A tight binding model of the bond dissociation mechanism in thermochromic leuco dyes

Abstract

It has long been known that heat, UV light, or protonization will cause the bond in Leuco dye's to break. The underlying microscopic mechanism is a still a matter of discussion. It stands to reason that a local model should correctly predict the temperature behavior of the heat capacity at the thermochromic phase transition. Here we develop a tight binding (TB) model for the variation of the energy of the bond as it breaks as a function of temperature. We use a statistical mechanical approach to incorporate this variation in a partition function, calculate a free energy, and obtain the heat capacity of the phase transition. A distribution of such heat capacities is shown to fit the data for the leuco dye. The heat capacity distribution is compared to the experimentally determined heat capacity by differential scanning calorimetry (DSC).