Emission Tuning of Ir(N^∧C)₂(pic)-Based Complexes via Torsional Twisting of Picolinate Substituents

Pyridine-2-carboxylate (pic) has been employed extensively as a blue-shifting ancillary ligand in the production of cyclometalated iridium complexes used in OLEDs, but surprisingly, further elaboration of this ligand has largely been unexplored. In this work we demonstrate a simple and versatile route for modifying picolinate ligands coordinated to iridium. Reacting a μ-chloro iridium­(C∧N) dimer (where C∧N is a phenylpyridine-based ligand) with 4-bromopicolinic acid (HpicBr) yields the corresponding iridium­(C∧N)2(picBr) complexes, which were readily modified by a Suzuki–Miyaura reaction to give the corresponding aryl-substituted picolinate complexes. The luminescent behavior of these complexes shows that by restricting the torsional angle between the substituent and pic the emission can be shifted by up to 77 nm

Emission Tuning of Ir(N^∧C)₂(pic)-Based Complexes via Torsional Twisting of Picolinate Substituents

Pyridine-2-carboxylate (pic) has been employed extensively as a blue-shifting ancillary ligand in the production of cyclometalated iridium complexes used in OLEDs, but surprisingly, further elaboration of this ligand has largely been unexplored. In this work we demonstrate a simple and versatile route for modifying picolinate ligands coordinated to iridium. Reacting a μ-chloro iridium­(C^∧N) dimer (where C^∧N is a phenylpyridine-based ligand) with 4-bromopicolinic acid (HpicBr) yields the corresponding iridium­(C^∧N)₂(picBr) complexes, which were readily modified by a Suzuki–Miyaura reaction to give the corresponding aryl-substituted picolinate complexes. The luminescent behavior of these complexes shows that by restricting the torsional angle between the substituent and pic the emission can be shifted by up to 77 nm

Encapsulation of the Be^II Cation: Spectroscopic and Computational Study

The structures of a series of tetracoordinate beryllium­(II) complexes with ligands derived from tertiary-substituted amines have been computationally modeled and their 9Be magnetic shielding values determined using the gauge-including atomic orbital (GIAO) method at the 6-311++g­(2d,p) level. A good correlation was observed between calculated 9Be NMR chemical shifts when compared to experimental values in polar protic solvents, less so for the values recorded in polar aprotic solvents. A number of alternative complex structures were modeled, resulting in an improvement in experimental versus computational 9Be NMR chemical shifts, suggesting that in some cases full encapsulation on the beryllium atom was not occurring. Several of the synthesized complexes gave rise to unexpected fluorescence, and inspection of the calculated molecular orbital diagrams associated with the electronic transitions suggested that the rigidity imparted by the locking of certain conformations upon BeII coordination allowed delocalization across adjacent aligned aromatic rings bridged by BeII

Single-Molecule Junction Formation in Deep Eutectic Solvents with Highly Effective Gate Coupling

The environment surrounding a molecular junction affects its charge-transport properties and, therefore, must be chosen with care. In the case of measurements in liquid media, the solvent must provide good solvation, grant junction stability, and, in the case of electrolyte gating experiments, allow efficient electrical coupling to the gate electrodes through control of the electrical double layer. We evaluated in this study the deep eutectic solvent mixture (DES) ethaline, which is a mixture of choline chloride and ethylene glycol (1:2), for single-molecule junction fabrication with break-junction techniques. In ethaline, we were able to (i) measure challenging and poorly soluble molecular wires, exploiting the improved solvation capabilities offered by DESs, and (ii) efficiently apply an electrostatic gate able to modulate the conductance of the junction by approximately an order of magnitude within a ∼1 V potential window. The electrochemical gating results on a Au–VDP–Au junction follow exceptionally well the single-level modeling with strong gate coupling (where VDP is 1,2-di­(pyridine-4-yl)­ethene). Ethaline is also an ideal solvent for the measurement of very short molecular junctions, as it grants a greatly reduced snapback distance of the metallic electrodes upon point-contact rupture. Our work demonstrates that DESs are viable alternatives to often relatively expensive ionic liquids, offering good versatility for single-molecule electrical measurements

Metal−Metal Communication in Copper(II) Complexes of Cyclotetraphosphazene Ligands

Copper(II) chloride and bromide react with the pyridyloxy-substituted cyclotetraphosphazene ligands, octakis(2-pyridyloxy)cyclotetraphosphazene (L1), and octakis(4-methyl-2-pyridyloxy)cyclotetraphosphazene (L2), to form the dimetallic complexes, [L(CuX2)2] (L = L1, X = Br; L = L2, X = Cl or Br), and [{L1(CuCl2)2}n]. Single crystal X-ray crystallography shows the complex [{L1(CuCl2)2}n] to be a coordination polymer propagated by interligand “Cu(μ-Cl)2Cu” bridges whereas [L2(CuCl2)2] forms discrete dimetallic cyclotetraphosphazene-based moieties. The variable temperature magnetic susceptibility data for [{L1(CuCl2)2}n] are consistent with a weak antiferromagnetic exchange interaction between the copper(II) centers occurring via the bridging chloride ions. [L2(CuCl2)2] and [L(CuBr2)2] (L = L1 and L2) exhibit normal Curie-like susceptibilities. The abstraction of a chloride ion, using [Ag(MeCN)4](PF6), from each copper site in [L2(CuCl2)2], affords the new complex, [L2(CuCl)2](PF6)2, in which the two copper(II) ions are separated by “NPNPN” phosphazene bridges. Electron spin resonance and variable temperature magnetic measurements indicate the occurrence of weak antiferromagnetic coupling between the unpaired electrons on the copper(II) centers. Density Functional Theory (DFT) calculations on the [L2(CuCl)2]2+ dication and the related cyclotriphosphazene complex, [L4(CuCl2)2] (L4 = hexakis(4-methyl-2-pyridyloxy)cyclotriphosphazene), have identified “electron-density-bridge” molecular orbitals which involve Cu 3d orbitals overlapping with the non-bonding N-based molecular orbitals on the phosphazene rings as the pathway for this interaction

Metal−Metal Communication in Copper(II) Complexes of Cyclotetraphosphazene Ligands

Copper(II) chloride and bromide react with the pyridyloxy-substituted cyclotetraphosphazene ligands, octakis(2-pyridyloxy)cyclotetraphosphazene (L1), and octakis(4-methyl-2-pyridyloxy)cyclotetraphosphazene (L2), to form the dimetallic complexes, [L(CuX2)2] (L = L1, X = Br; L = L2, X = Cl or Br), and [{L1(CuCl2)2}n]. Single crystal X-ray crystallography shows the complex [{L1(CuCl2)2}n] to be a coordination polymer propagated by interligand “Cu(μ-Cl)2Cu” bridges whereas [L2(CuCl2)2] forms discrete dimetallic cyclotetraphosphazene-based moieties. The variable temperature magnetic susceptibility data for [{L1(CuCl2)2}n] are consistent with a weak antiferromagnetic exchange interaction between the copper(II) centers occurring via the bridging chloride ions. [L2(CuCl2)2] and [L(CuBr2)2] (L = L1 and L2) exhibit normal Curie-like susceptibilities. The abstraction of a chloride ion, using [Ag(MeCN)4](PF6), from each copper site in [L2(CuCl2)2], affords the new complex, [L2(CuCl)2](PF6)2, in which the two copper(II) ions are separated by “NPNPN” phosphazene bridges. Electron spin resonance and variable temperature magnetic measurements indicate the occurrence of weak antiferromagnetic coupling between the unpaired electrons on the copper(II) centers. Density Functional Theory (DFT) calculations on the [L2(CuCl)2]2+ dication and the related cyclotriphosphazene complex, [L4(CuCl2)2] (L4 = hexakis(4-methyl-2-pyridyloxy)cyclotriphosphazene), have identified “electron-density-bridge” molecular orbitals which involve Cu 3d orbitals overlapping with the non-bonding N-based molecular orbitals on the phosphazene rings as the pathway for this interaction

Connectivity-Dependent Conductance of 2,2′-Bipyridine-Based Metal Complexes

The present work provides an insight into the effect of connectivity isomerization of metal-2,2′-bipyridine complexes. For that purpose, two new 2,2′-bipyridine (bpy) ligand systems, 4,4′-bis(4-(methylthio)phenyl)-2,2′-bipyridine (Lmeta) and 5,5′-bis(3,3-dimethyl-2,3-dihydrobenzothiophen-5-yl)-2,2′-bipyridine (Lpara) were synthesized and coordinated to rhenium and manganese to obtain the corresponding complexes MnLmeta(CO)3Br, ReLmeta(CO)3Br, MnLpara(CO)3Br, MoLpara(CO)4 and ReLpara(CO)3Br. The experimental and theoretical results revealed that coordination to the para system, i.e., the metal ion peripheral to the conductance path, gave a slightly increased conductance compared to the free ligand attributed to the reduced highest occupied molecular orbital (HOMO)–least unoccupied molecular orbital (LUMO) gap. The meta-based system formed a destructive quantum interference feature that reduced the conductance of a S···S contacted junction to below 10–5.5 Go, reinforcing the importance of contact group connectivity for molecular wire conductance

Connectivity-Dependent Conductance of 2,2′-Bipyridine-Based Metal Complexes

The present work provides an insight into the effect of connectivity isomerization of metal-2,2′-bipyridine complexes. For that purpose, two new 2,2′-bipyridine (bpy) ligand systems, 4,4′-bis(4-(methylthio)phenyl)-2,2′-bipyridine (Lmeta) and 5,5′-bis(3,3-dimethyl-2,3-dihydrobenzothiophen-5-yl)-2,2′-bipyridine (Lpara) were synthesized and coordinated to rhenium and manganese to obtain the corresponding complexes MnLmeta(CO)3Br, ReLmeta(CO)3Br, MnLpara(CO)3Br, MoLpara(CO)4 and ReLpara(CO)3Br. The experimental and theoretical results revealed that coordination to the para system, i.e., the metal ion peripheral to the conductance path, gave a slightly increased conductance compared to the free ligand attributed to the reduced highest occupied molecular orbital (HOMO)–least unoccupied molecular orbital (LUMO) gap. The meta-based system formed a destructive quantum interference feature that reduced the conductance of a S···S contacted junction to below 10–5.5 Go, reinforcing the importance of contact group connectivity for molecular wire conductance

Conductance Behavior of Tetraphenyl-Aza-BODIPYs

We studied the electrical conductance of single-molecule junctions formed from molecular wires with four anchor groups. Three tetraphenyl-aza-BODIPYs with four or two thiomethyl anchor groups were synthesized, and their single-molecule conductance was measured using break-junction-STM. Using DFT based calculations these compounds were shown to display a combination of a high and low conductance, depending on the molecule’s connectivity in the junction. A scissor correction is employed to obtain the corrected HOMO–LUMO gaps and a tight binding model (TBM) is used to highlight the role of transport through the pi system of the tetraphenyl-aza-BODIPY central unit. The three higher-conductance geometries follow the sequence 3 > 4 > 2, which demonstrates that their conductances are correlated with the number of anchors

Connectivity-Dependent Conductance of 2,2′-Bipyridine-Based Metal Complexes

The present work provides an insight into the effect of connectivity isomerization of metal-2,2′-bipyridine complexes. For that purpose, two new 2,2′-bipyridine (bpy) ligand systems, 4,4′-bis(4-(methylthio)phenyl)-2,2′-bipyridine (Lmeta) and 5,5′-bis(3,3-dimethyl-2,3-dihydrobenzothiophen-5-yl)-2,2′-bipyridine (Lpara) were synthesized and coordinated to rhenium and manganese to obtain the corresponding complexes MnLmeta(CO)3Br, ReLmeta(CO)3Br, MnLpara(CO)3Br, MoLpara(CO)4 and ReLpara(CO)3Br. The experimental and theoretical results revealed that coordination to the para system, i.e., the metal ion peripheral to the conductance path, gave a slightly increased conductance compared to the free ligand attributed to the reduced highest occupied molecular orbital (HOMO)–least unoccupied molecular orbital (LUMO) gap. The meta-based system formed a destructive quantum interference feature that reduced the conductance of a S···S contacted junction to below 10–5.5 Go, reinforcing the importance of contact group connectivity for molecular wire conductance