First-principles theoretical investigation of the electronic couplings in single crystals of phenanthroline-based organic semiconductors

The electronic couplings between adjacent molecules in the phenanthroline-based bathocuproine (BCP) and bathophenanthroline (Bphen) crystals have been studied using density functional theory on model dimers. Within the frame of the "two-state model" of charge-transfer theory, a generalized definition of the "effective transfer integral" is proposed. This definition addresses the issue arising when the lowest unoccupied molecular orbital (LUMO) [highest occupied molecular orbital (HOMO)] and LUMO+1 (HOMO-1) of the single molecules both have significant contributions to the dimer LUMO (HOMO) level. Charge-transfer integrals based on the new definition are compared with those from previous models; significant differences are found. The authors' results indicate that, within a simple Marcus theory approach, the charge-transport parameters of the BCP and Bphen crystals are expected to be similar.