Influence of water on photophysical properties of N-bromobenzyl- or nitrobenzyl derivatives of substituted 4-hydroxystilbazolium hemicyanines
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.Photochemical and Photobiological Sciences 06/2012; 11(9):1454-64. DOI:10.1039/c2pp25063b · 2.94 Impact Factor
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ABSTRACT: Photophysical properties of (E)-1-(4-bromobenzyl)-4-(4-hydroxy-3-methoxystyryl) pyridinium bromide existing in equilibrium of its protonated (BSHB) and deprotonated (BS) forms at different pH were characterized by UV–Vis spectroscopy, and steady-state, and time-resolved fluorescence techniques. Absorption, steady-state, and time-resolved emission spectra, and synchronous scanning spectra were used to explain the observed phenomena. The pKa of BSHB in the ground state was determined. We also found that the deprotonated form – BS, being a weak base, is protonated upon addition of water. The pKa* of BSHB in the excited state was determined in two different ways giving different results. This inspires the discussion on the applicability of the use of the thermodynamic Förster cycle for calculation of the excited-state pKa* for hydroxystilbazolium dyes. Solvatochromism of both BSHB and BS in polar aprotic and protic solvents was analyzed using the four-parameter Catalán solvent scale and the Δf scale. This analysis indicates that both compounds in their emissive states have benzenoid-like (or zwitterionic for BS) structure in protic solvents. However, in aprotic polar solvents the contribution of the zwitterionic structure to their emissive state is reduced in favor of the quinoid structure. Importantly, the Catalán scale shows a strong influence of solvent basicity (hydrogen-bond accepting ability) on the absorption properties of BSHB, indicating strong stabilization of its ground and vertical excited state by hydrogen-bonded complexes. Contrary to BSHB, the absorption spectra of BS are affected by solvent acidity. Hydrogen – bonded complexes stabilize the ground state of this compound, although their energy decreases strongly upon excitation. The hydrogen-bonded complexes influenced the structures of both compounds in their excited states.Dyes and Pigments 09/2014; 108:126–139. DOI:10.1016/j.dyepig.2014.04.013 · 3.47 Impact Factor