Exploring the Origins of Low-Temperature Thermochromism in Polydiacetylenes.

This review article delves into the intriguing phenomenon of low-temperature thermochromism, whereby materials change color in response to temperature variations, with a particular focus on its applications in temperature-sensitive fields like medical storage. By closely examining thermochromic materials, this article highlights their potential to offer innovative solutions for monitoring and preserving thermolabile products that require strict temperature control. This leads to a special emphasis on polydiacetylenes (PDAs), a class of conjugated polymers with unique low-temperature thermochromic properties, positioning them as promising candidates for reliable temperature indicators. This article then explores the underlying mechanisms for fine-tuning the thermochromic behavior of PDAs, particularly discussing recent advancements in PDA design, such as structural alterations of monomers to achieve low-temperature thermochromism. These modifications, influenced by factors like side-chain length, hydrogen-bonding interactions, and the use of copolymers, are intended to result in irreversible color transitions at specific low temperatures, which is crucial to maintaining the integrity of thermally sensitive products. Finally, this article discusses the potential applications of PDAs as thermochromic sensors in tissue biobanking, where their ability to provide visual indications of temperature fluctuations could significantly enhance the monitoring and management of biological samples.

Ionic Liquid Crystals as Chromogenic Materials.

Ionic liquid crystals (ILCs), a class of soft matter materials whose properties can be tuned by the wise pairing of the cation and anion, have recently emerged as promising candidates for different applications, combining the characteristics of ionic liquids and liquid crystals. Among those potential uses, this review aims to cover chromogenic ILCs. In this context, examples of photo-, electro- and thermochromism based on ILCs are provided. Furthermore, thermotropic and lyotropic ionic liquid crystals are also summarised, including the most common chemical and phase structures, as well as the advantages of confining these materials. This manuscript also comprises the following main experimental techniques used to characterise ILCs: Differential Scanning Calorimetry (DSC), Polarised Optical Microscopy (POM) and X-Ray Powder Diffraction (XRD). Chromogenic ILCs can be interesting smart materials for energy and health purposes.

Coordination Compound Guided a Novel Composite Film with Zn2V2O7 Nanoflake and VO2@Zn2V2O7 Nanoparticle for Enhanced Thermochromism Smart Window.

Thermochromic vanadium dioxide (VO2) can intelligently modulate the transmittance of indoor solar radiation to reduce the energy consumption of air conditioning in buildings. Nevertheless, it remains a great challenge to simultaneously improve the luminous transmittance (Tlum) and solar modulation ability (ΔTsol) of VO2. In this study, a novel approach is employed utilizing a coordination compound to finely tune the growth of a VO2 based composite film, yielding a hierarchical film comprising Zn2V2O7 nanoflakes and VO2@Zn2V2O7 core-shell nanoparticles. Remarkably, the resulting composite films showcase exceptional optical performance, achieving a Tlum of up to 73.0% and ΔTsol of 15.7%. These outcomes are attributed to the antireflection properties inherent in the nanoflake structure and the localized surface plasmon resonance of well-dispersed VO2 nanoparticles. In addition, the Zn2V2O7-VO2 film demonstrates remarkable environmental durability, retaining 90% of its initial ΔTsol even after undergoing aging at 100 °C under 50% relative humidity for a substantial period of 30 days - a durability equivalent to ≈20 years under ambient conditions. This work not only achieves a harmonious balance between Tlum and ΔTsol but also introduces a promising avenue for the design of distinctive composite nanostructures.

The Impact of Boron Nitride Additive on Thermal and Thermochromic Properties of Organic Thermochromic Phase Change Materials.

Thermochromic phase change materials (TPCMs) are gaining increasing interest among scientists. These multifunctional materials can store thermal energy but also, at the same time, during the phase transition, they can change colour. Thermal conductivity is also extremely important for this type of material, which is why various additives are used for this purpose. This work aimed to study the properties of thermochromic phase change materials with an inorganic modifier. Stearic acid, behenyl alcohol, and bromocresol purple were used as thermochromic system components, while boron nitride particles were used as an additive. The key tests for such systems are thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), which allow determining the thermal stability of the materials (at around 170 °C) and phase transition parameters (thermal energy storage of 300 J/g in the range of 40-75 °C). The thermochromic properties were tested, and satisfactory results were obtained. In the end, laser flash analysis (LFA) tests indicated that boron nitride improves the thermal conductivity of the organic thermochromic phase change material by almost 30%. The results showed that the tested materials have great potential as thermochromic phase change materials for thermal energy storage.

Photochromism, Thermochromism, and Electrochromism in Solid-State Host-Guest Inclusion Complexes of ß-Cyclodextrin with Dialkylcarboxyl-Substituted Viologens.

Multi-stimuli-responsive chromic materials have immense potential for utilization. Herein, two supramolecular inclusion complexes were prepared by self-assembly of ß-cyclodextrin (ß-CD) with dialkylcarboxyl-substituted viologens, N,N'-di(3-carboxy-propyl)-4,4'-bipyridinium dichloride (CPV·Cl2) and N,N'-di(6-carboxy-hexyl)-4,4'-bipyridinium dibromide (CHV·Br2). The self-assembled inclusion complexes CPV2+@ß-CD and CHV2+@ß-CD2 in the solid-state exhibited naked-eye photochromism, thermochromism, and electrochromism in response to multiple external stimuli including light, temperature, and electric field, respectively. Solid-state UV-vis diffuse reflectance and electron spin resonance (ESR) spectroscopy revealed that the observed photochromism, thermochromism and electrochromism are attributed to the formation of viologen free radicals induced by electron transfer under external stimuli. The excellent stimuli-response chromic properties of the title inclusion complexes support their practical utility in visual display, multiple anticounterfeiting, and multilevel information encryption.

Ultra-high Performance Thermochromic Polymers via a Solid-solid Phase Transition Mechanism and Their Applications.

Thermochromic materials are substances that change color in response to temperature variations. Today, sustainability concerns are the main drivers of thermochromic research, with smart, energy-efficient windows being one of the primary applications. While vanadium oxides and leuco dyes are traditionally the main thermochromic materials, hydrogels operating based on change of solvation have risen as some of the most promising materials due to their high optical transparency and good solar modulating abilities. In this work, a distinct mechanism for thermochromism arising from the crystalline solid to amorphous solid polymer transition, with a corresponding transition from an opaque state to a transparent state is disclosed. Both ultra-high optical transparency (Tlum up to 99%) and ultra-high solar modulation (ΔTsolar up to 87%) are observed. The transition temperature is tunable from 11 to 61 °C by tuning the polymer structure. When incorporated into applications such as greenhouse materials and thermoelectric devices, significant performance enhancement is observed, due to the thermochromic material functioning as a thermal valve, speeding up solar heat absorbance while inhibiting the cooling process via its phase transition.

Thermochromic Polymer Nanocomposites for the Heat Detection System: Recent Progress on Properties, Applications, and Challenges.

Reversible thermochromic polymers have emerged as compelling candidates in recent years, captivating attention for their application in heat detection systems. This comprehensive review navigates through the multifaceted landscape, intricately exploring both the virtues and hurdles inherent in their integration within these systems. Their innate capacity to change colour in response to temperature fluctuations renders reversible thermochromic nanocomposites promising assets for heat detection technologies. However, despite their inherent potential, certain barriers hinder their widespread adoption. Factors such as a restricted colour spectrum, reliance on external triggers, and cost considerations have restrained their pervasive use. For instance, these polymer-based materials exhibit utility in the domain of building insulation, where their colour-changing ability serves as a beacon, flagging areas of heat loss or inadequate insulation, thus alerting building managers and homeowners to potential energy inefficiencies. Nevertheless, the limited range of discernible colours may impede precise temperature differentiation. Additionally, dependency on external stimuli, such as electricity or UV light, can complicate implementation and inflate costs. Realising the full potential of these polymer-based materials in heat detection systems necessitates addressing these challenges head-on. Continuous research endeavours aimed at augmenting colour diversity and diminishing reliance on external stimuli offer promising avenues to enhance their efficacy. Hence, this review aims to delve into the intricate nuances surrounding reversible thermochromic nanocomposites, highlighting their transformative potential in heat detection and sensing. By exploring their mechanisms, properties, and current applications, this manuscript endeavours to shed light on their significance, providing insights crucial for further research and potential applications.

Nanocellulose-based cobalt(II) coordinated malonic acid hybrid aerogels exhibiting reversible thermochromism and moisture sensor properties.

The emergence of sustainable polymers and technologies has led to the development of innovative materials with minimal carbon emissions which find extensive applications in wearable electronics, biomedical sensors, and Internet of Things (IoT)-based monitoring systems. Nanocellulose which can be generated from abundant biomass materials has been widely recognized as a sustainable alternative for a diverse range of applications due to its remarkable properties and eco-friendly nature. By making use of the unique and easily accessible coordination transformation property of Co(II) ions and associated visible light absorption changes, we report a novel Co(II) cation-incorporated nanocellulose/malonic acid hybrid aerogel material that exhibits reversible thermochromism induced by thermal stimulus in the presence of atmospheric moisture. This effect is accentuated by the highly porous nature of the nanocellulose aerogel material we have developed. Besides the reversible thermochromic property which Co(II) ions exhibit, the metal ions act as very efficient reinforcing units contributing significantly to the structural stability and rigidity of the hierarchical aerogels by coordinative cross-linking through carboxylate moieties present in the TEMPO-oxidized cellulose nanofibers (TCNF) and additionally adding malonic acid to provide sufficient COO- for cross-linking. Thorough characterization and detailed investigation of as-prepared hybrid aerogels was conducted to evaluate their overall properties including reversible thermochromism and moisture sensor behaviour. Further, an Android mobile-based application was developed to demonstrate the real-world application of the aerogels for atmospheric humidity sensing.

Tunable and Switchable Thermochromism in Cholesteric Liquid Crystalline Elastomers.

Thermochromic materials have found widespread commercial use in packaging as temperature indicators. Often, these products utilize leuco dyes that are mixed into conventional polymeric resins to prepare coatings or films that exhibit temperature-dependent color change. Here, we consider a distinctive approach to thermochromism via the selective reflection of liquid crystalline elastomers that retain the helicoidal structure of the cholesteric phase (CLCEs). Upon heating, the order of the CLCEs reduces and approaches zero, resulting in a change in birefringence as well as material thickness, both of which manifest as changes in the selective reflection to heating. This examination systematically prepares CLCEs capable of reversible thermochromic response as a function of cross-link density and liquid crystalline composition. A particular focus of this examination is the preparation of CLCEs composed of chiral and achiral liquid crystalline monomers that reduce the strength of the mesogen-mesogen interaction and accordingly reduce the nematic-isotropic transition temperature. The low birefringence of some of the CLCE compositions facilitates thermochromic reflection tuning, followed by switching.

Polymer Network Film with Double Reflection Bands Prepared Using a Thermochromic Cholesteric Liquid Crystal Mixture.

Cholesteric liquid crystal polymer network (CLCN) films with a single reflection band have found applications for decoration and anticounterfeiting. The CLCN films with double reflection bands were more suitable for these applications. Herein, they were prepared by using thermochromic cholesteric liquid crystals (CLCs) through a two-step photopolymerization approach. At the first step, due to oxygen inhibition, the CLC monomers near the substrate surface were polymerized at a certain temperature. At the second step, those near the air were polymerized at another temperature. The wavelengths of these two reflection bands of the CLCN film were dominated by the two polymerization temperatures. Based on this approach, patterns with composite colors were prepared, which were suitably applied for decoration. Moreover, a double-layered CLCN film with a broad reflection band was prepared that could potentially be applied for displays.

Thermally Induced Lattice-Defective Oxygen Breathing in Perovskite-Structure Stannates with High-Contrast Reversible Thermochromism.

Inorganic thermochromic materials exhibit a tunable color gamut and a wide chromatic temperature range, indicating their potential for intelligent adaptive applications in thermal warning, temperature indication, thermal regulation, and interactive light-to-thermal energy conversion. However, most metal-oxide-based thermochromic materials show weak chromaticity adaption with the change of temperature, which needs further understanding of the microscopic principle to clarify the potential route to improve the contrast and identifiability for fabricating better thermochromic materials. Using perovskite-structure (AMO3) alkaline earth metal stannate (Ba1-xSrxSnO3, 0.0 ≤ x ≤ 1.0) as a model system, this paper reports for the first time the mechanism of the properties of thermally induced defect-enhanced charge transfer-type (CTT) thermochromic materials and the strategy for regulating their thermochromic properties by A-site cations. BaSnO3 exhibits continuously reversible thermochromic properties with high contrast from weak light yellow (b* = 11) to strong bright yellow (b* = 58) between room temperature and 550 °C. In-situ high-temperature X-ray diffraction (in-situ XRD), in-situ UV-vis absorption spectroscopy (in-situ UV-vis), thermogravimetric (TG), and electron paramagnetic resonance (EPR) spectra indicate that this excellent thermochromic phenomenon is attributed to the weakening of Sn-O bond hybridization at high temperatures, as well as the formation of a large number of oxygen vacancies at the top of the valence band, and the enhanced charge transfer resulting from the generation of impurity levels in the Sn2+ 5s2 intermediate. Replacing Ba2+ by Sr2+ in Ba1-xSrxSnO3 successfully tuned the thermochromic properties, which is attributed to the Sr2+ doping level-directed oxygen defect concentration and deoxygenation rate. The demonstrated defect-enhanced charge transfer behavior promotes a feasible route for lattice oxygen-mediated thermochromic materials and provides a fundamental relationship between thermally induced defects and colorimetry.

Programmable Dynamic Information Storage Composite Film with Highly Sensitive Thermochromism and Gradually Adjustable Fluorescence.

The development of an integrated material system capable of effectively organizing and combining multisource information, such as dynamic pigmentary, structural, and fluorescent colors, is significant and challenging. Achieving such programmable dynamic information storage can considerably enhance the diversity and security of information deliveries. Here, a polymer-stabilized cholesteric liquid crystal system with highly temperature-sensitive structural color and light-sensitive pigmentary and fluorescence colors is presented. The prepared cholesteric liquid crystals (clcs) can reversibly change their structural color from red to blue within variational 3 °C near room temperature, and exhibit a gradually adjustable fluorescence which can transform from blue to pink and finally to bright red. All this dynamic information is programmable and tailored, hundreds of thousands of (>540 000) pattern combinations can easily be achieved by optical writing with a "bagua" pattern photomask. Therefore, if the corresponding code combinations to the pattern are assigned particular meanings, encrypted transmission of information with very high security can be achieved by utilizing applicable information encoding tables and decryption rules.

Ferroelectricity and Thermochromism in a 2D Dion-Jacobson Organic-Inorganic Hybrid Perovskite.

2D organic-inorganic hybrid perovskites (OIHPs) have become one of the hottest research topics due to their excellent environmental stability and unique optoelectronic properties. Recently, the ferroelectricity and thermochromism of 2D OIHPs have attracted increasing interests. Integrating ferroelectricity and thermochromism into perovskites can significantly promote the development of multichannel intelligent devices. Here, a novel 2D Dion-Jacobson OIHP of the formula (3AMP)PbI4 (where 3AMP is 3-(aminomethyl)pyridinium) is reported, which has a remarkable spontaneous polarization value (Ps) of 15.6 µC cm-2 and interesting thermochromism. As far it is known, such a large Ps value is the highest for 2D OIHPs recorded so far. These findings will inspire further exploration and application of multifunctional perovskites.

Transition Temperature of Color Change in Thermochromic Systems and Its Description Using Sigmoidal Models.

BACKGROUND: Symmetric sigmoidal models with four parameters based on an idealized S/Z-shaped curve are commonly used to analyze the optical parameters of thermochromic materials. However, our experimental findings show that this approach leads to systematic errors involving the incorrect estimation of the transition temperature or the possibility of a virtual indication of the hysteresis nature of a reversible thermochromic change. For this reason, we sought to find a five-parameter model that would appropriately avoid this problem. METHODS: Two commercial thermochromic pigments were used for the test and applied to a textile substrate at different concentrations. The optical properties were measured using reflectance spectrophotometry and then converted to Kubelka-Munk function values and colorimetric coordinates. The following statistics were used to assess the quality of the selected sigmoidal models: coefficient of determination, R2; adjusted coefficient of determination, AR2; root mean square error, RMSE; and Akaike Information Criterion, AIC. RESULTS: The four-parameter models were compared with each other and with the five-parameter models using nested F-tests based on residual variance to obtain a statistical measure of superior performance. For all thermochromic color change data examined, the five-parameter models resulted in significantly better fitting. It could be shown that the five-parameter model showed significantly higher accuracy and precision in determining the transition temperature, like non-sigmoidal quantification methods. CONCLUSIONS: We concluded that the asymmetric five-parameter model is a valuable extension of the symmetric model in the investigation of thermochromic color changes, providing better parameter estimates and a new approach to investigating the mechanisms contributing to the asymmetry of the thermochromic curve.

Pt(II) Complexes with Tetradentate C

A series of four regioisomeric Pt(II) complexes (PtLa-n and PtLb-n) bearing tetradentate luminophores as dianionic ligands were synthesized. Hence, both classes of cyclometallating chelators were decorated with three n-hexyl (n = 6) or n-dodecyl (n = 12) chains. The new compounds were unambiguously characterized by means of multiple NMR spectroscopies and mass spectrometry. Steady-state and time-resolved photoluminescence spectroscopy as well quantum chemical calculations show that the effect of the regioisomerism on the emission colour and on the deactivation rate constants can be correlated with the participation of the Pt atom on the excited state. The thermal properties of the complexes were studied by DSC, POM and temperature-dependent steady-state photoluminescence spectroscopy. Three of the four complexes (PtLa-12, PtLb-6 and PtLb-12) present an intriguing thermochromism resulting from the responsive metal-metal interactions involving adjacent monomeric units. Each material has different transition temperatures and memory capabilities, which can be tuned at the intermolecular level. Hence, dipole-dipole interactions between the luminophores and disruption of the crystalline packing by the alkyl groups are responsible for the final properties of the resulting materials.

Thermochromic Conductive Fibers with Modifiable Solar Absorption for Personal Thermal Management and Temperature Visualization.

Thermal management textiles provide an energy-efficient strategy for personal thermal comfort by regulating heat flow between the human body and the environment. However, textiles with a single heating or cooling mode cannot realize temperature regulation under dynamic weather. Furthermore, monocolor textiles do not satisfy aesthetic requirements in a garment. Here, we develop a thermochromic (TC) conductive fiber with a coaxial structure composed of a conductive core and thermochromic shell. The TC conductive fiber-woven fabric has the ability of low-energy dynamic thermal management by combining Joule heating and modulation of solar absorption. Compared with commercial white fabrics, TC conductive fabrics exhibit a maximum temperature drop of 2.5 K, while the temperature of colored commercial fabrics is 7.5-16 K higher than that of commercial white fabrics in the hot. In the cold, the combination of Joule heating and the photothermal effect can provide desired thermal comfort for humans. Meanwhile, heat obtained from solar absorption brings the temperature of a fabric to a predetermined level, which saves energy of 625 W/m2 compared to a conductive-fiber-based textile. In addition, TC conductive fabrics with trichromatic evolution provide a sensitive and instant temperature visualization capable of identification of invisible and intense infrared radiation. These results provide another path to expand potential applications of wearable, flexible electronics.

Bioinspired Thermochromic Textile Based on Robust Cellulose Aerogel Fiber for Self-Adaptive Thermal Management and Dynamic Labels.

Aerogel fiber has emerged recently for incorporation in personal thermal management textiles due to its flexibility, scalability, and ultrahigh porosity, which allows the body to keep warm via thermal isolation without energy consumption. However, the functionalization and intellectualization of cellulose-based aerogel fibers have not yet been fully developed. Herein, we propose a biomimicking design inspired by polar bear and Siamese cat hair that combines porous cellulose aerogel fiber (CAF) with reversible thermochromic microcapsules to mimic biological sensory and adaptive thermoregulation functions. The produced CAF has a controllable pore structure, a large specific surface area (230 m2/g), and excellent mechanical strength (∼15 MPa). Low-temperature darkening microcapsules have been incorporated into the robust CAF to spontaneously adjust color by perceiving the ambient temperature. The functional aerogel fiber fabric achieves high thermal insulation and photothermal modulation simultaneously at temperatures below 18 °C. The temperature of the thermochromic fabric was higher by 6 °C than that of the sample without the microcapsules at a light intensity of 0.2 W/cm2. In addition, the aerogel fibers mixed with two types of thermochromic microcapsules exhibit three color switches with fast response, a color-control precision of 0.2 °C, and good cycling performance. This smart aerogel fibers hold great promise for self-adaptive thermal management, temperature indication, information transfer, and anticounterfeiting in textiles.

The Ambiguous Origin of Thermochromism in Molecular Crystals of Dichalcogenides: Chalcogen Bonds versus Dynamic Se-Se/Te-Te Bonds.

We report thermochromism in crystals of diphenyl diselenide (dpdSe) and diphenyl ditelluride (dpdTe), which is at variance with the commonly known mechanisms of thermochromism in molecular crystals. Variable temperature neutron diffraction studies indicated no conformational change, tautomerization or phase transition between 100 K and 295 K. High-pressure crystallography studies indicated no associated piezochromism in dpdSe and dpdTe crystals. The evolution of the crystal structures and their electronic band structure with pressure and temperature reveal the contributions of intramolecular and intermolecular factors towards the origin of thermochromism-especially the intermolecular Se⋅⋅⋅Se and Te⋅⋅⋅Te chalcogen bonds and torsional modes of vibrations around the dynamic Se-Se and Te-Te bonds. Further, a co-crystal of dpdSe with iodine (dpdSe-I2 ) and an alloy crystal of dpdSe and dpdTe implied a predominantly intramolecular origin of the observed thermochromism associated with vibronic coupling.

Freezing-resistant poly(N-isopropylacrylamide)-based hydrogel for thermochromic smart window with solar and thermal radiation regulation.

Adaptive regulation of solar and thermal radiation by windows in diverse (hot and cold) climates is essential to reduce building energy consumption. However, conventional hydrogel-based thermochromic smart windows lack thermal radiation regulation, and have difficulty to combine high solar regulation with excellent freezing resistance. It is challenging to integrate the above performance into one hydrogel-based thermochromic window. Here, we firstly prepared poly(N-isopropylacrylamide-co-N, N-dimethylacrylamide)/ethylene glycol (PNDE) hydrogels with tunable and excellent freezing resistance (below -100 °C) by adding the anti-freezing agent ethylene glycol, and assembled PNDE hydrogels, polyvinylidene fluoride and polymethyl methacrylate-silver nanowires panels into a freezing-resistant smart window with solar and thermal radiation regulation (STR). PNDE hydrogels had an excellent thermochromic performance with luminous transmittance (Tlum) of 89.3 %, solar regulation performance (ΔTsol) of 80.7 % and tunable phase change temperature (τc, 22-44 °C). The assembled STR window showed high Tlum of 68.2 %, high ΔTsol of 62.6 %, suitable τc of ∼30 °C and freezing resistance to low temperature of -27 °C. Moreover, the different thermal emissivity (0.94 and 0.68) of the two sides of the STR window gave it the ability of radiative cooling in hot climates and warm-keeping in cold climates. Compared to the conventional thermochromic windows, the STR window promotes heat dissipation in hot conditions while reduces heat loss in cold conditions and is applicable to diverse climates, which is a promising energy-saving device for reducing building energy consumption.

Multiresponsive Cylindrically Symmetric Cholesteric Liquid Crystal Elastomer Fibers Templated by Tubular Confinement.

Cylindrically symmetric cholesteric liquid crystal elastomer (CLCE) fibers templated by tubular confinement are reported, displaying mechanochromic, thermochromic, and thermomechanical responses. The synthesis inside a sacrificial tube secures radial orientation of the cholesteric helix, and the ground state retroreflection wavelength is easily tuned throughout the visible spectrum or into the near-infrared by varying the concentration of a chiral dopant. The fibers display continuous, repeatable, and quantitatively predictable mechanochromic response, reaching a blue shift of more than -220 nm for 180% elongation. The cylindrical symmetry renders the response identical in all directions perpendicular to the fiber axis, making them exceptionally useful for monitoring complex strains, as demonstrated in revealing local strain during tying of different knots. The CLCE reflection color can be revealed with high contrast against any background by taking advantage of the circularly polarized reflection. Upon heating, the fibers respond-fully reversibly-with red shift and radial expansion/axial contraction. However, there is no transition to an isotropic state, confirming a largely forgotten theoretical prediction by de Gennes. These fibers and the easy way of making them may open new windows for large-scale application in advanced wearable technology and beyond.