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).
A range of fluorescent alkynyl-naphthalimide fluorophores has been synthesized and their photophysical properties examined. The fluorescent ligands are based upon a 4-substituted 1,8-naphthalimide ...core and incorporate structural variations (at the 4-position) to tune the amphiphilic character: chloro (L1), 4-2-(2-aminoethoxy)ethanol (L2), 4-2-(2-methoxyethoxy)ethylamino (L3), piperidine (L4), morpholine (L5), 4-methylpiperidine (L6), and 4-piperidone ethylene ketal (L7) variants. The amino-substituted species (L2–L7) are fluorescent in the visible region at around 517–535 nm through a naphthalimide-localized intramolecular charge transfer (ICT), with appreciable Stokes’ shifts of ca. 6500 cm–1 and lifetimes up to 10.4 ns. Corresponding two-coordinate Au(I) complexes Au(L)(PPh3) were isolated, with X-ray structural studies revealing the expected coordination mode via the alkyne donor. The Au(I) complexes retain the visible fluorescence associated with the coordinated alkynyl-naphthalimide ligand. The ligands and complexes were investigated for their cytotoxicity across a range of cell lines (LOVO, MCF-7, A549, PC3, HEK) and their potential as cell imaging agents for HEK (human embryonic kidney) cells and Spironucleus vortens using confocal fluorescence microscopy. The images reveal that these fluorophores are highly compatible with fluorescence microscopy and show some clear intracellular localization patterns that are dependent upon the specific nature of the naphthalimide substituent.
Bis-terdentate (N^N^N) ligands coordinated to Cr(III) yield complexes that display near-IR emission under aerated solvent conditions at room temperature.
The synthesis and characterisation of eleven different 2-(thienyl)quinoxaline species that incorporate different points of functionality, including at the thiophene or quinoxaline rings, are ...described. These species display variable fluorescence properties in the visible region (
λ
em
= 401-491 nm) depending upon the molecular structures and extent of conjugation. The series of 2-(thienyl)quinoxaline species were then investigated as cyclometalating agents for Ir(
iii
) to yield Ir(C^N)
2
(bipy)PF
6
(where C^N = the cyclometalated ligand; bipy = 2,2′-bipyridine). Eight complexes were successfully isolated and fully characterised by an array of spectroscopic and analytical techniques. Two Ir(
iii
) examples were structurally characterised in the solid state using single crystal X-ray diffraction; both structures confirmed the proposed formulations and coordination spheres in each case showing that the thiophene coordinates
via
a Ir-C bond. The photophysical properties of the complexes revealed that each complex is luminescent under ambient conditions with a range of emission wavelengths observed (665-751 nm) indicating that electronic tuning can be achieved
via
both the thienyl and quinoxaline moieties.
The synthesis and characterisation of eleven different 2-(thienyl)quinoxaline species that incorporate different points of functionality are described. Cyclometalation with Ir(
iii
) was explored yielding a range of luminescent complexes.
Six disubstituted ligands based upon 2‐(2′‐pyridinyl/pyrazinyl)quinoline‐4‐carboxylic acids have been synthesised, solvent‐free, in one step from a range of commercially available isatin derivatives. ...These species behave as ancillary chelating ligands for Ir(III) complexes of the form Ir(C^N)2(N^N)PF6 (where C^N=cyclometalating ligand; N^N=2‐(2′‐pyridinyl/pyrazinyl)quinoline‐4‐carboxylic acids). An X‐ray crystallographic study on one complex shows a distorted octahedral geometry wherein a cis‐C,C and trans‐N,N coordination mode is observed for the cyclometalating ligands. DFT calculations predicted that variations in N^N ligand from 2,2′‐bipyridine to L1–6 should localise the LUMO on to the Ln ligand and that the complexes are predicted to display MLCT/LLCT character. All complexes displayed luminescence in the deep red part of the visible region (674–679 nm) and emit from triplet states, but with little apparent tuning as a function of L1–6. Further time‐resolved transient absorption spectroscopy supports the participation of these triplet states to the excited state character.
The synthesis of unsymmetrical, disubstituted quinoline‐based N^N ligands was achieved using a range of isatin reagents. Subsequent complexation within heteroleptic Ir(III) leads to the formation of luminescent complexes which emit ca. 675 nm.
A series of substituted 2‐phenylquinoxaline ligands have been explored to finely tune the visible emission properties of a corresponding set of cationic, cyclometallated iridium(III) complexes. The ...electronic and redox properties of the complexes were investigated through experimental (including time‐resolved luminescence and transient absorption spectroscopy) and theoretical methods. The complexes display absorption and phosphorescent emissions in the visible region that are attributed to metal to ligand charge‐transfer transitions. The different substitution patterns of the ligands induce variations in these parameters. Time‐dependent DFT studies support these assignments and show that there is likely to be a strong spin‐forbidden contribution to the visible absorption bands at λ=500–600 nm. Calculations also reliably predict the magnitude and trends in triplet emitting wavelengths for the series of complexes. The complexes were assessed as potential sensitisers in triplet–triplet annihilation upconversion experiments by using 9,10‐diphenylanthracene as the acceptor; the methylated variants performed especially well with impressive upconversion quantum yields of up to 39.3 %.
Sensitive to change: A series of substituted 2‐phenylquinoxaline ligands have been explored to finely tune the visible emission properties of a corresponding set of cationic, cyclometallated iridium(III) complexes. The complexes were assessed as sensitizers in triplet–triplet annihilation upconversion experiments (see figure), and demonstrated highly impressive upconversion quantum yields of up to 39 %.
A series of ligands based upon a 1,3-diimino-isoindoline framework have been synthesized and investigated as pincer-type (N∧N∧N) chelates for Pt(II). The synthetic route allows different ...combinations of heterocyclic moieties (including pyridyl, thiazole, and isoquinoline) to yield new unsymmetrical ligands. Pt(L 1–6 )Cl complexes were obtained and characterized using a range of spectroscopic and analytical techniques: 1H and 13C NMR, IR, UV–vis and luminescence spectroscopies, elemental analyses, high-resolution mass spectrometry, electrochemistry, and one example via X-ray crystallography which showed a distorted square planar environment at Pt(II). Cyclic voltammetry on the complexes showed one irreversible oxidation between +0.75 and +1 V (attributed to Pt2+/3+ couple) and a number of ligand-based reductions; in four complexes, two fully reversible reductions were noted between −1.4 and −1.9 V. Photophysical studies showed that Pt(L 1–6 )Cl absorbs efficiently in the visible region through a combination of ligand-based bands and metal-to-ligand charge-transfer features at 400–550 nm, with assignments supported by DFT calculations. Excitation at 500 nm led to luminescence (studied in both solutions and solid state) in all cases with different combinations of the heterocyclic donors providing tuning of the emission wavelength around 550–678 nm.
The syntheses of nine new monometallic heteroleptic platinum complexes Pt(L1–4)(acac), Pt(L1)(hmacac/hfacac), PtCl(L1)(py), Pt(L1)(8-Q), Pt(L1)(bpy)(PF6) (where L1 = ...2-phenyl-4-ethyl-quinolinecarboxylate; L2/L3 = N-functionalization of 2-phenyl-N-aryl/alkyl-quinoline-4-carboxamides; L4 = 2-phenyl-4-quinolinecarboxylic acid (cinchophen); acac = acetylacetonato; hmacac =2,2,6,6-tetramethyl-3,5-heptanedionate; hfacac = hexafluoroacetylacetonate; py = pyridine; 8-Q = 8-quinolinato; bpy =2,2′-bipyridine) are described from precursor dimeric Pt(II) species via an intermediate DMSO adduct of the general form PtCl(L1–4)(DMSO). Single crystal X-ray diffraction studies were undertaken on three complexes, Pt(L1)(acac), PtCl(L1)(DMSO), and Pt(L1)(bpy)(PF6). The structures show that the complexes each adopt a distorted square planar geometry (most severely in the case of Pt(L1)(bpy)(PF6)) with indications of intermolecular Pt–Pt interactions in one example. The complexes were investigated using 195Pt{1H} NMR spectroscopy, revealing varied chemical shifts that were strongly dependent upon the specific coordination environment of Pt(II). Luminescence studies showed the complexes possess a phosphorescent character with tunable emission wavelengths between 605 and 641 nm and luminescent lifetimes up to ∼450 ns. Supporting TD-DFT studies provided descriptions of the HOMO and LUMO energy levels of the key complex types, confirming an MLCT contribution to the lowest energy absorption that generally correlated well with the experimental spectra. The contribution of the Pt(5d) center to the calculated HOMOs was strongly ligand dependent, whereas the LUMOs are generally localized over the quinoline component of the cyclometalated ligand.
Six iridium(
iii
) complexes of the general form Ir(C^N)
2
(N^N)X (where C^N = cyclometalating ligand; N^N = disubstituted 2,2′-bipyridine), and incorporating alkyl chains of differing lengths (C8, ...C10, C12), have been synthesised and characterised. The complexes have been characterised using a variety of methods including spectroscopies (NMR, IR, UV-Vis, luminescence) and analytical techniques (high resolution mass spectrometry, cyclic voltammetry, X-ray diffraction). Two dodecyl-functionalised complexes were studied for their behaviour in aqueous solutions. Although the complexes did not possess sufficient solubility to determine their critical micelle concentrations (CMC) in water, they were amenable for use as emissive dopants in a
N
-methyl C12 substituted imidazolium salt microemulsion carrier system with a CMC = 36.5 mM. The investigation showed that the metal doped microemulsions had increased CMCs of 40.4 and 51.3 mM and luminescent properties characterised by the dopant.
Luminescent, alkyl chain functionalised Ir(
iii
) complexes have been synthesised, characterised and studied in microemulsion environments showing retention of emissive behaviour.
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