About monoradicals, triplet diradicals and higher spin states: Understanding electronic substituent effects through EPR and time-resolved UV spectroscopy

Electronic substituent effects on the alpha spin density (rho(alpha)) and radical stabilization (RSE) of benzyl-type monoradicals are reflected accurately by the D parameters of the triplet diradicals 1 and 2. Hence, the present EPR-spectroscopic Delta D scale offers a novel mechanistic tool to assess radical properties. This concept has been extended to heteroaromatic pi systems, i.e. pyridines, furanes and thiophenes, which show the general applicability of the model presented herein. Also the ISC rates of the triplet 1,3-cyclopentanediyls 1 and 2 depend on the type of aryl substitution. Whereas the D parameter of the localized triplet diradicals are determined by the spin densities at the radical sites and the distance of separation between the spin centers, the ISC rates are dictated by the balance of through-space and through-bond interactions, which are a function of the electronic properties of the NBMO's in the two-electron-two-orbital model. When two localized 1,3-cyclopentanediyl spin-bearing units are attached to meta-phenylene as ferromagnetic coupler, the novel quintet tetraradical 4 results, which not only displays interesting paramagnetic behavior, but also unexpected optical properties.