Photochromism of a Spiropyran and a Diarylethene in Bile Salt Aggregates in Aqueous Solution

Bile salt aggregates incorporate aqueous-insoluble photochromic compounds. The photochromism of a spiropyran (<b>1</b>, 1′,3′,3′-trimethyl-6-nitrospiro­[2<i>H</i>-1]-benzopyran-2,2′-indoline) and a diarylethene derivative (<b>2</b>, 1,2-bis­(2,4-dimethyl-5-phenyl-3-thienyl)-3,3,4,4,5,5-hexafluoro-1-cyclopentene) was quantified in different bile salt aggregates. These aggregates act as efficient hosts to solubilize aqueous insoluble photochromic compounds where either both isomers are nonpolar, for example, <b>2</b>, or compounds where one isomer is hydrophobic and the other is more polar, for example, <b>1</b>. Methodology was developed to determine molar absorptivity coefficients for solutions containing both isomers and to determine the photoconversion quantum yields under continuous irradiation. The methods were validated by determining parameters in homogeneous solution, which were the same as previously reported. In the case of the colored isomer of <b>1</b>, the molar extinction coefficient in ethanol at 537 nm ((3.68 ± 0.03) × 10⁴ cm^–1 M^–1) was determined with higher precision. The quantum yields for the photoconversion between the isomers of <b>2</b> were shown to be the same in cyclohexane and in the aggregates of sodium cholate (NaCh), deoxycholate (NaDC), and taurocholate (NaTC), showing that bile salt aggregates are not sufficiently rigid to affect the equilibrium between the two possible conformers of the colorless form. In contrast, for <b>1</b> the quantum yields for the conversion from the colorless to the colored isomer were higher in bile salts than in ethanol, and the quantum yield was highest in the more hydrophobic aggregates of NaDC, followed by NaCh and then NaTC. The structure of the bile salt had no effect on the quantum yield for the conversion of the colored to the colorless isomer of <b>1</b>, but these values were higher than in ethanol. For all three bile salts, the absorption maximum for the colored form of <b>1</b> suggested that this isomer was located in an environment that is more polar than ethanol