Complex structures arising from the self-assembly of a simple organic salt.

Molecular self-assembly is the spontaneous association of simple molecules into larger and ordered structures1. It is the basis of several natural processes, such as the formation of colloids, crystals, proteins, viruses and double-helical DNA2. Molecular self-assembly has inspired strategies for the rational design of materials with specific chemical and physical properties3, and is one of the most important concepts in supramolecular chemistry. Although molecular self-assembly has been extensively investigated, understanding the rules governing this phenomenon remains challenging. Here we report on a simple hydrochloride salt of fampridine that crystallizes as four different structures, two of which adopt unusual self-assemblies consisting of polyhedral clusters of chloride and pyridinium ions. These two structures represent Frank-Kasper (FK) phases of a small and rigid organic molecule. Although discovered in metal alloys4,5 more than 60 years ago, FK phases have recently been observed in several classes of supramolecular soft matter6-11 and in gold nanocrystal superlattices12 and remain the object of recent discoveries13. In these systems, atoms or spherical assemblies of molecules are packed to form polyhedra with coordination numbers 12, 14, 15 or 16. The two FK structures reported here crystallize from a dense liquid phase and show a complexity that is generally not observed in small rigid organic molecules. Investigation of the precursor dense liquid phase by cryogenic electron microscopy reveals the presence of spherical aggregates with sizes ranging between 1.5 and 4.6 nanometres. These structures, together with the experimental procedure used for their preparation, invite interesting speculation about their formation and open different perspectives for the design of organic crystalline materials.

Enzyme-like Acyl Transfer Catalysis in a Bifunctional Organic Cage.

Amide-based organic cage cavities are, in principle, ideal enzyme active site mimics. Yet, cage-promoted organocatalysis has remained elusive, in large part due to synthetic accessibility of robust and functional scaffolds. Herein, we report the acyl transfer catalysis properties of robust, hexaamide cages in organic solvent. Cage structural variation reveals that esterification catalysis with an acyl anhydride acyl carrier occurs only in bifunctional cages featuring internal pyridine motifs and two crucial antipodal carboxylic acid groups. 1H NMR data and X-ray crystallography show that the acyl carrier is rapidly activated inside the cavity as a covalent mixed-anhydride intermediate with an internal hydrogen bond. Michaelis-Menten (saturation) kinetics suggest weak binding (KM = 0.16 M) of the alcohol pronucleophile close to the internal anhydride. Finally, activation and delivery of the alcohol to the internal anhydride by the second carboxylic acid group forms ester product and releases the cage catalyst. Eyring analysis indicates a strong enthalpic stabilization of the transition state (5.5 kcal/mol) corresponding to a rate acceleration of 104 over background acylation, and an ordered, associative rate-determining attack by the alcohol, supported by DFT calculations. We conclude that internal bifunctional organocatalysis specific to the cage structural design is responsible for the enhancement over the background reaction. These results pave the way for organic-phase enzyme mimicry in self-assembled cavities with the potential for cavity elaboration to enact selective acylations.

Luminescent Pt(II) Complexes Using Unsymmetrical Bis(2-pyridylimino)isoindolate Analogues.

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(L1-6)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(L1-6)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.

Tuning Excited State Character in Iridium(III) Photosensitizers and Its Influence on TTA-UC.

A series of mixed ligand, photoluminescent organometallic Ir(III) complexes have been synthesized to incorporate substituted 2-phenyl-1H-naphtho[2,3-d]imidazole cyclometalating ligands. The structures of three example complexes were categorically confirmed using X-ray crystallography each sharing very similar structural traits including evidence of interligand hydrogen bond contacts that account for the shielding effects observed in the 1H NMR spectra. The structural iterations of the cyclometalated ligand provide tuning of the principal electronic transitions that determine the visible absorption and emission properties of the complexes: emission can be tuned in the visible region between 550 and 610 nm and with triplet lifetimes up to 10 µs. The nature of the emitting state varies across the series of complexes, with different admixtures of ligand-centered and metal-to-ligand charge transfer triplet levels evident. Finally, the use of the complexes as photosensitizers in triplet-triplet annihilation energy upconversion (TTA-UC) was investigated in the solution state. The study showed that the complexes possessing the longest triplet lifetimes showed good viability as photosensitizers in TTA-UC. Therefore, the use of an electron-withdrawing group on the 2-phenyl-1H-naphtho[2,3-d]imidazole ligand framework can be used to rationally promote TTA-UC using this class of complex.

Solid State and Solution Structures of Lanthanide Nitrate Complexes of Tris-(1-napthylphosphine oxide).

Coordination complexes of lanthanide metals with tris-1-naphthylphosphine oxide (Nap3PO, L) have not been previously reported in the literature. We describe here the formation of lanthanide(III) nitrate complexes Ln(NO3)3L4 (Ln = Eu to Lu) and the structures of [Ln(NO3)3L2]·2L (Ln = Eu, Dy, Ho, Er) and L. The core structure of the complexes is an eight-coordinate [Ln(NO3)3L2] with the third and fourth ligands H-bonded via their oxygen atoms to one of the naphthyl rings. The structures are compared with those of the analogous complexes of triphenylphosphine oxide and show that the Ln-O(P) bond in the Nap3PO complexes is slightly longer than expected on the basis of differences in coordination numbers. The reaction solutions, investigated by 31P and 13C NMR spectroscopy in CD3CN, show that coordination of L occurs across the lanthanide series, even though complexes can only be isolated from Eu onwards. Analysis of the 31P NMR paramagnetic shifts shows that there is a break in the solution structures with a difference between the lighter lanthanides (La-Eu) and heavier metals (Tb-Lu) which implies a minor difference in structures. The isolated complexes are very poorly soluble, but in CDCl3, NMR measurements show dissociation into [Ln(NO3)3L2] and 2L occurs.

Stellated cuboctahedron of FeIII.

The solvothermal reaction of FeCl2 ⋅ 4H2O and H4TBC[4] in a basic dmf/EtOH solution affords an [FeIII 18] Keplerate conforming to a stellated cuboctahedron. Magnetic and heat capacity measurements reveal spin frustration effects arising from the high symmetry. A crossover between inverse and direct magnetocaloric effects is observed at ~10 K for applied-field changes lower than 3 T.

ß,ß-Directly Linked Porphyrin Rings: Synthesis, Photophysical Properties, and Fullerene Binding.

Cyclic porphyrin oligomers have been studied as models for photosynthetic light-harvesting antenna complexes and as potential receptors for supramolecular chemistry. Here, we report the synthesis of unprecedented ß,ß-directly linked cyclic zinc porphyrin oligomers, the trimer (CP3) and tetramer (CP4), by Yamamoto coupling of a 2,3-dibromoporphyrin precursor. Their three-dimensional structures were confirmed by nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, and single-crystal X-ray diffraction analyses. The minimum-energy geometries of CP3 and CP4 have propeller and saddle shapes, respectively, as calculated using density functional theory. Their different geometries result in distinct photophysical and electrochemical properties. The smaller dihedral angles between the porphyrin units in CP3, compared with CP4, result in stronger π-conjugation, splitting the ultraviolet-vis absorption bands and shifting them to longer wavelengths. Analysis of the crystallographic bond lengths indicates that the central benzene ring of the CP3 is partially aromatic [harmonic oscillator model of aromaticity (HOMA) 0.52], whereas the central cyclooctatetraene ring of the CP4 is non-aromatic (HOMA -0.02). The saddle-shaped structure of CP4 makes it a ditopic receptor for fullerenes, with affinity constants of (1.1 ± 0.4) × 105 M-1 for C70 and (2.2 ± 0.1) × 104 M-1 for C60, respectively, in toluene solution at 298 K. The formation of a 1:2 complex with C60 is confirmed by NMR titration and single-crystal X-ray diffraction.

Organometallic Platinum(II) Photosensitisers that Demonstrate Ligand-Modulated Triplet-Triplet Annihilation Energy Upconversion Efficiencies.

A series of 2-phenylquinoxaline ligands have been synthesised that introduce either CF3 or OCF3 electron-withdrawing groups at different positions of the phenyl ring. These ligands were investigated as cyclometalating reagents for platinum(II) to give neutral complexes of the form [Pt(C^N)(acac)] (in which C^N=cyclometalating ligand; acac=acetyl acetonate). X-ray crystallographic studies on three examples showed that the complexes adopt an approximate square planar geometry. All examples revealed strong Pt-Pt linear contacts of 3.2041(6), 3.2199(3) and 3.2586(2) Å. The highly coloured complexes display efficient visible absorption at 400-500 nm (ϵ ≈5000 M-1  cm-1 ) and orange red photoluminescent characteristics (λem =603-620 nm; Φem ≤37 %), which were subtly tuned by the ligand. Triplet emitting character was confirmed by microsecond luminescence lifetimes and the photogeneration of singlet oxygen with quantum efficiencies up to 57 %. Each complex was investigated as a photosensitiser for triplet-triplet annihilation energy upconversion using 9,10-diphenylanthracene as the annihilator species: a range of good upconversion efficiencies (ΦUC 5.9-14.1 %) were observed and shown to be strongly influenced by the ligand structure in each case.

Hydroxypyridine/Pyridone Interconversions within Ruthenium Complexes and Their Application in the Catalytic Hydrogenation of CO2.

Reaction of a new ligand 6-DiPPon (6-diisopropylphosphino-2-pyridone) with 0.5 equiv of [RuCl2(p-cymene)]2 resulted in the formation of a mixture of [RuCl2(p-cymene)(κ1-P-6-DiPPon)]2 (1) and [RuCl(p-cymene)(κ2-P,N-6-DiPPin)]Cl ([2]Cl) (where 6-DiPPin = 6-diisopropylphosphino-2-hydroxypyridine). The ratio between the two products can be controlled by the nature of the solvent. The similar reaction between 6-DiPPon and [RuCl2(p-cymene)]2 in the presence of AgOTf and Na[BArF24] (where BArF24 = [{3,5-(CF3)2C6H3}4B]-) resulted in the formation of the complexes [RuCl(p-cymene)(κ2-P,N-6-DiPPin)]OTf, ([2]OTf) and [RuCl(p-cymene)(κ2-P,N-6-DiPPin)]BArF24 ([2]BArF24), respectively. Reactions between complex [2]Cl, [2]OTf, or [2]BArF24 and a base (either DBU or NaOMe) resulted in the deprotonation of the hydroxyl functional group to form a novel neutral orange-colored dearomatized complex, 3. The identity of complex 3 was confirmed as [RuCl(p-cymene)(κ2-P,N-6-DiPPon*)], where 6-DiPPon* is the anionic species (6-diisopropylphosphino-2-oxo-pyridinide), which contains the deprotonated moiety. The new 6-DiPPon ligand and its corresponding air stable half-sandwich derivative ruthenium complexes 1, [2]OTf, [2]BArF24, and 3 were all isolated in good yields and fully characterized by spectroscopic and analytical methods. The interconversions between the neutral and anionic forms of the ligands 6-DiPPon, 6-DiPPin, and 6-DiPPon* offer the potential for novel secondary sphere interactions and proton shuttling reactivity. The consequences for this have been explored in the activation of H2 and the subsequent catalytic hydrogenations of CO2 into formate salts in the presence of a base.

Chiral Co3Y Propeller-Shaped Chemosensory Platforms Based on 19F-NMR.

Two propeller-shaped chiral CoIII3YIII complexes built from fluorinated ligands are synthesized and characterized by single-crystal X-ray diffraction (SXRD), IR, UV-vis, circular dichroism (CD), elemental analysis, thermogravimetric analysis (TGA), electron spray ionization mass spectroscopy (ESI-MS), and NMR (1H, 13C, and 19F). This work explores the sensing and discrimination abilities of these complexes, thus providing an innovative sensing method using a 19F NMR chemosensory system and opening new directions in 3d/4f chemistry. Control experiments and theoretical studies shed light on the sensing mechanism, while the scope and limitations of this method are discussed and presented.

Structural (XRD) Characterization and an Analysis of H-Bonding Motifs in Some Tetrahydroxidohexaoxidopentaborate(1-) Salts of N-Substituted Guanidinium Cations.

The synthesis and characterization of six new substituted guanidium tetrahydroxidohexaoxidopentaborate(1-) salts are reported: [C(NH2)2(NHMe)][B5O6(OH)4]·H2O (1), [C(NH2)2(NH{NH2})][B5O6(OH)4] (2), [C(NH2)2(NMe2)][B5O6(OH)4] (3), [C(NH2)(NMe2)2][B5O6(OH)4] (4), [C(NHMe)(NMe2)2][B5O6(OH)4]·B(OH)3 (5), and [TBDH][B5O6(OH)4] (6) (TBD = 1,5,7-triazabicyclo [4.4.0]dec-5-ene). Compounds 1-6 were prepared as crystalline salts from basic aqueous solution via self-assembly processes from B(OH)3 and the appropriate substituted cation. Compounds 1-6 were characterized by spectroscopic (NMR and IR) and by single-crystal XRD studies. A thermal (TGA) analysis on compounds 1-3 and 6 demonstrated that they thermally decomposed via a multistage process to B2O3 at >650 °C. The low temperature stage (<250 °C) was endothermic and corresponded to a loss of H2O. Reactant stoichiometry, solid-state packing, and H-bonding interactions are all important in assembling these structures. An analysis of H-bonding motifs in known unsubstituted guanidinium salts [C(NH2)3]2[B4O5(OH)4]·2H2O, [C(NH2)3][B5O6(OH)4]·H2O, and [C(NH2)3]3[B9O12(OH)6] and in compounds 1-6 revealed that two important H-bonding R22(8) motifs competed to stabilize the observed structures. The guanidinium cation formed charge-assisted pincer cation-anion H-bonded rings as a major motif in [C(NH2)3]2[B4O5(OH)4]·2H2O and [C(NH2)3]3[B9O12(OH)6], whereas the anion-anion ring motif was dominant in [C(NH2)3][B5O6(OH)4]·H2O and in compounds 1-6. This behaviour was consistent with the stoichiometry of the salt and packing effects also strongly influencing their solid-state structures.

Synthesis and Structural Characterization of Copper Complexes Containing "R-Substituted" Bis-7-Azaindolyl Borate Ligands.

The coordination chemistry of scorpionate ligands based on borates containing the 7-azaindole heterocycle is relatively unexplored. Thus, there is a requirement to further understand their coordination chemistry. This article outlines the synthesis and characterization of a family of complexes containing anionic flexible scorpionate ligands of the type [(R)(bis-7-azaindolyl)borohydride]- ([RBai]-), where R = Me, Ph or naphthyl. The three ligands were coordinated to a series of copper(I) complexes containing a phosphine co-ligand to form the complexes, [Cu(MeBai)(PPh3)] (1), [Cu(PhBai)(PPh3)] (2), [Cu(NaphthBai)(PPh3)] (3), [Cu(MeBai)(PCy3)] (4), [Cu(PhBai)(PCy3)] (5) and [Cu(NaphthBai)(PCy3)] (6). Additional copper(II) complexes, namely, [Cu(MeBai)2] (7) and [Cu(PhBai)2] (8), were obtained during attempts to obtain single crystals from complexes 4 and 2, respectively. Complexes 7 and 8 were also prepared independently from CuCl2 and two equivalents of the corresponding Li[RBai] salt alongside an additional complex, namely, [Cu(NaphthBai)2] (9). The copper(I) and copper(II) complexes were characterized using spectroscopic and analytical methods. Furthermore, a crystal structure was obtained for eight of the nine complexes. In all cases, the boron-based ligand was found to bind to the metal centers via a κ3-N,N,H coordination mode.

Ammonium N-(pyridin-2-ylmethyl)oxamate (AmPicOxam): A Novel Precursor of Calcium Oxalate Coating for Carbonate Stone Substrates.

Ammonium N-(pyridin-2-ylmethyl)oxamate (AmPicOxam), synthesized from O-methyl-N-(pyridin-2-ylmethyl)oxamate, was spectroscopically and structurally characterized and assayed as a novel precursor for the protection and consolidation of carbonate stone substrates. An in-depth characterization of treated and untreated biomicritic limestone and white Carrara marble samples was carried out by means of SEM microscopy, X-ray powder diffraction, helium pycnometry, determination of water transport properties, and pull-off tests. The improved solubility (1.00 M, 16.5% w/w) of the title compound with respect to ammonium oxalate (0.4 M, 5% w/w) results in the formation of a thicker protective coating of calcium oxalate (CaOx) dihydrate (weddellite) on marble and biomicrite samples after the treatment with 5% and 12% w/w water solutions, producing a reduction in the stone porosity and increased cohesion. Theoretical calculations were carried out at the DFT level to investigate both the electronic structure of the N-(pyridin-2-ylmethyl)oxamate anion and the hydrolysis reaction leading from AmPicOxam to CaOx.

N=8 Armchair Graphene Nanoribbons: Solution Synthesis and High Charge Carrier Mobility.

Structurally defined graphene nanoribbons (GNRs) have emerged as promising candidates for nanoelectronic devices. Low band gap (<1 eV) GNRs are particularly important when considering the Schottky barrier in device performance. Here, we demonstrate the first solution synthesis of 8-AGNRs through a carefully designed arylated polynaphthalene precursor. The efficiency of the oxidative cyclodehydrogenation of the tailor-made polymer precursor into 8-AGNRs was validated by FT-IR, Raman, and UV/Vis-near-infrared (NIR) absorption spectroscopy, and further supported by the synthesis of naphtho[1,2,3,4-ghi]perylene derivatives (1 and 2) as subunits of 8-AGNR, with a width of 0.86 nm as suggested by the X-ray single crystal analysis. Low-temperature scanning tunneling microscopy (STM) and solid-state NMR analyses provided further structural support for 8-AGNR. The resulting 8-AGNR exhibited a remarkable NIR absorption extending up to ∼2400 nm, corresponding to an optical band gap as low as ∼0.52 eV. Moreover, optical-pump TeraHertz-probe spectroscopy revealed charge-carrier mobility in the dc limit of ∼270 cm2  V-1 s-1 for the 8-AGNR.

Solvent-Free Synthesis of Core-Functionalised Naphthalene Diimides by Using a Vibratory Ball Mill: Suzuki, Sonogashira and Buchwald-Hartwig Reactions.

Solvent-free synthesis by using a vibratory ball mill (VBM) offers the chance to access new chemical reactivity, whilst reducing solvent waste and minimising reaction times. Herein, we report the core functionalisation of N,N'-bis(2-ethylhexyl)-2,6-dibromo-1,4,5,8-naphthalenetetracarboxylic acid (Br2 -NDI) by using Suzuki, Sonogashira and Buchwald-Hartwig coupling reactions. The products of these reactions are important building blocks in many areas of organic electronics including organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic photovoltaic cells (OPVCs). The reactions proceed in as little as 1 h, use commercially available palladium sources (frequently Pd(OAc)2 ) and are tolerant to air and atmospheric moisture. Furthermore, the real-world potential of this green VBM protocol is demonstrated by the double Suzuki coupling of a monobromo(NDI) residue to a bis(thiophene) pinacol ester. The resulting dimeric NDI species has been demonstrated to behave as an electron acceptor in functioning OPVCs.

Scale-up and optimization of the synthesis of dual CBP/BRD4 inhibitor ISOX-DUAL.

ISOX-DUAL is a dual inhibitor of CBP/p300 (IC50 = 0.65 µM) and BRD4 (IC50 = 1.5 µM) bromodomains, and a useful chemical probe for epigenetic research. Aspects of the published synthetic route to this compound and its analogues are small-scale, poor-yielding or simply unamenable to scale-up without optimization. Herein we describe the development of a refined synthesis that circumvents the challenges of the original report, with notable improvements to several of the key synthetic transformations. Moreover, a general Suzuki Miyaura protocol for the late stage installation of alternative dimethyl-isoxazole acetyl-lysine (KAc) binding motifs is presented.

A Series of Crystallographically Characterized Linear and Branched σ-Alkane Complexes of Rhodium: From Propane to 3-Methylpentane.

Using solid-state molecular organometallic (SMOM) techniques, in particular solid/gas single-crystal to single-crystal reactivity, a series of σ-alkane complexes of the general formula [Rh(Cy2PCH2CH2PCy2)(ηn:ηm-alkane)][BArF4] have been prepared (alkane = propane, 2-methylbutane, hexane, 3-methylpentane; ArF = 3,5-(CF3)2C6H3). These new complexes have been characterized using single crystal X-ray diffraction, solid-state NMR spectroscopy and DFT computational techniques and present a variety of Rh(I)···H-C binding motifs at the metal coordination site: 1,2-η2:η2 (2-methylbutane), 1,3-η2:η2 (propane), 2,4-η2:η2 (hexane), and 1,4-η1:η2 (3-methylpentane). For the linear alkanes propane and hexane, some additional Rh(I)···H-C interactions with the geminal C-H bonds are also evident. The stability of these complexes with respect to alkane loss in the solid state varies with the identity of the alkane: from propane that decomposes rapidly at 295 K to 2-methylbutane that is stable and instead undergoes an acceptorless dehydrogenation to form a bound alkene complex. In each case the alkane sits in a binding pocket defined by the {Rh(Cy2PCH2CH2PCy2)}+ fragment and the surrounding array of [BArF4]- anions. For the propane complex, a small alkane binding energy, driven in part by a lack of stabilizing short contacts with the surrounding anions, correlates with the fleeting stability of this species. 2-Methylbutane forms more short contacts within the binding pocket, and as a result the complex is considerably more stable. However, the complex of the larger 3-methylpentane ligand shows lower stability. Empirically, there therefore appears to be an optimal fit between the size and shape of the alkane and overall stability. Such observations are related to guest/host interactions in solution supramolecular chemistry and the holistic role of 1°, 2°, and 3° environments in metalloenzymes.

Iridium(III) Sensitisers and Energy Upconversion: The Influence of Ligand Structure upon TTA-UC Performance.

Six substituted ligands based upon 2-(naphthalen-1-yl)quinoline-4-carboxylate and 2-(naphthalen-2-yl)quinoline-4-carboxylate have been synthesised in two steps from a range of commercially available isatin derivatives. These species are shown to be effective cyclometallating ligands for IrIII , yielding complexes of the form [Ir(C^N)2 (bipy)]PF6 (where C^N=cyclometallating ligand; bipy=2,2'-bipyridine). X-ray crystallographic studies on three examples demonstrate that the complexes adopt a distorted octahedral geometry wherein a cis-C,C and trans-N,N coordination mode is observed. Intraligand torsional distortions are evident in all cases. The IrIII complexes display photoluminescence in the red part of the visible region (668-693 nm), which is modestly tuneable through the ligand structure. The triplet lifetimes of the complexes are clearly influenced by the precise structure of the ligand in each case. Supporting computational (DFT) studies suggest that the differences in observed triplet lifetime are likely due to differing admixtures of ligand-centred versus MLCT character instilled by the facets of the ligand structure. Triplet-triplet annihilation upconversion (TTA-UC) measurements demonstrate that the complexes based upon the 1-naphthyl derived ligands are viable photosensitisers with upconversion quantum efficiencies of 1.6-6.7 %.

A Stereoselective aza-Prins Reaction: Rapid Access to Enantiopure Piperidines and Pipecolic Acids.

The aza-Prins reaction is a widely employed and highly efficient method for the preparation of saturated nitrogen-containing heterocycles. Its major drawback has always been a lack of diastereoselectivity and the formation of racemic products. Herein, we address these problems and report, for the first time, the synthesis of both diastereomerically and enantiopure multiply substituted piperidines via the aza-Prins reaction. This method is widely applicable for natural product synthesis and is exemplified here by the synthesis of enantiopure pipecolic acid derivatives.

Polysubstituted Ligand Framework for Color Tuning Phosphorescent Iridium(III) Complexes.

A series of ligands have been synthesized based upon a polysubstituted 2-phenylquinoxaline core structure. These ligands introduce different combinations of fluorine and methyl substituents on both the phenyl and quinoxaline constituent rings. The resultant investigation of these species as cyclometalating agents for Ir(III) gave cationic complexes of the form [Ir(C^N)2(bipy)]PF6 (where C^N = cyclometalating ligand; bipy = 2,2'-bipyridine). X-ray crystallographic studies were conducted on four complexes and each revealed the expected distorted octahedral geometry based upon a cis-C,C and trans-N,N ligand arrangement at Ir(III). Supporting computational studies predict that each of the complexes share the same general descriptions for the frontier orbitals. TD-DFT calculations suggest MLCT contributions to the lowest energy absorption and a likely MLCT/ILCT/LLCT nature to the emitting state. Experimentally, the complexes display tunable luminescence across the yellow-orange-red part of the visible spectrum (λem = 579-655 nm).