SURFACE-ENHANCED RESONANCE RAMAN-SPECTRA OF PHOTOCHROMIC CROWN-ETHER STYRYL DYES, THEIR MODEL CHROMOPHORES, AND THEIR COMPLEXES WITH MG2+

The complexation of the derivatives of crown ether styryl dyes betaine ,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclop in-16-yl)ethenyl]- 3-(3-sulfopropyl)benzothiazolium (1a) and perchlorate 2-[2-(2,3,5,6,8, 9,11,12-octahydro-1,4,7, lopentadecin-16-yl)ethenyl]-3-ethylbenzothiaz olium (1b) and their molecular fragment 2,3,5,6,8,9,11, 12-octahydro-1 ,4,7, 10,13-benzopentaoxacyclopentadecin (2) with magnesium cations ha s been studied by the surface-enhanced Raman scattering (SERS) spectro scopy at very low (down to 10(-8) M) concentrations. A detailed compar ison of the Raman and SERS spectra of 1a,b and their molecular fragmen ts 2 and perchlorate 2-Me-3-ethyl benzothiazolium (3) enabled one to a ssign the vibrational modes and propose the model of dye interaction w ith the silver SERS-active surface. Benzo-15-crown-5 chromophore of la ,b was found to be adsorbed through its crown ether part so that the p lane of benzene ring is nearly perpendicular to the surface of,silver electrode. The C=C bond as well as the benzothiazolium chromophore bei ng situated far from the electrode surface does not contribute noticea bly to the SERS spectra. The sulfonate group of la 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 complexatio n of Mg2+ leads to the remarkable frequency shifts of several bands as sociated primarily with the benzo-15-crown-5 fragment of 1a,b. The spe ctral effects which reflect the cation-induced intramolecular electron transfer were found to be very similar in both resonance Raman and SE RS spectra of the dyes. Hence, the adsorption on the surface of silver electrode does not perturb the complexation of the dyes with Mg2+. Th e 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 conce ntrations.