ENHANCED PHOTOREACTIVITY OF ACENAPHTHYLENE IN WATER - A PRODUCT AND MECHANISM STUDY

Acenaphthylene undergoes direct photolysis in dilute aqueous solutions at concentrations as low as 5 x 10(-7) M. The reactivity observed for acenaphthylene in water greatly exceeds that observed in organic solv ents. The major products from direct photolysis in aerated aqueous sol utions at concentrations above 5 x 10(-5) M are the cis and trans dime rs of acenaphthylene. Triplet-sensitized dimerization in water produce s a large excess of cis dimer (cis/trans = 14); however, direct photol ysis initially produces a small excess of cis dimer (cis/trans = 1.5) and upon further conversion the ratio changes to favor the trans dimer (cis/trans = 0.6). The origin of the trans dimer is assigned to photo lysis of crystalline acenaphthylene which is thought to form as a copr ecipitate with cis dimer during photolysis, The origin of the cis dime r is assigned to dimerization from the triplet manifold. The increased efficiency of acenaphthylene photolysis is attributed to a hydrophobi c effect and partly to an enhanced efficiency of dimerization in cryst alline acenaphthylene.