Metal Complexes with Azolate‐Functionalized Multidentate Ligands: Tactical Designs and Optoelectronic Applications

Abstract

This review is aimed at updating the recent development on the metal complexes bearing azolate-containing chelates that have received a growing attention from both the industrial and academic sectors. Particular emphasis is given to the luminescent metal complexes, for which tridentate and multidentate bonding interactions give rise to both higher ligand field strength and better rigidity versus their bidentate counterparts-consequently, this is beneficial to the chemical stability and emission efficiency needed for applications such as organic light-emitting diodes and bio-imaging. Their basic designs involve chelates, such as monoanionic 6-azolyl 2,2'-bipyridine, dianionic 2,6-diazolylpyridine, and 2-azolyl-6-phenylpyridine, and the core metal ion spanning from main group elements, such as Ga and In , to the late transition metal ions such as Ru , Os , Ir , and Pt and even the lanthanides. Furthermore, the great versatility of these azolate chelates for assembling the robust and emissive metal complexes, provides bright prospect in future optoelectronic investigations.

Recent advances in phosphorescent platinum complexes for organic light-emitting diodes

“…The combination of the strong σ-donor effect of the phenylate and the π-accepting character of the pyridine ring results in a high ligand-field for the coordinated metal, thus raising the energy of quenching d–d states while lowering emissive MLCT and LC excited states. Alternatively, the use of N -deprotonable azole units has also been largely explored due to the fact that it can exert similar effects to ppy-like ligands [16]. Nevertheless, easier deprotonation of the N–H site in comparison with ppy chelates notably widens applicability and increases the chemical structure diversity of the final luminophors, e.g., for complexating metal ions less prone to undergo cyclometallation reactions.…”

“…Nevertheless, easier deprotonation of the N–H site in comparison with ppy chelates notably widens applicability and increases the chemical structure diversity of the final luminophors, e.g., for complexating metal ions less prone to undergo cyclometallation reactions. Extensive work based on azolate-type of ligands has been developed by the group of Chi [16] who has recently described a series of neutral platinum(II) complexes bearing isoquinolylpyrazolates, complexes 4 – 7 in Fig. 3 [26].…”

“…As aforementioned, N-deprotonable azole units constitute a compelling alternative to C-cyclometalating ligands [16]. In this regard, dianionic tridentate N^N^N ligands bearing pyrazolate [69], triazolate [69–73] or tetrazolate [74] units have been used to successfully prepare highly luminescent neutral platinum(II) complexes in dilute solution and/or as aggregated state.…”

2,2′-Bipyridin-1′-ium 1-oxide bromide monohydrate

“…Bipyridine ligands are an important class of ligands with respect to the synthesis of transition metal complexes. They are especially well-known for their use in the development of complexes with specific photophysical (Thompson et al, 2013;Sun et al, 2015, Dongare et al, 2017 and/or photocatalytic (Wenger, 2013;Fukuzumi et al, 2016;Knoll et al, 2015;Duan et al, 2015;Pal & Hanan, 2014) properties or for the construction of dye-sensitized solar cells (Happ et al, 2012;Bomben et al, 2012;Robson et al, 2012;Adeloye & Ajibade, 2014;Lu et al, 2016;Omae, 2016). During our attempts to introduce substituents to 2,2 0 -bipyrdines that would allow us to use them as monomers in copolymerization reactions (Heintz et al, 2017), we treated 2,2 0 -bipyridine with a mixture of hydrobromic acid and hydrogen peroxide with the aim of getting direct access to 4-bromo-2,2 0 -bipyridine-1-oxide.…”

Potential strategy used for controlling the phosphorescent properties in tetradentate Pt(II) complexes: Effect of azole ligand

“…In the past few decades, transition metal complexes have aroused extensive interest and attention, they are widely used in the field of organic light‐emitting diodes (OLEDs) . Because Pt(II) and Ir(III) can introduce a strong spin–orbit coupling (SOC) effect, causing the effective intersystem crossing between singlet (S n ) and triplet (T n ) excited states and partial allowance of T n (usually, n = 1) → ground state transition, Pt(II) and Ir(III) complexes are regarded as the most popular emitters in OLEDs. For both Ir(III) and Pt(II) complexes, a wide emission color tunability and improvement of luminescence quantum yield can be achieved via wise design of reasonable ligands, which has been proved in experimental and theoretical studies.…”