Room temperature multiferroicity in hydrogenated triapentafulvalene and pentaheptafulvalene oligomers

To search for new organic multiferroics, we perform a systematic study on the magnetic and ferroelectric properties of fused triapentafulvalene and pentaheptafulvalene oligomers (n = 2-6), by using the density functional theory and quantum Monte Carlo method. It is found that the oligomers without hydrogenation always lie in the spin singlet (nonmagnetic) state, while a selective hydrogenation of carbon atoms at the ends of oligomers can result in the spin triplet (ferromagnetic) state, which is tens to hundreds meV lower than the nonmagnetic state. The formation of ferromagnetism can be attributed to the hydrogenation-induced near degeneracy between the highest occupied and lowest unoccupied molecular orbitals. Simultaneously, there exists a finite dipole moment in the ferromagnetic state, due to the breaking of the inversion symmetry of oligomers. Our results imply that the hydrogenated triapentafulvalene and pentaheptafulvalene oligomers could be promising candidates in the development of room temperature organic multiferroics.