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Original scientific paper

On the Existence of Biradical-Ionic States of Donor-Acceptor Cyclophanes. A Simple MO-Theoretical Study

Helmut Vogler ; MPI fur Medizinische Forschung, Abteilung Organische Chemie, Jahnstr. 29. D-6900 Heidelberg, West-Germany

Full text: english pdf 15.144 Kb

page 297-313

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The longest wavelength absorption in the UV-VIS spectra of
donor-acceptor cyclophanes corresponds to an excitation from the
donor D to the acceptor A with the transfer of one electron, i.e.
DA--+ D+A-. The size of the electron transfer should become
larger with increasing donor-acceptor strength already in the
ground state. One expects a biradical-ionic ground state D+Q A-•
with a large q, say q > 1/2, for certain donor-acceptor combinations
with a small enough difference between the ionization energy I0
of the donor and the electron affinity EA of the acceptor. In this
work we investigate the ground state and the lowest excited singlet
and triplet states of donor-acceptor cyclophanes within a semi-
empirical four-orbital configuration interaction model. The dependence
of energies and size of the electron transfer in these
states on molecular parameters of donor and acceptor and on their
mutual geometrical arrangement is elucidated. Our model leads
to the simple approximate condition v•" ;;;; O for the occurrence
of a biradical-ionic ground state. verr is a measure of the effective
donor-acceptor strength which does not only depend on the difference
I0 - EA but also on the Coulombic repulsion between an
electron in D and one in A and on the stabilizing effect of the
solvent. The symmetries of the frontier orbitals, i. e. the highest
occupied molecular orbital of the donor and the lowest unoccupied
molecular orbital of the acceptor, determine whether the biradical-
ionic ground state will be a triplet or a singlet. According to our
simple model a triplet biradical-ionic ground state should occur if
the frontier orbitals belong to different representations of the pointgroup of the phane. The transition from an ordinary ground state
without an appreciable electron transfer from D to A ·into the
biradical-ionic ground state is discontinuous. This transition is,
however, continuous if the frontier orbitals belong to the same
representation, although the magnitude of the electron transfer q
changes significantly only in a narrow range of v•rr whose width
is determined by the size of the transanular and through-bond
interaction between the frontier orbitals. Finally we discuss how
to choose donor and acceptor in a cyclophane in order to fulfill
the condition for a biradical-ionic ground state.


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