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Thermochromic performance of a new temperature sensitive pigment based on rhodamine derivative in both liquid and solid systems
Progress in Organic Coatings 137 (2019) 105280
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Thermochromic performance of a new temperature sensitive pigment based on rhodamine derivative in both liquid and solid systems
T
Zhang Wana, Ji Xiaoqiana, Chen Kunlina, Wang Chaoxiaa, , Sun Shiguob, ⁎
a b
⁎
Key Laboratory of Eco-Textile, Ministry of Education, School of Textiles and clothing, Jiangnan University, Wuxi 214122, China Key Laboratory of Xinjiang Phytomedicine Resource and Utilization, Minstry of Education, School of Pharmacy, Shihezi University, Shihezi 832000, China
ARTICLE INFO
ABSTRACT
Keywords: Thermochromism Turn-on fluorescence Visual color-transformation Rhodamine derivative Thermochromic coating
Rhodamine B octadecylamine (RBO) lipophilic derivative is developed as a new thermochromic pigment in both solid and liquid system with temperature controlled turn-on fluorescence and visible color-transformation performances. In liquid system, RBO exhibits color-transformation from light pink to rose red under illuminant D65 and presents temperature controlled turn-on fluorescence, which is attributed to the atom transmitting to an upper level by the thermal excitation. The log Intensity T fits a linear relationship in the body temperature range of 37 °C to 45 °C. In solid system, RBO displays reversible thermochromic performances from pink to yellow under visible light and transforms from fluorescence pink to colorless under UV light, which is originated from the conjugation structure conversion of RBO by monitoring the structure changes during heating via FT-IR spectra. The thermochromic rate in the repeated heatingcooling cycles is fast, calculated to be 0.17 s−1 (heating) and 0.05 s-1 (cooling), respectively. The RBO possesses a good thermostability up to 380 °C. The solid system RBO can be made into a hand-writable thermochromic pen. The deposition of RBOS on paper is easily performed by manual drawing. The image drawn using the pink RBO pen is visible as a pink coating, and showing a pink-to-colorlessness color change on paper under visible light with temperature increase. This new rhodamine derivative with special thermochromic property of turn-on fluorescence and visible color-transformation performance can be applied in both solid and liquid systems, which is promised as a thermochromic coating to replace the intrusive measurement techniques and play a role in external/internal temperature measurements.
1. Introduction Temperature sensors have attracted much attention for practical applications in diagnostics, [1,2] wearable devices, [3–5] foods preservation [6], security materials [7], and energy-saving materials [8], etc. In particular, thermochromic dyes have advantages as visible temperature sensors due to their distinct color change responding to external stimuli, which is easily detected by naked eyes [9]. Whereas, temperature measurements in microelectromechanical systems, marine research, diagnostics, shipbuilding and aircraft industries etc, need intrusive measurement techniques. Thermochromic dyes can not be observed exactly by naked eyes with temperature variation in such internal temperature measurements areas unless the dyes are exposed or in transparent media. Temperature-sensitive luminescence materials as thermochromic fluorescence sensors are applicable to internal temperature measurements by making use of the dependence between temperature and fluorescence intensities. Compared with intrusive measurement techniques, a thermochromic fluorescence sensor does not affect the temperature field and is particularly advantageous to work in electromagnetically and thermally
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harsh environments, such as electrical power stations, near high power electric transmission lines and remote temperature detection in buildings on fire. [10] Organic dyes, nanotube-based systems, quantum dots, inorganic phosphors and organic-inorganic hybrid materials have been used as thermochromic fluorescence sensors to measure temperature in internal environments [11–13]. Especially, rhodamine dyes possess excellent spectroscopic properties, such as large molar extinction coefficients, high fluorescence quantum yields, visible wavelength excitation, high stability against light, and so on [14–16]. Above all, the rhodamine groups can change their structure from non-fluorenscent spirolactam ring-closed structure to strongly fluorescent ring-open structure at certain conditions. Meanwhile, a vivid color change will emerge during the sensing event, which is an important feature that would facilitate “naked-eyes” detection [17]. It is highly expected to develop a novel thermochromic sensor with color-transformation performance by both fluorescence and naked-eyes detection based on rhodamine derivatives. Yuji Kubo and coworkers developed an emissive color turned nanothermometer by altering the amount of grafted rhodamine B, which is able to be used in water with naked-eyes detected thermo-responsive color change from 5 °C to 65 °C
Corresponding authors. E-mail addresses: [email protected] (C. Wang), [email protected] (S. Sun).
https://doi.org/10.1016/j.porgcoat.2019.105280 Received 24 June 2019; Received in revised form 13 August 2019; Accepted 16 August 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 137 (2019) 105280
W. Zhang, et al.
Scheme 1. Synthesis process of RBO.
[18]. Sung-Hoon Kim’s group prepared a temperature responsive hybrid rhodamine B polymer, whose fluorescence intensity decreased with the increase of temperature, accompanying a color change from pink to purple [19]. The above approaches detected the temperature by calculating intensity relying on fluorescence decay time, which is independent of temperature variation. Heron has developed several intrinsic thermochromic fluorescents based on rhodamine derivatives, which possess temperaturedependent thermochromism, but without fluorescence thermochromic performances. [20,21] In our previous work, ratiometric fluorescence sensitive pigments based on a rhodamine derivative, naked eyes detected thermochromic dye and thermochromic polyester fabrics were prepared. [22–24] To further develop a temperature sensitive pigment that can be not only detected by fluorescence and naked eyes, but can be also applicable in diverse conditions, a rhodamine B octadecylamine (RBO) lipophilic derivative is synthesized as a new rhodamine derivative thermochromic sensor with turn-on fluorescence and visible color-transformation performance, which is workable in both solid and liquid system. This fluorescence RBO thermochromic sensor shows turn-on fluorescence as a function of temperature. Observation of thermochromism is possible by fluorescence imaging and naked-eyes, indicating the RBO is applicable for detecting internal and external temperature. The thermochromic mechanism, structure changes and thermostability of the RBO are analyzed. The condition of color transition of RBO from nonfluorenscent spirolactam ring-closed structure to strongly fluorescent ring-open structure is investigated. Besides, thermochromic performances of RBO are presented, including turn-on fluorescence, thermochromic rate as well as thermochromic reversibility.
5.7 Hz, 4H (C6′, C6″, C2′, C2″)), 6.28 (dd, J = 8.9, 2.6 Hz, 2H (C5′, C5″)), 3.35 (q, J = 7.0 Hz, 8H (C1′-N(CH2CH3)2), C1″-N(CH2CH3)2), 3.15-3.05 (m, 2H (CONCH2CH2(CH2)15CH3)), 1.34-1.18 (m, 30H (CONCH2CH2(CH2)15CH3)), 1.18 (t, J = 7.1 Hz, 12H (C1′-N(CH2CH3)2), C1″-N(CH2CH3)2), 1.05 (s, 2H (CONCH2CH2(CH2)15CH3)), 0.89 (t, J = 6.8 Hz, 3H (CONCH2CH2(CH2)15CH3)). HRMS Calcd for C46H67N3O2 694.04. Found 694.05. The synthesis process of the RBO was shown in Scheme 1. 2.3. Preparation of thermochromic RBO systems The thermochromic RBO system in solid system (RBOS) was prepared by stirring 22 mg of RBO, 22 mg of bisphenol A and 257.6 mg of tetradecanol at 60 °C for 1 h. After cooling down, RBOS was obtained with pink color. The thermochromic RBO system in liquid system (RBOL) was prepared by adding RBO (5 × 10−4 M) into methanolwater solution (Vmethanol/Vwater = 1/4), then the pH was adjusted by hydrochloric acid (RBOL). 2.4. Characterization and measurement UV-vis spectra of RBOL was examined by a UV–vis Spectrophotometer (Cary 50, Varian) from 35 °C to 45 °C in the wavelength range of 500–600 nm. The photoluminescence (PL) excitation and emission spectra of RBOL were recorded from 35 °C to 45 °C with a fluorescence spectrophotometer (F-7000; Hitachi, Japan). The structures of RBOS during heating were characterized using a NICOLET is10 transform infrared instrument (Thermo Fisher Scientific, Co. Ltd, China). Measurements were carried out within the wavenumber range of 400-4000 cm−1. Thermal analysis of the colorant was carried out with a differential scanning calorimeter (DSC, Q200, TA Instruments, New Castle, USA). The colorant (less than 10 mg) was placed in an hermetically sealed aluminium crucible and scanning was done from 0 to 80 °C at a heating rate of 2 °C min−1 under 50 ml min−1 flowing nitrogen gas, with an empty pan as reference. Pictures were taken at an interval of 25 s to analyze the color parameters (L*, a*, b*) using adobe Photoshop CS. The chromatism (△E*) was used to express the color change between the tested sample and the initial sample. Color difference standards of samples were calculated by the following formula:
2. Experimental section 2.1. Materials Rhodamine B of analytical pure grade was obtained from Aladdin Industrial Corporation. Octadecylamine (98%) was obtained from Tianjing Guangfu fine chemical research institute. Bisphenol A, hydrochloric acid, acetone, ethyl acetate and tetradecanol of laboratory grade were provided from Sinopharm Chemical Reagent Co., Ltd. Methanol was from Tianjing Damao chemical research institute. Methylbenzene and dichloromethane were available from Beijing Chemical Works. All chemicals were used without further purification.
E=
( a*)2 + ( b*)2 + ( c*)2
(1)
where, a*, b*, L* are the color coordinates and Δa*, Δb*, ΔL* are the color coordinates contrast. The coordinates of L* represent the lightness of the color (L*, L* = 0 yields black and L* = 100 indicates white). The position of a* is between red/magenta and green (a*, negative values indicate green, while positive values indicate magenta). The position of b* is between yellow and blue (b*, negative values indicate blue while positive values indicate yellow). ΔE is the chromatism of samples between the tested temperature and the initial temperature.
2.2. Synthesis of rhodamine B octadecylamine lipophilic derivative (RBO) The mixture of 1.083 g (2.26 mmol) rhodamine B and 1.173 g (4.35 mmol) octadecylamine was dissolved in 5 ml toluene and stirred at 80 °C for 120 h under argon atmosphere, followed by filtration and evaporation of solvent. The orange residue was dissolved in dichloromethane and purified by column chromatography (silica gel 60, methanol/dichloromethane 1/19, volume fraction of methanol, ϕ = 5%). Pale orange oil, a neutral form of rhodamine B octadecylamide, was obtained (72% yield). IR spectrum (KBr) ν (cm−1): 1690s (st O = C–N), 1220 m, 1200 m (st C-O-C), 1360 m (δ C–N). 1H NMR (400 MHz,CDCl3) δ(ppm): 7.94-7.88 (m, 1H (C2)), 7.46-7.39 (m, 2H (C3, C4)), 7.12-7.05 (m, 1H (C5)), 6.42 (dd, J = 18.8,
3. Results and discussion 3.1. Color-Transformation mechanism To investigate the color-transformation mechanism, the color 2
Progress in Organic Coatings 137 (2019) 105280
W. Zhang, et al.
Fig. 1. (a) Color images of RBO in several solvents; (b) Absorption spectra of RBO at different pH values; (c) Optimized geomentry of RBO and RBO+.
properties of RBO (the concentration of RBO is 10−4 M) in several solvents were observed. In neutral polar and non-polar solvents such as methanol, acetone, ethyl acetate, methylene chloride and petroleum ether, RBO is dissolved and colorless (Fig. 1a), indicating the solvent polarity does not influence the color rendering and dissolution of RBO and these compounds substantially exist as colorless lactone structures in neutral solvents. In a hydrochloric acid solution (the solution pH is 2), the color of the solution turns to pink because the ring-opened lactone unit transforms into conjugated zwitterionic species. When sodium hydroxide solution is dropped into RBO methanol solution (the solution pH is 8), the solution turns to be turbid due to the separation of RBO from the solvent. [25] RBO is further dissolved in methanol-water solution (Vmethanol/Vwater = 1/4) with HCl and NaOH aqueous solution as pH adjuster to investigate the color properties at different pH values. From Fig. 1b, two absorption bands at 510 nm and 548 nm in the visible region are observed, which are attributed to the splitting of the excited singlet state of RBO, caused by aggregation of cationic RBO. The peak at 510 nm is assigned to the aggregation of opened form molecules and the peak at 548 nm is mainly the result of the absorption of monomers. [26,27] Moreover, a distinct decline of absorption intensity is observed without absorption position changes, indicating the cationic RBO is converting to the neutral form as the pH increases from 1 to 7. The structure change of RBO from pH 1 to 7 is shown in Fig. 1c. When the pH of the solution increases to 7, no absorption is presented, which means RBO is colorless in the status of the lactone structure. As a result, RBO displays pink color if it is on the ring-opened lactone structure, otherwise RBO is colorless in the status of the lactone structure.
photographs of the color change of RBOL under D65 light are also utilized to investigate the visible thermochromic performance of RBOL, as shown in Fig. 2c and Fig. 2e. As the temperature increases from 35 °C to 45 °C, the color of RBOL changes from light pink to rose red, and the absorption increases gradually at 548 nm without changing the absorbance position (Fig. 2c). The change of absorbance is increased, which is consistent with the change trend of fluorescence intensity. The phenomenon of turn-on fluorescence at 577 nm and the increased absorption at 548 nm are attributed to the atom transmitting to an upper level by the thermal excitation. [28] Hence, RBOL displays thermochromic performance under visible light and UV light, showing the potential to measure the temperature of internal objects by evaluating the fluorescence intensity. To describe the color change more accurately, the colors 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 spectrum. [29] 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¯ ( ) , y( ) , z¯ ( ) ) are required. The degree of simulation required to match the color of the given spectral power density (P(λ)) can be obtained using the below three functions. [30]
3.2. Thermochromic performance in liquid system The thermochromic performance of RBOL (the concentration of RBO in methanol-water mixture (Vmethanol/Vwater = 1/4) is 10−4 M, pH = 1) was investigated via testing the fluorescence intensity and UV–vis absorbance as a function of temperature. As it can be seen in Fig. 2a, the fluorescence intensity at 577 nm increases from 35 °C to 45 °C without change of the position of the emission peak, meaning that only the fluorescence intensity changes and the hue is unaffected, which is consistent with the photos taken under UV light shown in Fig. 2e. A linear calibration curve between the value of log Intensity T and temperature can be established (Fig. 2b). The log 2 Intensity T fits in a reasonably linear relationship (R = 0.9934) with a temperature range from 37 °C to 45 °C, and good accuracy and repeatability are obtained during sample detection. UV–vis absorption spectra and
X=
x¯ ( )p( )d
(2)
Y=
y( )p( )d
(3)
Z=
z¯ ( )p( )d
(4)
where, X, Y and Z are the tristimulus values called artificial colors, giving the simulation for each one of the three primary colors (blue, green and red) to match the color of a given spectral power density (P(λ)). When the color is examined under the illuminant D65 with a 10° standard observer, the X, Y and Z are calculated as following,
L*=116
a*=500
3
y y0
1 3
x x0
16 1 3
(5)
y y0
1 3
(6)
Progress in Organic Coatings 137 (2019) 105280
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Fig. 2. (a) Fluorescence spectra of RBOL from 35 °C to 45 °C. (b) a calibration curve between the log Intensity T and temperature in range of 37 °C to 45 °C. (c) UV–vis absorbance spectra of RBOL from 35 °C to 45 °C. (d) CIE 1931 chromaticity diagram of RBOL from 35 °C to 45 °C. (e) photographs of the color changing of RBOL under D65 light and UV light of 365 nm.
b*=200
y y0
1 3
z z0
3.3. Thermochromic performance in solid
1 3
(7)
RBO in solid system also exhibits thermochromic properties, endowing a broad application prospect. The thermochromic performance of RBOS under illuminant D65 and 365 nm UV light from 25 °C to 50 °C were tested and the results are shown in Fig. 3a. Under illuminant D65, the RBOS presents pink color at 25 °C, and turns to be yellow liquid at 45 °C. Under 365 nm UV light, the RBOS displays fluorescence pink at 25 °C and converts to liquid without fluorescence at 45 °C. The color of RBOS under illuminant D65 and 365 nm UV light remains unchanged with further temperature increasing from 45 °C to 50 °C, which can be explained by the thermochromic mechanism of RBOS. The color changing of RBOS is attributed to the conjugation structure conversion, which is connected with the reversible phase transitions of tetradecanol in the temperature range from 25 °C to 45 °C. When the temperature reaches the melting point (36 °C), tetradecanol undergoes a phase transition from solid to liquid. Meanwhile, the color of RBOS changes because the intermolecular interactions between the RBO and bisphenol A dissociate during tetradecanol melting. Inversely, when the
where, X0 is 94.825, Y0 is 100.00 and Z0 is 107.381. The x, y chromaticity coordinates of the samples can be further estimated from the tristimulus values, as follows:
X=
x x+ y+z
(8)
Y=
y x+ y+z
(9)
According to formula (8) and (9), it is calculated that the chromaticity coordinates of the RBOL at 35 °C are x = 0.33 and y = 0.31 in CIE 1931 chromaticity diagram (Fig. 2d), while the coordinates of the RBOL at 45 °C are x = 0.37 and y = 0.23. The chromatic changes from 35 °C to 45 °C in CIE 1931 diagram are consistent with the results of UV–vis absorption spectra in Fig. 2c. 4
Progress in Organic Coatings 137 (2019) 105280
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Fig. 3. (a) Photographs of the color changing of RBOs under D65 light and UV light of 365 nm. (b) CIE 1931 chromaticity diagram of RBOS between the temperature of 25 °C and 45 °C. (c) The thermochromism of RBOS during heating at 45 °C and cooling at 25 °C.
color transformation is much slower than the heating process, which is calculated to be average of 0.05 s−1. This may be associated with the octadecyl chain of RBO. The long branched chain hinders the forming of intermolecular interactions between the RBO and bisphenol A during the solid transitions of tetradecanol on cooling. To further demonstrate the thermochromic mechanism, the thermochromic structure of the RBOS was detected in the range of 25 °C to 45 °C recorded by FT-IR spectra, as displayed in Fig. 4. The peak of acidic carbonyl (−COOH) vibration shifts from 1666 cm−1 (25 °C) to 1674 cm−1 (45 °C), which is ascribed to the vibration of the closed lactone carbonyl group in the molten state. This phenomenon indicates that the quinoid structure of RBO shifts to lactone ring structure during heating, [33–35] which is in accordance with the color change principle in Fig. 1. The peak at 1548 cm-1 is in the overlapping region of asymmetric vibration of COOand the C]N vibration of RBO, indicating the lactone ring of RBO is opened and the quinoid structure forms at low temperature (25 °C). The peak of 1510 cm-1 is due to the CeC stretching in the aromatic ring of bisphenol A. All the phenomena indicate that RBO changes from a quinoid structure to a lactone ring from 25 °C to 45 °C, which leads to the color of RBO gradually changes from pink to yellow. Additionally, the peaks at 2970 cm-1, 2930 cm-1 and 2860 cm-1 are attributed to the vibrations of methyl and methylene groups in the RBOS thermochromic system. RBOS possesses the ability of coloring, and can be applied as a pen. RBOS was melted and poured into a pen mold. After cooling to room temperature, the “RBO pen” was obtained. The pink hand-writable RBO pen is shown in Fig. 5. The feasibility of writing on solid substrate using RBOS was investigated. The results show that the deposition of RBOS on paper is easily performed by manual drawing. The image drawn using the pink RBO pen is visible as a pink coating under visible light. To investigate the thermochromic property, a paper with the pink coating drawn by RBOS was stacked to the outer wall of a water filled beaker with a heating or cooling rate of 2 °C/min. The thermochromic process of the paper relative with the temperature was photographed. The color depth of the coating decreases gradually, leading to a pink-to-colorlessness color change when the paper is heated up to 50 °C. The notable color change from pink to colorlessness is attributed to the dissociation of intermolecular interactions between the RBO and bisphenol A owing to the melting of the tetradecanol in RBOS. Furthermore, it is found that the
Fig. 4. Thermochromic mechanism detected by FT-IR spectra in the range of 25 °C–45 °C.
temperature decreases to the freezing point (34 °C), tetradecanol converts to a solid system and the RBOS reverts to the original color because a proportion of the RBO becomes protonated during interaction with bisphenol A. [31,32] The color change of RBOS under illuminant D65 is also presented in Fig. 3b. It is calculated that in the CIE 1931 chromaticity diagram, the chromaticity coordinates of the RBOS at 25 °C are x = 0.45 and y = 0.33 and x = 0.39 and y = 0.41 at 45 °C. The thermochromic chromatism of RBOS was further monitored in the heating and cooling processes, as shown in Fig. 3c. On heating, the chromatism of RBOS gradually increases as the function of heating time. When RBOS is heated at 45 °C for 387 s, the total chromatism reaches 65.8, presenting a distinct color change. The thermochromic rate is calculated to be average of 0.17 s−1 according to (10), as following:
v=
E t
(10)
where, △E is the chromatism between the tested temperature and the initial temperature. t is the total chromatism time. In the case of the cooling process, the RBOS is cooled under 25 °C to test the rate of the reversible color change. The rate of the reversible 5
Progress in Organic Coatings 137 (2019) 105280
W. Zhang, et al.
Fig. 5. The visualization process of the handwritten RBO image on paper.
pink RBO appears again under 254 nm UV light and still remains after removing the UV light. The visualization and reversible thermochromic processes of the handwritten RBO coating image are promised to be applied in the field of privacy. RBOS images could also be generated on other substrates such as cotton fabric, filter paper, bank note and so on.
were evaluated from 100 to 600 °C by thermogravimetry in nitrogen atmosphere, the results of which are shown in Fig. 7. It can be observed that these three kinds of materials have different degradation processes. Among which, the TG curve of rhodanmine B has three degradation stages, which appeared in the range of 200-300 °C, 300-450 °C and 450600 °C, respectively. The TG curve of octadecylamine possesses two weight loss stages and appears at the range of 150–270 °C and 270–360 °C. Compared to rhodanmine B and octadecylamine, the RBO presents the best thermal stability and only has one degradation stage. The onset of thermal degradation of RBO is around 380 °C and the fastest thermal degradation presents at 445 °C. The thermal decomposition is probably attributed to the thermal debonding of alkyl chains. According to the above analysis, it can be concluded that RBO possesses excellent thermal stability until 380 °C.
3.4. Thermochromic reversibility The transition temperature and the thermal stability of the RBOS were tested by DSC. As shown in Fig. 6a, the thermochromic endothermic process starts at 26 °C and ends at 37 °C with the endothermic temperature of 36 °C, indicating the transition temperature of the RBOS is 36 °C. [24] On cooling, the exothermic process presents two peaks. Among which, the peak at 33 °C belongs to liquid-solid transition of the tetradecanol, meaning the RBOS changes from yellow liquid to yellow solid. The peak at 31 °C is the solid-solid transition for forming a crystalline structure of the co-solvent of tetradecanol, [36] indicating the RBOS changes from yellow solid to pink solid. This result illustrates that the thermochromic process of RBOS is reversible. The thermochromic reversibility of the RBOS was further tested by monitoring the changes of the color coordinate a* in 50 heating-cooling cycles. Fig. 6b reveals that RBOS has a high resistance to fatigue by showing the rapid and reproducible reversibility between the heating and cooling processes without significant loss of a* for at least 50 successive heating-cooling cycles. The excellent thermochromism to fatigue is due to the stable endothermic and exothermic process of tetradecanol. During the endothermic process, tetradecanol undergoes a phase transition, leading to the value of color coordinate a* decreasing due to the dissociation of intermolecular interactions between RBO and bisphenol A. [37] Inversely, during the exothermic process, the tetradecanol converts to the solid system with the forming of intermolecular interactions, and the value of color coordinate a* increases.
4. Conclusions Rhodamine B octadecylamine lipophilic derivative (RBO) was developed as a new rhodamine derivative thermochromic pigment with temperature controlled turn-on fluorescence and visible color-transformation performances. The color variation of RBO can be detected by fluorescence imaging and naked-eyes, which was used in both external and internal temperature measurement. In addition, RBO displayed thermochromism in both the solid and the liquid systems, possessing a wide application field. In the liquid system, RBO showed a turn-on fluorescence as a function of temperature. The log Intensity T fitted a reasonably linear relationship with a R2 = 0.9934 in the temperature range from 37 °C to 45 °C, which can be utilized to calculate internal temperature by the fluorescence intensity and to replace the intrusive measurement techniques. In the meantime, the RBO displayed a nakedeye observation color-transformation from light pink to rose red and the chromatic in CIE 1931 diagram changes from (0.33, 0.31) to (0.37, 0.23) with temperature increase. In the solid system, RBO depicted reversible thermochromic performance for more than 50 times, which in CIE 1931 diagram changed from (0.45, 0.33) to (0.39, 0.41) as temperature increases. The thermochromic rate was calculated to be
3.5. Thermal stability The thermal stabilities of RBO, rhodanmine B and octadecylamine
Fig. 6. (a) The color transition temperatures of the RBOS during heating and cooling; (b) Reversible thermochromic heating - cooling cycles of the RBOS represented by color coordinate a*. 6
Progress in Organic Coatings 137 (2019) 105280
W. Zhang, et al.
Fig. 7. The thermal stability tested by TGA (a) and DTG (b).
0.17 s−1, and the rate of reversible thermochromism was calculated to be 0.05 s-1, showing excellent sensitivity. The RBOS made hand-writable thermochromic pen displayed an obvious color change from pink to colorlessness on paper when raising the temperature. As a result, this new rhodamine derivative shows special thermochromism. It is promising to replace the intrusive measurement techniques and to be applied as coating or pigments in external temperature measurements as well as the privacy field.
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Acknowledgments The authors are grateful for the financial support of the National Natural Science Foundation of China (< GN1 > 21174055). Postgraduate Research & Practice Innovation Program of Jiangsu Provence (< GN2 > KYCX17_1448). The Excellent Doctoral Cultivation Project of Jiangnan University. A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. The Fundamental Research Funds for the Central Universities (JUSRP51724B). Financial support from the program of China Scholarships Council (No. 201806790050). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.porgcoat.2019.105280. References [1] A.C. Siegel, S.T. Phillips, B.J. Wiley, G.M. Whitesides, Thin, lightweight, foldable thermochromic displays on paper, Lab. Chip. 9 (2009) 2775–2781. [2] S. Sia, Cutting edge: thin, lightweight, foldable thermochromic displays on paper, Lab. Chip. 9 (2009) 2763-2763. [3] C. Yu, Y. Zhang, D. Cheng, X. Li, Y. Huang, J.A. Rogers, All-elastomeric, strainresponsive thermochromic color indicators, Small 10 (2014) 1266–1271. [4] M. Gao, L. Li, Y. Song, Inkjet printing wearable electronic devices, J. Mater. Chem. C. 5 (2017) 2971–2993. [5] T.Q. Trung, S. Ramasundaram, B.U. Hwang, N.E. Lee, An all-elastomeric transparent and stretchable temperature sensor for body-attachable wearable electronics, Adv. Mater. 28 (2016) 502–509. [6] D. Galliani, L. Mascheroni, M. Sassi, R. Turrisi, R. Lorenzi, A. Scaccabarozzi, N. Stingelin, L. Beverina, Thermochromic latent-pigment-based time-temperature indicators for perishable goods, Adv. Opt. Mater. 3 (2015) 1164–1168. [7] G. Monkman, Secure electronic tagging, Assem. Autom. 20 (2000) 24–27. [8] M. Miodownik, The time for thermochromics, Mater. Today. 11 (2008) 6–10. [9] D. Kitagawa, K. Tanaka, S. Kobatake, Thiophene-S, S-dioxidized diarylethenes for light-starting irreversible thermosensors that can detect a rise in heat at low temperature, J. Mater. Chem. C. 5 (2017) 6210–6215. [10] W. Xu, X. Gao, L. Zheng, Z. Zhang, W. Cao, An optical temperature sensor based on the upconversion luminescence from Tm3+/Yb3+ codoped oxyfluoride glass ceramic, Sens. Actuator B-Chem. 173 (2012) 250–253. [11] J. Feng, L. Xiong, S. Wang, S. Li, Y. Li, G. Yang, Fluorescent temperature sensing using triarylboron compounds and microcapsules for detection of a wide temperature range on the micro- and macroscale, Adv. Funct. Mater. 23 (2013)
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Thermochromic performance of a new temperature sensitive pigment based on rhodamine derivative in both liquid and solid systems
T
Zhang Wana, Ji Xiaoqiana, Chen Kunlina, Wang Chaoxiaa, , Sun Shiguob, ⁎
a b
⁎
Key Laboratory of Eco-Textile, Ministry of Education, School of Textiles and clothing, Jiangnan University, Wuxi 214122, China Key Laboratory of Xinjiang Phytomedicine Resource and Utilization, Minstry of Education, School of Pharmacy, Shihezi University, Shihezi 832000, China
ARTICLE INFO
ABSTRACT
Keywords: Thermochromism Turn-on fluorescence Visual color-transformation Rhodamine derivative Thermochromic coating
Rhodamine B octadecylamine (RBO) lipophilic derivative is developed as a new thermochromic pigment in both solid and liquid system with temperature controlled turn-on fluorescence and visible color-transformation performances. In liquid system, RBO exhibits color-transformation from light pink to rose red under illuminant D65 and presents temperature controlled turn-on fluorescence, which is attributed to the atom transmitting to an upper level by the thermal excitation. The log Intensity T fits a linear relationship in the body temperature range of 37 °C to 45 °C. In solid system, RBO displays reversible thermochromic performances from pink to yellow under visible light and transforms from fluorescence pink to colorless under UV light, which is originated from the conjugation structure conversion of RBO by monitoring the structure changes during heating via FT-IR spectra. The thermochromic rate in the repeated heatingcooling cycles is fast, calculated to be 0.17 s−1 (heating) and 0.05 s-1 (cooling), respectively. The RBO possesses a good thermostability up to 380 °C. The solid system RBO can be made into a hand-writable thermochromic pen. The deposition of RBOS on paper is easily performed by manual drawing. The image drawn using the pink RBO pen is visible as a pink coating, and showing a pink-to-colorlessness color change on paper under visible light with temperature increase. This new rhodamine derivative with special thermochromic property of turn-on fluorescence and visible color-transformation performance can be applied in both solid and liquid systems, which is promised as a thermochromic coating to replace the intrusive measurement techniques and play a role in external/internal temperature measurements.
1. Introduction Temperature sensors have attracted much attention for practical applications in diagnostics, [1,2] wearable devices, [3–5] foods preservation [6], security materials [7], and energy-saving materials [8], etc. In particular, thermochromic dyes have advantages as visible temperature sensors due to their distinct color change responding to external stimuli, which is easily detected by naked eyes [9]. Whereas, temperature measurements in microelectromechanical systems, marine research, diagnostics, shipbuilding and aircraft industries etc, need intrusive measurement techniques. Thermochromic dyes can not be observed exactly by naked eyes with temperature variation in such internal temperature measurements areas unless the dyes are exposed or in transparent media. Temperature-sensitive luminescence materials as thermochromic fluorescence sensors are applicable to internal temperature measurements by making use of the dependence between temperature and fluorescence intensities. Compared with intrusive measurement techniques, a thermochromic fluorescence sensor does not affect the temperature field and is particularly advantageous to work in electromagnetically and thermally
⁎
harsh environments, such as electrical power stations, near high power electric transmission lines and remote temperature detection in buildings on fire. [10] Organic dyes, nanotube-based systems, quantum dots, inorganic phosphors and organic-inorganic hybrid materials have been used as thermochromic fluorescence sensors to measure temperature in internal environments [11–13]. Especially, rhodamine dyes possess excellent spectroscopic properties, such as large molar extinction coefficients, high fluorescence quantum yields, visible wavelength excitation, high stability against light, and so on [14–16]. Above all, the rhodamine groups can change their structure from non-fluorenscent spirolactam ring-closed structure to strongly fluorescent ring-open structure at certain conditions. Meanwhile, a vivid color change will emerge during the sensing event, which is an important feature that would facilitate “naked-eyes” detection [17]. It is highly expected to develop a novel thermochromic sensor with color-transformation performance by both fluorescence and naked-eyes detection based on rhodamine derivatives. Yuji Kubo and coworkers developed an emissive color turned nanothermometer by altering the amount of grafted rhodamine B, which is able to be used in water with naked-eyes detected thermo-responsive color change from 5 °C to 65 °C
Corresponding authors. E-mail addresses: [email protected] (C. Wang), [email protected] (S. Sun).
https://doi.org/10.1016/j.porgcoat.2019.105280 Received 24 June 2019; Received in revised form 13 August 2019; Accepted 16 August 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 137 (2019) 105280
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Scheme 1. Synthesis process of RBO.
[18]. Sung-Hoon Kim’s group prepared a temperature responsive hybrid rhodamine B polymer, whose fluorescence intensity decreased with the increase of temperature, accompanying a color change from pink to purple [19]. The above approaches detected the temperature by calculating intensity relying on fluorescence decay time, which is independent of temperature variation. Heron has developed several intrinsic thermochromic fluorescents based on rhodamine derivatives, which possess temperaturedependent thermochromism, but without fluorescence thermochromic performances. [20,21] In our previous work, ratiometric fluorescence sensitive pigments based on a rhodamine derivative, naked eyes detected thermochromic dye and thermochromic polyester fabrics were prepared. [22–24] To further develop a temperature sensitive pigment that can be not only detected by fluorescence and naked eyes, but can be also applicable in diverse conditions, a rhodamine B octadecylamine (RBO) lipophilic derivative is synthesized as a new rhodamine derivative thermochromic sensor with turn-on fluorescence and visible color-transformation performance, which is workable in both solid and liquid system. This fluorescence RBO thermochromic sensor shows turn-on fluorescence as a function of temperature. Observation of thermochromism is possible by fluorescence imaging and naked-eyes, indicating the RBO is applicable for detecting internal and external temperature. The thermochromic mechanism, structure changes and thermostability of the RBO are analyzed. The condition of color transition of RBO from nonfluorenscent spirolactam ring-closed structure to strongly fluorescent ring-open structure is investigated. Besides, thermochromic performances of RBO are presented, including turn-on fluorescence, thermochromic rate as well as thermochromic reversibility.
5.7 Hz, 4H (C6′, C6″, C2′, C2″)), 6.28 (dd, J = 8.9, 2.6 Hz, 2H (C5′, C5″)), 3.35 (q, J = 7.0 Hz, 8H (C1′-N(CH2CH3)2), C1″-N(CH2CH3)2), 3.15-3.05 (m, 2H (CONCH2CH2(CH2)15CH3)), 1.34-1.18 (m, 30H (CONCH2CH2(CH2)15CH3)), 1.18 (t, J = 7.1 Hz, 12H (C1′-N(CH2CH3)2), C1″-N(CH2CH3)2), 1.05 (s, 2H (CONCH2CH2(CH2)15CH3)), 0.89 (t, J = 6.8 Hz, 3H (CONCH2CH2(CH2)15CH3)). HRMS Calcd for C46H67N3O2 694.04. Found 694.05. The synthesis process of the RBO was shown in Scheme 1. 2.3. Preparation of thermochromic RBO systems The thermochromic RBO system in solid system (RBOS) was prepared by stirring 22 mg of RBO, 22 mg of bisphenol A and 257.6 mg of tetradecanol at 60 °C for 1 h. After cooling down, RBOS was obtained with pink color. The thermochromic RBO system in liquid system (RBOL) was prepared by adding RBO (5 × 10−4 M) into methanolwater solution (Vmethanol/Vwater = 1/4), then the pH was adjusted by hydrochloric acid (RBOL). 2.4. Characterization and measurement UV-vis spectra of RBOL was examined by a UV–vis Spectrophotometer (Cary 50, Varian) from 35 °C to 45 °C in the wavelength range of 500–600 nm. The photoluminescence (PL) excitation and emission spectra of RBOL were recorded from 35 °C to 45 °C with a fluorescence spectrophotometer (F-7000; Hitachi, Japan). The structures of RBOS during heating were characterized using a NICOLET is10 transform infrared instrument (Thermo Fisher Scientific, Co. Ltd, China). Measurements were carried out within the wavenumber range of 400-4000 cm−1. Thermal analysis of the colorant was carried out with a differential scanning calorimeter (DSC, Q200, TA Instruments, New Castle, USA). The colorant (less than 10 mg) was placed in an hermetically sealed aluminium crucible and scanning was done from 0 to 80 °C at a heating rate of 2 °C min−1 under 50 ml min−1 flowing nitrogen gas, with an empty pan as reference. Pictures were taken at an interval of 25 s to analyze the color parameters (L*, a*, b*) using adobe Photoshop CS. The chromatism (△E*) was used to express the color change between the tested sample and the initial sample. Color difference standards of samples were calculated by the following formula:
2. Experimental section 2.1. Materials Rhodamine B of analytical pure grade was obtained from Aladdin Industrial Corporation. Octadecylamine (98%) was obtained from Tianjing Guangfu fine chemical research institute. Bisphenol A, hydrochloric acid, acetone, ethyl acetate and tetradecanol of laboratory grade were provided from Sinopharm Chemical Reagent Co., Ltd. Methanol was from Tianjing Damao chemical research institute. Methylbenzene and dichloromethane were available from Beijing Chemical Works. All chemicals were used without further purification.
E=
( a*)2 + ( b*)2 + ( c*)2
(1)
where, a*, b*, L* are the color coordinates and Δa*, Δb*, ΔL* are the color coordinates contrast. The coordinates of L* represent the lightness of the color (L*, L* = 0 yields black and L* = 100 indicates white). The position of a* is between red/magenta and green (a*, negative values indicate green, while positive values indicate magenta). The position of b* is between yellow and blue (b*, negative values indicate blue while positive values indicate yellow). ΔE is the chromatism of samples between the tested temperature and the initial temperature.
2.2. Synthesis of rhodamine B octadecylamine lipophilic derivative (RBO) The mixture of 1.083 g (2.26 mmol) rhodamine B and 1.173 g (4.35 mmol) octadecylamine was dissolved in 5 ml toluene and stirred at 80 °C for 120 h under argon atmosphere, followed by filtration and evaporation of solvent. The orange residue was dissolved in dichloromethane and purified by column chromatography (silica gel 60, methanol/dichloromethane 1/19, volume fraction of methanol, ϕ = 5%). Pale orange oil, a neutral form of rhodamine B octadecylamide, was obtained (72% yield). IR spectrum (KBr) ν (cm−1): 1690s (st O = C–N), 1220 m, 1200 m (st C-O-C), 1360 m (δ C–N). 1H NMR (400 MHz,CDCl3) δ(ppm): 7.94-7.88 (m, 1H (C2)), 7.46-7.39 (m, 2H (C3, C4)), 7.12-7.05 (m, 1H (C5)), 6.42 (dd, J = 18.8,
3. Results and discussion 3.1. Color-Transformation mechanism To investigate the color-transformation mechanism, the color 2
Progress in Organic Coatings 137 (2019) 105280
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Fig. 1. (a) Color images of RBO in several solvents; (b) Absorption spectra of RBO at different pH values; (c) Optimized geomentry of RBO and RBO+.
properties of RBO (the concentration of RBO is 10−4 M) in several solvents were observed. In neutral polar and non-polar solvents such as methanol, acetone, ethyl acetate, methylene chloride and petroleum ether, RBO is dissolved and colorless (Fig. 1a), indicating the solvent polarity does not influence the color rendering and dissolution of RBO and these compounds substantially exist as colorless lactone structures in neutral solvents. In a hydrochloric acid solution (the solution pH is 2), the color of the solution turns to pink because the ring-opened lactone unit transforms into conjugated zwitterionic species. When sodium hydroxide solution is dropped into RBO methanol solution (the solution pH is 8), the solution turns to be turbid due to the separation of RBO from the solvent. [25] RBO is further dissolved in methanol-water solution (Vmethanol/Vwater = 1/4) with HCl and NaOH aqueous solution as pH adjuster to investigate the color properties at different pH values. From Fig. 1b, two absorption bands at 510 nm and 548 nm in the visible region are observed, which are attributed to the splitting of the excited singlet state of RBO, caused by aggregation of cationic RBO. The peak at 510 nm is assigned to the aggregation of opened form molecules and the peak at 548 nm is mainly the result of the absorption of monomers. [26,27] Moreover, a distinct decline of absorption intensity is observed without absorption position changes, indicating the cationic RBO is converting to the neutral form as the pH increases from 1 to 7. The structure change of RBO from pH 1 to 7 is shown in Fig. 1c. When the pH of the solution increases to 7, no absorption is presented, which means RBO is colorless in the status of the lactone structure. As a result, RBO displays pink color if it is on the ring-opened lactone structure, otherwise RBO is colorless in the status of the lactone structure.
photographs of the color change of RBOL under D65 light are also utilized to investigate the visible thermochromic performance of RBOL, as shown in Fig. 2c and Fig. 2e. As the temperature increases from 35 °C to 45 °C, the color of RBOL changes from light pink to rose red, and the absorption increases gradually at 548 nm without changing the absorbance position (Fig. 2c). The change of absorbance is increased, which is consistent with the change trend of fluorescence intensity. The phenomenon of turn-on fluorescence at 577 nm and the increased absorption at 548 nm are attributed to the atom transmitting to an upper level by the thermal excitation. [28] Hence, RBOL displays thermochromic performance under visible light and UV light, showing the potential to measure the temperature of internal objects by evaluating the fluorescence intensity. To describe the color change more accurately, the colors 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 spectrum. [29] 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¯ ( ) , y( ) , z¯ ( ) ) are required. The degree of simulation required to match the color of the given spectral power density (P(λ)) can be obtained using the below three functions. [30]
3.2. Thermochromic performance in liquid system The thermochromic performance of RBOL (the concentration of RBO in methanol-water mixture (Vmethanol/Vwater = 1/4) is 10−4 M, pH = 1) was investigated via testing the fluorescence intensity and UV–vis absorbance as a function of temperature. As it can be seen in Fig. 2a, the fluorescence intensity at 577 nm increases from 35 °C to 45 °C without change of the position of the emission peak, meaning that only the fluorescence intensity changes and the hue is unaffected, which is consistent with the photos taken under UV light shown in Fig. 2e. A linear calibration curve between the value of log Intensity T and temperature can be established (Fig. 2b). The log 2 Intensity T fits in a reasonably linear relationship (R = 0.9934) with a temperature range from 37 °C to 45 °C, and good accuracy and repeatability are obtained during sample detection. UV–vis absorption spectra and
X=
x¯ ( )p( )d
(2)
Y=
y( )p( )d
(3)
Z=
z¯ ( )p( )d
(4)
where, X, Y and Z are the tristimulus values called artificial colors, giving the simulation for each one of the three primary colors (blue, green and red) to match the color of a given spectral power density (P(λ)). When the color is examined under the illuminant D65 with a 10° standard observer, the X, Y and Z are calculated as following,
L*=116
a*=500
3
y y0
1 3
x x0
16 1 3
(5)
y y0
1 3
(6)
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Fig. 2. (a) Fluorescence spectra of RBOL from 35 °C to 45 °C. (b) a calibration curve between the log Intensity T and temperature in range of 37 °C to 45 °C. (c) UV–vis absorbance spectra of RBOL from 35 °C to 45 °C. (d) CIE 1931 chromaticity diagram of RBOL from 35 °C to 45 °C. (e) photographs of the color changing of RBOL under D65 light and UV light of 365 nm.
b*=200
y y0
1 3
z z0
3.3. Thermochromic performance in solid
1 3
(7)
RBO in solid system also exhibits thermochromic properties, endowing a broad application prospect. The thermochromic performance of RBOS under illuminant D65 and 365 nm UV light from 25 °C to 50 °C were tested and the results are shown in Fig. 3a. Under illuminant D65, the RBOS presents pink color at 25 °C, and turns to be yellow liquid at 45 °C. Under 365 nm UV light, the RBOS displays fluorescence pink at 25 °C and converts to liquid without fluorescence at 45 °C. The color of RBOS under illuminant D65 and 365 nm UV light remains unchanged with further temperature increasing from 45 °C to 50 °C, which can be explained by the thermochromic mechanism of RBOS. The color changing of RBOS is attributed to the conjugation structure conversion, which is connected with the reversible phase transitions of tetradecanol in the temperature range from 25 °C to 45 °C. When the temperature reaches the melting point (36 °C), tetradecanol undergoes a phase transition from solid to liquid. Meanwhile, the color of RBOS changes because the intermolecular interactions between the RBO and bisphenol A dissociate during tetradecanol melting. Inversely, when the
where, X0 is 94.825, Y0 is 100.00 and Z0 is 107.381. The x, y chromaticity coordinates of the samples can be further estimated from the tristimulus values, as follows:
X=
x x+ y+z
(8)
Y=
y x+ y+z
(9)
According to formula (8) and (9), it is calculated that the chromaticity coordinates of the RBOL at 35 °C are x = 0.33 and y = 0.31 in CIE 1931 chromaticity diagram (Fig. 2d), while the coordinates of the RBOL at 45 °C are x = 0.37 and y = 0.23. The chromatic changes from 35 °C to 45 °C in CIE 1931 diagram are consistent with the results of UV–vis absorption spectra in Fig. 2c. 4
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Fig. 3. (a) Photographs of the color changing of RBOs under D65 light and UV light of 365 nm. (b) CIE 1931 chromaticity diagram of RBOS between the temperature of 25 °C and 45 °C. (c) The thermochromism of RBOS during heating at 45 °C and cooling at 25 °C.
color transformation is much slower than the heating process, which is calculated to be average of 0.05 s−1. This may be associated with the octadecyl chain of RBO. The long branched chain hinders the forming of intermolecular interactions between the RBO and bisphenol A during the solid transitions of tetradecanol on cooling. To further demonstrate the thermochromic mechanism, the thermochromic structure of the RBOS was detected in the range of 25 °C to 45 °C recorded by FT-IR spectra, as displayed in Fig. 4. The peak of acidic carbonyl (−COOH) vibration shifts from 1666 cm−1 (25 °C) to 1674 cm−1 (45 °C), which is ascribed to the vibration of the closed lactone carbonyl group in the molten state. This phenomenon indicates that the quinoid structure of RBO shifts to lactone ring structure during heating, [33–35] which is in accordance with the color change principle in Fig. 1. The peak at 1548 cm-1 is in the overlapping region of asymmetric vibration of COOand the C]N vibration of RBO, indicating the lactone ring of RBO is opened and the quinoid structure forms at low temperature (25 °C). The peak of 1510 cm-1 is due to the CeC stretching in the aromatic ring of bisphenol A. All the phenomena indicate that RBO changes from a quinoid structure to a lactone ring from 25 °C to 45 °C, which leads to the color of RBO gradually changes from pink to yellow. Additionally, the peaks at 2970 cm-1, 2930 cm-1 and 2860 cm-1 are attributed to the vibrations of methyl and methylene groups in the RBOS thermochromic system. RBOS possesses the ability of coloring, and can be applied as a pen. RBOS was melted and poured into a pen mold. After cooling to room temperature, the “RBO pen” was obtained. The pink hand-writable RBO pen is shown in Fig. 5. The feasibility of writing on solid substrate using RBOS was investigated. The results show that the deposition of RBOS on paper is easily performed by manual drawing. The image drawn using the pink RBO pen is visible as a pink coating under visible light. To investigate the thermochromic property, a paper with the pink coating drawn by RBOS was stacked to the outer wall of a water filled beaker with a heating or cooling rate of 2 °C/min. The thermochromic process of the paper relative with the temperature was photographed. The color depth of the coating decreases gradually, leading to a pink-to-colorlessness color change when the paper is heated up to 50 °C. The notable color change from pink to colorlessness is attributed to the dissociation of intermolecular interactions between the RBO and bisphenol A owing to the melting of the tetradecanol in RBOS. Furthermore, it is found that the
Fig. 4. Thermochromic mechanism detected by FT-IR spectra in the range of 25 °C–45 °C.
temperature decreases to the freezing point (34 °C), tetradecanol converts to a solid system and the RBOS reverts to the original color because a proportion of the RBO becomes protonated during interaction with bisphenol A. [31,32] The color change of RBOS under illuminant D65 is also presented in Fig. 3b. It is calculated that in the CIE 1931 chromaticity diagram, the chromaticity coordinates of the RBOS at 25 °C are x = 0.45 and y = 0.33 and x = 0.39 and y = 0.41 at 45 °C. The thermochromic chromatism of RBOS was further monitored in the heating and cooling processes, as shown in Fig. 3c. On heating, the chromatism of RBOS gradually increases as the function of heating time. When RBOS is heated at 45 °C for 387 s, the total chromatism reaches 65.8, presenting a distinct color change. The thermochromic rate is calculated to be average of 0.17 s−1 according to (10), as following:
v=
E t
(10)
where, △E is the chromatism between the tested temperature and the initial temperature. t is the total chromatism time. In the case of the cooling process, the RBOS is cooled under 25 °C to test the rate of the reversible color change. The rate of the reversible 5
Progress in Organic Coatings 137 (2019) 105280
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Fig. 5. The visualization process of the handwritten RBO image on paper.
pink RBO appears again under 254 nm UV light and still remains after removing the UV light. The visualization and reversible thermochromic processes of the handwritten RBO coating image are promised to be applied in the field of privacy. RBOS images could also be generated on other substrates such as cotton fabric, filter paper, bank note and so on.
were evaluated from 100 to 600 °C by thermogravimetry in nitrogen atmosphere, the results of which are shown in Fig. 7. It can be observed that these three kinds of materials have different degradation processes. Among which, the TG curve of rhodanmine B has three degradation stages, which appeared in the range of 200-300 °C, 300-450 °C and 450600 °C, respectively. The TG curve of octadecylamine possesses two weight loss stages and appears at the range of 150–270 °C and 270–360 °C. Compared to rhodanmine B and octadecylamine, the RBO presents the best thermal stability and only has one degradation stage. The onset of thermal degradation of RBO is around 380 °C and the fastest thermal degradation presents at 445 °C. The thermal decomposition is probably attributed to the thermal debonding of alkyl chains. According to the above analysis, it can be concluded that RBO possesses excellent thermal stability until 380 °C.
3.4. Thermochromic reversibility The transition temperature and the thermal stability of the RBOS were tested by DSC. As shown in Fig. 6a, the thermochromic endothermic process starts at 26 °C and ends at 37 °C with the endothermic temperature of 36 °C, indicating the transition temperature of the RBOS is 36 °C. [24] On cooling, the exothermic process presents two peaks. Among which, the peak at 33 °C belongs to liquid-solid transition of the tetradecanol, meaning the RBOS changes from yellow liquid to yellow solid. The peak at 31 °C is the solid-solid transition for forming a crystalline structure of the co-solvent of tetradecanol, [36] indicating the RBOS changes from yellow solid to pink solid. This result illustrates that the thermochromic process of RBOS is reversible. The thermochromic reversibility of the RBOS was further tested by monitoring the changes of the color coordinate a* in 50 heating-cooling cycles. Fig. 6b reveals that RBOS has a high resistance to fatigue by showing the rapid and reproducible reversibility between the heating and cooling processes without significant loss of a* for at least 50 successive heating-cooling cycles. The excellent thermochromism to fatigue is due to the stable endothermic and exothermic process of tetradecanol. During the endothermic process, tetradecanol undergoes a phase transition, leading to the value of color coordinate a* decreasing due to the dissociation of intermolecular interactions between RBO and bisphenol A. [37] Inversely, during the exothermic process, the tetradecanol converts to the solid system with the forming of intermolecular interactions, and the value of color coordinate a* increases.
4. Conclusions Rhodamine B octadecylamine lipophilic derivative (RBO) was developed as a new rhodamine derivative thermochromic pigment with temperature controlled turn-on fluorescence and visible color-transformation performances. The color variation of RBO can be detected by fluorescence imaging and naked-eyes, which was used in both external and internal temperature measurement. In addition, RBO displayed thermochromism in both the solid and the liquid systems, possessing a wide application field. In the liquid system, RBO showed a turn-on fluorescence as a function of temperature. The log Intensity T fitted a reasonably linear relationship with a R2 = 0.9934 in the temperature range from 37 °C to 45 °C, which can be utilized to calculate internal temperature by the fluorescence intensity and to replace the intrusive measurement techniques. In the meantime, the RBO displayed a nakedeye observation color-transformation from light pink to rose red and the chromatic in CIE 1931 diagram changes from (0.33, 0.31) to (0.37, 0.23) with temperature increase. In the solid system, RBO depicted reversible thermochromic performance for more than 50 times, which in CIE 1931 diagram changed from (0.45, 0.33) to (0.39, 0.41) as temperature increases. The thermochromic rate was calculated to be
3.5. Thermal stability The thermal stabilities of RBO, rhodanmine B and octadecylamine
Fig. 6. (a) The color transition temperatures of the RBOS during heating and cooling; (b) Reversible thermochromic heating - cooling cycles of the RBOS represented by color coordinate a*. 6
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Fig. 7. The thermal stability tested by TGA (a) and DTG (b).
0.17 s−1, and the rate of reversible thermochromism was calculated to be 0.05 s-1, showing excellent sensitivity. The RBOS made hand-writable thermochromic pen displayed an obvious color change from pink to colorlessness on paper when raising the temperature. As a result, this new rhodamine derivative shows special thermochromism. It is promising to replace the intrusive measurement techniques and to be applied as coating or pigments in external temperature measurements as well as the privacy field.
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