Exploration of Thermochromic Materials Using Experimental and

Laboratory Experiment pubs.acs.org/jchemeduc

Exploration of Thermochromic Materials Using Experimental and Theoretical Infrared and UV−Visible Spectroscopy Kelsey Costello, Kevin Thinh Doan, Kari Lynn Organtini, John Wilson, Morgan Boyer, Greglynn Gibbs, and Lorena Tribe* Division of Science, The Pennsylvania State University−Berks Campus, Reading, Pennsylvania 19610, United States S Supporting Information *

ABSTRACT: This laboratory was developed by undergraduate students in collaboration with the course instructor as part of a peer-developed and peer-led lab curriculum in a general chemistry course. The goal was to explore the hypothesis that crystal violet lactone was responsible for the thermochromic properties of a sipping straw using a FT-IR for experimental determinations of vibrational frequencies, UV−vis to quantify the color change, and the software packages Spartan and Gaussian 09 for theoretical calculations.

KEYWORDS: First-Year Undergraduate/General, Laboratory Instruction, Collaborative/Cooperative Learning, Inquiry-Based/Discovery Learning, Molecular Modeling, Computational Chemistry, Dyes/Pigments, Esters, Materials Science, Spectroscopy

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his laboratory experiment is provided as both an example of what can be achieved with a peer-developed and peerled (PDPL) pedagogy and as an experiment that can be performed by introductory level students. The laboratory was developed by undergraduate students, in collaboration with the course instructor, as part of a PDPL curriculum in a secondsemester general chemistry course. This pedagogical technique was published in detail previously1 and is summarized here. In a PDPL lab approach, students spend approximately half the semester developing laboratories in teams. The criteria for choosing a lab is that it should be aligned with the content of the concurrent lecture course, that students would engage in hands-on inquiry in the laboratory, and that it should be safe. During the second half of the semester, each team leads the rest of the class through the lab they have developed. While some teams focus on implementing laboratories from the literature (e.g., the Journal of Chemical Education), some compile experiments from demonstrations textbooks (e.g., Shakashiri2), and others develop their contribution from scratch. The lab described here is a product of the third approach: a question about a material encountered in a real-life setting led to some weeks of research and testing by the responsible team and developed into a meaningful learning experience for the rest of the class. © XXXX American Chemical Society and Division of Chemical Education, Inc.

LABORATORY DEVELOPMENT

Real-Life Question

Thermochromic products change colors with changes of temperature, making them attractive materials to engage the interest of undergraduate students. In this case, the material that prompted the research for and development of the lab was a color-changing sipping straw. These straws were originally found at a local restaurant and now can be purchased commercially (FrozenSolutions.com,3 Good Life Innovations,4 Drinkstuff.com,5 Aliexpress.com,6 Epromos.com7). A standard-looking white-to-pale yellow straw turned blue when dipped in a cold beverage. Removing the straw from the cold beverage led to a gradual change back to the white form, and touching the cold straw with a finger rapidly produced a pale area. This process could be repeated many times with consistent color changes. The lead team was interested in finding out the chemical basis for the observed change. Bibliographic research followed by exploration of various experimental and theoretical techniques led to a plausible explanation and to the lab described below, which was then completed by the rest of the class. Chemistry of Thermochromic Materials

There are several types of thermochromic materials, among them leuco dyes, inorganic crystals, and liquid crystals. A common application of the latter would be a mood ring, where

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dx.doi.org/10.1021/ed400578q | J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

FT-IR

a liquid crystal is combined with a quartz structure and used as a thermometer.8 Typical thermochromic inorganic materials include VO2 and are used in smart windows.9 In the case of a leuco dye, the color change is produced by an organic molecule that changes its structure, affecting its electronic structure and consequently its UV−vis absorption spectrum and leading to one form that is colorless and one that is colored. A commonly used compound of this kind is crystal violet lactone (CVL), 3,3bis(p-N,N-dimethylaminophenyl)-6-N,N-dimethylaminophthalide (C26H29N3O2), a spirolactone dye,10 which is the main focus of this lab. Lactones (cyclic esters) have been featured in laboratory experiments in the past as examples of organic synthesis.11−13 Different forms of crystal violet leucos have also drawn the attention of laboratory instructors to demonstrate kinetics14 and photochemistry.15 Examples of uses of CVL in everyday life are in plastics for novelty items such as Frisbees, baby bottles, and sipping straws or in microencapsulated versions used in fabrics for striking T-shirts and other garments.16 CVL is found in its colorless structure at room temperature (Figure 1A), changing to the blue form at lower temperatures

The infrared spectra were obtained with a Nicolet 6700 spectrometer over a wavelength range of 4000−400 cm−1, with a spectral resolution of 4 cm−1. Thirty-two acquisitions were collected for each spectrum during the original experiment, whereas 64 acquisitions were used more recently to check new straws. The spectrum was recorded at room temperature using a portion of the straw stapled to a rectangle of cardboard as a thin film, as seen in Figure 2.

Figure 2. Warm (left) and cold (right) sample for FT-IR.

A polyethylene bag with ice cubes was then held against the stapled sample for cooling until the sample turned blue. The sample was quickly dried with a tissue and a second spectrum obtained while the material was still blue. These measurements were repeated to achieve consistency. An alternative to using an ice bag is to cool the sample with liquid nitrogen. It should be noted that the color fades quickly so the spectra for the cool sample show features of the warm one. For comparison we also obtained a spectrum for CVL from a commercial powder, crystal violet lactone, CAS 1552-42-7, 95.9%, Tokyo Chemical Industry, Co., Ltd.

Figure 1. CVL at room temperature (A) and at low temperature (B).

(Figure 1B). The warm, colorless form includes a closed cyclic ester (lactone ring) with a π−π* transition in the ultraviolet region (λmax = 280 nm), whereas the cool, blue form has a carboxylate with increased conjugation in the system. The π−π* transition is shifted to the visible region (λmax = 620 nm)17 in the second case. For a first-year undergraduate audience, it suffices to know that a change in molecular structure will cause a change in the electronic structure of the compound, potentially leading to a distribution of energy levels that may cause absorption in the visible region of the electromagnetic spectrum

UV−Visible

The UV−vis spectra were obtained with a SpectroVis Plus spectrometer from Vernier Software & Technology. A blank was determined with a dry, empty cuvette. A small portion of the straw was placed inside the cuvette to determine the spectrum at room temperature. The sample was then removed with tweezers, dipped in liquid nitrogen, and placed in the cuvette again to determine the low temperature spectrum. As above, the color fades quickly, so it is essential to work fast to determine the spectrum of the cool sample.

Instructional Goals

The goal of this lab was for students to explore the hypothesis that CVL could be in the sipping straw sample while being introduced to methods in scientific inquiry. The available instrumentation was a FT-IR for experimental determinations of infrared vibrational frequencies, a UV−vis spectrometer to corroborate the change of color, the software package Spartan18 for molecular modeling and semiempirical calculations of the infrared vibrational frequencies and the UV−vis absorption, and the Gaussian 09 software package for more sophisticated firstprinciples electronic structure calculations of both the vibrational frequencies and the UV−vis absorption.

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Computational Calculations

An intrinsic part of this experiment was to model the system at the atomic level with software that provided appropriate visualization so that the students could observe the different structures (open and closed) for CVL and note the difference in the vibrational motions of both forms of the molecule. Originally the calculations were performed running the software package Spartan on a Dell workstation with Red Hat Linux as an operating system, but since then, similar calculations have been run on PCs in the chemistry laboratory. In addition, any computational package with semiempirical level calculations (e.g., Hyperchem, MOPAC, AMPAC, GAUSSIAN, CP2K, or PC GAMESS) could be used. The software models for both forms of CVL were prepared with the build feature. The models were energy minimized with

PROCEDURE AND MATERIALS

Thermochromic Plastic

A plastic sample cut from the sipping straw was provided for each team. The original sipping straws were gathered from a local restaurant, but similar materials may be purchased commercially. The experiments outlined below were repeated recently with straws from FrozenSolutions.com.3 B




Table 1. Example Schedule of the Class Dynamics Showing the Rotation between Stations Student Group

Lead student 1 Station 1: Background Information/Time

Lead student 2 Station 2: Experimental FT-IR/Time

Lead student 3 Station 3: Computational Calculations/ Time

Lead student 4 (or instructor, or laboratory technician) Station: Experimental UV−vis/Time

1 2 3 4

9:00 − 9:40 9:40 − 10:20 10:20 − 11:00 11:00 − 11:20

9:40 − 10:20 10:20 − 11:00 11:00 − 11:20 9:00 − 9:40

10:20 − 11:00 11:00 − 11:20 9:00 − 9:40 9:40 − 10:20

11:00 − 11:20 9:00 − 9:40 9:40 - 10:20 10:20 − 11:00

molecular mechanics using the MMFF potentials,19−23 and the same method as well as a PM324 level semiempirical quantum calculation were used to calculate the vibrational frequencies of both proposed species. For increased accuracy, we revisited the calculations after this experiment was done in class. At this later time, both target structures, the colorless lactone (ring closed) and the blue species (ring open), were modeled with the Gaussian 0925 package run at the Penn State Research Computing and Cyberinfrastructure Group on a Dell PowerEdge 1950 with 128 Intel Xeon E5450 Quad-Core 3.0 GHz processors with 32 Gb memory using density functional theory (DFT)26 and the B3LYP/6-31G(d) functional. The vibrational frequencies were scaled by a factor of 0.961.27 These calculations were performed by an undergraduate research student and may be more appropriate for advanced rather than introductory-level students. Similarly, the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were determined with the semiempirical and the firstprinciples calculations to provide an estimate of the UV−vis absorption frequencies.

calculations by K.O., the third student leader. This last step has been modified because computational chemistry software is now available in the laboratory. As each group completed their work, they moved on to the following station so everyone in the class had access to explanations on the theory behind the experiment, were able to actually obtain experimental FT-IR spectra, and manipulated the computational chemistry software. An example schedule is shown in Table 1. The lead team who developed this laboratory experiment worked on it during six laboratory periods; this included deciding on a topic, researching materials, learning to use equipment and software, testing the procedure, and writing a protocol for their classmates. The rest of the class used a 2.5-hlong laboratory to perform the whole experiment. The lead team provided a handout to guide the other teams thorough the lab. An example handout can be found in the Supporting Information. This laboratory procedure was used only once, the semester it was developed, as our second-semester chemistry laboratories are geared toward the peer-developed and peer-led pedagogy. Each new cohort of students has the opportunity to develop their own laboratories and to lead their peers through them.

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HAZARDS AND DISPOSAL The thermochromic straws were discarded in the recycling bin. Ice was disposed of in the sink. Used crystal violet lactone was stored in with nonhalogenated organic waste containers until collected and disposed of by University Park Environmental Health and Safety. Protective mitts should be used if any part of the experiment is done using liquid nitrogen.

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RESULTS AND DISCUSSION

IR and Computational Calculations

Slides of the warm and cold sample are shown in Figure 2, above. Figure 3 shows a typical example of the spectra obtained

GROUP DYNAMICS

This experiment was developed and conducted in a laboratory with 23 students that met weekly during the semester for a 2.5h-long laboratory periods. The students who developed the lab led the rest of the class through the experiment. The students that performed the lab were divided into three groups that rotated through three stations: (1) theory, (2) experimental FT-IR, (3) computational calculations. Since performing this laboratory, we acquired a UV−vis spectrometer as described above. We would now include a fourth station to do the experimental UV−vis spectroscopy and divide the students into four groups rather than three. Overall, five teams would be involved in the final form of this laboratory: the lead team and four teams performing the laboratory. One of the authors (J.W.) engaged group 1 in a discussion on what CVL was, why the color changes occurred, and how one could identify the compound (or at least infer its presence). Another author (T.D.) led group 2 to the instrumentation room and demonstrated how the FT-IR worked. Backgrounds were measured and measurements on the plastic straw were done by each group. Group 3 convened in the instructor’s (L.T.) office and was introduced to computational chemistry

Figure 3. Experimental FT-IR spectra determined by students for warm (solid line) and cool (dashed line) plastic sipping straw sample.

by students during the lab, focusing on the 1800−1660 cm−1 region of the electromagnetic spectrum. There was consistently only one peak that shifted appreciably in the IR spectrum each time the sample was cooled. This happened for the five teams that engaged in the experiment. The peak found at 1741 cm−1 in the colorless sample shifted to around 1734 cm−1 in the cool, blue sample. The value of 1741 cm−1 is within the range expected for a CO stretch.28 The decrease in frequency on cooling suggests that the ester ring is opening, giving rise to a carboxylate group that can be protonated.29 Enhanced C




match to the experimental observation, as seen in Table 2. The experimental frequencies for the warm (closed ring) CVL, the straw, and the calculated CVL are 1747, 1741, and 1750 cm−1, respectively, all in close agreement. The cool straw and the calculated open ring CVL have absorption frequencies of 1735 and 1720 cm−1, respectively, both presenting a shift toward longer wavelengths with respect to the closed ring, as expected. Overall, the theoretical calculations of vibrational spectra in this lab served to illustrate the fact that different bends and stretches of each moiety caused particular absorption bands and that not all models calculate properties with the same level of accuracy.

hydrogen bonding would decrease the frequency of the CO vibrations.28 These experiments are reported as Experimental I in Table 2. Similar values were obtained recently for a straw Table 2. Calculated and Experimental IR Absorption Values for Warm (Closed Ring) and Cool (Open Ring) Straw Samples Source Spartan/MMFF Spartan/PM3 Gaussian/DFT- B3LYP/631G(d) Experimental Ia Nicolet 6700 spectrometer Experimental IIb Nicolet 6700 spectrometer a b

Warm Straw IR Absorption/cm−1

Cool Straw IR Absorption/cm−1

1932 2036 1750

1761 1815 1720

1741

1735

1740

1736

UV−Vis and Computational Calculations

The experimental spectra for warm and cold sipping straws, obtained with a SpectroVis Plus spectrometer from Vernier Software & Technology, are shown in Figure 5. The peak at

Original sipping straw from Friendly’s Restaurant, Wyomissing, PA. Sipping straw from FrozenSolutions.com.3

from a new provider (Experimental II in Table 2). For the commercial sample of CVL powder a peak at 1747 cm−1 was obtained, in close agreement with the sipping straw at room temperature. The spectra for the cold and warm samples are included in the Supporting Information. The calculated results using both the Spartan software package on site and the Gaussian 09 package run remotely at a supercomputer center are reported in Table 2. Figure 4 shows the calculated spectra for the closed form of the molecule using the PM3 semiempirical approach. The calculation allows a broad view of the entire spectrum from 500 to 4000 cm−1, and the molecular structure of the energy minimized molecule, and is included here as an example of a typical output. The most compelling feature of this part of the lab was being able to see what motion of the molecule corresponded to each one of the absorption peaks. The MMFF and PM3 approximations are not accurate enough to predict vibrational frequencies absolutely, so it was not expected that the very small change in frequency detected experimentally would be reflected in the calculated numbers. It was interesting to note that given an experimental determination that did not coincide with the calculated results, most students thought they had performed the experimental infrared spectroscopy incorrectly. Bringing this misconception to light elicited a discussion about what one can learn from experiments and from theoretical calculations. Additionaly, it should be noted that the frequencies are sensitive to small variations in the orientation of the −COOH group for the open ring with these methods. The higher level electronic structure calculations performed remotely at the supercomputer center using DFT and the B3LYP/6-31G(d) potential provide a much improved

Figure 5. Experimental UV−vis spectra for the warm and cool plastic straw sample.

581 nm (yellow) for the cold sample is compatible with the blue color observed. An interesting feature of the UV−vis spectroscopy is that the spectrum can be set to refresh every few seconds, and the students can observe how this peak decreases and finally disappears as the straw turns quickly from blue to the original colorless form. In addition to an improved match with the higher-level theory, it was also possible to determine the energy of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The difference between these values can be used as an approximation to the absorption energy and can estimate the wavelength of the absorbed light E HOMO − E LUMO = ΔE = hν =

hc λ

where h is Planck’s constant, ν is the frequency of the electromagnetic radiation, c is the speed of light, and λ is the wavelength. The results are shown in Table 3. The theoretical approach, though not providing an exact determination of the

Figure 4. Example calculated infrared spectrum and molecule visualization for CVL; calculations with PM3; graphics from Spartan. D




Table 3. Absorbed Light for the Closed and the Open Ring, Calculated with DFT, the Experimental Determination, and Literature Values

a

Structure

EHOMO/Ha

ELUMO/Ha

ΔE/Ha

v/s−1

Calculated λ/nm

Experimental λ/nm

CVLa λ/nm

Closed ring DFT Open ring DFT

−0.180 −0.291

−0.0295 −0.194

0.151 0.0969

6.04 × 1014 3.88 × 1014

302 470

394 581

280 620

Closed ring PM3 Open ring PM3

EHOMO/eV −8.4 −10.1

ELUMO/eV −0.5 −5.3

ΔE/eV 7.9 4.8

1.90 × 1015 1.15 × 1015

158 260

394 581

280 620

Calculated from the literature data.17

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ACKNOWLEDGMENTS The authors gratefully acknowledge David Aurentz at The Pennsylvania State UniversityBerks Campus for the followup FT-IR measurements and the diligent reviewers who helped us improve this work.

absorption energy, reproduces the increase in wavelength associated with the opening of the lactone ring. As expected, the higher level of theory provides frequencies more in accordance with the experimental values.

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SUMMARY This lab was successfully developed by and delivered to firstyear students. It could be easily extended to organic, physical chemistry, or computational chemistry courses where students could look more in detail at both the IR vibrational spectra and at the electronic transitions leading to the UV−vis spectra. For first-year students it provided an exciting opportunity to explore an interesting material they encountered in the real world, it introduced the role of chemist as sleuth, it gave them a first exposure to infrared and UV−vis spectroscopy, and for many, it was their first opportunity to compare computational and experimental information. The students who developed the laboratory were able to provide a well-thought-out guide to lead their classmates through the experiment. The students who performed the laboratory obtained spectra, ran the computational software, and produced laboratory reports that indicated understanding of the phenomena involved. The general consensus was that CVL was a likely candidate to be the cause of the color change in the sipping straws. With the increased availability of thermochromic materials this exploration could be applied to other commercial items with potentially new and different outcomes, making it a true inquiry laboratory every time. Students respond well semester after semester to the peerdeveloped and peer-led pedagogy in our second-semester chemistry laboratory. Some typical comments are “... I also learned how incredibly joyful you become when you have success in the lab...”, “At first, I didn’t think I would like the format of the class, however, typing right now, I really enjoyed this class”, and “this course is a must, because like life in college, this course makes you take matter [sic] into your own hands and lets you make the decisions that can shape the process as a whole”.

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ASSOCIATED CONTENT

S Supporting Information *

Student handout. This material is available via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Tribe, L.; Kostka, K. Peer-Developed and Peer-Led Labs in General Chemistry. J. Chem. Educ. 2007, 84, 1031. (2) Shakashiri, B. Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volumes 1−4; University of Wisconsin Press: Madison, WI, 1983−1992 (3) Frozen Solutions. FrozenSolutions.com (accessed May 2014). (4) Good Life Innovations. http://www.amazon.co.uk/25-PartyDrinking-Straws-Change/dp/B001HZJRZG#productDescription (accessed May 2014). (5) Drinkstuff.com. http://www.drinkstuff.com/products/product. asp?ID=7957#.Ud73Pqy0Q2U (accessed May 2014). (6) Aliexpress.com. http://www.aliexpress.com/item/2012-UniqueFancy-Color-Changing-Straws-Color-change-while-put-in-cold-waterPerfect-Parties-Drinking/690245596.html (accessed May 2014). (7) Epromos.com. http://www.epromos.com/product/8841617/ color-changing-custom-drinking-straw.html;jsessionid= BA3BC35906B49ABC14F3874AE23F9996.eptom1 (accessed May 2014). (8) “Mood Ring Monitors Your State of Mind”. Chicago Tribune: Chicago, IL, Oct. 8, 1975; at C1. (9) Schwartz, M. M. Smart materials; CRC Press: Boca Raton, FL, 2008. (10) Aitken, D.; Burkinshaw, S. M.; Griffiths, J.; Towns, A. D. Rev. Prog. Color 1996, 26, 1−8. (11) Perrine, D. M. Synthesis of a lactone. J. Chem. Educ. 1998, 75 (7), 803−803. (12) Bozak, R. E.; Knittel, J.; Hicks, R. J. Convenient synthesis of a lactone, gamma-butyrolactone. J. Chem. Educ. 1998, 75 (1), 84−84. (13) Bunce, R. A.; Reves, H. D. Gamma-nonanoic lactone - Synthesis of a fragrance and flavor enhancer in the undergraduate laboratory. J. Chem. Educ. 1990, 67 (1), 69−70. (14) Cannon, J. F.; Gammon, S. D.; Hunsberger, L. R. KineticsLab: The Crystal Violet/Sodium Hydroxide reaction. J. Chem. Educ. 1994, 78 (1), 238. (15) Thyron, F. C. Photo Oxidation of Leuco Methyl Crystal Violet. J. Chem. Educ. 1971, 48, 766. (16) White, M. A.; LeBlanc, M. Thermochromism in Commercial Products. J. Chem. Educ. 1999, 76 (9), 1201. (17) Takaoka, K.; Maeda, S.; Miura, H.; Endo, K.; Chong, D. P. Bull. Chem. Soc. Jpn. 1998, 71, 807−816. (18) Wavefunction, Inc. Spartan’02, Wavefunction, Inc.: Irvine, CA, 2002. (19) Halgren, T. A. Merck molecular force field. I. Basis, Form, Scope, Parameterization, and Performance of MMFF94. J. Comput. Chem. 1996, 17, 490−519. (20) Halgren, T. A. Merck molecular force field. II. MMFF94 van der Waals and electrostatic parameters for intermolecular interactions. J. Comput. Chem. 1996, 17, 520−552.

AUTHOR INFORMATION

Corresponding Author

*L. Tribe. E-mail: [email protected]. Notes

The authors declare no competing financial interest. E




(21) Halgren, T. A. Merck molecular force field. III. Molecular geometries and vibrational frequencies for MMFF94. J. Comput. Chem. 1996, 17, 553−586. (22) Halgren, T. A.; Nachbar, R. B. Merck Molecular Force Field. IV. Conformational energies and geometries for MMFF94. J. Comput. Chem. 1996, 17, 587−615. (23) Halgren, T. A. Merck molecular force field. V. Extension of MMFF94 using experimenatal data, additional computational data and empirical rules. J. Comput. Chem. 1996, 17, 616−641. (24) Stewart, J. J. P. Optimization of parameters for semi-empirical methods. II. Applications. J. Comput. Chem. 1989, 10, 221−264. (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.1, Gaussian, Inc.: Wallingford, CT, 2009. (26) Hohenberg, P.; Kohn, W. Inhomogeneous electron gas. Phys. Rev. 1964, 136 (3B), B864−B871. (27) CCCBDB: Computational Chemistry Comparison and Benchmark Data Base. http://cccbdb.nist.gov/ (accessed May 2014). (28) Silverstein, R. M.; Calyton Bassler, G.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 5th ed., John Wiley and Sons: Weinheim, Germany, 1991. (29) MacLaren, D. C.; White, M. A. Dye-developer interactions in the crystal violet lactone-lauryl gallate binary system: implications in thermochromism. J. Mater. Chem. 2003, 13, 1695−1700.

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Exploration of Thermochromic Materials Using Experimental and

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