Planarity Control of Porphyrin Based Supramolecules for Enhanced Thermal Conductivity

As heat management becomes more important in the electronics industry, many researchers have tried to improve the low thermal conductivity of organic materials used in flexible and miniaturized devices. In this regard, many researches have enhanced thermal conductivity of soft materials through utilizing the mesogen with the extended π-conjugated length or increasing crystallinity. However, there are not enough studies on enhancing and adjusting the thermal conductivity of polymers by deep understanding the correlation between the supramolecular packing structure and thermal properties. For this kind of study, porphyrin-based reactive metallomesogens (PBRM-n; n= -2H, -Ni, -Cu, - Zn) were newly designed and synthesized. Porphyrin core is purposely selected due to the extended π-conjugated aromatic chemical structure and easy metal-substitution. Its substitution with different metals changes the molecular planarity, which can induce the different molecular packing structure and thermal conductivity. Based on thermal and scattering analyses, it is realized that PBRM-2H exhibits discotic columnar hexagonal structure that is tilted and obliquely stacked without π-π stacking. PBRM-Ni has discotic columnar hexagonal structure without π-π stacking. On the other hand, PBRM-Cu, and Zn exhibit columnar rectangular structure with π-π stacking. When we confirm the energy-minimized core geometry of PBRM-n compounds in the isolated gas phase by using molecular simulation and the measured macroscopic property, the tilted angle from molecular horizontal plane depending on the substituted metal atoms is 5.4°, 17°, 8.8°, 3° (PBRM-2H, -Ni, -Cu, and Zn, respectively). It indicates that PBRM-Zn has relatively small distortion because Zn atom matches well in the hole of the porphyrin core. Therefore, PBRM-Zn could have dense molecular packing structure and higher crystallinity than other species. PBRM-2H has relatively high planarity in the isolated gas phase, but it has a mesomorphic structure due to its large in-plane distortion in the solid state, resulting in relatively loose molecular packing and low crystallinity. PBRM-n films were prepared by irradiating UV light at 50 °C for 2 hours and then their thermal conductivity and several stabilities were evaluated. The fabricated PBRM-n films have no micro voids in the cross-section and endure scratch up to 2H (hardness). In addition, PBRM-n films show good chemical stability in various organic solvents and are thermally stable up to 350 °C due to polymer network structure. The thermal conductivity of PBRM-n films is measured to be 0.88, 1.02, 0.97, and 1.2 Wm^-1K^-1 (PBRM-2H, -Ni, -Cu, and -Zn, respectively). It is realized that the PBRM-Zn film has the highest thermal conductivity (1.2 Wm^-1K^-1) due to the flattest porphyrin core and high crystallinity. As a result, this study proves the importance of understanding the correlation between chemical structure, chemical conformation, supramolecular packing and thermal conductivity when we develop advanced thermal interface materials. This work was supported by the BK21 FOUR, Mid-Career Researcher Program (2021R1A2C2009423) and Basic Research Laboratory Program (2020R1A4A1018259).