A material comprising an electron-accepting unit of formula (I): wherein Ar1 and Ar2 independently is a 5- or 6-membered aromatic or heteroaromatic ring or is absent; and each X is independently H or a substituent with the proviso that at least one X is an electron-withdrawing group and wherein X groups bound to adjacent carbon atoms may be linked to form an electron-withdrawing group. The material further comprises an electron-donating unit D comprising a fused or unfused furan or thiophene. The material may be a polymer comprising repeat units of formula (I). The material may be a non- polymeric compound. An organic photodetector may contain a bulk heterojunction layer containing an electron acceptor or an electron donor wherein at least one of the electron acceptor and electron donor contains a unit of formula (I).