Markrete Krikorian

Columnar Liquid Crystallinity and Mechanochromism in Cationic Platinum(II) Complexes

Cationic square planar Pt(II) complexes are reported with high degrees of intermolecular association. These complexes display thermotropic columnar liquid crystalline behavior in spite of having only a single side chain. Crystals undergo mechanochromic transformations that can be reversed with solvent.

A Single-Molecule Potentiometer

Controlling electron transport through a single-molecule device is key to the realization of nanoscale electronic components. A design requirement for single molecule electrical devices is that the molecule must be both structurally and electrically connected to the metallic electrodes. Typically, the mechanical and electrical contacts are achieved by the same chemical moiety. In this study, we demonstrate that the structural role may be played by one group (for example, a sulfide) while the electrical role may be played by another (a conjugated chain of C═C π-bonds). We can specify the electrical conductance through the molecule by modulating to which particular site on the oligoene chain the electrode binds. The result is a device that functions as a potentiometer at the single-molecule level.

Importance of Direct Metal−π Coupling in Electronic Transport Through Conjugated Single-Molecule Junctions

We study the effects of molecular structure on the electronic transport and mechanical stability of single-molecule junctions formed with Au point contacts. Two types of linear conjugated molecular wires are compared: those functionalized with methylsulfide or amine aurophilic groups at (1) both or (2) only one of its phenyl termini. Using scanning tunneling and atomic force microscope break-junction techniques, the conductance of mono- and difunctionalized molecular wires and its dependence on junction elongation and rupture forces were studied. Charge transport through monofunctionalized wires is observed when the molecular bridge is coupled through a S-Au donor-acceptor bond on one end and a relatively weak Au-π interaction on the other end. For monofunctionalized molecular wires, junctions can be mechanically stabilized by installing a second aurophilic group at the meta position that, however, does not in itself contribute to a new conduction pathway. These results reveal the important interplay between electronic coupling through metal-π interactions and quantum mechanical effects introduced by chemical substitution on the conjugated system. This study affords a strategy to deterministically tune the electrical and mechanical properties through molecular wires.

Ultratrace Detection of Toxic Chemicals: Triggered Disassembly of Supramolecular Nanotube Wrappers.

Chemical sensors offer opportunities for improving personal security, safety, and health. To enable broad adoption of chemical sensors requires performance and cost advantages that are best realized from innovations in the design of the sensing (transduction) materials. Ideal materials are sensitive and selective to specific chemicals or chemical classes and provide a signal that is readily interfaced with portable electronic devices. Herein we report that wrapping single walled carbon nanotubes with metallo-supramolecular polymers creates sensory devices with a dosimetric (time- and concentration-integrated) increase in electrical conductivity that is triggered by electrophilic chemical substances such as diethylchlorophosphate, a nerve agent simulant. The mechanism of this process involves the disassembly of the supramolecular polymer, and we demonstrate its utility in a wireless inductively powered sensing system based on near-field communication technology. Specifically, the dosimeters can be powered and read wirelessly with conventional smartphones to create sensors with ultratrace detection limits.

Functionalizing molecular wires: a tunable class of α,ω-diphenyl-μ,ν-dicyano-oligoenes

We describe the synthesis and characterization of a new class of cyano-functionalized oligoenes and their derivatives. We have made the vinylogous series of α,ω-diphenyl-μ,ν-dicyano-oligoenes (DPDCn) comprised of each odd-numbered member from 3 to 13 linear conjugated olefins. Installing cyano groups onto the oligoene backbone lowers HOMO and LUMO energies by up to ∼0.7 eV, thereby stabilizing the molecule with respect to oxidative decomposition; this exemplifies a new approach to the stabilization of conjugated oligoenes. UV-vis absorption spectra and redox potentials across the DPDCn series reveal that the molecular band gap ranges from 2.80 to 1.75 eV. This gap can be further tuned by the facile installation of a variety of aryl end-groups. The choice of end-groups also greatly affects the physical properties such as solubility and the solid-state packing. We also present the longest oligoene crystal structure reported to date. Moreover, we find that the prototypical linear structure makes oligoenes suitable as molecular wires and connectors in the bottom-up construction of nanoscale architectures. As a proof of concept, carboxylic acid terminated oligoenes were used to position 10-nm Fe3O4 nanoparticles on a GaAs (100) substrate.

Smectic A mesophases from luminescent sandic platinum(II) mesogens

ABSTRACT Square planar platinum(II) thienyl pyridyl complexes with board-shaped structures assemble into lamellar (SmA) liquid crystal phases at elevated temperatures. Liquid crystals of this type are expected to have stronger biaxial correlations than typical calamitic mesogens. The mesophase stability improves with decreasing alkyl chain lengths with C8H17 having the widest range of stability. All complexes are luminescent in solution. GRAPHICAL ABSTRACT

Formation of an Unusual Four-Membered Nitrogen Ring

Significance: Above is one of three proposed mechanisms for the formation of the new radical cationic nitrogen four-membered ring 5. This tetrazetidine forms from allowing triphenylphosphine and diisopropyl azodicarboxylate to react. Previous studies had shown cursory evidence for the formation of a tetrazetidine by other methods, while this work offers more conclusive evidence in the form of EPR spectra and DFT calculations. Comment: The reaction was monitored by EPR spectroscopy, which showed the presence and disappearance of 3 followed by the formation of 5. Thus, the authors claim that the above mechanism is the most plausible. The DFT calculations show electron density on the methine or methylene protons (R = Et, i-Pr), which helps to rationalize hydrogen hyperfine coupling seen on the EPR spectra. RO N O

Tetrathiafulvalene-Based Microporous Metal-Organic Framework

Significance: The authors report the synthesis of a zinc(II) tetrathiafulvalene tetrabenzoate [Zn2(TTFTB)] metal-organic framework (MOF) with high charge mobility unlike conventional MOFs. This work shows that MOFs can be useful as electronic materials in addition to their more traditional roles as gas storage materials. Comment: The benzoate groups on the TTF ligand stack in such a way that they create a porosity that is permanent. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) was used to determine that the MOF has charge mobility comparable to organic conductors. The authors also show both powder and single crystal structures of the MOFs to complement their observation of permanent porosity. O