Elusive nature in spin state is a feature of singlet biradical molecules and arises from the weak coupling of unpaired electrons. This highlight review focuses on recent advances in experimental approaches for elucidating the electronic structure and properties of singlet biradical molecules having quinoid skeletons.

Although most third-order nonlinear optical (NLO) materials are closed-shell singlet systems, this Letter theoretically shows that the third-order NLO properties are drastically enhanced in symmetric open-shell diradical systems with intermediate diradical character and further reveals that this enhancement is associated with the ferromagnetic and antiferromagnetic interactions. This new paradigm opens up a promising area for designing new third-order NLO materials.