Photochemical & Photobiological Sciences Dynamic Article Links Cite this: Photochem. Photobiol. Sci., 2012, 11, 1454 www.rsc.org/pps PAPER Influence of pH on photophysical properties of (E)-1-(4-chlorobenzyl)- 4-(4-hydroxystyryl)pyridinium chloride† Dorota Prukala,* a Wieslaw Prukala, a Mateusz Gierszewski, a Jerzy Karolczak, b,c Igor Khmelinskii d and Marek Sikorski a Received 30th January 2012, Accepted 7th June 2012 DOI: 10.1039/c2pp25063b 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. Introduction We report the protonation/deprotonation equilibria in solutions of (E)-1-(4-chlorobenzyl)-4-(4-hydroxystyryl)pyridinium chloride (EPC), belonging to the hemicyanine class of compounds. The structure of this compound is shown in Fig. 1. Photophysical properties of this compound in polar protic solvents were described in our previous work. 1 Its spectral, photophysical and photochemical properties strongly depend on solvent polarity and also on pH. We previously demonstrated that this compound has negative solvatochromism, as a typical stilbazolium hemi- cyanine dye. 1 N-Substituted derivatives of (E)-stilbazols – quaternary stilba- zolium salts – also have a wide range of applications. Quaternary stilbazolium salts show moderate antimicrobial activity in vitro. We previously found that derivatives of (E)-stilbazols are active against various bacteria, including Staphylococcus aureus, Strep- tococcus faecalis, Bacillus subtilis and Escherichia coli. 2,3 Their mass and NMR spectra were discussed in our previous works. 3,4 Molecules of quaternary stilbazolium salts have a donor group (hydroxyl group) and an acceptor group (substituted pyridinium moiety) connected by an ethylene bridge with an extended con- jugated π-electronic system. For such type of compounds, similar to donor/acceptor (D/A)-substituted benzenes, dual fluor- escence is observed including fluorescence from an intramolecu- lar charge transfer state (ICT) and from the so-called locally excited state (LE). The fluorescence contributions of these states depend on solvent polarity. The photophysical properties of these compounds are also affected by the pH of the environment. 5 Fig. 2 shows two canonical structures of the protonated form of (E)-hydroxystilbazolium hemicyanines. The contribution of the first structure in neutral and acidic conditions is dominant in the ground state whereas the second structure contributes more to the excited state. 6,7 Such structural change upon passing from the ground to the excited state results in a large difference between the dipole moments in these states. Nakanishi and co- workers 8 found in their semiempirical calculations that the dipole moment in the ground state of (E)-N-methyl-4-hydroxy- stilbazolium cation amounts to 12.24 D and to 2.05 D in the excited state, with its direction switching to the opposite. The positive charge is localized on the nitrogen atom in the ground state and on an oxygen atom in the excited state. Fig. 1 Structure of the compound studied. † Electronic supplementary information (ESI) available. See DOI: 10.1039/c2pp25063b a Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznań, Poland. E-mail: dprukala@amu.edu.pl; Fax: +48 61 8658008; Tel: +48 61 8291309 b Faculty of Physics, A. Mickiewicz University, Umultowska 85, 61-614 Poznań, Poland c Centre of Ultrafast Laser Spectroscopy, A. Mickiewicz University, Umultowska 85, 61-614 Poznań, Poland d Universidade do Algarve, FCT, DQF and CIQA, Campus de Gambelas, 8005-139 Faro, Portugal 1454 | Photochem. Photobiol. Sci., 2012, 11, 1454–1464 This journal is © The Royal Society of Chemistry and Owner Societies 2012