a Synthetic scheme for the preparation of the free-standing flexible COF films. b The chemical structure of Ph2Pe-TAPB-COF, PhPeMa-TAPB-COF, and Ph2Ma-TAPB-COF. c XRD patterns of the COF films and the simulated patterns: AA stacking mode (dark cyan) and AB eclipsed stacking mode (magenta) with the experimental pattern (black) combined Pawley refined pattern (red) and the difference (blue).

The presence of rotationally labile imine bonds and the π-π stacking interactions between the layered conjugated structures can lead to significant emission quenching, attributed to thermal dissipation and aggregation-caused quenching²¹. The low luminescent characteristics and poor film-forming properties of imine-bonded COF hinder their practical application as emissive materials for optoelectronic devices. IPOG strategy has been developed to enhance the luminescent qualities of imine-bonded COFs by integrating different side chains into the linker. This approach effectively suppresses both intramolecular rotation and interlayer π-π interactions (Fig. 2a). As shown in Fig. 2b and Supplementary Fig. S24, the films of Ph2Pe-TAPB-COF, PhPeMa-TAPB-COF, and Ph2Ma-TAPB-COF exhibit continuous and uniform thin films with dimensions of ~20 cm². Under an optical microscope (Supplementary Fig. S25), the observed area appears to have no visible defects or fractures, except for a few wrinkles that are expected in the manipulation of thin films. In general, the hydrophilic and hydrophobic substituted COF films exhibit high integrity and smoothness on a macroscopic scale. Conversely, the film of the non-substituted analogs, Ph-TAPB-COF, displays numerous cracks, indicative of poor film formation. Notably, through the precise control of monomer concentrations and reaction times, the COFs can generate uniform free-standing films with controllable thickness. It was observed that the thickness of the COF films increased in response to elevated monomer concentrations and extended reaction times (Supplementary Fig. S26). Specifically, the thickness of the free-standing films can be effectively regulated by adjusting the monomer concentration. At a constant reaction time, an increase in monomer concentration resulted in a gradual transition of the film from transparent to opaque, indicating a corresponding increase in thickness. Scanning electron microscopy (SEM) was utilized for a more quantitative analysis, confirming a flat surface morphology. Top-view and cross-sectional SEM images illustrate the integrity and continuity of the COF films (Fig. 2c and Supplementary Fig. S27). Cross-sectional views revealed film thicknesses ranging from 100–300 nm (Supplementary Fig. S28), with the ability to adjust film thickness from nanometers to micrometers by modifying monomer concentrations and reaction times. Thus, the IPOG strategy facilitates the production of homogeneous films with considerable areas (around 20 cm²), representing an aspect ratio of 5 orders of magnitude between thickness and length scales. The surface properties of COF films were further characterized using atomic force microscopy (AFM) analysis (Supplementary Fig. S29). The results indicate that the amphiphilic PhPeMa-TAPB-COF films exhibit reduced roughness compared to the Ph2Pe-TAPB-COF and Ph2Ma-TAPB-COF films. This observation suggests that the introduction of amphiphilic chains facilitates the formation of a more uniform film morphology. Furthermore, the hydrophobicity of the synthesized COF films was assessed through water contact angle (CA) measurements. With increasing hydrophilic chains, the contact angle diminishes gradually, indicative of enhanced hydrophilicity (Supplementary Fig. S30). The different wetting behavior predominantly arises from the presence of hydrophilic and hydrophobic chains within the films. Amphiphilic PhPeMa-TAPB-COF films exhibit a significant difference in contact angle between water and oil surfaces, further demonstrating that the incorporation of hydrophilic and hydrophobic chains enhances the preassembly at the water-oil interface.

Fig. 2: Photophysical properties of COF films.

a Schematic diagram of imine-bonded COFs quenching emission and steric enhancing emission. b Photographs of PhPeMa-TAPB-COF film and red emission under 365 nm UV light. c Top-view (left) and cross-sectional view (right) SEM images of PhPeMa-TAPB-COF film. d Normalized PL spectra of Ph2Pe-TAPB-COF, PhPeMa-TAPB-COF, and Ph2Ma-TAPB-COF films. e The optimized crystal structures from side view of Ph-TAPB-COF, Ph2Pe-TAPB-COF, PhPeMa-TAPB-COF, and Ph2Ma-TAPB-COF.

Interestingly, the introduction of hydrophilic and hydrophobic chains not only enhances the properties of COF films but also improves their luminescence characteristics. Upon exposure to 365 nm UV light, free-standing films of PhPeMa-TAPB-COF and Ph2Ma-TAPB-COF exhibited strong red fluorescence emission, in contrast to the absence of fluorescence in Ph-TAPB-COF and Ph2Pe-TAPB-COF (Fig. 2b and Supplementary Fig. S24). The photophysical properties of Ph2Pe-TAPB-COF, PhPeMa-TAPB-COF, and Ph2Ma-TAPB-COF films were systematically investigated using ultraviolet-visible (UV-vis) and photoluminescence (PL) spectroscopy. UV-vis spectra reveal intense absorption bands between 290 nm due to the π-conjugated TAPB group (Supplementary Fig. S31), with PhPeMa-TAPB-COF exhibiting stronger absorption within the 350–450 nm range as compared to Ph2Pe-TAPB-COF and Ph2Ma-TAPB-COF. Notably, the introduction of diverse hydrophilic and hydrophobic chains results in substantial variations in luminescent intensity (Fig. 2d). Compared to the hydrophobic Ph2Pe-TAPB-COF and hydrophilic Ph2Ma-TAPB-COF, the amphiphilic PhPeMa-TAPB-COF exhibits stronger fluorescence emission. PLQY of PhPeMa-TAPB-COF (2.38%) is 60 times greater than that of Ph2Pe-TAPB-COF (0.04%) in solid film (Supplementary Table S1). The layer distance of non-substituted analogs, Ph-TAPB-COF, measures 3.52 Å, while the layer distance of amphiphilic PhPeMa-TAPB-COF extends to 4.01 Å (Fig. 2e). The moderate increase in COF layer distance can be attributed to steric repulsion from hydrophilic and hydrophobic chains. The incorporation of hydrophilic and hydrophobic chains limits the rotational movements of labile imine bonds and inhibits non-radiative transitions. Furthermore, the presence of hydrophilic and hydrophobic chains does not disrupt the interlayer interaction of COFs, aiding in enhancing charge carrier transport. To assess the porosity, N₂ adsorption analyses of the COF thin films were conducted at 77 K. The Brunauer-Emmett-Teller (BET) surface areas calculated for the three thin films were found to be 44 m²/g for Ph2Pe-TAPB-COF, 14 m²/g for PhPeMa-TAPB-COF, and 25 m²/g for Ph2Ma-TAPB-COF (Supplementary Fig. S32). The relatively low specific surface areas suggest that the introduction of hydrophilic and hydrophobic groups in these COFs results in the formation of dense films, thereby enhancing their flexibility. Additionally, the calculations using the Barret-Joyner-Halenda (BJH) method indicate narrow pore size distributions, with pore diameters of 3.7 nm for Ph2Pe-TAPB-COF, 3.4 nm for PhPeMa-TAPB-COF, and 3.3 nm for Ph2Ma-TAPB-COF thin films (Supplementary Fig. S33), which align well with the theoretically predicted pore sizes (about 3.1 nm). Thermogravimetric analysis indicates no weight loss for Ph2Pe-TAPB-COF, PhPeMa-TAPB-COF, and Ph2Ma-TAPB-COF until decomposition at 280 °C, demonstrating high thermal stability (Supplementary Fig. S34). The impressive luminescent qualities, film-forming properties, and thermal stability of the COF films position them as promising candidates for utilization in electroluminescent devices.

To demonstrate the universality of the IPOG strategy, a series of reactions were conducted using various amine and aldehyde monomers to synthesize additional flexible crystalline COF films. Eight additional flexible crystalline two- and three-dimensional COF films were synthesized. As depicted in Fig. 3a and Fig. 3b, all amphiphilic COFs exhibited exceptional film-forming properties and strong luminescent characteristics, indicating that the luminescence enhancement induced by amphiphilic chains is a universally applicable method, regardless of the topology and core skeleton of COFs. In comparison to the amphiphilic COFs, the hydrophilic COFs also demonstrated good film formation and luminescent qualities (Fig. 3c and Fig. 3d). This amphiphilic IPOG strategy is not restricted to two-dimensional COFs but can also be extended to three-dimensional COFs, offering a approach for fabricating luminescent three-dimensional crystalline COF films. Moreover, the incorporation of amphiphilic chains rendered the resulting COF films with high processability and flexibility. For example, amphiphilic COF films could be bent or twisted repeatedly without damage (Supplementary Fig. S35). In contrast, the non-substituted Ph-TAPB-COF (lacking amphiphilic chains) produced through the same process resulted in brittle film formation (Supplementary Fig. S24), indicative of weak mechanical properties. Intriguingly, the free-standing film of PhPeMa-TOB-COF not only exhibited remarkable flexibility but also demonstrated stretchability. The maximum stress and strain at the membrane break of PhPeMa-TOB-COF reached 10.8 MPa and 8.9%, respectively (Supplementary Fig. S36). These findings underscore the pivotal role of amphiphilic chains in enhancing the mechanical properties of free-standing COF films.

Fig. 3: Characterizations of COF films.

A Photographs (a) and PL spectra (b) of amphiphilic luminescent COF (PhPeMa-TTr-COF, PhPeMa-TPA-COF, PhPeMa-TOB-COF, and PhPeMa-Py-COF) films. Photographs (c) and PL spectra (d) of hydrophilic luminescent COF (Ph2Ma-TTr-COF, Ph2Ma-Py-COF, Ph2Ma-TOB-COF, and 3D-Ph2Ma-SpF-COF) films.

To elucidate the proposed mechanism for the formation of COF films utilizing the IPOG strategy (Fig. 4a and Supplementary Fig. S37a), time-dependent studies involving SEM, XRD, and FT-IR analyses were conducted to monitor the conversion into COF films. The interfacial preassembly process is influenced by oil-water interfacial tension and the hydrophilic and hydrophobic interactions. The results from time-dependent SEM studies demonstrate the controlled interface preassembly, oriented growth, and subsequent crystallization of the COFs. As the reaction time increased, the amphiphilic PhPeMa-TAPB-COF was progressively transformed from nanosheets into a uniform film (Fig. 4b). In contrast, the hydrophobic PhPeMa-TAPB-COF underwent preassembly into nanospheres, which subsequently transitioned into relatively rough thin films (Supplementary Fig. S37b). Notably, crystallinity was observed to increase over time, which can be attributed to enhanced π-π stacking during the crystal growth process (Fig. 4c). Further confirmation of the compositions was achieved through Fourier transform infrared (FT-IR) spectroscopy (Fig. 4d). It was noted that as the reaction progressed, the characteristic peaks of the precursor aldehyde and amine began to diminish, while new peaks corresponding to the formation of new bonds emerged. The disappearance of the N-H and aldehyde -C = O stretching bands at 3342 cm^-1 and 1682 cm^-1, respectively, indicates the absence of the starting precursors in the resulting films. The characteristic stretching frequencies for the COF film at 1609 cm^-1 (-C = N) and 1398 cm^-1 (N-Ar) exhibited a gradual increase corresponding to the extended reaction time.

Fig. 4: Proposed mechanism of COF films.

a Proposed mechanism for PhPeMa-TAPB-COF film formation using IPOG strategy. Time-dependent SEM (b), XRD (c), and FT-IR (d) studies provide support for the proposed mechanism.

To explore the potential for electroluminescent (EL) device applications, solution-processed COF-based organic light-emitting diodes (OLEDs) were fabricated with the configuration of ITO/PEDOT:PSS (50 nm)/PTAA:COF (100 nm)/TPBi (45 nm)/LiF (1 nm)/Al (100 nm) (Fig. 5a). As shown in Fig. 5b, all the COF-based OLEDs based on PhPeMa-TAPB-COF, Ph2Ma-TAPB-COF, and PhPeMa-Py-COF as emitting layers exhibited remarkably low turn-on voltages (V_on <3 V). The maximum luminance (L_max) values of the device based on PhPeMa-TAPB-COF, Ph2Ma-TAPB-COF, and PhPeMa-Py-COF are 1424 cd m^-2, 1031 cd m^-2, and 532 cd m^-2, respectively, surpassing the luminance levels of conventional commercial display devices (100 cd m^-2)²². Different batches of devices were prepared based on the three kinds of COFs, and the luminance of the devices remained basically the same in different manufacturing processes (Supplementary Fig. S38), indicating that the COF-based OLEDs have good repeatability. Under the same driving voltage, the current density of the device based on PhPeMa-TAPB-COF is observed to be 5‰ of that of the recently reported IISERP-COF7-based devices²³ and 2‰ of the strontium metal organic framework (MOF)-based devices²⁴, indicating that amphiphilic COFs show more efficient current injection. To the best of our knowledge, this represents the highest performance achieved to date in both MOF-based and COF-based devices (Supplementary Table S2)^25,26,27. The current efficiency of amphiphilic PhPeMa-TAPB-COF devices was measured at 0.91 cd A^-1, nearly double that of hydrophilic Ph2Ma-TAPB-COF devices (Fig. 5c), indicating the significant influence of hydrophilic and hydrophobic chains on the optoelectronic performance of COFs.

Fig. 5: Fabrication and characterizations of COF-based OLEDs.

a The configuration of COF-based OLEDs. b Current density-luminance-voltage (J-L-V) characteristics of COF-based OLEDs. c Current efficiency-voltage curves of COF-based OLEDs. EL spectra (d) and CIE diagram (e) of PhPeMa-TAPB-COF-based OLEDs with different driving voltages. f CCT of COF-based OLEDs with different driving voltage.

Notably, the EL spectra of COF-based OLEDs exhibited significant variation with increasing voltage, transitioning from deep blue to blue and finally to white emission. As the voltage increased, the short-wavelength peak ( ~ 415 nm) gradually diminished, while the long-wavelength peak ( ~ 650 nm) exhibited a corresponding increase (Fig. 5d). A comparison was conducted between the electroluminescence spectra of PhPeMa-TAPB-COF-based OLEDs and the photoluminescence spectra of PTAA and PhPeMa-TAPB-COF (Supplementary Fig. S39). It was observed that the short-wavelength emission aligns with the emission peak of PTAA, whereas the long-wavelength emission corresponds to the emission peak of PhPeMa-TAPB-COF. At high voltages, a greater number of charge carriers are delivered to the PhPeMa-TAPB-COF, resulting in enhanced emission. Additionally, OLEDs based on PhPeMa-TAPB-COF and PhPeMa-Py-COF also displayed exceptional voltage-dependent multicolor emission (Supplementary Fig. S40 and Fig. S41). The Commission International de l’Eclairage (CIE) coordinates indicated dominant blue emission from 4 V to 8 V, transitioning to white emission with further voltage increments (Fig. 5e, Supplementary Fig. S42 and Fig. S43). The CIE coordinates (Supplementary Table S2) for OLEDs using PhPeMa-TAPB-COF, Ph2Ma-TAPB-COF, and PhPeMa-Py-COF were determined to be (0.33, 0.39), (0.34, 0.40), and (0.32, 0.39), respectively, closely aligning with the standard white light coordinates (0.33, 0.33). The voltage-dependent electroluminescent devices offer a straightforward method for producing color-tunable displays. Furthermore, the correlated color temperature (CCT) of these COF-based OLEDs could be adjusted between 4500 K and 10,000 K with varying voltages (Fig. 5f). While possessing the same structure as PhPeMa-TAPB-COF synthesized via IPOG, OLEDs fabricated using sol-PhPeMa-TAPB-COF synthesized through the solvothermal method were found to be ineffective in operation. SEM images revealed that PTAA and PhPeMa-TAPB-COF blending films displayed lower roughness compared to sol-PhPeMa-TAPB-COF blending films (Supplementary Fig. S44). The presence of large nano-bulks in sol-PhPeMa-TAPB-COF films impacted the formation of the interface and electrode layers in the device, leading to operational failures. This highlights the advantageous role of the IPOG strategy in facilitating the formation of a two-dimensional ordered nanosheet structure in COFs for OLED preparation. The microstructure of the blended films was further characterized by AFM, and the COF films synthesized by solvothermal method showed higher roughness (Supplementary Fig. S45). Additionally, COFs synthesized using the IPOG strategy demonstrate promising potential for application in flexible OLED devices due to their structured two-dimensional nanosheets. The flexible COF-based OLEDs were successfully fabricated, showcasing stable electroluminescence performance even under external bending, internal bending, and folding conditions (Supplementary Fig. S46). These flexible OLEDs demonstrated low turn-on voltages (Supplementary Fig. S47) and appropriate current efficiency (Supplementary Fig. S48). Although the full potential of these COFs in flexible OLEDs has not yet been fully explored to optimize performance due to electrode and interface layer mismatches, the results present COFs as a highly promising material for constructing flexible OLEDs, opening up an avenue for future flexible display and lighting applications.

In conclusion, a comprehensive, straightforward, and effective strategy has been devised for fabricating diverse flexible crystalline COF films using the IPOG method for electroluminescent devices. By addressing concerns surrounding thin film production, luminescence, and electronic properties, the potential application of COFs as luminescent materials has been broadened. The maximum luminance of COF-based OLEDs reaches 1424 cd m^-2, representing unprecedented performance achieved by COF-based and MOF-based devices. The development of the flexible OLEDs based on COFs unlocks the application potential of COFs in flexible electronics. Through meticulous adjustment of hydrophilic/hydrophobic chains, conjugated backbones, topological geometries, and linked bonds, it is anticipated that the electroluminescent properties from COFs can be further enhanced to pave the way for high-performance devices. Of particular note, voltage-dependent multicolor OLEDs have been crafted using these luminescent COFs, providing a feasible scheme for the preparation of tunable multicolor display devices. Our study shed light on the design and development of next-generation flexible crystalline materials to combine excellent mechanical flexibility with high optoelectronic performance for flexible electronics.