Thienylpyridine-based cyclometallated iridium(iii) complexes and their use in solid state light-emitting electrochemical cells

Andreas M Bünzli,Jennifer A Zampese,Juan J Serrano-Pérez,José M Junquera-Hernández,Markus Neuburger,Henk J Bolink,Enrique Ortí,Daniel Tordera,Antonio Pertegás,Catherine E Housecroft,Edwin C Constable

doi:10.1039/c3dt52622d

Abstract

The synthesis and characterization of four iridium(iii) complexes [Ir(thpy)2(N^N)][PF6] where Hthpy = 2-(2'-thienyl)pyridine and N^N are 6-phenyl-2,2'-bipyridine (1), 4,4'-di-(t)butyl-2,2'-bipyridine (2), 4,4'-di-(t)butyl-6-phenyl-2,2'-bipyridine (3) or 4,4'-dimethylthio-2,2'-bipyridine (4) are described. The single crystal structures of ligand 4 and the complexes containing the [Ir(thpy)2(1)](+) and [Ir(thpy)2(4)](+) cations have been determined. In [Ir(thpy)2(1)](+), the pendant phenyl ring engages in an intra-cation π-stacking interaction with one of the thienyl rings in the solid state, and undergoes hindered rotation on the NMR timescale in [Ir(thpy)2(1)](+) and [Ir(thpy)2(3)](+). The solution spectra of [Ir(thpy)2(1)][PF6] and [Ir(thpy)2(4)][PF6] show emission maxima around 640 nm and are significantly red-shifted compared with [Ir(thpy)2(2)][PF6] and [Ir(thpy)2(3)][PF6] which have structured emission bands with maxima around 550 and 590 nm. In thin films, the emission spectra of the four complexes are similar with emission peaks around 550 and 590 nm and a shoulder around 640 nm that are reminiscent of the features observed in solution. In solution, quantum yields are low, but in thin films, values range from 29% for [Ir(thpy)2(1)][PF6] to 51% for [Ir(thpy)2(4)][PF6]. Density functional theory calculations rationalize the structured emission observed for the four complexes in terms of the (3)LC nature predicted for the lowest-energy triplet states that mainly involve the cyclometallated [thpy](-) ligands. Support for this theoretical result comes from the observed features of the low temperature (in frozen MeCN) photoluminescence spectra of the complexes. Photoluminescence and electroluminescence spectra of the complexes in a light-emitting electrochemical cell (LEC) device configuration have been investigated. The electroluminescence spectra are similar for all [Ir(thpy)2(N^N)][PF6] complexes with emission maxima at ≈600 nm, but device performances are relatively poor probably due to the poor charge-transporting properties of the complexes.

Highlights

Cationic iridium complexes have been extensively used in electroluminescence applications including organic lightemitting diodes (OLEDs) and light-emitting electrochemical cells (LECs).[1,2] LECs consist of a luminescent material that can be either a conjugated polymer with an added electrolyte in aWe describe the synthesis and characterization of a family of iridium(III) complexes of type [Ir(thpy)2(N^N)]+, in which Hthpy = 2-(2′-thienyl)pyridine and N^N are the 738 | Dalton Trans., 2014, 43, 738–750 PaperScheme 1 Iridium(III) complex cations and atom labelling for NMR spectroscopic assignments.2,2′-bipyridine-based ligands 1–4 (Scheme 1)
The solution spectra of [Ir(thpy)2(1)][PF6] and [Ir(thpy)2(4)][PF6] are red-shifted and the band shape significantly changes on going from solution to thin film
The similar structured emission observed in thin film for the four complexes can be understood in terms of the 3LC nature, involving the cyclometallated [thpy]− ligands, predicted for the lowest-energy triplet states using Density functional calculations (DFT)/Time-dependent DFT (TD-DFT) calculations

Summary

Introduction

Calculations reproduce the intra-cation face-to-face π-stacking of the pendant phenyl substituent with the adjacent thienyl ring of the [thpy]− ligand. This suggests that the lowest-energy triplet state should, in principle, originate from the HOMO → LUMO excitation, which gives rise to an electron transfer from the Ir-thpy environment to the diimino ligand.

Results

Conclusion