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Fluorescent-Cavity Host: An Efficient Probe to Study the Supramolecular Recognition Mechanisms Wei Cui, Lingyun Wang, Linxian Xu, Guozhen Zhang, Herbert Meier, Hao Tang, and Derong Cao J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00037 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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The Journal of Physical Chemistry Letters

Fluorescent-Cavity Host: an Efficient Probe to Study the Supramolecular Recognition Mechanisms Wei Cui,

,†

Lingyun Wang,

,†

Linxian Xu,† Guozhen Zhang,*,‡ Herbert Meier,⁑ Hao Tang,*,† Derong

Cao*,† †

State Key Laboratory of Luminescent Materials and Devices, School of Chemistry and Chemical Engi-

neering, South China University of Technology, Guangzhou 510641, China ‡

Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation

Center of Chemistry for Energy Materials), School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, China ⁑

Institute of Organic Chemistry, University of Mainz, D-55099 Mainz, Germany

AUTHOR INFORMATION Corresponding Author *

[email protected]; [email protected]; [email protected].

ABSTRACT: Using fluorometry to study the interactions between guests and host cavities is often challenging, especially for hosts with small cavities as the fluorophore may not be close to the binding site. Therefore, it is critical to overcome this hurdle to broaden the applicability of fluorometry in supramolecular chemistry. Herein, we designed a fluorescent-cavity host (H1) by conjugating the binding site of a pillar[5]arene cavity and studied its host-guest recognition mechanism in the cavity. Distinct fluores1 ACS Paragon Plus Environment

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cent responses of H1 were observed for cyano homologues: the fluorescence was enhanced for succinonitrile but quenched for malononitrile. Such an unusual phenomenon with so subtle difference in guest structure was attributed to the different host-guest interactions induced by the subtle difference of guest locations within the H1 cavity. Our results indicate that developing fluorescent-cavity hosts as probes will pave a powerful and insightful way to explore the exquisite details for host-guest recognition, self-assembly, and molecular machinery.

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Understanding and quantification of supramolecular interaction is essential for constructing highly organized and constitutionally dynamic systems.1-8 Common methods (including fluorometry, NMR, calorimetry, crystallography, etc.) have been widely employed to study the binding mechanism, e.g. the position of encapsulated guest within the host cavity, the binding affinity, the type of weak interactions between host and guest, etc. Fluorometry, aside from its direct, rapid, simple and sensitive characteristics, can provide unique information for the host-guest binding that cannot be obtained by other methods. However, using fluorometry to study the host-guest recognition mechanism is often challenging, especially for macrocycles with small cavities e.g. pillar[5]arene, cucurbit[6]uril, alpha-cyclodextrin, calix[4]arene. Two main reasons for this limitation are (1) that fluorescent molecules are too large to fit in the small cavities, and (2) that the fluorophore attached to the host molecule was outside the cavity and away from the binding site. Thus, introducing fluorescence labels into the binding site in a hostguest system is of great interest and yet a significant challenge. Conjugated materials with various light-emitting properties are constantly being developed and exploited in various applications, e.g. organic light-emitting diodes, sensors, color tunable emitters, cell imaging agents, photo-responsive materials, etc.9-19 Incorporating conjugated fragments into a supramolecular system has gained considerable interest for their tunability, responsiveness to stimuli, and the ease to constitute building blocks bearing a wide range of functionality.20-26 In particular, merging conjugated structures and the macrocyclic hosts has been achieved by (1) binding conjugated guests to hosts,27-29 (2) synthesizing fully conjugated macrocycles,30, 31 (3) attaching macrocyclic hosts onto conjugated backbone as pendant groups,32-36 or (4) incorporating macrocycles into the conjugated structure.37-43 The last approach, i.e. the strategy adopted here, may affect the volume, rigidity and electrostatic potential of the host cavity and, consequently, could tune the photophysical properties and the host−guest interactions. Specifically, pillar[5]arene, a promising class of macrocycles, was chosen for making the conjugated hosts due to its ease of synthesis and functionalization.44-53 Although few conju3 ACS Paragon Plus Environment


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gated pillararenes have been reported, none of the studies has taken advantage of the fluorescence signals for the evaluation of the host-guest recognition mechanism.38-41 As important organic solvents and starting materials, synthetic nitriles are wildly applied in the field of polymers, plastics, herbicides, and pharmaceuticals. Unfortunately, some nitriles are highly toxic for they are metabolized to cyanide, whereas some others are mildly toxic, e.g. the IC50 values of malononitrile (G1), succinonitrile (G2), and acetonitrile for rats are 14, 450, and 3800 mg/kg, respectively. 54, 55 Therefore, identification of nitrile homologues is crucial for the public health, environmental and industrial purposes. Conventional means to detect nitrile homologues, e.g. gas or column chromatography, rely on comprehensive methods, expensive instruments and trained personnel.56

To address all these ideas and issues, our strategy is to design a novel host by conjugating chromophore to the host cavity (i.e. a fluorescent-cavity host) and to use it to explore the host-guest recognition mechanism in the cavity. A pillar[5]arene H1 was designed as our first fluorescent-cavity host and was employed to bind and differentiate cyano homologues. As the fluorescent-cavity host was used to bind guests with a subtle difference of one methylene group, the fluorescence enhancement was observed for one guest whereas quenching for the other. The distinct response was due to the different host-guest interactions induced by the subtle difference of guest locations in H1 cavity. In other words, the fluorescence of H1 was very sensitive to the distance between the fluorescent unit of H1 and the electron withdrawing moiety of the guest, which provide an ideal signal to monitor the host-guest binding in situ and in real time, especially for the general case involving a small-cavity host or a nonfluorescent guest. H1 and its control molecule H2 were obtained by Suzuki reaction with high yields (see Scheme S1) and were characterized by NMR and HRMS (Supporting Information Figure S1 − S12). Briefly, the thi4 ACS Paragon Plus Environment

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ophene unit was linked to pillar[5]arene directly, followed by linking each 5-position of thiophene with a quinoxaline unit.57-60 H1 has better solubility in solvents (e.g. CHCl3, CH2Cl2, THF, etc.) than H2, suggesting that the presence of bulky pillar[5]arene moiety may suppress the aggregation of the conjugated segments. The absorption bands at 306 nm and 402 nm observed for the H1 in CHCl3 (Supporting Information Figure S13) were assigned to the π−π⁎ transition for pillararene unit38 and to the intramolecular charge transfer from the pillararene unit to the 2,3-di(pyridin-2-yl)quinoxaline unit, respectively. This intramolecular charge-transfer band was blue shifted relative to the band for H2 at 426 nm, which was consistent with the different HOMO/LUMO contour maps calculated for H1 and H2 (Supporting Information Figure S14). Both H1 and H2 in CHCl3 emit green light centered at 507 nm with the absolute fluorescence quantum yields of 11.1% and 64.3%, respectively. The higher value for H2 may be due to its larger spatial overlap between HOMO and LUMO. The absolute fluorescence quantum yields in solid state were 13.6% and 3.5% for H1 and H2, respectively, suggesting that the bulky pillar[5]arene moiety in H1 may suppress the π···π stacking in solid state and suppress the fluorescence quenching. Moreover, the solvent dependence of the fluorescence spectra was observed clearly for H1 and H2, although the solvent dependence of the absorption spectra was not very conclusive (Supporting Information Figure S15). The fluorescence peaks for H1 and H2 shifted to longer wavelength and the intensities decreased with the polarity of solvents increasing. In methanol, the fluorescence of H1 and H2 were completely quenched, showing a typical solvent-polarity-induced fluorescence quenching. Eleven different guests were selected to bind to H1 in CHCl3, since most of them have high binding affinities to well-studied ethoxypillar[5]arene or methoxypillar[5]arene.61 Upon addition of various guests at low concentration (0.1 mM), H1 displayed excellent selectivity towards G2 with 135% fluorescence enhancement and 16 nm blue shift (Figure 1a). The intensity increase and blue shift were also observed in the absorption spectra (Supporting Information Figure S16). Other guests induced no obvious absorption or fluorescence change under the same experimental condition (see red bars in Figure 5 ACS Paragon Plus Environment


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1b). As for H2, no significant fluorescence changes were observed in the presence of all guests (Supporting Information Figure S17), suggesting that the spectral change was related to the host-guest binding. To further confirm it, we studied the H1/benzeneacetonitrile system. Nearly no fluorescence change was observed for H1 with 50 mM benzeneacetonitrile, as the bulky benzene ring of the guest cannot be encapsulated into the cavity of pillar[5]arene. Moreover, no fluorescence signals were observed for nonfluorescent host system (e.g 1,4-dimethoxypillar[5]arene) in the presence or absence of guests.

Figure 1. (a) Fluorescence spectra of H1 with different guests (0.1 mM). Inset: Fluorescence of samples under a UV-lamp (365 nm). (b) Relative fluorescence intensity change (ΔI/I0) of H1 in the presence of guests. [guest] = 0.1 mM (red) or 20 mM (blue). (c) Binding isotherm of H1 with G2. (d) Binding isotherm of H1 with G1. [H1] = 5.0 μM for all samples and CHCl3 was used as the solvent.


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Upon addition of guests at high concentration (20 mM), only G1 induced the fluorescence quenching of H1, whereas almost all guests led to the fluorescence enhancement (see blue bars in Figure 1b and Supporting Information Figure S18). The emission enhancement upon the binding of most of guests was not surprising as the bound guests could restrict the motion of the pillararene moiety, suppress the collision between the excited H1 and the encapsulated solvent molecules, and thus hinder the non-radiative decay. However, the fluorescence quenching observed for H1 with G1 is interesting. To further investigate the host-guest binding, the guest-concentration dependence of the fluorescence spectra was determined (Figure 1c, 1d, and Supporting Information Figure S19). The binding isotherms were fit well with a 1:1 host−guest binding model (SI). The equilibrium binding constant (Ka) for G2 (1.1 × 104 M-1) was at least 10 times higher than for other guests (2.1 - 9.9 × 102 M-1, Table 1). The detection limit for G2 was (5.3 ± 0.2) × 10-8 M (Supporting Information Figure S20). The relative fluorescence quantum yield (ϕ), i.e. the ratio of the absolute fluorescence quantum yield of the bound host to that of the free host, varied between 1.96 and 4.46 for the binding of H1 to all guests except for G1 and acetonitrile. Therefore, the fluorescence enhancement of H1 selectively observed for G2 in Figure 2a was due to the high Ka for H1⊃G2 such that only G2 can bind to H1 at low concentration of guests (0.1 mM). Moreover, the selectivity of H1 toward G1 at high concentration, i.e. that only G1 quenched the considerable amount of H1 fluorescence, was due to the very low  (0.01) and the moderate binding affinity (66 M-1) for H1⊃G1. It is worth noticing that no fluorescence change was observed for H1 in 20 mM acetonitrile, although the  value for H1⊃acetonitrile is very low (0.04). This is due to the low Ka (2.1 M-1) for H1⊃acetonitrile such that 20 mM acetonitrile did not lead to the formation of H1⊃acetonitrile. Table 1 Ka and  for the binding of guests to H1 in CHCl3 at 298 K Guest

Ka / M-1



2.1 ±0.3

0.04 ±0.01

(6.6 ±0.2) × 10

0.01 ±0.01 7

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(1.11 ±0.04) × 104 3.80 ±0.03 (5.0 ±0.3) × 102

1.96 ±0.01

(2.5 ±0.1) × 102

3.72 ±0.02

(2.6 ±0.2) × 10

2.23 ±0.05

(9.9 ±0.3) × 102

4.46 ±0.04

(2.9 ±0.2) × 10

2.16 ±0.03

(7.0 ±0.2) × 10

3.89 ±0.02

(7.3 ±0.2) × 10

4.00 ±0.01

(2.8 ±0.1) × 10

2.25 ±0.03

The NMR, single-crystal studies and density functional theory (DFT) calculations were conducted for H1⊃G2 and H1⊃G1. The formation of H1⊃guest in CDCl3 was studied by 1H NMR (Supporting Information Figure S21 − 23). The proton signals of the quinoxaline moiety in H1, unlike the shifted proton signals of the pillar[5]arene moiety, remained unchanged, indicating no interaction between guest and the quinoxaline moiety. The methylene proton peaks for the encapsulated guests were broadened and shifted upfield relative to those for the free guest due to the inclusion-induced shielding effects.62, 63 Slow and fast exchanges on the NMR timescale were observed for the binding of H1 to G2 and G1, respectively, suggesting different complexation kinetics. Eight C−H···N interactions (H···N distances: 2.8 − 3.3 Å) and six C−H···π interactions (H···π distances: 2.5 − 3.3 Å) were observed for H1⊃G2 (Figure 2a and Supporting Information Figure S24), indicating that two cyano groups of G2 were both fixed to the cavity portal. In general, the C−H···N interactions between the guest’s cyano groups and the host’s alkoxyl hydrogens, together with the complementary charge distribution in the host and guest, led to a strong binding in a pillararene-G2 or pillararene-adiponitrile system.64, 65 Otherwise the C−H···π interactions solely led to a weak binding as in a pillararene-alkane system.66 8 ACS Paragon Plus Environment

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Figure 2. (a) Crystal structure of H1⊃G2. (b) Partial 2D NOESY spectrum (600 MHz, CDCl3, 298 K) for H1⊃G1 and the complex structure predicted by DFT calculation. The structure of H1⊃G2 optimized by DFT was similar to that determined by single-crystal X-ray diffraction (see the Cartesian coordinates listed in Supporting Information Figure S25), proving the adequacy and reliability of the computational method. As for G1, the optimized structure of H1⊃G1 calculated by DFT showed a deep encapsulation of the guest within the pillararene cavity (Figure 2b). Four C−H···N interactions (H···N distances: 2.9 − 3.2 Å) and four C−H···π interactions (H···π distances: 2.7 − 3.1 Å) were observed. The distance for each type of interaction is similar for H1⊃G1 and H1⊃G2. However, the number of the interactions for H1⊃G1 was much fewer than for H1⊃G2 and, therefore, led to the value of Ka for H1⊃G1 168 times lower than for H1⊃G2. Moreover, the correlations were observed between the methylene protons (Ha) of G1 and the aromatic protons (H5), methoxy protons (H12) as well as the methylene (H9) or methoxy protons (H14) of H1 in NOESY spectra (see dotted lines in the inset of Figure 2b). The consistency between the DFT calculations and the NOESY data 9 ACS Paragon Plus Environment


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strongly supports the validity of structure proposed for H1⊃G1, although the attempts to grow the single crystal of this complex unfortunately failed. Time-dependent DFT calculations showed that the S0-to-S1 transitions in both complexes were mainly contributed by the electronic transition from HOMO to LUMO. An intramolecular charge transfer process was observed for each complex (Supporting Information Figure S26), which was similar to that for the free H1 (Supporting Information Figure S14). Interestingly, the oscillator strength for S0-to-S1 transition in H1⊃G1 (0.002) is one order of magnitude smaller than in H1⊃G2 (0.02). The remarkably different oscillator strength, which originated from the reverse transition of the S1-to-S0 transition, is consistent to the different fluorescence response observed for these complexes. Furthermore, the electrostatic potential was significantly more negative in the cavity for H1⊃G1 than for H1⊃G2 (Figure 3), suggesting a polarity-induced fluorescence quenching for the former. The fluorescence quenching of H1 by the binding of G1 is consistent with the fluorescence quenching of H1 observed in methanol. In both cases, the fluorescence was quenched when an electron withdrawing group was closed to the fluorescent unit of H1. To further confirm the mechanism, a nitro compound nitromethane was employed as a guest or solvent for H1 (Supporting Information Figure S27), all leading to the fluorescence quenching due to the presence of the nitro group as a strong electron-withdrawing group. Furthermore, an interesting experiment was conducted by adding the high concentration of G2 (10 M) into H1 solution (Supporting Information Figure S28). The fluorescence peak of H1 in the presence of 10M G2 was blue-shifted to the same extent as the peak of H1 in the presence of 0.1 mM G2, both showing the binding of H1 to G2. However, unlike in the presence of 0.1 mM G2 where the fluorescence of H1 was enhanced (Figure 1a), the fluorescence of H1 was decreased in the presence of 10 M G2. This result is not surprising as the cyano group of G2 can approach the fluorescent unit of H1 easily when H1 was surrounded by the extensive amount of G2, leading to a polarity-induced fluorescence quenching.


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Figure 3. Electrostatic potentials mapped on the electron isodensity surface of H1⊃guests with a density of 0.001. To explore the applications, H1 was employed to prepare test papers. The fluorescence response of the test papers upon a drop of G1 or G2 aqueous solution (Supporting Information Figure S29) were similar to that observed in solutions (see the insets of Figure 2a and 2b). No signals were observed for the test papers prepared with nonfluorescent host (e.g 1,4-dimethoxyPillar[5]arene). These results demonstrate potential practical usefulness of H1 for detecting highly toxic pollutant G1 in water. It's worth noting that the sensitivity of the test paper is not as good as that of the H1-containing CHCl3, probably due to the low solubility of H1 in water. A following project with a water-soluble fluorescentcavity pillararene is currently underway. In conclusion, a conjugated fluorescent pillar[5]arene H1 was designed as our first example of fluorescent-cavity host, i.e. a host with a conjugated cavity for sensing guest molecules with fluorescence. The fluorescent-cavity host allows us to explore the exquisite details for host-guest recognition process in the cavity: upon the binding of H1 to two guests with a subtle difference of one methylene group, the fluorescence enhancement was observed for G2 whereas quenching for G1. Such an unusual fluorescence behavior is attributed to the different host-guest interactions induced by the subtle difference of 11 ACS Paragon Plus Environment


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the guest position in the fluorescent-cavity host. Deep encapsulation of cyano group of G1 in the host cavity significantly induced the negative electrostatic potential and led to a polarity-induced fluorescence quenching for the fluorescent-cavity host. The mechanism was confirmed by NMR, crystallography and DFT calculations. Thus, an effective and informative method by fluorometry is developed for study on the host-guest recognition mechanism. Based on the unusual fluorescence behavior caused by the host-guest interactions, easy discrimination between G2 and G1 were realized, which is very difficult in analytical chemistry. Future study of a bunch of fluorescent-cavity hosts with different conjugated structures, such as fluorescent-cavity pillar[6]arene and fluorescent-cavity calix[4]arene, and their different host-guest behaviors will aid the rational design of functional supramolecular systems and are currently underway.

ACKNOWLEDGMENTS This work was funded by the National Key Research Development Program of China (2016YFA0602900, 2016YFA0400904), the NSFC (21772045, 21572069), the Guangdong NSF (2015A030313209, 2016A030311034), and the Fundamental Research Funds for the Central Universities (2017ZD075). HT is grateful to the SCUT “Xinghua Scholar Talent Program”. GZ is grateful to High Performance Computing Center at USTC for providing computing resource. ASSOCIATED CONTENT Supporting Information Available: Experimental methods, synthetic procedures, and additional structural and spectroscopic data. AUTHOR INFORMATION ORCID 12 ACS Paragon Plus Environment

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Derong Cao: 0000-0002-5658-1145 Hao Tang: 0000-0003-1063-881X Guozhen Zhang: 0000-0003-0125-9666 Notes ∥These

authors contributed equally.

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