Theoretical Investigation of the β Value of the π-Conjugated Molecular Wires by Evaluating Exchange Interaction between Organic Radicals

The decay constant beta of pi-conjugated molecular wire corresponds to the efficiency of electron tunneling through the molecule. We have calculated beta values of intramolecular magnetic exchange interactions by density functional theory for organic bis(nitronyl nitroxide)s that have several molecular wire units, namely, oligo-p-phenylene (OPP), oligophenylene ethynylene (OPE), oligophenylene vinylene (OPV), oligophenylene imine (OPI), bicyclo[1.1.1]pentane oligomer (BCP), beta,meso,beta-triply fused porphyrin oligomer (TFP), and meso,meso-linked porphyrin oligomer (MLP). For OPP, OPE, OPV, OPI, BCP, TFP, and MLP, beta was calculated to be 0.41, 0.25, 0.22, 0.32, 0.77, 0.03, and 0.65 angstrom(-1), respectively. The obtained beta values are in good agreement with the experimental value of the molecular tunneling conductance, suggesting similarity of the mechanism between exchange interaction and molecular conductance. This method can be used to estimate the efficiency of electron tunneling through the molecular wire.