Characterizing Triplet States of Quinonoidal Dinitrenes as a Function of 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) parameters for the biradicals at 77 K are as follows: 1,4-phenylenedinitrene (| D / hc | = 0.169 cm - 1, | E / hc | = 0.004 cm - 1 ); 4,4‘-biphenyldinitrene (| D / hc | = 0.188 cm - 1, | E / hc | ≤ 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 model involving the π-electrons of the biradicals.