Organic Molecular Crystals: Charge-Transfer Complexes

Abstract

Solutions of electron donors (D) and acceptors (A) exhibit absorptions not associated with either individual molecule.(1) Mulliken(2) proposed that the new absorptions involve an electron transfer from D to A, as indicated by $${{\psi }_{complex}}\left( DA \right)\underrightarrow{h{{v}_{CT}}}{{\psi }_{complex}}\left( {{D}^{+}}{{A}^{-}} \right)$$ (1) Here hv CT is the charge-transfer (CT) excitation energy, and the CT band is explicitly associated with the molecular complex formed between a D and an A molecule. The nature of such DA complexes has been the subject of extensive experimental and theoretical studies.(3–5) The energetics of dimer formation; the geometry of the complex; the systematics of varying D, A, and solvents; and the nature of CT absorption have been explored. The CT excitation is greatly enhanced,(3,4,6) as suggested by Mulliken,(2) via small ad-mixtures of the excited singlet configuration 1|D+A−> into the ground state, |DA> However, the additional configuration-interaction stabilization due to this small admixture does not(6,7) usually dominate the binding energy or the geometry of the complex. As we will discuss in Section 2, the full quantum-mechanical treatment of a DA dimer thus requires explicit consideration of a variety of intermolecular forces.