Systems with CT Transitions

Solutions of a mixture of an electron donor D, such as hexamethylbenzene (40), and an electron acceptor A, such as chloranil (41), often exhibit an absorption band that is not present in solutions of the pure components. This band has been assigned to an electron transfer from the donor to the acceptor and is therefore referred to as a donor-acceptor or a CT (charge transfer) transition. Mulliken (1952) has developed a theory of CT transitions which can be summarized as follows If the ground configuration and the polar configuration produced by electron transfer are described by wave functions I)da and respectively, the interaction of these two configurations 

Under the assumption that the ZDO approximation is valid for an MO description of the components, the excitation energy of the CT band is easy to estimate. For the transition of an electron from the HOMO of the donor into the LUMO of the acceptor 

In many series the intensity of the CT band increases as the complex becomes more stable. This can be explained by using the wave functions 

Since the first integral vanishes if use is made of the ZDO approximation, the intensity is seen to be essentially due to the dipole moment of the polar CT configuration, and the transition moment is proportional to the coefficient A. But in reality the situation is much more complicated, as can be seen from the fact that the intensity can be markedly different from zero even in cases where the ground-state stabilization is very weak and A is therefore approximately zero. The reason may be that the wave functions Equation (2.28) and Equation (2.29) neglect locally excited states (Dewar and Thompson, 1966). Furthermore, it might be important, at least in cases of weak interaction, to take into account the overlap between donor and acceptor orbitals in calculating the transition moment from Equation (2.31). 

If a molecule consists of several weakly coupled chromophores it may be advantageous to speak of intramolecular CT transitions. The MIM method for calculating the absorption spectra of complex molecules, which has been mentioned in the discussion of substituent effects in Section 2.4.2, is based on this idea. 

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The p-isomer has superior properties to its o-/m-congeners, a behaviour that is consistent with chemical intuition Kekule resonance forms can be written only for the CT in the o- and p-isomers. The difference between ground- and excited-state dipole moments, is much greater in the p-case because the CT between donor and acceptor occurs over a larger distance. m-Isomers often have the undesirable properties of highest A ax and lowest s of the CT transition (Fabian and Hartmann, 1980). An inferior performance is thus also consistent with the expectations on the basis of the two-state model. However, m-nitroaniline is not a simple ID system as can be inferred from the different projections and as well as additional HRS measurements (Glania, 1996). Furthermore, the belief that m-substitution leads to inferior properties is too dogmatic as will be shown later (p. 204). 

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