Cationic Iridium(III) Complexes with Two Carbene-Based Cyclometalating Ligands: Cis Versus Trans Isomers

Abstract

A series of cationic iridium(III) complexes with two carbene-based cyclometalating ligands and five different N^N bipyridine and 1,10-phenanthroline ancillary ligands is presented. For the first time--in the frame of a rarely studied class of bis(heteroleptic) iridium complexes with two carbene-based cyclometalating ligands--a pair of cis and trans isomers has been isolated. All complexes (trans-1-5 and cis-3) were characterized by (1)H NMR, (13)C NMR, (31)P NMR, and HRMS (ESI-TOF); in addition, crystal structures of cis-3 and trans-4 are reported and discussed. Cyclic voltammetric studies show that the whole series exhibits highly reversible oxidation and reduction processes, suggesting promising potential for optoelectronic applications. Ground-state DFT and TD-DFT calculations nicely predict the blue shift experimentally observed in the room-temperature absorption and emission spectra of cis-3, compared to the trans complexes. In CH3CN, cis-3 displays a 4-fold increase in photoluminescence quantum yield (PLQY) with respect to trans-3, as a consequence of drastically slower nonradiative rate constant. By contrast, at 77 K, the emission properties of all the compounds, including the cis isomer, are much more similar, with a pronounced hypsochromic shift for the trans complexes. A similar behavior is found in solid state (1% w/w poly(methyl methacrylate) matrix), with all complexes displaying PLQY of ∼70-80%, comparable emission lifetimes (τ ≈ 1.3 μs), and a remarkable rigidochromic shift. To rationalize the more pronounced nonradiative deactivation (and smaller PLQY) observed for photoexcited trans complexes, comparative temperature-dependent emission studies in the range of 77-450 K for cis-3 and trans-3 were made in propylene glycol, showing that solvation effects are primarily responsible for the observed behavior.

Deep‐Red Phosphorescent Iridium(III) Complexes with Chromophoric N‐Heterocyclic Carbene Ligands: Design, Photophysical Properties, and DFT Calculations

“…[20] More recently, efforts have been devoted to preparing luminescent iridium complexes with N-heterocyclic carbene (NHC) ligands, which increase the stability of the complex and allow the preparation of complexes that emit at high energy. [13,[25][26][27][28][29][32][33][34][35][36][37][38][39][40][41] On the other hand, only few examples of heteroleptic iridium carbene complexes displaying red emissions have been reported. [13,[25][26][27][28][29][32][33][34][35][36][37][38][39][40][41] On the other hand, only few examples of heteroleptic iridium carbene complexes displaying red emissions have been reported.…”

“…[21][22][23] Therefore, homoleptic and heteroleptic cyclometalated iridium(III) complexes containing NHC units have been prepared that often display luminescence in the blue region, [13,15,[22][23][24][25][26][27][28][29][30][31] owing to the high triplet energy gaps of the carbene ligands. [34][35][36][37][38] Hence, by using a rational design approach, we undertook a research program targeted at stable cyclometalated carbene iridium complexes exhibiting red emission. [34][35][36][37][38] Hence, by using a rational design approach, we undertook a research program targeted at stable cyclometalated carbene iridium complexes exhibiting red emission.…”

Molecular Engineering of Iridium Blue Emitters Using Aryl N‐Heterocyclic Carbene Ligands

“…As was described in detail by Zuo and co‐workers very recently for the [(dfppy) 2 Ir(arylNHC)] scaffold (and by others for other ligand classes), the ancillary ligand also greatly impacts the photophysical properties . As a practical example, starting from [(ppy) 2 Ir(acac)] (ppy = 2‐phenylpyridine; acac = acetylacetonate) and changing the ancillary ligand can result in either a redshift (as in 2,2′‐bipyridine or phenanthrene) or a blueshift [as in (CN) 2 ,, bis(1‐methylimidazol‐3‐yl‐2‐idene)methane, Im 2 CH 2 , or 2‐picolinic acid] of the observed emission (Figure ).…”

“…Illustration of the ancillary ligand impact on the emitter energy level. HOMO and LUMO energy levels have been taken from the respective references: [Ir(ppy) 2 bpy], [Ir(ppy) 2 acac], [Ir(ppy) 2 (Im 2 CH 2 )], and [FIrPic] . The value of E ( T 1 ) was estimated by the following conversion: E ( T 1 ) = E (S+/S) – (1240/ λ em,onset ).…”

Water-soluble Ir(iii) complexes of deprotonated N-methylbipyridinium ligands: fluorine-free blue emitters

“…Their chemical structures bear some resemblance to the widely studied complexes of Nheterocyclic carbenes derived from imidazolium species, although such complexes are typically neutral or only +1 charged. [23][24][25][26] Scheme 1 Synthesis of the complex salts 1P-3P; their chloride counterparts 1C-3C were prepared by treating purified 1P-3P with [ n Bu4N]Cl in acetone. UV-vis absorption spectroscopic data are shown in Table 1.…”