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. Ab initio structure determination from powder diffraction and solid state C-13 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.
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