Composition having an organic semiconducting material and a triplet-accepting material of formula (I) with a triplet energy level lower than the triplet energy level of the organic semiconducting material, in which each Ar is optionally substituted aryl or heteroaryl group, n is 1-3, m is 1-5, q is 0 or 1, each R3 is H or a substituent, and each R4 is H or a substituent. Where R4 is not H, R4 and (Ar)m bound to the same carbon atom may be linked by a direct bond or a divalent group. Where n or m is at least 2, adjacent Ar groups may be linked by a divalent group. Where q=0, R3 is not H and is linked to (Ar)n by a direct bond or a divalent group.