An air- and thermally stable aryl dicyanomethyl radical is reported that switches between two dimeric forms-a σ dimer and a π dimer-by changing the solvent. The two dimer forms exhibit unique optical properties leading to solvatochromic behavior. The solvent-responsive behavior of these radicals can be explained by the higher polarizability of the pimer than the σ dimer that leads to pimer stabilization in polar solvents.