Femtosecond studies of barrierless rotamers in solution

Femtosecond fluorescence upconversion experiments have been performed for p hotoexcited auramine (a diphenylaminemethane dye) in liquid solution. Follo wing pulsed laser excitation, the fluorescence shows a multiexponential dec ay with an initial fast decay of 2 ps. The lifetime of the relaxed excited state is 30 ps or longer, depending on the viscosity of the solvent. Concom itant with the initial fast decay a dynamic Stokes shift is observed. From a study of the excited state dynamics as a function of solvent viscosity it is concluded that auramine undergoes a picosecond (quasi)-barrierless rela xation from an emissive to a non-emissive excited state. The relaxation wit hin the excited state arises from torsional diffusion motions of the phenyl rings of the auramine molecule.