Inchemistry,charge-transfer (CT)complex, orelectron donor-acceptor complex, describes a type ofsupramolecular assembly of two or more molecules or ions. The assembly consists of twomolecules that self-attract throughelectrostatic forces, i.e., one has at least partial negative charge and the partner has partial positive charge, referred to respectively as theelectron acceptor andelectron donor. In some cases, the degree of charge transfer is "complete", such that the CT complex can be classified as a salt. In other cases, the charge-transfer association is weak, and the interaction can be disrupted easily by polar solvents.

A number of organic compounds form charge-transfer complex, which are often described aselectron-donor-acceptor complexes (EDA complexes). Typical acceptors are nitrobenzenes or tetracyanoethylene (TCNE). The strength of their interaction with electron donors correlates with the ionization potentials of the components. For TCNE, thestability constants (L/mol) for its complexes with benzene derivatives correlates with the number of methyl groups:benzene (0.128),1,3,5-trimethylbenzene (1.11),1,2,4,5-tetramethylbenzene (3.4), andhexamethylbenzene (16.8).^[2] A simple example for a prototypicalelectron-donor-acceptor complexes isnitroaniline.^[3]

1,3,5-Trinitrobenzene and related polynitrated aromatic compounds, being electron-deficient, form charge-transfer complexes with many arenes. Such complexes form upon crystallization, but often dissociate in solution to the components. Characteristically, these CT salts crystallize in stacks of alternating donor and acceptor (nitro aromatic) molecules, i.e. A-B-A-B.^[4]

Early studies on donor-acceptor complexes focused on thesolvatochromism exhibited by iodine, which often results from I₂ forming adducts with electron donors such as amines andethers.^[5] Dihalogens X₂ (X = Cl, Br, I) and interhalogens XY(X = I; Y = Cl, Br) are Lewis acid species capable of forming a variety of products when reacted with donor species. Among these species (including oxidation or protonated products), CT adducts D·XY have been largely investigated. The CT interaction has been quantified and is the basis of many schemes for parameterizing donor and acceptor properties, such as those devised by Gutmann, Childs,^[6]Beckett, and theECW model.^[7]

Many organic species featuring chalcogen or pnictogen donor atoms form CT salts. The nature of the resulting adducts can be investigated both in solution and in the solid state.

In solution, the intensity of charge-transfer bands in the UV-Vis absorbance spectrum is strongly dependent upon the degree (equilibrium constant) of this association reaction. Methods have been developed to determine the equilibrium constant for these complexes in solution by measuring the intensity of absorption bands as a function of the concentration of donor and acceptor components in solution. TheBenesi-Hildebrand method, named for its developers, was first described for the association of iodine dissolved in aromatic hydrocarbons.^[8]

In 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.^[9] The elongation can be evaluated by means of structural determinations (XRD)^[10] and FT-Raman spectroscopy.^[11]

A well-known example is the complex formed byiodine when combined withstarch, which exhibits an intense purplecharge-transfer band. This has widespread use as a rough screen for counterfeit currency. Unlike most paper, the paper used in US currency is notsized with starch. Thus, formation of this purple color on application of an iodine solution indicates a counterfeit.

In 1954, charge-transfer salts derived fromperylene withiodine orbromine were reported with resistivities as low as 8 ohm·cm.^[4] In 1973, it was discovered that a combination oftetracyanoquinodimethane (TCNQ) andtetrathiafulvalene (TTF) forms a strong charge-transfer complex referred to asTTF-TCNQ.^[13] The solid shows almost metallic electrical conductance and was the first-discovered purely organicconductor. 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 andelectron 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.^[14]

Superconductivity is exhibited by tetramethyl-tetraselenafulvalene-hexafluorophosphate (TMTSF₂PF₆), which is a semi-conductor at ambient conditions, shows superconductivity at lowtemperature (critical temperature) and highpressure: 0.9K and 12 kbar. Critical current densities in these complexes are very small.

Many reactions involving nucleophiles attacking electrophiles can be usefully assessed from the perspective of an incipient charge-transfer complex. Examples includeelectrophilic aromatic substitution, the addition ofGrignard reagents to ketones, and brominolysis of metal-alkyl bonds.^[15]

• Exciplex – a special case where one of the molecules is in an excited state
• Organic semiconductor
• Organic superconductor

The electronic structure of a charge-transfer (CT) complex is a result ofthe electronic coupling between an electron donor (D) and an electronacceptor (A), in which partial or complete redistribution of electroniccharge can take place. Unlike isolated molecules, CT complexes are typicallycharacterized by electronic wavefunctions that are mixtures of neutral andionic structures rather than those of individual molecules.The electronic ground and excited states of a CT complex may be characterizedvia two limiting diabatic structures: a neutral one,${\displaystyle |D^0{A}^0⟩}$, and an ionic one,${\displaystyle |D^{+}{A}^{-}⟩}$, associated with electron transfer between thedonor and acceptor. The actual electronic ground and excited states are linearcombinations of these structures. Observable effects including the permanentdipole moment, optical absorption intensity, and degree of charge separationare thus dependent on the contribution of the ionic part to these states.^[16]

A traditional theoretical representation of the CT complex can be describedvia a two-state Hamiltonian, expressed in the basis of neutral and ionicdiabatic configurations,

where${\displaystyle E_N}$ and${\displaystyle E_I}$ are the diabatic energies of theneutral and ionic states, respectively, and${\displaystyle V_{DA}}$ indicates thedonor–acceptor electronic coupling. The diagonalized form of this Hamiltoniangives adiabatic electronic states, whose energies and charge-transfercharacter depend on both the energy separation of the diabatic states and thestrength of the donor–acceptor coupling.

A molecular orbital description of charge transfer is usually used, in whichthe donor and acceptor frontier molecular orbitals dominate the interaction.The HOMO of the donor and the LUMO of the acceptor generally govern the CTinteraction. Optical excitation corresponding to a CT transition can beconsidered as promotion of an electron from the donor HOMO to the acceptorLUMO, resulting in increased electronic charge separation and a largetransition dipole moment.

The energy difference between neutral and ionic diabatic structures isgoverned by key electronic parameters including the donor ionizationpotential, acceptor electron affinity, and the Coulomb interaction betweenthe resultant charges. Together, these quantities dictate the thermodynamicdriving force for charge transfer and the resulting equilibrium degree ofionicity of the CT complex.

Charge-transfer absorption bands arise from electronic transitions betweenmixed neutral and ionic states and are distinct from local excitations of theindividual donor or acceptor molecules. These CT absorption bands aredetermined by both the diabatic energy splitting and the electronic couplingstrength, and are typically analyzed using the Mulliken–Hush approach.^[17]

The environment of CT complexes has a strong influence on their electronicstructure. Increased solvation stabilizes the ionic configuration relativeto the neutral state and often enhances charge separation, shifting CTabsorption bands to lower energies. Differences in donor–acceptor distanceand mutual orientation similarly influence the electronic coupling and theobserved magnitude of charge transfer.^[18]

• Y. Okamoto and W. BrennerOrganic Semiconductors, Rheinhold (1964)
• H. Akamatsu; H. Inokuchi; Y. Matsunaga (1954). "Electrical Conductivity of the Perylene–Bromine Complex".Nature.173 (4395):168–169.Bibcode:1954Natur.173..168A.doi:10.1038/173168a0.S2CID 4275335.

• Physical organic chemistry
• Molecular electronics
• Organic semiconductors

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