Optical and structural dynamical behavior of Crystal Violet Lactone – Phenolphthalein binary thermochromic systems

• Composites based on Crystal Violet Lactone and Phenolphthalein were investigated.
• The stoichiometry of the colored complex was determined by molecular spectroscopy.
• The hysteresis phenomenon was evidenced for all thermochromic composites.
• Color hysteresis is described by four characteristic temperatures.
• The color path was described using CIELAB parameters and total color differences.

Several binary composites based on Crystal Violet Lactone (CVL) color former and Phenolphthalein (PPht) developing agent dissolved in 1-tetradecanol (TD) were studied in order to determine how different ratios between components influence the thermochromic behavior of the composites. The color dependence on temperature was investigated in direct relationship with structural changes and interactions between components. The stoichiometry of the colored complex was determined from diffuse reflectance data and confirmed by means of steady state fluorescence measurements and temperature modulated differential scanning calorimetry. The color former: developing agent molar ratio equal to 1:2.5 was determined to ensure the reversibility of the process in optimal conditions. The characteristics of the color path and the influence of thermal history during thermochromic transition were analyzed from the variations of CIELAB color parameters during successive heating/cooling cycles. Dynamic colorimetric properties of the thermochromic binary composite containing the components in optimum ratio are described by four characteristic temperatures in the hysteresis.

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