title: Electrolyte-induced mesoscopic aggregation of thiacarbocyanine dye in aqueous solution: counterion size specificity.
abstract: Countercation size specificity is presented for the electrolyte-induced aggregation of 3,3'-disulfopropyl-5,5'-dichloro-9-methyl thiacarbocyanine (TCC) dye in aqueous solution. Addition of electrolytes having a small monovalent cation (Na+, NH4+, or Cs+) induced pure H aggregates of TCC, whereas J aggregates were preferentially promoted by electrolytes with a large monovalent cation ([N(CH3)4]+ or [N(C2H5)4]+). The electrolyte-induced H aggregate (HS aggregate) differed spectroscopically from that spontaneously self-assembled in aqueous solution. Mesoscopic structure of the HS aggregates was revealed via polarized-light microscopy and atomic force microscopy; a rodlike morphology of 50-70 nm wide and tens to hundreds of micrometers long with very strong negative birefringence. A simple structural model based on semiempirical molecular orbital calculations can explain the aggregation behaviors: The anionic TCC monomer shows a considerable planar geometry between two benzothiazole end groups when it involves a sodium cation, which favors the H-type molecular arrangements in a face-to-face orientation. On the other hand, the TCC dye has a twisted conformation when it implicates a large tetramethylammonium cation, resulting in the formation of the J aggregates.
authors.affiliation: Graduate School of Material Science, University of Hyogo, Ako-gun, Hyogo, Japan. yao@sci.u-hyogo.ac.jp
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Yao
authors.forename: Takeshi
authors.initials: T
authors.lastname: Isohashi
authors.forename: Keisaku
authors.initials: K
authors.lastname: Kimura
mesh_terms: D002232:Carbocyanines / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D004396:Coloring Agents / Q000737:chemistry*; D004573:Electrolytes / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D012996:Solutions; D013053:Spectrophotometry; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D002232:Carbocyanines; D002412:Cations; D004396:Coloring Agents; D004573:Electrolytes; D012996:Solutions; D013438:Sulfhydryl Compounds; D014867:Water