In OLEDs, improved efficiency is obtained by using compounds of formula [Ar](CH═CH—R1)n wherein n is an integer from 1 to 4; [Ar] is a polycyclic aromatic or heteroaromatic scaffold optionally substituted with one or more alkyl or alkoxy groups; and R1 is a 5-membered heteroaryl group optionally substituted with methyl, methoxy, aryl or heteroaryl, or is phenyl or naphthyl optionally substituted with methyl, methoxy, trifluoromethyl or cyano or is biphenyl or substituted biphenyl. The compounds are believed to be novel and can be made by condensing a compound of formula wherein [Ar] and n are as defined above with a compound of formula [Ar](CH3)nR1CHO in the presence of an acid catalyst e.g. an anhydride of an organic acid.