Photoinduced Electron Transfer from ZrOCo Binuclear Light Absorber to Pyridine Elucidated by Transient Optical and Infrared Spectroscopy

Abstract

All-inorganic heterobinculear units can be excited to a metal-to-metal charge transfer (MMCT) state by visible and ultraviolet light, and form the core of multicomponent artificial photosystems that rely on conjugated molecular wires to transmit electrons and holes across ultrathin silica separation membranes to catalytic sites. To understand electron transfer from a heterobinuclear unit to an organic molecule, model systems consisting of pyridine interacting with Zr IV OCo II or Ti IV OCo II sites on silica nanoparticles were assembled and spectroscopically characterized.Transient optical absorption spectroscopy combined with infrared analysis revealed that visible light excitation of assemblies in which pyridine interacts with the Zr center of the ZrOCo unit results in sub-10 nanosecond separation by electron transfer to the organic moiety. Rapid-scan FT-IR spectroscopy supported by density functional theory confirmed the identity of the pyridine radical anion charge-separated state. This state shows a lifetime of 220 (±60) ms prior to complete recovery of the initial state, suggesting spin crossover along the electron transfer path.No hole transfer from excited Zr IV OCo II  Zr III OCo III to pyridine is observed. The rectifying charge transfer behavior is an important design aspect for developing efficient artificial photosystems.

Heterobinuclear Light Absorber Coupled to Molecular Wire for Charge Transport across Ultrathin Silica Membrane for Artificial Photosynthesis

“…Experimental evidence for the absence of metal oxide clusters of higher nuclearity beyond heterobinuclear, i.e., titania or Co oxide nanoclusters, is provided by Fourier transform Raman (FT-Raman) spectroscopy, as presented in a recent publication (Figure S1). The geometrical structure of closely related ZrOCo and TiOMn binuclear sites on silica surfaces was elucidated by extended X-ray absorption fine structure measurements in a previous work. , …”

Ultrathin oxide layers for nanoscale integration of molecular light absorbers, catalysts, and complete artificial photosystems

“…For the other, the organic is coordinated to Co. 32 -1300 cm -1 region observed upon excitation of the pyr-ZrOCo units with a 405 nm light pulse (120 ms duration) agreed well with pyridine radical anion vibrational spectrum calculated by density functional theory, but not with the computed spectrum of the cation. 32 The single exponential decay of the radical and recovery of the ground state gave a lifetime of 220 + 60 ms (room temperature) (Figure 5E). The very slow back electron transfer of the pyr --Zr IV -O-Co III charge-separated state is most likely due to spin crossover along the electron transfer path, with ultrafast flip of optically prepared high spin tetrahedral Co III (S=1) to low spin Co III (S=0) as a primary candidate, for which there are a number of precedents.…”

“…FT-IR spectroscopy revealed two types of weak complexes of pyridine with the binuclear light absorber: For one complex, pyridine interacts weakly with the Zr (Ti) center as shown in Figure 5A. For the other, the organic is coordinated to Co. 32 -1300 cm -1 region observed upon excitation of the pyr-ZrOCo units with a 405 nm light pulse (120 ms duration) agreed well with pyridine radical anion vibrational spectrum calculated by density functional theory, but not with the computed spectrum of the cation. 32 The single exponential decay of the radical and recovery of the ground state gave a lifetime of 220 + 60 ms (room temperature) (Figure 5E).…”

Exploring Resonance Raman Scattering with 4-Nitrophenol

“…Spectral interpretation was generally straightforward, but caution should be exercised to avoid misinterpreting background signals. As we have demonstrated in previous work, density functional theory is a natural companion to vibrational spectroscopies and vastly simplifies spectral interpretation. , It also provides a tool in helping students to visualize the modes being observed . Though predicting the frequencies of molecular vibrations with high accuracy remains challenging (particularly given their dependence on the solvent environment), the general order and pattern of vibrational modes tends to be determined correctly .…”