Driving forces for the phase transition of CuQ(2)-TCNQ molecular crystals

The driving forces for the phase transition and relative stability of the two forms of CuQ(2)-TCNQ molecular crystals have been studied using inelastic neutron scattering (INS), density functional theory (DFT), and Hirshfeld surface analysis. DFT molecular dynamics (MD) simulations show that form-II has a lower enthalpy, but with increasing temperature form-I becomes thermodynamically stable due to the greater entropy. INS and MD simulations both show that the entropy of the hydrogen-bond network that holds molecules together within layers is higher in form-I. The interlayer pi-pi interactions are also weaker in form-I, leading to an overall loosening of the structure. The phase transition is kinetically hindered by the requirement to re-optimize the orientation of the layers. The strong H-bond interactions keep the in-plane atomic arrangement stable, while the weak interlayer p-p interactions provide the coupling between layers during the phase-transition. This subtle interplay of the two interactions maintains the integrity of the crystal upon phase transition even with dramatic physical dimension changes.