A one-pot synthesis of oligo(arylene–ethynylene)-molecular wires and their use in the further verification of molecular circuit laws

Abstract

A convenient two-step, one-pot synthesis of oligo(arylene-ethynylene) (OAE) type molecular wires in yields of up to 70% via in situ desilylation of protected bis(alkynes) Me 3 SiC≡CArC≡CSiMe 3 (Ar = 2,5-thienyl, 1,4-naphthylene, 9,10-anthrylene) and subsequent Sonogashira cross-coupling with S-(4-iodophenyl) ethanethiolate, 4-iodothioanisole, or 5-bromo-3,3-dimethyl-2,3-dihydrobenzo[b]thiophene is described. The in situ desilylation avoids the manipulation of the sensitive terminal dialkynes (HC≡CArC≡CH), whilst the general approach presented has some advantages over alternative synthetic strategies based on coupling of aryl dihalides (XArX) by avoiding the multi-step preparation and purification of the terminal alkynes S-(4ethynylphenyl) ethanethiolate, 4-ethynylthioanisole and 5-ethynyl 3,3-dimethyl-2,3-dihydrobenzo [b]thiophene. The molecular conductance of the resulting thiolate or thioether functionalised OAE molecular wires has been determined using scanning tunneling microscope break junction (STM-BJ) methods. The trends in molecular conductance do not track simply with the degree of aromaticity of the molecular core despite the rather similar molecular lengths. Rather, the STM-BJ data are better correlated with the nature of the anchor group, highlighting the important role of electrode-molecule coupling on electron transport in a molecular junction. The experimental conductance data are in good agreement with recently described quantum circuit rules, further highlighting the potential for these relationships to be used as predictive tools in molecular electronics research.

Signatures of Room-Temperature Quantum Interference in Molecular Junctions

“…Further experimental verifications of this rule were recently presented. 46 ■ QUANTUM CIRCUIT RULES FOR…”

“…A more general extension of this rule was proposed and verified theoretically through DFT simulations, which also established a related circuit rule for Seebeck coefficients of organic molecules. Further experimental verifications of this rule were recently presented …”

Exploring relationships between chemical structure and molecular conductance: from α,ω-functionalised oligoynes to molecular circuits

“…From the QCR (eqn (1)), the a X parameters (Table 3) and the experimental conductance data presented as log(G/G 0 ) (Table 1), the backbone parameters, b B , for the various homologous members of the polyyne series investigated here (b CuC , b CuCCuC , b CuCCuCCuC and b CuCCuCCuCCuC ) are readily calculated (Table 3); these values differ slightly from those derived earlier from studies of related series, 12 but each Table 2 Experimental conductivity of 4,4'-bis(methylthiol)biphenyl, 5,5'-bis(3,3-dimethyl-2,3-dihydrobenzo[b]thiophenyl), 4,4'-diaminobiphenyl and 4,4'-bipyridine in mesitylene, the contact group conductance term from extrapolation of Fig. 2 to N = 0 expressed as log(G N 2C / G 0 ) (see also Table S4 †), and twice the value of the quantum circuit rule anchor parameter for each anchor group, 2a X , 12,14,15 for ease of comparison Compound log(G/G 0 ) (solvent) a log(G N 2C /G 0 ) 2a X −2.80 (TCB) 33 − Since the QCR treats each region of the molecule as an independent scattering region, an alternative analysis of the data can be made by incorporating the anchor groups within the scattering region associated with the junction electrode, and considering the conductance due to the (arbitrarily partitioned) bridge portion alone (Fig. 4).…”

“…From Fig. 5, the Table 3 Quantum circuit rule parameters, a X , for anchor groups and b B for backbones used in this work 12,14,15 Anchor group The QCR (eqn ( 1)) has immense potential for use in molecular circuit design, giving a simple algebraic expression that can estimate single-molecule conductance with surprising accuracy should the unique numerical parameters be known for the particular anchor group(s) (a X , a Y ) and bridge structure (b B ) in the molecule of interest. The strategies outlined above provide convenient methods to estimate these terms from a small set of experimental data.…”

“…If the independent parameters a X , a Y and b B are known, then eqn (1) allows an algebraic estimate of molecular conductance. 12,14,15 We report here a systematic study of a series of oligoynebased molecular wires, X-(CuC) N -X, featuring a range of anchor groups (X = 4-thioanisole ( ). The single-molecule conductance of each of these compounds, G, has been determined using the scanning tunnelling microscope based-break junction (STM-BJ) technique.…”