Here we report the resolution of phosphorescent light-emitting iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2]-picolinate into its respective enantiomers by using chiral HPLC and the photo-induced transformation of the isolated enantiomers

Harada, Takunori

Nakano, Tamaki

Ogasawara, Masamichi

Shiota, Yoshihito

Wang, Heng

Wang, Sheng

Wang, Yue

Ye, Xichong

Yoshizawa, Kazunari

English

Tokushima University Institutional Repository

RSC AdvancesPAPEROpen Access Article. Published on 06 June 2017. Downloaded on 2/12/2021 12:09:01 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View IssueIsolation and phoaInstitute for Catalysis (ICAT), Graduate SchHokkaido University, N 21, W 10, Kita-ku, Snakano@cat.hokudai.ac.jp; Fax: +81-11-706bDepartment of Applied Chemistry, FacultDannoharu, Oita, Oita 870-1124, JapancInstitute for Materials Chemistry and EnginNishi-ku, Fukuoka 819-0395, JapandGraduate School of Science and TecMinamijosanjima-cho, Tokushima 770-8506eIntegrated Research Consortium on ChCatalysis, Hokkaido University, N21W10, K† Electronic supplementary information (selected chromatograms and spectra. See‡ On leave from College of Chemistry & Mas visiting students.Cite this: RSC Adv., 2017, 7, 29550Received 12th April 2017Accepted 25th May 2017DOI: 10.1039/c7ra04141arsc.li/rsc-advances29550 | RSC Adv., 2017, 7, 29550–2955totransformation ofenantiomerically pure iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2]picolinate†Yue Wang,a Takunori Harada,b Yoshihito Shiota,c Kazunari Yoshizawa,c Heng Wang,aSheng Wang,‡a Xichong Ye,‡a Masamichi Ogasawarad and Tamaki Nakano *aeHere we report the resolution of phosphorescent light-emitting iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2]-picolinate into its respective enantiomers by using chiral HPLC and the photo-inducedtransformation of the isolated enantiomers.Phosphorescent light-emitting diodes (LEDs) are drawing moreattention than uorescent LEDs1–6 because the uorescent LEDshave shown maximum internal quantum yields of ca. 25 whilethe phosphorescent LEDs can in theory reach internal quantumyields of nearly 100%.1 Therefore, various iridium complexeshave been studied; with the blue light-emitting iridium bis[(4,6-diuorophenyl)pyridinato-N,C2]picolinate (FIrpic) beinga representative complex.2 FIrpic is a heteroleptic, octahedralcomplex, for which four geometric isomers are possible, and thecomplex from commercial sources is known to be the merisomer with cis C–C and trans N–N geometry, for which L- andD-enantiomers exist (Fig. 1, le pair).Herein we present the isolation of the L and D-enantiomersof FIrpic by preparative HPLC and their photo-induced trans-formation to the enantiomers of themer–cis isomer (Fig. 1, rightpair). Several examples of enantiomers of octahedral Ircomplexes have been reported.3–10 Recently, the isolation andcharacterization of chiroptical properties of FIrpic enantiomerswere reported by Cannazza and co-workers10 while the separa-tion of these enantiomers in the present work was conductedindependently. The phototransformation of an enantiomeri-cally pure FIrpic complex has not been previouslyool of Chemical Sciences and Engineering,apporo 001-0021, Japan. E-mail: tamaki.9156; Tel: +81-11-7069155y of Engineering, Oita University, 700,eering, Kyushu University, 744 Motooka,hnology, Tokushima University, 2-1,, Japanemical Sciences (IRCCS), Institute forita-ku, Sapporo 001-0021, JapanESI) available: Experimental details andDOI: 10.1039/c7ra04141aolecular Engineering, Peking University3accomplished, to the best of our knowledge; the ability to carryout this transformation is important for developing applica-tions for circularly polarized light (CPL) emitter designs whereexcited-state stability of light-emitting species would playa crucial role. In addition to LED applications, Ir complexeshave been used as valuable catalysts in excited states11 wherephototransformation of an Ir complex would have signicantinuences on the catalyzed reactions.Full details of the current experiments are given in the ESI.†Preparative chromatographic resolution experiments wereconducted using a Daicel ChiralPak IC column (25 cm  2 cm(i.d.)), and analytical resolution using analytical-scale ChiralPakIC, IA, and OD columns (25 cm 0.46 cm (i.d.)). These columnsuse polysaccharide derivatives as chiral selectors and have beenapplied for the resolution of a wide variety of racemiccompounds.12 When carrying out analytical-scale resolution onthe IA column, a mixture of hexane, 2-propanol, and dichloro-methane as eluent led to a clearly better separation than dida mixture of hexane and 2-propanol (Fig. S1 in ESI†), andslightly better resolution than did a mixture of hexane, 2-propanol, and chloroform (Fig. S2 in ESI†). In the analyticalresolution on the analytical IC column using a mixture ofhexane, 2-propanol, and dichloromethane, base-line separationwas achieved where the ratio of the three components affectedthe resolution proles (Fig. S3 in ESI†). On the basis of theseobservations, preparative resolution was performed using a 40/Fig. 1 Structures of FIrpic: L- and D-enantiomers of mer–cis-C–C–trans-N–N isomer (left pair) and those of the mer–cis isomer (rightpair) optimized by DFT calculations. Detailed conditions of calculationsare found in ESI.†This journal is © The Royal Society of Chemistry 2017Paper RSC AdvancesOpen Access Article. Published on 06 June 2017. Downloaded on 2/12/2021 12:09:01 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Online40/20 (v/v/v) mixture of hexane, 2-propanol, and dichloro-methane as the eluent.The Chiralcel OD column was also effective in resolvingFIrpic into its enantiomers; the circular dichroism (CD) signs at250 nm of the rst- and second-eluting enantiomers wereopposite those reported in ref. 10 under the same separationcondition (Fig. S4 in ESI†).Fig. 2A(1–4) show the analytical high-performance liquidchromatography (HPLC) proles obtained with the IA column,and CD, UV spectra, and CPL emission spectra13 of the isolatedenantiomers, and theoretical CD spectra of L- and D-enantio-mers obtained from density functional theory (DFT) calcula-tions. The HPLC chromatograms (Fig. 2A) indicated that theenantiomers obtained by preparative HPLC resolution of FIrpicwere almost enantiomerically pure; the separation factor inFig. 1A-2 (a) was 1.29. The isolated enantiomers exhibitedmirror-image CD spectra in methanol over the visible and UVranges (Fig. 2B).Furthermore, the isolated enantiomers showed clear, mirror-image CPL emission spectra13 in the range 450–550 nm onexcitation at 286 nm (Fig. 2D) where maximum anisotropy (glum)values were 2.0  103 at 489 nm (L-enantiomer) and 2.1 103 at 490 nm (D-enantiomer); anisotropy in emission (glum) isdened as 2(IL  IR)/(IL + IR) where IL and IR are L- and R-CPLemission intensities, respectively. These results showed theisolated enantiomers to have enantiomeric structures both inthe ground state and excited states. The clear CPL emissionspectra of the FIrpic enantiomers indicated they are potentiallyapplicable for CPL-emitting LEDs.Fig. 2 Analytical results on the racemic mer–cis-C–C–trans-N–NFIrpic isomer: HPLC profiles of the racemic mixture ((A-1) CD detec-tion at 254 nm; (A-2) UV detection at 254 nm), D-enantiomer ((A-3) UVdetection at 254 nm), and L-enantiomer ((A-4) UV detection at 254nm) and experimental CD ((B-1) L; (B-2) D) and UV ((B-3) L; (B-4) D)spectra, theoretical CD spectra ((C-1) L; (C-2) D), and CPL emissionspectra ((D-1) L; (D-2) D) with DC spectra (total emission spectra) ((D-3) L; (D-4) D) of the L- and D-enantiomers. Conditions: eluent ¼hexane/2-PrOH/CH2Cl2 mixture (65/15/20 (v/v/v)), flow rate ¼ 0.5 mLmin1 (A); solvent¼MeOH, conc.¼ 1.0 104 M, cell path¼ 1 mm (B);solvent ¼ THF, conc. 2.0  105 M (D).This journal is © The Royal Society of Chemistry 2017In addition, by comparing the experimental and theoreticalCD spectra, the absolute congurations of the resolved enan-tiomers were determined. The theoretical spectra in Fig. 2Cwere obtained by carrying out TDDFT calculations with theB3LYP functional14,15 and the SDD basis set16 for Ir using theGaussian 09 program package.17 The theoretical CD spectrumshowed a rather sharp and intense negative Cotton band anda broad, weaker negative Cotton band in the range of ca. 300–380 nm, which roughly corresponded to the spectrum of therst enantiomer eluted in the chiral HPLC resolution of FIrpic(Fig. 2B-1). On the basis of these observations, the rst-elutingenantiomer from the HPLC column was assigned to be the L-enantiomer and the second-eluting enantiomer was assigned tobe the D-enantiomer. Thus, the L- and D-enantiomers of themer–cis-C–C–trans-N–N isomer were successfully isolated andtheir absolute congurations were assigned.Furthermore, the isolated D- and L-enantiomers were foundto undergo photo-isomerization and degradation. Since opti-cally active Ir complexes may be used as CPL LED materials, thestability levels of such complexes in their excited states wouldbe an important criterion in molecular design. In order to shedlight on this aspect, the D- and L-enantiomers were indepen-dently irradiated in a MeOH solution (1.0  104 M) using non-polarized light (NPL) (ca. 200 mW cm2) from an Hg–Xe lamp ina MeOH solution under an N2 atmosphere.Fig. 3A and B show the changes in the CD and UV spectra ofthe L- and D-enantiomers, respectively, upon irradiation underan N2 atmosphere. CD spectral intensity remarkably decreasedupon irradiation, and the shapes of the UV spectra alsochanged. The changes in UV spectra suggested that the CDFig. 3 CD and UV spectral changes of the L-enantiomer (A) and D-enantiomer (B) (mer–cis-C–C–trans-N–N) in MeOH on NPL irradia-tion, and HPLC profile changes detected at UV 254 nm for the L-enantiomer (C) and D-enantiomer (D). Conditions: conc. ¼ 1.0 104 M, cell path ¼ 1 mm (A and B); eluent ¼ hexane/CH2Cl2/2-PrOHmixture (v/v/v), flow rate ¼ 0.5 mL min1, sample conc. before irra-diation ¼ 1.0  104 M, injected sample volume ¼ 10 mL (C and D). *denotes optically inactive impurity signals.RSC Adv., 2017, 7, 29550–29553 | 29551RSC Advances PaperOpen Access Article. Published on 06 June 2017. Downloaded on 2/12/2021 12:09:01 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinespectral changes were not due to simple racemization but someother structural transformation. This assumption was sup-ported by the HPLC proles of the irradiated samples (Fig. 3Cand D); upon irradiation, the intensity of the L- and D-enan-tiomer signals decreased and, at the same time, new signalsemerged at retention times of 16.5 min and 13.5 min, respec-tively, for the systems with the L- and D-enantiomers: the newspecies isolated using HPLC exhibited clear mirror image CDspectra (Fig. 4A-1 and A-2).Note the slightly different intensities of the CD spectra inFig. 2B-1 and B-2; theL-isomer yielded a slightly lower intensitythan did the D-isomer. A small amount of the L-isomer mighthave undergone the phototransformation before its CD spec-trum was acquired.In addition, a new species was obtained from the D-enan-tiomer as indicated from mass spectrometry results, whichshowed a main signal at a mass number (m/z) of 718, corre-sponding to an ion of FIrpic in the form of an [M + Na]+ sodiumadduct, along with other weaker signals (Fig. S6 in ESI†). Theseresults suggested that the L- and D-enantiomers of the mer–cis-C–C–trans-N–N isomer underwent, under the present experi-mental conditions, stereomutation to enantiomers of one of thethree other geometric isomers of FIrpic, i.e.,mer–cis, fac–cis, andmer–trans-C–C–cis-N–N, rather than simple racemization.Fig. 4 CD/UV spectra of the enantiomeric compounds obtained bychiral HPLC separation of the photo-irradiation products of the L-enantiomer ((A-1) CD; (A-3) UV) and D-enantiomer ((A-2) CD; (A-4) UV)of mer–cis-C–C–trans-N–N isomer, CPL/DC spectra of the photo-irradiation product of D-enantiomer of mer–cis-C–C–trans-N–Nisomer ((B-1) CPL; (B-2) DC), and theoretical CD spectra of enantiomersof the mer–cis isomer ((C-1) L; (C-2) D), of the fac–cis isomer ((D-1) L;(D-2) D) and of the mer–trans-C–C–cis-N–N isomer ((E-1) L; (E-2) D).29552 | RSC Adv., 2017, 7, 29550–29553Moreover, the phototransformation product of the D-enan-tiomers of the mer–cis-C–C–trans-N–N isomer yielded CPLemission spectra with a maximum glum value appearing to be atabout 2  103 (Fig. 4B-1 and B-2).Since attempts at crystallizing the isolated, transformedenantiomers for X-ray analysis failed, their structures wereidentied by acquiring and comparing their experimental andtheoretical CD spectra (Fig. 4C-1 and E-2). The theoretical CDspectra of the enantiomers of the three geometric isomersdiffered from one another. The shapes of the experimentalspectra in Fig. 4A-1 and A-2 appeared most similar to those ofthe theoretical spectra of the enantiomers of the mer–cis isomerin Fig. 4C-1 and C-2: both the theoretical spectrum of theL-enantiomer (in Fig. 4C-1) and the experimental spectrum (inFig. 4A-2) showed three positive and three negative Cottonbands in the range 200–400 nm and such a feature was not seenfor the enantiomers of fac–cis isomer and mer–trans-C–C–cis-N–N isomer (Fig. 4D-1 and E-1). We hence propose that theD-enantiomer of the mer–cis-C–C–trans-N–N isomer mutatedinto the L-enantiomer of the mer–cis isomer.Note that the intensities of all of the signals in the HPLCproles in Fig. 3C and D decreased as the irradiation time wasincreased, and, aer 180 min of irradiation, the overall inten-sities became much lower than those in the initial state (Fig. S7in ESI†). The decreases in the CD intensities in Fig. 3A and Bwere thus not due only to the generation of the optically activetransformed products but also to disappearance or decompo-sition of the enantiomers and their corresponding transformedderivatives.In addition, changes in the CD/UV spectra and in the HPLCproles of the D-enantiomer occurred more rapidly upon irra-diation using NPL with a relatively high irradiation power (of ca.200mW cm2) than when using LPL (ca. 40mW cm2) under anN2 atmosphere. Moreover, upon irradiation of the D-enan-tiomer using NPL, the changes in its CD and HPLC results werefaster in the presence of air than under N2 (Fig. S7 in ESI†).These results suggested that the stereomutation and decom-position of theL- and D-enantiomers occurred in excited states,and were accelerated by O2 contained in air, possibly throughinteractions between unpaired electrons of O2 with electrons ofFIrpic in its higher-energy orbitals. The photoreactions of FIrpicare assumed to have occurred through its triplet excited states.A photo-induced transformation of the Ir complex has beenreported for iridium(III) tris(2-phenylpyridinato-N,C20)(Ir(ppy)3)18 but has never been reported for FIrpic. The currentwork is the rst to report on the photo-isomerization of enan-tiomers of FIrpic.In conclusion, commercial FIrpic, i.e., the mer-isomer ofFIrpic with cis C–C and trans N–N arrangements, was resolvedinto optically pure L- and D-enantiomers by using chiral HPLC,and upon photo-irradiation, the enantiomers underwent ster-eomutation and decomposition: we propose that the L- and D-enantiomers of themer–cis-C–C–trans-N–N isomer mutated intothe D- and L-enantiomers of the mer–cis geometrical isomer,respectively. The mutation and decomposition occurred notonly in the presence of air but also under an N2 atmosphere.These results revealed an important cautionary note, thatThis journal is © The Royal Society of Chemistry 2017Paper RSC AdvancesOpen Access Article. Published on 06 June 2017. Downloaded on 2/12/2021 12:09:01 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineoptically active FIrpic may be transformed in excited states evenin a sealed device, which would need to be taken into consid-eration when designing LEDs using optically active FIrpicenantiomers. In addition, transformation of FIrpic may alsoplay a role in catalyses using Ir complexes.11AcknowledgementsThis work was supported in part by the Mitsubishi Foundationand in part by MEXT program of Integrated Research Consor-tium on Chemical Synthesis (IRCCS).Notes and references1 (a) M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley,M. E. Thompson and S. 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Isolation and phototransformation of enantiomerically pure iridium(iii) bis[(4,6-difluorophenyl)pyridinato-N,C2]picolinate

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Isolation and phototransformation of enantiomerically pure iridium(iii) bis[(4,6-difluorophenyl)pyridinato-N,C2]picolinate

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