RESUMO
The excited singlet state of an azomethine ylide or 'stable dipole' exhibits an ultrafast radiationless relaxation after femtosecond laser excitation. These transients are observed before the excited state decays in an almost activationless manner, the barrier is 440 cm-1, to the ground state with a 1.5 ps lifetime. Cooling of the hot ground state is also apparent in the transient absorption data and in methanol decays with a 5.7 ps lifetime. The viscosity dependence of the fluorescence yield and lifetime in different solvents is small and far less pronounced than in triphenylmethane dyes. Surprisingly, the excited state decay is not due to twisting about the C-N bond of the ylide but it is caused by buckling of one of the rings as the nitrogen atom changes character from sp2 to sp3 hybridisation.