Long-Range Charge Transport in Homogeneous and Alternating-Rigidity Chains

TL;DR

This study investigates long-range charge transport in molecular chains, revealing how electronic structure, environmental effects, and structural patterning influence conduction, with implications for designing efficient molecular electronic devices.

Contribution

It introduces a minimal model combining electronic and environmental factors to explain unconventional long-range charge transport in molecular chains.

Findings

Thermal effects can enhance or suppress conductance depending on chain length.

Unconventional size-dependent transport behavior emerges with strong monomer coupling.

Structural patterning affects charge delocalization but not overall electrical resistance.

Abstract

We study the interplay of intrinsic-electronic and environmental factors on long-range charge transport across molecular chains with up to $N\sim 80$ monomers. We describe the molecular electronic structure of the chain with a tight-binding Hamiltonian. Thermal effects in the form of electron decoherence and inelastic scatterings are incorporated with the Landauer-B\"uttiker probe method. In short chains of up to 10 units we observe the crossover between coherent (tunneling, ballistic) motion and thermally-assisted conduction, with thermal effects enhancing the current beyond the quantum coherent limit. We further show that unconventional (non monotonic with size) transport behavior emerges when monomer-to-monomer electronic coupling is made large. In long chains, we identify a different behavior, with thermal effects suppressing the conductance below the coherent-ballistic limit. With the goal to identify a minimal model for molecular chains displaying unconventional and effective long-range transport, we simulate a modular polymer with alternating regions of high and low rigidity. Simulations show that, surprisingly, while charge correlations are significantly affected by structuring environmental conditions, reflecting charge delocalization, the electrical resistance displays an averaging effect, and it is not sensitive to this patterning. We conclude by arguing that efficient long-range charge transport requires engineering both internal electronic parameters and environmental conditions.