LASER FLASH-PHOTOLYSIS OF DINAPHTHYL KETONES

The photophysics and photochemistry of 1,1'-, 1,2'-, and 2,2'-dinaphth yl ketones and a related 2,3,5,6-dibenzofluorenone were studied by las er flash photolysis techniques in organic solvents. Their photochemist ry is dominated by the triplet state, which has a pi,pi character. Th is is consistent with their phosphorescence spectra, as is the high ef ficiency of the sensitization of singlet oxygen generation, Phi=0.9-0. 96 in acetonitrile. Self-quenching plays an important role in determin ing triplet lifetimes in solution. The dinaphthyl ketone triplets are not photoreduced by 2-propanol, a common hydrogen donor toward n,pi k etone 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), triethyla mine, 1,4-cyclohexadiene and 2,4,6-trimethylphenol indicate their stro ng preference for electron transfer. For example, the electron transfe r from DABCO to 2,3,5,6-dibenzofluorenone triplet in methanol occurs w ith k=9.9X10(8) M-1 s(-1), and is almost two orders of magnitude faste r than H-atom abstraction from an excellent H-atom donor 1,4-cyclohexa diene (k=1.4X10(7) M-1 s(-1)). Hydrogen bonding between the reactants plays a major role in the reactions with 2,4,6-trimethylphenol. The re activities in non-hydrogen bonding hexane, are considerably higher tha n in methanol, where the hydrogen bonds with the solvent hinder the in teraction between the reactants. (C) 1997 Elsevier Science S.A.