On Hyperfine and Fine Structure of Excited Charge-Transfer-Complexes

The charge-transfer (CT)-crystal naphthalene (N)-1,2,4,5-tetracyanobenzene (TCNB) was doped with various guest acceptors. CT-complexes of these acceptors with N are formed acting as a triplet energy trap in the crystal. This provides a method to investigate oriented CT-complexes ESR-spectroscopically. In favorable cases the hyperfine structure (Hfs) in the ESR-spectra can be resolved and interpreted. The Hfs of the complexes N-hexacyanobenzene and N-pentacyanotoluene show that in the crystalline complex the triplet electrons are distributed over no more than two molecules. This finding suggests a strong exciton-phonon-coupling which causes a breakdown of the inversion symmetry upon excitation. The spectra of naphthalene-s-trinitrobenzene (TNB) yield the spin density on the three equivalent protons of TNB and demonstrate that the triplet electrons are also equally distributed over the three nitrogen atoms. As further systems, complexes of extremely large CT-character, ct2, in the triplet state are studied (e. g. N-chloranil). The data obtained from the measurements of the Hfsand of the zero-field-splitting (ZFS) parameters are compared, proving that previous models and approximations in deriving cx2 are essentially correct. It is also shown that the ZFS-parameter D (A+ D+) of the hypothetical purely ionic state is negative and for different complexes its value is very close to that obtained in earlier theoretical calculations ( — 0.027 cm 1) .