Fluorimetric Detection of Vapor Pollutants with Diketopyrrolopyrrole Polymer Microcavities

The increasing prevalence and detrimental effects of volatile organic compounds are driving the need for selective on-site sensors that do not require complex sampling or instrumentation. Broadband selective sensors exhibiting selectivity based on their distinct response mechanism is becoming of increasing technological relevance in both industrial and urban settings. In this context, we propose a label-free sensor based on a polymeric planar microcavity embedded with a fluorescent organic dye, designed to detect various pollutants in the vapor phase. The sensor consists of alternating layers of cellulose acetate and poly(N-vinylcarbazole) and contains a polystyrene defect layer doped with a quadrupolar diketopyrrolopyrrole. Both the structural properties of the polymer microcavity and the dye in the defect layer contribute to the sensor's response to analytes, creating a dual-probe system where a single photonic element translates chemical signals into optical signals, namely, transmission and fluorescence spectral variations. The discrimination capability of the photonic structure arises from the physicochemical interactions between the analytes and the polymer components. To validate our approach, we evaluate the sensor's response to four distinct volatile molecules and investigate the mechanisms influencing the optical response.

Generation of arbitrarily patterned polarizers using 2-photon polymerization

Patterned polarizers are prepared using liquid crystals (LC) doped with a black dichroic dye and in combination with a linear polarizer. The pattern is achieved with a nanostructured LC alignment surface, that is generated using a two-photon polymerization direct laser write (2PP-DLW). This technique creates a pattern of high-resolution grooves in the photoresist at any arbitrary angle. The angle governs the LC orientation at any substrate surface point, determining the transmitted light linear polarization angle. This paper presents the first use of a 2PP-DLW cured positive tone photoresist for dichroic dye-doped LC alignment. Two complementary photoresists have been employed: conventional negative tone SU-8 photoresist and, in this context novel, positive tone S1805 photoresist. The alignment quality of the polarizers has been assessed by analyzing the transmission using an additional polarizer. For SU-8, the resulting grayscale pattern and a contrast ratio (CR) of 14 has measured. The uniformity of the alignment has been measured to be 65% using normalized Shannon entropy (H). For S1805, a CR of 37 was measured, and a uniformity of 63% was obtained. 2PP-DLW allows for shaping complex patterns in submicron dimensions and for the fabrication of arbitrarily patterned polarizers and other LC devices.

A Halogen-Bonded Fluorescent Molecular Photoswitch: Transition from 3D Cubic Lattice to 1D Helical Superstructure for Polarization Inversion of Circularly Polarized Luminescence

The fabrication of materials that can switch between circularly polarized luminescence (CPL) signals is both essential and challenging. Here, two new halogen-bonded fluorescent molecular photoswitches, namely, HB-switch 1 and HB-switch 2, containing α-cyano-substituted diarylethene compounds with different end groups were developed. Upon exposure to specific UV or visible light wavelengths, they exhibited controllable and reversible Z/E photoisomerization. When these switches were integrated into blue-phase liquid crystals (BPLCs), the temperature range of BP significantly expanded. Notably, the BP system incorporating HB-switch 1 exclusively achieved reversible polarization inversion of CPL signals under irradiation with specific UV/Visible light and during cooling/heating. The photo/thermal dual-response behavior of the CPL signals can be attributed to the phase transition from a high-symmetry 3D BP Icubic lattice to a low-symmetry 1D helical superstructure induced by the Z/E photoisomerization of HB-switch 1 and temperature changes. This study underscores the significance of employing halogen-bond assembly strategies to design materials with switchable CPL signals, opening new possibilities for CPL-active systems.

Structural Color Colloidal Photonic Crystals for Biomedical Applications

Photonic crystals are a new class of optical microstructure materials characterized by a dielectric constant that varies periodically with space and features a photonic bandgap. Inspired by natural photonic crystals such as butterfly scales, a series of artificial photonic crystals are developed for use in integrated photonic platforms, biosensing, communication, and other fields. Among them, colloidal photonic crystals (CPCs) have gained widespread attention due to their excellent optical properties and advantages, such as ease of preparation and functionalization. This work reviews the classification and self-assembly principles of CPCs, details some of the latest biomedical applications of large-area, high-quality CPCs prepared using advanced self-assembly methods, summarizes the existing challenges in CPC construction and application, and anticipates future development directions and optimization strategy. With further advancements, CPCs are expected to play a more critical role in biosensors, drug delivery, cell research, and other fields, bringing significant benefits to biomedical research and clinical practice.

Directed crystalline symmetry transformation of blue-phase liquid crystals by reverse electrostriction

Soft-matter-based photonic crystals like blue-phase liquid crystals (BPLC) have potential applications in wide-ranging photonic and bio-chemical systems. To date, however, there are limitations in the fabrication of large monocrystalline BPLCs. Traditional crystal-growth process involves the transition from a high-temperature disordered phase to an ordered (blue) phase and is generally slow (takes hours) with limited achievable lattice structures, and efforts to improve molecular alignment through post-crystallization field application typically prove ineffective. Here we report a systematic study on the molecular self-assembly dynamics of BPLC starting from a highly ordered phase in which all molecules are unidirectionally aligned by a strong electric field. We have discovered that, near the high-temperature end of the blue phase, if the applied field strength is then switched to an intermediate level or simply turned off, large-area monocrystalline BPLCs of various symmetries (tetragonal, orthorhombic, cubic) can be formed in minutes. Subsequent temperature tuning of the single crystal at a fixed applied field allows access to different lattice parameters and the formation of never-before-seen monoclinic structures. The formed crystals remain stable upon field removal. The diversity of stable monocrystalline BPLCs with widely tunable crystalline symmetries, band structures, and optical dispersions will significantly improve and expand their application potentials.

When quantum dots meet blue phase liquid crystal elastomers: visualized full-color and mechanically-switchable circularly polarized luminescence

Polymer-based circularly polarized luminescence (CPL) materials with the advantage of diversified structure, easy fabrication, high thermal stability, and tunable properties have garnered considerable attention. However, adequate and precise tuning over CPL in polymer-based materials remains challenging due to the difficulty in regulating chiral structures. Herein, visualized full-color CPL is achieved by doping red, green, and blue quantum dots (QDs) into reconfigurable blue phase liquid crystal elastomers (BPLCEs). In contrast to the CPL signal observed in cholesteric liquid crystal elastomers (CLCEs), the chiral 3D cubic superstructure of BPLCEs induces an opposite CPL signal. Notably, this effect is entirely independent of photonic bandgaps (PBGs) and results in a high glum value, even without matching between PBGs and the emission bands of QDs. Meanwhile, the lattice structure of the BPLCEs can be reversibly switched via mechanical stretching force, inducing on-off switching of the CPL signals, and these variations can be further fixed using dynamic disulfide bonds in the BPLCEs. Moreover, the smart polymer-based CPL systems using the BPLCEs for anti-counterfeiting and information encryption have been demonstrated, suggesting the great potential of the BPLCEs-based CPL active materials.

The Interplay of Spacer Chirality and Parity in Mesogenic Dimers

Introducing chirality into soft materials, including liquid crystals (LCs), profoundly impacts their self-organization and physical properties. In this study, we synthesized a novel series of LC dimers with a chiral center as part of their flexible spacer. The dimers were prepared in racemic and enantiomerically pure forms. Their spacer length and parity were varied to investigate the effect of spacer chirality and parity on mesomorphic behavior and on chiral induction in the nematic phase of achiral mesogens. Our results show that the even-membered chiral dimers only have chiral nematic phases. In contrast, the odd-membered dimers display rich mesomorphism, including the intriguing blue phase (BP) and chiral form of the twist-bend nematic phase (N*TB). The observed significant difference in the 3D surface morphology between the racemic and chiral forms of the N*TB phase suggests that the chiral moiety in the spacer promotes a chiral hierarchy. Furthermore, the chiral dimers show a prominent odd-even effect in the helical twisting power in nematic hosts. These findings highlight the importance of the position of the chiral group within the dimeric molecule and provide new insights into how intrinsic chirality in the spacer affects the overall structural chirality.

Super-Wide Temperature Lasers Spanning from -180 to 240 °C Based on Fully-Polymerized Blue Phase Superstructures

Blue phase liquid crystal (BPLC) lasers have potential applications in displays, sensors, and anti-counterfeiting fields owing to their outstanding optical properties. However, there remain challenges on lasing below 0 °C, which significantly limits the potential application of BPLC lasers in low-temperature environments. In this work, BPLC lasing below 0 °C is realized for the first time in a super-wide temperature range of -180-240 °C using a well-designed fully-polymerized BPLC system with a narrow line width of 0.0881 nm and a low lasing threshold of 37 nJ pulse-1. This fully-polymerized BPLC both effectively avoids low-temperature random crystallization and has excellent compatibility with dye molecules that significantly widen the lasing temperature range below 0 °C. Besides, the variations of laser peak and threshold are also revealed below 0 °C, that is, redshifted laser wavelength and increased threshold value with decreasing temperature, which contribute to a blue-shifted laser signal and a U-shaped lasing threshold in -180-240 °C. These unique laser behaviors can be ascribed to the temperature-dependent anisotropically microstructural deformation of the BP lattice. This work not only opens a door to the development of low-temperature BPLC lasers but also sets out important insights in the design of novel organic optical devices.

Programming Thermochromic Liquid Crystal Hetero-Oligomers for Near-Infrared Reflectors: Unequal Incorporation of Similar Reactive Mesogens in Thiol-ene Oligomers

Cholesteric liquid crystal oligomers are widely researched for their interesting thermochromic properties. However, structure-property relationships to program the thermochromic properties of these oligomers have been rarely reported. In this work, we use the versatile thiol-ene click reaction to synthesize a series of hetero-oligomers and study the impact of different compositions on the thermochromic behavior of the resulting material. Characterization of the oligomers shows significantly different rates of reaction for the monomers despite their very similar structures, which leads to oligomer compositions that do not match the original reaction feed. The oligomers are then used to produce thin near-infrared reflecting coatings. The best-performing thermochromic reflector has a room-temperature reflection band that shifts a total of 510 nanometers upon heating to 120 °C. The shift is repeatable for up to 10 times with no appreciable degradation. The room temperature reflection of the coatings is shown to be tunable not only by adjusting the chiral dopant concentration but also by the ratio of the monomers. Finally, we show that the oligomers can be chemically modified by making their reactive end groups undergo a reaction with monothiol compounds. These modifications allow for further fine-tuning of liquid crystal oligomers for heat-regulating window films, for example.

Bioinspired Colloidal Photonic Composites: Fabrications and Emerging Applications

Organisms in nature have evolved unique structural colors and stimuli-responsive functions for camouflage, warning, and communication over millions of years, which are essential to their survival in harsh conditions. Inspired by these characteristics, colloidal photonic composites (CPCs) composed of colloidal photonic crystals embedded in the polymeric matrix are artificially prepared and show great promise in applications. This review focuses on the summary of building blocks, i.e., colloidal particles and polymeric matrices, and constructive strategies from the perspective of designing CPCs with robust performance and specific functionality. Furthermore, their state-of-the-art applications are also discussed, including colorful coatings, anti-counterfeiting, and regulation of photoluminescence, especially in the field of visualized sensing. Finally, current challenges and potential for future developments in this field are discussed. The purpose of this review is not only to clarify the design principle for artificial CPCs but also to serve as a roadmap for the exploration of next-generation photonic materials.

Over 200 °C Broad-Temperature Lasers Reconstructed from a Blue-Phase Polymer Scaffold

Blue-phase liquid crystal (BPLC) lasers have received extensive attention and have potential applications in sensors, displays, and anti-counterfeiting, owing to their unique 3D photonic bandgap. However, the working temperature range of such BPLC lasers is insufficient, and investigations are required to elucidate the underlying mechanism. Herein, a broad-temperature reconstructed laser is successfully achieved in dye-doped polymer-stabilized blue-phase liquid crystals (DD-PSBPLCs) with an unprecedented working temperature range of 25-230 °C based on a robust polymer scaffold, which combines the thermal stability and the tunability from the system. The broad-temperature lasing stems from the high thermal stability of the robust polymerized system used, which affords enough reflected and matched fluorescence signals. The temperature-tunable lasing behavior of the DD-PSBPLCs is associated with the phase transition of the unpolymerized content (≈60 wt%) in the system, which endows with a reconstructed characteristic of BP lasers including a U-shaped lasing threshold, a reversible lasing wavelength, and an obvious lasing enhancement at about 70 °C. This work not only provides a new idea for the design of broad-temperature BPLC lasers, but also sets out important insight in innovative microstructure changes for novel multifunctional organic optic devices.

Enantioselective Synthesis of 3-Aryl-3-hydroxypropanoic Esters as Subunits for Chiral Liquid Crystals

Chiral liquid crystals (LCs) with their unique optical and mechanical properties are perspective functional soft materials for fundamental science and advanced technological applications. Herein, we introduce the chiral 3-aryl-3-hydroxypropanoic ester moiety as a versatile building block for the preparation of LC compounds. Three chiral subunits differing in the aromatic part were obtained through asymmetric transfer hydrogenation using Ru(II) complexes with ee from 98% to >99%. Chiral LC compounds of diverse topologies were further prepared without deterioration of the ee during the synthesis. The mesomorphic behavior of rod-shaped, bent-shaped flexible dimeric, and polycatenar LCs is consistent with their topology─chiral nematic and smectic phases were identified as well as the rarely observed twist grain boundary A and blue phases. The utilization of synthetic chiral building blocks offers the possibility of fine tuning the intermolecular interactions by subtle changes in the molecular structure as well as the preparation of the corresponding racemic forms. This paves the way for the study of self-organization and the structure-property relationship in chiral soft materials.

Self-assembled liquid crystal architectures for soft matter photonics

Self-assembled architectures of soft matter have fascinated scientists for centuries due to their unique physical properties originated from controllable orientational and/or positional orders, and diverse optic and photonic applications. If one could know how to design, fabricate, and manipulate these optical microstructures in soft matter systems, such as liquid crystals (LCs), that would open new opportunities in both scientific research and practical applications, such as the interaction between light and soft matter, the intrinsic assembly of the topological patterns, and the multidimensional control of the light (polarization, phase, spatial distribution, propagation direction). Here, we summarize recent progresses in self-assembled optical architectures in typical thermotropic LCs and bio-based lyotropic LCs. After briefly introducing the basic definitions and properties of the materials, we present the manipulation schemes of various LC microstructures, especially the topological and topographic configurations. This work further illustrates external-stimuli-enabled dynamic controllability of self-assembled optical structures of these soft materials, and demonstrates several emerging applications. Lastly, we discuss the challenges and opportunities of these materials towards soft matter photonics, and envision future perspectives in this field.

Electrically switchable structural colors based on liquid-crystal-overlaid aluminum anisotropic nanoaperture arrays

Actively tunable or reconfigurable structural colors are highly promising in future development for high resolution imaging and displaying applications. To this end, we demonstrate switchable structural colors covering the entire visible range by integrating aluminum nanoaperture arrays with nematic liquid crystals. The geometrically anisotropic design of the nanoapertures provides strong polarization-dependent coloration. By overlaying a nematic liquid crystal layer, we further demonstrate switchable ability of the structural colors by either changing the polarization of the incident light or applying an external voltage. The switchable structural colors have a fast response time of 28 ms at a driving voltage of 6.5 V. Furthermore, colorful patterns are demonstrated by coding the colors with various dimensions of nanoaperture arrays with dual switching modes. Our proposed technique in this work provides a dual-mode switchable structural colors, which is highly promising for polarimetric displays, imaging sensors, and visual cryptography.

Liquid crystal-based structural color actuators

Animals can modify their body shape and/or color for protection, camouflage and communication. This adaptability has inspired fabrication of actuators with structural color changes to endow soft robots with additional functionalities. Using liquid crystal-based materials for actuators with structural color changes is a promising approach. In this review, we discuss the current state of liquid crystal-based actuators with structural color changes and the potential applications of these structural color actuators in soft robotic devices.

Liquid crystal-templated chiral nanomaterials: from chiral plasmonics to circularly polarized luminescence

Chiral nanomaterials with intrinsic chirality or spatial asymmetry at the nanoscale are currently in the limelight of both fundamental research and diverse important technological applications due to their unprecedented physicochemical characteristics such as intense light-matter interactions, enhanced circular dichroism, and strong circularly polarized luminescence. Herein, we provide a comprehensive overview of the state-of-the-art advances in liquid crystal-templated chiral nanomaterials. The chiroptical properties of chiral nanomaterials are touched, and their fundamental design principles and bottom-up synthesis strategies are discussed. Different chiral functional nanomaterials based on liquid-crystalline soft templates, including chiral plasmonic nanomaterials and chiral luminescent nanomaterials, are systematically introduced, and their underlying mechanisms, properties, and potential applications are emphasized. This review concludes with a perspective on the emerging applications, challenges, and future opportunities of such fascinating chiral nanomaterials. This review can not only deepen our understanding of the fundamentals of soft-matter chirality, but also shine light on the development of advanced chiral functional nanomaterials toward their versatile applications in optics, biology, catalysis, electronics, and beyond.

Advances in 4D printing of liquid crystalline elastomers: materials, techniques, and applications

Liquid crystalline elastomers (LCEs) are polymer networks exhibiting anisotropic liquid crystallinity while maintaining elastomeric properties. Owing to diverse polymeric forms and self-alignment molecular behaviors, LCEs have fascinated state-of-the-art efforts in various disciplines other than the traditional low-molar-mass display market. By patterning order to structures, LCEs demonstrate reversible high-speed and large-scale actuations in response to external stimuli, allowing for close integration with 4D printing and architectures of digital devices, which is scarcely observed in homogeneous soft polymer networks. In this review, we collect recent advances in 4D printing of LCEs, with emphases on synthesis and processing methods that enable microscopic changes in the molecular orientation and hence macroscopic changes in the properties of end-use objects. Promising potentials of printed complexes include fields of soft robotics, optics, and biomedical devices. Within this scope, we elucidate the relationships among external stimuli, tailorable morphologies in mesophases of liquid crystals, and programmable topological configurations of printed parts. Lastly, perspectives and potential challenges facing 4D printing of LCEs are discussed.

Single-, Dual-, Triple, and Quadruple-Wavelength Surface-Emitting Lasing in Blue-Phase Liquid Crystal

Soft organic lasers with multiwavelength output and high spectral purity are of crucial importance for versatile photonic devices, owing to their monochromaticity, coherence, and high intensity. However, there remain challenges for the achievement of surface-emitting multiwavelength lasing in soft photonic crystals, and the relative mechanisms need to be investigated. Herein, single-, dual-, triple-, and quadruple-wavelength lasers are successfully achieved in dye-doped blue-phase liquid crystal (BPLC) film. The number and wavelength of the lasing peaks can be manipulated by tuning the center of the bandgap, the order parameter of the laser dye, the quality of the resonance cavity, and even the pump energy. For single-wavelength lasing,  a lasing peak with an ultranarrow linewidth of 0.04 nm (Q-factor of 13 454) is achieved. Multiwavelength lasing is attained based on the following aspects: i) the narrow bandgaps of the BPLCs with full width at half maximum of 14-20 nm; ii) a laser dye with high gain over a wide wavelength band, having a low-order parameter in the liquid crystal matrix; iii) appropriate relative positions between the reflection and fluorescence peaks; and iv) the highly ordered crystal lattice of BPLC film. The proposed single-to-quadruple-wavelength surface-emitting lasers can be employed as coherent light sources for next-generation optical devices.

Liquid Crystals: Versatile Self-Organized Smart Soft Materials

Smart soft materials are envisioned to be the building blocks of the next generation of advanced devices and digitally augmented technologies. In this context, liquid crystals (LCs) owing to their responsive and adaptive attributes could serve as promising smart soft materials. LCs played a critical role in revolutionizing the information display industry in the 20th century. However, in the turn of the 21st century, numerous beyond-display applications of LCs have been demonstrated, which elegantly exploit their controllable stimuli-responsive and adaptive characteristics. For these applications, new LC materials have been rationally designed and developed. In this Review, we present the recent developments in light driven chiral LCs, i.e., cholesteric and blue phases, LC based smart windows that control the entrance of heat and light from outdoor to the interior of buildings and built environments depending on the weather conditions, LC elastomers for bioinspired, biological, and actuator applications, LC based biosensors for detection of proteins, nucleic acids, and viruses, LC based porous membranes for the separation of ions, molecules, and microbes, living LCs, and LCs under macro- and nanoscopic confinement. The Review concludes with a summary and perspectives on the challenges and opportunities for LCs as smart soft materials. This Review is anticipated to stimulate eclectic ideas toward the implementation of the nature's delicate phase of matter in future generations of smart and augmented devices and beyond.

Effect of Photonic Band Gap on Photoluminescence in a Dye-Doped Blue Phase Liquid Crystal

Tunability of fluorescence intensity is an essential parameter for enhancing the versatility of devices like emissive displays and solar cells. Soft photonic crystals, with their tunable photonic band gap (PBG), are highly sought-after systems for such purposes. Here, we report modulation of photoluminescence (PL) intensity in a fluorescent dye-doped blue phase liquid crystal, a 3D soft photonic crystal. On cooling, from the isotropic fluid phase, the PL intensity gets enhanced due to the overlapping of the emission wavelength of the dye with the photonic band edge. However, the PL intensity decreases on the application of an electric field, despite both thermal and electric fields having a similar effect (red shift) on the PBG. The contrasting behavior of PL intensity, also observed in composites obtained by varying the dye and the chiral dopant (handedness), is discussed in terms of scattering pathways for the emitted photons. The time-resolved PL studies show a reduction in the lifetime of the excited species upon cooling, validating the thermal dependence of PL intensity modulation due to Purcell effect. The facile modulation of PL intensity in the dye-doped blue phase system makes it appealing from the point of view of high-performance photonic applications.

Bioinspired Color-Changing Photonic Polymer Coatings Based on Three-Dimensional Blue Phase Liquid Crystal Networks

Photonic polymer coatings that can adaptively respond to the constant changes of surrounding environments are of profound significance for diverse applications such as optical sensors, information encryption, and adaptive camouflage. Here, we report the fabrication of humidity-driven color-changing photonic polymer coatings on the basis of judiciously designed hydrogen-bonded three-dimensional (3D) blue phase liquid crystal networks. Thanks to the inherent self-assembled 3D photonic nanostructures and tough covalent bonding between the polymers and substrate surfaces, the resulting polymer coatings are found to exhibit vivid structural colors, and humidity-driven reversible color changes across the visible spectrum of light can be achieved upon breaking the hydrogen bonds and subsequent conversion into a hygroscopic polymer coating. As the proof-of-concept applications, we demonstrate the information encryption, inkjet-printable photonic patterns, bioinspired adaptive camouflage, and colorimetric humidity sensor with such promising humidity-driven color-changing photonic polymer coatings. The results disclosed herein are expected to provide new insights into the development of stimuli-responsive advanced functional materials with tailorable 3D photonic nanostructures toward technological applications ranging from sensing, display, anticounterfeiting, and biomimetic camouflage.

Unique Optical Periodicity Assembly of Discrete Dendritic Lamellae and Pyramidal Single Crystals in Poly(ε-caprolactone)

A unique zig-zag banded morphology poly(ε-caprolactone) (PCL), crystallized at high T_c = 46-47 °C and confined in thin films (<1 μm), is found to be assembled of parallel cogrowth of two totally crystal entities: edge-on dendritic lamellar protrusion from surfaces and flat-on pyramidal single crystals. The alternating PCL optical bands are assembled as flat-on single crystals aligned as straight dendrites as a valley band in series with edge-on branches as a ridge band, leading to a unique assembly mechanism of periodic optical rings where these two crystal pieces are assembled in series. Detailed assembly mechanisms are proposed to explain how the composite bands in the PCL aggregates are correlated to optical birefringence periodicity. By techniques of melt crystallization via periodic species drainage, PCL single crystals could be prepared and tailor-made to a variety of nanopatterns as templates for applications.