Phenylpyridine Ligands Research Articles

Cyclometalated Pt(II)C^N phosphors bearing a bis(diphenylphorothioyl) amide ligand: synthesis and photophysical properties

Six Pt(II) complexes bearing a primary phenylpyridine (ppy) ligand or a ppy derivative and two Pt(II) complexes bearing a primary 8-hydroxyquinoline (hqnl) or 7-bromo-8-quinolinol ligand have been prepared via coordination with an auxiliary bis(diphenylphosphothionyl) amide (HStpip) ligand. All eight complexes were characterized by NMR (1H,13C, and 31P), HRMS, and elemental analysis. In addition, complexes 2, 4, 5, and 8 were characterized by single crystal XRD. The eight complexes were not emissive in solution under N2 at room temperature (r.t.). However, they all exhibited an intense emission in the solid state and in 2 wt.% polymethyl methacrylate (PMMA) films. In particular, 5 bearing a primary 2-([1,1'-biphenyl]-3-yl)-4-phenylpyridine ligand exhibited a triplet emission at 525 and 552 nm with a quantum yield of 80% in pure solid state. Density functional theory (DFT) and time dependent (TD) DFT calculations were performed for optimization and predicting the excitation properties of 2, 4, 5, and 8. The computational results reveal that the intrinsic triplet emissions of 2, 4, and 5 are the result of combinations of ligand-to-metal charge transfer (LMCT) (π*(ppy) → d(Pt)) and ligand-to-ligand charge transfer (LLCT) (π*(ppy) → p(Stpip)), whereas the triplet emission of 8 shows a π*–π character with a small LMCT (π*(hqnl) → d(Pt)) component.

Optoelectronic properties and charge transfer dynamics in graphene quantum dot/Ir(III) nanocomposites: Enhancing photocatalytic performance through strategic BTF conjugate binding and end-capping variations

The aim of this study is to analyze the optoelectronic properties and charge transfer kinetics in nanocomposites comprising graphene quantum dots (GQDs) and Ir(III) complexes, with a specific focus on their relevance in photocatalytic applications. The investigation explores how the extent of π-conjugation, variations in end-capping substituents, and the binding position of benzothiazole fluorene (BTF) conjugates influence the electronic structures. The results indicate that attaching GQDs alters the band gaps and results in different energy alignments. By adjusting the position of BTF conjugate binding and the end-capping substituent, optimal photocatalytic performance can be achieved. Binding the BTF conjugate at the para-position enhances electron injection into TiO2, thereby improving photocatalysis. Conversely, binding the BTF at the meta-position contributes to the regeneration of the ground state, leading to increased light absorption and photoelectric conversion. Furthermore, incorporating a BTF conjugate at the meta-position of the 2-phenyl pyridine ligand enhances light absorption and photoelectric conversion in the nanocomposites. These findings pave the way for further exploration of the effects of different conjugate binding positions and substituents on the overall properties and potential applications of these nanocomposites.

Cyclometalated Iridium(III) Complexes Containing Viologen-Modified Phenylpyridine Ligands as Electroluminochromic Active Molecules for Information Display.

Electroluminochromic (ELC) materials have garnered significant research interest because of their potential applications in lighting, displaying, and sensing. These materials exhibit reversible modulation of photoluminescence under low-voltage stimuli. Here five phosphorescent iridium(III) complexes are reported featuring viologen-substituted 2-phenylpyridine (Vppy) ligands acting as electroactive components. Four of the complexes are bis-cyclometalated and coordinated with either neutral bipyridine derivatives or negatively charged 2-picolinate. The remaining complex is heteroleptic tris-cyclometalated, containing one Vppy and two 2-phenylquinoline ligands. Upon photoexcitation, the bis-cyclometalated complexes exhibit orange to red phosphorescence originating from mixed triplet metal-to-ligand charge transfer (3MLCT) and intraligand (3IL) dπ(Ir)/π(Vppy) → π*(Vppy) state, whereas the tris-cyclometalated complex is non-emissive due to a low Ir(IV/III) oxidation potential favoring oxidative quenching by the viologen pendants. When the cationic viologens are electrochemically reduced to their neutral form, the bis-cyclometalated complexes show a remarkable blue-shift in their phosphorescence maxima due to increased energy levels of the Vppy molecular orbitals. In the case of the tris-cyclometalated complex, reduction of the viologen groups interrupts the quenching process, leading to a luminescence turn-on. These complexes are used to develop ELC devices, which exhibit reversible luminescence response in terms of color or on-off switching under a low voltage of 2V.

An Ir (III) complex with multiphoton absorption in the near-infrared region as a probe for mtDNA-specific recognition and mitochondrial imaging

The multiphoton absorbing (MPA) materials have been extensively investigated due to their crucial merits of long wavelength excitation and short wavelength emission, of which the organometallic complexes are the most promising but still challenging candidates for developing efficient MPA materials. Herein, a novel cyclometallated iridium complex (abbreviated as SQ) is reported as a multiphoton absorption phosphorescent probe, which is constructed by coordinating the central metal Ir with diethoxyphenyl terpyridine, phenyl pyridine and SCN ligands. The single crystal of SQ exhibits a twisted octahedral structure that can avoid phosphorescence quenching caused by π-π stacking, and the push-pull electron effect of the ligand endows SQ with excellent nonlinear optical (NLO) properties under near-infrared excitation. As a result, two-photon imaging with deep tissue penetration depths is realized at the excitation wavelength of 750 nm. Meanwhile, the planar conjugated structure of diethoxyphenyl terpyridine facilitates the insertion of SQ into base pairs of mitochondrial DNA (mtDNA), thus limiting the intramolecular rotation. Consequently, the emission intensity of the SQ solution enhanced approximately 10-fold after mtDNA was introduced, which led to mtDNA-specific recognition. Our work proves a new way of designing a phosphorescence probe that can specifically identify mtDNA, thus realizing dynamic monitoring of mitochondria.

Iridium-based Polymeric Multifunctional Imaging Tools for Biochemistry.

Herein, we report the development of a macromolecular multifunctional imaging tool for biological investigations, which is comprised of an N-(2-hydroxypropyl)methacrylamide backbone, iridium-based luminescent probe, glutamate carboxypeptidase II (GCPII) targeting ligand, and biotin affinity tag. The iridium luminophore is a tris-cyclometalated complex based on [Ir(ppy)3] with one of its 2-phenylpyridine ligands functionalized to allow conjugation. Synthesized macromolecular probes differed in the structure of the polymer and content of the iridium complex. The applicability of the developed imaging tools has been tested in flow cytometry (FACS) based assay, laser confocal microscopy, and fluorescence lifetime imaging microscopy (FLIM). The FACS analysis has shown that the targeted iBodies containing the iridium luminophore exhibit selective labelling of GCPII expressing cells. This observation was also confirmed in the imaging experiments with laser confocal microscopy. The FLIM experiment has shown that the iBodies with the iridium label exhibit a lifetime greater than 100 ns, which distinguishes them from typically used systems labelled with organic fluorophores exhibiting short fluorescence lifetimes. The results of this investigation indicate that the system exhibits interesting properties, which supports the development of additional biological tools utilizing the key components (iridium complexes, iBody concept), primarily focusing on the longer lifetime of the iridium emitter.

Detection and disaggregation of amyloid fibrils by luminescent amphiphilic platinum(II) complexes.

Cyclometallated Pt(II) complexes possessing hydrophobic 2-phenylpyridine (ppy) ligands and hydrophilic acetonylacetone (acac) ligands have been investigated for their ability to detect amyloid fibrils via luminescence response. Using hen egg-white lysozyme (HEWL) as a model amyloid protein, Pt(II) complexes featuring benzanilide-substituted ppy ligands and ethylene glycol-functionalized acac ligands demonstrated enhanced luminescence in the presence of HEWL fibrils, whereas Pt(II) complexes lacking complementary hydrophobic/hydrophilic ligand sets displayed little to no emission enhancement. An amphiphilic Pt(II) complex incorporating a bis(ethylene glycol)-derivatized acac ligand was additionally found to trigger restructuring of HEWL fibrils into smaller spherical aggregates. Amphiphilic Pt(II) complexes were generally non-toxic to SH-SY5Y neuroblastoma cells, and several complexes also exhibited enhanced luminescence in the presence of Aβ42 fibrils associated with Alzheimer's disease. This study demonstrates that easily prepared and robust (ppy)PtII(acac) complexes show promising reactivity toward amyloid fibrils and represent attractive molecular scaffolds for design of small-molecule probes targeting amyloid assemblies.

Investigation of photoluminescence properties of a DPA functionalized tris-cyclometalated iridium complex and its potential for metal sensing

Herein we report the synthesis and investigation of properties of a luminescent tris-cyclometalated complex [Ir(ppy)2ppy-dpa] (1), derived from fac-[Ir(ppy)3]. The complex carries a dipicolylamine (dpa) chelator on one of the 2-phenylpyridine (ppy) ligands. The dpa chelator exhibits minimal effect on the photophysical properties of the core iridium luminophore. The complex exhibits bright photoluminescence, which can be elicited with single photon and two-photon excitation. The two-photon excitation is achieved with light in the NIR region. Aggregation in aqueous solution has an effect on the photoluminescence properties of complex 1. The emission of complex 1 is quenched nearly completely in the presence of Cu2+ ions with a detection limit of 3.5 × 10−7 M. The luminescence of iridium complex 1 is unaffected by the presence of Co3+ and Zn2+. However, the emission of complex 1 is moderately quenched in the presence of Mn2+ and Fe2+ ions, and greatly quenched in the presence of Fe3+, Co2+, and Ni2+. However, Cu2+ causes its effect even when these competing cations are present. Complex 1 exhibits low cytotoxicity towards HeLa cells and can be imaged with single photon excitation at 405 nm or two-photon excitation at 800 nm.

Iridium-Catalyzed Hydrogenation of a Phenoxy Radical to the Phenol: Overcoming Catalyst Deactivation with Visible Light Irradiation.

Piano-stool iridium hydride complexes bearing phenylpyridine ligands are effective precatalysts for promoting the formation of element-hydrogen bonds using H2 as the stoichiometric H-atom source. Irradiation with blue light resulted in a profound enhancement of catalyst turnover for the iridium-catalyzed hydrogenation of the aryloxyl radical 2,4,6-tBu3-C6H2O• to the corresponding phenol. Monitoring the progress of the reaction revealed the formation of an iridium 3,3-dimethyl-2,3-dihydrobenzofuranyl compound arising from two C-H activation events following the proton-coupled electron transfer (PCET) step. Under thermal conditions, this compound was inactive for catalytic aryloxide hydrogenation, representing a deactivation pathway. Irradiation with blue light under H2 released the free heterocycle and regenerated the piano-stool iridium hydride precatalyst, establishing a pathway for catalyst recovery and overall enhanced turnover.

“Substituents optimization” and “push effect” dual strategy for design of cobalt phthalocyanine catalyst on oxygen reduction reaction

Molecular metallocycle electrocatalysts like metalloporphyrins and metallophthalocyanines were found to be effective for oxygen reduction reaction (ORR) due to their M–N4 active sites and large conjugated electronic molecular structures. Herein, the “substituents optimization” strategy combined with “push effect” modification was innovatively employed to target a single Co–N4 active site in three substituted phthalocyaninato cobalt complexes: tetranitrophthalocyaninato cobalt (CoTNPc), tetra(4-nitrophenoxy) phthalocyaninato cobalt (CoTPNPc), and tetraphenoxy phthalocyaninato cobalt (CoTPPc) electrocatalyst, also with 4-phenylpyridine axial coordination on Co–N4 unit. Through substituents screening, the half-wave potential (E1/2) for ORR increases in the order of CoTPNPc (0.75 V) < CoTPPc (0.80 V) < CoTNPc (0.83 V) along with decreased electron-withdrawing ability of their substituents from –OC6H4–NO2, –OC6H5 to –NO2 in the three cobalt phthalocyanine derivatives. CoTNPc with the weakest electron-withdrawing substituent exhibits the best ORR performance among the three compounds. This is attributed to its higher electron delocalization and lifted HOMO energy level with the lower energy barrier in the rate-determining step relative to the other two compounds, which facilitate the electron transfer and reduction of oxygen as evidenced by XPS, UPS, and DRS analysis combined with DFT calculations. Further coordination of 4-phenylpyridine shifts the E1/2 up to 0.78, 0.82, and 0.85 V for CoTPNPc, CoTPPc, and CoTNPc. DFT calculations demonstrate that the introduction of the electron-donating phenylpyridine ligand into the cobalt phthalocyanines breaks the symmetry of the Co–N4 center and also raises the electron density of Co sites, which promotes O2 adsorption and improves ORR performance. After comparing the two strategies, the substituents on metallophthalocyanine are more determined by the electroactivity than the axial group, which directly regulates the coordination environment and then the activation barrier of the ORR process. This work provides theoretical and experimental guidance by two coupling strategies for the design of highly active molecular CoPc-based ORR electrocatalysts in the practical application.

Platinum(II) diimine complexes containing phenylpyridine ligands decorated with anionic closo-monocarborane clusters [CB11H12

We report three Pt(II) diimine complexes containing ancillary ligands of phenylpyridine furnished with anionic closo-monocarborane clusters [CB11H12]-. Three neutral complexes exhibit intensive phosphorescence in the solid state and complex 1 was used to detect acetonitrile vapor in a quartz plate.

Cycling a Tether into Multiple Rings: Pt-Bridged Macrocycles for Differentiated Guest Recognition, Pseudorotaxane Transformations, and Guest Capture and Release.

Macrocycle engineering is a key topic in supramolecular chemistry. When synthesizing a ring, one can obtain either complex mixtures of macrocycles of different sizes or a single ring if a template is utilized. Here, we unite these approaches along with post-synthetic modifications to transform a single tether into multiple rings-up to five per tether. The macrocycles contain two bridged phenylpyridine ligands that are connected through a Pt atom, which defines the rings' shape, size, and host activity. All rings undergo redox reactions (between PtII and PtIV ) that allow for large conformational changes. Their reactivity, together with their host performance, is a convenient way to control the capture and release of guests, to mediate ring transformations, and to control pseudorotaxane-to-pseudorotaxane conversions. This novel approach could serve to assemble other libraries of small ring molecules, create cyclic polymers bridged by responsive-at-metal nodes, and produce processable mechanically interlocked molecules.

Cycling a Tether into Multiple Rings: Pt‐Bridged Macrocycles for Differentiated Guest Recognition, Pseudorotaxane Transformations, and Guest Capture and Release

AbstractMacrocycle engineering is a key topic in supramolecular chemistry. When synthesizing a ring, one can obtain either complex mixtures of macrocycles of different sizes or a single ring if a template is utilized. Here, we unite these approaches along with post‐synthetic modifications to transform a single tether into multiple rings—up to five per tether. The macrocycles contain two bridged phenylpyridine ligands that are connected through a Pt atom, which defines the rings’ shape, size, and host activity. All rings undergo redox reactions (between PtII and PtIV) that allow for large conformational changes. Their reactivity, together with their host performance, is a convenient way to control the capture and release of guests, to mediate ring transformations, and to control pseudorotaxane‐to‐pseudorotaxane conversions. This novel approach could serve to assemble other libraries of small ring molecules, create cyclic polymers bridged by responsive‐at‐metal nodes, and produce processable mechanically interlocked molecules.

Manipulating Excited State Properties of Iridium Phenylpyridine Complexes with "Push-Pull" Substituents.

We have prepared a series of complexes of the type [IrIII(ppy)2(L]n+ complexes (1-4), where ppy is a substituted 2-phenylpyridine and L is a chelating phosphine thioether ligand. The parent complex (1) comprises an unsubstituted phenylpyridine ligand, whereas complex 2 contains a nitro substituent on the pyridine ring, complex 3 features a diphenylamine group on the phenyl ring, and 4 has both nitro and diphenylamine groups. Crystallographic, 1H NMR, and elemental analysis data are consistent with each of the chemical formulae. DFT (density functional theory) computational results show a complicated electronic structure with contributions from Ir, ppy, and the PS ligand. Ultrafast pump-probe data show strong contributions from the phenylpyridine moieties as well as strong panchromatic excited state absorption transitions. The data show that nitro and/or diphenylamine substituents dominate the spectroscopy of this series of compounds.

Understanding the role of cyclometalating ligand regiochemistry on the photophysics of charged heteroleptic iridium(III) complexes

The ability to tune the emission energy of cyclometalated iridium(III) complexes through ligand modification is well understood. Changing the electronic profile of functional groups affords complexes that exhibit emission spanning the visible spectrum, but the regiochemistry of substitution, and its effect on photophysical properties beyond emission energy, remains less explored. In this paper we investigate the substitution pattern of an electron-donating (-OCH3) and electron-withdrawing (-CF3) functional group around the phenyl-ring of 2-phenylpyridine cyclometalating ligands. A family of heteroleptic [Ir(C∧N)2(phen)]PF6 complexes, containing C∧N as the cyclometalating and 1,10-phenanthroline (phen) as the ancillary ligand, were synthesized and characterized using a range of experimental and theoretical techniques. Optical spectroscopy revealed that iridium(III) complex photophysics were most significantly affected when substitution occurred at the 5 phenyl-ring position, para relative to the orthometalated carbon atom. Subtle differences were also observed when substitution occurred at either the 4 or the 6 phenyl-ring position, despite substitution at either site being meta relative to the organometallic bond.

Induction of Paraptosis by Cyclometalated Iridium Complex-Peptide Hybrids and CGP37157 via a Mitochondrial Ca2+ Overload Triggered by Membrane Fusion between Mitochondria and the Endoplasmic Reticulum.

We previously reported that a cyclometalated iridium (Ir) complex-peptide hybrid (IPH) 4 functionalized with a cationic KKKGG peptide unit on the 2-phenylpyridine ligand induces paraptosis, a relatively newly found programmed cell death, in cancer cells (Jurkat cells) via the direct transport of calcium (Ca2+) from the endoplasmic reticulum (ER) to mitochondria. Here, we describe that CGP37157, an inhibitor of a mitochondrial sodium (Na+)/Ca2+ exchanger, induces paraptosis in Jurkat cells via intracellular pathways similar to those induced by 4. The findings allow us to suggest that the induction of paraptosis by 4 and CGP37157 is associated with membrane fusion between mitochondria and the ER, subsequent Ca2+ influx from the ER to mitochondria, and a decrease in the mitochondrial membrane potential (ΔΨm). On the contrary, celastrol, a naturally occurring triterpenoid that had been reported as a paraptosis inducer in cancer cells, negligibly induces mitochondria-ER membrane fusion. Consequently, we conclude that the paraptosis induced by 4 and CGP37157 (termed paraptosis II herein) proceeds via a signaling pathway different from that of the previously known paraptosis induced by celastrol, a process that negligibly involves membrane fusion between mitochondria and the ER (termed paraptosis I herein).

Near-Infrared Emitting Ir(III) Complexes Bearing a Dipyrromethene Ligand for Oxygen Imaging of Deeper Tissues In Vivo.

Phosphorescence lifetime imaging microscopy (PLIM) using a phosphorescent oxygen probe is an innovative technique for elucidating the behavior of oxygen in living tissues. In this study, we designed and synthesized an Ir(III) complex, PPYDM-BBMD, that exhibits long-lived phosphorescence in the near-infrared region and enables in vivo oxygen imaging in deeper tissues. PPYDM-BBMD has a π-extended ligand based on a meso-mesityl dipyrromethene structure and phenylpyridine ligands with cationic dimethylamino groups to promote intracellular uptake. This complex gave a phosphorescence spectrum with a maximum at 773 nm in the wavelength range of the so-called biological window and exhibited an exceptionally long lifetime (18.5 μs in degassed acetonitrile), allowing for excellent oxygen sensitivity even in the near-infrared window. PPYDM-BBMD showed a high intracellular uptake in cultured cells and mainly accumulated in the endoplasmic reticulum. We evaluated the oxygen sensitivity of PPYDM-BBMD phosphorescence in alpha mouse liver 12 (AML12) cells based on the Stern-Volmer analysis, which gave an O2-induced quenching rate constant of 1.42 × 103 mmHg-1 s-1. PPYDM-BBMD was administered in the tail veins of anesthetized mice, and confocal one-photon PLIM images of hepatic tissues were measured at different depths from the liver surfaces. The PLIM images visualized the oxygen gradients in hepatic lobules up to a depth of about 100 μm from the liver surfaces with a cellular-level resolution, allowing for the quantification of oxygen partial pressure based on calibration results using AML12 cells.

Deciphering the photophysical kinetics, electronic configurations and structural conformations of iridium-cobalt hydrogen evolution photocatalysts.

Picosecond optical and X-ray absorption spectroscopies with time-dependent density functional theory revealed the reaction pathways, electronic and structural conformations of Ir-Co hydrogen evolution photocatalysts. The dyad bearing 2-phenylpyridine ancillary ligands produced more photoreduced Co(II) than its 2-phenylisoquinoline analogue. These findings are important for designs of earth-abundant photosensitizers for photocatalytic applications.

Photo- and Electrochemical Dual-Responsive Iridium Probe for Saccharide Detection.

Dual detection systems are of interest for rapid, accurate data collection in sensing systems and in vitro testing. We introduce an IrIII complex with a boronic acid receptor site attached to the 2‐phenylpyridine ligand as an ideal probe with photo‐ and electrochemical signals that is sensitive to monosaccharide binding in aqueous solution. The complex displays orange luminescence at 618 nm, which is reduced by 70 and 40 % upon binding of fructose and glucose, respectively. The electro‐chemiluminescent signal of the complex also shows a direct response to monosaccharide binding. The IrIII complex shows the same response upon incorporation into hydrogel matrices as in solution, thus demonstrating the potential of its integration into a device, as a nontoxic, simple‐to‐use tool to observe sugar binding over physiologically relevant pH ranges and saccharide concentrations. Moreover, the complex's luminescence is responsive to monosaccharide presence in cancer cells.

Dipyrrolyldiketone PtII Complexes: Ion-Pairing π-Electronic Systems with Various Anion-Binding Modes.

A variety of π-electronic ion-pairing assemblies can be constructed by combining anion complexes of π-electronic systems and countercations. In this study, a series of anion-responsive π-electronic molecules, dipyrrolyldiketone PtII complexes containing a phenylpyridine ligand, were synthesized. The resulting PtII complexes exhibited phosphorescence emission, with higher emission quantum yields (0.30-0.42) and microsecond-order lifetimes, and solution-state anion binding, as revealed by our spectroscopic analyses. These PtII complexes displayed solid-state ion-pairing assemblies, exhibiting various anion-binding modes, which derived from pyrrole-inverted and pyrrole-non-inverted conformations, and packing structures, with the contribution of charge-by-charge assemblies, which were dependent on the substituents in the PtII complexes and the geometries and electronic states of their countercations.

Radical Mechanism of Ir III /Ni II -Metallaphotoredox-Catalyzed C(sp 3 )–H Functionalization Triggered by Proton-Coupled Electron Transfer: Theoretical Insight

Photoredox catalysis can be induced to activate organic substrates or to modulate the oxidation state of transition-metal catalysts via unique single-electron transfer processes, so as to achieve c...