Insight into Water-Soluble Highly Fluorescent Low-Dimensional Host

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Insight into Water-Soluble Highly Fluorescent Low-Dimensional HostGuest Supramolecular Polymers: Structure and Energy Transfer Dynamics Revealed by Polarized Fluorescence Spectroscopy Paramjyothi C. Nandajan, Hyeong-Ju Kim, Santiago Casado, Soo Young Park, and Johannes Gierschner J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01562 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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The Journal of Physical Chemistry Letters

Insight into Water-Soluble Highly Fluorescent Low-Dimensional Host− −Guest Supramolecular Polymers: Structure and Energy Transfer Dynamics Revealed by Polarized Fluorescence Spectroscopy Paramjyothi C. Nandajan,† Hyeong-Ju Kim, ‡ Santiago Casado,† Soo Young Park, *,‡ Johannes Gierschner*,† †

Madrid Institute for Advanced Studies, IMDEA Nanoscience, Calle Faraday 9, Campus

Cantoblanco, 28049 Madrid, Spain. ‡

Center for Supramolecular Optoelectronic Materials, Department of Materials Science and

Engineering, Seoul National University, ENG 445, Seoul 151-744, Korea. *E-mail: [email protected], [email protected]. Abstract: Water soluble, highly fluorescent host−guest chromophore-cucurbit[8]uril supramolecular

polymer

bundles

are

investigated

by

polarized

time-resolved

photoluminescence spectroscopy, structural methods and quantum chemistry to fully reveal structural organization and heterogeneity, but in particular energy transfer dynamics, being of crucial importance for the design of supramolecular artificial light-harvesting systems. KEYWORDS supramolecular polymer, host−guest interactions, energy transfer, polarized fluorescence, quantum chemistry TOC Graph

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Supramolecular chemistry found immense interest during the past few decades due to applications in biology, materials etc,1-4 utilizing various directional and reversible noncovalent interactions.5-7 In particular, host−guest compounds (HGCs) have attracted much attention due to their high binding affinities, good selectivity and stimuli-responsive behavior, which makes these materials suitable candidates for various applications;8-10 among them molecular recognition, molecular machines, light emitting materials, and catalysis.11-19 Host can be either based on inorganic materials like zeolites and nanoporous silica,20-22 or on organics like perhydrotriphenylene (PHTP),23-25 cyclodextrins,26,27 deoxycholic acid,28 pillarenes29,30 and curbit[n]urils (CB[n]).31,32 The latter, in particular CB[7] and CB[8], have found special attention in the area of sensors, markers and probes in bio-environments due to a number of specific characteristics such as high binding affinities, nano-size, water solubility, stimuli responsivity, low toxicity and high photoluminescence (PL).33-36 Very recently, CB[8]-based quasi-1D37-41 and 2D42,43 supramolecular polymers (SPs) have found interest; in particular due to their bright emission as a combination of high intrinsic PL, extinction coefficient and concentration. In particular, Kim et. al. reported a water-soluble CB[8]-based SP HGC utilizing a cyano-stilbene platform, (G; Scheme 1), which showed a remarkable PL enhancement upon G@CB[8] SP formation with ΦF = 91 % compared to the practically non-fluorescent monomer.39 Bundles of one-dimensional SP strands with a G:CB[8] ratio of 1:1 were formed (Scheme 1), as unambiguously proven by a combination of experimental techniques (isothermal titration calorimetry; ITC, Job's plot analysis, dynamic light scattering; DLS, transmission electron microscopy; TEM) and quantum-chemical calculations. In a second contribution, further CB[8] SPs by cyano-stilbenes B, Y, R (Scheme 1) were prepared, in all covering the visible range for PL emission.40 Remarkably, the combination of B as energy donor (D) and R as acceptor (A) allowed for the first example of SP-based artificial light harvesting in water upon formation of B:R@CB[8]; however, this report did not detail the energy transfer (ET) dynamics of the initially created collective excited state (exciton). Earlier ET investigations had been carried out on a structurally related, highly defined D−A system, i.e. PHTP-based HGCs with a well understood host−guest arrangement.23,24,44-46 Here, polarized25,46-48 and time-resolved PL studies49-54, combined with quantum-chemical calculations,52-57 allowed to elucidate the ET dynamics of the weakly coupled excitons in an entirely unbiased approach.25,53,54


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Scheme 1. Chemical structures of the G chromophores and CB[8] (left), and of B, Y, R (bottom). Right: formation of SP strands and bundles. The ET process between neighboring strands is indicated.

For CB[8] SPs, although SP formation and the resulting PL emission features are well understood,39,40 there are a number of open questions, which have to be answered to achieve an in-depth (photo)physical understanding; the latter is considered as a crucial step for establishing a general rationale for such SP systems in materials and life science applications. One question concerns the role of the CB[8] host on the self-assembly ability of the chromophores and the resulting PL properties; this will be here investigated by a comparison of the G@CB[8] SP with the host-free self-assembled structure of G, through examining single crystals of pure G. Furthermore, although DLS and TEM proved the formation of µmsize 3D objects by bundling of the 1D SP strands, the homogeneity of the 3D bundles is not entirely clear; this concerns the presence of small 3D objects as well as the (spectral) inhomogeneity by (somewhat) varying SP formation. This will be answered by a combination of atomic force microscopy (AFM; in dynamic mode), white light and PL microscopy as well as polarized steady-state PL measurements on the water suspension and on immobilized particles. Finally, and most importantly, the ET mechanism in the 3D bundles is elucidated by polarized time-resolved PL and (time-dependent) density functional theory (TD-DFT) calculations; this allows to fully rationalize the ET dynamics and efficiency of the B:R@CB[8] artificial light harvesting system in comparison with the earlier reported related PHTP-based systems. Absorption and Emission Properties of the Single Chromophores. Absorption and PL spectra of G in water are shown in Fig. 1, revealing unstructured spectra which peak at λabs = 355 nm (3.49 eV) and λem = 435 nm (2.87 eV). According to our TD-DFT calculations, the main band is assigned to the S0→S1 transition (calculated at Evert = 2.75 eV), which is ACS Paragon Plus Environment

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described by a simple HOMO→LUMO excitation, and whose dipole moment is oriented along the long axis of the molecule; see Fig. S2. According to the DFT calculations, the G chromophore is substantially twisted around the vinyl-phenyl and phenyl-phenyl single bonds; the large twists are seen as the main source for the broad PL features.58 The molecule is practically non-fluorescent in solution (Φ F < 0.01),39 due to efficient non-radiative deactivation; i.e., very similar to other cyano-vinyl based compounds, which were recently discussed in detail.59 Similar arguments for the absorption and PL properties account also for chromophores B, Y, R.40

Figure 1. Absorption and PL spectra of G in aqueous medium (blue), as G@CB[8] aqueous suspension (red), and as single crystal (black). λex = 360 nm. Inset: PL image of the crystal.

Chromophore Self-Assembly and Resulting Emission Properties. As reported earlier,

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the G chromophore spontaneously forms 1:1 one-dimensional SP strands upon

interaction with cucurbit[8]uril (CB[8]) in aqueous medium. The interaction of G with CB[8] results in a broad PL spectrum (λem=534 nm), significantly red-shifted compared to the pure G solution (435 nm) along with a strong PL enhancement to Φ F = 0.91. The bathochromic shift may result from geometrical, excitonic, excimeric and polarizability contributions.59,60 The PL enhancement is assigned to a combined effect of the suppression of non-radiative decay channel knr and a sufficiently large radiative rate kF to give efficient PL via Φ F = kF/(kF + knr). Suppression of knr is driven by restricting the conformational space of large amplitude torsional motion around the double bond by host−guest interactions; such motions are the primary step to approach the conical intersection which leads to internal conversion (IC) and intersystem crossing (ISC) in this class of compounds.59 The radiative rate is determined by the intermolecular arrangement via H-/J-aggregation.60 Adjacent molecules arrange in a slipped-stacked arrangement along the long molecular axis (x-slip) within the G@CB[8]


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strand, see Figure 2. The pronounced x-slip, as induced by the CB[8] cavity, leads to a very weak excitonic coupling situation, close to the inversion point of H- vs. J-aggregates. To compare this arrangement with self-assembled chromophores of G. i.e. without CB[8], we grew needle-shaped, good quality crystals from the solution phase. Single crystal X-ray structure analysis revealed a triclinic crystal system (space group P1 ) with two molecules per unit cell (lattice parameters a = 6.6 Å, b = 14.6 Å, c = 15.6 Å, α = 64.9 °, β = 86.9 °, γ = 87.7°; CCDC No. 1585802, for details see SI). The molecules in the solid are effectively planarized due to packing constraints (Figure 2), demonstrating once more how cyano-stilbene react sensitively to packing effects by twist elasticity.58 Neighboring molecules are packed in a x-slipped π-stacked fashion with a separation of 3.5 Å; however, the x-slip is much smaller than that in the CB[8]-based SP, see Figure 2. This gives rise to substantial Htype interactions in the crystal. Nevertheless, the crystals are emissive with Φ F = 0.23 and a lifetime of 1.6 ns (Fig. S1 in the SI), demonstrating once more emissive H-aggregates under practically trap-free conditions,60,61 due to the restriction of access to the IC/ISC channel by dense packing.59 The crystal PL spectrum peaks at 495 nm, quite similar to the CB[8]-based SPs, however vibronically better structured. This is ascribed to the planar geometry of the molecules in the crystal, while the twisted structure in the SPs, which (partially) persists in the excited state (Fig. S2), gives rise to the featureless PL spectrum as discussed earlier for related systems.59

Figure 2. Nearest neighbor arrangement in the (a) crystal structure of G according to X-ray analysis, and (b) G@CB[8] SP according to quantum-chemical calculations.

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Structural Homogeneity. TEM measurements, as obtained from the drop-cast filtered suspension (pore size 0.45µm) showed the presence of rectangular particles having 1 µm length and varying width of 200–500 nm.39 This was confirmed by DLS, indicating again the particles of about 1 µm size.39 Nevertheless, the filtering could retain larger particles; moreover, the TEM images in Figure 3 also show the presence of smaller, i.e. sub-50 nm particles clearly, whose origin is not yet resolved. To answer these questions, we slowly evaporated an unfiltered suspension of G@CB[8] on a glass plate and performed AFM along with light and PL microscopy. The AFM images showed indeed the presence of long fibers with a length of tens of µm and a width of less than 0.5 µm, which are apparently breaking during filtering, giving the aforementioned rectangular µm-sized objects, see Fig. 2B and S3. The AFM images further reveal a textured surface (Fig. 3); in fact, white light (Fig. 3C) and PL (Fig. 3D) microscopy images demonstrate that the fibers are not completely homogeneous objects, but have kinks and defects, which lead to light refraction.

Figure 3. (A) TEM image; (B) AFM, (C) White light and (D) PL image of G@CB[8] drop casted on a glass plate.

The smaller nanoparticles, as seen in the TEM images, show essentially the same PL properties, we thus assign them to SP assemblies rather than crystals of the pure guests (i.e. undissolved or reprecipitated material) as the PL characteristics of the latter differ significantly (Figure 1). The presence of the small size bundled SPs in the aqueous suspension can be proven by PL anisotropy decay measurements rF(t). In fact, polarized PL stands as an excellent tool for indirect insights into the size62 and self-organization46,63-65 of supramolecular structures formed by emissive molecules; details on the technique are found in textbooks.66 The PL anisotropy rF was obtained from the measurements of the light intensity under vertical irradiation and vertical (horizontal) detection IVV (IVH): rF =

IVV − G ⋅ IVH IVV + 2 ⋅ G ⋅ IVH

(1)


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where the G-factor G = IHV/IHH was obtained from polarized measurements of dye solutions. In an isotropic system, the maximum value for rF is 0.4 if the transition dipole moments of absorption and emission are oriented in parallel (and rF = (3⋅cos2θ -1)/5 if they enclose an angle θ); in the current case, we irradiated at λ = 360 nm, i.e. in the main absorption band, so that rF,max = 0.4 is expected. Depolarization can take place through rotational motion during the fluorescence lifetime or by ET between translationally non-equivalent molecules. Rotational depolarization leads to (multi)exponential decay characterized by rotational correlation times τR,i. In the concurrent presence of ET between different molecules, the decay of rF takes the form rF (t ) = (r0 − r∞ ) ⋅ ∑ e

− t / τ R ,i

(2)

+ r∞

i

where r0 is the initial anisotropy (i.e. r0 = 0.4 for an isotropic system); r∞ is the residual anisotropy due to fixed orientations between the translationally non-equivalent molecules, between which ET takes place with a time constant τET. For immobilized particles (i.e. drop-casted on quartz), rotational diffusion is inhibited and thus, the rF kinetics are solely determined by ET with a time constant of about 0.4 ns, see Fig. 4; this will be discussed below in detail. In aqueous suspension an additional time constant of about 3 ± 0.3 ns is observed, which can be unambiguously assigned to rotational diffusion. According to Perrin this allows the determination of the particle volume V via66 τR =

η ⋅V 1 = 6 ⋅ DR kT

(3)

where DR is the rotational diffusion constant, η is the viscosity, k is the Boltzman constant and T is the temperature (in Kelvin). Inserting T = 298 K and η = 8.9·10-4 Pa⋅s (water, 25ºC), we obtain V ≈ 15 nm3; assuming a spherical shape we get an effective diameter of about 3 nm, which agrees reasonably well with the observations on the small sub-µm particles from the TEM graphs. Considering the structural inhomogeneities and size distribution (vide supra), the PL anisotropy decays are surprisingly simple, indicating that the size dispersion of the small size particles is quite narrow and independent on the concentration. On the other hand, we remind that µm-size particles (which are present in the suspension; vide supra) are not seen in the anisotropy decay as τR scales with the cube of the diameter (eq. 3), so that the resulting τR for µm particles is many orders of magnitude larger than τF. Anyway, it should be mentioned in this context, that the steady-state values of rF (which relates to τR via =


7


r0/(1+τF/τR)) for emission show a slight wavelength dependence (see Fig. S4), giving further evidence for structural inhomogeneity. 4 . 0

drop-casted τ = 0.4 ns

2 . 0

PL anisotropy rF

3 . 0 1 . 0 0 . 0

τ1 = 0.4 ns

1 . 0 -

0 5

τ2 = 3 ns

0 4

0 3

suspension

0 2

0 1

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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time / ns

Figure 4. Time-resolved fluorescence anisotropy rF(t) of G@CB[8] (λex = 405 nm, λem = 520 nm) in aqueous suspension (open symbols) and drop-casted on quartz (closed symbols); red lines are bi/mono-exponential fits, respectively.

Guest Orientation and ET. Time resolved PL anisotropy measurements permit a unique insight into the ET dynamics within the supramolecular assembly. It should be noted that within a single SP strand, energy migration might be quite effective, however this will not lead to noticeable PL depolarization as the molecules are arranged in a card packed motif, i.e. having a (approximate) translational symmetry. On the other side, depolarization will be induced by ET if translationally non-equivalent molecules are present, characterized by the inclination between the transition dipole moment responsible for the absorption/emission process (which are oriented parallel to the long molecular axis; vide supra). In fact, quantum chemical calculations reveal that the molecules enclose an angle of about α = 18º against the SP direction.40 Since it is not expected that an orientational correlation between neighboring SP strands in the bundle exists, the molecules are oriented on the surface of a cone with an opening angle of β = 2⋅α (see Scheme 2), which can be determined from the PL anisotropy measurement via67 r∞  1  = cos β (1 + cos β )  r0  2 

2

(4)

Inserting r0 = 0.4 and r∞ = 0.2 (Figure 4) a half-cone angle of α = 19º is obtained; this is in agreement with the calculated value, and demonstrates how the combination of polarized


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PL spectroscopy with structural and computational characterizations allows for a unique structural insight into the self-assembly of supramolecular systems. As shown in Fig. 4, the time-constant for this ET process is τET ≈ 0.4 ns, observed both for the particles in suspension, as well as immobilized on quartz.68 This is surprisingly long compared to related systems. For instance, in pure single crystals of the structurally related distyrylbenzene (DSB) molecule, sub-ps exciton dynamics within the herringbone layers were observed, along with a time constant of 43 ps for ET between the layers.61,62 Even more strikingly, the ET dynamics are much faster in the well-studied supramolecular host−guest assembly of DSB in PHTP.25 Here, the DSB molecules are aligned in a strictly co-linear (head-to-tail) fashion in the nanochannels of PHTP host; the latter are separated by ca. 1.5 nm (see Scheme 2).46 This is very similar to the average distance between G molecules of neighboring SP strands in the bundles of G@CB[8], i.e. 1.6 nm. In DSB@PHTP however, the time-constant for nearest-neighbor inter-channel ET was calculated to be only ~20 ps.53,54 To understand the difference between the both supramolecular systems, we have to recall the physical ingredients of the ET process, which can be essentially described by a Förster-type mechanism, however reformulated at a quantum-chemical basis because of the breakdown of the classical point dipole-based picture at short inter-chromophore separations as indeed observed in such supramolecular assemblies.53 In a general formulation one obtains k ET =

2 ⋅π 2 V J DA h

(5a)

for the rate constant kET of ET between a donor (D) and an acceptor (A) molecule, where V is the screened excitonic coupling, and JDA is the spectral overlap between donor emission and acceptor absorption, both area normalized on an energy scale. Rewriting all energies in wavenumbers, and explicitly considering the screening as n-4 (being a reasonable assumption for distances > ca. 1.5 nm)69 we obtain for homo-transfer (A = D) 2  cm  ∆ν~DD k ET = 1.1835 ⋅   ⋅ ⋅ J DD 4  ps  n

(5b)

with ∆ν~DD as the unscreened excitonic coupling (in cm-1). To estimate kET, we determined JDD from the experimental spectra; ∆νDD were obtained from TD-DFT calculations for molecules in neighboring SP strands without displacement along the SP axis (such displacement will largely reduce ∆νDD). The comparison of G@CB[8] and DSB@PHTP in Table 1 shows that


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∆νDD is essentially the same for a parallel alignment with ∆νDD ≈ 110 cm-1, and hardly change if the different orientations on the cone surface are considered. Thus, the difference in kET should mainly arise from the spectral overlap; in fact, in DSB, the spectral overlap is moderately large (J = 1.3 × 10-5 cm-1; see Scheme 2) due to medium structural reorganization energies (∆Ere ≈ 0.25 eV) as expected for a flexible molecule with planar equilibrium geometries in S0 and S1, respectively.70 On the contrary, G exhibits larger structural reorganization due to very non-planar equilibrium geometries of G in S0 and S1; this is directly visible in the large Stokes shift of 1 eV between the (approximately) mirrorsymmetrical absorption and PL spectra, so that ∆Ere ≈ 0.50 eV; i.e. double as large in DSB; see Scheme 2. This reduces the spectral overlaps roughly by a factor of forty to J = 2.9 × 10-7 cm-1. Taking into account the smaller refractive index in curcubiturils (n ≈ 1.2)71 compared to PHTP (n ≈ 1.4),53 the different dynamics in the two systems can be sufficiently explained. In fact, the calculated rate of kET = 2.1 ns-1 fits surprisingly well the experimental value kET = 2.5 ns-1, taking into account the simplicity of our computational approach.

Scheme 2: Comparison of the HGCs (left: G@CB[8], right DSB@PHTP): arrangements, PL (red) and absorption spectra (blue), type of interactions, inter- and intra-channel distances, inclinations, spectral overlaps and calculated ET rates.


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Table 1: Energy transfer dynamics in G@CB[8] vs. DSB@PHTP: measured ET transfer times (τET; rate constants kET = τET-1), spectral overlaps JDD', and calculated exciton couplings ∆νDD and kET; eq. 5b. measured

Calculated '

-1

SP

τET / ns

kET / ns-1

JDD / cm

∆νDD / cm-1

kET, n4 (calculated)

G@CB8

0.4

2.5

2.9 × 10-7

112

2.1 ns-1

DSB@PHTP





1.3 × 10-5

120

55 ns-1

Light-Harvesting. The different ET dynamics in G@CB[8] and DSB@PHTP have crucial consequences for the application of such systems, e.g. in artificial light-harvesting systems. Recently, a CB[8]-based system was reported by Kim et al. which is based on B as a donor and R as an acceptor (for molecular structures see Scheme 1; for optical spectra see Figs. S6, S7);40 this B:R@CB[8] system was shown to form block co-SP nano-bundles as sketched in Fig. S8. At a D:A ratio of 1:1, an energy transfer efficiency of η = 0.68 was observed for B:R@CB[8], obtained from the PL kinetics according to η = 1 -τDA/τD

(6)

in which τDA and τD are the amplitude-averaged PL lifetimes of D in the presence and absence of A, respectively. Although ET is apparently effective, comparison with the (however waterinsoluble) PHTP-based system shows that such efficiencies can be reached at much lower doping ratios. In fact, in the DSB@PHTP system doped with an appropriate energy acceptor, i.e. quinquethiophene (5T), an ET efficiency of η = 0.70 was reached at doping ratios as low as 1·10-3.53,54 The structural properties of B@CB[8] are quite similar to that of G@CB[8], with a somewhat enlarged orientational angle against the SP strand direction (αcalc = 27º, αexp = 26º). The time constant for the anisotropy decay for the immobilized nano-bundles of

B@CB[8] is 0.2 ns (Fig. S5), similarly small as in G@CB[8]; and indeed, both spectral overlap (1.7 × 10-7 cm-1; see Fig. S6) and exciton coupling (107 cm-1) of B@CB[8] are very similar to those in G@CB[8], rationalizing the slow ET dynamics in the B:R@CB[8] system. The prominence of DD overlap to the observed dynamics also gives further evidence for the modus of SP formation, i.e. block co-SPs as suggested earlier,40 since only in that case D-D ET steps are crucial for the overall ET efficiency even at high molar ratios of the acceptor as observed in the experiments.


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In conclusion, time-resolved polarized PL measurements of supramolecular polymers gave clear evidence for the structure of the CB[8]-based SPs, confirming the suggested arrangement from quantum chemistry. In combination with direct and indirect structural methods (TEM, AFM, PL microscopy), structural inhomogeneities both in size and bundle structure were revealed. Steady-state and time-resolved PL spectroscopic investigation of

G@CB[8] and a pure crystal of G elucidated the relationship between chromophore selfassembly and the resulting emission properties. Importantly, the PL anisotropy allowed for an understanding of the ET dynamics, underlining the critical importance of the spectral overlap on the time constant of the ET steps, and the resulting ET efficiency for CB[8]-based supramolecular donor−acceptor light-harvesting systems.

ASSOCIATED CONTENT Supporting Information: Experimental and computational details; PL decay of the G crystal; optimized geometries of G in S0, S1; zoomed AFM image, steady-state and polarized PL spectrum of G@CB[8]; absorption, steady-state and polarized PL spectrum, time-resolved PL anisotropy of B@CB[8]; absorption and PL spectra of R@CB[8]; sketch of the

B:R@CB[8] co-SP nanobundles. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].

ORCID Soo Young Park: 0000-0002-2272-8524 Johannes Gierschner: 0000-0001-8177-7919

Notes CCDC 1585802 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.


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Acknowledgments The work in Madrid was support by the 'Severo Ochoa' program of the Spanish Ministerio de Economía y Competitividad for Centers of Excellence in R&D (MINECO, Grant SEV-20160686), by the MINECO-FEDER projects CTQ2014-58801, CTQ2017-87054, by the Comunidad de Madrid (Project Mad2D, Grant No. S2013/MIT-3007) and by the Campus of International Excellence (CEI) UAM+CSIC. P. C. N. acknowledges funding from the EC via the COFUND program AMAROUT and the Severo Ochoa program. The authors especially thank R. Wannemacher (Madrid) for assistance in the microscope measurements, H. Bolink (Valencia) for access to the integrating sphere and J. P. Hernáez (Sidi, UAM, Madrid) for help in the crystal structure analysis. The work at IMDEA was performed further in the context of the European COST Action Nanospectroscopy, MP1302. The work at Seoul National University was supported in parts by the Creative Research Initiative Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP; Grant No. 2009‐0081571[RIAM0417‐20150013]) and by Basic Science Research Program through the NRF funded by the Ministry of Science, ICT and Future Planning (Grant No. 2017R1E1A1A01075372).

References (1) Ariga, K.; Kunitake, T. Supramolecular Chemistry: Fundamentals and Applications: Advanced Textbook. Springer: Heidelberg, 2006. (2) Schneider, H.-J. Applications of Supramolecular Chemistry for 21st Century Technology. Taylor & Francis: Boca Raton, FL, 2012. (3) Kolesnichenko, I. V.; Anslyn, E. V. Practical applications of supramolecular chemistry. Chem. Soc. Rev. 2017, 46, 2385-2390. (4) Sayed, M.; Pal, H. Supramolecularly assisted modulations in chromophoric properties and their possible applications: an overview. J. Mater. Chem. C 2016, 4, 2685-2706. (5) Ma, X; Zhao, Y. Biomedical Applications of Supramolecular Systems Based on Host− Guest Interactions. Chem. Rev. 2015, 115, 7794-7839. (6) Webber, M. J.; Appel, E. A.; Meijer, E. W.; Langer, R. Supramolecular biomaterials. Nat. Mater. 2016, 15, 13-26. (7) Wang, H.; Ji, X.; Li, Z.; Huang, F. Fluorescent Supramolecular Polymeric Materials. Adv. Mater. 2017, 29, 1606117.


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