Monocyclometalated Gold(III) Complexes Bearing π‑Accepting Cyanide Ligands: Syntheses, Structural, Photophysical, and Electrochemical Investigations

The synthesis, structural, photophysical,
and electrochemical investigations of a series of gold­(III) monocyclometalated
complexes bearing ancillary ligands with π-accepting properties
is reported. Complexes of the type [(C∧N)­Au­(CN)2] [C∧N = 2-phenylpyridine (ppy) (1), 2-(p-tolyl)­pyridine (tpy) (2), 2-(2-thienyl)-pyridine (thpy) (3), 2-(5-methyl-2-thienyl)­pyridine (5m-thpy) (4), 1-phenylisoquinoline (piq) (5)], and [(N∧N)­Au­(CN)2] [N∧N = 3,5-bis­(phenyl)-2-(2′-pyridyl)­pyrrole
(pyrpy) (6)] were prepared, and the
influence of both the cyanide as an ancillary ligand as well as the
different electronic properties of the cyclometalating ligands (1–5) and the chelating bidentate (6) on the triplet emission properties were studied. The physicochemical
properties were evaluated by a variety of physical methods, and the
structure of selected complexes was further confirmed by X-ray diffraction
studies. Complexes 1–5 display long-lived
emission in solution, neat solid, spin coated PMMA films, and at 77
K in 2-MeTHF. The emission energies were strongly dictated by the
cyclometalating ligands independent of the cyanide ligand, which is
in quite a contrast to the previously reported dicyano complexes of
iridium­(III) and the isoelectronic platinum­(II) complexes. The nonemissive
behavior of complex 6 in any medium further highlights
the importance that the good σ-donating properties of the cyclometalating
ligand alone is not decisive in rendering the gold complexes emissive,
but also the appropriate placement of the energy level of the ligand
orbitals is also important. Detailed photophysical studies in conjunction
with density functional theory and time-dependent density functional
theory calculations support the origin of the emission to be a metal
perturbed intra ligand 3IL (π–π*) delocalized
over the cyclometalating ligand. The stability of the complexes combined
with good emission quantum yields and tunability of the emission energies
makes these complexes suitable alternatives to the relatively less
stable monocyclometalated gold­(III) diaryl or dialkyne complexes for
organic light emitting device applications.