High‐Contrast Colorless‐to‐Colored Thermochromic Materials

Abstract

Herein, high‐contrast colorless‐to‐colored thermochromic materials are accomplished through novel 3‐component mixtures based on catechol/pyrogallol derivatives (color developer – CD), fatty acid phase change materials (PCM), and leuco dyes. Such components ensure: I) always highly colored hot liquid states, promoted by strong stabilization of the dye open form through hydrogen‐bonding interactions with the CDs’ vicinal ‐hydroxyl groups, as also demonstrated by density functional theory calculations; II) solid mixtures with fully controllable coloration state, which directly relies on the relative length of the alkyl chain of CD (C_CD) and PCM (C_PCM). As also rationalized by thermal and spectoscopic measurements, colorless solids are obtained when C_CD is larger than C_PCM (ΔC = C_PCM–C_CD<0), due to self‐assembly and phase segregation of CD molecules from PCMs, preventing their interaction with the dye. Instead, when ΔC≥0, the CD solubilizes in PCMs, developing the color of the co‐dissolved dye. The rule is extendable to several leuco dye families (lactams, lactons, fluorans) and acid PCMs, fostering a tunable hot state color palette and transition temperature. The target transition is also preserved in structured mixtures (solid lipid particles), granting for the preparation of polymer composites and screen‐printed thermochromic patterns, of interest for sensors and invisible security inks.

We developed new three‐component thermochromic systems showing reversible colorless‐to‐colored transition. Our general empirical rule applies to diverse fatty acid phase change materials and leuco dyes, enabling easily tunable transition temperature and activated colors (blue, cyan, green, pink, grey). Solid particles of these materials were integrated into polymer films or printed on papers, for applications like multistep thermal sensors and invisible patterns.

1. Introduction

The temperature‐induced color change of thermochromic materials is a property of strong interest for many different applications^{,[ 1} , ² , ^{3 ]} such as colorimetric temperature sensors,^{[ 4 ]} rewritable thermal printing,^{[ 5} , ^{6 ]} security inks,^{[ 7 ]} smart windows,^{[ 8} , ⁹ , ¹⁰ , ¹¹ , ¹² , ^{13 ]} logic gates,^{[ 14 ]} and energy storage materials and indicators.^{[ 15} , ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ , ^{20 ]} Among the several types of thermochromic systems reported in the literature, those based on phase change materials (PCMs) are already integrated in commercial products for their economic price, readily available constituting components, and easy integrability in polymer coatings, papers, or textile substrates.^{[ 3} , ²¹ , ²² , ^{23 ]} These are normally based on 3‐component mixtures of a leuco dye, a color developer, and a phase change material (PCM). Leuco dyes are compounds (e.g., fluorans, lactons, spiropyrans) able to undergo an equilibrium between their closed (colorless) and open (colored) forms depending on the pH, hydrogen‐bonding character, and polarity of the medium. The color developer is a compound with weak acidic or hydrogen‐bonding promoting properties (e.g., Bisphenol A) and develops the color when interacting with the dye. Finally, the PCM is the solvent that triggers dye‐color developer interaction depending on its solid or liquid state. Normally, in the solid state of the mixture, the low solubility of the dye (e.g., a lactone) and of an acidic color developer in the PCM promotes their phase separation and co‐precipitation through hydrogen‐bonding interactions and/or acid/base reaction, yielding the opening of the dye molecules and the color formation. In the liquid state, both dye and color developer are homogeneously solubilized in the PCM, which, due to the large excess, successfully competes against the dye‐color developer complex formation, shifting the equilibrium towards the closed colorless form.^{[ 21} , ²³ , ²⁴ , ²⁵ , ^{26 ]} As a consequence, these 3‐component systems normally manifest color in the solid (cold) state while becoming colorless in the liquid (hot) state (Scheme 1 ).

Scheme 1.

Scheme showing the molecular interaction mechanism for (a) the conventional colored‐to‐colorless thermochromic mixtures and (b) our colorless‐to‐colored thermochromic materials. In the solid state of the PCM, the catechol/pyrogallol‐based color developer is expected to segregate from the PCM through van der Waals and hydrogen‐bonding interactions and poor solubility in fatty acids, avoiding the interaction with the dye, which remains dissolved in the PCM in the closed colorless form. In the liquid state, both dye and color developer are homogeneously dissolved, and the competitive hydrogen‐bonding interaction between the color developer and the dye produces (with the help of the acid PCM) the opening of the ring and the formation of the colored form.

Temperature‐induced coloration, though, is far more desired than the opposite effect for many relevant applications. For instance, in smart windows, it is aimed to have a transparent state during cold days to allow the incident visible and/or near infrared light radiation crossing the window and illuminate and heat the interior of the rooms, decreasing the need for energy for artificial lighting and heating systems. On the contrary, during warmer days (e.g., above 35–40 °C), it is desired that the glass turns to a tinted or opaque state to filter or reflect, and thus block, the incident light, avoiding further heating due to solar heat gain and decreasing the need for air conditioning‐related‐energy^{.[ 8} , ⁹ , ¹⁰ , ¹¹ , ¹² , ²⁷ , ^{28 ]} Therefore, thermochromic smart windows that tint upon temperature increase are self‐responding dynamic filters that self‐regulate the light transmittance according to the external temperature. In this respect, it is also a challenge to develop thermochromic materials with easily tunable transition temperature so that they can be adapted to distinct geographical regions of different average temperatures. Colorless‐to‐colored transition is also preferred in rewritable thermal printing,^{[ 6 ]} anticounterfeiting labels, and colorimetric sensors. In these applications, such a transition is preferred because a color formation (even a little one) is visually easier to detect or see by the naked eye than a slight color reduction or change.^{[ 4 ]} Moreover, in security inks, thermochromic authentication marks exhibiting colorless‐to‐colored are highly desired as they remain invisible to most users (as it occurs with fluorescent invisible labels), while they become visible only upon controlled activation by specialized personnel. Finally, having a cold colorless state offers a fundamental advantage in terms of photostability: for those applications where the cold state is at room temperature, colored‐to‐colorless thermochromic materials would exhibit coloration most of the time. This implies more exposure of the predominant colored form of the thermochromic materials to continuous visible radiation of ambient light while not activated, which inevitably promotes photodegradation. On the contrary, colorless‐to‐colored thermochromic materials are not affected by ambient visible light when they are in the cold state, as they only absorb in the UV spectral region, which is not present indoors.

Despite so many advantages, very few works have been reported on PCM/leuco dye‐based colorless‐to‐colored thermochromic materials, mainly due to the difficulties in accomplishing such a transition in devices using readily available components. A promising approach involved bi‐component systems incorporating a dye and an acidic PCMs that acts both as solvent and color developer (e.g., dodecanoic acid);^{[ 29} , ³⁰ , ³¹ , ³² , ^{33 ]} and the colored form of the pH‐sensitive dye is only stabilized in the molten state, when the acid manifests its acidic character. More recently, we also reported a fully novel approach based on the negative thermochromic transition of PCM/acidic color developer bulk and microstructured mixtures of highly conjugated ketocyanine dyes that pass from the colorless (NIR absorbing) enol form to the colored keto form upon melting.^{[ 34 ]} Though exhibiting the desired colorless‐to‐colored transition, these approaches suffer one or more drawbacks that prevent their spreading in commercial products: they require PCM of high transition temperature, specific dye families (e.g., spiropyrans or cyanine dyes), not always commercially available (ketocyanine dyes), or are not fully reversible. Alternatively, White et al. developed 3‐component systems based on a crystal violet lactone dye and long alkyl chain alcohols (e.g., 1‐octadecanol), and aromatic esters (e.g., lauryl gallate) as PCM and color developer, respectively. They also attempted to establish a general rule based on the carbon number difference between the alkyl chain length of the PCM (C_PCM) and color developer (C_CD), according to which the colorless‐to‐colored transition upon heating is observed for matching chain length mixtures (i.e., ΔC = C_PCM – C_CD = 2 or less), because of the optimal interaction between color developer and PCM (which prevents the dye‐color developer interaction), while no thermochromism or the opposite transition is observed under mismatching chain length conditions (ΔC >6, which is verified for the majority of the mixtures).^{[ 35} , ³⁶ , ³⁷ , ³⁸ , ^{39 ]} However, besides being very specific for crystal violet lacton dye, exceptions for this rule were found (e.g., mixtures with ΔC = 0 with no thermochromism),^{[ 39 ]} while mixtures with ΔC = ± 4 (intermediate situation between matching and mismatching) gave unpredictable behaviors or were highly dependent on the components ratio, which had to be adapted case by case.^{[ 35} , ^{39 ]} Moreover, such thermochromic study was only carried out for bulk thermochromic mixtures, which are not practical for commercial applications due to the need of containers to avoid leakage of the liquid form of the PCMs; while no data are reported for their structured and confined form (e.g., as capsules or particles), of interest for inks formulation or composites preparation. All in all, and despite these pioneering examples, it is urgent to develop a more predictable, universal, and straightforward approach to accomplish materials for devices fabrication (e.g., films, coatings) exhibiting colorless‐to‐colored thermochromic transitions.

Herein we achieved this goal with the use of new 3‐component mixtures made of: I) leuco dyes of different families (lactams, spirolactons, fluorans); II) non‐toxic long chain fatty acid PCMs co‐stabilizing the dye open form in the molten state thanks to their higher acidity (than typical long alkyl chain alcohols), while their polarity avoids the dye co‐precipitation and phase segregation with the color developer in the solid state (and so the formation of color), and last but not least, III) long alkyl chain‐functionalized catechol or pyrogallol derivatives as color developers. In the molten state, the vicinal hydroxyl groups of the color developer are expected to strongly foster the open colored dye form through hydrogen‐bonding interactions (successfully competing with the PCM excess). On the contrary, in the solid state, the long alkyl chain and the hydroxyl (‐OH) groups are expected to promote their self‐assembly and phase segregation due to van der Waals and hydrogen‐bonding interactions, and poor solvation (of catechol derivatives) in fatty acids,^{[ 40} , ^{41 ]} avoiding the interaction with the dye remained dissolved in the PCM (Scheme 1). Besides, both catechol‐ and pyrogallol‐based compounds are bio‐inspired non‐toxic molecules that make the final thermochromic materials safer than those obtained from Bisphenol A, mostly used in current thermochromic systems. A fine composition tuning of these mixtures, along with a detailed study of the crystalline phase of the mixtures and Density Functional Theory (DFT) calculations, allowed us to obtain a general and robust rule to achieve bulk mixtures and particle‐loaded composites exhibiting the highly desired thermally‐induced colorless‐to‐colored transition, not only with the typical crystal violet lacton dye, but also with other spirolactons, lactams, and fluorans, giving access to easy transition temperature tuning and a broad color palette.

2. Results and Discussion

2.1. Design and Preparation of a Colorless‐to‐Colored Thermochromic Mixture

Our first dye of choice was Rhodamine B (RhB) as I) it is a very photostable dye, colorless in its lactone form and strongly (red/pink) colored and fluorescent in the open form,^{[ 42} , ⁴³ , ^{44 ]} and II) it has been reported to form a complex with ethyl gallate, stabilizing the open colored form.^{[ 45 ]} However, the lactone form is only stable in very non‐polar solvents (e.g., hexane, cyclohexane) where though its solubility is very limited. On the other hand, more polar PCMs (e.g., decanoic acid, DA) favor its dissolution, but also promote the open form even in the solid state (Figure S1, Supporting Information), making the mixture unsuitable for the target thermochromic transition.^{[ 46 ]} To circumvent this limitation, we designed a modified RhB (from now on RhB‐D), which we obtained by i) substituting the lactone ring with the lactam functionality, which requires more polar or acidic environments to open, and ii) using dodedecylamine to form the lactam to enhance the solubility in the PCM. The one‐step synthesis, adapted from previously reported works,^{[ 47} , ^{48 ]} gave RhB‐D with a 78% yield (for synthetic details see the Experimental Section and Scheme E1 (Supporting Information) and for characterization data Figures E1 and E2).

Afterward, a mixture of RhB‐D, DA as PCM (a 10‐carbon carboxylic acid with T_m ^DA = 31.4 °C),^{[ 49 ]} and Lauryl gallate (LG), a 12‐carbon ester of gallic acid as color developer (9 wt.%), was prepared dissolving the dye and the color developer in the molten PCM and letting the mixture cool down at room temperature. At high temperatures (T >32 °C), the liquid RhB‐D/LG@DA mixture showed a strong pink color and the typical absorption band at λ_abs = 560 nm, characteristic of the Rhodamine B and lactam derivative open forms,^{[ 42} , ^{47 ]} while cooling at room temperature (25 °C) the solid mixture turned colorless (with no absorption in the visible region, Figure 1a). Worth mentioning, the temperature‐induced color change was reversible over different cycles.

Figure 1.

Images, F(R) and absorption spectra of (a) RhB‐D/LG@DA at 25 °C and 50 °C and (b) RhB‐D/BA@DA, at 25 and 75 °C, c) F(R) and absorption spectra of the RhB‐D/LG@DA at 25 and 50 °C, at different concentrations of LG.

To demonstrate the requirement of these components to achieve the target transition, we performed a series of control experiments. First, the dye was directly dissolved in the color developer, without the PCM. The resulting RhB‐D@LG mixture turned pink in both the solid (25 °C) and liquid (90 °C) state, indicating that the vicinal ‐OH groups of the color developer were efficiently stabilizing the open form of the dissolved dye (Figure S2, Supporting Information). Indirectly, this result also confirms that in the RhB‐D/LG@DA solid state, the dye is not in contact with the color developer or is surrounded by too little amount of LG molecules to promote the opening. In a further experiment, removal of LG color developer from the mixture (RhB‐D@DA) resulted in no color formation or change (both phases were colorless), which suggests that the acidity of the liquid DA alone was not enough to stabilize the dye open form (Figure S3, Supporting Information), but it still plays an important co‐stabilization role in the dye coloration. Indeed, when DA was replaced by less acidic PCMs (i.e., tetradecanol, TD; hexadecanol, HD) conventionally used in the three‐component thermochromic materials (RhB‐D/LG@TD, RhB‐D/LG@HD), no color (and absorption in the visible region) was found in both phases (Figure S4, Supporting Information), confirming the need for synergism between the color developer and a proper acidity of the PCM. Worth mentioning is that in case of using TD, although the PCM and color developer do respect the chain length matching/mismatching experimental rule of White et al. for the colorless‐to‐colored transition to occur (ΔC = 2, i.e., matching),^{[ 36 ]} the RhB‐D‐loaded mixture does not manifest any thermochromic transition, confirming the specificity of the rule to crystal violet lacton. Also, attempts to use non‐acidic and non‐polar paraffins (i.e., Eicosane) as PCM failed due to the poor solubility of LG, which did not allow performing thermochromic studies. Finally, substitution of LG by Bisphenol A (BA, 9 wt.%) (RhB‐D/BA@DA) resulted in the opposite transition, from a pink‐colored solid mixture (λ_abs = 560 nm) to a faded liquid solution (with no absorption in the visible region) upon heating above the mixture melting point (Figure 1b), in agreement with previously reported works with similar RhB lactam derivatives,^{[ 50 ]} which shows how this new dye could be used to accomplish both types of transitions.

Aiming to maximize the color contrast along the thermochromic transition, the color developer concentration was systematically varied from ≈1 to 12 wt.%, while keeping constant the PCM and dye ratio. Overall, an LG concentration increase can be correlated with a stabilization of the open form and consequently with a stronger color in the liquid state (Figure 1c). However, an increase above 11.8 wt.% also starts to induce a light coloration in the solid mixture, most likely due to the interaction of the dye with the color developer, forced by the concentration increase. For this reason, an optimal color developer concentration was fixed at 9 wt.% of LG, which allows for the highest color contrast (ΔOD = 0.8), accomplished thanks to the maximum coloration in the molten state (optical density, OD_dark = 0.9, see Section S3, Supporting Information for the procedure of the OD determination), and negligible color for the solid mixture (OD_clear = 0.1). Worth noticing, the mixture kept the same thermochromic transition behaviour even after 1.5 years of storage at room temperature and ambient light (Figure S5, Supporting Information).

2.2. Alkyl Chain and ‐OH Group Effects in the Thermochromic Transition

To investigate the effect of the number and vicinity of ‐OH groups and of the alkyl chain in the color developer, produced on the stabilization of the open or closed forms of the RhB‐D dye, two new compounds with one (Ph‐E) and two vicinal ‐OHs (Cat‐E), bearing a 12‐carbon alkyl chain attached to the benzene ring through the ester functionality, were selected as color developer. Moreover, hexanediol (HD) was chosen as a non‐aromatic, flexible color developer molecule with an alkyl chain separating two non‐vicinal ‐OH groups. For comparison purposes, alkyl chain‐free color developers such as phenol (Ph, 1 OH), catechol (Cat, 2 OH), and pyrogallol (Pyr, 3 OH) were also studied. While most selected color developers were commercially available, Cat‐E was synthesized by us (66% yield) upon esterification of the corresponding acid (see Experimental Section and Scheme E2 for the synthesis and Figures E3, E4, for the characterization spectra, Supporting Information). The composite mixtures were prepared as previously described, maintaining the same concentration (9 wt.%) of the color developer. The results are summarized in Table 1 and Figures S6 and S7 (Supporting Information). For the two new mixtures obtained from Ph‐E (RhB‐D/Ph‐E@DA) and Cat‐E (RhB‐D/Cat‐E@DA) color developers, the desired colorless‐to‐colored transition upon heating was observed, though the color of the liquid solution decreased (becoming very faint) with the less number of ‐OHs in the benzene ring (Figure S6, Supporting Information). The color developers without alkyl chains (RhB‐D/Ph@DA, RhB‐D/Cat@DA, RhB‐D/Pyr@DA) did induce coloration in the liquid state (Table 1; Figure S7a–c, Supporting Information), but also in the solid mixture, while HD (non‐vicinal OH groups) did not develop color in any of the phases (Figure S7d, Supporting Information). These results corroborated that both, at least two vicinal ‐OH groups and the presence of an alkyl chain, are needed in the color developer to ensure the desired colorless‐to‐colored transition. To explain these results, we hypothesize that the ‐OH groups favor hydrogen bonding interaction with the dye in the liquid state of the PCM, while the alkyl chain and ‐OH groups promote the self‐assembly of color developer molecules and their phase segregation from the solid PCM, preventing their interaction with the dye that remains instead sequestered in the PCM. To confirm such a hypothesis, additional theoretical calculations and detailed experimental analyses of the solid mixtures were carried out.

Table 1.

Summary table of the thermally‐induced color change of the mixtures made by the color developer with different numbers of ‐iOH groups, with and without the 12‐carbon alkyl chain. White and pink boxes indicate the colorless or colored state, respectively, of the solid and liquid mixtures. “√” and “x” indicate if the desired colorless‐to‐colored transition is achieved or not.

2.2.1. Rational for the Open Form Stabilization: DFT Calculations

The effect of the ‐OH groups number (Ph, Cat, and Pyr, with 1, 2, and 3 OH, respectively) on the interaction with the RhB‐D dye and thus on the induced coloration, was rationalized through DFT calculations employing Gaussian 16.^{[ 51 ]} To this end, we have considered a series of geometry optimizations of the RhB‐D dye with different numbers of color developer molecules placed at different positions and orientations. To simplify the calculations, we did not consider the alkyl chain, since its role is minor for the color development in the liquid state. We outline here the main results of the calculations (see full details in Section S4 and Figures D1–D4, Supporting Information). The optimized geometry of the isolated RhB‐D dye corresponds to its closed state, which has two possible locations for hydrogen‐bonding interactions with ‐OH that affect the geometry of the dye, being these the carbonyl oxygen and the lactam nitrogen (Figure 2a). Our DFT calculations show that the interaction of the color developer with both hydrogen‐bonding sites involves two ‐OHs and very short donor‐acceptor distances (Figure 2b) typical of strong hydrogen bonds.^{[ 52 ]} The involvement of two vicinal ‐OH groups in these hydrogen bonds explains the weak or no effect of Ph (having 1‐OH), BA, HD (non‐vicinal ‐OHs), color developers, or DA (PCM) in the color formation. In addition, in the case of Pyr (3‐OH), the third ‐OH group not bonded to the dye allows for the formation of a supramolecular assembly involving the dye and three Pyr molecules, two of which bonded with the dye and the third interacting with the previous two (via hydrogen bonding), further stabilizing the supramolecular association (Figure 2b). Therefore, our DFT calculations indicate that color developers having 2 or more OHs (i.e., Pyr/Cat) and dye/color developer molecular ratio equal to or higher than 1:3 are required to stabilize the open colored form of the RhB‐D dye, as clearly validated by our experimental observations.

Figure 2.

Structures obtained in DFT calculations (licorice representation, color code): cyan (C), blue (N), red (O), white (H): a) optimized closed form of the RhB‐D dye with the possible sites for hydrogen bonding interactions, indicated with arrows; b) optimized structure of the open form of the dye, stabilized by three Pyr color developer molecules, two hydrogen‐bonded with the dye and the third with the previous two (one of the molecules is shown translucid for easier visualization of the structure, the distances in the H bonds are indicated in Å). Image made with VMD.^{[ 53 ]}

2.2.2. Rational for the Color of the Solid Mixture: Effect of the Relative Alkyl Chain Length of Color Developer and PCM

Next, to understand the relevance of the side alkyl chain of the color developer in the solid‐state color (and thus on the overall thermochromic transition), we studied the optical behavior provided by different color developers with shorter and longer alkyl chain than LG, while DA was fixed as PCM: octyl (OG), decyl (DG), myristyl (MG), cetyl (CG) and stearyl (SG) gallate of 8, 10, 14, 16, and 18‐carbon esters of gallic acid. Interestingly, while RhB‐D/MG@DA, RhB‐D/CG@DA, and RhB‐D/SG@DA, showed a clear thermally induced transition (Table 2 ; Figure S8, Supporting Information), RhB‐D/OG@DA and RhB‐D/DG@DA mixtures (obtained from color developers with shorter alkyl chains) did not as they remained colored (λ_abs = 560 nm) even in the solid state (Table 2; Figure S9, Supporting Information). This result suggests a threshold alkyl chain length for the color developer, below which the colourless‐to‐colored transition does not take place.

Table 2.

Summary table of the color‐changing behavior of the RhB‐D/color developer/PCM mixtures. Colorless‐to‐colored transition is observed for the mixtures with a chain length of the color developer longer (C_CD > C_PCM) than that of the PCM. White and pink boxes indicate the colorless or colored state, respectively, of the solid (at room temperature, RT) and liquid mixtures (above their melting point, T_m). “√” and “x” indicate if the desired colorless‐to‐colored transition is achieved or not.

Further experiments were done with two additional PCMs (to DA), bearing different alkyl chains, dodecanoic (DDA, T_m = 43.8 °C, 12‐carbon acid)^{[ 49 ]} and hexadecenoic acid (HDA, T_m = 62.5 °C, 16‐C acid)^{[ 49 ]} for each color developer. This study is also relevant for the tuning of the thermochromic transition temperature, dependent on the PCM melting point. OG‐ (8 C) and DG‐ (10 C) loaded mixtures always developed coloration in the solid and liquid state (i.e., no thermochromic transition), regardless the PCM chain length (RhB‐D/OG@DDA, RhB‐D/OG@HDA, RhB‐D/DG@DDA, and RhB‐D/DG@HDA) (Table 2; Figures S10 and S11, Supporting Information). LG (12 C) only promoted the colorless‐to‐colored transition in DA (10 C) mixture (RhB‐D/LG@DA), while induced coloration in the solid‐state DDA (12 C, RhB‐D/LG@DDA) and HDA (16 C, RhB‐D/LG@HDA) mixtures (Table 2; Figure S12, Supporting Information). MG‐ (14 C) and CG‐ (16 C) loaded mixtures showed colorless‐to‐colored transition in DDA (12 C) (RhB‐D/MG@DDA, RhB‐D/CG@DDA) (Figure S13, Supporting Information), whereas manifested color in both states of HDA (16 C) mixtures (RhB‐D/MG@HDA and RhB‐D/CG@HDA) (Figure S14, Supporting Information). Finally, SG induced the colorless‐to‐colored transition in the other two PCM mixtures too (RhB‐D/SG@DDA and RhB‐D/SG@HDA) (Table 2; Figure S15, Supporting Information). We rationalized this tendency as follow: when the alkyl chain length of the color developer is lower or equal than the length of the alkyl moiety of the PCM (C_CD ≤ C_PCM;, i.e., ΔC = C_PCM – C_CD ≥ 0), the mixtures manifest pink color (λ_abs = 560 nm) in both the liquid and solid state, with consequent lower or no color contrast during the thermochromic transition. On the contrary, when the alkyl chain of the color developer is longer than that of the PCM (C_CD > C_PCM, i.e., ΔC < 0), the desired colorless‐to‐colored thermochromic transition is always observed.

Worth to mention that besides allowing a straightforward transition temperature tuning, the use of different PCMs permit modulating the kinetics of the coloration at high temperatures and of the fading at room temperature: to show this, RhB‐D/SG@DA, RhB‐D/SG@DDA, and RhB‐D/SG@HDA mixtures were layered with their containers over a heating plate pre‐heated at 65 °C, i.e., just above the highest melting point for the selected PCMs. The sample containers started melting and changing their colors when the mixture's temperature reached the respective melting points. Therefore, RhB‐D/SG@DA activated faster than RhB‐D/SG@DDA, which changed its color before RhB‐D/SG@HDA (Figure S16, Supporting Information). The melting point of the PCM in the mixture also affected the fading rate: the higher the T_m of the PCM, the faster the recovery of the colorless state upon cooling, passing from 6 h for RhB‐D/SG@DA to less than 60 min for RhB‐D/SG@HDA mixture (Figure S17, Supporting Information). This was directly ascribed to the higher recrystallization point of the HDA mixture, which thus solidifies faster at room temperature, as already reported for thermofluorochromic materials.^{[ 54 ]} To confirm this, we carried out differential scanning calorimetry (DSC) experiments, where the three samples were first heated at 75 °C, then rapidly cooled down to 25 °C (50 °C min⁻¹), and finally monitored this temperature for 60 s (isothermal). This study simulates the experimental conditions for the colour and spectral measurements of the bulk mixtures. As expected, the crystallization occurred faster for the mixtures made by the PCM with a higher melting point (Figure S18a, Supporting Information). Actually, for RhB‐D/SG@HDA, the crystallization was so fast that it occurred before reaching 25 °C, and its exothermic peak appeared broad due to the fast cooling (Figure S18b, Supporting Information). RhB‐D/SG@DA mixture was slower (5 s) than RhB‐D/SG@DDA (12 s)  to crystallize. The time difference between the recovery of the colorless (solid) state of the bulk mixtures and the crystallization observed in the DSC experiments was ascribed to the different amounts of material used for each type of experiment and to a possible delay (in the fading experiment) caused by the dye and color developer to reorganize within the mixtures once they reached the solid state.

The cooling rate of the bulk RhB‐D/SG@HDA mixture was fast enough that we could quickly repeat 20 heating/cooling cycles without observing any significant change in the visual color and absorption properties of the colored and colorless states (Figures 3 ; S19, Supporting Information). The stability of these heating/cooling cycles was also confirmed by DSC experiments, which for the three mixtures showed identical curves (i.e., same melting and crystallization temperatures) upon 3 heating/cooling cycles (Figure S20, Supporting Information).

Figure 3.

F(R) at 560 nm during 20 heating/cooling cycles of the RhB‐D/SG@HDA mixture.

To get more insights on the mechanism involved in the color development of the mixtures, differential scanning calorimetry (DSC), X‐ray diffraction (XRD), and Fourier‐Transform infrared (FT‐IR) measurements were carried out for mixtures of RhB‐D, DA with three different color developers: the one owning the shortest (RhB‐D/OG@DA), longest (RhB‐D/SG@DA), and intermediate (RhB‐D/LG@DA) alkyl chain length. As general protocol, for XRD and FT‐IR measurements, the mixtures were initially melted and subsequently solidified onto a substrate to ensure the homogeneity of the samples.

DSC thermograms showed just a little variation of the melting point of the mixtures compared to the pure DA (31.2–33.3 °C), though the recrystallization point of RhB‐D/OG@DA was slightly lower (25.5 °C) than the other mixtures (27.2–28.0 °C), which might suggest the homogeneous OG dissolution in DA (acting as plasticizer), delaying its solidification (Figure 4a). More relevant are the differences in the polymorphic transitions of the mixtures, observed while cooling, with the intermediate phases being less rigid than the crystalline one, induced at the lowest temperature.^{[ 55 ]} These transitions were observed at increasing temperature as the chain length of the color developer enlarged: for RhB‐D/OG@DA were recorded at quite low temperatures (< 20 °C), close to those registered for pure DA (12–16 °C), while for RhB‐D/LG@DA, and RhB‐D/SG@DA they manifested at higher temperatures (>20 and 25 °C, respectively). From these data, we can postulate that LG‐ and SG‐loaded mixtures already reached a crystalline rigid phase at room temperature (20–25 °C), possibly promoting the phase segregation of the color developer. On the contrary, the formation of the crystalline phase of RhB‐D/OG@DA mixture is accomplished at too low values to induce the phase separation of OG at room temperature, and the less rigid intermediate polymorphs can still accommodate the color developer molecules, which then tend to remain within the PCM, interacting with the dye and promoting its coloration. To validate this, we heated two identical RhB‐D/OG@DA mixtures above the DA melting point and let them cool down at room temperature to slowly solidify in the colored form. Successively, one of the mixtures was transferred in the freezer (≈5 °C) for one week. As expected, while the mixture kept at room temperature for 1 week preserved the same coloration, the one in the freezer faded significantly (although not completely) due to the induced polymorphic transition of DA to the more rigid phase, causing the partial phase separation of the color developer (Figure S21, Supporting Information).

Figure 4.

a) DSC thermograms, b) XRD diffractogram, and (c) FT‐IR spectra of the RhB‐D/OG@DA, RhB‐D/LG@DA, and RhB‐D/SG@DA mixtures.

XRD diffractograms of RhB‐D/LG@DA and RhB‐D/SG@DA mixtures show the additive combination of the color developer (θ = 13–17°) and PCM peaks, while for RhB‐D/OG@DA the color developer peaks disappeared completely, despite being present in a higher molar ratio (i.e., same weight ratio as the other mixtures). In all cases, the amount of RhB‐D was too little to be detected. The absence of OG peaks suggests the formation of homogeneous solid solutions. Therefore, the lower chain length may reduce the OG molecules' tendency to self‐assemble and facilitate their dissolution even in the solid PCM, where they interact with the co‐dissolved dye, thus developing the color, as shown experimentally (Figures 4b; S22a, Supporting Information). On the contrary, the presence of the LG and SG color developer peaks (also evidenced by other additional peaks at 2θ = 6, 20, 22, 24°), indicates that at least some of these molecules preserve their own segregated crystalline phase within the mixtures, minimizing the interaction with the dye sequestrated in the PCM and the color formation (Figures 4b; S22b,c, Supporting Information). For comparison, we also measured the diffractogram of a mixture obtained from the physical mixing of the LG and DA powders, without melting and freezing, and with the same weight ratio as before (Figure S23, Supporting Information). As expected, the LG peaks of the physically separated powdered components could be easily detected, which confirms that in the mixtures obtained from melting, the observed XRD peaks originate from the phase segregation of the color developer occurring upon freezing. However, it cannot be discarded that some color developer molecules remain homogeneously dissolved in the PCM, though their concentration might not be sufficient to induce the opening of the dye. Actually, as described in the previous section (Figure 1c), above certain color developer concentrations, the interaction with the dye is forced, and residual coloration is also induced in the solid state.

To further correlate the phase segregation process with the absence of color in the solid‐state mixture, a fast cooling of the RhB‐D/LG@DA mixture was induced with liquid nitrogen, expecting to freeze the material in an amorphous state mimicking the liquid mixture. Immediately after cooling, the mixture preserved the pink color, and XRD showed no peaks of the color developer. Accordingly, the mixture eventually faded over time (24 h) as it equilibrated at room temperature (Figure S24, Supporting Information), and XRD peaks of the color developer increased their intensity (Figure S25, Supporting Information). Once at room temperature, the color developer molecules have enough energy to migrate and find the new thermodynamic equilibrium state as a segregated phase, not interacting with the RhB‐D.

FT‐IR spectroscopy measurements further confirmed our proposed hypothesis. The colored RhB‐D/OG@DA mixture showed strong color developer peaks (e.g., the ‐OH stretching bands at 3450 and 3340 cm⁻¹, C─O─C stretching band at 1025 cm⁻¹, C═C stretching of benzene ring at 1617 cm⁻¹ etc.), while for the colorless RhB‐D/LG@DA and RhB‐D/SG@DA solid mixtures, the respective color developer bands appeared more broaden and/or weaker (Figures  4c; S26 and S27). We ascribed the decrease of the FT‐IR peaks of the color developers to their phase segregation and migration towards the inner part of the mixtures, which start solidifying from the outer part, in contact with the substrate, pushing inwards the non‐soluble color developer aggregates.

In summary, segregation and formation of independent self‐assembled structures (through van der Waals and hydrogen‐bonding interactions) of the color developer in the acid PCMs is observed for the solid state colorless RhB‐D/LG@DA (ΔC = – 2) and RhB‐D/SG@DA (ΔC = – 8) mixtures. In contrast, the colored RhB‐D/OG@DA solid mixtures (ΔC = + 2) manifest an analogous situation as the liquid state, with homogeneous distribution of the color developer in PCM and consequent loss of its XRD peaks and stronger FT‐IR signals. Therefore, compared to White and collaborators’ rule the tendency to the precipitation or not of the color developer from the PCM is not given by the matching/mismatching of their alkyl chain length. Indeed, mixtures with ΔC =−2 (matching) and ΔC =−8 (mismatching) give the same effect (colorless solids), while those with ΔC =−2 and ΔC = + 2 (both matching) behave in the opposite way. Instead, in our systems, the tendency to the precipitation, or not, of the color developer from the acid PCM, relies on well‐defined ΔC values, being ≥ or <0. When the color developer has a shorter or equal chain length to the PCM, it dissolves well in it, forming a homogenous solid solution (as for the liquid), interacting with the dye molecules (i.e., developing color, Scheme 2a). On the contrary, when the color developer chain length overcomes that of the PCM, the molecules increase their tendency to phase separate from the PCM and the dye (i.e., no coloration is developed, Scheme 2b). This is also in agreement with the previously reported work based on a three‐component mixture made of a leuco dye, N‐acylaminophenols, and stearic acid as PCM. Although they had a similar composition as our systems, they did not observe the colorless‐to‐colored transition, possibly because the color developer had a shorter chain length than the PCM.^{[ 24 ]}

Scheme 2.

Schematic representation of thermochromic change in our dye/CD@PCM mixture during heating and cooling process in case of (a) C_CD ≤ C_PCM and (b) C_CD > C_PCM, where C is carbon chain length.

There is an important difference from most reported systems based on long alkyl chain alcohols as PCM, which grants for the achievement of the opposite thermochromic transition: the solid mixtures in which the color developer segregates, do not develop color, in contrast to conventional crystal violet lacton/color developer/alcohol systems, where the color is given by the complex formation of co‐precipitated dye and color developer. We ascribe this difference to the chosen acidic PCMs, which act as a better solvent for the dye than the alcohols, sequestering it in the solid state from the interaction with the segregated color developer.

This same tendency is also suggested by the observation of the coloration behavior of many other RhB‐D/color developer@acidic PCM mixtures. On the one hand, colorless mixtures RhB‐D/SG@HDA (ΔC =−2), RhB‐D/SG@DDA (ΔC = −6), are formed from color developer molecules with alkyl chain length longer than that of the PCM, being thus more prone to phase segregation, as corroborated by XRD and FT‐IR measurements (Figure S28, Supporting Information). On the other hand, in RhB‐D/LG@DDA (ΔC = 0), RhB‐D/OG@DDA, RhB‐D/LG@HDA (ΔC = + 4), and RhB‐D/OG@HDA (ΔC = + 8) mixtures, the color developer chain length is small enough compared to the PCM to become soluble in it (Figures S29 and S30, Supporting Information). This also applies to the mixtures made by alkyl chain‐free color developers, i.e., RhB‐D/Cat@DA, RhB‐D/Pyr@DA (ΔC = + 12), where Cat and Pyr are fully solubilized through hydrogen‐bonding interactions with the acidic PCM, as confirmed by the absence of XRD peaks in the measured mixture (Figure S31, Supporting Information).

2.3. Universality of the Strategy

After demonstrating the viability of the thermally‐induced colorless‐to‐colored transition with lactam derivative RhB‐D dye, and establishing a rationalized general rule to obtain the desired transition using different PCMs and color developers, we aimed to generalize this to other leuco compounds (i.e., lactons and fluorans), so that such a strategy would not only permit tuning the transition temperature, but also the color of the thermally‐activated state. Several Dye/LG@DA mixtures (ΔC =−2) were prepared as above, obtaining in all cases colorless solid materials at room temperature. Upon heating, the mixtures turned to blue (λ_max = 610 and 560 nm), cyan (λ_max = 600 and 490 nm), pink (λ_max = 540 nm), green (λ_max = 605, 460 and 435 nm), gray (λ_max = 585 and 445 nm), and black (λ_max = 575 and 450 nm), depending on the corresponding dyes used, i.e., crystal violet lactone (LAC1), 3‐(1,2‐dimethyl‐3‐indolyl)‐3‐[4‐(diethylamino)‐2‐methylphenyl]phthalide (LAC2), 3,3‐bis(2‐methyl‐1‐octyl‐1H‐indol‐3‐yl)isobenzofuran‐1(3H)‐one (LAC3), 2′‐(dibenzylamino)‐6′‐(diethylamino)fluoran (FLU1), 2′‐(2‐chloroanilino)‐6′‐(dibutylamino)fluoran (FLU2), and 2′‐anilino‐6′‐(dibutylamino)‐3′‐methylfluoran (FLU3), respectively (Figure 5 ).

Figure 5.

Chemical structure of different dyes, images, F(R), and absorption spectra of the corresponding thermochromic mixtures in the solid and liquid state: a) LAC1/LG@DA, b) LAC2/LG@DA, c) LAC3/LG@DA, d) FLU1/LG@DA, e) FLU2/LG@DA, and (f) FLU3/LG@DA.

Furthermore, colorless‐to‐colored transitions were preserved when the LG color developer was changed for SG (dye/SG@DA, ΔC =−6, Figure S32, Supporting Information), while solid‐state colored mixtures (and colored‐to‐colored transitions) were always observed when OG was employed (dye/OG@DA, (ΔC = + 2, Figure S33, Supporting Information). Noticeably, the new experimental rule extrapolated from this study, based on relative color developer/PCM chain length (ΔC > or ≤0) applies to all dyes, regardless they are lactam, lactone, and fluoran‐based dyes, and without needing specific optimization of the mixture composition. This also applies to LAC1 dye (i.e., crystal violet lacton), which most of the time is reported to undergo colored‐to‐colorless transition upon heating, in bulk and micro/nanostructured 3‐component mixtures, made by the dye, bisphenol A (color developer), and tetradecanol (PCM).^{[ 7} , ¹⁹ , ⁵⁶ , ⁵⁷ , ^{58 ]}

Taking advantage of the possibility that such a system allows for both color and transition temperature modulation, we build up a multistep, multicolor colorless‐to‐colored temperature sensor. For this, we have chosen three different systems, namely RhB‐D/LG@DA, LAC1/SG@DDA, and FLU2/SG@HDA mixtures for pink, blue, and gray coloration after heating them at different temperatures. As shown in (Figures 6 ; S34, Supporting Information), all mixtures were colorless at room temperature. When the temperature was increased above 40 °C, RhB‐D/LG@DA mixture started to melt, developing pink color, while at 52 and 62 °C, the LAC1/SG@DDA and FLU2/SG@HDA mixtures melted to exhibit blue and gray colors, respectively. The decoloration of each mixture was observed upon cooling the multi‐colored sensor below its respective recrystallization point (Figure S35, Supporting Information).

Figure 6.

Images of the colorimetric temperature sensor made of RhB‐D/LG@DA (top), LAC1/SG@DDA (middle), and FLU2/SG@HDA (bottom) thermochromic mixtures. The sensor is kept at different temperatures to induce the sequential coloration of each mixture.

2.4. Composite Materials Based on Structured Mixtures

Finally, we explored the possibility of integrating these mixtures in solid substrates, which is more relevant for practical applications. For this, we first microstructured the above FLU2/SG@HDA mixture as solid lipid particles (SLPs), by using the emulsification/cooling method previously optimized in our group (see Experimental Section).^{[ 34} , ⁵⁹ , ^{60 ]} SLPs of our FLU2/SG@HDA mixture were obtained as spherical microparticles of size ranging between 10 and 50 µm (Figure 7a,b), as demonstrated by optical and scanning electron microscopies (SEM). Compared to encapsulated thermochromic mixtures, where a portion of the particle structure is made by non‐active shell material, in SLPs all particle volume is thermochromic. This should maximize the loading in solid substrates and ensure better color contrast.

Figure 7.

a) Optical and (b) SEM images of FLU2/SG@HDA SLPs and (c) optical and (d) the cross‐section SEM images of the SLP‐loaded PVA film.

In suspension and in the powder form, the SLPs were colorless at room temperature, while they developed a grayish color upon heating over 70 °C, faithfully reproducing the behavior exhibited by the bulk mixture. Finally, both solid powders and colloidal suspensions recovered the colorless state once cooled to room temperature, despite the powdered particles losing their structure due to their melting and merging once in the liquid state (Figure S36, Supporting Information).

We then proceeded with the preparation of different solid thermochromic materials of relevance for practical applications. First, we prepared self‐standing composite films upon casting an aqueous suspension of the SLPs and polyvinyl alcohol (PVA), selected for its optical transparency and film‐forming properties, as previously demonstrated.^{[ 27} , ²⁸ , ^{61 ]} Optical microscopy imaging confirmed the distribution of the particles within the film (Figure 7c), while SEM inspection of the cryo‐fractured film cross‐section, showed particles and voids all over the thickness (Figure 7d). These voids, of the size of the particles, were left over by the SLPs, extracted away upon CHCl₃ treatment of the film. DSC measurements showed a slight decrease in the melting point and an even higher difference in the crystallization (Figure S37, Supporting Information). Such difference is due to the supercooling effect, which is known to increase when the PCMs are structured as particles or confined in micro/nano‐environments. Nevertheless, the thermogram of two heating/cooling cycles revealed reproducible melting and crystallization transitions, which corroborate the stability of the material during the thermal treatment.

The films were colorless at room temperature (with no absorption bands in the visible region) and developed grayish coloration (with two absorption bands at 450 and 580 nm upon heating at 70 °C, while faded back upon cooling at room temperature (Figures 8a; S38, Supporting Information). Noticeably, this process could be repeated several times with no leakage observed when the particles melt, suggesting the good trapping of the particles within the PVA polymer matrix (Figure S39, Supporting Information). These composite films could serve as colorimetric thermal sensors or thermal energy storage indicators. A proof of concept of this was obtained by immersing one of these films in a warm oil (eicosane at 65 °C), in which it immediately exhibited the tint color (Figure 8b; Video S1, Supporting Information). Noticeably, the produced transition is opposite to most previously described indicators (e.g., thermal energy storage indicators) based on structured thermochromic mixtures.^{[ 7} , ¹⁹ , ⁵⁶ , ⁵⁷ , ^{58 ]}

Figure 8.

a) Images and F(R) spectra of SLP‐loaded PVA film kept at 25 and 70 °C, b) color change of the SLP‐loaded PVA film used to sense, upon immersion, the temperature of a hot oil. The film is initially half‐immersed, exhibiting coloration only in half of it (snapshot 3 from the left) and then fully immersed. Immediately after being removed from the oil, the film is in its tinted state and then quickly fades in the air at room temperature.

Finally, we accomplished thermochromic printed patterns by integrating the SLPs in cellulose papers through screen‐printing. The papers resulted fully colorless (invisible pattern) at room temperature, while developing reversible and patterned color changes upon heating (Figure 9 ). Such materials could be used for sensors or invisible inks for anticounterfeiting applications.

Figure 9.

Screen‐printed thermochromic (a) square and (b) “ICN2” logo patterns obtained from a PVA suspension of the thermochromic FLU2/SG@HDA SLPs onto cellulose paper at 25 and 70 °C.

3. Conclusion

In summary, we designed and demonstrated a general methodology to prepare stable, reversible, and high color‐contrast colorless‐to‐colored thermochromic materials based on 3‐component mixtures. These materials rely on leuco dyes and on the rational selection of the color developer molecules, based on catechol/pyrogallol derivatives owning a long alkyl chain and at least 2 ‐OH groups, and fatty acid PCMs. The presence of at least two vicinal ‐OH groups (rationalized by also DFT calculation) in the color developer resulted in fundamental importance for the coloration of the hot (liquid) state, which is observed in all studied mixtures containing these molecules (except those with alcohol‐based PCMs). On the other hand, the acidic PCMs help co‐stabilize the open form of the leuco dye in the liquid state and enhance the dye solubility in the solid state, preventing its co‐precipitation with the color developer. This granted for a novel color formation mechanism in the solid state: indeed, while for most reported works, colored solid‐state mixtures form upon co‐precipitation and interaction of the dye with the color developer, in our systems, coloration is observed when the dye and color developer remain co‐dissolved in the PCM. Colorless solids are thus obtained when the color developer is selectively phase segregated. This allowed for the straightforward control of the solid‐state color and, overall, for the preparation of thermochromic mixtures undergoing colorless‐to‐colored transition. Indeed, because of the choice of these components, the solid‐state color becomes directly dependent on a well‐defined relationship of the chain length of the color developer and the PCM: colored solid mixtures are obtained from color developers with shorter or equal alkyl chain length than that of the PCMs (ΔC = C_PCM – C_CD ≥0), while the desired colorless mixtures are accomplished from color developers with alkyl chains of larger length than that of the PCMs (ΔC <0). XRD, FT‐IR, and DSC experiments allowed to correlate these experimental optical observations to the tendency of color developers to remain co‐dissolved (shorter alkyl chains) with the dye in the PCM (interacting and developing the color), or phase‐segregate (longer alkyl chains) from the crystalline PCM (interrupting the interaction with the dye). Such a rule proved quite solid and general, and so far, free of exceptions. Mixtures satisfying ΔC <0 rule provided stable, reproducible (over several cycles), and high‐contrast thermally‐induced colorless‐to‐colored transitions, and could be stored over one year under ambient light, without losing the functionality. Even more, such a rule could be exploited to easily tune the colorless‐to‐colored transition temperature and was extended to leuco dyes of different families, namely, lactams, lactons, and fluorans, without needing further composition optimization, enhancing the color variability in the hot state. Besides, our strategy does not require the typically used color developer Bisphenol A, which is well known to be toxic. Instead, it employs bioinspired and more biocompatible gallol and catechol derivatives.

Finally, we demonstrated that the thermochromic transition could be preserved in microstructured solid lipid particles, from which we have been able to prepare thermochromic composite films and patterned cellulose papers, of relevance for practical applications, such as sensing and security inks. In the future, such thermochromic composites could be proposed as coatings for smart window applications, as they undergo the desired temperature‐induced colorless‐to‐colored transition and allow straightforward tuning of the transition temperature, important to adapt the material to the different geographical regions. However, for this, higher transparency of the composites should be achieved, which can be accomplished by reducing the size of the SLPs down to the nanoscale, a strategy that we have previously adopted to obtain transparent photo/thermoresponsive smart window films and coatings.^{[ 27} , ²⁸ , ^{61 ]}

As far as we know, this is the most general rule established to achieve a thermochromic material with colorless‐to‐colored transition, which will be of future strong interest in emerging fields such as thermal energy storage sensing, textile engineering, anticounterfeiting, and smart window technologies.

4. Experimental Section

Materials

Decanoic acid (DA), dodecanoic acid (DDA), and hexadecanoic acid (HDA) were purchased from TCI chemicals. Rhodamine B (RhB) was acquired from Sigma–Aldrich. Other thermochromic dyes, i.e., Crystal Violet lactone (LAC1), 3‐(1,2‐Dimethyl‐3‐indolyl)‐3‐[4‐(diethylamino)‐2‐methylphenyl] phthalide (LAC2), 3,3‐Bis(2‐methyl‐1‐octyl‐1H‐indol‐3‐yl)isobenzofuran‐1(3H)‐one (LAC3), 2′‐(dibenzylamino)‐6′‐(diethylamino)fluoran (FLU1), 2′‐(2‐chloroanilino)‐6′‐(dibutylamino)fluoran (FLU2), and 2′‐anilino‐6′‐(dibutylamino)‐3′‐methylfluoran (FLU3) were taken from TCI chemicals. Octyl Gallate (OG), Lauryl Gallate (LG), Cetyl Gallate (CG), and Stearyl Gallate (SG) were purchased from TCI chemicals. Decyl Gallate (DG) and Myristyl Gallate (MG) were purchased from BIOSYNTH. Bisphenol A (BA), pyrocatechol (Cat), resorcinol (Res), pyrogallol (Pyr) were acquired from Sigma–Aldrich, and 1,6‐Hexanediol (HD) from Alfa Aesar. 3,4‐dihydroxybenzoic acid, dodecylamine, dodecanol, N, N'‐dicyclohexylcarbodiimide (DCC) Phosphorus(V) oxychloride (POCl₃), and anhydrous acetonitrile (ACN) were acquired from Sigma–Aldrich. Different organic solvents such as dichloromethane (DCM), tetrahydrofuran (THF), triethylamine (NEt₃), ethyl acetate (EA), hexane, and sodium bicarbonate (NaHCO₃) were taken from Scharlab. Deutered solvents were acquired from Euriso‐Top. All commercial solvents and chemicals were used without any further purification.

Characterization Techniques

UV–vis spectra were carried out with Cary 60 spectrophotometer. For the liquid solutions, the mixtures were characterized in transmittance mode upon heating the sample chamber above the T _m ^PCM with two Peltier plates (TEC1‐19906) connected to a power supply (RS‐3005D DC) and attached to the two sides of the sample chamber parallel to the detection light beam. The measurements were carried out with a Hellma macro‐cuvette 100‐QS of 2 mm optical path cuvette and using the PCM mixture without the dye as a blank. The real sample temperature was monitored through the RS‐206‐3738 digital thermometer. The diffuse reflectance (R) of solid PCM powders was measured at room temperature in the Cary 60 spectrophotometer through an integrating sphere connected to the spectrophotometer via fiber optic. For spectral comparison with liquid absorption measurements, the diffuse reflectance spectra were converted into F(R) function by Kubelka–Munk equation K/S = (1‐R)² /(2^* R) where K, S, and R are the light absorption coefficients, scattering coefficients, and measured reflectance data, respectively. The mixture powder obtained with the same components as the sample, but without the dye, was used as a blank. UV–vis spectral measurements of the mixtures during heating/cooling cycles were carried out using the Hamamatsu C9920‐02G spectrophotometer in reflectance mode. For the measurements, a MICQ quartz cylinder cuvette (diameter 12.5 mm, height 3.5 mm, and thickness 1 mm) open at the top was filled with the sample up to the top and kept on a Peltier plate (TEC1‐19906) connected to a power supply. The measurements were obtained in front face by placing the fiber optic of the excitation Xe lamp and the fiber optic of the detector at ≈90 ° respect to each other and 45 ° respect to the substrate plane. The fiber optics and the sample holder were maintained fixed along all heating/cooling cycles. The real sample temperature was monitored through the RS‐206‐3738 digital thermometer. Digital images of the thermochromic mixtures in the solid and liquid state were obtained from the mobile phone Samsung Galaxy S23. Image J software was used to obtain the RGB coordinates. The optical density (OD) was obtained by converting the RGB coordinates into the tristimulus XYZ and CIE Lab coordinates and applying the equations shown in the supporting information. Proton Nuclear Magnetic Resonance (¹ H‐NMR) spectra were achieved from Bruker DPX400 (400 MHz). FT‐IR spectra of the solid mixtures were obtained from a Tensor 27/PMA 50 Spectrometer (Bruker). XRD diffractograms of the mixture were carried out at room temperature in the Powder X‐Ray Diffractometer MALVERN PANalytical X´pert Pro MPD Theta–Theta system equipped with Cu Kα (1.54 Å) radiation. XRD and FT‐IR measurements were carried out by making a thin film over a glass slide. First, a drop of the molten thermochromic mixture was put on a glass slide substrate, and then it was let to cool down at room temperature, forming a film. For liquid N₂‐cooling experiments, the mixture of RhB‐D/LG@DA was initially melted over a hot plate and put a few drops of hot liquid over the XRD sample holder, which immediately after was frozen by pouring liquid N₂ over the solution. The frozen mixture was placed over the XRD instrument for measurement over time. DSC thermograms were obtained from the Parking Elmer, Pyris 9 after carrying on 2 heating and cooling cycles (2 °C min⁻¹) from 0 to 40 °C. For cycle experiments, the heating and cooling process was carried out at 5 °C min⁻¹. For the isothermal experiments, the mixtures were first heated at 75 °C and then rapidly cooled at 25 °C (50 °C min⁻¹) at which the samples were kept for over 60 s. SEM images of solid‐lipid particles (SLP) and cross‐sectional view of SLP containing PVA film were investigated with FEI Quanta 650 FESEM microscope. Aqueous dispersion of SLPs was deposited on the SEM metal stub by drop casting, allowing solvent evaporation in air at room temperature, and coated with 5 nm thick platinum layer prior to imaging. Optical microscopy images of the PVA‐SLP film were collected using a Zeiss Primo Star equipped with a camera AxioCam ERc 5s. A few drops of the suspension of SLP/PVA were poured on top of a glass slide and observed via optical microscope after drying.

Temperature Sensing Experiments

An equal amount of three optimized thermochromic mixtures, i.e., RhB‐D/LG@DA (top), LAC1/SG@DDA (middle), and FLU2/SG@HDA (bottom), were put in 1.5 cm³ spherical hole over a white colored poly(methyl methacrylate) template, where the holes were obtained by laser engraving. Gradually, the temperature was increased until reaching 70 °C and cooled them to room temperature to observe their color changes. This process was continued for five times to take a digital photo with the Samsung Galaxy S23 and noted transition temperatures for different color changes using RS‐206‐3738 digital thermometer.

Synthesis of RhB‐D

RhB‐D was obtained from RhB in two steps (Scheme E1, Supporting Information) by treating it with POCl₃ and dodecylamine. For the first step, 100 mg of RhB was dissolved in 5 mL of DCM, and 0.1 mL of POCl₃ was added dropwise to it, followed by stirring under N₂ atmosphere. Then the mixture was refluxed at 65 °C for 6 h. The resultant mixture was cooled down to room temperature, and the solvent was evaporated under reduced pressure. In the second step, the remaining mixture was transferred to 10 ml of anhydrous ACN in a 50 mL round bottom flux and mixed with 0.3 ml of NEt₃ and 1 ml of dodecylamine. Then the mixture was refluxed for 24 h under N₂ atmosphere. Finally, the solvent was evaporated under reduced pressure, and the product was purified by column chromatography (silica gel, DCM/MeOH from 1/0 to 20/1, v/v) to obtain the desired RhB‐D product with 78% yield as oil.

¹H‐NMR (400 MHz, CDCl₃, Figure E1, Supporting Information) of RhB‐D in δ (ppm): 7.94‐7.88 (1H, m, ArH), 7.46‐7.39 (2H, m, ArH), 7.12‐7.05 (1H, m, ArH), 6.42 (4H, m, ArH), 6.28 (2H, m, ArH), 3.35 (8H, q, J = 7,2 Hz, N(CH ₂CH₃)₂), 3.15‐3.05 (2H, m, J = 7,2 Hz, CONHCH ₂CH₂(CH₂)₉CH₃), 1.34‐1.18 (18H, m, CONHCH₂CH ₂(CH₂)₉CH₃), 1.18 (12H, t, J = 7,2 Hz, N(CH₂CH ₃)₂), 1.05 (2H, m, CONHCH₂CH ₂(CH₂)₉CH₃), 0.89 (3H, t, J = 7,2 Hz, CONHCH₂CH₂(CH₂)₉CH ₃). The additional peaks at lower δ values (1.34‐1.18, 1.05, and 0.89) belong to the protons of dodecyl amine. FT‐IR (Figure E2, Supporting Information): ν = 1680 cm⁻¹ (C═O stretching), 1426 cm⁻¹ (amide C─N stretching). These data confirm the attachment of dodecyl amine with RhB.

Synthesis of Cat‐E

200 mg of 3,4‐dihydroxybenzoic acid (1.3 mmol) were dissolved in 10 mL of THF. 242 mg of dodecanol (1.3 mmol) was added to it, and the solution was cooled at 0 °C. Then, 115 mg (2 mmol) of DCC was added to it and stirred overnight at room temperature. The solvent was removed under reduced pressure, and the resultant residue was extracted with ethyl acetate several times and filtered. The filtrate was washed successively with diluted aqueous citric acid solution, saturated aqueous NaHCO₃ solution, and water. The organic layer was dried over Na₂SO₄ and evaporated. The crude product was purified by silica gel chromatography (10:40 EA‐hexane mixture) until urea was completely removed. Finally, the desired dodecyl ester of pyrocatechol (Cat‐E) product was achieved with 66% yield as a white solid. The reaction scheme is reported in Scheme E2 (Supporting Information).

¹H‐NMR (400 MHz, CD₃COCD₃, Figure E3, Supporting Information): δ (ppm): 8.47 (2H, bs), 7.50 (1H, d, J = 2 Hz, ArH), 7.48 (1H, dd, J₁ = 2 Hz, J₂ = 8 Hz, ArH), 6.92 (1H, d, J = 8.0 Hz, ArH), 4.27 (2H, t, J = 6.8 Hz, COOCH ₂CH₂(CH₂)₉CH₃), 1.77 (2H, q, J = 6.8 Hz, COOCH₂CH ₂(CH₂)₉CH₃), 1.45 (18H, m, COOCH₂CH₂(CH ₂)₉CH₃), 0.91 (3H, t, J = 6.8 Hz, COOCH₂CH₂(CH₂)₉CH ₃). FT‐IR (Figure E4, Supporting Information): ν = 2920 and 2850 cm⁻¹ (CH₂ stretching of alkyl chain), 1450 cm⁻¹ (CH₂ bending of alkyl chain), 1100 cm⁻¹ (C─O─C stretching of ester). These data confirm the attachment of the alkyl chain with the acid to form long‐chain alkyl ester of Cat‐E.

Preparation of Thermochromic Mixtures

The thermochromic RhB‐D mixtures were prepared by weighing 4.5 g of solid PCM (i.e., DA, DDA, and HDA), heated above their T _m (32, 43, and 62 °C, respectively. To this, 50 µL of 10 mm RhB‐D solution in DCM was added. After 15 min, different color developers were added (from 50 to 600 mg), and the molten mixtures were stirred until homogenization. The mixtures were then cooled down and stored at room temperature (25 °C). The mixtures containing the other dyes were obtained in the same way, by adding 50 µL of 10 mm of LAC1, LAC2, LAC3, FLU1, FLU2, and FLU3 dyes in DCM, respectively. The blank samples, prepared without the dyes, were obtained following the same procedure, but without adding the dye.

Preparation of Solid Lipid Particles (SLPs)

600 mg of SG was dissolved in molten HDA (9 g) at 70 °C followed by the addition of 200 µL of 10 mm of FLU2 dye in DCM. The final melted mixture was subsequently added to a previously prepared CTAB aqueous solution (20 mL, 0.5 wt.%) and was homogenized at 80 °C using IKA T25 Ultra–Turrax for 10 min at 6000 rpm. Then, the whole emulsion was quickly transferred to 30 mL of cold water (2–5 °C) to induce the rapid solidification of the HDA droplets with SG/dye and generate a suspension of the corresponding SLPs, and kept overnight in the refrigerator to flocculate. Finally, the white flocculated SLPs were decanted and freeze‐dried overnight to get a white powder.

Preparation of Polymer Composites

Dried white SLP powder (250 mg) was mixed with previously prepared 10 wt.% PVA (20‐98) solution (750 mg) in a flask and mixed homogeneously with a spatula. Then the white suspension was poured into a 2 cm × 2 cm polyethylene terephthalate (PET) mould and dried at 40 °C overnight to evaporate the water under ambient conditions, yielding a white PVA film embedding the SLPs. Finally, a free‐standing film was obtained by easily peeling it off the PET mould container.

Screen‐Printing of Cellulose Papers

200 mg of white SLP powder was mixed with previously prepared 10 wt.% PVA (20‐98) solution (1 g) in a vial and mixed homogeneously with a spatula. Then the white suspension was poured over a pre‐patterned poly(methyl methacrylate) mask placed over a cellulose paper to obtain the desired printed pattern.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Supplemental Video 1

Dinda D., Pérez N. M., Faraudo J., Ruiz‐Molina D., and Roscini C., “High‐Contrast Colorless‐to‐Colored Thermochromic Materials.” Small 21, no. 51 (2025): e11454. 10.1002/smll.202511454

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.