Molecular and Spacer Engineering of Carbazolyl-Substituted Polydithienopyrrole Polymers with Memristive Response

This work combines experimental and theoretical approaches to examine how the length of an alkyl linker can be used to tune the electrochemical, optical, and device-related electrical properties of carbazolylalkyl-substituted polydithienopyrrole polymers (pCxDTP). Using a scalable four-step synthetic route, a series of monomers with systematically varied alkyl spacers was prepared and subsequently electropolymerized directly onto conductive substrates, yielding uniform polymer films suitable for device integration. The polymer band gap ranges from to 2.4 for pC3DTP to 2.0 eV for pC12DTP. This trend arises from the concurrent increase of the HOMO and lowering of the LUMO levels, with both effects depending on the alkyl chain length. Sandwich devices with an ITO | pCxDTP | Al architecture show reproducible memristive behavior, characterized by pinched hysteresis loops and pronounced frequency dependence. Longer alkyl linkers result in a more pronounced hysteresis in current-voltage already at higher sweep rates, revealing a clear relationship between molecular structure and device performance. Density functional theory and time-dependent DFT calculations are consistent with the experimental findings and indicate that alkyl linker length subtly influences frontier orbital energies, ionization potentials, and charge delocalization. Oligomer-level modeling further accounts for the observed band gap narrowing and spectral broadening through increased conjugation length and chain dispersity. Overall, these results highlight alkyl linker engineering as a practical strategy for optimizing conjugated polymers for electrochromic and memristive applications.