Phenyl–Acetylene Bond Assembly: A Powerful Tool for the Construction of Nanoscale Architectures

A review of oligo(arylene ethynylene) derivatives in molecular junctions

Oligo(arylene ethynylene) (OAE) derivatives are the "workhorse" molecules of molecular electronics. Their ease of synthesis and flexibility of functionalisation mean that a diverse array of OAE molecular wires have been designed, synthesised and studied theoretically and experimentally in molecular junctions using both single-molecule and ensemble methods. This review summarises the breadth of molecular designs that have been investigated with emphasis on structure-property relationships with respect to the electronic conductance of OAEs. The factors considered include molecular length, connectivity, conjugation, (anti)aromaticity, heteroatom effects and quantum interference (QI). Growing interest in the thermoelectric properties of OAE derivatives, which are expected to be at the forefront of research into organic thermoelectric devices, is also explored.

Electronic conductance and thermopower of single-molecule junctions of oligo(phenyleneethynylene) derivatives

We report the synthesis and the single-molecule transport properties of three new oligo(phenyleneethynylene) (OPE3) derivatives possessing terminal dihydrobenzo[b]thiophene (DHBT) anchoring groups and various core substituents (phenylene, 2,5-dimethoxyphenylene and 9,10-anthracenyl). Their electronic conductance and their Seebeck coefficient have been determined using scanning tunneling microscopy-based break junction (STM-BJ) experiments between gold electrodes. The transport properties of the molecular junctions have been modelled using DFT-based computational methods which reveal a specific binding of the sulfur atom of the DHBT anchor to the electrodes. The experimentally determined Seebeck coefficient varies between -7.9 and -11.4 μV K^-1 in the series and the negative sign is consistent with charge transport through the LUMO levels of the molecules.

Exploring antiaromaticity in single-molecule junctions formed from biphenylene derivatives

We report the synthesis of a series of oligophenylene-ethynylene (OPE) derivatives with biphenylene core units, designed to assess the effects of biphenylene antiaromaticity on charge transport in molecular junctions. Analogues with naphthalene, anthracene, fluorene and biphenyl cores are studied for comparison. The molecules are terminated with pyridyl or methylthio units. Single-molecule conductance data were obtained using the mechanically controllable break junction (MCBJ) technique. It is found that when electrons pass from one electrode to the other via a phenylene ring, the electrical conductance is almost independent of the nature of the pendant π-systems attached to the phenylene ring and is rather insensitive to antiaromaticity. When electrons pass through the cyclobutadiene core of the biphenylene unit, transport is sensitive to the presence of the relatively weak single bonds connecting the two phenylene rings of biphenylene, which arise from partial antiaromaticity within the cyclobutadiene core. This leads to a negligible difference in the molecular conductance compared to the fluorene or biphenyl analogues which have standard single bonds. This ability to tune the conductance of molecular cores has no analogue in junctions formed from artificial quantum dots and reflects the quantum nature of electron transport in molecular junctions, even at room temperature.

Cross-conjugation increases the conductance of meta-connected fluorenones

Charge transport is strongly suppressed by destructive quantum interference (DQI) in meta-connected 1,1'-biphenyl-containing molecules, resulting in low electrical conductance. Surprisingly, we have found that DQI is almost entirely overcome by adding a bridging carbonyl, to yield a cross-conjugated fluorenone. This contrasts with other π-systems, such as para-connected anthraquinone, where cross-conjugation results in low conductance.

Symmetrically backfolded molecules emulating the self-similar features of a Sierpinski triangle

We synthesized self-similar molecules (G3 and G2; based on phenylalkynyl backbones) with symmetrically backfolded shapes inspired by the famous fractal of a Sierpinski triangle. Unlike the more traditional, starburst dendrimers, the centripetal-shaped Sierpinski molecules feature side branches symmetrically bent away from the growth direction of the main branch, thus contrasting the natural-tree shape. Molecule G3 exhibits three distinct levels of the structural hierarchy comprising the primary, secondary and tertiary branches, while the smaller G2 contains only features of the 1^st and 2^nd orders. In spite of the much larger conjugated backbone of G3, its solution UV-vis absorption and fluorescence exhibit no red shift relative to G2. In a test of nitrobenzene sensing, a thin film of G3 deposited from THF was more sensitively quenched in fluorescence than the smaller G2.

Room-Temperature, Copper-Free Sonogashira Reactions Facilitated by Air-Stable, Monoligated Precatalyst [DTBNpP] Pd(crotyl)Cl

A novel application of [DTBNpP] Pd(crotyl)Cl (DTBNpP = di-tert-butylneopentylphosphine) (P2), an air-stable, commercially available palladium precatalyst that allows rapid access to a monoligated state, has been identified for room-temperature, copper-free Sonogashira couplings of challenging aryl bromides and alkynes. The mild reaction conditions with TMP in dimethyl sulfoxide afford up to 97% yields, excellent functional group tolerability, and broad reaction compatibility with access to one-pot indole formation.

Sequential nested assembly at the liquid/solid interface

Studying the stepwise assembly of a four component hybrid structure on Au(111)/mica, the pores of a hydrogen bonded bimolecular network of 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) and 1,3,5-triazine-2,4,6-triamine (melamine) were partitioned by three and four-armed molecules based on oligo([biphenyl]-4-ylethynyl)benzene, followed by the templated adsorption of either C₆₀ fullerene or adamantane thiol molecules. The characterisation by ambient scanning tunneling microscopy (STM) reveals that the pore modifiers exhibit dynamics which pronouncedly depend on the molecular structure. The three-armed molecule 1,3,5-tris([1,1'-biphenyl]-4-ylethynyl)benzene (3BPEB) switches between two symmetry equivalent configurations on a time scale fast compared to the temporal resolution of the STM. Derivatisation of 3BPEB by hydroxyl groups substantially reduces the switching rate. For the four-armed molecule configurational changes are observed only occasionally. The observation of isolated fullerenes and small clusters of adamantane thiol molecules, which are arranged in a characteristic fashion, reveals the templating effect of the trimolecular supramolecular network. However, the fraction of compartments filled by guest molecules is significantly below one for both the thermodynamically controlled adsorption of C₆₀ and the kinetically controlled adsorption of the thiol with the latter causing partial removal of the pore modifier. The experiments, on the one hand, demonstrate the feasibility of templating by nested assembly but, on the other hand, also pinpoint the requirement for the energy landscape to be tolerant to variations in the assembly process.

Quantum interference and heteroaromaticity of para- and meta-linked bridged biphenyl units in single molecular conductance measurements

Is there a correlation between the (hetero)aromaticity of the core of a molecule and its conductance in a single molecular junction? To address this question, which is of fundamental interest in molecular electronics, oligo(arylene-ethynylene) (OAE) molecular wires have been synthesized with core units comprising dibenzothiophene, carbazole, dibenzofuran and fluorene. The biphenyl core has been studied for comparison. Two isomeric series have been obtained with 4-ethynylpyridine units linked to the core either at para-para positions (para series 1-5) or meta-meta positions (meta series 6-10). A combined experimental and computational study, using mechanically controlled break junction measurements and density functional theory calculations, demonstrates consistently higher conductance in the para series compared to the meta series: this is in agreement with increased conjugation of the π-system in the para series. Within the para series conductance increases in the order of decreasing heteroaromaticity (dibenzothiophene < carbazole < dibenzofuran). However, the sequence is very different in the meta series, where dibenzothiophene ≈ dibenzofuran < carbazole. Excellent agreement between theoretical and experimental conductance values is obtained. Our study establishes that both quantum interference and heteroaromaticity in the molecular core units play important and inter-related roles in determining the conductance of single molecular junctions.

Ligand- and copper-free Sonogashira and Heck couplings of (Het)aryl chlorides and bromides catalyzed by palladium nanoparticles supported on in situ generated Al(OH)3

The ligand- and copper-free Sonogashira and Heck couplings catalyzed by palladium nanoparticles supported on newly generated Al(OH)₃, were developed.

Pd-catalyzed one-pot sequential unsymmetrical cross-coupling reactions of aryl/heteroaryl 1,2-dihalides

The present methodology allows the conversion of easily available aryl/heteroaryl 1,2-dihalides into synthetically useful unsymmetrically substituted arenes/heteroarenes in good to excellent yields under mild reaction conditions.

Knitting up 2,7-disubstituted carbazole based oligomers through supramolecular interactions for their application in organic thin film transistors

A specific molecule has been knitted up to form a supramolecular architecture and applied to organic thin film transistors.

Palladium precatalysts containing meta-terarylphosphine ligands for expedient copper-free Sonogashira cross-coupling reactions

Novel precatalysts containing meta-terarylphosphine ligands have set a new standard for high performance and practicality in copper-free Sonogashira cross-coupling catalysis using simple and reliable protocols.

Bestowing structure upon the pores of a supramolecular network

Trigonal molecules compartmentalise the pores of a honeycomb network of 3,4:9,10-tetracarboxylic diimide (PTCDI) and 1,3,5-triazine-2,4,6-triamine (melamine). Extending the 1,3,5-tri(phenylene-ethynylene)benzene core by a phenyl group allows for a well-defined accommodation of the molecule into two symmetry equivalent positions in the pore. The corresponding styryl or phenylene-ethynylene derivatives exceed the pore size and, thus, impede pore modification.