By choosing the right combination of monomers a polymer with a small energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), i.e. the HOMO-LUMO gap, can be achieved. This leads to degenerate or near-degenerate frontier molecular orbitals and allows for the unpairing of the normally paired HOMO atoms into two singly-occupied molecular orbital (SOMO) with a biradical nature .