Evidence of Low-Temperature Phase Transition in Tetracene-Tetracyanoquinodimethane Complex

Multitemperature X-ray diffraction and vibrational spectroscopic studies of tetracene-tetracyanoquinodimethane (TCNQ) charge transfer cocrystals have revealed the existence of a reversible phase transition in the temperature interval 150-200 K. It was shown that a new monoclinic polymorph of the cocrystal exists above that temperature range, with a structure that is very similar to the one previously described in the literature as being triclinic. The crystal structures of the two polymorphs were also calculated with periodic density functional theory quantum chemical calculations, and their geometries were compared to the experimental ones. The degree of charge transfer in both polymorphs was characterized using alteration in the bond lengths of the TCNQ molecule and frequency shifts obtained from vibrational spectroscopy data.