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

Abstract

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.

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

“…The approach we take is based on the fact that the strength of the Coulombic interaction between polarons on the conjugated polymer backbone and dopant counterions is actually recorded in the vibrational spectra of certain dopant anions. Our first hint that this might be possible comes from previous work by both our group and others − who have used the nitrile stretching vibrational spectrum of the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano­quinodimethane (F 4 TCNQ) dopant as a diagnostic tool for tracking the doping mechanism in conjugated polymers, which takes place either via ICT or by the formation of charge transfer complexes (CTCs). Nitrile stretching modes have the advantage that they occur in a mid-IR spectral “window” where no other molecular vibrations appear, so that they are relatively easy to measure and characterize.…”

Microscopic Insights into Charge Formation and Energetics in n-Doped Organic Semiconductors

“…On a fully microscopic level, on the other hand, valuable insights can be obtained from first-principles electronic structure calculations. Given the size of realistic systems, those are typically based on density functional theory (DFT) and mostly concentrate on the principle bimolecular CTC building blocks ,− or just the polaron states for individual molecules. − Attempts for larger systems are still limited, and possible trends in the transition from micro- to more macroscopic systems are rarely discussed. − In the present work, we particularly consider dopant:host complexes that form ICTCs with individual and stacked dopant molecules and varying number of host molecules, as illustrated in Figure . On the basis of detailed DFT calculations, we investigate the energetics and charge-transfer behavior in these systems with and without considering the dielectric environment.…”

Exploring organic semiconductors in solution: The effects of solvation, alkylization, and doping

“…As discussed in Section III A 3, the ground-state electron transfer from the 6T to the F4-TCNQ in vacuo amounts to 0.28 e. In the first excited state, corresponding to P 1 , it is increased to 0.60 e. This behavior can be understood by analyzing the character of the HOMO and the LUMO of the complex, which are involved in this transition. While they correspond to bonding and antibonding superpositions of the HOMO of the 6T and of the LUMO of the F4-TCNQ, the HOMO of the complex has predominantly the character of the HOMO of the 6T, while the LUMO of the complex is more resemblant of the LUMO of the F4-TCNQ [114]. This is a general feature of CT complexes; hence, the excited state corresponding to the antibonding superposition has a more ionic character [115].…”

“…We now proceed to analyzing the electronic properties of the CT complex. Its frontier orbitals are dominated by the hybridization between the HOMO of the donor and the LUMO of the acceptor, which give rise to an occupied bonding and an unoccupied antibonding orbital superposition [13,18,19,114]. Also other valence orbitals show signs of hybridization, i.e.…”