The PRI first proposed in the 1800s, was initially employed to elucidate the spontaneous break-up of a water column into discrete droplets. More generally, it describes the constant-volume transformations of 1D liquids or solids in order to reduce the total surface tension or energy. The PRI phenomenon has now been broadly extended to various artificial 1D viscous fluid systems, including liquids, polymers, inorganic solids, and liquid metals. These instability-driven transformations, which facilitate the formation of uniform droplets, ordered microparticles, and periodic fibrous structures, have found widespread practical applications ranging from water collection systems to sensor technologies. However, the full color-tunable heterostructures driven by the PRI-based on small organic molecules in dilute solutions remain largely unexplored, in which the abundant interfacial interactions can be leveraged to create luminescent 1D barcode-like heterostructures. The spatial stripe patterns are precisely positioned, aligned, and oriented on the 1D substrates. To achieve such striped heterostructures, two key requirements must be met: (i) spontaneous formation and regioselective on-surface crystallization of the separated, well-aligned, and distinguishable stripes and (ii) strong interaction between spatial stripes and 1D templates to ensure effective integration. The former can be realized by the developed PRI at the micro/nanoscale, while the latter demands elaborate selection of luminescent molecules and compatible 1D templates.
With these considerations in mind, naphthalene-based MOFs (Naph-MOFs) were selected as a prominent template for the construction of organic striped super-heterostructures, owing to their 1D assembly characteristics, specific network topology and high-environmental stability. Simulated morphology reveals that Naph-MOFs possess a rod-shaped microstructure with a longitudinal orientation along the [001] direction (Supplementary Fig. 1). The nanoscale pores in Naph-MOFs effectively suppress intermolecular π-π stacking between adjacent naphthalene units that would otherwise preferentially growth along the π-π interactions, which results in the naphthyl planes being prominently exposed on the MOFs surfaces (Fig. 1b and Supplementary Fig. 2). Moreover, the electron-donating naphthalene motifs would attract the external electron-deficient molecules via electrostatic interaction, facilitating the subsequent formation of CT complexes on the surface of the MOFs microstructures.
To form effective CT complexes, 9,10-dicyanoanthracene (DCA) was employed as a building block due to its efficient electron-withdrawing properties. The electrostatic potential (ESP) of DCA reveals that the negative electron density is primarily concentrated on the cyano groups, attributed to the higher electronegativity of nitrogen compared to carbon atoms (Supplementary Fig. 3a). Conversely, the positive charges were mainly occupying the surface of the anthracene rings. In contrast, the naphthyl rings in MOFs present negative charges, while the peripheral H atoms display slightly positive potentials due to carbon's higher electronegativity compared to hydrogen (Supplementary Fig. 3b). As a result, the opposite electrostatic polarities of the two chromophore units enhance the intermolecular recognition and facilitate the formation of new frontier molecular orbitals (FMOs) during the self-assembly process (Supplementary Fig. 4).
The synthesis strategy for constructing heterostructures with well-aligned stripe patterns is schematically illustrated in Fig. 1b-d. First, 1D single-crystal Naph-MOFs microrods were synthesized using a facile solvothermal method, which exhibit solvent stability (Supplementary Fig. 5), facilitating the subsequent on-surface crystallization. Moreover, the length of the MOFs was finely controlled by regulating the concentrations of the capping reagents with the same chemical functionality as the linkers (Supplementary Figs. 6 and 7). Next, these Naph-MOFs microrods were used as substrates for the secondary growth of luminescent DCA bars via PRI self-assembly. As the solvent evaporated, the liquid film on the MOFs surface would break into separated droplets to minimize the total surface tension. Solvent evaporation causes the supersaturation of solute molecules and drives the DCA nucleation on the MOFs surfaces through effective CT recognition interactions. Ultimately, we obtained well-defined and large-scale barcode-like microstructures with recognizable color stripes (Supplementary Figs. 8 and 9).
The as-prepared super-heterostructures display a series of color stripes quasi-periodically distributed along the longitudinal axis of the microrods, indicating the effective deposition of DCA on MOFs surface (Fig. 1e). The assembled stripe patterns exhibit yellow-green emission color under UV radiation. The fluctuating emission intensity along longitudinal direction of the microrods implies that the position-dependent luminescent characteristic (Fig. 1f). Fourier-transform Infrared (FTIR) spectra of striped microcrystals exhibit the symmetric C = O stretching vibrations (ν) and asymmetric C = O stretching vibrations (ν) of the carboxylic group from the host MOFs and C-N stretching vibrations from DCA molecules (Fig. 1g), revealing that DCA units are effectively anchored on the Naph-MOFs. Additionally, the cyano group's stretching mode at 2230 cm in the DCA@MOFs microrods shows a 9 cm high-wavenumber shift compared to pure DCA, indicating strong interfacial interactions between host MOFs and DCA molecules. Scanning electron microscopy (SEM) images of the heterostructures show an almost smooth surface with undetectable stripes (Supplementary Fig. 10). Nevertheless, atomic force microscopy (AFM) images reveal stripe heights of around 20-25 nm, thus yielding nanoscopic molecular thin-stripes (Supplementary Fig. 11). These results combined with the pore size analysis suggest that the DCA molecules form a co-axial belt on-surface of MOFs microstructures (Supplementary Fig. 12).
On the basis of the above experimental observations, we propose a two-stage growth model for the formation of the barcode-like heterostructures (Fig. 2a). During solvent evaporation, a thin 1D liquid column forms on the MOFs microstructures via structural confinement driven by the solid-liquid interface interaction. As evaporation continues, the cumulative cohesive energy becomes insufficient to maintain a uniform film due to interfacial-tension effect. Accordingly, a periodic Plateau-Rayleigh oscillation is developed, which is initially triggered by small perturbations in the liquid film and further transformed into sinusoidal waves. As the curvature radius at the peak (R) is larger than that at the trough (R), the resulting Laplace pressure difference drives the liquid moving from the trough to the peak (Supplementary Fig. 13). Ultimately, the unstable molecular film breaks into a series of droplets at specific locations on the surface of MOFs microrods to minimize the total surface energy (Supplementary Fig. 14).
In the second stage, the continuous solvent evaporation induced the saturation of DCA molecules in the pre-formed droplets. The DCA molecules then regioselectively nucleate at the MOFs interfaces via CT interactions (Fig. 2b). The inter-naphthalene distance (d ≈ 8.27 Å) along the [001] direction closely matches the inter-anthracene spacing (d ≈ 8.39 Å) between adjacent DCA molecules along the [010] direction (Supplementary Table 1 and Supplementary Fig. 15). This near-perfect spatial correspondence strongly facilitates the lattice matching for on-surface crystallization (Fig. 2c and Supplementary Fig. 16). Moreover, the strong CT interactions at the interface, driven by the opposite electrostatic polarities of the two chromophore units, influence nucleation and growth processes. Therefore, the matching crystal structures at the interface, coupled with strong CT interactions, serve as key driving forces for the crystallization growth of DCA color stripes on Naph-MOF surfaces. Consequently, the resulting sedimentary coarse sites with a higher surface energy than undeposited bare regions can serve as nucleation centers to collect extra DCA molecules. Simultaneously, such concave-convex configuration of the non-uniform striped microstructure could generate differential Laplace pressure, driving DCA movement toward the stripe sites. As a result, the molecular units are neatly arranged into well-defined and regioselective striped heterostructures guided by strong CT interactions coupled with the physical driving force (Supplementary Figs. 17-19). Given that the growth process is kinetically controlled, we assume that the imperfect-periodic distribution of the stripes may arise from the gravity-induced uneven liquid film and inhomogeneous assembly environment. This explains the random features of as-fabricated 1D organic barcode-like structures. From an anti-counterfeiting perspective, this imperfect periodic distribution allows stripe patterns to function as security labels with physical unclonable functions, which leverage the stochastic nature of their physical microstructure to generate truly unique, non-reproducible identifiers.
Based on the assembly mechanism discussed above, the number of the color stripes is highly associated with the amounts of separated droplets during the de-wetting processes, which is generally inversely proportional to the thickness of the liquid films. As a result, the spacing of the organic stripe patterns can be relatively controllable through tailoring the self-assembly parameters. In principle, molecular evaporation and diffusion could be accelerated at elevated temperature. Therefore, the critical thickness of the solvent films would be thinner and thus disintegrated into more separated domains to minimize the surface energy (Fig. 2d). Accordingly, we have investigated the temperature-dependent evolution of the barcoding microstructures. The number of stripes increased from the original ≈3 to ≈8 bars as the temperature increased from 20 to 80 °C, accompanying with obvious reduction of the average inter-stripe distance (Fig. 2e, f). The structural scalability of the Ln-MOFs could also enable the isomorphic Eu-MOFs as the templates to construct the stripe patterns, in which the combination of the characteristic Eu luminescence and the organic stripes could strongly enhance the encoding capability of the barcode-like microstructures (Supplementary Fig. 20). Moreover, we have also investigated the influence of MOFs template parameters on the formation of the barcoding microstructures (Supplementary Figs. 21-23). For example, the controlled variation in Naph-MOF aspect ratios from ≈8.2 to ≈11.4 was effectively achieved through precise modulation of the molar ratio of sodium acetate modulators/ligand. Such morphological modulation of the MOFs templates directly translated to enhanced pattern complexity, with the average stripe count increasing from ≈3 to ≈8 distinct bars (Fig. 2g-j).
We also find that the stripe height is strongly correlated with the DCA concentrations, where the average height was increased from ≈10-15 nm to ≈40-50 nm through tuning the DCA concentrations ranging from 0.05 to 1 mM (Supplementary Fig. 24). Such nanoscale height of the stripes prompted us to further explore the size-dependent optical properties (Fig. 2k). Interestingly, the as-prepared stripe patterns exhibit a charming emission-color evolution from initial green to intermediate yellow and further to orange as the concentration increases gradually, exhibiting a special size-dependent characteristics (Fig. 2l, m and Supplementary Figs. 25 and 26). The broadband emission spectra at different wavelengths exhibit different fluorescence lifetimes, implying that these emissive photons arise from distinct excited states (Supplementary Fig. 27). The lower-lying excited states are dominated with increase of the stripe height, which attribute to the extended charge-transfer state. In contrast, the high-energy band in thinner stripes is primarily resulting from the localized exciton state due to the exciton confinement effect. Additionally, the decay process in high-energy band is accelerated with increasing the stripe height (Supplementary Fig. 28), revealing efficient energy transfer from high-energy states to lower-lying excited states (Supplementary Fig. 29).
The characteristic CT interactions between electron-donating and electron-withdrawing components endow the complexes with electron delocalization and adjustable energy levels. Accordingly, abundant emissive colors can be achieved through finely tailoring the CT strengths, thus enabling the fulfillment of full-color stripe patterns. Here, perylene (Pe) molecules with planar π-conjugated skeleton serve as the electron donors aiming to form CT pairs with DCA molecules (Fig. 3a). The highest occupied molecular orbital (HOMO) of the DCA-Pe (-5.24 eV) is close to that of Pe (-5.19 eV), while the lowest unoccupied molecular orbital (LUMO) of DCA-Pe (-3.00 eV) approaches that of DCA (-3.29 eV). These theoretical calculations confirm the CT process occurs from the HOMO of Pe donors to the LUMO of DCA acceptors, facilitating electron cloud rearrangement and formation of new FMOs (Supplementary Fig. 30). The CT complexation narrow down the HOMO-LUMO gap from 3.04 eV (Pe) or 3.11 eV (DCA) to 2.24 eV, which is further validated by the red-shift PL and absorption spectra after doping with Pe (Supplementary Fig. 31). On this basis, the red emissive stripe patterns were effectively acquired through collaborative sedimentation of DCA and Pe via the developed regioselective PRI assisted assembly method (Fig. 3b), wherein energy transfer from the DCA host to DCA-Pe complexes is effectively established (Fig. 3c and Supplementary Figs. 32 and 33).
To further create full-color stripe patterns and enlarge the multivariate information loading, 1,4-dicyanonaphthalene (DCN) molecules with blue-emissive characteristic were selected as the host stripes due to their molecular similarity to DCA and smaller π-conjugated structure (Fig. 3d). The symmetric bonding of two CN groups on DCN's molecular skeleton allows for effective integration with Naph-MOFs templates. Using the PRI assisted assembly strategy, well-aligned blue-emissive stripe patterns were fabricated, where blue-color information was effectively encoded in the well-aligned striped microstructures (Fig. 3e). Furthermore, cyan and green emissive stripe patterns could further be acquired by introducing varying amounts of Pe molecules into the DCN host, thereby creating a characteristic CT transition from HOMO of DCN donor to LUMO of Pe acceptor (Fig. 3f, g and Supplementary Figs. 34 and 35). Through careful modulation of the CT strength between different electron donors and acceptors, full-color stripe patterns covering the entire visible spectra were achieved (Fig. 3e-j and Supplementary Fig. 36). These findings imply that the developed assembly strategy, driven by PRI and CT interactions, can effectively control the growth of periodic stripe patterns.
Patterning spatially resolved heterogeneous stripes with varying emissive colors along 1D templates could carry abundant coding information at micro/nanoscale. Drawing inspiration from the PRI-assisted regioselective on-surface crystallization process, we speculate that if the donor (D) and acceptor (A) units were alternately nucleated and sequentially deposited on the MOF templates, the pre-formed stripes with different widths behave difference in Laplace pressure. This differential pressure would drive solutes movement and result in differential sedimentation (Fig. 3k). Consequently, the spatial arrangement of stripes along the 1D templates would exhibit an inhomogeneous distribution, thereby forming the spatially resolved heterogeneous stripe patterns.
Although conceptually simple, the pre-formed organic color stripes are prone to damage owing to the serious dissolution of existing seeds during subsequent assembly steps. To eliminate the destruction of the existing organic stripes, we adopt n-hexane as the solvents in the second-growth stage, which offers higher solubility for Pe molecules while having lower solubility for DCA molecules (Supplementary Fig. 37). Additionally, its low boiling point and high evaporation rate accelerate crystallization, helping to preserve the integrity of pre-existing stripes. The pre-formed stripes, differing in width or height, generate variations in Laplace pressure, which drive the mass transport of Pe molecules and facilitate targeted nucleation. As a result, multicolor luminescent organic stripes were obtained (Fig. 3l), in which the assembled barcoded stripes exhibit spatially resolvable heterogeneous features, validating the design principles stated above.
Apart from the high encoding capacity and easy readability, the compatibility of the supramolecular materials endows the hetero-stripes with smart responsiveness to external stimuli by incorporating intelligent responsive molecules. This spatially tunable feature is highly desired in applications of advanced information encryption, anti-counterfeiting, and so on (Fig. 4a). As a typical photochromic dye, o-BCB molecules can be functioned as an intelligent material owing to their photoisomerization process towards UV stimuli (Supplementary Fig. 38). With the electron-withdrawing and photochromic characteristics, the o-BCB molecules allow for the effective construction of smart responsive heterogeneous color stripes. Such a barcode-like heterostructure, with multiple responsive sites and diverse color combinations, are expected to enhance the security of anti-counterfeiting labels. Towards this end, the green emissive o-BCB stripes were effectively prepared through the developed PRI-assisted assembly strategy (Supplementary Fig. 39). The spatial emissive colors in each stripes exhibits obvious distinct changes from green to blue under UV stimulation, creating alternating blue-emissive patterns. Such behavior provides a platform for constructing the smart responsive heterogeneous stripe patterns.
To create versatile barcode-like microstructures with site-selective adjustable feature, we create the striped heterostructures featuring multi-color responsive characteristics through multi-step dewetting growth, in which distinct formation behaviors of the DCA-Pe heterogeneous stripe patterns and the DCA-o-BCB heterogeneous stripe patterns might stem from the different intermolecular interaction strengths as driving forces (Supplementary Figs. 40-42). The as-prepared heterostructures display alternating red-green-black pattern under UV excitation. Interestingly, the spatial emissive colors in green block exhibits obvious distinct changes from green to cyan and then to blue under continuous UV stimulation (Fig. 4b, c), finally forming the alternating red-bule-black patterns. Accordingly, the emissive color of stripes in the 1D heterogeneous microrod can be selectively regulated, while the other segments remain almost unchanged. The combination of dynamic and static modules in barcoding structure can simultaneously enhance the capabilities of encoding capacity and hierarchical security levels. The static modules of the PRI-assisted 1D barcode with adjustable color design could improve the encoding capacity to meet normal coding requirements. In scenarios requiring high-security-level labels, the dynamic modules incorporating o-BCB would be launched to meet the high security level requirements. Therefore, through multi-step on-surface crystallization and elaborate component design, smart responsive heterogeneous stripe patterns with multi-responsive colour sites and improved security strength have been achieved. Moreover, the flexibility of the PRI growth strategy enabled realization of the heterostructures onto diverse substrates, including the flexible polymer substrates (Supplementary Figs. 43 and 44). Additionally, the as-prepared stripe patterns display optical and environmental stability, guaranteeing their practical applications (Supplementary Figs. 45 and 46).
The responsive heterogeneous heterostructures provide a platform to achieve two-dimensional (2D) photonic barcodes based on the abundant optical outputs (emission colors) in spatial and temporal dimensions, which is a promising building block for advanced anti-counterfeiting and information encryption applications. As shown in Fig. 4d, the horizontal axis in the 2D barcode represents the sequential emissive channels (O-O) from left to right, while the vertical axis corresponds to red, green, cyan, and blue colors from the top to the bottom. In addition, UV exposure time could be further introduced as the Z-axis to temporally record the color evolution information (e.g., typical UV exposure time of 0, 20, 40 s from the bottom to the top were marked, respectively). Accordingly, distinct 2D barcodes were obtained accompanying with the evolution of the emissive colors under different UV exposure time (Fig. 4e). Finally, the built-up 2D photonic barcodes were effectively achieved by integrating the three subset 2D photonic barcodes, where the gray and white positions are identified as binary values of 1 and 0, respectively. Therefore, abundant distinct encoding information in the responsive heterogeneous stripe patterns could be effectively stored, which strongly promotes the anti-counterfeiting applications in organic integrated photonic devices compared with the existing encoding technologies (Supplementary Table 2).
The responsive heterogeneous stripe patterns provide a platform for achieving advanced anti-counterfeiting applications. The high design reliability and flexibility of the self-assembly technique enabled the construction of organic stripe patterns onto flexible polymer substrates, thus allowing us to tear off and attach the barcodes as the anti-counterfeiting tags onto the watch. As a proof-of-concept demonstration, manufacturers embedded a stimuli-responsive heterogeneous stripe pattern into smartwatches as a covert "security tag" (Fig. 5a). The verification process involves two stages. (1) Initial verification: The tag is exposed to UV light (low-power reading mode, 330-380 nm, ≈31.85 mW cm). If the expected luminescent stripe pattern emerges, the process advances (Fig. 5b). (2) Dynamic validation: The tag is then subjected to continuous high-power UV stimulation (330-380 nm, ≈254.8 mW cm). Distinct 2D barcodes were expected to obtain according to the evolution of the emissive colors under different UV exposure time (Fig. 5c-f). The observers verify if the pattern demonstrates time-dependent color evolution. If the 2D barcode information based on the heterogeneous stripe pattern corresponds to pre-designed coding information, the matching result is authentic. Otherwise, the product is considered counterfeit. The covert 2D barcodes derived from intelligent responsive heterogeneous stripe patterns offer enhanced security features, with potential applications extending to various confidential media, including identity documents and currency.
In summary, we have presented an interface-recognition assembly strategy assisted by a PRI process to regioselectively construct organic stripe patterns directed on 1D MOF templates, where the striped periodicity and height are elaborately controlled through modulating the de-wetting process. More importantly, full-color and heterogeneous stripe patterns were effectively achieved by tuning the CT strength in the organic stripe composites, thus facilitating the realization of high-quality barcode-like structures. Attractively, with the integration of responsive stripes into the stripe patterns, 2D covert photonic barcodes featuring with responsive graphical patterns were achieved based on the light-controlled photochromic reaction in the organic color stripes. Our work provides a comprehensive understanding for the rational construction of 1D organic patterned heterostructures and delicate design of such critical building blocks to enable the emerging technology of nanophotonics and nanoelectronics.