Vibrational Stark Effect Mapping of Polaron Delocalization in Chemically Doped Conjugated Polymers

The nitrile vibrational modes of the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) anion occur across a range of different frequencies in doped conjugated polymers. We show that these shifts can be understood as resulting from the vibrational Stark effect. F4TCNQ– serves as a sensitive vibrational Stark probe, and its nitrile stretches provide a direct read-out of the locally experienced electric fields from the nearby polaron on the doped polymer backbone. The shift of the F4TCNQ– B1u mode and broadening of the B2u mode in doped poly(3-hexylthiophene-2,5-diyl) (P3HT) confirm that the dopant anion’s long axis is oriented perpendicular to the P3HT backbone. The magnitude of the vibrational shifts prove that the anion–polaron distance in F4TCNQ–-doped P3HT films is ∼6 Å. We also show that the intrachain polaron coherence can be varied over a range spanning 6 to about 7.5 P3HT monomer units by controlling the local polymer order and crystallinity. At the highest degree of local order, polaron delocalization is limited by the minimum intrinsic strength of the anion–polaron Coulombic interaction. This work provides the first direct experimental measurement of polaron delocalization in doped conjugated polymer films and verifies theoretical models relating polaron coherence to the shape of the doped polymer’s mid-IR electronic absorption spectrum.