Soft versus hard junction formation for α-terthiophene molecular wires and their charge transfer complexes

Spin-crossover in iron(II)-phenylene ethynylene-2,6-di(pyrazol-1-yl) pyridine hybrids: toward switchable molecular wire-like architectures

Luminescent oligo(p-phenylene ethynylene) (OPE) and spin-crossover (SCO) active Fe(II)-2,6-di(pyrazol-1-yl) pyridine (BPP) systems are prominent examples proposed to develop functional materials such as molecular wires/memories. A marriage between OPE and Fe(II)-BPP systems is a strategy to obtain supramolecular luminescent ligands capable of metal coordination useful to produce novel spin-switchable hybrids with synergistic coupling between spin-state of Fe(II) and a physical property associated with the OPE skeleton, for example, electronic conductivity or luminescence. To begin in this direction, two novel ditopic ligands, namely L¹ and L², featuring OPE-type backbone end-capped with metal coordinating BPP were designed and synthetized. The ligand L² tailored with 2-ethylhexyloxy chains at the 2 and 5 positions of the OPE skeleton shows modulated optical properties and improved solubility in common organic solvents relative to the parent ligand L¹. Solution phase complexation of L¹ and L² with Fe(BF₄)₂·6H₂O resulted in the formation of insoluble materials of the composition [Fe(L¹)] _n (BF₄)_2n and [Fe(L²)] _n (BF₄)_2n as inferred from elemental analyses. Complex [Fe(L¹)] _n (BF₄)_2n underwent thermal SCO centred at T _1/2  =  275 K as well as photoinduced low-spin to high-spin transition with the existence of the metastable high-spin state up to 52 K. On the other hand, complex [Fe(L²)] _n (BF₄)_2n , tethered with 2-ethylhexyloxy groups, showed gradual and half-complete SCO with 50% of the Fe(II)-centres permanently blocked in the high-spin state due to intermolecular steric interactions. The small angle x-ray scattering (SAXS) pattern of the as-prepared solid complex [Fe(L¹)] _n (BF₄)_2n revealed the presence of nm-sized crystallites implying a possible methodology towards the template-free synthesis of functional-SCO nanostructures.

Charge transfer complexation boosts molecular conductance through Fermi level pinning

Interference features in the transmission spectra can dominate charge transport in metal-molecule-metal junctions when they occur close to the contact Fermi energy (E F). Here, we show that by forming a charge-transfer complex with tetracyanoethylene (TCNE) we can introduce new constructive interference features in the transmission profile of electron-rich, thiophene-based molecular wires that almost coincide with E F. Complexation can result in a large enhancement of junction conductance, with very efficient charge transport even at relatively large molecular lengths. For instance, we report a conductance of 10-3 G 0 (∼78 nS) for the ∼2 nm long α-quaterthiophene:TCNE complex, almost two orders of magnitude higher than the conductance of the bare molecular wire. As the conductance of the complexes is remarkably independent of features such as the molecular backbone and the nature of the contacts to the electrodes, our results strongly suggest that the interference features are consistently pinned near to the Fermi energy of the metallic leads. Theoretical studies indicate that the semi-occupied nature of the charge-transfer orbital is not only important in giving rise to the latter effect, but also could result in spin-dependent transport for the charge-transfer complexes. These results therefore present a simple yet effective way to increase charge transport efficiency in long and poorly conductive molecular wires, with important repercussions in single-entity thermoelectronics and spintronics.