EFFECTS OF ADDITIONAL LINKERS IN BIPHENYL-4,4'-DINITRENE ON THE LOW-LYING SINGLET-TRIPLET ENERGY-GAP AND ZERO-FIELD SPLITTING

Perturbation effects of additional linkers on the spin-spin coupling i n biphenyl-4,4'-dinitrene (1) were examined by introducing a linking g roup between 2- and 2'-positions of 1. Five different doubly linked sy stems showed triplet ESR spectra corresponding to quinonoid dinitrenes . Curie law analyses suggested that all those tripler states were ther mally excited triplet states. In addition, the singlet-tripler energy gaps, which were determined by the Curie law analyses, were well, corr elated with their corresponding zero-field-splitting (zfs) D values. T he result could be explained by the stability of dinitrene character w hich is estimated from the resonance energy of the intervening pi-syst em. Our semiempirical molecular orbital calculations supported the exp erimental correlation between the singlet-triplet energy gap and the z fs D value.