Enabling photocatalytic activity of [Ru(2,2′:6′,2′′-terpyridine)2]2+ integrated into a metal–organic framework

Dong Luo a, Tao Zuo a, Ji Zheng a, Zi-Hao Long a, Xue-Zhi Wang a, Yong-Liang Huang b, Xiao-Ping Zhou *a and Dan Li *a
aCollege of Chemistry and Materials Science, Guangdong Provincial Key Laboratory of Functional Supramolecular Coordination Materials and Applications, Jinan University, Guangzhou, Guangdong 510632, P. R. China. E-mail: zhouxp@jnu.edu.cn; danli@jnu.edu.cn
bDepartment of Chemistry, Shantou University Medical College, Shantou, Guangdong 515041, P. R. China

Received 5th January 2021 , Accepted 29th January 2021

First published on 1st February 2021


As a creation platform for multifunctional materials, crystalline metal–organic frameworks (MOFs) can integrate different chromophores through reticular chemistry to adjust their spatial arrangement and intermolecular interaction, in turn achieving the purpose of improving the nature of optoelectronic properties. Herein, a stepwise reticular synthesis approach is successfully used to construct a multicomponent MOF, in which the well-known bis-terpyridyl ruthenium chromophore is orderly arranged into the skeleton of the material. Remarkably, this method promotes the excited state lifetime of the bis-terpyridyl ruthenium core, by two orders of magnitude (from 0.39 to 22.09 ns), to the extent that it can produce singlet oxygen under visible light irradiation at room temperature. Meanwhile, the obtained multicomponent MOF has been established to have considerable porosity for exposure of substrates to the catalytic sites, rendering it suitable for heterogeneous photocatalysis, including as a photooxidation detoxifier for sulfur mustard simulant. Moreover, DFT and TDDFT calculations reveal that the synergistic charge transfer among different components in the MOF may play a crucial role in improving the excited state properties of the bis-terpyridyl ruthenium motif.


Terpyridine, as a classical family of N-donor ligands, has abundant coordination modes with various metals to successfully construct an enormous number of discrete and polymeric coordination complexes.1–10 Thereinto, [Ru(tpy)2]2+ (tpy = 2,2′:6′,2′′-terpyridine) chromophore has the advantage of convenient ligand modification1 and interesting physicochemical properties2,11 and has long been considered in molecular design, supramolecular self-assembly and material creation.2–6 However, the [Ru(tpy)2]2+ motif suffers from a fatal weakness in photophysical properties, with an awfully short lifetime (τ ∼ 0.25 ns) and a negligible quantum yield (Φ ≤ 5 × 10−6) at room temperature,11 which limits its potential for optoelectrical applications. For example, it generally requires excited species having nanosecond lifetimes (τ > 1 ns) to be able to generate singlet oxygen under light radiation for subsequent catalytic uses,12 which precludes the use of [Ru(tpy)2]2+ for this purpose. Such a disadvantage of [Ru(tpy)2]2+ is a consequence of the close equilibrium between the emissive triplet metal-to-ligand charge transfer (3MLCT) state and the non-emissive d–d metal-centered (3MC) state due to the weakened ligand field caused by the distorted octahedral coordination geometry of chelating tpy ligands around the Ru centre.13,14 Although there were substantial studies on prolonging the excited state lifetimes of [Ru(tpy)2]2+-type complexes via molecular design, including modification of ligands to increase the 3MLCT–3MC energy gap or incorporation of another chromophore that can repopulate the 3MLCT state, the synthetic procedures can be very complicated.11,14

Metal–organic frameworks (MOFs),15 as a new class of crystalline porous materials, are typically composed of two types of components, in which the metal ion/cluster and organic ligand are connected through coordination bonds and the reticulation is guided by its underlying nets.16,17 For MOF materials with light-harvesting ability, the spatial arrangement and intermolecular interaction of the embedded homogenic chromophores has an important impact on the excited state behaviours, which will ultimately affect the photophysical properties of the material (e.g. prolonging luminescent lifetime, enhancing quantum yield, etc.).18–20 In order to optimize the functionality and increase the complexity of the targeted MOF materials, several strategies guided by reticular chemistry have been developed for rationally constructing multicomponent metal–organic framework (MMOF) materials which are composed of at least three different components.21–25 When the heterogenic chromophores are introduced into the MOFs, the changes of the chromophores’ concentration26 or the formed donor–acceptor system27 in the frameworks, for instance, will affect the interaction between the chromophores, inducing unique exciton interaction and dynamics, arising novel energy and charge transfer pathways.26–29 Although the above factors can be used to adjust the photoelectric properties of MOFs,30,31 it is still very challenging to rationally adjust the excited state lifetime of the materials. Moreover, it is worth noting that the [Ru(tpy)2]2+ chromophore has been introduced recently into a few MOFs32–35 for exploring the photoactivities. Nevertheless, these results either had no crystal-structure evidence,33 or failed to improve the lifetime performance,34 or possessed limited surface area due to the lack of reticular design.35

In this work, an MMOF (named RuZn-MMOF-1) with a pillared-layer architecture36–39 is designed and constructed through stepwise reticular synthesis (Scheme 1), in which the pre-assembled “pillar”, [Ru(pytpy)2]2+ (pytpy = 4′-(pyridin-4-yl)-2,2′:6′,2′′-terpyridine), holds up the “layer” with a square lattice composed of [Zn2NDC2] (H2NDC = naphthalene-2,6-dicarboxylic acid). The photoluminescence measurements show that RuZn-MMOF-1 has obviously improved emission intensity and quantum yield compared with metalloligand Ru(pytpy)2(PF6)2. Meanwhile, the lifetime of its low-energy 3MLCT emission has also been significantly increased at room temperature, by two orders of magnitude. As a result, the photoactivity of the [Ru(tpy)2]2+ chromophores is activated and the material can be used to generate singlet oxygen under visible light irradiation. Subsequently, the RuZn-MMOF-1 has established sufficient porosity for diffusing substrates, facilitating the photocatalytic oxidation of organic matter under mild conditions (e.g. as a detoxifier for sulfur mustard simulant).

image file: d1qm00024a-s1.tif
Scheme 1 Stepwise reticular synthesis of 4-component, photo-catalytically active RuZn-MMOF-1 based on [Ru(tpy)2]2+ chromophore.

Results and discussion

The reaction of Ru(pytpy)2(PF6)2, H2NDC and Zn(NO3)2·6H2O was performed under solvothermal conditions within DMF/EtOH mixed solvent at 120 °C for 72 h, forming a suitable single crystal of RuZn-MMOF-1 for X-ray diffraction analysis (XRD) (see the ESI for experimental details and crystallographic information, Table S1).40–42 RuZn-MMOF-1 crystallised in the tetragonal space group P42/nnm, and the NDC2− components exhibited symmetry disorder (Fig. S5c, ESI). The only crystallographically independent Zn(II) ion has a tetragonal pyramidal coordination geometry fulfilled by four deprotonated NDC2− (Zn–O 1.897(18) Å) and one pyridine group (Zn–N 2.021(7) Å) from pytpy (Fig. 1a). Two Zn(II) ions and four carboxyl groups form a Zn2(CO2)4 paddle-wheel secondary building unit (SBU), which is connected by NDC2− linkers in the ab plane to form the two-dimensional layer with an sql net. The [Ru(pytpy)2]2+ metalloligands are distributed on the poles of paddle-wheel SBUs (Fig. S5b, ESI) and join the layers into a three-dimensional framework with pcu topology (Fig. 1b). Because pcu is a self-dual net and there is plenty room for another set of identical frameworks, the final RuZn-MMOF-1 has a 2-fold interpenetrated topology and therefore the underlying net is pcu-c (Fig. 1c).43
image file: d1qm00024a-f1.tif
Fig. 1 (a) Coordination environment around Zn2(CO2)4 paddle-wheel containing [Ru(pytpy)2]2+ and NDC2−. (b) Crystal structure of one set of RuZn-MMOF-1 with a pcu net. (c) The underlying topology of the 2-fold interpenetrated RuZn-MMOF-1 with a pcu-c net. (d) Reduced density gradient isosurfaces (isovalue = 0.5 a.u.) showing the weak interactions between [Ru(pytpy)2]2+ and the NDC2−-based wall. The color scale (in a.u.) shows a range of interaction strengths from strong attraction (blue), close to van der Waals radii (green) to strong repulsion (red).

Interestingly, the shape and size of the [Ru(pytpy)2]2+ pillar and a grid of the [Zn2NDC2] layer are compatible. Therefore the pillar penetrates the square grid, and the [Ru(tpy)2]2+ motif is exactly located in the center of the grid with a surrounding NDC2−-based wall (Fig. S5a, ESI). The reduced density gradient analysis (RDG)44 was used to identify the noncovalent interactions between the [Ru(pytpy)2]2+ and NDC2−-based wall (Fig. 1d). The results indicated that there were abundant van der Waals interactions evenly distributed between the pytpy and NDC2− ligands; however, no strong attractive or repulsive force was identified therein. Notably, when replacing H2NDC with other dicarboxylic acid linkers (e.g. 1,4-naphthalenedicarboxylic acid, 4,4′-biphenyldicarboxylic acid), isoreticular MMOFs could not be obtained under similar synthetic conditions, suggesting that these weak van der Waals interactions may play a certain role in the reticulation process.

The bulk phase purity of RuZn-MMOF-1 is confirmed by the CHN element analysis and powder X-ray diffraction (PXRD, see the ESI and Fig. S6), and the Fourier transform infrared (FT-IR) spectrum of RuZn-MMOF-1 shows that NO3 (absorbance band at about 1348 cm−1) and PF6 (around 791 cm−1) coexist as the counter anions in the porous network (Fig. S7, ESI). In addition, the scanning electron microscopy (SEM) recorded the stratiform crystal morphology of RuZn-MMOF-1, and energy-dispersive X-ray (EDX) mapping confirmed the coexistence and homogeneity of Ru and Zn in the crystal (Fig. S8, ESI). The molar ratio of Ru: Zn in the bulk sample of RuZn-MMOF-1 is close to 1[thin space (1/6-em)]:[thin space (1/6-em)]2, which is measured by inductively coupled plasma atomic emission spectrometry (ICP-AES, see Table S2, ESI), consistent with the single crystal data, showing the advantage of reticular chemistry for obtaining uniform mixed-metal MOF materials.

RuZn-MMOF-1 showed good thermostability up to about 350 °C under N2 and air atmosphere, demonstrated by thermogravimetric analysis (TGA) and variable temperature PXRD (Fig. S9 and S10, ESI), respectively. When the samples of RuZn-MMOF-1 were immersed in various common organic solvents for 5 days at room temperature, it is found, through PXRD (Fig. S11, ESI), that the framework crystallinity was mostly maintained in several solvents, except in water, which is commonly observed for such Zn-based pillar-layer structures.45–47 However, it should be noted that, except for isopropanol, other solvents showed a certain degree of degradation of the RuZn-MMOF-1 samples (Fig. S12, ESI), indicating that subsequent heterogeneous photocatalysis studies had better be performed in isopropanol. Moreover, although activated RuZn-MMOF-1 had no adsorption of N2 at 77 K, it had obvious uptake of CO2 at 196 K and shows a typical type I adsorption isotherm, indicating a microporous structure (Fig. S13 and S14, ESI). The calculated Brunauer–Emmett–Teller (BET) and Langmuir surface area of RuZn-MMOF-1 are 340.3 m2 g−1 and 457.3 m2 g−1 (Fig. S15, ESI), respectively, which are much larger than that of previously reported MOF materials containing [Ru(tpy)2]2+ motif (e.g. with a BET surface area of 20 and 50 m2 g−1, respectively).33,35

Remarkably, through reticular installing [Ru(pytpy)2]2+ into the 4-component MMOF the photophysical properties, especially the excited state lifetime of the [Ru(tpy)2]2+ core, are greatly improved. By comparing the solid ultraviolet visible (UV-vis) absorption and emission spectra of RuZn-MMOF-1 and Ru(pytpy)2(PF6)2 (Fig. 2a), it is observed that although the typical MLCT absorption band (ca. 500 nm) is relatively weaker than that of Ru(pytpy)2(PF6)2, the emission intensity of the MMOF is much stronger, with a characteristic 3MLCT band at 686 nm, indicating much better light-harvesting ability of the MMOF compared with the isolated chromophore at room temperature. In fact, the absolute Φ of Ru(pytpy)2(PF6)2 could not be detected due to its faint photoluminescence, while RuZn-MMOF-1 has a Φ = 0.6% under optimum excitation at 465 nm. Most importantly, the excited state lifetime, obtained from fitting the luminescence decay curves (Fig. 2b and Table S3, ESI), of RuZn-MMOF-1 (τav = 22.09 ns) is almost two orders of magnitude longer than that of Ru(pytpy)2(PF6)2 (τav = 0.39 ns) at room temperature. In the most recent literature, the introduction of a [Ru(tpy)2]2+-based precursor into the MOF materials did not significantly prolong (precursor τav = 12.2 ns vs. MOF τav = 34.7 ns)35 or even shorten (precursor τav = 1.78 ns vs. MOF τav = 1.49 ns)34 the lifetime values.

image file: d1qm00024a-f2.tif
Fig. 2 (a) Normalized solid UV-vis spectra (dotted line) and photoluminescence spectra (solid line, λex = 465 nm) of RuZn-MMOF-1 (black line) and Ru(pytpy)2(PF6)2 (red line), respectively. (b) Photoluminescence decay profiles and lifetime fitting curves of RuZn-MMOF-1 and Ru(pytpy)2(PF6)2. (c) In situ EPR spectra for detecting formed 1O2 when suspending RuZn-MMOF-1 in isopropanol with TEMPO as a spin-trapping agent under visible-light irradiation at different times. (d) UV-vis absorption spectra of 1,3-diphenylisobenzofuran upon visible light irradiation at different times under an O2 atmosphere and room temperature.

The improvement of lifetime performance of RuZn-MMOF-1 to exceed the nanosecond threshold makes it capable of generating singlet oxygen (1O2) under visible light. The electron paramagnetic resonance (EPR) spectra were used to verify the active oxygen radicals produced when exposed to visible light (Fig. 2c). In the presence of 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TMP) as the 1O2 capture agent, a characteristic triplet EPR signal of 4-oxo-TMPO was observed, indicating that RuZn-MMOF-1 has the photocatalytic ability to transform 3O2 into 1O2.48,49 The longer the time exposed to visible light, the more 1O2 was produced. Moreover, the photooxidation reaction of 1,3-diphenylisobenzofuran (1,3-DPBF) was used to evaluate the production of 1O2. As shown in Fig. 2d, the decrease of the characteristic UV-vis absorption band at 410 nm indicated that the formation of 1O2 can oxidize the substrate. Also, it is found that 1,5-dihydroxynaphthalene (1,5-DHN) could transformed into juglone in the presence of oxygen and under white light irradiation,38 evidenced by the gradual emergence of the absorption band at 425 nm with increasing illumination time (Fig. 3a).50–52

image file: d1qm00024a-f3.tif
Fig. 3 (a) UV-vis absorption spectra of 1,5-dihydroxynaphthalene upon visible light irradiation at different times. (b) Photooxidation kinetic curves of CEES transformed to CEESO and CEESO2 in isopropanol using RuZn-MMOF-1 as a photocatalyst. (c) Reusability of RuZn-MMOF-1 for photooxidation of CEES to CEESO. (d) Proposed Jablonski diagram including S1 and T1 excited states of a fragment of RuZn-MMOF-1 and relevant 1O2 active pathway. ISC = intersystem crossing.

It is of great significance to study the photocatalytic oxidation and detoxification of sulfur mustard, an extremely dangerous chemical warfare agent.53,54 On account of the ability of RuZn-MMOF-1 to generate 1O2, it is further developed as a photooxidation detoxifier for sulfur mustard simulant (2-chloroethyl ethyl sulfide, CEES) under mild reaction conditions. The kinetics of the oxidation reaction of CEES with 5 mol% RuZn-MMOF-1 as a catalyst under white light irradiation and room temperature was monitored by gas chromatography – mass spectrometry (GC-MS, Fig. S18 and see the ESI for experimental details). During the 18 hour reaction processes, RuZn-MMOF-1 exhibited a much faster reaction rate than that of Ru(pytpy)2(PF6)2 (Fig. 3b). The conversion of oxidation products of CEESO (2-chloroethyl ethyl sulfoxide, nontoxic) and CEESO2 (2-chloroethyl ethyl sulfone) reached close to 90%. In addition, when the light irradiation was stopped in the middle of the reaction, it could be found that the CEES ceased to be oxidized, indicating that the oxidation reaction was indeed a photocatalytic process. Further reusability experiments showed that RuZn-MMOF-1 maintained the conversion close to 90% in four catalytic cycles, and the selectivity of CEESO over CEESO2 was also maintained above 90% (Fig. 3c). The crystalline phase of the catalyst was mostly maintained after four catalytic cycles (Fig. S19, ESI). These experimental results indicated that RuZn-MMOF-1 is potentially an excellent CEES detoxifier.

In order to understand the effect of RuZn-MMOF-1 in photocatalytic production of 1O2, DFT and TDDFT calculations were performed by using a simplified model (a fragment of the MOF, see the ESI for computational details). The results showed that the energy of the T1 state (2.42 eV) is adequately higher than that of the first excited state of 1O2 (1.63 eV), confirming the unobstructed energy transfer pathway (Fig. 3d).55–57 Interestingly, it is further observed that the low-lying singlet excited states (S1 to S10) mostly originate from ligand-to-ligand charge transfer (1LLCT), while the first three triplet excited states (T1 to T3), very close in energy level (2.42–2.47 eV), have a mixed origin of 3MLCT/3LLCT (Table S4 and S5, ESI). This means that the NDC2− component also plays a significant role in the excited state electronic structure that governs the photophysical properties of the overall MMOF materials. In light of the quasi-isoenergetic T1 to T3 with a mixed 3MLCT/3LLCT origin, it is possible that the 3LLCT can act as an energy reservoir, which repopulates the 3MLCT state undergoing a 3MLCT–3MC oscillating equilibrium, thus prolonging the excited state lifetime. Such a strategy is reminiscent of the bichromophoric effects developed in molecular [Ru(tpy)2]2+-type systems,11 although here the construction of MMOF guided by reticular chemistry is straightforward and much easier in synthesis.


In summary, a multicomponent metal–organic framework RuZn-MMOF-1 has been successfully constructed, through stepwise reticular synthesis, to endow the [Ru(tpy)2]2+ chromophore with the advantage of ordered arrangement and homodispersity. Not only has this strategy proved to be convenient and effective in turning the [Ru(tpy)2]2+ core out of photoactive silence through synergistic charge transfer among different components, prolonging the excited state lifetime by almost two orders of magnitude at room temperature, but also it renders RuZn-MMOF-1 useful as a heterogeneous photocatalyst capable of producing singlet oxygen under visible light, as well as having good recyclability. Moreover, the photooxidation ability of the RuZn-MMOF-1 can be used to catalyze useful organic reactions, especially as a potential sulfur mustard detoxifier. In this way, the nature of the photosensitive component has turned from the silent state at the molecular level to the active state at the framework scale, which provides a new way of improving the molecular excited state properties through reticular chemistry. Further ongoing studies include developing more MOF materials containing M(tpy)2 cores and expanding the scope of photocatalytic reactions.

Conflicts of interest

There are no conflicts to declare.


This work was supported by the National Natural Science Foundation of China (no. 21731002, 21975104, 21871172, and 21801095), the Guangdong Major Project of Basic and Applied Research (no. 2019B030302009), the Fundamental Research Funds for the Central Universities (21619405), the Guangzhou Science and Technology Program (202002030411), and Jinan University. The computation in this work was supported by the high-performance public computing service platform of Jinan University.

Notes and references

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Electronic supplementary information (ESI) available: Experimental section, physical measurements and photocatalytic characterization. CCDC 2026257. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1qm00024a
These authors contributed equally to this work.

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