New insights concerning the mechanism of reversible thermochromic mixtures Hong Tang, Douglas C. MacLaren, and Mary Anne White Abstract: Three-component organic thermochromic materials, consisting of a leuco dye, a weak acid acting as the colour developer, and a low-melting organic solvent, can change their colour in response to temperature changes. Although widely used in applications, their detailed thermochromic mechanism is not fully understood. The present study delineates the role of subtle changes in the solvent���s molecular structure and concentration in the crystal violet lactone (CVL, dye)/ lauryl gallate (LG, developer)/1-alcohol (dodecanol (DD), tetradecanol (TD), hexadecanol (HD), or octadecanol (OD) sol- vent) system. Through inkjet printing of the components directly onto a substrate, combinatorial approaches reveal differ- ences when the alkyl chain length of the alcohol solvent is changed slightly. During the process of heating to the melt, followed by cooling to room temperature, CVL/LG/DD showed no colour change. On the other hand, CVL/LG/TD exhib- ited reversible thermochromism with colour forming in the molten state and colour loss in the solid state. In the composi- tion range investigated, the CVL/LG/HD system showed no colour change during heating, but on cooling from the molten state, at first a blue colour appeared just below the freezing point, and this was followed by a slow colour fading on fur- ther cooling. A significant new finding is that the orientationally disordered a-phase of the solvent is required to support the dye���developer complex that provides colour. Furthermore, there is an optimal solvent chain length for thermochrom- ism: if too short, there is no disordered phase and no colour if too long, the formation of the coloured developer���dye complex is prevented in the melt. Key words: thermochromism, polymorphism, dynamical disorder. Resume �� �� : Des materiaux �� thermochromiques organiques a ` trois composants formes �� d���un colorant leuco, d���un acide faible agissant comme agent de developpement �� de la couleur et d���un solvant organique de faible point de fusion peuvent changer de couleur en reponse �� a ` des changements de temperature. �� Le present �� travail permet de delimiter �� le role �� de changements subtils dans la structure moleculaire �� du solvant et les concentrations dans le systeme ` forme �� par le colorant, la lactone du cristal violet (LCV), l���acide faible agissant comme agent de developpement �� de la couleur (gallate de lauryle, GL) et les di- vers solvants [dodecan-1-ol �� (DD) tetradecan-1-ol �� �� (TD) hexadecan-1-ol �� (HD) octadecan-1-ol �� (OC)]. En procedant �� a ` une impression a ` jet d���encre directement sur un substrat, des approches combinatoires permettent d���identifier les differences �� re-�� sultant d���un faible changement dans la longueur des cha��nes �� alkyles de l���alcool agissant comme solvant. Durant le proces- sus de chauffage conduisant au liquide suivi d���un refroidissement a ` la temperature �� ambiante, on n���observe aucun changement de couleur pour le systeme ` LCV/GL/DD. Par ailleurs, pour le systeme ` LCV/GL/TD, on observe une thermo- chromie reversible �� avec une formation de couleur dans l���etat �� liquide et une perte de couleur a ` l���etat �� solide. Pour le sys- teme ` LCV/GL/HD, il n���y a pas de changement de couleur durant le chauffage mais, lorsqu���on refroidit a ` partir de l���etat�� fondu, il y a d���abord apparition d���une couleur bleue juste en dessous du point de fusion et ceci est suivi d���une disparition lente de la couleur par refroidissement subsequent. �� Une observation significative a trait au fait que la phase adu solvant, desordonnee �� �� d���un point de vue de l���orientation, a besoin du support du complexe colorant/agent de developpement �� de la couleur pour conduire au developpement �� d���une couleur. De plus, l���apparition d���une thermochromie ne se produit qu���avec un solvant comportant une cha��ne �� carbonee �� de longueur optimale : si la cha��ne �� est trop courte, il n���y a pas de phase desor-�� donnee �� et il n���y a pas de couleur si elle est trop longue, le complexe colorant/agent de developpement �� de la couleur ne peut se produire. Mots-cles �� : thermochromie, polymorphie, desordre �� dynamique. Introduction Thermochromism, i.e., temperature-induced colour changes, can be the basis of functional materials such as sensors, thermometers, or thermal printing. Some thermochromic processes, such as chemical degradation, are irreversible, while others are reversible. The mechanisms of thermo- Received 18 January 2010. Accepted 25 May 2010. Published on the NRC Research Press Web site at canjchem.nrc.ca on 25 August 2010. This article is part of a Special Issue dedicated to Professor R. J. Boyd. We dedicate this manuscript to our colleague, Professor Russ Boyd, on the occasion of his 65th birthday. H. Tang, D.C. MacLaren, and M.A. White.1 Department of Chemistry and Institute for Research in Materials, Dalhousie University, Halifax, NS B4H 4J3, Canada. 1Corresponding author (e-mail: mawhite@dal.ca). Pagination not final/Pagination non finale 1 Can. J. Chem. 88: 1���8 (2010) doi:10.1139/V10-069 Published by NRC Research Press

chromism are varied, including phase-transition induced changes in the crystal field and a change in wavelength of constructively interfered light due to thermal expansion of layered structures such as liquid crystals,1,2 and changes in luminescence in sol-gel films,3 or polymer structures,4 or the shape of gold or silver nanorods,5 or isomer distribu- tion.6 Spiropyrans are known to have potential for thermo- chromism, whether they are adsorbed on a surface,7,8 embedded in a polymer,9 part of a polymer backbone,10 or a component in an organic thermochromic mixture. In current materials applications, the latter is most common thermo- chromism arises from the change in interaction of a spiro- pyran leuco dye with a developer in the presence of a low- melting solvent.1,11���13 Often, heating results in colour loss and such mixtures are particularly promising as components of thermally erasable toner for printers, allowing the same paper to be used, thermally erased, and then used again.14 A recent development is thermochromic fibers, with polymeric shells and thermoresponsive cores based on such mixtures.15 Such materials have recently been proposed as the basis for low-cost ($0.10/m2) displays.16 Although we now have some insights into the molecular processes involved in thermochromic organic mixtures, im- portant issues remain unresolved, such as control of the con- trast between the coloured and uncoloured states, the kinetics of decolourization, and the long-term stability of the coloured state.17 Early mechanistic studies of ternary organic thermochro- mic mixtures outlined the main features of the mechanism,18 but left open important questions. More detailed studies showed the importance of interactions of spiropyrans with the developer,19���21 and how the competition between the de- veloper���dye complex (metastable, coloured state) and the developer���solvent complex (equilibrium, uncoloured state) leads to thermochromism.22���24 However, considering the range of possible compositions in the ternary system and the many possible dyes, developers, and solvents, the gener- ality of the mechanism has not been tested. Combinatorial approaches are known to lead to rapid advances when there are many compositional variables a relevant example is col- orimetric differentiation of polydiacetylene sensors.25 To ad- vance understanding of thermochromism mechanisms, we have embarked on combinatorial investigations of three- component thermochromic mixtures, delivered directly on paper by an inkjet printer. The dye was crystal violet lactone (CVL), the developer was lauryl gallate (LG), and the sol- vent was a long-chain alcohol. In the present studies we ex- plore the question: How does the solvent���s molecular structure influence reversible, rewritable thermochromism? Although the solvent was initially thought to be most impor- tant for its thermochromism-initiating melting point, or as a decolourization acceleration agent in which the solvent pro- vides nucleation sites for the phase separation of developer molecules,26,27 detailed studies have revealed that the nature of the solvent controls decolourization, through formation of a solvent���developer complex.23 We now show that the role of the solvent is more subtle and complex, and is important for both colour formation and thermal erasure. Although the present system usually shows colour formation on heating rather than the more usual colour loss, the conclusions pre- sented here apply to both types of thermochromic systems. Experimental methods Chemicals Crystal violet lactone (3,3-bis(p-N,N-dimethylamino-phenyl)- 6-N,N-dimethyl-aminophthalide, CVL, 97%), lauryl gallate (dodecyl(3,4,5-trihydroxy)benzoate, LG, 97%), 1-dodecanol (DD, 96%), 1-tetradecanol (TD, 96%), 1-hexadacanol (HD, 99%), and 1-octadecanol (OD, 99%) were obtained from Sigma-Aldrich. All compounds were used without further purification. Combinatorial sample preparation Small quantities of the dye (CVL), developer (LG), and solvent (DD, TD, HD, or OD) were delivered onto paper via a combinatorial inkjet printing technique. In brief, the dye (0.025 mol/L), developer (0.3 mol/L), and solvent (0.5 mol/L), each in an ethyl acetate/1-butanol (16:9 volume ratio) were placed in the cleaned ������ink������ cartridges of a drop- on-demand thermal roof-shooter inkjet printer (HP 5940c). The mixed solvent system of ethyl acetate (EMD, 99.5%) and 1-butanol (ACP, 99.4%) was selected as the volatile sol- vent system to deliver the thermochromic components to the paper (ordinary printer paper) and allow mixing before its evaporation. (Note that we refer here to ethyl acetate/1-buta- nol as the volatile solvent, to distinguish it from the solvent required for the thermochromic mixture.) The ratio in the mixed volatile solvent system was optimized to give rheo- logical properties in the same range as the commercial ink for which the printer was designed. In particular, it should not be so viscous that it cannot flow, or so low in viscosity that leakage is a problem, yet it needs appropriate spreading (no splashing). The Ohnesorge number28 for this system was about 12. Quantitative delivery of the components (dye, de- veloper, and solvent) to the various regions of the paper was handled through the software (Adobe Photoshop 5.0) con- trolling the printer. Optimal parameters included using 90% ������colour������ density and 10���30 passes per location. (Fewer passes led to too little deposition more passes increased the Pagination not final/Pagination non finale 2 Can. J. Chem. Vol. 88, 2010 Published by NRC Research Press