Development of the molecular level descripton for nickel(II)-based ligand-exchange thermochromism

Abstract

Coordination compound-based nickel(II) thermochromic systems rely on a temperature-dependent equilibrium shift between different coordination environments of the central nickel ion. These systems are found in thermochromic "smart windows" that tint reversibly in response to temperature increases in their environment providing the benefit of energy savings in commercial and private buildings. Despite the stoichiometrically simple equilibrium for these ligand exchange systems, there is a complex and delicate network of chemical interactions that determine the color, and thermodynamic performance. Accurate computational modeling of nickel(II) ligand exchange thermochromic systems is an important first step in the direction of understanding the parameter space that determines whether a given metal ligand system is thermochromic, the color of the high and low temperature species, the temperature at which the system will change color. The research presented in this dissertation uses experimental results to evaluate theoretical models. Core and valence electronic spectroscopies probe the ground and excited state electronic structures of high temperature nickel(II) thermochromic chromophores which range from the very covalent nickel tetrathiocyclotetradecane thiocrownether to the highly ionic nickel dibromodi(1-pentylbenzimidazole)nickel(II). The experimental electronic structures of these high temperature species combined with experimental ligand exchange thermodynamics are used to guide the evaluation of computational modeling methods in search of methods that reproduces the experimental observables. It is found that commercially relevant nickel(II) thermochromism takes place on an extremely flat potential energy surface governed by ion pairing, hydrogen bonding and dispersion interactions. The modeling of these surfaces requires the explicit consideration of ion pairing and solvent-solute interactions.