This contribution investigates the role of the metal-metal bond in paddle-wheel complexes for molecular conductance. To these ends, we compare two pairs of Mo and Rh paddle-wheel complexes M2(LN)4 and M2(LSMe)4 (M = Rh, Mo), each with four lateral 4-pyridyl-(LN-) or 4-(methylthio)-(LSMe-) functionalized benzamidinate ligands mutually disposed at 90° angles. These complexes represent bond orders of 4 (Mo) and 1 (Rh), while offering metal-based highest occupied frontier orbitals of the same δ symmetry. The structural features of the complexes were established by X-ray diffraction on single crystals. Molecular conductance measurements were performed with the aid of a scanning-tunnelling microscopy break-junction setup and revealed that the Mo complexes surpass their Rh congeners. Decoration of the paddle-wheel complexes with four laterally disposed anchor groups results in four different possible modes of molecule attachment to the Au electrodes with two, three or even four anchoring points. The conductances of possible junction geometries were assessed quantum chemically with the DFT+Σ approach using Au slab electrodes. Calculated variations by about one order of magnitude for the different anchoring geometries can explain the rather broad conductance distributions observed in our experiments. Further transport calculations considered two-point molecular attachment to sharp or blunt nanoelectrodes for mutual cis and trans dispositions of the anchor groups. Our computational results indicate that the better performance of the Mo complexes originates from superior conjugation between the Mo2 δ-binding and the ligand π orbitals as compared to the Rh2 δ* orbital, rather than from the larger metal-metal bond order. Upon increasing the bias voltage, we observed a new conductance feature associated with a nearly 100 times higher G value, which we ascribe tentatively to oxidation of the molecule inside the junction.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

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).

We report the synthesis and X-ray structures of four Rh 2 (OAc) 4 (L Ax ) 2 (OAc = acetate, CH 3 COO (cid:0) ) paddle-wheel complexes ( C1 (cid:0) C4 ) with methylthio-modified axial ligands L Ax derived from benzamidine ( L1 ), anilinopyrimidine ( L2 ) or isothiourea ( L3 , L4 ) that are capable of forming N (cid:0) 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 stabilises axial ligand binding by 15 kJ/mol to 30 kJ/mol and that the HOMO of the rhodium paddle-wheels is closer to the Au work function than the LUMO, so that the rhodium paddle-wheels are expected to constitute hole conductors.

We report on the photophysical properties of three dyads that combine a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (bodipy, BDP) and a mercaptopyrene (SPyr) dye ligand at a Pt(PEt3)2 fragment. σ-Bonding of the dyes to the Pt ion promotes intersystem crossing (ISC) via the external heavy atom effect. The coupling of efficient ISC with charge-transfer from the electron-rich mercaptopyrene to the electron-accepting BDP ligand (PB-CT) gives rise to a multitude of (potentially) emissive states. This culminates in the presence of four different emissions for the mono- and dinuclear complexes BPtSPyr and BPtSPyrSPtB with an unsubstituted BDP ligand and either a terminal 1-mercaptopyrene or a bridging pyrene-1,6-dithiolate ligand. Thus, in fluid solution, near IR emission at 724 nm from the 3PB-CT state is observed with a quantum yield of up to 15%. Excitation into the BDP-based 1ππ* or the pyrene-based 1ππ* band additionally trigger fluorescence and phosphorescence emissions from the BDP-centred 1ππ* and 3ππ* states. In frozen solution, at 77 K, phosphorescence from the pyrene ligand becomes the prominent emission channel, while PB-CT emission is absent. Alkylation of the BDP ligand in KBPtSPyr funnels all excitation energy into fluorescence and phosphorescence emissions from the KBDP ligand. The assignments of the various excited states and the deactivation cascades were probed by absorption and emission spectroscopy, transient absorption spectroscopy, electrochemical and UV/Vis/NIR spectroelectrochemical measurements, and by quantum chemical calculations. Our conclusions are further corroborated with the aid of suitable reference compounds comprising of just one chromophore. All dyads are triplet sensitizers and are able to generate singlet oxygen.

Ruthenocene-vinylruthenium conjugates Rc/Rc*-CH=CH-Ru(CO)(L)((PPr3)-Pr-i)(2) (Rc = (eta(5)-C5H5)Ru(eta(5)-C5H4); Rc* = (eta(5)-C5Me5)Ru(eta(5)-C5H4); L = Cl or kappa O,O'-acetylacetonato) have been prepared and investigated in their neutral, mono-, and dioxidized states by cyclic voltammetry, IR and UV/vis/NIR spectroelectrochemistry, and EPR spectroscopy. Their corresponding radical cations are (almost) completely delocalized mixed-valent systems as indicated by the low half-widths, the absence of solvatochromism, and the low-energy cutoff of their IVCT bands in the near-infrared (NIR) and their IR and EPR spectroscopic signatures. The degree of electronic coupling even exceeds that of their ferrocene analogs despite comparable differences between the intrinsic half-wave potentials of the vinylruthenium and the metallocenyl entities and substantially smaller half-wave potential splittings, Delta E-1/2, in the ruthenocene congeners. All experimental results are backed by quantum chemical calculations.

Surfactants are functional molecules comprising a water-compatible head group and a hydrophobic tail. One of their features is the formation of self-assembled structures in contact with water, for instance, micelles, vesicles, or lyotropic liquid crystals. One way to increase the functionality of surfactants is to implement moieties containing transition-metal species. Ferrocene-based surfactants represent an excellent example because of the distinguished redox features. In most existing ferrocene-based amphiphiles, an alkyl chain is classically used as the hydrophobic tail. We report the synthesis and properties of 1-triisopropylsilylethynyl-1'-trimethylammoniummethylferrocene (FcNMe3TIPS). In FcNMe3TIPS, ferrocene is part of the head group (Gemini design) but is also attached to a (protected) π-conjugated ethynyl group. Although this architecture differs from that of classical amphiphiles and those of other ferrocene-based amphiphiles, the compound shows marked surfactant properties comparable to those of lipids, exhibiting a very low value of critical aggregation concentration in water (cac = 0.03 mM). It forms classical micelles only in a very narrow concentration range, which then convert into monolayer vesicles. Unlike classical surfactants, aggregates already form at a very low concentration, far beneath that required for the formation of a monolayer at the air-water interface. At even higher concentration, FcNMe3TIPS forms lyotropic liquid crystals, not only in contact with water, but also in a variety of organic solvents. As an additional intriguing feature, FcNMe3TIPS is amenable to a range of further modification reactions. The TIPS group is easily cleaved, and the resulting ethynyl function can be used to construct heterobimetallic platinum-ferrocene conjugates with trans-Pt(PEt3)2X (X = Cl, I) complex entities, leading to a heterobimetallic surfactant. We also found that the benzylic α-position of FcNMe3TIPS is rather reactive and that the attached ammonium group can be exchanged by other substituents (e.g., -CN), which offers additional opportunities for further functionalization. Although FcNMe3TIPS is reversibly oxidized in voltammetric and UV-vis spectroelectrochemical experiments, the high reactivity at the α-position is also responsible for the instability of the corresponding ferrocenium ion, leading to a polymerization reaction.