Electronic Structure Changes of an Aromatic Amine Photoacid along the Förster Cycle

Abstract

Photoacids show a strong increase in acidity in the first electronic excited state, enabling real-time studies of proton transfer in acid-base reactions, proton transport in energy storage devices and biomolecular sensor protein systems. Several explanations have been proposed for what determines photoacidity, ranging from variations in solvation free energy to changes in electronic structure occurring along the four stages of the Förster cycle. Here we use picosecond nitrogen K-edge spectroscopy to monitor the electronic structure changes of the proton donating group in a protonated aromatic amine photoacid in solution upon photoexcitation and subsequent proton transfer dynamics. Probing core-to-valence transitions locally at the amine functional group and with orbital specificity, we clearly reveal pronounced electronic structure, dipole moment and energetic changes on the conjugate photobase side. This result paves the way for a detailed electronic structural characterization of the photoacidity phenomenon.

Keywords: Aromaticity; Electronic Structure; Orbital Interactions; Photoacids; Time-Resolved Soft x-Ray Spectroscopy.

Figure 1

a) Förster cycle of APTS, showing the electronic states involved. As the pK _a*<0 the S₁ energy level of APTS in its base form is lower than in its acid form, while for weaker photoacids with pK _a*>0 the opposite occurs. Molecular structures of APTS in its acid and base forms are shown underneath; b) Electronic absorption and fluorescence spectra of APTS in its acid and base forms. Fluorescence emission of APTS, initially excited in its acid form, is almost exclusively due to APTS in its base form, as proton transfer occurs on a much shorter time scale than the electronic excited state decay.

Figure 2

Nitrogen K‐edge absorption spectra of electronic ground state APTS in its R−ND₃ ⁺ acid and R−ND₂ base forms (solid lines) at a concentration of 50 mM. Dashed lines at 397.5 eV, 399.7 eV, 402.1 eV, 403.8 eV, and 406.3 eV are relevant for the comparison with transient absorption spectral features as displayed in Figure 3. Calculated spectra spanning the whole energy window, obtained with TDDFT CAM‐B3LYP/def2‐SVP/CPCM level of theory (see Supporting Information), indicate that the first strong transitions for the acid are of 1s→σ* character, whereas for the base form we also observe distinct pre‐edge 1s→π* transitions.

Figure 3

Accessing the Förster cycle of APTS through UV pump—X‐ray absorption probe spectroscopy. a) The S₁ state, characterized by a dominant $\pi$ →$\pi$ * transition, is populated by a photo‐excitation of the HOMO→LUMO (S₀→S₁) transition. Consecutive absorbance changes in the nitrogen K‐edge spectral range probing the transient population kinetics of the $\pi$ * and other electronic states monitor the dynamics of APTS along the Förster cycle. Transient X‐ray absorption spectra of the APTS photo‐excited in its photoacid (b) and in its photobase (c) forms measured for specific pulse delays, and the time dependence of the transient changes as recorded for APTS photo‐excited in its acid form (d), (e) enable us to monitor both the deprotonation process, as well as the transient population of the orbitals involved in the excitation. Dashed lines at 397.5 eV, 399.7 eV, 402.1 eV, 403.8 eV, and 406.3 eV in panels (b), (c) indicate the energies of the kinetic curves with the same colour codes as displayed in panel (d), (e). For comparison we show the build‐up dynamics of the excited photobase population obtained from UV/IR pump‐probe measurements (see Supporting Information), and have added the transient response of Fe(bipy)₃ ²⁺ at 399.5 eV, also photoexcited at 343 nm, that has a bleach signal appearance that is instrument‐response limited. ^[15b] The slower in‐growth of the bleach signal at 406.3 eV for APTS reflects its delayed conversion of APTS R−ND₃ ⁺ form into APTS R−ND₂ forms by deuteron transfer in the S₁‐state.

Figure 4

a) Comparison of the excited states involved the X‐ray probing of APTS in its photoacid (red curves) and its conjugate photobase forms (blue curves) for the electronic S₀ ground and S₁ and S₂ electronic excited states. Please note the 20‐fold increase in absorption strength of the 1s core→LUMO excitations for APTS in its photobase form compared to those of APTS in its photoacid form. The theoretical spectra, calculated with NEVPT2, have been calibrated by a comparison of the lowest energy pre‐edge transition of APTS photobase in the S₀ state as measured experimentally (blue shaded spectrum). b) With the same energy calibration a satisfying comparison results for the calculated and experimental spectra of APTS photobase in the S₁ state. c) Graphical representation of the most important involvement of the HOMOs and LUMOs of APTS for the particular 1s core excitations probed with (transient) N K‐edge spectroscopy. Electronic charge distribution changes upon S₀→S₁ electronic excitation of APTS in its photoacid (d) and photobase (e) forms (Isovalue 0.0006 with red being positive). Electrical dipole moment changes from the CASSCF(6,7) level upon S₀→S₁ electronic excitation of APTS in its photoacid (f) and photobase (g) forms are 0.334 D and 4.701 D, respectively.