Vibronic coupling and intersystem crossing in aromatic amines

Abstract

The fluorescence and phosphorescence spectra of the aromatic amines acridan, iminobibenzyl, and carbazole have been measured in Shpolskii matrices at ∼10 K. Under these conditions the emission exhibits a detailed vibrational structure which has been analyzed. The change of the polarization degree observed within the fluorescence spectra at 77 K, particularly pronounced in acridan and iminobibenzyl, is attributed to vibronic interaction between the closely lying S 1( 1A 1) and S 2( 1B 1) excited states. This process activates a b 1 vibration with a frequency of ∼1200 cm −1 in the ground state. The appearance of a long-axis ( b 1) polarized vibration (∼700 cm −1) following the out-of-plane polarized 0-0 band of the phosphorescence of these amines at 77 K is suggested to arise from vibronic interactions in the triplet manifold. This second-order spin-orbit coupling (soc) process is superimposed upon the dominant first-order electronic soc mechanism, which couples the lowest π, π ∗ triplet with high-energy (σ, π) ∗ singlet states.