Q8(e). Charge-transfer spectra of donor–acceptor complexes

Introduction. When an electron donor D (low ionisation energy, high-lying HOMO) meets an electron acceptor A (high electron affinity, low-lying LUMO), a weak charge-transfer (CT) complex D·A forms. It shows a new, broad, intense absorption band that neither D nor A has alone: the CT band, in which light moves an electron from D to A.

1. Mulliken theory: ground state Ψ_N = a ψ(D,A) + b ψ(D⁺A⁻) with a ≫ b (mostly no-bond); excited state Ψ_E is mostly the ion pair D⁺A⁻.
2. The CT transition Ψ_N→Ψ_E transfers an electron from the HOMO of D to the LUMO of A. Its energy is hν_(CT) ≈ I_D - E_A - C (I_D ionisation energy of D, E_A electron affinity of A, C Coulomb attraction in D⁺A⁻).

Figure: The CT transition (red) needs less energy than either partner's own transition (dashed).

Example: benzene (or hexamethylbenzene) with iodine

1. Iodine is violet in CCl₄ but brown in benzene. Benzene (π donor) and I₂ (σ* acceptor) form a 1: 1 complex with a strong new band in the near UV (near 290–300 nm), absent in both components.
2. With tetracyanoethylene (strong π acceptor) and methylbenzenes the CT band moves into the visible and the solutions are coloured; with more methyl groups (lower I_D) the band shifts to longer wavelength: a straight line of hν_(CT) against I_D is the proof of charge transfer.
3. CT bands are broad (the complex has many geometries) and intense (ε of the order of 10³–10⁴), because the electron moves a long distance (large transition dipole).

Conclusion. A CT spectrum is the absorption of light that moves an electron from a donor to an acceptor in their complex; its energy follows hν_(CT) ≈ I_D - E_A - C. Benzene–I₂ (brown, new UV band) and TCNE–arene complexes (coloured) are typical; lower donor ionisation energy gives longer CT wavelength.

Application. CT complexes explain the colours of picrates used to identify arenes, starch–iodine, organic conductors (TTF–TCNQ) and many inorganic colours (MnO₄⁻, ligand-to-metal CT); CT bands are used to measure ionisation energies of donors.