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Supporting Information Femtosecond Infrared Spectroscopy Reveals The Electronic Supplementary Material (ESI) for Physical Chemistry Chemical Physics. This journal is © the Owner Societies 2018 Supporting Information Femtosecond infrared spectroscopy reveals the primary events of the ferrioxalate actinometer Steffen Straub, Paul Brünker, Jörg Lindner and Peter Vöhringer†*. †Institut für Physikalische und Theoretische Chemie, Rheinische Friedrich-Wilhelms-Universität, Wegelerstraße 12, 53115 Bonn, Germany. 1. Various photochemical mechanisms for ferrioxalate The chemical processes that are initiated upon ultraviolet excitation of an aqueous solution of potassium trisoxalatoferrate(III) provide the molecular basis of the “gold standard” of photochemical actinometry;1 namely, the ferrioxalate actinometer. In the field of photochemistry, actinometers are widespread analytical tools that allow for a precise measurement of the absolute internal radiative flux of a photochemical reactor. The ferrioxalate actinometer rests on the net light-induced photocoversion of ferric into ferrous iron III 3‒ II 2‒ 2‒ 2 [Fe (ox)3] + hν 2 [Fe (ox)2] + 2 CO2 + ox , (1) 2 where ox = C2O4. Hatchard and Parker originally proposed the photochemical processes following an optical excitation (eq. (2) below) to proceed via a sequence of inner-sphere (3a) and outer-sphere electron transfer reactions, (3b) and (3c), each of which are coupled to the loss of an oxalate ligand from the Fe(III) center: III 3‒ III 3‒ [Fe (ox)3] + hν *[Fe (ox)3] (2) III 3‒ II 2‒ ●‒ *[Fe (ox)3] [Fe (ox)2] + ox (3a) ●‒ ●‒ ox ⇋ CO2 + CO2 (4) III 3‒ ●‒ II 2‒ 2‒ [Fe (ox)3] + ox [Fe (ox)2] + 2 CO2 + ox (3b) III 3‒ ●‒ II 2‒ 2‒ [Fe (ox)3] + CO2 [Fe (ox)2] + CO2 + ox (3c) ●‒ In bulk water, the intermediate oxalate radical anions, C2O4 , are known to be in equilibrium ●‒ 3 with neutral CO2 and carbon dioxide radical anions, CO2 (eq. (4)). Both open-shell carbonaceous species can then transfer their unpaired electron to another ferric center (3b and c) II 2‒ 2‒ to yield a ferrooxalate, [Fe (C2O4)2] , the closed-shell free oxalate dianion, C2O4 , and neutral carbon dioxide. Figure S1: Correlation of the electronic absorption spectrum of aqueous ferrioxalate and the photochemical quantum yield of the ferrioxalate actinometer. The quantum yield data (symbols) are compiled from the literature. Circles from Ref. 2, squares from Ref. 4, diamonds from Ref. 5. This Hatchard-Parker mechanism fully accounts for a photochemical quantum yield for Fe(II) production that exceeds a value of one as was observed in many independent experiments reported in the literature so far. 1 Thus, a single photon is capable of reducing more than a single iron(III) complex (cf. Figure S1) as is expressed in the net actinometer reaction (1).5 In addition, the actinometer’s photochemical quantum yield correlates in a unique fashion with the electronic spectrum of the ferrioxalate complex in aqueous solution. Formally, the UV/Vis spectrum can be divided into two regions (see Figure S1): ligand-field (d-d) transitions are observed for wavelengths longer than ~450 nm whereas only ligand-to-metal charge-transfer (LMCT) transitions can occur for shorter wavelength. A steep increase of the photochemical quantum yield of ferrioxalate at photon energies of 20000 cm‒1 was observed,1 which can clearly be correlated with the onset of the intense charge-transfer excitations. The first flash photolysis experiments6 were carried out with a time resolution in the microsecond regime and relied exclusively on a near-UV-to-visible (UV/Vis) detection. These studies were targeted at exploring the UV-induced photochemical kinetics of ferrioxalate in H2O solution and were able to detect a long-lived intermediate, which absorbed around 400 nm and which was found to decay according to first-order kinetics. Such a result provided some evidence that the rate controlling step cannot be the expected bimolecular reactions (3b and 3c) of the ferrioxalate ●‒ ●‒ with either of the two carbon-containing open-shell fragments, C2O4 or CO2 . Instead, Hatchard and Parker suggested that this intermediate is generated directly from the optically III 3‒ prepared excited state, *[Fe (C2O4)3] , and that is features some ligand-centered radical character, e.g. a ferrous oxalate to which an oxalate radical anion is still attached (cf. Figure S2, structure b): III 3‒ II ● 3‒ II 2‒ ●‒ *[Fe (ox)3] [(ox)2Fe (OCOCO2 )] [Fe (ox)2] + ox , (5) The sequence (5) is then followed by the equilibration (4). In such a mechanism, the formation of ●‒ ●‒ the free radicals, C2O4 and CO2 , occurs after a metal-oxygen bond cleavage and requires a prior electron transfer from the oxalate ligand to the iron center. Figure S2. Lewis structures. Ferric parent complex (a); a penta-coordinated ferrous intermediate bearing the oxalate radical anion as a monodentate ligand (b); a hexa-coordinated ferric intermediate bearing two carbon dioxide radicals as monodentate ligands (c); a ferric dioxalate (d); a ferric trioxalate with one of the three oxalate ligands bound in a monodentate fashion (e), and the ferrous dioxalate with a O- coordinated and bent carbon dioxide radical anion as presented in this paper (f). Perone and coworkers7-8 conducted a spectro-electrochemical study on flash-photolyzed ferrioxalate solutions and proposed that the same intermediate corresponds to a species, which arises from a homolytic C‒C bond rupture and which features a ligand-centered biradical character as schematically depicted by structure c in Figure S2: III 3‒ III ● 3‒ II 2‒ ●‒ *[Fe (ox)3] [(ox)2Fe (OC O)2] [Fe (ox)2] + ox . (6) In this alternative mechanism, the carbonaceous free radicals emerge in parallel to the dissociation of the metal-oxygen bond and their formation does not require a prior full ligand-to- metal one-electron transfer, i.e. quite in contrast to the original Hatchard-Parker mechanism. Later, a re-examination of the flash-induced kinetics in the near-UV/Vis was carried out by Cooper and DeGraff.9-10 The authors concluded that the first process of sequence (5) is practically indistinguishable from the three-body ligand dissociation III 3‒ III ‒ ●‒ II 2‒ ●‒ *[Fe (ox)3] [Fe (ox)2] + 2 CO2 [Fe (ox)2] + CO2 + CO2 (7) during which the oxidation state +III at the metal is fully preserved. In contrast to reactions (5) and (6), the reduction of the metal takes place here in a secondary bimolecular electron-transfer ●‒ from a CO2 primary fragment to the ferric center dressed with only two oxalate ligands (cf. structure d, Figure S2). Thus, the photochemical mechanisms of aqueous ferrioxalate can broadly be classified into “prompt” (sequence 5) and “delayed” (sequences 6 and 7) electron-transfer mechanisms. Near-UV/Vis flash-photolysis experiments with a much better time resolution of a few tens of nanoseconds have appeared in the literature only recently11-13 and their results seem to favor the prompt sequence (5). Yet, a direct pathway, which avoids the formation of an intermediate ●¯ through the direct coupling of the photoreduction with the C2O4 dissociation, also needs to be invoked to quantitatively reproduce the experimentally determined optical transients. At this stage, it becomes quite obvious that experiments with an even better time resolution, preferably on the ultimate femtosecond scale, are required to differentiate unmistakably between “prompt” and “delayed” electron transfer mechanisms. It is quite remarkable however, that published reports on such femtosecond spectroscopic experiments on ferrioxalate are very scarce. To date, the only existing literature, which investigates the ultrafast optical spectroscopy of ferrioxalate, is limited to probing in the UV/Vis.p14-17 (see also Ref. 18 for a summary of ultrafast UV/Vis pump-probe studies of ferric dicarboxylates containing citrate, tartarate, and lactate ligands). Interestingly, Rentzepis et al. used such femtosecond optical data only to complement ultrafast extended x-ray absorption fine structure (EXAFS) spectroscopy and to facilitate a data analysis of their x-ray data. Iron-K post-edge EXAFS between 7 keV and 8 keV is known to deliver very valuable structural information such as bond lengths and coordination numbers pertaining to the coordination sphere surrounding the metal. In its electronic ground state, the ferrioxalate parent complex features a high-spin electronic configuration (FeIII, d5) with S = 5/2 (sextet) and a molecular structure, in which all Fe‒O bond lengths are equal to 2.00 Å. During the first 2 ps after UV absorption, the EXAFS data are indicative of an increase of the Fe‒O bond distances to 2.16 Å. This structural III 3‒ expansion was attributed to the excited state, *[Fe (C2O4)3] ; in which the metal’s oxidation state is preserved. Most importantly, this period of bond length increase is purely non-reactive, i.e. it is not accompanied by an electron transfer or a dissociation. Subsequently, the Fe‒O distances decrease to 1.93 Å at 4 ps and 1.87 Å at 9 ps. Aided by electronic structure calculations, the authors assigned this phase of bond-length contraction to a heterolytic cleavage of a Fe‒O bond and not (!) to a homolytic cleavage as in expressed sequence 5) followed by the full ligand fragmentation. Instead, the ferric nature of the parent is conserved throughout the entire sequence III 3‒ III 3‒ III ‒ ●‒ *[Fe (ox)3] [(ox)2Fe (OCOCO2)] [Fe (ox)2] + 2CO2 . (8) The evolution from five-fold to four-fold coordination of the iron(III) center (i.e. from structure e to structure d in Figure S2) takes place within less than 10 ps, whereas the necessary reduction of the metal is purely bimolecular in nature and as such occurs only on the microsecond scale. These results initiated additional time-resolved studies at the x-ray free electron laser facility, SACLA (XFEL), at RIKEN using advanced core-level spectroscopy.19 Specifically, utilizing the total fluorescence yield method, the precise spectral position of the iron-K edge was tracked as a function of time after UV-excitation.
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