Surface-Enhanced Resonance Raman Spectra of Photochromic Crown Ether Styryl Dyes, Their Model Chromophores, and Their Complexes with Mg2+

Abstract

The complexation of the derivatives of crown ether styryl dyes 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 (1a) and perchlorate 2-[2-(2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin-16-yl)ethenyl]-3-ethylbenzothiazolium (1b) and their molecular fragment 2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin (2) with magnesium cations has been studied by the surface-enhanced Raman scattering (SERS) spectroscopy at very low (down to 10-8 M) concentrations. A detailed comparison of the Raman and SERS spectra of 1a,b and their molecular fragments 2 and perchlorate 2-Me-3-ethyl benzothiazolium (3) enabled one to assign the vibrational modes and propose the model of dye interaction with the silver SERS-active surface. Benzo-15-crown-5 chromophore of 1a,b was found to be adsorbed through its crown ether part so that the plane of benzene ring is nearly perpendicular to the surface of silver electrode. The CC bond as well as the benzothiazolium chromophore being situated far from the electrode surface does not contribute noticeably to the SERS spectra. The sulfonate group of 1a and alkyl group of 1b do not affect the geometry of adsorption. The SERS spectra of trans-1a,b and their complexes with Mg2+ are observed selectively studying the photostationary mixtures of cis and trans isomers. The complexation of Mg2+ leads to the remarkable frequency shifts of several bands associated primarily with the benzo-15-crown-5 fragment of 1a,b. The spectral effects which reflect the cation-induced intramolecular electron transfer were found to be very similar in both resonance Raman and SERS spectra of the dyes. Hence, the adsorption on the surface of silver electrode does not perturb the complexation of the dyes with Mg2+. The results demonstrate that the SERS spectroscopy enables one to study the complexation of the photochromic crown ether styryl dyes with the metal cations, ensuring selective detection of trans isomers and high sensitivity to structural changes of the dyes in a wide range of concentrations.