Inversion of the Ground-State Spin Multiplicity by Electron-Withdrawing Groups in Trimethylenemethane Derivatives Generated Photochemically from Methylenequadricyclane Derivatives

An inversion of the ground-state spin multiplicity of trimethylenemethane (TMM) by electron-withdrawing groups was investigated by EPR spectroscopy, for a series of TMMs 2b-g generated photochemically from methylenequadricyclanes 1b-g. Curie law analyses between 4 and 50 K proved the triplet ground states of the monophenyl derivative 2b and the monocyano derivatives 2c-d as well as that of the diphenyl derivative 2a. The dicarbomethoxy derivative 2e also exhibited the EPR signal, but the Curie plot was nonlinear and the signal disappeared irreversibly above 10K. The triplet ground state was inverted to the singlet by the carbomethoxy-cyano and dicyano substitutions, and 2f and 2g were EPR silent species similar to the singlet oxyallyl (OA) derivative 2h. The singlet ground state of 2g was demonstrated by its chemical behaviors which resemble those of 2h.