Concerning the band structure of D(M(dmit) 2) 2 (D=TTF,Cs,NMe 4); M=Ni,Pd) molecular conductors and superconductors: Role of the M(dmit) 2 Homo and Lumo

Abstract

We report tight-binding band structure calculations showing that the partially filled band of D[M(dmit) 2] 2 molecular conductors containing dimeric building blocks can originate either from the LUMO (lowest unoccupied molecular orbital) or the HOMO (highest occupied molecular orbital) of the M(dmit) 2 acceptor, depending upon the strength of the intra-dimer transfer integrals (t HOMO and t LUMO) with respect to the HOMO-LUMO splitting. The first situation is found for D=Me 4N and M=Ni whereas the second one is found for D=Cs or TTF ( δ phase) and M=Pd. Both the HOMO and LUMO bands are partially filled in the TTF[Ni(dmit) 2] 2 and α′-TTF[Pd(dmit) 2] 2 salts which contain M(dmit) 2 stacks with uniform chains. The structural reasons behind these surprising results are explained.