We experimentally and theoretically investigate the thermal conductivity and mechanical properties of polycrystalline HKUST-1 metal−organic frameworks (MOFs) infiltrated with three guest molecules: tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 -TCNQ), and (cyclohexane-1,4-diylidene)dimalononitrile (H 4 -TCNQ). This allows for modification of the interaction strength between the guest and host, presenting an opportunity to study the fundamental atomic scale mechanisms of how guest molecules impact the thermal conductivity of large unit cell porous crystals. The thermal conductivities of the guest@MOF systems decrease significantly, by on average a factor of 4, for all infiltrated samples as compared to the uninfiltrated, pristine HKUST-1. This reduction in thermal conductivity goes in tandem with an increase in density of 38% and corresponding increase in heat capacity of ∼48%, defying conventional effective medium scaling of thermal properties of porous materials. We explore the origin of this reduction by experimentally investigating the guest molecules' effects on the mechanical properties of the MOF and performing atomistic simulations to elucidate the roles of the mass and bonding environments on thermal conductivity. The reduction in thermal conductivity can be ascribed to an increase in vibrational scattering introduced by extrinsic guest-MOF collisions as well as guest molecule-induced modifications to the intrinsic vibrational structure of the MOF in the form of hybridization of low frequency modes that is concomitant with an enhanced population of localized modes. The concentration of localized modes and resulting reduction in thermal conductivity do not seem to be significantly affected by the mass or bonding strength of the guest species.

A novel n‐type copolymer dopant polystyrene–poly(4‐vinyl‐N‐hexylpyridinium fluoride) (PSpF) with fluoride anions is designed and synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization. This is thought to be the first polymeric fluoride dopant. Electrical conductivity of 4.2 S cm–1 and high power factor of 67 µW m–1 K–2 are achieved for PSpF‐doped polymer films, with a corresponding decrease in thermal conductivity as the PSpF concentration is increased, giving the highest ZT of 0.1. An especially high electrical conductivity of 58 S cm–1 at 88 °C and outstanding thermal stability are recorded. Further, organic transistors of PSpF‐doped thin films exhibit high electron mobility and Hall mobility of 0.86 and 1.70 cm2 V–1 s–1, respectively. The results suggest that polystyrene–poly(vinylpyridinium) salt copolymers with fluoride anions are promising for high‐performance n‐type all‐polymer thermoelectrics. This work provides a new way to realize organic thermoelectrics with high conductivity relative to the Seebeck coefficient, high power factor, thermal stability, and broad processing window.

MOFs are of great interest for applications in gas storage, separation, catalysis and more recently, thermoelectrics (TE) due to their extremely high porosity (surface area), large chemical and mechanical tailorability defined by the choice of the metal node, the organic ligand, and infiltrated guest molecules, and low thermal conductivities. In particular, new emergent properties, such as electronic conductivity and energy transfer, have been achieved by infiltrating MOF pores with specialized ‘guest’ molecules. The thermal transport in infiltrated MOFs is critically important for understanding the efficiency of both gas adsorption and thermoelectric applications. Understanding the impacts on thermal transport upon MOF infiltration with an electrically conductive guest molecule is critical for realizing an efficient TE material. This work experimentally investigates the impacts on the thermal properties of HKUST-1 MOFs infiltrated with charge accepting TCNQ and F4-TCNQ guest molecules. We provide experimental evidence paired with Molecular Dynamics (MD) results to describe the thermal transport mechanism as a loading-dependent structural change within the MOF. Since HKUST-1 exhibits negative thermal expansion (NTE), we note the unusual phenomena that the material softens as it becomes denser with increasing temperatures. We show this is achieved by which the ligands take on a contorted configuration as they absorb more energy at higher temperatures. Upon infiltration, the MOF softens, however the softening (and therefore reduction in phonon group velocity) cannot be completely responsible for the large reduction in thermal conductivity that is experimentally observed (~71% at room temperature). Spectral Energy Density (SED) and MD results indicate that localized phonon modes from the adsorbates act as additional scattering mechanisms to reduce the thermal conductivity. Further, experimental evidence of the existence of low frequency ZA-like flexural modes (usually found only in 2D materials) within the highly porous 3D MOF structure likely contributes to a large portion of the thermal conductivity, and is suppressed upon infiltration with a guest molecule, causing the large reduction in thermal conductivity.