A density-functional-theory study of biradicals from benzene to hexacene

► Calculated singlet-triplet energy gap of biradicals in polyacenes by means of density-functional theory. ► The singlet-triplet gap monotonically increases with increasing the length of polyacene. ► Spin delocalization affects the singlet-triplet gap. ► Spin delocalization of singlet state is mediated by a spin polarization of pz orbitals.