The present invention relates to a compound having a four-coordinate boron atom to which is connected a first C3-C25 substituent, a second C3-C25 substituent and a bridging substituent comprising fromeight to forty non-hydrogen atoms and having two bonds to the boron atom; wherein: (i) each of the first and second C3-C25 substituents is bonded to the boron atom through a carbon, nitrogen or oxygen atom, and, optionally the first and second C3-C25 substituents are connected to form a single substituent having two bonds to the boron atom; (ii) at least one of the first and second C3-C25 substituents is a C6-C25 aromatic substituent; (iii) said aromatic substituent comprises at least one nitrogen atom which is bonded only to carbon or boron atoms; and (iv) the bridging substituent has at least one oxygen, nitrogen, sulfur or phosphorus atom bonded to the boron atom and at least one aromatic ring.