Shift from an inverse to a normal isotope effect on the non-radiative decay rate of anthracene-d₁₀ observed under partially jet-cooled conditions

The absorption and fluorescence excitation spectra of anthracene-d₁₀ in the first excited singlet state were measured under partially jet-cooled conditions. From the ratio of the corresponding peak intensities of both spectra the fluorescence quantum yields were evaluated for a few sequence bands as well as single vibronic levels. In contrast to the case of anthracene-h₁₀, the values for the sequence bands (Φ_f=0.17-0.24) exceed that of the 0₀⁰ band (Φ_f=0.13), suggesting a resume of normal isotope effect on intersystem crossing. This novel finding is discussed in relation to the fluorescence lifetimes of anthracene-d₁₀ measured in the gas phase.