Photodimerization of 1-Phenylcyclohexene. A Novel Transient−Transient Component

Low-temperature irradiations of 1-phenylcyclohexene (PC6) yield
two singlet state 2 + 2 photodimers
and two 4 + 2 cycloadducts. The 4 + 2 adducts are not observed
in low intensity lamp irradiations at room temperature
but are produced in significant quantities when PC6 solutions are
subjected to higher intensity laser irradiation. The
results indicate a reaction mechanism involving two
trans-PC6 molecules. The findings are consistent with
earlier
kinetic observations that the decay of trans-PC6 adopts a
significant second-order component at lower temperatures.
The transient−transient reaction also occurs under
triplet-sensitized low-temperature/low-intensity or room
temperature/high-intensity irradiation conditions. Triplet-sensitized
irradiations also yield significant amounts of the 2 + 2
photodimers produced by attack of the PC6 triplet on its ground state.
Quantum yield studies reveal that the rate of
addition of 3PC6* to its ground state is in the range
0.28 to 4.0 × 105 L mol-1 s-1
consistent with the 1,2-biradical
model for alkene triplet reactivity.