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Inspired by the unique reactivity of the surface-bound quinone and copper cofactors in copper-containing amine oxidases, we investigated the interactions of pyrroloquinoline quinone (PQQ) with copper ions in solution and adsorbed on indium-doped tin oxide (ITO) electrodes. Characterizations based on atomic force microscopy, electrochemical mode scanning tunneling microscopy, and emission spectroscopy showed that when PQQ was reduced, the resulting two-electron transfer product, PQQH2, could couple to 3-aminophenylboronic acid and therefore be tethered to ITO. PQQH2 was also noticed to form multilayer adsorption on the electrodes, featuring reversible changes in adsorption and desorption with potential switching. X-ray photoelectron spectroscopic analysis and X-ray absorption near-edge-structure spectral measurements showed that both PQQ and PQQH2 could interact with copper ions through the N-1 and N-6 sites. Because of this reactivity, the copper ion exhibited quenching effects on the photoexcited PQQ and PQQH2 in solution and on ITO. In addition, current enhancement for PQQ0/2− was also noticed during the reduction of PQQ as copper ions were added, indicating that PQQH2 could transfer electrons to Cu2+ ions. The electron transfer rate constant was estimated to be ∼1012 cm6 mol−2 s−1 at pH 3. This electron transfer reaction, however, was less influential than the complexation counterpart in quenching the excited PQQH2. We thus deduce that the electron transfer process may be less energetic or slower than the complexation counterpart or that it takes place only after the latter is complete.

In this work, we report a photoinduced aziridination reaction sensitized by lead oxide-containing zeolite colloids (NaY|PbO x ). When N-aminophthalimide and 3-cinnamoyl-2-oxazolidinone were photolyzed with NaY|PbO x , (2R,3S)- and (2S,3R)-N-phthalimidoaziridines were observed as the major products. Under similar conditions, bare zeolite Y showed an insignificant catalytic effect on this reaction. These results highlight that the contained PbO x is a potential catalyst. According to X-ray photoelectron spectroscopy (XPS) and electrochemical characterizations, the contained lead oxide is firmly deposited on the host and behaves like an n-type semiconductor. The associated conduction band and valence-band edges are located at −0.5 ± 0.2 and 0.9 ± 0.2 V versus a saturated calomel electrode (SCE), respectively. Since the valence-band edge is positive enough to oxidize N-aminophthalimide, N-aminophthalimide radicals can be formed on the illuminated NaY|PbO x and react with 3-cinnamoyl-2-oxazolidinone to produce aziridine products. A 1-h irradiation provided the aziridine products in 3% yield and 93% selectivity. For the low yield, calculations based on the doping density of NaY|PbO x suggest that the penetration depth of light may not be sufficient to excite the majority carriers relative to the thickness of the space-charge layer. Due to this or another reason, such as light blocking by the metallic silver formed on the surface of NaY|PbO x when silver nitrate is used as the sacrificial acceptor, the efficiency in light harvesting may be limited in this case. Although limited by these drawbacks and the low concentration within NaY (<20 μg/g of NaY), the prepared PbO x (NaY) still behaves as an effective aziridination catalyst compared to some other photosensitizers. Noticeably, incorporating appropriate chiral ligands can advance the enantiomeric excess (ee) for the (2S,3R) isomer. X-ray absorption near-edge-structure (XANES) spectral measurements show that the incorporated chiral ligands may function as bidentate agents to interact with PbO x (NaY) and cause the binding energy of Pb (LIII edge) in NaY|PbO x to shift to a higher energy by ∼2.6 eV. Likely due to this chelating effect, an asymmetric oxidation of N-aminophthalimide results. A preferential formation for the (2S,3R) isomer becomes possible.