Field-induced Conductance Switching by Charge-state Alternation in Organometallic Single-Molecule Junctions

Charge transport through single molecules can be influenced by the charge and spin states of redox-active metal centres placed in the transport pathway. These molecular intrinsic properties are usually addressed by varying the molecules electrochemical and magnetic environment, a procedure that requires complex setups with multiple terminals. Here we show that oxidation and reduction of organometallic compounds containing either Fe, Ru or Mo centres can solely be triggered by the electric field applied to a two-terminal molecular junction. Whereas all compounds exhibit bias-dependent hysteresis, the Mo-containing compound additionally shows an abrupt voltage-induced conductance switching, yielding high to low current ratios exceeding 1000 at voltage stimuli of less than 1.0 V. DFT calculations identify a localized, redox active molecular orbital that is weakly coupled to the electrodes and closely aligned with the Fermi energy of the leads because of the spin-polarised ground state unique to the Mo centre. This situation opens an additional slow and incoherent hopping channel for transport, triggering a transient charging effect of the entire molecule and a strong hysteresis with unprecedented high low-to-high current ratios.Comment: 9 pages, 4 figure

Stable and color tunable emission properties based on non-cyclometalated gold(iii) complexes

Stable and emission tunable non-cyclometalated gold(III) triaryl complexes of the type [(L)Au(C6F5)3] [L = 2-(2,4-difluorophenylpyridine) (1), 4-phenylpyridine (2), 2-phenylpyridine (3), 2-phenylisoquinoline (4), 2-thienylpyridine (5)] were synthesized starting from a common precursor complex [(THT)Au(C6F5)3] [THT = tetrahydrothiophene] in good to modest yields. Extensive characterization of the complexes by various nuclear magnetic resonance spectroscopy techniques and elemental analysis further corroborated the single-crystal X-ray diffraction studies. The complexes displayed room temperature phosphorescence in the neat solid and in 2-MeTHF at 77 K. Detailed photophysical investigations of the complexes in the neat solid and at 77 K revealed the successful tuning of the emission maxima with modest quantum yields across the visible part of the electromagnetic spectrum depending on the electronic properties of the heterocyclic ligands. DFT (Density Functional Theory) and TDDFT (Time Dependent Density Functional Theory) calculations were performed to discern the composition of the excited state as well as confirm the obtained relative emission energies upon substitution with electronically different ligands. The obtained diverse emissive behavior of the complexes combined with the ease of synthesis illustrate the generality and applicability of the design approach to obtain emissive gold(III) complexes devoid of cyclometalation

Luminescent monocyclometalated cationic gold(III) complexes: synthesis, photophysical characterization and catalytic investigations

Stable, luminescent, and cationic monocyclometalated gold(III) monoaryl complexes of the type [(ppy)Au(FMes)(L)]+[OTf]− [L = 4-phenylpyridine (3), quinoline (4), 4-fluoroaniline (5), P(OMe)3 (6), PPh3 (7)], bearing different ancillary ligands, synthesized starting from the precursor complex [(ppy)Au(FMes)(OH2)]+[OTf]− (2) are reported. The preliminary assignment of the structure of the complexes by various nuclear magnetic resonance spectroscopy techniques and elemental analysis has been further corroborated by single-crystal X-ray diffraction studies. The complexes exhibit room temperature phosphorescence in solution, in neat solids and in doped PMMA films. Detailed photophysical investigations of the complexes in solution, in neat solids and in PMMA films revealed the successful tuning of the emission quantum yield (ϕp) based on the electronic properties of the ancillary ligands. The catalytic photo-oxidation of benzylic amines to their corresponding imines using molecular oxygen as the oxidant was successfully achieved in the presence of the luminescent Au(III) complexes. It is also established that the photocatalytic performance was strongly governed by the electronic properties of the ancillary ligands on the photosensitizer as well as by the steric bulk of the substrates

Structural and Electronic Variations of sp/sp2Carbon-Based Bridges in Di- and Trinuclear Redox-Active Iron Complexes Bearing Fe(diphosphine)2X (X = I, NCS) Moieties

Starting from the mononuclear precursor trans-Fe(depe)2I2 (depe = 1,2-bis(diethylphosphino)ethane), four dinuclear complexes IFe(depe)2–R–Fe(depe)2I, with R = 1,4-(−C≡C–C6H4–C≡C−) 1, 1,3-(−C≡C–C6H4–C≡C−) 2, 4,4′-(−C≡C–C6H4–C6H4–C≡C−) 3, and 2,5-(−C≡C–thiophene–C≡C−) 4, as well as a trinuclear complex, {I–Fe(depe)2(C≡C−)}3(1,3,5-C6H3)} 5, were prepared in a facile way by transmetalation from stannylated precursors. Substitution of the terminal iodides applying an excess of NaSCN yielded the corresponding isothiocyanate complexes 6–10 in very good yields. All complexes 1–10 are intrinsically functional due to the redox-active Fe centers embedded in a structurally rigid and covalent sp/sp2 framework. 1–10 were characterized by NMR, IR, and Raman spectroscopy, as well as elemental analyses. X-ray diffraction studies were carried out for 1, 2, 4, 5, 6, 8, and 9. Cyclic voltammetry was employed to explore the redox behavior of 1–10. The 1,4-(−C≡C–C6H4–C≡C−) and the 2,5-(−C≡C–thiophene–C≡C−) bridged compounds 1, 4, 6, and 9 exhibit two fully reversible oxidation waves, while the 1,3-(−C≡C–C6H4–C≡C−) and 4,4′-(−C≡C–C6H4–C6H4–C≡C−) bridged dinuclear complexes and the trinuclear complexes show only one reversible oxidation wave corresponding to 2 e– and 3 e– processes, respectively. Calculations were carried out for truncated model complexes to determine the HOMO/LUMO energies. The DFT results confirmed that by changing the sp/sp2 bridging ligand, tuning of the energies of the molecular orbitals and modifying of the HOMO–LUMO gap ΔE(H-L) and the chemical hardness are possible

Towards blue emitting monocyclometalated gold(iii) complexes – synthesis, characterization and photophysical investigations

lue emitting phosphorescent materials with high efficiency and high stability are a key requirement for the wider adoption of organic light emitting devices (OLEDs). In order to achieve triplet derived emission at the higher energy region of the electromagnetic spectrum, a series of neutral, monocyclometalated gold(III) complexes with trifluoromethyl and trifluoromethoxy functionalized 2-phenylpyridine (ppy) derivatives, 2-anilinopyridine (apy), 2-benzoylpyridine (bpy) and 2-benzylpyridine (bepy), as the cyclometalating framework and diaryl or monoaryl alkyne as ancillary ligands have been designed and synthesized. Extensive photophysical and chemical characterization by various nuclear magnetic resonance spectroscopy techniques, elemental analysis and single crystal X-ray diffraction studies of selected compounds was carried out to confirm the structural and the chemical identity of the complexes. The emission wavelength maxima of the complexes appear in the blue/sky blue region of the electromagnetic spectrum. Detailed photophysical investigations revealed that the different emission properties of the complexes are predominantly dictated by the electronic properties of the cyclometalating ligand. The interesting photoluminescence properties along with the facile synthetic access makes this a promising concept to obtain highly suitable blue emitting gold(III) complexes

Monocyclometalated (C N) Gold(III) Metallacycles: Tunable Emission and Singlet Oxygen (1O2) Generation Properties

The synthesis, characterization and photoluminescent properties of four cyclometalated (C N)-type gold(III) complexes bearing a bidentate diacetylide ligand, tolan-2,2'-diacetylide (tda), are reported. The complexes exhibit highly tunable excited state properties and show photoluminescence (PL) across the entire visible spectrum from sky-blue (lambda(PL)=493 nm) to red (lambda(PL)=675 nm) with absolute PL quantum yields (PLQY) of up to 75 % in solution, the highest PLQY found for any monocyclometalated Au(III) complex in solution. As a consequence of the use of the strongly rigidifying diacetylide bidentate ligand, a significant increase in the excited state lifetimes (tau(0)=16-258 mu s) was found in solution and in thin films. The complexes showed remarkable singlet oxygen generation in aerated solution with absolute singlet oxygen quantum yield (phi(1 Delta)) values reaching up to 7.5x10(-5) and singlet oxygen lifetimes (tau(1 Delta)(0)) in the range of 66-95 mu s. Furthermore, the radiative and non-radiative rates of singlet oxygen were determined using the phi(1 Delta) and tau(1 Delta)(0) values and correlations are drawn between the formation of singlet oxygen and its interaction with cyclometalated (C N) gold(III) complexes

Tunable and Efficient White Light Phosphorescent Emission Based on Single Component N-Heterocyclic Carbene Platinum(II) Complexes

A new class of cyclometalated pyridine N-heterocyclic carbene (NHC) Pt(II) complexes with electronically different alkyne derivatives (C≡CR; R = C6H4C(CH3)3 (1), C6H5 (2), C6H4F (3), C6H3(CF3)2 (4)) as ancillary ligands were synthesized, and the consequences of the electronic properties of the different substituted phenylacetylene ligands on the phosphorescent emission efficiencies were studied, where C≡CC6H4C(CH3)3 = 4-tert-butylphenylacetylene, C≡CC6H5 = phenylacetylene, C≡CC6H4F = 4-fluorophenylacetylene, and C≡CC6H3(CF3)2 = 3,5-bis(trifluoromethyl)phenylacetylene. Structural characterization, electrochemistry, and photophysical investigations were performed for all four compounds. Moreover, the emission quantum efficiencies and wavelength emission intensities of the complexes were also recorded in different weight percents in poly(methyl methacrylate) films (PMMA) and evaluated in the CIE-1931 chromaticity diagram. The square planar coordination geometry with the alkynyl ligands was corroborated for complexes 1, 2, and 3 by single crystal X-ray diffraction studies. These complexes show tunable monomeric high energy triplet emission and an additional concentration-dependent low-energy excimer-based phosphorescence. While adopting weight percent concentrations between 15 and 25%, the two emission bands covering the entire visible spectrum were obtained with these particular complexes displaying the properties of an efficient white light triplet emitter with excellent CIE-1931 coordinates (0.31, 0.33). On the basis of the high luminescent quantum efficiency of over 50% for white light emission, these compounds could be potentially useful for white organic light-emitting diodes (WOLEDs) based applications

Syntheses and Tunable Emission Properties of 2-Alkynyl Azulenes

Various substituted 2-azulenes have been synthesized via Sonogashira coupling. Doping with superacids allows tunable emission from 443 to 750 nm depending on the substitution. The proton doped compounds are the first azulene alkyne based systems that show emission originating only from the S₁ excited state

Monocyclometalated Gold(III) Complexes Bearing π-Accepting Cyanide Ligands: Syntheses, Structural, Photophysical, and Electrochemical Investigations

The synthesis, structural, photophysical, and electrochemical investigations of a series of gold(III) monocyclometalated complexes bearing ancillary ligands with π-accepting properties is reported. Complexes of the type [(C∧N)Au(C≡N)2] [C∧N = 2-phenylpyridine (ppy) (1), 2-(p-tolyl)pyridine (tpy) (2), 2-(2-thienyl)-pyridine (thpy) (3), 2-(5-methyl-2-thienyl)pyridine (5m-thpy) (4), 1-phenylisoquinoline (piq) (5)], and [(N∧N)Au(C≡N)2] [N∧N = 3,5-bis(phenyl)-2-(2′-pyridyl)pyrrole (pyrpy) (6)] were prepared, and the influence of both the cyanide as an ancillary ligand as well as the different electronic properties of the cyclometalating ligands (1–5) and the chelating bidentate (6) on the triplet emission properties were studied. The physicochemical properties were evaluated by a variety of physical methods, and the structure of selected complexes was further confirmed by X-ray diffraction studies. Complexes 1–5 display long-lived emission in solution, neat solid, spin coated PMMA films, and at 77 K in 2-MeTHF. The emission energies were strongly dictated by the cyclometalating ligands independent of the cyanide ligand, which is in quite a contrast to the previously reported dicyano complexes of iridium(III) and the isoelectronic platinum(II) complexes. The nonemissive behavior of complex 6 in any medium further highlights the importance that the good σ-donating properties of the cyclometalating ligand alone is not decisive in rendering the gold complexes emissive, but also the appropriate placement of the energy level of the ligand orbitals is also important. Detailed photophysical studies in conjunction with density functional theory and time-dependent density functional theory calculations support the origin of the emission to be a metal perturbed intra ligand 3IL (π–π*) delocalized over the cyclometalating ligand. The stability of the complexes combined with good emission quantum yields and tunability of the emission energies makes these complexes suitable alternatives to the relatively less stable monocyclometalated gold(III) diaryl or dialkyne complexes for organic light emitting device applications