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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Supramolecular Chirality Issues in Unorthodox Naphthalene Diimide Gelators.

Herein, the self-assembly of a few 1,3-dihydroxyl functionalized naphthalene diimide (NDI) derivatives has been reported with particular emphasis on the impact of chirality on gelation and the effect of self-assembly on charge-carrier mobility. A nonconventional gelator (R)-NDI, devoid of any long alkyl chains, exhibited spontaneous gelation in tetra-chloroethylene (TCE). Based on X-ray crystallography and powder X-ray diffraction studies, it was established that a ladder-like hydrogen-bonded chain formation between the 1,3-dihydroxyl group leads to the fibrillar structures with preferential helicity. Likewise the (S)-isomer also exhibited identical gelation and mesoscopic structure but produced fibrils with the opposite handedness. Intriguingly, an equimolar mixture of the (R)- and (S)-isomers did not show any gelation ability, rather a macroscopic precipitation was observed and, in sharp contrast to the individual isomers, the morphology of the mixture showed ill-defined near spherical agglomerates. Differential scanning calorimetry (DSC) studies revealed an identical crystallization peak for the supramolecular polymer produced from the enantiopure gelators ((R)- or (S)-isomer), which was absent in their equimolar mixture. However, mixtures of the two isomers with enantiomeric excess retrieved the gelation ability and preferential helicity demonstrating that chiral amplification is operative in the present system through the so-called "majority rule" effect. Chirality induction was also realized by the "sergeant and soldier" principle in the supramolecular assembly of an achiral NDI gelator in the presence of either the (R)- or (S)-isomer as the chiral dopant. However, the strong helical bias induced by the chiral gelator was found to be opposite in nature compared to that found in the self-assembly of the pure chiral gelator that has been rarely reported in the literature. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements indicated the strong positive impact of the gelation on the electrical conductivity.

Peripherally Cyanated Subphthalocyanines as Potential n-Type Organic Semiconductors.

A series of peripherally dicyano-, tricyano-, and tetracyano-substituted subphthalocyanines (SubPcs) have been prepared through microwave-assisted, palladium-mediated cyanation of iodinated precursors. The introduction of π-accepting cyano groups in the macrocycle clearly influences its electronic and redox properties, which are dependent on the number and relative position of these substituents. Additional functionalization of the periphery of SubPcs with electron-donating or -withdrawing groups allows for a further fine-tuning of their features, leading to intensely absorbing, strongly electron-accepting panchromatic dyes with low-lying LUMO energy levels. Flash-photolysis time-resolved microwave conductivity measurements on vapor-deposited films demonstrate that some of these novel SubPc derivatives display remarkable intrinsic charge-carrier mobilities that are comparable to or larger than those of other known well-performing acceptor SubPcs; thus confirming their potential as n-type organic semiconductors for application in the fabrication of photovoltaic devices.

Topologically Directed Assemblies of Semiconducting Sphere-Rod Conjugates.

Spontaneous organizations of designed elements with explicit shape and symmetry are essential for developing useful structures and materials. We report the topologically directed assemblies of four categories (a total of 24) of sphere-rod conjugates, composed of a sphere-like fullerene (C60) derivative and a rod-like oligofluorene(s) (OF), both of which are promising organic semiconductor materials. Although the packing of either spheres or rods has been well-studied, conjugates having both shapes substantially enrich resultant assembled structures. Mandated by their shapes and topologies, directed assemblies of these conjugates result not only in diverse unconventional semiconducting supramolecular lattices with controlled domain sizes but also in tunable charge transport properties of the resulting structures. These results demonstrate the importance of persistent molecular topology on hierarchically assembled structures and their final properties.

Cooperative supramolecular polymerization of a perylene diimide derivative and its impact on electron-transporting properties.

H-bonding-promoted supramolecular polymerization of a perylene diimide (PDI) building block and its impact on charge carrier mobility were studied. PDI-1, containing a carboxylic acid group, exhibits H-aggregation in a non-polar solvent decalin while in THF or chloroform it remains in the monomeric form. In contrast, the control molecule PDI-2, containing an ester group does not show aggregation even in decalin, indicating that H-bonding among the carboxylic acid is primarily responsible for H-aggregation, which is further verified by the FT-IR study. A variable temperature UV/Vis study establishes a cooperative pathway for the supramolecular polymerization of PDI-1. Microscopy images show a short fibrillar morphology. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements reveal significantly higher electrical conductivity for the PDI-1 film prepared from decalin compared with that prepared from THF/MeOH or the film of non-aggregated PDI-2 prepared from decalin. By combining the transient absorption spectroscopy data (that estimate the charge carrier generation efficiency) and the TRMC evaluated conductivity, the 1D charge carrier mobility of PDI-1 (µe,1D) is estimated to be 0.24 cm2 V-1 s-1, which is among the top values reported for any PDI derivative measured using the same technique.

Ion-Pairing Assemblies Based on Pentacyano-Substituted Cyclopentadienide as a π-Electronic Anion.

Pentacyanocyclopentadienide (PCCp(-) ), a stable π-electronic anion, provided various ion-pairing assemblies in combination with various cations. PCCp(-) -based assemblies exist as single crystals and mesophases owing to interionic interactions with π-electronic and aliphatic cations with a variety of geometries, substituents, and electronic structures. Single-crystal X-ray analysis revealed that PCCp(-) formed cation-dependent arrangements with contributions from charge-by-charge and charge-segregated assembly modes for ion pairs with π-electronic and aliphatic cations, respectively. Furthermore, some aliphatic cations gave dimension-controlled organized structures with PCCp(-) , as observed in the mesophases, for which synchrotron XRD analysis suggested the formation of charge-segregated modes. Noncontact evaluation of conductivity for (C12 H25 )3 MeN(+) ⋅PCCp(-) films revealed potential hole-transporting properties, yielding a local-scale hole mobility of 0.4 cm(2) V(-1) s(-1) at semiconductor-insulator interfaces.

π-Electron Systems That Form Planar and Interlocked Anion Complexes and Their Ion-Pairing Assemblies.

Interactions between designed charged species are important for the ordered arrangements of π-electron systems in assembled structures. As precursors of π-electron anion units, new arylethynyl-substituted dipyrrolyldiketone boron complexes, which showed anion-responsive behavior, were synthesized. They formed a variety of receptor-anion complexes ([1+1] and [2+1] types) in solution, and the stabilities of these complexes were discussed in terms of their thermodynamic parameters. Solid-state ion-pairing assemblies of [1+1]- and [2+1]-type complexes with countercations were also revealed by single-crystal X-ray analysis. In particular, a totally charge-segregated assembly was constructed based on negatively and positively charged layers fabricated from [2+1]-type receptor-anion complexes and tetrabutylammonium cations, respectively. Furthermore, the [1+1]-type anion complex of the receptor possessing long alkyl chains exhibited mesophases based on columnar assembled structures with contributions from charge-by-charge and charge-segregated arrangements, which exhibited charge-carrier transporting properties.

Cyclobuteno[60]fullerenes as Efficient n-Type Organic Semiconductors.

Cyclobuteno[3,4:1,2][60]fullerenes have been prepared in a straightforward manner by a simple reaction between [60]fullerene and readily available allenoates or alkynoates as organic reagents under basic and mild conditions. The chemical structure of the new modified fullerenes has been determined by standard spectroscopic techniques and confirmed by X-ray diffraction analysis. Some of these new fullerene derivatives exhibit a remarkable intrinsic electron mobility (determined by using flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements), which surpasses that of the well-known phenyl-C61-butyric acid methyl ester, thus behaving as promising n-type organic semiconductors.

Fusing Porphyrins and Phospholes: Synthesis and Analysis of a Phosphorus-Containing Porphyrin.

A phosphole-fused porphyrin dimer, as a representative of a new class of porphyrins with a phosphorus atom, was synthesized for the first time. The porphyrin dimer exhibits remarkably broadened absorption, indicating effective π-conjugation over the two porphyrins through the phosphole moiety. The porphyrin dimer possesses excellent electron-accepting character, which is comparable to that of a representative electron-accepting material, [60]PCBM. These results provide access to a new class of phosphorus-containing porphyrins with unique optoelectronic properties.

Functional Sulfur-Doped Buckybowls and Their Concave-Convex Supramolecular Assembly with Fullerenes.

Buckybowls are fascinating components of supramolecular assemblies owing to their unique bowl-shaped π-surfaces. Herein we present a protocol for the functionalization of a sulfur-doped buckybowl, trithiasumanene, via a brominated intermediate, from which thiolated trithiasumanenes were derived. The curved surface and electron-donating properties of thiolated trithiasumanenes promote their ready assembly with fullerenes to form concave-convex complexes. The supramolecular assembly behavior in solution was investigated by NMR analysis. The structures of supramolecular complexes were unambiguously characterized by crystallography. The crystals of the concave-convex complexes showed high thermal stability and photoconductivity.

Carboxylate-Driven Supramolecular Assemblies of Protonated meso-Aryl-Substituted Dipyrrolylpyrazoles.

Dipyrrolylpyrazole (dpp) derivatives possessing an aryl ring at the pyrazole 4-position were synthesized. Upon protonation, modified dpp derivatives formed a variety of assembled structures through complexation with carboxylates, as observed by single-crystal X-ray and synchrotron XRD analyses. In particular, the complexation of protonated dpp species possessing long alkyl chains with dicarboxylates resulted in highly ordered assembled structures, the packing modes of which as lamellar structures were controlled by the lengths of the spacer units between two carboxylate moieties. The charge-carrier transporting properties of the solid materials were also controlled by bound anions, including dicarboxylates.