A Multivariate Toolbox for Donor–Acceptor Alignment: MOFs and COFs

Donor–acceptor (D-A) alignment is a powerful concept to tailor the properties of metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) through the integration of different D-A pairs through distinct pathways: periodic ligand ordering within the framework, host–guest interactions, or covalent post-synthetic grafting.D-A architectures inside MOFs and COFs have gained esteem for the construction of materials with enhanced optical, photocatalytic, conducting, sensing, and magnetic properties due to numerous advantages demonstrated through the utilization of crystalline matrices of MOFs and COFs and a number of distinct pathways for the precise control of D-A alignment inside porous scaffolds.The modularity and versatility of COFs and MOFs can promote directional energy and charge transport through tuning the geometrical parameters (e.g., distances or angles between D and A) or electronic structures of D-A architectures, thus propelling the advancement of framework-based optoelectronics and photocatalysis devices.Multivariable scaffolds such as D-A MOFs and COFs provide a pathway for synergistic correlations among charge, spin, and lattice. The modularity of multivariant scaffolds such as metal–organic frameworks (MOFs) and covalent–organic frameworks (COFs) provides an unprecedented level of control in the alignment of donor (D) and acceptor (A) units, a demand that is driven by the production of optoelectronic, photonic, and spintronic devices. The foray into novel motifs bearing D-A ensembles in frameworks has been applied towards material property modulation for device performance enhancement. This review surveys an emerging trend in the development of D-A interfaces by highlighting recent advances probing D-A interactions in porous crystalline matrices, with a focus on energy transfer (EnT) and charge transfer (CT) as well as spin alignment. Thus, we anticipate that this review is timely due to the burgeoning field of D-A porous scaffolds that have recently come to light. The modularity of multivariant scaffolds such as metal–organic frameworks (MOFs) and covalent–organic frameworks (COFs) provides an unprecedented level of control in the alignment of donor (D) and acceptor (A) units, a demand that is driven by the production of optoelectronic, photonic, and spintronic devices. The foray into novel motifs bearing D-A ensembles in frameworks has been applied towards material property modulation for device performance enhancement. This review surveys an emerging trend in the development of D-A interfaces by highlighting recent advances probing D-A interactions in porous crystalline matrices, with a focus on energy transfer (EnT) and charge transfer (CT) as well as spin alignment. Thus, we anticipate that this review is timely due to the burgeoning field of D-A porous scaffolds that have recently come to light. Precise D-A alignment is an important material-development-driven target for device performance enhancement. For instance, mutual D-A orientation can promote directional CT and EnT, allowing tailoring of the electrical and optical properties of materials and therefore significantly affecting their behavior [1.Goswami S. et al.Anisotropic redox conductivity within a metal-organic framework material.J. Am. Chem. Soc. 2019; 141: 17696-17702Crossref PubMed Scopus (1) Google Scholar, 2.Rieth J. et al.Hydrogen bonding structure of confined water templated by a metal–organic framework with open metal sites.Nat. Commun. 2019; 10: 4771Crossref PubMed Scopus (0) Google Scholar, 3.Rice A.M. et al.Hierarchical corannulene-based materials: energy transfer and solid-state photophysics.Angew. Chem. Int. 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Res. 2017; 50: 1654-1662Crossref PubMed Scopus (0) Google Scholar, 18.Zang L. Interfacial donor-acceptor engineering of nanofiber materials to achieve photoconductivity and applications.Acc. Chem. Res. 2015; 48: 2705-2714Crossref PubMed Scopus (55) Google Scholar, 19.Müllen K. Pisula W. Donor–acceptor polymers.J. Am. Chem. Soc. 2015; 137: 9503-9505Crossref PubMed Scopus (85) Google Scholar]. This review describes the advantages of MOFs and COFs to achieve preferential D-A orientation and possibly provide a glimpse of how such D-A alignment may advance the field of MOF- and COF-based devices. In general, crystalline modular scaffolds such as MOFs and COFs provide an unprecedented level of control of D-A arrangement through the versatility of the metal nodes and organic linkers, the modularity of the framework topology, and the variety of incorporated guests (Figure 1, Key Figure) [1.Goswami S. et al.Anisotropic redox conductivity within a metal-organic framework material.J. Am. Chem. Soc. 2019; 141: 17696-17702Crossref PubMed Scopus (1) Google Scholar,2.Rieth J. et al.Hydrogen bonding structure of confined water templated by a metal–organic framework with open metal sites.Nat. Commun. 2019; 10: 4771Crossref PubMed Scopus (0) Google Scholar,20.Chen Z. et al.Reticular chemistry in the rational synthesis of functional zirconium cluster-based MOFs. Coord.Chem. Rev. 2019; 386: 32-49Google Scholar, 21.Otake K-I. et al.Enhanced activity of heterogeneous Pd(II) catalysts on acid-functionalized metal–organic frameworks.ACS Catal. 2019; 9: 5383-5390Crossref Scopus (3) Google Scholar, 22.Luo T-Y. et al.Multivariate stratified metal-organic frameworks: diversification using domain building blocks.J. Am. Chem. Soc. 2019; 141: 2161-2168Crossref PubMed Scopus (6) Google Scholar]. MOFs comprise metal nodes coordinated to organic linkers to afford 1D, 2D, or 3D frameworks, while COFs are covalently connected solely through organic linkers usually in a 2D fashion. Due to a signficant number of variables in each framework and, therefore, great opportunities for tunable properties, D-A alignment could play a critical role in a large number of applications including organic binary system electronics, organic field-effect transistors, and optical devices (Figure 1) [10.Burns D.A. et al.2D oligosilyl metal–organic frameworks as multi-state switchable materials.Angew. Chem. Int. Ed. 2020; 59: 763-768Crossref PubMed Scopus (0) Google Scholar,23.Ma L. et al.Recent advances of covalent organic frameworks in electronic and optical applications.Chin. Chem. Lett. 2016; 27: 1383-1394Crossref Scopus (31) Google Scholar,24.Mandal A.K. et al.Two-dimensional covalent organic frameworks for optoelectronics and energy storage.ChemNanoMat. 2017; 3: 373-391Crossref Scopus (29) Google Scholar]. Herein, we utilize three main areas – D-A alignment-dependent EnT, modular D-A architectures promoting charge transport, and spin alignment as a function of D-A organization – as blueprints to accentuate the overall potential of crystalline scaffolds for the precise control of D-A interfaces. One of the main reasons that drives the enormous interest in the areas of energy and charge transport is that hybrid frameworks such as COFs and MOFs provide a ‘three-in-one’ platform for simultaneous light harvesting and EnT and the possibility of performing photocatalytic transformations, thereby mimicking the main components of a natural photosystem. There are a number of pathways available for EnT to occur in MOFs entailing organic linkers, metal nodes, or guests. Both Förster and Dexter mechanisms were proposed to be responsible for EnT in such extended structures [25.Williams D.E. Shustova N.B. Metal–organic frameworks as a versatile tool to study and model energy transfer processes.Chem. Eur. J. 2015; 21: 15474-15479Crossref PubMed Scopus (52) Google Scholar, 26.So M.C. et al.Metal–organic framework materials for light-harvesting and energy transfer.Chem. Commun. 2015; 51: 3501-3510Crossref PubMed Google Scholar, 27.Zhang Q. et al.Förster energy transport in metal–organic frameworks is beyond step-by-step hopping.J. Am. Chem. Soc. 2016; 138: 5308-5315Crossref PubMed Scopus (66) Google Scholar]. In the case of Förster resonance EnT (FRET), the excitation energy of an excited fluorophore (D) transfers to a nearby chromophore (see Glossary) (A) in a non-radiative fashion through long-range dipole–dipole interactions [25.Williams D.E. Shustova N.B. Metal–organic frameworks as a versatile tool to study and model energy transfer processes.Chem. Eur. J. 2015; 21: 15474-15479Crossref PubMed Scopus (52) Google Scholar]. By contrast, the Dexter mechanism for EnT could be described in terms of the interactions due to intermolecular orbital overlap, including electron exchange, which requires the D and A to be less than ~1 nm away from each other to occur [25.Williams D.E. Shustova N.B. Metal–organic frameworks as a versatile tool to study and model energy transfer processes.Chem. Eur. J. 2015; 21: 15474-15479Crossref PubMed Scopus (52) Google Scholar]. Later are three sections that describe different advantages in achieving D-A alignment to promote (i) EnT-coupled sensing and (ii) light harvesting and directional EnT, as well as different avenues for (iii) engineering EnT-driven optoelectronic devices. MOF- and COF-based sensors are in great demand due to their tunable porosity and therefore possible size-exclusion selectivity, in addition to the availability of the utilization of different parts of the frameworks for selectivity enhancement [8.Khatun A. et al.Thiazolothiazole-based luminescent metal–organic frameworks with ligand-to-ligand energy transfer and Hg2+-sensing capabilities.Inorg. Chem. 2019; 58: 12707-12715Crossref PubMed Scopus (2) Google Scholar,28.Karmakar A. et al.Bimodal functionality in a porous covalent triazine framework by rational integration of an electron-rich and -deficient pore surface.Chem. Eur. J. 2016; 22: 4931-4937Crossref PubMed Scopus (20) Google Scholar, 29.Fang X. et al.Metal–organic framework-based sensors for environmental contaminant sensing.Nano Micro Lett. 2018; 10: 64Crossref PubMed Scopus (53) Google Scholar, 30.Dolgopolova E.A. et al.Photochemistry and photophysics of MOFs: steps towards MOF-based sensing enhancements.Chem. Soc. Rev. 2018; 47: 4710-4728Crossref PubMed Google Scholar]. Prior to D-A exploration in MOFs for EnT-coupled sensing, the importance of D-A interactions was well illustrated by the example of quantum dots (QDs) acting as D and a fluorescent dye, Cy3, as A to sense botulinum neurotoxins, bacterial toxins capable of causing neuroparalytic disease [31.Sapsford K.E. et al.Monitoring botulinum neurotoxin A activity with peptide-functionalized quantum dot resonance energy transfer sensors.ACS Nano. 2011; 5: 2687-2699Crossref PubMed Scopus (96) Google Scholar]. This strategy allows for construction of ‘lab-on-a-chip’ devices for field deployment [31.Sapsford K.E. et al.Monitoring botulinum neurotoxin A activity with peptide-functionalized quantum dot resonance energy transfer sensors.ACS Nano. 2011; 5: 2687-2699Crossref PubMed Scopus (96) Google Scholar], and to some extent paved the way to a closer look at EnT-coupled sensing performed on the basis of multidimensional porous materials such as COFs and MOFs. In the latter case, MOF modularity can provide an unprecedented level of control over other porous structures through an almost unlimited selection of inorganic and organic building blocks [20.Chen Z. et al.Reticular chemistry in the rational synthesis of functional zirconium cluster-based MOFs. Coord.Chem. Rev. 2019; 386: 32-49Google Scholar,22.Luo T-Y. et al.Multivariate stratified metal-organic frameworks: diversification using domain building blocks.J. Am. Chem. 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EnT-coupled sensing achieved through specific D-A orientation can be considered an additional avenue to enhance the sensing capability of MOFs, which is discussed in the section later. The first investigations to tune D-A moieties were performed on a layered MOF, [Zn2(OX)3]·(H2DAB) (OX2- = oxalate and H2DAB2+ = butane-1,4-diaminium) that selectively adsorbed ethanol over acetonitrile and acetaldehyde, demonstrating both size-dependent selectivity and recognition of protic solvents through D-A interactions [34.Sadakiyo M. et al.Hydroxyl group recognition by hydrogen-bonding donor and acceptor sites embedded in a layered metal–organic framework.J. Am. Chem. Soc. 2011; 133: 11050-11053Crossref PubMed Scopus (63) Google Scholar]. Similarly, another zinc-based scaffold, Zn2(NDC)2(DPTTZ) [NDC2– = 2,6-naphthalenedicarboxylate and DPTTZ = N,N′-di(4-pyridyl)thiazolo-(5,4-d)thiazole], constructed from NDC struts (D) and DPTTZ pillars (A), showed potential for sensing Hg2+ at the parts-per-million level [8.Khatun A. et al.Thiazolothiazole-based luminescent metal–organic frameworks with ligand-to-ligand energy transfer and Hg2+-sensing capabilities.Inorg. Chem. 2019; 58: 12707-12715Crossref PubMed Scopus (2) Google Scholar]. Furthermore, the MOF sensor could be recovered after removal of the toxic element, potentially paving a way for recyclable EnT-coupled MOF sensors. Typically, more traditional EnT-coupled sensing MOFs are based on lanthanide (Ln)-containing MOFs due to their propitious luminescence properties (e.g., sharp emission bands, EnT-regulated quantum yields, high sensitivity to changes of chemical environment) [35.Tang Q. et al.Cation sensing by a luminescent metal-organic framework with multiple Lewis basic sites.Inorg. Chem. 2013; 52: 2799-2801Crossref PubMed Scopus (315) Google Scholar,36.Chen H. et al.Photo- and thermo-activated electron transfer system based on a luminescent europium organic framework with spectral response from UV to visible range.Chem. Commun. 2014; 50: 13544-13546Crossref PubMed Google Scholar]; however, attractive strategies to utilize D-A pairing inside Ln frameworks for sensitivity enhancement have only just begun to be developed [29.Fang X. et al.Metal–organic framework-based sensors for environmental contaminant sensing.Nano Micro Lett. 2018; 10: 64Crossref PubMed Scopus (53) Google Scholar,31.Sapsford K.E. et al.Monitoring botulinum neurotoxin A activity with peptide-functionalized quantum dot resonance energy transfer sensors.ACS Nano. 2011; 5: 2687-2699Crossref PubMed Scopus (96) Google Scholar,37.Shen X. Yan B. Photofunctional hybrids of lanthanide functionalized bio-MOF-1 for fluorescence tuning and sensing.J. Colloid Interface Sci. 2015; 451: 63-68Crossref PubMed Scopus (27) Google Scholar]. For instance, the Zheng research team engineered a scaffold constructed from Eu3+ ions bridged by a tritopic carboxylate ligand with a triazine core, resulting in the formation of a 3D network, Eu(BTPCA) [BTPCA3- = 1,1′,1′′-(benzene-1,3,5-triyl)tripiperidine-4-carboxylate] [35.Tang Q. et al.Cation sensing by a luminescent metal-organic framework with multiple Lewis basic sites.Inorg. Chem. 2013; 52: 2799-2801Crossref PubMed Scopus (315) Google Scholar]. The Lewis basic triazinyl nitrogen atoms of the ligand (D) decorate the inner channels of the framework, presenting the possibility of interactions with Lewis acidic guest molecules (A). In this case, interactions between N-containing linkers and metal cations, Fe3+ and Zn2+, resulted in drastic changes in material photoluminescence (PL) profiles attributed to guest-promoted EnT [35.Tang Q. et al.Cation sensing by a luminescent metal-organic framework with multiple Lewis basic sites.Inorg. Chem. 2013; 52: 2799-2801Crossref PubMed Scopus (315) Google Scholar]. On immersion of the Eu(BTPCA) into an Fe3+ solution in DMF (3:1 mole ratio, Fe3+:MOF), emission was essentially quenched, while on immersion of Eu(BTPCA) into a Zn2+ solution (3:1 mol ratio, Zn2+:MOF), the PL intensity was enhanced by greater than threefold compared with that of the parent framework. The Fe3+@MOF exhibited a strong absorption band centered at 344 nm resulting in diminished transfer of excitation energy to Eu3+, and therefore led to a decreased average luminescent lifetime by greater than four orders of magnitude (7.8 × 10-1 ms vs. 5.2 × 10-5 ms). Due to relatively weak absorption of Zn2+@MOF at the same wavelength, the average luminescent lifetime was enhanced 1.2 times (0.78 ms vs. 0.92 ms), suggesting that ligand-to-metal EnT was likely to be occurring. Analogously, Yan and coworkers reported tuning of EnT between a zinc-based MOF constructed from adeninate (AD–) and 4,4’-biphenyldicarboxylate (BPDC2-) ligands and embedded lanthanide cations [37.Shen X. Yan B. Photofunctional hybrids of lanthanide functionalized bio-MOF-1 for fluorescence tuning and sensing.J. Colloid Interface Sci. 2015; 451: 63-68Crossref PubMed Scopus (27) Google Scholar]. The latter (Ln3+ = Eu3+ and Tb3+) was introduced into the anionic framework using post-synthetic cation exchange, resulting in Ln3+@bio-MOF-1 {bio-MOF-1 = [Zn8O(AD)4(BPDC)6]·2Me2NH2}. In this case, the MOF (host) behaves as D and Ln3+ (guest) behaves as A, facilitating guest-induced EnT. Interestingly, the EnT efficiency from the host to the Eu3+ guest molecules was estimated to be 66.5%, but in the case of host to Tb3+ guest molecules, the efficiency decreased to 32.4% according to spectroscopic studies. As a next step, Ln3+@bio-MOF-1 was probed as a sensor for various cations. In the case of Cr3+, Al3+, Na+, Mg2+, Co2+, and Cd2+ encapsulation, there were almost no changes in the emission profile of the framework. By contrast, Fe3+ integration inside the europium-containing framework virtually quenched emission with an associated Stern–Volmer quenching constant (Ksv) of 3.82 × 103 M-1. Analysis of Eu3+ content after the Fe3+ treatment revealed a decrease of Eu3+ concentration inside the framework, implying that a cation exchange reaction between Eu3+ and Fe3+ may occur. In a similar vein, Gao and coworkers reported their efforts to synthesize a Ln-viologen MOF, LVMOF-1 {LVMOF-1 = [Eu2(OH)2L]Cl2; L2− = 5,5′-[(4,4′-bipyridine)-1,1′-diium-1,1′-diylbis(methylene)]diisophthalate}, a system that integrates Eu3+ ions (D) and viologen moieties (A1) [38.Gong T. et al.A stable electron-deficient metal–organic framework for colorimetric and luminescence sensing of phenols and anilines.J. Mater. Chem. A. 2018; 6: 9236-9244Crossref Google Scholar]. Their elegant design produced a 3D framework fashioned from [Eu(OH)(COO)2]n columns and tetratopic viologen-based arrays forming infinite stacks with electron-deficient cavities between viologen chromophores (A1). This arrangement is ideal for sandwiching electron-rich aromatic guest molecules (D1) and thus capable of forming a CT complex, D1-A1. The scaffold selectively adsorbs aniline (D1), resulting in an infinitely alternating D1-A1-D1-A1 array that is also a CT complex with π···π stacking between the aromatic core from the guest (D1) and the viologen core (A1). Such D-A alignment is possible from the rigidity imposed by the framework and can allow the newly formed CT complex (aniline and viologen) to act in tandem as A and facilitate EnT from the excited Eu3+ (D). Such EnT from Eu3+ led to quenching of PL, which was attributed to FRET by the authors. Overall, Gao’s team proposed utilizing the designed system as a portable qualitative sensor for the detection of phenols and anilines (D1) with very low detection limits, within a 1–8 parts-per-billion range. Moreover, further work in this field could significantly modify the material landscape of multipurpose orthogonal sensory devices. Light harvesting and directional EnT are attracting continued attention since they are key components for efficient solar energy utilization. Biobased ordered structures such as proteins, viruses, DNAs, and lipids [39.Spillmann C.M. Medintz I.L. Use of biomolecular scaffolds for assembling multistep light harvesting and energy transfer devices.J. Photochem. Photobiol. C Photochem. 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Rev. 2015; 23: 1-24Crossref Scopus (28) Google Scholar, 40.Li L. et al.Quantum dot–aluminum phthalocyanine conjugates perform photodynamic reactions to kill cancer cells via fluorescence resonance energy transfer.Nanoscale Res. Lett. 2012; 7: 386Crossref PubMed Scopus (0) Google Scholar, 41.Rao K.V. et al.Organic–inorganic light-harvesting scaffolds for luminescent hybrids.J. Mater. Chem. C. 2014; 2: 3055-3064Crossref Google Scholar] through control over D-A alignment. However, there are several advantages of MOFs and COFs that make them invaluable assets in the energy sector. For instance, framework modularity allows tuning of MOF and COF light harvesting profiles over a wide spectral range [42.Lee C.Y. et al.Light-harvesting metal–organic frameworks (MOFs): efficient strut-to-strut energy transfer in BODIPY and porphyrin-based MOFs.J. Am. Chem. 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Soc. 2016; 138: 5308-5315Crossref PubMed Scopus (66) Google Scholar,42.Lee C.Y. et al.Light-harvesting metal–organic frameworks (MOFs): efficient strut-to-strut energy transfer in BODIPY and porphyrin-based MOFs.J. Am. Chem. Soc. 2011; 133: 15858-15861Crossref PubMed Scopus (0) Google Scholar,44.Dolgopolova E.A. et al.A bio-inspired approach for chromophore communication: ligand-to-ligand and host-to-guest energy transfer in hybrid crystalline scaffolds.Angew. Chem. Int. Ed. 2015; 54: 13639-13643Crossref PubMed Scopus (26) Google Scholar,45.Cheng Y-C. Fleming G.R. Dynamics of light harvesting in photosynthesis.Annu. Rev. Phys. Chem. 2009; 60: 241-262Crossref PubMed Scopus (0) Google Scholar]. Moreover, a combination of porosity and structural modularity in these scaffolds provides a unique opportunity to utilize various pathways for EnT based on organic linkers, metal nodes, or guest molecules. Furthermore, a self-assembly approach applied to scaffold preparation allows the coupling of hundreds of chromophores around the catalytic center(s). A number of research teams pioneered the development of MOFs capable of harvesting light across the entire visible spectrum [25.Williams D.E. Shustova N.B. Metal–organic frameworks as a versatile tool to study and model energy transfer processes.Chem. Eur. J. 2015; 21: 15474-15479Crossref PubMed Scopus (52) Google Scholar,26.So M.C. et al.Metal–organic framework materials for light-harvesting and energy transfer.Chem. Commun. 2015; 51: 3501-3510Crossref PubMed Google Scholar,42.Lee C.Y. et al.Light-harvesting metal–organic frameworks (MOFs): efficient strut-to-strut energy transfer in BODIPY and porphyrin-based MOFs.J. Am. Chem. Soc. 2011; 133: 15858-15861Crossref PubMed Scopus (0) Google Scholar]. The goal of this review is not to provide a comprehensive analysis, which can be found elsewhere [25.Williams D.E. Shustova N.B. Metal–organic frameworks as a versatile tool to study and model energy transfer processes.Chem. Eur. J. 2015; 21: 15474-15479Crossref PubMed Scopus (52) Google Scholar, 26.So M.C. et al.Metal–organic framework materials for light-harvesting and energy transfer.Chem. Commun. 2015; 51: 3501-3510Crossref PubMed Google Scholar, 27.Zhang Q. et al.Förster energy transport in metal–organic frameworks is beyond step-by-step hopping.J. Am. Chem. Soc. 2016; 138: 5308-5315Crossref PubMed Scopus (66) Google Scholar,30.Dolgopolova E.A. et al.Photochemistry and photophysics of MOFs: steps towards MOF-based sensing enhancements.Chem. Soc. Rev. 2018; 47: 4710-4728Crossref PubMed Google Scholar], but instead to describe this direction as one of the trends in MOF chemistry that will be further developed. One of the examples reported by the Hupp team describes nearly quantitative, strut-to-strut EnT in a well-ordered D-A MOF with linkers that could be considered mimics of carotenoids and chlorophylls [42.Lee C.Y. et al.Light-harvesting metal–organic frameworks (MOFs): efficient strut-to-strut energy transfer in BODIPY and