Carbazole–Benzonitrile–Norbornadiene Conjugates for Photothermally Reversible Ambient Phosphorescence

Organic photoswitches have attracted
significant attention across
various fields, such as sensing, bioimaging, photopharmacology, molecular
machines, and solar energy storage. However, as a result of design
complexities, achieving photothermally reversible ambient phosphorescence
switching in the condensed state remains elusive. Herein, we explore
the impact of norbornadiene (NBD)/quadricyclane (QC) substitution
at position 5 of the benzonitrile acceptor covalently attached to
the carbazole donor on photothermally reversible luminescence switching.
Experimental investigations demonstrated that the CzN and TBCzN switches
exhibited photothermally reversible fluorescence switching in solution.
Moreover, in the condensed state, fluorescence and ambient phosphorescence
switching were observed as a result of a low singlet–triplet
(ΔEST) gap (CzN ⇆ CzQ, ΔESTCzN/CzQ = 0.05/0.28 eV; TBCzN ⇆ TBCzQ, ΔESTTBCzN/TBCzQ = 0.06/0.09
eV). Reversible ambient phosphorescence switching is primarily influenced
by modulation of acceptor conjugation resulting from NBD ⇆
QC switching. This approach may provide important clues for the design
of visible-light-absorbing photothermally reversible phosphorescent
materials.