This article is licensed under CC-BY 4.0 pubs.acs.org/JACS Communication Assembly of Covalent Organic Frameworks into Colloidal Photonic Crystals Javier Fonseca, Lingxin Meng, Pedro Moronta, Inhar Imaz,* Cefe López,* and Daniel Maspoch* Cite This: J. Am. Chem. Soc. 2023, 145, 20163−20168 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Self-assembly of colloidal particles into ordered superstructures is an important strategy to discover new materials, such as catalysts, plasmonic sensing materials, storage systems, and photonic crystals (PhCs). Here we show that porous covalent organic frameworks (COFs) can be used as colloidal building particles to fabricate porous PhCs with an underlying face-centered cubic (fcc) arrangement. We demonstrate that the Bragg reflection of these can be tuned by controlling the size of the COF particles and that species can be adsorbed within the pores of the COF particles, which in turn alters the Bragg reflection. Given the vast number of existing COFs, with their rich properties and broad modularity, we expect that our discovery will enable the development of colloidal PhCs with unprecedented functionality. C olloidal photonic crystals (PhCs) or opals are periodi- cally structured materials composed of assembled particles that can control the propagation and emission of harvesting, separation, and contaminant removal.10,11 How- ever, unlike MOFs, they are difficult to obtain as single- crystalline particles with homogeneous sizes and shapes, which photons.1 These self-assembled materials exhibit excellent would be crucial for their self-assembly into ordered super- optical properties, making them promising for applications structures. To circumvent this barrier, we first reflected on such as photonics, optics and optoelectronics, sensing, solar earlier observations that COFs could be synthesized in the photovoltaics, energy storage, biomedical engineering, environ- form of submicrometer spherical particles comprising mental remediation, communications, and even quantum aggregated nanocrystallites.12 We reasoned that by optimizing computing.2 the synthesis of these spherical COF particles in terms of size Colloidal PhCs were first formed in the 1990s by self- monodispersity and colloidal stability, we could provoke self- assembly of spherical polystyrene colloidal particles.3 Almost assembly of COF particles into three-dimensional super- simultaneously, silica particles, which are naturally forming structures via depletion interactions. We further envisioned opals, were also postulated as building particles for assembling that precise control of the particle size would enable fabrication colloidal PhCs.4 Significantly, the use of silica evidenced the of colloidal PhCs with tunable photonic bandgaps. Here we possibility to impart to PhCs new functionalities (e.g., report the results of experiments performed to prove these mesoporosity) coming from the colloidal building particles. hypotheses. However, to further extend the functions of PhCs, there is a We began by synthesizing monodisperse, submicrometer- need to discover and apply new functional materials as sized colloidal spherical particles of the imine-linked TAPB- colloidal building particles into PhCs. In the past few years, BTCA-COF. 12a Both 1,3,5-tris(4-aminophenyl)benzene more sophisticated polymeric (e.g., hydrogel) spheres, (TAPB) (0.04 mmol) and benzene-1,3,5-tricarbaldehyde including functionalized polymeric particles and composite, (BTCA) (0.04 mmol) were first dissolved in 5 mL of Janus, or core−shell polymeric particles, have been incorpo- acetonitrile containing acetic acid (AcOH) (12 M). This rated into PhCs, thus opening the application of these crystals solution was then stirred vigorously for 10 s and left to displays, barcodes and sensors.5,6 In the field of porous undisturbed for 72 h at room temperature, resulting in the PhCs, our group has introduced porous metal−organic formation of a yellow-brown colloid (Figures 1a and S1). With frameworks (MOFs) as a new type of colloidal polyhedral this synthetic protocol, the size of TAPB-BTCA-COF particles particles to form porous PhCs.7,8 To date, however, the range composing this colloid was systematically tuned by mod- of available porous colloidal particles for developing porous ification of the amount of AcOH (12 M) from 1.2 to 0.2 mL. PhCs remains rather limited. Alternatively, researchers have Field-emission scanning electron microscopy (FE-SEM) explored other strategies to construct porous PhCs, such as etching of nonporous colloidal polymeric particles.9 Herein we introduce a new type of colloidal porous particles Received: June 14, 2023 to assemble porous PhCs: purely organic crystalline covalent Published: September 6, 2023 organic frameworks (COFs). COFs are a widely known class of porous crystalline materials that exhibit high surface areas and tunable pore sizes and compositions, and that have found a broad variety of applications, including catalysis, water © 2023 The Authors. Published by American Chemical Society https://doi.org/10.1021/jacs.3c06265 20163 J. Am. Chem. Soc. 2023, 145, 20163−20168 Journal of the American Chemical Society pubs.acs.org/JACS Communication Figure 1. (a) Schematic of the formation of the imine-linked TAPB-BTCA-COF. (b) PXRD patterns, (c) FE-SEM images, and (d) size distribution histograms of spherical TAPB-BTCA-COF particles of different diameters. From bottom to top in (b) and left to right in (c): black, simulated; violet, 179 ± 6 nm; sky blue, 203 ± 3 nm; green, 220 ± 4 nm; red, 277 ± 5 nm; yellow, 416 ± 7 nm; orange, 785 ± 12 nm. (Figure 1c) and dynamic light scattering (DLS) analysis three-dimensional, ordered superstructures (Figure 2). As for (Figure S2) of the resulting colloids revealed the formation of the majority of spherical colloidal particles (e.g., silica, monodisperse spherical TAPB-BTCA-COF particles with polystyrene, and acrylates),14 the TAPB-BTCA-COF particles diameters of 179 ± 6 nm (1.2 mL of AcOH), 203 ± 3 nm self-assemble into the entropically favored face-centered cubic (1.0 mL of AcOH), 220 ± 4 nm (0.8 mL of AcOH), 277 ± 5 (fcc) lattice. The formation of this lattice was clearly illustrated nm (0.6 mL of AcOH), 416 ± 7 nm (0.4 mL of AcOH), and by the characteristic top- and cross-sectional views of triangular 785 ± 12 nm (0.2 mL of AcOH) (Figure 1c,d and Table S1). arrangements of the spheres, which correspond to the (111) Powder X-ray diffraction (PXRD) of all synthesized TAPB- and (111̅) planes of an fcc arrangement, respectively (Figure BTCA-COF particles indicated that they are crystalline, with 2b). FE-SEM also showed that the arrangement of these patterns matching those reported for the AA-eclipsed stacking spheres reaches lengths of up to 300 μm without any cracks structure (Figure 1b). Nitrogen sorption measurements on and with only minimal point defects, stacking faults, and these particles proved their microporosity, showing Brunauer− dislocations (Figure 2). Note that the evaporation conditions Emmett−Teller surface areas (SABET) up to 1238 m2 g−1 are critical to controlling the order of the self-assembled (Figures S4−S21 and Table S1).13 superstructures. For instance, we found that the degree of Next, we explored the self-assembly of TAPB-BTCA-COF three-dimensional ordering of TAPB-BTCA-COF particles particles into ordered three-dimensional superstructures on a decreased when the solvent was evaporated off at temperatures solvophobic surface using an evaporation-induced self- lower than or higher than 120 °C (Figures S24−S28). assembly method. To this end, Sylgard 184 (polydimethylsi- Similarly, we confirmed that other self-assembly approaches, loxane, PDMS) was initially spread onto the surface of clean including heat-assisted vertical deposition and centrifugation, glass slides and subsequently cured at 120 °C to form a could also yield COF-superstructures, albeit with a lower solvophobic surface on which the self-assembly reactions were degree of ordering than that obtained with the solvent- to occur. Then a droplet of one TAPB-BTCA-COF colloid was evaporation method at 120 °C (Figures S29 and S30). placed onto this solvophobic surface and dried at 120 °C for 6 Another crucial factor that we identified is that the presence min, resulting in the formation of a colored film. FE-SEM of unreacted linkers in the colloid is essential for colloidal images of the evaporated films revealed the formation of large, stability and therefore for self-assembly of the TAPB-BTCA- 20164 https://doi.org/10.1021/jacs.3c06265 J. Am. Chem. Soc. 2023, 145, 20163−20168 Journal of the American Chemical Society pubs.acs.org/JACS Communication Figure 2. (a) FE-SEM images of a self-assembled superstructure comprising TAPB-BTCA-COF particles (179 ± 6 nm). The two magnified sections (left) show that order is maintained throughout the superstructure. The magnified section (right) is the edge of the superstructure, revealing the order in all three dimensions. (111) and (111̅) surfaces can be seen. (b) Simulation of the formation of an fcc lattice. (c−g) FE-SEM images of superstructures comprising TAPB-BTCA-COF particles of (c) 785 ± 12 nm, (d) 416 ± 7 nm, (e) 277 ± 5 nm, (f) 220 ± 4 nm, and (g) 203 ± 3 nm. Figure 3. (a) Optical images and (b) optical reflectance spectra at normal incidence (θ = 0°) of the PhCs comprising TAPB-BTCA-COF particles of different sizes. The peaks marked with an asterisk correspond to high-energy photon bands. (c) Bragg reflection maximum (λc) plotted against the interplanar distance (d111) and fitted to the Bragg−Snell law. (d) Optical reflectance spectra at normal incidence (θ = 0°) for the PhC comprising TAPB-BTCA-COF particles (277 ± 5 nm) as-made and after exposure to alcohol vapors. 20165 https://doi.org/10.1021/jacs.3c06265 J. Am. Chem. Soc. 2023, 145, 20163−20168 Journal of the American Chemical Society pubs.acs.org/JACS Communication COF particles. The presence of unreacted linkers was close to those exhibited by their TAPB-BTCA-COF building confirmed by 1H NMR analysis of the supernatant (Figure particles (up to 990 m2 g−1; Figures S36−S53 and Table S1). S31) once the TAPB-BTCA-COF particles had been separated Having confirmed the microporous character of the photonic out from the colloid by centrifugation. Clear evidence of the structures, we next exposed an evacuated PhC comprising importance of the unreacted linkers was found when such TAPB-BTCA-COF particles (diameter: 277 ± 5 nm) to vapors linkers were removed from the colloids by consecutive of one of various alcohols, including methanol, ethanol, centrifugation/washing steps. Under these new conditions, isopropanol, and 1-butanol. In each case, we found a red the resulting TAPB-BTCA-COF particles exhibited a lower shift in λc when the PhC was exposed to the alcohol vapors colloidal stability, which precluded their ability to form ordered (methanol, λc = 651 nm; ethanol, λc = 654 nm; isopropanol, λc assembled structures. Thus, unreacted linkers are cardinal for = 657 nm; 1-butanol, λc = 658 nm; Figure 3d and Table S3). stabilizing TAPB-BTCA-COF colloids and for their subse- Significantly, after the alcohol molecules had been evacuated quent self-assembly into ordered structures. We attributed this from the COF pores by heating at 120 °C for 30 min, λc to the notion that the linkers also act as depletants to induce shifted back to the initial value of the original PhC (S54). attraction among the TAPB-BTCA-COF particles. Having demonstrated the self-assembly of spherical TAPB- Another remarkable feature of the various self-assembled BTCA-COF particles into colloidal PhCs, we extended the COF particle films that we observed is their structural colors, formation of COF-based PhCs to a second imine-linked COF, ranging from violet to red depending on their constituent hereafter termed TAPB-TP-COF (where TP is terephthalalde- particle size, as typically seen upon formation of colloidal PhCs hyde; Figure 4a). Using a synthetic protocol similar to that (Figure 3a). To optically characterize the PhCs comprising TAPB-BTCA-COF spheres with different diameters, the optical reflections were recorded using reflectance spectropho- tometry at normal incidence (θ = 0°), as shown in Figures 3b, S33 and S34. The Bragg reflection or photonic bandgaps caused by the (111) crystallographic planes of the fcc lattices were evidenced by a clear band in each spectrum. As expected, these Bragg reflection spectral positions shifted linearly toward shorter wavelengths as the diameter of TAPB-BTCA-COF particles decreased (Table S2). Figure 3c shows the Bragg reflection maxima or bandgap centers (λc) plotted against the interplanar distance (d111). For an fcc lattice, the interplanar spacing for (111) planes is given by d111 = 0.816·D, where D is the diameter of the TAPB- BTCA-COF particles. We fitted λc using the Bragg−Snell law for normal incidence: λc = 2·n·d111, where n is the effective index of refraction of the PhC. From the slope of the fitted curve, it is possible to calculate the effective refractive index (neff) to be 1.46. Next, we calculated the refractive index of COF particles by averaging the effective dielectric function: ϵeff = neff2 = nc2·Vc + nm2·Vm, where nc and nm are the refractive indices of the COF particles and surrounding medium, respectively, and Vc and Vm are their corresponding volume fractions. For (111) planes in the fcc lattice, the volume fractions of COF particles and the surrounding medium are approximately 0.74 and 0.26, respectively.15 Since the surrounding medium was air and nair is 1.00,16 the refractive index (nc) of COF particles was determined to be 1.59, which is close to the values found in polymeric materials.17 Moreover, colloidal PhCs comprising COF particles with diameters larger than 277 ± 5 nm (specifically, 416 ± 7 or 785 ± 12 nm) exhibited optical reflectance in the spectral range corresponding to higher-energy photonic bands, usually Figure 4. (a) Schematic of the formation of TAPB-TP-COF. (b) FE- SEM images of a superstructure comprising urchin-like-shaped TAPB- referred to as second-order diffraction (Figures 3b and S33). TP-COF particles. (c) Optical image and (d) optical reflectance In this range, a more complex photonic band structure spectrum at normal incidence (θ = 0°) of the PhC comprising TAPB- describes the states accessible to photons.18,19 The reflectance TP-COF particles. spectra exhibited by these PhCs exhibit a set of spectral fluctuations that do not always correspond to the presence of forbidden bands. used for TAPB-BTCA-COF, a colloid composed of mono- An interesting property of PhCs built from porous materials disperse urchin-like-shaped TAPB-TP-COF particles with a is their potential to adsorb species within their pores. We diameter of 282 ± 14 nm was synthesized (Figures S55−S59, sought to test examples of this adsorption, exploring any S61−S63 and Table S4). Afterward, we self-assembled these consequential changes in the Bragg reflection maximum. To COF particles on a solvophobic surface using the evaporation- this end, we first measured the N2 adsorption−desorption induced method. Despite not exhibiting a perfect spherical isotherm of these PhCs, from which we calculated SABET values shape, FE-SEM of the resulting red film revealed that these 20166 https://doi.org/10.1021/jacs.3c06265 J. Am. Chem. Soc. 2023, 145, 20163−20168 Journal of the American Chemical Society pubs.acs.org/JACS Communication urchin-like particles can also self-assemble into an fcc Complete contact information is available at: arrangement, forming a colloidal PhC exhibiting a photonic https://pubs.acs.org/10.1021/jacs.3c06265 bandgap at λc = 664 nm together with a high-energy photonic band feature (Figures 4b−d and S65−S67). Notes In conclusion, we have reported the first-ever example of the The authors declare no competing financial interest. self-assembly of monodisperse COF particles into porous colloidal PhCs. The resulting PhCs exhibit an fcc lattice and corresponding photonic bandgap that can be tuned either by ■ ACKNOWLEDGMENTS This work received funding from Grants PID2021-123265NB- controlling the size of the COF particles or by changing the I00 and PID2021-124814NB-C21 funded by MCIN/AEI/ species adsorbed in the pores of the COF particles. Our 10.13039/501100011033/ and by “ERDF A way of making findings suggest the potential applicability of COF-based PhCs Europe” and the Catalan AGAUR (Project 2021 SGR 00458). as responsive materials or sensors. We believe that this first It was also funded by the CERCA Program/Generalitat de demonstration of the possibility to use COFs to create long- Catalunya. ICN2 is supported by the Severo Ochoa Centres of range-ordered superstructures opens the door to the future Excellence Programme, Grant CEX2021-001214-S, funded by design of new sensing materials (e.g., through the use of COF- MCIN/AEI/10.13039.501100011033. L.M. acknowledges the 10 or COF-DL229 for ammonia and iodine sensing),20 China Scholarship Council for scholarship support. P.M. catalysts, PhCs, and storage systems. acknowledges FPI Grant PRE2019-091452 funded by MCIN/ ■ * ASSOCIATED CONTENT sı Supporting Information AEI/10.13039.501100011033. ■ REFERENCES The Supporting Information is available free of charge at (1) Marlow, F.; Muldarisnur; Sharifi, P.; Brinkmann, R.; Mendive, C. https://pubs.acs.org/doi/10.1021/jacs.3c06265. Opals: Status and Prospects. Angew. Chem., Int. Ed. 2009, 48, 6212− 6233. Detailed synthesis, DLS analysis, FT-IR spectra, FE- (2) (a) Furumi, S.; Fudouzi, H.; Sawada, T. 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