The EPR D parameter as a measure of spin delocalization in cyclopentane-1,3-diyl triplet diradicals with extended conjugation π-type substituents

Abstract

π-Substituted cyclopentane-1,3-diyl triplet diradicals 2 were readily prepared from the corresponding azoalkanes 1 by photodenitrogenation in a 2-methyltetrahydrofuran matrix at 77 K. The D values of these triplet diradicals, obtained from the EPR spectra under matrix isolation, depend on the π substituent in the order butadienyl > 2-anthryl > vinyl > 2-naphthyl > phenyl. Good linear correlations of the D values have been obtained with the reported hyperfine coupling constants (α-hfc), with the ab initio (B3LYP) spin densities for the corresponding monoradicals 3, and with the radical stabilization energies (RSE). The present results for these extended π systems are compared with those for previously reported heteroaryl-substituted triplet diradicals.