Spectral properties and structure of fluorescein and its alkyl derivatives in micelles

The absorption and fluorescence spectra, the fluorescence quantum yields and lifetimes of fluorescein and three series of alkylated fluorescein, namely, esters, ethers and esterethers of fluorescein in SDS, CTAB and TX-100 micelles, were investigated. The interactions between the dyes and micelles, and the existing structure forms of the dyes in micelles were also studied. All alkylated fluorescein could bind to the three micelles because of the hydrophobic interaction between their alkyl(s) and the hydrophobic region of the micelles, while the aqueous soluble fluorescein could not bind to SDS and TX-100 micelles. However, fluorescein could bind to the cationic CTAB micelles and existed mainly as dianion forms due to the electrostatic attraction. As a supplement to the hydrophobic interaction between the dyes and micelles, the electrostatic attraction enhanced the binding of esters and ethers of fluorescein to CTAB micelles and stabilized their anion forms. In SDS and TX-100 micelles, the esters of fluorescein existed mainly as neutral quinoid forms, while the ethers as neutral lactonic forms. As the alkylation of both the carboxyl and the hydroxyl, the esterethers of fluorescein could only exist as quinoid forms in all micelles. In each series, the hexadecylated fluorescein bound to micelles stronger than the butylated.