Laser flash photolysis of dinapthyl ketones

Abstract

The photophysics and photochemistry of 1,1′-, 1,2′-, and 2,2′-dinaphthyl ketones and a related 2,3,5,6-dibenzofluorenone were studied by laser flash photolysis techniques in organic solvents. Their photochemistry is dominated by the triplet state, which has a π, π ∗ character. This is consistent with their phosphorescence spectra, as is the high efficiency of the sensitization of singlet oxygen generation, φ = 0.9–0.96 in acetonitrile. Self-quenching plays an important role in determining triplet lifetimes in solution. The dinaphthyl ketone triplets are not photoreduced by 2-propanol, a common hydrogen donor toward n,π ∗ ketone triplets. The energies of dinaphthyl ketone triplets range from E T = 53.8 for rigid 2,3,5,6-dibenzofluorenone to 59.2 kcal mol −1 for flexible 2,2′-dinaphthyl ketone, and reflect the degree of conjugation of the carbonyl group and naphthalene rings promoted by their ability to approach a coplanar conformation. The reactions of the dinaphthyl. ketone triplets with 1,4-diazabicyclo[2.2.2]octane (DABCO), triethylamine, 1,4-cyclohexadiene and 2,4,6-trimethylphenol indicate their strong preference for electron transfer. For example, the electron transfer from DABCO to 2,3,5,6-dibenzofluorenone triplet in methanol occurs with k = 9.9 × 10 8 M −1 s −1, and is almost two orders of magnitude faster than H-atom abstraction from an excellent H-atom donor 1,4-cyclohexadiene ( k = 1.4 × 10 7 M −1 s −1). Hydrogen bonding between the reactants plays a major role in the reactions with 2,4,6-trimethylphenol. The reactivities in non-hydrogen bonding hexane, are considerably higher than in methanol, where the hydrogen bonds with the solvent hinder the interaction between the reactants.