Synthesis and photoluminescence of iridium(iii) arylacetylide complexes with acetylide-localized emissive excited states

TL;DR

This paper explores the synthesis and light-emitting properties of iridium complexes with acetylide ligands, focusing on their weak luminescence and the role of ligand structure.

Contribution

The study introduces new iridium complexes with acetylide-localized emissive states and investigates their photoluminescence behavior.

Findings

The cyclometalated complexes show weak luminescence from acetylide-localized excited states.

The pentamethylcyclopentadienyl complex does not emit light under tested conditions.

Emissive states are localized on the arylacetylide ligand, not the iridium center.

Abstract

Herein, we report a series of three bis-cyclometalated iridium(iii) complexes each bearing an arylacetylide and an N-heterocyclic carbene (NHC) ligand. The cyclometalating ligand is 2-(2,4-difluorophenyl)pyrazole (F2ppz), chosen to ensure the 5d(Ir) → π*(F2ppz) 3MLCT excited-state is higher in energy than the 3(π → π*) state of the acetylide, allowing luminescence to originate from the latter. The strong-field NHC is included to alleviate photoluminescence (PL) quenching caused by the thermal population of the triplet metal-centered (3MC) states, but nevertheless, the three compounds are weakly luminescent. The parent phenylacetylide complex only emits blue luminescence in dichloromethane solution at 77 K, with a peak wavelength of 424 nm. Complexes bearing substituted phenylacetylides that engender emission in the green and yellow regions are also not emissive in solution at room temperature, but phosphoresce weakly in poly(methylmethacrylate) film and also emit at 77 K in solution. Since the emissive excited states in these compounds are acetylide-localized, we tested whether cyclometalating ligands are needed at all by preparing a new 1,2,3,4,5-pentamethylcyclopentadienyl Ir(iii) acetylide complex, but this complex does not luminesce under any of the conditions tested. The four reported complexes are structurally characterized by multinuclear NMR and one of them by single-crystal X-ray diffraction. Thorough electrochemical and photophysical studies of the complexes were also carried out, complemented by time-dependent density functional theory (TD-DFT) calculations.

Two types of Ir(iii) acetylide complexes are reported. The cyclometalated complexes exhibit weak luminescence from a localized 3(π → π*) state on the arylacetylide, whereas the pentamethylcyclopentadienyl-supported complex is not luminescent.

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