A compound of formula: wherein: M is a transition metal; Ar 1 is a polycyclic heteroaromatic group; Ar 2 is a polycyclic group comprising two or more rings selected from aromatic and heteroaromatic rings; Ar 1 and Ar 2 may be linked by a direct bond or divalent linking group; L is a ligand other than a ligand of formula: x is at least 1; y is 0 or a positive integer; a is 0 or a positive integer; b is 0 or a positive integer; R 1 and R 2 are each independently in each occurrence a substituent selected from the group consisting of: - C 1-20 alkyl wherein one or more non-adjacent C atoms of the C 1-20 alkyl may be replaced with -O-, -S-, C=O or -COO- and one or more H atoms may be replaced with F; and - a group of formula -(Ar 3)p wherein Ar 3 in each occurrence is a C 6-20 aryl group or a 5-20 membered heteroaryl group that may be unsubstituted or substituted with one or more substituents selected from the group consisting of F, CN, NO 2 and C 1 -20 alkyl wherein one or more non-adjacent C atoms of the C 1-20 alkyl may be replaced with -O-, -S-, C=O or -COO- and one or more H atoms may be replaced with F; and p is at least 1; and R 3 in each occurrence is independently selected from the group consisting of: - C 1-20 alkyl wherein one or more non-adjacent C atoms of the C 1-20 alkyl may be replaced with -O-, -S-, C=O or -COO- and one or more H atoms may be replaced with F; and - a group of formula -(Ar 4)q wherein Ar 4 in each occurrence is a C 6-20 aryl group or a 5-20 membered heteroaryl group that may be unsubstituted or substituted with one or more substituents selected from the group consisting of F, CN, NO 2 and C 1 -20 alkyl wherein one or more non-adjacent C atoms of the C 1-20 alkyl may be replaced with -O-, -S-, C=O or -COO- and one or more H atoms may be replaced with F; and q is at least 1. M is a transition metal; Ar 1 is a polycyclic heteroaromatic group; Ar 2 is a polycyclic group comprising two or more rings selected from aromatic and heteroaromatic rings; Ar 1 and Ar 2 may be linked by a direct bond or divalent linking group; L is a ligand other than a ligand of formula: x is at least 1; y is 0 or a positive integer; a is 0 or a positive integer; b is 0 or a positive integer; R 1 and R 2 are each independently in each occurrence a substituent selected from the group consisting of: - C 1-20 alkyl wherein one or more non-adjacent C atoms of the C 1-20 alkyl may be replaced with -O-, -S-, C=O or -COO- and one or more H atoms may be replaced with F; and - a group of formula -(Ar 3)p wherein Ar 3 in each occurrence is a C 6-20 aryl group or a 5-20 membered heteroaryl group that may be unsubstituted or substituted with one or more substituents selected from the group consisting of F, CN, NO 2 and C 1 -20 alkyl wherein one or more non-adjacent C atoms of the C 1-20 alkyl may be replaced with -O-, -S-, C=O or -COO- and one or more H atoms may be replaced with F; and p is at least 1; and - C 1-20 alkyl wherein one or more non-adjacent C atoms of the C 1-20 alkyl may be replaced with -O-, -S-, C=O or -COO- and one or more H atoms may be replaced with F; and - a group of formula -(Ar 3)p wherein Ar 3 in each occurrence is a C 6-20 aryl group or a 5-20 membered heteroaryl group that may be unsubstituted or substituted with one or more substituents selected from the group consisting of F, CN, NO 2 and C 1 -20 alkyl wherein one or more non-adjacent C atoms of the C 1-20 alkyl may be replaced with -O-, -S-, C=O or -COO- and one or more H atoms may be replaced with F; and p is at least 1; and R 3 in each occurrence is independently selected from the group consisting of: - C 1-20 alkyl wherein one or more non-adjacent C atoms of the C 1-20 alkyl may be replaced with -O-, -S-, C=O or -COO- and one or more H atoms may be replaced with F; and - a group of formula -(Ar 4)q wherein Ar 4 in each occurrence is a C 6-20 aryl group or a 5-20 membered heteroaryl group that may be unsubstituted or substituted with one or more substituents selected from the group consisting of F, CN, NO 2 and C 1 -20 alkyl wherein one or more non-adjacent C atoms of the C 1-20 alkyl may be replaced with -O-, -S-, C=O or -COO- and one or more H atoms may be replaced with F; and q is at least 1. - C 1-20 alkyl wherein one or more non-adjacent C atoms of the C 1-20 alkyl may be replaced with -O-, -S-, C=O or -COO- and one or more H atoms may be replaced with F; and - a group of formula -(Ar 4)q wherein Ar 4 in each occurrence is a C 6-20 aryl group or a 5-20 membered heteroaryl group that may be unsubstituted or substituted with one or more substituents selected from the group consisting of F, CN, NO 2 and C 1 -20 alkyl wherein one or more non-adjacent C atoms of the C 1-20 alkyl may be replaced with -O-, -S-, C=O or -COO- and one or more H atoms may be replaced with F; and q is at least 1.