Sensitizers) 1,3-Di(4-pyridiyl)propane was alkylated with benzyl bromide or n-octyl iodide to form dipyridinium salts, which were condensed readily with a-diketones in the presence of piperidine to give bridged cyanine dyes containing hydrophobic alkyl groups. In acidic solution, the entire 1H-NMR spectra of the cyanines were shifted lower in field 0.4-0.7 ppm, and the CH singlet corresponding to the fulvene ring proton of the cyanines was replaced by a 2 proton singlet at 4.25-4.70 ppm for the methylene groups of dications. A dramatic hypochromic shift and a very strong hypochromic effect in their visible spectra were observed in acidic media. The bridged cyanine dyes were protonated at the central ring to form dications with resulting loss of the cyanine conjugation. The pKb values of the cyanines were evaluated from the pH dependence of dye absorption spectra. Substituents at the 4,5-position of the central cyclopentadiene ring greatly affected the pKb, but the N-alkyl groups and counterion played no role on the pKb of the cyanines. The presence of micelle, amylose, and b-cyclodextrin had very small effects on the visible absorptions and protonation processes of the cyanines. The bridged cyanines showed fluorescence emissions at ~600 nm with moderate fluorescence quantum yield.

Key words: PYRIDINE P, CONDENSATION REACTION, CYCLOPENTADIENE, HALOHYDROCARBON, PROPANE P, N-ALKYL GROUP, BRIDGE COMPOUNDS, FLUOROSPECTROPHOTOMETRY, PROTONATION, CYANINE DYES