A Molecular Orbital Study on the Hole Transport Property of Organic Amine Compounds

Reorganization energy (λ 1 ) in the ionization process of organic amines and that (λ 2 ) of the electron attaching process of amine cation radicals are evaluated with AM1, ab initio MO, and DFT methods, where dimethylaniline, methyldiphenylamine, and triphenylamine are adopted as a model of a hole transport material. The total λ value (=λ 1 + λ 2 ) decreases in the order dimethylaniline > methyldiphenylamine > triphenylamine, which agrees well with an increasing order of experimentally reported hole transport mobility of diamines that are dimers of above-mentioned amines, N, N ‘-tetraphenyl-[1,1‘-biphenyl]-4,4‘-diamine < N, N ‘-dimethyl-N,N‘-diphenyl-[1,1‘-biphenyl]-diamine < N, N ‘-tetramethyl-[1,1‘-biphenyl]-4,4‘-diamine. This relation is reasonably explained with Marcus theory, since the λ value is directly related to the activation energy of hole transfer from one amine cation radical to a neighboring neutral amine, according to Marcus theory. The geometry changes in the ionization process are inspected to find a determining factor for λ. The large λ value of dimethylaniline arises from the fact that the pyramidal structure of dimethylaniline changes to the planar structure upon the ionization. On the other hand, the small λ value of triphenylamine might be attributed to the fact that the bond angle about the N atom changes little upon the ionization because both neutral triphenylamine and its cation radical are planar about the N atom. From these results, we might provide a prediction that a planar amine is a good candidate for a hole transport material.