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

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.