A composition comprising an electron acceptor material and an electron donor material wherein the electron acceptor material is a compound of formula (I): EAG-EDG-EAG (I) wherein each EAG is an electron-accepting group and EDG is a group of formula (II): (II) wherein: n is at least 1; each m is independently 0 or at least 1; each X, Y and A is independently O, S or Se; Z, independently in each occurrence if n is greater than 1, is O, S, C═O or NR9 wherein R9 is H or a substituent; and R1-R8 are each independently selected from H or a substituent. The composition may be used as photosensitive organic layer of an organic photodetector.