Amplified spontaneous emission from a liquid crystalline phase: anisotropic property and active modulation.

Amplified spontaneous emission (ASE) is considered to be a primary indication of optical gain in active media without an external resonator. Molecular materials with ASE are expected to be one of the suitable light sources for specific applications such as optical coherent tomography owing to their low coherence and flexible tunability. Concentration quenching of emissive excited states has been a critical issue to boost the quantum efficiency of molecular materials in their condensed phases. The rod-like design of molecules with excited state intramolecular proton transfer (ESIPT) has been demonstrated to overcome this issue in highly-concentrated molecularly-doped systems, as represented by C4alkyne-HBT (2-(4-(1-hexynyl)-2-hydroxyphenyl)-benzothiazole). We designed an ESIPT molecule-doped liquid crystalline (LC) system for optical amplification via the ASE regime with its wide tunability of emission intensity. Detailed ASE behaviour and optical gain of a LC blend of C4alkyne-HBT and 4-pentyl-4'-cyano biphenyl (5CB) was evaluated to afford a maximum optical gain of 16.5 cm-1 with an estimated ASE threshold of optical pumping at 0.6-0.7 mJ cm-2. Although most ASE studies focus on homogeneous solutions, solids, or crystalline states, ASE from a soft-flexible LC phase is quite limited and advantageous for the design of an external optical resonator/cavity structure. Optical excitation parallel and perpendicular to the director resulted in the strong modulation of the ASE. By using the benefits of a LC phase, the ASE was actively modulated under the external electric field by the reorientation of the molecular dipole moment.

Ultra-fast single-crystal polymerization of large-sized covalent organic frameworks.

In principle, polymerization tends to produce amorphous or poorly crystalline materials. Efficiently producing high-quality single crystals by polymerization in solvent remains as an unsolved issue in chemistry, especially for covalent organic frameworks (COFs) with highly complex structures. To produce µm-sized single crystals, the growth time is prolonged to >15 days, far away from the requirements in practical applications. Here, we find supercritical CO2 (sc-CO2) accelerates single-crystal polymerization by 10,000,000 folds, and produces two-dimensional (2D) COF single crystals with size up to 0.2 mm within 2~5 min. Although it is the fastest single-crystal polymerization, the growth in sc-CO2 leads to not only the largest crystal size of 2D COFs, but also higher quality with improved photoconductivity performance. This work overcomes traditional concept on low efficiency of single-crystal polymerization, and holds great promise for future applications owing to its efficiency, industrial compatibility, environmental friendliness and universality for different crystalline structures and linkage bonds.

Ubiquitous organic molecule-based free-standing nanowires with ultra-high aspect ratios.

The critical dimension of semiconductor devices is approaching the single-nm regime, and a variety of practical devices of this scale are targeted for production. Planar structures of nano-devices are still the center of fabrication techniques, which limit further integration of devices into a chip. Extension into 3D space is a promising strategy for future; however, the surface interaction in 3D nanospace make it hard to integrate nanostructures with ultrahigh aspect ratios. Here we report a unique technique using high-energy charged particles to produce free-standing 1D organic nanostructures with high aspect ratios over 100 and controlled number density. Along the straight trajectory of particles penetrating the films of various sublimable organic molecules, 1D nanowires were formed with approximately 10~15 nm thickness and controlled length. An all-dry process was developed to isolate the nanowires, and planar or coaxial heterojunction structures were built into the nanowires. Electrical and structural functions of the developed standing nanowire arrays were investigated, demonstrating the potential of the present ultrathin organic nanowire systems.

Metal-Free Organic Luminophores that Exhibit Dual Fluorescence and Phosphorescence Emission at Room Temperature.

Dual-fluorescent-phosphorescent compounds have attracted increasing attention in various fields, such as bio-imaging, data protection/encryption, ratiometric luminescence sensing, and white-light emission. Conventional dual-emissive compounds contain a phosphorescent organometallic complex of a precious metal, such as iridium or platinum. However, the use of precious metals in organic materials has several drawbacks. This Minireview focuses on precious-metal-free organic light-emitting materials that exhibit dual fluorescence and phosphorescence emission in the solid state at room temperature to produce bimodal steady-state emission spectra. The dual emitters presented herein are categorized into the following six compound classes: (1) difluoroboron diaroylmethanes, (2) diarylketones, (3) diarylsulfones, (4) triazines and pyrimidines, (5) fused phenazines, and (6) N-arylcarbazoles.

A structural parameter to link molecular geometry to macroscopic orientation in discotic liquid crystals: study of metalloporphyrin tapes.

Novel liquid crystalline (LC) molecules were prepared from a dimeric porphyrin tape. A series of metal complexes (1Zn, 1Pd, 1Cu, and 1Ni) and the free-base form (12H) of the porphyrin tape formed a columnar LC phase. Although only the central metal ions were different among these compounds, 1Ni, 12H, and 1Cu aligned homeotropically in a sandwiched glass cell, while 1Zn and 1Pd exhibited a random orientation at a macroscopic scale. The strength of the π-π interactions, tunable by the distortion of the porphyrin cores through metallation, is a key factor for the observed macroscopic orientation difference.

Towards Macroscopically Anisotropic Functionality: Oriented Metallo-supramolecular Polymeric Materials Induced by Magnetic Fields.

Based on the predesigned self-selective complexation, metallo-supramolecular P3HT-b-PEO diblock copolymers with varying block ratios were synthesized, and their oriented polymer films generated during solvent evaporation in a 9 T magnetic field were investigated. An anisotropic, ordered layer structure was achieved using [P3HT20 -Zn-PEO107 ] and carefully characterized by polarized optical microscopy (POM), AFM, polarized UV/Vis spectroscopy, and GI-SAXS/WAXS. The PEO-removed [P3HT20 -Zn-PEO107 ] film was obtained after decomplexation with TEA-EDTA under mild conditions, and the selective removal of PEO domains was evidenced by UV/Vis and ATR-FTIR spectroscopy. Anisotropic photoconductivity of the magnetically aligned film was evaluated by flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements. The results indicated that the presence of insulating crystalline PEO segments diminished the photoconductivity along the P3HT backbone direction.

Efficient Emission of Ultraviolet Light by Solid State Organic Fluorophores: Synthesis and Characterization of 1,4-Dialkeny-2,5-dioxybenzenes.

The design and development of organic luminophores that exhibit efficient ultraviolet (UV) fluorescence in the solid state remains underexplored. Here, we report that 1,4-dialkenyl-2,5-dialkoxybenzenes and 1,4-dialkenyl-2,5-disiloxybenzenes act as such UV-emissive fluorophores. The dialkenyldioxybenzenes were readily prepared in three steps from 2,5-dimethoxy-1,4-diacetylbenzene or 2,5-dimethoxy-1,4-diformylbenzene via two to four steps from 1,4-bis(diethoxyphosphonylmethyl)-2,5-dimethoxybenzene. The dialkenyldioxybenzenes emit UV light in solution (λem =350-387 nm) and in the solid state (λem =328-388 nm). In addition, the quantum yields in the solid state were generally higher than those in solution. In particular, the adamantylidene-substituted benzenes fluoresced in the UV region with high quantum yields (Φ=0.37-0.55) in the solid state. Thin films of poly(methyl methacrylate) doped with the adamantylidene-substituted benzenes also exhibited UV emission with good efficiency (Φ=0.27-0.45). Density functional theory calculations revealed that the optical excitation of the dialkenyldimethoxybenzenes involves intramolecular charge-transfer from the ether oxygen atoms to the twisted alkenyl-benzene-alkenyl moiety, whereas the dialkenylbis(triphenylsiloxy)benzenes were optically excited through intramolecular charge-transfer from the oxygen atoms and twisted π-system to the phenyl-Si moieties of each triphenylsilyl group.

Extended conjugation of ESIPT-type dopants in nematic liquid crystalline phase for enhancing fluorescence efficiency and anisotropy.

Organic compounds capable of excited-state intramolecular proton transfer (ESIPT) show fluorescence with a large Stokes shift and serve as solid-state emitters, luminescent dopants, and fluorescence-based sensing materials. Fluorescence of ESIPT molecules is usually increased in the solid state, but is weak in solvents due to the accelerated non-radiative decays by rotational motions of a part of the molecular core in these environments. Here we report, using a representative ESIPT motif 2-(2-hydroxyphenyl)benzothiazole (HBT), the extended-conjugation strategy of keeping sufficient fluorescence efficiency both in the solid state and in organic media. The introduction of an alkyl-terminated phenylene-ethynylene group into the HBT molecule dramatically enhances the fluorescence quantum yield from 0.01 to 0.20 in toluene and from 0.07 to 0.32 in a representative room-temperature nematic liquid crystal, 4-pentyl-4'-cyano biphenyl (5CB). The newly-synthesized CnP-C[triple bond, length as m-dash]C-HBT (n = 5 or 8) serves as a fluorescent dopant in 5CB and exhibits anisotropic fluorescence with the order parameter of 0.48, where the luminescence is controlled by the applied electric-field. The enhanced emission efficiency is rationalized by the larger height of energy barrier for the ESIPT process due to the introduction of phenylene-ethynylene groups.

Frustrated Layered Self-Assembly Induced Superlattice from Two-Dimensional Nanosheets.

Here we reported a hierarchical self-assembly approach toward well-defined superlattices in supramolecular liquid crystals by fullerene-based sphere-cone block molecules. The fullerenes crystallize to form monolayer nanosheets intercalated by the attached soft hydrocarbon cones. The frustration caused by cross-sectional area mismatch between the spheres and the somewhat oversize cones leads to a unique lamellar superlattice whereby each stack of six pairs of alternating sphere-cone sublayers is followed by a cone double layer. While such areal mismatch problems in soft matter are usually solved by interface curvature, the lamellar superlattice solution is best suited to systems with rigid layers. Meanwhile, formation of the superlattice significantly improves the material's transient electron conductivity, with the maximum value being among the highest for π-conjugated organic materials. The design principle of solving steric frustration by forming a superlattice opens a new avenue toward self-assembled optoelectronic materials.

Ultrathin Two Dimensional (2D) Supramolecular Assembly and Anisotropic Conductivity of an Amphiphilic Naphthalene-Diimide.

Two-dimensional (2D)-supramolecular assemblies of π-conjugated chromophores are relatively less common compared to a large number of recent examples on their low dimensional (0D or 1D) assemblies or 3D architectures. This article reports a rational design for the 2D supramolecular assembly of an amphiphilic core-substituted naphthalene-diimide derivative (cNDI-1). The building block contains a naphthalene-diimide (NDI) chromophore, symmetrically substituted with two dodecyl chains from the aromatic core while the imide positions are functionalized with two hydrophilic wedges containing oligo-oxyethylene chains. In water, it exhibits entropically favorable self-assembly with a critical aggregation concentration of 1.5 × 10-5 M and a lower critical solution temperature of 55 °C. The UV/vis absorption spectrum in water shows bathochromically shifted absorption bands compared to that of the monomeric dye in THF, indicating offset π-stacking among the NDI chromophores. C-H symmetric and asymmetric stretching frequencies in the FT-IR spectrum support the presence of organized hydrocarbon chains in trans conformation in the self-assembled state, similar to that in the crystalline n-alkanes, which is further supported by studying the general polarization (GP) values of a noncovalently entrapped Laurdan dye. The atomic force microscopy (AFM) image shows the formation of ultrathin (height < 2.0 nm) ribbons for the spontaneously assembled sample which eventually produces a large-area 2D nanosheet by the lateral organization. The powder X-ray diffraction pattern of the drop-casted film, prepared from the preformed aggregates, reveals sharp peaks that indicate a crystalline lamellar packing along the direction of the 2D growth. Differential scanning calorimetry trace shows the melting of the crystalline alkyl chain domain at T > 75 °C, which destroys the 2D assembly. Local-scale photoconductivity of the ordered 2D assembly, studied by the flash-photolysis time-resolved microwave conductivity (FP-TRMC) technique, reveals an anisotropic conductivity with ∼3 times larger conductivity along the parallel direction compared to that along the perpendicular one.

Protein-directed crystalline 2D fullerene assemblies.

Water soluble 2D crystalline monolayers of fullerenes grow on planar assemblies of engineered consensus tetratricopeptide repeat proteins. Designed fullerene-coordinating tyrosine clamps on the protein introduce specific fullerene binding sites, which facilitate fullerene nucleation. Through reciprocal interactions between the components, the hybrid material assembles into two-dimensional 2 nm thick structures with crystalline order, that conduct photo-generated charges. Thus, the protein-fullerene hybrid material is a demonstration of the developments toward functional materials with protein-based precision control of functional elements.

Systematic Synthesis of Tetrathia[8]circulenes: The Influence of Peripheral Substituents on the Structures and Properties in Solution and Solid States.

We developed the diversity-oriented approach for the synthesis of tetrathia[8]circulenes with a variety of peripheral substituents. Iridium-catalyzed direct C-H borylation of tetrathienylene provided 1,4,7,10-tetraboryltetrathienylene as a major product. 1,4,7,10-Tetraboryltetrathienylene served as an a key intermediate to achieve the selective synthesis of octasubstituted or tetrasubstituted tetrathia[8]circulenes via rhodium-catalyzed annulation with symmetric internal alkynes or sequential Sonogashira-Hagihara coupling and base-promoted intramolecular cyclization. A variety of substituents were installed at the peripheral positions of tetrathia[8]circulenes systematically. The self-assembling behavior of tetrathia[8]circulenes was investigated using 1H NMR and AFM measurements. The number and the chain length of alkyl groups exerted a significant influence on the aggregation ability and the crystal packing structures of tetrathia[8]circulenes in both solution and solid states. We also found that the molecular arrangement of the self-assembled tetrathia[8]circulene molecules affected the hole mobility assessed by the FP-TRMC method.

Highly Miscible Hybrid Liquid-Crystal Systems Containing Fluorescent Excited-State Intramolecular Proton Transfer Molecules.

Doping of luminescent molecules in a nematic liquid-crystal (LC) host is a convenient approach to develop light-emitting LC displays that would be a promising alternative to conventional LC displays. The requirements for the luminescent guest molecules include high miscibility in the host LC, high-order parameters in the host LC media to show anisotropic luminescence, lack of self-absorption, transparency in the visible region, and a large photoluminescence quantum yield independent of its concentration. To address these issues, here, we newly synthesize a highly miscible and fluorescent excited-state intramolecular proton transfer molecule, C4-C≡C-HBT, based on 2-(2-hydroxyphenyl)benzothiazole (HBT). This compound is highly miscible in a conventional room-temperature nematic LC 4-pentyl-4'-cyano biphenyl (5CB) up to 14 wt % (∼12 mol %) and exhibits a large photoluminescence quantum yield of ΦFL = 0.32 in the 5CB host, both of which were achieved by the introduction of an alkynyl group into the HBT core. C4-C≡C-HBT possesses a high-order parameter of S = 0.46 in 5CB, and the C4-C≡C-HBT/5CB mixtures show anisotropic fluorescence whose intensity is controlled by the applied electric field. A patterned image is demonstrated, which is not visible under an ambient environment but is readable upon UV illumination, relying on the orientational differences of ordered C4-C≡C-HBT molecules.

Rod-like transition first or chain aggregation first? ordered aggregation of rod-like poly(p-phenyleneethynylene) chains in solution.

The rod-like configuration of conjugated polymer chains with its low energetic disorder is the key to utilizing the backbone as a highly electrically-conductive wire. An energetic disorder that is higher than 0.1 eV, coupled with vibronic modes of the chains, leads to the localization of charges. Herein, we have tracked precisely the rod-like transition of poly(p-phenyleneethynylene) (PPE) chains as a function of temperature in diluted solutions, and shown a steep increase in persistence length at 230 K. The resulting rod-like configuration of the PPE chains with its extended electronic conjugation exhibited an extremely small energetic disorder of ∼70 meV, and was stabilized by subsequent polymer aggregate formation.

Cold Crystallization of Ferrocene-Hinged π-Conjugated Molecule Induced by the Limited Conformational Freedom of Ferrocene.

Crystallization that proceeds above the glass transition temperature upon heating the glassy or amorphous state is referred to as "cold crystallization", which has often been observed in supercooled phases of molecular materials followed by the transition to the thermodynamically stable crystalline phase. Although this behavior is common for the macromolecules with high structural flexibility among segments preserving the wide temperature range of corresponding glassy phases, cold crystallization of small molecules is relatively rare and there is still less knowledge on the design guideline of such molecules. Here we report a ferrocene-hinged molecule DC12 carrying two units of didodecyl-substituted pentathiophenes at the 1,1'-positions. Due to the rotational freedom of the ferrocene unit, DC12 forms amorphous solid on cooling from its isotropic melt. The amorphous state was realized even by slow cooling such as 0.1 °C min-1. The possible reason for the easy formation of the amorphous phase is the coexistence of various conformations of DC12 originating from the open conformers of the ferrocene. On heating from the amorphous phase, DC12 shows cold crystallization with the estimated activation energy of 61 kJ mol-1. The crystalline phase is composed of the closed form of DC12 molecules packed in a lamellar fashion, and the degree of crystallinity is remarkably high compared with the case of macromolecular materials. The crystallization is triggered by the intermolecular interactions among the dodecyl chains and rotational flexibility of the ferrocene unit. This work provides an unprecedented example that ferrocenes act as heat-controllable rotational hinges in condensed phases. Considering that the cold crystallization phenomenon is related to heat-storage materials, the design strategy presented in this work will be novel to realize both easily formed amorphous phases and highly crystalline phases formed through cold crystallization.

Anomalous Photoinduced Hole Transport in Type I Core/Mesoporous-Shell Nanocrystals for Efficient Photocatalytic H2 Evolution.

Controlling the carrier dynamics in a semiconductor nanoparticulate photocatalyst is the key to developing catalytic activity. Generally, type I band alignment is unsuitable for photocatalysts because the photoinduced carriers accumulate in the narrow bandgap semiconductor. To avoid the termination of reactions and/or photocorrosion of materials caused by carrier accumulation, it is common to employ type II band alignment for photoenergy conversion systems instead of type I. However, CdS/ZnS core/mesoporous-shell heterostructures show superior photocatalytic activity despite having type I band alignment that is generally unfavorable for photocatalytic reactions. Transient absorption spectroscopy and time-resolved microwave conductivity revealed efficient photoinduced hole transfer from the CdS phase to the ZnS phase. The defect-mediated hole transfer from the CdS to the ZnS phase resulted in long-lived charge separation (>2.4 ms) leading to high photocatalytic performance. This study provides insight into defect-mediated carrier transfer in nanoparticulate photocatalysts, which could be used as a guideline for the design of highly active and stable nanoparticulate photocatalysts.

Pluripotent Features of Doubly Thiophene-Fused Benzodiphospholes as Organic Functional Materials.

Linear ladder-type π-conjugated molecules have attracted much interest because of their intriguing physicochemical properties. To modulate their electronic structures, an effective strategy is to incorporate main-group elements into ladder-type π-conjugated molecules. In line with this strategy, a variety of ladder-type π-conjugated molecules with main-group elements have been synthesized to explore their potential utility as organic functional materials. In this context, phosphole-based π-conjugated molecules are highly attractive, owing to their unique optical and electrochemical properties, which arise from the phosphorus atom. Herein, the synthesis and physicochemical properties of doubly thiophene-fused benzodiphospholes, as a new class of phosphole-based ladder-type π-conjugated molecule, are reported. Systematic investigations into the physicochemical properties of doubly thiophene-fused benzodiphospholes revealed their pluripotent features: intense near-infrared fluorescence, excellent two-photon absorption property, and remarkably high electron-transporting ability. This study demonstrates the potential utility of doubly thiophene-fused benzodiphospholes as organic functional materials for biological imaging, nonlinear optics, organic transistors, and organic photovoltaics.

Observation of dissociative quasi-free electron attachment to nucleoside via excited anion radical in solution.

Damage to DNA via dissociative electron attachment has been well-studied in both the gas and condensed phases; however, understanding this process in bulk solution at a fundamental level is still a challenge. Here, we use a picosecond pulse of a high energy electron beam to generate electrons in liquid diethylene glycol and observe the electron attachment dynamics to ribothymidine at different stages of electron relaxation. Our transient spectroscopic results reveal that the quasi-free electron with energy near the conduction band effectively attaches to ribothymidine leading to a new absorbing species that is characterized in the UV-visible region. This species exhibits a nearly concentration-independent decay with a time constant of ~350 ps. From time-resolved studies under different conditions, combined with data analysis and theoretical calculations, we assign this intermediate to an excited anion radical that undergoes N1-C1' glycosidic bond dissociation rather than relaxation to its ground state.

Conjugated Nanowire Sensors via High-Energy Single-Particle-Induced Linear Polymerization of 9,9'-Spirobi[9 H-fluorene] Derivatives.

Nanostructures composed of conjugated polymers or π-conjugated molecules provide sensing platforms with large specific surface areas. One of the feasible approaches to accessing such nanostructured miniaturized sensors with ultrahigh sensitivity is to develop a network of organic nanowires with optical/electronic properties that can measure signals upon interacting with the analytes at their surfaces. In this work, organic nanowires with controlled number density and uniform length were fabricated by one-dimensional solid-state polymerization of 9,9'-spirobi[9 H-fluorene] (SBF) derivatives triggered by high-energy single particles. SBF was chosen as a conjugated molecular motif with the interplay of high density of π-electrons, high solubility, and uniform solid-state structures, allowing us to fabricate sensing platforms via solution processing. The as-deposited energy density in linear polymerization nanospace was theoretically analyzed by a collision model, interpreting nanowire sizes at subnanometer levels. The substitution of bromine atoms was confirmed to be effective not only for the higher collision probability of the incident particles but also for the remarkable increase in radiolytic neutral radical yield via C-Br cleavages or electron-dissociative attachments onto the bromine atoms. The fluorescence spectra of SBF-based nanowires were different from those of SBF derivatives due to extended bond formation as a result of polymerization reactions. Fluorescence was quenched by the addition of nitrobenzene, indicating the potential use of our nanowires for fluorometric sensing applications. Microwave-based conductivity measurements revealed that the SBF-based nanowires exhibited charge carrier transport property upon photoexcitation, and that the conductivity was changed upon treatment with nitrobenzene vapors. The presented strategy of bromination of aromatic rings for efficient fabrication of controlled nanowire networks with favorable fluorescent and charge transport properties of nanowires advances the development of nanostructured sensing systems.

Electron delocalization and charge mobility as a function of reduction in a metal-organic framework.

Conductive metal-organic frameworks are an emerging class of three-dimensional architectures with degrees of modularity, synthetic flexibility and structural predictability that are unprecedented in other porous materials. However, engendering long-range charge delocalization and establishing synthetic strategies that are broadly applicable to the diverse range of structures encountered for this class of materials remain challenging. Here, we report the synthesis of K x Fe2(BDP)3 (0 ≤ x ≤ 2; BDP2- = 1,4-benzenedipyrazolate), which exhibits full charge delocalization within the parent framework and charge mobilities comparable to technologically relevant polymers and ceramics. Through a battery of spectroscopic methods, computational techniques and single-microcrystal field-effect transistor measurements, we demonstrate that fractional reduction of Fe2(BDP)3 results in a metal-organic framework that displays a nearly 10,000-fold enhancement in conductivity along a single crystallographic axis. The attainment of such properties in a K x Fe2(BDP)3 field-effect transistor represents the realization of a general synthetic strategy for the creation of new porous conductor-based devices.