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‐4Cl. Benefiting from the broadened absorption and promoted nonradiative transition, LY1 and LY2 display enhanced photothermal conversion performance, presenting as promising pure organic photothermal materials.