Design of All-Fused-Ring Electron Acceptors with High Thermal, Chemical, and Photochemical Stability for Organic Photovoltaics

High-performance donor-acceptor electron acceptors containing 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN)-type terminals are labile toward photooxidation and basic conditions, and new molecular designs toward electron acceptors that can achieve both high power conversion efficiencies and high stability are urgently needed. By replacing the central benzene ring in the classical ladder-type n-type semiconductor, 2,2'-(indeno[1,2-b]fluorene-6,12-diylidene)dimalononitrile, with the electron-rich 4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene, we report herein the design of 2,2'-(7,7,15,15-tetrahexyl-7,15-dihydro-sindaceno[1,2-b:5,6-b']diindeno[1,2-d]thiophene-2,10 (2H)-diylidene)dimalononitrile (ITYM), a new type of all-fused-ring electron acceptor (AFRA). A threestep reaction including a key Pd-catalyzed double C-H activation/intramolecular cyclization is established for the efficient synthesis of such type of electron acceptors. ITYM is confirmed by singlecrystal X-ray analysis, which shows a planar nonacyclic structure with strong pi-pi stacking. Compared with the classical carbon-bridged INCN-type acceptors, ITYM exhibits extraordinary stability with very promising performance. The AFRA concept opens a new avenue toward high-efficiency and -stability organic photovoltaics (OPVs).