Ion-Pair Oligomerization of Chromogenic ... - ACS Publications

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Ion-Pair Oligomerization of Chromogenic Triangulenium Cations with Cyanostar-Modified Anions That Controls Emission in Hierarchical Materials Bo Qiao,† Brandon E. Hirsch,† Semin Lee,† Maren Pink,† Chun-Hsing Chen,† Bo W. Laursen,*,‡ and Amar H. Flood*,† †

Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States Nano-Science Center & Department of Chemistry, University of Copenhagen, Universitetsparken 5, København Ø 2100, Denmark

‡

S Supporting Information *

ABSTRACT: The hierarchical assembly of colored cationic molecules with receptor-modified counteranions can be used to control optical properties in materials. However, our knowledge of when the optical properties emerge in the hierarchical organization and the variety of cation−anion salts that are available to create these materials is limited. In this work, we extend the salts from small halides to large inorganic anions and determine how the structure coevolves with the emission properties using solution assemblies. We study the chromogenic trioxatriangulenium (TOTA+) cation and its coassembly with cyanostar (CS) macrocycles selected to modify tetrafluoroborate (BF4−) counteranions through formation of 2:1 sandwich complexes. In the solid state, the TOTA+ cation stacks in an alternating manner with the sandwich complexes producing new red-shifted emission and absorption bands. Critical to assigning the structural origin of the new optical features across the four levels of organization (1° → 4°) is the selection of specific solvents to produce and characterize different assemblies present in the hierarchical structure. A key species is the electrostatically stabilized ion pair between the TOTA+ cation and sandwich complex. The red-shifted features only emerge when the ion pairs oligomerize together into larger (TOTA·[CS2BF4])n assemblies. New electronic states emerge as a result of multiple copies of the TOTA+ making π-contact with cyanostar−anion complexes. Our findings and the ease with which the materials can be prepared as crystals and films by mixing the salt with a receptor provide a strong platform for the de novo design of new optical materials.

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INTRODUCTION The self-assembly of chromophores is a common strategy1−4 for creating materials with optical properties that depend on the hierarchical packing of molecules.5−7 Understanding when the optical properties emerge in the hierarchy is rarely investigated despite being essential for predictive materials design. When investigated, most insights have come from studies of neutral chromophores8 and have led to an understanding of packing effects,9,10 the thermodynamics10−12 and mechanisms of assembly,13−16 as well as the coevolution of structures and properties.17,18 This understanding allows optical properties to be controlled using noncovalent contacts like hydrogen bonds,19,20 and the π−π interactions commonly seen in H-/Jaggregations.1,21,22 Among these self-assembled systems, materials with two components are not common. When employed, complementary functionalities from the two different components foster opportunities to access new amalgamated behaviors. The challenge associated with preparing twocomponent materials reduces to predictably coassembling two different molecules rather than letting them phase-separate. One solution to this challenge is to use charged chromophores © 2017 American Chemical Society

and exploit the fact that charge balance always enforces a 1:1 stoichiometry.23−32 One variation of the charge-by-charge approach uses chromogenic cations paired with anions captured inside synthetic receptors.4,33−38 In these systems, the impact of the counteranion’s form factor (size, shape) and its electronics on the material shifts from the anion to the complex.28,33 This cation−anion−receptor approach has the potential to leverage the already extensive varieties of anion receptors39,40 to engineer optical properties by simply mixing the building blocks together (Figure 1). However, examples of these materials are still rare; for example, they have been restricted to halide salts.33 Furthermore, while a lot of new properties have been described,4 an understanding of how the optical properties emerge in the material is limited. This limitation may be overcome if it is possible to isolate and investigate the elementary species seen in the hierarchical structure. Charge-based self-assembly41 has long been used to control the structures of molecular materials. Recent examples include Received: February 24, 2017 Published: April 24, 2017 6226

DOI: 10.1021/jacs.7b01937 J. Am. Chem. Soc. 2017, 139, 6226−6233

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Journal of the American Chemical Society

pyrrolo-receptors. It is striking that the resulting cation− anion−receptor materials display emissive properties not observed in either of the two parent chromophores. Fundamentally, the assignment of these new emissive properties to a specific stage of the structural hierarchy and to an electronic origin (lattice Stark effects,32,44 coupling between neighboring molecules) is currently unaddressed. Furthermore, these novel properties are only available with halide salts. Use of salts of larger anions, like BF4− and PF6−, failed33 to generate new optical properties. This finding was attributed to the anion’s poor size-match to the receptor, which then inhibits planar stacking of chromophores. This problem places BF4− salts45 and their congeners beyond modification despite their wide use in compounds for optoelectronic materials.46,47 These types of salts have a range of favorable properties for materials creation including being redox inert, electronically silent, and nonhygroscopic. To broaden the applicability of cation−anion−receptor materials and to deepen our understanding of their structures and properties, we provide the first modification of the BF4− salt and first identification of the species giving rise to new emission in the material (Figure 2). The cation−anion−receptor material is formed by self-assembly between the trioxatriangulenium cation48 (TOTA+, Figure 1) and the anion-based complex of the macrocyclic receptor, cyanostar49 (CS, Figure 1). We selected TOTA+ as the chromophore on account of its flat and symmetric shape, and relatively high charge density on the center carbon atom to favor alternating stacks instead of segregated ones.30,50,51 We selected cyanostar as the receptor because it opens the first opportunity to modify anions like BF4− by forming sandwich complexes,49,52−55 for example, [CS2BF4]−, of persistent planarity.56 We show that a Coulombically driven and π-stacked assembly involving the TOTA+ and the cyanostar complex controls the emission properties. The crystal gives rise to new electronic states and corresponding red-shifted absorption and emission bands. The origin of the new optical features of the crystalline state is probed in solution. Use of solution allows us to isolate different species corresponding to different levels of hierarchical organization (Figure 2) for the first time. We use different solvents to favor the assembly of different species representing the four levels in the organizational hierarchy. After anion binding (1°), an elementary and putative growth species (2°), proposed to be (TOTA·[CS2BF4])n, was studied using lowpolarity solvents. We find that the degree of association, n,

Figure 1. (a) Molecular structure of TOTA·BF4 and cyanostar (CS). (b) Solution mixtures and (c) drop-cast films of the two components produce red-shifted emission. (d) Single-crystal structure of TOTA· [CS2BF4] harvested from the solution mixture highlights charge-bycharge cofacial stacking.

K+-templated G-quadruplexes with lengths of cylindrical nanostructures controlled by counteranions,23 and charged amphiphiles that can induce nanoscale phase segregation and order in dye structures.24,42,43 For charged discotics, variations in either the size of the counterions or the extent of ion− solvent interactions have been used to form organized nanostructures such as sheets, rods, and tubes.25−27,29 For cation−anion−receptor materials, the approach taken to control assembly and optical properties is to modify the counteranion. This modification occurs by in situ encapsulation of the anion inside a complementary receptor.28,33 Maeda33−38 has used chromogenic triazatriangulenium cations (TATA+) and modified the associated counteranions with optically active

Figure 2. Structure−property coevolution of TOTA·BF4 with cyanostar across the hierarchy (1° → 4°) of assembled species from solution to the solid-state cocrystal of TOTA·[CS2BF4]. 6227


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Figure 3. Normalized diffuse reflectance and emission spectra of microcrystalline samples of the (a) TOTA·BF4 crystal and (b) TOTA·[CS2BF4] cocrystal. Insets: Fluorescence microscope images and packing mode of the sample. Excited at 470 nm; 510 nm long-pass emission filter.

should be greater than 1 to produce the new red-emitting, hierarchically assembled state (3° and 4°). Our study, therefore, helps shed light on the underlying character of the early steps of charge-by-charge assembly and the role of the anion-complex in cation−anion−receptor materials.

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RESULTS AND DISCUSSION New Emission from the Cocrystallization of the Chromogenic TOTA+ and Cyanostar. Crystal samples obtained from a 2:1 solution mixture of cyanostar and TOTA·BF4 have a new emission feature at 642 nm (black trace, Figure 3b) not seen in the parent compounds. This band is shifted 100 nm to the red relative to the parent TOTA·BF4 crystals (548 nm, Figure 3a). While the crystal’s diffuse reflectance spectrum (red trace, Figure 3b) has a peak at the same place as TOTA·BF4, it features a new red-edge absorption that is also shifted almost 100 nm relative to TOTA·BF4 alone. The diffuse reflectance and fluorescence spectra obtained from the parent salt, TOTA·BF4 (Figure 3a), are similar to the spectra of TOTA+ when recorded in solution under conditions where the monomolecular species dominates.57 Thus, the origin of the red-shifted absorption and emission features seen in the crystal of the cation−anion−receptor material must be related to intermolecular interactions. To understand these intermolecular interactions, we obtained a crystal structure of the TOTA·[CS2BF4] cocrystals by X-ray diffraction and compared it to the crystal structure of TOTA·BF4.50 The crystal structure of the cocrystal shows (Figure 4) cofacial stacks between the TOTA+ cations and the cyanostar-based sandwich complexes [CS2BF4]− that produce an alternating array of cations and anions. The same alternating cation−anion stacking is also found for the parent crystal of TOTA+ and BF4− (Figure 4d)50 and several other TOTA+ salts. This stacking mode is likely to be both stabilized and ordered by Coulombic interactions involving the relatively large positive charge density on the central carbon atom of TOTA+. When the charge is more delocalized, as with the nitrogen-containing TATA + cations and amino-substituted versions of TOTA+,29−31,51 segregated stacking of cations and anions is preferred instead. The crystal structure exhibits whole molecule disorder. For the 2:1 sandwich of cyanostars around BF4−, its characteristics are consistent with prior studies, for example, cyanostar-based whole molecule disorder, and static disorder in the orientation of the anion within the cyanostar cavity.49,52−55 We observe that the TOTA+ cation is tilted relative to the cyanostars. The

Figure 4. Different packing modes in the crystal of TOTA·[CS2BF4] resulting from the two disordered positions of TOTA+ (green and blue). (a) 100% occupancy of green positions. (b) 100% occupancy of blue positions. (c) Random occupancy of either positions. (d) Packing mode in the crystal structure of TOTA·BF4.

TOTA+ is disordered over two locations (Figure 4a−c) between anionic complexes; we cannot distinguish from the data between either an equal mixture of domains constituted by packing modes 1 and 2 or that of a single crystal with random mode 3. A comparison of the packing in the cocrystal (Figure 4c) and the parent salt (Figure 4d) gives some indication of the origin of the new emissive properties. Both structures have an alternating array of cations and anions with minimal TOTA+− TOTA+ contact, yet they have very different optical properties. As a consequence, any lattice Stark effects would be expected to be common to both crystals. Thus, these effects are unlikely to account for the 100 nm red shift in the fluorescence of the TOTA·[CS2BF4] crystal. Similarly, any TOTA+−TOTA+ exciton couplings, which would be expected to be stronger in the parent salt, are unlikely to give rise to the new emission. Thus, the novel optical properties must originate from specific electronic interactions between the π-system of TOTA+ and the π-system of the anionic [CS2BF4]− sandwich complex. 6228


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Figure 5. (a) Emission spectra and (b) the corresponding Stern−Volmer plot and lifetime of a solution of TOTA·BF4 (10 μM) with increasing amounts of cyanostar (90:10 toluene:dichloromethane, excited at 455 nm; 0.25 equiv of CF3COOH added to stabilize51 TOTA+).

Evaluating the Model of Hierarchical Self-Assembly. With some insights on the origin of the red-shifted emission, we next consider how the optical properties might emerge at different stages of the hierarchical organization. Given that the crystal of TOTA·[CS2BF4] grew from solution, we envisioned that cofacially stacked assemblies of (TOTA·[CS2BF4])n, where n = 1, 2, etc., exist in solution as precursors to the cocrystal. Our expectation is that the cyanostar will form highly stable sandwich complexes with the anion49 in the form of [CS2BF4]− (1° organization) and that TOTA+ will form an ion pair with those complexes in solution (2° organization). The elementary ion pair (n = 1; 3° organization) will further assemble to form higher order species (n > 1) when the concentration is higher and solvent properties enhance Coulombic interactions. We do not know whether the new emission emerges with the elementary ion pair (n = 1) or only comes with its higher order assemblies (n > 1), that is, at the 2° or 3° level of organization. To test these ideas, we conducted a series of solution-phase studies to produce and interrogate the simple ion-paired species, n = 1, and the higher order species, n > 1. We selected three representative solvent systems to control ion pairing and to alter the stability of π-stacked assemblies. The three solvent systems are a 90:10 toluene:dichloromethane (toluene:CH2Cl2) mixture of low polarity to enhance ion pairing but to disfavor π-stacking,58 a 50:50 acetonitrile:dichloromethane (MeCN:CH2Cl2) mixture of highest polarity to dissociate all ions, and chloroform (CHCl3) of low polarity to facilitate both Coulombic and π−π interactions. These three solvent systems allow us to probe the hierarchical formation of the (TOTA·[CS2BF4])n species at different stages: free ions (eq 1), complexed anions (eq 2), a single ion pair (eq 3), and assembled ion pairs (eq 4). TOTA·BF4 ⇌ TOTA+ + BF4 −

green emission

2CS + BF4 − ⇌ [CS2BF4 ]− [CS2BF4 ]− + TOTA+ ⇌ TOTA+[CS2BF4 ]−

The low polarity (εeff = 3) of the solvent mixture can enhance electrostatic attractions, while the toluene can disfavor large πstacked assemblies.58 In the preparation of the TOTA+ samples in this and all other solution phase experiments, the solubility limit of TOTA+ had to be taken into consideration. All concentrated stock solutions were filtered to remove undissolved material, and their concentrations were subsequently standardized. Concentrated mixtures of the TOTA+ and cyanostar were also filtered as needed. In other cases, highly diluted solutions were prepared and used directly. Titrations show that addition of 0−8 equiv of cyanostar to the solution of TOTA+ (10 μM) leads to the quenching of the fluorescence of TOTA+ at 530 nm (Figure 5a). The Stern− Volmer plots of fluorescence intensity ratio, I0/I, and lifetime, τ (Figure 5b), are consistent with static quenching; the fluorescence lifetime is unaffected, while the intensity ratio depends linearly on cyanostar concentration. This static quenching is attributed to ion pairing (eq 3) and leads to the interpretation that the TOTA+[CS2BF4]− ion pair is nonemissive. We observed that the quenching (Figure 5a) does not saturate with 2 equiv of cyanostar. More cyanostar is needed to produce the 2:1 complex and consequently the quenched ion pair on account of multiple equilibria (eqs 1−4) distributing the cyanostar among other species. The red emission of the crystals does not emerge under conditions that allow for formation of the simple TOTA+[CS2BF4]− ion pair alone. To access higher order assemblies (n > 1, with 3° level of organization), different conditions are required. π-Stack Enhanced Oligomerization Turns on New Emission in Solution. We show that the red-emitting state seen in the solid state can also be produced in solution. We believe this red-emitting assembly, (TOTA·[CS2BF4])n, is the putative crystal precursor and thus representative of the fluorescent cofacial stacks seen in the solid state. Chloroform (ε = 5) was selected as the low-dielectric medium to better propagate π-stacked oligomerization (eq 4) of the TOTA+ [CS2BF4]− ion pairs. We repeated the cyanostar titration in chloroform (20 μM, Figure S4) and observed the same quenching seen with the toluene mixture, but it is more effective and shows saturation with only ∼3 equiv of added cyanostar. Critically, we are now able to observe a new redshifted emission band emerge at 630 nm during the titration that we assign to the assembled species (TOTA·[CS2BF4])n. The 630-emission also displays a fluorescent lifetime of ∼25 ns (Figure S5) that is much longer than that seen from the quenched TOTA+ emission at ∼530 nm (Table S2). Consistent with the existence of the assembled species (n > 1), the concentration was increased from 20 to 200 μM to drive

(1) (2)

quenched green (3)

+

−

nTOTA [CS2BF4 ] ⇌ (TOTA·[CS2BF4 ])n

red emission

(4)

Ion-Pair Complexation Leads to Quenching of the Parent TOTA+ Cations in Solution. Starting with solutions of TOTA·BF4 in 90:10 toluene:CH2Cl2, we see the addition of cyanostar causes static quenching of the TOTA+ emission. We show this behavior is consistent with the formation of a single ion-paired complex, TOTA+[CS2BF4]−, that is, where n = 1. 6229


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Figure 6. (a) UV−vis absorption and (b) emission spectra of 3:1 CS:TOTA·BF4 (red) and TOTA·BF4 (black) at 200 μM in CHCl3 with 10 equiv of CF3COOH as stabilizer51 of TOTA+, excited at 530 nm. (c) Excitation spectra of 3:1 CS:TOTA·BF4 under the same solvent conditions at 2, 20, and 200 μM; emission detected at 620 nm. (d) Emission spectra of 3:1 CS:TOTA·BF4 at 200 μM in the same solvent, excited at 330 (green) and 375 (red) nm.

3b). As a consequence, the characteristic TOTA+ excitation peak at 330 nm can be used to selectively sensitize (Figure 6d) pure emission from the unassembled TOTA+ cation present in solution. By contrast, emission from the new (TOTA· [CS2BF4])n assemblies can be selectively excited at 375 nm (Figure 6d) and in the red edge beyond 500 nm (Figure 6d). The red emission can also be selected in the time-resolved fluorescence spectra using longer delay times (Figure S7), consistent with the observation that the assembled (TOTA· [CS2BF4])n species has a longer fluorescent lifetime. The observation of multiple emitting species in chloroform is consistent with the presence of the monomolecular TOTA+ cation, the nonemissive ion pairs, TOTA+[CS2BF4]−, and the red emissive oligomeric species (TOTA·[CS2BF4])n. The new features in the optical spectra show that a wholly new excitedstate manifold is accessed in the (TOTA·[CS 2 BF 4 ]) n assemblies. The correspondence between the emissive state (630 nm) seen in solution from (TOTA·[CS2BF4])n and the crystalline state is strong. The alternative hypothesis is that the emission arises from crystallites (4° organization, Figure 2). Consequently, the same solutions that were prepared for optical measurements were examined using light and X-ray scattering techniques that are sensitive to crystals. We saw no evidence for crystallites, which provides strong indications that emission stems from solution assemblies and the 3° level of hierarchical organization. We also know ion pairing is critical because in the more polar solvent, for example, 50:50 v:v MeCN:CH2Cl2, the cyanostar and TOTA·BF4 mixture only shows dissociated ions

further association. We selected 3 equiv of cyanostar to mix with TOTA·BF4 as this appeared to be the minimum amount needed to ensure complete formation of the [CS2BF4]− complex. First, it is satisfying to observe at high concentration (200 μM) a red-shifted UV−vis absorption edge (Figure 6a) relative to the TOTA·BF4 absorption. This feature also resembles the diffuse reflectance data seen with the TOTA· [CS2BF4] cocrystal. Second, the red emission peak at 630 nm grows (Figure S8a) and becomes distinguishable from the 525 nm emission.59 For completeness, we note that the red emission peak is not observed with TOTA·BF4 across the same concentration range (2−200 μM, Figure S8b). Consistently, the excitation of the 3:1 CS:TOTA·BF4 mixture at the 530 nm red edge leads to emission at 630 nm (Figure 6b, red trace), while with TOTA·BF4 alone, we do not see this peak; we only see residual emission from the 530 nm band (Figure 6b, black trace). To further understand the nature of the red emissive state observed in solution, we examined the excitation spectra corresponding to both the parent 525 nm and the new 630 nm emission peaks. The 525 nm emission shows excitation peaks at 330 and 400−500 nm (Figure S9), nicely aligned with the UV− vis absorption of TOTA+. In strong contrast, the excitation spectrum associated with the 630 nm emission shows two new excitation features (Figure 6c): a band at 375 nm and a shoulder on the red side of the parent TOTA+ band beyond 500 nm. While the 375 nm band is new, the 500 nm red edge matches the high-concentration UV−vis absorption spectrum of the CS:TOTA·BF4 mixture (Figure 6a) and the diffuse reflectance spectrum of the TOTA·[CS2BF4] cocrystals (Figure 6230


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Journal of the American Chemical Society with green emission (Figure S10) from monomolecular TOTA+ cations. On the basis of the observations from solid state and solution, we assign the new emission properties in the cation− anion−receptor materials to originate from a wholly new excited state resulting from electronic interactions mediated by π−π contacts between TOTA+ and cyanostar sandwich complex. The red emission seen in the solid state emerges when cation−anion−receptor ion pairs start to oligomerize in solution (n > 1). On the basis of the accumulated information on the emission characteristics, we hypothesize that the quenching of TOTA+ emission arises when the anion complex [CS2BF4]− (1° organization) engages as an ion pair (2° organization) with TOTA+ resulting from either a photoinduced electron transfer process or deactivation via a nonemissive exciplex. Further oligomerization of these ion-pairs into an assembly (3° organization) constrains the cyanostar and TOTA+, which permits direct charge transfer (CT) transitions between TOTA+ and the [CS2BF4]− to become more allowed. This outcome is consistent with the new absorption features seen from the assemblies (3°) and the solid state (4°) samples. The new red emission likely results from this CT state, which can now compete with nonradiative deactivation. The long lifetime of the red emission (25 ns) shows that the fluorescence transition is only weakly allowed (in agreement with a CT-like transition that operates orthogonal to the π systems). Comprehensive theoretical studies, and time-resolved and polarized spectroscopies would be needed to verify these ideas in the future. Solution 1H NMR Spectroscopy Shows π-Stacked IonPairs. Solution 1H NMR spectroscopy was utilized as an independent approach to verify the formation of the (TOTA· [CS2BF4])n species in chloroform. Solution 1H NMR spectra of cyanostar and TOTA·BF4 were compared to a 3:1 mixture of CS and TOTA·BF4. We see the peak position of the two TOTA+ protons, He and Hf (Figure 7, 200 μM, CDCl3), shift ∼0.4 ppm upfield upon addition of 3 equiv of cyanostar indicative of strong intermolecular shielding by the macrocycle’s π surface. The cyanostar protons (Ha, Hb, Hc, Hd) move in a way that is consistent with forming a 2:1 [CS2BF4]− sandwich.49,53,54,60,61 Specifically, Ha shifts downfield upon C−H···BF4− hydrogen bonding, while Hb and Hc shift upfield on account of π stacking. Consistent with prior observations, the [CS2BF4]− exists as a pair of meso and chiral diastereomers; the major and minor diastereomeric peaks are marked as i and i′, where i = a, b, c, and d. In addition, the broadening seen for all of the cyanostar peaks was not seen49,54,60 in previous binding studies with salts comprised of BF4− anions and ammonium cations. We attribute the broadening to exchange between various π-stacked species on the NMR time scale. Consistent with our observation (Figure S10) that polar solvents destabilize ion pairing, the 1H NMR spectrum of a 2:1 solution of the cyanostar and TOTA·BF4 in the 50:50 MeCN:CH2Cl2 mixture shows less dramatic upfield shifts and significantly less broadening (Figure 7b). This result suggests that electrostatic stabilization is a crucial factor for the formation of the ion-paired assemblies observed in chloroform. Solution Self-Assembly of Emissive Species Allows for Easy Processing as Drop-Cast Films. Our data indicate that the emissive species (TOTA·[CS2BF4])n can preassemble prior to crystallization. On the basis of this idea, we prepared dropcast films from a 3:1 solution mixture of cyanostar and TOTA·

Figure 7. 1H NMR spectra of cyanostar, TOTA·BF4, and CS:TOTA· BF4 mixture in (a) CDCl3 and (b) 50% v/v CD3CN in CD2Cl2 (200 μM, 298 K, 600 MHz, with 10 equiv of CF3COOH as stabilizer51 of TOTA+).

BF4. We used a 3:1 ratio between cyanostar and TOTA+ to match the solution-phase experiments. As expected, the thin film also shows the red-shifted emission (Figure 8). The emission of the drop-cast film (608 nm) is more orange than that of the cocrystal (642 nm). The powder X-ray diffraction (P-XRD) pattern obtained from the drop-cast sample (Figure S15) does not directly match that predicted from the single crystal. However, the estimated unit cell’s volume obtained from the P-XRD (3200 Å3) is consistent with that of the single crystal (3600 Å3) after solvent loss. The unit cell parameters of the P-XRD are similar to those from the single-crystal polymorph with all cell axes contracted to some extent. This observation may suggest that the structure of the putative solvent-free sample obtained from drop casting is similar to that of the single crystal. The general packing of the main components (cyanostar, TOTA+, BF4−) in the drop-cast solvent-free sample must have been preserved to some degree to allow retention of the red-shifted emission. Drop-cast films from the more polar CH3CN:CH2Cl2 solutions at 0.5 mM, in which the preassembled species (TOTA·[CS2·BF4])n do not appear to exist, were prepared and also show similar red-shifted 608 nm emission. This observation indicates that the solvent drying process allows for the stacked species to form despite not being strongly favored in solution. Our modification of a chromogenic cation was the first to involve a nonhalide anion. Consequently, while we cannot directly compare to other tetrafluoroborate salts, we can compare it to halides. In almost all other cases,4 the anion 6231



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b01937. General methods, X-ray data, additional UV−vis and fluorescence spectra, and equilibria modeling (PDF) X-ray crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *afl[email protected] ORCID

Amar H. Flood: 0000-0002-2764-9155 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Sidsel Bogh (University of Copenhagen), Irina B. Tsvetkova (Indiana University), Byeongdu Lee (Argonne National Laboratory), and Dennis P. Chen (Indiana University) for their help with the data measurements. We thank the National Science Foundation (DMR 1533988) for financial support. We acknowledge early support from the Indiana University Institute for Advanced Study to initiate the collaboration. Crystal data were acquired using ChemMatCARS Sector 15, principally supported by the National Science Foundation/Department of Energy under grant number CHE0535644. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC0206CH11357. X-ray scattering experiments were carried out at beamline 12-ID-B at the Advanced Photon Source (APS), a U.S. DOE Office of Science User Facility operated by Argonne National Laboratory under contract no. DE-AC02-06CH11357.

Figure 8. (a) Emission and excitation spectra of (a) TOTA·BF4 and (b) 3:1 CS:TOTA·BF4 films drop-cast from CHCl3 solution. (c) Image of TOTA·BF4, 3:1 CS:TOTA·BF4, and cyanostar films dropcast from CHCl3 solution under UV light.

receptor was emissive, which is in contrast to the low emissive character of cyanostars. With those emissive receptors, it was observed that the emission was modified to greater or lesser degrees across a variety of solid-state packing modes.4,33−38 In some cases,34 the emissive state is able to produce circularly polarized light from the solid-state structures. However, in none of those other cases were the emissive properties correlated to the various levels of organizations of the molecular building blocks seen in the final solid-state packing arrangements. As such, it is not yet possible to use other literature examples to generalize our findings on how hierarchical structure correlates with the emission properties.

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REFERENCES

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CONCLUSIONS We discovered that complexes of the large BF4− anions sandwiched by macrocyclic cyanostar receptors form hierarchical assemblies with the chromogenic cation, TOTA+, to produce a novel red-emission resulting from π contacts between TOTA+ and cyanostar. The red-emitting assembly can be readily produced in the solid state as single crystals and drop-cast films as well as in solution. For the first time, the hierarchical coevolution of the assembly and the emission properties were studied in solution to identify the onset of red emission. We found that the TOTA+ cation exists in solution as a green emissive monomolecular cation, a nonemissive single ion pair, TOTA+[CS2·BF4]−, and red-emitting oligomeric assembly, (TOTA·[CS2·BF4])n. We demonstrated that the self-assembled species (TOTA·[CS2·BF4])n has its own excited state manifold differing from the parent TOTA+ and that it can be selectively excited. We postulated that emergence of a new charge-transfer state when the assemblies are formed give rise to the red-shifted emission. This work takes a critical step toward understanding how properties are generated at different hierarchical levels in cation−anion−receptor materials. 6232


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