Spectroscopic and Theoretical Investigation of Color Tuning in Deep-Red Luminescent Iridium(III) Complexes

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 1 H, 13 C, and 19 F 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 (λ em = 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 spinforbidden 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).

Deep-Red Luminescence from Platinum(II) Complexes of N^N–^N-Amido Ligands with Benzannulated N-Heterocyclic Donor Arms

“…The IUPAC numbering system for quinolines and phenanthridines is illustrated for L1 R,R and L3 R,R . The proligands were characterized by 1 H, 13 C and 19 F (for the CF 3 derivatives) NMR spectroscopy in solution (see Table S1 for selected 1 H NMR resonances). The crystal structure of a representative proligand (L3 CF3,CF3 ) was obtained to verify the solution-state structure assigned by NMR (Figure 1).…”

Unveiling Luminescent IrI and RhI N‐Heterocyclic Carbene Complexes: Structure, Photophysical Specifics, and Cellular Localization in the Endoplasmic Reticulum

“…In analogy to similar metal–ligand systems, [46] the 340 nm absorption band presumably corresponds to the ligand‐centered π–π* transition (compare Figure S7 in the Supporting Information and closely related species [53] ), as the metalation of the NHC ligand does not affect the spectral position of the absorption maximum significantly (Table 2). Furthermore, the low‐frequency absorption above 400 nm is indicative of a metal‐to‐ligand charge transfer (MLCT) transition, as known, for example, for Ir III complexes [54–61] . Excitation of the free ligand 4 at 340 nm displays an emission band with its maximum at 440 nm.…”

Metal Peptide Conjugates in Cell and Tissue Imaging and Biosensing

“…The photophysical properties of such complexes can be tuned via a number of strategies via π-extension of the coordinated ligands, or their modification with electron donating/withdrawing substituents to cyclometalated ligands or by introducing an ancillary ligand, e.g. N, N coordinating ligands that have σ-donating or п-accepting properties [ 24 – 27 ]. A recent review on NIR-emitting Ir(III) complexes discusses in detail the different methods that can be utilised to tune the photophysics of Ir(III) complexes [ 28 ].…”