A novel class of luminescent cyclometalated gold(III) alkynyl complexes has been demonstrated to possess EL properties and has been employed in the roles of electrophosphorescent emitters or dopants of organic light-emitting diodes (OLEDs) with high brightness and efficiency.
In contrast to the related gold(i) [1][2][3] and the isoelectronic platinum(ii) compounds, [4,5] which are known to show rich luminescence properties, luminescent gold(iii) compounds are rare, with very few exceptions that emit at room temperature in solution.[6] The reasons for the lack of luminescence in gold(iii) species are probably the presence of low-energy d-d ligand field (LF) states and the electrophilicity of the gold(iii) center. The presence of a nonemissive low-lying d-d state would quench the luminescence excited state by thermal equilibration or energy transfer.[7] Coupling of strong sdonating alkynyl ligands to gold(iii) should render the metal center more electron rich, with the additional advantage of raising the energy of the d-d states, which would result in enhanced luminescence by increasing the chances of populating the emissive state for the construction of luminescent organometallic materials. Despite the fact that alkynyl complexes of gold(i) [3] and the isoelectronic platinum(ii) [5] are known and have been shown to display rich photoluminescence properties, to our surprise, alkynyl complexes of gold(iii) are extremely rare.[8] Although there is increasing interest in the use of gold(iii) compounds for catalysis of organic synthetic reactions of alkynes, [9] the chemistry of gold(iii) alkynyls is essentially unexplored and underdeveloped, and their luminescence properties are virtually unknown.Herein we describe the synthesis of a novel series of biscyclometalated gold(iii) alkynyl complexes [Au(C^N^C)-(C CR)] [HC^N^CH = 2,6-diphenylpyridine, R = C 6 H 5 (1), C 6 H 4 -Cl-p (2), C 6 H 4 -OCH 3 -p (3), C 6 H 4 -NH 2 -p (4); HC^N^CH = 2,6-bis(4-tert-butylphenyl)pyridine, R = C 6 H 5 (5)], which are the first of their kind (Scheme 1). The molecular structure of 1 was determined by X-ray crystallography. The photophysical properties of 1-5 were also studied.Unlike most other gold(iii) compounds, which exhibit luminescence only at low temperature or are nonemissive, complexes 1-5 display luminescence in various media at both low and ambient temperature. Figure 1 shows the crystal structure of 1, in which the gold(iii) center adopts a distorted square-planar coordination geometry with C (9)
A new class of luminescent cyclometalated alkynylgold(III) complexes, [Au(RC=N(R')=CR)(CCR' ')], i.e., [Au(C=N=C)(C triple bond CR'')] (HC=N=CH = 2,6-diphenylpyridine) R' ' = C6H5 1, C6H4-Cl-p 2, C6H4-NO2-p 3, C6H4-OCH3-p 4, C6H4-NH2-p 5, C6H4-C6H13-p 6, C6H13 7, [Au(tBuC=N=CtBu)(C triple bond CC6H5)] 8 (HtBuC=N=CtBuH = 2,6-bis(4-tert-butylphenyl)pyridine), and [Au(C=NTol=C)(CCC6H4-C6H13-p)] 9 (HC=NTol=CH = 2,6-diphenyl-4-p-tolylpyridine), have been synthesized and characterized. The X-ray crystal structures of most of the complexes have also been determined. Electrochemical studies show that, in general, the first oxidation wave is an alkynyl ligand-centered oxidation, while the first reduction couple is ascribed to a ligand-centered reduction of the cyclometalated ligand with the exception of 3 in which the first reduction couple is assigned as an alkynyl ligand-centered reduction. Their electronic absorption and luminescence behaviors have also been investigated. In dichloromethane solution at room temperature, the low-energy absorption bands are assigned as the pi-pi* intraligand (IL) transition of the cyclometalated RC=N(R')=CR ligand with some mixing of a [pi(C triple bond CR'') --> pi*(RC=N(R')=CR)] ligand-to-ligand charge transfer (LLCT) character. The low-energy emission bands of all the complexes, with the exception of 5, are ascribed to origins mainly derived from the pi-pi* IL transition of the cyclometalated RC=N(R')=CR ligand. In the case of 5 that contains an electron-rich amino substituent on the alkynyl ligand, the low-energy emission band was found to show an obvious shift to the red. A change in the origin of emission is evident, and the emission of 5 is tentatively ascribed to a [pi(CCC6H4NH2) --> pi*(C=N=C)] LLCT excited-state origin. DFT and TDDFT computational studies have been performed to verify and elucidate the results of the electrochemical and photophysical studies.
Ultraviolet photoelectron spectroscopy has been applied to the investigation of modified hole injection barriers in organic light-emitting devices (OLEDs). Different from those reported previously, the indium tin oxide (ITO) surface treated in situ by oxygen plasma possesses a work function of 5.2 eV, and the organic ITO interface thereafter formed shows a 0.5 eV smaller hole injection barrier compared to that on untreated ITO. Insertion of an ultrathin SiO2 layer between the organic and ITO results in a similar reduction of the barrier. This indicates that improved hole injection favors efficient operation of OLEDs, as manifested by enhanced efficiency by the SiO2 insertion.
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