Molecular concepts of normal and superconducting states in acenes and B, N-substituted acenes: A theoretical study

Abstract

Electron–phonon coupling and the normal and possible superconducting states in the monoanions of B, N-substituted acenes such as B3N3H6, B5N5H8, and B7N7H10 are studied. The results for B, N-substituted acenes are compared with those for acenes. The B–N stretching modes around 1500 cm−1 and the low-frequency modes, less and more, respectively, strongly couple to the lowest unoccupied molecular orbitals (LUMO) with an increase in molecular size from B3N3H6 to B7N7H10. The relationship between the intrinsic intramolecular conductivity and the intramolecular electronic structures is investigated, and it is found that the intrinsic high conductivity needs small energy difference between the highest occupied molecular orbitals and the LUMO, the high frequency modes which play an essential role in the electron–phonon interactions, and large number of atoms. The relationship between the normal and superconducting states in acenes is compared with that in B, N-substituted acenes. Both possible superconducting transition temperatures and the intrinsic normal conductivity in acenes are estimated to be larger than those in the same size of B, N-substituted acenes. These results argue against the interesting apparent paradox in conventional superconductivity; the higher resistivity at room temperature, the more likely it is that a metal will be a superconductor when cooled. That is because the frequencies of the vibrational modes which play an important role in the electron–phonon interactions in negatively charged B, N-substituted acenes are much lower than those in negatively charged acenes because of electronegativity perturbation. In conventional superconductivity, such frequencies depend mainly on the atomic masses. However, such frequencies depend on the detailed intramolecular electronic structures as well as the molecular weights in nano-sized molecular systems.