Interrogating the photogenerated Ir(iv) state of a water oxidation catalyst using ultrafast optical and X-ray absorption spectroscopy

Abstract

Using sunlight to drive molecular water oxidation catalysts for fuel formation requires understanding the single electron transfer events involved in catalyst activation. In an effort to photogenerate and characterize the highly reactive Ir(IV) state of the Ir(III)-based water oxidation catalyst Cp*Ir(ppy)Cl (ppy ¼ 2-phenylpyridine), we have incorporated the complex into a covalent electron acceptor-chromophore-Cp*Ir(ppy)Cl triad, in which naphthalene-1,8:4,5-bis(dicarboximide) (NDI) is the electron acceptor and perylene-3,4-dicarboximide (PMI) is the chromophore. Photoexcitation of the PMI chromophore in dichloromethane results in two competitive reactions: NDI-1 *PMI-Ir(III) / NDI-PMI_ À -Ir(IV) and NDI-1 *PMI-Ir(III) / NDI_ À -PMI_ + -Ir(III) that each proceed with s < 5 ps, as determined by femtosecond transient absorption spectroscopy. Both intermediate ion pairs undergo charge shift reactions to produce NDI_ À -PMI-Ir(IV). The fully charge-separated ion pair has a lifetime of 17.2 AE 0.1 ns, and its photophysical behavior is similar in the more polar solvent benzonitrile. Time-resolved X-ray absorption measurements on the triad at 100 ps following PMI photoexcitation show a new absorption feature at the L III -edge of Ir and a blue-shifted white-line peak, which provides direct evidence of a change in the Ir oxidation state from Ir(III) to Ir(IV), consistent with the photophysical measurements. Our work underscores the utility of ultrafast spectroscopy performed on covalent assemblies of electron donoracceptor systems with solar fuels catalysts to generate and probe their higher valence states in ways that complement chemical or electrochemical oxidation and establish the nature of key intermediates implicated in their catalytic mechanisms.

Solar energy for electricity and fuels

“…Molecular and semiconductor nanostructure processes. The light-driven catalyst can be molecular (Hammarström and Hammes-Schiffer 2009 ; Sundström 2009 ; Nocera and Guldi 2009 ; Kamat and Bisquert 2013 ) or non-molecular (Cook et al 2010 ; Poddutoori et al 2011 ; Vagnini et al 2013 ; Joya et al 2013 ). The physical limitations are equal and the scientific problems are of equal magnitude.…”

Highly Soluble Benzo[ghi]perylenetriimide Derivatives: Stable and Air‐Insensitive Electron Acceptors for Artificial Photosynthesis

“…[42–44] In other words, a rather poor electron donor should be employed. The reaction free energy criteria to achieve a high quantum yield of photoinduced charge separation in the assembled D/A molecular system then dictates the use of a strong electron acceptor.…”

“…[38][39][40][41] To accomplish photosynthetic water oxidation in moleculebased artificial photosynthetic devices,ahigh electrochemical reduction potential of the electron donor (photosensitizer) radical cation, E(D/DC + + )i sr equired. [42][43][44] In other words, ar ather poor electron donor should be employed. The reaction free energy criteria to achieve ah igh quantum yield of photoinduced charges eparation in the assembled D/A molecular system then dictates the use of as trong electron acceptor.…”

Triperyleno[3,3,3]propellane triimides: achieving a new generation of quasi-D3h symmetric nanostructures in organic electronics

“…Scheme 1 shows the synthesis of the rigid 3D-symmetric rylene imide. The key chemistry is the synthesis of the 3D perylenetetracarboxylic trianhydrides ( 5 ), which is unlike the synthesis of perylene dicarboxylic monoanhydride via decarboxylation of 3,4,9,10-perylenetetracarboxylic dianhydrides at high temperature 48,49. We designed an efficient synthetic protocol based on acid-promoted cyclodehydrogenation of binaphthyl derivatives bearing dicarboxylic anhydride at the peri-position to prepare the key intermediate compound 5 .…”