Stable acceptor-donor-acceptor type open-shell diradical with ultra-narrow bandgap and efficient photothermal conversion

It has been reported recently that open-shell diradical character could exist in narrow bandgap donor-acceptor (D-A) organic semiconductors in our previous work. The A-D-A type molecules play as important role in the organic electronic such as donors and acceptors (ITIC and Y6) in organic solar cells. However, their relatively poor chemical and photostability prevent their industrial application. In this work, we reported a stable A-D-A type open-shell diradical, named LY1-4Cl, by replacing the 2-(3-oxo-2,3-dihydro-1H-inden-1-yli-dene)malononitrile (IC)-series end group of typical closed-shell non-fullerene acceptor IT-4Cl. The new electron-withdrawing building block could give LY1-4Cl the redshift near-infrared (NIR) absorption, the smaller bandgap and highly enhanced photostability comparing with IT-4Cl. The open-shell diradical character of LY1-4Cl originates from the formation of quinoid diradical form. Moreover, benefiting from the broad NIR absorption and radical-promoted nonradiative transition, LY1-4Cl displayed high photothermal properties that an increasing temperature of 188℃ was recorded within 60 s under 808 nm laser irradiation of 0.8 W cm-2. This research provides a new strategy to design IC-free A-D-A type open-shell quinoid diradical with ultra-narrow bandgap, good photostability and efficient photothermal conversion.