Through-space Charge Transfer Research Articles

Double charge transfer processes enable a green multiple resonance-induced thermally activated delayed fluorescence emitter for an efficient narrowband OLED.

Benefitting from short-range charge transfer (SR-CT) and through-space charge transfer (TSCT) effects, an efficient green narrowband emitter, BNDCN, was developed. Owing to the synergistic effect of double CT processes, a BNDCN-based organic light-emitting diode showed a high external quantum efficiency of 32.3%.

X-ray-triggered through-space charge transfer and photochromism in silver nanoclusters.

Materials exhibiting X-ray-induced photochromism have consistently piqued the interest of researchers. Exploring the photochromic properties of such materials is valuable for understanding the structural changes and electron transfer processes that occur under high energy radiation, such as X-ray irradiation. Here, a crystalline silver(I) nanocluster synthesized from tert-butylacetylene silver was found to have the ability to exhibit color and photoluminescence changes upon exposure to X-ray radiation. The responsive behavior was observed across a wide temperature range of 100-300 K, with the ability to respond particularly well to soft X-rays (λ > 1 Å) and exhibit light responsiveness to hard X-rays (λ < 1 Å). By combining experimental findings including X-ray diffraction, X-ray photoelectron spectroscopy, electron spin resonance, etc. with theoretical calculations, we have proposed that X-ray irradiation induces electron transfer from chloride (Cl-) located in the center of the silver(I) nanocluster to the surrounding Ag14 in the skeleton. This represents the first documented example in which electron transfer induced by X-ray excitation has been observed, accompanied by a photochromism process, in silver nanoclusters. This study contributes to our understanding of X-ray-induced photochromism and the electron transfer process in silver cluster compounds. It also provides valuable insights and potential design strategies for applications such as photochromism, photoluminescence color change, and photoenergy conversion.

Simultaneously enhancing organic phosphorescence quantum yields and lifetimes for triphenylphosphine salt doped polymer films.

Simultaneously enhancing the quantum yields and luminescence lifetimes of organic persistent room temperature phosphorescence (RTP) molecules is a priority in the organic photonic area, but it remains a formidable challenge. Here, an effective strategy was proposed to improve both quantum efficiencies and emission decay times for phosphorescent triphenylphosphine salts. This approach involves integrating an electron donor unit into a triphenylphosphine salt via an alkyl chain. This structure facilitates an intermediate through-space charge transfer excited state, which enhances the intersystem crossing process to boost RTP performance. Moreover, the electron donor moiety contributes additional triplet excitons to the triphenylphosphine salts through triplet-to-triplet energy transfer, substantially increasing the population of triplet excitons. Specifically, compared to butyl(naphthalen-1-yl) diphenylphosphonium bromide (Φphos. = 4.9% and τ = 255.79 ms), (2-(9H-carbazol-9-yl)ethyl)(naphthalen-1-yl)diphenylphosphonium bromide demonstrates a higher phosphorescence quantum yield of 19.6% and an extended emission lifetime of 800.59 ms. This advancement lays the groundwork for developing high-performance organic RTP materials, unlocking new possibilities for advanced photonic applications.

A donor-acceptor cage for circularly polarized TADF emission.

Herein, we report the first example of chiral donor-acceptor cage DA-2 displaying efficient circularly polarized thermally activated delayed fluorescence (CP-TADF) with |glum| values up to 2.1 × 10-3 and PLQY of 32%. A small ΔEST of 0.051 eV and quasi-parallel (θ = 6°) transition electric and magnetic dipole moments were realized from the through-space charge transfer interaction between the parallelly aligned donor and acceptor in DA-2. This D-A cage configuration has provided a novel design strategy for discovering potential efficient CP-TADF emitters.

A through-space charge transfer pyrene-based fluorophore with anti-quenching behavior for deep-blue organic light-emitting devices.

A through-space charge transfer pyrene-based fluorophore has been developed for organic light-emitting devices (OLEDs). This material exhibits deep-blue fluorescence, bipolar characteristics, and anti-quenching behavior in the solid state. It proves to be an effective emitter for both doped and nondoped deep-blue OLEDs.

Intramolecular through-space charge transfer between benzofuran and ynone groups on a naphthalene spacer.

We have developed a new scaffold that exhibits an efficient intramolecular through-space charge transfer (CT). In this design, electron-rich benzofuran and electron-deficient ynone groups are placed strategically in proximity via a naphthalene spacer. Charge transfer is supported by distinct CT bands in the visible region (>500 nm) in their UV-vis absorption and emission spectra.

Unveiling the potential of triphenylphosphine salts in tuning organic room temperature phosphorescence.

Triphenylphosphine (TPP) salt derivatives, with their rich chemistry of core-substitution, have emerged as promising candidates for ultralong room temperature phosphorescence (RTP) materials owing to their distinct molecular structures, high quantum efficiency and exceptional phosphorescence properties. This feature article highlights the vast potential of TPP salt derivatives in tunable RTP properties by exploring some factors such as the alkyl chains, halogen anions, through-space charge transfer states, etc., and recent advancements in multi-level information encryption, high-level anticounterfeiting tags and X-ray imaging applications. We anticipate that this article will assist in directing future analyses based on the mechanisms underlying the RTP behavior of TPP derivatives and offer guidance for the rational design of high-performance RTP materials.

Unveiling the potential of nonconjugate linkers (sp3-cores) in through-space charge transfer emitters and host materials for thermally activated delayed fluorescence organic light emitting diodes

Nonconjugate linkers (sp3 cores) are a versatile platform for molecular design for TADF OLEDs. The introduction of an sp3 core disrupts direct conjugation between donor and acceptor units, preventing immediate charge transfer between them.

Intramolecular exciplex featuring a bis-sp3 C-locked acceptor-donor-acceptor sandwich.

Intramolecular exciplex systems featuring thermally activated delayed fluorescence (TADF) have garnered significant attention in the realm of organic light-emitting diodes (OLEDs). Nonetheless, the occurrence of organic sandwich intramolecular exciplexes remains rare due to structural limitations and synthetic challenges. Herein, we present a novel rigid acceptor-donor-acceptor (A-D-A) sandwich complex, dSFQP, characterized by two sp3 C-locking moieties. This compound exhibits TADF characteristics facilitated by a multiple through-space charge-transfer process. X-ray crystallographic analysis confirms the distinctive sandwich configuration. The parallel spatial arrangement and minimized A-D-A configuration enhance electronic interactions, resulting in a high photoluminescence quantum yield, rapid reverse intersystem crossing rate, and sluggish nonradiative decay rate. OLEDs employing dSFQP as the dopant achieve a maximum external quantum efficiency (EQE) of 28.5% with a low efficiency roll-off of merely 2.8% at 1000 cd m-2. Even at a high brightness of 10 000 cd m-2, the EQE remains notably high at 17.5%. Our current results provide an effective way to further innovate the design of new organic charge-transfer complexes.

Regulation of Multiple Resonance Delayed Fluorescence via Through‐Space Charge Transfer Excited State towards High‐Efficiency and Stable Narrowband Electroluminescence

AbstractB‐ and N‐embedded multiple resonance (MR) type thermally activated delayed fluorescence (TADF) emitters usually suffer from slow reverse intersystem crossing (RISC) process and aggregation‐caused emission quenching. Here, we report the design of a sandwich structure by placing the B−N MR core between two electron‐donating moieties, inducing through‐space charge transfer (TSCT) states. The proper adjusting of the energy levels brings about a 10‐fold higher RISC rate in comparison with the parent B−N molecule. In the meantime, a high photoluminescence quantum yield of 91 % and a good color purity were maintained. Organic light‐emitting diodes based on the new MR emitter achieved a maximum external quantum efficiency of 31.7 % and small roll‐offs at high brightness. High device efficiencies were also obtained for a wide range of doping concentrations of up to 20 wt % thanks to the steric shielding of the B−N core. A good operational stability with LT95 of 85.2 h has also been revealed. The dual steric and electronic effects resulting from the introduction of a TSCT state offer an effective molecular design to address the critical challenges of MR‐TADF emitters.

Regulation of Multiple Resonance Delayed Fluorescence via Through-Space Charge Transfer Excited State towards High-Efficiency and Stable Narrowband Electroluminescence.

B- and N-embedded multiple resonance (MR) type thermally activated delayed fluorescence (TADF) emitters usually suffer from slow reverse intersystem crossing (RISC) process and aggregation-caused emission quenching. Here, we report the design of a sandwich structure by placing the B-N MR core between two electron-donating moieties, inducing through-space charge transfer (TSCT) states. The proper adjusting of the energy levels brings about a 10-fold higher RISC rate in comparison with the parent B-N molecule. In the meantime, a high photoluminescence quantum yield of 91 % and a good color purity were maintained. Organic light-emitting diodes based on the new MR emitter achieved a maximum external quantum efficiency of 31.7 % and small roll-offs at high brightness. High device efficiencies were also obtained for a wide range of doping concentrations of up to 20 wt % thanks to the steric shielding of the B-N core. A good operational stability with LT95 of 85.2 h has also been revealed. The dual steric and electronic effects resulting from the introduction of a TSCT state offer an effective molecular design to address the critical challenges of MR-TADF emitters.

Long-range Through-space Charge Transfer in a pH-responsive Mixed Cyclophane of Pyridine and Teropyrene.

A large, strained (SE=44.2 kcal/mol) and conformationally flexible mixed cyclophane of pyridine and teropyrene was synthesized using two intramolecular Wurtz coupling reactions and an unprecedented Scholl reaction between the unreactive 2 positions of the pyrene systems in a triply bridged pyrenophane. Protonation of the pyridine unit results in a greatly enhanced preference for nesting in the cavity of the highly bent teropyrene system (θcalc =162.6°) and emergence of a charge transfer absorption band (λmax =592 nm) due to a long range (5.0-5.5 Å), through-space intramolecular transition between the teropyrene and pyridinium units, which does not exist in the neutral cyclophane.

Highly efficient color-tunable organic co-crystals unveiling polymorphism, isomerism, delayed fluorescence for optical waveguides and cell-imaging

Photofunctional co-crystal engineering strategies based on donor-acceptor π-conjugated system facilitates expedient molecular packing, consistent morphology, and switchable optical properties, conferring synergic ‘structure-property relationship’ for optoelectronic and biological functions. In this work, a series of organic co-crystals were formulated using a twisted aromatic hydrocarbon (TAH) donor and three diverse planar acceptors, resulting in color-tunable solid and aggregated state emission via variable packing and through-space charge-transfer interactions. While, adjusting the strength of acceptors, a structural transformation into hybrid stacking modes ultimately results in color-specific polymorphs, a configurational cis-isomer with very high photoluminescence quantum yield. The cis-isomeric co-crystal exhibits triplet-harvesting thermally activated delayed fluorescence (TADF) characteristics, presenting a key discovery in hydrocarbon-based multicomponent systems. Further, 1D-microrod-shaped co-crystal acts as an efficient photon-transducing optical waveguides, and their excellent dispersibility in water endows efficient cellular internalization with bright cell imaging performances. These salient approaches may open more avenues for the design and applications of TAH based co-crystals.

Uncovering the substituted-position effect on excited-state evolution of benzophenone-phenothiazine dyads.

Photofunctional materials based on donor-acceptor molecules have drawn intense attention due to their unique optical properties. Importantly, Systematic investigation of substitution effects on excited-state charge transfer dynamics of donor-acceptor molecules is a powerful approach for identifying application-relevant design principles. Here, by coupling phenothiazine (PTZ) at the ortho-, meta-, and para-positions of the benzene ring of benzophenone (BP), three regioisomeric BP-PTZ dyads were designed to understand the relationship between substituted positions and excited-state evolution channels. Ultrafast transient absorption is used to detect and trace the transient species and related evolution channels of BP-PTZ dyads at excited state. In a non-polar solvent, BP-o-PTZ undergoes the through-space charge transfer process to produce a singlet charge-transfer (1CT) state, which subsequently proceeds the intersystem crossing process and transforms into a triplet charge-transfer (3CT) state; BP-m-PTZ experiences intramolecular charge transfer (ICT) process to generate the 1CT state, which subsequently transforms into the 3CT state by the intersystem crossing (ISC) and finally converts into the local-excited triplet (3LE) state; as for BP-p-PTZ, only 3LE states can be detected after the ISC process from the 1CT state. On the other hand, the twisted ICT states are generated via twisted motion between the donor and acceptor for all BP-PTZ dyads or planarization of the PTZ unit in high polar solvents. The excited-state theoretical calculations unveil that the features of ICT and intramolecular interaction between the three dyads play a decisive role in determining the through-bond charge transfer and through-space charge transfer processes. Also, these results demonstrate that the excited-state evolution channels of PTZ derivatives could be modified by tuning the substituted positions of the donor-acceptor dyads. This study provides a deep perspective for the substitute-position effect on donor-acceptor-type PTZ derivatives.

Phosphine-Oxide-Balanced Intra- and Interchain Through-Space Charge Transfer in Thermally Activated Delayed Fluorescence Polymers: Beyond 30% External Quantum Efficiency.

Through-space charge transfer (TSCT) is crucial for developing highly efficient thermally activated delayed fluorescence polymers. The balance of intra- and interchain TSCT can markedly improve performance, but it is still a big challenge. In this work, an effective strategy for "intra- and interchain TSCT balance" is demonstrated by way of a series of non-conjugated copolymers containing a 9,9-dimethylacridine donor and triazine-phosphine oxide (PO)-based acceptors. Steady-state and transient emission spectra indicate that compared to the corresponding blends, the copolymers can indeed achieve balanced intra- and interchain TSCT by accurately optimizing the inductive and steric effects of the acceptors. The DPOT acceptor with the strongest electron-withdrawing ability and the second bigger steric hindrance endows its copolymers with state-of-the-art photoluminescence and electroluminescence quantum efficiencies beyond 95% and 32%, respectively. This demonstrates that, compared to other congeners, the synergistic inductive and steric effects effectively enhance TSCT in DPOT-based copolymers for radiation, and suppress singlet and triplet quenching. The record-high efficiencies of its devices make this kind of copolymers hold the potential for low-cost, large-scale, and high-efficiency applications.

Design and Synthesis of Red Through-Space Charge Transfer Thermally Activated Delayed Fluorescence Emitters with Donor/Acceptor/Donor Stacking.

Red through-space charge transfer thermally activated delayed fluorescence (TSCT TADF) materials named SAF36DCPP and SAF27DCPP with sandwiched structures were synthesized. Single crystals indicated that the intramolecular C-H···π interactions play a vital role in rigidifying the sandwiched structure, which results in a fluorescence yield of 63% for SAF36DCPP compared to 40% for SAF27DCPP. Organic light-emitting diodes with SAF36DCPP as the emitter realized a maximum external quantum efficiency of 16.12%.

Space-confined through-space charge-transfer emitters based on CN-substituted fluorene acceptors: Design, synthesis, and thermally-activated delayed fluorescence

Space-confined through-space charge-transfer (TSCT) emitters have been extensively explored for developing thermally-activated delayed fluorescence (TADF), but their donor/acceptor units need to be expanded for tuning the emission properties. Herein we report TSCT-TADF emitters 1–3 by chemically fixing cyano-substituted fluorenes as the acceptors. For 1 and 2, 9H-fluorene-2,5-dicarbonitrile (2,5-CN-FL) and 9H-fluorene-2,7-dicarbonitrile (2,7-CN-FL) are used as the acceptors, respectively, and 1,3,6,8-tetramethyl-9H-carbazole is used as the donor. For emitter 3, 2,7-CN-FL and benzo[5,6][1,4]oxazino[2,3,4-kl]phenoxazine are used as the acceptor and the donor, respectively. Single-crystal structures and theoretical calculations reveal close, cofacial donor/acceptor alignments and thus effective TSCT in 1–3. In doped 2,8-bis(diphenylphosphoryl) dibenzo[b,d]furan films, emitters 1 and 2 show similar blue-green emission centered at around 510 nm, and emitter 3 shows yellow emission centered at 562 nm, with photoluminescent efficiencies at 0.66−0.76. Compared to emitters 1 and 2, emitter 3 shows much faster reverse intersystem crossing due to its much smaller singlet-triplet energy splitting. Organic light-emitting diodes (OLEDs) using 1–3 as the emitters afford blue-green or yellow electroluminescence with high external quantum efficiencies up to 14.9%. The work demonstrates that cyano-substituted fluorenes with a planar and rigid geometry are promising acceptors for developing TSCT-TADF emitters for OLED applications.

Spiro[fluorene-9,9ʹ-xanthene] based deep-blue emitter and the hidden electroplex for solution-processed non-doped white organic light-emitting devices

A deep-blue fluorescent emitter with an A−D−A-skeleton, named pTRASX, constructed from two diphenyltriazine acceptors and one spiro[fluorene-9,9ʹ-xanthene] (SFX) donor, was deliberately designed and synthesized. The blocking effect of O-atom in xanthene moiety of SFX and the rigid A−D−A-skeleton endow a deep-blue emission at 410 nm with a photoluminescence quantum yield of 43% in solid film. The solution-processed non-doped white organic light-emitting diode (OLED) was demonstrated by merging the emission of pTRASX and its electroplex. Based on the intermolecular through-space charge-transfer, the electroplex mechanism resulted in the non-doped white OLED with the wide spectral range from 400 to 800 nm, the color coordinate of (0.33, 0.27), the high color rendering index of 85, and the external quantum efficiency of 0.49%. The electro- and photo-luminance measurements clarify that the electroplex emission originates from the intermolecular interaction between pTRASX and the adjacent electron transporting layer, ranging from ca. 540 to 800 nm. This study illustrates that the rational molecular design cooperated with the innovative electroluminescent mechanism blazes a trail for simple solution-processed and metal-free blue and white OLED.

Alternating Thermally Activated Delayed Fluorescence Copolymers Featuring Through-Space Charge Transfer for Efficient Electroluminescence

Alternating Thermally Activated Delayed Fluorescence Copolymers Featuring Through-Space Charge Transfer for Efficient Electroluminescence

Molecular design strategy for through-space charge transfer blue polyimides with rigid non-conjugated backbone and the role of alicyclic imide linker

The exploration of blue polymeric emitters plays an important role in the application of solution-processed polymer light-emitting diodes (PLEDs) in full color display. But the development of highly efficient blue thermally activated delayed fluorescence (TADF) polymers is still extremely challenging. Here, a series of rigid non-conjugated polyimide (PI)-based blue thermally activated delayed fluorescence (TADF) polymers with through-space charge transfer effect were firstly reported based on our novel design strategy. Among them, TRZ-oC-ph (λem = 497 nm) with a typical face-to-face structure as TADF unit was introduced into a polyimide backbone using twisted alicyclic imide as Linker unit. The twisted Linker unit with different structures can effectively regulate the d-spacing between donor and acceptor in the TADF unit, thus obtaining a series of blue TADF polyimides with λem at 485 nm, and high thermal stability (Tg > 277 °C, Td > 477 °C). Remarkably, highly-efficient solution-processing blue polymer light emitting diodes (PLEDs) based on the above polyimides were successfully assembled, with a maximal external quantum efficiency of 23.2%. Such outstanding efficiency is amongst the state-of-the-art performance of non-conjugated PLEDs. It confirms the effectiveness of the structural design strategy, and provides useful and valuable guidance for the development of highly-efficient blue TADF polymer materials and PLEDs.