Effect of Twisting on the Capture and Release of Singlet Oxygen by Tethered Twisted Acenes

The use of polyaromatic hydrocarbons
to capture and release singlet oxygen is of considerable importance
in materials chemistry, synthesis, and photodynamic therapy. Here
we studied the ability of a series of tethered twistacenes, possessing
different degrees of backbone twist, to capture and release singlet
oxygen via the reversible Diels–Alder reaction. When the twistacene
acts as both a sensitizer and a diene, the photo-oxidation rate depends
on the extinction coefficient of the irradiation wavelength. However,
when the twistacenes function solely as a diene, the rate of photo-oxidation
increases with increasing twist. The rate of the reverse reaction,
the singlet oxygen release, also increases with increasing twist.
The calculated transition state energy decreases with increasing twist,
which can explain the observed trend. The presence of the tether significantly
increases the reversibility of the reaction, which can proceed in
repeated forward and reverse cycles in very high yield under mild
conditions, as required for molecular switches.