Ligand-scope mechanism of tris-cyclometalation and the microscopic origin of aggregation-controlled solid-state emission in octahedral cyclometalated Ir(III) complexes

The 2001 Inorganic Chemistry paper of Lamansky, Djurovich, Thompson and co-workers on phosphorescent cyclometalated iridium complexes (Inorg. Chem. 2001, 40, 1704) established the now-standard chemistry of neutral Ir(III) phosphors, $(C^{\wedge}N)_2\mathrm{Ir}(\mathrm{LX})$ and fac-$\mathrm{Ir}(C^{\wedge}N)_3$, and left two unresolved scientific questions that survive today. First, a mechanistic one: only some cyclometalating ligands (phenylpyridines, benzoquinoline, 2-(2-thienyl)pyridine) could be converted into tris-cyclometalated Ir(III) complexes, while 2-phenylbenzothiazole (bt), 2-(1-naphthyl)benzothiazole (bsn) and 2-phenylquinoline (pq) failed under the reported conditions even though molecular modeling showed no steric barrier; the authors could only speculate that heterocycle (imine) basicity or electronic stabilization of intermediates controls the formation of the third Ir–C bond. Later work overcame the bt case empirically — homoleptic tris(2-phenylbenzothiazolato)iridium(III) was finally made in 2011 using silver triflate as a halide abstractor, and homoleptic tris(2-phenylquinoline)iridium(III) has since been prepared and photophysically characterized (Endo and Adachi, 2009) — but no kinetic or computational study establishes why the benzothiazolyl and quinolyl ligand classes fail in the unactivated routes, homoleptic tris(2-(1-naphthyl)benzothiazolato)iridium(III) has not been reported, and the condition-dependence of the ligand scope remains unexplained. Second, a photophysical one: crystalline $(\mathrm{tpy})_2\mathrm{Ir}(\mathrm{acac})$ shows a structured, red-shifted orange emission (575 nm vs 512 nm in solution) that the authors tentatively ascribed to ground-state dimer traps or another intermolecular emissive mechanism, distinct from the broad excimer-like emission of powdered $(\mathrm{ppy})_2\mathrm{Ir}(\mathrm{acac})$; 25 years of aggregation-emission research on Ir(III) complexes (excimer-based OLEDs, aggregation-induced phosphorescence, polymorphism-controlled dual emission) has documented the phenomenon and produced contested mechanistic assignments, but a definitive case-specific assignment for the structured emission of $(\mathrm{tpy})_2\mathrm{Ir}(\mathrm{acac})$, and a general predictive account of how $\pi$–$\pi$ stacking and aggregation control the solid-state emission of octahedral Ir complexes, are still missing. The problem is to close both gaps: a mechanistic, predictive account of ligand-dependent $\mathrm{Ir}(C^{\wedge}N)_3$ formation, and a definitive microscopic assignment of packing-controlled emission in octahedral cyclometalated iridium complexes.

Background

Cyclometalated iridium(III) complexes are octahedral $d^6$ organometallics in which one or more bidentate $C^{\wedge}N$ ligands — aromatic ligands such as 2-phenylpyridine (ppy) bound through one carbon and one nitrogen donor after C–H activation, a process called cyclometalation — chelate the metal. Because the heavy Ir center strongly couples spin and orbital angular momentum (spin–orbit coupling), the formally spin-forbidden triplet-to-singlet emission is partially allowed, so these complexes phosphoresce efficiently at room temperature from a mixture of metal-to-ligand charge-transfer (MLCT) and ligand-centered ($\pi$–$\pi^*$) excited states. Two families dominate applications: heteroleptic $(C^{\wedge}N)_2\mathrm{Ir}(\mathrm{acac})$, where acac is the monoanionic acetylacetonate ancillary ligand, and homoleptic tris complexes $\mathrm{Ir}(C^{\wedge}N)_3$, which exist as facial (fac) and meridional (mer) geometric isomers. Their room-temperature phosphorescence made them the workhorse dopants of phosphorescent organic light-emitting diodes (PHOLEDs), where an external quantum efficiency (EQE) well above the 25 percent ceiling of fluorescent emitters is achieved by harvesting both singlet and triplet excitons. The classical synthesis, introduced by Nonoyama (Bull. Chem. Soc. Jpn. 1974, 47, 767), cyclometalates the ligand onto $\mathrm{IrCl}_3$ to give a chloride-bridged dimer, $(C^{\wedge}N)_2\mathrm{Ir}(\mu\text{-}\mathrm{Cl})_2\mathrm{Ir}(C^{\wedge}N)_2$, which is then cleaved with an ancillary ligand (e.g. acetylacetone) or converted, by heating with excess free ligand in high-boiling glycerol, into the tris complex. Lamansky, Djurovich, Murphy, Abdel-Razzaq, Kwong, Tsyba, Bortz, Mui, Bau and Thompson (Inorg. Chem. 2001, 40, 1704-1711) systematized this chemistry across six ligands — 2-phenylpyridine (ppy), 2-(p-tolyl)pyridine (tpy), benzoquinoline (bzq), 2-phenylbenzothiazole (bt), 2-(1-naphthyl)benzothiazole (bsn) and 2-phenylquinoline (pq) — characterized the $(C^{\wedge}N)_2\mathrm{Ir}(\mathrm{acac})$ family photophysically and electrochemically, and solved the crystal structures of $(\mathrm{ppy})_2\mathrm{Ir}(\mathrm{acac})$ and $(\mathrm{tpy})_2\mathrm{Ir}(\mathrm{acac})$. Two of their observations define this problem. (1) When $(C^{\wedge}N)_2\mathrm{Ir}(\mathrm{acac})$ precursors were heated with excess ligand in refluxing glycerol, only phenylpyridines, benzoquinoline and 2-(2-thienyl)pyridine gave $\mathrm{Ir}(C^{\wedge}N)_3$; bt, bsn and pq did not, and molecular modeling revealed no obvious steric barrier, leading the authors to suggest that the controlling factors are electronic — for example the basicity of the heterocyclic imine nitrogen, or the electronic stabilization of reaction intermediates. (2) Both crystal structures show close intermolecular aromatic contacts (ca. 3.4–3.5 Å), and the emission of the crystalline powders is strongly red-shifted relative to solution (ppy: 516 to 540 nm, broad and structureless, characteristic of an excimer — an emissive dimer formed only in the excited state between an excited and a ground-state molecule; tpy: 512 to 575 nm, structured and orange, which the authors tentatively attributed to ground-state dimer traps, i.e. pre-associated pairs in the crystal that act as lower-energy emissive sites, or to a different intermolecular emissive mechanism). The authors noted that for square-planar cyclometalated Pt(II) complexes the control of emission by $\pi$–$\pi$ stacking and aggregation was already well documented (Brooks et al., Inorg. Chem. 2002, 41, 3055), but for octahedral Ir complexes this was terra incognita. The device-relevant part of their agenda (OLED metrics for these dopants) was discharged within months by the companion paper of the same group (Lamansky et al., J. Am. Chem. Soc. 2001, 123, 4304), which reported $(\mathrm{ppy})_2\mathrm{Ir}(\mathrm{acac})$-, $(\mathrm{bt})_2\mathrm{Ir}(\mathrm{acac})$- and $(\mathrm{btp})_2\mathrm{Ir}(\mathrm{acac})$-doped OLEDs (btp = 2-(2-thienyl)pyridine) with EQE of 6-12.3 percent, so it forms no part of the open question.

Problem Statement

Two linked questions left open by Lamansky et al. (Inorg. Chem. 2001, 40, 1704-1711) for octahedral cyclometalated iridium(III) complexes (Ir(III) complexes in which bidentate $C^{\wedge}N$ ligands — aromatic C,N donors such as 2-phenylpyridine, ppy, or 2-(p-tolyl)pyridine, tpy — chelate the metal; acac denotes the monoanionic acetylacetonate ancillary ligand) remain unresolved. (1) Synthetic-mechanistic: establish a predictive mechanistic account of tris-cyclometalation — which $C^{\wedge}N$ ligands form $\mathrm{Ir}(C^{\wedge}N)_3$ under which conditions, and why. Such an account determines the origin (electronic, kinetic or thermodynamic) of the failure of 2-phenylbenzothiazole (bt), 2-(1-naphthyl)benzothiazole (bsn) and 2-phenylquinoline (pq) to give tris-cyclometalated Ir(III) complexes under the conventional glycerol routes from $(C^{\wedge}N)_2\mathrm{Ir}$ precursors, given that molecular modeling shows no steric barrier, that silver-triflate-mediated conditions have since been shown to overcome at least the bt case, and that homoleptic $\mathrm{Ir}(\mathrm{pq})_3$ has since been prepared by other routes while homoleptic $\mathrm{Ir}(\mathrm{bsn})_3$ has not been reported, identifies the intermediates and transition states of the third Ir–C bond formation, tests the source paper's imine-basicity and electronic-stabilization hypotheses, and derives rules predicting the ligand scope under each class of conditions. (2) Photophysical: assign the microscopic origin of the structured, red-shifted orange emission of crystalline $(\mathrm{tpy})_2\mathrm{Ir}(\mathrm{acac})$ (575 nm maximum versus 512 nm in solution) — ground-state dimer traps, an excited-state dimer (excimer) of a distinct geometry, or an intermolecular aggregate/charge-transfer emissive state — in contrast to the broad excimer-like emission of crystalline $(\mathrm{ppy})_2\mathrm{Ir}(\mathrm{acac})$; and, building on that assignment, establish how $\pi$–$\pi$ stacking, metal–metal contacts and aggregate formation generally control the solid-state photophysics of octahedral cyclometalated Ir(III) complexes, resolving the still-contested mechanistic alternatives (new intermolecular ${}^3\mathrm{MLLCT}$/aggregate states versus suppression of nonradiative decay) into a predictive structure–emission relationship. The device-characterization item of the original formulation is excluded because it was resolved by the companion paper of the same group (J. Am. Chem. Soc. 2001, 123, 4304-4312).

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