The differences are not large, the bond most sensitive to charge being the extra-ring double bond ( c), while bond length differences for adjacent single-bond double-bond pairs (intra-ring ( b – a) and extra-ring ( c – d)) also provide a way of distinguishing different charge states. Bond lengths have been surveyed for the various types of TCNQ moiety. The several probes are (i) X-ray diffraction, giving moiety geometries (bond lengths and deviations from planarity) (ii) infra-red and resonance Raman spectroscopy for identification of charge states (iii) electron spin resonance and magnetic measurements (where applicable) to give further details of moiety structure. First, the crystal chemistry is described, then various probes of increasing sensitivity are applied to establish the charge state of the moieties and their relation to the packing arrangement. Each structural group has been analysed in the same way.
Each family is found with a characteristic isostructural packing arrangement, there being differences of detail among the members there are some exceptions to the overall rule, usually with combinations of standard packing elements forming a non-standard overall arrangement. In the crystalline state the 7,7,8,8-tetracyanoquinodimethane (TCNQ) moiety is found in mixed stack π–π* molecular compounds with neutral TCNQ as acceptor and a variety of neutral donors, and as five types of salt, three of which contain only TCNQ 1− and have either (i) segregated monad TCNQ stacks, (ii) mixed-stack.
More than 50 crystals containing 7,7,8,8-tetracyanoquinodimethane (TCNQ) in various guises are classified into a limited number of structural types and the possible assignment of charge states by diffraction and spectroscopic methods is compared.
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