Radiationless transitions in azulene: Evidence for the ultrafast S2→S internal conversion

Abstract

The energy and isotope dependence of S₂→S₀ fluorescence of azulene vapor suggest that the S₂→S₀ internal conversion, already competitive with the S₂→S₁ internal conversion at zero excess energy, becomes the dominant photophysical process with vibrational excitation of molecules in S₂. In azulene‐d₀, the lifetime of the vibrationless level of the S₂ appears to be already governed largely by the S₂→S₀ internal conversion. The rate of the S₂→S₀ internal conversion at zero excess energy is estimated to be ∼2.2×10⁸ sec⁻¹ for azulene‐d₀ and ∼1.0×10⁸ sec⁻¹ for ‐d₈. These are almost three orders of magnitude greater than predicted by the energy gap correlation. The origin of this ultrafast S₂→S₀ internal conversion is attributed to the large configurational change associated with the transition.