A compound of formula (I): (Core)n-(X)m wherein Core is a core group; n is 0 and m is 1, or n is 1 and m is at least 1; and X is a group of formula (II): wherein: R1, R3 and R5 are each independently H or a substituent; R2 and R4 are each a substituent; one of R1-R5 is a direct bond or divalent linking group linking the group of formula (II) to Core in the case where n is 1; x and y are 0, 1, 2, 3 or 4; and the compound of formula (I) is substituted with at least one ionic substituent. The compound may be used as an n-dopant to dope an organic semiconductor.