Plug-and-Play Optical Materials from Fluorescent Dyes and Macrocycles
Optical materials are needed for various applications that emit light. Highly emissive dyes are expected to be widespread in materials creation but they display emission quenching in the solid state. Flood, Laursen, and colleagues discovered the first universal solution to this 150-year-old problem. They report a class of fluorescent materials and the design rules that allow cationic dyes to be plugged into an ionic lattice to reinstate their bright emission.
INTRODUCTION<!>Article<!>RESULTS AND DISCUSSION<!>Generality of the SMILES Effect<!>Spatial Isolation of the Dyes in the Ionic Crystals<!>Defining the Dual Role of Cyanostar Macrocycles in Enforcing Electronic Isolation<!>Article<!>Unprecedented Brightness from SMILES Materials<!>Conclusion<!>EXPERIMENTAL PROCEDURES<!>Chemicals and Materials<!>SUPPLEMENTAL INFORMATION
<p>Fluorescence 1 is a property of matter that is critical for many technologies, such as OLEDs, 2,3 as well as new applications, such as triplet-triplet annihilation upconversion. [4][5][6][7][8] Fluorescence has been the subject of a large number of discoveries including the revolutionary lead halide perovskites 9 made by mixing and matching ionic building blocks to control optical properties. Fluorescence is also readily seen in many classes of dye molecules that display bright emission in dilute solution, which has motivated their widespread use as biolabels. 10,11 However, almost none of the 100,000+ dyes 12 can be used as molecular building blocks simply by mixing and matching them in a predictable way for the tailored creation of optical materials. Perkin made the first synthetic dye, 13 and as first reported in 1888, 14 dyes lose their properties in the solid state (Figure 1A) by displaying fluorescence quenching (Figure 1B), 15,16 shifts in color, 17 and degraded quantum efficiencies. 18 These problems stem from strong excitonic coupling 17 when dyes pack together. 15,19 Despite this knowledge and the growing advances in crystal engineering, 20 it remains almost impossible to predict the packing, coupling, and the resulting optical properties of fluorescent dyes in materials. Thus, the challenge of transferring the optical properties from high-performance dyes to programmed materials with superior optical properties requires predictable strategies to circumvent exciton coupling and emission quenching. We report a simple and general solution to this long-standing problem with the discovery of a new class of materials that we are calling small-molecule</p><p>The Bigger Picture Optical materials span multiple applications from bioimaging to solar energy harvesting. They can be made from various building blocks; however, traditional fluorescent dyes have a hard time being among them. While dyes can be readily made with bright fluorescence of any color, those properties cannot be transferred to materials without losing their fluorescence and changing their color. It is more art than science to obtain specific optical properties from dyes in the solid state. The problem occurs when dyes pack together in solids to become electronically coupled, which impacts the vast majority of the 100,000+ dyes in existence and constitutes a serious bottleneck in materials creation. The class of materials discovered here circumvents these issues allowing high-fidelity transfer of dyes to solids. By removing the guesswork, these plug-and-play optical materials can now be logically programmed with potential impacts spanning human health and solar energy harvesting. ionic isolation lattices (SMILES; Figure 1C). Therein, cationic dyes form well-defined lattices by co-crystallization with the wide-band-gap macrocyclic anion receptor called cyanostar 21 to turn-on emission and transfer the optical properties (Figure 1D) from solution to solids and materials with high fidelity. The fact that commercial dyes work perfectly in the material when first used, without adjustment, mark these materials as plug and play.</p><p>The organization of molecular fluorophores determine the optical properties of a crystal. Kasha taught us that small changes in distances (R) and angles (q) between transition dipole moments (m) of neighboring dyes significantly alters inter-dye electronic coupling 17 (J): J = m 2 ð1 À 3cos 2 qÞ 4peR 3 (Equation 1)</p><p>Thus, close-packed dyes couple together (Figure 2A) to produce new excitonic states with red or blue shifts in optical spectra 22,23 and produce fluorescence quenching. Solids also support efficient exciton migration 24 to quenched defect sites. While new states and energy migration are attractive for advanced materials, 22 the underlying coupling and the resulting aggregation-caused quenching 25,26 is in general not controllable.</p><p>Many strategies to circumvent quenching rely on fluorophore isolation. 27,28 This idea originates from the 1/R 3 distance dependence (Figure 2B) of exciton coupling. 17 For example, extending inter-dye distances (R) from 3.5 A ˚of p-stacked fluorophores to 14 A ˚is expected to reduce coupling to $1%. Presumably, weak coupling will reinstate both the color and intensity of the dye's emission in solids (Figure 2C). Installation of bulky substituents on fluorophores is one approach 29,30 to isolation but demands time-consuming modifications of each and every dye.</p><!><p>Other approaches are to leverage host-guest complexation that are believed to enhance fluorescence as a result of increased rigidity, planarity, and deaggregation of the guest dyes. 31 Bulky and contorted structures are also a common feature of dyes that display aggregation-induced emission (AIE), 26,32 which represents an alternative approach to generate emissive solid-state molecular materials. However, the AIE dyes are limited to specific structural motifs instead of activating all dye classes. Their optical properties are also subject to substantial packing effects, 33 therefore, given the fact that AIE dyes are not emissive in dilute solution, an understanding of how the optical properties stem from the resulting molecular packing is still outstanding. Thus, AIE materials are essentially no more predictable than traditional dye crystals. A key part of the problem is the lack of structural control in the solid state.</p><p>Recent efforts to address structural ordering of molecular building blocks rely on the principles of self-assembly. These include the revolutions in programmable molecular materials, such as metal-organic frameworks (MOFs), 34 nanoporous solids, 20,35 and covalent organic frameworks (COFs), 36 which all emphasize the importance of strong structure-directing units. Toward fluorescent materials, new approaches have employed the co-assembly of cationic dyes with large anions. [37][38][39] Most successful are the works of Reisch and Klymchenko that obtained very high brightnesses using a covalently modified cationic rhodamine dye that was also ion paired with bulky fluorinated anions to become lipophilic and then loaded into polymer nanoparticles. [40][41][42] Despite the ability to translate optical properties, under the appropriate conditions, most of the assemblies do not display the crystalline ordering expected from use of strong structure-directing units. Toward reliable crystallinity, Maeda has pioneered use of receptor-modified anions 43 to produce alternating ionic packing of cationic dyes (+ -+ -). In principle, the spatially isolated dyes could be decoupled from each other. However, in all those prior works, the materials failed to transfer the optical properties of the dye from solution to solid state and as a consequence did not display the property of isolated dyes. We have also previously employed this approach 44 to create charge-by-charge materials with the same outcomes as Maeda. In that work, we observed that a totally new emission color was produced that did not match the parent triangulenium dye. For that reason, we were able to study the stepwise build-up of hierarchical order in the charge-by-charge materials. Nevertheless, Maeda's and our findings perpetuate the 150-year-old problem.</p><p>We now report the first general approach to transfer a fluorophore's optical properties to the crystalline state by formulation of cationic dyes into ionic lattices called SMILES materials. The materials are made using a simple two-pot mix of a cationic dye with an ion-binding macrocycle. We use commercially available rhodamine 3B perchlorate (R3B$ClO 4 ; Figure 2D) combined with two equivalents of the anion-binding cyanostar 21 (Figure 2E). The key discovery is that ionic crystallization with cyanostar produces both spatial and electronic isolation of the fluorophores. Borrowing from prior work, spatial isolation comes from hierarchical assembly (Figure 2F): the counteranions for the dye form complexes with the cyanostar and the resulting disc-like 2:1 cyanostar-anion complexes direct packing into an alternating charge-by-charge assembly. 43,44 In a major departure from those prior works on structural control, we discovered that electronic isolation of dyes is the all-critical missing feature needed to resolve the 150-year-old problem of emission quenching. Isolation arises from the use of colorless cyanostar-anion complexes with wide 3.45 eV band gap. We verified the generality using ll Chem 6, 1978-1997, August 6, 2020 1981</p><p>Article commercial dyes (xanthenes, oxazines, styryls, and cyanines), representing over 1,000 commercial homologs and synthetic dyes (triangulenium). 45 Control studies helped establish our mechanistic understanding and provided the basis for two key design rules: the dyes must be (1) spatially and (2) electronically isolated.</p><p>No other method offers the degree of reliability or simplicity of SMILES; see Table S1 for a comparison of related charge-by-charge crystalline materials. No other method confers the breadth of use enabled by the wide band gap of cyanostar complexes; therefore, this property enables us to plug almost any cationic dye into the material. As a proof of concept for the impact beyond molecular crystals, we prepared fluorescent samples of major classes of industrial polymers and plastics with SMILES by extrusion and compounding bulk materials. We verified that SMILES circumvent the persistent problem of concentration quenching often expressed by dyes in commodity polymers. Overall, this approach is expected to valorize the vast back catalog of cationic dyes 12 and all future ones that follow the design rules to program targeted optical properties seamlessly from molecules to materials.</p><!><p>Rhodamine's Optical Properties Can Be Rescued in SMILES Crystals One of the primary discoveries is that SMILES materials can rescue the emission intensity and reinstate the spectral properties of the dye seen in a dilute solution. Rhodamine 3B perchlorate is exemplary (Figure 3). Simple mixing of the dye with cyanostar produces SMILES materials with superior fluorescence properties. Solids of the dye are highly quenched while the solids of SMILES analogs are brightly fluorescent (Figures 3B and S1).</p><p>The spectral data reinforce this finding. The absorption spectrum of the dye alone in solution (black trace; Figure 3C) is poorly reproduced by spin-coated films (blue; Figure 3C) with 25-nm red-shifting and substantial broadening. Fluorescence shows the same red-shift (blue trace; Figure 3D) with significant aggregation-caused quenching (Figure 3C). This quenching can be quantified by the comparison of the emission quantum yields. The dye in solution is highly emissive, F = 73%, but in the thin film it is too low to be quantified, F < 1%. Consistently, crystals of R3B$ClO 4 show the quantum yield is much reduced, F = 3%. However, when SMILES crystals are made of R3B dyes by co-crystallization with cyanostar, we see the quantum yield increased 10-fold, F = 29%. The same 310 turn-on is seen in the emission intensity of the thin films (Figure 3A) where the SMILES film shows an emission quantum yield of F = 9%. In addition, the solution-phase properties of the dye are reinstated in thin films upon addition of cyanostar: compare the narrow spectra from the thin films of the SMILES materials (red traces; Figures 3E and 3F) that better match solution spectra of the dye (black traces).</p><!><p>We verified the broad generality of the SMILES effect using a range of dye classes (Figure 4). We examined xanthenes as the R3B$ClO 4 dye; oxazines in the form of oxazine 720$ClO 4 ; styryls as pyridine 1$ClO 4 ; cyanines as DIOC 2 $PF 6 ; and a broad class of synthetic 45 dyes called trianguleniums using N,N 0 -dipropyl-diazaoxatriangulenium tetrafluoroborate, DAOTA$BF 4 . In all cases, the fluorophore's properties were transferred seamlessly to the solid state.</p><p>We see emission intensity turned on in every one of the SMILES films (red traces; Figure 4). This behavior contrasts with non-SMILES films of the dyes alone that were either completely quenched in the case of the oxazine and styryl dyes (Figures 4A  and 4B; blue) or partially quenched with the rhodamine, cyanine, and triangulenium dyes (Figures 3B, 4C, and 4D; blue). Photographs of the thin films recorded under UV illumination confirm the large emission turn-on (Figure 5A). Furthermore, spectra of SMILES films closely resemble solution (compare the red and black traces, respectively; Figures S2 and S3). The absorption spectra of SMILES materials show modest red shifts averaging 15 nm. These small shifts stand in stark contrast to films of dyes alone (Figure S3; blue), which barely resemble solution spectra. They show unpredictably large shifts either to the red (38 nm) or blue (-62 nm) and invariably show Article substantial band broadening. These red and blue shifts are indicative of uncontrolled J-and H-type aggregate coupling explained in general by Kasha's work. 17 However, the origins of the shifts for each specific dye are not directly explicable without detailed study. 22,23 Another manifestation of electronic decoupling in SMILES is high-fidelity color reproduction. This is seen with R3B SMILES materials (Figure 5B) but is more dramatic with cyanine (Figure 5B). When the green cyanine solution is spun down into a neat film of the dye, it looks orange as a result of significant red-shifting in small crystallites with strong dye coupling. By contrast, the original green color is perfectly reproduced in SMILES film when cyanostar is added to the deposition solution. The data all clearly show that the optical properties of dyes in the solid state are completely unpredictable while formulation as SMILES are predictable entirely from their solution-phase properties.</p><!><p>A series of crucial mechanistic features are associated with the reliable transfer of optical properties from solution to solids. Specifically, spatial and electronic structures of the SMILES materials and their components. Single crystal structures of all SMILES materials were acquired (Methods S1). Key to the reliability, we see the same alternating charge-by-charge packing of cationic dyes with anionic complexes (Figures 2F and 4). In each case there are similar dye-dye distances across the different dye families (Table S2). The similarity of all the structures is striking and indicates that the disc-like cyanostar-anion complex has a very strong structure-directing role essential to the formation and performance of SMILES materials.</p><p>We assign the remarkable structure-directing ability of the cyanostar-anion complexes to their large flat p surface and the Coulomb interactions with the cationic dyes. These factors appear to act exclusively along the stacking direction leading to the columnar charge-by-charge packing. These factors also help rationalize the high predictability of the general packing motif seen in SMILES crystals. In contrast, side-on crystal packing of the cyanostar complexes appears to be mediated by the macrocycle's tert-butyl groups, which appear to support weak interactions and simple space-filling requirements. The packing of the stacked and tilted columns appears somewhat alike to what is seen for columnar liquid crystals. Consistent with the ordering in liquid crystals, the SMILES materials display higher disorder than is typical for small-molecule structures. 46 This outcome is a known pathology that stems from large, solvent-accessible volumes as well as a unique form of whole molecule disorder that is already well-described in cyanostar crystal structures 21,44,46 and outlined in more detail in the Supplemental Information.</p><p>The dyes are isolated from each other in the SMILES crystals. Along the stacking axis they range from 14-18 A ˚and laterally span from 13-16 A ˚(Tables S2-S8). This isolation is in stark contrast to the dyes alone. We see significantly shorter inter-dye distances of 8 A ˚for R3B (Figure 2D) and 3.5 A ˚for pyridine 1 (CCDC: 200780). These spatial changes are consistent with predictions from Kasha's model (Figures 2B  and 2C) that excitonic decoupling occurs at larger distances.</p><p>Anion-Binding Cyanostar Macrocycles Guide Ionic Crystallization and Spatial Isolation Control compounds (Figures 6 and S4) verify the roles of individual SMILES components. We used controls to make thin films and compared the spectra to thin films of SMILES and dyes alone. To evaluate the role of the cyanostar, we exchanged it with 2 equiv of (Figure 6A) either a weak anion-binding bis-amide receptor, nonanion-binding cyclodextrin, uncharged tetra-phenyl methane, or uncharged pyrene. Unlike R3B SMILES materials, none of the resulting films displayed emission turn-on  S4D). These controls verify that cyanostar provides anion binding of a sufficient strength 21 to reliably form the hierarchical ionic lattice in SMILES materials.</p><p>To reinforce the importance of anion binding, we exchanged other components in the SMILES composition. We prepared a cyanine dye with the bulky tetraphenylborate anion (Figure 6B), which is too large to bind to cyanostar. Consequently, we expected and found no benefits to the fluorescence (Figure S4E). Only with anions that are complementary to cyanostar, 21 such as ClO 4 -, BF 4 -, and PF 6 -, do we see the SMILES effect.</p><p>To test for ionic crystallization, we used the non-cationic dye anthraquinone (Figure 6B). When mixed with cyanostar, the emission did not increase (Figure S4F). Without the ionic building blocks, there is no driving force to isolate the fluorophores by forming a lattice. Thus, SMILES materials require cationic dyes, macrocycles with strong anion affinities, and complementary anions to form charge-by-charge packing as a precursor to reinstate the dye fluorescence in solids.</p><!><p>All the examples that we have investigated display predictable emission when cationic dyes are mixed with cyanostar. However, one of the reasons why this predictability is so remarkable is that all the prior examples 43,47 with similar dyes, similar packing, and similar receptor-bound anions failed to show this behavior; see Table S1. While emission was usually obtained in those cases, the charge-by-charge materials did not show high-fidelity transfer of optical properties from solution to solid. These counter examples motivated further tests of the mechanism underpinning the SMILES effect we observed.</p><!><p>We discovered that the transfer of optical properties from solution to solid resulting from the decoupling of dyes at increased inter-dye distances (Figure 2B) only manifests when the dye's frontier molecular orbitals are located inside those of the cyanostar-anion complex (Figure 7A). This orbital alignment also decouples the dyes from the anion complex. This finding establishes an electronic design rule of orbital alignment that is required to achieve the SMILES effect.</p><p>To test this electronic design rule, the energy of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels (Figure 7A) of SMILES components were mapped using electrochemistry (Figure 8) and density functional theory (DFT). The 2:1 cyanostar-ClO 4 complex defines a wide 3.45 V window with reduction and oxidation processes at -2.07 Article and +1.43 V versus ferrocene, respectively. For a full description of the electrochemistry results, please see Methods S2 and Tables S9 and S10. Consistently, the HOMO and LUMO levels of all five dyes (Figure 7A) reside inside this window. DFT calculations were conducted on an elementary unit of the R3B SMILES materials composed of the dye and the 2:1 cyanostar-ClO 4 complex. These calculations show the HOMO and LUMO orbitals of the assembly localized on the rhodamine dye (Figure 7A; left) in prefect agreement with the electronic design rule.</p><p>Our understanding of the electronic design rule is supported by a negative control.</p><p>For this purpose, we needed a dye with one of its frontier orbitals located outside of the complex's electronic window. Examples of fluorescent dyes with such properties are uncommon. Such dyes would need to have either an extremely deep HOMO or high LUMO, which also makes them prone to redox chemistry and photoinduced electron-transfer quenching. Fortunately, the triangulenium dye called TOTA (Figure 7B) has an unusually low-lying HOMO orbital. 48 The TOTA dye is also a structural analog of DAOTA making it a perfect test case.</p><p>The TOTA dye does not comply with the electronic design rule. Its HOMO sits below that of the anion complex. For this reason, this control compound is not expected to show the SMILES effect. Consistently, in a previous study, 44 we found that the green emission seen from the TOTA in dilute solution was not transferred to the solid state. The color of emission from the co-crystals formed with cyanostar instead changed to orange. The origin of the orange color was not clear at that time. With the advent and understanding of SMILES, we are now in a position to explain the orange color.</p><p>Given the isosteric geometry of the TOTA dye relative to DAOTA and its isostructural crystal packing with cyanostar (Figure 7B), the failure to reproduce the color of TOTA seen from dilute solution must stem from the energy-mismatched HOMO. Consistently, DFT calculations show (Figure 7B) the HOMO is located on the cyanostar-anion complex instead of the dye. Given that the LUMO of the ionpaired assembly is situated on the TOTA dye, these results suggest that the orange emission stems from a charge-transfer transition.</p><p>Even though TOTA and its charge-by-charge packing is isostructural with all the SMILES crystals, its spatial isolation fails to produce a material that reproduces the optical properties of TOTA in solution on account of its charge-transfer coupling to the cyanostar-anion complex. Such charge-transfer states are expected to be more common when using anion receptors having smaller optical band gaps. Presumably, Maeda's materials 43,49,50 do not fulfill this design rule and this might explain why they did not show the types of SMILES behaviors reported herein.</p><p>The Design Rules Reinstate Optical Spectra and High Emission Quantum Yields Both structural and electronic design rules must be followed to see high-fidelity reproduction of the optical spectra and reinstated emission intensity. We see this outcome in every one of the solid-state forms we investigated. In the thin films, however, emission quantum yields were modest, likely resulting from higher densities of defects that quench emission.</p><p>High-quantum yields from SMILES materials are observed in crystals with their lower defect densities. Again, R3B$ClO 4 is exemplary. Prior to determining the emission quantum yield of the crystals, we needed to account for inner filter effects. 25,51 This effect arises from the reabsorption of emitted light by the red tail Article of the absorption band. This filtering is pronounced in solid-state materials with high optical densities and leads to the appearance of an artificially red-shifted emission maximum and attenuated intensity. The inner filter effect was reduced as the light path was made shorter by reducing the crystal size. Consistently, we saw the emission and excitation maxima and spectral band shapes approach the values seen in dilute solution when inner filter effects are reduced. The closest correspondence is reached with the smallest crystals we could produce, $2 mm (Figure 9; Tables S11 and S12).</p><p>SMILES crystals have superior quantum yields relative to crystals of the R3B$ClO 4 dyes alone. Micron-sized SMILES crystals display fluorescence quantum yields of 29%. This value is already 10 times higher than the crystals of the R3B$ClO 4 alone (3%; Figure 2D; Table 1). The intrinsic quantum yield of SMILES materials in the ideal defect-free state is actually higher. First, residual inner filter effects are an artifact of the measurement that will attenuate the quantum yield. Second, the high density of dyes in solids results in efficient energy migration through the bulk material allowing Article just a few defects to quench many emitters. Our SMILES crystals are expected to conform to this general model. Consistently, the presence of defects in the SMILES crystals is evidenced from the multi-exponential fluorescence decay (Tables 1 and S13).</p><p>The z50 mm SMILES crystals provided a dataset suitable for analysis of the emission decay, which is fitted by a three-exponential model. We see a reasonable fraction of the excited states decaying quickly (0.9 ns), presumably by excitation energy migration to significantly quenched defect sites. Based on the decay time, we estimate that the average fluorescence quantum yield of this subpopulation is F z 15%. The second and largest component of 1.9 ns correspond to F z 30%. The longest decay component of 4.6 ns are assigned to a subpopulation of emitters located far from defects with high-quantum yields, F z 78%. These data show that the intrinsic fluorescence of dyes in defect-free SMILES environments can reach high values on par with or higher than that in solution (3.7 ns, F = 73%). Inhomogeneity in the fluorescence decay was found to be general and common to dyes in SMILES crystals (Table S13). This inhomogeneity indicated that, while every single one of the dyes showed large turn-on effects when converted into SMILES materials, the final quantum yields (Table 1) are reduced by exciton migration to quenching defect sites. For pyridine 1 and cyanine in particular, the solution-state quantum yields are relatively low on account of conformational flexibility that is known 52 to enhance non-radiative deactivation (internal conversion) in liquid solutions. In SMILES, these fluorophores gain higher relative fluorescence enhancement as a result of the additional effect of confinement in the solid state. In the case of oxazine, we see that its formulation as a SMILES material is turned on relative to the dye alone but still substantially quenched relative to solution. We attribute this outcome to a high defect density and more efficient energy migration as a result of the large Fo ¨rster distance for homo-transfer in this dye with its small Stoke's shift.</p><!><p>The high densities of dyes in SMILES materials (1 dye per $4 nm 3 ; Table S2), combined with intensely absorbing rhodamines and cyanines (e max z 100,000 M -1 cm -1 ) displaying $30% quantum yields will produce a volume-normalized brightness (F 3 e/Vol) exceeding 7,000 M -1 cm -1 /nm 3 . This brightness surpasses the brightest organic and inorganic materials optimized for nanoparticle bioimaging: 53 AIEs, 54 rhodamines Article with large counter ions in polymer nanoparticles, 41 and conjugated polymers 55 having brightnesses of z1,500, z4,500, and z5,700 M -1 cm -1 /nm 3 , respectively. It is remarkable that this very high brightness in the SMILES crystals is achieved without optimization (purification, annealing, and surface passivation) to reduce defect densities.</p><p>Blending High-Fluorophore Concentrations Using SMILES into Polymeric Materials The reliable transfer of the optical properties of a dye from dilute solution to highdensity solids without quenching and chromaticity shifts strongly suggests that SMILES could be used to program optical properties into a range of materials. Polymers, in particular, are attractive matrices in which to incorporate optically active fluorophores on account of their ease of processability relative to molecular crystals. Thin film applications, such as, exciton upconversion, 8,18 and solid-state dye lasers, 56 typically, require high loading of dyes into polymers to generate the desired effect. However, achieving the corresponding high loading using traditional dyes can prove challenging due to the dye-dye interactions (aggregation-caused quenching) described as concentration quenching. 27 The latter is characterized by loss of emission intensity, shorter emission lifetimes, and lowered quantum yields. Being able to transfer the benefits of fluorescent SMILES to industrially relevant polymers would represent immediate positive impact on materials that have widespread use across all sectors of modern life. For this reason, we undertook proof-of-principle demonstrations showing the ability to deliver the SMILES advantage to nine important polymer classes and to verify that they circumvent concentration quenching.</p><p>Al-Kaysi has shown that the loading of dyes into polymers give rise to concentration quenching. 27 Therein, any attempts to increase the dye concentration will lead to undesirable dye self-association. When dye loadings surpass a given threshold concentration, emission quenching begins to occur. The thresholds quoted center around $3% w/w ($30 mM). 57 The thresholds depend on the dye, polymer, and method of analysis, e.g., losses in quantum yield and shortened lifetimes 58 and the losses in fluorescence intensity, 59 are all lower ($10 mM) than thresholds in material gain properties of solid-state dye lasers ($85 mM). 60 Among the polymers, poly(methyl methacrylate) (PMMA) is highly represented, presumably resulting from higher concentration thresholds, lower processing temperatures, optical transparency, and glassy texture. For example, Reisch and Klymchenko were able to load $200 mM ($30% w/w) of a covalently modified rhodamine ion paired with a large fluorinated anion into PMMA with high-fidelity properties transfer. 41 Thus, the impact of SMILES is to confer the same high thresholds but to other industrial polymers and by using standard commercial dyes without further modification, thereby enabling a broader range of polymer classes to be considered alongside PMMA for similar optical applications.</p><p>Polyurethane resins (PU) possess attractive optical transparency, hardness, and viscoelastic properties for use in a variety of applications, ranging from ''low-tech'' applications, such as coatings for wood or concrete, to more advanced systems, the likes of solid-state dye lasers. 61 Like many other doped matrices, PU is prone to concentration quenching (Figure 10A). First, we added rhodamine 3B perchlorate to a polyurethane resin at various loadings 0.1%, 1.0%, 2.5%, 5%, and 10% w/w (R3B$ClO 4 concentrations: 1.66, 16.6, 41.5, 83, and 166 mM). Images of the doped polyurethane recorded under UV light irradiation (Figure 10A) clearly show concentration quenching of the dye alone in PU. As the dye loading increased from 0.1% to 1% w/w (1.66-16.6 mM), we observed benefits accrued in the emission intensity but further increase in dye concentration to 2.5% (41.5 mM) led to an apparent decrease in emission ll Chem 6, 1978-1997, August 6, 2020 1991</p><p>Article intensity, which we attribute to the onset of concentration quenching. At 5% and 10% w/w (83 and 166 mM), the quenching is so efficient that the film is no longer distinguishable under UV light irradiation. By contrast, films doped with SMILES at the same dye loadings did not exhibit the same behavior. Instead, the emission intensity initially grew with dye loading and persisted even at the highest loadings we examined 10% w/w (166 mM), far above the threshold seen in PU films doped with the dye alone.</p><p>To further demonstrate the broad applicability of our approach to crosslinked resins, we also demonstrated the doping of a methacrylate-based stereolithography resin used in 3D printing (Figure 1C) and verified its compatibility with the fabrication process to make a cube-shaped gyroid. It is noteworthy that the photocuring process was unhindered by the presence of high concentrations of dye and cyanostar in the resin.</p><p>Polystyrene (PS), PMMA, polypropylene (PP), and polycarbonate (PC) are commodity plastics that represent a significant proportion of all industrial polymers in production. They are particularly attractive for optical applications on account of their characteristic transparency. The latter is the result of the inherent amorphous nature of PS, PMMA, and PC materials. PP can also be translucent by tuning tacticity and/or by using nucleating agents to limit the size of crystalline domains. We successfully blended each with rhodamine 3B SMILES and observed bright emission (Figures 10C-10F), enabling these to behave at doping levels matching PMMA.</p><p>With both PS (0.5% and 1.0% w/w; Figures 10C-10F) and PP (0.5% w/w; Figures 10K  and 10L), the traditional dye-doped samples were all quenched and above the quenching threshold. By contrast, the SMILES-doped polymers all reinstated the emission at the same dye loadings by weight. The PMMA shows only modest PC is used in demanding applications requiring strong and tough materials. Its high processing temperatures, e.g., 300 C, preclude doping of many fluorescent dyes. For example, rhodamine 3B perchlorate is shown by thermogravimetric analysis (TGA) to decompose above 250 C (Figure S5). However, when the same dye was formulated as a SMILES compound, we observed a significant elevation in the thermal stability such that the onset of decomposition occured at 369 C. For this reason, and to demonstrate the ability of SMILES to raise the thermal stability of dyes, we extruded a sample of PC with the rhodamine 3B SMILES (0.5% w/w, 8.5 mM) at 300 C. Gratifyingly, the resulting hot-extruded sample displayed bright fluorescence (Figure 10M). We did not attempt to prepare the corresponding material by compounding it with the rhodamine 3B perchlorate dye alone because it would have likely decomposed during extrusion based on the TGA data (Figure S5).</p><p>The introduction of dyes into water-soluble polymers is particularly relevant for biological or ecological applications. To that end, we showed compatibility with SMILES doping with polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), and poly(acrylic acid) (PAA) (Figures 10N-10P). Overall, we demonstrated the universality of doping crosslinked resins and commodity plastics to water-soluble polymers. We anticipate that this strategy can be further extended to other types of matrices, such as gels, block copolymers, and other multicomponent systems.</p><!><p>SMILES are a new class of materials that offer direct access to brightly emissive solids as designed and with immediate impact in the preparation of fluorescent commercial polymers. SMILES materials with predictable crystalline packing can be prepared by following general design rules to produce spatial and electronic isolation. Spatial isolation can be achieved using a variety of cationic dyes with a range of anions complementary to cyanostar. Electronic isolation is fundamental to the discoveries reported herein. It can be fulfilled by ensuring that the dyes HOMOs and LUMOs are nested inside those of the structure-directing cyanostar-anion complex. The core of the materials chemistry discovery is that the use of cyanostar macrocycles fulfills both design rules and can be realized by addition of just one component to the dye. In this way, this approach is expected to valorize the vast back catalog of dyes 12 and all future ones that follow the design rules. Thus, these are the first class of plug-and-play optical materials composed of fluorescent dyes. As with modern computer componentry, the dyes can be taken off the shelf and used directly in SMILES without optimization or adjustment. This strategy was demonstrated by doping polymers with SMILES materials to circumvent concentration quenching in nine major polymer classes. The overall consequence of the discovery reported here is one of simple plug-and-play materials creation to confer superior and predictable functionality. Of broader relevance, SMILES offer use of the wide range of powerful cationic dyes as building blocks for the rational design, understanding, and discovery of advanced optical materials.</p><!><p>Resource Availability Lead Contact Further information and requests for reagents should be directed to and will be fulfilled by the Lead Contact, Amar H. Flood (aflood@indiana.edu).</p><p>ll Chem 6, 1978-1997 The spectroscopic data for the solution UV-vis and emission spectra of the dyes are available free of charge from Fluorophores.org (http://www.fluorophores.tugraz.at); entries listed under rhodamine 3B ClO4, oxazine 720, pyridin 1, cyanine PF6, and DAOTA BF4.</p><p>Typical Preparations of SMILES Materials SMILES were made by mixing one equivalent of the dyes with two equivalents of the cyanostar in an appropriate solvent for making thin films on glass by spin coating or crystal growing. Rhodamine 3B SMILES powders were made by mixing the dye with two equivalents of the cyanostar in an appropriate solvent and removing the solvent.</p><!><p>Commercial dyes were purchased. Triangulenium dyes were provided from the University of Copenhagen and cyanostar from Indiana University. Polymers were from commercial suppliers.</p><p>Preparations of Thin Films, Single Crystals, and Polymer Blends Thin films were made by spin coating using standard methods. Crystal growing was conducted using standard vapor diffusion or evaporation methods. Polymer films were made by mixing rhodamine SMILES powders into liquid forms of the polyurethane, compounding rhodamine SMILES with polystyrene, PC, poly(methyl methacrylate), and PP using a heat press and extruder, mixed into solid forms of the polymer with the aid of heat or physically mixed into photopolymer resin prior to fabricating the gyroid cube.</p><!><p>Supplemental Information can be found online at https://doi.org/10.1016/j.chempr. 2020.06.029.</p>