Radical Anions of Porphyrin\nMolecular Wires: Delocalization\nand Dynamics

The delocalization length of charge carriers in organic\nsemiconductors\ninfluences their mobility and is an important factor in the design\nof functional materials. Here, we have studied the radical anions\nof a series of linear and cyclic butadiyne-linked porphyrin oligomers\nusing CW-EPR, <sup>1</sup>H Mims ENDOR and NIR/MIR spectroelectrochemistry\ntogether with DFT calculations and multiscale molecular modeling.\nLow-temperature hyperfine EPR spectroscopy and optical data show that\npolarons are delocalized nonuniformly over about four porphyrins with\nmost of the spin density on just two units even in the cyclic structures,\nin which all porphyrin sites are identical. Room temperature CW-EPR\nspectra indicate a larger spatial distribution of spin density on\nthe EPR time scale. We introduce a combined molecular dynamics simulations\nand DFT approach to demonstrate that dynamic migration of delocalized\npolarons can occur in porphyrin oligomers and that this fully accounts\nfor the apparent spin density distribution at room temperature. This\nmethod is a powerful tool in both the study and development of molecular\nwires and molecular electronics.