Charge Transfer Doping of Conjugated Polymers with Large Vibrational Activities: Insights into the Regime of Partial Charge Transfer

Effective charge transfer (CT) doping of conjugated polymers depends on electronic and structural factors alike, though the former receives the most attention in design and mechanistic considerations. We investigate CT doping in chalcogenophene-vinylene polymers with similar frontier orbital energies and packing characteristics as other semicrystalline polythiophenes frequently used in doping studies, for example, poly(3-hexylthiophene), or P3HT. However, unlike P3HT, these systems experience large vibrational displacements along many coordinates during the course of an electronic transition, which affects doping yields. Poly(3-decylthienylene-vinylene) and poly(3-decylselenylene-vinylene) (P3DSV) undergo CT doping in the ground electronic state when combined with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ) in a solution. Electronic absorption spectra indicate efficient CT doping evident from the emergence of F₄-TCNQ– transitions concomitant with losses of pristine-type polymer absorption strength. Resonance Raman spectra excited on-resonance with the pristine, dominant aggregate polymer forms reveal appreciable contributions from F₄-TCNQ– in the fundamental region. Conversely, excitation light pre-resonant with polymer aggregate absorption transitions exposes changes in the overtone-combination region, indicating appreciable coupling between ionized polymer segments and dopants. Density functional theoretical simulations were performed on pristine and ionized polymer surrogates (i.e., small oligomers) and dopants in addition to a model CT complex of both ionized forms. Simulated Raman spectra reveal that the CT complex lineshape most closely resemble experimental data confirming that the dopant anion proximity influences Raman transitions on doped polymer segments. We propose that the preponderance of CT complexes originates from rapid charge (hole) localization due to large, multi-mode vibrational displacements accompanying the CT event. Furthermore, this case resembles the regime of partial CT where significant fractions of injected charges do not contribute to conductivity unless the complex dissociates. We also find that despite similar structural qualities of both polymers, P3DSV exhibits stronger apparent CT-doping responses that we attribute to greater crystallinity due to the heavier selenium atom. We next take advantage of the rich vibrational activity of both polymers to spatially map CT interactions using resonance Raman spectroscopic imaging for the first time. Raman images are constructed using ionized polymer and dopant transitions, revealing morphology-dependent polymer–dopant interactions. Larger contrast ratios in doped P3DSV thin films are observed consistent with greater crystallinity and doping efficiency.