The correlation of electron transfer with molecular conductance ( g : electron transport through single molecules) by Nitzan and others has contributed to a fundamental understanding of single-molecule electronic materials. When an unsymmetric, dipolar molecule spans two electrodes, the possibility exists for different conductance values at equal, but opposite electrode biases. In the device configuration, these molecules serve as rectifiers of the current and the efficiency of the device is given by the rectification ratio (RR = g forward / g reverse ). Experimental determination of the RR is challenging since the orientation of the rectifying molecule with respect to the electrodes and with respect to the electrode bias direction is difficult to establish. Thus, while two different values of g can be measured and a RR calculated, one cannot easily assign each conductance value as being aligned with or opposed to the molecular dipole, and calculations are often required to resolve the uncertainty. Herein, we describe the properties of two isomeric, triplet ground state biradical molecules that serve as constant-bias analogs of single-molecule electronic devices. Through established theoretical relationships between g and electronic coupling, H 2 , and between H 2 and magnetic exchange coupling, J ( g ∝ H 2 ∝ J ), we use the ratio of experimental J -values for our two isomers to calculate a RR for an unsymmetric bridge molecule with known geometry relative to the two radical fragments of the molecule and at a spectroscopically-defined potential bias. Our experimental results are compared with device transport calculations.
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Transferrable property relationships between magnetic exchange coupling and molecular conductance
Calculated conductance through Au n –S–Bridge–S–Au n (Bridge = organic σ/π-system) constructs are compared to experimentally-determined magnetic exchange coupling parameters in a series of Tp Cum,Me ZnSQ–Bridge–NN complexes, where Tp Cum,Me = hydro-tris(3-cumenyl-1-methylpyrazolyl)borate ancillary ligand, Zn = diamagnetic zinc( ii ), SQ = semiquinone ( S = 1/2), and NN = nitronylnitroxide radical ( S = 1/2). We find that there is a nonlinear functional relationship between the biradical magnetic exchange coupling, J D→A , and the computed conductance, g mb . Although different bridge types (monomer vs. dimer) do not lie on the same J D→A vs. g mb , curve, there is a scale invariance between the monomeric and dimeric bridges which shows that the two data sets are related by a proportionate scaling of J D→A . For exchange and conductance mediated by a given bridge fragment, we find that the ratio of distance dependent decay constants for conductance ( β g ) and magnetic exchange coupling ( β J ) does not equal unity, indicating that inherent differences in the tunneling energy gaps, Δ ε , and the bridge–bridge electronic coupling, H BB , are not directly transferrable properties as they relate to exchange and conductance. The results of these observations are described in valence bond terms, with resonance structure contributions to the ground state bridge wavefunction being different for SQ–Bridge–NN and Au n –S–Bridge–S–Au n systems.
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- NSF-PAR ID:
- 10269516
- Date Published:
- Journal Name:
- Chemical Science
- Volume:
- 11
- Issue:
- 42
- ISSN:
- 2041-6520
- Page Range / eLocation ID:
- 11425 to 11434
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d0sc04350h
