Spectral-fluorescent and photochemical properties of polymethine dyes anionic 3,3'-di(g-sulfopropyl)-4,5,4',5'-dibenzo-9-ethylthiacarbocyanine-betaine (DEC) and cationic 3,3',9-trimethylthiacarbocyanine (Cyan 2), 3,3'-di(b-hydroxy)-9-methylthiacarbocyanine, 3,3'-di(b-hydroxy)-5,5’-dimethoxy-9-ethylthiacarbocyanine in the presence of biological surfactants (bile salts, cholates) sodium cholate (NaC), sodium deoxycholate (NaDC), sodium taurocholate (NaTC) and, for comparison, sodium dodecyl sulfate (SDS), were studied. Upon introduction of surfactants into DEC solution, changes of dye properties are observed due to decomposition of dye dimers into cis -monomers and cis-trans conversion of the resulting monomers. In the presence of SDS, both processes occur in parallel due to noncovalent interaction of dye monomers with micelles, and mainly occur near the critical micelle concentration (CMC). In contrast, upon introduction of cholates, decomposition of dye dimers into monomers begins at much lower concentrations than cis-trans conversion. The former process is completed at cholate concentrations close to CMC of secondary micelles (CMC2), while the latter occurs even at concentrations >> CMC2. It is concluded that DEC can serve as a probe for estimating CMC2; spectral changes of DEC indicate reorganization of secondary micelles with increasing cholate concentration. Aggregation of DEC and Cyan 2 on cholates is also observed. Since it occurs at concentrations < CMC2, monomeric molecules of cholates, their small associates, and primary micelles serve as matrices for aggregation. Decomposition of the aggregates begins at cholate concentrations > CMC2 and continues at higher cholate concentrations. Upon flash photolysis of solutions of DEC and Cyan 2 in the presence of cholates, photoisomerization and triplet state formation are observed.
polymethine dyes, dimers, aggregates, bile salts (cholates), micelles
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