Excited-State Engineering in Heteroleptic Ionic Iridium(III) Complexes

Abstract

Conspectus Iridium­(III) complexes have assumed a prominent role in the areas of photochemistry and photophysics due to the peculiar properties of both the metal itself and the ligand environment that can be assembled around it. Ir­(III) is larger, heavier, and bears a higher ionic charge than its analogue and widely used d6 ions such as Fe­(II) and Ru­(II). Accordingly, its complexes exhibit wider ligand-field d–d orbital splitting with electronic levels centered on the metal, typically nonemissive and photodissociative, not playing a relevant role in excited-state deactivations. In other words, iridium complexes are typically more stable and/or more emissive than Fe­(II) and Ru­(II) systems. Additionally, the particularly strong heavy-atom effect of iridium promotes singlet–triplet transitions, with characteristic absorption features in the UV–vis and relatively short excited-state lifetimes of emissive triplet levels. Ir­(III) is also a platform for anchoring ligands of rather different sorts. Its versatile chemistry includes not only coordination with classic N∧N neutral ligands but also the binding of negatively charged chelators, typically having a cyclometalating C∧N anchor. The carbon–metal bond in these systems has some degree of covalent character, but this does not preclude a localized description of the excited states of the related complexes, which can be designated as metal-centered (MC), ligand-centered (LC), or charge transfer (CT), allowing a simplified description of electronic and photophysical properties. The possibility of binding different types of ligands and making heteroleptic complexes is a formidable tool for finely tuning the nature and energy of the lowest electronic excited state of cationic Ir­(III) complexes by ligand design. Herein we give an account of our work on several families of iridium complexes typically equipped with two cyclometalating bidentate ligands (C∧N), in combination with mono or bidentate “ancillary” ligands with N∧N, C∧N, and C∧C motifs. We have explored new synthesis routes for both cyclometalating and ancillary ligands, obtaining primarily cationic complexes but also some neutral or even negatively charged systems. In the domain of the ancillary ligands, we have explored isocyanides, carbenes, mesoionic triazolylidenes, and bis-tetrazolic ligands. For the cyclometalating moiety, we have investigated carbene, mesoionic triazolylidene, and tetrazolic systems. Key results of our work include new strategies to modify both cyclometalating and ancillary ligands by relocating ionic charges, the determination of new factors affecting the stability of complexes, a demonstration of subtle structural effects that strongly modify the photophysical properties, new options to get blue-greenish emitters for optoelectronic devices, and a set of ligand modifications allowing the optimization of electrochemical and excited-state properties to obtain new promising Ir­(III) complexes for photoredox catalysis. These results constitute a step forward in the preparation of custom i...

External Magnetic Field Driven, Ambidextrous Circularly Polarized Electroluminescence from Organic Light Emitting Diodes Containing Racemic Cyclometalated Iridium(III) Complexes

“…In recent years, chiral emitters exhibiting circularly polarized luminescence (CPL), with high quantum yield (Φ PL ) and high anisotropy factor (g CPL ), upon excitation with unpolarized light have been used in numerous applications such as circularly polarized electroluminescence (CPEL) devices and security inks. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Several chiral p-block organic [16][17][18][19][20][21][22] and d-block/f-block organometallic compounds [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] are efficient fluorescent and phosphorescent emitters, respectively, with narrow CPL bandwidths and high g CPL values in the visible region, making themselves suitable for producing high-performance CPELs. However, it is difficult to obtain a high yield of enantiomerically pure isomers in less time, at a reasonable cost, using green methods.…”

“…Considerable numbers of homoleptic tris-cyclometalated Ir(III) [23,[26][27][28]31,35,36] (as Δ-/Λ-forms in each fac/mer-isomer) and heteroleptic bis-cyclometalated Ir(III) [24,25,[28][29][30][33][34][35][36] (as Δ-/Λ-forms) complexes with achiral organic ligands have been investigated. They exhibit phosphorescence-based EL with high Φ PL and short photoluminescence (PL) lifetimes, owing to strong spin-orbit coupling in Ir(III).…”

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“…The most prominent feature of iTMC materials is that they catalyse charge transfer reactions with their tunable oxidation and reduction properties. Ir(III) complex-based iTMC-LEC devices allow to create high electric fields in the device, thanks to their ability to work with air-resistant electrodes [12][13][14]. Hole injection at the oxide/organometallic interfaces, which form the basic structure of the LEC device and allow the light to be formed and emitted here, is a crucial parameter that affects the performance of the device [15][16][17][18][19][20][21].…”

Luminescence properties of [Ir(C^N)2(N^N)]+ complexes: relations between DFT computation results and emission band-shape analysis data

“…Due to their unprecedented luminescence properties, particularly high photoluminescence quantum yields with tuneable emission wavelengths, there are objectives of many expressive studies performed on the homoleptic and heteroleptic Ir 3+ chelates. [74][75][76][77][78][79] From the latter class, these containing the cyclometalated C^N and ancillary a-diimine N^N ligands attached to the Ir 3+ core, can be regarded as particularly interesting. The wide-ranging combinations of C^N and N^N ligands, easily attachable using the well-known Nonoyama synthetic strategy, 80,81 allow tuning the emissive properties of the [Ir(C^N) 2 (N^N)] + complexes in very simple way.…”