Probing Host-Guest Interactions via Conductance and Thermopower Measurements

Host-guest interactions play a crucial role in molecular recognition, self-assembly, and the development of molecular electronic devices, which requires a fundamental understanding of their structure-property relationships. Significant progress has been made in studying charge transport within such systems; however, the correlation between molecular structure, conductance, and thermoelectric properties remains largely unexplored. Here, we present a systematic investigation of the conductance and thermoelectric properties of low-symmetry M₈L^un ₄ molecular barrel (UNMB) and its fullerene-encapsulated derivatives (C₆₀⊂UNMB and C₇₀⊂UNMB). Conductance measurements performed using EGaIn setup reveal ≈1.5-order-of-magnitude increase in current density upon guest-encapsulation, indicating a strong modulation in the electronic structure of UNMB framework. Additionally, thermopower measurements demonstrates an opposite trend to the conductance values-a decrease from +174.4 ± 19.0 µV K^-1 for pristine UNMB to +85.3 ± 9.7 µV K^-1 for C₆₀⊂UNMB and +90.9 ± 7.0 µV K^-1 for C₇₀⊂UNMB. The positive sign of Seebeck coefficient indicates HOMO-dominated charge transport. We employed Ultraviolet photoelectron spectroscopy measurements, which show a reduction in HOMO energy offset upon guest-encapsulation that validates the trends in conductance and thermopower measurements. To understand the underlying charge transport mechanism, we performed temperature-dependent conductance measurements that indicated a thermally activated hopping mechanism.