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

Abstract

Bis -heteroleptic cyclometalated iridium complexes of the form Ir(L a ) 2 (acac), where L a 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(L a )(L b )(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(L a )(L b )(acac).

Aligning π-Extended π-Deficient Ligands to Afford Submicrosecond Phosphorescence Radiative Decay Time of Mononuclear Ir(III) Complexes

“…24 There is only one example of a mononuclear Ir(III) complex emitting in the red range with sub-μs radiative decay time, reported very recently by Beeby, Davidson, and co-authors. 25 The structure of that complex complies with the design principles discussed in this work. Therefore, achieving a submicrosecond radiative decay time of phosphorescence in mononuclear complexes remains a challenge, and the few reported cases are rather lucky findings than results of targeted molecular design.…”

“…The emission rate of the heteroleptic complex Ir(dppm) 2 (acac) is notably higher than that of its both analogues: the homoleptic fac-Ir(dppm) 3 (k r = 0.86 × 10 6 s −1 ) and the heteroleptic Ir(ppy) 2 (acac), (k r = 0.53 × 10 6 s −1 ) compromising less π-extended ppy ligands. In fact, Ir(dppm) 2 (acac) is currently one of only two 25 highly emissive mononuclear Ir(III) complexes, which show orange to red emission with a radiative decay time below 1 μs. Detailed information on T 1 properties of Ir(dppm) 2 (acac), fac-Ir(dppm) 3 , and Ir(ppy) 2 (acac) was elucidated from decay times measured in frozen glassy toluene at different temperatures in the range 1.6 K < T < 130 K (Figures 2 and 3).…”

“…Although much higher phosphorescence rates can be achieved with multinuclear molecular structures, ,− there are only few examples of mononuclear Ir­(III) complexes with a radiative decay time below 1 μs. − Those mononuclear compounds emit in the blue spectral range where the radiative rate is enhanced due to a relatively large Einstein’s coefficient of spontaneous emission, increasing with the cube of the emission energy . There is only one example of a mononuclear Ir­(III) complex emitting in the red range with sub-μs radiative decay time, reported very recently by Beeby, Davidson, and co-authors . The structure of that complex complies with the design principles discussed in this work.…”

Tuning Emission Lifetimes of Ir(C^N)2(acac) Complexes with Oligo(phenyleneethynylene) Groups

“…Unlike the acac chelate, the position of substitution on the ppy ligand is a factor that must be considered, as any modifications to the pyridyl group directly impact the LUMO, while modifications to the phenylene group alter the HOMO. The complexes were prepared using a known approach, 27,32 To provide a series of complexes, where OPE3 is attached to the ppy ligand but also electronically decoupled, a series of durylene-linked complexes dOPE3 (Figure 4) were synthesized. Using the same synthetic approach as that used for the pOPE3 complexes, duryl-ethynyl-TIPS ligands were first formed.…”

“…To explore whether one OPE3 unit instead of two OPE3 units in a dOPE3 complex significantly influences the lifetimes of these dOPE complexes, a tris-heteroleptic complex precursor was synthesized. This mixed ligand system was achieved using the Edkins method, 32,34 reacting a 1:1 mixture of ppyH:L 10 H with 1 equiv of IrCl 3 •3H 2 O under standard conditions to give the dimer mixture. This was treated with acetylacetone and K 2 CO 3 , yielding the three complexes Ir(ppy) 2 (acac), Ir(L 10 ) 2 (acac) (12), and the desired complex Ir(L 10 )(ppy)(acac) (13) with a 21% yield.…”