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Stereospecific Synthesis of Tris-heteroleptic Tris-cyclometalated Iridium(III) Complexes via Different Heteroleptic Halogen-Bridged Iridium(III) Dimers and Their Photophysical Properties
Herein,
we report on the stereospecific synthesis of two single
isomers of tris-heteroleptic tris-cyclometalated iridium(III) (Ir(III))
complexes composed of three different nonsymmetric cyclometalating
ligands via heteroleptic halogen-bridged Ir dimers [Ir(tpy)(F<sub>2</sub>ppy)(μ-Br)]<sub>2</sub> <b>17b</b> and [Ir(mpiq)(F<sub>2</sub>ppy)(μ-Br)]<sub>2</sub> <b>27b</b> (tpyH:
(2-(4′-tolyl)pyridine) and F<sub>2</sub>ppyH: (2-(4′,6′-difluorophenyl)pyridine),
and mpiqH: (1-(4′-methylphenyl)isoquinoline))
prepared by Zn<sup>2+</sup>-promoted degradation of Ir(tpy)<sub>2</sub>(F<sub>2</sub>ppy) <b>21</b> and Ir(mpiq)<sub>2</sub>(F<sub>2</sub>ppy) <b>26</b>, as reported by us.
Subsequently, <b>17b</b> and <b>27b</b> were converted
to the tris-heteroleptic tris-cyclometalated Ir complexes Ir(tpy)(F<sub>2</sub>ppy)(mpiq) <b>25</b> consisting of tpy, F<sub>2</sub>ppy, and mpiq, as confirmed by spectroscopic data and X-ray
crystal structure analysis. The first important point in this work
is the selective synthesis of specific isomers among eight possible
stereoisomers of Ir complexes having the same combination of three
cyclometalating ligands. Namely, two meridional forms of <b>25</b> were synthesized and isolated. The second finding is that the different
stereoisomers of <b>25</b> have different stability. Finally,
different stereoisomers exhibit different emission spectra. Namely,
one of its stereoisomers <b>25a</b> exhibits a single broad
emission from <i>ca</i>. 550 nm to <i>ca</i>.
650 nm (orange emission), while stereoisomer <b>25c</b> emits
dual emission at <i>ca</i>. 509 nm and <i>ca</i>. 600 nm (pale pink emission), as supported by time-dependent density
functional theory calculation. To the best of our knowledge, this
is the first report of the selective and efficient synthesis of different
stereoisomers of tris-heteroleptic tris-cyclometalated Ir(III) complexes
that have different stabilities and different photophysical properties