Luminescent cyclometalated gold (III) complexes with ancillary N-heterocyclic carbene and alkynyl ligands : from design, synthesis, photophysics to supramolecular assembly and functions

Abstract

A library of luminescent cyclometalated gold(III) complexes containing

various bis-cyclometalating R-C^N^C ligands derived from 2,6-diphenylpyridine

and ancillary alkynyl ligands, [Au(R-C^N^C)(C≡CR’)], has been synthesized.

Some of the complexes have been structurally determined by X-ray

crystallography. These complexes were found to exhibit intense emission in

dichloromethane solution at 298 K, originating from metal-perturbed triplet

intraligand (IL) excited states of the R-C^N^C ligand, with substantial charge

transfer character from the aryl moiety to the pyridyl ring. In the presence of

electron-rich alkynyl ligands, the emission origin could be switched to an

alkynyl-to-cyclometalating ligand ligand-to-ligand charge transfer (LLCT) excited

state. This class of complexes was also demonstrated to show rich

electroluminescence properties as emitters in organic light-emitting devices. In

addition, the supramolecular assembly of this class of complexes has also been

revealed in gelation studies.

N-Heterocyclic carbenes have been incorporated into the gold(III) metal

center to prepare a series of luminescent mononuclear and dinuclear gold(III)

complexes, [{Au(C^N^C)}n(NHC)](PF6)n and [{Au(tBuC^N^CtBu)}n(NHC)]-

(PF6)n

(n = 1, 2). The X-ray crystal structures of most of the complexes have been

determined. The emissions of these complexes were assigned to originate from the

metal-perturbed intraligand excited state of the bis-cyclometalating ligand. One of

the C2-bridged dinuclear complexes was found to exhibit two distinct reduction

couples, tentatively correlated to the presence of significant intramolecular π-π

interaction in the complex.

Two novel series of luminescent mononuclear alkynylgold(III) complexes,

[Au(C^N)(C≡CR)2] (HC^N = 2-phenylpyridine (Hppy) and derivatives) and

[Au(C^N^N)(C≡CR)]PF6 (HC^N^N = 6-phenyl-2,2’-bipyridine and derivatives),

have been synthesized. Some of the X-ray crystal structures have been determined.

The former class of complexes with bidentate C^N ligands has been observed to

show tunable emission spanning across the visible spectrum from 462 to 697 nm.

With the exception of [Au(ppy)(C≡C-C6H4-NH2-p)2] which showed a low-energy

band originated from a 3LLCT [π(C≡C-C6H4-NH2)→π*(C^N)] excited state, all

complexes exhibited vibronic-structured emission bands originated from the

intraligand transition of the cyclometalating C^N ligand in dichloromethane

solution at 298 K. On the other hand, most of the complexes with the tridentate

C^N^N-type ligand have been observed to exhibit vibronic-structured emission

bands attributed to the intraligand transition of the C^N^N ligand in

low-temperature butyronitrile glass.