto-conductance and magneto-electroluminescence in exciplex-based organic light-emitting diodes at room temperature

In this work, we report on large magneto-conductance (MC) over 60% and magneto-electroluminescence (MEL) as high as 112% at room temperature in an exciplex-based organic light-emitting diode (OLED) with efficient reverse intersystem crossing (ISC). The large MC and MEL are individually confirmed by the current density-voltage characteristics and the electroluminescence spectra under various magnetic fields. We proposed that this type of magnetic field effect (MFE) is governed by the field-modulated reverse ISC between the singlet and triplet exciplex. The temperature-dependent MFEs reveal that the small activation energy of reverse ISC accounts for the large MFEs in the present exciplex-based OLEDs.

Triplet Harvesting: Triplet Harvesting by a Conventional Fluorescent Emitter Using Reverse Intersystem Crossing of Host Triplet Exciplex (Advanced Optical Materials 7/2015)

Triplet Harvesting: Triplet Harvesting by a Conventional Fluorescent Emitter Using Reverse Intersystem Crossing of Host Triplet Exciplex (Advanced Optical Materials 7/2015)

Simple structured hybrid WOLEDs based on incomplete energy transfer mechanism: from blue exciplex to orange dopant.

Exciplex is well known as a charge transfer state formed between electron-donating and electron-accepting molecules. However, exciplex based organic light emitting diodes (OLED) often performed low efficiencies relative to pure phosphorescent OLED and could hardly be used to construct white OLED (WOLED). In this work, a new mechanism is developed to realize efficient WOLED with extremely simple structure by redistributing the energy of triplet exciplex to both singlet exciplex and the orange dopant. The micro process of energy transfer could be directly examined by detailed photoluminescence decay measurement and time resolved photoluminescence analysis. This strategy overcomes the low reverse intersystem crossing efficiency of blue exciplex and complicated device structure of traditional WOLED, enables us to achieve efficient hybrid WOLEDs. Based on this mechanism, we have successfully constructed both exciplex-fluorescence and exciplex-phosphorescence hybrid WOLEDs with remarkable efficiencies.

Triplet Harvesting by a Conventional Fluorescent Emitter Using Reverse Intersystem Crossing of Host Triplet Exciplex

A red fluorescent OLED with an unprecedented external quantum efficiency of 10.6% is created using an exciplex forming system as a host for a conventional fluorescent dopant not showing delayed fluorescence. Quantitative analysis indicates that over 35% of the total excitons are converted to emission light, clearly indicating the triplet harvesting in the system.

Mixing of phosphorescent and exciplex emission in efficient organic electroluminescent devices.

We fabricated a yellow organic light-emitting diode (OLED) based on the star-shaped donor compound tri(9-hexylcarbazol-3-yl)amine, which provides formation of the interface exciplexes with the iridium(III) bis[4,6-difluorophenyl]-pyridinato-N,C2']picolinate (FIrpic). The exciplex emission is characterized by a broad band and provides a condition to realize the highly effective white OLED. It consists of a combination of the blue phosphorescent emission from the FIrpic complex and a broad efficient delayed fluorescence induced by thermal activation with additional direct phosphorescence from the triplet exciplex formed at the interface. The fabricated exciplex-type device exhibits a high brightness of 38 000 cd/m(2) and a high external quantum efficiency.

Synthesis and characterization of heterofluorenes containing four-coordinated group 13 elements: theoretical and experimental analyses and comparison of structures, optical properties and electronic states.

Herein, we report the syntheses of dibenzoheteroles, namely, heterofluorenes, containing four-coordinated group 13 elements (boron Bf; aluminum Alf; gallium Gaf; indium Inf) and the relationship between their structures and optical properties. The electronic states of the compounds were considered theoretically by the density functional theory (DFT) calculation. In particular, we focused on their emission behaviors and electronic structures in the excited states. Initially, we confirmed that Bf and Gaf showed high stability in water and air, while Alf and Inf were sensitive. The structures of heterofluorenes, involving the heavier elements in the 13(th) group, tend to form trigonal planar structures even in the presence of coordination by nitrogen. Next, in their emissions, larger contribution from the triplet excited states was observed in the heterofluorenes with heavier elements. The major emission of Inf at 77 K was attributed to phosphorescence. These phosphorescence properties can be explained by the heavy atom effect. In Gaf and Inf, their excited states were deactivated by vibrational relaxation in their triplet excited states at room temperature. In Bf, Alf and Gaf when adding B(C6F5)3, the emissions oriented from the triplet exciplex were observed. Time-dependent DFT (TD-DFT) calculations revealed that the optimized structure of Bf in the excited S1 state has a considerably different geometry from those of Alf and Gaf. Finally, we obtained the data that the B-N bond could be cleaved in the excited S1 of Bf according to the B-N bond length and bond order. As a result, the lower intensity of the emission of Bf was comparable to that of Alf. This bond cleavage could be caused by an increase of the anti-bonding property in the B-N bond in the Franck-Condon (FC) S1 state and by weak electrostatic interaction between boron and nitrogen atoms. In Alf and Gaf, although the anti-bonding character of the M-N bonds (M = Al or Ga) in the FC S1 states also increases, the M-N bonds survive because of their stronger electrostatic interaction. The subsequent stronger emission in Alf and Gaf could be observed by suppressing the molecular motion in the excited states.

Efficient “Warm-White” OLEDs Based on the Phosphorescent bis-Cyclometalated iridium(III) Complex

The starburst carbazole derivative and phosphorescent bis-cyclometalated iridium(III) complex (IC2) were used for the preparation of multilayered “warm-white” organic light-emitting diodes (OLEDs), the emission spectra of which are modulated by the thickness of the phosphorescent layer. It was shown that the electroluminescence spectra of the fabricated devices are more extended into the visible region compared with the photoluminescence spectra of both component materials. The observed extension of the electroluminescence spectra can be assigned to the phosphorescent emission of the low-energy exciplex formed at the interface of the emissive layers. The quantum-chemical calculations performed by the DFT and (TD) DFT methods support the formation of the low-energy triplet exciplex at the interface of the IC2 layer and the neighboring layer of the starshaped carbazole-based compound, (4,4′,4″-tris[3-methylphenyl(phenyl)amino] triphenylamine, tri(9-hexylcarbazol-3-yl)amine (THCA). Indeed, the triplet excite...

A Mechanistic Study of Photoinduced Electron Transfer from Triplet Erythrosin to Various Quinones Using Time Resolved Absorption and ESR-CIDEP Measurements

Abstract Photoinduced electron transfer reactions from the triplet erythrosin (Ery2−) dianion to various quinones (1,4-benzoquinone, 2,5-di-t-butyl-1,4-benzoquinone, duroquinone, 2,5-dichloro-1,4-benzoquinone, chloranil, bromanil, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and 9,10-anthraquinone) and other organic acceptors like 7,7,8,8-tetracyanoquinodimethane, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane and with tetracyano-1,4-benzoquinone (cyanil), one of the strongest oxidizing agent reported in literature (E ○ = 0.90 V vs. SCE), were studied by laser flash photolysis in acetonitrile/water mixtures at room temperature. Quenching rate constants are obtained from Stern-Volmer plots. The measured bimolecular quenching rate constants are close to the diffusion controlled rates. Excitation of a contact radical-pair or a triplet exciplex as an intermediate in the photoinduced electron transfer reaction in a slightly polar solvent (1-propanol) is confirmed by the observation of the net absorptive chemically induced dynamic electron polarization (CIDEP) spectra. The unusual net–absorptive CIDEP spectrum is explained by spin-orbit coupling interactions due to presence of four iodine atoms in the structure of erythrosin (heavy atom effect). The dependence of the electron transfer rate constants, k et, on the driving force, ΔG et is slightly parabolic and indicates the Marcus Inverted Region.

Efficient triplet harvesting by fluorescent molecules through exciplexes for high efficiency organic light-emitting diodes

Efficient triplet harvesting from exciplexes by reverse intersystem crossing (RISC) is reported using a fluorescent molecular system composed of the 4,4′,4″-tris(N-carbazolyl)-triphenylamine and bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine. The exciplex forming material system shows the efficient delayed fluorescence emission. As a result, almost 100% PL efficiency at 35 K and 10% external quantum efficiency at 195 K are achieved from the exciplex. The delayed fluorescence of the exciplex clearly demonstrates that a significant proportion of the triplet exciplexes is harvested through the RISC.

Synthesis and Optical Properties of Stable Gallafluorene Derivatives: Investigation of Their Emission via Triplet States

We designed and synthesized air- and moisture-stable gallafluorenes in which two benzene rings were bridged by the four-coordinate gallium atoms. The series of gallafluorenes were prepared by introducing electron-donating and -withdrawing groups through Suzuki-Miyaura coupling reactions. The gallafluorenes showed unique emissions via their triplet states in the presence of B(C6F5)3. These emissions were obtained via the triplet exciplex of gallafluorene and B(C6F5)3.

A host material with a small singlet–triplet exchange energy for phosphorescent organic light-emitting diodes: Guest, host, and exciplex emission

A host material containing a triazine core and three phenylcarbazole arms, called 2,4,6-tris(3-(carbazol-9-yl)phenyl)-triazine (TCPZ), was developed for phosphorescent organic light-emitting diodes (OLEDs). Ultra-low driving voltages were achieved by utilizing TCPZ as the host due to its decreased singlet–triplet exchange energy (ΔEST) and low-lying lowest unoccupied molecular orbital (LUMO) energy level. Interaction between the RGB triplet emitters and TCPZ were studied in both photoluminescent and electroluminescent processes. Transient photoluminescence (PL) measurement of the co-deposited film of fac-tris(2-phenylpyridine) iridium (Ir(PPy)3):TCPZ exhibits a shoulder at 565nm whose lifetime is about two times longer than that of the Ir(PPy)3 triplet excitons and can be attributed to the triplet exciplex formed between Ir(PPy)3 and TCPZ. Such exciplex was also found for the green phosphorescent OLED, giving the most efficient phosphorescent OLED with triplet exciplex emission hitherto. Different from the PL process, a broad featureless band with a maximum at 535nm was found for the OLED based on an EML of iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C2′)picolinate (FIrpic):TCPZ, which can be attributed to the emission from the singlet excited state of TCPZ formed by direct hole-electron recombination. A multi-emitting-layer white OLED was also fabricated by utilizing FIrpic and tris(1-phenylisoquinolinolato-C2,N)iridium(III) (Ir(piq)3) as the complementary triplet emitters and TCPZ as the host. Different from most of ever reported white OLEDs fabricated with blue/red complementary triplet emitters that exhibit color rendering index (CRI) lower than 70, a high CRI of 82 is achieved due to the combination of blue and red phosphorescence emissions from FIrpic and Ir(piq)3, and the emerging green fluorescence emission from TCPZ.

The nature of trapping sites and recombination centres in PVK and PVK–PBD electroluminescent matrices seen by spectrally resolved thermoluminescence

Two electroluminescent polymer matrices poly(N-vinylcarbazole) (PVK) and PVK with 40 wt% of 2-tert-butylphenyl-5-biphenyl–1,3,4-oxadiazole (PBD) were studied using spectrally resolved thermoluminescence (SRTL) in the temperature range 15–325 K. The comparison of the SRTL results with the electroluminescence (EL) spectra has allowed identification of the localized (trapping) sites and the radiative recombination centres present in the investigated matrices. In the neat PVK films deep traps with a depth about 200 meV, related to triplet excimers dominate, while in the PVK–PBD (40 wt%) blend films the traps that are related to triplet exciplexes formed by the carbazole groups and the PBD molecules dominate. Depth of the traps in the PVK–PBD blend is somewhat lower than that in the neat PVK. An analysis of the EL spectra shows that in the PVK and in the PVK–PBD blend the dominant radiative centres are singlet excimers and singlet exciplexes, respectively. However, in the neat PVK some contributions of the triplet monomer and the triplet excimer states in the EL were also detected.

White organic light-emitting diodes via mixing exciplex and electroplex emissions

The authors report the fabrication of white organic light-emitting devices and discuss their electroluminescence (EL) properties. The device structure is ITO/TPD (50 nm)/BCP (8 nm)/Rubrene (0.5 nm)/BCP (10 nm)/Alq 3 (20 nm)/LiF (1 nm)/Al. In the EL spectra of this device, two new emissions peaking at 590 and 630 nm have been observed. These two emissions should be attributed to triplet exciplex and electroplex occurring at TPD/BCP interface. White emission was obtained based on this device under 12 V driving voltage, the Commission Internationale de l’Eclairage (CIE) coordinates arrives to (0.31, 0.33).