A Comprehensive Study of Extended Tetrathiafulvalene Cruciform Molecules for Molecular Electronics: Synthesis and Electrical Transport Measurements

Abstract

Cruciform-like molecules with two orthogonally placed π-conjugated systems have in recent years attracted significant interest for their potential use as molecular wires in molecular electronics. Here we present synthetic protocols for a large selection of cruciform molecules based on oligo(phenyleneethynylene) (OPE) and tetrathiafulvalene (TTF) scaffolds, end-capped with acetyl-protected thiolates as electrode anchoring groups. The molecules were subjected to a comprehensive study of their conducting properties as well as their photophysical and electrochemical properties in solution. The complex nature of the molecules and their possible binding in different configurations in junctions called for different techniques of conductance measurements: (1) conducting-probe atomic force microscopy (CP-AFM) measurements on self-assembled monolayers (SAMs), (2) mechanically controlled break-junction (MCBJ) measurements, and (3) scanning tunneling microscopy break-junction (STM-BJ) measurements. The CP-AFM measurements showed structure-property relationships from SAMs of series of OPE3 and OPE5 cruciform molecules; the conductance of the SAM increased with the number of dithiafulvene (DTF) units (0, 1, 2) along the wire, and it increased when substituting two arylethynyl end groups of the OPE3 backbone with two DTF units. The MCBJ and STM-BJ studies on single molecules both showed that DTFs decreased the junction formation probability, but, in contrast, no significant influence on the single-molecule conductance was observed. We suggest that the origins of the difference between SAM and single-molecule measurements lie in the nature of the molecule-electrode interface as well as in effects arising from molecular packing in the SAMs. This comprehensive study shows that for complex molecules care should be taken when directly comparing single-molecule measurements and measurements of SAMs and solid-state devices thereof.

Carbazole‐Based Tetrapodal Anchor Groups for Gold Surfaces: Synthesis and Conductance Properties

“…It can be concluded, however, that it is reasonable to compare our G M values with conductances determined using MCBJ, with the caveat that the G M values may be slight underestimates. As MCBJ conductances have been shown to be comparable with those obtained using STM‐BJ, it follows that comparisons with STM‐BJ data are also valid.…”

Carbazole‐Based Tetrapodal Anchor Groups for Gold Surfaces: Synthesis and Conductance Properties

“…[21] Our cAFM method results in slightly lower conductances for both SpP (log G M /G 0 = À3.78 vs.log G/G 0 = À3.2 [21] )and SmP (log G M / G 0 = À4.17 vs.l og G/G 0 = À3.9 [21] ); ac omparable deviation between cAFM and MCBJ has been observed previously. [22] Such discrepancies may result from differences in the nature of the junctions or assumptions made when calculating G M from G J .Itcan be concluded, however, that it is reasonable to compare our G M values with conductances determined using MCBJ,w ith the caveat that the G M values may be slight underestimates.AsMCBJ conductances have been shown to be comparable with those obtained using STM-BJ, [15,23] it follows that comparisons with STM-BJ data are also valid.…”

Tuning crystal polymorphs of a π-extended tetrathiafulvalene-based cruciform molecule towards high-performance organic field-effect transistors

“…One aim was to achieve good and tunable conducting properties by virtue of the different conjugation pathways that exist between the individual parts of the molecule [17][18][19][20][21][22][23]. These DTF-based cruciform molecular wires have demonstrated potential promising applications in molecular electronics and also provided a good platform for the investigation of the dependence of conducting properties on their molecular structures [24][25][26]. As a continuation of our work on such extended TTF compounds, it is interesting to further investigate their applications in organic electronic devices, such as OFETs, the basic component of organic circuits.…”