Bias-Driven Conductance Increase with Length in Porphyrin Tapes

Abstract

A key goal in molecular electronics has been to find molecules that facilitate efficient charge transport over long distances. Normally, molecular wires become less conductive with increasing length. Here, we report a series of fused porphyrin oligomers for which the conductance increases substantially with length by >10-fold at a bias of 0.7 V. This exceptional behavior can be attributed to the rapid decrease of the HOMO-LUMO gap with the length of fused porphyrins. In contrast, for butadiyne-linked porphyrin oligomers with moderate inter-ring coupling, a normal conductance decrease with length is found for all bias voltages explored (±1 V), although the attenuation factor (β) decreases from ca. 2 nm at low bias to <1 nm at 0.9 V, highlighting that β is not an intrinsic molecular property. Further theoretical analysis using density functional theory underlines the role of intersite coupling and indicates that this large increase in conductance with length at increasing voltages can be generalized to other molecular oligomers.

Unusual Length Dependence of the Conductance in Cumulene Molecular Wires

“…[a] Experimental conductance peak positions from data in Figure f; the run‐to‐run variation in peak position is about 0.02, see Figure S19. [b] The lengths were calibrated by adding 0.4 nm to the peak position of a Gaussian fit to the total distribution of plateau lengths . Values in parentheses are derived from the 95th percentile.…”

Constitution and Conductivity of Metalloporphyrin Tapes

“…The excellent conductivity of the Zn(DMP) oCVD film (Figure ; Supporting Information, Figure S17) likely arises from the exclusive triple linkages in the porphyrin tapes,, , , the good polymerization yield (LDI‐HRMS, UV/Vis/NIR) and the absence of redox active metal centers and strongly bound axial ligands.…”

“…Porphyrin polymers and especially porphyrin tapes (Scheme ) are promising materials for applications as near‐infrared absorbers, in non linear optics, and as molecular wires and electrodes , . However, their integration in optoelectronic devices is often hindered by their tedious multistep synthesis and poor solubility.…”

“…Zinc(II) porphyrins yield triply‐linked porphyrin tapes with high regioselectivity. Such triply linked tapes exhibit highly interesting properties for optoelectronic device fabrication , , . Other cations preferably form doubly‐linked ( β ‐ meso, meso ‐ β ) or mixed porphyrin tapes with lower regioselectivity.…”

Controlled Growth of Porphyrin Wires at a Solid‐Liquid Interface

“…[15 -17] More recently, for a series of fused porphyrin oligomers, a substantial increase of conduction was correlated with increasing molecular length at increasing voltages. [18] Multi-porphyrin wires are thus of interest, but their preparation is known to be limited in quantities, even using iterative methods such as oxidative coupling. [9,12] As a result, conductance measurements on covalent, multi-porphyrin wires are still scarce and primarily concern covalent scaffolds.…”