Why does 1,1-diphenyl-2-picrylhydrazyl exist as a stable violet solid?

1,1-diphenyl-2-picrylhydrazyl (DPPH) exists as a stable violet solid primarily due to the phenomenon of resonance, also known as delocalization.1

In radical chemistry, stability is often achieved when the odd electron is not confined to a single atom but is instead distributed across the molecule. According to quantum mechanics, this delocalization of the electron lowers the energy of the system, significantly increasing the substance's stability and its probability of existence. While many early free radicals were only stable in specific organic solvents or were highly reactive in the presence of air or water, the structural configuration of DPPH allows it to persist as a stable solid.1