A Mechanistic Study of Halogen Addition and Photoelimination from π‑Conjugated Tellurophenes

The ability to drive
reactivity using visible light is of importance
for many disciplines of chemistry and has significant implications
for sustainable chemistry. Identifying photochemically active compounds
and understanding photochemical mechanisms is important for the development
of useful materials for synthesis and catalysis. Here we report a
series of photoactive diphenyltellurophene compounds bearing electron-withdrawing
and electron-donating substituents synthesized by alkyne coupling/ring
closing or palladium-catalyzed ipso-arylation chemistry. The redox
chemistry of these compounds was studied with respect to oxidative
addition and photoelimination of bromine, which is of importance for
energy storage reactions involving X2. The oxidative addition
reaction mechanism was studied using density functional theory, the
results of which support a three-step mechanism involving the formation
of an initial η1 association complex, a monobrominated
intermediate, and finally the dibrominated product. All of the tellurophene
derivatives undergo photoreduction using 430, 447, or 617 nm light
depending on the absorption properties of the compound. Compounds
bearing electron-withdrawing substituents have the highest photochemical
quantum efficiencies in the presence of an alkene trap, with efficiencies
of up to 42.4% for a pentafluorophenyl-functionalized tellurophene.
The photoelimination reaction was studied in detail through bromine
trapping experiments and laser flash photolysis, and a mechanism is
proposed. The photoreaction, which occurs by release of bromine radicals,
is competitive with intersystem crossing to the triplet state of the
brominated species, as evidenced by the formation of singlet oxygen.
These findings should be useful for the design of new photochemically
active compounds supported by main-group elements.