Chapter 8 - Azobenzene photoisomerization: Two states and two relaxation pathways explain the violation of Kasha's rule

Abstract

Azobenzene is a photochromic molecule, which undergoes trans-cis isomerization upon irradiation in the near-UV. The photo-isomerization reaction is ultrafast, efficient, and clean and has attracted much attention for its possible application in molecular electronics, data storage and nonlinear optics. In violation of Kasha's rule, the isomerization yields vary anomalously with the excitation wavelength. Two possible reasons have been proposed to explain the violation of Kasha's rule. Based on the investigation of quantum yields in substituted azobenzenes, Rau suggested a large deformation of the molecular structure along the torsional coordinate in S2, which would quench the isomerization reaction proceeding along the inversion coordinate in S1. Fujino et al. could not find evidence for torsional motion in time-resolved fluorescence and resonance-Raman spectra but observed ultrafast S2→ S1 internal conversion. To explain the violation of Kasha's rule, Fujino assumed that additional relaxation pathways for high vibrational levels in Si could quench the observed quantum yield after excitation of S2. The photoisomerization reaction was studied by time resolved photoelectron spectroscopy (TRPES). It proposes a new relaxation pathway, which can reconcile the two conflicting models. First results were recently published in a communication. This chapter presents an improved data analysis and a detailed discussion of the experimental data.