Quantum chemical study on the mechanism for the cycloreversion of anthracene–benzene and naphthalene–benzene [4 + 4] cycloadducts

The reaction mechanisms for cycloreversion of anthracene–benzene and naphthalene–benzene [4 + 4] cycloadducts have been studied by CASPT2//CASSCF calculations. Upon UV excitation, the initially populated 1(π, π∗) state is converted to the 1(σ, σ∗) state, which undergoes the cycloreversion adiabatically to give the product in the singlet excited state. The barrierless and downhill nature of the potential energy curve accounts for the high quantum yield of anthracene in the 1La state. In the naphthalene–benzene system, the 1Lb state of naphthalene is formed via internal conversion from the 1La state that is directly generated in the reaction.