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.