Cross-couplings in the elaboration of luminescent bis-terpyridyl iridium complexes: the effect of extended or inhibited conjugation on emission

Abstract

The utility of Suzuki cross-coupling methodology for the in situ elaboration of bromo-functionalised bis-terpyridyl iridium(III) complexes has been explored. The complex [Ir(tpy)(tpy-ϕ-Br)]³⁺ {tpy-ϕ-Br = 4′-(4-bromophenyl)-2,2′:6′,2″-terpyridine} undergoes palladium-catalysed cross-coupling with aryl boronic acids to yield biaryl-substituted complexes directly. The biphenyl and 4-cyanobiphenyl-substituted products display relatively intense, long-lived (τ > 100 µs) yellow emission in degassed aqueous solution at room temperature, assigned to a ³π–π* state. A 4-aminobiphenyl-substituted analogue displays an additional low energy absorbance band, attributed to an intraligand charge-transfer (ILCT) excited state, and is scarcely emissive under the same conditions. The iridium(III) complex of 4′-mesityl-terpyridine is also reported. Its emission is much shorter-lived, with a spectral profile resembling that of unsubstituted [Ir(tpy)₂]³⁺, confirming the need for the attainment of a roughly coplanar geometry for stabilisation of the ³π–π* excited state.