Supercooling of functional alkyl-π molecular liquids

Robust Supercooled Liquid Formation Enables All-Optical Switching Between Liquid and Solid Phases of TEMPO

Organic molecules that undergo supercooling can provide the basis for novel stimuli-responsive materials, but the number of such compounds is limited. Results in this paper show that the stable organic radical 2,2,6,6-tetramethyl-1-piperidine-1-oxyl (TEMPO) can form a stable supercooled liquid (SCL). Upon melting and cooling back to room temperature, the TEMPO SCL can persist for months, even after mild physical agitation. Its high vapor pressure can enable crystal growth at remote locations within the sample container over the course of days. Optical, electron paramagnetic resonance, and birefringence measurements show no evidence of new chemical species or partially ordered phases in the supercooled liquid. TEMPO's free radical character permits absorption of visible light that can drive photothermal melting to form the SCL, while a single nanosecond light pulse can initiate recrystallization of the SCL at some later time. This capability enables all-optical switching between the solid and the SCL phases. The physical origin of TEMPO's remarkable stability as an SCL remains an open question, but these results suggest that organic radicals comprise a new class of molecules that can form SCLs with potentially useful properties.

Solvent-Free Organic Liquids: An Efficient Fluid Matrix for Unexplored Functional Hybrid Materials

ConspectusThe invention of solvent-free organic liquids (SOLs) was serendipitous. However, the curiosity-driven research in the later stage delivered new soft materials with exciting optical, and optoelectronic properties along with appealing physical characteristics suitable for the futuristic applications. A slight change in the molecular design resulted in a drastic change in the physical state of molecules demonstrating monomer-like features in the bulk. The basic idea of core isolation has been successful in delivering new SOLs with attractive functional properties. The unique fluid matrix associated with SOLs offers a tremendous opportunity for making hybrid materials by simple mixing. The chance to study the fundamentally important electron transfer, energy transfer, charge transfer interactions, triplet-state emissions, and even detailed NMR experiments in the solvent-free neat state is the major attraction of SOLs. Usually, solvents and their polarity control such molecular properties, and in the case of SOLs, it avoids the use of solvents to study such fundamentally important properties. Besides, SOLs protect the triplet emitters and excited state processes involving triplet states from quenchers and make the analysis possible under ambient conditions.Our effort in this direction was focused on tuning the ground and excited state properties by transforming conventional organic molecules to SOLs and further value addition by preparing the hybrid SOLs. We developed a series of hybrid SOLs, exploring room-temperature phosphorescence, thermally activated delayed fluorescence, charge or energy transfer between donor and acceptor SOLs, selective explosive sensing, etc. A slight variation in the chemical structure or optoelectronic properties of the individual components imparted exciting optical features for the hybrid SOLs. It includes nonemissive charge transfer, tunable emission exciplex, room temperature phosphorescence, and thermally activated delayed fluorescence SOLs. The liquid matrix of donor SOLs accommodated varying amounts of acceptor SOLs to tune the ground and excited state features. In all examples of donor-acceptor-based hybrid SOLs, even a low amount of acceptor, such as a donor-acceptor ratio of 1000:1, can cause pronounced optical properties. Hence, the evaluation of the optical properties of SOLs, especially, in the absence of solvents is so special that it avoids the interference of solvent molecules. Still, the major drawback of SOLs remains unsolved until we report polymerizable SOLs. Although a large variety of SOLs have been reported in the literature, the long-lasting problem of surface stickiness of SOLs was resolved by polymerizable SOLs. It enabled the development of flexible, foldable, and stretchable large-area luminescent films suitable for lighting and display devices. In this Account, we summarize our work on SOLs, hybrid SOLs, polymerizable SOLs, and the application of SOLs in selective sensing of explosives. Finally, an outlook on the feasibility of luminescent polymerizable SOLs in futuristic applications is provided.

Luminescent solvent-free liquids based on Schiff-base boron difluoride complexes with polyethylene glycol chains

A series of Schiff-base boron difluoride complexes with polyethylene glycol chains were synthesized and their photophysical properties were examined. These complexes maintained the solvent-free liquid state even at room temperature and their glass transition temperatures (Tg) were determined to be around -40 °C. The complexes showed blue to yellow luminescence under UV irradiation in the solvent-free liquid state with good emission quantum yields (Φ) of up to 0.26. The luminescence colour could also be tuned by dissolving organic dyes in the blue luminescent liquid sample. Density functional theory (DFT) and time-dependent (TD) DFT calculations were performed to further understand the photophysical properties.

Chiral Polymer Dots Show Unexpected Versatility of Highly Ordered Self-Assembly into Chiroptical Liquid Crystals, Ultra-Thin Films, and Long-Ribbons

Compared to the organic counterparts, chiral self-assembly of nanomaterials shows persistency to kinetic factors such as solvent environments, and consequently, dynamic modulation of self-assembly and functions remains major challenge. Here, it is shown that alkylated, chiral polymer dots (c-PDs) give highly ordered self-assemblies with amplified chirality adaptive to solvent environments, and one-to-many hierarchical aggregation can be realized. The c-PDs tended to self-assemble into nanohelices with cubic packing in the solid state, which, thanks to the thermo-responsiveness, transformed into thermic liquid crystals upon heating. Cotton effects and circularly polarized luminescence evidenced the chirality transfer from central chirality to supramolecular chirality. At the air-water interface, the c-PDs are self-assembled into monolayers, which further stack into multiple layers with chirality transfer and highly ordered packing. In addition, undergoing a good/poor solvent exchange, the c-PDs afforded ultra-long microribbons up to a length scale of millimeters, which are constituted by the bilayer lamellar stacking. The versatile chiral self-assembly modalities with long-range ordered packing arrays of carbonized c-PDs via solvent strategy are realized. This feature is comparable to the organic species, although the c-PDs have no atomic precise structures. This work would surely expand the applications of quantum dot ordered self-assembly with adaptiveness to kinetic factors.

Alkyl-C60 liquid electrets as deformable mechanoelectric generators

Special attention is being paid to the potential applicability of various soft electronics in deformable/wearable devices. These devices must be constantly connected to energy sources to ensure their uninterrupted operation. Electrets, which are capable of retaining quasi-permanent electric charges inside or on the surface of materials, are expected to be a battery-less power source. Here, we present a strategy for harvesting the charges in alkyl-C60 liquids. Suitable substitution of bulky yet flexible branched long-alkyl chains generated C60-mono-adducts and regioisomeric bis-adducts as room-temperature solvent-free liquids. These alkyl-C60 liquids were negatively poled by the corona-discharging and soaked in nylon fabric. The liquid of the C60 bis-adduct exhibited better charge retention in comparison to the liquid of the C60 mono-adduct. This suggests that the bulky long-alkyl chains provided proper insulation for the C60 core and charge trapping in the liquid. This charge-trapping behaviour and the inherent fluidity of the alkyl-C60 liquids enabled their fabrication into deformable mechanoelectric generator (MEG) devices. The MEG exhibited applicability as a deformable micropower source or vibration sensor by generating output voltage pulses even under folded/twisted/rolled conditions. The alkylated-liquid-based MEGs worked at frequencies similar to human body motion, showing promising potential for body motion sensors and healthcare applications.

Liquid Structures and Diffusion Dynamics of Alkyl-Pyrene Liquids Studied by Molecular Dynamics Simulations

Functional molecular liquids (FMLs) based on alkylated π-conjugated molecules have attracted attention as solvent-free and nonvolatile liquid materials with prominent optoelectronic features. Recently, novel FML compounds containing pyrene as the functional core were synthesized, and their rheological and photochemical properties were investigated. Although the molecules differ only in the number of alkyl chain substituents and their substitution positions, their viscosity coefficients are largely different beyond the Stokes-Einstein relation on the assumption of identical microscopic friction, indicating that local microscopic molecular interactions are crucial for the macroscopic rheological properties. Here, we report a theoretical study on the rheological properties of the alkyl-pyrene liquids by means of atomistic molecular dynamics (MD) simulations. We performed long-time MD simulations for tens of microseconds to obtain ample statistical samples of the alkyl-pyrene liquids and analyzed their liquid structures and diffusion dynamics based on spatiotemporal correlation functions. We found the formation of characteristic local liquid structures of π-π stacking of the pyrene moieties and locally anisotropic and anomalous diffusion dynamics, which remarkably vary depending on the alkyl substituent patterns. The present results provide an atomistic insight into the macroscopic rheological properties of alkyl-π FMLs and molecular design strategy for them.

Organogel of Acai Oil in Cosmetics: Microstructure, Stability, Rheology and Mechanical Properties

Organogel (OG) is a semi-solid material composed of gelling molecules organized in the presence of an appropriate organic solvent, through physical or chemical interactions, in a continuous net. This investigation aimed at preparing and characterizing an organogel from acai oil with hyaluronic acid (HA) structured by 12-hydroxystearic acid (12-HSA), aiming at topical anti-aging application. Organogels containing or not containing HA were analyzed by Fourier-transform Infrared Spectroscopy, polarized light optical microscopy, thermal analysis, texture analysis, rheology, HA quantification and oxidative stability. The organogel containing hyaluronic acid (OG + HA) has a spherulitic texture morphology with a net-like structure and absorption bands that evidenced the presence of HA in the three-dimensional net of organogel. The thermal analysis confirmed the gelation and the insertion of HA, as well as a good thermal stability, which is also confirmed by the study of oxidative stability carried out under different temperature conditions for 90 days. The texture and rheology studies indicated a viscoelastic behavior. HA quantification shows the efficiency of the HA cross-linking process in the three-dimensional net of organogel with 11.22 µg/mL for cross-linked HA. Thus, it is concluded that OG + HA shows potentially promising physicochemical characteristics for the development of a cosmetic system.

Artificial Light-Harvesting System with White-Light Emission in a Bicontinuous Ionic Medium

Artificial light-harvesting systems (ALHSs), which are closely related to Förster resonance energy transfer (FRET), are among the most attractive scientific topics during the past few decades. Specifically, binary ALHSs that are composed of a fluid donor and acceptor have a simplified composition and high number density of the donor units. However, largely due to the difficulty in obtaining a fluid donor, investigation of these systems is still quite limited, especially for the ionic systems. Herein, we report a new type of binary ALHS using an ionic naphthalimide (NPI) derivative as a donor, which shows greatly improved photoluminescence for its bicontinuous liquid structure. When blending with an acceptor such as rhodamine 6G or trans-4-[4-(dimethylamino)styryl]-methylpyridinium iodide, efficient FRET was confirmed by both experimental results and molecular dynamics simulations, with an energy transfer efficiency up to ∼90%. Tunable color, including white-light emission, was achieved by tuning the acceptor/donor ratio, opening the door for a variety of applications such as light-emitting diodes and photoluminescent inks.

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.

Real-Time Fluorescence Visualization of Nanoparticle Aggregation and the Polymorph-Transition Process of a Mechanofluorochromic Difluoroboron-β-Diketone Derivative

The use of organic nanomaterials in biomedical and optical devices has been widely studied. The key to improving the performance and stability of these devices is to control the fabrication process, which determines the phase stability and photophysical properties. In this study, fluorescence changes were observed during the reprecipitation process of mechanofluorochromic molecules of dibenzoyl(methanato)boron difluoride. The cyan-emission phase (C-phase) was first identified. The time evolution of the resolved fluorescence spectra revealed that the green-emission phase (G-phase) was formed from the amorphous phase with yellow emission via the C-phase, in addition to the direct formation of the G-phase. Combined with the results of the investigation into the thermal properties, the fluorescence changes clearly indicate a two-step nucleation process and Ostwald's rule of stages for polymorph transition, which enables us to not only provide guidance for controlling the fabrication process but also propose the ripening process for organic nanoparticle formation.

Peripherally Modified Tetraphenylethene: Emerging as a Room-Temperature Luminescent Disc-Like Nematic Liquid Crystal

A blue-light-emitting liquid crystalline (LC) material was designed and prepared. By employing a twisted luminescent core (i.e., tetraphenylethene), four peripheral LC units with long alkyl chains and the small polar benzyl-ether-typed linking groups, the resulting material displayed a hexagonal columnar phase near room temperature and a disc-like nematic phase between 32 and 70 °C. The columnar LC showed a high quantum yield of 0.49 at 20 °C, and the efficient luminescence property was retained even in the isotropic phase at high temperature. Additionally, the fluidity of the nematic phase rendered the LC a non-volatile solvent, and the proper addition of a red dye led to the achievement of polarized white-light emission, which revealed a promising application prospect in LC display fabrication.

Circularly Polarized Luminescence Liquids Based on Siloxybinaphthyls: Best Binaphthyl Dihedral Angle in the Excited State

A series of axially chiral 1,1'-binaphthyls with trialkylsiloxy (OSiR₃ ) groups were synthesized. Among them, 1 a-c possessing OSiR₃ groups at the 7,7'-positions and methyl groups at the 2,2'-positions were liquids at room temperature, and the neat liquids showed circularly polarized luminescence (CPL) (R=Bu; Φ_fl,liquid =0.21, |g_lum,liquid |=1.6×10^-3 ). The |g_lum,liquid | value is the highest of pure liquids. These compounds remained liquid over a broad range of temperatures, down to -50 °C. Time-dependent DFT calculations indicated that in the excited state, the binaphthyls adopt a transoid conformation with a small angle between the electric and magnetic transition dipole moments (θ_μ,m =77°), which is a key factor in their CPL activity. The best binaphthyl dihedral angle in the excited state is approximately 110°.

Stimulus-Responsive Supercooled π-Conjugated Liquid and Its Application in Rewritable Media

Herein, we report a stimulus-responsive supercooled π-conjugated liquid and the possibility of its application in rewritable media. Supercooled liquid 1 showed a dramatic change in its photoluminescent color upon the transformation from liquid 1l (yellow emission) to solid 1s (green emission). These phenomena were revealed by fluorescence spectra as well as lifetime decay profiles.

Circularly Polarized Luminescence from Solvent-Free Chiral Organic π-Liquids

The solvent-free organic π-liquids have been attracting increasing attentions owing to the inherent optoelectronic properties accompanied by the advantages of non-volatility and high processability. Herein, we reported a series of naphthalene derivatives substituted with chiral branched alkyl chains, which are present as liquids (Nap1-3) or solid (Nap4) at room temperature, depending on the substitution positions. Circular dichroism (CD) and circularly polarized luminescence (CPL) were only observed for enantiomeric Nap2 (2,3-substituted) liquid. It is suggested that the chiral aggregation in the π-liquid leads to the CD signal and the chiral excimer resulting in the CPL performance. When achiral anthracene or pyrene was dissolved in Nap2, the π-liquid could serve as chirality and energy transfer media in which both CD and CPL emerged from the achiral anthracene. A CPL dissymmetry factor (|g_lum |) of anthracene reached to 5.2×10^-2 when dissolved in chiral Nap2 liquid, which is nearly two orders of magnitude higher than that of the pure Nap2 π-liquid.

Cooperatively assembled liquid crystals enable temperature-controlled Förster resonance energy transfer

Balancing the rigidity of a π-conjugated structure for strong emission and the flexibility of liquid crystals for self-assembly is the key to realizing highly emissive liquid crystals (HELCs). Here we show that (1) integrating organization-induced emission into dual molecular cooperatively-assembled liquid crystals, (2) amplifying mesogens, and (3) elongating the spacer linking the emitter and the mesogen create advanced materials with desired thermal–optical properties. Impressively, assembling the fluorescent acceptor Nile red into its host donor designed according to the aforementioned strategies results in a temperature-controlled Förster resonance energy transfer (FRET) system. Indeed, FRET exhibits strong S-curve dependence as temperature sweeps through the liquid crystal phase transformation. Such thermochromic materials, suitable for dynamic thermo-optical sensing and modulation, are anticipated to unlock new and smart approaches for controlling and directing light in stimuli-responsive devices.

A temperature-sensitive Förster resonance energy transfer system was constructed using a highly emissive liquid crystal co-assembled with Nile red, enabling thermo-optical modulation for controlling and directing light in stimuli-responsive devices.

Highly Ordered 2D-Assemblies of Phase-Segregated Block Molecules for Upconverted Linearly Polarized Emission

Materials based on the laminar ordering of self-assembled molecules have a unique potential for applications requiring efficient energy migration through densely packed chromophores. Here, employing molecular assemblies of coil-rod-coil block molecules for triplet-triplet annihilation upconversion (TTA-UC) based on triplet energy migration with linearly polarized emission is reported. By covalently attaching discrete-length oligodimethylsiloxane (oDMS) to 9,10-diphenylanthracene (DPA), highly ordered 2D crystalline DPA sheets separated by oDMS layers are obtained. Transparent films of this material doped with small amounts of triplet sensitizer Pt^II octaethylporphyrin show air-stable TTA-UC under non-coherent excitation. Upon annealing, an increase in TTA-UC up to two orders of magnitude is observed originating from both an improved molecular ordering of DPA and an increased dispersion of the sensitizer. The molecular alignment in millimeter-sized domains leads to upconverted linearly polarized emission without alignment layers. By using a novel technique, upconversion imaging microscopy, the TTA-UC intensity is spatially resolved on a micrometer scale to visually demonstrate the importance of molecular dispersion of sensitizer molecules for efficient TTA-UC. The reported results are promising for anti-counterfeiting and 3D night-vision applications, but also exemplify the potential of discrete oligodimethylsiloxane functionalized chromophores for highly aligned and densely packed molecular materials.

Stacking Control by Molecular Symmetry of Sterically Protected Phthalocyanines

The synthesis and characterization of two phthalocyanine (Pc) structural isomers, 1 and 2, in which four 2,6-di(hexyloxy)phenyl units were attached directly to the 1,8,15,22- or 1,4,15,18-positions of the Pc rings, are described. Both Pcs 1 and 2 exhibited low melting points, i.e., 120 and 130 °C respectively, due to the reduction in intermolecular π-π interaction among the Pc rings caused by the steric hindrance of 2,6-dihexyloxybenzene units. The thermal behaviors were investigated with temperature-controlled polarizing optical microscopy, differential scanning calorimetry, powder X-ray diffraction, and absorption spectral analyses. Pc 1, having C_4h molecular symmetry, organized into a lamellar structure containing lateral assemblies of Pc rings. In contrast, the other Pc 2 revealed the formation of metastable crystalline phases, including disordered stacks of Pcs due to rapid cooling from a melted liquid.

Synthesis of Oligo(1,8-pyrenylene)s: A Series of Functional Molecular Liquids

A monomer-through-pentamer series of oligo(1,8-pyrenylene)s was synthesized using a two-step iterative synthetic strategy. The trimer, tetramer, and pentamer are mixtures of atropisomers that interconvert slowly at room temperature (as shown by variable-temperature NMR analysis). They are liquids well below room temperature, as indicated by POM, DSC and SWAXS analysis. These oligomers are highly fluorescent both in the liquid state and in dilute solution (λ_F,max = 444-457 nm, φ_F = 0.80) and an investigation of their photophysical properties demonstrated that delocalization plays a larger role in their excited states than it does in related pyrene-based oligomers.

Kinetics of Excimer Electrogenerated Chemiluminescence of Pyrene and 1-Pyrenebutyricacid 2-Ethylhexylester in Acetonitrile and an Ionic Liquid, Triethylpentylphosphonium Bis(trifluoromethanesulfonyl)imide

We describe the kinetics of excimer electrogenerated chemiluminescence (ECL) of a liquid pyrene derivative, 1-pyrenebutyricacid 2-ethylhexylester (PLQ) dissolved in a molecular solvent, acetonitrile (MeCN), and an ionic liquid, triethylpentylphosphonium bis(trifluoromethanesulfonyl)imide ([P₂₂₂₅][TFSI]). Pyrene was also used for comparison. To discuss the kinetics of the excimer ECLs, the photophysical and electrochemical properties and electronic states of PLQ and pyrene were revealed. The photoluminescence (PL) spectra, rate constants for the radiative transitions, and redox potentials of PLQ and pyrene dissolved in MeCN and [P₂₂₂₅][TFSI] suggest that as a solvent, [P₂₂₂₅][TFSI] behaves more polar than MeCN. By analyzing the PL decay curves, the rate constants to form the excimer were determined to be on the order of 10⁹ and 10⁷ M^-1 s^-1 in MeCN and [P₂₂₂₅][TFSI], respectively, which were limited by the diffusion. For neat PLQ (1.6 M), a delay of 0.3-0.4 ns for the excimer emission compared to the monomer emission was observed. It is likely that the delay corresponds to the timescale for arranging the conformation to form the excimer. The ECL of PLQ was generated by applying a square wave voltage to produce the radical anion and cation, and on the ECL spectra, the excimer emission was more prevailed compared to the PL spectra. Kinetic analysis for the electron transfer reaction between the radical ions based on Marcus theory indicates that the electron transfer is limited by the diffusion of the radical ions. Moreover, the electron transfer distance (d_et) between the radical cation and anion to generate excited states was calculated with a framework of the theory. Kinetically, the electron transfer can take place at d_et < ∼11 Å in MeCN and d_et < ∼12 Å in [P₂₂₂₅][TFSI]. The density functional theory (DFT) and time-dependent DFT calculations show that the potential energy curve of the excimer against the distance between the pyrene rings reaches a minimum at 3.50 Å. This suggests that through the electron transfer, the process of the direct formation of the monomer S₁ state followed by the excimer formation is more prevailed than that of the direct excimer formation.

Self-Assembled and Nonassembled Alkylated-Fullerene Materials

Fullerene (C₆₀), a π-conjugated cage molecule consisting of 60 sp²-hybridized carbon atoms that are arranged into perfect icosahedral symmetry, is one of the most extensively studied nanocarbon materials by virtue of its characteristic spherical structure, fascinating optoelectronic properties, and widespread applications in material science. To implement practical applications, C₆₀ is generally used as a building motif to assemble into various ordered superstructures. Unlike the controllable face-to-face π-π interactions of planar π-conjugated molecules, the π-π interactions between the three-dimensional spherical C₆₀ units are random and directionless, which generally lead to complicated aggregated structures and unpredictable properties. The primary target of our research is to produce a robust design strategy for functional C₆₀ materials, by which the single C₆₀ molecules can be engineered into desirable self-organized architectures with optimized functions. To this end, we focused on alkylated fullerene (alk-C₆₀) derivatives, a simple molecular system whose two components, alkyl chains and C₆₀, exhibit both hydrophobicity yet different affinities to organic solvents. As a result, the alk-C₆₀ derivatives present an unusual "hydrophobic amphiphile" system. Through systematic tuning of the substitution pattern of a series of alkyl side chains (number, length, branching, and substitution position) and external experimental conditions, the factors influencing alk-C₆₀ self-assembly behaviors were determined. In addition, the feasibility of forming hybrid coassemblies with alk-C₆₀ and other nanocarbon materials was demonstrated. By taking full advantage of the hydrophobic nature and active optoelectronic properties of these self- or hybrid-assemblies, various superhydrophobic materials and/or optoelectronic devices were developed. However, supported only by weak noncovalent interactions, these ordered superstructures are intrinsically fragile under various external stimuli. To improve the structural stability and achieve consistent optoelectronic performance of these novel materials, we strengthened the ordered structures via metallization and plasticization. Both approaches gave rise to robust and endurable materials with functions inherited from the pristine assemblies but at the cost of their former softness and facile processability. Thereafter, we focused on amorphous materials in view of their consistent and predictable optoelectronic properties that are independent of their geometry and physical environment. Unexpectedly, the amorphous materials obtained were liquids at room temperature, whose excellent deformability might enable applications in flexible/wearable optoelectronic devices. However, the lack of sufficient molecular order impaired their optoelectronic performance. To address this, we devised a straightforward strategy toward the directed ordered self-assembly of the alk-C₆₀ liquids by adding molecular cofactors (n-alkanes or C₆₀) into the liquids. Using this strategy, the balance between intermolecular order and material softness can be readily adjusted to meet different application requirements. Through iterative refinements to our novel alk-C₆₀ system, we have demonstrated its power in generating numerous self-assembled, hybrid-assembled, and nonassembled materials toward versatile applications. We believe such a comprehensive description of these alk-C₆₀-based functional materials provides deep insights into these still-evolving materials, which will underpin more advanced applications in near future.

Colored Ionic Liquid Based on Stable Polycyclic Anion Salt Showing Halochromism with HCl Vapor

A sodium salt of a polycyclic trioxotriangulene (TOT) anion with six triethylene glycol chains exhibiting the formation of a colored ionic liquid at room temperature was synthesized. The ionic liquid is air- and water-stable, reflecting thermodynamic stabilization of a charge-delocalized TOT anion. Upon protonation of the TOT anion, the salt shows halochromic behaviors in solution and even in the neat liquid state with HCl vapor. The ionic liquid shows no morphological change with the chromism, presumably as a result of poor intermolecular interactions between π skeletons.

Stimuli-Responsive Room-Temperature N-Heteroacene Liquid: In Situ Observation of the Self-Assembling Process and Its Multiple Properties

A novel stimuli-responsive room-temperature photoluminescent liquid 1 based on the N-heteroacene framework is developed and analyzed by several experiments such as differential scanning calorimetry, X-ray diffraction, dynamic viscoelasticity measurement, in situ observation by optical and polarized optical microscopes, UV-vis absorption and fluorescence spectroscopy, and by theoretical methods such as ab initio calculation and molecular dynamics (MD) computer simulation techniques. In contrast to stimuli-responsive solid materials reported previously, liquid 1 in response to HCl vapor as a single stimulus can involve dramatically multiple changes in physical properties such as rheological behavior, morphology, as well as photoluminescence. The present ab initio calculation and microsecond-timescale MD simulations reveal that the complexation of 1 and HCl molecules induces a large dipole moment, leading to the formation of stacking structures because of their dipole-dipole interaction. Upon exposure to HCl vapor, in situ microscopic observation of the stimuli-responsive liquid elucidates a self-assembling process involving the formation of the wrinkle structure in a micrometer scale, indicating disorder-order phase transition. Further exposure of 1 to HCl vapor from seconds to hours has an influence on the macroscopic physical properties such as viscosity, viscoelasticity, and photoluminescent colors. The synergy between the experimental and theoretical investigations opens a new strategy to develop a novel class of stimuli-responsive materials showing multiple changes in physical properties.

A Record Chromophore Density in High-Entropy Liquids of Two Low-Melting Perylenes: A New Strategy for Liquid Chromophores

Liquid chromophores constitute a rare but intriguing class of molecules that are in high demand for the design of luminescent inks, liquid semiconductors, and solar energy storage materials. The most common way to achieve liquid chromophores involves the introduction of long alkyl chains, which, however, significantly reduces the chromophore density. Here, strategy is presented that allows for the preparation of liquid chromophores with a minimal increase in molecular weight, using the important class of perylenes as an example. Two synergistic effects are harnessed: (1) the judicious positioning of short alkyl substituents, and (2) equimolar mixing, which in unison results in a liquid material. A series of 1-alkyl perylene derivatives is synthesized and it is found that short ethyl or butyl chains reduce the melting temperature from 278 °C to as little as 70 °C. Then, two low-melting derivatives are mixed, which results in materials that do not crystallize due to the increased configurational entropy of the system. As a result, liquid chromophores with the lowest reported molecular weight increase compared to the neat chromophore are obtained. The mixing strategy is readily applicable to other π-conjugated systems and, hence, promises to yield a wide range of low molecular weight liquid chromophores.

Synthesis of Furan-Substituted N-Heteroacene-Based Liquid Material and Its Acid-Recognizing Behavior

In this study, we synthesized a novel N-heteroacene-based liquid material 6,7-bis(3,7,11-trimethyl-1-dodecyloxy)-2,3-difurylquinoxaline (RPNL 1), containing two furan rings. We revealed that RPNL 1 adopted a disordered liquid at 25 ∘ C, determined by polarized optical microscopic observation, differential scanning calorimetry, and X-ray diffraction measurements. The fluorescent spectrum measurement revealed that RPNL 1 showed a blue emission at 25 ∘ C. Dissolving benzene sulfonic acid (BSA) in RPNL 1 brought about dramatic changes in its physical properties, such as emission colors, as well as sample states. Upon recognizing BSA, photoluminescent color was changed into orange, as well as phase transition occurred from liquid to a liquid-crystalline phase. RPNL 1 can function as an acid-recognizing material, accompanied with the color changes in emission.