Complexation of Photochromic Crown Ether Styryl Dyes with Mg2+ As Probed by Surface-Enhanced Raman Scattering Spectroscopy

Abstract

Complexation with magnesium cations of 2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin (1), as well as derivatives of crown ether styryl dyes perchlorate 2-[2-(2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin-16-yl)ethenyl]-3-ethylbenzothiazolium (2), betaine 2-[2-(2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin-16-yl)ethenyl]-3-(3-sulfopropyl)benzothiazolium (3), betaine 2-[2-(2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin-16-yl)ethenyl]-3-(3-sulfoethyl)benzothiazolium (4) was studied by surface-enhanced Raman scattering (SERS) spectroscopy. Comparative studies of isomerization and complexation of photochromic ionophores 2−4 with Mg2+ in solution and when adsorbed on the silver surface were performed. The aim was to characterize parameters of these processes depending on the nature (alkyl versus sulfo-alkyl) as well as the length (sulfoethyl versus sulfopropyl) of the N substituent in the benzothiazolium residue. The data obtained indicate stabilization of the trans conformation and prevention of the trans−cis isomerization for the adsorbed molecules as well as for their complexes with Mg2+ due to fast nonradiative energy transfer from the dye excited state to metal surface. Adsorption of a dye having initially cis conformation in solution is accompanied by surface-induced cis−trans relaxation. Adsorption constants calculated from the Langmuir isotherms revealed slightly preferential adsorption of 2−4 complexed with Mg2+ as compared to the free dyes. Desorption of the dyes was found to occur on the hours time scale, whereas reversible capture/release of metal ions by adsorbed molecules is quite fast and may serve as an indicator of a cation level and/or complexation processes in bulk solution. The SERS titration procedure was developed permitting the complexation of the ionophores with metal cations to be followed and equilibrium constants of these processes to be measured in a wide range (down to 10-7 M) of ligand concentrations. By the measurement of the equilibrium constants, adsorption on the silver surface was concluded not to affect the complexation of 1 and trans-2 with Mg2+. Association of the Mg2+ cation with the sulfonate group was found to be more favorable than binding with the crown ether moiety for the adsorbed (trans-3)Mg2+ and (trans-4)Mg2+ complexes. Appearance of the (trans-3)(Mg2+)2 and (trans-4)(Mg2+)2 species was detected in the solution at increased metal/ligand molar ratios. These complexes were found to be unperturbed if absorbed on the silver surface. Since the measurement of equilibrium constants are intractable by other techniques, measuring equilibrium constants for (trans-3)(Mg2+)2 and (trans-4)(Mg2+)2 complexe formation is readily acquired by SERS spectroscopy.