Suppression of Dexter transfer by covalent encapsulation for efficient matrix-free narrowband deep blue hyperfluorescent OLEDs.

Hyperfluorescence shows great promise for the next generation of commercially feasible blue organic light-emitting diodes, for which eliminating the Dexter transfer to terminal emitter triplet states is key to efficiency and stability. Current devices rely on high-gap matrices to prevent Dexter transfer, which unfortunately leads to overly complex devices from a fabrication standpoint. Here we introduce a molecular design where ultranarrowband blue emitters are covalently encapsulated by insulating alkylene straps. Organic light-emitting diodes with simple emissive layers consisting of pristine thermally activated delayed fluorescence hosts doped with encapsulated terminal emitters exhibit negligible external quantum efficiency drops compared with non-doped devices, enabling a maximum external quantum efficiency of 21.5%. To explain the high efficiency in the absence of high-gap matrices, we turn to transient absorption spectroscopy. It is directly observed that Dexter transfer from a pristine thermally activated delayed fluorescence sensitizer host can be substantially reduced by an encapsulated terminal emitter, opening the door to highly efficient 'matrix-free' blue hyperfluorescence.

Singlet Fission in Pechmann Dyes: Planar Chromophore Design and Understanding.

Singlet fission in organic chromophores holds the potential for enhancing photovoltaic efficiencies beyond the single-junction limit. The most basic requirement of a singlet fission material is that it has a large energy gap between its first singlet and triplet excited states. Identifying such compounds is not simple and has been accomplished either through computational screening or by subtle modifications of previously known fission materials. Here, we propose an approach that leverages ground and excited-state aromaticity combined with double-bond conformation to establish simple qualitative design rules for predicting fundamental optical properties without the need for computational modeling. By investigating two Pechmann dye isomers, we demonstrate that although their planarity and degree of charge transfer are similar, singlet fission is active in the isomer with a trans-conformation, while the cis-isomer exhibits greater favorability for polaronic processes, experimentally validated using ultrafast and electron spin resonance spectroscopy. Our results offer a new design perspective that provides a rational framework for tailoring optoelectronic systems to specific applications such as singlet fission or triplet-triplet annihilation.

Steric Control of Luminescence in Phenyl-Substituted Trityl Radicals.

Triphenylmethyl (trityl) radicals have shown potential for use in organic optoelectronic applications, but the design of practical trityl structures has been limited to donor/radical charge-transfer systems due to the poor luminescence of alternant symmetry hydrocarbons. Here, we circumvent the symmetry-forbidden transition of alternant hydrocarbons via excited-state symmetry breaking in a series of phenyl-substituted tris(2,4,6-trichlorophenyl)methyl (TTM) radicals. We show that 3-fold phenyl substitution enhances the emission of the TTM radical and that steric control modulates the optical properties in these systems. Simple ortho-methylphenyl substitution boosts the photoluminescence quantum efficiency from 1% (for TTM) to 65% at a peak wavelength of 612 nm (for 2-T3TTM) in solution. In the crystalline solid state, the neat 2-T3TTM radical shows a remarkably high photoluminescence quantum efficiency of 25% for emission peaking at 706 nm. This has implications in the design of aryl-substituted radical structures where the electronic coupling of the substituents influences variables such as emission, charge transfer, and spin interaction.

Soluble Diphenylhexatriene Dimers for Intramolecular Singlet Fission with High Triplet Energy.

Intramolecular singlet fission (iSF) facilitates single-molecule exciton multiplication, converting an excited singlet state to a pair of triplet states within a single molecule. A critical parameter in determining the feasibility of SF-enhanced photovoltaic designs is the triplet energy; many existing iSF materials have triplet energies too low for efficient transfer to silicon via a photon multiplier scheme. In this work, a series of six novel dimers based upon the high-triplet-energy, SF-active chromophore, 1,6-diphenyl-1,3,5-hexatriene (DPH) [E(T1) ∼ 1.5 eV], were designed, synthesized, and characterized. Transient absorption spectroscopy and fluorescence lifetime studies reveal that five of the dimers display iSF activity, with time constants for singlet fission varying between 7 ± 2 ps and 2.2 ± 0.2 ns and a high triplet yield of 163 ± 63% in the best-performing dimer. A strong dependence of the rate of fission on the coupling geometry is demonstrated. For optimized iSF behavior, close spatial proximity and minimal through-bond communication are found to be crucial for balancing the rate of SF against the reverse recombination process.

Aza-Cibalackrot: Turning on Singlet Fission Through Crystal Engineering.

Singlet fission is a photophysical process that provides a pathway for more efficient harvesting of solar energy in photovoltaic devices. The design of singlet fission candidates is non-trivial and requires careful optimization of two key criteria: (1) correct energetic alignment and (2) appropriate intermolecular coupling. Meanwhile, this optimization must not come at the cost of molecular stability or feasibility for device applications. Cibalackrot is a historic and stable organic dye which, although it has been suggested to have ideal energetics, does not undergo singlet fission due to large interchromophore distances, as suggested by single crystal analysis. Thus, while the energetic alignment is satisfactory, the molecule does not have the desired intermolecular coupling. Herein, we improve this characteristic through molecular engineering with the first synthesis of an aza-cibalackrot and show, using ultrafast transient spectroscopy, that singlet fission is successfully "turned on."

Re-Thinking Dimer Design Principles with Indolonaphthyridine Intramolecular Singlet Fission.

Singlet fission is a phenomenon that could significantly improve the efficiency of photovoltaic devices. Indolonaphthyridine thiophene (INDT) is a photostable singlet fission material that could potentially be utilised in singlet fission-based photovoltaic devices. This study investigates the intramolecular singlet fission (i-SF) mechanism of INDT dimers linked via para-phenyl, meta-phenyl and fluorene bridging groups. Using ultra-fast spectroscopy the highest rate of singlet fission is found in the para-phenyl linked dimer. Quantum calculations show the para-phenyl linker encourages enhanced monomer electronic coupling. Increased rates of singlet fission were also observed in the higher polarity o-dichlorobenzene, relative to toluene, indicating that charge-transfer states have a role in mediating the process. The mechanistic picture of polarisable singlet fission materials, such as INDT, extends beyond the traditional mechanistic landscape.

N-Doped Nonalternant Nanoribbons with Excellent Nonlinear Optical Performance.

Two novel N-doped nonalternant nanoribbons (NNNR-1 and NNNR-2) featuring multiple fused N-heterocycles and bulky solubilizing groups were prepared via bottom-up solution synthesis. NNNR-2 achieves a total molecular length of 33.8 Å, which represents the longest soluble N-doped nonalternant nanoribbon reported to date. The pentagon subunits and doping of N atoms in NNNR-1 and NNNR-2 have successfully regulated their electronic properties, achieving high electron affinity and good chemical stability enabled by the nonalternant conjugation and electronic effects. When applied a laser pulse of 532 nm, the 13-rings nanoribbon NNNR-2 shows outstanding nonlinear optical (NLO) responses, with the nonlinear extinction coefficient of 374 cm GW-1 , much higher than those of NNNR-1 (96 cm GW-1 ) and the well-known NLO material C60 (153 cm GW-1 ). Our findings indicate that the N-doping of nonalternant nanoribbons is an effective strategy to access another type of excellent material system for high-performance NLO applications, which can be extended to construct numerous heteroatom-doped nonalternant nanoribbons with fine-tunable electronic properties.

An In-Situ Cyanidation Strategy To Access Tetracyanodiacenaphthoanthracene Diimides with High Electron Mobilities Exceeding 10†cm2 †V-1 s-1.

A family of novel highly π-extended tetracyano-substituted acene diimides, named as tetracyanodiacenaphthoanthracene diimides (TCDADIs), have been synthesized using a facile four-fold Knoevenagel condensation strategy. Unlike conventional cyano substitution reactions, our approach enables access to a large π-conjugated backbone with the in-situ formation of four cyano substitutents at room temperature while avoiding extra cyano-functionalization reactions. TCDADIs decorated with different N-alkyl substituents present good solubility, near-coplanar backbones, good crystallinity, and low-lying lowest unoccupied molecular orbital energies of -4.33 eV, all of which contribute to desirable electron-transport performance when applied in organic field-effect transistors (OFET). The highest electron mobility of an OFET based on a 2-hexyldecyl-substituted TCDADI single crystal reaches 12.6 cm2  V-1 s-1 , which is not only among the highest values for the reported n-type organic semiconductor materials (OSMs) but also exceeds that of most n-type OSMs decorated with imide units.

Quantitative Singlet Fission in Solution-Processable Dithienohexatrienes.

Singlet fission (SF) is a promising strategy to overcome thermalization losses and enhance the efficiency of single junction photovoltaics (PVs). The development of this field has been strongly material-limited, with a paucity of materials able to undergo SF. Rarer still are examples that can produce excitons of sufficient energy to be coupled to silicon PVs (>1.1 eV). Herein, we examine a series of a short-chain polyene, dithienohexatriene (DTH), with tailored material properties and triplet (T1) energy levels greater than 1.1 eV. We find that these highly soluble materials can be easily spin-cast to create thin films of high crystallinity that exhibit ultrafast singlet fission with near perfect triplet yields of up to 192%. We believe that these materials are the first solution-processable singlet fission materials with quantitative triplet formation and energy levels appropriate for use in conjunction with silicon PVs.

Molecular Encapsulation of Naphthalene Diimide (NDI) Based π-Conjugated Polymers: A Tool for Understanding Photoluminescence.

Conjugated polymers are an important class of chromophores for optoelectronic devices. Understanding and controlling their excited state properties, in particular, radiative and non-radiative recombination processes are among the greatest challenges that must be overcome. We report the synthesis and characterization of a molecularly encapsulated naphthalene diimide-based polymer, one of the most successfully used motifs, and explore its structural and optical properties. The molecular encapsulation enables a detailed understanding of the effect of interpolymer interactions. We reveal that the non-encapsulated analogue P(NDI-2OD-T) undergoes aggregation enhanced emission; an effect that is suppressed upon encapsulation due to an increasing π-interchain stacking distance. This suggests that decreasing π-stacking distances may be an attractive method to enhance the radiative properties of conjugated polymers in contrast to the current paradigm where it is viewed as a source of optical quenching.

Doubly Encapsulated Perylene Diimides: Effect of Molecular Encapsulation on Photophysical Properties.

Intermolecular interactions play a fundamental role on the performance of conjugated materials in organic electronic devices, as they heavily influence their optoelectronic properties. Synthetic control over the solid state properties of organic optoelectronic materials is crucial to access real life applications. Perylene diimides (PDIs) are one of the most highly studied classes of organic fluorescent dyes. In the solid state, π-π stacking suppresses their emission, limiting their use in a variety of applications. Here, we report the synthesis of a novel PDI dye that is encapsulated by four alkylene straps. X-ray crystallography indicates that intermolecular π-π stacking is completely suppressed in the crystalline state. This is further validated by the photophysical properties of the dye in both solution and solid state and supported by theoretical calculations. However, we find that the introduction of the encapsulating "arms" results in the creation of charge-transfer states which modify the excited state properties. This article demonstrates that molecular encapsulation can be used as a powerful tool to tune intermolecular interactions and thereby gain an extra level of control over the solid state properties of organic optoelectronic materials.

Double [4]Helicene-like Naphthobisbenzothiophene Diimides and Their Thienyl-S,S-dioxidized Derivatives with Attractive Solid-State Fluorescence and High Electron Affinity.

A series of double [4]helicene-like naphthobisbenzothiophene diimides and their thienyl-S,S-dioxidized derivatives are synthesized via MoCl5-catalyzed cyclization and m-CPBA-mediated oxidation reactions. The functional five-membered ring diimides show a helicene-like geometry, strong solid-state fluorescence, and deep LUMO of -4.37 eV.

Mesitylated trityl radicals, a platform for doublet emission: symmetry breaking, charge-transfer states and conjugated polymers.

Neutral π-radicals have potential for use as light emitters in optoelectronic devices due to the absence of energetically low-lying non-emissive states. Here, we report a defect-free synthetic methodology via mesityl substitution at the para-positions of tris(2,4,6-trichlorophenyl)methyl radical. These materials reveal a number of novel optoelectronic properties. Firstly, mesityl substituted radicals show strongly enhanced photoluminescence arising from symmetry breaking in the excited state. Secondly, photoexcitation of thin films of 8 wt% radical in 4,4'-bis(carbazol-9-yl)-1,1'-biphenyl host matrix produces long lived (in the order of microseconds) intermolecular charge transfer states, following hole transfer to the host, that can show unexpectedly efficient red-shifted emission. Thirdly, covalent attachment of carbazole into the mesitylated radical gives very high photoluminescence yield of 93% in 4,4'-bis(carbazol-9-yl)-1,1'-biphenyl films and light-emitting diodes with maximum external quantum efficiency of 28% at a wavelength of 689 nm. Fourthly, a main-chain copolymer of the mesitylated radical and 9,9-dioctyl-9H-fluorene shows red-shifted emission beyond 800 nm.

Clinical features, surgical outcomes and genetic analysis of ectodermal dysplasia with ocular diseases.

AIM: To report on the clinical features, surgical outcomes and gene mutation analysis of three ectodermal dysplasia probands with ocular diseases. METHODS: A case-note review of three unrelated probands diagnosing with ectodermal dysplasia with ocular diseases was undertaken. Patient clinical features and the outcomes of surgery were analysed. The suspected pathogenic genes were analysed by whole exome sequencing from patients with ectodermal dysplasia and Sanger sequencing from family members. RESULTS: The ocular clinical features of ectodermal dysplasia with ocular diseases mainly include eyelid ectropion, lagophthalmos and absence of lacrimal punctum. All the probands underwent surgeries of full-thickness free skin flap grafting to correct ectropion. They achieved good recovery, and there were no obvious complications during the follow-up. The gene sequencing results did not show any meaningful genetic mutations. CONCLUSION: Lid ectropion is one of the key clinical traits of ectodermal dysplasia with ocular diseases. Ectropion correction with full-thickness free skin flap grafting is an effective procedure to correct ectropion for ectodermal dysplasia patients with ichthyosis-like tissue. The suspected pathogenic genes of ectodermal dysplasia with ectropion should be further verified or confirmed by large samples of the family.

Triplet transfer from PbS quantum dots to tetracene ligands: is faster always better?

Quantum dot-organic semiconductor hybrid materials are gaining increasing attention as spin mixers for applications ranging from solar harvesting to spin memories. Triplet energy transfer between the inorganic quantum dot (QD) and organic semiconductor is a key step to understand in order to develop these applications. Here we report on the triplet energy transfer from PbS QDs to four energetically and structurally similar tetracene ligands. Even with similar ligands we find that the triplet energy transfer dynamics can vary significantly. For TIPS-tetracene derivatives with carboxylic acid, acetic acid and methanethiol anchoring groups on the short pro-cata side we find that triplet transfer occurs through a stepwise process, mediated via a surface state, whereas for monosubstituted TIPS-tetracene derivative 5-(4-benzoic acid)-12-triisopropylsilylethynyl tetracene (BAT) triplet transfer occurs directly, albeit slower, via a Dexter exchange mechanism. Even though triplet transfer is slower with BAT the overall yield is greater, as determined from upconverted emission using rubrene emitters. This work highlights that the surface-mediated transfer mechanism is plagued with parasitic loss pathways and that materials with direct Dexter-like triplet transfer are preferred for high-efficiency applications.

Excited state character of Cibalackrot-type compounds interpreted in terms of Hückel-aromaticity: a rationale for singlet fission chromophore design.

The exact energies of the lowest singlet and triplet excited states in organic chromophores are crucial to their performance in optoelectronic devices. The possibility of utilizing singlet fission to enhance the performance of photovoltaic devices has resulted in a wide demand for tuneable, stable organic chromophores with wide S1-T1 energy gaps (>1 eV). Cibalackrot-type compounds were recently considered to have favorably positioned excited state energies for singlet fission, and they were found to have a degree of aromaticity in the lowest triplet excited state (T1). This work reports on a revised and deepened theoretical analysis taking into account the excited state Hückel-aromatic (instead of Baird-aromatic) as well as diradical characters, with the aim to design new organic chromophores based on this scaffold in a rational way starting from qualitative theory. We demonstrate that the substituent strategy can effectively adjust the spin distribution on the chromophore and thereby manipulate the excited state energy levels. Additionally, the improved understanding of the aromatic characters enables us to demonstrate a feasible design strategy to vary the excited state energy levels by tuning the number and nature of Hückel-aromatic units in the excited state. Finally, our study elucidates the complications and pitfalls of the excited state aromaticity and antiaromaticity concepts, highlighting that quantitative results from quantum chemical calculations of various aromaticity indices must be linked with qualitative theoretical analysis of the character of the excited states.

Bis-acenaphthoquinone diimides with high electron deficiency and good coplanar conformation.

A series of novel bis-acenaphthoquinone diimides featuring a highly electron-deficient bis-acenaphthoquinone core are facilely synthesized via Knoevenagel condensation reaction. The diimides show high electron deficiency and good coplanar conformation, together with one of them having a maximum electron mobility up to 0.038 cm2 V-1 s-1.

Interface engineering in CeO2 (111) facets decorated with CdSe quantum dots for photocatalytic hydrogen evolution.

The interfaces of heterostructures have been widely studied in the field of photocatalytic H2 evolution reaction (HER). In the present study, the CdSe QDs/CeO2(111) heterostructures were synthesized by wet chemistry method. The CdSe QDs/CeO2(111)-0.075 showed higher photocatalytic H2 evolution with 283.32 µmol g-1h-1, because of the enhanced light absorbance intensity and edge, lower recombination, higher separation and transfer, as well as longer lifetime of the photogenerated carrier. Density functional theory (DFT) calculations further confirmed that the enhanced HER activity of CdSe QDs/CeO2(111) heterostructures is resulted from a stronger water adsorption, a lower energy barrier of water dissociation and a more optimal free energy of hydrogen adsorption than CdSe and CeO2. The strategy of construction heterostructures provides a promising pathway for enhancing the performance of photocatalytic H2 evolution as well as other catalytic reactions.

Acceptor plane expansion enhances horizontal orientation of thermally activated delayed fluorescence emitters.

Manipulating orientation of organic emitters remains a formidable challenge in organic light-emitting diodes (OLEDs). Here, expansion of the acceptor plane of thermally activated delayed fluorescence (TADF) emitters was demonstrated to selectively modulate emitting dipole orientation. Two proof-of-the-concept molecules, PXZPyPM and PXZTAZPM, were prepared by introducing a planar 2-phenylpyridine or 2,4,6-triphenyl-1,3,5-triazine substituent into a prototypical molecule (PXZPM) bearing a pyrimidine core and two phenoxazine donors. This design approach suppressed the influence of substituents on electronic structures and associated optoelectronic properties. Accordingly, PXZPyPM and PXZTAZPM preserved almost the same excited states and similar emission characteristics as PXZPM. The expanded acceptor plane of PXZPyPM and PXZTAZPM resulted in a 15 to 18% increase in horizontal ratios of emitting dipole orientation. PXZPyPM supported its green device exhibiting an external quantum efficiency of 33.9% and a power efficiency of 118.9 lumen per watt, competitive with the most efficient green TADF OLEDs reported so far.

Boosting charge transfer via molybdenum doping and electric-field effect in bismuth tungstate: Density function theory calculation and potential applications.

Regulating internal electronic structure of photocatalysts via elements doping holds huge potential in tuning efficient charge transfer and boosting high-performance. Herein, molybdenum embedded bismuth tungstate (Bi2WO6) is employed to explore the electronic structures and various performances via the assistance of experimental verification and density function theory (DFT) simulation. The band structures and Mo ions doping behaviors of Bi2MoxW1-xO6 are systematically measured. Doping can induce the distortion of intrinsic electric density and internal electric-field, resulted in efficient charge transfer of Bi2Mo0.4W0.6O6. It exhibits much efficient photocatalytic activities under visible-light irradiation, also manifests huge potential as an anode material in lithium-ion batteries (LIBs) which is rarely reported before. This work may provide insights in the development of bismuth-based semiconductors in energy related applications.