Oriented Single-Crystal-to-Single-Crystal Phase Transition with Dramatic Changes in the Dimensions of Crystals

We report here a new polymorph of
cocrystal CuQ2-TCNQ
that shows an oriented single-crystal-to-single-crystal phase transition
along its a-axis at ambient conditions. Upon mechanical
stimulation, it converts into another polymorph accompanied by almost
doubling its length and halving its thickness. Our crystallographic
studies indicate the dramatic changes in crystal dimensions resulted
from the prominent changes of molecular stacking patterns. A reasonable
mechanism for the phenomenon was proposed on the basis of the structural,
microscopic, and thermal analysis.