Supramolecular Photocatalyst for the Reduction of Au(III) to Au(I) and High-Turnover Generation of Gold Nanocrystals

We report a photocatalyst composed of a diphenylanthracene core appended with two lipophilic thioether side-chains that binds gold(III) chloride. Upon excitation using visible light, the AuIII ions are smoothly reduced to AuI which, in the presence of water, lead to the formation of crystalline gold nanoparticles of 20–50 nm diameter that are devoid of sulfur-containing capping agents. Ultrafast transient absorption spectroscopy shows that the anthracene excited state is quenched with a rate k = 3.5 × 1010 s–1, assigned to intramolecular energy transfer to the bound gold ions, which then oxidizes the solvent to produce an intermediate low-valency gold(I) species. In the absence of water, the latter is stable and can be used as a homogeneous AuI catalyst. When employed in a biphasic reactor, the photocatalyst shows average turnover numbers of 150 atoms of AuIII reduced to Au0 per molecule of photocatalyst.


INTRODUCTION
Supramolecular photocatalysts combine a molecular recognition motif with a photosensitizer designed to absorb light and promote a chemical transformation using a photoinduced electron or energy transfer process. 1,2 Several such systems have been explored toward cycloaddition reactions, isomer ization, and free radical transformations. 3−5 In some cases, it has been possible to achieve turnovers greater than unity and mimic enzymatic behavior by combining hydrogen bonding interac tions with photosensitizers for the oxidation of alcohols 6 or for catalyzing intramolecular cyclization. 7−10 The design of supra molecular photocatalysts has so far focused on the trans formation of organic molecules as they can readily combine a reaction center and a molecular recognition motif within a single species. In contrast, the design of supramolecular photocatalysts for the generation of inorganic species remains undocumented despite the potential of using light for smoothly preparing metal based catalytic intermediates or metal frameworks 11 endowed with novel properties.
The photoinduced reduction of AuCl 3 by triplet benzophe none was recently investigated in detail by the group of Scaiano and shown to proceed via a diffusion limited photoinduced electron transfer to generate Au nanoparticles (AuNP) and benzophenone oxidation products. 12 We reasoned that a photosensitizer might be able to drive the photoreduction of a metal ion such as Au III either by pumping electrons from the solvent to the gold ions, or by exciting the gold ions via an energy transfer mechanism. Such a system would be very useful for providing a photochemical route toward low valent Au species using long wavelength light that is not absorbed by common small organic molecules, as well as providing an example of the use of an organic photocatalyst for the generation of an inorganic material. Based on this, we designed a supramolecular photo catalyst containing a diphenylanthracene chromophore ap pended with two metal binding thioether side chains as such systems can promote strong interactions between the photo active moiety and the metal ions. 13 The thioether side chains are very lipophilic and render the gold−photocatalyst assembly soluble in organic solvents, thus allowing its selective photo excitation while preventing background irradiation of nonbound Au III that remains in the aqueous phase. We show that the system can be readily steered toward either the production of a homogeneous Au I catalyst that can be directly used in organic synthesis, or toward the formation of uncapped gold nanocryst als. The latter can be produced in large quantities through the use of a recirculating reactor in which the catalyst reaches up to 170 catalytic cycles. Such turnover numbers are unprecedented for a supramolecular photocatalyst and demonstrate that photo catalytic generation of an inorganic material by an organic catalyst can be highly efficient. The reaction mechanism was elucidated by a combination of ultrafast absorption spectroscopy, product analysis, and dynamic 1 H NMR spectroscopy. 1

EXPERIMENTAL SECTION
2.1. General Protocols. Reaction quantum yield measurements were measured at low conversion using monochromatic radiation from a medium pressure Xe−Hg lamp equipped with a monochromator. The samples (aerated) possessed absorbance ≥2 at the excitation wavelength and were stirred throughout the irradiation. Ferrioxalate actinometry was used to determine the photon flux (using Φ(Fe III → Fe II ) = 1.20 and 1.13 at 313 and 380 nm, respectively) 14 and the Au III concentration was determined spectroscopically. Fluorescence quantum yields were determined by comparison to a secondary standard (quinine sulfate in 2 N H 2 SO 4 , Φ F = 0.54). 14

Preparation of Gold(III) and Gold(I) Complexes.
Complex 1·2AuCl 3 was obtained by two methods that gave identical results: (i) dissolution of solid HAuCl 4 or KAuCl 4 into a solution of 1 (toluene or dichloromethane) or (ii) extraction of AuCl 3 from an aqueous solution of HAuCl 4 by a solution of 1 in toluene or dichloromethane. The binding of AuCl 3 by 1 was followed spectroscopically by monitoring the absorption of Au(III) at 330 nm, and the complexes were characterized using mass spectroscopy (FD + ). Complex 1·2AuCl was obtained by irradiating a solution of 1·2AuCl 3 in toluene (0.1−1 mM), at 20°C , using a TLC lamp (365 nm, 6W) or LED (400 nm, 10W), for 15−30 min.
2.2. Preparation of AuNP. A solution of 1·2AuCl 3 in dichloromethane or toluene (0.1 to 1 mM, 10 mL) was prepared in a centrifugation tube (50 mL). Water (10 mL) was added and the biphasic system was agitated on an orbital stirring table at 300 rpm while irradiated using a TLC lamp (365 nm, 6W) or a LED (400 nm, 10W) for 15 min. The reaction mixture was then centrifuged at 1500 rpm for 15 min and the aqueous phase, containing gold nanoparticles, was collected.
2.3. Photoreduction Reactor and Calculation of Turnover Number. To estimate the maximum number of turnover cycles that can be performed by 1, the recirculating reactor described in Figure 9 was loaded with 1 (initial concentration = 15 μM, Vol = 40 mL) and an excess of HAuCl 4 (initial concentration = 6 mM, Vol = 50 mL). The absorption spectrum of the aqueous phase was monitored at t = 0 and 15 h (end of conversion) by withdrawing an aliquot (100 μL) and diluting it in 2 mL H 2 O. Using the extinction coefficient of the HAuCl 4 solution at 290 nm, the concentration of Au III remaining in solution can be determined. The turnover number is calculated in terms of moles of consumed Au III ions per moles of 1 consumed.

Synthesis and Characterization of Au Complex.
The photocatalyst used in this study is easily prepared in three steps from commercially available reagents (72% overall yield). All spectroscopic analyses are consistent with the structures shown in Scheme 1. As expected for diphenylanthracene derivatives, 1 is highly fluorescent in nonpolar organic solvents (Φ F = 0.96, τ = 7.5 ns in toluene, λ ex = 365 nm), although photooxidation of the thioether groups occurs in solvents of moderate polarity via a known photoinduced electron transfer mechanism. 15,16 The thioethers in 1 are designed to bind two AuCl 3 and maintain them in the vicinity of the anthracene chromophore. With this in mind, flexible ethylene glycol subunits were introduced in the linker to alleviate unfavorable gauche interaction associated with folding of the chain. To examine compound 1's ability to bind AuCl 3 , a solution of 1 in dichloromethane or toluene was placed in contact with an aqueous solution of HAuCl 4 or with solid AuCl 3 . Upon gentle shaking, the yellow color of the Au III was transferred to the organic phase which became nonfluorescent. Complexation can be readily followed by UV−vis or emission spectroscopy ( Figure  1). In all cases, whether by titration of 1 by the Au III salt or by measuring the amount of Au III salt lost from the aqueous phase upon incremental addition of 1, the stoichiometry was found to be 1:2 (1: AuCl 3 ).
In agreement with the formation of a 1:2 complex, mass spectrometric analysis of the organic phase following extraction of AuCl 3 by 1 evidenced the presence of a molecular ion with m/z = 1630 Da corresponding to a species with the formula 1·2AuCl 3 (Figure 2A). Irradiation of anhydrous 1·2AuCl 3 solutions in toluene or dichloromethane (aerated or Ar saturated) at 365 or 400 nm, where the diphenylanthracene chromophores absorbs, results in the rapid reduction of the Au III into Au I , as evidenced by the discoloration of the solution and by MS analysis of the photolyzed solution which now only shows signals corresponding to the dissociation of 1·2AuCl 3 ( Figure 2B).
The solid state structure of the 1·2AuCl 3 complex is not currently known, as it is a waxy solid that is not amenable to X ray diffraction analysis. However, binding of AuCl 3 is likely to occur at the sulfur positions in 1 as generally observed for thioethers (Scheme 1). 17 Diffusion ordered NMR spectroscopy (DOSY) 18 was used to obtain information regarding the diffusion constant of 1 and the 1·2AuCl 3 complex in dichloromethane ( Figure 3). The latter gives a narrow distribution of diffusional rates centered around the value of 5.44 × 10 −10 m 2 /s, compared to a diffusion constant of 1.75 × 10 −9 m 2 /s for 1 under the same conditions. In the case of the 1·2AuCl 3 complex, the observed diffusion constant corresponds to a calculated Stokes radius of R S = 8.9 Å (V S = 2950 Å 3 ), which is in good agreement with the solvent excluded volume calculated for a compact structure using a PM5 semiempirical model (3250 Å 3 ). The observation that the 1 H NMR spectrum of the 1·2AuCl 3 complex is sharp, and that its diffusion coefficient is only reduced by one half with respect to 1 strongly argues against the formation of coordination polymer networks, which would be expected to exhibit broadened NMR signals and considerably reduced diffusional constants.
The formation of coordination polymers or aggregates would be expected to greatly influence the reduction of Au species by accelerating the formation of Au clusters and seeding. 19 To further verify that binding to AuCl 3 did not induce aggregation of the complex in solution, experiments using high field (800 MHz) and variable temperature 1 H NMR were undertaken. At room temperature, the 1 H NMR spectrum of the 1·2AuCl 3 complex in CD 2 Cl 2 is sharp, with the exception of the aromatic protons of the diphenylanthracene core which are somewhat broadened (see Figure 4A and B). No broadening of the dodecyl or ethylene glycol chains is observed, suggesting that the complex is not aggregated in solution and that the aromatic core experiences slow interconversion between different environments. Hindered rotation of the diphenylanthracene moiety in 1 would be expected upon complexation of Au III if the complex adopts a compact conformation as suggested by the results from DOSY. In agreement with this, we find that these signals sharpen upon lowering the temperature to 203 K ( Figure 4C). From the coalescence temperature of the aromatic protons at 7.4 ppm, an activation barrier for equilibration of ca. 60 kJ/mol is calculated. This behavior is contrary to that expected for aggregation, which is instead favored at low temperatures and generally induces increased broadening of NMR signals.
3.2. Photoreduction of Au III and Catalytic Behavior of Au I Complex. As mentioned above, irradiation of the 1·2AuCl 3 complex in toluene or dichloromethane (λ ≥ 365 nm) in the absence of water rapidly leads to the formation of AuCl species as expected for the photoreduction of the AuCl 3 salt. The efficiency of the reduction process can be monitored from the disappearance of the absorption band associated with Au III bound to 1 upon selective irradiation of the anthracene chromophore. Irradiation at 380 nm of aerated toluene or dichloromethane solutions of 1·2AuCl 3 led to the rapid disappearance of the absorption band of Au III with quantum yields of 0.07 and 0.06, respectively. Because Au II salts possess similar electronic transitions as Au III salts, we believe that these values correspond to the formation of Au I species,  which are transparent in the visible and near UV region of the spectrum. 20 In this case, the observed quantum yields represent the overall reduction process of Au III to Au I , which would be two consecutive reduction steps. Thus, assuming that each of the two photoreduction steps leading to Au I from Au III occurs with similar efficiency, the overall quantum efficiency of each photoreduction process would be 0.26. This indicates that the sequential energy transfer/solvent oxidation process is efficient and comparable to that observed by reductive photoinduced electron transfer to Au III from an excited donor. 12 Energy wasting processes, possibly through spin exchange quenching of the anthracene excited state and vibronic relaxation, therefore amount to a quantum yield of 0.74.
The photolyzed solution can be directly used to catalyze organic reactions promoted by Au I . To demonstrate this, we investigated the intramolecular ring closure of t butyl prop argylamide using photolyzed and nonphotolyzed solutions of 1· 2AuCl 3 . It is known that the reaction is catalyzed by Au III species to form the corresponding oxazole, whereas the use of an Au I catalyst leads to the formation of the methylene dihydroox azole. 21 As expected from the proposed photoreduction of Au III to Au I , the use of the irradiated 1·2AuCl 3 solution led to the rapid and exclusive formation of the dihydrooxazole, whereas use of nonirradiated 1·2AuCl 3 complex led to formation of the oxazole (Scheme 2). Thus, the supramolecular complex behaves as a light sensitive catalyst in which it is possible to control the nature of the catalytic species (Au I or Au III ) simply by irradiation with visible light.
3.3. Photogeneration of Au Nanocrystals. In the absence of water, the 1·2AuCl complex is relatively stable and undergoes slow dismutation over several days to form Au 0 and AuCl 3 . In contrast, the presence of water promotes the formation of gold nanoparticles (AuNP), which is complete in 45−60 min in water/toluene or occurs during irradiation if water/ dichloromethane is used as a reaction medium. The appearance of a ruby red coloration of the aqueous phase (λ max = 535 nm) observed is typical of the absorption from the plasmon resonance of AuNP that are ca. 30 nm in size. 22,23 Transmission electron microscopy (TEM) images of the particles suspended in the aqueous phase are shown in Figure 5.
They unambiguously identify the nanoparticles as crystalline AuNP of dimensions that are between 20 and 50 nm. Further inspection shows that they are formed by the agglomeration along crystal growth axes of smaller (ca. 5 nm) AuNP that are also crystalline.
The process leading to the formation of AuNP is broadly similar to that previously observed for the photoreduction of aqueous solutions of Au complexes. 24−26 However, we observe that in this case the Au 0 atoms formed undergo nucleation and eventually migrate to the aqueous phase due to the hydrophilic nature of pristine gold, 27 thereby releasing the photocatalyst which remains in the organic phase. In support of this, X ray photoelectron spectroscopy (XPS) analysis of the AuNP reveals the presence of chlorine atoms along with carbon and oxygen, whereas the amount of sulfur atoms is below the detection limit ( Figures 6). This result shows that 1 is not extracted into the aqueous phase along with the nanoparticles, which are instead stabilized by chloride anions and traces of organic molecules. The XPS analysis also revealed the presence of cationic Au species, which may also contribute to the stabilization  of the AuNP and provide further opportunities for applications in catalysis. 28 3.4. Mechanistic Investigation. Several pathways have been reported for the photoreduction of Au III salts into AuNP, the most common of which being the use of pulse radiolysis or photoinduced fragmentation of a suitable precursor to generate a chemical reductant. 29,30 Direct irradiation of Au III in the presence of ethylene glycol and a capping agent using broad wavelength UV light (250−400 nm) yields small (7 nm diameter) spherical AuNP through a mechanism involving glycol oxidation. 31 Uncapped AuNP can be obtained by reduction of HAuCl 4 by photogenerated ketyl radicals in water, 24 direct excitation of AuCl 4 − in the presence of a reductant, 25 or via an electron transfer process from excited benzophenone. 12 However, all of these mechanisms result in the loss of the absorbing chromophore, which therefore does not exhibit catalytic activity. In contrast, we find that when toluene is used as the organic phase, there is no degradation of 1 and that the toluene solution of 1 can be isolated and reused to prepare another batch of AuNP. This cycle was repeated five times with no apparent loss of efficiency, confirming that 1 acts as a photocatalyst in contrast to previously reported photoreductants.
The mechanism by which AuCl 3 is reduced to Au I or to Au 0 involves initial excitation of the diphenyanthracene chromo phore as this absorbs the majority of the incident radiation at λ irr ≥ 365 nm. Oxidative electron transfer quenching of the diphenylanthracene excited state by AuCl 3 would be expected to proceed through a single electron transfer to generate Au II Cl 2 and is calculated to be thermodynamically favorable by Δ ET G 0 = −0.5 eV using eq 1, 32 where N A is the Avrogado constant, e is the elementary charge, ΔE 00 is the energy level of the singlet excited state of 1 (3.08 eV), and E 0 (D +• /D) and E 0 (A −• /A) refer to the oxidation and reduction potential of 1 (+1.46 V vs V NHE ) and Au III Cl 3 , respectively. The reduction potential of the Au III /Au I couple was bracketed between +0.35 and +0.71 V by Scaiano and co workers, 12 and a value of +0.6 V vs V NHE as estimated by Gachard et al. from pulse radiolysis experiments 33 was used in the calculation. The Coulombic term was calculated according to w(D +• A −• ) = 2e 2 /(4πε 0 ε r a), where ε 0 and ε r are the vacuum and relative electric permittivity, respectively, and a is the separation between D +• and A −• (0.7 nm). Photoinduced electron transfer in 1·2AuCl 3 would be expected to be fast as it is intramolecular and not limited by the diffusion of the reactants. Femto second time resolved transient absorption spectroscopy was therefore used to identify the initially formed excited state as well as the primary photoinduced processes upon light absorption. In the case of electron transfer quenching, the ensuing diphenylanthracene radical cation possesses a characteristic spectral signature at 500−600 nm that is easily identified. 34 Upon pulsed excitation (λ ex = 380 nm, 40 fs), the absorption spectrum of the S 1 state of the diphenylanthracene chromophore is clearly visible (λ max = 560 nm, Figure 7), thereby excluding direct population of the anthracene triplet state owing to strong spin−orbit coupling induced by the Au atoms. The anthracene S 1 signal decreases rapidly with a time constant of 27 ps (in either toluene or dichloromethane), from which a rate constant for intramolecular quenching of the singlet excited state of 1 by the bound gold ions k q = 3.7 × 10 10 s −1 can be determined. However, no signal corresponding to the absorption of the radical cation of 1 or of the diphenylanthracene T 1 state can be detected, and the repopulation of the ground state was found to occur on the same time scale as the depopulation of the S 1 excited state. 35 This implies that either the ensuing reaction of the diphenylan  thracene radical cation occurs with a rate ≥3.7 × 10 10 s −1 or, more likely, that the quenching mechanism occurs via an energy transfer pathway as this would directly repopulate S 0 without any intermediate states. This latter possibility is intriguing as one might expect energy transfer from the diphenylanthracene singlet excited state to be spin forbidden in view of the predominantly triplet character of the Au III excited state, and the process is likely facilitated by strong spin−orbit coupling or may proceed through a spin exchange mechanism. 36 Intramolecular energy transfer from the excited diphenylan thracene chromophore to Au III could occur via a spin exchange processor via dipole−dipole interactions (FRET mechanism). The former requires orbital overlap between the donor and acceptor which is not apparent from the 1 H NMR or electronic absorption spectrum of the complex. In the case of FRET, the distance separating the donor−acceptor pair can be calculated from the observed energy transfer rate and the overlap integral between the emission spectrum of the donor and the absorption spectrum of the acceptor using eq 2, 37 where τ D is the lifetime of the excited donor and R 0 is the Forster radius (calculated from the spectral overlap to be 2.3). From eq 2, an average Au− anthracene distance of 9 Å is calculated for quenching in the 1· 2AuCl 3 complex, which is consistent with an intramolecular energy transfer process. 38 Comparison of dichloromethane and toluene as an organic medium reveals important differences in behavior. When dichloromethane is used, MS and NMR analysis of the organic phase after formation of the AuNP show that 1 has decomposed, principally by cleavage of the chains in the benzylic position and α to the thioethers. In contrast, when toluene is used as an organic phase, 1 does not decompose and the presence of toluene byproducts (benzyl chloride, benzaldehyde, chloroto luene) is detected instead. These may be formed through a mechanism involving oxidation of toluene by photoexcited Au III or through the release of chloride radicals from the 1·2AuCl 3 complex analogously to the photoejection of Cl· from AuCl 4 − as described by McGillivray et al. 25 From the experimental observations, a reaction mechanism can be proposed for the photoreduction of Au III into Au I , which in the presence of water continues to the reduction of Au I into AuNP (Figure 8). Catalyst 1 binds two AuCl 3 molecules to form a 1·2AuCl 3 complex that is soluble in the organic phase. Upon excitation at 400 nm, the anthracene chromophore is excited preferentially and ultrafast energy transfer populates the Au III * excited state which is a strong photooxidant. 39,40 If the reaction is conducted in toluene, the excited Au III complex oxidizes the solvent to produce Au II ions and toluene oxidation byproducts. If toluene is replaced by dichloromethane, then the excited Au III complex oxidizes 1, eventually leading to its decomposition. It is commonly assumed that Au II species formed during the reduction of Au III ions undergo dismutation to form Au I and Au III . 29 This is indeed observed if the irradiation is stopped after 1−2 min, but the process is relatively slow as the concentrations involved are low (several hours). This is not entirely compatible with the rapid transformation of Au III that is observed upon continued irradiation and it is possible that a light induced process for the consumption of Au II is also operating in competition with dismutation of Au II . Because it is known that  ). Excitation of the diphenylanthracene chromophore is followed by ultrafast energy transfer to the AuCl 3 center (step ii), which oxidized the solvent (toluene) and is consequently reduced. The entire process occurs twice per bound Au III atom to produce Au I which then forms Au 0 in the presence of water. Alternatively, it is possible that the Au II produced undergoes dismutation to form Au III and Au I , and the process is repeated (not shown, see text). In the presence of water, nucleation of the Au 0 atoms leads to the formation of AuNP which migrate to the aqueous phase and release 1 into organic phase to continue the reduction cycle.
Au II species absorb more intensely and at longer wavelengths than the corresponding Au III complexes, 20 it is possible that energy transfer to the Au II complex from the diphenylanthracene S 1 state is as fast or faster than energy transfer to the Au III species. The excited Au II * ion would once again oxidize the solvent to generate Au I . The latter does not absorb in the visible range, 20 thereby stopping the energy transfer quenching of the S 1 (1) excited state.
3.5. Catalytic Turnover. In order to calculate the maximum turnover cycles of 1 in toluene, we constructed a recirculating reactor that allows the selective excitation of the organic phase while protecting the aqueous phase from light to avoid possible background photoreduction of excess Au III (Figure 9). The reactor is composed of an extractor in which the catalyst in the toluene organic phase is exposed to a large excess of HAuCl 4 contained in the aqueous phase. The supernatant organic solution is pumped through a glass tube that is exposed to two 10 W 400 nm LED sources and then circulated back into the aqueous subphase through a fritted glass membrane. The pumping rate was adjusted to allow a 10−15 min exposure to the irradiation, and it was immediately apparent that the solution leading into the photochemical reactor was nonfluorescent, whereas that exiting the photochemical reactor was highly fluorescent. The amount of Au III Cl 4 − remaining in the aqueous phase was analyzed by UV−vis absorption at various times. Upon operation, the aqueous phase became red and a reddish precipitate formed on the fritted glass as expected for the formation of AuNP. The experiment was repeated 5 times, yielding an average turnover number (TON) of Au III atoms reduced per molecule of 1 consumed of TON = 146 (σ = 20, TON max = 174, TON min = 120, see Table 1). Continued irradiation after all 1 had been consumed did not lead to further reduction of Au III . It is likely that optimization of the experimental conditions could further increase the turnover of the catalyst, but it should be noted that this figure is already significant in comparison with previous reports of supra molecular photocatalysts.

CONCLUSIONS
Photoinduced processes are inherently quantized and, unlike chemical reduction of Au III , photoreduction using 1 as a photocatalyst allows the straightforward preparation of low valent Au I salts that can be of interest for applications in catalysis of organic reactions. We have shown that bound Au III is efficiently and selectively reduced to Au I using long wavelength irradiation (λ ≥ 380 nm) and that the resulting Au I complex is catalytically active toward a prototypical organic transformation. In this respect, the system is very different from those previously reported that make use of a photogenerated reductant and which invariably lead to the production of AuNP. By avoiding the presence of water, it is possible to steer the photoreduction toward the exclusive generation of Au I species. In this case, the supramolecular complex becomes a light sensitive catalyst that makes it possible to control the nature of the catalytic species (Au I or Au III ) by irradiation with visible light. The use of energy transfer from an organic sensitizer allows longer wavelength irradiation to be used, which is desirable to avoid direct excitation of the substrates or products that are present in the reaction mixture.
The synthesis of uncapped crystalline AuNP is interesting for their development for applications in medicine, 41 plasmonics, and catalysis. 42,43 It has been noted that the presence of cationic Au species, 28 or that of organic stabilizers, 44 can profoundly influence their catalytic activity. Although several routes for the photochemical preparation of AuNP are already known, 12,24,25,31,45−51 it remains remarkable that this can be efficiently accomplished using an organic photocatalyst and this is an important milestone in the development of supramolecular systems for materials synthesis. Furthermore, it has been recently shown that the presence of organic stabilizers influences the catalytic activities of AuNP and there is therefore interest in the development of facile approaches to the preparation of uncapped AuNP. We are currently exploring this approach toward other transition metal ions possessing low lying excited states by adapting the energy level of the photosensitizer and the nature of the chelating sites. Figure 9. Recirculating reactor constructed to determine the maximum turnover cycles attainable by 1. The biphasic reactor draws the toluene solution of the 1·2AuCl 3 complex from the supernatant, which is then sent to a photoreactor enclosure composed of glass tubing that is irradiated by two 400 nm 10 W LED sources. The irradiated solution is then pumped back into the reactor through a glass frit located in the aqueous subphase in which an excess of HAuCl 4 is present. The amount of Au III present in the aqueous phase is determined spectroscopically at various intervals.

Notes
The authors declare no competing financial interest.