the solid state a valuable parameter is the elongation of the X–X or X–Y bond length, resulting from the antibonding nature of the σ* LUMO. The elongation can be evaluated by means of structural determinations (XRD) and FT-Raman spectroscopy. A well-known example is the complex formed by iodine when combined with starch, which exhibits an intense purple charge-transfer band. This has widespread use as a rough screen for counterfeit currency. Unlike most paper, the paper used in US currency is not sized with starch. Thus, formation of this purple color on application of an iodine solution indicates a counterfeit. TTF-TCNQ: prototype for electrically conducting complexes In 1954, charge-transfer salts derived from perylene with iodine or bromine were reported with resistivities as low as 8 ohm·cm. In 1973, it was discovered that a combination tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (TTF) form a strong charge-transfer complex, referred to as TTF-TCNQ. The solid shows almost metallic electrical conductance and was the first discovered purely organic 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 holes 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. Superconductivity is exhibited by tetramethyl-tetraselenafulvalene-hexafluorophosphate (TMTSF2PF6), which is a semi-conductor at ambient conditions, shows superconductivity at low temperature (critical temperature) and high pressure: 0.9 K and 12 kbar. Critical current densities in these complexes are very small. Mechanistic implications Many reactions involving nucleophiles attacking electrophiles can be usefully assessed from the perspective of an incipient charge-transfer complex. Examples include electrophilic aromatic substitution, the addition of Grignard reagents to ketones, and brominolysis of metal-alkyl bonds. See also Organic semiconductor Organic superconductor Exciplex - a special case where one of the molecules is in an excited state Historical sources Y. Okamoto and W. Brenner Organic Semiconductors, Rheinhold (1964) References Physical organic chemistry Molecular electronics Organic semiconductors