Synthesis, Structure, and Ion Selective Complexation of Trans and Cis Isomers of Photochromic Dithia-18-crown-6 Ethers

Photochromic styryl dyes containing benzodithia-18-crown-6 and N-substituted benzothiazolium
moieties are synthesized and characterized spectroscopically and, for a specific case, by X-ray crystallography.
The effects of the N-substituent and of sulfur atom substitution in the crown ether cavity on metal cation
complex formation and on photochromism due to reversible trans−cis photoisomerization are evaluated. High
selectivities for heavy metal ions, such as Ag+ and Hg2+, are observed, in contrast to the benzo-18-crown-6
dyes which selectively bind alkali and alkali earth cations. Relative complex formation stability constants,
determined spectrophotometrically, are converted to absolute values by polarographic measurements. The stability
constant for Hg2+ complex formation by the trans dye is enhanced 47-fold upon substitution of the
4-butylsulfonate group for methyl at the nitrogen of the benzothiazolium moiety. Photoisomerization to the
cis dye is accompanied by a further 11-fold stability constant increase and by spectral changes that are consistent
with formation of an ion-“capped” complex, involving intramolecular coordination of the sulfonate anion to
Hg2+. Addition of a large excess of Mg2+ ions disrupts this intramolecular coordination bond without displacing
the Hg2+ ion from the dithiacrown ether cavity.