Locally excited configuration

The delocalization of excessive a- (or P-) spins and the bond polarization can take place among radical orbitals, p and q, and the central n (or o) and n (or o ) orbitals, resulting in the electron transferred configurations (T) and locally excited configurations (E), respectively (Fig. 5a). The delocalization-polarization mechanisms are different between singlet and triplet states, as addressed in the following subsections. 

We note at this point that, when the two fragments become identical, the charge transfer configurations and the locally excited configurations will have to be replaced by charge-resonance and exciton-resonance configurations, respectively. 

In an electron donor-acceptor system (D-A) in which an electron donor (D) and an electron acceptor (A) are separated by a carbon chain, the exciplex state may be considered as a resonance hybrid of the electron transfer configuration (D A" ) mixed with the locally excited configuration (D A) or (DA ) [211]. 

Reconsidering the optical activity of calycanthine, we shall focus on two chromo-phores which maintain the pertinent distance and geometry. It is assumed that the electronic states of these chromophores do not mix with those of other chromo-phores. The linear combinations of the locally excited configurations were taken to be ... 

In this simple qualitative consideration we have neglected the interaction with the ground-state configuration AB). Let us for simplicity assume that the overlap between the two subunits is negligible. Although both subunits are equal, we will keep notation A and B for them. The excitation within subunits A and B is labeled by A and B, respectively, and is represented by promoting one electron from the MO a to the MO a and from the MO b to the MO b, respectively. The matrix element between two locally excited configurations is... 

The matrix elements between dot-dot and locally excited configurations are ... 

The comparison of the Mfiu values with those of Mabs allows one to obtain information about the changes in the electronic structure and molecular conformation between the Franck-Condon excited state initially reached upon excitation and the solvent-equilibrated fluorescent state [14]. Electronic transition dipole moments are mainly determined by the direct interactions between the lowest CT state and the ground state (So), and by the contributions from the locally excited configurations [14, 54, 56, 57]. For example, for the fluorescent CT state one can obtain... 

Often, however, it is sufficient to distinguish between o and tc orbitals in order to decide whether a barrier is likely to occur. For instance, the T, surface of toluene along the path of nuclear geometries that leads to dissociation to CjHjCHj- + H- can be viewed as originating from interaction of a locally excited configuration of the benzene chromophore... 

The LUMOs are it of alkenes and of ketenes. The LUMO-LUMO interaction occurs between the C=C bond of alkenes and the C=0 bond of ketenes, promoting the reaction across the C=0 bond of kentenes. The important pseudoex-cited configuration D A is the locally-excited nn configuration of alkenes. 

In the present work the Cl procedure ( ) employed includes single, double, and higher excited configurations, and can treat in a natural way, using the localized monomer basis orbitals, intra- as well as inter-molecular excitations. States which arise from such monomer-based excitations can then be interpreted in terms of their locally-excited, excitonic, CT, or "mixed" character. Details as to the nature of the dimer states have been discussed in our earlier works (7, 8). 

An exciplex can be described as a hybrid of locally excited and charge transfer configurations, eq. 13 ... 

It must be pointed out that the classification into MC, LC and CT transitions (or excited states) is somewhat arbitrary and loses its meaning whenever the states involved cannot be described with localized MO configurations. The chemical and physical properties of these orbitally different excited states have been examined in detail by several authors2 3>6> 8-10) and will not be further discussed here. 

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