Excited State Behavior of Aromatic Amines in Solution: Dihydrophenazine Derivatives

Abstract

AbstractAbsorption and emission spectra as well as emission quantum yields and lifetimes of 5,10‐dimethylphenazine (DMP), 5‐methyl‐10‐phenylphenazine, and 5,10‐diphenylphenazine were measured in 3‐methylpentane at room temperature and 77°K. By means of relative polarization measurements in conjunction with CNDO/2‐CI calculations, the excited states were assigned. The long natural lifetime of the fluorescence (τf0 (DMP) ∼ 210 ns), together with the results of the calculation, reveals that the first transition is a symmetry forbidden 1A1 → 1A2 transition in a C2v geometry bent along the N‐N axis. This transition has an oscillator strength f0 ∼ 0.006 for all three compounds as determined from the fluorescence data. We propose that it acquires most of its intensity through a vibronic interaction with S2(1B1) (Herzberg‐Teller coupling). Since intersystem crossing between S1(1A2) and T1(3A2) is also orbitally forbidden, we suggest, in accordance with the phosphorescence polarization, an electronic spin‐orbit coupling between S2 and T1. Replacement of the methyl substituents by phenyl groups increases kisc and enhances the temperature dependent behavior of this parameter. Non‐radiative transitions occuring from the lowest triplet state account for nearly 40% of the absorbed quanta.