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-substitute d benzothiazolium moieties are synthesized and characterized spectroscopica lly 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 i ons, 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 com plex formation stability constants, determined spectrophotometrically, are converted to absolute values by polarographic measurements. The stability c onstant for Hg2+ complex formation by the trans dye is enhanced 47-fold upo n substitution of the 4-butylsulfonate group for methyl at the nitrogen of the benzothiazolium moiety. Photoisomerization to the cis dye is accompanie d by a further 11-fold stability constant increase and by spectral changes that are consistent with formation of an ion-"capped" complex, involving in tramolecular coordination of the sulfonate anion to Hg2+. Addition of a lar ge excess of Mg2+ ions disrupts this intramolecular coordination bond witho ut displacing the Hg2+ ion from the dithiacrown ether cavity.