Delayed fluorescence from inverted singlet and triplet excited states.

Hund's multiplicity rule states that a higher spin state has a lower energy for a given electronic configuration1. Rephrasing this rule for molecular excited states predicts a positive energy gap between spin-singlet and spin-triplet excited states, as has been consistent with numerous experimental observations over almost a century. Here we report a fluorescent molecule that disobeys Hund's rule and has a negative singlet-triplet energy gap of -11 ± 2 meV. The energy inversion of the singlet and triplet excited states results in delayed fluorescence with short time constants of 0.2 µs, which anomalously decrease with decreasing temperature owing to the emissive singlet character of the lowest-energy excited state. Organic light-emitting diodes (OLEDs) using this molecule exhibited a fast transient electroluminescence decay with a peak external quantum efficiency of 17%, demonstrating its potential implications for optoelectronic devices, including displays, lighting and lasers.

Possibilities and Limitations in Monomer Combinations for Ternary Two-Dimensional Covalent Organic Frameworks.

The diversity and complexity of covalent organic frameworks (COFs) can be largely increased by incorporating multiple types of monomers with different topologies or sizes. However, an increase in the number of monomer types significantly complicates the COF formation process. Accordingly, much remains unclear regarding the viability of monomer combinations for ternary or higher-arity COFs. Herein, we show that, through an extensive examination of 12 two-nodes-one-linker ([2 + 1]) combinations, monomer-set viability is determined primarily by the conformational strain originating from disordered monomer arrangements, rather than other factors such as the difference in COF formation kinetics between monomers. When monomers cannot accommodate the strain associated with the formation of a locally disordered, yet crystalline framework, the corresponding [2 + 1] condensation yields a mixture of different COFs or an amorphous polymer. We also demonstrate that a node-linker pair that does not form a binary COF can be integrated to generate a single-phase framework upon addition of a small amount of the third component. These results will clarify the factors behind the successful formation of multicomponent COFs and refine their design by enabling accurate differentiation between allowed and disallowed monomer combinations.

Synthesis of Anthracene Derivatives with Azaacene-Containing Iptycene Wings and the Utilization as a Dopant for Solution-Processed Organic Light-Emitting Diodes.

Substituted acene derivatives are regarded as promising materials for organic electronic devices such as organic light-emitting diodes (OLEDs). In particular, anthracene derivatives are known to exhibit good fluorescence property, with the air stability and solubility in common organic solvents expected to give advantages for solution-processed device fabrication. In this study, a series of bistriisopropylsilyl(TIPS)ethynyl anthracene derivatives with azaacene-containing iptycene wings have been synthesized by using condensation reactions. Effects of size of azaacenes on optical properties and packing structures were investigated. UV/Vis absorption and fluorescence spectra indicate that the π-elongation of iptycene units has small effects on the overall π-system, which is also supported by electrochemical measurements. Secondly, single-crystal X-ray analysis implies that the molecules likely have interactions with the iptycene units of adjacent molecules, while the iptycene wings and TIPSethynyl groups can prevent the central anthracene unit from undesirable non-radiative energy loss. Finally, the most emissive derivative was used as a dopant for solution-processed OLEDs, showing obvious electroluminescence with a luminance of over 920 cd m-2 .

Room-Temperature Phosphorescence from a Series of 3-Pyridylcarbazole Derivatives.

Exploration of pure metal-free organic molecules that exhibit strong room-temperature phosphorescence (RTP) is an emerging research topic. In this regard, unveiling the design principles for an efficient RTP molecule is an essential, but challenging, task. A small molecule is an ideal platform to precisely understand the fundamental role of each functional component because the parent molecule can be easily derivatized. Here, the RTP behaviors of a series of 3-pyridylcarbazole derivatives are presented. Experimental studies in combination with theoretical calculations reveal the crucial role of the n orbital on the central pyridine ring in the dramatic enhancement of the intersystem crossing between the charge-transfer-excited singlet state and the locally excited triplet states. Single-crystal X-ray crystallographic studies apparently indicate that both the pyridine ring and fluorine atom contribute to the enhancement of the RTP because of the restricted motion owing to weak C-H⋅⋅⋅N and H⋅⋅⋅F hydrogen-bonding interactions. The single crystal of the fluorine-substituted derivative shows an ultra-long phosphorescent lifetime (τP ) of 1.1 s and a phosphorescence quantum yield (ΦP ) of 1.2 %, whereas the bromine-substituted derivative exhibits τP of 0.15 s with a ΦP of 7.9 %. We believe that this work provides a fundamental and universal guideline for the generation of pure organic molecules exhibiting strong RTP.

Solution-based electrical doping of semiconducting polymer films over a limited depth.

Solution-based electrical doping protocols may allow more versatility in the design of organic electronic devices; yet, controlling the diffusion of dopants in organic semiconductors and their stability has proven challenging. Here we present a solution-based approach for electrical p-doping of films of donor conjugated organic semiconductors and their blends with acceptors over a limited depth with a decay constant of 10-20 nm by post-process immersion into a polyoxometalate solution (phosphomolybdic acid, PMA) in nitromethane. PMA-doped films show increased electrical conductivity and work function, reduced solubility in the processing solvent, and improved photo-oxidative stability in air. This approach is applicable to a variety of organic semiconductors used in photovoltaics and field-effect transistors. PMA doping over a limited depth of bulk heterojunction polymeric films, in which amine-containing polymers were mixed in the solution used for film formation, enables single-layer organic photovoltaic devices, processed at room temperature, with power conversion efficiencies up to 5.9 ± 0.2% and stable performance on shelf-lifetime studies at 60 °C for at least 280 h.

Modulating Photo- and Electroluminescence in a Stimuli-Responsive π-Conjugated Donor-Acceptor Molecular System.

Functional organic materials that display reversible changes in fluorescence in response to external stimuli are of immense interest owing to their potential applications in sensors, probes, and security links. While earlier studies mainly focused on changes in photoluminescence (PL) color in response to external stimuli, stimuli-responsive electroluminescence (EL) has not yet been explored for color-tunable emitters in organic light-emitting diodes (OLEDs). Here a stimuli-responsive fluorophoric molecular system is reported that is capable of switching its emission color between green and orange in the solid state upon grinding, heating, and exposure to chemical vapor. A mechanistic study combining X-ray diffraction analysis and quantum chemical calculations reveals that the tunable green/orange emissions originate from the fluorophore's alternating excited-state conformers formed in the crystalline and amorphous phases. By taking advantage of this stimuli-responsive fluorescence behavior, two-color emissive OLEDs were produced using the same fluorophore in different solid phases.

Thermal equilibration between singlet and triplet excited states in organic fluorophore for submicrosecond delayed fluorescence.

In any complex molecular system, electronic excited states with different spin multiplicities can be described via a simple statistical thermodynamic formalism if the states are in thermal equilibrium. However, this ideal situation has hitherto been infeasible for efficient fluorescent organic molecules. Here, we report a highly emissive metal-free purely organic fluorophore that enables thermal equilibration between singlet and triplet excited states. The key to this unconventional excitonic behavior is the exceptionally fast spin-flipping reverse intersystem crossing from the triplet to singlet excited states with a rate constant exceeding 108 per second, which is considerably higher than that of radiative decay (fluorescence) from the singlet excited state. The present fluorophoric system exhibits an emission lifetime as short as 750 nanoseconds and, therefore, allows organic light-emitting diodes to demonstrate external electroluminescence quantum efficiency exceeding 20% even at a practical high luminance of more than 10,000 candelas per square meter.

Regulation of Multicolor Fluorescence Changes Found in Donor-acceptor-type Mechanochromic Fluorescent Dyes.

The regulation of multicolor fluorescence changes in mechanochromic fluorescence (MCF) remains a challenging task. Herein, we report the regulation of MCF using a donor-acceptor structure. Two crystal polymorphs, BTD-pCHO(O) and BTD-pCHO(R) produced by the introduction of formyl groups to an MCF dye, respond to a mechanical stimulus, allowing a three-color fluorescence change. Specifically, the orange-colored fluorescence of the metastable BTD-pCHO(O) polymorph changed to a deep-red color in the amorphous-like state to finally give a red color in the stable BTD-pCHO(R) polymorph. This change occurred by mechanical grinding followed by vapor fuming. The two different crystal packing patterns were selectively regulated by the electronic effect of the introduced functional groups. The two types of selectively formed crystals in BTD(F)-pCHO bearing fluorine atoms, and BTD(OMe)-pCHO bearing methoxy groups, respond to mechanical grinding, allowing for the regulation of multicolor MCL from a three-color change to two different types of two-color changes.

Electron spin resonance resolves intermediate triplet states in delayed fluorescence.

Molecular organic fluorophores are currently used in organic light-emitting diodes, though non-emissive triplet excitons generated in devices incorporating conventional fluorophores limit the efficiency. This limit can be overcome in materials that have intramolecular charge-transfer excitonic states and associated small singlet-triplet energy separations; triplets can then be converted to emissive singlet excitons resulting in efficient delayed fluorescence. However, the mechanistic details of the spin interconversion have not yet been fully resolved. We report transient electron spin resonance studies that allow direct probing of the spin conversion in a series of delayed fluorescence fluorophores with varying energy gaps between local excitation and charge-transfer triplet states. The observation of distinct triplet signals, unusual in transient electron spin resonance, suggests that multiple triplet states mediate the photophysics for efficient light emission in delayed fluorescence emitters. We reveal that as the energy separation between local excitation and charge-transfer triplet states decreases, spin interconversion changes from a direct, singlet-triplet mechanism to an indirect mechanism involving intermediate states.

Kinetic prediction of reverse intersystem crossing in organic donor-acceptor molecules.

Reverse intersystem crossing (RISC), the uphill spin-flip process from a triplet to a singlet excited state, plays a key role in a wide range of photochemical applications. Understanding and predicting the kinetics of such processes in vastly different molecular structures would facilitate the rational material design. Here, we demonstrate a theoretical expression that successfully reproduces experimental RISC rate constants ranging over five orders of magnitude in twenty different molecules. We show that the spin flip occurs across the singlet-triplet crossing seam involving a higher-lying triplet excited state where the semi-classical Marcus parabola is no longer valid. The present model explains the counterintuitive substitution effects of bromine on the RISC rate constants of previously unknown molecules, providing a predictive tool for material design.

Exciplex emissions derived from exceptionally long-distance donor and acceptor molecules.

Intermolecular electron-hole coupling in organic semiconductor excited states plays important roles in organic light-emitting diodes and organic photovoltaics, and the distance of the coupling is typically only on the order of a few nanometers. Here, we report exceptionally long-distance coupled exciplex emissions between electron-donor and electron-acceptor molecules even with a 70 nm-thick spacer layer. Donor/spacer (∼70 nm)/acceptor-type stacked films showed a low-energy band emission, which is not ascribed to the emission of the donor, spacer, and acceptor themselves, but well corresponds to the energy difference between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor. Delayed transient photoluminescence (PL) and electroluminescence (EL) decays and PL quenching by oxygen at the low-energy band were observed and are consistent with the characteristics of the exciplex species.

High-Crystallinity π-Conjugated Small Molecules Based on Thienylene-Vinylene-Thienylene: Critical Role of Self-Organization in Photovoltaic, Charge-Transport, and Morphological Properties.

Narrow-band-gap small molecules with π-extended backbones are promising donor materials for solution-processed bulk-heterojunction (BHJ) organic solar cells (OSCs). Herein, a series of acceptor-donor-acceptor (A-D-A) photovoltaic small molecules incorporating thienylene-vinylene-thienylene (TVT) as a central D unit and alkyl-substituted rhodanine or 2-(1,1-dicyanomethylene)rhodanine as terminal A units are designed and synthesized. Their physical properties including photoabsorption, electronic energy levels, hole mobility, and morphological characteristics are systematically investigated. Using solvent vapor annealing (SVA), the morphologies of the BHJ photoactive layers composed of these small-molecule donors and a [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) acceptor can be properly modulated. As a result of increased crystallinity of the donors and desired phase segregation between the donors and PC71BM upon rapid SVA treatment, the photovoltaic performances of the resultant OSC devices undergo drastic enhancement. The results reported here indicate that high-efficiency small-molecule OSCs can be achieved through rational design of the TVT-based molecular framework and optimization of the nanoscale phase-segregated morphology via proper SVA treatment.

Versatile Molecular Functionalization for Inhibiting Concentration Quenching of Thermally Activated Delayed Fluorescence.

Concentration quenching of thermally activated delayed fluorescence is found to be dominated by electron-exchange interactions, as described by the Dexter energy-transfer model. Owing to the short-range nature of the electron-exchange interactions, even a small modulation in the molecular geometric structure drastically affects the concentration-quenching, leading to enhanced solid-state photoluminescence and electroluminescence quantum efficiencies.

Simultaneous cross-linking and p-doping of a polymeric semiconductor film by immersion into a phosphomolybdic acid solution for use in organic solar cells.

Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) is shown to be simultaneously cross-linked and p-doped when immersed into a phosphomolybdic acid solution, yielding conductive films with low solubility that can withstand the solution processing of subsequent photoactive layers. Such a modified PCDTBT film serves to improve hole collection and limit carrier recombination in organic solar cells.

Fabrication of organic light-emitting devices comprising stacked light-emitting units by solution-based processes.

Multi-organic light-emitting devices comprising two light-emitting units stacked in series through a charge-generation layer are fabricated by solution processes. A zinc oxide nanoparticles/polyethylene-imine bilayer is used as the electron-injection layer and phosphomolybdic acid is used as the charge-generation layer. Appropriate choice of solvents during spin-coating of each layer ensures the nine-layered structure fabricated by solution processes.

Instant low-temperature cross-linking of poly(N-vinylcarbazole) for solution-processed multilayer blue phosphorescent organic light-emitting devices.

Poly(N-vinylcarbazole) undergoes cross-linking to highly solvent-resistant films through an oxidative coupling reaction, for which an annealing process takes only 3 min at 110 °C. This reaction allows the construction of a solution-processed multilayer OLED without a time-consuming annealing process. The maximum external quantum efficiency reaches 18%, and remains at 17%, even at a high brightness of 10 000 cd m(-2) for all-solution-processed blue OLEDs.

2,6-Bis(arylsulfonyl)anilines as Fluorescent Scaffolds through Intramolecular Hydrogen Bonds: Solid-State Fluorescence Materials and Turn-On-Type Probes Based on Aggregation-Induced Emission.

A series of 2,6-bis[aryl(alkyl)sulfonyl]anilines were synthesized by nucleophilic aromatic substitution of 2,6-dichloronitrobenzene with various aryl or alkyl thiolates (benzyl-, phenyl-, 2-naphthyl-, and 2-aminophenyl thiolate), followed by hydrogenation and subsequent oxidation. All prepared 2,6-bis[aryl-(alkyl)sulfonyl]anilines showed high fluorescence emissions in the solid state; X-ray structures revealed well-defined intramolecular hydrogen bonds, which served to immobilize the rotatable amino group and generate a fluorescence enhancement in addition to improved photostability. Moreover, absorption and fluorescence spectra showed redshifts in the order of benzyl

Solution-processed multilayer small-molecule light-emitting devices with high-efficiency white-light emission.

Recent developments in the field of π-conjugated polymers have led to considerable improvements in the performance of solution-processed organic light-emitting devices (OLEDs). However, further improving efficiency is still required to compete with other traditional light sources. Here we demonstrate efficient solution-processed multilayer OLEDs using small molecules. On the basis of estimates from a solvent resistance test of small host molecules, we demonstrate that covalent dimerization or trimerization instead of polymerization can afford conventional small host molecules sufficient resistance to alcohols used for processing upper layers. This allows us to construct multilayer OLEDs through subsequent solution-processing steps, achieving record-high power efficiencies of 36, 52 and 34 lm W(-1) at 100 cd m(-2) for solution-processed blue, green and white OLEDs, respectively, with stable electroluminescence spectra under varying current density. We also show that the composition at the resulting interface of solution-processed layers is a critical factor in determining device performance.