Fig. 1. The transient absorption spectra obtained immediately after the laser excitation of HO-Bp-NO₂ in acetonitrile (MeCN) solution without acid (orange) and with 0.38 mol/mL HClO\begin{document}$ _4 $\end{document} (green). Blue bars represent vertical transitions of \begin{document}$ ^3 $\end{document}HO-Bp-NO₂H\begin{document}$ ^+ $\end{document} predicted by TDDFT calculation. A scaled UV-Vis spectrum of HO-Bp-NO₂ in MeCN (dashed line) is also provided for comparison.

Fig. 1. The molecular structure diagrams corresponding to the abbreviations mentioned in text.

Fig. 1. Plot of HOMO and LUMO of HO-Bp-NO₂ corresponding to S₀\begin{document}$ \to $\end{document} S₁ transition obtained with TDDFT calculation at the PBE//B3LYP/6-311++G(d, p) with PCM solvent model in acetonitrile.

Fig. 2. Transient absorption spectrum obtained immediately after 355 nm laser pulse photolysis of HO-Bp-NO₂ in cyclohexane (CHX) compared to that in acetonitrile (MeCN).

Fig. 2. [in Chinese]

Fig. 2. Nanosecond transient absorption spectra upon laser excitation of HO-Bp-NO₂ in acetonitrile in the presence of 0.1 mmol/L acid. Insert: (black) kinetics monitored at 660 and 450 nm, and (red) exponential curves fitted with a single decay (660 nm) and a combined (growth and decay) functions respectively.

Fig. 3. Nanosecond transient absorption spectra obtained immediately after laser excitation of Bp-NO₂ in acetonitrile solution containing various concentrations of HClO\begin{document}$ _4 $\end{document}.

Fig. 3. [in Chinese]

Fig. 3. Molecular structure of (top) close-shell and (middle) open-shell singlet of O-Bp-NO₂H. (bottom) The spin density contribution for O-Bp-NO₂H, plotted with Multiwfn software¹.

Fig. 4. Nanosecond transient absorption spectra obtained immediately after laser excitation of HO-Bp-NO₂ in acetonitrile solution containing various concentrations of acid. Symbol star indicates the residual of excitation laser pulse.

Fig. 4. Nanosecond transient absorption spectra upon laser excitation of HO-Bp-NO₂ in acetonitrile in the presence of various concentration of acid.

Fig. 5. (a) Nanosecond transient absorption spectra recorded at various time delays after laser pulse excitation of HO-Bp-NO₂ (0.05 mmol/L) in neat acetonitrile. (b) Difference spectra obtained by subtraction of scaled 0 ns spectra from those recorded at post time delays which have been labeled in the graph, with the criterion of completely removing the absorption of \begin{document}$ ^3 $\end{document}HO-Bp-NO₂ at 650 nm, and 1.0 \begin{document}$ {\rm{ \mathsf{ μ} }} $\end{document}s spectrum (orange) in solution in the presence of 0.1 mmol/L NpOH.

Fig. 5. Kinetics (black) monitored at 450 nm obtained upon excitation of HO-Bp-NO₂ in acetonitrile solution containing various concentration of acid, and simulated curves (red) fitted with exponential functions.

Fig. 6. pK_a^*=3.1 for 3Bp-NO₂H⁺ obtained by using the dual-wavelength spectrophotometry measured at characteristic absorption bands at 625 (neutral T₁) and 740 nm (triplet cation).

Fig. 7. Kinetics (black) monitored at 650, 450 and 380 nm obtained in neat acetonitrile solution with different concentrations of HO-Bp-NO₂, and simulated curves (red) fitted with exponential functions.

Fig. 8. (a) UV-visible absorption spectra of HO-Bp-NO₂ in solutions containing MeCN:buffered water (9:1/v:v), and (b) pK_a=10.7 for HO-Bp-NO₂ obtained by using the dual-wavelength spectrophotometry measured at characteristic absorptions 328 and 426 nm.