Stable Open-Shell IC-Fused Fluorenyl Enabling Efficient Photothermal Conversion

2-(3-Oxo-2,3-dihydro-1H-inden-1-yli-dene)malononitrile (IC) derivatives are popular and crucial acceptor building blocks for the construction of highly efficient n-type organic semiconductors. However, it is still challenging to increase their relatively poor chemical stability and photostability, originating from their unstable vinyl protons. Herein, two stable acceptor-donor-acceptor narrow-bandgap materials LY1 and LY2 via replacing the IC series end groups of typical nonfullerene acceptors with IC-fused fluorenyls are reported. The enhanced conjugation and strong electron-withdrawing capability enable LY1 and LY2 to show lower bandgap, a deeper lowest unoccupied molecular orbital energy level, enhanced electrochemical stability, and significantly improved photostability compared with IT-4F and IT-4CI. Benefiting from the broadened absorption and promoted nonradiative transition, LY1 and LY2 display enhanced photothermal conversion performance, presenting as promising pure organic photothermal materials.