Ultrafast photodriven intramolecular electron transfer from an iridium-based water-oxidation catalyst to perylene diimide derivatives

Abstract

Photodriving the activity of water-oxidation catalysts is a critical step toward generating fuel from sunlight. The design of a system with optimal energetics and kinetics requires a mechanistic understanding of the single-electron transfer events in catalyst activation. To this end, we report here the synthesis and photophysical characterization of two covalently bound chromophore-catalyst electron transfer dyads, in which the dyes are derivatives of the strong photooxidant perylene-3,4:9,10-bis(dicarboximide) (PDI) and the molecular catalyst is the Cp à IrðppyÞCl metal complex, where ppy ¼ 2-phenylpyridine. Photoexcitation of the PDI in each dyad results in reduction of the chromophore to PDI •− in less than 10 ps, a process that outcompetes any generation of 3à PDI by spin-orbit-induced intersystem crossing. Biexponential charge recombination largely to the PDI-Ir(III) ground state is suggestive of multiple populations of the PDI •− -IrðIVÞ ion-pair, whose relative abundance varies with solvent polarity. Electrochemical studies of the dyads show strong irreversible oxidation current similar to that seen for model catalysts, indicating that the catalytic integrity of the metal complex is maintained upon attachment to the high molecular weight photosensitizer.photoinduced electron transfer | solar fuels | ultrafast optical spectroscopy | water oxidation A rtificial photosynthetic systems for solar fuels generation must integrate the functions of light harvesting, charge separation, and catalysis, with water as the source of electrons for reductive fuel-forming chemistry (1-5). The design of a lightdriven water-splitting system based on molecular catalysts requires a fundamental understanding of the individual electron transfer steps involved in multielectron catalyst activation. To investigate the energetic and kinetic demands of coupling photodriven charge separation and catalysis, many research groups have studied the photophysical properties of covalently linked redox-active organic dyes and transition metal complexes (6-16). In several cases, electron transfer to or from the metal has been observed and characterized, though energy transfer and intersystem crossing to the chromophore triplet state can be significant competing processes. In this work, we use derivatives of perylene-3,4∶9,10-bis(dicarboximide) (PDI) linked to an iridium complex of the type Cp à IrðN-CÞX to demonstrate light-driven single-electron oxidation of a highly active molecular catalyst precursor for water oxidation.Derivatives of PDI are useful organic chromophores for solar device applications (5,17,18). Their high molar absorptivity, stability, low cost, ease of synthetic manipulation, and often advantageous self-assembly properties have led to their use in molecular electronics and a variety of solar energy conversion systems (19). For solar fuels applications, the mild reduction potentials of PDI derivatives make their excited states powerful photooxidants, though there are only a few literature examples in which 1à PDI is used to ...

Perylene Bisimide Dye Assemblies as Archetype Functional Supramolecular Materials

“…Besides the focus topic of this Review, that is, supramolecular materials systems, fundamental studies that rely on PBI dyes were in particular devoted to conventional and single molecule fluorescence spectroscopy, 82−84 photoinduced energy, 9,85−88 and electron transfer processes, 89,90 and more recently to singlet fission 62 and artificial photosynthesis. 91,92 The traditional areas Figure 4. Conversion between propeller-like distorted PBI atropisomers and relationship between observed dihedral angles and the sterical demands of the bay substituents (for exact calculation of the apparent overlap parameters ∑r*, see ref 66).…”

Solar-driven proton and carbon dioxide reduction to fuels — lessons from metalloenzymes

“…Overall rates of 1:1 fusion systems, comparable to molecular 'dyads' [48,49], will always be controlled by the slower component, a limitation that need not apply if enzymes are attached to nanoparticles. The amount of immobilised catalyst (and, if necessary, sensitiser dye) are varied, allowing control over the reaction rates of the different components.…”

A Dynamic View of Proton-Coupled Electron Transfer in Photocatalytic Water Splitting

“…Furthermore, the coupling between the WOC, the chromophore and an electron accepting semiconductor into a photoanode has been achieved through co-absorption of both the catalyst and the chromophore 16,[29][30][31][32] or through dye-WOC supramolecular complexes. [33][34][35][36] Acquiring a fundamental understanding of the electron transfer processes and catalytic water oxidation mechanism following light excitation of the photoanode is essential for the design and the optimization of solar fuel cells. However, this is a challenging task mainly because of the different time scales of the processes occurring upon photoexcitation.…”