Comparison of the Photochemistry of Acyclic and Cyclic 4‑(4-Methoxy-phenyl)-4-oxo-but-2-enoate Ester Derivatives

To clarify the cis–trans
isomerization mechanism of simple
alkenes on the triplet excited state surface, the photochemistry of
acyclic and cyclic vinyl ketones with a p-methoxyacetophenone
moiety as a built-in triplet sensitizer (1 and 2, respectively) was compared. When irradiated, ketone 1 produces its cis-isomer, whereas ketone 2 does
not yield any photoproducts. Laser flash photolysis of ketone 1 yields a transient spectrum with λmax ∼
400 nm (τ ∼ 125 ns). This transient is assigned to the
first triplet excited state (T1) of 1, which
presumably decays to form a triplet biradical (1BR) that
is shorter lived than the triplet ketone. In comparison, laser flash
photolysis of 2 reveals two transients (τ ∼
20 and 440 ns), which are assigned to T1 of 2 and triplet biradical 2BR, respectively. Density functional
theory calculations support the characterization of the triplet excited
states and the biradical intermediates formed upon irradiation of
ketones 1 and 2 and allow a comparison of
the physical properties of the biradical intermediates. As the biradical
centers in 2BR are stabilized by conjugation, 2BR is more rigid than 1BR. Therefore, the longer lifetime
of 2BR can be attributed to less-efficient intersystem
crossing to the ground state.