Fig 1.
Panel A: Electronic absorption spectra of SAL-3 in aqueous solutions with various pH. The arrow marks the isosbestic point at the wavelength of ~305 nm. The figure presents spectra for pH: 3; 6; 6.6; 7.1; 7.4; 8; 8.6, and 11. The experiments were conducted at 23 oC. Panel B: Correlation between the absorbance ratio at the wavelength of 296 to 326 nm relative and the pH of the aqueous solution, based on the spectra in Panel A.
Table 1.
Characteristics of the investigated compound (Fig 2).
Fig 2.
Chemical structure of the SAL-3 molecule (A–non-ionized form, B–form with an H-bond between the–OH and C = O group, C–form after proton transfer between the–OH and C = O group, in this case a carbonyl group is formed and an–OH group emerges where the carbonyl group used to be).
Fig 3.
Fluorescence emission spectra for SAL-3 in aqueous solutions with various pH.
The figure presents the results for pH: 1; 6.5; 7.1; 8; 8.9; and 11. The spectra were obtained at 23°C in the spectral range from 305 to 600 nm.
Fig 4.
Panel A: Resonance light scattering (RLS) spectra for selected values of aqueous solution pH, analogically to the previous experiments. Panel B: RLS spectra for selected mixtures of H2O and propan-2-ol.
Fig 5.
Fluorescence excitation spectra for selected pH of the aqueous solution.
Excitation emission was set at the respective maximum of the longwave fluorescence emission spectra. The spectra were obtained at 23°C in the spectral range from 270 to 390 nm.
Fig 6.
Panel A: Electronic absorption spectra for selected ratios of water to propan-2-ol, normalized at the wavelength maximum. Panel B: Position of the maximum of the electronic absorption band for the spectra presented in Panel A.
Fig 7.
Panel A: Fluorescence emission spectra corresponding to the spectra in Fig 6A. The excitation wavelength corresponded to the maximum of the absorption band. Panel B: Fluorescence emission spectra analogous to those in Panel A but with the excitation wavelength corresponding to the slopes of the absorption band–as described in the paper.
Fig 8.
Fluorescence excitation spectra corresponding exactly to the emission spectra in Fig 7.
Panel A: Excitation spectrum with emission detected at 331 nm; Panel B: Excitation spectrum with emission detected at 429 nm.
Fig 9.
Panels A-B: TCSPC analysis for SAL3 in water at various pH. The top panel presents the decay of fluorescence intensity. Black color denotes the apparatus profile. Points correspond to measurement results and the continuous line to the double component analysis. The bottom panel presents the distribution of residuals. The excitation wavelength was 294 nm. Panels C-D: TCSPC analysis for SAL-3 in water in various solution compositions. The top panel presents the decay of fluorescence intensity for a selected propanol content. Black color denotes the apparatus profile. Points correspond to measurement results and the continuous line to the double component analysis. The bottom panel presents the distribution of residuals. The excitation wavelength was 294 nm.
Fig 10.
Panels A-C: Effect of pH on the fluorescence lifetime for SAL3 in water. Panel A presents fluorescence lifetimes of the mean values observed at various pH levels, while panel B shows fluorescence lifetimes of the first (τ1) and the second (τ2) component, observed for various pH. The dependences of the fraction of both components in pH are presented in Panel C. The data analysis was carried out for the two-component model (Eq (1) for i = 2) for data presented in Fig 9A and 9B. Panels: D-F: Effect of the content of propan-2-ol on SAL-3 fluorescence lifetime. Panel A presents fluorescence lifetimes of the mean values observed in various propanol mixtures, while panel B shows fluorescence lifetimes of the first (τ1) and the second (τ2) component. The dependences of the fraction of both components in solution composition are presented in Panel C. The data analysis was carried out for the two component model (Eq (1) for i = 2) for data presented in Fig 9C and 9D.
Table 2.
Fluorescence lifetimes of SAL-3 in different aqueous solvents at different pH with F408 cut-off filter and diode 294 nm.
Table 3.
Fluorescence lifetimes of SAL-3 in different aqueous solvents at different pH with F408 cut-off filter and diode 294 nm.
Fig 11.
Ground state structures of various protonation forms of the SAL-3 molecule as optimized using B3LYP/aug-cc-pVDZ and PCM treatment with water as a solvent.
Hydrogen bonds depicted by red dashed lines. Panel A: SAL-3H+ form, Panel B: SAL-3 form and Panel C: SAL-3- form.
Table 4.
TD-DFT predictions for various protonation forms of the SAL-3 molecule in a solvent (water) treated as per the PCM approach.
Energetics given in a nm scale, dimensionless oscillatory strengths given in parentheses. Low-lying absorbing states shown as well as the lowest excited state for emission.
Table 5.
Positions of the absorption spectra and fluorescence maxima for SAL-3 in different solvents.
Table 6.
Solvent parameters F1(ε,n), F2(ε,n), dielectric constant ε, index of refraction n and for different solvents.
Fig 12.
Stokes shift to F1(ε,n) using Bakhshiev’s equation (panel A), to F2(ε,n) using Kawski-Chamma-Viallet’s equation (panel B) for SAL-3 dissolved in different solvents (1—n-heptane, 2—n-hexane, 3 –chloroform, 4—butan-1-ol, 5—propan-2-ol, 6—ethanol, 7—DMF, 8—DMSO, 9—methanol, 10 –acetonitrile, 11—H2O). Black square–Em 1 (maximum of fluorescence ~ 330 nm), blue circle–Em 2 (maximum of fluorescence ~ 430 nm).
Fig 13.
Stokes shift variation with the normalized value of solvent polarity for SAL-3.
Black square–I Stokes Shift, red circle–II stokes shift in different solvents (1—n-hexane, 2—n-heptane, 3 –chloroform, 4—DMF, 5—DMSO, 6—acetonitrile, 7 –propan-2-ol, 8 –butan-1-ol, 9—ethanol, 10 –methanol, 11—H2O).
Table 7.
Onsager cavity radius a0 for SAL3.
Slopes S1 and S2 determined by Stokes shift and in the function of F1(ε,n) and F2(ε,n), ground state μg, excited state μe, change in dipole moment Δμ and coefficient of determination r2 for SAL-3.
Table 8.
Slope m, change in dipole moment Δμ and coefficient of determination r2 for SAL-3.