Tris-cyclometalated Ir(III) complexes Ir(C^N)3, workhorse phosphors of organic light-emitting diodes and photoredox catalysis, exist as facial (fac) and meridional (mer) geometrical isomers with strikingly different photophysics: the mer isomers combine order-of-magnitude larger nonradiative decay rates with the ability to convert thermally and photochemically to the thermodynamically preferred fac form. Since Tamayo, Alleyne, Djurovich, Lamansky, Tsyba, Ho, Bau, and Thompson established controlled synthetic access to both isomers (JACS 2003), the central open question has been the mechanistic and energetic account of these phenomena: the reaction pathways, intermediates, transition states, and barriers of the thermal and photochemical mer-to-fac isomerizations; the microscopic origin and timescales of the very large nonradiative decay rates of the mer isomers; how the fac-mer energy difference and interconversion barriers are tuned by the electronic and steric properties of the C^N ligand; and why coordinating solvents accelerate the photochemical conversion. Later work has substantially advanced but not closed this question: the thermal route in coordinating media is now known to be solvent-catalyzed through iridium alkoxide intermediates, and the photochemical route is attributed to a dissociative triplet metal-centered (3MC) state with an elongated Ir-N bond, largely on computational and indirect experimental evidence. What remains open is direct experimental characterization of the transient intermediates and their timescales, a quantitative general map of fac-mer energetics and barriers across ligand and solvent classes, and the mechanistic basis of solvent acceleration of photoconversion.

Background

Cyclometalation is the formation of a metal-carbon bond by activation of a C-H bond in an aromatic ligand; in the complexes at issue, each monoanionic C^N ligand (for example 2-phenylpyridyl, ppy, or 1-phenylpyrazolyl, ppz) binds iridium through one carbon and one nitrogen donor. Ir(III) is a low-spin d6 center, and three such bidentate ligands fill an octahedral coordination sphere. When the three chelates are identical but unsymmetrical, two geometrical isomers exist: in the facial (fac) isomer the three carbon donors occupy one face of the octahedron, so every Ir-C bond is trans to an Ir-N bond; in the meridional (mer) isomer two carbon donors are trans to each other. Because an aryl carbon donor exerts a strong trans influence (the lengthening and weakening of the bond opposite a strongly trans-influencing ligand), the mer isomers have systematically elongated Ir-C and Ir-N bonds trans to phenyl groups, a higher-lying HOMO, and easier oxidation than the fac isomers. These complexes phosphoresce from a triplet state of mixed metal-to-ligand charge-transfer (MLCT, an excited state in which electron density moves from the metal to the ligand) and ligand-centered (3LC) character; the emission efficiency is governed by the competition between the radiative rate k_r and the nonradiative rate k_nr. A key deactivation channel in such complexes is the population of triplet metal-centered (3MC) states, d-d excited states with elongated Ir-ligand bonds that are dissociative along an Ir-N or Ir-C coordinate. In 2003 Tamayo et al. (JACS, DOI 10.1021/ja034537z) reported the first controlled syntheses of both isomers across a ligand series (mer isomers from dichloro-bridged dimers with excess ligand at 140-150 °C; fac isomers at higher temperature), single-crystal structures, electrochemistry, and photophysics. They established that the mer isomers are the kinetically favored products while the fac isomers are thermodynamically preferred (their density functional theory (DFT, a quantum-chemical method that computes energies from the electron density) calculations put the fac preference at roughly 30 kJ per mol), that isolated mer samples convert to fac both thermally (refluxing glycerol) and photochemically (near-quantitative conversion within 2 h under UV irradiation in coordinating d6-DMSO, versus about 72 h in weakly coordinating toluene-d8), and that the mer isomers have enormously enhanced nonradiative decay (for Ir(ppy)3, k_nr rises from 3.2e5 s^-1 in fac to 6.4e6 s^-1 in mer, dropping the photoluminescence quantum yield from 0.40 to 0.036). These observations define the problem: what, at the level of intermediates, transition states, barriers, and excited-state potential-energy surfaces, actually happens during the thermal and photochemical mer-to-fac conversions and during the excited-state decay of the mer isomers, and how do ligand and solvent control these processes. The question matters practically because fac/mer isomerism controls the efficiency and operational stability of phosphorescent organic light-emitting diode (PhOLED) emitters and the behavior of Ir(III) photoredox catalysts, and synthetic routes to either isomer depend on the kinetic-versus-thermodynamic balance that the mechanism determines. A general framework for 3MC-state-mediated nonradiative decay in cyclometalated Ir(III) complexes was later established (notably by Sajoto, Djurovich, Tamayo, Oxgaard, Goddard, and Thompson, JACS 2009), but largely for facial emitters.

Problem Statement

Establish a complete mechanistic and energetic account, for tris-cyclometalated Ir(III) complexes Ir(C^N)3, of the fac/mer (facial versus meridional geometrical) isomerization and of the excited-state deactivation of the mer isomers, addressing four linked questions: (1) identify the reaction pathways of the thermal and photochemical mer-to-fac isomerizations, i.e., determine whether they proceed through discrete bond-rupture/coordination intermediates, concerted ligand rearrangements, or solvent-assisted dissociative steps, and characterize the relevant intermediates, transition states, and activation barriers for different C^N ligand types; (2) determine the microscopic origin of the very large nonradiative decay rates of the meridional isomers, including which excited-state distortion (Ir-N or Ir-C bond elongation or cleavage) is the primary quenching channel, on what timescales these bond distortions occur in the excited state, and how substituents on the C^N ligands modulate these processes; (3) map quantitatively how ligand electronic and steric properties tune the fac-mer thermodynamic energy difference and the kinetic barriers to interconversion across at least three chemically distinct C^N ligand classes spanning electron-poor to electron-rich, at least one of which lies outside the C^N families already characterized in the cited literature (the 2-phenylpyridyl, 1-phenylpyrazolyl, phenylisoquinoline, phenyltriazole, and N-heterocyclic carbene families); and (4) characterize mechanistically and kinetically the solvent dependence of the photoconversion rates, in particular why coordinating solvents accelerate the photochemical mer-to-fac conversion and how solvent participation affects the intermediates. The common scientific objective uniting these questions is a predictive, experimentally grounded structure-mechanism relationship connecting the geometrical isomerism of the Ir(C^N)3 core to its isomerization kinetics, its excited-state decay channels, and their ligand and solvent dependence. A full account of the mechanism requires all four questions to be addressed, with the intermediates and barriers identified and validated (not merely inferred computationally) and with the fac-mer energetic and kinetic landscape predictable for ligands and solvents beyond the already-characterized baseline. That baseline is the set characterized in the cited references: the 2-phenylpyridyl, 1-phenylpyrazolyl, phenylisoquinoline, phenyltriazole, and N-heterocyclic carbene families for the ligands, and glycerol, d6-DMSO, and toluene-d8 for the solvents. The extrapolation requirement adds no ligand class beyond the one required by question (3): the same additional class that question (3) requires outside this baseline, together with at least one solvent outside it, must be covered by the predictions, and the extrapolated predictions must be validated against independent measurements on that ligand class or solvent.

References

  1. Tamayo A. B., Alleyne B. D., Djurovich P. I., Lamansky S., Tsyba I., Ho N. N., Bau R., Thompson M. E. (2003). Synthesis and Characterization of Facial and Meridional Tris-cyclometalated Iridium(III) Complexes. Journal of the American Chemical Society 125, 7377-7387. DOI 10.1021/ja034537z. https://doi.org/10.1021/ja034537z
  2. Karatsu T., Nakamura T., Yagai S., Kitamura A., Yamaguchi K., Matsushima Y., Iwata T., Hori Y., Hagiwara T. (2003). Photochemical mer → fac One-way Isomerization of Phosphorescent Material. Studies by Time-resolved Spectroscopy for Tris[2-(4′,6′-difluorophenyl)pyridine]iridium(III) in Solution. Chemistry Letters 32, 886-887. DOI 10.1246/cl.2003.886. https://doi.org/10.1246/cl.2003.886
  3. McDonald A., Lutz M., von Chrzanowski L., van Klink G., Spek A., van Koten G. (2008). Probing the mer- to fac-Isomerization of Tris-Cyclometallated Homo- and Heteroleptic (C,N)3 Iridium(III) Complexes. Inorganic Chemistry 47, 6681-6691. DOI 10.1021/ic800169n. https://doi.org/10.1021/ic800169n
  4. Tsuchiya K., Ito E., Yagai S., Kitamura A., Karatsu T. (2009). Chirality in the Photochemical mer→fac Geometrical Isomerization of Tris(1-phenylpyrazolato,N,C2′)iridium(III). European Journal of Inorganic Chemistry 2009, 2104-2109. DOI 10.1002/ejic.200801254. https://doi.org/10.1002/ejic.200801254
  5. Sajoto T., Djurovich P., Tamayo A., Oxgaard J., Goddard W., Thompson M. (2009). Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III) Complexes. Journal of the American Chemical Society 131, 9813-9822. DOI 10.1021/ja903317w. https://doi.org/10.1021/ja903317w
  6. Deaton J., Young R., Lenhard J., Rajeswaran M., Huo S. (2010). Photophysical Properties of the Series fac- and mer-(1-Phenylisoquinolinato-N^C2′)x(2-phenylpyridinato-N^C2′)3−xIridium(III) (x = 1−3). Inorganic Chemistry 49, 9151-9161. DOI 10.1021/ic1002594. https://doi.org/10.1021/ic1002594
  7. Zheng Y., Batsanov A., Edkins R., Beeby A., Bryce M. (2012). Thermally Induced Defluorination during a mer to fac Transformation of a Blue-Green Phosphorescent Cyclometalated Iridium(III) Complex. Inorganic Chemistry 51, 290-297. DOI 10.1021/ic201655n. https://doi.org/10.1021/ic201655n
  8. Setzer T., Lennartz C., Dreuw A. (2017). A theoretical study on the mechanistic highlights behind the Brønsted-acid dependent mer–fac isomerization of homoleptic carbenic iridium complexes for PhOLEDs. Dalton Transactions 46, 7194-7209. DOI 10.1039/c7dt01201b. https://doi.org/10.1039/c7dt01201b
  9. Arroliga-Rocha S., Escudero D. (2018). Facial and Meridional Isomers of Tris(bidentate) Ir(III) Complexes: Unravelling Their Different Excited State Reactivity. Inorganic Chemistry 57, 12106-12112. DOI 10.1021/acs.inorgchem.8b01675. https://doi.org/10.1021/acs.inorgchem.8b01675
  10. Wu B., Huang M., Lai C., Cheng C., Chen I. (2018). Isomerization Reaction of mer- to fac-Tris(2-phenylpyridinato-N,C2′)Iridium(III) Monitored by Using Surface-Enhanced Raman Spectroscopy. Inorganic Chemistry 57, 4448-4455. DOI 10.1021/acs.inorgchem.8b00113. https://doi.org/10.1021/acs.inorgchem.8b00113
  11. Escudero D. (2019). Mer-Ir(ppy)3 to Fac-Ir(ppy)3 Photoisomerization. ChemPhotoChem 3, 697-701. DOI 10.1002/cptc.201900029. https://doi.org/10.1002/cptc.201900029
  12. Song C., Chen Y., Li J., Zhao F., Zhang H. (2019). Unraveling the marked differences of the phosphorescence efficiencies of blue-emitting iridium complexes with isomerized phenyltriazole ligands. Inorganic Chemistry Frontiers 6, 2776-2787. DOI 10.1039/C9QI00844F. https://doi.org/10.1039/C9QI00844F
  13. Gitlina A., Fadaei-Tirani F., Ruggi A., Plaice C., Severin K. (2022). Acid-base-induced fac → mer isomerization of luminescent iridium(III) complexes. Chemical Science 13, 10370-10374. DOI 10.1039/d2sc02808e. https://doi.org/10.1039/d2sc02808e
  14. Gitlina A., Fadaei-Tirani F., Severin K. (2023). The acid-mediated isomerization of iridium(III) complexes with cyclometalated NHC ligands: kinetic vs. thermodynamic control. Dalton Transactions 52, 2833-2837. DOI 10.1039/d2dt04039e. https://doi.org/10.1039/d2dt04039e
  15. Gómez Bustos D., Sreenivasan S. T., Pinter B. (2025). A computational study on the effect of structural isomerism on the excited state lifetime and redox energetics of archetype iridium photoredox catalyst platforms [Ir(ppy)2(bpy)]+ and Ir(ppy)3. The Journal of Chemical Physics 162. DOI 10.1063/5.0239293. https://doi.org/10.1063/5.0239293
  16. McGee K. A., Mann K. R. (2007). Selective Low-Temperature Syntheses of Facial and Meridional Tris-cyclometalated Iridium(III) Complexes. Inorganic Chemistry 46, 7800-7809. DOI 10.1021/ic700440c. https://doi.org/10.1021/ic700440c