Conjugated, rigidified bibenzimidazole ancillary ligands for enhanced photoluminescence quantum yields of orange/red-emitting iridium(iii) complexes

A series of six novel [Ir(C<^>N)(2)(N<^>N)](PF6) complexes (C<^>N is one of two cyclometalating ligands: 2-phenyl-4-(2,4,6-trimethylphenyl)pyridine, MesppyH, or 2-(napthalen-1-yl)-4-(2,4,6-trimethylphenyl)pyridine, MesnpyH; N<^>N denotes one of four neutral diamine ligands: 4,4 '-di-tert-butyl-2,2 '-bipyridine, dtbubpy, 1H,1 ' H-2,2 '-bibenzimiazole, H(2)bibenz, 1,1 '-(alpha,alpha '-o-xylylene)-2,2 '-bibenzimidazole, o-xylbibenz or 2,2 '-biquinoline, biq) were synthesised and their structural, electrochemical and photophysical properties comprehensively characterised. The more conjugated MesnpyH ligands confer a red-shift in the emission compared to MesppyH but maintain high photoluminescence quantum yields due to the steric bulk of the mesityl groups. The H(2)bibenz and o-xylbibenz ligands are shown to be electronically indistinct to dtbubpy but give complexes with higher quantum yields than analogous complexes bearing dtbubpy. In particular, the rigidity of the o-xylbibenz ligand, combined with the steric bulk of the MesnpyH C<^>N ligands, gives a red-emitting complex 4 (lambda(PL) = 586, 623 nm) with a very high photoluminescence quantum yield (phi(PL) = 44%) for an emitter in that region of the visible spectrum. These results suggest that employing these ligands is a viable strategy for designing more efficient orange-red emitters for use in a variety of photophysical applications.