Dexter Energy Transfer Research Articles

Two-channel singlet exciton harvesting for high efficiency fluorescent organic light-emitting diodes using dual thermally activated delayed fluorescence mechanism

A two-channel singlet exciton harvesting device was developed to improve the external quantum efficiency (EQE) of the fluorescent organic light-emitting diodes (OLEDs) using a dual, thermally activated delayed fluorescence (TADF) mechanism. The emitting layer consisted of two TADF materials, the 10,10'-(sulfonylbis(4,1-phenylene))bis(9,9-dimethyl-9,10-dihydroacridine) (DMAC-DPS) TADF emitter and the TADF exciplex formed between DMAC-DPS and ((1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) (POT2T), and a green fluorophore. The DMAC-DPS serves as the main TADF matrix, which transports carriers, forms excitons, and harvests the singlet excitons of the fluorophore, whereas the DMAC-DPS:POT2T exciplex played a role of a second TADF material harvesting the singlet excitons of the fluorophore while suppressing the Dexter energy transfer. The exciplex was dispersed in the DMAC-DPS by managing the POT2T content, which reduced triplet-triplet-annihilation of the exciplex while suppressing Dexter energy transfer to the fluorophore. The two-channel singlet exciton harvesting mechanism maximized the singlet exciton formation, which enhanced the EQE of the fluorescent OLEDs from 10.3% to 15.0% compared to the conventional singlet exciton harvesting device using either TADF compound or TADF exciplex.

Improving the thermoelectric performance of solution-processed polymer nanocomposites by introducing platinum acetylides with tailored intermolecular interactions

Hybrid thermoelectric (TE) materials incorporated with the conjugated polymers and single-walled carbon nanotubes (SWCNTs) are promising to decouple the intractable interrelation of the key TE parameters, and therefore yield considerable composites for solution-processable flexible TE generators. However, the disordered intermolecular interactions and inefficient overlap of the frontier molecular orbitals between the polymer backbones and the SWCNTs largely degenerate the TE properties of the composites. Herein, we demonstrate an efficient strategy to regulate their intermolecular π-π interactions by introducing heavy metal atoms (platinum) along with tailored π-ligands to reconstruct the main chain of the incorporated polymers. Consequently, both the calculated binding energy and the observed Dexter energy transfer process between the SWCNTs and the polymers are remarkably increased with the insertion of platinum, indicating significantly enhanced π-π interactions between the two. Meanwhile, their self-aggregation can be effectively restrained by the large steric hindrance groups of tributyl phosphine ligands synchronously. Therefore, a high power factors of over 464 μW m−1 K−2 (r.t.) and a high output power approaching to 2200 nW (ΔT = 70 K, 10 legs) can be achieved by the P(HTh-Pt)/SWCNT-based TE generators, which is almost two times as high as the homologous P3HT one without platinum.

Partial energy transfer from blue TADF sensitizer to orange fluorescent dopant for prolonging device lifetime

White organic light-emitting diodes (WOLEDs) with thermally activated delayed fluorescence-sensitized fluorescence (TSF) have aroused wide attention, considering their potential for attaining ultimate high efficiency and long lifetime with pure organic fluorescent emitters. To develop high-performance TSF-based WOLEDs, an efficient energy transfer system is built by combining a conventional host, a stable blue delayed fluorescence sensitizer, and an orange fluorescent dopant in a single-emissive-layer. Transient photoluminescence measurements decipher the Förster resonance energy transfer process with the high exciton utilization and the suppressed Dexter energy transfer in the single-emissive-layer. The TSF-based WOLEDs achieve a maximum external quantum efficiency of 20.9% and the balanced white emission with color coordinates of (0.31, 0.41) because of the partial energy transfer from blue sensitizer to the fluorescent dopant. Moreover, the TSF-WOLEDs exhibit a half-lifetime of ~2,800 h at an initial brightness of 1,000 cd m−2, which can be further improved to >12,000 h by implementing the stacked structure and light outcoupling enhancement techniques. The prolonged device stability is because the TSF system can effectively prevent exciton leakage and suppress the accumulation-caused exciton-annihilation.

Photon Upconversion in a Glowing Metal-Organic Framework.

The interaction of low-energy light with matter that leads to the production of high-energy light is known as photon upconversion. This phenomenon is of importance because of its potential applications in optoelectronics, energy harvesting, and the biomedical arena. Herein, we report a pillared-paddlewheel metal-organic framework (MOF), constructed from a tetrakis(4-carboxyphenyl)porphyrin sensitizer and a dipyridyl thiazolothiazole annihilator, designed for efficient triplet-triplet annihilation upconversion (TTA-UC). Single-crystal X-ray diffraction studies reveal that the Zn-metalated sensitizers are coordinated to Zn2 nodes in a paddlewheel fashion, forming 2D sheets, to which are linked annihilators, such that each sensitizer is connected to five of them. The precise arrangements of sensitizers with respect to annihilators, and the high annihilator-to-sensitizer ratio, facilitate Dexter energy transfer. This level of organization in an extended structure leads to a high TTA-UC efficiency of 1.95% (theoretical maximum = 50%) at an excitation power density of 25 mW cm-2.

Design Rule of Assistant Dopant for High External Quantum Efficiency in Hyperfluorescence Organic Light‐Emitting Diodes

The design rule of an assistant dopant for enhanced external quantum efficiency (EQE) in hyperfluorescence organic light‐emitting diodes is investigated by synthesizing two assistant dopants with thermally activated delayed fluorescent (TADF) features, BPAc and BPAcCz, by combining a benzophenone acceptor and a dimethylacridane donor. BPAc has only the benzophenone acceptor and the dimethylacridane donor, whereas BPAcCz has an extra carbazole donor added to the BPAc structure. Comparison of the two assistant dopants reveals that the incorporation of the donor unit with a small dihedral angle degrades the efficiency of the hyperfluorescence devices by Dexter energy transfer. The hyperfluorescence devices with the BPAc and BPAcCz assistant dopants exhibit EQE of 16.6% and 15.0%, respectively.

Bulk Transparent Photon Upconverting Films by Dispersing High-Concentration Ionic Emitters in Epoxy Resins

It remains challenging to achieve efficient and air-stable photon upconversion (UC) in rigid, technologically valuable transparent films. Here, we report the first example of epoxy resins that show an air-stable and efficient triplet-triplet annihilation (TTA)-based UC. Epoxy resins are thermally cross-linked polymers widely used as coating and sealing materials in actual devices. To achieve efficient TTA-UC in rigid epoxy films, it is essential to execute both the triplet sensitization and triplet exciton diffusion processes without relying on molecular diffusion. This requires homogeneously dispersing emitter molecules without aggregation in three-dimensionally cross-linked rigid polymer networks at a high concentration (ca. 1000 mM) such that the inter-emitter distance is less than 1 nm, where dexter energy transfer can occur. This difficult requirement is solved by employing an ionic liquid emitter that consists of 9,10-diphenylanthracene sulfonate and lipophilic phosphonium ions bearing long alkyl chains. The obtained epoxy resins show a high TTA-UC efficiency (ηUC = 3.8%) and low threshold excitation intensity (Ith = 40 mW cm-2) in air. These UC parameters are achieved by virtue of a very high sensitizer-to-emitter triplet energy-transfer efficiency (92.8%) and a significantly long emitter triplet lifetime (17.8 ms) that reflect the high emitter concentration and the rigid chromophore environment, respectively. The bulk transparent upconverting resins can be prepared in air and function in air, which opens a new avenue toward a wide range of real-world applications.

Manipulating Complementarity of Binary White Thermally Activated Delayed Fluorescence Systems for 100% Exciton Harvesting in OLEDs

AbstractDifferent to fluorescent and phosphorescent counterparts, white thermally activated delayed fluorescence (TADF) involves in multiple reverse intersystem crossing (RISC), leading to the correlation but competition between blue and other color components in both singlet and triplet allocations. Herein, three blue TADF emitters SSFAPO, DSFAPO, and TSFAPO, collectively named xSFAPO are developed, featuring a moderately electron‐withdrawing phosphine oxide (PO) acceptor respectively linked 1‐3 donors. Despite nearly identical blue emissions, photoluminescence quantum yields of xSFAPO are proportional to donor number. But, their RISC efficiencies are below 70%, markedly less than 85% of a conventional yellow TADF emitter 2,3,5,6‐tetrakis(3,6‐di‐(tert‐butyl)carbazol‐9‐yl)‐1,4‐dicyanobenzene (4CzTPNBu). Furthermore, sp3 hybrid configuration of PO enlarges steric hindrance of peripheral donor groups. So, Dexter energy transfer is impeded by increasing donor numbers. Among xSFAPO and 4CzTPNBu dually doped white‐emitting films, yellow emission from SSFAPO‐based film is the strongest, reflecting the predominance of fast Dexter energy transfer in triplet allocation. Therefore, SSFAPO endowed its warm‐white organic light‐emitting diodes (WOLEDs) with an external quantum efficiency of 25.1%, corresponding to 100% internal quantum efficiency, which are 1.25 and 1.60 folds of those of DSFAPO and TSFAPO‐based WOLEDs. These results suggest advantage complementarity of different components is crucial for developing white‐emitting systems with 100% exciton utilization.

Improved performance of fluorescent/phosphorescent hybrid white organic light-emitting diodes with enhanced chromatic stability utilizing bipolar interlayer

Fluorescent/phosphorescent hybrid white organic light-emitting diodes (WOLEDs) employing a bipolar CBP interlayer with different thickness have been fabricated. The two-color hybrid WOLEDs at the optimal interlayer thickness exhibit a maximum efficiency of 11.47 cd A−1. Compared with the device without a CBP interlayer, the efficiency of the device is increased by 38.5% and the chromatic stability is also improved. This work found that appropriate thickness of the CBP interlayer with a higher triplet energy level can effectively block the Dexter energy transfer between the phosphorescent and fluorescent layers, which can reduce the energy of triplet in the exciton recombination zone and improve the efficiency of the device. Furthermore, the introduction of the bipolar interlayer with a suitable energy level adjusts the balance of exciton distribution, thereby achieving white light emission with good color stability. The present research could provide valuable strategy in developing hybrid WOLEDs with stable chromaticity.

Design Approach of Lifetime Extending Thermally Activated Delayed Fluorescence Sensitizers for Highly Efficient Fluorescence Devices.

In this work, a design approach of three thermally activated delayed fluorescence (TADF) emitters to extend the device lifetime of the TADF sensitized fluorescent devices was studied. Three TADF materials, 5-{4,6-bis[4-(tert-butyl)phenyl]-1,3,5-triazin-2-yl}-2-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)benzonitrile (tTCNTruX), 4-[3-cyano-4-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)phenyl]-2,6-diphenylpyrimidine-5-carbonitrile (PCNTruX) and 4-(4-{10,15-bis[4-(tert-butyl)phenyl]-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl}-3-cyanophenyl)-2,6-diphenylpyrimidine-5-carbonitrile (PCNtTruX), were synthesized as sensitizers for TADF-sensitized fluorescent organic light-emitting diodes. The two tTCNTruX and PCNtTruX TADF emitters were designed to have Dexter energy transfer with blocking groups either in the donor or acceptor unit of the donor-acceptor-type TADF sensitizer. The TADF materials showed small singlet-triplet energy splitting and a high reverse intersystem crossing (RISC) rate for effective sensitization of the fluorescent emission of the fluorescent emitter. tTCNTruX- and PCNtTruX-sensitized fluorescent devices showed maximum external quantum efficiencies (EQEs) of 17.7 % and 11.5 % in the yellow and red devices, respectively, which were higher than those of TADF-sensitized devices with the corresponding TADF sensitizer without a blocking group. Moreover, the device lifetime was also extended by employing the tTCNTruX and PCNtTruX sensitizers. This work demonstrated that the tTCNTruX and PCNtTruX sensitizers are effective to improve the maximum EQE and device lifetime of TADF-sensitized fluorescent devices.

Color-tunable emission and non-contact optical temperature sensing performance in NaY9Si6O26: Ce3+, Eu3+ phosphors

In this article, the tunable-color emission properties of Ce3+/Eu3+ in NaY9Si6O26 phosphors were reported. Ce3+ and Eu3+ ions had effectively incorporated the sites of Na+ and Y3+ ions, which were revealed by Rietveld refinements. The site occupation preference of Ce3+ in cationic sites were discussed. The energy transfer from Ce3+ to Eu3+ ion under the 312 nm excitation was ascribed to dipole-dipole interaction mechanism based on Dexter's energy transfer theory of multipolar interaction, which makes NaY9Si6O26: Ce3+, Eu3+ produce tunable-color emission. Additionally, the temperature dependent emission spectra of the powders were studied. Temperature sensing performances were reflected by the fluorescence intensity ratio based on different temperature dependent variation tendency of Ce3+ and Eu3+ ions. The maximum values of the absolute and relative sensor sensitivities of NaY9Si6O26: Ce3+, Eu3+ phosphors were about 0.0328 K−1 and 0.95 % K−1, respectively. Above all, NaY9Si6O26: Ce3+, Eu3+ phosphors show a promising prospect for optical thermometer applications.

Excited state mechanisms in crystalline carbazole: the role of aggregation and isomeric defects

The role of aggregation and isomeric impurities on the excited state mechanisms in crystalline carbazole are revisited considering exciton, Dexter energy transfer and electron transport based on Marcus and Marcus–Levich–Jortner theories.

Matrix‐Free Hyperfluorescent Organic Light‐Emitting Diodes Based on Carbene–Metal–Amides

AbstractA wide‐gap host matrix is a major obstacle detrimentally influencing the performance of hyperfluorescent organic light‐emitting diodes since it substantially increases driving voltage. Moreover, these hyperfluorescent devices typically require at least three components in their emitting layer, which is unfavorable for mass production. To tackle the issue, hyperfluorescent organic light‐emitting diodes are reported based on a two‐component emissive system of carbene–metal–amide donors and conventional fluorescent acceptors. A significant reduction of the driving voltage versus three‐component hyperfluorescent devices at practical brightness (1000 cd m–2) is demonstrated, leading to a doubling of power conversion efficiency for some composites. From an analysis of thin‐film photophysics, it is shown that operational efficiency is limited by Dexter energy transfer between donors and acceptors, which may be reduced by tert‐butyl steric substituents, providing new targets for molecular design. While reducing driving voltage, matrix‐free hyperfluorescent devices also achieve a maximum external quantum efficiency of 16.5%.

The role of the bulky blocking unit of the fluorescent emitter in efficient green hyper-fluorescent organic light-emitting diodes

The efficiency loss process of hyper-fluorescent organic light-emitting diodes (OLEDs) was analyzed with different fluorescent emitters. Two blocking units of tert-butyl and 2-phenylpropan-2-yl in fluorescent emitters suppressed the Dexter energy transfer from the host and thermally activated delayed fluorescent (TADF) sensitizer to the fluorescent emitter and concentration quenching caused by intermolecular interaction. The performance of the green hyper-fluorescent OLEDs showed that the device of the fluorescent emitter with the 2-phenylpropan-2-yl unit achieved a high external quantum efficiency of 18.1% with a low efficiency roll-off, compared to the device of the fluorescent emitter with the tert-butyl unit. This study demonstrated the usefulness of the 2-phenylpropan-2-yl blocking unit of the fluorescent emitter for hyper-fluorescent OLEDs, and exhibited the directionality of the development of hyper-fluorescent OLEDs.

Terpyridine-containing porphyrin and coordination assembly with fullerene-based pyridine for enhanced electrocatalytic oxygen evolution and photocurrent response

A novel terpyridine-modified porphyrin, ZnP-Pr-tpy, has been synthesized and characterized successfully using elemental analysis, mass spectrometry (MS), nuclear magnetic resonance spectroscopy (NMR), infrared spectroscopy (IR) and ultraviolet/visible spectroscopy (UV–Vis). Studies by steady-state fluorescence and UV–visible spectroscopies show that a supramolecular dyad is formed by fullerene-based pyridine (C60-py) and the zinc porphyrin via a coordination bond, with a binding constant K of 1.18 × 105 M−1. The photoexcitation of ZnP-Pr-tpy causes the ground state electrons of the tpy moiety to shift to the ZnP unit via an electron-exchange process following the Dexter energy transfer mechanism. Thus, a donor–acceptor composite film, ZnP-Pr-tpy/C60-py, was fabricated by combining the photosensitizer ZnP-Pr-tpy with C60-py. A comparison of the over-potential at 10 mA/cm2, the Tafel slopes and the photocurrent response under visible light irradiation showed the electrocatalytic oxygen evolution reaction (OER) and the photocurrent response to be clearly improved by the covalent modification of the porphyrin with terpyridine, and further improved by the axial coordination of C60-py to ZnP-Pr-tpy. These improvements are attributed to increased intra- and intermolecular electron transfer, the decrease in the electrochemical impedance resulting from the axial coordination of C60-py to ZnP-Pr-tpy and the electron-exchange in ZnP-Pr-tpy through the Dexter energy transfer mechanism under light excitation.

Photophysics of graphene quantum dot assemblies with axially coordinated cobaloxime catalysts.

We report a study of chromophore-catalyst assemblies composed of light harvesting hexabenzocoronene (HBC) chromophores axially coordinated to two cobaloxime complexes. The chromophore-catalyst assemblies were prepared using bottom-up synthetic methodology and characterized using solid-state NMR, IR, and x-ray absorption spectroscopy. Detailed steady-state and time-resolved laser spectroscopy was utilized to identify the photophysical properties of the assemblies, coupled with time-dependent DFT calculations to characterize the relevant excited states. The HBC chromophores tend to assemble into aggregates that exhibit high exciton diffusion length (D = 18.5 molecule2/ps), indicating that over 50 chromophores can be sampled within their excited state lifetime. We find that the axial coordination of cobaloximes leads to a significant reduction in the excited state lifetime of the HBC moiety, and this finding was discussed in terms of possible electron and energy transfer pathways. By comparing the experimental quenching rate constant (1.0 × 109 s-1) with the rate constant estimates for Marcus electron transfer (5.7 × 108 s-1) and Förster/Dexter energy transfers (8.1 × 106 s-1 and 1.0 × 1010 s-1), we conclude that both Dexter energy and Marcus electron transfer process are possible deactivation pathways in CoQD-A. No charge transfer or energy transfer intermediate was detected in transient absorption spectroscopy, indicating fast, subpicosecond return to the ground state. These results provide important insights into the factors that control the photophysical properties of photocatalytic chromophore-catalyst assemblies.

Full Confinement of High‐Lying Triplet States to Achieve High‐Level Reverse Intersystem Crossing in Rubrene: A Strategy for Obtaining the Record‐High EQE of 16.1% with Low Efficiency Roll‐Off

AbstractA high‐level reverse intersystem crossing (HL‐RISC, T2 → S1 → S0 + hν) process has recently been discovered as a promising route for achieving highly efficient organic light‐emitting diodes (OLEDs), but the prerequisites for the occurrence of HL‐RISC in rubrene is still vague and the reported external quantum efficiencies (EQEs) of rubrene‐doped OLEDs are typically limited to several percent. Herein, using the fingerprint magneto‐electroluminescence tools, it is found that the energy confinement of high‐lying triplet states (T2, rub) is of great importance for the achievement of the HL‐RISC process. Namely, when the triplet energies of hosts satisfy the criterion of E(T1, host) ≥ E(T2, rub), the high‐level Dexter energy transfer channel (T1, host → T2, rub) can facilitate the occurrence of HL‐RISC (T2, rub → S1, rub) in rubrene. Most importantly, through selecting an exciplex with a high triplet energy as the co‐host for rubrene dopant so as to simultaneously utilize the HL‐RISC of the dopant and the RISC of the host, a record high EQE up to 16.1% is achieved and no obvious efficiency roll‐off is observed at high luminance due to the absence of triplet‐charge annihilation. Accordingly, this work not only deepens the physical understanding of this amazing HL‐RISC channel, but also provides a new direction for designing a series of highly efficient OLEDs.

Intense red emission via energy transfer from (Ce3+/Eu3+):P2O5+NaF+CaF2+AlF3 glasses for warm light sources

Europium (Eu3+)-doped fluorophosphate (PNCA:P2O5+NaF + CaF2+AlF3) glasses with the addition of cerium (Ce3+) ions were fabricated by the melt-quenching technique to know their ability for the bright red (615 nm) luminescence. The emission (PL) and excitation (PLE) spectra, decay curve measurements as well as energy transfer (ET) process of Ce3+→ Eu3+ were studied in detail. An excitation spectrum related to the 7F0→ 5D2 level of Eu3+ is used to estimate the phonon energy (1121 cm−1) of the title glass host. Under ultraviolet (UV) irradiation of 299 nm, the PL spectra of (Ce3+/Eu3+):PNCA glasses show intense red emission at 615 nm whereas the lifetime decrease with respect to increase of Eu3+ that could support the observed efficient ET from Ce3+ to Eu3+. The ET:Ce3+ →Eu3+ via quadrupole-quadrupole process was confirmed by Reisfeld's approximation and Dexter's ET formula. The ET efficiency (ηET) and critical distance (Rc) were also calculated. Interestingly, the (Ce3+/Eu3+):PNCA glasses showed intense red light emission with low correlated color temperatures and the corresponding color purity reached as great as 99%, indicating its potentiality as a red component for warm light sources.

Trap state mediated triplet energy transfer from CdSe quantum dots to molecular acceptors

Triplet energy transfer (TET) from quantum dots (QDs) to molecular acceptors has received intense research interest because of its promising application as triplet sensitizers in photon up-conversion. Compared to QD band edge excitons, the role and mechanism of trap state mediated TET in QD-acceptor complexes have not been well understood despite the prevalence of trap states in many QDs. Herein, TET from trap states in CdSe QDs to adsorbed 9-anthracene carboxylic acid (ACA) is studied with steady state photoluminescence, transient absorption spectroscopy, and time-resolved photoluminescence. We show that both band edge and trap excitons undergo direct Dexter energy transfer to form the triplet excited state of ACA. The rate of TET decreases from (0.340 ± 0.002) ns-1 to (0.124 ± 0.004) ns-1 for trap excitons with decreasing energy from 2.25 eV to 1.57 eV, while the TET rate from band edge excitons is 13-37 times faster than trapped excitons. Despite slightly higher TET quantum efficiency from band edge excitons (∼100%) than trapped excitons (∼95%), the overall TET process from CdSe to ACA is dominated by trapped excitons because of their larger relative populations. This result demonstrates the important role of trap state mediated TET in nanocrystal sensitized triplet generation.

Interplay between Förster and Dexter Energy Transfer Rates in Isomeric Donor-Bridge-Acceptor Systems.

The ability to direct the flow of excitons enable molecular systems to perform highly advanced functions. Intramolecular energy transfer in donor-bridge-acceptor systems can occur by different mechanisms, and the ability to control the excited state energy pathways depends on the capacity to favor one process over another. Here, we show an anticorrelation between the rates of Förster and Dexter types of energy transfer in two isomeric donor-bridge-acceptor systems. Both dyads display intramolecular Förster triplet-to-singlet and Dexter triplet-to-triplet energy transfers. However, as the bridge-acceptor connection point changes, the rate of one energy transfer process increases at the same time as the other one decreases, allowing us to control the energy flow direction. This work shows how rational design can be used to tune excited state energy pathways in molecular dyads, which is of importance for advanced functions such as multiplicity conversion in future molecular materials.

Predicting Dexter Energy Transfer Interactions from Molecular Orbital Overlaps

Dexter energy transfer (DET), also known as spin forbidden energy transfer (e.g., 3D1A  1D3A), is undergoing a resurgence of interest because of its utility in energy science, photocatalysis, and ...