Crystal-to-Crystal Synthesis of Photocatalytic Metal–Organic Frameworks for Visible-Light Reductive Coupling and Mechanistic Investigations

Article abstract of DOI:10.1002/cssc.202000465

Postmodification of reticular materials with well-defined catalysts is an appealing approach to produce new catalytic functional materials with improved stability and recyclability, but also to study catalysis in confined spaces. A promising strategy to this end is the postfunctionalization of crystalline and robust metal–organic frameworks (MOFs) to exploit the potential of crystal-to-crystal transformations for further characterization of the catalysts. In this regard, two new photocatalytic materials, MOF-520-PC1 and MOF-520-PC2, are straightforwardly obtained by the postfunctionalization of MOF-520 with perylene-3-carboxylic acid (PC1) and perylene-3-butyric acid (PC2). The single crystal-to-crystal transformation yielded the X-ray diffraction structure of catalytic MOF-520-PC2. The well-defined disposition of the perylenes inside the MOF served as suitable model systems to gain insights into the photophysical properties and mechanism by combining steady-state, time-resolved, and transient absorption spectroscopy. The resulting materials are active organophotoredox catalysts in the reductive dimerization of aromatic aldehydes, benzophenones, and imines under mild reaction conditions. Moreover, MOF-520-PC2 can be applied for synthesizing gram-scale quantities of products in continuous-flow conditions under steady-state light irradiation. This work provides an alternative approach for the construction of well-defined, metal-free, MOF-based catalysts.

Full text of DOI:10.1002/cssc.202000465

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Crystal-to-Crystal synthesis of photocatalytic MOFs for visible-
light reductive coupling and mechanistic investigations
Authors: Julio Lloret-Fillol, Luis Gutiérrez, Suvendu Sekhar Mondal,
Alberto Bucci, Noufal Kandoth, Eduardo C. Escudero-Adán,
and Alexandr Shafir
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.202000465
Link to VoR: https://doi.org/10.1002/cssc.202000465
01/2020

10.1002/cssc.202000465
ChemSusChem
Graphical abstract:
Dyes into crystals do not go with the flow:
Crystal-to-crystal post-functionalization of
MOF-520 with perylene yield a photocatalytic
active material. The functionalised MOF
promoted by light, the reductive coupling of
aromatic aldehydes, ketones and imines; even
under aerobic conditions under batch and flow.
The dyes into the highly regular X-ray-
diffractive MOF behave like the homogeneous
systems in solution, which help to clarify the reaction mechanism.
KEYWORDS: MOF, Photoredox Catalysis, Crystal-to-Crystal, Pinacol Coupling, Flow.
1
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10.1002/cssc.202000465
ChemSusChem
Crystal-to-Crystal synthesis of photocatalytic MOFs for
visible-light reductive coupling and mechanistic investigations
Luis Gutiérrez,a,¥ Suvendu Sekhar Mondal,a,¥ Alberto Bucci,a Noufal Kandoth,a Eduardo C.
Escudero-Adán,a Alexandr Shafir,b and Julio Lloret-Fillola,c,*
a Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and
Technology, Avinguda Païos Catalans 16, 43007 Tarragona, Spain
b Institute of Advanced Chemistry of Catalonia (IQAC-CSIC), c/Jordi Girona 18-26, 08034,
Spain.
c Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluïs Companys, 23,
08010, Barcelona, Spain
Email: jlloret@iciq.es
ABSTRACT:
Post-modification of reticular materials with well-defined catalysts is an appealing approach to
produce new catalytic functional materials with improved stability and recyclability, but also to
study the catalysis phenomena in confined spaces. A promising strategy to this end is the post-
functionalization of crystalline and robust metal-organic frameworks (MOFs) exploiting the
potential of crystal-to-crystal transformations for further the characterisation of catalysts. In this
regard, two new photocatalytic materials, MOF-520-PC1 and MOF-520-PC2, are
straightforwardly obtained by the post-functionalization of MOF-520 with perylene-3-carboxylic
acid (PC1) and perylene-3-butyric acid (PC2). The single crystal-to-crystal transformation
yielded the X-ray diffraction structure of catalytic MOF-520-PC1. The well-defined disposition
of the perylenes inside the MOF served as suitable model systems to get insights into the
photophysical properties and mechanism by combining steady-state, time-resolved and transient
absorption spectroscopy. The resulting materials are active photoredox catalysts in the reductive
dimerisation of aromatic aldehydes, benzophenones, and imines, under mild reaction conditions.
Moreover, MOF-520-PC2 can be applied for synthesising gram-scale quantities of products in
continuous-flow conditions under steady-state light irradiation. This work provides an alternative
approach for the construction of well-defined metal-free MOF based catalysts.
2
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INTRODUCTION.
Photoredox catalysis has emerged as a powerful synthetic alternative as compared to
classical thermal organic transformations.[1] An important family of photoredox catalysts (PCs)
are based on Ir, Ru and Cu transition metals due to their robustness and long-lived MLCT excited
states.[1e, 2] As a cost-effective alternative, organic dyes PCs such as benzophenones, cyanoarenes,
quinolinius, pyrilium salts have emerged as promising photoredox catalysts based on earth-
abundant elements.[3] Additionally, simple perylene can also efficiently promote redox
transformations under light irradiation.[4]
A promising approach to further develop catalysts is to combine outstanding intrinsic
activities of homogeneous systems with the chemical stability of heterogeneous ones by
immobilisation of catalytic sites. In some cases, catalyst immobilisation leads to a reduction of
performance due to the inaccessibility of active sites and mass transport issues. On the other hand,
the recyclability and stability of heterogenised catalysts generally increase. Successful examples
include the coordination of PCs on porous zeolites,[5] polymers,[6] and metal-organic frameworks
(MOFs).[7],[8] Among them, MOFs are versatile materials since they are built from organic
bridging ligands and inorganic connecting points (referred as secondary building units or SBUs).10
Not only the large number of accessible SBUs,[9] metalloligands,[10] and organic linkers,[11],[12] but
also the post-modification by encapsulation of guest molecules offer a further possibility of
functionalisation, such as the incorporation of photoactive molecules.[13] As a result, the properties
of the final material can be modulated to broaden the range of possible applications, from gas
adsorption,[14] sensing,[15] light-emitting devices,[16] biochemical systems[17] to heterogeneous
catalysis.[18] Therefore, MOFs are unique to design earth-abundant metal-based single-site solid
catalyst.[18i] Still, only a limited number of MOFs have been reported to catalyse light-driven
organic transformation,[19] despite the wide variety of homogeneous photoredox catalysts
counterparts.[19] Those examples rely on demanding synthetic procedures, such as the synthesis
of Ru(II) and Ir(III)-polypyridyl complex into UiO-type frameworks or metalloporphyrins
MOFs,[10] or the use of photoredox organocatalyst for direct synthesis of polymers[11-12]
A pioneering work by Yaghi and co. demonstrated that the coordinative alignment (CAL) of
a guest molecule to the MOF-520’s SBU is an excellent methodology allowing the post-synthetic
modification and determination of a single-crystal X-ray structure and absolute configuration of
bound molecules.[20] This crystal-to-crystal modification involves the replacement of the bridging
formate ligands at the SBUs with guest molecules possessing either a carboxylate, phenol, diol,
azolate, sulfur-containing oxoacid, and a phosphorus-containing oxoacid.[20]
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Inspired by these studies, we hypothesised the advantage of including functional guest
molecules into MOF-520, such as photoredox catalysts. The coordination of the guest molecule
into the SBU places the photoredox catalyst in a confined environment, which may allow for the
crystallographic characterisation and therefore aiding into the mechanistic understanding.
Another interesting aspect of the coordination of guest molecules into MOF-520 is that can be
considered as isolated units and therefore acting as single-site catalysts in a heterogeneous
material. In general, mechanistic studies of heterogeneous phases are more challenging than
homogeneous ones. However, heterogeneous reticular materials, with well-defined single
catalytic sites, may serve to clarify mechanistic aspect that otherwise is tough to address in a
homogeneous phase. For instance, it could be useful to elucidate the active catalytic species in
photoredox catalysts based on -conjugated molecules. The tendency to form -stacking
aggregates in solution introduces an uncertainty like catalytic species since the dynamics of the
excited states are very complex.[21] In those systems, the catalytic activity can take place from the
monomer, the dimer, larger aggregates, or from a combination of them.
As proof of concept, we have anchored two different photoredox catalysts (PCs) in MOF-
520. Perylene-3-carboxylic acid (PC1) and perylene-3-butyric acid (PC2) reacted with MOF-520
and incorporated in the structure in a crystal-to-crystal transformation. The resulting MOFs
(MOF-520-PC1 and MOF-520-PC2) showed photoredox catalytic activity for the C–C bond
reductive dimerisation of aromatic aldehydes, ketones and imines, under mild reaction conditions.
Diols and diamines obtained are structural motifs in natural products,[22] pharmacologically active
compounds, and auxiliaries in asymmetric syntheses.[23] The reaction was also performed under
continuous flow irradiation and aerobic conditions to generate bulk amount of desired products.
Furthermore, the obtained MOF-520 based photocatalysts are suitable model systems to perform
photophysical studies. The perylenes within the MOFs behave as single units, in contrast to the
expected behaviour in the homogeneous phase. We also present the insights on the photocatalytic
cycle obtained with the help of steady-state, time-resolved spectroscopic studies and transient
absorption spectroscopic studies.
RESULTS AND DISCUSSION
Synthesis and structure determination:
Single crystals of MOF-520 were obtained in good yield following the procedure previously
reported.[20a] As previously reported, the X-ray analysis showed the SBUs ([Al8(-
OH)8(HCOO)4(BTB)4]) constituted by a ring of eight aluminium octahedra shared corners
through eight -OHs, twelve BTB (4,4′,4′′,-benzene-1,3,5-triyl-tris(benzoiate)) and four formate
ligands, with wide window openings (13.7 Å) and cavities. Each BTB is connected to three SBUs
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to form a three dimensional (3D) porous framework. The formates are exchangeable and offer
anchoring points for molecules derivate with carboxylate acids.
The SBU was straightforwardly modified by incubating the crystalline white MOF-520 in
DMF at 100 C for five days in the presence of an excess amount of perylene-3-carboxylic acid
(PC1) or perylene-3-butyric acid (PC2) (Scheme 1 and see the Supporting Information (SI) for
details). At the end of the reaction, dark-reddish crystals were severely washed with DMF and
acetone and collected by filtration (Figure S1).
Scheme 1. Presentation of the crystal-to-crystal reaction by ligand exchange. Formate ligands are
replaced by carboxylate derivate perylenes (PC1 and PC2) into the SBU of MOF-520. Incoming
perylene and outgoing formic acid are highlighted with red and blue colours, respectively.
The powder X-Ray diffraction (PXRD) pattern of the coloured MOF-520-PC1 and MOF-
520-PC2 exhibited sharp diffraction peaks identical to the ones of the as-synthesized sample,
indicating that the porous framework maintained the crystalline integrity (Figure 1a). Dinitrogen
(N2) adsorption/ desorption isotherms (77 K) of MOF-520, MOF-520-PC1, and MOF-520-PC2,
indicate a decrease in porosity (Figure 1b), with Brunauer−Emmett−Teller (BET) surface areas
determined to be 2438, 1448, and 1081 m2/g, respectively. The loss of BET surface area is
consistent with the steric bulk of guest perylene ligands residing in the pores of the MOF-520.
These MOFs exhibited reversible type I sorption curves, which are characteristic of microporous
materials.
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b)
a)
600
450
300
MOF-520
MOF-520-PC2
MOF-520-PC2
MOF-520-PC1
MOF-520 (as synth)
MOF-520 (simulated)
MOF-520-PC1
0.0
0.2
0.4
0.6
0.8
1.0
10
20
30
40
P/P0
2q (deg)
Figure 1. a) PXRD patterns and b) N2 sorption isotherms at 77 K of MOF-520 (blue), MOF-520-
PC1 (red), and MOF-520-PC2 (green). Adsorption and desorption branches are indicated in
closed and open symbols, respectively.
Liquid-phase 1H NMR spectroscopy of digested MOF-520-PC1 and MOF-520-PC2 samples
was performed in order to quantify the amount of guest catalyst binding in the bulk samples. The
samples (1 mg) were transferred to a GC vial. Deuterated dimethyl sulfoxide (d6-DMSO) (0.4
mL) was added to the vial followed by the addition of 0.1 mL of NaOH (1 M in D2O). The solution
was sonicated for 5 min to digest the crystals. The vial was capped and placed in a preheated 120
°C oven for approved h to dissolve the crystals completely. The final clear solution was used for
1
recording H-NMR spectrum. The perylene incorporation was determined by the ratio between
the perylene and BTB linker measured in the digestion mixture (Figure S2-S4). Based on above
results, we observed the 44% and 71% of incorporation of PC1 and PC2, respectively, postulating
the formulas [Al8(-OH)8(HCOO)2(PC1)2(BTB)4] and [Al8(-OH)8(HCOO)(PC2)3(BTB)4],
respectively.
Single crystal diffraction of MOF-520-PC1 (Figure 2) confirms the presence of PC1 ligand
coordinated through the carboxylate bridging to two aluminium atoms (Figure S5a). The X-ray
occupancy factor of PC1 ligand was 40%, in agreement with nearly 50% of formate replaced
determined 1H NMR. We speculate that the full replacement of formates by PC1 is not possible
due to the rigidity and steric hindrance of PC1 as judged by the potential collision between PC1
neighbouring positions within the same SBU (Figure S5b). MOF-520-PC1 exhibits two types of
ellipsoidal pores, formed from elongated arrangements of SBUs (Figure S5a). The first type is an
octahedral pore of 10.01 Å × 10.01 Å × 23.23 Å, while the second type can accommodate an
elongated tetrahedral of 5.89 Å × 5.89 Å × 6.21 Å (given the van der Waals radii of the nearest
atoms).[20a] A PLATON calculation indicates that the solvent-accessible void space of MOF-520-
PC1 is approx. 6000 Å3, that accounts for ca. 50 % of the crystal volume (See SI animation
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highlighting the MOF cavity, windows and PC1 included). In the case of the MOF-520-PC2,
although the single crystal data collection was successful the detection of PC2, it was not possible
the resolution of the structure, most probably be due to the high degree of freedom of the flexible
-(CH2)3-unit.
Figure 2. a) Synthesis of MOF-520 from Al-based SBUs and organic BTB linkers, Al-based SBU
is orange polyhedral. b) The reaction of PC1 with MOF-520 leads to MOF-520-PC1. Atom colour
scheme: C: grey; O: red; Al: orange; perylene: green; and H atoms are omitted for clarity.
Figure 3. Representation of the X-ray single-crystal diffraction structure of MOF-520-PC1. The
MOF cavity and framework ligands are emphasized with the green translucid plains. PC1 moieties
are represented in blue ORTEP and Al-based SBUs in orange polyhedral.
The strong coordination bond between Al3+ and the carboxylate groups, along with the high
connectivity between the [Al8] clusters infers an expected of MOF-520-PC1 and MOF-520-PC2
stability under different conditions. In this regard, we tested the stability of dispersed MOF
samples in acetone, tetrahydrofuran, methanol, acetonitrile, dichloromethane, and DMF for three
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days at room temperature. After such a treatment, the material maintained the fully crystalline
integrity as confirmed by PXRD (Figure S6 and S7).
Catalysis.
To proof the photocatalytic ability of the encapsulated PC inside the MOFs, we selected the
light-driven pinacol coupling of a model substrate 4-methylbenzaldehyde (1a). We find that both
suspension of MOF-520-PC1 or MOF-520-PC2 (0.5 mol %) in CH3CN and after irradiation with
visible light (447±20 nm) for 16 h in the presence of DIPEA under N2 atmosphere, yielded the
corresponding pinacol (2b) in low yields (9 % and 32 %, respectively). These results encourage
us to optimise the conditions of the reaction (Table S2-S4).[3a] After the optimising the solvent
mixture (MeOH:CH3CN, 3:2 v:v), the yield increase up to 74 and 82 % for MOF-520-PC1 and
MOF-520-PC2, respectively, when using 1 mol % of MOF-520-PC. Loadings of 1 mol % of
MOF-520-PC is substantially lower than previously used for homogeneous perylene
organocatalysis (ca. 8-10%,).[3a] Blank experiments under dark, without DIPEA, or without PC
(MOF-520-PC2) did not produce 2b (Table S3), and MOF-520 (without PC) was inactive.
Interestingly, the catalytic reaction mediated by MOF-520-PC2 was also effective under aerobic
environment, giving a similar catalytic performance as under anaerobic conditions (Table S3).
Reductive transformations under aerobic conditions are uncommon,[24] and usually need to
elaborate sophisticated strategies to avoid the reactivity of the O2 with the reduced catalyst.[25]
Following, we directly compared the MOF-520-PCs with the corresponding homogeneous
PCs under the best reaction conditions (Table 1). Interestingly, the heterogeneous materials
exhibit similar activity than homogeneous counterparts. These results suggested that most of the
catalytic centres within the MOF-520-PCs are accessible and active or that the incorporation
enhances the activity of the perylenes in the MOF structure. Besides, leaching experiments
suggested that MOFs are robust and the primary source of the catalytic activity. The amount of
PC1 and PC2 leach measured by UV-Vis absorption at the end of the catalytic run was only about
2.3 % and 1.4 % of PC1 and PC2 (Figure S9 and S10).
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Table 1. Summary of control experiments for the reductive coupling reactions.
Photocatalyst (PC) PC (mol %) yield (%)a
PC1
0.5
68
63
75
81
74
82
PC2
0.5
PC1
2
PC2
3
0.9 (1.6)b
0.9 (2.8)b
MOF-520-PC1
MOF-520-PC2
Conditions: 4-methylbenzaldehyde (1a, 0.1 mmol), DIPEA (1.4 mmol) and different
photocatalysts in CH3CN/CH3OH (v:v = 3:2, 2 mL) at 30℃, under N2 were irradiated for 16 h
(LED 447 nm). a Yield is calculated by 1H-NMR using 1,3,5-trimethoxybenzene, as an internal
b
standard (I.S.). The PC incorporation within the structure is 44% and 77% for PC1 and PC2,
respectively. Therefore, it should be considered that the PC concentration is estimated to be about
1.6 mol % and 2.8 mol % for PC1 and PC2, respectively.
Following, we extended the scope to a broad range of aromatic aldehydes with electron-
donating, electron-withdrawing and bulky substituents. MOF-520-PC2 was the chosen catalyst
for the isolation of products because of the better yield. In order to facilitate the isolation of the
products, the formed diols 2(a-m) were converted into the corresponding diacetate 3(a-m) using
standard conditions (see SI). The formation of products was obtained from moderate to excellent
yields (around 70%) under the optimised conditions (Table 2). Interestingly, heteroaromatic
aldehydes 1l and 1m were found to be compatible with the reaction conditions, although it gave
only from moderate to low yield. The developed methodology was not compatible with aliphatic
aldehyde 1n. Having outlined the scope for aldehydes, we next applied MOF-520-PC2 catalyst
to benzophenones and imines without further optimisation (Table 3). In the case of imine
reduction, up to 79% of yield was found. The limit of the catalytic system starts to appear in the
coupling of more bulky substrates and the more challenging to reduce substrates, such as ketones
and aliphatic aldehydes.[26]
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Table 2. Summary of the results obtained for the reductive coupling reactions of aromatic
aldehydes, catalysed by MOF-520-PC2.
Reaction conditions: MOF-520-PC2 (1 mol%), aldehyde (0.1 mmol), DIPEA (1.4 mmol), in
CH3CN/CH3OH (v:v = 3:2, 2 mL) at 30 ℃, under N2 during 16 h of irradiation (LED 447 nm).
Yields are referred to isolated products; each substrate was run in three vials in parallel reactions
that were combined after the end of the reaction for isolation.
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On the other hand, although the significant advances produced in the pinacol coupling
reaction,[27] the intermolecular cross-pinacol coupling that produces a single cross-coupled 1,2-
diol selectively still remains a challenge.[28] Strategies to address the chemoselective control
include pre-functionalization with stoichiometric quantities of metal salts, employing one
coupling partner in large quantities, using highly functionalized carbonyl compounds,[29] or
exploiting differences in reactivity between coupling partners via an ionic mechanism.[30] In this
regard, to proof the effect of the photocatalysts encapsulation into the MOF-520-PC2 into
selectivity, we preformed the hetero-coupling of two different aldehydes, 1b and 1j, with marked
differences in steric effects and compared with the homogeneous PC2.
The reactions were carried out using an equimolar mixture of the two aldehydes. Moreover,
irradiated for 8 h to better appreciate the differences in reactivity. The crude of the reaction,
1
without further manipulation, was analyzed by H-NMR using an internal standard for
quantification. In this way, the selectivity is not altered during the isolation procedure (Table S7).
Although in both cases a mixture of homo-coupling (2j, 2b) and hetero-coupling (2o) products
were obtained there is a remarkable difference in the selectivity for the bulkier substrate 2j. A
homolytic product selectivity 2b/2j value of 4.6 fold was obtained for MOF-520-PC2, while is
1.0 for PC2. This 4.6 fold increase for the 2b formation versus 2j suggests a size-exclusion effect
of the cavities of MOF-520-PC2.
Table 3. Summary of the results obtained for the reductive coupling reactions of benzophenones
and aromatic imines, catalysed by MOF-520-PC2.
Reaction conditions: MOF-520-PC2 (1 mol%), ketone or imine (0.1 mmol), DIPEA (1.4 mmol),
CH3CN/CH3OH (v:v = 3:2, 2 mL) at 30 ℃, under N2 during 16 h of irradiation (LED 447 nm).
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Yields referred to isolated products. Each substrate was run in two vials in parallel reactions that
were combined after the end of the reaction for isolation.
Recycling in batch: MOF-520-PC2 can be easily separated and recovered from the reaction
mixture through filtration. Therefore, we studied the potential recycling of the catalysts after
catalysing the pinacol coupling of 1a. After washing twice with methanol and acetone, the
recycled MOF-520-PC2 was subjected to a following up catalytic run with the same substrate.
The catalytic activity was maintained for the first two cycles (79 and 75 %) and dropped in the
third one to 34 % of 2a (Table S5). This reduction of the yield was accompanied by a significant
reduction in the crystallinity of the catalyst, as confirmed by its PXRD patterns (Figure S11). We
rationalise that the prolonged stirring used under the catalytic conditions may be the responsible
for the structure degradation of MOF-520-PC2, which then diminish the catalytic activity since
the catalytic sites are no longer reachable by the organic substrates.
Photocatalysis in flow: Taking advantage of the heterogeneous nature of MOF catalyst, we
studied the potential use of MOF-520-PC2 as photocatalyst in flow.[31],[32] Indeed, flow chemistry
has many advantages, such as easy up-scaling and automatisation.[33] However, the scarce light
penetration is most probably the reason behind the few applications of photocatalytic
transformation using heterogeneous materials under flow.
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Figure 4. a) Consecutive photocatalytic reduction of aldehydes 1i and 1a to the corresponding
pinacols in flow with the same MOF-520-PC2/celite® packed column. b) Schematics of the
experimental setup.
The nature of the developed material (MOF-520-PC2) encouraged us to develop a heterogeneous
photocatalytic system operative in flow. We selected prototype substrates with different electronic
properties, 1i and 1a, for the testing. To set up the flow reactor, we prepared a 1 % dilution of the
MOF-520-PC2 in celite® and directly packed into a column (20 x 0.6 cm) with glass beads to
increase the light-harvesting of the photocatalyst (Figure 4, see SI for further details). The packing
was carried out under aerobic conditions without any particular precaution. The reaction mixture
contained the substrate and the electron donor under identical conditions to those already
discussed optimised for batch reactions. The photoreactor was irradiated perpendicular to the
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surface from two light sources (Kessil lamp 450 nm) at 180º one to each other. The first solution
containing 0.55 g of 1i was pumped through the photoreactor (flow rate of 1.5 mL·h-1 Figure
S12), followed by 0.55 g of 1a after washing. Noteworthy, the sample preparation and the
photoreactions were done under air atmosphere, further demonstrating the robustness of the
catalytic system. As a result, the two consecutive 0.55 g scale reactions produced good overall
isolated yields of products 3i and 3a (0.49 and 0.38 g corresponding to 70 and 51 % yield after
two reactions, respectively, Figure 4), validating the effectiveness of the system.
Photophysical and electrochemical characterization.
To better understand the nature of the catalytic sites, we have investigated the photophysical and
electrochemical properties of the as-synthesized MOFs and PCs.
First, optical spectroscopic studies to give insights into potential interactions between perylenes,
or perylenes and the MOF matrix. Diffused reflectance spectra of solid PC1 and PC2 precursors
(Figure S13a and S13b) showed a broad characteristic perylene monomer absorption band with a
peak at about 430 nm.[34] In both cases, encapsulation in the MOF-520 only induced minor
changes in the absorbance, suggesting that the electronic structure of the PC are not essentially
altered in the solid-state. Absorption and luminescence studies in solution or suspension in the
case of MOFs were more informative regarding the potential aggregation processes (Figure 5 and
S14). The UV-Vis absorption spectrum of PC1 and PC2 shows broad bands (360-470 nm). In the
case of the PC1 the characteristic vibrational fine structure of the perylene is not resolved, even
at concentrations as low as 1 M, while in the case of PC2, the characteristic vibrionic spectra of
perylene monomers are present (1 – 25 M). Presumably, the difference raised from the different
capacity of PC1 and PC2 to form -staking structures, which is likely more favourable for PC1
since the formation of carboxylic acid homodimers duplicate the size of the -conjugated structure
(See Figure 5). This is in agreement with the fact that the presence of a base such as N,N-
Diisopropylethylamine (DIPEA, 5 mM) resolves the characteristic vibrionic spectra of perylene
monomers for PC1, since it disrupts that formation of homodimers. The UV-Vis features of PC1
and PC2 are located in the visible region, without overlapping the MOF-520 absorption of the
BTB linkers (276 nm), facilitating the analysis of the post-modified MOFs (Figure S14).
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0.5
0.5
0.4
0.3
0.2
0.1
0.0
a)
c)
0.4
0.3
0.2
0.1
0.0
PC1 @ different concentration
1 M
2 M
3 M
6.25 M
12.5 M
PC1 (1M) + DIPEA
MOF-520-PC1
PC2 @ different concentration
1 M
3 M
6 M
12.5 M
25 M
MOF-520-PC2
300 350 400 450 500 550 600 650
300 350 400 450 500 550 600 650
Wavelength / nm
Wavelength / nm
PC2 @ different concentration
PC1 @ different concentration
d)1.0
1.0
b)
1 M
2 M
3 M
6.25 M
12.5 M
25 M
1 M
PC1 (1 M)+ DIPEA
2 M
3 M
6.25 M
12.5 M
25 M
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
50 M
100 M
150 M
250 M
MOF-520-PC2
50 M
100 M
MOF-520-PC1
450
500
550
600
650
450
500
550
600
Wavelength / nm
Wavelength / nm
Figure 5. (a-b) Absorption and fluorescence spectra of PC1 measured upon increasing its
concentration and further compared with a suspension of MOF-520-PC1 (1 mg/mL), (c-d)
Absorption and fluorescence spectra of PC2 measured upon increasing its concentration and
further compared with the same in the presence of DIPEA and MOF-520-PC2 (1 mg/mL) in
suspension. Solvent CH3CN, DIPEA (5 mM), ex= 410 nm.
Interestingly, in the absorption spectra of the MOF-520-PCs, perylene vibronic features are
present for both cases, suggesting negligible perylene − perylene interactions or MOF structure −
perylene interactions. To further understand the aggregation behaviour of perylenes inside the
MOF matrix, we also recorded fluorescence spectra of PCs in the concentration range from 1 to
100 μM, as well as MOF-520-PCs suspensions. Like in the UV-Vis absorption spectra, PC1
fluorescence spectra showed unresolved vibronic features, which appeared in the presence of
DIPEA and perfectly matched to the spectrum of the suspended MOF-520-PC1. Also informative
was the fluorescence spectra in the case of PC2 and MOF-520-PC2. While the concentration
increases, the luminescence vibronic bands at 445 nm decrease and the feature at 475 nm rises,
revealing an aggregation process between perylene units, which was not revealed in the studied
UV-Vis spectra range. (Figure 5). Likewise in the UV-Vis, the fluorescence spectrum of MOF-
520-PC2 matches with that of PC2 only at low concentration (1 μM of photocatalyst), indicating
that PC2 behaves as a monomer inside of the MOF-520.[35]
15
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10.1002/cssc.202000465
ChemSusChem
Cyclic voltammetry (CV) experiments showed apparent differences between perylene precursors.
The first reduction wave of PC2 is shifted by 340 mV to more negative potentials concerning PC1
(E0PC2 = -1.80 V and E0PC1 = -1.45 V, respectively). All redox values are given vs SCE otherwise
notified (Table 4, Figure S13c). Although the CVs of the MOFs follow a similar trend than the
perylene precursors, the interpretation is not straightforward. The CV of the MOF-520 shows two
reduction waves that shifted to positive and negative redox potentials in MOF-520-PC1 and MOF-
520-PC2, respectively (Figure S13d). We noticed that the extension of the shifts depends on the
perylene. The shift was more significant in the case of MOF-520-PC1, where the perylene is
electronically connected to the carboxylate and then to the aluminium centres. Conversely, for
MOF-520-PC2, the perylene – Al centres are electronically disconnected by an alkyl spacer, and
barely affects the redox process of the MOF.[34] Nevertheless, since the absorption spectra and
fluorescence do not suffer essential changes, we estimate the redox potentials of PCs within the
MOF would not significantly deviate respect to the free PCs. Therefore, we hypothesize that the
redox difference between the two materials can be the ascribed as the main factor of the
differences in catalysis.
Finally, we characterised the excited states by time-correlated single-photon counting in solid-
state (Figure 6, and Table S8). In both cases, the multiexponential lifetime decay of PCs is similar
to the MOF-520-PCs (Table 4), being the slowest  about 6 ns for PC1 and 8 ns for
PC2.[36],[35a],[37] This result together with the previous ones are in agreement with the fact that both
PCs behave as isolated units inside the MOF-520.
a)
b)
PC2
PC1
IRF
IRF
MOF-520-PC2
MOF-520-PC1
0
10
20
Time / ns
30
40
0
10
20
30
40
Time / ns
Figure 6. Normalised solid-state lifetime decay of a) PC1, MOF-520-PC1, and b) PC2, MOF-
520-PC2 measured by time-correlated single-photon counting and its exponential fits. ex = 470
nm laser, em = 560 nm. (IRF = instrument response function from the laser source).
Mechanistic studies.
16
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10.1002/cssc.202000465
ChemSusChem
To get insights into the reductive coupling reaction of aldehydes, we examined key steps of the
catalytic cycle with the help of steady-state, time-resolved fluorescence quenching studies and
transient spectroscopy experiments. To this end, we have focused the efforts on the prototype
reaction of 1a with MOF-520-PC2 under suspension, in the absence and presence of DIPEA. As
presented above, the perylene PC2 inside the MOF-520 behaves as a monomer as judged by the
absorption and emission spectra (Figure S15, S17).
[DIPEA] / mM
0
IRF
10
20
30
50
70
150
200
300
a)
(0/)-1 vs. [DIPEA]
b)
8
6
4
2
0


y= 0.035 x+ 0.51
R2= 0.984
10
20
30
40
0
50
100
150
200
Time / s
[DIPEA] / mM
c)
d)
[DIPEA] / mM
0
10
20
(0/)-1 vs. [DIPEA]
3
30
50
70
100
150
200
300
2
1
0

y= 0.0284 x + 0.041
2
R = 0.998
0
10
20
30
40
0
20
40
60
80
100
Time / ns
[DIPEA] / mM
Figure 7. a) Titration lifetime decay of MOF-520-PC1 (a) and MOF-520-PC2 (c) with DIPEA in
CH3CN/CH3OH solvent mixture (v:v = 3:2, 2 mL). ex = 405 nm laser, em = 460 nm. Stern-Volmer
quenching analysis of the lifetimes' changes as a function of [DIPEA] for MOF-520-PC1 (b) and MOF-
520-PC2 (d). The sample prepared as fine suspension after ball milling, cell path length 1 cm, T = 25 oC.
17
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10.1002/cssc.202000465
ChemSusChem
Table 4. Summary of the redox processes and excited-state lifetimes in solution.
(0/-)
E(*/-)
a
b
c
Sample
E1/2
abs
em
1
[SCE]
[nm] [nm]
[ns]
PC1
442
440
275
445
440
452
446
390
454
448
4.5
4.2
3.5
6
-1.45
1.25
PC2
-1.80
1.0
MOF-520
MOF-520-PC1
MOF-520-PC2
-
-
-
-
-
-
5.2
b
a Values obtained from the maximum absorption peak; values obtained from the maximum
c
fluorescence peak; lifetime obtained from the single exponential function fit at the em.
Absorption and fluorescence spectra are collected CH3CN/CH3OH (v:v = 3:2).
The lifetime of the MOF-520-PC2* exited state was long enough to be quenched by the
DIPEA as observed by the fluorescence (Figure S15c-d) and excited-state lifetime quenching
studies (Figure 7c-d). Similar bimolecular quenching rate constants (kq) were obtained by time-
correlated single-photon counting for MOF-520-PC1 (6.0 ± 0.3 109 M-1s-1, Table S9 and see SI
for calculation details) and for MOF-520-PC2 (5.9 ± 0.3 109 M-1s-1, Table S10). When studying
1a as a quencher, we could not observe quenching at concentration relevant for catalysis (Figure
S16). Redox values of DIPEA and PC2* suggests that a single electron transfer (SET) between
-
them is thermodynamically feasible (E(DIPEA)0 = 0.72 V[38] and E(*PC2/PC2 ) = 1.0 V vs SCE, Figure
S17b,d). Likewise, a SET from DIPEA to MOF-520-PC2* should also be thermodynamically
feasible.[39] Nevertheless, the formation of the radical anion of the photosensitiser can be observed
by millisecond transient absorption spectroscopy. Indeed, in the case of PC2, the PC2* exited
state follows the formation of the radical anion PC2•- in the presence of DIPEA (Figure S18a)
with a clear absorption peak at 570 nm. Monitoring the radical anion at 570 nm presented a
biexponential decay time of ca. 0.75 and 3.6 ms (Table S11), which can be rationalized as
recombination in ms-time scale. Under the same condition but in the presence of 1a the decays
are reduced to 0.44 and 3.0 ms (Table S11).
Likewise, in the case of MOF-520-PC2 the evolution of the radical anion can be followed at 570
nm in the presence of DIPEA, but with faster decay time than in the case of the PC2 in solution
(<> = 0.25 ms). By adding 1a to the same cuvette, the intensity of the signal at 570 nm is reduced
18
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10.1002/cssc.202000465
ChemSusChem
to more than half, suggesting a SET between the radical anion by 1a. (Figure S18d). Therefore,
the proposed mechanism of the photocatalytic reductive coupling reaction starts with the
excitation of the PC ( = 447 nm), which undergoes reductive quenching with DIPEA to afford
MOF-520-PC•-. The successive SET to the substrate results in the reduction of the carbonyl group,
followed by the C-C homocoupling reaction (Scheme 2).[26]
Scheme 2. Proposed mechanism of visible-light-driven reductive coupling reaction of aromatic
aldehydes, acetophenones, and imines using MOF-520-PC2 photoredox catalyst.
CONCLUSION
In the present work, we reported a straightforward post-synthetic transformation of MOF-520
with two different substituted perylene molecules introducing a photoredox functionality to the
material. As judged by the redox potential, the differences in catalytic activity between the two
materials can be ascribed to the redox potential differences between the PCs reduced state, being
PC2 more reducing than PC1. The new heterogeneous organo-photocatalysts revealed efficient
light-driven reductive coupling of aldehydes, ketones and imines, to give 1,2-diols and 1,2-
diamines, with similar catalytic activity to the homogeneous counterpart, but with recyclable
capacity. Moreover, continuous flow photocatalytic conditions highlight the potential by proving
gram scale catalytic transformation and reusability. Combining steady-state and time-resolved
spectroscopy revealed that immobilised perylenes at the MOF-520 act as an isolated unit, and
therefore the catalytic activity could be exclusively ascribed to perylene monomer, whereas in
homogeneous phase the potential aggregation of perylenes complicates the assignment of the real
catalytic active species. We envision that this study will open new perspectives in the design of
heterogeneous photoredox catalysts by further developments on photocatalytic active materials
based on the straightforward crystal-to-crystal transformation of MOF-520 as well as potential
derived light-driven organic transformations under flow.
19
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10.1002/cssc.202000465
ChemSusChem
ASSOCIATED CONTENT
Supporting Information.
Figures, tables, text, and CIF files giving crystal data, experimental details, and the catalysis
results.
The crystal structure depositing number for MOF-520-PC1 is CCDC-1972303.
AUTHOR INFORMATION
Author Contributions
The manuscript was written through the contributions of all authors. / All authors have given
approval to the final version of the manuscript. ¥ This authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank the ICIQ Foundation, the European Research Foundation for project ERC-2014-CoG
648304 (J.L.-F.), MINECO (CTQ2016-80038-R; J.L-F.) and AGAUR 2017-SGR-1647 (J.L.-F.).
S.S.M. and N.K. thank Marie-Curie COFUND and JyC for postdoctoral scholarships,
respectively.
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