Inkjet-Printed Photoluminescent Patterns of Aggregation-Induced

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Inkjet-Printed Photoluminescent Patterns of Aggregation-InducedEmission Chromophores on Surface-Anchored Metal−Organic Frameworks Nicolò Baroni,† Andrey Turshatov,† Michael Adams,† Ekaterina A. Dolgopolova,‡ Stefan Schlisske,§,∥ Gerardo Hernandez-Sosa,§,∥ Christof Wöll,⊥ Natalia B. Shustova,‡ Bryce S. Richards,†,∥ and Ian A. Howard*,†,∥ Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 20, 2018 at 23:06:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Institute of Microstructure Technology (IMT) and ⊥Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡ Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States § Light Technology Institute (LTI), Karlsruhe Institute of Technology, Engesserstrasse 13, 76131 Karlsruhe, Germany ∥ InnovationLab, Speyererstr. 4, 69115 Heidelberg, Germany S Supporting Information *

ABSTRACT: Organic chromophores that exhibit aggregationinduced emission (AIE) are of interest for applications in displays, lighting, and sensing, because they can maintain efficient emission at high molecular concentrations in the solid state. Such advantages over conventional chromophores could allow thinner conversion layers of AIE chromophores to be realized, with benefits in terms of the efficiency of the optical outcoupling, thermal management, and response times. However, it is difficult to create large-area optical quality thin films of efficiently performing AIE chromophores. Here, we demonstrate that this can be achieved by using a surface-anchored metal−organic framework (SURMOF) thin film coating as a host substrate, into which the tetraphenylethylene (TPE)-based AIE chromophore can be printed. We demonstrate that the SURMOF constrains the AIE-chromophore molecular conformation, affording efficient performance even at low loading densities in the SURMOF. As the loading density of the AIE chromophore in the SURMOF is increased, its absorption and emission spectra are tuned due to increased interaction between AIE molecules, but the high photoluminescent quantum yield (PLQY = 50% for this AIE chromophore) is maintained. Lastly, we demonstrate that patterns of the AIE chromophore with 70 μm feature sizes can be easily created by inkjet printing onto the SURMOF substrate. These results foreshadow novel possibilities for the creation of patterned phosphor thin films utilizing AIE chromophores for display or lighting applications. KEYWORDS: metal−organic frameworks, aggregation-induced emission, inkjet printing, photoluminescence quantum yield

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INTRODUCTION Typically, when organic chromophores come into close proximity with one another, the internal photoluminescence quantum yield (i-PLQY) of the chromophore decreases, for example, in concentrated solutions, in solutions of a poor solvent, or in the solid state.1 The i-PLQY is defined as the number of photons emitted by a material divided by the number of absorbed photons or equivalently the probability of a photon being emitted per absorbed photon. The drastic decrease in i-PLQY when molecules are in close proximity is usually caused by an increase in nonradiative rates in interacting molecules and/or by an increase in energy transfer to sites with fast nonradiative decay channels.2 There are two notable exceptions wherein the i-PLQY does not decrease when the chromophores come into close proximity. The first exception is found in some chromophores that exhibit J-type aggregation. In such cases, the radiative rate in the aggregate © XXXX American Chemical Society

state can be faster than that in the momoner, so-called superradiance.3 This increased radiative rate can compensate for increased nonradiative rates and result in aggregate states with high i-PLQYs.2,4 A second exception is observed in molecules that exhibit aggregation-induced emission (AIE).1,5 In the case of AIE, the chromophores in solution have rotational and vibrational modes that allow the excited state to return rapidly and nonradiatively to the ground state. However, in the solid state, the molecular environment constrains the rotational and vibrational freedom of AIE chromophores, thus increasing the i-PLQY. AIE chromophores need a constrained molecular environment for their emission to turn on.1 A prototypical AIE Received: April 11, 2018 Accepted: July 9, 2018

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DOI: 10.1021/acsami.8b05568 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces chromophore is tetraphenylethylene (TPE). For this compound, photoluminescence of single, solvated molecules is weak, while crystallization results in bright blue photoluminescence (PL). The higher yield for photon emission has been ascribed to the restriction of intramolecular rotation and vibration modes of the aromatic rotors (phenyl rings) in the solid state.6,7 This molecular class has been thoroughly investigated. Recently, intramolecular rotations and vibrations have been restricted in isolated TPE molecules in solution in order to study whether this would turn on the PL. The restriction of intramolecular rotation was achieved through the derivatization of individual molecules with short tethering olefinic double bonds between the phenyl units that restrict their rotation even in isolated molecules.8 The addition of these tethering olefinic double bonds lead to efficient PL of the isolated molecules (an i-PLQY approaching unity being realized in dilute solution).8 This demonstration of how PL can be switched on for individual molecules provides strong experimental support for the hypothesis that the mechanism responsible for AIE is the restriction of intramolecular motion.8 Theoretical work also supports this explanation, suggesting that ultrafast nonradiative relaxation occurs either through a conical intersection between the excited and ground state or through photocyclization, with both pathways being gated by molecular vibrations.9 There is also a rich body of work exploring the formation of larger molecules and supramolecular assemblies of TPE in order to provide new avenues to constrain the AIE chromophore’s vibrational/rotational behavior and therefore tune the PL of the material. For example, Yan et al. synthesized metallacages that resulted in formation of constrained dimers of a TPE-based chromophore.10 In this case, even the solvated molecules (in the good solvent CH2Cl2) showed PL due to the constraints imposed on the TPE within the individual dimers. This PL stemming from individual dimers has an emission spectrum with a maximum at 2.25 eV (550 nm, λex = 365 nm). Dynamic light scattering measurements of the 10 μM dichloromethane solution showed that the individual dimers were well separated; the average hydrodynamic diameter was 4.18 nm.10 When a solvent mixture of dichloromethane and hexane was used to dissolve the metallacages, the hydrodynamic radius increased with increasing hexane fraction, demonstrating that metallacages aggregated into larger clusters with higher hexane fractions.10 As the hexane fraction increased and many metallacages came together to form larger aggregates, the emission peak blue-shifted to 2.5 eV (495 nm, λex = 365 nm).10 These results indicate that AIE is already present in the individual metallacage but also that further aggregation can tune the nature of the emitting state (the latter point is evidenced by the changing emission wavelength).10 We will revisit these results later, as we observe very similar shifts in emission while increasing the density of a TPE-based chromophore in surface-anchored metal−organic frameworks (SURMOFs). We also note in this context the recent work of Peng et al. wherein they observed a difference in the emission spectrum between two isomers of a TPE-based chromophores that form different supramolecular assemblies.11 TPE-based linkers have also been used in metal organic frameworks (MOFs) and covalent-organic frameworks (COFs), wherein the PL properties are also altered through the implementation of rigid constrains on the TPE core caused by its covalent bonding into the network.12−17 Such novel TPE-based materials have been investigated for a variety of applications,4

but most advanced is the utilization of AIE chromophores for sensing applications, wherein analytes are detected by the turnon of the AIE fluorescence. For instance, some examples of sensing include the detection of dissolved carbon dioxide,18 amyloid protein fibrillation,19 nitroaromatics,20 and mercury.11 Inspired by these supramolecular approaches, we focus on developing a thin film technology for easily depositing optical quality luminescent thin films over a large area that exploits AIE chromophores. We present a new method of printing a TPE-based chromophore into the constrained molecular environment of a prefabricated SURMOF.21 We grew the SURMOFs on a modified quartz substrate, creating a uniform thin film over a large area by spray-coating.21 The AIE chromophore (which can act as a Lewis base) can then be easily loaded into the already fabricated SURMOF thin films by drop-casting or inkjet printing, resulting in efficient largearea thin films of the AIE chromophore. The fabrication and loading process builds on our previous work, wherein it was demonstrated that it was possible to easily load porphyrinbased chromophores that exhibit Lewis-base functionality between the sheets of a SURMOF-2 structure by postsynthetic drop-casting.22 In the present study, we demonstrate that 150 nm thick SURMOF films can be loaded with high concentrations of AIE chromophores (up to 2.55 mmol/ cm3), creating optical quality thin films that allow significant fractions of excitation light to be absorbed (30%) while maintaining a high i-PLQY (around 50%). The strong absorption and high i-PLQY allows excellent external photoluminescence quantum yields (e-PLQY, defined as the ratio of emitted photons to incident photons) to be reached. Furthermore, we demonstrate two approaches for creating patterned films of the AIE chromophore. In the first approach, the SURMOF was patterned by spray deposition through a mask. Although the feature quality is rough, this approach reinforces the role the SURMOF plays in taking up the AIE chromophore (no chromophores are present on areas of the sample on which no SURMOF was deposited). The second approach, inkjet printing the AIE chromophore onto the prefabricated SURMOF substrate, leads to facile creation of features down to 70 μm, with further reduction in feature size likely possible upon further optimization. Altogether, this work demonstrates the use of a SURMOF for creating efficient patterned thin films of AIE chromophores and suggests new avenues for the implementation of MOFs and AIE in display and illumination applications.23,24

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EXPERIMENTAL SECTION

Substrate Preparation. Quartz substrates were used for the preparation of SURMOF samples because of their high transparency to ultraviolet (UV) light. First, the quartz samples were cleaned by immersion into an ultrasonic acetone bath for 10 min. Subsequently, the quartz surfaces were treated with an oxygen plasma (4-TEC plasma etcher) in order to (i) remove the organic surface contaminants and (ii) increase the number of hydroxyl groups (OH−) present on the surface that act as anchoring points for SURMOF growth.25 SURMOF Deposition. SURMOFs were deposited by spraycoating, which is one of the fastest SURMOF deposition methods that gives good results in terms of crystallinity and allows SURMOF deposition over a large area.21 In this case, monolithic MOF thin films were grown by repeated cycles of spraying of metal ion solution (1 mM Zn(Ac)2 in ethanol), a pure ethanol rinse, a linker solution (0.2 mM terephthalic acid (H2BDC) in ethanol), and a pure ethanol rinse. The SURMOF thickness could be controlled by varying the number B


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Figure 1. (a) Chemical structures of the zinc paddle-wheel metal node (red square) and terephthalic acid linker (blue line) used to create the ZnBDC structure as well as the tetraphenylethylene-based AIE chromophore (H4ETTC, green bow tie) that is loaded into the SURMOF. (b) The process of postfabrication SURMOF loading by drop-casting with the AIE chromophore. A SURMOF is fabricated (and optionally patterned) on a substrate. Following this, a solution of the chromophore in ethanol is drop-cast onto the SURMOF/substrate. The solution is allowed to dry, and then, the surface is briefly rinsed with ethanol to remove chromophores not loaded into the SURMOF. In order to vary the concentration of the AIE chromophore in the SURMOF, differing volumes of the chromophore solution can be drop-cast. (c) A uniform SURMOF thin film loaded with the AIE chromophore exhibiting bright emission under UV light. (d) A patterned SURMOF film with a “K”-shape loaded with AIE chromophore under UV excitation. No emission is seen outside the area of the SURMOF, indicating that only AIE chromophores that are taken up as guests in the SURMOF remain on the sample. (e) Magnified view of structured SURMOF loaded with AIE chromophore shown in (d) taken under UV illumination in an optical microscope. of spraying cycles. In this work, 30 cycles were sprayed resulting in a SURMOF thickness of around 150 nm. In order to fabricate patterned SURMOFs, the spray-coating was done through a mask cut in a plastic foil that was clipped to the front surface of the substrate. The sample structures were characterized using X-ray diffraction (XRD) using both out-of-plane and in-plane geometries (Figures S1 and S2). Synthesis of 4′,4‴,4⁗′,4⁗⁗-(Ethene-1,1,2,2-tetrayl)tetrakis(([1,1′-biphenyl]-3-carboxylic acid)) (H 4 ETTC). H4ETTC was synthesized following the route presented by Wei et al.15 The prepared compound and its precursors were characterized via proton nuclear magnetic resonance (1H NMR) spectroscopy (Figures S3 and S4). Postsynthetic Loading of SURMOF Films with AIE Chromophore: Drop-Casting. On the basis of the previously reported chromophore-loading procedure,22 we drop-cast various volumes (from 0.2 to 4.5 mL) of 0.1 mM solution in ethanol of the AIE chromophore containing Lewis-base moieties (H4ETTC) onto the SURMOF-coated substrate. The solution was spread over the surface of the substrate and allowed to completely dry. During drying, chromophore molecules diffuse from the solution and intercalate between the SURMOF sheets, as previously demonstrated using timeof-flight secondary ion mass spectrometry for three different types of porphyrins.22 Depending on the volume of the solution deposited on the substrate, different amounts of chromophore diffused into the SURMOF, with larger volumes of solution leading to higher concentrations of chromophore in the SURMOF. After the deposited solution was completely dry, the sample was rinsed in 20 mL of flowing ethanol for 30 s. This removes all chromophore molecules that are surface adsorbed or coated onto areas of the blank substrate where no SURMOF was present (in the case of a patterned SURMOF). The concentration of the AIE chromophore in the SURMOF was determined using UV−visible spectroscopy of the digested samples as discussed below. Postsynthetic Loading of SURMOF Films with AIE Chromophore: Inkjet Printing. To adjust the viscosity and jetability of the ink, 30% vol. 2-butoxyethanol (SigmaAldrich) was added to the above-described 0.1 mM stock solution. All structures were printed with a Dimatix DMP-2831 inkjet printer using a Fujifilm Dimatix 10 pl cartridge at a maximum jetting frequency of 1 kHz with a custom designed waveform and a drop spacing of 25 μm at ambient

conditions. Both the printhead and substrate temperatures were kept at 25 °C. The inkjet printing experiments were executed under cleanroom conditions (20 °C, 50% RH). The printed substrates were transferred to a vacuum oven and dried for 10 min at 15 mbar. Chromophore Density Determination. In order to determine the AIE-chromophore concentration in the SURMOFs, the AIEchromophore-loaded SURMOFs were digested using 1 mL of 0.1 M ethylenediaminetetraacetic acid (EDTA) solution in water. In order to measure the dye concentration, the absorbance of these solutions was compared to the absorbance of standard solutions prepared by dissolving an unloaded SURMOF and then adding a known amount of dye. Three separate amounts of dye were added to create a standard series, via which the molar extinction coefficient of the AIE chromophore in this solution could be estimated (Figure S5). This allowed the number of moles of dye in each digested AIE-loaded SURMOF to be determined. The volume of the SURMOF was estimated by taking the area of the glass (measured by weighing the glass slide and knowing the thickness and density) and multiplying it by the thickness of the SURMOF. The SURMOF thickness of all the samples has been considered constant over the entire sample area. i-PLQY and e-PLQY Determination. The PLQYs were determined using the three-measurement method described by Mello et al.26 The samples were placed inside an integrating sphere of 150 mm and excited by a LED with a peak emission of 375 nm. Optical Characterization. The absorption spectra were recorded with a spectrophotometer (PerkinElmer Lambda 950) in the transmittance mode. The emission spectra were measured with a spectrofluorometer (Varian Cary Eclipse) using an excitation wavelength of 350 nm. The absolute i-PLQY was measured as described in the literature, 26,27 using an integrating sphere (Sphereoptics) with a 150 mm diameter, a fiber-coupled spectrometer (Avantes AvaSpec-HERO), and a UV light emitting diode (LED, 370 nm peak emission). In order to measure the i-PLQY of the powder, a small volume of powder was pressed between two microscope coverslips, yielding a circle of powder around 1 cm in diameter. Timeresolved PL data were measured using a streak camera (Hamamatsu Universal Streak Camera C10910) using pulsed excitation at 375 nm from the frequency-doubled output of a titanium:sapphire oscillator (Coherent Chameleon). C


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ACS Applied Materials & Interfaces Film Thickness Determination. The thicknesses of the samples were measured and established with a profilometer (Dektak 220-Si, Veeco). The samples were scratched, and the depth of the film was established by scanning over the scratch and analyzing on the machine.

magnified view taken with an optical microscope under UV excitation (Figure 1e). Having established that AIE chromophores can be infiltrated into the SURMOF in a straightforward fashion, we now turn to examine how the density of AIE chromophores in the SURMOF can be varied and how the photophysical properties of the H4ETTC@SURMOF depend on the AIE-chromophore concentration. In order to vary the concentration of the H4ETTC inside the SURMOF, we drop-cast different volumes of 0.1 mM ethanol solutions onto a series of samples. For every sample, we rinsed the surface with ethanol to remove the nonintercalated AIE chromophores. Figure 2a shows a series of

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RESULTS AND DISCUSSION The procedure for preparation of the AIE-chromophoreloaded SURMOF thin films is illustrated in Figure 1 alongside optical images of the resulting thin films luminescing under UV excitation. The chemical structures of the zinc-based paddlewheel metal node, the linker (benzene-1,4-dicarboxylic acid, H2BDC) used for the synthesis of the ZnBDC SURMOF, and H4ETTC (the TPE-based AIE chromophore loaded inside the SURMOF scaffold) are shown in Figure 1a. In Figure 1b, the 2D sheet-like structure of the SURMOF is illustrated. Within the sheets, the distance between the metal centers established by XRD corresponds to the length of the organic linkers (1.1 nm). The separation between the stacked sheets is shorter (0.6 nm). Given these geometric constraints, in the loading process, the H4ETTC molecule likely intercalates in the 0.6 nm thick planar void between the 2D SURMOF sheets. Such intercalation is expected to result in restricting the rotational and vibrational modes of H4ETTC, in analogy to the situation in the H4ETTC solid. We explain the efficient PL shown in Figure 1c−e to such packing effects. The process of postsynthetic H4ETTC loading into the SURMOF film by drop-casting is schematically shown in Figure 1b. After drop-casting a given volume of a 0.1 mM solution of H4ETTC in ethanol onto the ZnBDC SURMOF, the solvent is allowed to evaporate. This process leads to a diffusion of the chromophores into the porous SURMOF but also leaves a thin coating of solid H4ETTC covering the SURMOFs. In the case of the patterned SURMOFs, this coating will also be present on the blank areas of the substrate, between the SURMOF patches. As a final step, the sample is rinsed with ethanol, washing away the chromophore coatings on the surface of the SURMOF and on the blank areas of the substrate. The successful removal of the H4ETTC coating by rinsing was evidenced by the absence of the characteristic emission spectrum of H4ETTC solids after rinsing. Note that the chromophore embedded into the SURMOF could not be removed by rinsing with ethanol or a prolonged immersion into ethanol. In fact, we had to disassemble the SURMOF by immersion into an EDTA solution to recover the guest chromophores from the porous MOF materials. Figure 1c depicts an image of the PL under UV excitation of a square substrate fully coated with a ZnBDC SURMOF that has been loaded with H4ETTC. In contrast to the nonluminescent solution, H4ETTC intercalated into the SURMOF shows a strong PL under a UV lamp (365 nm). Furthermore, in Figure 1d, we show PL of a patterned SURMOF substrate, wherein the SURMOF was spray-cast through a mask with a “K”-shape. It is possible to see how the AIE chromophore is retained only where the SURMOF is present after it was dropcast over the entire surface of the glass then rinsed. This demonstrates that the chromophore is loaded inside the SURMOF but not retained on the uncoated quartz surface. Chromophore molecules deposited on directly on the quartz where there is no SURMOF are washed away during the rinsing procedure. This is further shown in the abrupt disappearance of the PL outside the SURMOF region in the

Figure 2. (a) Photographs showing under UV light the PL of SURMOF substrates loaded with increasing (left-to-right) concentrations of AIE chromophore. The concentration of the AIE chromophore is controlled by varying the volume of solution dropcast onto the substrate. On the far left, the AIE chromophore is shown in solution, and on the far right, the AIE chromophore is shown as powder. (b) Values of dye density in mmol/cm3 for the different samples, showing that increasing the drop-casted solution increases the dye density inside the SURMOF. (c) The internal and external PLQY of the AIE-loaded SURMOFs compared with the AIE chromophore in solution and in the solid-state powder form. The iPLQY of the AIE-loaded SURMOFs rivals that of the AIE powder for a wide variety of loading concentrations within the SURMOF. In green, the percentage of absorbed light in the ZnBDC samples loaded with different densities of AIE chromophores. The absorbed light is increasing with the increasing of drop-casted solution until it reached a plateau, when no more dyes can be loaded inside the SURMOF scaffold.

six samples produced by drop-casting an increasing volume (0.2, 0.8, 1.2, 2.5, 3.5, and 4.5 mL) of solution onto the substrate. In order to determine the concentration of the chromophore in the SURMOFs, we digested the SURMOFs using EDTA to recover the AIE chromophore into solution. Comparing the optical absorption in these recovered chromophore solutions with a set of standards, we were able to determine the amount of chromophore intercalated for each volume of solution deposited. By taking the area of the glass substrate and the height of the SURMOF film, we could D


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LED emission spectrum for high loading densities (as visible in the absorption curve in Figure 2b). Figure 3a illustrates the change in absorbance and emission spectra for the varying concentrations of AIE chromophore

calculate the volume of SURMOF for each sample. The corresponding concentrations amounted to 0.2, 0.3, 0.6, 1.0, 1.8, and 2.6 mmol/cm3 for the samples fabricated by dropcasting 0.2, 0.8, 1.2, 2.5, 3.5, and 4.5 mL of solution, respectively. We note that prolonged soaking of an AIE-chromophoreloaded sample in ethanol led to a negligible change in absorption of the film and a negligible amount of AIE chromophore in the ethanol. This fact demonstrates that the intercalation energy of H4ETTC inside the SURMOF pores is considerably larger than the solvation energy in ethanol. In order to extract the chromophores from the SURMOF, it was necessary to dissolve the molecular framework in a solution containing EDTA. Figure 2 shows the comparison in terms of i-PLQY, absorption, and e-PLQY of the samples with different concentrations of AIE chromophores. Solution and powder values are also shown for reference as well as optical images of all samples under UV excitation. The SURMOF samples are labeled on the x-axis according to the loading density of the AIE chromophore relative to the highest loading, for example, with 0.07 AIE@ZnBDC indicating that the concentration of the AIE chromophore in this sample was 7% of the concentration in the most densely loaded sample. Analyzing the data presented in Figure 2, we see that in solution, the AIE chromophore has a very low PLQY, with iPLQY = 2.0% and e-PLQY = 0.4% (measurements were performed in a 1 cm thick cuvette). On the other hand, the SURMOF with the lowest concentration of AIE chromophore (0.07 AIE@ZnBDC) exhibits an i-PLQY of 40.2% and ePLQY of 2.6% (excitation provided with a UV LED 370 nm, 3.35 eV). Thus, we have a 20-fold increase in i-PLQY. The smaller increase of the e-PLQY is attributed to the moderate extinction of the excitation light in the film. Increasing the chromophore concentration inside the SURMOF (from 0.2 up to 1.0 mmol/cm3) slightly enhances the i-PLQY (25% relative increase) but drastically increases the e-PLQY (650% relative increase). This observation indicates that the constraint of the AIE chromophore inside the SURMOF effectively turns on the emission of individual chromophores. As the chromophore concentration is increased, the individual chromophores become slightly more efficient. We explain this observation by the additional steric hindrance created by the interactions between adjacent AIE chromophores at higher densities. However, the e-PLQY increases rapidly as the absorption is increased because of the denser loading of the chromophore. We note that in this wavelength regime, the BDC SURMOF linkers do not show any absorption and thus do not interfere with the luminescence of the guest chromophores (see Figure S5). These e-PLQY values are directly related to the excitation and emission from the guest AIE dye without any direct contribution from the host molecular framework. Further increasing the e-PLQY by using appropriately absorbing linkers in the SURMOF and enabling energy transfer from the host lattice to the embedded chromophores28,29 is the subject of ongoing work. The decrease in the e-PLQY for the highest density loading of AIE chromophores will be discussed in the next section; we will find it is due to a shift of the absorption band of the AIE chromophore at high densities. This shift reduces the fraction of photons from the excitation LED absorbed because of a poorer match between the AIE absorption spectrum and the

Figure 3. (a) Normalized PL and absorbance spectra of the series of SURMOF films loaded with increasing amounts of AIE chromophore compared with the properties in solution and solid state (powder). (b) The peak emission energy as a function of amount of chromophore loaded into the SURMOF. The emission peak first shifts toward lower energy until it resembles the solid-state-like peak, and then, at further increased loading, the peak shifts back to higher energy (bluer color) now resembling the emission in solution but maintaining the high i-PLQY. (c) The shift in energy between the half-rise of the absorbance and the peak of the emission energy as a function of the amount of AIE chromophore loaded into the SURMOF. This metric of the amount of relaxation in energy between the absorbing and emitting states steadily decreases as the concentration of chromophore in the SURMOF increases, indicating that the chromophores become increasingly constrained.

inside the SURMOFs. Increasing the concentration of the chromophore inside the SURMOF leads to the red-shift of absorbance. Along with the slight increase in i-PLQY with AIE density, this shift to lower wavelengths indicates that at higher loadings, the constraints cannot result only from host−guest interactions but must involve direct chromophore−chromophore contact. The peaks of the chromophore absorption spectra in the SURMOFs match well to vibronic peaks visible in the spectrum of the molecule in solution. The relative strength of the lower energy vibronic peaks show a strong increase with higher concentrations of the molecule in the E


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when moving from solution into the SURMOF and then steadily decreases within the SURMOF samples as the concentration of loaded chromophores is increased. As the energy transfer efficiency would increase with increasing concentration and into the solid state, if inhomogeneous broadening and energy transfer dictated the energy difference between the absorption and emission onset, then the energy difference should increase as the AIE dyes become more densely loaded. As this is not the casethe energy gap actually decreaseswe can conclude that the first mechanism, reorganization in geometry between the ground- and excitedstate, must dominate the Stokes shift. Therefore, the decreasing shift between absorption and emission with increased loading again demonstrates that the molecules must be effectively stiffened as their concentration increases in the SURMOF and upon moving to the powder form. This evidence of effective stiffening of the molecules with increasing concentration as a result of increased steric interaction supports our interpretation of the absorption data discussed above and the similar conclusion drawn from that analysis. The optical data presented in Figure 3 provides conclusive evidence that the AIE chromophore is intercalated inside the SURMOF structure rather than adsorbed onto its surface. If the chromophore were just adsorbed in the form of a dense coating covering the SURMOF, its properties should correspond to that of the solid-state powder. Although the iPLQY value is indeed close to that of the solid-state powder (demonstrating that the SURMOF-loading approach is promising for making highly emissive thin films), the absorption and emission spectra of the highly loaded SURMOF samples show characteristic differences to the powder (solid) form of H4ETTC. This observation again confirms that the chromophores are held in distinct molecular conformations inside the SURMOF pores and are not simply aggregated in their normal crystal form. To further illustrate this point of the difference between the nonintegrated chromophores in the solid state and the H4ETTC@ SURMOF heterosystem, we inspect the timeresolved PL measurements displayed in Figure 4. The blue and gray lines in the PL decay plots correspond to the AIE chromophore in solution and nonintegrated solid state, respectively. The PL lifetime in solution (monoexponential decay with τ = 150 ps) is substantially shorter than in the solid state (biexponential decay with τ1 = 139 ps and τ2 = 614 ps). The lifetimes of the lowest and highest concentrations of AIE chromophore inside the SURMOF are shown by the brown (biexponential decay with τ1 = 124 ps and τ2 = 630 ps) and pink lines (biexponential decay with τ1 = 93 ps and τ2 = 375 ps), respectively. At the lowest concentration, the PL lifetime is similar to that in the nonintegrated solid state but varies strongly when the number of embedded chromophores is increased. It is also at this lowest concentration that the PL most closely matches that in the solid state. When the SURMOF is loaded with a higher concentration of the AIE chromophore, the lifetime shortens and approaches the value observed for the solvated molecule. Interestingly, the PL emission peak also moves to higher energies, thus approaching the value seen in solution. Importantly, the highly loaded SURMOF exhibits pronounced differences to the solution properties in that the quantum yields are more than an order of magnitude higher. This indicates that the radiative rate must be increased in the excited-state geometry achieved at high chromophore concentrations. Although further investigation

SURMOF. This observation is consistent with the molecule being held in a restricted geometry by the SURMOF host and further restricted by neighboring AIE chromophores as the loading density is increased. The absorption spectrum of the chromophores in the SURMOF is significantly different from powder solid-state absorption spectrum. We relate this observation to the H4ETTC conformation inside the SURMOF differing substantially from that in the chromophore solid. We can rationalize this finding by pointing out that in order to intercalate between the SURMOF sheets, a relatively planar configuration of the TPE chromophore should be preferential. Meanwhile, in the normal solid-state structure,8 the rotation of the phenyl units relative to the ethylene bridge to form the “propeller-like” configuration results in a molecule that is not very planar. Therefore, it would be reasonable to suppose that a configuration wherein the phenyl units are less rotated and the molecule is more planar is favored for the intercalated chromophores, explaining their absorption and emission spectrum being different to those observed in a neat powder. The emission spectrum of the H4ETTC@SURMOF heterostructure also shifts with increasing chromophore concentration. However, this shift, as shown in Figure 3b, is not monotonic. In Figure 3b, the emission energy of the TPEbased chromophore is shown first, as a reference, for the solvated molecule (the highest energy emission and upper dashed gray line) and then in the solid state (the lowest energy emission and the lower dashed gray line). The emission energy of the chromophores can be tuned across 200 meV by varying the concentration inside the SURMOF and made to closely resemble the spectra in solution or in the solid state. Interestingly, at high concentrations of chromophore in the SURMOF, the spectrum of the chromophore emission is very similar to the spectrum of the chromophore in solution but with an i-PLQY approaching 50% rather than the 2% in solution. This tuning of the emission color as a function of the concentration while maintaining a high i-PLQY is an interesting feature of the chromophores intercalated inside the SURMOF. As discussed in the introduction, a similar modification of emission color from lower to higher energy was observed when individual metallacages containing a TPE chromophore were aggregated/clustered by adding a poor solvent to the solution.10 Thus, such spectral shifting upon denser packing of already constrained TPE-based chromophores seems to be a general phenomenon. We hypothesize that the geometry of the molecule is differently constrained at the high concentrations, giving rise to the change in the properties of the emissive state. In Figure 3c, we plot the change in energy between the halfrise of the absorption spectrum and the half-rise of the emission spectrum. Using the half-rise rather than the position of the maximum allows us to establish a quantity similar to the Stokes shift but one that more accurately characterizes the gradual sequential shift in this case. Like the Stokes shift, this quantity characterizes the loss in energy between an absorbed and emitted photon and is predominantly affected by two factors. First, the difference in energy between absorption and emission could be increased by an increasing reorganization of the molecule between the ground and exited-state geometries. Second, the energy difference can be increased with increasing inhomogeneous broadening of the energy levels and energy transfer in the solid state. We observe that the energy difference between the absorption and emission decreases F


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Figure 4. Time-resolved PL decays showing the lifetime of the emissive state in the AIE-chromophore-loaded SURMOFs compared to that in solution and solid state (powder). At low concentration, the excited-state lifetime is similar to that in the solid state (as is the emission wavelength), whereas at high concentrations, the lifetime is shorter. There is no loss of i-PLQY at the higher concentration, so the observation of the shortened lifetime is a clear indication that the nature of the emitting state is changed at the high loading concentrations (this is also supported by the shift in absorption and emission spectra).

into the precise origins of the photophysics of these chromophores in the highly constrained environment of a SURMOF are desirable, these first results clearly indicate that the conformation of the molecule when intercalated into the SURMOF must be different from that adopted in the bulk. Finally, we turn to investigate how AIE chromophores can be infused into SURMOF substrates by inkjet printing. The fabrication of such lateral chromophore gradients inside an otherwise homogeneous MOF coating is of particular interest for applications such as in the display field wherein precise patterns of differing AIE dyes might be useful, for example, competing with printed quantum dot technologies.30 Single printed droplets of the AIE chromophores led to circular dots with a diameter of about 70 μm (Figure 5a). Alongside further test patterns, inkjet-printed logo of the Institute of Microstructure Technology (IMT) can be seen in Figure 5b. The photographs of the AIE-loaded SURMOFs were taken under illumination with a 365 nm UV lamp. The further increase of resolution of the inkjet printing of AIE chromophores into the SURMOF structure is the focus of ongoing optimization work. The unique loading/drying characteristics of printing into the “molecular sponge” of the SURMOF substrate are also a topic of ongoing interest.

Figure 5. (a) Magnified image from optical microscope of test pattern prepared by inkjet printing with circular features of 70 μm. (b) Photograph of test patterns of AIE chromophores printed into SURMOF substrates under 365 nm UV illumination. Note that substrates were rinsed with ethanol after printing to remove any surface-adsorbed chromophores.

SURMOF rivals that of the chromophore in its powder form (50%). We demonstrate that high densities of AIE chromophores can be intercalated, up to 2.6 mmol/cm3, which leads to significant absorption in films only 150 nm thin thick and e-PLQYs in excess of 10%. These e-PLQYs could be further enhanced by moving to slightly thicker SURMOF films. In terms of mechanistic insight, our work shows that the SURMOF structure confines the AIE chromophores and switches on their emission. Chromophore photophysics are modified by interactions with neighboring chromophores, as the loading is increased. This causes distinct shifting of the absorption and emission of the SURMOF-loaded chromophores as a function of their concentration. Interestingly, while maintaining a high PLQY, the emission spectrum of the AIE chromophore can be tuned from that of the solid-state powder to that of the solution as a function of AIE-chromophore concentration inside the SURMOF. Finally, the AIE chromophores can be easily introduced into a SURMOF substrate by inkjet printing. Without exhaustive optimization, we can achieve feature sizes down to 70 μm in this fashion. Further reduction is the subject of ongoing work. Further development of these concepts that enable AIE chromophores to be brought into high quality, large-area, thin films wherein their photophysical properties can be optimized provides interesting new possibilities for exploiting the properties of AIE chromophores in display and lighting applications.

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CONCLUSION In this work, we demonstrate that AIE chromophores can be loaded into a ZnBDC SURMOF thin film by simply coating the MOF thin film with a solution of the chromophore. This process allows for a straightforward fabrication of large-area thin films, in which AIE chromophores can be introduced and emit with high quantum efficiency. We varied the concentration of the AIE chromophore in the SURMOF and demonstrated that high values for the i-PLQY can be obtained across all concentrations. The high i-PLQY even at low concentration indicates that intercalation into the SURMOF substrate effectively constrains the AIE molecule and turns on its PL. The maximum i-PLQY of the AIE chromophores in the G


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(8) Xiong, J.-B.; Feng, H.-T.; Sun, J.-P.; Xie, W.-Z.; Yang, D.; Liu, M.; Zheng, Y.-S. The Fixed Propeller-Like Conformation of Tetraphenylethylene That Reveals Aggregation-Induced Emission Effect, Chiral Recognition, and Enhanced Chiroptical Property. J. Am. Chem. Soc. 2016, 138, 11469−11472. (9) Prlj, A.; Doslic, N.; Corminboeuf, C. How Does Tetraphenylethylene Relax from Its Excited States? Phys. Chem. Chem. Phys. 2016, 18, 11606−11609. (10) Yan, X.; Cook, T. R.; Wang, P.; Huang, F.; Stang, P. J. Highly Emissive Platinum(Ii) Metallacages. Nat. Chem. 2015, 7, 342−348. (11) Peng, H.-Q.; Zheng, X.; Han, T.; Kwok, R. T. K.; Lam, J. W. Y.; Huang, X.; Tang, B. Z. Dramatic Differences in Aggregation-Induced Emission and Supramolecular Polymerizability of TetraphenyletheneBased Stereoisomers. J. Am. Chem. Soc. 2017, 139, 10150−10156. (12) Shustova, N. B.; McCarthy, B. D.; Dincă, M. Turn-on Fluorescence in Tetraphenylethylene-Based Metal−Organic Frameworks: An Alternative to Aggregation-Induced Emission. J. Am. Chem. Soc. 2011, 133, 20126−20129. (13) Shustova, N. B.; Cozzolino, A. F.; Dincă, M. Conformational Locking by Design: Relating Strain Energy with Luminescence and Stability in Rigid Metal−Organic Frameworks. J. Am. Chem. Soc. 2012, 134, 19596−19599. (14) Shustova, N. B.; Ong, T.-C.; Cozzolino, A. F.; Michaelis, V. K.; Griffin, R. G.; Dincă, M. Phenyl Ring Dynamics in a Tetraphenylethylene-Bridged Metal−Organic Framework: Implications for the Mechanism of Aggregation-Induced Emission. J. Am. Chem. Soc. 2012, 134, 15061−15070. (15) Wei, Z.; Gu, Z.-Y.; Arvapally, R. K.; Chen, Y.-P.; McDougald, R. N.; Ivy, J. F.; Yakovenko, A. A.; Feng, D.; Omary, M. A.; Zhou, H.C. Rigidifying Fluorescent Linkers by Metal−Organic Framework Formation for Fluorescence Blue Shift and Quantum Yield Enhancement. J. Am. Chem. Soc. 2014, 136, 8269−8276. (16) Gong, Q.; Hu, Z.; Deibert, B. J.; Emge, T. J.; Teat, S. J.; Banerjee, D.; Mussman, B.; Rudd, N. D.; Li, J. Solution Processable Mof Yellow Phosphor with Exceptionally High Quantum Efficiency. J. Am. Chem. Soc. 2014, 136, 16724−16727. (17) Dalapati, S.; Jin, E.; Addicoat, M.; Heine, T.; Jiang, D. Highly Emissive Covalent Organic Frameworks. J. Am. Chem. Soc. 2016, 138, 5797−5800. (18) Khandare, D. G.; Joshi, H.; Banerjee, M.; Majik, M. S.; Chatterjee, A. Fluorescence Turn-on Chemosensor for the Detection of Dissolved Co2 Based on Ion-Induced Aggregation of Tetraphenylethylene Derivative. Anal. Chem. 2015, 87, 10871−10877. (19) Pradhan, N.; Jana, D.; Ghorai, B. K.; Jana, N. R. Detection and Monitoring of Amyloid Fibrillation Using a Fluorescence “Switch-on” Probe. ACS Appl. Mater. Interfaces 2015, 7, 25813−25820. (20) Yan, X.; Wang, H.; Hauke, C. E.; Cook, T. R.; Wang, M.; Saha, M. L.; Zhou, Z.; Zhang, M.; Li, X.; Huang, F.; Stang, P. J. A Suite of Tetraphenylethylene-Based Discrete Organoplatinum(Ii) Metallacycles: Controllable Structure and Stoichiometry, AggregationInduced Emission, and Nitroaromatics Sensing. J. Am. Chem. Soc. 2015, 137, 15276−15286. (21) Liu, J.; Woll, C. Surface-Supported Metal-Organic Framework Thin Films: Fabrication Methods, Applications, and Challenges. Chem. Soc. Rev. 2017, 46, 5730−5770. (22) Baroni, N.; Turshatov, A.; Oldenburg, M.; Busko, D.; Adams, M.; Haldar, R.; Welle, A.; Redel, E.; Woll, C.; Richards, B. S.; et al. Facile Loading of Thin-Film Surface-Anchored Metal-Organic Frameworks with Lewis-Base Guest Molecules. Mater. Chem. Front. 2017, 1, 1888−1894. (23) Sun, C.-Y.; Wang, X.-L.; Zhang, X.; Qin, C.; Li, P.; Su, Z.-M.; Zhu, D.-X.; Shan, G.-G.; Shao, K.-Z.; Wu, H.; et al. Efficient and Tunable White-Light Emission of Metal−Organic Frameworks by Iridium-Complex Encapsulation. Nat. Commun. 2013, 4, 2717. (24) Cui, Y.; Song, T.; Yu, J.; Yang, Y.; Wang, Z.; Qian, G. Dye Encapsulated Metal-Organic Framework for Warm-White Led with High Color-Rendering Index. Adv. Funct. Mater. 2015, 25, 4796− 4802.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05568. ZnBDC XRD in and out of plane; 1H NMR of tetramethyl 4',4''',4''''',4'''''''-(ethene-1,1,2,2-tetrayl)tetrakis([1,1' biphenyl]-4-carboxylate); 1H NMR of 4',4''',4''''',4'''''''-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1'-biphenyl]-4-carboxylic acid)); calibration curve used to determine dye density in SURMOF samples; emission and absorbance of empty ZnBDC and 0.39 AIE@ ZnBDC loaded (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Andrey Turshatov: 0000-0002-8004-098X Gerardo Hernandez-Sosa: 0000-0002-2871-6401 Christof Wöll: 0000-0003-1078-3304 Natalia B. Shustova: 0000-0003-3952-1949 Bryce S. Richards: 0000-0001-5469-048X Ian A. Howard: 0000-0002-7327-7356 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support by the Helmholtz Association via: (i) the Recruitment Initiative for B.S.R.; (ii) the Science and Technology of Nanosystems (STN) research programme; and (iii) the Helmholtz Energy Materials Foundry (HEMF). N.B. also gratefully acknowledges the financial support of Karlsruhe School of Optics & Photonics (KSOP) and for the research travel grant obtained from Karlsruhe House of Young Scientists (KHYS). N.B.S. and E.A.D. thank the National Science Foundation Career Award (DMR1553634) and Cottrell Scholar Award from the Research Corporation for Science Advancement. S.S. and G.H.S. acknowledge the financial support of the German Federal Ministry for Education and Research through the grant FKZ 13N13691.

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REFERENCES

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Inkjet-Printed Photoluminescent Patterns of Aggregation-Induced

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