Metal-metal To Ligand Charge Transfer Research Articles

Nucleobase Stacking Sustained by Pt−Pt Interactions

AbstractThis article reports the use of metal–metal interactions as a way to induce nucleobase stacking in a dinucleotide chain. We synthetized and study a series of C,N‐cyclometallated platinum(II) binuclear complexes derived from 6‐phenylpurine dinucleotide 1 as proligand, and β‐diketonates, 2,2'‐bipyridines and 1,10‐phenanthroline as ancillary ligands. The X‐ray diffraction analysis revealed that the diketonate derivatives 7 and 8 are chiral helixes sustained by Pt−Pt intramolecular interactions that allow the nucleobase stacking. The folded helical structures are maintained in solution, as demonstrated by the appearance of metal–metal‐to‐ligand charge‐transfer (MMLCT) transitions in the UV‐vis spectra. The influence of the ancillary ligand in the molecular structure was evidenced by the absence of bands derived from M−M interactions in the absorption spectra of 2,2’‐bipyridine and and 1,10‐phenanthroline derivatives 12–14. The existence of Pt−Pt in the complexes of this study can be also detected by electrochemistry.

Vibrational Coherence in the Metal–Metal-Bonded Excited State of Pt(II) Complexes

In the past decade, there have been significant advancements in the investigation of coherence-related phenomena in organic systems such as biological photosynthetic reaction centers. The d8 Pt(II) dinuclear complex or molecular aggregate with a metal–metal-to-ligand charge transfer (MMLCT) or metal-centered (MC) excited state was reported to show the vibrational coherence phenomenon in the intersystem crossing (ISC) process, due to the Metal–metal (M-M) interaction at excited state. In this study, we review the coherence effect in the Pt(II)-Pt(II) complexes which are speculated to be a coherent energy conversion system. The impacts of coherence on the photo-physics of Pt(II) dinuclear complexes have been discussed and reviewed, including the intersystem crossing process and vibrational wavepacket dynamics.

Octanuclear heterometallic one-dimensional complex extended by metal–metal bonds showing MMLCT in the visible region

In this study, [{Rh2(O2CCH3)4(OH2)}{Rh2(O2CCH3)4(O2CCF3)}{Pt2(acam)2(bpy)2}2](PF6)3 (2, acam = acetamidate, bpy = 2,2ʹ-bipyridine), a one-dimensional (1D) octanuclear complex aligned as Rh–Rh–Pt–Pt–Pt–Pt–Rh–Rh, was successfully synthesized and characterized. Compound 2 contained a tetranuclear platinum complex where each Pt atom was coordinated with bpy as the co-ligand, exhibiting metal–metal to ligand charge transfer (MMLCT) around 2.0 eV. It admixed with three characteristic bands attributed to heterometallic Pt–Rh bonds in the range of 1.3–2.5 eV. This success paves the way for the development of further novel heterometallic one-dimensional chains containing co-ligands that affect the σ-type orbitals of metal–metal bonds.

Visible Light Absorption and Long-Lived Excited States in Dinuclear Silver(I) Complexes with Redox-Active Ligands.

Well-defined dinuclear silver(I) complexes have been targeted for applications in catalysis and materials chemistry, and the effect of close silver-silver interactions on electronic structure remains an area of active inquiry. In this study, we describe the synthesis, structure, and photophysical properties of dimeric silver complexes featuring a redox-active naphthyridine diimine ligand. Unusually for silver(I), these complexes display absorption features in the visible region due to metal-metal to ligand charge transfer (MMLCT) transitions, which arise from the combination of close silver-silver interactions and low-lying ligand π* orbitals. The complexes' photophysical properties are explored via a combination of spectroscopic and computational studies, revealing MMLCT excited state lifetimes that exceed 1 μs. These results portend previously unforeseen applications of silver(I) dimers in visible light absorption and excited state reactivity.

Versatile Pt(II) Pyrazolate Complexes: Emission Tuning via Interplay of Chelate Designs and Stacking Assemblies.

Three homoleptic Pt(II) metal complexes [Pt(imPz)2] (1), [Pt(imiz)2] (2), and [Pt(imMz)2] (3) were synthesized from the treatment of Pt(DMSO)2Cl2 and functional imidazolyl pyrazole in refluxing tetrahydrofuran (THF). Alternatively, the heteroleptic Pt(II) complexes [Pt(imPz)(fppz)] (4), [Pt(imiz)(fppz)] (5), and [Pt(imMz)(fppz)] (6) were obtained from the treatment of a common intermediate [Pt(fppzH)Cl2] with a corresponding imidazolyl chelate. Pt(II) complexes 1, 2, and 5 were studied by single-crystal X-ray diffraction to reveal the corresponding packing arrangement in their crystal lattices, among which both homoleptic complexes 1 and 2 formed monomeric species, while heteroleptic 5 aligned as a dimer with a nonbonding Pt···Pt contact of 3.574 Å. Subsequent photophysical examinations showed that the homoleptic 1-3 and heteroleptic 4-6 exhibited the structured sky-blue ππ* emission and structureless light-green-emitting metal-metal-to-ligand charge transfer (MMLCT) emission in the solid state, respectively. A shortened Pt···Pt interaction of approximately 0.34-0.35 nm was confirmed in thin films of all heteroleptic Pt(II) complexes 4-6 by grazing-incidence X-ray diffraction (GIXD) analyses. Finally, Pt(II) complex 6 was employed as a dopant in the fabrication of organic light-emitting diode (OLED) devices with varied doping ratios, among which OLEDs with only 1 wt % 6 in the SimCP host exhibited a maximum external quantum efficiency (EQE) of 5.8% and CIEx,y values of 0.20, 0.31. In contrast, OLEDs using a nondoped architecture (i.e., 100% of 6 in the emitting layer (EML)) achieved a maximum EQE of 26.8%, current efficiency (CE) of 91.7 cd A-1, and power efficiency (PE) of 80.1 lm·W-1 and CIEx,y values of 0.41, 0.55, manifesting their versatility in various degrees of stacking assemblies and hence facile color-tuning capability on OLEDs.

Near-Infrared Emission Induced by Shortened Pt-Pt Contact: Diplatinum(II) Complexes with Pyridyl Pyrimidinato Cyclometalates.

Four diplatinum(II) complexes with the formula [Pt(pypm)(μ-Fn)]2 (2, 3a-c) bearing both a pyridine-pyrimidinate chelate and formamidinate bridge, where (pypm)H and FnH stand for 5-(pyridin-2-yl)-2-(trifluoromethyl)pyrimidine and functional formamidines with various substituents of iPr (n = 1), Ph (n = 2), C6H4tBu (n = 3), and C6H4CF3 (n = 4), were synthesized en route from a mononuclear intermediate represented by [Pt(pypm)Cl(F1H)] (1). Single-crystal X-ray diffraction studies confirmed the structure of 1 and 3a comprised of an individual "Pt(pypm)" unit and two "Pt(pypm)" units with a Pt···Pt distance of 2.8845(2) Å, respectively. Therefore, in contrast to the structured emission of mononuclear 1 with the first vibronic peak wavelength at 475 nm, all other diplatinum complexes with shortened Pt···Pt separation exhibited greatly red shifted and structureless metal-metal to ligand charge transfer (MMLCT) emission that extended into the near-infrared region in solid states. Their photophysical characteristics were measured under three distinctive morphological states (i.e., crystals, sublimed powders, and vacuum-deposited thin films) by steady-state UV-vis spectroscopy, while retention of Pt···Pt interactions in deposited thin films of 2 and 3a-c was confirmed using Raman spectroscopy, demonstrating lowered Pt···Pt stretching at 80-200 cm-1. Most importantly, complexes 3a-c exhibited a gradual red shift with the trends crystals < sublimed powders < vacuum-deposited thin films, a result of increased intermolecular π-π stacking interactions and Pt···Pt interactions, while crystalline samples exhibited the highest luminescence among all three morphological states due to the fewest defects in comparison to other morphologies. Finally, 3b was selected as a nondoped emitter for the fabrication of NIR-emitting OLEDs, giving an electroluminescence peak at 767 nm and a maximum external quantum efficiency of 0.14% with negligible roll-off.

Coherent Vibrational Wavepacket Dynamics in Platinum(II) Dimers and Their Implications

Vibrational coherence in the metal–metal-to-ligand-charge transfer (MMLCT) excited state of cyclometalated platinum dimers with a pseudo C2 symmetry was investigated where two nearly degenerate transitions from the highest occupied molecular orbital (metal–metal σ* orbital) to higher energy ligand π* orbitals could be simultaneously induced. We observed oscillatory features in femtosecond degenerate transient absorption (TA) signals from complexes [(ppy)Pt(μ-tBu2pz)]2 (1) and anti-[(ppy)Pt(μ-pyt)]2 (2), which are attributed to coherent nuclear motions that modulate the HOMO (antibonding σ*) energy level, and hence, the energy for the MMLCT transition. The characteristics of such coherent nuclear motions, such as the oscillatory frequency and the dephasing time, differ between 1 and 2 and are explained by mainly two structural factors that could influence the vibrational coherence: the Pt–Pt distance (2.97 A for 1 vs 2.85 A for 2) and molecular shape (1 in an “A” frame vs 2 in an “H” frame). Because the el...

A Hydrido η1-Alkynyl Diplatinum Complex Obtained from a Phosphinito Phosphanido Complex and Trimethylsilylacetylene

The reaction of (trimethylsilyl)acetylene with the phosphinito phosphanido Pt(I) complex [(PHCy2)Pt(μ-PCy2){κ2P,O-μ-P(O)Cy2}Pt(PHCy2)](Pt-Pt) (1) results in the protonation of the Pt-Pt bond with the formation of the bridging hydride complex [(PHCy2)(Me3SiC≡C)Pt(μ-PCy2)(μ-H) Pt(PHCy2){κP-P(O)Cy2}](Pt-Pt) (2), which was characterized by spectroscopic, spectrometric and XRD analyses. Complex 2 exhibits in the solid state at 77 K a long-lived, weak, orange emission assigned as metal-metal to ligand charge transfer (MMLCT) (L = alkynyl) due to the presence of a very short Pt···Pt distance [2.8209(2) Å]. Reaction of 2 with etherate HBF4 results in the selective protonation of the phosphinito ligand to afford the species [(PHCy2)(Me3SiC≡C)Pt(μ-PCy2)(μ-H) Pt(PHCy2){κP-P(OH)Cy2}](Pt-Pt)[BF4] ([3]BF4).

Analyzing nanostructures in mesogenic host–guest systems for polarized phosphorescence

Feasibility of polarized phosphorescent organic light-emitting devices (OLEDs) had been previously demonstrated by combining a discotic Pt(II) complex with a glassy-nematic oligofluorene host to form a mesogenic host–guest phosphorescent emitting system. Previous photophysical studies suggested that in the host–guest film, the Pt(II) complex tended to aggregate into columnar stacks, exhibiting metal–metal-to-ligand charge transfer (MMLCT) emission. Both host molecules and guest aggregates in the host–guest films could be oriented by a conductive alignment layer, giving rise to polarized phosphorescence from the Pt(II) aggregates. Nevertheless, film morphologies and nanostructures of the mesogenic host–guest systems have remained to be elucidated. In this work, grazing incidence X-ray scattering (GIXS) was carried out to analyze nanostructures in both neat films of the discotic Pt(II) complex and mesogenic host–guest mixture films. In addition, confocal laser scanning microscopy (CLSM) was also utilized for visualization of the morphologies of mesogenic host–guest systems. The columnar axes of nanostructured Pt(II) stacks lying on the alignment-treated surfaces were found to be preferentially oriented perpendicular to the rubbing direction, which is responsible for the observed linearly polarized phosphorescence.

Structural, photophysical, and mesomorphic properties of luminescent platinum(II)-salen Schiff base complexes

A series of square planar platinum(II) salen complexes containing 4-substituted alkoxy chains of aromatic rings, [Pt((4-CnH2n+1O)2salen)] (n=3 (1), 4 (2), 6 (3), 8 (4), 10 (5), 12 (6), 14 (7), 16 (8), and 18 (9)) has been prepared, and photophysical and mesomorphic properties have been investigated. Complexes 1–9 emitted intense phosphorescence from the mixed exited-state between the MLCT (MLCT=metal-to-ligand charge transfer) and LLCT (LLCT=ligand-to-ligand charge transfer) states both in dichloromethane solution and in solid states at room temperature. The red-shifted solid-state emission spectra from monomers in solution are caused by the supramolecular contact with one-dimensional stacking between neighboring chromophores (salen moieties) through the weak C–H⋯O type hydrogen bonding and van der Waals interactions in the solid state revealed by an X-ray crystallographic analysis for 2. Though 1–4 did not exhibit any mesophases (liquid–crystalline phases), 5–9 showed a lamello-columnar (ColL) mesophase with the high thermal stability, leading to the different phosphorescence spectra from both in solution and in solid states. This is affiliated with the appearance of the intermolecular dz2Pt(II)-dz2Pt(II) orbital interaction in a one-dimensional stacking distance with ca. 3.0Å due to self-assemblies in the liquid crystal which gives rise to the MMLCT (MMLCT=metal–metal-to-ligand charge transfer) transition. This is supported by absorption and emission measurements in the wide range from room temperature in the solid state to the clearing points (around 280°C). The relationship between molecular assemblies and photophysical properties is discussed.

Polarized phosphorescent organic light-emitting devices adopting mesogenic host–guest systems

We report successful realization of workable polarized phosphorescent organic light-emitting devices (OLEDs) by combining a recently developed mesogenic phosphorescent Pt(II) complex and a mesogenic oligofluorene host to form the corresponding mesogenic host–guest emitting system. In the host–guest film, the mesogenic Pt(II) complex tends to aggregate and self-assemble into the columnar stacking arrangement, exhibiting metal–metal-to-ligand charge transfer (MMLCT) emission of the Pt(II) complex. Both the host molecules and guest aggregates in the host–guest films were successfully aligned by using the rubbed conducting polymer as the alignment layer. With such aligned host–guest films, polarized red-emitting OLEDs were successfully implemented, showing an EL polarization ratio of ∼2, a maximal brightness exceeding 2000 cd/m 2, and a cd/A efficiency up to 2.4 cd/A. Due to the particular alignment mechanism of guest aggregates and the unique MMLCT emission mechanism, the polarized OLED exhibiting stronger electroluminescence perpendicular to the rubbing direction than along the rubbing direction.

Theoretical Study of Excited States of Pyrazolate- and Pyridinethiolate-Bridged Dinuclear Platinum(II) Complexes: Relationship between Geometries of Excited States and Phosphorescence Spectra

Dinuclear platinum(II) complexes [Pt(2)(μ-pz)(2)(bpym)(2)](2+) (1; pz = pyrazolate and bpym = 2,2'-bipyrimidine) and [Pt(2)(μ-pyt)(2)(ppy)(2)] (2; pyt = pyridine-2-thiolate and Hppy = 2-phenylpyridine) were theoretically investigated with density functional theory (DFT) to clarify the reasons why the phosphorescence of 1 is not observed in the acetonitrile (CH(3)CN) solution at room temperature (RT) but observed in the solid state at RT and why the phosphorescence of 2 is observed in both the CH(3)CN solution and the solid state at RT. The S(1) and T(1) states of 1 in the CH(3)CN solution are assigned as a metal-metal-to-ligand charge-transfer (MMLCT) excited state. Their geometries are C(2v) symmetrical, in which spin-orbit interaction between the S(1) and T(1) excited states is absent because the direct product of irreducible representations of the singly occupied molecular orbitals (SOMOs) of these excited states and the orbital angular momentum (l) operator involved in the Hamiltonian for spin-orbit interaction does not belong to the a(1) representation. As a result, the S(1) → T(1) intersystem crossing hardly occurs, leading to the absence of T(1) → S(0) phosphorescence in the CH(3)CN solution at RT. In the solid state, the geometry of the S(1) state does not reach the global minimum but stays in the C(1)-symmetrical local minimum. This S(1) excited state is assigned as a mixture of the ligand-centered π-π* excited state and the metal-to-ligand charge-transfer excited state. Spin-orbit interaction between the S(1) and T(1) excited states operates to induce the S(1) → T(1) intersystem crossing because the direct product of the irreducible representations of the SOMOs of these excited states and the l operator belongs to the "a" representation. As a result, T(1) → S(0) phosphorescence occurs in the solid state. In 2, the S(1) and T(1) excited states are assigned as the MMLCT excited state. Their geometries are C(2)-symmetrical in both the CH(3)CN solution and the solid state, in which spin-orbit interaction between the S(1) and T(1) states operates to induce the S(1) → T(1) intersystem crossing because the direct product of the irreducible representations of the SOMOs and the l operator belongs to the "a" representation. Thus, T(1) → S(0) phosphorescence occurs in both the CH(3)CN solution and the solid state at RT, unlike 1.

Synthesis and Structural Characterization of a New Vapochromic Pt(II) Complex Based on the 1-Terpyridyl-2,3,4,5,6-pentaphenylbenzene (TPPPB) Ligand

A novel terpyridine ligand containing a pentaphenylphenyl moiety linked to the terpyridyl core (1-terpyridyl-2,3,4,5,6-pentaphenyl-benzene (TPPPB)) has been synthesized in good yield and reacted with Pt(DMSO)2Cl2, to produce the cationic complex [Pt(TPPPB)Cl]Cl (5). 5 was studied structurally and spectroscopically. It is observed to be brightly luminescent in the solid state at room temperature and at 77 K, with no change in lambdaem(max). The complex exhibits reversible vapochromic behavior upon exposure to methylene chloride vapors, changing color from red (5-R) to green (5-G). The shift to higher energy in the emission maximum from 654 to 514 nm is the largest vapochromic shift (140 nm) yet reported. The [Pt(TPPPB)Cl]Cl complex exhibits high selectivity for certain volatile organic compounds (VOCs) including methylene chloride, ethanol, ethyl acetate, and acetonitrile. The crystal structures of both the green and red forms have been determined by single-crystal X-ray diffraction. In both forms, the cationic Pt(II) complex possesses the anticipated square-planar coordination geometry that is distorted as a consequence of constraints from the terpyridyl binding. Analysis of the crystal packing of the green form (5-G) reveals the presence of non-interacting Pt...Pt separations with distances of 3.9092(9) and 4.5483(11) A and a zigzag arrangement between neighboring Pt(II) ions. The red form (5-R) contains complexes that are stacked with Pt...Pt separations of 3.2981(14) and 3.3427(14) A, indicative of metallophilic interaction. The change in the emitting state, as a consequence of the effect of the volatile organic compounds, results from a disruption of the d8-d8 metallophilic interactions in the red form and its metal-metal-to-ligand charge transfer (MMLCT) excited state to a more-localized Pt(dpi)-to-tpy(pi*) metal-to-ligand charge transfer (MLCT) excited state in the green form.

Observation of Low-Energy Metal−Metal-to-Ligand Charge Transfer Absorption and Emission: Electronic Spectroscopy of Cyclometalated Platinum(II) Complexes with Isocyanide Ligands

A series of luminescent mononuclear [(C∧N∧N)Pt(C⋮NR)]X (HC∧N∧N = 6-phenyl-2,2‘-bipyridine; R = tBu (1), nBu (2), iPr (3), cyclohexyl (Cy, 4), X = ClO4; R = 2,6-Me2C6H3 (Ar‘, 5), X = PF6) and [(C∧N∧N)Pt(CO)]CF3SO3 (6(CF3SO3)) and binuclear [{(C∧N∧N)Pt}2(μ-C⋮N(CH2)3N⋮C)](PF6)2 (7(PF6)2) complexes were synthesized, and their spectroscopic and photophysical properties have been investigated. The crystal lattices of 1(ClO4) and 5(PF6) reveal close [C∧N∧N] π−π intermolecular contacts (3.4−3.6 A). Additional metal−metal interactions are evident in 5(PF6) with intermolecular Pt−Pt distances of 3.3831(9) A. At complex concentration greater than 5 × 10-3 mol dm-3, the UV−vis absorption spectrum of 2 displays a weak shoulder at 511 nm (e = 120 dm3 mol-1 cm-1), which does not obey Beer's law. A metal−metal-to-ligand charge transfer (MMLCT) [dσ*(Pt−Pt) → π*(C∧N∧)] absorption of a dimeric species via 2[Pt(C∧N∧N)(C⋮NnBu)]+ ⇋ [(C∧N∧N)Pt(C⋮NnBu)]22+ is proposed. The room-temperature emission spectrum of 4 for complex conc...