Cyclometalated Ir(III) complexes of deprotonated N-methylbipyridinium ligands: effects of quaternised N centre position on luminescence.

Benjamin J Coe,Sergio Sánchez,Martyn K Peers,James Raftery,Nigel S Scrutton,Madeleine Helliwell

doi:10.1039/c5dt03753k

Abstract

Six new tricationic Ir(III) complexes of cyclometalating ligands derived from 1-methyl-2-(2'-pyridyl)pyridinium or 1-methyl-4-(2'-pyridyl)pyridinium are described. These complexes of the form [Ir(III)(C^N)2(N^N)](3+) (C^N = cyclometalating ligand; N^N = α-diimine) have been isolated and characterised as their PF6(-) and Cl(-) salts. Four of the PF6(-) salts have been studied by X-ray crystallography, and structures have been obtained also for two complex salts containing MeCN and Cl(-) or two Cl(-) ligands instead of N^N. The influence of the position of the quaternised N atom in C^N and the substituents on N^N on the electronic/optical properties are compared with those of the analogous complexes where C^N derives from 1-methyl-3-(2'-pyridyl)pyridinium (B. J. Coe, et al., Dalton Trans., 2015, 44, 15420). Voltammetric studies reveal one irreversible oxidation and multiple reduction processes which are mostly reversible. The new complexes show intramolecular charge-transfer absorptions between 350 and 450 nm, and exhibit bright green luminescence, with λmax values in the range 508-530 nm in both aqueous and acetonitrile solutions. In order to gain insights into the factors that govern the emission properties, density functional theory (DFT) and time-dependent DFT calculations have been carried out. The results confirm that the emission arises largely from triplet excited states of the C^N ligand ((3)LC), with some triplet metal-to-ligand charge-transfer ((3)MLCT) contributions.

Highlights

Cyclometalated IrIII complexes have attracted attention for many years, with a primary focus being their fascinating optical emission properties.1–7 Such species show high stability, large quantum yields and long-lived excited states, due to strong spin–orbit coupling
The new complexes 1–3 and 7–9 (Fig. 1) were synthesised by following the procedure used previously for 4–6.45 The cyclometalated chloride-bridged dimeric intermediates were isolated in crude form only, but identified by 1H NMR spectroscopy
The identities and purities of the new complex salts are confirmed by 1H NMR spectroscopy, elemental analyses and +electrospray mass spectrometry, and in several cases by X-ray crystallography

Summary

Introduction

Cyclometalated IrIII complexes have attracted attention for many years, with a primary focus being their fascinating optical emission properties.1–7 Such species show high stability, large quantum yields and long-lived excited states, due to strong spin–orbit coupling. C^N derived from 2-phenylpyridine ( ppy) or its derivatives have been characterised crystallographically.66 apparently the only reported structure of a related monometallic dichloride complex is that of cyclometalated 2-(2,4-difluorophenyl)pyridine, crystallised as a monoanion with the cation [IrIII(C^N)2(bpym)]+ (bpym = 2,2′-bipyrimidine) containing the same ligand.67 The average Ir–Cl distance in this known compound of 2.493(4) Å is slightly longer than the corresponding distance in 11P (2.464(1) Å, Table 2). Electronic absorption spectroscopy The absorption spectral data for the PF6− salts of the new complexes 1–3 and 7–9 recorded in acetonitrile are shown in

Results

Conclusion