Colour tuning by the ring roundabout: [Ir(C^N)2(N^N)]+ emitters with sulfonyl-substituted cyclometallating ligands

Abstract

A series of cationic bis-cyclometallated iridium(III) complexes [Ir(C^N)₂(N^N)]⁺ is reported. Cyclometallating C^N ligands are based on 2-phenylpyridine with electron-withdrawing sulfone substituents in the phenyl ring: 2-(4-methylsulfonylphenyl)pyridine (H1) and 2-(3-methylsulfonylphenyl)pyridine (H2). 2-(1H-Pyrazol-1-yl)pyridine (pzpy) and 2-(3,5-dimethyl-1H-pyrazol-1-yl)pyridine (dmpzpy) are used as electron-rich ancillary N^N ligands. The complexes have been fully characterized and the single crystal structure of [Ir(2)₂(dmpzpy)][PF₆]·MeCN has been determined. Depending on the position of the methylsulfonyl group, the complexes are green or blue emitters with vibrationally structured emission maxima at 491, 523 nm for [Ir(1)₂(N^N)][PF₆] or 463, 493 nm for [Ir(2)₂(N^N)][PF₆] in MeCN solution. The marked vibrational structure and the absence of a rigidochromic shift, together with theoretical predictions based on density functional theory calculations, confirm the ³LC nature of the emitting triplet state. All four complexes have relatively high photoluminescence quantum yields in de-aerated solution (53 to 77%). On going from solution to powder samples, the emission is red-shifted and the quantum yields are considerably lower (≤11%). The complexes were tested in light-emitting electrochemical cells (LECs) achieving maximum luminances of 141 cd m⁻² when operated at 100 A m⁻² using pulsed current driving conditions.