Effect of Molecular Geometry on the Electronic Structure of Aromatic Amines: Diphenylamine, Iminodibenzyl, Acridan, and Carbazole

Abstract

AbstractAbsorption and emission properties of a series of aromatic amines (Diphenylamine (DPA), Iminodibenzyl (IBB). Acridan (AC) and Carbazole (C)) whose molecular geometries gradually change from a distinctly non‐planar to a planar geometry have been studied in EPA glass at 77°K. Using polarization results in conjunction with CNDO/2‐CI calculations it is shown that the first broad absorption band of DPA, IBB, and AC is a superposition of two electronic transitions. These transitions have been assigned as 1A1 → 1A1 (30 kK) and 1A1 → 1B1 (34 kK) ππ*‐transitions for IBB, AC, and C (all of dominant C2v symmetry) and 1A → 1B transitions in DPA (C2 symmetry). The first transitions, of lower intensity, are related to the benzene A1g → B2u transition, but contain some charge‐transfer character. The intensities of the second transitions decrease from DPA to C. In the former case, the major component involves charge‐transfer, but in the latter, electron density rearrangements are restricted to the hydrocarbon skeleton alone.Direct spin‐orbit coupling mechanism to 1B2 (1A in DPA) perturbing singlets appears to dominate intersystem crossing but the strong vibronic activity in the phosphorescence spectra suggests that spin‐vibronic routes are also important contributors for AC, IBB, and DPA.