Excited State Behavior of Aromatic Amines in Solution: Dihydrophenazine Derivatives

Abstract

Abstract Absorption 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 (τ f 0 (DMP) ∼ 210 ns), together with the results of the calculation, reveals that the first transition is a symmetry forbidden 1 A 1 → 1 A 2 transition in a C 2v geometry bent along the N‐N axis. This transition has an oscillator strength f 0 ∼ 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 S 2 ( 1 B 1 ) (Herzberg‐Teller coupling). Since intersystem crossing between S 1 ( 1 A 2 ) and T 1 ( 3 A 2 ) is also orbitally forbidden, we suggest, in accordance with the phosphorescence polarization, an electronic spin‐orbit coupling between S 2 and T 1 . Replacement of the methyl substituents by phenyl groups increases k isc 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.