Mechanism of photodimerization of acenaphthylene

The mechanism of photodimerization of acenaphthylene (ACN) has been investigated in order to elucidate the roles of the singlet and the triplet excited states of ACN in the formation of the Z- and E-dimers in several solvents. The quantum yields and the ratio of the produced Z- to E-dimer were determined under irradiation of ACN at 435.8 nm in several solvents (1,2-dichloroethane, acetonitrile, cyclohexane, benzene, methanol, and DMF) over a wide concentration range (2.0 x 10(-4)-2.0 M) in the absence of additives and in the presence of 1,2-dibromoethane, a heavy-atom-containing solvent; Eosin-Y, a triplet sensitizer (irradiated at 546.1 nm); and ferrocene, a triplet quencher. On irradiation of ACN in dilute solution, the initially generated S-1 state crosses over to the T-1 state, though with low quantum yield, before reaction with the ground state ACN can occur due to the very short lifetime of the S-1 state, and the resultant T-1 state reacts with ground state ACN to give a mixture of the Z- and E-dimers in a ratio that depends on the solvent employed. In more concentrated solution, the S-1 state reacts with ACN before intersystem crossing to the T-1 state can occur, affording exclusively the Z-dimer.