Regioisomerism in cationic sulfonyl-substituted [Ir(C^N)2(N^N)]+ complexes: its influence on photophysical properties and LEC performance

Abstract

A series of regioisomeric cationic iridium complexes of the type [Ir(C^N)₂(bpy)][PF₆] (bpy = 2,2′-bipyridine) is reported. The complexes contain 2-phenylpyridine-based cyclometallating ligands with a methylsulfonyl group in either the 3-, 4- or 5-position of the phenyl ring. All the complexes have been fully characterized, including their crystal structures. In acetonitrile solution, all the compounds are green emitters with emission maxima between 493 and 517 nm. Whereas substitution meta to the Ir–C bond leads to vibrationally structured emission profiles and photoluminescence quantum yields of 74 and 77%, placing a sulfone substituent in a para position results in a broad, featureless emission band, an enhanced quantum yield of 92% and a shorter excited-state lifetime. These results suggest a larger ligand-centred (³LC) character of the emissive triplet state in the case of meta substitution and a more pronounced charge transfer (CT) character in the case of para substitution. Going from solution to the solid state (powder samples and thin films), the emission maxima are red-shifted for all the complexes, resulting in green-yellow emission. Data obtained from electrochemical measurements and density functional theory calculations parallel the photophysical trends. Light-emitting electrochemical cells (LECs) based on the complexes were fabricated and evaluated. A maximum efficiency of 4.5 lm W⁻¹ at a maximum luminance of 940 cd m⁻² was observed for the LEC with the complex incorporating the sulfone substituent in the 4-position when operated under pulsed current driving conditions.