Per‐6‐Thiol‐Cyclodextrin Engineered [FeFe]‐Hydrogenase Mimic/CdSe Quantum Dot Assembly for Photocatalytic Hydrogen Production

Abstract

[FeFe]‐hydrogenase ([FeFe]‐H2ase) mimics, inspired by hydrogenases in nature, have emerged as an important class of molecular catalysts toward hydrogen (H2) evolution. Herein, a per‐6‐thiol‐cyclodextrin (CDSH) engineered [FeFe]‐H2ase mimic/CdSe quantum dot (QD) assembly is established for effective photocatalytic H2 evolution. In the assembly, CDSH is first anchored on CdSe QDs via the strong multidentate Cd‐thiol coordination. Then, [FeFe]‐H2ase mimic is assembled on the QD surface via the host–guest supramolecular interaction between the hydrophobic cavity of cyclodextrin (CD) and the adamantyl (AD) group of [FeFe]‐H2ase mimic. In neutral aqueous solution of isopropanol, the Fe2S2‐AD/CDSH/CdSe assembly exhibits a turnover number (TON) of 1.12 × 105 (based on Fe2S2‐AD) under visible light, which is about 12‐fold to the corresponding system without CDSH. Mechanistic insights indicate that the CDSH can not only prevent the aggregation of QDs in acidic conditions, but also balance the interfacial charge (electron and hole) transfer to avoid the photocorrosion of QDs, thereby giving the highest TON of H2 photogeneration based on [FeFe]‐H2ase mimics up to date.

Ultrafast Electron Transfer from CdSe Quantum Dots to an [FeFe]-Hydrogenase Mimic

“…These mostly focused on nanosecond or longer timescales. [14][15][16][17][18][19][20][21][22] Charge transfer to H2ase enzymes was found to occur on the nanosecond timescale with rate constants of electron transfer in the range of 10 6 -10 9 s -1 . [16][17][18][19]23 Here, the slow interfacial electron transfer from the nanoparticle surface to the enzyme is limiting the efficiency of hydrogen generation due to the competition with exciton recombination on the same timescale.…”

“…25 Thus, combining QDs and so-called H2ase mimics, which are simple and small molecular structures derived from the catalytic center of H2ase, 11 has become a promising approach and TONs of 10 4 -10 5 for CdSe QDs as photosensitizer have been reported. 14,15,22 Reported rates of electron transfer to H2ase mimics are also in the range of nanoseconds but remain only estimates based on Stern-Volmer analyses of photoluminescence (PL) quenching and time-resolved PL or transient absorption (TA) spectroscopy with nanosecond resolution. In the Stern-Volmer analysis, the quenching process is usually treated as a dynamic (diffusion controlled) process, yielding bimolecular quenching constants at low H2ase mimic concentrations, 14,26 while static quenching due to adsorption of the mimic to the surface is often not considered.…”

“…In particular, time-resolved spectroscopic studies elucidating the relevant charge transfer processes have only gained traction in the past few years. These mostly focused on nanosecond or longer time scales. − Charge transfer to H 2 ase enzymes was found to occur on the nanosecond time scale with rate constants of electron transfer in the range 10 6 –10 9 s –1 . − , Here, the slow interfacial electron transfer from the nanoparticle surface to the enzyme is limiting the efficiency of hydrogen generation due to the competition with exciton recombination on the same time scale. Small molecular redox-mediators, which shuttle charges from the excited semiconductor particle to the enzyme, have been successfully used to enhance hydrogen conversion efficiency. ,,, Critical parameters to consider are the distance to the catalytic center, which is influenced by the nature of the surface-capping ligands, the structure of the enzyme itself, and the number of adsorbed enzymes per nanocrystal.…”

“…This becomes particularly important for quantum dots (QDs), which are comparable in size to H 2 ases and have only limited surface area for enzyme adsorption . Thus, combining QDs and so-called H 2 ase mimics, which are simple and small molecular structures derived from the catalytic center of H 2 ase, has become a promising approach, and TONs of 10 4 –10 5 for CdSe QDs as a photosensitizer have been reported. ,, …”

“…In the Stern–Volmer analysis, the quenching process is usually treated as a dynamic (diffusion controlled) process, yielding bimolecular quenching constants at low H 2 ase mimic concentrations, , while static quenching due to adsorption of the mimic to the surface is often not considered. For a CdSe QD-H 2 ase mimic assembly, a rate constant of ∼10 7 s –1 was estimated based on ns-TA spectroscopy, but this value only offers a lower limit of the actual electron transfer rate, as the recorded decay trace of the QD:H 2 ase mimic complex lay within the response function of the detector. In particular, it is known that electron transfer from photoexcited cadmium chalcogenide nanoparticles to surface-proximal small organic molecules can occur on a 10s of picoseconds or faster time scale. − However, this time scale has never been probed for a system of small [FeFe]-H 2 ase mimics and CdSe QDs.…”