Amino acid-appended pyromellitic diimide liquid materials, their photoluminescence, and the thermal response that turns the photoluminescence off

We report a liquid material based on an L-valine-appended pyromellitic diimide framework. This liquid adopts a room-temperature liquid with Tg at -50 °C and can dissolve naphthalene derivatives to show various photoluminescent colors. Furthermore, the on/off photoluminescence of these solutions can be controlled by heating.

Alkyl-π Functional Molecular Gels: Control of Elastic Modulus and Improvement of Electret Performance

The development of optoelectronically-active soft materials is drawing attention to the application of soft electronics. A room-temperature solvent-free liquid obtained by modifying a π-conjugated moiety with flexible yet bulky alkyl chains is a promising functional soft material. Tuning the elastic modulus (G') is essential for employing optoelectronically-active alkyl-π liquids in deformable devices. However, the range of G' achieved through the molecular design of alkyl-π liquids is limited. We report herein a method for controlling G' of alkyl-π liquids by gelation. Adding 1 wt % low-molecular-weight gelator formed the alkyl-π functional molecular gel (FMG) and increased G' of alkyl-π liquids by up to seven orders of magnitude while retaining the optical properties. Because alkyl-π FMGs have functional π-moieties in the gel medium, this new class of gels has a much higher content of π-moieties of up to 59 wt % compared to conventional π-gels of only a few wt %. More importantly, the gel state has a 23 % higher charge-retention capacity than the liquid, providing better performance in deformable mechanoelectric generator-electret devices. The strategy used in this study is a novel approach for developing next-generation optoelectronically-active FMG materials.

Exciplex Formation by Complexation of an Electron-Accepting Guest in an Electron-Donating Pillar[5]arene Host Liquid

We present a novel system, a liquid-state pillar[5]arene decorated with tri(ethylene oxide) chains, that brings electron-donor and electron-acceptor molecules into proximity for efficient exciplex formation. The electron-accepting guests exhibit a blue-purple emission from a localized excited state upon excitation in common solvents. However, directly dissolving the guests in the electron-donating pillar[5]arene liquid (a bulk system) results in visible green emission from the formed exciplexes. In the bulk system, the guest molecules are always surrounded by excess pillar[5]arene molecules, resulting in the formation of mainly inclusion-type exciplexes. In the bulk system, energy migration occurs between the pillar[5]arene molecules. Excitation of the pillar[5]arenes results in a more intense green exciplex emission than that observed upon direct excitation of the guests. In summary, the pillar[5]arene liquid is a novel system for achieving efficient exciplex formation and energy migration that is different from typical solvent and solid systems.

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.

Control of the Organization of 4,4'-bis(carbazole)-1,1'-biphenyl (CBP) Molecular Materials through Siloxane Functionalization

We show that through the introduction of short dimethylsiloxane chains, it was possible to suppress the crystalline state of CBP in favor of various types of organization, transitioning from a soft crystal to a fluid liquid crystal mesophase, then to a liquid state. Characterized by X-ray scattering, all organizations reveal a similar layered configuration in which layers of edge-on lying CBP cores alternate with siloxane. The difference between all CBP organizations essentially lay on the regularity of the molecular packing that modulates the interactions of neighboring conjugated cores. As a result, the materials show quite different thin film absorption and emission properties, which could be correlated to the features of the chemical architectures and the molecular organizations.

Biomolecule-Based Optical Metamaterials: Design and Applications

Metamaterials are broadly defined as artificial, electromagnetically homogeneous structures that exhibit unusual physical properties that are not present in nature. They possess extraordinary capabilities to bend electromagnetic waves. Their size, shape and composition can be engineered to modify their characteristics, such as iridescence, color shift, absorbance at different wavelengths, etc., and harness them as biosensors. Metamaterial construction from biological sources such as carbohydrates, proteins and nucleic acids represents a low-cost alternative, rendering high quantities and yields. In addition, the malleability of these biomaterials makes it possible to fabricate an endless number of structured materials such as composited nanoparticles, biofilms, nanofibers, quantum dots, and many others, with very specific, invaluable and tremendously useful optical characteristics. The intrinsic characteristics observed in biomaterials make them suitable for biomedical applications. This review addresses the optical characteristics of metamaterials obtained from the major macromolecules found in nature: carbohydrates, proteins and DNA, highlighting their biosensor field use, and pointing out their physical properties and production paths.

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.

Ultraslow Biological Water-Like Dynamics in Waterless Liquid Protein

Fluorescence correlation spectroscopy and time-dependent fluorescence Stokes shift have been employed to elucidate dynamics in different time scales, ranging from picoseconds to nanoseconds, for human serum albumin, in its native and cationized forms as well as in the self-assembled complex of the cationized protein with the polymer surfactant (PS) glycolic acid ethoxylate lauryl ether. The effect of crowding in this complex, especially in the waterless condition, is of prime importance in this context. Excellent correlation of the dynamics with the structures, obtained by circular dichroism and Fourier transform infrared spectroscopy, has been observed. Slow solvation, associated classically with biological water, has been observed in these systems, even in the waterless condition. This apparently intriguing observation has been rationalized by the relaxation of segments of the protein and the PS in the microenvironment of the fluorescent probe.

Interplay between Polaritonic and Molecular Trap States

Strong exciton-photon coupling exhibits the possibility to modify the photophysical properties of organic molecules. This is due to the introduction of hybrid light-matter states, called polaritons, which have unique physical and optical properties. Those strongly coupled systems provide altered excited-state dynamics in comparison to the bare molecule case. In this study, we investigate the interplay between polaritonic and molecular trap states, such as excimers. The molecules used in this study show either prompt or delayed emission from trap states. For both cases, a clear dependency on the exciton-photon energy tuning was observed. Polaritonic emission gradually increased with a concurrent removal of aggregation-induced emission when the systems were tuned toward lower energies. For prompt emission, it is not clear whether the experimental results are best explained by a predominant relaxation toward the lower polariton after excitation or by a direct excimer to polariton transition. However, for the delayed emission case, trap states are formed on the initially formed triplet manifold, making it evident that an excimer-to-polariton transition has occurred. These results unveil the possibility to control the trap state population by creating a strongly coupled system, which may form a mitigation strategy to counteract detrimental trap states in photonic applications.

Functionalization of Biphenylcarbazole (CBP) with Siloxane-Hybrid Chains for Solvent-Free Liquid Materials

We report herein the synthesis of siloxane-functionalized CBP molecules (4,4′-bis(carbazole)-1,1′-biphenyl) for liquid optoelectronic applications. The room-temperature liquid state is obtained through a convenient functionalization of the molecules with heptamethyltrisiloxane chains via hydrosilylation of alkenyl spacers. The synthesis comprises screening of metal-catalyzed methodologies to introduce alkenyl linkers into carbazoles (Stille and Suzuki Miyaura cross-couplings), incorporate the alkenylcarbazoles to dihalobiphenyls (Ullmann coupling), and finally introduce the siloxane chains. The used conditions allowed the synthesis of the target compounds, despite the high reactivity of the alkenyl moieties bound to π-conjugated systems toward undesired side reactions such as polymerization, isomerization, and hydrogenation. The features of these solvent-free liquid CBP derivatives make them potentially interesting for fluidic optoelectronic applications.

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.

Fluorescent Liquid Tetrazines

Tetrazines with branched alkoxy substituents are liquids at ambient temperature that despite the high chromophore density retain the bright orange fluorescence that is characteristic of this exceptional fluorophore. Here, we study the photophysical properties of a series of alkoxy-tetrazines in solution and as neat liquids. We also correlate the size of the alkoxy substituents with the viscosity of the liquids. We show using time-resolved spectroscopy that intersystem crossing is an important decay pathway competing with fluorescence, and that its rate is higher for 3,6-dialkoxy derivatives than for 3-chloro-6-alkoxytetrazines, explaining the higher fluorescence quantum yields for the latter. Quantum chemical calculations suggest that the difference in rate is due to the activation energy required to distort the tetrazine core such that the nπ*S1 and the higher-lying ππ*T2 states cross, at which point the spin-orbit coupling exceeding 10 cm-1 allows for efficient intersystem crossing to occur. Femtosecond time-resolved anisotropy studies in solution allow us to measure a positive relationship between the alkoxy chain lengths and their rotational correlation times, and studies in the neat liquids show a fast decay of the anisotropy consistent with fast exciton migration in the neat liquid films.

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°.

Stimuli-responsive Behaviors for Room-temperature Fluorescent Liquid Materials based on N-Heteroacenes and their Mixtures in Response to HCl Vapor and their Facile Synthesis

In this paper, we report on stimuli-responsive behaviors for room temperature fluorescent liquid materials based on N-heteroacene frameworks in response to HCl vapor. These liquid materials as well as their mixtures prepared by varying the combination can provide various emission colors and stimuli-responsive properties in liquid states. Also, we achieved an improvement in total synthetic yield (>40%) by redesigning the molecular structures of liquid materials as compared to previous liquid materials (<10%).

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.

Transforming Metal-Organic Frameworks into Porous Liquids via a Covalent Linkage Strategy for CO2 Capture

Porous liquids (PLs), an emerging kind of liquid materials with permanent porosity, have attracted increasing attention in gas capture. However, directly turning metal-organic frameworks (MOFs) into PLs via a covalent linkage surface engineering strategy has not been reported. Additionally, challenges including reducing the cost and simplifying the preparation process are daunting. Herein, we proposed a general method to transform Universitetet i Oslo (UiO)-66-OH MOFs into PLs by surface engineering with organosilane (OS) and oligomer species via covalent bonding linkage. The oligomer species endow UiO-66-OH with superior fluidity at room temperature. Meanwhile, the resulting PLs showed great potential in both CO₂ adsorption and CO₂/N₂ selective separation. The residual porosity of PLs was verified by diverse characterizations and molecular simulations. Besides, CO₂ selective capture sites were determined by grand canonical Monte Carlo (GCMC) simulation. Furthermore, the universality of the covalent linkage surface engineering strategy was confirmed using different classes of oligomer species and another MOF (ZIF-8-bearing amino groups). Notably, this strategy can be extended to construct other PLs by taking advantages of the rich library of oligomer species, thus making PLs promising candidates for further applications in energy and environment-related fields, such as gas capture, separation, and catalysis.

Room-Temperature Pentacene Fluids: Oligoethylene Glycol Substituent-Controlled Morphologies and Singlet Fission

We report the first synthesis of solvent-free pentacene fluids at room temperature together with observation of singlet fission (SF). Three pentacenes with different number of ethylene glycol (EG) side chains (n) were employed (denoted as (EG)_n-Pc-(EG)_n: n = 2, 3, and 4). The morphologies of these pentacenes largely depend on the lengths of EG chains (n). (EG)₃-Pc-(EG)₃ and (EG)₄-Pc-(EG)₄ indicate fluid compounds at room temperature, whereas (EG)₂-Pc-(EG)₂ is a solid compound. Microscopic clustering with short-range interactions between pentacene chromophores was confirmed in X-ray diffraction profiles of solvent-free fluids. Such a structural trend is an important origin of SF and consistent with the steady-state spectroscopic results. To one's surprise, femtosecond transient absorption spectroscopy demonstrated that SF occurred in thin films prepared from solvent-free fluids of (EG)₃-Pc-(EG)₃ and (EG)₄-Pc-(EG)₄ in spite of such excessive EG chains.

Glassy Porphyrin/C60 Composites: Morphological Engineering of C60 Fullerene with Liquefied Porphyrins

Morphological control of C₆₀ fullerene using liquefied porphyrins (1 and 2) as the host matrices was explored. Slow evaporation of the solvent of the equimolar mixture of porphyrin and C₆₀ in toluene afforded the porphyrin/C₆₀ composite with a 3:1 molar ratio. The stoichiometric binding behaviors suggest that specific porphyrin-C₆₀ interactions operate the formation of the porphyrin/C₆₀ composites, as corroborated by spectroscopic and thermal properties, and glazing-incidence wide-angle X-ray diffraction. Under the bulk conditions, the conventional thermodynamic advantage of multiple binding cooperativity for molecular recognition is unlikely to explain the stoichiometric binding behaviors. Instead, we propose a size-matching effect on the porphyrin-C₆₀ interaction in the bulk porphyrin matrices, i.e., "supramolecular solvation". The glassy nature of the porphyrin matrices was transmitted to C₆₀ through the specific interaction, and the porphyrin/C₆₀ composites adopted glassy states at room temperature.

Entropic Mixing Allows Monomeric-Like Absorption in Neat BODIPY Films

Intermolecular interactions play a crucial role in materials chemistry because they govern thin film morphology. The photophysical properties of films of organic dyes are highly sensitive to the local environment, and a considerable effort has therefore been dedicated to engineering the morphology of organic thin films. Solubilizing side chains can successfully spatially separate chromophores, reducing detrimental intermolecular interactions. However, this strategy is also significantly decreasing achievable dye concentration. Here, five BODIPY derivatives containing small alkyl chains in the α-position were synthesized and photophysically characterized. By blending two or more derivatives, the increase in entropy reduces aggregation and therefore produces films with extreme dye concentration and, at the same time almost solution like absorption properties. Such a film was placed inside an optical cavity and the achieved system was demonstrated to reach the strong exciton-photon coupling regime by virtue of the achieved dye concentration and sharp absorption features of the film.

Bright Solvent-Free Luminescent Liquid with Magnetism Composed of a Thiocyanate Complex of Ce(III)

Ionic liquids composed of a thiocyanate complex of Ce(III) exhibit bright cyan photoluminescence with a quantum yield close to 40% in addition to paramagnetism. The morphology of a droplet of ionic liquid changes in response to solvent vapor as a stimulus. The emission lifetime and thermal property are characterized. The Weiss temperature is evaluated from the magnetic property measurements, which indicates that antiferromagnetic exchange interaction exists between Ce(III) ions. Insight into the characteristics of the electronic transitions in the Ce(III) complex is obtained using quantum chemical calculations. Thiocyanate complexes of Ce(III) are demonstrated as promising building blocks to produce solvent-free luminescent functional materials.

Liquid/solution-based microfluidic quantum dots light-emitting diodes for high-colour-purity light emission

Organic light-emitting diodes (OLEDs) using a liquid organic semiconductor (LOS) are expected to provide extremely flexible displays. Recently, microfluidic OLEDs were developed to integrate and control a LOS in a device combined with microfluidic technology. However, LOS-based OLEDs show poor-colour-purity light emissions owing to their wide full width at half maximum (FWHM). Here we report liquid/solution-based microfluidic quantum dots light-emitting diodes (QLEDs) for high-colour-purity light emission. Microfluidic QLEDs contain liquid materials of LOS for a backlight and QDs solutions as luminophores. The microfluidic QLED exhibits red, green, and blue light emissions and achieves the highest light colour purity ever reported among LOS-based devices for green and red lights with narrow FWHMs of 26.2 nm and 25.0 nm, respectively. Additionally, the effect of the channel depth for the luminophore on the peak wavelength and FWHM is revealed. The developed device extends the capabilities of flexible microfluidic OLEDs-based and QDs-based displays.

Asymmetric Preparation of α-Quaternary Fluorinated β-keto Esters. Review

In this review, recent advances over the past decade in the preparation of fluorinated stereogenic quaternary centers on β-keto esters compounds are analyzed. Since the incorporation of fluorine and fluorinated groups is of special interest in pharmaceutical chemistry, a range of metal-catalyzed and organocatalyzed methods have been developed. Herein, we review the enantioselective fluorination, trifluoromethylation and trifluoromethylthiolation of 3-oxo esters. The scope, the induction of enantioselectivity and mechanistic investigations are presented.

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.

A near-infrared fluorescent phthalocyanine liquid developed through controlling intermolecular interactions

A near-infrared fluorescent phthalocyanine (Pc) liquid was developed through introducing bulky yet flexible units onto the Pc skeleton.

Structures, Phase Behavior, and Fluorescent Properties of 3-Phenyl-1-(pyridin-2-yl)-1H-pyrazol-5-amine and Its ZnCl2 Complex

The synthesis of the asymmetric ligand 3-phenyl-1-(pyridin-2-yl)-1H-pyrazol-5-amine (L1) and its single-crystal X-ray structure are reported. L1 displays crystallographic symmetry (orthorhombic, Pccn) higher than its molecular symmetry (point group C₁) and also displays supercooling, with a difference in the melting and solidification points of over 100 °C. Upon complexation with ZnCl₂, L1 engages in both primary cation and secondary anion coordination via hydrogen bonding, and the complex exhibits a room-to-low-temperature single crystal-to-crystal phase transition. The ZnCl₂ complex becomes a birefringent fluid mixed with crystalline domains at high temperatures, as detected by polarized optical microscopy. Examination of the photoluminescence properties showed that the emission intensity increased and a pronounced bathochromic shift was observed in the emission maximum upon going from solution to the solid state, for both the ligand and complex, consistent with aggregation-induced emission behavior.

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.

Soft chromophore featured liquid porphyrins and their utilization toward liquid electret applications

Optoelectronically active viscous liquids are ideal for fabricating foldable/stretchable electronics owing to their excellent deformability and predictable π-unit–based optoelectronic functions, which are independent of the device shape and geometry. Here we show, unprecedented ‘liquid electret’ devices that exhibit mechanoelectrical and electroacoustic functions, as well as stretchability, have been prepared using solvent-free liquid porphyrins. The fluidic nature of the free-base alkylated-tetraphenylporphyrins was controlled by attaching flexible and bulky branched alkyl chains at different positions. Furthermore, a subtle porphyrin ring distortion that originated from the bulkiness of alkyl chains was observed. Its consequences on the electronic perturbation of the porphyrin-unit were precisely elucidated by spectroscopic techniques and theoretical modelling. This molecular design allows shielding of the porphyrin unit by insulating alkyl chains, which facilitates its corona-charged state for a long period under ambient conditions.

Though electret materials are attractive for realizing flexible mechanoelectrical devices, these materials are typically solid films. Here, the authors report stretchable ‘liquid-electret’ devices consisting solvent-free liquid porphyrins that show piezoelectric and electroacoustic functionality.

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.

Viscoelastic Conjugated Polymer Fluids

The introduction of optoelectronic functions into viscoelastic polymers can yield highly sophisticated soft materials for biomedical devices and autonomous robotics. However, viscoelasticity and excellent optoelectronic properties are difficult to achieve because the presence of a large number of π-conjugated moieties drastically stiffens a polymer. Here, we report a variation of additive-free viscoelastic conjugated polymers (VE-CPs) at room temperature by using an intact π-conjugated backbone and bulky, yet flexible, alkyl side chains as "internal plasticizers." Some of these polymers exhibit gel- and elastomer-like rheological behaviors without cross-linking or entanglement. Furthermore, binary blends of these VE-CPs exhibit a never-seen-before dynamic miscibility with self-restorable and mechanically induced fluorescence color changes.

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.

Catalytic Asymmetric Fluorination of Alkyl 1-indanone-2-carboxylates Ruled by Pybox-Eu(III) Combination

A highly enantioselective catalytic method for the synthesis of quaternary α-fluoro derivatives of 3-oxo esters is described. The reaction uses europium (III) triflate and commercially available chiral pybox-type C2-symmetric ligand. Excellent results in terms of yields and enantioselectivities were assured using the electrophilic NFSI reagent under mild reaction conditions.

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.

Paintable Room-Temperature Phosphorescent Liquid Formulations of Alkylated Bromonaphthalimide

Organic phosphors have been widely explored with an understanding that crystalline molecular ordering is a requisite for enhanced intersystem crossing. In this context, we explored the room-temperature phosphorescence features of a solvent-free organic liquid phosphor in air. While alkyl chain substitution varied the physical states of the bromonaphthalimides, the phosphorescence remained unaltered for the solvent-free liquid in air. As the first report, a solvent-free liquid of a long swallow-tailed bromonaphthalimide exhibits room-temperature phosphorescence in air. Doping of the phosphor with carbonyl guests resulted in enhanced phosphorescence, and hence a large-area paintable phosphorescent liquid composite with improved lifetime and quantum yield was developed.

Charge Carrier Polarity Modulation in Diketopyrrolopyrrole-Based Low Band Gap Semiconductors by Terminal Functionalization

Organic semiconductors with variable charge carrier polarity are required for optoelectronic applications. Herein, we report the synthesis of three novel diketopyrrolopyrrole (DPP)-based D-A molecules having three different terminal groups (amide, ester, and dicyano) and study their electronic properties. An increase in electron acceptor strength from amide to dicyano leads to a bathochromic shift in absorption. Photoconductivity and field effect transistor (FET) measurements confirmed that a small increase in acceptor strength can result in a large change in the charge transport properties from p-type to n-type. The molecule with an amide group, DPP-amide, exhibited a moderate p-type mobility (1.3 × 10^-2 cm² V^-1 s^-1), whereas good n-type mobilities were observed for molecules with an ester moiety, DPP-ester (1.5 × 10^-2 cm² V^-1 s^-1), and with a dicyano group, DPP-DCV (1 × 10^-2 cm² V^-1 s^-1). The terminal functional group modification approach presented here is a simple and efficient method to alter the charge carrier polarity of organic semiconductors.

Luminescent imidazolium–naphthalene salts in liquid and solid states

This article describes highly luminescent ionic compounds in liquid and crystalline states, where a naphthalene moiety is conjugated to an imidazolium center decorated with two different lengths of alkyl chains.

Fast 3D Microscopy Imaging of Contacts Between Surfaces Using a Fluorescent Liquid

A novel method is presented for the rapid direct 3D visualization of the contact between two surfaces by means of fluorescence microscopy using a fluorescent liquid. Distances between the surfaces of up to several hundred nanometers can be determined with subnanometer accuracy in 3D and within seconds of measurement time. The method opens new possibilities for research in the areas of contact mechanics, friction, wear, and lubrication.

Scope of surface-modified molecular and nanomaterials in gel/liquid forms for developing mechanically flexible DSSCs/QDSSCs

The advanced lifestyle of the human race involves heavy usage of various gadgets which require copious supplies of energy for uninterrupted functioning. Due to the ongoing depletion of fossil fuels and the accelerating demand for other energy resources, renewable energy sources, especially solar cells, are being extensively explored as viable alternatives. Flexible solar cells have recently emerged as an advanced member of the photovoltaic family; the flexibility and pliability of these photovoltaic materials are advantageous from a practical point of view. Conventional flexible solar cell materials, when dispersed in solvents, are usually volatile and create severe stability issues when incorporated in devices. Recently, non-volatile, less viscous functional molecular liquids/gels have been proposed as potential materials for use in foldable device applications. This perspective article discusses the scope of surface-modified non-volatile molecular and nanomaterials in liquid/gel forms in the manufacturing and deployment of flexible photovoltaics.

Supercooling of functional alkyl-π molecular liquids

Metastable states of soft matters are extensively used in designing stimuli-responsive materials. However, the non-steady properties may obstruct consistent performance. Here we report an approach to eradicate the indistinguishable metastable supercooled state of functional molecular liquids (FMLs), which remains as a liquid for weeks or months before crystallizing, via rational molecular design. The phases (solid, kinetically stable liquid, and supercooled liquid) of a model FML, branched alkyl chain-substituted 9,10-diphenylanthracene (DPA), are found to be governed by subtle alterations of the molecular structure (alkyl-DPA ratio and bulkiness of the DPA unit). We thus outline molecular design principles to avoid supercooled FML formation. Moreover, we demonstrate a practical technique to rapidly discriminate supercooled FMLs (within 5 h) by accelerating their crystallization in differential scanning calorimetry heating via pre-annealing or relatively slow scanning.