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 compoundsla,b with a silver surface during their adsorption on an electrochemically treated electrode was suggested. Fastcis-trans relaxation of the adsorbed molecules1a,b and complexes (1a,b)Mg2+ was found. It was shown that at [1a] = 10−5 mol L−1 and moderate molar ratios (CMg/[1a] = 3/1 to 9/1) in acetonitrile solutions, (trans-1a)Mg2+ complexes are joined into head-to-tail type dimers. An excess of Mg2+ cations (CMg/[1a] > 100) leads to dissociation of the dimers yielding (trans-1a)(Mg2+)2 complexes. The formation of dimers from complexes (trans-1a)Mg2+ is accompanied by a substantial distortion of the planar structure oftrans-1a. This may be an important factor influencing the efficiency of photocycloaddition involving dimers of (trans-1a)Mg2+.