| Popis: |
The partially localized tetraradical 22 has been generated at temperatures of 77 K and below in 2-methyltetrahydrofuran and poly(methylmethacrylate) matrices, and directly observed by EPR spectroscopy. The evolution of the EPR spectrum resulting from prolonged photolysis of the bisdiazene precursor 20 indicates a two-photon process, wherein a triplet spectrum (assigned unambiguously to the monoazobiradical) gradually gives way to a quintet spectrum, ascribed to the target tetraradical. The observed quintet state is tentatively identified as the ground state of the tetraradical, based on design considerations, the fact that the quintet spectrum is intense at temperatures as low as 3.8 K, and preliminary Curie Law studies. The thermal stability of 22 (indefinitely stable in the dark at 77 K) is an unexpected attribute of this molecule; no other cyclobutanediyl studied thus far has exhibited similar sturdiness at temperatures above ca. 60 K. Two synthetic routes to the prerequisite bisfulvene 15 are discussed. |
