Abstract: In dimethyl sulfoxide-water (DMSO-H₂O) mixed solvent the fluorescene spectrum of △⁵ androstene-3β-(1-naphthyl) acetoxy-17-dansyl (1) shifts to the red side as the volume fraction of water in the mixture (Φ) increases.However, the spectrum suddenly shifts from λ_max=550nm to λ_max=500nm when Φ>0.4, suggesting aggregate formation of 1 This is accompanied by the increase in dansyl fluorescence and the decrease in naphthyl fluorescence if excited at 280nm.due to the enhancement of energy transfer from naphthyl to dansyl group in aggregate.The efficiencies of the energy transfer are 18% and 90% before and after the aggregate formation respectively.Measurements of fluorescence lifetime of dansyl group and the experients by using different excitation wavelength demonstrate the existence of the equilibrium of 1 btween inside and outside of the aggregate.

Key words: hydrophobic interaction, aggregation of rigid molecule, △⁵ androstene-3β-(1-anphthyl)acetoxy-17-dansyl, inter- and intramolecular energy transfer