A molecular orbital study on the hole transport property of organic amine compounds

Reorganization energy (lambda(1)) in the ionization process of organic amines and that (lambda(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 lambda value (= lambda(1) + lambda(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 lambda 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 lambda. The large lambda 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 lambda 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.