Limits of delocalization in through-conjugated dinitrenes: Aromatization or bond formation?

Dinitrenes 4 and 5 both can form quinonoidal structures by conjugative bond formation. However, ESR spectroscopy detects a thermally populated, excited-state, triplet quinonoidal structure only for 4, with a zero-field splitting of \D/hc\ = 0.0822 cm(-1), \E/hc\ congruent to 0.0 cm(-1). The tendency to maintain aromaticity in the additional ring of 5 favors a dinitrene structure (with one less formal x-bond) over a quinonoidal structure. The thermally populated quintet state of 5 has a zero-field splitting of \D/hc\ = 0.287 cm(-1), \E/hc\ less than or equal to 0.002 cm(-1).