Synthesis, Photophysical Properties, and Biomolecular Labeling Studies of Luminescent Platinum(II)-Terpyridyl Alkynyl Complexes

Self-assembled heterometallic complexes showing enhanced two-photon absorption and their distribution in living cells

Heterometallic complexes were prepared via self-assembly, showing enhanced TPA ability and preferable localization into lysosomes.

Luminescent Pt(2,6-bis(N-methylbenzimidazol-2-yl)pyridine)X+: a comparison with the spectroscopic and electrochemical properties of Pt(tpy)X+ (X = Cl, CCPh, Ph, or CH3)

A series of platinum(ii) pincer complexes of the formula Pt(mbzimpy)X+, 1(a-d), (mbzimpy = 2,6-bis(N-methylbenzimidazol-2-yl)pyridine; X = Cl; (a), CCPh; (b), Ph; (c), or CH3; (d), CCPh = phenylacetylide, and Ph = Phenyl) have been synthesized and characterized. Electronic absorption and emission, as well as electrochemical properties of these compounds, have been investigated. Pt(tpy)X+ analogs (tpy = 2,2';6'2''-terpyridine), 2(a-d), have also been investigated and compared. Electrochemistry shows that 1 and 2 analogs undergo two chemically reversible one-electron reduction processes that are shifted cathodically along the a < b < c < d series. Notably, these reductions occur at slightly higher negative potentials in the case of 1. The absorption spectra of 1 and 2 in acetonitrile exhibit ligand-centered (1LC) transitions (ε ≈ 104 M-1 cm-1) in the UV region and metal-to-ligand-charge transfer (1MLCT) transitions (ε ≈ 103 M-1 cm-1) in the visible region. The corresponding visible bands of 1b and 2b have been assigned to 1(LLCT/MLCT) mixed state (LLCT: ligand-to-ligand-charge transfer). The preceding 1LC and 1MLCT transitions of 1 occur at lower energies than that of 2. These 1LC transitions have distinctly been blue-shifted along a < c < d in 2, but occur at nearly identical energies in 1. Conversely, 1MLCT transitions are red-shifted along a < c < d in both the analogs. The 77 K glassy solutions of 1 and 2 exhibit an intense vibronically-structured emission band at λmax(0-0) in the 470-560 nm range. This band is red-shifted along b < a ≤ c < d in 1 and along a ≤ d ≈ c ≪ b in 2. The main character of these emissions is assigned to 3LLCT emissive state in 1b and 2b, whereas to 3LC in the rest of the compounds. Relative stabilization of these spin-forbidden emissive states is discussed by invoking configuration mixing with the higher-lying 3MLCT state.

Solution aggregation of platinum(ii) triimine methyl complexes

Self-assembly processes are investigated in two model compounds, Pt(L)(CH₃)⁺ (where, L = 2,6-bis(N-methylbenzimidazol-2-yl)pyridine OR 2,2′;6′2′′-terpyridine), employing NMR. Structural conformations of formed assemblies are confirmed by 2D NOESY.

Multiresponsive Luminescent Cationic Cyclometalated Gold(III) Amphiphiles and Their Supramolecular Assembly

A new class of amphiphilic tridentate cyclometalated gold(III) complexes has been designed and synthesized as luminescent supramolecular building blocks. Positively charged trimethylammonium (-CH₂NMe₃⁺) containing alkynyl ligands have been incorporated to introduce the electrostatic interactions. The X-ray crystal structures of two of the complexes have been determined, and the existence of π-π interactions between molecules has been observed. Steady-state and time-resolved absorption and emission studies have been carried out to investigate the nature of the excited states. The complexes are found to exhibit self-assembly properties with the assistance of π-π stacking and hydrophobic interactions and possibly weak Au···Au interaction, resulting in notable emergence of low-energy absorption bands and luminescence changes. The presence of a large hydrophobic moiety is found to be crucial for the formation of aggregates, especially in polar media where hydrophobic interactions play an important role. The nature of the counterion has been shown to have a significant effect on the extent of self-assembly in different media. Upon aggregation, nanofibers are formed in polar media, while nanorods are observed in nonpolar media in one of the representative complexes. Interestingly, a small modification on the alkynyl ligand resulted in the formation of nanoribbons instead. Intriguing luminescence mechanochromic properties have also been observed. This orthogonal and rational molecular design strategy has been shown to be effective in the construction of gold(III)-based smart and multiresponsive materials.

Amyloid Protein-Induced Supramolecular Self-Assembly of Water-Soluble Platinum(II) Complexes: A Luminescence Assay for Amyloid Fibrillation Detection and Inhibitor Screening

Amyloid fibrillation has been acknowledged as a hallmark of a number of neurodegenerative ailments such as Alzheimer's disease. Accordingly, efficient detection of amyloid fibrillation will allow for great advances in the field of biomedical applications as well as in achieving early medical diagnosis. In this work, a luminescence assay for the sensitive and specific detection of amyloid fibrillation was developed by using platinum(II) complexes as sensing platforms. Supramolecular self-assembly of platinum(II) complexes was induced upon addition of amyloid, leading to alterations in the spectroscopic and luminescence properties of the complexes. As compared to fluorescent dyes, luminescent platinum(II) complexes exhibit attractive large Stokes shifts, phosphorescence lifetimes in the microsecond to submicrosecond regime, and low-energy red emission after aggregation, which are advantageous to biological imaging. At the same time, the platinum(II) complex adopted herein was found to have high photostability, high selectivity and specificity, and low cytotoxicity. The proposed design is the very first approach to detect amyloid fibrillation through the supramolecular self-assembly of luminescent platinum(II) complexes.

Long-lived triplet excited state in a platinum(ii) perylene monoimide complex

We report the synthesis and solution based photophysical properties of a new Pt(ii)-terpyridine complex coupled to a perylene monoimide (PMI) chromophoric unit through an acetylene linkage. This structural arrangement resulted in quantitative quenching of the highly fluorescent PMI chromophore by introducing metal character into the lowest energy singlet state, thereby leading to the formation of a long-lived PMI-ligand localized triplet excited state (τ = 8.4 μs). Even though the phosphorescence from this triplet state was not observed, highly efficient quenching of this excited state by dissolved oxygen and the observation of singlet oxygen photoluminescence in the near-IR at 1270 nm initially pointed towards triplet excited state character. Additionally, the coincidence of the excited state absorbance difference spectra from the sensitized PMI ligand using a triplet donor and the Pt-PMI complex provided strong evidence for this triplet state assignment, which was further supported by TD-DFT calculations.

Temperature dependence of photophysical properties of a dinuclear C^N-cyclometalated Pt(ii) complex with an intimate Pt–Pt contact. Zero-field splitting and sub-state decay rates of the lowest triplet

A dinuclear Pt(ii) complexes exhibits an unusually large zero field splitting in its metal–metal-to-ligand charge transfer triplet excited state.

Spectroscopy, electrochemistry and antiproliferative properties of Au(iii), Pt(ii) and Cu(ii) complexes bearing modified 2,2′:6′,2′′-terpyridine ligands

Impact of the metal centre and the substituent incorporated into a terpy framework.

Supramolecular assembly of platinum-containing polyhedral oligomeric silsesquioxanes: an interplay of intermolecular interactions and a correlation between structural modifications and morphological transformations

A series of alkynylplatinum(ii) terpyridine complexes functionalized with polyhedral oligomeric silsesquioxane (POSS) moieties has been demonstrated to exhibit drastic color changes and give various distinctive nanostructures with interesting multi-stage morphological transformations from spheres to nanoplates in response to solvent conditions through the interplay of various intermolecular interactions, including hydrophilic-hydrophilic, hydrophobic-hydrophobic, Pt···Pt and π-π stacking interactions. These supramolecular architectures can be systematically modified and controlled through the molecular design and the variation of solvent compositions. In particular, drastic changes in color in response to solvent polarity were observed through the incorporation of the charged moieties, representing a new class of potential candidates for functional materials with sensing or imaging capabilities. This class of complexes has been studied by 1H NMR spectroscopy, electron microscopy, UV-vis absorption and emission spectroscopy.

Platinum and Gold Complexes for OLEDs

Encouraging efforts on the design of high-performance organic materials and smart architecture during the past two decades have made organic light-emitting device (OLED) technology an important competitor for the existing liquid crystal displays. Particularly, the development of phosphorescent materials based on transition metals plays a crucial role for this success. Apart from the extensively studied iridium(III) complexes with d(6) electronic configuration and octahedral geometry, the coordination-unsaturated nature of d(8) transition metal complexes with square-planar structures has been found to provide intriguing spectroscopic and luminescence properties. This article briefly summarizes the development of d(8) platinum(II) and gold(III) complexes and their application studies in the fabrication of phosphorescent OLEDs. An in-depth understanding of the nature of the excited states has offered a great opportunity to fine-tune the emission colors covering the entire visible spectrum as well as to improve their photophysical properties. With good device engineering, high performance vacuum-deposited OLEDs with external quantum efficiencies (EQEs) of up to 30 % and solution-processable OLEDs with EQEs of up to 10 % have been realized by modifying the cyclometalated or pincer ligands of these metal complexes. These impressive demonstrations reveal that d(8) metal complexes are promising candidates as phosphorescent materials for OLED applications in displays as well as in solid-state lighting in the future.

Multiaddressable molecular rectangles with reversible host-guest interactions: modulation of pH-controlled guest release and capture

A series of multiaddressable platinum(II) molecular rectangles with different rigidities and cavity sizes has been synthesized by endcapping the U-shaped diplatinum(II) terpyridine moiety with various bis-alkynyl ligands. The studies of the host-guest association with various square planar platinum(II), palladium(II), and gold(III) complexes and the related low-dimensional gold(I) complexes, most of which are potential anticancer therapeutics, have been performed. Excellent guest confinement and selectivity of the rectangular architecture have been shown. Introduction of pH-responsive functionalities to the ligand backbone generates multifunctional molecular rectangles that exhibit reversible guest release and capture on the addition of acids and bases, indicating their potential in controlled therapeutics delivery on pH modulation. The reversible host-guest interactions are found to be strongly perturbed by metal-metal and π-π interactions and to a certain extent, electrostatic interactions, giving rise to various spectroscopic changes depending on the nature of the guest molecules. Their binding mode and thermodynamic parameters have been determined by 2D NMR and van't Hoff analysis and supported by computational study.

A luminescent cyclometalated gold(iii)–avidin conjugate with a long-lived emissive excited state that binds to proteins and DNA and possesses anti-proliferation capacity

Here we describe a luminescent cyclometalated Au(iii)–avidin conjugate that exhibits a 520 nm emission with a lifetime of 1.8 μs in PBS solution in open air.

Synthesis, characterization, photophysics, and anion binding properties of platinum(ii) acetylide complexes with urea group

The relationship between the structure and anion-binding ability of platinum(ii) acetylide complexes with urea group has been studied.

Phosphorescent proteins for bio-imaging and site selective bio-conjugation of peptides and proteins with luminescent cyclometalated iridium(III) complexes

A new bio-conjugation reaction for site selective modification of proteins and peptides with phosphorescent iridium(III) complexes has been developed; the Ir(III)-modified proteins and peptides display long emission lifetimes and large Stoke shifts that can be used for bio-imaging studies.

Synthesis and characterization of luminescent cyclometalated platinum(II) complexes of 1,3-bis-hetero-azolylbenzenes with tunable color for applications in organic light-emitting devices through extension of π conjugation by variation of the heteroatom

A series of luminescent cyclometalated platinum(II) complexes of N^C^N ligands [N^C^N=2,6-bis(benzoxazol-2'-yl)benzene (bzoxb), 2,6-bis(benzothiazol-2'-yl)benzene (bzthb), and 2,6-bis(N-alkylnaphthoimidazol-2'-yl)benzene (naphimb)] has been synthesized and characterized. Two of the platinum(II) complexes have been structurally characterized by X-ray crystallography. Their electrochemical, electronic absorption, and luminescence properties have been investigated. In dichloromethane solution at room temperature, the cyclometalated N^C^N platinum(II) complexes exhibited rich luminescence with well-resolved vibronic-structured emission bands. The emission energies of the complexes are found to be closely related to the electronic properties of the N^C^N ligands. By varying the electronic properties of the cyclometalated ligands, a fine-tuning of the emission energies can be achieved, as supported by computational studies. Multilayer organic light-emitting devices have been prepared by utilizing two of these platinum(II) complexes as phosphorescent dopants, in which a saturated yellow emission with Commission International de I'Eclairage coordinates of (0.50, 0.49) was achieved.

Synthesis, characterization and electrochemiluminescent properties of cyclometalated platinum(II) complexes with substituted 2-phenylpyridine ligands

Two neutral cyclometalated platinum(II) complexes, Pt(DPP)(acac) and Pt(BPP)(acac) (DPP = 2,4-diphenylpyridine, BPP = 2-(4-tert-butylphenyl)-4-phenylpyridine, acac = acetylacetone), have been synthesized and characterized by (1)H NMR spectroscopy, mass spectrometry, elemental analyses and by X-ray crystallography for Pt(DPP)(acac). Electrogenerated chemiluminescence (ECL) of the two complexes in the absence or presence of coreactant tri-n-propylamine (TPrA) in different solvents (CH(3)CN, CH(2)Cl(2), DMF, CH(3)CN/H(2)O (V, 50 : 50)) has been studied. The ECL spectra are identical to their own PL spectra, indicating that ECL processes lead to the same metal-to-ligand charge-transfer ((3)MLCT) excited state that is generated by light excitation. The ECL potentials of Pt(DPP)(acac) and Pt(BPP)(acac)/TPrA in CH(3)CN and CH(3)CN/H(2)O solution were at ~0.75 V vs. SCE, and significantly negatively shifted by about 0.6 V compared to that of the Ru(bpy)(3)(2+)/TPrA system. The ECL quantum efficiencies of the complexes are comparable to that of the Ru(bpy)(3)(2+)/TPrA system. The significant increase of the ECL signal in the coreactant system is due to the formation of the strongly reducing intermediate TPrA˙. It is noteworthy that the ECL efficiencies of the synthesized compounds are much higher than that of the tridentate polypyridyl ligands.