Spectroscopy of electron donor-acceptor systems

The formation of complexes between electron donors and acceptors has long been recognized a s an important phenomenon. Many of themolecular complexes are colored and give rise to new absorption bands in the electronic spectra. These molecular complexes have been investigated by employing a variety of physical methods such as optical spectroscopy, magnetic resonance spectroscopy, calorimetry, dielectric measurements, and X-ray diffraction. Studies of donor-acceptor complexes are of value in understanding various types of reaction mechanisms, including molecular phenomena in biological systems. Some of the relatively recent developments in the study of molecular complexes are: the classification of donors and acceptors by Mulliken [l], vibrational spectroscopy, particularly in the very-low-frequency region [2,3], estimation of the per cent charge transfer by dipole moment measurements [4-6], studies of weak interactions (contact pairs) [7], spectra and thermodynamics of complexes in vapor phase [8,9], study of radical ions produced by charge-transfer intermediates [10,11], charge-transfer processes in biological systems [12], and thermal as well as photochemical reactions involving charge-transfer processes [10]