Multistimuli-Responsive Enaminitrile Molecular Switches Displaying H

Communication Cite This: J. Am. Chem. Soc. 2018, 140, 13640−13643

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Multistimuli-Responsive Enaminitrile Molecular Switches Displaying H+‑Induced Aggregate Emission, Metal Ion-Induced Turn-On Fluorescence, and Organogelation Properties Yansong Ren,† Sheng Xie,†,‡ Erik Svensson Grape,§ A. Ken Inge,§ and Olof Ramström*,†,∥,⊥ †

Department of Chemistry, Royal Institute of Technology, Teknikringen 36, S-10044 Stockholm, Sweden College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, People’s Republic of China § Department of Materials and Environmental Chemistry, Stockholm University, SE-10691, Stockholm, Sweden ∥ Department of Chemistry, University of Massachusetts Lowell, One University Avenue, Lowell, Massachusetts 01854, United States ⊥ Department of Chemistry and Biomedical Sciences, Linnaeus University, SE-39182 Kalmar, Sweden

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‡

S Supporting Information *

by intramolecular hydrogen bonding (IMHB), and which could undergo constitutional exchange.5 Such controllable properties can potentially lead to the construction of “smart materials” for various applications. For example, switches displaying stimuli-responsive fluorescence under external control display high potential as, e.g., molecular probes, sensors, and organic light-emitting diodes.6 Moreover, molecular machines and robots with switchable fluorescent output hold great promise for different applications.4d,7 However, to achieve distinct and differentiated properties of the switching states is still challenging. In addition, the fidelity and efficiency of the applications are often affected by the conditions of establishing switching, as well as the isomeric E/ Z-ratio formed in the process. Ideally, switching should take place rapidly and reversibly under mild conditions, with close to complete E-to-Z- or Z-to-E-transitions. The process should also be stable over many cycles. Herein, these challenges have been addressed, and we report a novel type of enaminitrile-based rotary molecular switches ((1−3)-E, Figure 1). The structures could be precisely regulated by acid/base, and the switching states were associated with aggregation-induced emission (AIE),8 and CuII-responsive fluorescence. A switchable, fluorescent organogelator could furthermore be demonstrated.

ABSTRACT: Multistimuli-responsive enaminitrile-based configurational switches displaying aggregation-induced emission (AIE), fluorescence turn-on effects, and supergelation properties are presented. The E-isomers dominated (>97%) in neutral/basic solution, and the structures underwent precisely controlled switching around the enamine CC bond upon addition of acid/base. Specific fluorescence output was observed in response to different external input in the solution and solid states. In response to H+, configurational switching resulted in complete formation of the nonemissive Z-H+-isomers in solution, however displaying deep-blue to blue fluorescence (ΦF up to 0.41) in the solid state. In response to CuII in the solution state, the E-isomers exhibited intense, turn-on, blue-green fluorescence, which could be turned off by addition of competitive coordination. The acid/baseactivated switching, together with the induced AIE-effects, further enabled the accomplishment of a responsive superorganogelator. In nonpolar solvents, a blue-fluorescent supramolecular gel was formed upon addition of acid to the E-isomer suspension. The gelation could be reversed by addition of base, and the overall, reversible process could be repeated at least five cycles.

A

rtificial molecular switches with distinct properties related to their different states have drawn considerable attention over the past few decades.1 Configurational transitions, especially cis/trans-isomerization of double bonds, have thus been widely studied, resulting in a variety of intriguing architectures that are able to perform configuration-specific functions.2 In particular, photoinduced processes have been extensively employed to manipulate the structure and properties of various entities.1c,e,3 Chemically activated configurational switches are in this context considerably less explored, for example demonstrated in acyl hydrazone-based processes leading to a range of structurally interesting systems.4 Recently, we also demonstrated a multiresponsive enamine-based molecular switch, in which the states in part were controlled © 2018 American Chemical Society

Figure 1. Acid/base-activated switching process of enaminitrile-based molecular switches; ORTEP drawings of enaminitrile 1-E (left) and 1-Z-H+ (right) (thermal ellipsoid plots at the 50% probability level). Received: September 18, 2018 Published: October 11, 2018 13640

DOI: 10.1021/jacs.8b09843 J. Am. Chem. Soc. 2018, 140, 13640−13643

Communication

Journal of the American Chemical Society

Conversely, the solids of protonated isomers 1-Z-H+ and 2Z-H+ showed high quantum yields (ΦF = 0.37 and 0.41, respectively) with strong blue to deep-blue emission (λem= 460 and 443 nm, respectively; Figure 2). The fluorescence of

The enaminitrile switches 1−3 were obtained through a condensation process with isolated yields of 48−75%.9 NMR spectroscopy indicated that the purified products displayed exclusive E-configuration in organic solvents. This was identified by the broad NH-signals at 10.4 to 10.6 ppm, implying formation of an IMHB between the pyridine nitrogen and the enamine NH group. In solution, small amounts of the corresponding Z-isomers were slowly formed, stabilizing at approximately 3%. Upon addition of methylsulfonic acid (MSA) to the neutral solutions, compounds 1−3 underwent switching (Figures S2−S4). For example, addition of 1.0 equiv of MSA to a solution of compound 1 in CD3CN resulted in switching of isomer 1-E to a high degree (93%) of the protonated isomer 1-Z-H+. The 1H NMR spectra indicated that the pyridinium signals of the Z-H+-isomer generally shifted downfield, that a broad N−H-signal appeared at 7.64 ppm, and that the alkene CH-signal showed a distinguished shift to 8.69 ppm. Further addition of MSA to 2.0 equiv led to full conversion to enaminitrile 1-Z-H+, as shown by the disappearance of the signals of isomer 1-E in the 1H NMR spectrum, in which the N−H signal of enaminitrile 1-Z-H+ shifted to 7.76 ppm. Subsequent addition of 4.0 equiv of Et3N resulted in rapid restoration of the original E/Z-ratio of the enaminitrile solution. The configurations of the two states of enaminitrile switch 1 could be confirmed by 2D NOESY NMR spectroscopy and single crystal X-ray diffraction (cf. Supporting Information). The XRD results explicitly demonstrated the configurations of the two isomers in the crystalline states (Figure 1). The structure determined from the crystal grown from the solution of compound 1-E showed a distinct IMHB between the enaminitrile N−H hydrogen atom and the pyridyl nitrogen atom (N−H···N, 2.085 Å), whereas no IMHB effect was observed in the structure of protonated compound 1-Z-H+. An interesting AIE effect was unexpectedly observed for protonated species (1−3)-Z-H+. After switching of the enaminitrile E-isomers to their Z-H+-counterparts, the solutions did not show obvious fluorescence under UV-light. However, the white solids obtained by evaporation of the solvents of the Z-H+-isomers exhibited strong deep-blue fluorescence under UV light. The photophysical properties of the two isomers of enaminitriles 1−3 in the solid state were subsequently determined (Table 1). The quantum yields (ΦF) and quantum lifetimes (τ) of compounds (1−3)-E in the solid states proved inaccessible due to their low sensitivities.

Figure 2. Fluorescence spectra of films of enaminitrile 1-E (black) and 1-Z-H+ (blue); Inset: photographs of enaminitrile 1-E (left, solid), 1-Z-H+ (middle, solid), and 1-Z-H+ (right, 0.1 mM suspension in THF/MeOH (10:90)) taken under hand-held UV-light (365 nm).

enaminitrile 1-Z-H+ could also be observed in the film state (λem= 454 nm), and as a suspension in MeOH/THF (90/10). These results clearly demonstrate that enaminitriles (1−3)-ZH+ represent a series of novel blue, pH-responsive AIEgens. To elucidate the cause of the fluorescence behavior of the two enaminitrile states upon aggregation, the crystal structures of compounds 1-E/1-Z-H+ and 2-E/2-Z-H+ were examined. This revealed that the compounds adopted perpendicular structures that were highly twisted around the C−N single bond. The nonemissive isomers 1-E and 2-E displayed a “headto-head” stacking arrangement in which large degrees of π−πinteractions could be observed (Figures S13, S15). In contrast, the strongly emissive enaminitrile 1-Z-H+ adopted a “head-totail” stacking arrangement with a low degree of π−πinteractions between the pyridinium and the phenyl rings thereby decreasing a potential quenching effect (Figure S14). The crystal structure of strongly emissive enaminitrile 2-Z-H+ exhibited only a low degree of π−π-interactions between the pyridinium moieties, and no interactions between the phenyl rings (Figure S16). Although protonation would enhance the acceptor effect in the enaminitriles, the slight hypsochromic shift indicates that the configurational restrictions in the solid state play an important role in the Z-isomers. Because of these conspicuous fluorescence results, we envisioned that the enaminitrile switches could be applied to controllable gelation with concomitant luminescence generation. Sol-to-gel fluorescence has been reported for other AIE systems,10 and the unique responsive properties of the enaminitrile switches would make the systems adaptive. Thus, a switchable low-molecular-weight-gelator (LMWG) was designed and synthesized by employing a rotor based on a glutamic acid derivative carrying long aliphatic chains (enaminitrile 4, Figure 3). Initially, different solvents were screened with isomer 4-E (1.0% (w/v)), however not resulting in any gel formation (Table S1). Conversely, switching to enaminitrile 4-Z-H+ by addition of 2.0 equiv of MSA to the suspension of isomer 4-E, followed by heating−cooling, resulted in stable gels as observed by vial inversion (Figure 3, bottom, inset). This effect could be observed in several nonpolar solvents, and gels were successfully obtained with enaminitrile 4-Z-H+ (1.0% (w/

Table 1. Photophysical Properties of Enaminitriles 1−3 in the Solid State λem (nm)b Compound

λabs (nm)a

solid

film

ΦF (%)c

1-E 1-Z-H+ 2-E 2-Z-H+ 3-E 3-Z-H+

331 361 329 358 324 352

ND 443 ND 460 ND 435

ND 454 ND 448 ND 451

ND 37.3 ± 1.1 ND 41.3 ± 0.4 ND 3.6 ± 1.0

τ (ns)d 3.9 2.9 0.7

Maximum absorption wavelength, concentration: 10 μM. bMaximum emission wavelength, solid: amorphous powder, film: solution spin-coated on glass slide. cAbsolute fluorescence quantum yield of amorphous powder. dFluorescence lifetime of amorphous powder; ND: Not detectable. a

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DOI: 10.1021/jacs.8b09843 J. Am. Chem. Soc. 2018, 140, 13640−13643

Communication

Journal of the American Chemical Society

Figure 4. Fluorescence spectra of enaminitriles (1−3)-E (10 μM) after addition of 1.0 equiv. CuII in CH3CN; photographs under UV light (365 nm) of solutions of enaminitrile 3-E (10 μM) after addition of 1.0 equiv. metal ions in CH3CN.

proton relay process.12 Subsequent addition of excess amounts of cyanide then “turned off” the fluorescence while restoring the absorption band to 325 nm (Figure S21). In summary, a series of functional, enaminitrile-based molecular switches were designed, synthesized, and evaluated. The E-isomers were present to a very high degree under neutral/basic conditions, and configurational switching could be regulated by acid/base, leading to complete formation of the Z-isomers and restoring the original E/Z-ratio after a switching cycle. The Z-H+-isomers displayed an AIE effect, in which high quantum yields could be recorded. A fluorescent, switchable supergelator was also demonstrated, by which a fluorescent, organogel could be formed upon H+-induced switching, and the process could be reversed by addition of base. Furthermore, the E-isomers displayed selective CuIIinduced, intense fluorescence in CH3CN, an effect that could be turned off by competitive coordination. The very high E/Zratios observed for this novel family of configurational switches enables it promising for constructing molecular machines and robots with high fidelity and output. In addition, the intriguing multistimuli-responsive “on−off” fluorescence effects, selectively triggered by metal coordination in the solution state or by acid/base in the solid state, provides possibilities for future applications. These controllable enaminitrile switches may thus become integral parts in bioimaging devices, environmental analysis, optical recording systems, electronic devices, etc.

Figure 3. Top: Acid/base-activated switching of enaminitrile 4. Bottom: Fluorescence spectra of enaminitrile 4-Z-H+ in hot solution (black) and as gel (blue); Inset: photographs of gelation cycle of compound 4 in TBME, 0.5% (w/v) under ambient or hand-held UV light (365 nm).

v)) in hexane, cyclohexane, toluene, diethyl ether, and tertbutyl methyl ether (TBME) (Table S1). The gelation process was furthermore associated with enhanced luminescence. Under UV light, the suspension of enaminitrile 4-E showed very weak green fluorescence, and a clear, UV-silent solution formed upon addition of MSA and heating. Cooling to ambient temperature then resulted in the formation of a supramolecular gel, which displayed deep-blue fluorescence (λem = 438 nm; Figure 3, blue line). The critical gelation concentration was estimated to 0.2% (w/v) in TBME, thus representing a supergelator in this solvent.11 After removal of the solvent, the resulting xerogel displayed a blue fluorescent 3D network-structure of connected nanofibers (Figures S17, S18). Moreover, the adaptive nature of the enaminitrile switch enabled the gel with responsive features. The formed gel thus collapsed immediately upon addition of base (Et3N) to the system, and the overall sol−gel process could be repeated at least five cycles. In addition to the fluorescence observed for the Z-H+isomers in the solid state, the potential for selective switching and fluorescence generation by metal ions was evaluated.5 Thus, 1.0 equiv of different metal ions was added to separate solutions of enaminitriles (1−3)-E (10 μM) in CH3CN. The obtained results showed that selective, turn-on fluorescence was observed for CuII whereas all other metal ions showed no or low effects (Figures 4, S19, S20). Enaminitrile 3 displayed especially strong blue-green fluorescence under UV illumination (Figure 4), whereas lower degrees of fluorescence intensity were observed for the corresponding compounds 1 and 2 (Figures 4, S19, S20). Furthermore, this turn-on effect could be reversed by addition of coordinating agents, such as TREN and CN−, to the mixture. The turn-on/turn-off process was furthermore monitored by UV−vis spectroscopy, revealing that addition of CuII (3.0 equiv) resulted in a shift of the maximum absorption wavelength from 325 to 354 nm, similar to the shift observed when enaminitrile 3-E was titrated with MSA. A broad shoulder was furthermore observed at higher wavelength, indicating that excess CuII to some extent may induce switching to the Z-H+-isomer through a Lewis acid-activated

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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.8b09843. Detailed experimental procedures and characterizations of all new compounds, enamine switching, AIE, crystallography, gelation, metal sensing and crystallographic data (PDF) Crystallographic data for C15H13N3 (CIF) Crystallographic data for C16H17N3O3S (CIF) Crystallographic data for C18H17N3 (CIF) Crystallographic data for C19H21N3O3S (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

A. Ken Inge: 0000-0001-9118-1342 Olof Ramström: 0000-0002-1533-6514 13642

DOI: 10.1021/jacs.8b09843 J. Am. Chem. Soc. 2018, 140, 13640−13643

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The study was in part supported by the Swedish Research Council. Y.R. thanks the China Scholarship Council for a special scholarship award. E.S.G. and A.K.I. acknowledge support from the Swedish Foundation for Strategic Research. X. Wang and Z. Feng are thanked for assistance with analyses.

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DOI: 10.1021/jacs.8b09843 J. Am. Chem. Soc. 2018, 140, 13640−13643