Bulk charge-transfer coupling and tunable dielectric relaxation in benzoquinone-doped nematic liquid crystals

Abstract

Developing next-generation electro-optic devices demands precise control over the dielectric dynamics of soft materials. To address this challenge, we report a systematic investigation of the nematic liquid crystal (LC) Merck IV doped with 1,4-benzoquinone (BQ) as a redox-active modulator. Through a combination of calorimetry, optical spectroscopy, and broadband impedance spectroscopy, rigorously analyzed via the electric modulus formalism, we identify a sharp concentration-dependent crossover between localized charge-transfer (CT) coupling and disorder-driven relaxation. Crucially, polarized optical microscopy confirms the preservation of the nematic phase across the explored concentration range. We find that at 1 wt%, BQ functions as an energetic modulator: it facilitates CT interactions that drastically lower the activation barrier for hopping conduction (∼0.025 eV), although this comes with a slight increase in relaxation time due to steric constraints. This behavior highlights a critical trade-off where thermodynamic efficiency improves even as dipolar reorientation kinetically slows down. Conversely, increasing the load to 2 wt% shifts the system into a disorder-dominated regime. Here, π-stacked aggregates form deep traps and trigger Maxwell–Wagner–Sillars (MWS) interfacial polarization. Such disorder appears as a dynamic broadening of the relaxation spectrum, characteristic of charge transport within a rugged energy landscape. By demonstrating a decoupling between fast electronic transport and slow molecular reorientation, this study establishes BQ as a versatile, non-destructive dopant suitable for developing tunable dielectric LC materials.