Influence of water on photophysical properties of N-bromobenzyl- or nitrobenzyl derivatives of substituted 4-hydroxystilbazolium hemicyanines.

Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780, Poznań, Poland.
Photochemical and Photobiological Sciences (Impact Factor: 2.92). 08/2011; 10(10):1670-9. DOI: 10.1039/c1pp05119a
Source: PubMed

ABSTRACT Absorption, steady-state and timed-resolved fluorescence spectra of nine N-p-bromobenzyl substituted (E)-4'-hydroxy-4-stilbazolium bromides and N-p-bromo- (or nitro-) benzyl substituted (E)-4'-hydroxy-3'methoxy-4-stilbazolium bromides, belonging to the hemicyanine class of compounds, were studied in dry and water-containing polar solvents and in water. All of the studied compounds displayed negative solvatochromism. In solvents with small amounts of water the solutions of each of the compounds change color to red, blue-green or blue, while in extra dry solvents they are all yellow. The new band causing the change in color is interpreted as belonging to the deprotonated form of the respective compound. The absorption and emission spectra of protonated and deprotonated forms of hemicyanines in solvents are presented in comparison with those of selected, isolated deprotonated forms (merocyanines) of the same compounds. The discrimination between the quinoid and zwitterionic deprotonated forms was achieved based on the absorption band location. Time-resolved fluorescence measurements in selected dry and water-containing solvents were also performed.

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    ABSTRACT: The protonation/deprotonation equilibrium was investigated for N-p-chlorobenzyl-substituted (E)-4'-hydroxy stilbazolium halide, namely (E)-1-(4-chlorobenzyl)-4-(4-hydroxystyryl)pyridinium chloride (EPC). Absorption, emission and synchronous scanning spectra were used to explain the observed phenomena. The excited state lifetimes of the protonated and deprotonated forms of EPC were measured and discussed. Absorption spectra were used to determine its pK(a) value in the ground state. We conclude that the protonation/deprotonation equilibrium is not attained in the first excited state of EPC, for kinetic reasons. The quinoid and benzenoid structures of EPC in the ground and excited state are discussed in acidic and basic range of pH. Aqueous solutions of EPC were yellow at pH < 7 and red at pH > 7, and addition of alcohols (methanol or 2-propanol) enhanced this change. Therefore, quaternary stilbazolium salts were investigated for application as acid-base indicators.
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