FRAMEWORK MATERIALS

Seeded growth of single-crystaltwo-dimensional covalentorganic frameworksAustin M. Evans1, Lucas R. Parent1,2,3, Nathan C. Flanders1, Ryan P. Bisbey1,4,Edon Vitaku1, Matthew S. Kirschner1, Richard D. Schaller1,5, Lin X. Chen1,6,Nathan C. Gianneschi1,2,3, William R. Dichtel1*

Polymerization of monomers into periodic two-dimensional networks provides structurallyprecise, layered macromolecular sheets that exhibit desirable mechanical, optoelectronic,and molecular transport properties.Two-dimensional covalent organic frameworks(2D COFs) offer broad monomer scope but are generally isolated as powders comprisingaggregated nanometer-scale crystallites.We found that 2D COF formation could becontrolled using a two-step procedure in which monomers are added slowly to preformednanoparticle seeds. The resulting 2D COFs are isolated as single-crystalline, micrometer-sized particles. Transient absorption spectroscopy of the dispersed COF nanoparticlesrevealed improvement in signal quality by two to three orders of magnitude relative topolycrystalline powder samples, and suggests exciton diffusion over longer length scalesthan those obtained through previous approaches.These findings should enable a broadexploration of synthetic 2D polymer structures and properties.

Polymerizing monomers into periodic two-dimensional (2D) networks yields macro-molecular sheets linked by robust covalentbonds (1–4). Such polymers differ from es-tablished linear, branched, or cross-linked

polymer architectures and promise distinct com-binations of properties that emerge from theirdesigned structures and uniform, nanometer-scale pores (4, 5). This promise has been hinderedby the limited materials quality provided by cur-rent 2D polymerization methods (4, 6). Previouswork has resulted in 2D polymers with largecrystalline domains, obtained by crystallizing tri-topic monomers into layered crystals capable oftopochemical photopolymerizations, some ofwhich have been characterized as single-crystalto single-crystal transformations (7, 8). But thestrict geometric requirements of topochemicalpolymerizations and the difficulty of designingmolecules that crystallize appropriately have lim-ited the generality of this approach to just ninemonomers across three design motifs.In contrast, the simultaneous polymerization

and crystallization of monomers into 2D poly-mers known as covalent organic frameworks(COFs) is more general, with more than 200 re-ported examples (9–20). However, 2D COFs typ-

ically form as insoluble, polycrystalline powdersor films with small crystalline domains (typical-ly smaller than 50 nm), which we attribute topoorly controlled nucleation and growth pro-cesses (21–23). Wuest and co-workers reportedthree 3D COFs isolated as large single crystals(24), but these are linked by weak azodioxybonds, are unstable to elevated temperaturesand solvent removal, and have not been general-ized to 2D COF single crystals. Here, we report2D COFs as discrete particles comprising single-crystalline domains with sizes ranging from500 nm to 1.5 mm. They were prepared through atwo-step approach that separates the nucleationand growth processes. These 2D COF sampleshave superior properties and can be studiedmore rigorously than polycrystalline samples,as demonstrated by a transient absorption studythat provides improved signal quality by threeorders of magnitude and offers evidence for ex-citon delocalization at length scales larger thanthe crystalline domains of the powders. These find-ings represent a major advance in 2D COF mate-rials quality thatwill greatly broaden themonomerscope available for accessing well-defined 2Dpolymers.2D COF formation is poorly understood be-

cause effective reaction conditions are identifiedby empirically screening for the formation of thedesiredmaterials as polycrystalline powders. Theappropriate rates of polymerization and exchangeprocesses needed for defect correction are un-known, and we postulate that any attempt toidentify reactions that produce polycrystallinepowders is inherently predisposed to identifyconditions with uncontrolled nucleation. Ourrecent mechanistic studies of boronate ester–linked 2D COF formation revealed a nucleation-elongation process that was interrupted by the

aggregation and precipitation ofmicrocrystallineCOF powders (22). We later found that nitrile-containing cosolvents prevented aggregation andprecipitation, resulting in stable colloidal suspen-sions of 30-nm crystalline 2D COF nanoparticlesthat could be solution-processed into films (25).Here, we enlarged the COF colloids into facetedsingle crystals with lateral dimensions greaterthan 1.5 mm through a second growth step inwhich additional monomers were introducedslowly to suppress further nucleation. This ap-proach is shown to be general for three boronateester–linked COFs, including a pyrene-containingstructure previously shown to be photoconductive(26). These seeded 2D COF polymerizations pro-vide 2D COFs as nonaggregated single crystalsand demonstrate a means to obtain high-quality2D polymers using modular and versatile direc-tional bonding approaches.The solvothermal condensation of 1,4-phenylene-

bis(boronic acid) (PBBA) and 2,3,6,7,10,11-hexahydro-xytriphenylene (HHTP) in a mixture of 1,4-dioxaneand mesitylene (4:1 v/v) affords a boronate ester–linked 2D COF (COF-5), which precipitates withinminutes (22). However, when 80 volume percent(vol %) of CH3CN is included as a cosolvent, astable colloidal suspension of COF-5 nanopar-ticles is obtained instead (25). Having identified2D COF polymerization conditions that preventaggregation and precipitation, we hypothesizedthat newly introduced monomers would add tothe existing nanoparticles rather than nucleatingnew particles under appropriate conditions. In-deed, adding monomers gradually to the reac-tion mixture limited the steady-state monomerconcentration, and the colloids grew withoutforming new particles. In contrast, when themonomers were introduced quickly, their con-centration increased above a critical nucleationconcentration, and the reaction was dominatedby the formation of new particles (Fig. 1).We prepared an initial COF-5 colloidal sus-

pension by heating HHTP and PBBA to 90°C inCH3CN:1,4-dioxane:mesitylene (80:16:4 vol %)for 18 hours. The formation of crystalline COF-5colloids with an average diameter of 30 nm wasconfirmed by dynamic light scattering (DLS) andsynchrotron small-angle and wide-angle x-rayscattering (SAXS/WAXS) experiments. At first,we screened conditions that provide seededgrowth by adding monomers at various concen-trations and rates and monitoring colloid size byDLS. These exploratory experiments were suc-cessful and provided COF-5 nanoparticles thathad grown from 30 nm to 400 nm (see supple-mentary materials) (fig. S2). The 400-nm particleswereused inmore careful growth studies (depictedin Fig. 2, A to C) because they are sufficientlylarge to enable easy differentiation of newlyformed colloids from the preexisting seeds usingDLS and other tools.We observed both seeded growth and new

particle formation regimes by adding monomersat different rates to dispersions of the 400-nmcolloids. A COF-5 colloidal suspension was heatedto 85°C and separate solutions of PBBA (6 mM)and HHTP (4 mM) were simultaneously added

RESEARCH

Evans et al., Science 361, 52–57 (2018) 6 July 2018 1 of 5

1Department of Chemistry, Northwestern University,Evanston, IL 60208, USA. 2Department of Materials Scienceand Engineering, Northwestern University, Evanston, IL60208, USA. 3Department of Biomedical Engineering,Northwestern University, Evanston, IL 60208, USA.4Department of Chemistry and Chemical Biology, BakerLaboratory, Cornell University, Ithaca, NY 14853, USA.5Center for Nanoscale Materials, Argonne NationalLaboratory, Argonne, IL 60439, USA. 6Chemical Sciencesand Engineering Division, Argonne National Laboratory,Argonne, IL 60439, USA.*Corresponding author. Email: wdichtel@northwestern.edu

on April 2, 2020

http://science.sciencem

ag.org/D

ownloaded from

using a syringe pump. The size distributions ofthe COF-5 nanoparticles were monitored by DLSand are plotted as a function of molar equiva-lents of the monomers added relative to thosepresent in the initial colloids (equiv HHTPadded/equiv HHTP in the COF-5 seed solution;Fig. 2). When monomers were added slowly(0.10 equiv hour−1), the average particle sizesteadily increased (Fig. 2A); the size distribu-tion remained monomodal as particle size in-creased (Fig. 2C). The added monomers wereincorporated into the existing colloids, whichgrew from 400 to 1000 nm after addition of 4.0equiv HHTP, and no evidence for new nano-particle formation was observed by DLS (Fig. 2C),x-ray diffraction, or microscopy (see below). Incontrast, when the rate of monomer additionwas increased by a factor of 10 (1.0 equiv hour−1),the average particle size began to decrease afteraddition of 0.5 equiv of monomers (Fig. 2A). Theparticle size distributionwas bimodal with a newpopulation at 50 nm after addition of 0.8 equivHHTP, and this population of smaller particlesdeveloped into the dominant signal in the DLSmeasurements as more monomers were added(Fig. 2C). Control experiments in which the re-action solvent lacking HHTP and PBBA wasadded to the colloids resulted in no change ofthe size distribution of the colloids (fig. S3), indi-cating that the increased particle size observedduring slow monomer addition is not attribut-able to Ostwald ripening or other particle fusionprocesses.These experiments indicate that the faster ad-

dition rate causes themonomer concentration toexceed that needed to nucleate new nanoparticles.As indicated by the point at which the DLS curvesfor the fast and slow addition experiments di-verge, HHTP and PBBA concentrations of ~1mM

represent an upper bound needed for nucleationof new COF-5 particles under these conditions.However, this estimate of the critical nucleationconcentration is likely to be influenced by severalreaction parameters. These experiments demon-strate that COF-5 colloidal suspensions remainstable and available for continued 2D polymer-ization when the monomer concentrations aresufficiently low.Simultaneous WAXS, which characterizes the

crystallinity of the colloids, and SAXS, whichtracks the average particle size, also differentiatethe seeded growth and new particle formationregimes. In the seeded growth regime, the aver-age crystallite size and particle size increase asmonomers are added slowly. In contrast, theaverage crystallite size and particle size decreasewhen monomers are added quickly. The WAXSdata indicate that the initial COF seeds diffractedat q = 0.24 and 0.42 Å−1, corresponding to the100 and 010 Bragg directions of a hexagonallattice with in-plane lattice parameters a = b =29.9 Å (fig. S6). These parameters match wellwith calculated (30.0 Å) and measured (29.7 Å)values previously reported for COF-5 powders(27). When monomers were added slowly, the100 peak intensified and sharpened, and the200 and 210 Bragg peaks became visible at q =0.48 and 0.64 Å−1 after addition of 0.8 equivHHTP. These observations indicate that the sizeof the COF-5 crystalline domains and the crys-tallinity within the domain both increased as themonomers were added to the solution. However,when monomers were added at 1.0 equiv hour−1,the WAXS peaks broadened and decreased inintensity; this observation is consistent with thenucleation of new particles with smaller crys-talline domains. The SAXS results were fullyconsistent with the DLS studies described above,

validating that the WAXS experiments weredone under conditions relevant to each growthregime. The SAXS traces (fig. S13) trended towardhigher intensities at low scattering angles at theslow monomer addition rate (0.10 equiv hour−1),which indicates an increased average particle size.SAXS of solutions with rapid monomer addition(1.0 equiv hour−1) trended to lower intensities atlow scattering angles, consistent with decreasedaverage particle size. Together, these experimentsdemonstrate that the average crystalline domainsize of the COF-5 colloids increases along with theparticle size as monomers are added sufficientlyslowly to suppress nucleation.We demonstrated the generality of the seeded

growth approach for two other boronate ester–linked 2D COFs, COF-10 and TP-COF (Fig. 1),which are synthesized by the condensation ofHHTP with 4,4′-biphenylbis(boronic acid) and2,7-pyrenebis(boronic acid), respectively. Bothsystems exhibited growth behavior similar tothat of COF-5. For each network, an initial col-loidal suspension was generated by heating HHTPand the corresponding boronic acid in a mixtureof CH3CN:1,4-dioxane:mesitylene (80:16:4 vol %)for 18 hours at 90°C. Once the colloids wereformed, separate solutions of HHTP and thediboronic acid were added at 0.10 equiv hour−1

to the 2D COF nanoparticle suspensions. DLSof the reaction solutions showed monomodalparticle size distributions that shifted to largeraverage sizes asmonomers were added. The DLSparticle size of the COF-10 colloids shifted from80 nm to 190 nm after 1.5 equiv HHTP and 2.25equiv 4,4′-biphenylbis(boronic acid) were added(Fig. 2D). The TP-COF particle size (Fig. 2F) in-creased from 400 nm to 750 nm after 1.50 equivHHTP and 2.25 equiv 2,7-pyrenebis(boronic) acidwere added at 0.10 equiv HHTP hour−1. These

Evans et al., Science 361, 52–57 (2018) 6 July 2018 2 of 5

Fig. 1. Schematic of controlled 2D polymerization. A two-step seededgrowth approach provides 2D COFsingle crystals.When HHTP and a linear bis(boronic acid)monomer are condensed in a solventmixture containing CH3CN,crystalline 2D COF nanoparticles are formed as stable colloidal suspensions.

These nanoparticles are enlarged in a second polymerization step in which themonomers are slowly added to the solution. If the monomers are addedmore quickly, their concentration increases above a critical nucleation thresh-old, which leads to uncontrolled nucleation and smaller average particle size.

RESEARCH | REPORTon A

pril 2, 2020

http://science.sciencemag.org/

Dow

nloaded from

experiments suggest that nucleation is suppressedunder these conditions and that the existing COFdomains are enlarged through addition of theadded monomers. Furthermore, nucleation pre-dominated when the monomer solutions wereadded quickly (1.0 equiv hour−1), resulting insmaller COFparticle sizes of approximately 50 nm(figs. S4 and S5).We also used in-solution x-ray diffraction tech-

niques to interrogate the crystallinity of thesecolloidal suspensions. For TP-COF, WAXS peakswere observed at q = 0.19, 0.34, 0.39, and 0.52 Å−1

(Fig. 2G), corresponding to the Bragg directions100, 110, 200, and 210 that were reported in itspowder pattern (fig. S8) (26). Likewise, COF-10colloid diffraction peaks were observed at q =0.20, 0.34, and 0.52 Å−1 (Fig. 2E), which corre-spond to the 100, 110, and 210 Bragg diffractions(fig. S8) (28). These observations are consistentwith those of the seeded growth of COF-5, inwhich the intensity of the diffraction peaksincreased and the width (as judged by their fullwidth at half of themaximum intensity) decreased.Here again, the 200 and 210 diffraction peaksbecame visible after addition of 0.8 equiv HHTPto each initial sample. These observations indi-cate that as monomers are introduced to COFnanoparticles sufficiently slowly, they add to andenlarge existing crystalline domains.The enlarged COF nanoparticles were heated

at 85°C for 14 days and then analyzed by low-dose,high-resolution transmission electron microscopy(HRTEM) to visualize theirmorphology, size, aspectratio, and crystallinity (Fig. 3). TEM imaging ofdiscrete particles was possible for COF-5 andCOF-10, with TP-COF appearing aggregatedwhenprepared on TEM substrates; this precluded closeanalysis of its lattice structure by microscopy (fig.S9). TEM imaging at low magnification revealedthat the COF-5 particles (Fig. 3A) have uniformsix-fold symmetry and hexagonal faceting in pro-jection. Most individual particles were 300 to500 nm in diameter, with some reaching >1 mm.The COF-5 particles were observed at randomorientations by TEM (no preferential orientationwhen drop-cast on TEM substrates), and all

orientations appeared dimensionally isotropic.Lattice-resolution images of individual COF-5particles show that they are single-crystalline(Fig. 3, B to D), as consistent and continuouslattice fringes extend throughout the particles.

For the particle selected in Fig. 3, B to D, which istilted just off a zone axis, the fringe spacing is~10 Å (fitting d210 for COF-5), as measured byfast Fourier transform (FFT; Fig. 3B, inset) andthe left-to-right intensity profile (Fig. 3D) of the

Evans et al., Science 361, 52–57 (2018) 6 July 2018 3 of 5

A

C

D E

F G

BFig. 2. The monomer addition rate definesseeded growth and nucleation regimes.(A) The DLS number-average size of COF-5particles as a function of added monomerequivalents. (B) Wide-angle x-ray scatteringof COF-5 particles as a function of the amountof added monomers at the two monomeraddition rates. (C) DLS number-average sizedistributions obtained at three points shownin (A). (D) DLS number-average size distribu-tions for the initial and final particle sizesof COF-10 during slow monomer addition.(E) WAXS traces of COF-10 particles as a functionof the amount of added monomers. (F) DLSnumber-average size distributions for the initialand final particle sizes of TP-COF duringslow monomer addition. (G) WAXS traces ofTP-COF particles as a function of the amount ofadded monomers.

RESEARCH | REPORTon A

pril 2, 2020

http://science.sciencemag.org/

Dow

nloaded from

particle. The magnified lattice images of verticallyaligned regions in this particle show the contin-uous single-crystalline structure that extendsvertically (parallel with the fringes) from edgeto edge, as shown in several regions of a particlejuxtaposed inFig. 3C. The intensity profile (Fig. 3D)similarly shows that the single-crystalline struc-ture is continuous horizontally (perpendicularto the fringes) from edge to edge.The COF-10 particles (Fig. 3, E to I) were also

highly uniform in size and morphology. Theywere hexagonally faceted in projection yet lackedthe six-fold symmetry of the COF-5 particles;their sizes were predominantly 4 to 5 mm acrosstheir major length and ~3 mm across their minorlength (Fig. 3E). When prepared on TEM sub-strates, the particles preferentially oriented withtheir intersheet stacking dimension normal tothe substrate, which was likely caused by theirz-dimension (particle thickness) being smaller thantheir lateral dimension (fig. S10). Because of thelarge size of these particles (thickness ~0.5 to1 mm), analysis was limited to their edges; manyparticles were too large for high-resolution TEMcharacterization. Therefore, we acquired high-magnification images at various regions of in-terest around the perimeter of a single particlethat was sufficiently thin (Fig. 3, F to H). Lat-tice fringes (Fig. 3, G and H) could not be vis-ually resolved in the images for all regions, but

the FFTs all contained more subtle periodic in-formation, which revealed that all regions wereconsistently crystalline throughout the particle(Fig. 3, H and I). The FFTs of all four regionscontained ~8.3 Å fringe spots (fitting d400 forCOF-10) at the same radial location (Fig. 3I), in-dicating consistency of the crystal structure andcrystal orientation at each of the four regions ofinterest (fig. S11). These observations stronglysuggest that the particle is a single continuouscrystalline domain.Wenote that low-magnificationimages of the COF-10 particles contain very pro-minent diffraction contrast with no detectablegrain boundaries, which is consistent with theparticles being single crystals (Fig. 3E). Collect-ively, the TEM characterization of the individual,single-crystalline COF-5 and COF-10 particles fur-ther indicates that the well-controlled, seededgrowth procedure represents a major advancein COF materials quality and the modular syn-thesis of 2D polymers.The well-dispersed, single-crystalline COF col-

loids enable characterization of emergent elec-tronic properties of these 2D layered assembliesthat were not previously possible in polycrystal-line aggregates. The optical signatures and excitondynamics of the COF-5 colloids were characterizedusing optical transient absorption (TA) spectros-copy measurements. Delocalized excitons andcharge carriers in 2D COFs have been of interest

since the earliest reports of these materials, buttheir small crystalline domain sizes and aggre-gated powder forms have severely limited theircharacterization to qualitative photoconductivitymeasurements (11, 29). Previous TA experimentson 2D COFs were performed either on polycrys-talline samples dispersed in solvent or on poly-crystalline thin films (30–32). Although the firstreported TAmeasurements showed reasonabledata quality, they were inconsistent with morerecent results from the same group on the samesystems, which were noisy and dominated byscattering (30, 32). Similar noise sources werealso observed in our TAmeasurements on COF-5powder samples obtained by solvothermal growth(fig. S12). In comparison, the TA data from ourdispersed COF-5 colloids show signal-to-noiseenhancement by approximately three orders ofmagnitude relative to the powder sample mea-sured under the same experimental conditions(Fig. 4B). The well-defined structures of thesecolloids now make it possible to correlate theiroptical TA spectra to exciton dynamics and decaypathways inherent in the nature of the COF itself.TA spectroscopy was performed on COF-5 col-

loids of varying particle size and as a function ofpump fluence. These spectra were dominated bya broad excited-state absorption that decayedon a time scale of hundreds of picoseconds whileremaining spectrally consistent, as well as a

Evans et al., Science 361, 52–57 (2018) 6 July 2018 4 of 5

Fig. 3. Low-dose HRTEM characterizationof COF single-crystalline particles. Cumulativedose per image: ~25 e– Å–2 s–1. (A and E)Low-magnification images of COF-5 andCOF-10 particles, respectively. (B) Lattice-resolution HRTEM image of a COF-5 particlewith consistent lattice fringes extending acrossthe entire particle. Inset: FFTof the image, croppedat the predominant fringe spacing (~10 Å, d210).(C) Four regions of interest at highermagnification corresponding to the green,teal, red, and magenta boxed regions of theparticle in (B), which are aligned verticallyand parallel with the 10 Å fringes. (D) Intensityprofile plot (left to right across the particle)of the image in (B) after applying a bandpassfilter. The periodicity of the intensity profileis constant and continuous with a periodof ~10 Å. (F) Overview image of one COF-10particle, from which higher-magnificationimages were acquired at four regionsof interest (green, teal, magenta, and red).(G) Lattice-resolution HRTEM image atthe lower right edge of the particle in(F) where 8.3 Å lattice fringes are resolved,corresponding to d400. (H) The HRTEMimage in (G) after applying a Fourier filterto select the central spot and the two 8.3 Åspots in the FFT. (I) The FFTs of the fourhigh-magnification images from the fourcolor-coded regions of interest in (F).

RESEARCH | REPORTon A

pril 2, 2020

http://science.sciencemag.org/

Dow

nloaded from

stimulated emission feature that was longer thanthe time scale of the experiment and was sub-tracted to ensure consistent baselines for the timescale of observation. The identity of this featureis likely related to the singlet excitons of the tri-phenylene cores. However, dynamics clearlydiffered according to the particle size for highphoton fluence (Fig. 4C), with the emergence offaster decays that were not observed at the lowphoton fluences (Fig. 4C, inset). We attribute thishigh fluence, fast exciton decay kinetics, and par-ticle size dependence at the high excitation pho-ton fluences to exciton-exciton annihilation,whichoccurs when multiple excitons within a diffusionlength of each other are excited within the sameparticle sufficiently close to each other so as todiffuse and interact (33–35). At a given numberdensity, excitons in smaller COF crystallitesmorereadily undergo exciton-exciton annihilation be-cause they are confined to a smaller effectivevolume (Fig. 4A). These combined observationsdemonstrate that the optical quality of our COFcolloid nanoparticles enables high-quality spectro-scopic measurements that were previously inac-cessible. The fluence-sensitive exciton decay

dynamics convey size-dependent photophysicalproperties and will lead to an improved under-standing and leveraging of emergent electronicprocesses in these materials.

REFERENCES AND NOTES

1. J. Sakamoto, J. van Heijst, O. Lukin, A. D. Schlüter, Angew.Chem. Int. Ed. 48, 1030–1069 (2009).

2. C. S. Diercks, O. M. Yaghi, Science 355, eaal1585 (2017).3. J. W. Colson, W. R. Dichtel, Nat. Chem. 5, 453–465 (2013).4. R. P. Bisbey, W. R. Dichtel, ACS Cent. Sci. 3, 533–543 (2017).5. M. Servalli, A. D. Schlüter, Annu. Rev. Mater. Res. 47, 361–389

(2017).6. R. Bhola et al., J. Am. Chem. Soc. 135, 14134–14141 (2013).7. P. Kissel, D. J. Murray, W. J. Wulftange, V. J. Catalano,

B. T. King, Nat. Chem. 6, 774–778 (2014).8. M. J. Kory et al., Nat. Chem. 6, 779–784 (2014).9. S. Wan, J. Guo, J. Kim, H. Ihee, D. Jiang, Angew. Chem. Int. Ed.

48, 5439–5442 (2009).10. X. Ding et al., Angew. Chem. Int. Ed. 50, 1289–1293 (2011).11. X. Feng et al., Angew. Chem. Int. Ed. 51, 2618–2622 (2012).12. H. Ding et al., Chem. Eur. J. 20, 14614–14618 (2014).13. S.-Y. Ding et al., J. Am. Chem. Soc. 133, 19816–19822 (2011).14. Y. Zeng et al., J. Am. Chem. Soc. 137, 1020–1023 (2015).15. S. Kandambeth et al., Angew. Chem. Int. Ed. 52, 13052–13056

(2013).16. S. Chandra et al., J. Am. Chem. Soc. 136, 6570–6573 (2014).17. M. Dogru et al., Angew. Chem. Int. Ed. 52, 2920–2924 (2013).18. T. Sick et al., J. Am. Chem. Soc. 140, 2085–2092 (2018).

19. D. Bessinger, L. Ascherl, F. Auras, T. Bein, J. Am. Chem. Soc.139, 12035–12042 (2017).

20. C. R. DeBlase, K. E. Silberstein, T.-T. Truong, H. D. Abruña,W. R. Dichtel, J. Am. Chem. Soc. 135, 16821–16824 (2013).

21. D. D. Medina et al., J. Am. Chem. Soc. 137, 1016–1019 (2015).22. B. J. Smith, W. R. Dichtel, J. Am. Chem. Soc. 136, 8783–8789

(2014).23. B. J. Smith, N. Hwang, A. D. Chavez, J. L. Novotney,

W. R. Dichtel, Chem. Commun. 51, 7532–7535 (2015).24. D Beaudoin, T. Maris, J. D. Wuest, Nat. Chem. 5, 830–834 (2013).25. B. J. Smith et al., ACS Cent. Sci. 3, 58–65 (2017).26. S. Wan, J. Guo, J. Kim, H. Ihee, D. Jiang, Angew. Chem. Int. Ed.

47, 8826–8830 (2008).27. J. W. Colson et al., Science 332, 228–231 (2011).28. C. J. Doonan, D. J. Tranchemontagne, T. G. Glover, J. R. Hunt,

O. M. Yaghi, Nat. Chem. 2, 235–238 (2010).29. E. L. Spitler, W. R. Dichtel, Nat. Chem. 2, 672–677 (2010).30. S. Jin et al., Angew. Chem. Int. Ed. 52, 2017–2021 (2013).31. D. D. Medina et al., ACS Nano 8, 4042–4052 (2014).32. S. Jin et al., J. Am. Chem. Soc. 137, 7817–7827 (2015).33. Y. Tamai, Y. Matsuura, H. Ohkita, H. Benten, S. Ito, J. Phys.

Chem. Lett. 5, 399–403 (2014).34. A. Suna, Phys. Rev. B 1, 1716–1739 (1970).35. D. Sun et al., Nano Lett. 14, 5625–5629 (2014).

ACKNOWLEDGMENTS

We thank J. Bower and the cryo-electron microscopy facilityat UC San Diego for assistance in low-dose HRTEM imaging.Funding: Supported by the Army Research Office for aMultidisciplinary University Research Initiatives (MURI) awardunder grant W911NF-15-1-0447. A.M.E. is supported by theNSF Graduate Research Fellowship under grant DGE-1324585,the Ryan Fellowship, and the Northwestern UniversityInternational Institute for Nanotechnology. This study madeuse of the IMSERC and EPIC at Northwestern University,both of which have received support from the Soft andHybrid Nanotechnology Experimental (SHyNE) Resource(NSF NNCI-1542205 and NSF ECCS1542205, respectively),the State of Illinois, and the International Institute forNanotechnology (IIN). L.R.P. is supported by the NationalInstitute of Biomedical Imaging and Bioengineering underaward F32EB021859. N.C.G. was supported through aMURI through the Army Research Office under awardW911NF-5-1-0568. Portions of this work were performed atthe DuPont-Northwestern-Dow Collaborative Access Team(DND-CAT) located at Sector 5 of the Advanced Photon Source(APS). DND-CAT is supported by Northwestern University,E.I. DuPont de Nemours & Co., and the Dow Chemical Company.This research used resources of the Advanced Photon Sourceand Center for Nanoscale Materials, both U.S. Departmentof Energy (DOE) Office of Science User Facilities operated forthe DOE Office of Science by Argonne National Laboratoryunder contract DE-AC02-06CH11357. N.C.F. and L.X.C. arepartially supported by Basic Energy Science, CBG Division,DOE through Argonne National Laboratory under contractDE-AC02-06CH11357. Resources at the Advanced PhotonSource were funded by NSF under award 0960140. Authorcontributions: A.M.E., N.C.F., R.P.B., E.V., and W.R.D. performedand interpreted the COF growth and structural characterizationexperiments; L.R.P. and N.C.G. performed and interpretedthe TEM experiments; N.C.F., M.S.K., R.D.S., and L.X.C.performed and interpreted the time-resolved spectroscopyexperiments; and all authors wrote and revised the manuscript.Competing interests: None declared. Data and materialsavailability: All data needed to evaluate the conclusions in thepaper are present in the paper or the supplementary materials.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/361/6397/52/suppl/DC1Materials and MethodsSupplementary TextFigs. S1 to S13Tables S1 to S4

18 December 2017; resubmitted 9 March 2018Accepted 3 May 2018Published online 21 June 201810.1126/science.aar7883

Evans et al., Science 361, 52–57 (2018) 6 July 2018 5 of 5

Fig. 4. Exciton diffusion studies of COF-5. (A) Depiction of exciton distribution in COF-5 singlecrystals of different sizes. Exciton interactions at boundaries are highlighted. (B) Transientabsorption spectra (excitation wavelength 360 nm) as a function of indicated probe delay timefor 110-nm COF-5 colloids. (C) Exciton decay kinetics (dots) and fits (lines) of COF-5 colloidsof different sizes at high photon fluence (91 mJ/pulse) observed at 410 nm. Inset: Exciton decaykinetics (dots) and fit (line) at low photon fluence (3 mJ/pulse) observed at 410 nm.

RESEARCH | REPORTon A

pril 2, 2020

http://science.sciencemag.org/

Dow

nloaded from

Seeded growth of single-crystal two-dimensional covalent organic frameworks

Schaller, Lin X. Chen, Nathan C. Gianneschi and William R. DichtelAustin M. Evans, Lucas R. Parent, Nathan C. Flanders, Ryan P. Bisbey, Edon Vitaku, Matthew S. Kirschner, Richard D.

originally published online June 21, 2018DOI: 10.1126/science.aar7883 (6397), 52-57.361Science 

, this issue p. 48, p. 52; see also p. 35Sciencerevealed superior charge transport in these crystallites compared with that observed in conventional powders.crystals by using a solvent that suppresses the nucleation of additional nanoparticles. Transient absorption spectroscopy

linked two-dimensional COFs can be grown into micrometer-scale single−which nanoscale seeds of boronate ester describe a two-step process in et al.the growth of crystals large enough for single-crystal x-ray diffraction studies. Evans

imine linkages (see the Perspective by Navarro). They found that the addition of aniline inhibited nucleation and allowed studied a variety of three-dimensional COFs based onet al.they tend to form powders or amorphous materials. Ma

Covalent organic framework (COF) materials have been difficult to characterize structurally and to exploit becauseCovalent organic frameworks writ large

ARTICLE TOOLS http://science.sciencemag.org/content/361/6397/52

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2018/06/20/science.aar7883.DC1

REFERENCES

http://science.sciencemag.org/content/361/6397/52#BIBLThis article cites 35 articles, 2 of which you can access for free

PERMISSIONS http://www.sciencemag.org/help/reprints-and-permissions

Terms of ServiceUse of this article is subject to the

is a registered trademark of AAAS.ScienceScience, 1200 New York Avenue NW, Washington, DC 20005. The title (print ISSN 0036-8075; online ISSN 1095-9203) is published by the American Association for the Advancement ofScience

Science. No claim to original U.S. Government WorksCopyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of

on April 2, 2020

http://science.sciencem

ag.org/D

ownloaded from