Structure-property studies toward the stimuli-responsive behavior of benzyl-phospholium acenes

A series of new phospholium acenes, quaternized with benzyl groups, was synthesized. Both different π-conjugated backbones and electron-donating/-withdrawing benzyl groups were systematically studied to reveal details on the nature of their structural dynamics. Extensive NMR studies (including variable concentration/temperature and 2D) suggested that the systems undergo intramolecular conformation changes in solution that are strongly affected by the electronic nature of the benzyl group, and thereby significantly affecting the phosphole-typical σ*-π* interaction. This class of "smart" phosphole system exhibits enhanced emission in the solid state and at low temperature in solution, due to aggregation-induced enhanced emission (AIEE). The dynamic features of these smart phospholes also endow the systems with external-stimuli (thermal and mechanical force) responsive photophysical properties. Crystallographic studies and theoretical calculations confirmed that the thermal response of the phospholium system is mainly due to the conformation changes in solution, while the mechanical response of the system can be attributed to both the intramolecular conformation and the intermolecular organization changes in the solid state.     

Induction and stabilization of liquid crystal order in H-bond complexes containing non-mesogenic species

A new approach to create mesomorphic ordered systems is discussed. It is based on the idea to use noncovalent interactions in the designing of thermotropic structures by modelling lyotropic liquid crystal (LC) when the water interlayer is changed by appropriate solid substances that are able to interact with the polar part of an amphiphilic molecule without any strong change in the intermolecular interactions between the non-polar “tails”.     

Fluorescence study of a thermotropic liquid crystal: Bis (p-hexyloxyphenyl) terephthalate

The microstructure of the low molecular weight thermotropic liquid crystal, bis(p-hexyloxyphenyl)terephthalate (PP6), was analysed by fluorescence. The material has an emission around 400 nm in its various liquid crystalline phases, but this emission disappears in the isotropic phase. The emission was attributed to fluorescence from an intermolecular ground state complex between two different chromophores of PP6 based on fluorescence experiments of model compounds and dilute PP6 solutions. No peak shifts were observed on changing temperature, while the fluorescence intensity decreased as the temperature was increased. The fluorescence intensity changed dramatically at the phase transitions. An abrupt change in the fluorescence intensity was observed on changing depending on the temperature especially at the smectic-nematic transition temperature. Thus, the intermolecular fluorescence of the liquid crystal is greatly affected by the phase structure and the order of liquid crystalline phase.     

Induced amphotropic and thermotropic ionic liquid crystallinity in phosphonium halides: "lubrication" by hydroxyl groups

The influence of covalently attaching hydroxymethylene to the methyl groups of methyl-tri-n-alkylphosphonium halides (where the alkyl chains are decyl, tetradecyl, or octadecyl and the halide is chloride or bromide) or adding methanol as a solute to the salts on their solid, liquid-crystalline (smectic A2), and isotropic phases has been investigated using a variety of experimental techniques. These structural and compositional changes are found to induce liquid crystallinity in some cases and to enhance the temperature range and lower the onset temperature of the liquid-crystalline phases in some others. The results are interpreted in terms of the lengths of the three n-alkyl chains attached to the phosphorus cation, the nature of the halide anion, the influence of H-bonding interactions at the head group regions of the layered phases, and other solvent-solute interactions. The fact that at least 1 molar equiv of methanol must be added to effect complete (isothermal) conversion of a solid methyl-tri-n-alkylphosphonium salt to a liquid crystal demonstrates a direct and strong association between individual methanol molecules and the phosphonium salts. Possible applications of such systems are suggested.     

胆甾醇修饰羟基喹啉锌的聚集诱导荧光蓝移

用稳态光谱以及皮秒瞬态荧光光谱研究了新型有机电致发光分子胆甾醇修饰羟基喹啉锌(Zn(ChQ)2)的聚集诱导荧光蓝移性质. 在Zn(ChQ)2的极性溶剂溶液中, 分子激发后会发生从胆甾醇基团向喹啉环的光致电子转移, 转移后形成了“扭转的分子内电荷转移态”作为新的荧光发射态. 而在薄膜态中, 分子由于聚集产生空间位阻, 不能形成新的荧光发射态, 相对于极性溶剂中, 产生聚集荧光增强效应, 荧光发射峰会蓝移, 发射强度会增强. 在薄膜态中, 全波长上的超快荧光衰减说明存在分子间光致能量转移过程.     

Room-temperature phosphorescence of a supercooled liquid: kinetic stabilisation by desymmetrisation

Achieving organic room-temperature phosphorescence (RTP) in a solvent-free liquid state is a challenging task because the liquid state provides a less rigid environment than the crystal. Here, we report that an unsymmetrical heteroaromatic 1,2-diketone forms an organic RTP liquid. This diketone exists as a kinetically stable supercooled liquid, which resists crystallisation even under pricking or shearing stresses, and remains as a liquid for several months. The unsymmetrical diketone core is flexible, with eight distinct conformers possible, which prevents nucleation and growth for the liquid–solid transition. Interestingly, the thermodynamically stable crystalline solid-state was non-emissive. Thus, the RTP of the diketone was found to be liquiefaction-induced. Single-crystal X-ray structure analysis revealed that the diminished RTP of the crystal is due to insufficient intermolecular interactions and restricted access to an emissive conformer. Our work demonstrates that flexible unsymmetrical skeletons are promising motifs for bistable liquid–solid molecular systems, which are useful for the further development of stimuli-responsive materials that use phase transitions.

Metal-free, single-component, unsymmetrical 1,2-diketone exhibits liquefaction-induced room-temperature phosphorescence. Desymmetrisation provides the supercooled liquid with notable kinetic stability and phase-dependent phosphorescence properties.     

Self‐Assembly and Aggregate‐Induced Enhanced Emission of Amphiphilic Fluorescence Dyes in Water and in the Solid State

1‐Cyano‐1,2‐bis(biphenyl)ethene (CNBE) derivatives with a hexa(ethylene glycol) group as an amphiphilic side chain were synthesized and the self‐assembling character and fluorescence behavior were investigated. The amphiphilic derivatives showed aggregate‐induced enhanced emission (AIEE) in water and in the solid state. The fluorescence quantum yield increased as the rigidity of the aggregates increased (i.e., in ethyl acetate<in water<in the solid state). As determined from measurements of fluorescence spectra, fluorescence quantum yields, and fluorescence lifetimes, a key factor for the enhanced emission is suppression of the nonradiative decay process arising from restricted molecular motion. Additionally, the difference in the emission rate constant is not negligible and can be used to interpret the difference in fluorescence quantum yield in water and in the solid state.     

Large and Emissive Crystals from Carbon Quantum Dots onto Interfacial Organized Templates

Here, we report template-assisted assembly of emissive carbon quantum dot (CQD) microcrystals on organized cellulose nanocrystals templates at the liquid–air interface. This large-scale assembly is facilitated by the complementary amphiphilic character of CQDs and cellulose nanocrystals in the organized nematic phase. The resulting large microcrystals up to 200 μm across show unusually high emission that is not observed for limited CQDs aggregates. The dense crystal packing of CQDs in the layered fashion suppresses local molecular rotations and vibrations, thus restricting the intermolecular energy transfer and corresponding quenching phenomena. The as-prepared crystals are mechanically stable and can be exploited for recyclable catalysis, enabling applications beyond the individual nanoparticles or disordered aggregates. The ligand-templated assembly can be used to diversify CQD crystal architectures to guide formation of fibers, microplates, and micro-flowers.     

A Two‐Tailed Phosphopeptide Crystallizes to Form a Lamellar Structure

The crystal structure of a designed phospholipid‐inspired amphiphilic phosphopeptide at 0.8 Å resolution is presented. The phosphorylated β‐hairpin peptide crystallizes to form a lamellar structure that is stabilized by intra‐ and intermolecular hydrogen bonding, including an extended β‐sheet structure, as well as aromatic interactions. This first reported crystal structure of a two‐tailed peptidic bilayer reveals similarities in thickness to a typical phospholipid bilayer. However, water molecules interact with the phosphopeptide in the hydrophilic region of the lattice. Additionally, solid‐state NMR was used to demonstrate correlation between the crystal structure and supramolecular nanostructures. The phosphopeptide was shown to self‐assemble into semi‐elliptical nanosheets, and solid‐state NMR provides insight into the self‐assembly mechanisms. This work brings a new dimension to the structural study of biomimetic amphiphilic peptides with determination of molecular organization at the atomic level.     

2D Nanoarchitectonics: Soft Interfacial Media as Playgrounds for Microobjects,Molecular Machines,and Living Cells

Soft and flexible two-dimensional (2D) systems, such as liquid interfaces, would have much more potentials in dynamic regulation on nano–macro connected functions. In this Minireview article, we focus especially on dynamic motional functions at liquid dynamic interfaces as 2D material systems. Several recent examples are selected to be explained for overviewing features and importance of dynamic soft interfaces in a wide range of action systems. The exemplified research systems are mainly classified into three categories: (i) control of microobjects with motional regulations; (ii) control of molecular machines with functions of target discrimination and optical outputs; (iii) control of living cells including molecular machine functions at cell membranes and cell/biomolecular behaviors at liquid interface. Sciences on soft 2D media with motional freedom and their nanoarchitectonics constructions will have increased importance in future technology in addition to popular rigid solid 2D materials.     

Solid-state thermochromism and phase transitions of charge transfer 1,3-diamino-4,6-dinitrobenzene dyes

The lower 1,3-bis(hydroxyalkylamino) homologues of the strong intramolecular X-type charge transfer (CT) system 1,3-diamino-4,6-dinitrobenzene (DADNB) exhibit reversible color change in the solid state from yellow at room temperature (RT) to orange and red at high temperature (HT). To investigate the structural prerequisites for occurrence of this phenomenon, we prepared 10 new derivatives of DADNB where the hydroxyalkyl arms at the amino groups were replaced with substituents having different electronic and steric profiles. Two of the new materials exhibit sharp and reversible thermochromic change in the solid state: when heated, the bis(aminoethyl) derivative (DADNB-1) undergoes color change from orange-red to brown, while one of the three polymorphs of the bisphenyl product (DADNB-2) changes its color from red to yellow. The physicochemical analysis and the crystal structures of seven of these compounds, one of which is trimorphic, confirmed that both phenomena are due to solid-solid phase transitions. The brown high-temperature phase of DADNB-1 presents the first example where the absorption is shifted beyond the red region. Form C of DADNB-2 is the first material of this group that exhibits "negative" thermochromism, where the high-temperature phase absorbs at lower wavelength than the low-temperature one. The results demonstrate the potentials of these simple and easily accessible organic molecular materials for thermal switching of the optical properties by utility of intermolecular interactions to modulate the intramolecular CT.     

金属Cu液固转变及晶体生长的分子动力学模拟

采用分子动力学模拟研究了液态Ｃｕ在不同冷却速度下的凝固特点,模拟采用ＥＡＭ作用势,计算了不同温度,不同冷却速度下Ｃｕ的偶相关函数,结果表明ＥＡＭ作用势能很好地描述液态Ｃｕ的结构特征,当冷却速度较快时,液Ｃｕ形成非晶;当冷却速度较慢时,液Ｃｕ形成晶体,分析了不同冷却速度下体系的相变热力学及相变动力学过程,最后采用液固两层构型法,描述了Ｃｕ晶体的生长过程。     

'Remote' adiabatic photoinduced deprotonation and aggregate formation of amphiphilic N-alkyl-N-methyl-3-(pyren-1-yl)propan-1-ammonium chloride salts

The absorption and emission properties of a series of amphiphilic N-alkyl-N-methyl-3-(pyren-1-yl)propan-1-ammonium chloride salts were investigated in solvents of different polarities and over a wide concentration range. For example, at 10(-5) M concentrations in tetrahydrofuran (THF), salts with at least one N-H bond exhibited broad, structureless emissions even though time-correlated single photon counting (TCSPC) experiments indicated negligible static or dynamic intermolecular interactions. Salts with a butylene spacer or lacking an N-H bond showed no discernible structureless emission; their emission spectra were dominated by the normal monomeric fluorescence of a pyrenyl group and the TCSPC histograms could be interpreted on the basis of intramolecular photophysics. The broad, structureless emission is attributed to an unprecedented, rapid, adiabatic proton-transfer to the medium, followed by the formation of an intramolecular exciplex consisting of amine and pyrenyl groups. The proposed mechanism involves excitation of a ground-state conformer of the salts in which the ammonium group sits over the pyrenyl ring due to electrostatic stabilization. At higher concentrations, with longer N-alkyl groups, or in selected solvents, electronic excitation of the salts led to dynamic and static excimeric emissions. For example, whereas the emission spectrum of 10(-3) M N-hexyl-N-methyl-3-(pyren-1-yl)propan-1-ammonium chloride in THF consisted of comparable amounts of monomeric and excimeric emission, the emission from 10(-5) M N-dodecyl-N-methyl-3-(pyren-1-yl)propan-1-ammonium chloride in 1:9 (v:v) ethanol/water solutions was dominated by excimeric emission, and discrete particles near micrometer size were discernible from confocal microscopy and dynamic light scattering experiments. Comparison of the static and dynamic emission characteristics of the particles and of the neat solid of N-dodecyl-N-methyl-3-(pyren-1-yl)propan-1-ammonium chloride indicate that molecular packing in the microparticles and in the single crystal are very similar if not the same. It is suggested that other examples of the adiabatic proton transfer found in the dilute concentration regime with the pyrenyl salts may be occurring in very different systems, such as in proteins where conformational constraints hold ammonium groups over aromatic rings of peptide units.     

Supramolecular polar thin films built by surfactant liquid crystals: polarization-tunable multilayer self-assemblies with in-plane ferroelectric ordering of ion-based dipoles.

Polar order in the phosphonium liquid crystal thin films, which are composed of two-dimensional ion-pair-based domains separated by the insulating hydrocarbon layers, was probed by second harmonic generation (SHG) analysis. Despite the ordinary amphiphilic self-assemblies containing no pi-electron moieties, the solid-state thin films retaining a smectic layer structure showed clearly an SHG activity, while the disordered films without the layer structure were not active at all for the SHG. It was found that the multilayer structure plays a crucial role for the SHG from the phosphonium thin films and the ionic layers act as an SHG-active site. The most significant characteristic of this system is to possess an ability to control SHG intensity electrically. The efficiency of the SHG process in the thin-film assemblies was enhanced by applying an external electric field parallel to the layer plane. Furthermore, through evaluation of thermal stability of the sample films, it was revealed that the SHG signals were detected only in the solid-state temperature range and the disappearance of the SHG occurs earlier than the solid-to-liquid crystalline phase transition. These results demonstrated that the origin of polar order in the phosphonium thin films is due to in-plane noncentrosymmetric arrangement (ferroelectric ordering) of ion pairs as an electric dipole, that is, dipole symmetry in an ionic layer.     

芘衍生物发光行为的研究

本工作合成了新型的芘衍生物-芘基查尔酮(PD), 研究了其在溶液及胶束中的稳态光物理行为。结果表明: 即使在较低浓度条件下, PD分子间也容易生成基态电荷转移配合物(Inter-CT), 其荧光光谱表现为芘单体荧光、分子内电荷转移态(Intra-CT)荧光以及Inter-CT荧光, 并且具有较好的溶致变色效应、浓度效应以及温度效应, 是一良好的探针分子化合物。     

Insights into the AIEE of 1,8‐Naphthalimides (NPIs): Inverse Effects of Intermolecular Interactions in Solution and Aggregates

Systematic structural perturbation has been used to fine‐tune and understand the luminescence properties of three new 1,8‐naphthalimides (NPIs) in solution and aggregates. The NPIs show blue emission in the solution state and their fluorescence quantum yields are dependent upon their molecular rigidity. In concentrated solutions of the NPIs, intermolecular interactions were found to quench the fluorescence due to the formation of excimers. In contrast, upon aggregation (in THF / H₂O mixtures), the NPIs show aggregation‐induced emission enhancement (AIEE). The NPIs also show moderately high solid‐state emission quantum yields (ca. 10–12.7 %). The AIEE behaviour of the NPIs depends on their molecular rigidity and the nature of their intermolecular interactions. The NPIs 1 – 3 show different extents of intermolecular (π–π and C?H???O) interactions in their solid‐state crystal structures depending on their substituents. Detailed photophysical, computational and structural investigations suggest that an optimal balance of structural flexibility and intermolecular communication is necessary for achieving AIEE characteristics in these NPIs.     

一种基于二芘基苯的高效有机固态蓝色荧光化合物

利用Suzuki偶联反应,合成了一种新的二芘基苯的衍生物2,5-二甲基-1,4-双(7-特丁基-1-)芘基苯,并研究了其吸收和发光性质随着溶剂和温度等条件的变化情况.实验表明,与不含特丁基的类似化合物相比,引入特丁基可以明显减弱分子间作用并避免激基缔合物的形成,因而在保持吸收和发射光谱的谱形及峰位基本不变的同时显著地增强了荧光发光效率.该化合物的固态荧光量子效率高达0.82.     

Controlling the solid-state luminescence of gold(I) <Emphasis Type="Italic">N</Emphasis>-heterocyclic carbene complexes through changes in the structure of molecular aggregates

Thermally stable, solid-state luminescent organic materials are highly desired for the development of practical applications. Herein we synthesized new gold(I) complexes with N-heterocyclic carbene ligands, which have the ability to form strong metal-organic bond. Consequently, their thermochemical stability is enhanced at temperatures around 300 °C. Precise design of the molecular structure of the ligands, with a focus on ensuring low steric hindrance around Au atoms in order to limit disturbances to Au/Au interactions, provided a complex with a densely packed crystal with a shorter intermolecular Au–Au distance (3.17 Å) than the typical distance. In the solid state, this complex exhibited strong aurophilic interactions, which generated intense phosphorescence even in air at room temperature (quantum yield=16%) in spite of absence of any phosphorescence in solution. This behavior is characteristic for solid-state luminescence referred to as aggregation-controlled emission. Furthermore, the gold (I) complex displays capacity for mechano- and vapo-chromism—that is, the ability to change color reversibly in response to the application of external stimuli. We believe that the proposed design framework, which involves controlling thermal stability and luminescence property separately, provides a new opportunity for the development of practical applications using solid-state luminescent organic molecules.     

Direct Anisotropic Growth of CdS Nanocrystals in Thermotropic Liquid Crystal Templates for Heterojunction Optoelectronics

The direct growth of CdS nanocrystals in functional solid‐state thermotropic liquid crystal (LC) small molecules and a conjugated LC polymer by in situ thermal decomposition of a single‐source cadmium xanthate precursor to fabricate LC/CdS hybrid nanocomposites is described. The influence of thermal annealing temperature of the LC/CdS precursors upon the nanomorphology, photophysics, and optoelectronic properties of the LC/CdS nanocomposites is systematically studied. Steady‐state PL and ultrafast emission dynamics studies show that the charge‐transfer rates are strongly dependent on the thermal annealing temperature. Notably, annealing at liquid‐crystal state temperature promotes a more organized nanomorphology of the LC/CdS nanocomposites with improved photophysics and optoelectronic properties. The results confirm that thermotropic LCs can be ideal candidates as organization templates for the control of organic/inorganic hybrid nanocomposites at the nanoscale level. The results also demonstrate that in situ growth of semiconducting nanocrystals in thermotropic LCs is a versatile route to hybrid organic/inorganic nanocomposites and optoelectronic devices.     

Solid-emissive rhodamine: hydrogen bonding-assisted efficient intermolecular fluorescence resonance energy transfer in the solid state

Solid-emissive rhodamine complexes are obtained by mixing commercial rhodamine B (RhB) with the recently developed solid-emissive boron 2-(2′-pyridyl)imidazole (BOPIM) derivatives. The formation of intermolecular hydrogen bonds between RhB and BOPIM dyes plays a key role in the emission of RhB in the solid state. The disappearance of emissions from BOPIM dyes indicates the occurrence of efficient intermolecular fluorescence resonance energy transfer (FRET). The hydrogen bond also helps prevent the intermolecular interaction between the carboxyl moieties on RhB to alleviate concentration-induced fluorescence quenching because the emission of the complexes can be directly lightened by excitation at the RhB absorption (510 nm). Our results indicate that intermolecular FRET assisted by non-covalent interactions can be an efficient tool for constructing red or near-infrared solid emitters.