Introduction

Thermochromic materials have garnered significant attention in various research domains due to their versatile applications1,2. These materials find utility in anti-counterfeiting marks, temperature indicators, wearable electronics, medical diagnostics, security features, aerospace technology, and beyond3. Thermochromism is characterized by color transition phenomena observed upon heating or cooling substances4. Thermochromic materials exhibit a unique thermal memory function, wherein their color undergoes pronounced changes during thermal excursions5,6. Various types of materials have been explored for the development of thermochromic sensors for internal temperature measurements. These include organic dyes, nanotube-based systems, quantum dots, inorganic phosphors, and organic-inorganic hybrid materials, each offering unique advantages and capabilities in temperature sensing applications7,8,9.

Notably, rhodamine dyes have emerged as particularly promising candidates due to their exceptional spectroscopic characteristics. These dyes are characterized by large molar extinction coefficients and high fluorescence quantum yields, allowing for efficient absorption and emission of light. They can be excited by visible light wavelengths and exhibit remarkable stability against light-induced degradation, making them durable for various applications10,11,12. These attributes make rhodamine dyes highly suitable for use in thermochromic sensors, facilitating precise and reliable temperature measurements in diverse environmental conditions. Rhodamine groups demonstrate an exceptional ability to undergo structural transformation, shifting from a non-fluorescent spirolactam ring-closed form to a highly fluorescent ring-open form under certain conditions. This transition is accompanied by a vivid color change, which serves as a crucial feature facilitating “naked-eye” detection13. Ozawa and colleagues were the first to create a color-tuned nano-thermometer by modifying the level of conjugated rhodamine B. This groundbreaking nanothermometer was extremely efficient in water and displays a visible color change in response to heat in the temperature range of 5–65 °C14. This advancement holds promise for the creation of highly sensitive and visually intuitive thermochromic sensors for various applications, including temperature monitoring in aqueous environments. Lee et al.. has successfully synthesized a temperature-responsive hybrid rhodamine B-based polymer. Fluorescence intensity of this innovative material diminished as temperature increased, accompanied by a visually perceptible color transition from pink to purple15.

Visible light interacting with matter is responsible for the color we perceive in objects. Sunlight, with its high reflectivity, significantly contributes to rising interior temperatures. Cold dyes offer an effective solution by absorbing less solar energy in the infrared spectrum, thereby maintaining lower surface temperatures and reducing the energy transferred to the interior, thus alleviating the burden on air conditioning systems16. Roughly 43% of the solar energy that reaches Earth comes in the form of visible light, which can heat up objects when absorbed. To counteract this, cool pigments are used. These pigments are created to absorb the majority of visible light while reflecting near-infrared (NIR) radiation from the sun, thus reducing heat absorption. The use of these cool pigments shows potential for saving energy in cooling systems17. Cool dyes can be classified into two categories: organic and inorganic, where inorganic dyes are more commonly used in commercial applications. However, there are fewer options available for organic cool dyes. Organic cool dyes often include black dyes derived from compounds such as phthalocyanine, azo dyes, and certain perylene-based dyes like Lumogen, Dye Black 3, and Dye Black 32. In recent years, thermochromic and cold pigments have been introduced to modify surface temperatures by responding dynamically to environmental temperatures and sunlight exposure. Thermochromic pigments change color with temperature variations, enabling surfaces to reflect more or less heat depending on their color state. Similarly, cold pigments maintain lower temperatures by reflecting near-infrared (NIR) radiation, helping to reduce heat absorption even in darker colors. This dual approach, combining the dynamic properties of thermochromic pigments with the stable NIR-reflecting properties of cold pigments, theoretically provides a highly effective means of managing surface temperatures. Thermochromic pigments, like rhodamine B, and cold pigments, such as coumarin, offer innovative methods for regulating surface temperatures. Rhodamine B, a thermochromic pigment, shifts color in response to temperature changes, enabling it to modulate heat absorption by reflecting more or less light depending on its color state. Conversely, coumarin, as a cold pigment, provides a stable means of reducing heat absorption by reflecting near-infrared (NIR) radiation, even in higher temperature environments. Theoretically, combining the dynamic heat-response properties of thermochromic rhodamine B with the consistent NIR-reflective performance of coumarin could allow for more efficient control of surface temperatures, adapting to environmental changes while maintaining lower heat absorption overall. While these dyes have been thoroughly studied for their fluorescence and near-infrared (NIR) reflectance properties, 7-hydroxy-2H-chromen-2-one (HC) has not yet been explored as a cold dye in the scientific literature. This study endeavors to use a reflective cool pigment and manufacture a thermal sensor strip capable of data detection without the need for supplementary tests. The synthesis of amino-functionalized rhodamine B is prepared through the modification of rhodamine B using ethylenediamine (MA-RB) (Fig. 1).

Fig. 1
figure 1

Schematic of MA-RB and HC as temperature sensors.

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Experimental Methods

Materials

7-hydroxy-2H-chromen-2-one (HC, Aldrich, 99.5%), rhodamine B (Aldrich, 99%), acetone (Merck, 99%), ethylenediamine (EDA, Aldrich, 99%), ethanol (Merck, 99.9%), 4-(dimethylamino)pyridine (DMAP, Aldrich, 99%), nitrocellulose resin, N,N'-dicyclohexylcarbodiimide (DCC, Merck, 99%), and dye thinner (Dr. Mojallali, 20000) were used as received.

Synthesis of amino-functionalized rhodamine B (MA-RB)

The process involved dissolving 2.3 g (4.8 mmol) rhodamine B in 90 mL ethanol. Subsequently, 1.8 mL (27 mmol) EDA was added and the reaction was carried out for 5 h at 65 °C. Once the reaction was completed, the mixture was diluted with 25 mL distilled water and subsequently filtered. Finally, amino-functionalized rhodamine B (MA-RB) was subjected to drying in a vacuum oven at 65 °C for 24 h. The yield of the reaction was determined gravimetrically, resulting in an approximate yield of 88%.

Film preparation

A mixture was prepared by combining nitrocellulose resin (64 wt%), pigments (16 wt%), and solvent (20 wt%, dye thinner), which was then subjected to ball milling for 5 h. The resulting paints were subsequently applied to Leneta checkerboard charts using a film applicator, resulting in a paint film with a thickness of 120 μm. This application method aimed to prevent the reflection of the substrate, allowing for the exclusive detection of paint reflection.

Instrumentation

Proton nuclear magnetic resonance (1H NMR) and carbon nuclear magnetic resonance (13 C NMR) spectra were recorded on a Varian Unity Inova 500 MHz using deuterated dimethyl sulfoxide ((CD3)2SO) as solvents and tetramethylsilane (TMS) as an internal standard. Fourier transform infrared (FT-IR) spectroscopy was performed by means of a Bruker Tensor 27 FT‐IR spectrophotometer, in the range between 500 and 4000 cm−1 with a resolution of 4 cm−1. Ultraviolet visible (UV–visible) absorption spectra from liquid samples were recorded by a Hanon instrument. UV–Vis–NIR reflectance spectra were obtained over the wavelength range of 250–2500 nm using a PerkinElmer lambda 1050.

To describe the color change more accurately, the color are presented in CIE (Commission International d’Eclairage) 1931 chromaticity diagram for clear observation. In the color perception study, the CIE 1931 chromaticity diagram is usually utilized to mathematically define the chromatic sensation of human eyes to a specific optical spectrum18. The standard equal energy point (x = 0.33, y = 0.33) at the center of the CIE 1931 diagram is attributed to the white light emission. To examine the color of any light source, three dimensionless quantities called color matching functions (x¯ (λ), y (λ), z¯(λ)) are required which are used in Eqs. (1) and (2)19.

$$\:X=\frac{x}{x+y+z}$$
(1)
$$\:Y=\frac{y}{x+y+z}$$
(2)

All films were tested at different temperatures (5, 15, 25, 35, 45, 55, 65, 75 and 85 °C). The color of the indicator corresponds to the total color difference (TCD or ∆E) value under different temperatures. All film samples were checked every 24 h with a color meter (Minolta CR-10, Japan). The L*, a*, and b* chroma system was used to analyze the dynamic change in the indicator’s color, which is the color difference (TCD), following Eq. (3):

$$\:\varDelta\:{\rm\:E}=\left[\right({\varDelta\:L}^{*}{)}^{2}+({\varDelta\:a}^{*}{)}^{2}+({\varDelta\:b}^{*}{)}^{2}{]}^{1/2}$$
(3)

where ∆L* is the brightness difference between initiation and each time interval; ∆a* is the redness-greenness difference between initiation and each time interval; and ∆b* is the yellowness- blueness difference between initiation and each time interval. According to the indicator kinetics characterized by Taoukis and Labuza20, the total color difference value X = ∆E or TCD of the indicator maybe expressed in terms of a response function as follows:

$$\:F\left(X\right)=\left(\frac{1}{{X}_{0}}-\frac{1}{{X}_{t}}\right)=kt$$
(4)

where k is the rate constant of the reaction that is correlated with temperature and t is the storage time. Plotting a curve between the response function of TCD F(X) and time generated a straight line, and the k of different storage temperatures may be calculated from the slope. One can take the logarithm or natural logarithm on both sides of the Arrhenius function (Eq. (5)).

$$\:Ln\:k=\frac{{E}_{a}}{RT}+\text{ln}A$$
(5)

Plotting a curve between lnk and 1/T generates a straight line. The activation energy (Ea) may be calculated from the slope, and A can be determined directly from the intercept.

Results and discussion

Investigating the chemical structure of thermochromic and cool pigments

An amidation reaction is carried out by reacting EDA with RB, resulting in the production of MA-RB. The reaction pathway is illustrated in Fig. 221.

Fig. 2
figure 2

Synthesis mechanism of MA-RB.

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The chemical structures of MA-RB and HC were thoroughly characterized using FT-IR, and the results are illustrated in Fig. 3. The successful synthesis of MA-RB was confirmed by the emergence of the N − H stretching band at 3370 cm−1. Additionally, the observed shift in the carbonyl peak of COOH groups from 1590 to 1610 cm−1 in amide groups provides supporting evidence for the successful synthesis of MA-RB. HC was validated by the appearance of the aromatic (C = C) stretching bands at 1508 and 1606 cm−1, as shown in Fig. 3.

Fig. 3
figure 3

FTIR spectra of MA-RB and HC.

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To further validate the structure of MA-RB and HC, their chemical structures were investigated using 1H NMR and 13C NMR in DMSO-d6, and the results are presented in Figs. 4 and 5. Based on the results, the peak at 10.38 ppm in Fig. 4a is attributed to the proton of the hydroxyl group of HC attached to the ring. The 13C NMR spectrum of HC (Fig. 5a) is provided in detail in Supporting information (S1. Spectral analysis). The observed chemical shifts for aromatic carbons were noted at 160.8, 115.4, 143.5, 111.1, 129.4, 159.1, 70.9, and 47.8 ppm. The 1H NMR spectrum for MA-RB is illustrated in Fig. 4c, confirming the structure of MA-RB. According to the 13C NMR spectrum (Fig. 5b), chemical shifts for aromatic and aliphatic carbons were observed at 16, 42, 99, 107, 130, 124, 132, 169, 150, and 152 ppm.

Fig. 4
figure 4

1H NMR spectra of (a) HC, (b) rhodamine B, and (c) MA-RB.

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Fig. 5
figure 5

13C NMR spectra of (a) HC and (b) MA-RB.

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To evaluate the reflective properties of the HC pigment, a UV–Vis-NIR reflectance analysis was conducted, as results depicted in Fig. 6. Measurements were taken on black and white substrates to assess the amount of transmitted radiation. The black substrate, which absorbs light well, was contrasted with the highly reflective white substrate to classify pigments based on their NIR behavior. Pigments were categorized as NIR reflective22, NIR transparent, or NIR absorptive depending on their reflectance on these substrates. Significant differences in reflectance suggest high transmittance and transparency in the NIR region, while similar reflectance on both substrates indicates high reflectivity, and low reflectance points to high absorption. Some pigments may exhibit intermediate reflectance and absorption. In the NIR range, HC demonstrated high reflectance on the white substrate and low reflectance on the black substrate, indicating its transparency in this wavelength range. Quantitative analysis, as shown in Fig. 6 and Table S1, revealed that HC reflects 98% of light in the 700–1000 nm and 1500–1000 nm ranges on a white substrate. In contrast, on a black substrate, light absorption in all three ranges (700–1000 nm, 1000–1500 nm, and 1500–2500 nm) was below 2%, confirming its exceptional transparency.

Fig. 6
figure 6

Reflectance UV-Vis-NIR integral of HC.

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Thermochromic performance of MA-RB in solution and solid states

The study examined the thermochromic properties of MA-RB in water at 0.1 mM concentration by analyzing its UV-vis absorbance across different temperatures (Fig. 7a-d). Observations from the UV-vis spectra and color change images demonstrated that MA-RB exhibits visible thermochromic behavior (Fig. 7a and d). As the temperature increased from 30 to 60 °C, MA-RB color transitioned from light pink to rose red, with a corresponding gradual rise in absorption at 473 and 525 nm, while the absorbance peak positions remained unchanged (Fig. 7a and b). The enhanced absorption at 473 and 525 nm is attributed to the thermal excitation causing atoms to transition to higher energy levels23,24. Thermochromism in rhodamine B relies on structural changes at the molecular level that alter its light absorption properties, resulting in a visible color change with temperature variation. Rhodamine B is a dye with a structure that can switch between open and closed forms in response to heat. At lower temperatures, the molecule is typically in a closed, lactone form, which absorbs light in a way that produces a lighter color. As the temperature increases, rhodamine B undergoes a structural rearrangement, shifting to an open, zwitterionic form that enhances conjugation within the molecule. This increased conjugation in the open form allows rhodamine B to absorb longer wavelengths of light, giving rise to a more intense color, often shifting toward red or pink hues. This color change is reversible: as the temperature decreases, the molecule reverts to its closed form, reducing conjugation and thus altering the absorption spectrum back to its original color. The color change in rhodamine B is therefore driven by temperature-dependent shifts in its molecular structure, impacting the dye’s optical properties and resulting in its characteristic thermochromic behavior25,26,27,28. Rhodamine B is a fluorescent dye that exhibits color and fluorescence shifts influenced by several factors: (1) In polar solvents, rhodamine B adopts a fluorescent form due to favorable interactions, while in acidic conditions, it becomes protonated, enhancing fluorescence and shifting to a pink/magenta color. In neutral or basic conditions, deprotonation reduces fluorescence and can cause a colorless state. (2) At high concentrations, rhodamine B molecules can aggregate, leading to reduced fluorescence and red-shifting of spectra. This aggregation can cause fluorescence quenching due to self-interactions. (3) Rhodamine B exists in two forms: a fluorescent zwitterionic (open ring) form, favored in polar solvents, and a non-fluorescent lactone (closed ring) form, favored in non-polar solvents. The open form enhances fluorescence, while the closed form results in weak fluorescence or colorlessness. (4) Exposure to light can excite electrons in rhodamine B, leading to color shifts or fluorescence quenching. Interactions with reducing agents or nanoparticles can also alter its fluorescence. (5) These factors affect the equilibrium between the open and closed forms of rhodamine B. High temperatures may favor the non-fluorescent form, while high pressure stabilizes the fluorescent open form24,29.

Figure 7b and c illustrate that MA-RB maintains its thermochromic properties over multiple heating and cooling cycles without significant loss. Subsequent absorption measurements were conducted after one day, indicating the enduring performance of this property. This characteristic makes MA-RB suitable for applications as a temperature sensor in various fields, such as monitoring exothermic reactions, regulating the heating of smart windows, coating for vehicles, hardware, and batteries in smart devices during operation25,26,30. Based on Eqs. (1) and (2), the chromaticity coordinates of MA-RB at 30 °C were calculated to be x = 3.275 and y = 10.51 in the CIE 1931 chromaticity diagram (Fig. 7e). At 55 °C, these coordinates shifted to x = 5.02 and y = 9.906. The change in chromaticity from 30 to 55 °C in the CIE 1931 diagram aligns with the findings from the UV-vis absorption spectra shown in Fig. 7a. Optical properties are one of the key features of communication and information, which illustrates the purpose of this material innovation. MA-RB was reviewed for important optical properties, including color shade, hue, and ΔE*ab as well as gloss, in conjunction with the dried-ink film (MA-RB without HC) and MA-RB (with HC) ink at different concentrations. Figure S1 and Table S2show a continuously increasing color shade of MA-RB without HC and MA-RB with HC as a function of the MA-RB concentration. However, MA-RB (with HC) showed a slightly higher rate of increase, which can be attributed to the presence of cold pigment, causing the color change in the film to occur more gradually31.

Fig. 7
figure 7

(a) UV–vis absorbance spectra of MA-RB from 20 to 60 °C, (b) Cooling spectra of MA-RB from 60 to 20 °C, (c) Reversible thermochromic behavior of MA-RB during heating and cooling, (d) Photographs showing the color change of MA-RB, and (e) CIE 1931 chromaticity diagram of MA-RB between 30 and 55 °C.

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In the solid system, MA-RB also displays thermochromic properties, providing a broad range of potential applications. The thermochromic performance of MA-RB under an NIR lamp with a wavelength of 700 nm was tested from 20 to 60 °C, and the results are depicted in Fig. 8a. When exposed to the NIR, MA-RB becomes colorful and exhibits a rose red color at 60 °C. To assess the impact of the cool pigment, HC sample was applied to another side of an aluminum plate, while the opposite side of the aluminum plate was coated with thermochromic pigment. As shown in Fig. 8b, the results indicate that upon exposure to an NIR lamp at 60 °C, the thermochromic color undergoes a distinct change, transitioning to a pale rose red. This transformation effectively underscores the efficacy of the cool pigment, clearly demonstrating its substantial influence on the observed thermochromic color change.

Over a 14-day period, the nitrocellulose film containing rhodamine B modified with ethylenediamine was examined under different temperatures and environmental conditions (Figure S2). As time increased, no significant TCD changes were observed in the film samples at 5, 15, 25, 35, and 45 °C. The most significant color changes occurred between 55 and 85 °C in a normal environment (Figure S2a). However, as shown in Figure S2b, under humid conditions, the curve exhibited a reduced intensity in certain sections compared to the normal environment, particularly at lower temperatures, where the effect of moisture became more noticeable. This is because nitrocellulose has some ability to absorb moisture, and this absorption leads to physical changes in the coating film, such as swelling or reduced adhesion to the underlying surface, which in turn decreased the color transparency. To highlight the uniqueness and significance of the MA-RB development, it is essential to compare our findings with past studies on thermochromic pigments. Peng et al.32designed the first ratiometric optical thermometers based on core − shell γ-cyclodextrin metal-organic frameworks (γ-CD-MOFs). These frameworks encapsulated a combination of the temperature-sensitive RhB and the less sensitive fluorescein (FL). RhB, highly responsive to temperature changes, was paired with FL to provide a stable luminescent reference. The resulting dye-encapsulated γ-CD-MOF materials demonstrated excellent thermometric performance, achieving a relative sensitivity of up to 5%, one of the highest reported for dual-luminescent thermometers. This unique approach, with controlled incorporation of dyes in the γ-CD-MOFs, improved both the macro and nano forms’ efficiency, enhancing their thermometric performance and extending their potential applications in sensitive and reliable temperature sensing. In contrast, the development of RBO24 introduces a novel lipophilic thermochromic pigment with both solid and liquid system applications. In the liquid system, RBO exhibits a color shift from light pink to rose red under standard lighting, accompanied by temperature-sensitive fluorescence activation. The fluorescence intensity varies linearly with temperature between 37 and 45 °C, suggesting its potential for body temperature monitoring. In the solid system, RBO shows reversible thermochromic behavior, shifting from pink to yellow with visible light and from fluorescent pink to colorless under UV light, driven by structural changes in the molecular framework. RBO rapid thermochromic response and stability up to 380 °C make it suitable for a wide range of applications, including as a hand-writable thermochromic pigment. When compared to these advanced thermochromic systems, the MA-RB development stands out with its distinct properties. While both γ-CD-MOFs and RBO focus on color shifts driven by temperature changes, MA-RB offers innovative characteristics such as gradual color change due to the inclusion of cold pigments, which allows for more controlled and stable optical responses. The MA-RB system also demonstrates versatility in terms of concentration-dependent optical properties like color shade, gloss, and hue, offering applications that may be more adaptable in various fields, such as coatings and sensors.

Fig. 8
figure 8

(a) Photographs of the color change in MA-RB under an NIR lamp without the presence of cold pigment, (b) Photographs of the color change in MA-RB under an NIR lamp with the inclusion of cold pigment.

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Conclusions and outlook

In this study, a novel strip was fabricated to enable facile determination of the presence of cold pigment without necessitating supplementary tests. Utilizing NIR light illumination, the strip undergoes a color change, indicative of the presence of cold pigment. Conversely, the absence of a color change signifies the absence of cold pigment. This methodology offers a rapid and efficient means of cold pigment detection, obviating the need for additional analytical procedures. Thermochromic coatings were developed by integrating thermochromic pigments into an appropriate binder system. These coatings were categorized into two groups: one without heat-reflective (HC) additives and one with HC additives, enabling the assessment of the thermochromic pigments’ properties both independently and in combination with HC. The results revealed that thermochromic samples containing cold pigments had lower surface temperatures than those without cold pigments. Specifically, the thermochromic coatings with HC additives (lighter tones) showed lower temperatures compared to the darker-toned samples without HC. The study concludes that combining thermochromic systems with cold pigments can serve as an energy-saving solution. In high temperatures, such as during the summer, these coatings reflect solar energy, thereby reducing surface temperatures. Conversely, in colder periods, they absorb solar energy, increasing surface temperatures as the coatings undergo reversible color changes.