Room-Temperature Stable Noncovalent Charge-Transfer Dianion Biradical to Produce Singlet Oxygen by Visible or Near-Infrared Light Photoexcitation

Noncovalent
interaction between small molecules can generate a
charge-transfer (CT) state, achieving the effect of a conjugated large
molecule as well as a transition-metal complex. Herein, we demonstrate
a room-temperature stable dianion biradical conveniently produced
by noncovalent intermolecular CT interaction between anthraquinone
(AQ) and potassium tert-butoxide (KOtBu). Essentially,
CT from KOtBu to AQ boosts absorption bands from the UV to visible
and near-infrared (NIR) range, enabling AQ-KOtBu to have new absorption
bands around 400, 550, and 900 nm. The absorption bands of AQ-KOtBu
are dramatically enhanced after blue-to-green or NIR light excitation.
Interestingly, both ground state AQ–KOtBu (C(1))
and photoexcited AQ–KOtBu (C(2)) are quenched by
oxygen to produce singlet oxygen. Furthermore, C(1) can
be photoactivated by purged nitrogen in solution, and C(2) can be regenerated after the photoexcitation and purged nitrogen
in solution, which may serve as a photosensitizer under visible and
NIR light excitation.