Polymorphism in the $\bf [(TTF)_2(TCNE)_2]$ Charge-Transfer Complex

The reaction between tetrathiafulvalene (TTF) and tetracyanoethylene (TCNE) yields two polymorphs ($\alpha$ and $ \beta$) both having formula $\rm [(TTF)_2^{2\delta +}(TCNE)_2^{2\delta -}]$ with similar molecular structure. Both polymorphs display a stack structure of the type $\cdots$(TTF-TTF)(TCNE-TCNE)(TTF-TTF) (TCNE-TCNE)$\cdots$ but they differ in crystal structure for hydrogen bonding system and lamellar sheets. These two CT complexes possess an high resistivity ($\cong 10^9~\Omega$.cm) and a degree of charge-transfer of $\delta =0.54e$ estimated through the C-S bond length