FULL PAPER DOI: 10.1002/ejic.200801254 Chirality in the Photochemical merfac Geometrical Isomerization of Tris(1-phenylpyrazolato,N,C 2' )iridium(III) Kazuyoshi Tsuchiya, [a] Etsuko Ito, [a] Shiki Yagai, [a] Akihide Kitamura, [a] and Takashi Karatsu* [a] Keywords: Iridium / Isomerization / Chirality / Photochemistry Irradiation of the optically resolved mer -Δ isomer of tris(1- phenylpyrazolato,N,C 2' )iridium(III) with 366-nm light in CH 3 CN purged by argon at 25 °C gave 59 % fac-Δ and 41 % fac-Λ (18 % ee) at the end of geometrical isomerization. For- mation of the intermediate mer -Λ species was not observed, which is quite characteristic when compared with the corre- sponding thermal isomerization reaction. This enantiomeric Introduction Iridium triscyclometalated complexes have recently at- tracted significant attention because of their supreme phos- phorescence performance for OLEDs (organic light-emit- ting diodes). [1–5] For those materials, understanding of the behavior of the excited states including radiative and nonra- diative processes are very important to prepare complexes with high emission efficiency and stability. Triscyclometalated complexes such as tris(2-phenylpyrid- inato,N,C 2' )iridium(III) [Ir(ppy) 3 ] have meridional (mer) and facial (fac) geometrical isomers. The fac isomer is the thermodynamically controlled product in their synthesis and is generally strongly phosphorescent, whereas the cor- responding mer isomer is the kinetically controlled product and weakly phosphorescent in solution at ambient tempera- ture. Although several reports on the preparation of mer isomers have recently appeared for iridium complexes, their photochemical properties remains unclear. [6,7] Among them, we are interested in tris(1-phenylpyrazolato,N,C 2' )- iridium(III) [Ir(ppz) 3 , Scheme 1], as both the mer and fac isomers give no phosphorescence in solution at ambient temperature. [6,7] The main reason for this was reported to be the location of the thermally equilibrated nonradiative excited state just above the emissive state, [8,9] and the ab- sence of phosphorescence from the mer isomer is partly due to the merfac geometrical isomerization. [6,7] Ir(ppz) 3 is reasonably phosphorescent in the solid phase, and there- fore, a blue-emitting OLED device has been fabricated. [10] [a] Department of Applied Chemistry and Biotechnology, Gradu- ate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan Fax: +81-43-290-3401 E-mail: karatsu@faculty.chiba-u.jp © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2009, 2104–2109 2104 photoisomerization is rationally explained by a mechanism based on Ir–N bond dissociation at the top or bottom axial ligand. This reaction mechanism is explained by the poten- tial energy surface of the triplet excited state. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) In addition, Ir(ppz) 3 is not only used as a part of a dopant to give white emission, [11] but it is also used as a material for electron blocking layer. [12] Scheme 1. Enantiomers and geometrical isomers in the photochem- ical isomerization of Ir(ppz) 3 . In this work we addressed how the chirality of the mer isomer is transferred to the fac isomer in the photochemical merfac geometrical isomerization of Ir(ppz) 3 . Interest- ingly, we found that the intermediate mer-Λ is not generated upon irradiation of mer-Δ. This cannot be explained by faster merfac isomerization than mer-Δmer-Λ isomer- ization, that is, the two processes are not parallel. The isom- erization seems to proceed through a ligand dissociation– association mechanism, and as a result, the geometrical isomerization must accompany the optical isomerization. Once mer-Λ is produced by the irradiation of mer-Δ, both isomers behave similarly. Thus, selective disappearance of only mer-Λ does not occur. We propose a plausible mecha- nism based on DFT calculations. Results and Discussion Ir(ppz) 3 was synthesized according to the method re- ported previously. [8] The mer and the fac isomers were sepa-