In OLEDs, improved efficiency is obtained by compounds which can form inter alia electron injection layers of the formula (I) wherein R1 is a 1-5 ring aryl (including polycyclic), aralkyl or heteroaryl group which is optionally substituted with one or more C1-C4 alkyl, alkoxy or cyano; R2 and R3 together form a 1-5 ring aryl (including polycyclic), aralkyl or heteroaryl group which is optionally substituted with C1-C4 alkyl, alkoxy or cyano; R4 is hydrogen, C1-C4 alkyl or aryl; and Ar is monocyclic, bicyclic or tricyclic aryl or heteroaryl which is optionally substituted with one or more C1-C4-alkyl or alkoxy groups, or an oligomer thereof.