Femtosecond studies of twisting dynamics of auramine in solution

Femtosecond fluorescence upconversion experiments as performed for photoexc ited auramine (a diphenylaminemethane dye) in liquid solution are discussed . Upon pulsed laser excitation the solute shows a dynamic Stokes shift as w ell as a drop in fluorescence intensity in only a few picoseconds. From a s tudy of the excited state dynamics as a function of solvent viscosity it is concluded that auramine undergoes a picosecond (quasi)-barrierless relaxat ion from an emissive state to a non-emissive excited state that decays to t he ground state in about 30 ps. The relaxation is controlled by the torsion al motions of the phenyl rings of the auramine molecule. (C) 2000 Elsevier Science B.V. All rights reserved.