Abstract
A series of heteroleptic, neutral iridium(III) complexes of the form [Ir(L)2(N^O)] (where L = cyclometalated 2,3-disubstituted quinoxaline and N^O = ancillary picolinate or pyrazinoate) are described in terms of their synthesis and spectroscopic properties, with supporting computational analyses providing additional insight into the electronic properties. The 10 [Ir(L)2(N^O)] complexes were characterized using a range of analytical techniques (including 1H, 13C, and 19F NMR and IR spectroscopies and mass spectrometry). One of the examples was structurally characterized using X-ray diffraction. The redox properties were determined using cyclic voltammetry, and the electronic properties were investigated using UV–vis, time-resolved luminescence, and transient absorption spectroscopies. The complexes are phosphorescent in the red region of the visible spectrum (λem = 633–680 nm), with lifetimes typically of hundreds of nanoseconds and quantum yields ca. 5% in aerated chloroform. A combination of spectroscopic and computational analyses suggests that the long-wavelength absorption and emission properties of these complexes are strongly characterized by a combination of spin-forbidden metal-to-ligand charge-transfer and quinoxaline-centered transitions. The emission wavelength in these complexes can thus be controlled in two ways: first, substitution of the cyclometalating quinoxaline ligand can perturb both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital levels (LUMO, Cl atoms on the ligand induce the largest bathochromic shift), and second, the choice of the ancillary ligand can influence the HOMO energy (pyrazinoate stabilizes the HOMO, inducing hypsochromic shifts).
Highlights
The development and study of luminescent cyclometalated iridium(III) complexes continue to attract significant attention.[1]
This study has shown that heteroleptic neutral iridium(III) complexes, [Ir(L)2(N^O)], incorporating cyclometalated quinoxaline ligands can demonstrate tunable emission in the long-wavelength part of the visible spectrum
The emission from these complexes likely arises from a mixture of quinoxaline-centered and 3MLCT excited states
Summary
The development and study of luminescent cyclometalated iridium(III) complexes continue to attract significant attention.[1]. The structured absorption profiles in the region 400 nm < λ < 500 nm (notably in Figure 9) are ascribed to the pseudodegeneracy of the LUMO noted in the Density Functional Theory (DFT) section, resulting in numerous absorption features very close to each other in energy This can be observed faintly in the ground-state absorption spectra of some of the complexes but is far more prominent in the TA spectra, highlighting the sensitivity of this technique as a means of characterization. This structure to the absorption feature was not visible in the cationic variants [Ir(L)2(bipy)]PF6.7b As with the emission spectra, the TA spectra display a band shift (Figure 10) relative to [Ir(L1)2(pic)] depending on the ligand type. It is noteworthy that very few examples of the TA spectra of organometallic complexes have been reported in the literature.[27]
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