A compound of formula (I): EAG-EDG-EAG (I) wherein EDG is an electron-donating group comprising a polycyclic heteroaromatic group and each EAG is an electron-accepting group of formula (II): (II) wherein R10 in each occurrence is H or a substituent; --- is a bond to EDG; and each X1-X4 is independently CR11 or N wherein R11 in each occurrence is H or a substituent, with the proviso that at least one occurrence of at least one of X1-X4 is N. The compound may be used as an acceptor in a bulk heterojunction layer of an organic photodetector.