Single-molecule conductance studies on quasi- and metallaaromatic dibenzoylmethane coordination compounds and their aromatic analogs

The ability to predict the conductive behaviour of molecules, connected to macroscopic electrodes, represents a crucial prerequisite for the design of nanoscale electronic devices. In this work, we investigate whether the notion of a negative relation between conductance and aromaticity (the so-called NRCA rule) also pertains to quasi-aromatic and metallaaromatic chelates derived from dibenzoylmethane (DBM) and Lewis acids (LAs) that either do or do not contribute two extra dπ electrons to the central resonance-stabilised β-ketoenolate binding pocket. We therefore synthesised a family of methylthio-functionalised DBM coordination compounds and subjected them, along with their truly aromatic terphenyl and 4,6-diphenylpyrimidine congeners, to scanning tunneling microscope break-junction (STM-BJ) experiments on gold nanoelectrodes. All molecules share the common motif of three π-conjugated, six-membered, planar rings with a meta-configuration at the central ring. According to our results, their molecular conductances fall within a factor of ca. 9 in an ordering aromatic < metallaaromatic < quasi-aromatic. The experimental trends are rationalised by quantum transport calculations based on density functional theory (DFT)

Gauge to simultaneously determine the electrical conductivity, the Hall constant, and the Seebeck coefficient up to 800&thinsp;°C

A new high temperature gauge to simultaneously determine the electrical conductivity, the Hall constant, and the Seebeck coefficient has been developed. Screen-printed heating structures on a ceramic sample holder are used to generate temperatures up to 800 ∘C by Joule heating. The heating structures were designed using the finite element method (FEM) simulations and the temperature distribution was validated by thermal imaging. To measure the Seebeck coefficient, Au/Pt thermocouples with different geometries were investigated and successfully integrated into the gauge. Measurements on constantan, a typical Seebeck coefficient reference material with high electrical conductivity, high charge carrier concentration, and a known Seebeck coefficient, as well as on a well-described boron-doped silicon wafer confirm the functionality of the gauge up to 800 ∘C.</p

Enhanced control of self-doping in halide perovskites for improved thermoelectric performance

Metal halide perovskites have emerged as promising photovoltaic materials, but, despite ultralow thermal conductivity, progress on developing them for thermoelectrics has been limited. Here, we report the thermoelectric properties of all-inorganic tin based perovskites with enhanced air stability. Fine tuning the thermoelectric properties of the films is achieved by self-doping through the oxidation of tin (ΙΙ) to tin (ΙV) in a thin surface-layer that transfers charge to the bulk. This separates the doping defects from the transport region, enabling enhanced electrical conductivity. We show that this arises due to a chlorine-rich surface layer that acts simultaneously as the source of free charges and a sacrificial layer protecting the bulk from oxidation. Moreover, we achieve a figure-of-merit (ZT) of 0.14 ± 0.01 when chlorine-doping and degree of the oxidation are optimised in tandem

Synthesis and crystal structures of rhodium acetate paddle‐wheel complexes with anchor group‐functionalized and hydrogen bond‐supported axial ligands

We report the synthesis and X-ray structures of four Rh 2 (O 2 Ac) 4 (L Ax ) 2 (OAc = acteate, CH 3 COO - ) paddle-wheel complexes ( C1 - C4 ) with methylthio-modified axial ligands L Ax derived from benzamidine ( L1 ), anilinopyrimidine ( L2 ) or isothiourea ( L3 , L4 ) that are capable of forming N‑H×××O hydrogen bonds to the equatorially bridging acetate ligands. This was done with the aim to suppress dissociation of the axial ligands and to make the complexes amenable to single-molecule conductance measurements in a scanning tunneling microscope break-junction (STM-BJ) setup. The characteristic spectroscopic features (NMR, IR, vis), crystal structures, the hydrogen-bonding motifs and a DFT-based screening of their frontier molecular orbitals are presented. Our calculations suggest that hydrogen-bonding stabilizes axial ligand binding by 20kJ/mol to 30 kJ/mol and that the HOMO of the rhodium paddle-wheels is closer to the Au Fermi level than the LUMO, so that the rhodium paddle-wheels are expected to constitute hole conductors.publishe

Optimization of a sensor for a Tian–Calvet calorimeter with LTCC-based sensor discs

In this work, it is shown how a finite element method (FEM) model of a Tian–Calvet calorimeter is used to find improvements in the sensor design to increase the sensitivity of the calorimeter. By changing the layout of the basic part of the sensor, which is a low temperature co-fired ceramics (LTCC) based sensor disc, an improvement by a factor of 3 was achieved. The model was validated and the sensors were calibrated with a set of measurements that were later used to determine the melting enthalpies and melting temperatures of indium and tin samples. Melting temperatures showed a maximum deviation of 0.2 K while the enthalpy was measured with a precision better than 1 % for most samples. The values for tin deviate by less than 2 % from literature data

Macrocyclic Triruthenium Complexes Having Electronically Coupled Mixed-Valent States

5-Ethynyl-2-furancarboxylic acid and 3-ethynylbenzoic acid self-assemble with [HRu(CO)Cl(PiPr(3))(2)] to form macrocyclic C-3-symmetric triangular triruthenium alkenyl complexes [{Ru(CO)(PiPr(3))(2)(CH=CHArCOO)}(3)] (Ar=C6H4: 1-B, Ar=C4H2O: 1-F), which were characterized by multinuclear NMR spectroscopy, high-resolution ESI mass spectrometry, and, in the case of 1-B, by X-ray crystallography. Electrochemical studies indicate that the macrocycles are oxidized in three consecutive one-electron steps. The mixed-valent states obtained by electrochemical or chemical oxidation show signs of valence delocalization, which makes these complexes rare examples of molecule-based conductive loops with through-bond charge delocalization

Turning-On of Coumarin Phosphorescence in Acetylacetonato Platinum Complexes of Cyclometalated Pyridyl-Substituted Coumarins

Two pyridine-functionalized coumarins differing with respect to the site of pyridine attachment to the coumarin dye (3 in L1 or 7 in L2) and with respect to the presence (L1) or absence (L2) of a peripheral NMe2 donor were prepared and used as cyclometalating ligands towards the Pt(acac) fragment. X-ray crystal structures of complexes 1 and 2 show strong intermolecular interactions by π-stacking and short Pt∙∙∙Pt or C-H∙∙∙O hydrogen bonding that result in the formation of sheetlike packing patterns. The NMe2 donor substituent has a profound influence on the absorption and emission properties of the free coumarin dyes; L1 emits strongly while L2 is only weakly emissive. On binding to Pt(acac) the strong fluorescence of L1 is partially quenched while coumarin phosphorescence is observed from cyclometalated L1 and L2. The ligand-centered nature of the LUMO was confirmed by IR spectroelectrochemistry while the assignment of the phosphorescence emission as ligand-based rests on the vibrational structuring, the negligible solvatochromism, the small temperature-induced Stokes shifts on cooling to 77 K, the emission lifetimes, and strong oxygen quenching. (TD-)DFT calculations confirm our experimental results and provide an assignment of the electronic transitions and the spin density distributions in the T1 state

Tetraruthenium Metallamacrocycles with Potentially Coordinating Appended Functionalities

We present four new tetraruthenium macrocycles built from two 1,4-divinylphenylene diruthenium and two isophthalic acid building blocks with peripheral, potentially mono- or tridentate donor functions attached to the isophthalic linkers. These macrocycles are characterized by multinuclear NMR spectroscopy, mass spectrometry and, in the case of the thioacetyl-appended complex 4, by X-ray crystallography. Cyclic and square wave voltammetry establish that the macrocycles can be oxidized in four consecutive redox steps that come as two pairs of two closely spaced one-electron waves. Spectroscopic changes observed during IR and UV/Vis/NIR spectroelectrochemical experiments (NIR = near infrared) show that the isophthalate linkers insulate the electroactive divinylphenylene diruthenium moieties against each other. The macrocycles exhibit nevertheless pronounced polyelectrochromism with highly intense absorptions in the Vis (2+/4+ states) and the NIR (2+ states) with extinction coefficients of up to &gt;100,000 M&minus;1&middot;cm&minus;1. The strong absorptivity enhancement with respect to the individual divinylphenylene diruthenium building blocks is attributed to conformational restrictions imposed by the macrocycle backbone. Moreover, the di- and tetracations of these macrocycles are paramagnetic as revealed by EPR spectroscopy

Five shades of green : substituent influence on the (spectro-) electrochemical properties of diferrocenyl(phenyl)methylium dyes

Five new, intensely green diferrocenylphenylmethylium complexes 1+-5+ with electron donating (EDG: 4-MeO, 4-Me, 4-Br) or withdrawing (EWG: 3,5-CF3, 4-nC6F13) substituents were synthesized and fully characterized. The substituent influence on their electrochemical and spectroscopic properties was investigated by cyclic voltammetry, UV/Vis/NIR and T-dependent EPR spectroscopy of the cationic as well as the oxidized (12+-52+) and reduced (1˙-5˙) species. The reduced forms equilibrate with their corresponding dimers (65-83%) with a clear substituent influence as expressed by their Hammett parameters in an ordering 4+ > 5+ > 3+ > 2+ > 1+. The structures of all five precursor carbinols 1-OH-5-OH and those of three of the diferrocenylphenylmethylium cations (1+, 4+-5+) were established by X-ray crystallography.publishe