ABSTRACT
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(â§)N)Au(C≡N)2] [C(â§)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(â§)N)Au(C≡N)2] [N(â§)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 (3)IL (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.
ABSTRACT
Highly tunable and rich phosphorescent emission properties based on the stable monocyclometalated gold(III) monoaryl structural motif are reported. Monochloro complexes of the type cis-[(N^C)Au(C6 H2 (CF3)3)(Cl)] N^C=2-phenylpyridine (ppy)] (1), [N^C=benzo[h]quinoline (bzq)] (2), [N^C=2-(5-Methyl-2-thienyl)pyridine (5m-thpy)] (3) were successfully prepared in modest to good yields by reacting an excess of 2, 4, 6-tris(trifluoromethyl)phenyl lithium (LiFmes) with the corresponding dichloride complexes cis-[(N^C)AuCl2]. Subsequent replacement of the chloride ligand in 1 with strong ligand field strength such as cyanide and terminal alkynes resulted in complexes of the type cis-[(ppy)Au(Fmes)(R)] R=CN (4), I (5), C≡C-C6 H5 (6) and C≡C-C6 H4 N(C6 H5)-p (7). Single crystal X-ray diffraction studies of all the complexes except 7 were performed to further corroborate their chemical identity. Thermogravimetric analysis (TGA) studies of the uncommon cis configured aryl alkyne complex 7 confirmed the high stability of this complex. Detailed photophysical investigations carried out in solution at room temperature, at 77â K (2-MeTHF) in rigidified media, solid state and 5â wt % PMMA revealed the phosphorescent nature of emission in these complexes. Additionally, their behavior was found to be governed based on both the nature of the cyclometalated ligand and the electronic properties of the ancillary ligands. Highly efficient interligand charge transfer in complex 7 provides access to a wide range of emission colors (solvent-dependent) from deep blue to red with phosphorescence emission quantum yield of 30 % (441â nm) and 39 % (622â nm) in solution and solid state, respectively, and is the highest reported for any Au(III) complexes. DFT and TDDFT calculations carried out further validated the observations and assignments based on the photophysical experimental findings.
