Radical Cation π-Dimers of Conjugated Oligomers as Molecular Wires: An Analysis Based on Nitronyl Nitroxide Spin Labels.

Nitronyl nitroxide (NN)-substituted conjugated oligomers, which were expected to self-associate in biradical cation states, were designed to analyze the capability of π-dimers as molecular wires. The oligomer moieties were composed of dithienyl-N-methylpyrrole with methoxy substituents at the inner β-position of thiophene rings (DTP-NN. ) and its propylenedioxythiophene (ProDOT) inserted derivative (DTP-P-NN. ), or two ethylenedioxythiophene (EDOT) and two ProDOT units (E2 P2 -NN. ). Among them, chemical one-electron oxidation gave biradical cations (DTP-P).+ -NN. and (E2 P2 ).+ -NN. that formed π-dimers (DTP-P-NN. )22+ and (E2 P2 -NN. )22+ in dichloromethane at low temperatures. ESR studies of (DTP-P-NN. )22+ and (E2 P2 -NN. )22+ showed the presence of a relatively strong exchange interaction between two NN radicals through the radical cation π-dimer moieties. DFT calculations supported these experimental results and predicted that exchange interactions between two NN radicals were comparable or stronger than those through covalently linked quaterthiophene. Thus, the conjugated oligomer radical cation π-dimers acted as efficient molecular wires for electronic communication.