Editing Charge-transfer complex (section)

===TTF-TCNQ: prototype for electrically conducting complexes===
{{Anchor|TTF-TCNQ}}
[[File:SegStackEdgeOnHMTFCQ.jpg|thumb|Edge-on view of portion of crystal structure of hexamethylene[[Tetrathiafulvene|TTF]]/TCNQ charge transfer salt, highlighting the segregated stacking.<ref>{{cite journal|author1=D. Chasseau|author2=G. Comberton|author3=J. Gaultier|author4=C. Hauw|journal=Acta Crystallographica Section B|title=Réexamen de la structure du complexe hexaméthylène-tétrathiafulvalène-tétracyanoquinodiméthane|year=1978| volume=34|issue=2|page=689|doi=10.1107/S0567740878003830|doi-access=|bibcode=1978AcCrB..34..689C }}</ref>]]
[[File:SegStackEndOnHMTFCQ.jpg|thumb|End-on view of portion of crystal structure of hexamethylene[[Tetrathiafulvene|TTF]]/TCNQ charge transfer salt.  The distance between the TTF planes is 3.55 Å.]]
In 1954, charge-transfer salts derived from [[perylene]] with [[iodine]] or [[bromine]] were reported with resistivities as low as 8 ohm·cm.<ref name=Goetz/> In 1973, it was discovered that a combination of [[TCNQ|tetracyanoquinodimethane]] (TCNQ) and [[tetrathiafulvalene]] (TTF) forms a strong charge-transfer complex referred to as ''TTF-TCNQ''.<ref>{{cite journal |author1=P. W. Anderson |author2=P. A. Lee |author3=M. Saitoh | journal = [[Solid State Communications]] | volume = 13 | year = 1973 | pages = 595–598 | doi = 10.1016/S0038-1098(73)80020-1 | title = Remarks on giant conductivity in TTF-TCNQ |issue=5 | bibcode=1973SSCom..13..595A}}</ref> The solid shows almost metallic electrical conductance and was the first-discovered purely organic [[conductor (material)|conductor]]. In a TTF-TCNQ crystal, TTF and TCNQ molecules are arranged independently in separate parallel-aligned stacks, and an electron transfer occurs from donor (TTF) to acceptor (TCNQ) stacks. Hence, electrons and [[electron hole]]s are separated and concentrated in the stacks and can traverse in a one-dimensional direction along the TCNQ and TTF columns, respectively, when an electric potential is applied to the ends of a crystal in the stack direction.<ref>{{cite journal |doi=10.1021/acs.jchemed.5b00340|title=Opposites Attract: Organic Charge Transfer Salts |year=2015 |last1=Van De Wouw |first1=Heidi L. |last2=Chamorro |first2=Juan |last3=Quintero |first3=Michael |last4=Klausen |first4=Rebekka S. |journal=Journal of Chemical Education |volume=92 |issue=12 |pages=2134–2139 |bibcode=2015JChEd..92.2134V }}</ref>

[[Superconductivity]] is exhibited by tetramethyl-tetraselenafulvalene-hexafluorophosphate (TMTSF<sub>2</sub>PF<sub>6</sub>), which is a semi-conductor at ambient conditions, shows superconductivity at low [[temperature]] ([[critical temperature]]) and high [[pressure]]: 0.9 [[Kelvin|K]] and 12 k[[bar (unit)|bar]]. Critical current densities in these complexes are very small.