title: Solvatochromic azamethine dyes for probing the polarity of gold-cluster-functionalized silica particles.
abstract: Azamethine dyes of the merocyanine type [4-(N,N-di-n-butylamino)-2-methylphenyl][2,4-di-keto-3-[N'-(n-hexyl)]-5-cyano-6-methyl-3-pyridinio]-1-azamethine (1) and [4-(N,N-diethylamino)-2-(N'-tert-butylcarboxy)-amidophenyl]-[2,4-diketo-3-[N"-(n-hexyl)]-5-cyano-6-methyl-3-pyridinio]-1-azamethine (2) have been used as surface-polarity indicators for gold-cluster-functionalized silica particles. Their UV/Vis absorption maxima range from about lambda=600 to 700 nm as a function of solvent polarity and are clearly separated from the surface plasmon UV/Vis absorption band of gold (lambda approximately 520-540 nm). Solvatochromism of both dyes has been investigated in 26 solvents of different polarity. The positive solvatochromic band shifts of 1 and 2 can be well expressed in terms of the empirical Kamlet-Taft solvent polarity parameters alpha and pi*. They are mainly sensitive to the dipolarity/polarizability (pi* term; 70-75 %) and HBD (hydrogen-bond donating) acidity (alpha term) of the solvent. Both dyes adsorb readily on functionalized silica samples from solutions in 1,2-dichloroethane or cyclohexane. The surface polarities of gold-cluster-functionalized silica particles, with and without co-adsorbed L-cysteine and poly(ethylenimine), have been investigated by using these solvatochromic dyes. The specific interaction of dye 2 with cysteine has been examined independently by quantum-chemical calculations by using the AM1 and PM3 methods.
authors.affiliation: Polymer Chemistry, Institute of Chemistry Chemnitz University of Technology, Strasse der Nationen 62 09111 Chemnitz, Germany. stefan.spange@chemie.tu-chemnitz.de
authors.forename: Stefan
authors.initials: S
authors.lastname: Spange
authors.forename: Dagmar
authors.initials: D
authors.lastname: Kunzmann
authors.forename: Rüdiger
authors.initials: R
authors.lastname: Sens
authors.forename: Isabelle
authors.initials: I
authors.lastname: Roth
authors.forename: Andreas
authors.initials: A
authors.lastname: Seifert
authors.forename: Werner R
authors.initials: WR
authors.lastname: Thiel
mesh_terms: D000327:Adsorption; D001372:Aza Compounds / Q000737:chemistry*; D004396:Coloring Agents / Q000737:chemistry*; D003545:Cysteine / Q000737:chemistry; D017979:Gold Colloid / Q000737:chemistry*; D008958:Models, Molecular; D012822:Silicon Dioxide / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet
chemical_list: D001372:Aza Compounds; D004396:Coloring Agents; D017979:Gold Colloid; D012997:Solvents; D012822:Silicon Dioxide; D003545:Cysteine