Powder Structure Analysis of Vapochromic Quinolone Antibacterial Agent Crystals

Vapochromic materials, or those that show a reversible color change induced by vapor, are expected to serve as valuable sensors for volatile organic compounds or humidity. Crystals of pipemidic acid (PPA), a quinolone antibacterial agent, were found to exhibit vapochromism, as they undergo a reversible color change in the presence of acetonitrile vapor. The colorless trihydrate phase transformed into a yellow anhydrous phase upon exposure to acetonitrile vapor and returned to the trihydrate phase under high humidity. <i>Ab initio</i> structure determination from powder diffraction and solid state ¹³C NMR measurements revealed that the molecule exists in its zwitterionic form in the colorless trihydrate phase, whereas it is non-zwitterionic in the anhydrous phase because of the rearrangement of hydrogen bonds, due to dehydration in the crystal state. Theoretical calculations revealed that the color change in PPA is due to the change in the molecular electronic state upon taking the non-zwitterionic form, which generates a new highest occupied molecular orbital (HOMO) state, thus leading to a HOMO–lowest unoccupied molecular orbital (LUMO) transition with a lower energy

Mechanochromic Luminescence Based on Crystal-to-Crystal Transformation Mediated by a Transient Amorphous State

Photoluminescent materials that exhibit tunable emission properties when subjected to mechanical stimuli have numerous potential applications. Although many organic/inorganic and organometallic compounds display this property, called mechanochromic luminescence, most of these materials undergo a crystalline-to-amorphous (C → A) phase transition; examples of crystalline-to-crystalline (C₁ → C₂) transformation are rare. Single-crystal X-ray diffraction may allow direct analysis of the molecular packing of mechanochromic luminescence materials before and after C₁ → C₂ transformation, which may help to understand the underlying mechanism of this transformation. Reported herein is a mechanochromic luminescence material that displays an unprecedented type of C₁ → C₂ transformation mediated by a transient amorphous phase (C₁ → [A] → C₂). This mechanochromic luminescence material was developed by introducing soft triethylene glycol side chains in a crystalline gold­(I) complex that exhibits mechanochromic luminescence based on a C → A phase transition. When this new gold­(I) complex bearing triethylene glycol chains was subjected to a mechanical or thermal stimulus, dynamic phase changes were observed with irreversible luminescence color changes from blue to yellow to green in both the cases. The crystallinity of the mechanically generated C₂ phase was lower than that of the thermally generated C₂ phase. This is because the mechanically induced C₁ → [A] → C₂ process was finished within seconds, whereas the thermal C₁ → [A] → C₂ process occurred over a few minutes. To control the C₁ → [A] → C₂ transformation, we doped the complex with an inactive soft component. This successfully made the transformation reversible (from green to blue) upon thermal annealing of the mechanically obtained C₂ phase. This approach allowed the development of an imaging process involving invisible information storage even under UV illumination