Interaction between Excited β-Naphthol and Pyridine. Hydrogenbond Formation in S* and H-Transfer Reaction in T

Abstract

The interaction between the excited β-naphthol and pyridine in cyclohexane has been investigated by an absorption-emission flash technique. The triplet naphthol has been found to be deactivated by pyridine with the rate constant of 1.5–1.9×10⁹ M⁻¹sec⁻¹, yielding naphthoxyl radical. With 10⁻⁴ M pyridine the triplet naphthol is completely transformed into naphthoxyl radical. Above 10⁻¹ M pyridine naphthol exists almost exclusively as a H-bonded species in the ground state, which is non-fluorescent and produces no transient species detectable by an ordinary flash apparatus. For 10⁻⁴–10⁻¹ M pyridine, the singlet excited naphthol undergoes collisional deactivation which most likely leads to a H-bonded species (rate constant, 1.2–1.4×10¹⁰ M⁻¹sec⁻¹). At the same time naphthoxyl radical is also produced with about the same rate constant as that for the triplet state. The results strongly suggest that the mechanism for the internal conversion of the singlet excited state via H-bonding is a transient H-atom transfer.