azacrowns to Design Materials with Improved Emission and Stimuli

Article pubs.acs.org/JPCC

Modulating the Self-Assembly of Calix[4]azacrowns to Design Materials with Improved Emission and Stimuli-Responsive Behavior Issam Oueslati,*,† José A. Paixaõ ,† Vitor H. Rodrigues,† Aleksander Shkurenko,‡ Barbara Leśniewska,‡ Kinga Suwinska,‡ M. Ermelinda S. Eusébio,§ Teresa M. R. Maria,§ and J. Sérgio Seixas de Melo§ †

CEMDRX, University of Coimbra, P-3004-516 Coimbra, Portugal Institute of Physical Chemistry, Polish Academy of Sciences, 44/52 Kasprzaka, PL-01-224 Warsaw, Poland § Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal ‡

S Supporting Information *

ABSTRACT: 1,3-[Ethylene-bis(aminocarbonylmethyl)]-ptert-butylcalix[4]arene is analyzed and its optical properties are investigated, showing luminescence upon aggregation at low temperature or in the solid state described as aggregationinduced blue emission. The shift of the self-assembly from nanoporous to helical framework in the crystal is associated with a remarkable blue-shift and 33-fold increase in the fluorescence intensity. The purple-blue light emission is highly efficient (ΦF ∼ 60%) and is switched to thermally activated yellow emission as a stimuli-responsive behavior of the helical assembly. The emitting material has been characterized by single crystal and powder X-ray diffraction, ATR-FTIR spectroscopy, and DSC and PLTM analysis, and its optical properties are investigated.

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INTRODUCTION

1 was prepared following a previously described procedure.24 Crystallization of 1 in acetonitrile/ethanol mixture gives nanoporous molecular crystals of 1b (A·CH3CN·C2H5OH) with the monoclinic space group C2/c.25 On the contrary, the crystallization in acetonitrile produces crystals of 1a (1· 2CH3CN) in the trigonal space group P322. In both polymorphs, 1 adopts a distorted cone conformation (Table 2 of the Supporting Information), including acetonitrile in the hydrophobic cavity via the C−H···N hydrogen bond [ ∼ 2.6 Å] in 1a (Figure 1c and the structure with atom numbering in the Supporting Information) and C−H···π interactions [ ∼ 2.8 Å] in 1b.26 The cone conformation in 1a is stabilized by intramolecular hydrogen-bonding networks O−H···O [1.97 Å] and N−H···O [2.26 Å],26 which have a considerable effect on the infrared (IR) O−H stretching vibration zone of 1a, when compared to p-tert-butylcalix[4]arene 2 (Figure 2a). A single band is observed, centered at 3362 cm−1, that must also include N−H stretching contributions, and is blue-shifted when compared to 2. The strong band at 1032 cm−1 (absent in the spectrum of 2) is tentatively assigned to the N−H bending vibration (Figure 2b).27 Its strong red shift (compared to 1157 cm−1 of N−H bending vibration of crystalline azophenine,

Understanding how molecular structures, conformations of molecules, and packing arrangements affect photophysical processes in the solid state is a prerequisite for the rational design of new luminophoric systems with enhanced efficiencies and improved performances. In this sense, the aggregationinduced emission (AIE)1,2 effect is useful for providing information on structure−property relationships, from which insights into working mechanisms may be gained.3,4 Most fluorescent organic materials suffer from the notorious aggregation-caused quenching (ACQ) effect, which has limited their applications in the real world.5 Nevertheless, AIE materials have been successfully applied in the construction of OLEDs,6−8 bioimaging systems,9 and chemical sensors.10,11 However, combining AIE with control of packing arrangement to produce materials that exhibit stimuli-responsive switching between two or more solid phases with different emission characteristics still remains a challenge.12−16 Although calixarene-based luminescent molecules have been widely studied,3,17−20 no calixarene without appended fluorophores has yet been shown to exhibit AIE.21−23 Here, we report what we believe to be the first example of AIE for such a calixarene, demonstrating also its facility to assemble from nanoporous to helically nanostructured material providing increased blue emission and a stimuli-responsive behavior. The calixarene concerned is the cone-calix[4]azacrown derivative 1 (Figure 1a). © 2014 American Chemical Society

RESULTS AND DISCUSSION

Received: April 22, 2014 Revised: May 21, 2014 Published: May 23, 2014 13118

dx.doi.org/10.1021/jp5039392 | J. Phys. Chem. C 2014, 118, 13118−13125

The Journal of Physical Chemistry C

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Figure 1. Structural Formulas of (a) 1 and (b) 2. (c) Crystal structure of 1a.

Figure 2. (a) νO−H and (b) δN−H bands in the IR spectra of 2 (gray line) and 1a (black line).

where N−H···N hydrogen bonding occurs)27 is in accordance with N−H···O intramolecular hydrogen bonding. The geometry of the cone conformation and that of the −NH−(CH2)2−NH bridge (Figure 3) are relatively similar in

Figure 3. Structural geometry of azacrown bridge in (a) 1a and (b) 1b.

Figure 4. (a-c) Left-handed helical chains in 1a. (d) O···H bonding between calixarenes.

both polymorphs (Tables 2 and 3 of the Supporting Information), yet the location of acetonitrile, instead of ethanol, adjacent to 1 is associated with a different assembly in 1a. Such solvent-controlled structural manipulation has been previously studied.28−30 Calixarene units in 1a are assembled in a one-dimensional left-handed helical chain around the crystallographic 32 axis (Figure 4, panels a−c), which is constructed by acetonitrile bridging adjacent calixarenes via C− H···N hydrogen bonding [C−H(23A)···N(3A) 2.41 Å] on one side and H···H intermolecular hydrogen bonding [C−H(29F)···H(17A)−C 1.96 Å] on the opposite side (Figure 4a). This latter hint is in agreement with the evidenced role that H···H intermolecular hydrogen bond plays in stabilizing organized self-assemblies in the crystal.31−34 Each helix in the chain consists of three calix[4]azacrown units, with an helix pitch of 28.32 Å and radius of 10.47 Å. A similar helical arrangement with comparable helix pitch was observed for calix[4]azacrown bearing an ester group on the bridge.35 The overall crystal lattice is stabilized by a periodic set of O···H [O(2)···H(21A)−C 2.55 Å] hydrogen bonds between adjacent calixarenes in each layer (Figure 4d). These arrays are extended along a and b axes. The long O···H distance is characteristic of weak hydrogen bonding,36−39 although C−H vibration bands do not shift in the infrared region. Indeed, the lack of a

significant shift does not necessarily indicate the absence of such hydrogen bonding.27,36 Control of the handedness was always a hot topic for the construction of helical nanostructures, as the optical and biological properties of the helices differ a lot according to their handedness.40,41 In most cases, the formation of single handedness is directed by a chiral head in organic molecules. Here, the enantiomerically pure left-handed helical nanostructure results from interplay of achiral calix[4]azacrown and acetonitrile molecules. Optical Properties in Solid State. Optical characterization of both nanoporous and helical-like solid-state arrays of 1 was addressed to investigate the structure-optical property relationship in both self-assembled materials. The absorption spectrum of 1 in acetonitrile (1.1 × 10−4 M) shows a broad band, with two maxima at 281 and 290 nm, similar to that observed for its precursor 1,3-bis(methoxycarbonylmethyl)-ptert-butylcalix[4]arene,3 and corresponds to the first π,π* transitions of the aromatic moiety (Figure 5a).42 The mirrorlike image of absorption and fluorescence spectra with the excitation spectrum being closely similar to the absorption spectrum indicate that the emission is fluorescence of the aryl moieties (monomer). The emission spectrum of 1 in the crystal lattice is much more complex (Figure 5b). In addition to the monomer emission at 340 nm,43 the porous assembly in 1b is 13119



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Figure 5. Normalized (a) absorption, (b) fluorescence (λexc = 280 nm, spectra normalized to the intensity of monomer emission), and (c) excitation spectra of 1 (1.1 × 10−4 M) in acetonitrile (gray line) and in the crystals 1a and 1b (black line) at room temperature. Fluorescence images of (d) 1a and (e) 1b under 330−385 nm illumination. Reflected light images of (f) 1a and (g) 1b. Partial structure showing the benzene dimer in (h) 1a and (i) 1b. Molecularly stacked structure showing the network of benzene dimers in the crystal structure of (j) 1a and (k) 1b.

component (ca. 17%). This fluorescence is still observed when exciting crystals 1a at 365 nm (excitation spectrum of the emission at 440 nm, Figure 5c), where the monomer does not absorb, suggesting an AIE process to generate purple-blueemitting crystals (Figure 5, panels d and f). Given that the amorphous powder is not emissive, thus the AIE depends on the crystal packing and is caused by conformational twisting and structural rigidification of calixarene molecules in the crystal phase.4 Correspondingly, the AIE process generates in 1b blue emission, but of relatively low intensity (Figure 5, panels e and g). The appealing result of blue shift (20 nm) and improvement (33-fold) of aggregate emission from 1b to 1a (Figure 5b) suggests that helical assembly is associated with AIEE effect against an AIE effect associated with porous

associated with appearance of a weak emission in the visible range centered at 460 nm. The latter emission is blue-shifted to 440 nm and is 33-fold intense in the helical-like assembly in 1a, which exhibits a fluorescence efficiency (ΦF = 0.66) higher than that of its precursor 1,3-bis-methoxycarbonylmethyl-p-tertbutylcalix[4]arene (ΦF = 0.29) or its analogous bridged with propylene-diamide (ΦF = 0.15) in acetonitrile.3,44 The monomer emission decay in 1a is a single exponential with a lifetime of τ = 480 ps being comparable to that observed in the crystalline precursor.3 The large purple-blue emission (440 nm) in 1a is due to fluorescence45 that decays with a biexponential decay law, where the preexponential coefficients indicate that the fluorescence is due to two longer-lived components: the 1.07 ns major component (ca. 82%) and the 4.64 ns minor 13120



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Figure 6. Fluorescence spectra of 1 in acetonitrile (1.1 × 10−4 M) at variable temperatures (a) 270−180 K, (b) 180−80 K. (c) Shift and (d) intensity variation traces of fluorescence of monomer (■) and aggregate (○).

spheres with diameters ranging from 1.0 to 5.6 μm and respective heights from 0.1 to 0.6 μm. The microspheres were agglomerated and fused in some places with a coralloid morphology, which is also observed by fluorescence microscopy (Figures 7, panels c and f). This motif is composed of small spheres connected through a two-dimensional pattern as was recently observed in the organogel of biscalix[4]arene, which is analogous to our system and exhibits an aggregation-induced blue-light emission.23 The microspheres, observed under fluorescence microscope, are hollow spherical aggregates (Figure 7b), as evidenced by AFM (Figures 7, panels e and f). The cross section of topography provides information about the wall thickness of the hollow microspheres, which is about 1.4 μm (Figures 7, panels f and g). On the basis of these findings, the blue-shift observed on cooling the solution of 1 to 80 K (Figure 6b) may be correlated to growth of the spherical aggregates.53 Stimuli Response. The solid state array of the molecules can easily be tuned by external stimuli such as pressure, heat, and vapor. These unique features would provide potential applications for smart materials.55,56 The purple-blue emitting crystals of 1a suffered thermal treatment for the sake of stimuliresponsive behavior. The thermal behavior investigation was monitored by differential scanning calorimetry (DSC), IR, powder X-ray diffraction (PXRD), and polarized light thermal microscopy (PLTM) techniques (Figure 8). Upon heating 1a up to 260 °C, DSC trace shows a low energetic endothermic event from 100 to 105 °C and an exothermic event from 180 to 200 °C. After heating to 105 °C, no changes were observed in the IR spectrum of 1a; however, some peaks (2θ 16−18°) disappear in the structured PXRD pattern and the others experience a little shift. This is in accordance with solvent removal associated with partial loss of crystallinity but not destruction of the crystalline order. PLTM observation only

assembly of 1. Such an outcome is more striking in reflected light images of 1a and 1b (Figures 5, panels f and g). The shift in the color of the polymorph is usually attributed to different intramolecular conformation instead of π−π stacking and H- or J-aggregation.16,46−48 Although 1 adopts the same conformation in the polymorphs 1a and 1b, the geometric configuration of the benzene dimer (Figure 5, panels h and i, dihedral angle in the benzene dimer is 26 and 7 degree in 1a and 1b, respectively) maximizes π−π overlap in 2 by formation of parallel-type contact associated with C−H···O hydrogen-bonding interactions.25 The supramolecular stacking architecture in 1b (Figure 5k) could in turn explain the shift in the emission.49 Fluorescence spectra of a solution of 1 in acetonitrile (1.1 × 10−4 M) at low temperatures (Figure 6, panels a and b) show 24-fold quenching of the monomer fluorescence and 2.1-fold enhancement of the “aggregate” emission.50 The cooling of the solution of 1 to 180 K quenches the fluorescence of the monomer and red shifts its emission with 9 nm (Figure 6, panels c and d). Below this critical temperature, variation of monomer fluorescence is almost stopped (quenching drops 1.8 times and the shift disappears), and the blue emission, centered at 454 nm, is enhanced 2.1 times and is 16 nm blue shifted on cooling the solution to 80 K (Figure 6, panels c and d). These results are in agreement with an AIE effect to be caused by restriction of the intramolecular rotation process of aromatics in the aggregate state.4,51−54 In order to prove that aggregation occurs, we prepared at room temperature a drop-cast film of a concentrated acetonitrile solution of 1 (1.1 × 10−2 M), showing the aggregation emission band described above (Figure 7a). The film, examined with atomic force microscopy (AFM) and fluorescence microscope, has a vesicular structure (Figure 7, panels b−f). AFM image (Figures 7d) showed many micro13121



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Figure 7. (a) Normalized fluorescence spectra at 270 K of the drop-cast film (full line) of the concentrated solution of 1 (1.1 × 10−2 M) and a diluted solution of 1 (dashed line). Fluorescence images of the drop-cast film under 330−385 nm illumination showing (b) the microspheres and (c) the coralloid-like motif. AFM images of the drop-cast film: (d) height image, (e) amplitude image of a microsphere connected to the network, and (f) three-dimensional image. (g) Cross section of topography.

(start at 170 °C and end at 200 °C), indicating the formation of a new solid phase. This transformation is not reversible as shown by DSC, PXRD, and PLTM. With consideration that the blue emission of 1a is caused by the AIE effect, the new crystalline order should, in principle, affect the optical properties.49 Interestingly, the crystal-tocrystal transformation produces yellow-emitting materials (Figure 9c). The thermally activated yellow-emission is not reversible and again is a consequence of the AIE effect. This stimuli-responsive behavior in 1a is absent in 1b, as the crystalto-crystal transformation in the porous assembly of 125 does not show any notable emission in 1b at 220 °C (Figure 9d).

shows few changes on the surface of the crystal (Figure 8d), and as expected, the blue emission is maintained at 105 °C with appearance of some little green spots (Figure 9b). When heating up to 200 °C, the PXRD pattern suffers pronounced changes: new peaks and others vanish, in particular the main peak at 2θ = 6°. This pattern shows the establishment of a different crystalline order, which is compatible with a monoclinic cell with a = 15.87 Å, b = 15.14 Å, c = 13.27 Å, α = 90.0°, β = 103.6°, and γ = 90.0°, and a cell volume = 3099.50 Å3. IR spectrum at 220 °C shows N−H and O−H stretching vibrations to become more structured, and the C O stretching vibration is 13 cm−1 red shifted and split into two bands 1699 and 1683 cm−1 (Figure 8b). This band splitting may be attributed to an interaction between neighboring electric transition dipoles and, hence, could suggest that carbonyl groups are close to each other in an ordered arrangement.57 PLTM observations show dramatic alteration

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CONCLUSIONS Here, we report the assembly of p-tert-butylcalix[4]azacrown molecules in a helical-like framework. The overall crystal lattice 13122



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Figure 8. (a) DSC trace, (b) IR spectra, (c) PXRD patterns, and (d) PLTM images.

Figure 9. Fluorescence images under 330−385 nm illumination of 1a (a) before heat, (b) heat to 105 °C, (c) heat to 220 °C, and of 1b (d) heat to 220 °C.

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is stabilized by both a 2D periodic set of O···H hydrogenbonding between adjacent calixarenes and an acetonitrile bridging role. The helical-like assembly in 1a promotes an AIEE effect causing efficient blue emission at 440 nm. The latter is detected as consequence of aggregation in acetonitrile and at low temperatures. Interestingly, the blue-emitting material possesses stimuli-responsive behavior that is thermally switched to a yellow-emitting material as direct photophysical response to solid phase transformation. Additionally, we verify that luminogenic aggregates “containing intrinsic chromophores” are reliable to afford information on structure− property relationships, as 1 shows different optical properties when assembled in helical or in nanoporous solid-state framework. In a first application, we considerably enhanced the emission of the porous materials of 1 by recrystallizing them in acetonitrile. The resulting materials assemble in a helical-like architecture and are 33-times more emissive. Moreover, this procedure is totally reversible.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+351) 239 829 158. Tel: (+351) 239 410 637. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by funds from FEDER (Programa Operacional Factores de Competitividade COMPETE) and from Fundaçaõ para a Ciência e a Tecnologia (FCT) under the project PEst-OE/FIS/UI0036/2014. Access to TAIL-UC facility funded under QREN-Mais Centro project ICT_2009_02_012_1890 is gratefully acknowledged. The authors thank V. Khomchenko for help in running the AFM experiments.

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ASSOCIATED CONTENT

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S Supporting Information *

Details of experimental section; crystallographic data; DSC, FTIR, absorption, fluorescence, and powder X-ray spectra; PLTM images. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC 960094. 13123



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