Synthesis,photophysical, and electrochemical properties of two polynuclear ruthenium(II) polypyridyl complexes containing diazafluorene

Two polypodands, tetrakis[2-(4,5-diazafluoren-9-ylimino)phenoxymethyl]methane (L¹) and 1,1,1-tris[2-(4,5-diazafluoren-9-ylimino)phenoxymethyl]propane (L²), and their corresponding Ru(II) polypyridyl complexes have been synthesized and characterized. The photophysical behaviors of the two complexes were investigated by UV–vis absorption and emission spectroscopy. They display metal-to-ligand charge transfer (MLCT) absorptions at around 443 nm in MeCN solution at room temperature and emission at around 573 nm in EtOH:MeOH (4:1) glassy matrix at 77 K. Electrochemical studies of the two complexes show one Ru(II)-centered oxidation at around 1.35 V and three ligand-centered reductions.     

Synthesis,photophysical, electrochemical,and ion-binding properties of Ru(II) polypyridyl complexes containing benzo-15-crown-5

Two polypyridyl ligands, 5-(4′-ethynylbenzo-15-crown-5)-2,2′-bipyridine (L¹) and 3-bromo-8-(4′-ethynylbenzo-15-crown-5)-1,10-phenanthroline (L²), and their Ru(II) complexes [(bpy)₂RuL](PF₆)₂ have been prepared and characterized. Both complexes exhibit metal-to-ligand charge transfer absorption at around 452 nm and emission at around 640 nm in MeCN solution. Electrochemical studies of the complexes reveal a Ru(II)-centered oxidation at around 1.31 V and three ligand-centered reductions. The binding ability of the complexes with Na⁺ has been investigated by UV/Vis absorption, emission, and electrochemical titrations. Addition of Na⁺ to MeCN solutions of both complexes results in a progressive enhancement of the emission, a red-shift of the UV/Vis absorption, and a progressive cathodic shift of the Ru(II)-centered E _1/2 couple. The stability constants for the 1:1 stoichiometry adducts of the complexes with Na⁺ have been obtained from the UV/Vis absorption titrations.     

Preparation, photophysical, and electrochemical properties of two tetranuclear ruthenium(II) polypyridyl complexes containing 4,5-diazafluorene

Two tetrapodal ligands L¹ and L² containing 4,5-diazafluorene units have been synthesized and characterized. Both ligands are composed of two kinds of nonequivalent coordinating sites: one involves the 4-(4,5-diazafluoren-9-ylimino)phenoxy moiety, and the other one involves the 2-(4,5-diazafluoren-9-ylimino)phenoxy moiety. The Ru(II) complexes [(bpy)₈Ru₄(L¹)](PF₆)₈ and [(bpy)₈Ru₄(L²)](PF₆)₈ (bpy = 2,2′-bipyridine) have been obtained by refluxing Ru(bpy)₂Cl₂·2H₂O and each ligand in 2-methoxyethanol. Both complexes exhibit metal-to-ligand charge transfer (MLCT) absorptions at around 443 nm and emission at around 574 nm. Electrochemical studies of both complexes display one Ru(II)-centered oxidation at around 1.33 V and three ligand-centered reductions.     

Synthesis, characterization, luminescence and nonlinear optical (NLO) properties of truxene-containing platinum(II) alkynyl complexes

A series of luminescent trinuclear platinum(II) alkynyl complexes containing dihydro-5H-diindeno[1,2-a;1′,2′-c]fluorene (truxene) as the core and aryl alkynyl ligands with different electronic properties at the periphery has been successfully synthesized and characterized. The electronic absorption, emission, nanosecond transient absorption and electrochemical properties of these complexes have been reported. These complexes showed long-lived emissions in degassed benzene solution at room temperature, and their emissions have been assigned to originate from triplet states of intraligand (IL) character with some mixing of metal-to-ligand charge-transfer (MLCT) character. The luminescent platinum(II) alkynyl complexes are found to show two-photon absorption (2PA) and two-photon induced luminescence (TPIL) properties, and their two-photon absorption cross-sections have been determined to be 6-51 GM upon excitation at 720 nm.     

Synthesis and properties of 6,6′-dithienyl-4,4′-bipyrimidine and its hetero- and homo-leptic Ru(II) complexes

Hetero- and homo-leptic Ru(II) complexes of a new 4,4′-bipyrimidine ligand, th₂bpm (6,6′-di(2″-thienyl)-4,4′-bipyrimidine), have been synthesized and characterized. The parent ligand th₂bpm has electron rich thiophene units on the periphery of a bidentate ligand which is capable of binding to metal ions. The heteroleptic complex of th₂bpm [Ru(bpy)₂th₂bpm]²⁺ (bpy = 2,2′-bipyridine) exhibits a Ru-to-bpm metal-to-ligand charge transfer (MLCT) absorption centered at 547 nm and a Ru-to-bpy MLCT absorption centered at 438 nm. The assignment of the low energy absorption is supported by the relative ease of electrochemical reduction of the new complex as compared to [Ru(bpy)₃]²⁺. The homoleptic complex, [Ru(th₂bpm)₃]²⁺, exhibits a Ru-to-bpm MLCT absorption at slightly higher energy (544 nm). Both complexes are emissive at room temperature in fluid solution and 5 is one of the lowest energy emitters based on tris-bidentate Ru(II) complexes known (λ_max = 770 nm). The luminescence spectra is red-shifted compared to [Ru(bpy)₃]²⁺ and this effect is ascribed to the delocalization in the acceptor ligand.     

Tetradentate cyclometalated platinum complex enables high-performance near-infrared electroluminescence with excellent device stability

Due to the high decay rate of the non-radiative transition of long wavelengths, the molecular design of efficient and stable near-infrared (NIR) electroluminescent materials remains a big challenge. Herein, a new tetradentate cyclometalated platinum(II) complex with an N^∧C^∧C^∧N coordinated framework has been developed and used as a dopant for NIR organic light-emitting diodes (OLEDs). The complex exhibited a short-lived (0.5–1.5 µs) metal-to-ligand charge transfer (MLCT) excited state in doped and neat films. The resulting NIR OLEDs (λ_EL = 730 nm) achieved maximum external quantum efficiency (EQE_max) of 5.2% and radiance of 74626 mW sr^?1 m^–2. Of note, the device exhibited excellent stability with operational lifetime of 119 h for LT₉₀. This work demonstrated the great potential of tetradentate platinum(II) complexes in the field of NIR OLEDs.     

Solvent dependence of charge-transfer transitions in binary aqueous mixtures as an indicator of preferential solvation

Summary Wavelengths of maximum absorptions are reported for the main metal-to-ligand charge transfer-bands (MLCT) of complexes [Fe(CN)₅(4-phpy)]^3– and [Fe(CN)₅(4-t-bupy)]^3– (4-phpy=4-phenylpyridine and 4-t-bupy=4-t-butylpyridine) in several series of mixed solvents. Correlations of the shifts in MLCT energies as a function of dielectric constant and polarizability of the medium have been investigated. The interpretation of the medium effects on E_op (MLCT) takes into account the process of preferential solvation of solutes by solvent molecules.     

Light-induced spin crossover in Fe(II)-based complexes: The full photocycle unraveled by ultrafast optical and X-ray spectroscopies

The light-induced spin and structure changes upon excitation of the singlet metal-to-ligand charge transfer (¹MLCT) state of Fe(II)-polypyridine complexes are investigated in detail in the case of aqueous iron(II)-tris-bipyridine ([Fe^II(bpy)₃]²⁺) by a combination of ultrafast optical and X-ray spectroscopies. Polychromatic femtosecond fluorescence up-conversion, transient absorption studies in the 290–600 nm region and femtosecond X-ray absorption spectroscopy allow us to retrieve the entire photocycle upon excitation of the ¹MLCT state from the singlet low-spin ground state (¹GS) as the following sequence: ^1,3MLCT → ⁵T → ¹GS, which does not involve intermediate singlet and triplet ligand-field states. The population time of the HS state is found to be ～150 fs, leaving it in a vibrationally hot state that relaxes in 2–3 ps, before decaying to the ground state in 650 ps. We also determine the structure of the high-spin quintet excited state by picosecond X-ray absorption spectroscopy at the K-edge of Fe. We argue that given the many common electronic (ordering of electronic states) and structural (Fe–N bond elongation in the high-spin state, Fe–N mode frequencies, etc.) similarities between all Fe(II)-polypyridine complexes, the results on the electronic relaxation processes reported in the case of [Fe^II(bpy)₃]²⁺ are of general validity to the entire family of Fe(II)-polypyridine complexes.     

Structures and spectral properties of heteroleptic copper (I) complexes: A theoretical study based on density functional theory

The structures and electronic absorption spectra of newly synthesized heteroleptic copper (I) complexes [CuL₁L₂]⁺ (L₁ = phen-imidazole and/or L₂ = dipyrido [3,2-a:2’,3’-c] phenazine derivatives) are analyzed under the light of density functional theory (DFT) and time-dependent DFT (TD-DFT). The ground states geometries, characterized by π-stacking interactions, have been optimized using PBE-D functional taking into account dispersion correction. The UV-visible theoretical absorption spectra have been calculated using B3LYP functional in vacuum and taking into account solvent corrections by means of the polarized continuum model (PCM). Whereas the PBE-D functional is well adapted to the determination of the structures, it does underestimate drastically the transition energies. The spectra are characterized by high density of states, mainly metal-to-ligand-charge-transfer (MLCT) and intra-ligand (IL), between 600 nm and 250 nm. Most of the complexes show an intense band in the near-UV energy domain (～320 nm) corresponding to an IL transition. The lowest part of the absorption spectra, starting at 600 nm, corresponds to MLCT transitions leading to a shoulder observed experimentally between 400 and 500 nm. The upper part of the spectra, beyond 300 nm, puts in evidence strong mixing between ligand-to-ligand-charge-transfer (LLCT), IL and MLCT states.     

Design and synthesis of Ru(II-bipyridyl-containing conjugated polymers

A series of novel π-conjugated polymers containing ruthenium bipyridine complexes was synthesized by a cross-coupling reaction and characterized. These polymers exhibit absorption maxima around 330–350 nm (π-π*) and 460–500 nm metal-to-ligand charge transfer (MLCT), respectively. They are soluble in common organic solvents, and all polymers can be converted into transparent films. We investigated the influence of different donating and acceptor diethynylarenes of the ultraviolet-visible spectra. The oxidation potential, which was measured by cyclic- and square-wave voltametry, showed a typical Ru^2+/3+ exhibited at 1.25 V versus the saturated calomel electrode. The polymers were further characterized with photoluminescence measurements. When excited at 442 nm ( 11a ), the polymer exhibited an emission peak at 690 nm. This peak was attributed to the MLCT states. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 722–732, 2004     

Phosphorescent Cationic Heterodinuclear IrIII/MI Complexes (M=CuI,AuI) with a Hybrid Janus-Type N-Heterocyclic Carbene Bridge

A novel class of phosphorescent cationic heterobimetallic Ir^III/M^I complexes, where M^I=Cu^I ( 4 ) and Au^I ( 5 ), is reported. The two metal centers are connected by the hybrid bridging 1,3-dimesityl-5-acetylimidazol-2-ylidene-4-olate (IMesAcac) ligand that combines both a chelating acetylacetonato-like and a monodentate N-heterocyclic carbene site coordinated onto an Ir^III and a M^I center, respectively. Complexes 4 and 5 have been prepared straightforwardly by a stepwise site-selective metalation with the zwitterionic [(IPr)M^I(IMesAcac)] metalloproligand (IPr=1,3-(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene) and they have been fully characterized by spectroscopic, electrochemical, and computational investigation. Complexes 4 and 5 display intense red emission arising from a low-energy excited state that is located onto the “Ir(C^N)” moiety featuring an admixed triplet ligand-centered/metal-to-ligand charge transfer (³IL/¹MLCT) character. Comparison with the benchmark mononuclear complexes reveals negligible electronic coupling between the two distal metal centers at the electronic ground state. The bimetallic systems display enhanced photophysical properties in comparison with the parental congeners. Noteworthy, similar non-radiative rate constants have been determined along with a two-fold increase of radiative rate, yielding brightly red-emitting cyclometalating Ir^III complexes. This finding is ascribed to the increased MLCT character of the emitting state in complexes 4 and 5 due to the smaller energy gap between the ³IL and ¹MLCT manifolds, which mix via spin–orbit coupling.     

Theoretical studies on the structural and optical properties of a series of Os(II) diimine complexes [Os(NN)(CO)2I2] (NN = 2,2′-bipyridine(bpy), 4,4′-di-tert-butyl-2,2′-bipyridine(dbubpy), 4,4′-dichlorine-2,2′-bipyridine(dclbpy))

The ground- and excited-state structures for a series of Os(II) diimine complexes [Os(NN)(CO)₂I₂] (NN = 2,2′-bipyridine (bpy) (1), 4,4′-di-tert-butyl-2,2′-bipyridine (dbubpy) (2), and 4,4′-dichlorine-2,2′-bipyridine (dclbpy) (3)) were optimized by the MP2 and CIS methods, respectively. The spectroscopic properties in dichloromethane solution were predicted at the time-dependent density functional theory (TD-DFT, B3LYP) level associated with the PCM solvent effect model. It was shown that the lowest-energy absorptions at 488, 469 and 539 nm for 1–3, respectively, were attributed to the admixture of the [d_xy (Os) → π^*(bpy)] (metal-to-ligand charge transfer, MLCT) and [p(I) → π^*(bpy)] (interligand charge transfer, LLCT) transitions; their lowest-energy phosphorescent emissions at 610, 537 and 687 nm also have the ³MLCT/³LLCT transition characters. These results agree well with the experimental reports. The present investigation revealed that the variation of the substituents from H → t-Bu → Cl on the bipyridine ligand changes the emission energies by altering the energy level of HOMO and LUMO but does not change the transition natures.     

Preparation, photophysical, and electrochemical properties of two polynuclear Ru(II) polypyridyl complexes containing imidazole-based ligands

A tripodal ligand L¹ and dipodal ligand L² containing imidazole rings have been synthesized by the reaction of 1,10-phenanthroline-5,6-dione with 2,2??-bipyridine-4,4??-dicarbaldehyde and 4-methyl-2,2??-bipyridine-4??-carbaldehyde, respectively, in the presence of ammonium acetate. Both ligands have two kinds of nonequivalent coordinating sites: one involving the phenanthroline moiety and the other involving the 2,2??-bipyridine moiety. The Ru(II) complexes, [(bpy)₆Ru₃(L¹)](PF₆)₆ and [(bpy)₄Ru₂(L²)](PF₆)₄ (bpy?=?2,2??-bipyridine), have been obtained by refluxing Ru(bpy)₂Cl₂·2H₂O with each ligand in solution. The two complexes display MLCT absorptions at 465 and 480?nm, respectively, and emission at 665 and 675?nm, respectively, in CH₃CN solution. Electrochemical studies of both complexes show one Ru(II)-centered oxidation at around 1.29?V and three ligand-centered reductions.     

Two trinuclear Ru(II) complexes of hetero-tritopic bridging ligands: synthesis,characterization, theoretical calculations,photophysical and electrochemical properties

Two hetero-tritopic bridging ligands L¹ and L² based on 2,2′-bipyridine and 1,10-phenanthroline moieties, and their corresponding Ru(II) complexes [{Ru(bpy)₂}₃(µ₃?L¹)](PF₆)₆ and [{Ru(bpy)₂}₃(µ₃?L²)](PF₆)₆ (bpy = 2,2′-bipyridine), were synthesized. The molecular structures of both complexes were deduced by ¹H NMR, ESI-MS, ESI-HRMS, elemental analyses, and IR spectroscopy. Quantum calculations on the free bridging ligands and their complexes are also presented. Both complexes display MLCT absorptions at around 454 nm, and emissions at around 613 nm in CH₃CN solution at room temperature and at around 590 nm in EtOH–MeOH glassy matrix at 77 K. Cyclic and differential pulse voltammetry studies of both complexes reveal one reversible Ru(II)-centered oxidation and three reversible ligand-centered reductions, in each case.     

Synthesis,crystal structure and photophysical study of luminescent three-coordinate cuprous bromide complexes based on pyrazole derivatives

The 1?:?2 M-ratio reaction between cuprous bromide and pyrazole derivatives in toluene results in mononuclear Cu(I) complexes [CuBr(pyrazole)₂]. The complexes have been characterized by ¹H NMR spectroscopy and elemental analysis. The molecular structure, established by single-crystal X-ray diffraction, features a trigonal planar geometry around copper, with monodentate pyrazole derivatives. All the Cu(I) complexes are luminescent in the solid state at ambient temperature. Intense blue or blue-green emission in the solid state is observed for these complexes, with the maxima ranging from 431 to 493 nm. The observed photoluminescence could be ascribed to the metal-to-ligand charge-transfer excited states, probably mixed with some halide-to-ligand character. The microsecond lifetime scale of the complexes implies that these transitions arise from the triplet excited states.     

Syntheses and properties of complexes with bis(2,2′-bipyridyl)ruthenium(II) moieties coordinated to 4,4′:2′,2′′:4′′,4′′′-quaterpyridinium ligands

Eleven new complexes of the form cis-[Ru^II(bpy)₂(L^A)]⁴⁺ (bpy = 2,2′-bipyridyl; L^A = a pyridinium-substituted bpy derivative) have been prepared and isolated as their PF₆⁻ salts. Characterisation involved various techniques including ¹H NMR spectroscopy and MALDI mass spectrometry. The UV-Vis spectra show intense intraligand π → π^∗ absorptions and metal-to-ligand charge-transfer (MLCT) bands with two distinct maxima in the visible region. Small shifts in the MLCT bands correlate with the electron-withdrawing strength of the ligand L^A. Cyclic voltammograms show quasi-reversible or reversible Ru^III/II oxidation waves, and two or more ligand-based reductions with varying degrees of reversibility. The variations in the redox potentials correlate with changes in the structure of L^A, and also with the MLCT energies. Differential pulse voltammetry allows the first reduction process for two of the complex salts to be resolved into two peaks. Single-crystal X-ray structures have been solved for three of the new complex salts and also for a pro-ligand salt. Two carboxylate-functionalised compounds have been tested as photosensitizers on TiO₂-coated electrodes, but show only negligible efficiencies, in accord with expectations.     

[Bis(trispivalatodimolybdenum (II))-μ-bis(4′-carboxylato-2,2′:6′,2″-terpyridine) ruthenium (II)] (2+) Tetrafluoroborate: Photophysical Studies

The photophysical properties of the title compound have been studied by fs and ns transient absorption spectroscopy. The electronic absorption spectrum consists of three principle absorptions assigned to terpy ¹LLCT at ~300 nm, ruthenium (II) t_2g⁶ to terpy ¹MLCT at ~470 nm and Mo₂ δ to terpycarboxylate at ~670 nm. The compound shows weak room temperature emission in THF solution at ~1,100 nm when excited into each of the aforementioned bands. This emission is assigned to the T₁ state, ³MMδδ*. Transient absorption spectroscopy indicates a lifetime for T₁ of 9.6 μs. This paper is dedicated to Prof. C. N. R. Rao.     

Tuning the Photophysical and Excited State Properties of Phosphorescent Iridium(III) Complexes by Polycyclic Unit Substitution

Two novel N-embedded polycyclic units functionalized phosphorescent iridium(III) complexes ( Ir-1 and Ir-2 ) with substituents in different positions have been prepared. Complex Ir-1 bearing the substituent at the 3-position shows a distinct blue shift single-peak emission (524 nm) with a higher luminescence efficiency (Φ_PL=42 %) and shorter emission lifetime (τ=282 ns) by comparison with 4-position substitution based complex Ir-2 (Φ_PL=23 %, τ=562 ns), which exhibits a dual-peak emission (564 nm and 602 nm), and phosphorescence color can be tuned from green to yellow. In addition, DFT calculations demonstrate that unusual ligand-to-metal charge transfer (³LMCT) excited state property can be found in Ir-2 , which is in contrast to metal-to-ligand charge transfer (³MLCT) excited state character in Ir-1 . This result can be attribute to strong electron-donating character and 4-position substitution effect of the unit.     

Absorption Spectra and Luminescence Properties of Isomeric Platinum (II) and Palladium (II) Complexes containing 1,1′-biphenyldiyl, 2-phenylpyridine,and 2,2′-bijpyridine as ligands

The absorption spectra, luminescence spectra, and luminescence lifetimes of the isomeric [M(bph)(bpy)] and [M(phpy)₂] complexes M = Pt(II) or Pd(II), bph^2? = 1,1′-biphenyl-2,2′-diyl dianion, phpy^? = 2-phenylpyridine-2′-yl anion, and bpy = 2,2′-bipyridine have been investigated and compared with those of [M(bpy)₂]²⁺ complexes and of the free protonated ligands H₂bph, Hbpy⁺, and Hphpy. In the absorption spectra, the region below 320 mm is dominated by ligand-centered (LC) transitions, whereas metal-to-ligand charge transfer (MLCT) transitions are responsible for the bands present in the near UV/VIS region. The MLCT bands move to higher energies on replacing Pt with Pd and in going from [M(bph)(bpy)] to the [M(phpy)₂] isomer. For the mixed-ligand complexes, evidence for both M → bph^2? (at higher energies) and M → bpy bands is found. The structured luminescence observed at 77 K shows lifetimes of 4.0 and 1.1 μs for [Pt(phpy)₂] and [Pt(bph)(bpy)], respectively, and 480 and 250 μs for the analogous Pd complexes. On the basis of the energy and lifetime data, the luminescence of the Pt(II) complexes is assigned to the lowest triplet MLCT excited state, whereas for the Pd complexes the luminescent state is thought to result from a mixture of MLCT and LC triplet levels.