INTRODUCTION
The interaction between materials and light, known as the optical behavior, is critical for their application in both industry and our daily life,[] such as optical devices,[] luminescent sensors,[] bioimaging,[] and phototherapy.[] The optical behavior of molecules dissolved in solvent is mainly governed by the functional groups in the molecules.[] When molecular building blocks are aggregated to form solid-state materials, their macroscopic optical properties are not only dictated by the functional groups in the structure,[] but also strongly influenced by the specific geometric arrangement, including the connection, orientation, and alignment of the molecular building blocks. The development in polymer chemistry has already shown that linking of monomers into larger size can effectively tune their optical property by restricting intramolecular rotation.[] In the field of molecular crystals, the control of packing between molecules in three-dimensional (3D) space, manifested in the orientation and alignment of the building blocks, provides additional means to further optimize their light absorption and emission behavior.
Covalent organic frameworks (COFs), an emerging class of porous crystalline materials constructed by linking molecular building blocks through covalent bond,[] enjoy features from both classic polymers and molecular crystals,[] where all these parameters, functional groups, connection, orientation, and alignment can be used to customize their optical properties. Furthermore, the molecularly defined pore space offers an additional parameter to play with. The pores surrounded by molecular backbones not only provide fixed distance between functional groups protruding into the pores, or sometimes contact between them,[] but also allow for the inclusion of guest materials into the pores, inducing various interactions to the COF backbone.[] In comparison to the other properties of COFs that have been extensively studied, such as gas absorption,[] separation and conversion,[] catalysis,[] electro-chemistry, the investigation of optical properties started to emerge lately,[] but the attention has been gradually increasing, especially in the recent years.[] In this review, we summarized the influence from all these aspects for the optical properties of COFs (Figure ), including photo absorption and emission. Another critical feature of COFs is molecular feature, where both the framework backbone and the pore environment can be tailored by introducing a large variety of functional groups. Therefore, the influence of electro-static effect can be studied systematically with fixed distance and defined pore geometry, an aspect inaccessible in traditional polymers and molecular crystals. The precise control of crystal size also allowed for the investigation of domain size. The packing modes between the layers are also found to be critical for their optical properties.[] The detail of this will be reviewed from both energy and dynamic aspects to provide insight into underlying mechanism. In this review, the discussions will be focused on potentials using COFs to enhance their optical properties involving photo absorption and emission. Photocatalytic and optoelectronics applications, involving the photon–electron energy transfer process, are not the main topic of this review.
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TABLE 1 The linkage, topology, and application of various COFs discussed in this review
COF | Geometry of monomer | Linkage | Topology | Application | Ref. |
sp2c-COF-5 | 4, 2 | Olefin | 2D (eclipsed) | Fluorescence | |
2D CCP-1, 2 | 6, 2 | Olefin | 2D (eclipsed) | Fluorescence | |
TPE-COF-OH, OMe | 4, 2 | Imine | 2D (eclipsed) | Fluorescence | |
BCTB–BCTA COFs | 4, 4 | Imine | 2D (eclipsed) | Fluorescence | |
TpBDH | 3, 2 | Imine | 2D (eclipsed) | Fluorescence | |
IMDEA-COF-1, 2 | 3, 2 | Imine | 2D (staggered) | Fluorescence | |
TTA-DFP COF | 3, 2 | Imine | 2D (eclipsed) | White-Light Emission | |
Tf-DHzDPr series | 3, 2 | Imine | 2D (eclipsed) | White-Light Emission | |
sp2c-COF-1, 2, 3 | 4, 2 | Olefin | 2D (eclipsed) | White-Light Emission | |
3D-TPE-COF | 4, 4 | Imine | 3D (sevenfold interpenetrated) | White-Light Emission | |
NUS 30–32 | 4, 2 | Imine | 2D (eclipsed) | Sensor | |
TFPPy-DETHz-COF | 3, 2 | Imine | 2D (eclipsed) | Sensor | |
TPE-Ph COF | 4, 2 | Boronate | 2D (eclipsed) | Sensor | |
COF-808, 909 | 4, 2 | Imine | 2D (eclipsed) | Cancer therapy | |
LZU-1-BODIPY-2X | 3, 2 | Imine | 2D (eclipsed) | Cancer therapy | |
COF-LZU1⊃DPYNS | 3, 2 | Imine | 2D (eclipsed) | Fingerprint detection | |
COF-601 to 606 | 4, 2 | Imine | 2D (serrated) | Two photon absorption | |
cyano-sp2c-COF | 3, 2 | Olefin | 2D (eclipsed) | Two photon absorption | |
TPI-COF | 4, 2 | Imine | 2D (eclipsed) | Bioimaging | |
3D-Py-COF | 4, 4 | Imine | 3D (twofold interpenetration) | Explosive detection | |
PyTA-BC-Ph | 4, 4 | Imine | 2D (eclipsed) | H2 evolution | |
PEG-COF-42 | 3, 2 | Imine | 2D (eclipsed) | Artificial muscles | |
P2PV, P3PcB | 3, 2 | Olefin | 2D (eclipsed) transfer to 3D | Proton conductivity |
STRUCTURE DESIGN STRATEGIES OF COFs FOR OPTICAL APPLICATIONS
The design of COF structure is initiated by the sketch of its molecular building blocks, a process not much different from the design of molecular crystals and polymers. The use of multiple types of molecular building blocks allows for the introduction of various types of electro-static effect within the same structure.[] These building blocks are connected to form an extended and crystalline network, where the connection is determined by both the linkage and the shape of building blocks, and provides a set of possible topologies for the COF design.[] The angles within and between these molecules are imparted by their conformation, leading to a preferred network topology and specific orientations of these building blocks. These networks, either in 2D or 3D, can be further aligned through different packing modes and interpenetration, respectively, to yield the final COF structure.[] In this way, the connection (C), orientation (O), and alignment (A) of molecules are precisely controlled within these crystalline materials. The detailed methods to tune the three critical factors above, C, O, and A, are discussed in the following subsections, where each method might involve multiple factors as labeled in Figure .
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Integrating functional groups into the backbone of COFs
Molecular building blocks could be divided into two categories according to their different emission mechanism. One type includes organic molecules with strong fluorescence in diluted solution, but exhibiting aggregation caused quenching (ACQ) effect in solid state, and are known as ACQphors. The other type, on the contrary, involves organic molecules none or weakly emissive in disperse-state, while showing aggregation-induced emission (AIE) to give efficient solid-state emission, and are known as AIEgens. Both AIEgens and ACQphors types of molecules have been used as building blocks to construct COFs with enhanced optical properties. The following parts will discuss in detail how these two types of molecular building blocks achieve enhanced fluorescence through controlling C, O, and A factors, where different mechanisms are applied.
Construction of luminesce COF by AIE building blocks
AIE effect can be interpreted by the mechanism of restriction of intramolecular motion, including restricted intramolecular rotation (RIR) and intramolecular vibration (RIV), of fluorophores aggregated in solid state. This significantly reduces the excited-state energy dissipation through nonradiative pathways and results in enhancement of fluorescence.[] Actually, the intramolecular motion of AIE building blocks could be efficiently restricted after forming COF structures with precise control of C, O, and A factors. For example, in 2D COF systems, the intramolecular motion is strongly confined by forming tight interlayer packing, utilizing framework restriction, extending intralayer π-conjugation, and covalently cyclization, therefore enhancing the fluorescence of these COFs (Figures ).[] As for 3D COFs, the intramolecular motion is further restricted by the formation of interpenetration structure (Figures ).[]
Tight interlayer packing of 2D COFs
In 2D COF system, one way to restrict the intramolecular motion of AIE building blocks is the utilization of their 2D layered structures for the construction of tightly eclipsed interlayer packing. Such eclipsed interlayer packing makes the molecular AIE units aligned (tuning A) in the same direction, thus forming AIE columns. In 2016, a highly emissive 2D-COF with boronate ester linkage, TPE-Ph COF, was synthesized by the cocondensation of tetraphenylethylene-based AIE unit with linear benzene monomer (Figure ). Benefit from the strong π–π interactions between the 2D layered structures, periodic columnar TPE π-arrays was successfully formed, which significantly reduced the rotation-induced excitation energy dissipation, leading to excellent luminescence property of TPE-Ph COF. The photoluminescence quantum yield (PLQY) of the TPE-Ph COF solid samples reached 32%, two- to threefold higher compared to the model compound. In 2019, DL-COF was constructed by linking linear anthracene and tetraphenylethylqene building blocks. The characteristic fluorescence of AIE units in this COF was promoted by the tight AA stacking of anthracene units, where intramolecular rotation was restricted effectively.
Interpenetrated structure of 3D COFs
Similarly, in 3D COF systems, the interpenetrated structure could be used to create an aggregated state for the AIE units, and enhance their conformation locking as a result of collective alignment of these AIE units (tuning A). In 2018, an imine-linked 3D-COF, 3D-TPE-COF, was designed using tetraphenylethylene and Td-symmetric tetraphenylmethane building blocks. A sevenfold interpenetrated pts topology with P2/c space group (Figure ) was identified, which efficiently suppressed the intramolecular rotation of AIE units, hence achieving bright yellow emission in solid state with a PLQY of 20%, much higher than that of the model compound (6%).
Framework restriction of 2D COFs
The intramolecular rotation of AIE units was strongly restricted by the formation of 2D framework of COF structures (tunning C). In 2019, an imine-linked 2D COF, NUS-32, was constructed to eliminate the influence of interlayer π–π stacking. The AIE test results revealed efficient fluorescence for this COF in bulk, which was attributed to the interlayer π–π interaction. The COF in nanosheet orphology also exhibited strong fluorescent emission. The formation of extended covalent linking in framework provided considerable conformation locking effect, leading to restricted rotation of the AIE units (Figure ). Similarly, Py-TPE COF with PLQY up to 21% was successfully designed using tetraphenylethylene and pyrene units. The excellent fluorescence properties of Py-TPE COF can also be ascribed to the framework restriction, where the rotation of AIE unit was limited.
Linkage of COFs through cyclization reaction
It is usually rare for the AIEgens-based COF with imine linkage to exhibit strong emission, due to the rotational freedom of the imine bonds in excited state. To overcome this challenge, intramolecular cycling strategy was introduced to further lock the conformation. The linkage formed by cyclization reaction not only changed the connection between AIE unites (tuning C) but also uniformed the orientation of building blocks (tuning O). In 2020, tetraphenylethylene unit and linear 2,3-dihydroxyterephthalaldehyde were used as building blocks to construct TPE-COF-OH. TPE-COF-OH only exhibited weak fluorescence, though the rotation of AIE units was partially restricted by the hydrogen bond between ydroxyl and imine linkage. However, drastic enhancement in emission was achieved after the addition of boron trifluoride etherate (BF3·Oet2). This was attributed to the formation of a six-member ring with covalent bond and a cis-configuration at both ydroxyl and imine linkage, which precluded the rotation of imine bonds (Figure ).
Extended π-conjugation across the intralayer of COFs
The enhanced and extended π-conjugation of COF structures were utilized to provide a more uniform and rigid conformation to confine the rotation of AIE units in the backbone, thus promoting the fluorescent emission of the COFs. The connection between AIE units varied from boronate, imine, and olefin by choosing different reversible reactions in the synthesis (tuning C). In 2020, a topology-templated method was developed to construct a series of olefin-linked sp2c-COFs using tetraphenylethylene and acetonitrile functionalized biphenyl building blocks. These COFs with entirely sp2 carbon offered topology-dependent π-conjugation, which was inaccessible in imine-linked COFs previously. Among them, sp2c-COF-5 exhibited the best PLQY (20 %), due to the enhanced π-conjugation along (100), (010), and (110) directions across each single layer based on its Kagome topology (Figure ).
Construction of luminescent COF by ACQ building blocks
ACQ effect was induced by the face-to-face π–π stacking of adjacent aromatic rings in chromophores in aggregate state, where excimers were usually formed upon light absorption. The excimers led to quick decay of excited states back to the ground state via nonradiative channels, hence the quench of fluorescence. However, by linking ACQphors into 3D COF structures or 2D COFs with interlayer packing, where C, O, and A factors were precisely controlled, the π–π stacking interactions between ACQphors can be remarkably reduced, thus avoiding the formation of excimers and turning on the fluorescence of these COFs.[] Another way to enhance the fluorescence intensity of ACQphors was tuning the behavior of excited-state intramolecular charge transfer (ICT). Such ICT behavior can be precisely controlled by deprotonation or protonation of specific active sites in the COF backbones.[]
Isolated ACQ building blocks in 3D framework
One direct method to eliminate π–π stacking between molecular building blocks was constructing 3D structure instead of 2D layered structure. In this way, the ACQ units were isolated and uniformly aligned in the 3D COFs (tuning A). In 2016, a pyrene-based fluorescent 3D COF, 3D-Py-COF, was designed using tetrahedral tetraphenylmethane and rectangle pyrene as building blocks. The twofold interpenetrated pts topology isolated pyrene units from intense π–π interaction between them, successfully avoiding ACQ effect, thus these COFs exhibited bright green fluorescence (Figure ). In 2020, a 3D-LCOF was formed by the cocondensation of pyrene and D2d-symmetric tetrakisphenylbimesityl monomers. Bright blue fluorescence was observed with a high PLQY of 29%, attributing to the efficient reduction of ACQ effect by isolating Py units in the quadrangular pores.
Staggered stacking mode in 2D COFs
Controlling 2D-COFs to form a staggered AB stacking mode instead of eclipsed AA mode was also an effective strategy to weaken interlayer π–π packing of ACQphors. AB stacking allowed for the staggered alignment of ACQ units in adjacent layers (tuning A). Two 2D-COFs with imine linkage, IMDEA-COF-1 and IMDEA-COF-2, were constructed using linear pyrene monomer and two triangular benzene ligands with different substituent groups, respectively. IMDEA-COF-1, in staggered stacking mode, showed strong fluorescence due to the dramatically reduced π–π interaction, while IMDEA-COF-2, in eclipsed stacking mode, was nonemissive, due to the ACQ effect induced by strong interlayer π–π interaction of pyrene units (Figure ). In 2020, ANCOF was synthesized by the solvothermal reaction between triphenoxycyanurate and hydrazine hydrate. The strong fluorescence of ANCOF was ascribed to the suppressed ACQ effect by the formation of staggered packing, where the flexibility of TFPC monomer played a critical role.
Enlarged interlayer distance in 2D COFs
Another method for weakening π–π stacking of ACQphors was enlarging the packing distance between interlayer by inducing steric-hinderance to the COF backbones (tuning A). 2D CCP-1 and 2D CCP-2 were formed by hexakisphenyltriphenylene and two linear acetonitrile-functionalized monomers, respectively. The existence of cyan groups led to the twisted and rigid conformation at each single layer, hence hindered close π–π stacking of ACQ units. The interlayer distance of 2D CCP-1 and 2D CCP-2 was calculated more than 6 Å, beyond the required distance for interlayer [2 + 2] photocycloaddition (4.2 Å) (Figure ). With suppressed ACQ effect, 2D CCP-1 and 2D CCP-2 displayed PLQY of 24.9% and 32.3%, respectively. This strategy was also applied in the design of COF-JLU3 with efficient orange fluorescence. The trisphenylbenzene monomer offered large steric-hinderance at the tert-butyl groups. When integrated into COF-JLU3 by condensation with hydrazine hydrate, the existence of tert-butyl groups significantly increased the interlayer distance, thus reducing ACQ effect.
Adjusted electron distribution in COF structures
When ACQphores were integrated in COFs using linkages containing nitrogen atoms, such as hydrazone, imine, triazine, and bipyridine, the ICT was usually observed between ACQ units and the nitrogen atoms in the linkage, and resulted in fluorescence quenching. One strategy to turn on the fluorescence was blocking ICT by the deprotonation of N−H, which not only changed the connection environment (tuning C) but also shifted the ICT orientation (tuning O). Another strategy for avoiding quenching was introducing an electron-deficient building block together with ACQphors in COF backbones to change the orientation of ICT, involving the control of O. In 2018, the pinpoint surgery method was developed to block ICT and improved the fluorescence of an almost nonemissive COF, TFPPy-DETHz-COF. The ICT pathway in TFPPy-DETHz-COF was eliminated by the deprotonation of N−H into N anion upon the addition of fluoride anion, leading to a 3.8-fold increase in fluorescence intensity and a PLQY of 17% (Figure ). In 2019, Bpy COF nanosheets (Bpy-NSs) was obtained by exfoliating bulk Bpy-COF, which was synthesized from linear bipyridine and rectangular phloroglucinol. Bpy-NSs only exhibited weak fluorescence, due to the ICT occurring between Bpy units and Tp units. However, strong fluorescence was achieved by adding Al3+, which interacted with the nitrogen atoms of Tp units, and cut off ICT pathway. In addition, the enhancement in ICT process may also promote the fluorescence intensity of COFs. For example, PyTA-BC was constructed by using pyrene and an electron-deficient group, bicarbazole, as building blocks. In PyTA-BC, excited state electrons transfer occured from Py moieties toward bicarbazole units instead of imine linkers, thus avoiding fluorescence quenching. By protonaizing imine, ICT was enhanced (Figure ), giving rise to strong fluorescence at around 600 nm, the characteristic wavelength of BC unit. Similarly, three COFs based on bicarbazole, BCTB-PD, BCTA-TP, and BCTB-BCTA, were designed. The BC unites in BCTB-BCTA were linked with asymmetric linkage, thus different electron distribution were observed at BC units of different chemical environments. The difference in electron density enhanced ICT process in BCTB-BCTA, and led to the most intense fluorescence among three COFs in the same series.
Introducing functional groups into the surface or pores of COFs
In addition to de novo design of COFs using fluorophores as building blocks as discussed above, the optical properties of COFs can also be tuned by the postsynthetic modification (PSM) of pore interior and/or the inclusion of fluorophores as guest molecules into the pores.[] In the following sections, the discussion involves the postmodification of fluorophores, such as lanthanide ions, quantum dots (QDs), and fluorescence dyes, via coordination or covalent bond formation with the backbone of COFs, as well as inclusion fluorescent molecules into COF pores.
Grafting metal complex onto COF backbone
Lanthanide ions were well-known luminogens with narrow emission peaks and various emission wavelengths. Lanthanide complexes and lanthanide salts were widely used to construct light-emitting materials. For example, two kinds of lanthanide grafted COFs, Ln@TTA-DFP COF and TpBpy-Ln_acac were prepared in 2019 and 2020, respectively.[] Ln@TTA-DFP COF, obtained by mixing Ln(hfac)3(H2O)2 (Ln = Eu3+, Tb3+; hfac = hexafluoroacetylacetone) complex with TTA-DFP-COF and heating, exhibited characteristic transition peaks of Eu3+ and Tb3+. Furthermore, white light emission was achieved by tuning the ratio of Eu3+ and Tb3+ and the excitation wavelength (Figure ). TpBpy-Ln_acac was gained with similar process, by mixing TpBpy COF with Ln(acac)3 complexes (Ln = Eu3+, Tb3+, Eu3+/Tb3+, Dy3+; acac = acetylacetonate), and exhibited bright characteristic Ln3+ luminescence.
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Loading fluorophores as guests into the pores
Fluorescence dyes are a main class of fluorescent molecules that have been widely used in both industry and daily life. In 2019, a carboxymethyl cellulose COF, CMC-COF-LZU1, was formed by using amino-functionalized carboxymethyl cellulose, triformylbenzene, and p-phenylenediamine as building blocks. Then, fluorescence dyes, 5-(dimethylamino)-N, N-bis-(pyridin-2-ylmethyl)napthalene-1-sulfonamide, were added to form CMC-COF-LZU1⊃DPYNS. CMC-COF-LZU1⊃DPYNS exhibited bright green fluorescence and could be turned on or off by adding H2O or Cu2+, respectively (Figure ). In 2020, TpPa-1 COF, was used to include a well-known fluorescence dye, fluorescein sodium, and gained TpPa-1@Dye. TpPa-1@Dye exhibited strong emission around 514 nm, which originated from fluorescein sodium dye, and revealed the success of inclusion.
Postmodifying fluorescent molecules onto the pores or surface
The PSM method is another excellent strategy to decorate the COF backbone by using the condensation reaction of free −NH2 or −CHO group with aldehyde or amino-substituted guest. Luminescent molecules that are hard to integrate into COFs as building blocks can be introduced onto the backbone (pores or surface) of COFs via PSM. PSM method can efficiently avoid unwanted aggregation of luminescent molecules and promote the optical properties of molecules. These luminescent units became part of COF structure, and thus equipped the COFs with their specific optical properties. In 2017, the T-COF-OH was obtained from the condensation of triphenylene and linear dihydroxy-terphenyl building blocks. Then, a PSM reaction with fluorescein-isothiocyanate (FITC) was carried out. The resulting T-COF-OFITC revealed strong fluorescence in solution and fluorescence of rather small particles was observed, while FITC modified COF-5 exhibited fluorescence only on boundary of the crystalline COF domains. The results indicated that FTIC was successfully modified onto the wall of pore channels. In 2019, BODIPY-2I and BODIPY-2H were decorated onto the defects (both the pores and surface) of a nanoscale COF (NCOF), LZU-1, to form LZU-1-BODIPY-2I and LZU-1-BODIPY, respectively. The resulting LZU-1-BODIPY-2I and LZU-1-BODIPY displayed bright green and red fluorescence, which were originated from BODIPY-2I and BODIPY-2H, respectively. After coating with glycosaminoglycan (GAG) and delivered into biological tissue, the COFs still exhibited fluorescence activity (Figure ). QDs are semiconductor particles with size of about 1 to 10 nm and can generate highly efficient luminescence through radiative transition between the energy levels that originate from quantum confinement effect. In 2018, CdSe/ZnS QD-grafted TpPa COFs with strong luminescence (PLQY = 31.6%) was gained by grafting 3-aminopropyltriethoxysilane (APTEs)-modified CdSe/ZnS QDs onto TpPa COF.
PRECISE CONTROL ON THE ABSORPTION AND EMISSION PROPERTIES OF COFs
The interaction between COFs and light usually gives rise to two significant macroscopic optical phenomena, in comparison to their corresponding building blocks (Figure ).[] One phenomenon is the increase in absorption or emission intensity, representing higher energy absorption and conversion efficiency, which offers COFs as a good candidate for fluorescence-related applications. The other one is the shift in emission wavelength, which enables COFs to emit light with a wide range of colors, and even white light from the combination of emitting units with different maximum emission wavelengths. Improvement in macroscopic optical properties, both absorption and emission, originated from the specific geometric arrangement, including the C, O, and A factors of the molecular building blocks in COF structures.
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Increasing the light absorption and emission intensity
The fluorescence intensity enhancement induced by the formation of COFs and the underlying mechanism has attracted lot of attention. In 2020, BCTB-PD COF, BCTA-TP COF, and BCTB-BCTA COF were gained through Schiff base condensations of a series of bicarbazole monomers. These COFs exhibited different enhancement in emission intensity compared to their monomers, where the type of connection and the orientation of linkage were two key factors. BCTB-BCTA showed the strongest emission among these three COFs, and moderate protonation can enhance ICT to further increase its fluorescence.
Linking molecules into COFs could also promote the light absorption property. A series of two-photo absorption (2PA) COFs, COF-601 to COF-606 was developed using monomers with low 2PA efficiency (Figure ). All six COFs exhibited remarkable increase in two-photon action cross-section (2PACS), from 16.7 folds to 110 folds in comparison to their corresponding monomers. Such excellent 2PACS performance could be attributed to three key aspects. First, from the aspect of O, the functional groups are collectively aligned across the COF structure, making the orientation of their transition dipole moment to be in the same direction to maximize their coherent dipole moment. Second, from the aspect of A, structure determination and theoretical calculation proved that the serrated packing offered the higher 2PACS than other possible packing mode such as staggered packing and eclipsed packing. Third, from the aspect of both O and A, the formation of large crystals provides enlarged coherent domains, thus magnifying the transition dipoles across the crystal, which also give rise to fluorescence intensity enhancement (Figure ). One of these COFs, COF-606, is shown suitable for two-photon induced biomedical imaging.
Increasing in both emission and absorption intensity could also be achieve by the control of C and A. Cyano-sp2c-COF was synthesized by linking electron donor with electron acceptor with sp2c linkage, which provides good π-conjugation. The collectively aligned D-π-A structure in this COF gave raise to its enhanced two photo fluorescence. Because of better penetrability of low-frequency light, the TPF COFs provide potential application in the field of photo therapy.
Tuning the fluorescence wavelength
The capability to tune emission wavelength is another advantage of COFs interacting with light, due to their molecular and structure features. The possible influence of connection was studied through the design and synthesis of sp2c-COF, sp2c-COF-2, and sp2c-COF-3. These COFs are linked with π-conjugated sp2c linkages of different length and their red shift of emission wavelength exhibited good dependency on the length of linkages. This result revealed that the connection could lead to controllable red shift of emission wavelength of COFs through tuning the degree of π-conjugation. There is another example of emission wavelength shift that involves an integrated control of C, O, and A. A series of luminescent 2D-COFs with intra- and interlayer hydrogen bonds were designed by linking 2,5-disubstituted terephthalohydrazide (DHzDR), where R was varied from methoxy, propoxy, allyloxy to 3-(ethylthio)propoxy, with three benzene or triphenylbenzene-based monomers, respectively. (Figure ). This COF series exhibited different emission wavelength varying from 440 to 500 nm (Figure ), due to the connection of different monomers and the orientation of intra- and interlayer hydrogen bonds. In addition, a dual emission character of the COFs, with R = allyloxy, was observed, and attributed to a unique COF-triggered excited-state interlayer proton shift (ESIPS) mechanism. The ESIPS mechanism, induced by the existence of two conformers with different interlayer–intralayer hydrogen environments, is closely related to the alignment and the orientation of different building blocks. The tunable fluorescent wavelength enables controllable emitting color, providing potential application on light emitting especially white light emitting devices, which ask for quantitative combination of different colors.
POTENTIAL OF COFS AS BIOMEDICAL AND SMART MATERIALS
The structure–function relationship between COF structure and their optical properties have been studied in depth by researchers through the precise control of C, O, and A, as mentioned above. Nevertheless, in addition to fluorescence properties, when interaction with light, COFs could display some special properties that are hardly accessible for traditional materials. For example, after interact with light, COFs can efficiently absorb the energy of photons realizing excellent reactive oxygen species (ROS) generation activity, while their corresponding monomers are inactive in generating ROS (Figure ). Another example is visible macroscopic photo-mechanic response of polyethylene COF membrane after UV light irradiation. Such kind of reversible microscopic intralayer contraction shows potential in fabricating artificial muscles (Figure ). In addition, researchers have also found that carbon–carbon double bonds at specific location on the COF backbones underwent the [2+2] cyclization by absorption light with specific wavelength. This led to the successful transfer from 2D COFs to 3D COF structures (Figure ). These examples demonstrate the potential of tuning microscopic structure of COFs to influence its macroscopic optical properties.
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In the field of biomedical materials, COFs with excellent optical properties were promising candidates. For example, two dicarbaldehyde-based molecules L-3C and L-3N, inactive to generating ROS, were used to form two COFs, COF-808 and COF-909, respectively, exhibiting excellent ROS production efficiency (Figure ). This change from ROS inactive to active was originated from the shifting of energy level. The energy level of the small molecules used as building blocks mismatched the band gap of superoxide, while COF-909 exhibits energy overlap with a bandgap of 1.96 eV, where the band structure met the requirement of ROS generation. Furthermore, the high permanent porosity of these COFs (surface areas 2270 and 2610 m2·g–1) promoted both the diffusion of oxygen and the release of ROS in cells, which is also critical in realizing efficient ROS generation. This, combined with the excellent photostability and biocompatibility, led to excellent PDT performance.
However, the synthesis of nanoscale COFs was challenging, thus limiting their further use in biomedical applications. The strategies on addressing this problem can be classified into two categories, the bottom-up method and the top-down method. Top-down methods can be further divided into ultrasonic exfoliation, mechanical exfoliation, chemical exfoliation, gas-driven exfoliation and charge-mediated self-exfoliation through ultrasonic. Bottom-up methods include reaction kinetics regulation, surfactant-assisted synthesis, colloid formation, interfacial synthesis, and templated methods. These methods have been utilized to obtain nanosized COFs with specific optical properties, which is suitable for biomedical applications. For example, TPB-DMTP-COF nanoparticles were prepared through colloid formation method by adding acetonitrile, which enabled large-scale (gram-level) synthesis under ambient conditions. Then, the nanoscale CaCO3@COF-BODIPY-2I@GAG, diameters around 200 nm, was obtained by loading fluorescent dye BODIPY and CaCO3 onto nanoscale TPB-DMTP-COF and exhibited antitumor efficiency via photo dynamic therapy (PDT) and Ca2+ overload synergistic therapy. Another example was ICG@COF-1@PDA nanoparticles, which were prepared by ultrasonically exfoliating NIR dye indocyanine green (ICG) loaded COF-1, and coating with polydopamine (PDA). ICG@COF-1@PDA exhibited notable phototherapeutic efficacy against 4T1 murine breast to lung metastasis.
For the study of dynamic effect induced by light, PEG-COF-42 was synthesized using rectangular benzene- monomer and phthalohydrazide dimer with PEG bridging linkers as building blocks (Figure ). The interfacial polycondensation methods yielded uniform and freestanding membranes of PEG-COF-42. The PEG-COF-42 membranes exhibited reversible blending behavior in response to UV irradiation, that the strip can bent toward the direction of light source and reach a maximum bending angle of θ = 30° in only 20 s. Such photomechanical properties can be ascribed to two key factors. First, PEG-COF-42 membranes bear better mechanical properties, including bending, lifting, and crimping, enabled by the existence of flexible PEG bridges, in comparison to COF-42. At the same time, PEG-COF-42 membranes consist aligned photo-responsive structures with uniform orientation, which requires good crystallinity and large coherent domain. The configurational change of acylhydrazone offers microscopic photo-induced intralayer structural transfer, while suitable length of PEG (× = 400 or 600) offered considerable crystallinity, ensuring the orientation and alignment of acylhydrazone. Those favorable properties make PEG-COF-42 suited for applications, such as artificial muscles fabrication.
Another example of dynamic structure change is photochemical reaction on COFs. 2D P2PV and P2NV COFs was designed and synthesized from the base-catalyzed aldol condensation of trimethyltriazine with terephthaldehyde and naphthalene-2,6-dicarbaldehyde, respectively (Figure ). These 2D COFs exhibited eclipsed AA interlayer stacking mode and all the olefinic linker were of same configuration, which are the two key factors that contribute to the interlayer [2 + 2] cycloaddition activity of 2D P2PV and P2NV COFs. For instance, eclipse AA stacking provides suitable distance between adjacent interlayer olefin bonds, making orbital overlap possible, while the same configuration ensured proper orbital orientation for [2 + 2] cycloaddition. With UV irradiation, 2D P2PV and P2NV COFs undergo microscopic structure transfer induced by the formation of new interlayer carbon–carbon single bonds in the process of [2 + 2] cycloaddition, and resulting P3PcB and P3NcB COFs was demonstrated to adapt 3D network and kept good crystallinity. Furthermore, by heating 3D P3PcB and P3NcB COFs up to 100°C, the structure can change back to 2D, and again transfer to 3D by UV irradiation, exhibiting excellent reversibility in the structure transfer process.
CONCLUSION
In summary, COFs started to attract lots of attention in the field of optical application, benefited by their crystalline and porous nature, as well as the abundant choices of molecular building blocks to influence their macroscopic optical properties. Other than the influence of functional groups that has been frequently discussed in previous studies, the impact from the microscopic structure features of COFs gradually emerged. The detailed analysis of these works, combined with our own experimental experience in this field, allow for the extraction of three key factors, connection, orientation and alignment of molecular building blocks in COF structures. These factors are found to play critical role in the promotion of the optical properties. Design strategies are also reviewed based on their structural differences in building blocks, backbones, and pores, and all these strategies are efficient in enhancing either fluorescent emission or light absorption. The interaction between COFs and light is also viewed from aspects of energy and dynamics. Although such analysis is more qualitative rather than quantitative, it brings out the initiative to further unveil the profound mechanism for these promising solid-state materials in the future.
In the cases discussed in this review, the fine tuning of connection, orientation, and alignment of molecular building blocks in COF structures are correlated to the improvement of their optical properties. However, we are at the very beginning to try to understand the physics and chemistry behind these porous crystals, and to potentially control their optical behaviors at will. There are still quite a few challenges for the further development of COFs to achieve excellent optical properties. First, the number of optical COFs are still scarce, making it difficult to derive the concrete relationship between the energy level and microscopic structure of COFs, as well as its underlying mechanism. Second, although others and we found that the packing mode between COF layers drastically influence the optical properties of COFs, there are few studies on the precise control of packing mode of 2D COFs for further improving their optical performance. Third, only a few numbers of 3D optical COFs has been reported and their linkage types hardly go beyond imine bond. Last but not least, it is imperative to further develop methods to grow COF crystal with high quality, precisely tune crystal domain size, and fabricate membranes in large scale. Improvements toward these directions are likely to gradually transfer their potentials into practical applications in the field of optical materials. We believe these challenges will be undertaken with joined effort from researchers of different backgrounds, and COFs will find its appropriate position in the development of solid-state materials.
With our limited knowledge, five specific directions are envisioned for the future exploration in this fascinating field, include but not limited to: (i) rational design of optical COFs and quantitative study on the correlation between the macroscopic optical properties, such as room-temperature phosphorescence and nonlinear optical behaviors, and the band structure of COFs as well as their molecular dynamics; (ii) revealing new packing modes other than the classic eclipsed and staggered ways, such as serrated or antiparallel, to systematically study the impact from interlayer packing; (iii) employment of new linkages, that is, carbon–carbon, carbon–oxygen, and boron–nitrogen bonds, to discover the electronic effects at the bridge between molecular building blocks of various kinds; (iv) exploration of 3D COFs with different topology or controllable interpenetration folds and enriching the knowledge of anisotropic effect in 3D space; (v) precisely control of crystal morphology, such as particle size, crystallinity, and membranes quality to fill the gap between the potential of these COFs and their practical optic applications. We believe the important structural factors of COFs mentioned throughout this review will provide different angles to look at this emerging class of solid-state optical materials, and hopefully efforts toward the above challenges will bring out new understanding from both physical and chemical aspects, in turn providing new guideline for the design of COFs.
ACKNOWLEDGMENTS
We acknowledge support from the National Natural Science Foundation of China (22025106, 91545205, 91622103, 21971199, 82072996, 81874131, and 82002879), National Key Research and Development Project (2018YFA0704000), and Innovation Team of Wuhan University (2042017kf0232).
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Abstract
Linking molecules into extended crystalline networks to construct covalent organic frameworks (COFs) added in variety to the readily thriving research on molecule‐based solid‐state materials, featured by classic polymers and molecular crystals. Compared to the development of COFs for gas separation, energy storage, and conversion, where the porosity feature of COFs is utilized, the optical applications, such as fluorescence, white light emission, and photodynamic therapy, involving the molecular and crystalline feature of COFs, are much less explored. In this review, we focused on the optical properties of COFs, and how do these macroscopic properties correlate with the microscopic structure of COFs. Other than the influence from organic functional groups in previous reviews on COFs, here, three critical structure factors, the connection, orientation, and alignment of the molecular building blocks, are outlined and associated with the optical properties of COFs. We also analyze the properties of COFs from both energy and dynamic aspects in an attempt to provide further insight into the possible underlying mechanism. At the end of this review, we also discuss the remaining challenges and future directions for the design of COFs for optical applications, and unveil the potential of COFs toward this direction.
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1 The State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei‐MOST) & Key Laboratory of Oral Biomedicine Ministry of Education, School and Hospital of Stomatology, and, Department of Oral Maxillofacial‐Head Neck Oncology, School and Hospital of Stomatology, Wuhan University, Wuhan, China
2 Key Laboratory of Biomedical Polymers‐Ministry of Education, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, China