Photoinduced Water Oxidation in Chitosan Nanostructures Containing Covalently Linked Ru^II Chromophores and Encapsulated Iridium Oxide Nanoparticles

Abstract

The luminophore Ru(bpy)2(dcbpy)2+ (bpy=2,2’‐bipyridine; dcbpy=4,4’‐dicarboxy‐2,2’‐bipyridine) is covalently linked to a chitosan polymer; crosslinking by tripolyphosphate produced Ru‐decorated chitosan fibers (NS‐RuCh), with a 20 : 1 ratio between chitosan repeating units and RuII chromophores. The properties of the RuII compound are unperturbed by the chitosan structure, with NS‐RuCh exhibiting the typical metal‐to‐ligand charge‐transfer (MLCT) absorption and emission bands of RuII complexes. When crosslinks are made in the presence of IrO2 nanoparticles, such species are encapsulated within the nanofibers, thus generating the IrO2⊂NS‐RuCh system, in which both RuII photosensitizers and IrO2 water oxidation catalysts are within the nanofiber structures. NS‐RuCh and IrO2⊂NS‐RuCh have been characterized by dynamic light scattering, scanning electronic microscopy, and energy‐dispersive X‐ray analysis, which indicated a 2 : 1 ratio between RuII chromophores and IrO2 species. Photochemical water oxidation has been investigated by using IrO2⊂NS‐RuCh as the chromophore/catalyst assembly and persulfate anions as the sacrificial species: photochemical water oxidation yields O2 with a quantum yield (Φ) of 0.21, definitely higher than the Φ obtained with a similar solution containing separated Ru(bpy)32+ and IrO2 nanoparticles (0.05) or with respect to that obtained when using NS‐RuCh and “free” IrO2 nanoparticles (0.10). A fast hole‐scavenging process (rate constant, 7×104 s−1) involving the oxidized photosensitizer and the IrO2 catalyst within the IrO2⊂NS‐RuCh system is behind the improved photochemical quantum yield of IrO2⊂NS‐RuCh.

Self-assembled systems for artificial photosynthesis

“…Indeed, flash photolysis experiments showed that such a hole-scavenging process is faster in IrO 2 ⊂NP-RuCh . For this latter species, the rate constant of the hole scavenging process is 7.4 × 10 4 s −1 , 25 a remarkable value when compared to the rate constant for the hole scavenging process in the [Ru(bpy) 3 ] 2+ /IrO 2 /Na 2 S 2 O 8 separated system, which is reported to be 8 × 10 2 s −1 , 26 that is two orders of magnitude slower. The relevant acceleration of the hole scavenging in IrO 2 ⊂NP-RuCh is attributed to the increase in the proximity of both photosensitizer and catalyst subunits, thanks to the restricted environment promoted by the chitosan nanofibers, which play the role of concentrators.…”

“…In particular, a hydrogel formed by polymerized chitosan, functionalized with bpy moieties serving to anchor Ru(bpy) 3 2+ -type chromophores on the chitosan structure, has been prepared in the presence of iridium oxide nanoparticles, the water oxidation catalysts. 25 In these conditions, iridium oxide nanoparticles are incorporated within the hydrogel, forming a nanofiber-like structure, IrO 2 NP-RuCh. 25 Figure 11 shows the structural formula of the chitosan polymer containing the Ru(II) chromophore.…”

“…25 In these conditions, iridium oxide nanoparticles are incorporated within the hydrogel, forming a nanofiber-like structure, IrO 2 NP-RuCh. 25 Figure 11 shows the structural formula of the chitosan polymer containing the Ru(II) chromophore. Dynamic light scattering, scanning electronic microscopy (SEM), and energy-dispersive X-ray analysis (EDX)…”

“…25 In these conditions, iridium oxide nanoparticles are incorporated within the hydrogel, forming a nanofiber-like structure, IrO 2 ⊂NP-RuCh . 25 Fig. 11 shows the structural formula of the chitosan polymer containing the Ru( ii ) chromophore.…”

“…Photoinduced water oxidation with production of molecular oxygen takes place under visible irradiation in the presence of persulfate anions, with a quantum yield of 0.21. 25 Interestingly, the quantum yield of an analogous solution containing the same amount of photosensitizers, catalysts, and sacrificial agents is 0.05. The better performance of the hydrogel, nano-fiber system is attributed to a kinetic effect, in particular to the faster hole scavenging process between the oxidized form of the photosensitizer (produced by photoinduced oxidative electron transfer from the excited Ru-based chromophore to the sacrificial electron acceptor) and the catalyst, which regenerate the ground state of the photosensitizer and contribute to producing the active form of the catalyst.…”