Time-Resolved Infrared Spectroscopy with Multivariate Analysis on Photoinduced Proton Transfer in Aromatic...

1-Anthracen-2-yl-3-phenylurea (2PUA) is an aromatic urea compound, which forms a hydrogen-bonded complex with an acetate anion (AcO^-), 2PUA-AcO^- complex. We investigated the photoinduced reaction of the 2PUA-AcO^- complex in dimethyl sulfoxide (DMSO) by nanosecond time-resolved infrared (TR-IR) spectroscopy. TR-IR spectra obtained after the photoexcitation of 2PUA with the equal concentration of AcO^- were consistently explained by a photoinduced proton transfer model. The spectral and temporal profiles of the TR-IR spectra largely depended on concentration conditions of 2PUA and AcO^-. Under the condition where excessive amounts of AcO^- existed, the TR-IR spectra contained an unexpected signal whose amplitude was related to the concentration of free AcO^- in the solution. Using singular value decomposition analysis of the concentration-dependent TR-IR spectra, we extracted the spectral component that reflects the photoinduced reaction of the 2PUA-AcO^- complex. The extracted spectrum resembled the TR-IR spectrum obtained under the equal concentration condition, indicating that the same proton transfer occurs during the photoinduced reaction of the 2PUA-AcO^- complex irrespective of the concentration conditions. Comparing the steady-state and transient IR spectra of the 2PUA with AcO^- in DMSO with density functional theory calculations suggests that both 2PUA-AcO^- complex and tautomer species interact with solvent DMSO molecules in their electronic ground states to a large extent.