Dexter Energy Transfer Research Articles

Evolution Between Exciton and Exciplex Emission in Planar Heterojunction OLEDs with Different Hole-Injection Characteristics

Planar heterojunction-based organic light-emitting diodes (PHJ OLEDs) usually have simultaneous exciton and exciplex emissions and understanding their evolution processes is crucial for designing high-performance white OLEDs, but relevant evolution mechanisms are still unclear. Here, unreported competitions between exciton and exciplex emissions and an undiscovered energy-transfer channel from triplet exciton to triplet exciplex states in PHJ OLEDs with different hole-injection abilities are investigated by separately measuring their current-dependent electroluminescence (EL) spectra and magneto-EL (MEL) traces at different temperatures. Interestingly, the ratio of emission intensity between exciton and exciplex states ($R={I}_{\mathrm{exciton}}/{I}_{\mathrm{exciplex}}$) from the device with a good hole-injection ability rises with increasing bias current at each temperature, whereas that from the device with a poor hole-injection ability hardly changes. In addition, R from the devices with good and poor hole-injection abilities show nonmonotonic variation and monotonic reduction with decreasing operational temperature at each bias current, respectively. These various current- and temperature-dependent competitions between exciton and exciplex emissions are attributed to distinct current- and temperature-dependent electron-tunneling effects occurring at organic heterojunction interfaces of devices with different hole-injection abilities. More intriguingly, using MEL as a fingerprint detection tool, we find there is a Dexter energy-transfer (DET) channel from the triplet exciton to triplet exciplex (${T}_{1}\ensuremath{\rightarrow}{\mathrm{EX}}_{3}$) state and the DET can enhance the reverse intersystem crossing (RISC) process from triplet to singlet exciplexes (${\mathrm{EX}}_{3}\ensuremath{\rightarrow}{\mathrm{EX}}_{1}$), which cannot be found using the conventional probing tool, EL spectra. Because the numbers of ${T}_{1}$ excitons in these devices have different current and temperature dependencies, various current- and temperature-dependent DET and RISC processes happen, which cause abundant MEL behaviors. Obviously, this work deepens the physical understanding of the competition and DET processes between exciton and exciplex states in PHJ OLEDs.

Stable Thermally Activated Delayed Fluorescence‐Sensitized Red Fluorescent Devices through Physical Suppression of Dexter Energy Transfer

AbstractTo date, thermally activated delayed fluorescence‐sensitized fluorescent organic light‐emitting diodes (TSF‐OLEDs) have undergone substantial research to achieve high efficiency and good operational stability in wide color gamut regions. Usually, to achieve a highly efficient TSF device, the Förster resonance energy transfer rate (kFRET) should be enhanced, whereas the Dexter energy transfer rate (kDET) should be suppressed. Even though highly efficient devices are achieved in all RGB color regions by satisfying the BT2020 requirements, achieving long device lifetimes is still challenging. Herein, a highly stable red‐TSF device is reported by adopting a new Dexter energy transfer suppressive layer (DSL) adjacent to the main emissive layer. Here, the DSL can improve the distance between the excitons generated from the host‐TADF layer and the final dopant (FD) of the TSF device, which allows for suppressing the kDET. Furthermore, the detailed device mechanistic pathways are analyzed by varying the DSL doping concentration with different thicknesses in different positions. Among the fabricated devices, the DSL‐TSF device manifested a longer operational lifetime (LT95) over 370 h at 5000 cd m‐2 and reduced efficiency roll‐off compared with TSF devices. Such long lifetime and high stability in DSL‐TSF OLEDs are owing to the decreased kDET than TSF devices.

Suppression of Dexter Energy Transfer through Modulating Donor Segments of Thermally Activated Delayed Fluorescence Assistant Dopants.

Device degradation in red hyperfluorescent organic light-emitting diodes is primarily caused by exciton energy loss due to Dexter energy transfer (DET) from a thermally activated delayed fluorescence (TADF) assistant dopant to a fluorescent dopant. In this work, the donor segments in the TADF assistant dopants were delicately modulated to suppress DET for high efficiency. The derived benzothienocarbazole donors were introduced to the TADF assistant dopants instead of carbazole, and they accelerated the reverse intersystem crossing of the TADF assistant dopant and managed the DET from the TADF assistant dopant to the fluorescent dopant. As a result, the red TADF-assisted device showed a high external quantum efficiency of 14.7% and improved the device lifetime by 70% compared to a well-known TADF-assisted device.

Tracking chromatin state changes using nanoscale photo-proximity labelling.

Interactions between biomolecules underlie all cellular processes and ultimately control cell fate. Perturbation of native interactions through mutation, changes in expression levels or external stimuli leads to altered cellular physiology and can result in either disease or therapeutic effects1,2. Mapping these interactions and determining how they respond to stimulus is the genesis of many drug development efforts, leading to new therapeutic targets and improvements in human health1. However, in the complex environment of the nucleus, it is challenging to determine protein-protein interactions owing to low abundance, transient or multivalent binding and a lack of technologies that are able to interrogate these interactions without disrupting the protein-binding surface under study3. Here, we describe a method for the traceless incorporation of iridium-photosensitizers into the nuclear micro-environment using engineered split inteins. These Ir-catalysts can activate diazirine warheads through Dexter energy transfer to form reactive carbenes within an approximately 10 nm radius, cross-linking with proteins in the immediate micro-environment (a process termed µMap) for analysis using quantitative chemoproteomics4. We show that this nanoscale proximity-labelling method can reveal the critical changes in interactomes in the presence of cancer-associated mutations, as well as treatment with small-molecule inhibitors. µMap improves our fundamental understanding of nuclear protein-protein interactions and, in doing so, is expected to have a significant effect on the field of epigenetic drug discovery in both academia and industry.

2D Nano-Sonosensitizers Facilitate Energy Transfer to Enhance Sonodynamic Therapy.

Although sonodynamic therapy (SDT) has shown promise for cancer treatment, the lack of efficient sonosensitizers (SSs) has limited the clinical application of SDT. Here, a new strategy is reported for designing efficient nano-sonosensitizers based on 2D nanoscale metal-organic layers (MOLs). Composed of Hf-oxo secondary building units (SBUs) and iridium-based linkers, the MOL is anchored with 5,10,15,20-tetra(p-benzoato)porphyrin (TBP) sensitizers on the SBUs to afford TBP@MOL. TBP@MOL shows 14.1- and 7.4-fold higher singlet oxygen (1 O2 ) generation than free TBP ligands and Hf-TBP, a 3D nanoscale metal-organic framework, respectively. The 1 O2 generation of TBP@MOL is enhanced by isolating TBP SSs on the SBUs of the MOL, which preventsaggregation-induced quenching of the excited sensitizers, and by triplet-triplet Dexter energy transfer between excited iridium-based linkers and TBP SSs, which more efficiently harnesses broad-spectrum sonoluminescence. Anchoring TBP on the MOL surface also enhances the energy transfer between the excited sensitizer and ground-state triplet oxygen to increase 1 O2 generation efficacy. In mouse models of colorectal and breast cancer, TBP@MOL demonstrates significantly higher SDT efficacy than Hf-TBP and TBP. This work uncovers a new strategy to design effective nano-sonosensitizers by facilitating energy transfer to efficiently capture broad-spectrum sonoluminescence and enhance 1 O2 generation.

Multiexciton Generation from a 2D Organic-Inorganic Hybrid Perovskite with Nearly 200% Quantum Yield of Red Phosphorescence.

2D organic-inorganic hybrid perovskites (OIHPs) show obvious advantages in the field of optoelectronics due to their high luminescent stability and good solution processability. However, the thermal quenching and self-absorption of excitons caused by the strong interaction between the inorganic metal ions lead to a low luminescence efficiency of 2D perovskites. Herein, a 2D Cd-based OIHP phenylammonium cadmium chloride(PACC) with a weak red phosphorescence (ΦP < 6%) at 620nm and a blue afterglow is reported. Interestingly, the Mn-doped PACC exhibits very strong red emission with nearly 200% quantum yield and 15ms lifetime, thus resulting in a red afterglow. The experimental data prove that the doping of Mn2+ not only induces the multiexciton generation (MEG) process of the perovskite, avoiding the energy loss of inorganic excitons, but also promotes the Dexter energy transfer from organic triplet excitons to inorganic excitons, thus realizing the superefficient red-light emission of Cd2+ . This work suggests that guest metal ions can induce host metal ions to realize MEG in 2D bulk OIHPs, which provides a new idea for the development of optoelectronic materials and devices with ultrahigh energy utilization.

Roles of Molecular Spatial Arrangement in Exciton Energy Transfer in Organic Light-Emitting Diodes: A Theoretical Study

Revealing the mechanism of exciton energy transfer (EET) in the emitting layer (EML) of organic light-emitting devices is significant for improving the device performance. In this paper, we explore how the host-TADF combinations influence EET in the amorphous film, especially for the Dexter energy transfer (DET) process. With the help of density functional theory calculations and molecular dynamics simulations, we study the Förster resonance energy transfer and the DET of two thin films (2,6-2CzBN:4tCzBN and 3,5-2CzBN:4tCzBN) to determine how the intermolecular interaction influences the energy transfer. We find that the exciton coupling value is the decisive factor that leads to the external quantum efficiency difference between the two amorphous films by analyzing the variable in Marcus theory. The molecular spatial arrangement in 3,5-2CzBN is beneficial to enhance exciton coupling. The more space around benzonitriles helps to strengthen fragment interaction between benzonitriles, which plays a vital role in exciton coupling and spatial distribution. This work reveals how the molecular spatial arrangement and interaction influence energy transfer.

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

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

Ultrafast Energy Transfer from Photoexcited Tryptophan to the Haem in Cytochrome c.

We report femtosecond Fe K-edge absorption (XAS) and nonresonant X-ray emission (XES) spectra of ferric cytochrome C (Cyt c) upon excitation of the haem (>300 nm) or mixed excitation of the haem and tryptophan (<300 nm). The XAS and XES transients obtained in both excitation energy ranges show no evidence for electron transfer processes between photoexcited tryptophan (Trp) and the haem, but rather an ultrafast energy transfer, in agreement with previous ultrafast optical fluorescence and transient absorption studies. The reported (J. Phys. Chem. B 2011, 115 (46), 13723-13730) decay times of Trp fluorescence in ferrous (∼350 fs) and ferric (∼700 fs) Cyt c are among the shortest ever reported for Trp in a protein. The observed time scales cannot be rationalized in terms of Förster or Dexter energy transfer mechanisms and call for a more thorough theoretical investigation.

Suppressing the Undesirable Energy Loss in Solution‐Processed Hyperfluorescent OLEDs Employing BODIPY‐Based Hybridized Local and Charge‐Transfer Emitter

Hyperfluorescent organic light‐emitting diodes (HF‐OLEDs) approach has made it possible to achieve excellent device performance and color purity with low roll‐off using noble‐metal‐free pure organic emitter. Despite significant progress, the performance of HF‐OLEDs is still unsatisfactory due to the existence of a competitive dexter energy transfer (DET) pathway. In this contribution, two boron dipyrromethene (BODIPY)‐based donor‐acceptor emitters (BDP‐C‐Cz and BDP‐N‐Cz) with hybridized local and charge transfer characteristics (HLCT) are introduced in the HF‐OLED to suppress the exciton loss by dexter mechanism, and a breakthrough performance with low‐efficiency roll‐off (0.3%) even at high brightness (1000 cd m−2) is achieved. It is demonstrated that the energy loss via the DET channel can be suppressed in HF‐OLEDs utilizing the HLCT emitter, as the excitons from the dark triplet state of such emitters are funneled to its emissive singlet state following the hot‐exciton mechanism. The developed HF‐OLED device has realized a good maximum external quantum efficiency (EQE) of 19.25% at brightness of 1000 cd m−2 and maximum luminance over 60 000 cd m−2, with an emission peak at 602 nm and Commission International de L'Eclairage (CIE) coordinates (0.57, 0.41), which is among the best‐achieved results in solution‐processed HF‐OLEDs. This work presents a viable methodology to suppress energy loss and achieve high performance in the HF‐OLEDs.

Modeling Excited States of Molecular Organic Aggregates for Optoelectronics.

Light-driven phenomena in organic molecular aggregates underpin several mechanisms relevant to optoelectronic applications. Modeling these processes is essential for aiding the design of new materials and optimizing optoelectronic devices. In this review, we cover the use of different atomistic models, excited-state dynamics, and transport approaches for understanding light-activated phenomena in molecular aggregates, including radiative and nonradiative decay pathways. We consider both intra- and intermolecular mechanisms and focus on the role of conical intersections as facilitators of internal conversion. We explore the use of the exciton models for Frenkel and charge transfer states and the electronic structure methods and algorithms commonly applied for excited-state dynamics. Throughout the review, we analyze the approximations employed for the simulation of internal conversion, intersystem crossing, and reverse intersystem crossing rates and analyze the molecular processes behind single fission, triplet-triplet annihilation, Dexter energy transfer, and Förster energy transfer.

Visible DC approach by controlling the UV light in (Eu3+/Tb3+) co-activated TBZN glasses for w-LEDs and a-Si solar cells

Borotellurite (TBZN: TeO2–B2O3–ZnO–Na2O) glasses co-activated with Eu3+ and Tb3+ ions were fabricated by a melt quenching method to know their ability for white light sources and solar converters. Amorphous nature, morphology, elemental mapping of the chemical composition, and functional groups of glasses were investigated. The down conversion (DC) process was studied on the luminescence characteristics of Eu3+/Tb3+ co-activated TBZN glasses by ultraviolet light (UV:351 & 378 nm) conversion and these glasses exhibit the characteristic red (5D0→7F1,2,3,4) luminescence of Eu3+ along with green (5D4→7F5) and blue (5D4→7F6) luminescence of Tb3+ ions in the visible region. Upon 378 nm of UV radiation, the co-activating of Tb3+ with Eu3+ ions improves the green and red DC emissions by 2 and 4 orders of magnitude due to energy transfer (ET) from Eu3+ to Tb3+, respectively. The evidence of ET: Eu3+ → Tb3+ is stated by the declining of red emission (%) and decay times from the 5D0 level of Eu3+ ions, and the reduction in decay times was confirmed to be dipole-dipole interaction in the means of Reisfeld and Dexter's ET process. The solar spectral performance of the excitation and de-excitation spectra of (Eu3+/Tb3+):TBZN glass system is imparted with the standard solar spectrum. Results are also discussed in terms of integrated intensity of red to green emission in (%), critical transfer distance (Rc), (x, y) color coordinates, correlated color temperatures (CCT), color purity (CP), and quantum efficiency (ηQE) values to discriminate the potentiality of (Eu3+/Tb3+):TBZN glass system in white light emitting diodes and solar cells.

Efficient pure blue hyperfluorescence devices utilizing quadrupolar donor-acceptor-donor type of thermally activated delayed fluorescence sensitizers

The hyperfluorescence (HF) system has drawn great attention in display technology. However, the energy loss mechanism by low reverse intersystem crossing rate (kRISC) and the Dexter energy transfer (DET) channel is still challenging. Here, we demonstrate that this can be mitigated by the quadrupolar donor-acceptor-donor (D-A-D) type of thermally activated delayed fluorescence (TADF) sensitizer materials, DBA-DmICz and DBA-DTMCz. Further, the HF device with DBA-DTMCz and ν-DABNA exhibited 43.9% of high maximum external quantum efficiency (EQEmax) with the Commission Internationale de l'Éclairage coordinates of (0.12, 0.16). The efficiency values recorded for the device are among the highest reported for HF devices. Such high efficiency is assisted by hindered DET process through i) high kRISC, and ii) shielded lowest unoccupied molecular orbital with the presence of two donors in D-A-D type of skeleton. Our current study provides an effective way of designing TADF sensitizer for future HF technology.

Isomer engineering to adjusting full width at half maximum and emission wavelength for efficient solution-processed red OLEDs

Given the great effect of red dyes with high color purity on ultrahigh-definition displays, exploring new red fluorophores remains urgent. In this work, two novel boron dipyrromethene derivatives (BODIPYs), named o-OMB and p-OMB, were designed and synthesized by adjusting the substitution position of methoxy group. The introduction of tert-butylbenzene units increases steric hindrance and adjacent distance between the host and guest to inhibit adverse Dexter energy transfer. Regioisomerization of methoxy group has a certain impact on photophysical properties, including the distribution of frontier molecular orbitals and recombination energy. The recombination energy of p-OMB is much smaller than that of o-OMB, and the Eg of the former is 1.94 eV less than that of the latter 2.02 eV. Compared with o-OMB, p-OMB with para-substituent has narrower emission spectrum with a FWHM of 37 nm and red-shifted emission wavelength of 634 nm. The solution-processed based-p-OMB OLEDs show a maximum EQE of 1.3% and standard red emission with CIE coordinates of (0.67, 0.33) when using PhCz-4CzTPN as a sensitizer.

A Surface Engineering Approach for Promoting Dexter Energy Transfer from Lead Halide Perovskite Nanocrystals

Efficient Dexter energy transfer from colloidal semiconductor nanocrystals lays the foundation for applications like photocatalysis and photon upconversion. We design a two-step functionalization strategy based on aromatic zwitterionic anchoring motifs to fix molecular triplet acceptors to the surface of CsPbBr3 perovskite nanocrystal (PNC), which increases the energy transfer rate by 120-fold. The binding condition of triplet acceptors and the influence on energy transfer during each surface treatment step are illustrated using steady-state and time-resolved optical spectroscopies and NMR. The results show that this new strategy can preclude competitive adsorption from residual synthetic ligands and achieve stable anchoring for triplet acceptors. Compared to acceptors that undergo rapid exchange on and off the dynamic PNC surface, the fixation significantly strengthens the electronic coupling with the PNC and hence promotes the Dexter energy transfer. Our strategy can be extended for quantitatively grafting other functional groups that require strong electronic coupling with lead halide perovskite surface.

Detection of Dexter energy transfer process in interface-type OLED via utilizing the characteristic magneto-electroluminescence response of hot exciton reverse intersystem crossing

The maximum external quantum efficiency of the host-guest-type organic light-emitting diodes (OLEDs) with interface exciplex as the host has been over 36%. However, studies about the energy transfer processes occurring from the host to guest remain lacking. Herein, a strategy is proposed to probe the energy transfer processes in interface-type OLEDs by utilizing the characteristic magneto-electroluminescence (MEL) response from the hot exciton reverse intersystem crossing (T&lt;sub&gt;2,Rub&lt;/sub&gt; → S&lt;sub&gt;1,Rub&lt;/sub&gt;) of rubrene. Specifically, a donor/spacer/accepter (D/S/A)-type interface exciplex device and a D/spacer:&lt;i&gt;x&lt;/i&gt;% Emitter/A (D/S:3% Rubrene/A)-type Rubrene-doped device are fabricated. The Förster resonance energy transfer (FRET) process occurring between the singlet state of the exciplex-host and the singlet state of Rubrene-guest is demonstrated by characterizing the photophysical properties of the donor, accepter, and guest materials. The Dexter energy transfer (DET, T&lt;sub&gt;1,Host&lt;/sub&gt; → T&lt;sub&gt;2,Rub&lt;/sub&gt;) process between the triplet state of the host and the triplet state of guest is visualized by the comparative studying of the current- and temperature-dependent MEL response curves of D/S/A and D/S:3% Rubrene/A devices, respectively. More importantly, the occurrence of the DET process greatly promotes the electroluminescence intensity of the D/S:3% Rubrene/A device. Furthermore, we also investigate the differences in the electroluminescence performance of devices at low temperature to demonstrate again the co-existence of FRET and DET process in the D/S:3% Rubrene/A system. Obviously, this work not only provides a promising strategy for probing the DET process in OLEDs, but also paves a new way for designing high-performance “hot exciton” type OLEDs.

High-efficiency all fluorescence white OLEDs with high color rendering index by manipulating excitons in co-host recombination layers.

All fluorescence white organic light-emitting diodes (WOLEDs) based on thermally activated delayed fluorescence (TADF) emitters are an attractive route to realize highly efficient and high color quality white light sources. However, harvesting triplet excitons in these devices remains a formidable challenge, particularly for WOLEDs involving conventional fluorescent emitters. Herein, we report a universal design strategy based on a co-host system and a cascaded exciton transfer configuration. The co-host system furnishes a broad and charge-balanced exciton generation zone, which simultaneously endows the devices with low efficiency roll-off and good color stability. A yellow TADF layer is put forward as an intermediate sensitizer layer between the blue TADF light-emitting layer (EML) and the red fluorescence EML, which not only constructs an efficient cascaded Förster energy transfer route but also blocks the triplet exciton loss channel through Dexter energy transfer. With the proposed design strategy, three-color all fluorescence WOLEDs reach a maximum external quantum efficiency (EQE) of 22.4% with a remarkable color rendering index (CRI) of 92 and CIE coordinates of (0.37, 0.40). Detailed optical simulation confirms the high exciton utilization efficiency. Finally, by introducing an efficient blue emitter 5Cz-TRZ, a maximum EQE of 30.1% is achieved with CIE coordinates of (0.42, 0.42) and a CRI of 84 at 1000 cd m-2. These outstanding results demonstrate the great potential of all fluorescence WOLEDs in solid-state lighting and display panels.

Red-light-mediated Barton decarboxylation reaction and one-pot wavelength-selective transformations.

In organic chemistry, selecting mild conditions for transformations and saving energy are increasingly important for achieving sustainable development goals. Herein, we describe a red-light-mediated Barton decarboxylation using readily available red-light-emitting diodes as the energy source and zinc tetraphenylporphyrin as the catalyst, avoiding explosive or hazardous reagents or external heating. Mechanistic studies suggest that the reaction probably proceeds via Dexter energy transfer between the activated catalyst and the Barton ester. Furthermore, a one-pot wavelength-selective reaction within the visible light range is developed in combination with a blue-light-mediated photoredox reaction, demonstrating the compatibility of two photochemical transformations based on mechanistic differences. This one-pot process expands the limits of the decarboxylative Giese reaction beyond polarity matching.

The Study on the Relationship between Dynamic Balance Energy Distribution and Spectral Stability with Voltage Change in White Organic Light Emitting Diode

The stable spectrum can be obtained when the voltage changes, which is a necessary condition for the white organic light emitting diode (WOLED) device to be widely used in the field of solid-state lighting. However, with the increase of voltage, the movement of the recombination zone (RZ) is inevitable because the perfect bipolar host material is difficult to obtain, which will redistribute the energy in the light emitting layer (EML) and affect the stability of the spectrum. We fabricate a series of ternary hybrid WOLEDs with a simple structure by inserting ultra-thin PO-T2T into the blue exciplex (TCTA:TPBi) to form the green interface exciplex. Without considering the movement of RZ, device B2 realizes the dynamic balance energy distribution in EML and stable spectrum by controlling two processes of the Dexter energy transfer and exciton capture. By modifying the doping ratio of the host material, we also find that the broadened RZ is helpful to further improve the spectral stability of the device. When the voltage changes from 3 V to 7 V, the change range of color coordinates is only (0.026, 0.025).

Benzophenone as a cheap and effective photosensitizer for the photocatalytic synthesis of dimethyl cubane-1,4-dicarboxylate.

The key intramolecular [2 + 2] photochemical cycloaddition step in the synthesis of dimethyl cubane-1,4-dicarboxylate is performed with substoichiometric amounts of the photosensitizer benzophenone. The reaction proceeds via a Dexter energy transfer process between the triplet excited state benzophenone and a well-known cubane precursor diene. The use of the cheap and widely available benzophenone as the photosensitizer enables lower energy light to be used than the traditional photochemical process.