Integrated Circular Dichroism and Circularly Polarized Luminescence Measurements.

Circularly polarized luminescent (CPL) systems have a plethora of potential applications owing to their interesting excited-state properties. However, the progress in developing new chiral luminescence systems is significantly hindered by the lack of available instrumentation for the broader chemistry and materials science community to perform routine, reproducible measurements of chiral spectroscopies. In this work, we present data from an easy-to-use custom-built instrument based on a Jasco circular dichroism (CD) spectropolarimeter coupled with a CPL emission monochromator (CD/CPL hybrid system). The hybrid system measures CPL, fluorescence, CD, and absorbance on the same part of the sample without the need to move between the CD and CPL measurements. The instrument uses a xenon arc lamp as the light source, enabling a wide range of excitation wavelengths to support flexible development of new molecules and materials. Data obtained and presented for camphor, ruthenium metal complexes, the peptide gramicidin, and a DNA-ligand (4',6-diamidino-2-phenylindole, DAPI) system in this work highlight the ease of use and reproducibility of the results. The g-factors for CD and CPL obtained for the different compounds are shown to be the same for isolated transitions and some examples of how to use variations of g-factors with wavelength are demonstrated. The reliable and excellent benchmark results obtained from a custom-built commercial wavelength scanning CPL/CD hybrid instrument open up new avenues for the broader chemical and materials science community to intensify research on chiral luminescent systems.

Metallic nanoparticle contacts for high-yield, ambient-stable molecular-monolayer devices.

Accessing the intrinsic functionality of molecules for electronic applications1-3, light emission4 or sensing5 requires reliable electrical contacts to those molecules. A self-assembled monolayer (SAM) sandwich architecture6 is advantageous for technological applications, but requires a non-destructive, top-contact fabrication method. Various approaches ranging from direct metal evaporation6 over poly(3,4-ethylenedioxythiophene) polystyrene sulfonate7 (PEDOT:PSS) or graphene8 interlayers to metal transfer printing9 have been proposed. Nevertheless, it has not yet been possible to fabricate SAM-based devices without compromising film integrity, intrinsic functionality or mass-fabrication compatibility. Here we develop a top-contact approach to SAM-based devices that simultaneously addresses all these issues, by exploiting the fact that a metallic nanoparticle can provide a reliable electrical contact to individual molecules10. Our fabrication route involves first the conformal and non-destructive deposition of a layer of metallic nanoparticles directly onto the SAM (itself laterally constrained within circular pores in a dielectric matrix, with diameters ranging from 60 nanometres to 70 micrometres), and then the reinforcement of this top contact by direct metal evaporation. This approach enables the fabrication of thousands of identical, ambient-stable metal-molecule-metal devices. Systematic variation of the composition of the SAM demonstrates that the intrinsic molecular properties are not affected by the nanoparticle layer and subsequent top metallization. Our concept is generic to densely packed layers of molecules equipped with two anchor groups, and provides a route to the large-scale integration of molecular compounds into solid-state devices that can be scaled down to the single-molecule level.

Monocyclometalated (C

Invited for the cover of this issue are Koushik Venkatesan and co-workers at Macquarie University and the University of Zurich. The image depicts the conversion of 3 O2 to 1 O2 upon photoexcitation by new monocyclometalated gold(III) metallacycles. Read the full text of the article at 10.1002/chem.202102331.

Monocyclometalated (C N) Gold(III) Metallacycles: Tunable Emission and Singlet Oxygen (1 O2 ) 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 (λPL =493 nm) to red (λPL =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 (τ0 =16-258 µs) was found in solution and in thin films. The complexes showed remarkable singlet oxygen generation in aerated solution with absolute singlet oxygen quantum yield (ϕ1Δ ) values reaching up to 7.5×10-5 and singlet oxygen lifetimes (τ0 1Δ ) in the range of 66-95 µs. Furthermore, the radiative and non-radiative rates of singlet oxygen were determined using the ϕ1Δ and τ0 1Δ values and correlations are drawn between the formation of singlet oxygen and its interaction with cyclometalated (C N) gold(III) complexes.

Tunable Light-Emission Properties of Solution-Processable N-Heterocyclic Carbene Cyclometalated Gold(III) Complexes for Organic Light-Emitting Diodes.

N-Heterocyclic carbene (NHC) cyclometalated gold(III) complexes remain very scarce and therefore their photophysical properties remain currently underexplored. Moreover, gold(III) complexes emitting in the blue region of the electromagnetic spectrum are rare. In this work, a series of four phosphorescent gold(III) complexes was investigated bearing four different NHC monocyclometalated (C^C*)-type ligands and a dianionic (N^N)-type ancillary ligand ((N^N)=5,5'-(propane-2,2-diyl)bis(3-(trifluoromethyl)-1 H-pyrazole) (mepzH2 )). The complexes exhibit strong phosphorescence when doped in poly(methyl methacrylate) (PMMA) at room temperature, which were systematically tuned from sky-blue [λPL =456 nm, CIE coordinates: (0.20, 034)] to green [λPL =516 nm, CIE coordinates: (0.31, 0.54)] by varying the monocyclometalated (C^C*) ligand framework. The complexes revealed high quantum efficiencies (ϕPL ) of up to 43 % and excited-state lifetimes (τ0 ) between 15-266 µs. The radiative rate constant values found for these complexes (kr =103 -104  s-1 ) are the highest found in comparison to previously known best-performing monocyclometalated gold(III) complexes. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations of these complexes further lend support to the excited-state nature of these complexes. The calculations showed a significant contribution of the gold(III) metal center in the lowest unoccupied molecular orbitals (LUMOs) of up to 18 %, which was found to be unique for this class of cyclometalated gold(III) complexes. Additionally, organic light-emitting diodes (OLEDs) were fabricated by using a solution process to provide the first insight into the electroluminescent (EL) properties of this new class of gold(III) complexes.

Thermally Robust and Tuneable Phosphorescent Gold(III) Complexes Bearing (N

Phosphorescent mono-cyclometalated gold(III) complexes and their possible applications in organic light emitting diodes (OLEDs) can be significantly enhanced with their improved thermal stability by suppressing the reductive elimination of the respective ancillary ligands. A rational tuning of the π-conjugation of the cyclometalating ligand in conjunction with the non-conjugated 5,5'-(1-methylethylidene)bis(3-trifluoromethyl)-1H-pyrazole were used as a strategy to achieve room-temperature phosphorescence emission in a new series of gold(III) complexes. Photophysical studies of the newly synthesised and characterised complexes revealed phosphorescent emission of the complexes at room temperature in solution, thin films when doped in poly(methyl methacrylate) (PMMA) as well as in 2-Me-THF at 77 K. The complexes exhibit highly tuneable emission behaviour with photoluminescent quantum efficiencies up to 22 % and excited state lifetimes in the range of 63-300 µs. Detailed photophysical investigations in combination with DFT and TD-DFT calculations support the conclusion that the emission properties are strongly dictated by both the cyclometalating ligand and the ancillary chelating ligand. Thermogravimetric studies further show that the thermal stability of the AuIII complexes has been drastically enhanced, making these complexes more attractive for OLED applications.

Highly Stable and Strongly Emitting N-Heterocyclic Carbene Platinum(II) Biaryl Complexes.

C^C cyclometalated platinum(II) triplet emitters bearing electronically different N-heterocyclic carbenes-(1,3-diisopropyl-4-(trifluoromethyl)-imidazol-2-ylidene (d), 1,3-diisopropyl-benzimidazol-2-ylidene (e), and 1,3-diisopropyl-imidazol-2-ylidene (f))-as neutral ligands and biphenyl (bph) as well as its fluorinated derivative octafluorobiphenyl (oFbph) as dianionic cyclometalating ancillary ligand were synthesized and structurally characterized by 1H, 13C, 19F, and 195Pt NMR, single crystal X-ray diffraction, and HR-ESI-MS studies. Detailed photophysical investigations carried out reveal a strong influence on the excited-state properties exerted by the electronic nature of the N-heterocyclic carbenes as well as the fluorine functional groups on the ancillary biphenyl moiety. The solid-state structures of all complexes reveal a nearly planar and slightly distorted square planar geometry around the platinum center. Introduction of fluorine groups into the ligand framework leads to a less structured emission centered at 513 nm in poly(methyl methacrylate) (PMMA) thin films, compared to the highly structured emission profile of the bph analogues. Additionally, a hypsochromic shift of approximately 10-12 nm was found in the absorption as well as in the emission profiles and is attributed to the electron deficient nature of the oFbph ligand. Three wt % of the compounds doped in PMMA exhibit photoluminescence efficiencies as high as 92% in thin films. DFT and TD-DFT calculations on selected molecules revealed the charge transfer to be an admixture of intraligand (3ILCT) and metal-to-ligand charge transfer (3MLCT) and the frontier orbitals corresponding to the emission to be mainly localized on the bph and oFbph ligands, which is consistent with the observations from the photophysical investigations. The thermal stability of the complexes evaluated by thermogravimetric analysis (TGA) shows an enhanced thermal stability for the complexes bearing fluorine functional groups.

Harnessing White-Light Luminescence via Tunable Singlet-and Triplet-Derived Emissions Based on Gold(III) Complexes.

White light emitting gold(III) complexes were synthesized by tuning the percentage of metal dπ contribution in the charge transfer. This was achieved through specific tailoring of the ligand scaffold, which led to increase in the HOMO π-energy properties, resulting in a decrease of efficiency on the intersystem crossing (ISC). As a consequence, monomolecular based singlet- and triplet-derived emission covering the entire visible spectrum with quantum yield up to 28 % and CIE-1931 chromaticity coordinates of (0.29, 0.33) to (0.32, 0.40) could be obtained. Furthermore, two complexes displayed excitation-dependent emission property due to hyper-ISC allowing the regulation of the ratio between fluorescence versus phosphorescence intensity and accomplish precise tuning of white light emission.

Rationally Designed Blue Triplet Emitting Gold(III) Complexes Based on a Phenylpyridine-Derived Framework.

A series of blue-emitting phosphorescent mono-cyclometalated AuIII complexes have been successfully synthesized. Tailoring the substitutions on the phenylpyridine (ppy) ligand scaffold with electron-withdrawing fluorine groups on the phenyl ring to achieve stabilization of the HOMO and an electron-donating dimethylamino group on the pyridine ring to destabilize the LUMO resulted in a large energy gap and bestowed on the gold(III) complexes high-energy emission and high quantum efficiencies. The results of cyclic voltammetry studies suggested a predominantly redox event localized on the cyclometalated ligand. Thermogravimetric analysis of selected complexes revealed a high stability up to 280 °C, thus the complexes are suitable for device fabrication through vacuum-deposition. Photophysical investigations performed on all the derivatives revealed phosphorescence emission in neat solid, solution, doped in poly(methyl methacrylate) (PMMA) films at room temperature as well as in rigidified glass media (2-MeTHF) at 77 K. A high photoluminescent quantum efficiency of 28 % was obtained for a complex in PMMA, the highest quantum yield reported for a blue-emitting gold(III) complex.

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.

Charge Transport and Conductance Switching of Redox-Active Azulene Derivatives.

Azulene (Az) is a non-alternating, aromatic hydrocarbon composed of a five-membered, electron-rich and a seven-membered, electron-poor ring; an electron distribution that provides intrinsic redox activity. By varying the attachment points of the two electrode-bridging substituents to the Az center, the influence of the redox functionality on charge transport is evaluated. The conductance of the 1,3 Az derivative is at least one order of magnitude lower than those of the 2,6 Az and 4,7 Az derivatives, in agreement with density functional theory (DFT) calculations. In addition, only 1,3 Az exhibits pronounced nonlinear current-voltage characteristics with hysteresis, indicating a bias-dependent conductance switching. DFT identifies the LUMO to be nearest to the Fermi energy of the electrodes, but to be an active transport channel only in the case of the 2,6 and the 4,7 Az derivatives, whereas the 1,3 Az derivative uses the HOMO at low and the LUMO+1 at high bias. In return, the localized, weakly coupled LUMO of 1,3 Az creates a slow electron-hopping channel responsible for the voltage-induced switching due to the occupation of a single molecular orbital (MO).

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 (3)IL (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.

High-conductive organometallic molecular wires with delocalized electron systems strongly coupled to metal electrodes.

Besides active, functional molecular building blocks such as diodes or switches, passive components, for example, molecular wires, are required to realize molecular-scale electronics. Incorporating metal centers in the molecular backbone enables the molecular energy levels to be tuned in respect to the Fermi energy of the electrodes. Furthermore, by using more than one metal center and sp-bridging ligands, a strongly delocalized electron system is formed between these metallic "dopants", facilitating transport along the molecular backbone. Here, we study the influence of molecule-metal coupling on charge transport of dinuclear X(PP)2FeC4Fe(PP)2X molecular wires (PP = Et2PCH2CH2PEt2); X = CN (1), NCS (2), NCSe (3), C4SnMe3 (4), and C2SnMe3 (5) under ultrahigh vacuum and variable temperature conditions. In contrast to 1, which showed unstable junctions at very low conductance (8.1 × 10(-7) G0), 4 formed a Au-C4FeC4FeC4-Au junction 4' after SnMe3 extrusion, which revealed a conductance of 8.9 × 10(-3) G0, 3 orders of magnitude higher than for 2 (7.9 × 10(-6) G0) and 2 orders of magnitude higher than for 3 (3.8 × 10(-4) G0). Density functional theory (DFT) confirmed the experimental trend in the conductance for the various anchoring motifs. The strong hybridization of molecular and metal states found in the C-Au coupling case enables the delocalized electronic system of the organometallic Fe2 backbone to be extended over the molecule-metal interfaces to the metal electrodes to establish high-conductive molecular wires.

Organometallic single-molecule electronics: tuning electron transport through X(diphosphine)2FeC4Fe(diphosphine)2X building blocks by varying the Fe-X-Au anchoring scheme from coordinative to covalent.

A series of X(depe)2FeC≡C-C≡CFe(depe)2X complexes (depe =1,2-bis(diethylphosphino)ethane; X = I 1, NCMe 2, N2 3, C2H 4, C2SnMe3 5, C4SnMe3 6, NCSe 7, NCS 8, CN 9, SH 10, and NO2 11) was designed to study the influence of the anchor group on organometallic molecular transport junctions to achieve high-conductive molecular wires. The FeC4Fe core is electronically functional due to the redox-active Fe centers and sp-bridging ligands allowing a strong electronic delocalization. 1-11 were characterized by elemental analyses, X-ray diffraction, cyclic voltammetry, NMR, IR, and Raman spectroscopy. DFT calculations on model compounds gave the HOMO/LUMO energies. 5-9 were investigated in mechanically controllable break-junctions. For 9, unincisive features at 8.1 × 10(-7) G0 indicate that sterical reasons prevent stable junctions to form or that the coordinative binding motif prohibits electron injection. 7 and 8 with the hitherto unexploited coordinatively binding end groups NCSe and NCS yielded currents of 1.3 × 10(-9) A (7) and 1.8 × 10(-10) A (8) at ±1.0 V. The SnMe3 in 5 and 6 splits off, yielding junctions with covalent C-Au bonds and currents of 6.5 × 10(-7) A (Au-5'-Au) or 2.1 × 10(-7) A (Au-6'-Au). Despite of a length of almost 2 nm, the Au-5'-Au junction reaches 1% of the maximum current assuming one conductance channel in quantum point contacts. Additionally, the current noise in the transport data is considerably reduced for the covalent C-Au coupling compared to the coordinative anchoring of 7-9, endorsing C-Au coupled organometallic complexes as excellent candidates for low-ohmic molecular wires.

Monocyclometalated gold(III) monoaryl complexes--a new class of triplet phosphors with highly tunable and efficient emission properties.

Highly tunable and rich phosphorescent emission properties based on the stable monocyclometalated gold(III) monoaryl structural motif are reported. Monochloro complexes of the type cis-[(N^C)Au(C6 H2 (CF3)3)(Cl)] N^C=2-phenylpyridine (ppy)] (1), [N^C=benzo[h]quinoline (bzq)] (2), [N^C=2-(5-Methyl-2-thienyl)pyridine (5m-thpy)] (3) were successfully prepared in modest to good yields by reacting an excess of 2, 4, 6-tris(trifluoromethyl)phenyl lithium (LiFmes) with the corresponding dichloride complexes cis-[(N^C)AuCl2]. Subsequent replacement of the chloride ligand in 1 with strong ligand field strength such as cyanide and terminal alkynes resulted in complexes of the type cis-[(ppy)Au(Fmes)(R)] R=CN (4), I (5), C≡C-C6 H5 (6) and C≡C-C6 H4 N(C6 H5)-p (7). Single crystal X-ray diffraction studies of all the complexes except 7 were performed to further corroborate their chemical identity. Thermogravimetric analysis (TGA) studies of the uncommon cis configured aryl alkyne complex 7 confirmed the high stability of this complex. Detailed photophysical investigations carried out in solution at room temperature, at 77 K (2-MeTHF) in rigidified media, solid state and 5 wt % PMMA revealed the phosphorescent nature of emission in these complexes. Additionally, their behavior was found to be governed based on both the nature of the cyclometalated ligand and the electronic properties of the ancillary ligands. Highly efficient interligand charge transfer in complex 7 provides access to a wide range of emission colors (solvent-dependent) from deep blue to red with phosphorescence emission quantum yield of 30 % (441 nm) and 39 % (622 nm) in solution and solid state, respectively, and is the highest reported for any Au(III) complexes. DFT and TDDFT calculations carried out further validated the observations and assignments based on the photophysical experimental findings.

Tuning the luminescent properties of Pt(II) acetylide complexes through varying the electronic properties of N-heterocyclic carbene ligands.

This Article reports the synthesis, structural characterization, electrochemistry, and photophysical investigations of five groups of luminescent Pt(II) alkynyl complexes bearing N-heterocyclic carbene (NHC) ligands with varying electronic properties. Complexes of the type [Pt(pmdb)(C≡CR)2] 1a-c, [Pt(pm2tz)(C≡CR)2] 2a-d, [Pt(pm3tz)(C≡CR)2] 3a-c, [Pt(ppim)(C≡CR)2] 4(a, b, e), and [Pt(ppbim)(C≡CR)2] 5(a, b, e), where pmdb =1,1'-dipentyl-3,3'-methylene-dibenzimidazoline-2,2'-diylidene, pm2tz = 1,1'-dipentyl-3,3'-methylene-di-1,2,4-triazoline-5,5'-diylidene, pm3tz = 1,1'-dipentyl-3,3'-methylene-di-1,3,4-triazoline-5,5'-diylidene, ppim = 3-pentyl-1-picolylimidazoline-2-ylidene, and ppbim = 3-pentyl-1-picolylbenzimidazoline-2-ylidene, and R = 4-C6H4F, C6H5, 4-C6H4OMe, SiMe3, and 4-C6H4N(C6H5)2, were prepared, and the consequences of the electronic properties of the NHC ligands on the phosphorescent emission efficiencies were studied. Moreover, the emission quantum efficiencies of the previously reported complexes [Pt(pmim)(C≡CR)2] where pmim = 1,1'-dipentyl-3,3'-methylene-diimidazoline-2,2'-diylidene and R = 4-C6H4F 6a, C6H5 6b, and 4-C6H4OMe 6c were also recorded in neat solid and in 10 wt % PMMA film. The square planar coordination geometry with the alkynyl ligands in cis configuration was corroborated for selected complexes by single crystal X-ray diffraction studies. The observed moderate difference in emission efficiencies of the bis-carbene complexes 6a-c, 1a-c, 2a-c, and 3a-c in conjunction with the decreasing electron-donating nature of the NHC ligands, pmim > pmdb > pm2tz ≈ pm3tz, can be attributed to the slight modification of the triplet emission parentage among the different complexes. The quantum efficiencies of complexes 4(a, b) and 5(a, b) bearing pyridyl-NHC ligand were significantly low in comparison to the bis-carbene complexes owing to the significant change in the charge transfer character of the triplet manifold. Complexes 4e and 5e bearing diarylamine phenylacetylenes display high ϕem of 27% and 33% in 10 wt % PMMA film, respectively.

Metal-free triplet phosphors with high emission efficiency and high tunability.

Design of highly efficient phosphorescent emitters based on metal- and heavy atom-free boron compounds has been demonstrated by taking advantage of the singlet fission process. The combination of a suitable molecular scaffold and appropriate electronic nature of the substituents has been utilized to tailor the phosphorescence emission properties in solution, neat solid, and in doped PMMA thin films.

Stepwise construction of an iron-substituted rigid-rod molecular wire: targeting a tetraferra-tetracosa-decayne.

trans-Fe(depe)2I2 (depe =1,2-bis(diethylphosphino)ethane) was employed to stepwise incorporate Fe(II) centers into a rigid-rod butadiyne based 5,10,15,20-tetraferratetracosa-1,3,6,8,11,13,16,18,21,23-decayne. The iterative synthesis first connects two Fe(II) centers via a central butadiynediyl ligand to provide I-Fe(depe)2-C4-Fe(depe)2-I (2), then extends the system by substituting the terminal halides of 2 to yield Me3SiC4-Fe(depe)2-C4-Fe(depe)2-C4SiMe3 (3). Further modification of the termini gives the deprotected and stannylated compounds RC4-Fe(depe)2-C4-Fe(depe)2-C4R (4 and 5; R = H, Sn(CH3)3, respectively). Transmetalation with two more mononuclear units furnishes the homometallic tetranuclear compound I-Fe(depe)2-C4-Fe(depe)2-C4-Fe(depe)2-C4-Fe(depe)2-I (6), to which two more butadiynyl units were attached to give Me3SiC4-Fe(depe)2-C4-Fe(depe)2-C4-Fe(depe)2-C4-Fe(depe)2-C4SiMe3 (7). All compounds were characterized by NMR, IR, and Raman spectroscopies and by elemental analyses. X-ray diffraction studies were carried out on the dinuclear complexes revealing highly symmetrical rigid-rod structures. Cyclic voltammetric studies showed that compounds 2-7 undergo reversible and well-defined oxidations with high Kc values indicating thermodynamically stable mixed valence species. While the number of the oxidation waves of compounds 2, 6, and 7 are equivalent to the number of metal centers, the dinuclear complexes 3, 4, and 5 exhibit three reversible oxidation waves, one at significantly more positive potential. Two redox waves were attributed to the oxidation of the metal centers, while the remaining one is due to the oxidation of the butadiynediyl ligand. The electronic properties of complexes 2, 3, and 7 were investigated by spectroelectrochemical measurements.

Highly efficient deep-blue emitters based on cis and trans N-heterocyclic carbene Pt(II) acetylide complexes: synthesis, photophysical properties, and mechanistic studies.

We have synthesized cis and trans N-heterocyclic carbene (NHC) platinum(II) complexes bearing σ-alkynyl ancillary ligands, namely [Pt(dbim)2 (CCR)2 ] [DBIM=N,N'-didodecylbenzimidazoline-2-ylidene; R=C6 H4 F (4), C6 H5 (5), C6 H2 (OMe)3 (6), C4 H3 S (7), and C6 H4 CCC6 H5 (8)] and [Pt(ibim)2 (CCC6 H5 )2 ] (9) (ibim=N,N'-diisopropylbenzimidazoline-2-ylidene), starting from [Pt(cod)(CCR)2 ] (COD=cyclooctadiene) and 2 equivalents of [dbimH]Br ([ibimH]Br for complexes 9) in the presence of tBuOK and THF. Mechanistic investigations aimed at uncovering the cis to trans isomerization reaction have been performed on the representative cis complex 5 a [Pt(dbim)2 (CCC6 H5 )2 ] and revealed the isomerization to progress smoothly in good yield when 5 a was treated with catalytic amounts of [Pt(cod)(CCR)2 ] at 75 °C in THF or when 5 a was heated at 200 °C in the solid state under an inert atmosphere. Detailed examination of the reactions points to the possible involvement, in a catalytic fashion, of a solvent-stabilized Pt(II) dialkyne complex in the former case and a Pt(0) NHC complex in the latter case, for the transformation of the cis isomer to the corresponding trans complex. Thermal stability and the isomerization process in the solid state have been further investigated on the basis of TGA and DSC measurements. X-ray diffraction studies have been carried out to confirm the solid-state structures of 4 b, 5 a, 5 b, and 9 b. All of the synthesized dialkyne complexes 4-9 exhibit phosphorescence in solution, in the solid state at room temperature (RT), and also in frozen solvent glasses at 77 K. The emission wavelengths and quantum yields have been found to be highly tunable as a function of the alkynyl ligand. In particular, the trans isomer of complex 9 in a spin-coated film (10 wt % in poly(methyl methacrylate), PMMA) exhibits a high phosphorescence quantum yield of 80 %, which is the highest reported for Pt(II) -based deep-blue emitters. Experimental observations and time-dependent density functional theory (TD-DFT) calculations are strongly indicative of the emission being mainly governed by metal-perturbed interligand ((3) IL) charge transfer.

New Approaches to Stretched Film Sample Alignment and Data Collection for Vibrational Linear Dichroism.

Rapid measurements of vibrational linear dichroism (VLD) infrared spectra are shown to be possible by using stretched polymer films and an extension of existing instrumentation designed for vibrational circular dichroism spectroscopy. Earlier techniques can be extended using additional inexpensive polymer substrates to record good-quality VLD spectra of a significantly wider range of compounds with comparatively short sample-preparation times. The polymer substrates used, polyethylene and polytetrafluoroethylene, are commonly available and inexpensive, and samples are more easily prepared than that for many earlier stretched-film and crystal studies. Data are presented for neutral hydrophobic organic molecules on hydrophobic films including acridine, anthracene, fluorene, and recently synthesized S-(4-((4-cyanophenyl)ethynyl)phenyl)ethanethioate. We extend the approach to polar or ionic species, including 2,2'-bipyridine, 1,10-phenanthroline, and sodium dodecyl sulfate, by oxidizing polyethylene films to change their wetting properties. The combination of new instrumentation and modified sample preparation methods is useful in basic spectroscopy for untangling and assigning complicated infrared spectra. Nevertheless, it is not a panacea as surface-adsorbed molecules are often not monodispersed, and higher analyte concentrations can lead to aggregation and resonance phenomena that have previously been observed for infrared spectra on surfaces. These effects can be assessed by varying the sample concentration. The focus of this paper is experimental, and detailed analysis of most of the spectra lies outside its scope, including some well-studied compounds such as acridine and anthracene that allow comparisons with earlier research.