Abstract: The fluorescence intensity and maximum wavelength of 4-dimethylaminochalcone(DMAC)in aqueous solution are observed to increase with the concentration of sodium deoxycholate (SDOC), a steroidal surfactant,to their maxima and then decrease,while in thesolution of an aliphatic surfactant sedium laurate(NaL),both the parameters increase withincreasing NaL concentration and level off after micelle formation. Although the fluores-cence maximum wavelength of DMAC is shorter in SDOC system than that in NaL system,the fluorescence intensity is much higher in SDOC system. The ultraordinarily high enhancement of DMAC fluorescence in SDOC system is rationalized as being due to the aggregation of rigid SDOC molecule in aqueous solution. The SDOC aggregate provides DMAC with an also rigid microenvironment where the formation of the photoinduced nonemissivebiradical state P. of DMAC is hindered and thus the emission is much strongly enhanced.Before the formation of micelle,SDOC molecule is shown to exist mainly in dimer which forms a sandwich supramolecular complex with DMAC by an association constant of 9 3.2L/mol.

Key words: biradical excited-state, fluorescence probe, cholic acid-type steroidal surfactant, aggregation