Classification of closed shell TCNQ salts into structural families and comparison of diffraction and spectroscopic methods of assigning charge states to TCNQ moieties

  • Herbstein F
  • Kapon M
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Abstract

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. 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 TCNQ1? and have either (i) segregated monad TCNQ stacks, (ii) mixed-stack {[cation+][TCNQ1?]} arrangements or (iii) isolated {2[cation+] ?·?[(TCNQ1?)2]} π-dimers, while the remaining two contain both TCNQ0 and TCNQ1? and have compositions (iv) {[cation+] ]} and (v) {[cation+]2 ?·? }. 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. Each structural group has been analysed in the same way. 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. 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. Bond lengths have been surveyed for the various types of TCNQ moiety. 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. Although bond lengths have provided, for almost 40 years, a popular way to assess moiety charge, our present review suggests that much caution is required in the application of this method not only to individual structures determined at room temperature but even to those determined at very low temperatures. Resonance Raman spectroscopy provides an alternative method of assessing moiety charge and is determinative with singly charged TCNQ moieties but not with other TCNQ species; both methodology and correlation with diffraction results require further investigation. Most studies of electrical conductivity lack detail about orientation and temperature dependence and thus are difficult to relate to packing arrangements. ESR measurements generally confirm strong antiferromagnetic coupling within π-dimers. 1An abbreviated version was presented at the 2007 Salt Lake City Meeting of the American Crystallographic Association on the occasion of the Fankuchen Award to FHH.
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. 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 TCNQ1? and have either (i) segregated monad TCNQ stacks, (ii) mixed-stack {[cation+][TCNQ1?]} arrangements or (iii) isolated {2[cation+] ?·?[(TCNQ1?)2]} π-dimers, while the remaining two contain both TCNQ0 and TCNQ1? and have compositions (iv) {[cation+] ]} and (v) {[cation+]2 ?·? }. 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. Each structural group has been analysed in the same way. 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. 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. Bond lengths have been surveyed for the various types of TCNQ moiety. 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. Although bond lengths have provided, for almost 40 years, a popular way to assess moiety charge, our present review suggests that much caution is required in the application of this method not only to individual structures determined at room temperature but even to those determined at very low temperatures. Resonance Raman spectroscopy provides an alternative method of assessing moiety charge and is determinative with singly charged TCNQ moieties but not with other TCNQ species; both methodology and correlation with diffraction results require further investigation. Most studies of electrical conductivity lack detail about orientation and temperature dependence and thus are difficult to relate to packing arrangements. ESR measurements generally confirm strong antiferromagnetic coupling within π-dimers. 1An abbreviated version was presented at the 2007 Salt Lake City Meeting of the American Crystallographic Association on the occasion of the Fankuchen Award to FHH.

Author-supplied keywords

  • Assessing moiety charge
  • Raman resonance spectroscopy
  • TCNQ structural families
  • X-ray diffraction

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Authors

  • Frank H. Herbstein

  • Moshe Kapon

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