The structure phase transition is responsible for these properties, as it leads to changes in the π-orbital overlap between the anion and cation in a mixed stack. Additionally, 1 exhibits irreversible mechanochromism, intense near-IR absorbance, and a dielectric anomaly. This is the first report of a green-colored crystal of bis(maleonitriledithiolato)nickelate(II) salt. Upon heating, 1 undergoes a reversible phase transition at around 340/320 K during the first heating/cooling cycle, resulting in rapid thermochromism with a color change from green (stable state) to red (metastable state) within a few seconds. These mixed stacks then combine to form a molecular solid through Coulomb and van der Waals interactions. In this study, we investigated a Ni-dithiolene dianion salt, 1,1'-diheptyl-4,4'-bipyridinium bis(maleonitriledithiolato)nickelate ( 1), and found that its cations and anions stack alternately to form a uniform mixed stack. A thermochromic or mechanochromic material can switch between at least two stable states in response to changes in temperature or static pressure/strain.

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