Polar-axis-deformation induced molecular pyroelectrics for waste heat harvesting

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18-crown-6-based supramolecular crystals can exhibit structural flexibility or polarity. For example, nitromethane solvate 18C6 is highly structurally flexible due to Van der Waal interactions between nitromethane and 18C6. Additionally, (4-methoxyanilinium)(18C6)(BF4) shows polarity due to the polar -OCH3 group, allowing the crystal to exhibit piezoelectricity, pyroelectricity, and ferroelectricity. Combining polarity and structural flexibility in an 18C6 crystal is achievable using polar ammoniums with an increased number of -CH2 groups.Our preliminary study found that that the combination of 3-hydroxyl-adamantane-1-aminium (HADA-OH) with 18C6 produced a crystal that was both structurally flexible and polar due to the -OH group. The polar (HADA-OH)(18C6)ClO4 crystal was found to exhibit both the highest structural flexibility among organic salts and large pyroelectricity (20 times that of PVDF).This study focuses on HADA-R derivatives (R = H, F, Cl, Br) and examines the effects of different -R groups and counter anions on crystal packing, flexibility, and pyroelectricity. The goal is to establish a relationship between design, structure, and performance in this system and propose a novel strategy for high-performance pyroelectrics for waste heat harvesting.