Photodissociation of α-naphthol in solution: influence of hydrogen bonding

Abstract

The photodissociation of α-naphthol in liquid solutions was studied by flash photolysis. The transient absorption spectra in n-hexane, isooctane and carbon tetrachloride solutions decay bimolecularly and are attributed mainly to the α-naphthoxyl radical: the spectra obtained on adding proton acceptors Et₂O, dioxan or THF to n-hexane are different and decay unimolecularly; they are assigned to α-naphthol triplet-triplet transitions. Results with added acetophenone as triplet energy donor to excite α-naphthol to the triplet state directly, with piperylene as triplet quencher, and with varying flash intensity show that radical formation proceeds monophotonically from the lowest triplet state of α-naphthol. Addition of Et₂O to n-hexane solutions reduced radical formation but enhanced triplet formation and lifetime; these results along with a kinetic analysis suggest that photodissociation is suppressed by hydrogen bonding of triplet α-naphthol to proton acceptors such as Et₂O.