Temperature-Induced Switching of the Mechanism for Intramolecular Energy Transfer in a 2,2‘:6‘,2‘ ‘-Terpyridine-Based Ru(II)−Os(II) Trinuclear Array

Construction of Luminescent Terpyridine-Based Metallo-Bowties with Alkyl Chain-Bridged Dimerized Building Blocks

Numerous metallo-supramolecules with well-defined sizes and shapes have been successfully constructed via the strong coordination interaction between terpyridine (TPY) moieties and ruthenium cations. However, the pseudo-octahedral geometry of unit hampers the luminescent properties of such metallo-architectures, thus limiting their applications as optical materials. To address this issue, we herein use a flexible alkyl chain to bridge TPY building blocks, replacing conventional linkage. The introduction of alkyl chain guides the self-assembly into desired architecture while simultaneously eliminating the quenching effects typically associated with the linkage. More importantly, this design strategy enables the precise construction of bowtie-shaped metallo-supramolecules with significantly enhanced emission. The incorporation of alkyl chain linkage not only maintains structural integrity but also enhances optical performance, making these metallo-supramolecular assemblies highly promising for applications in advanced photonic and luminescent materials. This study offers a versatile approach to construct complex metallo-supramolecular architectures with desired optical properties.

Ru(II)-Ru(II) and Ru(II)-Os(II) Homo-/Heterodinuclear Complexes and Ru3(II)-Ru(II) Homotetranuclear Complexes Based on Heteroditopic Bridging Ligands: Synthesis, Photophysics, and Effective Energy Transfer

In this paper, the synthesis, photophysics, electrochemistry, and intramolecular energy transfer of two series of dinuclear and tetranuclear metallic complexes [(bpy)2M1LxM2(bpy)2]4+ (x = 1, 2; M1 = Ru, M2 = Ru/Os; M1 = Os, M2 = Ru) and {[Ru(bpy)2(Lx)]3Ru}8+ based on new heteroditopic bridging ligands (L1 = 6-phenyl-4-Hpip-2-2'-bipyridine, L2 = 6-Hpip-2-2'-bipyridine, Hpip = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline) are reported. The dimetallic and tetrametallic complexes exhibit rich redox properties with successive reversible metal-centered oxidation and ligand-centered reduction couples. All complexes display intense absorption in the entire ultraviolet-visible spectral regions. The mononuclear [LxRu(bpy)2]2+ and homodinuclear [(bpy)2RuLxRu(bpy)2]4+ complexes display strong Ru-based characteristic emission at room temperature. Interestingly, the optical studies of heterodinuclear complexes reveal almost complete quenching of the RuII-based emission and efficient photoinduced energy transfer, resulting in an OsII-based emission in the near-infrared region. As a result of the intramolecular energy transfer from the center to the periphery and steric hindrance quenching of the peripheral RuII-centered emissive triplet metal-to-ligand charge transfer states, the tetranuclear complexes exhibit weak RuII-based emission with a short lifetime. Since the light absorbed by several chromophores is efficiently directed to the subunit with the lowest-energy excited state, the present multinuclear complexes can be used as well-visible-light-absorption antennas.

Single-molecule nano-optoelectronics: insights from physics

Single-molecule optoelectronic devices promise a potential solution for miniaturization and functionalization of silicon-based microelectronic circuits in the future. For decades of its fast development, this field has made significant progress in the synthesis of optoelectronic materials, the fabrication of single-molecule devices and the realization of optoelectronic functions. On the other hand, single-molecule optoelectronic devices offer a reliable platform to investigate the intrinsic physical phenomena and regulation rules of matters at the single-molecule level. To further realize and regulate the optoelectronic functions toward practical applications, it is necessary to clarify the intrinsic physical mechanisms of single-molecule optoelectronic nanodevices. Here, we provide a timely review to survey the physical phenomena and laws involved in single-molecule optoelectronic materials and devices, including charge effects, spin effects, exciton effects, vibronic effects, structural and orbital effects. In particular, we will systematically summarize the basics of molecular optoelectronic materials, and the physical effects and manipulations of single-molecule optoelectronic nanodevices. In addition, fundamentals of single-molecule electronics, which are basic of single-molecule optoelectronics, can also be found in this review. At last, we tend to focus the discussion on the opportunities and challenges arising in the field of single-molecule optoelectronics, and propose further potential breakthroughs.

"Chemistry-on-the-complex": functional RuII polypyridyl-type sensitizers as divergent building blocks

Ruthenium polypyridyl type complexes are potent photoactive compounds, and have found - among others - a broad range of important applications in the fields of biomedical diagnosis and phototherapy, energy conversion schemes such as dye-sensitized solar cells (DSSCs) and molecular assemblies for tailored photo-initiated processes. In this regard, the linkage of RuII polypyridyl-type complexes with specific functional moieties is highly desirable to enhance their inherent photophysical properties, e.g., with a targeting function to achieve cell selectivity, or with a dye or redox-active subunits for energy- and electron-transfer. However, the classical approach of performing ligand syntheses first and the formation of Ru complexes in the last steps imposes synthetic limitations with regard to tolerating functional groups or moieties as well as requiring lengthy convergent routes. Alternatively, the diversification of Ru complexes after coordination (termed "chemistry-on-the-complex") provides an elegant complementary approach. In addition to the Click chemistry concept, the rapidly developing synthesis and purification methodologies permit the preparation of Ru conjugates via amidation, alkylation and cross-coupling reactions. In this regard, recent developments in chromatography shifted the limits of purification, e.g., by using new commercialized surface-modified silica gels and automated instrumentation. This review provides detailed insights into applying the "chemistry-on-the-complex" concept, which is believed to stimulate the modular preparation of unpreceded molecular assemblies as well as functional materials based on Ru-based building blocks, including combinatorial approaches.

Mapping Bridge Conformational Effects on Electronic Coupling in Mo2-Mo2 Mixed-Valence Systems

The large bridging ligand 9,10-anthracenedicarboxylate and its thiolated derivatives have been employed to assemble two dimolybdenum complex units and develop three Mo₂ dimers, [Mo₂(DAniF)₃]₂(μ-9,10-O₂CC₁₄H₈CO₂), [Mo₂(DAniF)₃]₂(μ-9,10-OSCC₁₄H₈COS), and [Mo₂(DAniF)₃]₂(μ-9,10-S₂CC₁₄H₈CS₂) (DAniF = N, N'-di( p-anisyl)formamidinate), for the study of conformation dependence of the electronic coupling between the two Mo₂ centers. These compounds feature a large deviation of the central anthracene ring from the plane defined by the Mo-Mo bond vectors, with the torsion angles (ϕ = 50-76°) increasing as the chelating atoms of the bridging ligand vary from O to S. Consequently, the corresponding mixed-valence complexes do not exhibit characteristic intervalence charge transfer absorptions in the near-IR spectra, in contrast to the phenylene and naphthalene analogues, from which these systems are assigned to the Class I in Robin-Day's scheme. Together with the phenylene and naphthalene series, the nine total mixed-valence complexes in three series complete a transition from the electronically uncoupled Class I to the strongly coupled Class II-III borderline via moderately coupled Class II and permit a systematic mapping of the bridge conformation effects on electronic coupling. Density functional theory calculations show that the HOMO-LUMO energy gap, corresponding to the metal (δ) to ligand (π*) transition energy, and the magnitude of HOMO-HOMO-1 splitting in energy are linearly related to cos² ϕ. Therefore, our experimental and theoretical results concur to indicate that the coupling strength decreases in the order of the bridging units: phenylene > naphthalene > anthracene, which verifies the through-bond superexchange mechanism for electronic coupling and electron transfer.

Design of Ru(II) Complexes Based on Anthraimidazoledione-Functionalized Terpyridine Ligand for Improvement of Room-Temperature Luminescence Characteristics and Recognition of Selective Anions: Experimental and DFT/TD-DFT Study

In this work we report synthesis and characterization of three rigid and linear rodlike monometallic Ru(II) complexes based on a terpyridine ligand tightly connected to 9,10-anthraquinone electron-acceptor unit through phenyl-imidazole spacer. The motivation of designing these complexes is to enhance their excited-state lifetimes at room temperature. Interestingly it is found that all three complexes exhibit luminescence at room temperature with excited-state lifetimes in the range of 1.6-52.8 ns, depending upon the coligand as well as the solvent. Temperature-dependent luminescence investigations indicate that the energy gap between the emitting ³MLCT state and nonemitting metal-centered state ³MC in the complexes increased enormously compared with parent [Ru(tpy)₂]²⁺. In addition, by taking advantage of the imidazole NH proton(s), which became appreciably acidic upon combined effect of electron accepting anthraquinone moiety as well as metal ion coordination, we also examined anion recognition and sensing behaviors of the complexes in organic, mixed aqueous-organic as well as in solid medium through different optical channels such as absorption, steady-state and time-resolved emission, and ¹H NMR spectroscopic techniques. In conjunction with the experiment, computational investigation was also employed to examine the electronic structures of the complexes and accurate assignment of experimentally observed spectral and redox behaviors.

Preparation, Characterization, and Oxygenase Activity of a Photocatalytic Artificial Enzyme

A bicyclo[6,1,0]nonyne-substituted 9-mesityl-10-methyl-acridinium cofactor was prepared and covalently linked to a prolyl oligopeptidase scaffold containing a genetically encoded 4-azido-L-phenylalanine residue in its active site. The resulting artificial enzyme catalyzed sulfoxidation when irradiated with visible light in the presence of air. This reaction proceeds by initial electron abstraction from the sulfide within the enzyme active site, and the protein scaffold extended the fluorescence lifetime of the acridium cofactor. The mode of sulfide activation and placement of the acridinium cofactor (5) in POP-ZA₄ -5 make this artificial enzyme a promising platform for developing selective photocatalytic transformations.

Providing power for miniaturized medical implants: triplet sensitization of semiconductor surfaces

Here, we recognize the growing significance of miniaturized devices as medical diagnostic tools and highlight the need to provide a convenient means of powering such instruments when implanted into the body. One of the most promising approaches to this end involves using a light-collection facility to absorb incident white light and transfer the photonic energy to a tiny semiconductor embedded on the device. Although fluorescent organic molecules offer strong potential as modules for such solar collectors, we emphasize the promise offered by transition metal complexes. Thus, an extended series of binuclear Ru(II)/Os(II) poly(pyridine) complexes has been shown to be highly promising sensitizers for amorphous silicon solar cells. These materials absorb a high fraction of visible light while the Ru(II)-based units possess triplet energies that are comparable to those of the naphthalene-based bridge. The metal complex injects a triplet exciton into the bridge and this, in turn, is trapped by the Os(II)-based terminal. The result is extremely efficacious triplet-energy transfer; at room temperature the rate of energy transfer is independent of distance over some 6 nm and only weakly dependent on temperature.

Synthesis, structural characterization, and photophysical, spectroelectrochemical, and anion-sensing studies of heteroleptic ruthenium(II) complexes derived from 4'-polyaromatic-substituted terpyridine derivatives and 2,6-bis(benzimidazol-2-yl)pyridine

Heteroleptic bis-tridentate ruthenium(II) complexes of composition [(H2pbbzim)Ru(tpy-Ar)](ClO4)2, where H2pbbzim = 2,6-bis(benzimidazol-2-yl)pyridine and tpy-Ar = 4'-substituted terpyridine ligands with Ar = phenyl (2), 2-naphthyl (3), 9-anthryl (4), and 1-pyrenyl (5) groups, have been synthesized and characterized by using standard analytical and spectroscopic techniques. The X-ray crystal structures of the complexes [(H2pbbzim)Ru(tpy-Naph)](ClO4)2 (3), [(pbbzim)Ru(tpy-Naph)]·(CH3)2CO·H2O (3a), and [(H2pbbzim)Ru(tpy-Py)](ClO4)2 (5) have been determined. The absorption, steady-state, and time-resolved luminescence spectral properties of the complexes were thoroughly investigated in dichloromethane. The compounds display strong luminescence at room temperature with lifetimes (τ2) in the range of 5.5-62 ns, depending upon the nature of the polycyclic aromatic moiety as well as the solvents. The complexes are found to undergo one reversible oxidation in the positive potential window (0 to +1.5 V) and four successive quasi-reversible reductions in the negative potential window (0 to -2.4 V). The anion-sensing properties of the receptors were thoroughly investigated in acetonitrile/dichloromethane (1/9 v/v) solutions (2 × 10(-5) M) using absorption, steady-state, and time-resolved emission spectroscopic studies. (1)H NMR titration experiments, on the other hand, were carried out in either CD3CN or DMSO-d6. The anion-sensing studies revealed that the receptors act as sensors for F(-), CN(-), AcO(-), and SO4(2-) and to some extent for HSO4(-) and H2PO4(-). It is evident that, in the presence of excess anions, deprotonation of the imidazole N-H fragments of the receptors occurs, which is signaled by the change of color from yellow-orange to violet visible with the naked eye. From the absorption and emission titration studies the binding/equilibrium constants of the receptors with the anions have also been determined. Anion-induced lifetime quenching and/or enhancement make the receptors suitable lifetime-based sensors for selective anions. Cyclic voltammetric (CV) measurements of the compounds carried out in acetonitrile have provided evidence in favor of anion-dependent electrochemical responses with F(-) and AcO(-) ions. Spectroelectrochemical studies have also been carried out for both the protonated and deprotonated forms of the complexes in the range of 300-1200 nm. With successive oxidation of the Ru(II) center, replacement of MLCT bands by LMCT bands occurs gradually with observation of sharp isosbestic points in all cases.

2,3-Di(2-pyridyl)-5-phenylpyrazine: a NN-CNN-type bridging ligand for dinuclear transition-metal complexes

A new bridging ligand, 2,3-di(2-pyridyl)-5-phenylpyrazine (dpppzH), has been synthesized. This ligand was designed so that it could bind two metals through a NN-CNN-type coordination mode. The reaction of dpppzH with cis-[(bpy)2RuCl2] (bpy = 2,2'-bipyridine) affords monoruthenium complex [(bpy)2Ru(dpppzH)](2+) (1(2+)) in 64 % yield, in which dpppzH behaves as a NN bidentate ligand. The asymmetric biruthenium complex [(bpy)2Ru(dpppz)Ru(Mebip)](3+) (2(3+)) was prepared from complex 1(2+) and [(Mebip)RuCl3] (Mebip = bis(N-methylbenzimidazolyl)pyridine), in which one hydrogen atom on the phenyl ring of dpppzH is lost and the bridging ligand binds to the second ruthenium atom in a CNN tridentate fashion. In addition, the RuPt heterobimetallic complex [(bpy)2Ru(dpppz)Pt(C≡CPh)](2+) (4(2+)) has been prepared from complex 1(2+), in which the bridging ligand binds to the platinum atom through a CNN binding mode. The electronic properties of these complexes have been probed by using electrochemical and spectroscopic techniques and studied by theoretical calculations. Complex 1(2+) is emissive at room temperature, with an emission λmax = 695 nm. No emission was detected for complex 2(3+) at room temperature in MeCN, whereas complex 4(2+) displayed an emission at about 750 nm. The emission properties of these complexes are compared to those of previously reported Ru and RuPt bimetallic complexes with a related ligand, 2,3-di(2-pyridyl)-5,6-diphenylpyrazine.

Influence of applied pressure on the probability of electronic energy transfer across a molecular dyad

A pair of covalently linked molecular dyads is described in which two disparate boron dipyrromethene dyes are separated by a tolane-like spacer. Efficient electronic energy transfer (EET) occurs across the dyad; the mechanism involves important contributions from both Förster-type coulombic interactions and Dexter-type electron exchange processes. The energy acceptor is equipped with long paraffinic chains that favor aggregation at high concentration or at low temperature. The aggregate displays red-shifted absorption and emission spectral profiles, relative to the monomer, such that EET is less efficient because of a weaker overlap integral. The donor unit is insensitive to applied pressure but this is not so for the acceptor, which has extended π-conjugation associated with appended styryl groups. Here, pressure reduces the effective π-conjugation length, leading to a new absorption band at higher energy. With increasing pressure, the overall EET probability falls but this effect is nonlinear and at modest pressure there is only a small recovery of donor fluorescence. This situation likely arises from compensatory phenomena such as restricted rotation and decreased dipole screening by the solvent. However, the probability of EET falls dramatically over the regime where the π-conjugation length is reduced owing to the presumed conformational exchange. It appears that the pressure-induced conformer is a poor energy acceptor.

(4'-Ethynyl-2,2':6',2''-terpyridine)(2,2':6',2''-terpyridine)-ruthenium(II) bis-(hexa-fluoridophosphate) acetonitrile disolvate

The title heteroleptic bis­-terpyridine complex, [Ru(C₁₅H₁₁N₃)(C₁₇H₁₁N₃)](PF₆)₂·2CH₃CN, crystallized from an acetonitrile solution as a salt containing two hexa­fluoridophosphate counter-ions and two acetonitrile solvent mol­ecules. The Ru^II atom has a distorted octa­hedral geometry due to the restricted bite angle [157.7 (3)°] of the two mer-arranged N,N′,N′′-tridendate ligands, viz. 2,2′:6′,2′′-terpyridine (tpy) and 4′-ethynyl-2,2′:6′,2′′-terpyridine (tpy′), which are essentially perpendicular to each other, with a dihedral angle of 87.75 (12)° between their terpyridyl planes. The rod-like acetyl­ene group lies in the same plane as its adjacent terpyridyl moiety, with a maximum deviation of only 0.071 (11) Å from coplanarity with the pyridine rings. The mean Ru—N bond length involving the outer N atoms trans to each other is 2.069 (6) Å at tpy and 2.070 (6) Å at tpy′. The Ru—N bond length involving the central N atom is 1.964 (6) Å at tpy and 1.967 (6) Å at tpy′. Two of the three counter anions were refined as half-occupied.

Compensation of Coulomb blocking and energy transfer in the current voltage characteristic of molecular conduction junctions

We have studied the influence of both exciton effects and Coulomb repulsion on current in molecular nanojunctions. We show that dipolar energy-transfer interactions between the sites in the wire can at high voltage compensate Coulomb blocking for particular relationships between their values. Tuning this relationship may be achieved by using the effect of plasmonic nanostructure on dipolar energy-transfer interactions.

Luminescent bis-tridentate ruthenium(II) and osmium(II) complexes based on terpyridyl-imidazole ligand: synthesis, structural characterization, photophysical, electrochemical, and solvent dependence studies

Homo- and heteroleptic bis-tridentate ruthenium(II) and osmium(II) complexes of compositions [(tpy-PhCH(3))Ru(tpy-HImzPh(3))](ClO(4))(2) (1), [(H(2)pbbzim)Ru(tpy-HImzPh(3))] (ClO(4))(2) (2) and [M(tpy-HImzPh(3))(2)](ClO(4))(2) [M = Ru(II) (3) and Os(II) (4)], where tpy-PhCH(3) = p-methylphenyl terpyridine, H(2)pbbzim = 2,6-bis(benzimidazole-2-yl)pyridine and tpy-HImzPh(3) = 4'-[4-(4,5-diphenyl-1H-imidazol-2-yl)-phenyl]-[2,2':6',2'']terpyridine, have been synthesized and characterized by using standard analytical and spectroscopic techniques. These compounds were designed to increase the room temperature excited-state lifetimes of bisterpyridine-type ruthenium(II) and osmium(II) complexes. The X-ray crystal structures of two homoleptic complexes 3 and 4 have been determined and show that both the compounds crystallized in orthorhombic form with space group Fddd. The photophysical and redox properties of the complexes have been thoroughly investigated. All the complexes display moderately strong luminescence at room temperature with lifetimes in the range of 6-35 ns. The complexes are found to undergo one reversible oxidation in the positive potential window (0 to +1.6 V) and one irreversible and two successive quasi-reversible reductions in the negative potential window (0 to -2.0 V). The influence of solvents on the photophysical properties of the complexes has also been investigated in detail.

Ruthenium(II)-bis(4′-(4-ethynylphenyl)-2,2′:6′, 2″-terpyridine) — A versatile synthon in supramolecular chemistry. Synthesis and characterization

A homoleptic ethynyl-substituted ruthenium(II)-bisterpyridine complex representing a versatile synthon in supramolecular chemistry was synthesized and analyzed by NMR spectroscopy, mass spectrometry and X-ray diffractometry. Furthermore, its photophysical properties were detailed by UV/Vis absorption, emission and resonance Raman spectroscopy. In order to place the results obtained in the context of the vast family of ruthenium coordination compounds, two structurally related complexes were investigated accordingly. These reference compounds bear either no or an increased chromophore in the 4′-position. The spectroscopic investigations reveal a systematic bathochromic shift of the absorption and emission maximum upon increasing chromophore size. This bathochromic shift of the steady state spectra occurs hand in hand with increasing resonance Raman intensities upon excitation of the metal-to-ligand charge-transfer transition. The latter feature is accompanied by an increased excitation delocalization over the chromophore in the 4′-position of the terpyridine. Thus, the results presented allow for a detailed investigation of the electronic effects of the ethynyl substituent on the metal-to-ligand charge-transfer states in the synthon for click reactions leading to coordination polymers.