Metal–organic framework composites with luminescent gold(iii) complexes. Strongly emissive and long-lived excited states in open air and photo-catalysis

Encapsulation of luminescent gold(iii) complexes by metal–organic frameworks results in enhanced phosphorescence in open air, solid state two-photon-induced phosphorescence, and reusable photo-catalysts under aerobic conditions.


Introduction
Metal-organic frameworks (MOFs) have received burgeoning interest over the past decades, with applications in gas storage/ separation, proton/electrical conductivity, biomedicines, sensing, and catalysis. 1,2 The tuning of both the metal nodes and the organic linkers in MOFs, by either pre-or postsynthetic 1,3 methods involving covalent/dative bond formation, leads to extraordinary versatility in framework structure, porosity, and function. Encapsulation of a functional cargo into the pores of MOFs via non-covalent and/or dative interactions, 4 such as the use of MOFs to encapsulate metal catalysts or luminescent metal ions/complexes by the cation exchange method, 4a is an alternative approach to MOF modication. Pores with dened environments inside MOFs constitute a unique platform to conne guest species, and as a result, new functions/performance of the encapsulated guest species may emerge.
We are attracted to using MOFs for housing metal complexes with long-lived emissive excited states for heterogeneous photochemical catalysis and luminescence applications. In the literature, other porous materials besides MOFs, such as mesoporous silica, have also been employed to encapsulate photoluminescent and/or photochemically active compounds, 5,6 including connement of a photoluminescent trinuclear gold(I) pyrazolate complex in the channels of mesoporous silica. 6b Our attention has been directed towards recently reported luminescent gold(III) complexes which display strongly emissive excited states with long lifetimes of up to 506 ms (emission quantum yields: up to 11.4%) in degassed solutions at room temperature. 7 These complexes may have useful applications in photo-catalysis and emission sensing, particularly if they are located in an aerobic environment in which their photo-physical characteristics can be retained, and if the photochemical reactions they catalyse can be endowed with specicity. Here we report our ndings on the encapsulation of strongly luminescent gold(III) complexes with various dimensions (Au1-Au4, 7,8 Scheme 1; Table S1 in the ESI †) by two MOFs with differing porous structures. The encapsulation was performed using a cation exchange method. 4a,9 The Au III -MOF composites (Au III @MOFs) display long emission lifetimes of up to 48.8 ms at room temperature and in open air, exhibit solid state twophoton-induced phosphorescence, and can act as selective and recyclable catalysts for light-induced electron transfer reactions and aerobic C-N/C-C/C-O bond forming reactions. Of note, Corma and co-workers 10 have described a recyclable Au IIIfunctionalized MOF catalyst for domino coupling/cyclization and alkene hydrogenation, prepared by post-synthetic dative bond formation.

Syntheses and structures of MOF1 and MOF2
Two MOFs, MOF1 and MOF2, built from similar ligands but possessing different types of pores with suitable window sizes, were chosen in this work to allow investigation of the effect of porous structure on the luminescence of Au(III) complexes.
Syntheses, characterization and photo-physical properties of Au III @MOF composites Au III @MOFs, including Au III @MOF1 (Au III ¼ Au1-Au4) and Au III @MOF2 (Au III ¼ Au1, Au2), were obtained as yellow or pale yellow solids (Fig. S4 Table S2, ESI †), are likely to reside in the inner pores of the MOF host materials based on the following: (i) the MOF hosts of these Au III @MOFs contain nanochannels/nanocages with window sizes larger than the sizes of the incorporated Au III Scheme 1 Au III complexes Au1-Au4 used in this work. complexes (Table S1, ESI †); (ii) complex Au3, which is larger than the window sizes of the nano-cages in MOF2, did not give Au3@MOF2 (under similar conditions the MOF2 in the reaction mixture remained colourless, and ICP analysis revealed no Au3 incorporation into MOF2), suggesting that adsorption of the Au III complex outside the pores (or in gaps/cracks between crystal grains) of the MOF is negligible; (iii) the PXRD data of the Au III @MOFs were nearly identical to those of the corresponding MOF matrices (Fig. S5, ESI †); (iv) examination of the cross section of a crystal of Au2@MOF2 (1.43 wt% Au2) by optical microscopy under UV light irradiation at 365 nm showed the characteristic yellowish green emission of Au2 (ref . 7), congruent with incorporation of Au2 inside the crystal (Fig. S8, ESI †); (v) further analysis of the cross section of Au2@MOF2 by scanning electron microscopy (SEM) and electron dispersive X-ray spectroscopy (EDX) revealed that Au2 was disorderedly dispersed at the cross section ( Fig. S9, ESI †) and that no F was detected in Au2@MOF2 by EDX, even when the content of incorporated Au2 increased from 1.43 to 8.26 wt% (Fig. S10, ESI †); (vi) N 2 adsorption experiments ( Fig. S11, ESI †) showed that, from free MOF2 to Au2@MOF2 (1.43 wt% Au2), the pore volume decreased from 0.303 to 0.276 cm 3 g À1 , with a decrease in BET surface area from 1112 to 1011 m 2 g À1 . Increasing the Au2 content of Au2@MOF2 to 8.26 wt% led to a reduction in the pore volume and BET surface area to 0.183 cm 3 g À1 and 656 m 2 g À1 , respectively. The decreases in the pore volume and BET surface area are attributed to the encapsulation of Au2 in the pores of MOF2. Au III @MOFs display strong emission under air at room temperature. As depicted in their emission spectra (Fig. 2), two emission bands are observed. The one at $450 nm corresponds to the MOF host. The other is a vibronic structured emission with peak l max at 478-550, 520-610, 540-630 and 510-580 nm for Au III ¼ Au1, Au2, Au3 and Au4, respectively. These bands are all similar to those observed in the emission spectra of the corresponding Au III complexes in degassed solution. 7,8 The measured emission lifetimes (s 0 ), quantum yields (f em ), and estimated radiative (k r ) and non-radiative (k nr ) decay rate constants of Au III @MOFs and Au1-Au4 under different conditions are compiled in Table 1. Encapsulation of Au1-Au4 by MOF1 or MOF2 was observed to cause signicant increases in the intensity and lifetime of the emission of these Au III complexes ( Fig. 2 and Table 1; Fig. S6 and S7, ESI †). As an example, at room temperature, Au2 is nonemissive in the solid state, and the intensity of its emission at 520 nm in degassed dichloromethane solution decreased more than 100-fold upon exposure of the solution to air. 7 Remarkably, Au2@MOF2 is luminescent under aerobic conditions, 16 and the quantum yield decreased only $5-fold compared with that measured under argon (Table 1). For Au2@MOF1 and Au4@MOF1, the decrease in emission quantum yield on changing the atmosphere from argon to air is $10and $20-fold, respectively. Moreover, while Au2 is virtually nonemissive in DMF solution in air, Au2@MOF2 (1.43 wt% Au2), Au2@MOF2 (8.26 wt% Au2), and Au2@MOF1 are luminescent in air, with emission lifetimes of 14.5, 15.8, and 7.9 ms, respectively. Notably, the emission lifetime of Au1@MOF2 in air is 19.4 ms, which is >60-fold longer than that of Au1 in the solid state (0.20 ms) and in DMF solution (0.32 ms) under aerobic conditions (Table 1). 17 The emission lifetime of Au3@MOF1 in open air is 48.8 ms, the longest emission lifetime reported for luminescent Au III complexes under aerobic conditions. Compared with Au III @MOF2, the Au III @MOF1 composites are more susceptible to luminescence quenching under aerobic conditions ( Fig. 2 and Table 1), due probably to the larger window size of the pores in MOF1 than in MOF2. The intrinsic triplet excited state properties and molecular sizes of the Au III complexes could also affect the luminescence quenching behaviour of Au III @MOFs upon exposure to air. The elevated intensities and lifetimes of the emissions of Au III @MOFs in the solid state under aerobic conditions are attributed to the following: (i) the emission lifetimes of Au III @MOFs under argon are longer, and their non-radiative decay rate constants (k nr ) are smaller, than those of the corresponding Au III complexes in degassed solution (Table 1). Electrostatic binding of the Au III complexes on the surfaces of the inner pores of the MOFs would restrict the molecular motion of the Au III complexes, thereby slowing down non-radiative decay and self-quenching (by diffusion) of the emissive excited state. 18 The radiative decay rate constants (k r ) of Au III @MOFs and Au1-Au4 are in the order of 10 2 to 10 3 s À1 (Table 1), typical of 3 IL excited states of luminescent gold(III) complexes. 8a (ii) The steady state concentration of oxygen in the pores of Au III @MOFs may be lower than that of the oxygen dissolved in the solvent and in free atmosphere. This may reduce oxygen quenching of the emission of Au III @MOFs in air. 19 Given that the activity of O 2 in the matrix would equal that in the external environment if the matrix and the ambient environment are at equilibrium, and considering the use of O 2 as a terminal oxidant in photo-catalytic reactions within the matrix (see the section below on the photo-catalytic properties of the Au III @MOF composites) as well as the partial luminescence quenching under aerobic conditions, O 2 diffusion in the matrix of Au III @MOFs is likely to occur at a nite rate, with the mass transport of O 2 through the matrix being sufficiently slow that the above-mentioned equilibrium did not fully occur.

Light-induced electron transfer reactivity of Au2@MOF2
Au III @MOFs were observed to display light-induced electron transfer reactivity. As an example, Au2@MOF2 (1.43 wt% Au2) was rst immersed into a solution of methyl viologen dication (MV 2+ ) in DMF at room temperature for 12 h to allow encapsulation of MV 2+ inside the pores of the MOF composite, followed by removal of the solution, washing with DMF and drying. Upon xenon lamp (l > 370 nm) irradiation of a mixture of the resulting Au III @MOF-MV 2+ and a drop of Et 3 N liquid in air, the characteristic blue colour of the MV + c radical appeared within a few seconds (the colour intensity increased with irradiation time), as depicted in Fig. 3. Heating the mixture (aer removing the xenon lamp) to remove Et 3 N was observed to revert MV + c back to MV 2+ , and the photolysis was recycled three times. For comparison, irradiation of a mixture of Au2, MV 2+ and Et 3 N in MeCN under the same aerobic conditions did not give a similar net change in colour of the solution mixture. In fact, a recently reported photochemical reaction of a Au III complex with MV 2+ was conducted under degassed conditions. 8a We measured the N 2 adsorption of Au2@MOF2-MV 2+ without sample pretreatment, and found that very little N 2 could be adsorbed by the sample ($3.7 cm 3 g À1 ). The corresponding BET surface area is 6.3 m 2 g À1 (Fig. S12, ESI †), which is quite small compared with the BET surface area of the pretreated Au2@MOF2 (1011 m 2 g À1 ). Thus, for Au2@MOF2, the observed photochemical reaction with MV 2+ and Et 3 N in air can be attributed to the occurrence of the reaction inside the pores of the MOF composite, which could be occupied by solvent and Et 3 N molecules, thus decreasing the quenching of MV + c by O 2 .

Two-photon absorption of Au3@MOF1 in the solid state
Two-photon absorption has useful applications in chemistry and biological science; 20,21 the two-photon-excited emission of a phosphorescent Au III complex in solution has recently been reported. 8a However, there are few reported examples of this property in the solid state, owing to aggregation-induced quenching. 21 In the present study, we observed that Au III @ MOFs display two-photon-induced phosphorescence 22 in the  solid state. For Au3@MOF1, excitation at 756 nm with a focused laser beam gave bright yellow emission (Fig. 4a) with peak maxima at 545 and 585 nm (characteristic of the emission of Au3) and an intensity showing a quadratic dependence (y ¼ x 2 ) on the laser power density (Fig. 4b and c).

Photo-catalytic properties of Au III @MOF composites
Cyclometalated Au III complexes have recently been reported to sensitize the formation of 1 O 2 upon light excitation for the oxidation of secondary amines to imines and for oxidative cyanation of tertiary amines. 7,8a,23 Using the former as a paradigm, we examined the photo-catalytic activities of Au III @ MOFs. The photochemical reactions were performed in MeCN. The solution mixture was under light irradiation (l > 400 nm) with constant bubbling of oxygen. Control experiments showed that both MOF1 and MOF2 are stable in MeCN for at least 24 h, as revealed by PXRD measurements (see Fig. S13 of ESI †). The following was observed: (i) for the photo-reaction with dibenzylamine (Fig. 5), Au1@MOF1 gave the imine product with a turnover number of 692, exceeding the turnover number of 390 obtained with free Au1. The photo-activity of Au1@MOF1 showed little variation over a period of 10 h, whereas free Au1 exhibited a signicant decrease in photo-activity aer 2 h, with almost no imine product obtained aer 6 h (Fig. 5b). (ii) The Au1@MOF1 catalyst could be recycled by washing with MeCN. Aer ve cycles, the substrate conversion retained a value of $70% (Fig. 5c). (iii) The photo-catalytic activity is attributed to Au1 encapsulated by MOF1 rather than Au1 leached into solution. This is conrmed by the nding that the solution phase of the Au1@MOF1-catalysed reaction mixture remained colourless during the course of photolysis. This is in contrast to the reaction catalysed by free Au1, in which there was a colour change attributed to the decomposition of the Au1 catalyst aer 6 h of photolysis. ICP measurements revealed nearly the same content of gold in the Au1@MOF1 sample before and aer photocatalysis; control experiments with MOF1 (free of Au) as the catalyst gave few product turnovers (Fig. 5b). (iv) Substrate size selectivity 1b,2d was observed for the Au1@MOF1-catalysed competitive photochemical oxidation of dibenzylamine (S1) in the presence of another secondary amine of larger size (S2, Scheme 2). The photochemical reaction consumed S1 rapidly without signicant consumption of S2, and the imine products P1 and P2 were formed in a yield ratio of $11 : 1 for a reaction time of 1.5 h. Under the same conditions, catalyst Au1 showed no selectivity in solution, with a P1/P2 yield ratio of $1 : 1 (Scheme 2). The drastic difference between the yields of P1 and P2 in the Au1@MOF1-catalysed reaction can be attributed to the reaction mainly occurring inside the channels of the MOF framework. The smaller substrate, S1, could enter the channels of MOF1, but the larger one, S2, could not; the formation of a small amount of P2 in the reaction mixture is attributed to the external diffusion of singlet oxygen (produced inside the MOF) and its subsequent reaction with S2. Au2@MOF1 and Au3@MOF1 were also found to catalyse the light-induced aerobic oxidation of dibenzylamine ( Fig. S14 and S15, ESI †). The total turnover numbers of the imine product furnished by Au2@MOF1 (0.91 wt% Au2) and Au3@MOF1 (2.34 wt% Au3) were 557 and 920, respectively, and both were higher than the values of 510 and 760 obtained with catalysts Au2 and Au3, respectively. The Au2@MOF2 (1.43 wt% Au2) catalyst gave the imine product with turnover number of 610; changing the catalyst to Au2@MOF2 (8.26 wt% Au2) led to a $2-fold increase in conversion rate, albeit with a decrease in product turnover  Scheme 2 Competitive photo-catalytic oxidation of secondary amines.
number (calculated based on % wt of Au2 in Au2@MOF2; Fig. S16, ESI †). The photochemical oxidation of dibenzylamine was also catalysed by Au4 and Au4@MOF1 (Au4: 6 Â 10 À7 mol in both cases). Au4@MOF1 was a more active catalyst with a turnover number of >143 attained within 2 h (Fig. 6a). Notably, the initial reaction rate of the Au4@MOF1 system was two-fold higher than that of Au4 alone. Aer recycling ve times, the photoactivity of the Au4@MOF1 catalyst maintained $80% of its initial value (Fig. 6b). Increasing the content of Au4 in solution from 6 Â 10 À7 to 6 Â 10 À6 mol decreased the product turnover number from 90 to 18 (Fig. 6a).
The improvement in photo-catalytic activity of Au III @MOFs relative to the free Au III complexes could be extended to the oxidative cyanation of a tertiary amine, a Mannich-type reaction, the aza-Henry reaction, the hydroxylation of 4-chlorophenylboronic acid, and the reductive cyclization of alkyl iodide. 24 These ndings are depicted in Scheme 3 and in Fig. S17 and Tables S3-5 of the ESI. † For example, the steady formation of product was observed over 8 h in the Au1@MOF1-catalysed oxidative cyanation of N-phenyl-1,2,3,4-tetrahydroisoquinoline, in contrast to a signicant decrease in the activity of the free Au1 catalyst aer 2.5 h of photolysis (Fig. S17, ESI †). The product yield obtained with the Au1@MOF1 catalyst was $81% (turnover number of 923). For the other photochemical reactions with Au2@MOF2 (1.43 wt% Au2) as catalyst, the products were obtained in 63-86% yields with turnover numbers of 137-549. These values were higher than the corresponding product yields and turnover numbers obtained with Au2 alone (Tables S3-S5, ESI †).
To gain further insight into the photochemical oxidation reactions catalysed by Au III @MOFs under aerobic conditions, which are likely to involve the formation of 1 O 2 , 7,23b we measured the reduction potentials, E(Au 0/À ), of Au1-Au4 (Table S6, ESI †) and estimated the excited state reduction potentials, E(Au*/Au À ), from the electrochemical and emission data. The estimated E(Au*/Au À ) values of Au1-Au4 range from +0.74 to +1.38 V vs. Cp 2 Fe +/0 (Table S6, ESI †), indicating that they are strong oxidants in the excited state. The potentials of dibenzylamine and N-phenyl-1,2,3,4-tetrahydroisoquinoline, E(amine +/0 ), were measured to be +0.50 V and +0.25 V vs. Cp 2 Fe +/0 , respectively. Thus, the thermodynamic driving force for the reaction of the excited Au1-Au4 with dibenzylamine was estimated to be in the range of +0.24 to +0.88 V, whereas that of the excited Au2 complex with N-phenyl-1,2,3,4-tetrahydroisoquinoline is +0.49 V. Nanosecond time-resolved transient absorption (TA) measurements (Table S7, ESI †) revealed that, in the presence of dibenzylamine (0.1 M) in degassed DMF, the TA of Au2 and Au3 was quickly quenched (s TA : 1.4 ms for Au2, 2.2 ms for Au3) and returned to the baseline, whereas the TA of Au1 and Au4 evolved to give long-lived species (s TA : 145 ms for Au1, 24 ms for Au4), attributable to the formation of Au1 À and Au4 À . 25 The TA spectrum of Au2 in the presence of N-phenyl-1,2,3,4-tetrahydroisoquinoline (0.01 M) shows the formation of a long-lived species (s ¼ 65 ms) assignable to Au2 À (Table S8, ESI †). Thus, in the photochemical oxidation of dibenzylamine catalysed by Au1 and Au4, and that of N-phenyl-1,2,3,4-tetrahydroisoquinoline catalysed by Au2, the excited gold(III) complexes may undergo electron transfer with the amines (to give amine radical cations) besides producing singlet oxygen for oxidation.

Conclusions
A series of luminescent Au III -encapsulated MOF composites (Au III @MOFs) have been synthesized. These Au III @MOF solids exhibit long emission lifetimes in air, display solid state twophoton-induced phosphorescence, and function as reusable and size-selective heterogeneous photo-catalysts. The simple approach of incorporating phosphorescent metal complexes with long-lived emissive excited states into MOFs provides a means of developing new classes of heterogeneous photofunctional materials/photo-catalysts with useful applications.