The excited state relaxation processes of a nitrogen-cored distyrylbenzene-stilbene (A-DSB) dendrimers and the analogous linear model compound (bis-MSB) were investigated by polarized fluorescence upconversion spectroscopy. The fluorescence anisotropy (FA) of A-DSB was found to decay to a value close to zero in less than 200 fs after excitation. For the model compound bis-MSB the FA initial value was close to 0.4 and showed a relatively slow decay (82 ps) corresponding to the overall rotational diffusion of the molecule. The results are interpreted in terms of fast transition dipole reorientation during relaxation of the excited states of the branched molecules.

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