title: Direct and sensitized photoprocesses of bis-benzimidazole dyes and the effects of surfactants and DNA.
abstract: The photoprocesses of two bis-benzimidazole dyes, Hoechst 33258 (1) and an analog, where the phenolic group in p-position is replaced by an ethoxy group, Hoechst 33342 (2), were studied. For 1 and 2 in aqueous solution the quantum yield of fluorescence is strongly pH dependent; it decreases from a maximum value of phi f = 0.4 at pH 5 to phi f = 0.02 at pH 8. The effects of absorption and fluorescence, induced by sodium dodecyl sulfate surfactants below and above the critical micelle concentration and by double-stranded DNA, are interpreted by assuming that in bulk aqueous solution the dyes are essentially present as monomers. The strong enhancement of phi f, when the dye is bound to double-stranded DNA or solubilized in micelles, is suggested to be due to different environments at the benzimidazole rings. A quinoid intermediate with absorption maximum at 380 nm is formed for 1 at neutral pH using lambda exc = 248 or 308 nm. N-centered radicals of 1 or 2 in aqueous solution were observed by laser flash photolysis after electron ejection using wavelengths of 193 or 248 nm (mono and biphotonic, respectively). The precursor radical cation escaped observation but is transformed into the above radicals by deprotonation. Electron transfer from 1 in aqueous solution to triplet acetone takes place, and subsequent deprotonation is proposed to yield N-centered radicals. In addition, energy transfer from acetone to 1 is suggested, leading to T-T absorption with the maximum at 700 nm. The photoprocesses are discussed and the results compared with those known from pulse radiolysis.
authors.affiliation: Max-Planck-Institut für Strahlenchemie, Mülheim an der Ruhr, D-45413, Germany. goerner@mpi-muelheim.mpg.de
authors.forename: H
authors.initials: H
authors.lastname: Görner
mesh_terms: D000818:Animals; D001562:Benzimidazoles / Q000737:chemistry*; D006690:Bisbenzimidazole / Q000737:chemistry; D002417:Cattle; D004247:DNA / Q000737:chemistry*; D015394:Molecular Structure; D010777:Photochemistry; D013050:Spectrometry, Fluorescence; D013054:Spectrophotometry, Atomic; D013329:Structure-Activity Relationship; D013501:Surface-Active Agents / Q000737:chemistry*; D013950:Thymus Gland / Q000737:chemistry
chemical_list: D001562:Benzimidazoles; D013501:Surface-Active Agents; D004247:DNA; D006690:Bisbenzimidazole; C017807:bisbenzimide ethoxide trihydrochloride