The nature of pi-conjugated wire is characterized by the decay constant beta of electron tunneling. To evaluate the decay constant, intramolecular magnetic exchange interaction was calculated by density functional theory for organic biradicals: nitronyl nitroxide and verdazyl were chosen for the radical substituent and oligophenylene and oligo(phenylene ethynylene) were chosen for the wire. For phenylene and phenylene ethynylene units, beta was calculated to be 0.42 angstrom(-1) and 0.24 angstrom(-1), respectively, and the beta value was independent of radical species. The obtained beta values are in good agreement with the experimental value of the molecular tunneling conductance. (C) 2012 Elsevier B. V. All rights reserved.