A concept to explain the variation of the triplet zero-field splitting parameters with the structure of aromatic hydrocarbons in terms of local benzenoid characteristics

The zero-field splitting (ZFS) parameters D and E of the lowest electronically excited triplet state T₁ of a series of systematically selected pyrene derivatives have been determined by ODMR techniques. With this class of compounds as an example we intended to gain some insight into the variation of the ZFS-parameters with the structure and topology of aromatic hydrocarbons. The variation of the experimental ZFS-parameters could be explained in terms of the dipole—dipole model together with criteria describing the local benzene likeness of single C₆-rings in the molecule. For these the character orders according to Polansky were calculated on an MO-basis and the local benzenoid indices according to Randi? on a graph-theoretical basis. As these quantities for the local aromaticity were in accordance with Clar's qualitative sextet concept a qualitative principle could be formulated explaining the variation of the D-parameter with molecular structure. This principle correctly describes all known D-parameters of aromatic molecules measured so far. So for the first time, a clear and consistent explanation for the variation of D with molecular structure can be given. Also for the E-parameter a connection with molecular structure could be established; it could be demonstrated in which way the criteria of local aromaticity mentioned can be applied to decide on the sign of E. For the acene-analogue pyrene derivatives linear correlations could be established between the averaged local quantities and the ZFS-parameters D and E. These correlations allow a quantitative determination of the ZFS-parameters of further analogue pyrene compounds. Also for other series of aromatic hydrocarbons such correlations could be found as shown for the polyacene series as an example. For the latter also a strong similarity to the series of the acene-analogue pyrene derivatives was found. D- and E-values are given for the six molecules pentacene to decacene. These were calculated from both the correlation with the character orders and that with the Randi?-indices, and the result of both methods are compared.