Short lead(II) soaps: from weakly fluorescent crystals to strongly phosphorescent and structurally varied vitreous phases. A thermal, structural and spectroscopic study

摘要

Short lead(II) alkanoates, from propionate to heptanoate, show a very intricate and reversible thermal behaviour, presenting crystalline phases and three different glass states (regular or amorphous, liquid crystal and rotator glasses) with different degrees of ordering depending on the alkyl chain length. A thorough thermal study was carried out in order to study the different phases and to analyze the thermodynamic parameters. The crystal structures of the compounds were solved by X-ray diffraction, showing similar arrangements of the 2D molecular stacking. PDF analyses of the local order in the glass structures showed shorter first neighbour lead-lead interatomic distances than in the crystalline structures. This allows establishment of a direct relationship between the structure and optical properties. Luminescence properties are, in fact, impressively enhanced in the glass states, passing from weak fluorescence at 77 K in the crystal phase to strong phosphorescence in the frozen glasses, which persists at room temperature. The high variability and the structure-property relationship described here pave the way for the design of materials with varied luminescence properties based on fine-tuning of their local structure.