Divergent Approach for Tris-Heteroleptic Cyclometalated Iridium Complexes Using Triisopropylsilylethynyl-Substituted Synthons

Bis-heteroleptic cyclometalated iridium complexes of the form Ir(La)2(acac), where La is a substituted 2-phenylpyridine derivative and acac is an acetylacetonato ligand, are a useful class of luminescent organometallic complexes for a range of applications. Related tris-heteroleptic complexes of the form Ir(La)(Lb)(acac) offer the potential advantage of greater functionality through the use of two different cyclometalated ligands but are, in general, more difficult to obtain. We report the synthesis of divergent bis- and tris-heteroleptic triisopropylsilylethynyl-substituted intermediate complexes that can be diversified using a “chemistry-on-the-complex” approach. We demonstrate the methodology through one-pot deprotection and Sonogashira cross-coupling of the intermediate complexes with para-R-aryliodides (R = H, SMe, and CN). The photophysical and electrochemical behaviors of the resultant bis- and tris-heteroleptic complexes are compared, and it is shown that the tris-heteroleptic complexes exhibit subtly different emission and redox properties to the bis-heteroleptic complexes, such as further red-shifted emission maxima and lower extinction coefficients, which can be attributed to the reduced symmetry. It is demonstrated, supported by DFT and time-dependent DFT calculations, that the charge-transfer character of the emission can be altered via variation of the terminal substituent; the introduction of an electron-withdrawing cyano group in the terminal position leads to a significant red shift, while the introduction of an SMe group can substantially increase the emission quantum yield. Most notably, this convenient synthetic approach reduces the need to perform the often challenging isolation of tris-heteroleptic complexes to a single divergent intermediate, which will simplify access to families of complexes of the form Ir(La)(Lb)(acac).


General method for Sonogashira coupling with 2
Tetrahydrofuran (50 mL) was added to a suspension containing Et3N (5 mL), 3 (0.10 mmol), ArI (0.15 mmol), Pd(PPh3)4 (11 mg, 0.01 mmol), and CuI (1.9 mg, 0.01 mmol). The resultant solution was degassed by three freeze-pump-thaw cycles, before tetrabutylammonium fluoride (1.0 M in THF, 0.25 mL, 0.25 mmol) was added. The solution was then stirred at room temperature for 12 h before the solvent was removed. The residue was purified by silica chromatography eluted by either 1:2 hexane:DCM or neat DCM, collecting the emissive band. The solvent was removed to yield an orange solid.

S3. Crystallographic data
The X-ray single crystal data have been collected using λMoKα radiation (λ = 0.71073 Å) on a Bruker D8Venture (Photon100 CMOS detector, IμS-microsource, focusing mirrors) diffractometer equipped with a Cryostream (Oxford Cryosystems) open-flow nitrogen cryostats at the temperature 120.0(2) K. The structure was solved by direct method and refined by full-matrix least squares on F 2 for all data using Olex2 2 and SHELXTL 3 software.
All non-hydrogen atoms were refined in anisotropic approximation; hydrogen atoms were placed in the calculated positions and refined in riding mode. The structure of 6 contains several severely disordered solvent molecules, most probably three DMF molecules, which could not be modelled and refined reliably. The contribution of disordered solvent molecules into diffraction intensities was taken into account by application of the MASK procedure of the Olex2 program package.
Crystal data and parameters of refinement are listed in Table S1. Crystallographic data for the structure have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication CCDC-2169136. S17

S5. Photophysical measurements
All photophysical measurements of iridium complexes (except solvatochromism) were performed using DCM as the solvent. The ultraviolet (UV)-visible spectra were measured on a UV2-100 spectrometer operated with Vison software (Unicam Digital, Leeds, UK). Samples were held in quartz cuvettes, with path length l = 1 cm. Excitation and emission photoluminescence spectra were recorded on a Spex ® Fluorolog ® 3-22 spectrofluorometer (Horiba Scientific, Irvine, CA, USA). Samples were degassed in quartz cuvettes of l = 1 cm fitted with Teflon J Young taps by repeated freeze-pump-thaw cycles using a turbomolecular pump until the pressure was stable. The solutions' absorbance was < 0.15 to minimize inner filter effects.
Photoluminescence quantum yields (PLQYs) were measured following our previously reported method (see below). 4 Quinine sulfate in 0.1 M H2SO4 (ΦF: 0.546) 5 or rhodamine 101 in ethanol (ΦF: 1.00) 6 as the references, the emission spectra of quinine sulfate were collected by exciting the samples at 360 nm. The PLQY of each complex was measured in duplicate and determined by the following method: 1. The UV-vis absorbance spectrum was recorded in quartz cuvette with path length l = 2 cm (to improve spectrum signal-to-noise), recording the absorbance at the excitation wavelength.
2. The same sample solution was transferred to the quartz fluorescence cell (a standard 1 cm cell modified with a Teflon Young's tap) degassed via repeated freeze-pump-thaw cycles before the emission spectrum was recorded.
3. The fully corrected fluorescence spectrum was integrated and the integrated intensity (the area of the fluorescence spectrum) was recorded.
4. Steps 1 to 3 were repeated for five additional solutions with increasing concentrations (with absorbance ranging from 0.02 to 0.1).
5. The integrated fluorescence intensity was plotted verse absorbance, which resulted a linear plot with gradient X (GradX).
7. Fluorescence quantum yield for each complex was calculated using the following Where ST and X denote standard and the measured complex, Φ is the fluorescence quantum yield, Grad the gradient from the plot of integrated fluorescence intensity vs.
absorbance, and η the refractive index of the solvent.

S6. Calculations
Hybrid density functional theory (DFT) calculations were performed using the Gaussian16 package (Gaussian, Inc) 7 All calculations used the B3LYP 8 functional with a mixed basis set LANL2DZ/3-21G* where the pseudopotential LANL2DZ 9, 10 was applied to iridium and the 3-21G* basis set 11,12 for all other atoms. The fully optimised geometries were confirmed as true minima with no imaginary frequencies found from frequency calculations. The rotational barriers were estimated from constrained geometries with fixed torsional angles so that the two rings attached to the ethynyl bridge are perpendicular to each other. A frequency calculation on each constrained geometry showed one imaginary frequency indicating a rotation barrier transition state. Orbital figures were drawn using GabEdit 13 and the group contributions to each orbital were estimated as a percentage (%) with GaussSum 14 .     Figure S47. Frontier molecular orbitals for the fully optimised geometry 6 and the constrained geometry 6 (90°). Isocontours at 0.055 e bohr -3/2 . Figure S48. Frontier molecular orbitals for the fully optimised geometry 7 and the constrained geometry 7 (90°). Isocontours at 0.055 e bohr -3/2 .