Single-molecule fluorescence autocorrelation experiments on pentacene: The dependence of intersystem crossing on isotopic composition

Single pentacene molecules containing C13 or H1 in a pentacene-d14 doped p-terphenyl crystal have been studied by fluorescence autocorrelation. The triplet dynamics has been analyzed and a systematic dependence of the S1→T1 intersystem crossing rate on isotopic composition was found. This variation is discussed in terms of a modulation of the near resonance of the first excited singlet state S1 and vibrational levels of an intermediating triplet state T3 which results from the distinct isotope dependence of the zero-point energy of both electronic states.