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

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.

Facile preparation and charge retention mechanism of polymer-based deformable electret

Electret materials with high deformability largely extend their applications such as wearable devices and actuators. Meanwhile, the deformability of currently reported electrets is somewhat limited except for a liquid electret that requires synthetic procedures with relatively low product yield. Here, we report a polymer-based electret with infinite deformability, which is simply prepared by corona-discharging on the mixture of two commercially available polymers, i.e., polybutenes (PB) as a liquid polyolefin and polypropylene-graft-maleic anhydride (MPP) as a solute. The charge retention mechanism of the PB/MPP electret was both experimentally and computationally elucidated from the views of molecular and nanoscale structures, and transport properties. Contrary to the ease of the preparation, the charge retention mechanism was complicated. The results of quantum chemical calculations and X-ray scattering indicated that the succinic anhydride polar moieties in MPP act as a charge trap site while how they distribute in the non-polar matrix also matters. Transport property measurements revealed the strong connection between complex viscosity and the relaxation time of the charge decay of the PB/MPP electret. Finally, we fabricated a simple piezoelectric device consisting of the PB/MPP electret. It was demonstrated that the piezoelectric performance of the PB/MPP electret is comparable to that of a conventional solid electret.

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.

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.

A Stretchable Expanded Polytetrafluorethylene-Silicone Elastomer Composite Electret for Wearable Sensor

Soaring developments in wearable electronics raise an urgent need for stretchable electrets. However, achieving soft electrets simultaneously possessing excellent stretchability, longevity, and high charge density is still challenging. Herein, a facile approach is proposed to prepare an all-polymer hybrid composite electret based on the coupling of elastomer and ePTFE membrane. The composite electrets are fabricated via a facile casting and thermal curing process. The obtained soft composite electrets exhibit constantly high surface potential (-0.38 kV) over a long time (30 days), large strain (450%), low hysteresis, and excellent durability (15,000 cycles). To demonstrate the applications, the stretchable electret is utilized to assemble a self-powered flexible sensor based on the electrostatic induction effect for the monitoring of human activities. Additionally, output signals in the pressure mode almost two orders of magnitude larger than those in the strain mode are observed and the sensing mechanism in each mode is investigated.

Stretchable π-conjugated polymer electrets for mechanoelectric generators

Electrets are materials that retain quasi-permanent electric charges and are attracting attention as key components of batteryless micropower supplies. A chemical structure that facilitates ionization and that can stabilize these charges, such as a π-conjugated system, is expected to increase the charge density compared with that of conventional insulating polymers. Here, we report a mechanoelectric generator (MEG) (vibrational energy harvester) that uses alkylated π-conjugated polymers (Alk-CPs), which can be monopolarized either into positive or negative mode electrets. With the attachment of insulating, bulky, yet flexible alkyl side chains to the π-conjugated backbone, the poled Alk-CPs showed long charge lifetime suitable for MEGs. The elastic modulus of the electret was adjusted to approximately match that of the stretchable polyurethane substrate by blending two miscible Alk-CPs with different elastic moduli, producing a laminated film that could be stretched up to 300%. The MEG presented showed conformability when applied to a deformable object.

Multiplying the Stable Electrostatic Field of Electret Based on the Heterocharge-Synergy and Superposition Effect

Owing to magic charge storage behavior, an electret can exhibit an external electrostatic field, which is widely used in numerous domains such as electronics, energy, healthcare, and environment. However, the theory of the charge storage mechanism still needs further development to enhance the performance and stability of the electret. Herein, a stable charge storage model known as the heterocharge-synergy model (HSM) in electrets is proposed and verified, and the electrostatic field superposition effect of electrets is also proved. Based on the HSM and superposition effect, the stable electrostatic field intensity (average of ≈22.49 kV cm^-1 and maximum of ≈29.58 kV cm^-1 , which is close to the minimum air breakdown field intensity of ≈30 kV cm^-1 ) of the composite electret film is multiplied by simple layer-by-layer stacking. Utilizing the multilayer composite electret films and designing a two-sided electrostatic induction structure, a two-sided bipolar single-electrode non-contact nanogenerator is constructed with transferred charge density up to ≈132.61 µC m^-2 , which is twice as large as that of the non-contact nanogenerators with one-sided electrostatic induction structure. Clearing and utilizing the charge behaviors of the electret can boost the performance and enhance the stability of electret-based devices in various domains.

Porphyrin Photoabsorption and Fluorescence Variation with Adsorptive Loading on Gold Nanoparticles

Here, we report the photophysical structure–property relationship of porphyrins adsorbed on gold nanoparticles. The number of porphyrin–alkanethiolate adsorbates per particle was adjusted by a post-synthetic thiol/thiolate exchange reaction on 1-dodecanethiolate–protected gold nanoparticles. Even with a low loading level of adsorbates (<10% of all thiolate sites on gold nanoparticles), the shoulder absorption at the Soret band was intensified, indicating the formation of aggregates of porphyrin adsorbates on the nanoparticles. Steady-state fluorescence quantum yields could be adjusted by the bulkiness of substituents at the meso-positions of the porphyrin or the methylene linker chain length, regardless of the porphyrin loading level and the nanoparticle diameter.

Droplet-based nanogenerators for energy harvesting and self-powered sensing

The energy crisis is a continuing topic for all human beings, threatening the development of human society. Accordingly, harvesting energy from the surrounding environment, such as wind, water flow and solar power, has become a promising direction for the research community. Water contains tremendous energy in a variety of forms, such as rivers, ocean waves, tides, and raindrops. Among them, raindrop energy is the most abundant. Raindrop energy not only can complement other forms of energy, such as solar energy, but also have potential applications in wearable and universal energy collectors. Over the past few years, droplet-based electricity nanogenerators (DENG) have attracted significant attention due to their advantages of small size and high power. To date, a variety of fundamental materials and ingenious structural designs have been proposed to achieve efficient droplet-based energy harvesting. The research and application of DENG in various fields have received widespread attention. In this review, we focus on the fundamental mechanism and recent progress of droplet-based nanogenerators in the following three aspects: droplet properties, energy harvesting and self-powered sensing. Finally, some challenges and further outlook for droplet-based nanogenerators are discussed to boost the future development of this promising field.

An excimer to exciplex transition through realization of donor-acceptor interactions in luminescent solvent-free liquids

Luminescent solvent-free organic liquids are known for their enhanced quantum yield, color tunability, and availability of a matrix for other dopants to generate hybrid luminescent materials with improved features for newer applications. Herein, we report a donor-acceptor based luminescent "exciplex liquid" by utilizing the slightly different electron affinity of the acceptor molecules. A red-shifted broad exciplex emission exhibited by the donor-acceptor pair even at a lower concentration of the acceptor (0.001 equiv.) indicates high efficiency in the solvent-free state. A detailed NMR study revealed weak intermolecular interactions between the donor and acceptor in the solvent-free matrix that stabilizes the exciplex liquid. The failure of structurally similar solid counterparts to form an exciplex confirms the advantage of the available supportive liquid matrix. Besides, the luminescent exciplex liquid is found efficient in sensing application, which is unachievable by either the individual liquids or their solid counterparts. Here, a transition of a donor-acceptor pair from a solid to solvent-free liquid results in a new hybrid liquid that can be an alternative for solid sensor materials.

Ionic Charge-Transfer Liquid Crystals Formed by Alternating Supramolecular Copolymerization of Liquid π-Donors and TCNQ

A new family of liquid π-donors, lipophilic dihydrophenazine (DHP) derivatives, show remarkably high π-electron-donor property which exhibit supramolecular alternating copolymerization with 7,7,8,8-tetracyanoquinodimethane (TCNQ), giving ionic charge-transfer (ICT) complexes. The ICT complexes form distinct columnar liquid crystalline (LC) mesophases with well-defined alternating molecular alignment as demonstrated by UV-Vis-NIR spectra, IR spectra, and X-ray diffraction (XRD) patterns. These liquid crystalline ICT complexes display unique phase transitions in response to mechanical stress: the columnar ICT phase is converted to macroscopically oriented smectic-like mesophases upon applying shear force. Although there exist reports on the formation of ICT in the crystalline state, this study provides the first rational identification of ICT mesophases based on the spectroscopic and structural data. The liquid crystalline ICT phases are generated by strong electronic interactions between the liquid π-donors and solid acceptors. It clearly shows the significance of simultaneous fulfillment of strong π-donating ability and ordered self-assembly of the stable ICT pairs. The flexible, stimuli-responsive structural transformation of the ICT complexes offer a new perspective for designing processable CT systems with controlled hierarchical self-assembly and electronic structures.

Evaluation-oriented exploration of photo energy conversion systems: from fundamental optoelectronics and material screening to the combination with data science

Light is a form of energy that can be converted to electric and chemical energies. Thus, organic photovoltaics (OPVs), perovskite solar cells (PSCs), photocatalysts, and photodetectors have evolved as scientific and commercial enterprises. However, the complex photochemical reactions and multicomponent materials involved in these systems have hampered rapid progress in their fundamental understanding and material design. This review showcases the evaluation-oriented exploration of photo energy conversion materials by using electrodeless time-resolved microwave conductivity (TRMC) and materials informatics (MI). TRMC with its unique options (excitation sources, environmental control, frequency modulation, etc.) provides not only accelerated experimental screening of OPV and PSC materials but also a versatile route toward shedding light on their charge carrier dynamics. Furthermore, MI powered by machine learning is shown to allow extremely high-throughput exploration in the large molecular space, which is compatible with experimental screening and combinatorial synthesis.

Stretchable Electrets: Nanoparticle-Elastomer Composites

Manipulating charges is fundamental to numerous systems, and this ability is achieved through materials of diverse characteristics. Electrets are dielectrics that trap charges or dipoles. Applications include electrophotography, microphones, air filters, and energy harvesters. To trap charges or dipoles for a long time, electrets are commonly made of hard dielectrics. Stretchable dielectrics are short-lived electrets. The two properties, longevity and stretchability, conflict; existing electrets struggle to attain both. This work describes an approach to developing stretchable electrets. Nanoparticles of a hard electret are immobilized in a matrix of dielectric elastomer. The composite divides the labor of two functions: the particles trap charges with longevity, and the matrix enables stretchability. The design considerably broadens the choice of materials to enable stretchable electrets. Silica nanoparticles in the polydimethylsiloxane elastomer achieve a charge density ∼ 4 × 10^-5 C m^-2 and a lifetime beyond 60 days. Long-lived, stretchable electrets open extensive opportunities.

Electronic Component Mounting for Durable E-Textiles: Direct Soldering of Components onto Textile-Based Deeply Permeated Conductive Patterns

For the improvement of the performance and function of electronic textiles (e-textiles), methods for electronic component mounting of textile circuits with electrical and mechanical durability are necessary. This manuscript presents a component mounting method for durable e-textiles, with a simpler implementation and increased compatibility with conventional electronics manufacturing processes. In this process, conductive patterns are directly formed on a textile by the printing of conductive ink with deep permeation and, then, components are directly soldered on the patterns. The stiffness of patterns is enhanced by the deep permeation, and the enhancement prevents electrical and mechanical breakages due to the stress concentration between the pattern and solder. This allows components to be directly mounting on textile circuits with electrical and mechanical durability. In this study, a chip resistor was soldered on printed patterns with different permeation depths, and the durability of the samples were evaluated by measuring the variation in resistance based on cyclic tensile tests and shear tests. The experiments confirmed that the durability was improved by the deep permeation, and that the samples with solder and deep permeation exhibited superior durability as compared with the samples based on commercially available elastic conductive adhesives for component mounting. In addition, a radio circuit was fabricated on a textile to demonstrate that various types of components can be mounted based on the proposed methods.

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.