Crown-ether styryl dyes .16. Betaines of photochromic 15-crown-5 ethers and their complexes with Mg2+: A surface enhanced Raman scattering spectroscopy study

The surface enhanced Raman scattering (SERS) spectra of styryl dyes containing a crown-ether group and a heteroaromatic residue with sulfoalkyl (1a) or alkyl (1b) N-substituent and of their complexes with Mg2+ cations were recorded in the 10(-4)-10(-8) mol L(-1) concentration range. A model for the interaction of compounds 1a,b with a silver surface during their adsorption on an electrochemically treated electrode was suggested. Fast cis-trans relaxation of the adsorbed molecules 1a,b and complexes (1a,bMg2+ was found. It was shown that at [1a] = 10(-5) mol L(-1) and moderate molar ratios (C-Mg/[1a] = 3/1 to 9/1) in acetonitrile solutions, (trans-1aMg2+ complexes are joined into head-to-tail type dimers. An excess of Mg2+ cations (c(Mg)/[1a] > 100) leads to dissociation of the dimers yielding (trans-1a)(Mg2+)(2) complexes. The formation of dimers from complexes (trans-1aMg2+ is accompanied by a substantial distortion of the planar structure of trans-1a. This may be an important factor influencing the efficiency of photocycloaddition involving dimers of (trans-1a) Mg2+.