Solid-state dynamics and single-crystal to single-crystal structural transformations in...

Reactive octahedral silsesquioxanes of rod-like [octakis(3-chloropropyl)octasilsesquioxane - T₈(CH₂CH₂CH₂Cl)₈] and spherical [octavinyloctasilsesquioxane - T₈(CH[double bond, length as m-dash]CH₂)₈] structure can undergo reversible thermally induced phase transitions in the solid state. The phase behaviour has been studied with differential scanning calorimetry (DSC, including temperature modulated DSC), X-ray diffraction, dielectric relaxation spectroscopy (DRS), and nuclear magnetic resonance spectroscopy in the solid state (SS NMR), as well as positron annihilation lifetime spectroscopy (PALS) and polarized optical microscopy (POM). The mechanisms involving fitting the molecules into most symmetrical crystal lattices vary for species of different structure. Thermal energy can be used to expand the crystal lattice leading to thermochromism in the case of T₈(CH[double bond, length as m-dash]CH₂)₈ or conversely to an unusual negative thermal expansion of crystals of T₈(CH₂CH₂CH₂Cl)₈ that results in their self-actuation. The complex behaviour is reflected in unusual changes in the capacitance and fractional free volume of the material. These phenomena can be used for molecular design of advanced well-defined hybrid materials capable of reversible thermally induced structural transformations. The findings present a new perspective for POSS-based flexible metal-organic frameworks (MOF) of cooperative structural transformability via entropy-based translational sub-net sliding.