No abstract

Reorganization energy (λ1) in the ionization process of organic amines and that (λ2) 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 (=λ1 + λ2) 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.

Fluorescence of fac-AlQ3 (Q=8-quinolinolato), mer-AlQ3, mer-Al(mQ)3 (mQ=4-methyl-8-quinolinolato), and BeQ2 were investigated with electronic structure calculations. The molecular structure of the first singlet excited state (the emission state) was optimized with the ab initio “configuration interaction with single excitations” (CIS) method. Ab initio CIS and semiempirical “Zerner’s intermediate neglect of differential overlap” (ZINDO) methods were used to calculate the emission energies (ΔE) and also the corresponding absorption energies. Although the ab initio CIS method overestimated the experimental value of ΔE by 1.09–1.16 eV, the ZINDO method reproduced it to a reasonable accuracy (within 0.26 eV). The optimized excited-state structure has an interesting feature in that one of the equivalent ligands distorts appreciably, while the thers keep their ground-state structures. As a result the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO) are localized on the distorted ligand. Since the excited state is characterized as the HOMO–LUMO transition, the emission from AlQ3 (and its analogues) directly reflects that of the ligand. In order to analyze this intrigung excited-state structure, exciton transfer theory was applied, where the exciton coupling between ligands and the structural relaxation of the ligand upon excitation were taken into account. By examining these two factors for BeQ2, it is shown that the exciton localization results from weak exciton coupling and/or large structural relaxation energy.

Substituent dependence of a fluorescence energy of the 8-quinolinolato anion (Q−) was systematically investigated with electronic structure calculations. Large red- and blueshifts of the fluorescence were predicted for Q− with the NO2 and CN groups, respectively. For bis(8-quinolinolato)berylium with the NO2 and CN groups, the emission energies were calculated to be 1.56 eV (795 nm) and 2.66 eV (466 nm), respectively, indicating that they are potential candidates as red- and blue-light-emitting compounds.