CHARACTERIZING TRIPLET-STATES OF QUINONOIDAL DINITRENES AS A FUNCTIONOF CONJUGATION LENGTH

Photolysis of para,para'-diazide precursors in 2-methyltetrahydrofuran gives biradical ESR spectra assignable to quinonoidal dinitrenes with excited triplet biradical states. The zero field splitting (zfs) para meters for the biradicals at 77 K are as follows: 1,4-phenylenedinitre ne (\D/hc\ = 0.169 dm(-1), \E/hc\ = 0.004 cm(-1)); 4,4'-biphenyldinitr ene (\D/hc\ = 0.188 cm(-1), \E/hc\ less than or equal to 0.002 cm(-1)) ; 4,4'-stilbenedinitrene (\D/hc\ = 0.122 cm(-1), \E/hc\ < 0.002 cm(-1) ); 1,4-bis(p-nitrenophenyl)buta-1,3-diene (\D/hc\ = 0.0865 cm(-1), \E/ hc\ < 0.002 cm(-1)); 1,8-bis(p-nitrenophenyl)octa-1,3,5,7-tetraene (\D /hc\ = 0.0442 cm(-1), \E/hc\ < 0.001 cm(-1)). These zfs parameters are inconsistent with simple dipole-dipole analysis of the ESR spectra in localized biradicals, but are reconcilable with a spin-polarized mode l involving the x-electrons of the biradicals.