Effect of Substituents on the Bond Strength of Air-Stable Dicyanomethyl Radical Thermochromes

A series of substituted aryl dicyanomethyl radicals were synthesized, and the bonding thermodynamic parameters for self-dimerization were determined from van’t Hoff plots obtained from variable-temperature electron paramagnetic resonance and ultraviolet–visible spectroscopy. At low temperatures, the radicals dimerize, but the colored, air-stable free radicals return upon heating. Heating and cooling cycles (5–95 °C) can be repeated without radical degradation and with striking thermochromic behavior. We find a linear free energy relationship between the Hammett para substituent parameter and the dimerization equilibrium constant, with para electron-donating substituents leading to a weaker bond and electron-withdrawing substituents leading to stronger bonds, following a captodative effect. Density functional theory investigations [B98D/6-31+G­(d,p)] reveal that the dimers prefer a slip-stacked geometry and feature elongated bonds.