The effects of pH and hydrogen bonds on photophysical properties of N-(4-bromobenzyl) substituted hydroxystilbazolium hemicyanine and merocyanine

• The effect of pH on the color of solutions of hydroxystilbazolium dye is explained.
• Acid/base equilibria for dye in the ground and in the excited state are discussed.
• The structures of absorptive and emissive species of dye are identified.
• The hydrogen-bonded properties of the protonated and deprotonated dye are studied.

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.