Green and blue polymorphs: A single-crystal-to-single-crystal (SCSC) phase transition of phenyl(3,5-dimethylphenyl isocyanide)gold(I) was triggered by mechanical picking or solid seeding and propagated spontaneously with a domino-like mechanism. As a result, one phase with intense green emission was transformed to another phase with weaker blue emission.

We report the first photoinduced single-crystal-to-single-crystal phase transition of a gold complex that involves shortening of intermolecular aurophilic bonds. The gold(i) isocyanide complex also shows a photosalient effect.

Mechano-induced phase transitions in organic crystalline materials that alter their properties have received much attention. However, most mechano-responsive molecular crystals exhibit crystal to amorphous phase transitions on mechanical stimulation, and the intermolecular interaction patterns in the daughter phase are difficult to characterize. We investigated phenyl(phenylisocyanide)gold(I) (1) and phenyl(3,5-dimethylphenylisocyanide)gold(I) (2) complexes that exhibit a mechanotriggered single crystal to single crystal phase transition. Previous reports of 1 and 2 focused on relationships between the crystalline structures and photoluminescence properties; in this work we have focused on other aspects. The face index measurements of 1 and 2 before and after the mechano-induced phase transitions indicate they undergo non-epitaxial phase transitions without a rigorous orientational relationship between the mother and daughter phases. Differential scanning calorimetry analyses revealed the phase transition of 1 to be enthalpically driven by the formation of new aurophilic interactions. In contrast, the phase transition of 2 is entropically driven, with an empty void in the mother phase, not present in the daughter phase. Scanning electron microscopy observation showed that the degree of the charging effect of both 1 and 2 was changed by the phase transitions, suggesting that the formation of the aurophilic interactions affords more effective conductive pathways. Moreover, flashphotolysis time resolved microwave conductivity measurements revealed that 1 increased in conductivity after the phase change, whereas the conductivity of 2 decreased. These contrasting results were explained by the different patterns in the aurophilic interactions. Finally, an intriguing disappearing polymorphism of 2 is reported, in which a polymorph form could not be obtained again after some period of time even with repeated trials. The present studies provide us with a variety of hitherto unknown insights into mechano-responsive molecular crystals, which help us understand the phase transition behaviors upon mechanical stimulation and establish rational design principles.