Manipulating Thermal Transport in Porous Organic Frameworks through Pore Wall Engineering
Authors: Dionne Connor , Giri Ashutosh 
Published: December 2024

The design of porous crystals has garnered extreme interest in the past few decades due to their potential applications in electronics, gas storage, sensing, and catalysis, among others. With the unique “building-block” nature of metallic and covalent organic frameworks (MOF and COF, respectively), a huge number of porous crystals can be fabricated to serve a wide variety of purposes. However, heat dissipation in these materials poses a major challenge as many of their application-relevant properties are highly temperature dependent. Previous works have investigated the effect of guest-framework interactions on the thermal transport properties of MOFs and COFs for gas storage applications, uncovering a strong effect on the framework thermal conductivity upon gas infiltration. These effects have been attributed to the collisions of guest molecules with the pore wall acting as a strong phonon scattering source, as well as complex bonding interactions between the guest and framework leading to the hybridization of certain phonon modes and the localization of other phonon modes. In all, the thermal transport properties of organic frameworks are hindered upon infiltration with guest molecules.
In gas storage and sensing, the ability to attach functional groups to the organic linkers leads to selective gas-framework interactions and provides an additional mechanism to tailor the properties of the framework towards specific applications. Despite their potential for targeted applications, the role of attached functional groups on the framework thermal conductivity has not been well studied. In this work, utilizing extensive atomistic simulations, we study the thermal transport properties of organic frameworks with attached functional groups. By varying the length of the attached functional groups, we systematically study the effect the functional group has on the framework as a whole. Using molecular dynamics, we calculate the temperature dependent directional thermal conductivities of the framework and find a noticeable change in the dominant scattering mechanisms dictating heat transfer in the framework based on the length of the attached functional groups. By calculating the phonon density of states of these systems, we observe strong evidence of phonon hybridization that further increases when the length of the functional groups increases. We also use lattice dynamics simulations to investigate the role that functional group length has on phonon localization within the framework. We observe significant localization, even for small functional groups, and this localization is primarily restricted to the functional groups themselves. Despite this enhanced hybridization and localization, which has been attributed to the reduction of thermal conductivity in gas infiltrated MOFs, the thermal conductivity of these systems is not significantly altered by varying the length of the attached functional group and may even be enhanced for longer functional groups.
This work provides a comprehensive computational insight into the effect of attached functional groups on the thermal transport properties of organic frameworks and will assist in the further development of organic frameworks utilizing functional groups with targeted applications towards gas storage and sensing.