Topochemical Reaction Induces Anisotropy, Decreasing Solid-State Thermal Conductivity

Realizing organic materials that exhibit a dynamic thermal conductivity requires a fundamental understanding of how molecular structure and processing affect thermal transport. Herein, we demonstrate that the photoinduced polymerization of [2,2'-bi-1H-indene]-1,1'-dione-3,3'-diheptylcarboxylate (BIT) into polyBIT results in over a 4-fold decrease in thermal conductivity as measured on polycrystalline thin-films in the through-plane direction, mostly perpendicular to the chain growth direction. Experimental determination of the material's decreased heat capacity supports this view. Through theoretical calculations, we attribute this decrease in thermal conductivity in part to induced anisotropy in the polymer. We also discuss the non-negligible changes in morphology, phase transitions, and thermal degradation that serve to limit the thermal depolymerization reaction. This work highlights the different contributions one must consider when designing an organic thermal switch that operates in the solid-state.