Supramolecular Polymer Networks Based on Calix [5] arene Tethered

Aug 31, 2012 - Dipartimento di Scienze Chimiche, Università di Catania, Viale Andrea Doria 6, 95125 Catania, Italy. ‡. Istituto per i Processi Chim...
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Supramolecular Polymer Networks Based on Calix[5]arene Tethered Poly(p‑phenyleneethynylene) Andrea Pappalardo,*,† Francesco P. Ballistreri,† Giovanni Li Destri,† Placido G. Mineo,†,‡ Gaetano A. Tomaselli,† Rosa M. Toscano,† and Giuseppe Trusso Sfrazzetto† †

Dipartimento di Scienze Chimiche, Università di Catania, Viale Andrea Doria 6, 95125 Catania, Italy Istituto per i Processi Chimico Fisici - CNR, Viale Ferdinando Stagno D’Alcontres, 37, 98158 Messina, Italy

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S Supporting Information *

ABSTRACT: A poly(p-phenyleneethynylene) polymer (PC[5]), featuring two π-rich cone-like calix[5]arene cavities (assembling cores) connected to a rigid p-phenyleneethynylene spacer, was synthesized by a Pd-catalyzed cross-coupling reaction. NMR investigations provide support for the selfassembly dynamics of PC[5] (homopolytopic host molecule) and complementary 1,10-decanediyldiammonium (C10) guest in solution in the construction of supramolecular polymer networks. Upon variation of the host/guest molar ratio, two distinct types of assemblies were identified in solution: a 1:2 bis-endo-cavity assembly C10⊂PC[5]⊃C10 (PC[5]cap) and a polycapsular polymer network assembly PC[5]⊃C10⊂PC[5] (PC[5]net). The reversibility of the assembly/disassembly processes of PC[5]net and PC[5]cap upon successive additions of Et3N and TFA was also investigated. AFM analysis of PC[5]net revealed the formation of a homogeneous continuous network on surface.

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optoelectronic devices.4,7 Poly(phenyleneethynylene)s (PPEs) are a class of conjugated polymers with applications in the field of optics and electronics.8 However, as a general rule, properties of organic semiconductors not only depend on the molecular structure but are also strongly influenced by supramolecular ordering and aggregation phenomena,9 both in solution and in the solid state, which have to be carefully considered in designing novel materials. Since PPEs show a propensity to form aggregates in solution of some solvents or solvent mixtures,10 with strong influence on properties, the control of the assembly of the polymer chains is a mandatory step to tune the optoelectronic properties of the conductive polymer-based devices. In this context, calixarenes are attractive building blocks in supramolecular chemistry for their availability and easy chemical modification at both the wide and narrow rims. These characteristics make them ideal molecules in the assembly of supramolecular polymers.5a,11−13 In particular, ptert-butylcalix[5]arenes, in a fixed cone-like arrangement, selectively form strong 1:1 inclusion complexes with linear alkylammonium ions14 and discrete dimeric capsules in the presence of long-chained α,ω-alkanediyldiammonium ions, in which a single ditopic guest of appropriate length coordinates to a pair of calix[5]arene units.15−17 The introduction of calix[5]arenes as substituents in the PPE main chain implies at

INTRODUCTION Dynamic polymers (dynamers)1 are polymeric materials displaying reversible formation and component exchange. They are mainly subdivided into two categoriessupramolecular polymers and molecular polymersdue either to the presence of noncovalent or to the introduction of covalent reversibility moieties. Compared with conventional polymers, supramolecular polymers possess many dynamic or precisely controllable properties arising from dynamic linking between the constituent monomers and have the ability to respond to their environment as adaptive materials,2 producing unique properties, which are not shown by the individual components. In view of the ability of dynamers to build up by self-assembly and to select their components in response to external stimuli or to environmental factors, they are often generated by molecular monomers featuring complementary binding groups capable of connecting through noncovalent interactions:3 hydrogen bonding, donor−acceptor, electrostatic, van der Waals, and metal ion coordination.4 With the aim of developing sophisticated highly ordered self-assembled structures from small building blocks by molecular recognition, self-replication, and self-organization based on noncovalent interactions, chemists have developed a variety of strategies for constructing supramolecular polymers, in which host−guest interaction5,6 has become one of the most important and convenient approaches. Over the past decade, exploiting and tailoring novel materials by utilizing supramolecular approaches in conjunction with pconjugated structures is one of the key ways to design © 2012 American Chemical Society

Received: July 20, 2012 Revised: August 25, 2012 Published: August 31, 2012 7549

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the AFM measurements were purchased by NanoAndMore GmbH (Wetzlar; Germany) model TAP300-G with spring constant of around 40 N/m, a resonant frequency of ∼300 kHz, and a typical tip radius of less than 10 nm. Synthesis of 1,4-Bis(4-Methylpentyloxy)-2,5-diiodobenzene (4). A solution of 2,5-diiodohydroquinone (3) (0.45 g, 1.24 mmol) and anhydrous K2CO3 (0.687 g, 4.97 mmol) in of dry CH3CN (15 mL) was stirred under a N2 atmosphere. After 20 min, 1-bromo-4methylpentane (0.452 g, 2.73 mmol) was added dropwise, and then the system was heated to reflux. After 36 h, the solvent was evaporated, and the residue was extracted with dichloromethane. The organic phase was washed with 0.1 N HCl and then dried over anhydrous Na2SO4. The solvent was evaporated at reduced pressure. The crude product was purified by column chromatography (CC) (SiO2, hexane/ EtOAc 15:1 v/v as an eluent) to give the desired product 4 as a white solid after precipitation from MeOH (0.395 g, 60% yield). 1H NMR: δ = 0.92−0.95 (m, 12H), 1.35−1.40 (m, 4H), 1.61−1.64 (m, 2H), 1.79−1.82 (m, 4H), 3.91 (t, 4H, J = 7.0 Hz), 7.17 (s, 2H) ppm. 13C NMR: δ = 22.6, 27.0, 27.7, 35.1, 70.7, 86.3, 122.8, 152.9 ppm. Elemental analysis calcd (%) for C18H28I2O2: C 40.77, H 5.32; found: C, 40.89; H, 5.29. Synthesis of 1,4-Bis[(trimethylsilyl)ethynyl]-2,5-bis(4methylpentyloxy)benzene (5). A solution of 4 (0.395 g, 0.75 mmol), trimethylsilylacetylene (0.146 g, 1.5 mmol), Pd(PPh3)Cl2 (2.6 mg, 0.0375 mmol), and CuI (0.72 mg, 0.044 mmol) in 10 mL of diisopropylamine was refluxed under a N2 atmosphere for 16 h. After cooling, toluene (10 mL) was added and the white precipitate (ammonium iodide) was filtered off. The solution was passed through a 4 cm plug of silica gel using toluene as eluent. The evaporation of the solvent led to a yellow oil which crystallized upon standing. Recrystallization from hexane yielded white needles (0.324 g, 92%). 1 H NMR: δ = 0.25 (s, 18H), 0.92 (d, J = 7.0 Hz, 12H), 1.36−1.41 (m, 4H), 1.58−1.63 (m, 2H), 1.76−1.82 (m, 4H), 3.93 (t, J = 6.0 Hz, 4H), 6.89 (s, 2H) ppm. 13C NMR: δ = −0.02, 22.6, 27.2, 27.8, 35.2, 69.7, 100.0, 101.0, 113.9, 117.2, 154.0 ppm. Elemental analysis calcd (%) for C28H46O2Si2: C 71.43, H 9.85; found: C 71.16, H 9.88. Synthesis of 1,4-Diethynyl-2,5-bis(4-methylpentyloxy)benzene (1). Methanol (5 mL) and aqueous KOH (300 μL, 20%) were added at room temperature to a stirred solution of 5 (200 mg, 0.424 mmol) in THF (10 mL), and the reaction mixture was stirred for 4 h. The mixture was filtered, and after evaporation of the solvent, a yellow solid was obtained. Recrystallization from hexane with charcoal yielded pale yellow crystals (111 mg, 80%). 1H NMR: δ = 0.91 (d, J = 6.5 Hz, 12H), 1.32−1.37 (m, 4H), 1.57−1.64 (m, 2H), 1.77−1.83 (m, 4H), 3.33 (s, 2H), 3.96 (t, J = 6.5 Hz, 4H), 6.95 (s, 2H) ppm. 13C NMR: δ = 22.5, 27.0, 27.7, 35.1, 70.0, 82.3, 82.4, 113.3, 117.8, 154.1 ppm. Elemental analysis calcd (%) for C22H30O2: C 80.94, H 9.26; found: C 81.16, H 9.84. Synthesis of 1,4-Bis(6-chlorohexyloxy)-2,5-diiodobenzene (7). A solution of 6 (536 mg, 0.953 mmol) in 15 mL of CH2Cl2 was stirred in an ice bath for 20 min. SOCl2 (454 mg, 3.813 mmol) and Et3N (386 mg, 3.813 mmol) were added, and the mixture was allowed to stir for 3 h. The organic phase was washed with H2O and then dried over anhydrous Na2SO4. The solvent was evaporated at reduced pressure, and the crude product was purified by CC (eluent: hexane/CH2Cl2 3:1) to afford the desired product that was precipitated from hexane to give 350 mg of 7 (61% yield). 1H NMR: δ = 1.50−1.57 (m, 8H), 1.79−1.86 (m, 8H), 3.56 (t, J = 6.5 Hz, 4H), 3.94 (t, J = 6.5 Hz, 4H), 7.17 (s, 2H) ppm. 13C NMR: δ = 25.4, 26.5, 29.0, 32.5, 45.0, 70.1, 86.3, 122.8, 152.8 ppm. Elemental analysis calcd (%) for C18H26I2Cl2O2: C 36.09, H 4.37; found: C 35.88, H 4.41. Synthesis of Bis-calix[5]arene (2). A solution of 7 (100 mg, 0.163 mmol), calix[5]arene 8 (411 mg, 0.358 mmol), and anhydrous K2CO3 (395 mg, 2.86 mmol) in CH3CN (15 mL) was stirred under a N2 atmosphere for 48 h. The solvent was evaporated, and the residue was extracted with dichloromethane, washed with 0.1 N HCl, and then dried over anhydrous Na2SO4. The solvent was evaporated at reduced pressure. The crude product was purified by CC (eluent: hexane/ CH2Cl2 3:1, v/v) to afford 2 (208 mg, 45% yield) as a white solid. 1H NMR: δ = 0.94 (d, J = 7.0 Hz, 48H), 1.01, 1.03, 1.05 (s, 1:2:2, 90H),

least two consequences: it is expected to break the symmetry of the PPEs preventing their tendency to form linear aggregates and, moreover, may give the unique opportunity of investigating the optical properties of novel supramolecular architectures (polymer networks) descending from its selfassembly dynamics with complementary α,ω-alkanediyldiammonium ions. Therefore, in this contribution we illustrate the formation of higher order polycapsular supramolecular polymer network assemblies achieved from a poly(p-phenyleneethynylene) polymer, bearing two π-rich cone-like calix[5]arene cavities (PC[5]), with the complementary 1,10-decanediyldiammonium ion (C10). Our data, however, most significantly show that the PC[5]/C10 complementary pairs showed dynamic behavior, responding to changes in molar ratio by rearranging into different assemblies, demonstrating that is possible to control the properties of an entire supramolecular assembly (i.e., fluorescence) by simply fixing the host/guest molar ratio. Because of the well-known conductivity properties of PPE polymers, these polymer networks possess the potential for fabricating novel switchable electrochemical sensors and electronic devices.

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EXPERIMENTAL SECTION

Materials. 5,11,17,23,29-Penta-(1,1-dimethylethyl)-31-hydroxy32,33,34,35-tetra(4-methylpentyloxy)calix[5]arene (8),18 2,5-diiodohydroquinone (3),19 and 6-[4-(6-hydroxyhexyloxy)-2,5-diiodophenoxy]-hexan-1-ol (6)20 were prepared as reported in the literature. Trimethylsilylacetylene, 6-bromo-1-hexanol, 1-bromo-4-methylpentane, Pd(PPh3)4, Pd(PPh3)2Cl2, iodobenzene, TFA, triethylamine, and CuI were commercial products. Characterization Methods. GPC analyses were performed on a PL-GPC 110 (Polymer Laboratories) thermostated system, equipped with three PL-gel 5 μm columns (two Mixed-D and one Mixed-E) attached in series. The analyses were performed at 35 ± 0.1 °C using THF as eluent at a flow rate of 1 mL/min. A differential refractometer (Polymer Laboratories) was used as detector. The instrument was calibrated with a mixture of polystyrene standards (Polysciences; molecular masses between 2000 and 1 200 000) using PL-Caliber GPC software for the determination of average molecular masses and polydispersity of the polymer samples. Positive MALDI-TOF mass spectra were acquired by a Voyager DE (PerSeptive Biosystem) using a delay extraction procedure (25 kV applied after 2600 ns with a potential gradient of 454 V/mm and a wire voltage of 25 V) with ion detection in linear mode. The instrument was equipped with a nitrogen laser (emission at 337 nm for 3 ns) and a flash AD converter (time base 2 ns). trans-2-[3-(4-tert-Butylphenyl)-2-methyl-2propenylidene]malonitrile (DCTB) was used as a matrix; mass spectrometer calibration was performed as reported in previous cases.21 NMR experiments were carried out at 27 °C on a Varian UNITY Inova 500 MHz spectrometer (1H at 499.88 MHz, 13C NMR at 125.7 MHz) equipped with pulse field gradient module (Z-axis) and a tunable 5 mm Varian inverse detection probe (ID-PFG). Unless otherwise stated, NMR spectra were obtained for CDCl3 solutions. The chemical shifts (ppm) were referenced using the residual solvent signal as the internal standard. A JASCO V-630 UV/vis spectrophotometer equipped with a 1 cm path-length cell was used for the UV/vis measurement. Luminescence measurements were carried out using a Cary Eclipse fluorescence spectrophotometer with a λex of 440 nm and a 0.5 nm resolution, at room temperature. The emission was recorded at 90° with respect to the exciting line beam using 5: 5 slit widths for all measurements. Samples for atomic force microscopy (AFM) analysis were prepared by casting 20 μL of 1.4 × 10−5 M solutions of each analyte onto freshly cleaved mica substrates and by letting the solvent to completely evaporate. Topographical images were taken in tapping mode using a Digital Instruments (DI) Nanoscope IIIA with 100 × 100 μm scanner under ambient conditions. The cantilevers for 7550

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Scheme 1. Polymerization Reaction

1.28−1.36 (m, 24H), 1.58−1.66 (m, 8H), 1.81−1.93 (m, 24H), 3.25 and 4.54 (AX system, J = 13.5 Hz, 20H), 3.55−3.66 (m, 20H), 3.94 (t, J = 6.0 Hz, 4H), 7.01, 7.02, 7.04 (pseudo-s, 1:2:2, 20H), 7.17 (s, 2H) ppm. 13C NMR: δ = 22.7, 22.83, 22.86, 26.0, 26.5, 28.0, 28.3, 28.4, 29.3, 29.4, 29.7, 30.4, 30.9, 31.0, 31.4, 31.7, 33.8, 34.0, 35.19, 35.23, 73.7, 74.0, 74.2, 75.0, 86.3, 124.8, 125.3, 125.4, 125.6, 126.0, 126.8, 132.8, 133.4, 133.8, 133.9, 134.0, 144.5, 144.8, 145.5, 151.8, 152.7, 152.9 ppm. MALDI-TOF (m/z): 2821 [MH+]; 2843 [MNa+]. Elemental analysis calcd (%) for C176H260I2O12: C 74.91, H 9.29; found: C 74.36, H 9.48. Synthesis of PC[5]. Monomer 1 (10 mg, 0.0312 mmol), biscalix[5]arene (2) (81 mg, 0.0283 mmol), Pd(PPh3)4 (2.1 mg, 0.0018 mmol), and CuI (0.22 mg, 0.00116 mmol) were combined in toluene (7 mL) and diisopropylamine (3 mL). The reaction mixture was then stirred at 70 °C for 10 days. Ammonium iodide salt formed immediately after the start of the reaction, and the mixture became highly fluorescent. Iodobenzene (6.7 mg, 0.034 mmol) was added, and the mixture allowed to stir for 4 days. After a total reaction time of 14 days, the reaction mixture was cooled to room temperature and added dropwise to rapidly stirred methanol (30 mL). After stirring for 2 h, the precipitate was collected and washed with hot methanol and hot acetonitrile. After drying overnight at room temperature, PC[5] was purified by a short CC (eluent: CH2Cl2/MeOH 6:1, v/v) to afford a dark red solid (56 mg, 69%). 1H NMR: δ = 0.94 (d, J = 6.5 Hz, 60H), 1.02, 1.04, 1.05 (s, 2:1:2, 90H), 1.30−1.36 (m, 28H), 1.59−1.65 (m, 10H), 1.84−1.92 (m, 28H), 3.25 and 4.55 (AX system, J = 14.0 Hz, 20H), 3.58−3.67 (m, 24H), 3.92−3.98 (m, 4H), 6.91−6.96 (m, 22H), 7.17 (s, 2H) ppm. 13C NMR: δ = 22.4, 22.8, 25.9, 26.3, 26.9, 27.4, 27.7, 28.1, 28.2, 28.6, 29.2, 29.3, 29.7, 30.1, 30.2, 33.4, 34.6, 35.1, 74.5, 86.2, 116.4, 122.5, 125.6, 125.7, 133.17, 133.26, 133.4, 144.9, 152.6 ppm. GPC (THF, 35 °C, Polystyrene standards as calibrant): Mw = 61 800; Mn = 46 100; PDI = 1.34 (degree of polymerization [DP] ≈16). Elemental analysis calcd (%) for C197H284O14: C 82.26, H 9.95; found: C 81.93, H 10.26.

Monomer 1 was prepared in three steps starting from 2,5diiodohydroquinone, 3 (Scheme 2).19 An excess of 1-bromo-4Scheme 2. Synthesis of Monomer 1

methylpentane was treated with 3 to yield the derivative 4, which was then reacted with trimethylsilylacetylene by the palladium-catalyzed cross-coupling condensation to produce the precursor 5. Finally, the hydrolysis of the trimethylsilylprotecting group produced the monomer precursor 1. Monomer 2 was synthesized in two steps from 6-[4-(6hydroxyhexyloxy)-2,5-diiodophenoxy]-hexan-1-ol (6) 20 (Scheme 3), which was treated with SOCl2 to give the 1,4bis(6-chlorohexyloxy)-2,5-diiodobenzene (7). The reaction of derivative 7 with the calix[5]arene (8)18 afforded the monomer 2. NMR Titration and Diffusion Studies. Extensive NMR investigations were carried out to elucidate the self-assembly dynamics of PC[5] and complementary 1,10-decanediyldiammonium guest in solution. NMR studies were focused on 1,10decanediyldiammonium dipicrate (C10·2Pic) as the length of this dication was judged to be ideal for the formation of a

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RESULTS AND DISCUSSION The synthesis of the PC[5] was performed by Pd-catalyzed cross-coupling reaction of 1,4-diethynyl-2,5-bis(4methylpentyloxy)benzene (1) with the suitable bis-calix[5]arene (2) (Scheme 1). 1H and 13C NMR spectra of PC[5] demonstrate the complete absence of butadiyne structural defects. In fact, the integration data related to the proton signals of the polymer show that the two structural units derived from monomers 1 and 2 are present in a 1:1 ratio. Furthermore, the 13 C resonances of butadiyne carbon atoms at about 80 ppm are absent. The weight (Mw) and number (Mn) average molecular weight of PC[5] (measured by GPC in THF, using polystyrene as standards; see Figure S5) were 61 800 and 46 100 (PDI = 1.34; DP ≈ 16), respectively. 7551

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Scheme 3. Synthesis of Monomer 2

variety of supramolecular species.15,22 As expected, the PC[5]/ C10 complementary pairs showed dynamic behavior, responding to changes in molar ratio by rearranging into different types of assemblies. According to an inevitably simplified overall picture, which of course does not account for all the possible coexisting species, two predominant distinct species were identified: a 1:2 bis-endo-cavity assembly C10⊂PC[5]⊃C10 (hereafter PC[5]cap) and a polycapsular polymer network assembly PC[5]⊃C10⊂PC[5] (hereafter PC[5]net) (Figure 1). 1 H NMR spectroscopy proved to be an excellent tool to assess the structural features of these types of assemblies and monitor the dynamics of formation/dissociation. Host and guest components were found to possess diagnostic probe signals (see Figure 2) undergoing (in slow exchange regime on the NMR time scale) substantial and distinctive chemical shift changes upon self-assembly as a function of the symmetry elements present within a given assembled species. Complex formation does induce symmetrization/desymmetrization of the guest. Accordingly, when the two methylene groups at the far ends of C10 (α-CH2 and α′-CH2) are both cavity-included, they are equivalent andas a result of the shielding induced by the aromatic walls of the calixarene moietiesresonate as a high-field broad singlet at δ = −1.30 ppm (i.e., PC[5]net), whereas they appear as two distinct resonances at very different field strengths when only half of the dication is embedded into the cavity of a calixarene subunit (i.e., PC[5]cap).22 In the latter case, the included α-CH2 signal resonates at δ = −1.27 ppm, whereas the α′-CH2, at the opposite end of the dication, appears as a triplet at δ = 2.82 ppm. In order to identify the different types of assemblies, a series of 1H NMR experiments were devised, so as to induce a progressive evolution of the self-assembled species upon variation of the host/guest molar ratio (Figure 2). When C10 (0.5 equiv) was added to a 1 mM (per repeat unit for the polymer)23 CDCl3/CD3OD (4:1, v/v) solution of PC[5] (Figure 2, trace b), so as to reach a 2:1 host/guest ratio, the spectrum revealed the formation of the PC[5]net assembly

Figure 1. Schematic representation of the polycapsular polymer network assembly PC[5]net and the 1:2 bis-endo-cavity assembly PC[5]cap.

Figure 2. Selected regions of the spectrum in a typical 1H NMR titration of the host PC[5] with the guest C10·2Pic. Constant concentration of host PC[5] (1.00 × 10−3 M) with addition of various concentrations (0 to 2.50 × 10−3 M) of C10·2Pic. (a) PC[5]; (b) [H]/ [G] 2:1; (c) [H]/[G] 1:1; (d) [H]/[G] 1:2.5.

as demonstrated by the presencein the high-field region (−2.0 to 0.3 ppm)of a single set of peaks, for the α- to ε-CH2 and the symmetry-related α′- to ε′-CH2 groups of the guest, 7552

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shielded by the π-rich cavities of two host molecules arranged in a polycapsular fashion. Note that resonances compatible with the presence of either free guest (α-CH2 ≡ α′-CH2) or 1:2 bis-endo-cavity assembly (exocavity α′-CH2) are barely observed in the diagnostic δ = 2.82 ppm region. Upon further addition of 0.5 equiv of C10 to the preformed PC[5]net assembly, a second more-spread set of signals for the included guest became evident in the high-field region of the spectra (δ = −1.9 to 0.4 ppm), as a result of the incipient formation of the 1:2 bis-endo-cavity assembly (Figure 2, trace c). This conclusion is also validated by the presence in the spectrum of two equally intense AX signals (4.3−4.5 and 3.3−3.8 ppm, respectively) relative to the calix[5]arene methylenes and by the triplet detected at δ = 2.82 ppm, which is assigned to the α′-CH2 protons of the guest and is judged of diagnostic value for the 1:2 host/guest complex.18 Further addition of 1.5 equiv of C10 to the above solution, so as to reach PC[5]/[C10] 1:2.5, mainly led to the formation of the latter architecture with the presence of a small fraction of the polymer network assembly as deduced by the persistence of the signals belonging to the α- to α′-CH2 groups of C10 in the −2.0 to 0.3 ppm high-field region (Figure 2, trace d). To shed further light on the dynamics and modes of assembly between PC[5] and C10·2Pic and validate at the same time the 1H NMR results with a size-sensitive technique, extensive diffusion NMR studies were undertaken. The diffusion NMR technique provides a means to simultaneously obtain the diffusion coefficients (D) of the various supramolecular species coexisting in solution, as long as they display distinct and not superimposed peaks.24 Diffusion experiments of monomer 2, PC[5], and PC[5] with added C10·2Pic were carried out in CDCl3/CD3OD (4:1, v/v) solution at a fixed 1 mM concentration (see Figures S7 and S8). The D values of the monomer 2 and PC[5] are D = (4.00 ± 0.01) × 10−6 cm2 s−1 and D = (2.06 ± 0.01) × 10−6 cm2 s−1, respectively. As already reported, upon addition of 1 equiv of C10·2Pic to PC[5], in solution are present both the polymeric network assembly PC[5]net and the 1:2 bis-endo-cavity assembly PC[5]cap (Figure 2, trace c). The diffusion coefficients associated with the α- to ε-CH2 and the symmetry-related α′to ε′-CH2 groups of the host/guest complementary pair were found to be significantly lower (D = (5.27 ± 0.01) × 10−7 cm2 s−1 for PC[5]net and D = (1.75 ± 0.01) × 10−6 cm2 s−1 for PC[5]cap) than those measured for the PC[5]. As expected, binding of guest molecules to PC[5] (i.e., PC[5]net and PC[5]cap, respectively) increases the molar mass, and as a result, the diffusion coefficients of the new species decrease. Optical Characterization of Assemblies. The absorption spectrum of PC[5] in CHCl3/CH3OH (4/1 v/v) shows a broad π−π* conjugation band with maximum at 433 nm, with an ε = 29 800 (see Figure S9), in keeping with other 2,5dialkoxy-substituted poly(aryleneethynylenes).8b,10 The photoluminescence spectrum of PC[5] in CHCl3/CH3OH (4/1 v/ v), upon excitation at 440 nm, displays a main emission band at 480 nm with a fluorescence quantum yield ΦF = 0.052 and with a long-wavelength (probably vibronic) shoulder (see Figure S9). The observed Stokes shift with respect to the main absorption band is 43 nm, a somewhat large value in the dialkoxypoly(aryleneethynylene) family.8b Fluorescence titration experiments were performed to determine the affinity constants for the assemblies PC[5]net and PC[5]cap by addition of various aliquots of C10·2Pic (0.02−20.0 equiv). Interestingly, in the range of 0.02−0.5 equiv

of C10·2Pic added, the fluorescence emission increases with respect to the free PC[5] and remains nearly constant (Figure 3).

Figure 3. Normalized emissions of PC[5] (5.0 × 10−6 M) with different equivalents of C10·2Pic (0, 0.02, 0.04, 0.06, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9) in CHCl3/CH3OH (4/1 v/v); λex = 440 nm, λem = 480 nm.

The pertinent constant value for this initial association was estimated log Knet = 9.88 ± 0.85 (see Figure S10). On the other hand, upon further addition of 0.5 equiv of C10·2Pic, a progressive quenching of emission was observed (Figure 4). The log Kcap value for this second association is 7.08 ± 0.15 (see Figure S11).25

Figure 4. Normalized fluorescence emissions of PC[5] (5.0 × 10−6 M) with different equivalents of C10·2Pic (0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.6, 1.8, 2.0, 2.5, 3.0, 3.5, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 12.0, 15.0, 18.0, and 20.0) in CHCl3/CH3OH (4/1 v/v); λex = 440 nm.

This trend confirms the formation of two different species in these host−guest ratios: the initial increasing of emission (0.02−0.5 equiv of C10·2Pic) suggests the formation of a more ordered species with respect to the starting polymer PC[5] (Figure 3, PC[5]net), while the decrease of emission after further addition of 0.5 equiv indicates the formation of a less ordered species (Figure 3, PC[5]cap). One of the attractive features of noncovalent polymers over covalently bound ones is their ability to reversibly vary the degree of polymerization in response to changes in the environmental conditions. In this respect, the PC[5]/C10 complementary pair is, in principle, an extremely useful precursor for polymer formation because the 1,10-decanediyldiammonium promptly responds to simple chemical stimuli (i.e., acid/base treatment) by activating or deactivating the selfassembly process. In particular, the C10 diammonium salt is easily deprotonated to the free diamino derivative by treatment 7553

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with a mild organic base (Et3N). Furthermore, this diamino derivative can be successfully reconverted to the C 10 diammonium salt upon subsequent exposure to a TFA solution. Hence, we tested the reversibility assemblies of PC[5]net and PC[5]cap upon successive additions of Et3N and TFA (Figure 5).

Figure 6. 5 × 5 μm2 AFM height images of solvent cast film of (1a) PC[5], (2a) PC[5]net, and (3a) PC[5]cap. 3D images of (1b) PC[5], (2b) PC[5]net, and (3b) PC[5]cap.

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Figure 5. Normalized change emissions of PC[5]net (■) and PC[5]cap (●) (5.0 × 10−6 M) during the cyclic addition of 1 equiv of base (Et3N) and acid (TFA) in CHCl3/CH3OH (4/1, v/v); λex = 440 nm, λem = 480 nm. Black and red lines refer to PC[5]net and PC[5]cap, respectively.

CONCLUSION A poly(p-phenyleneethynylene) polymer, featuring two conelike calix[5]arene cavities connected to a rigid p-phenyleneethynylene spacer, was synthesized by a Pd-catalyzed cross-coupling reaction. By NMR investigations of the polymer with complementary 1,10-decanediyldiammonium guest, two distinct species were identified: a 1:2 bis-endo-cavity assembly and a polycapsular polymer network assembly. The affinity constants of the two supramolecular species were determined by fluorescence titration, and the reversibility of the assembly processes of the two species reveals a strong stability of the polycapsular polymer network upon successive additions of Et3N and TFA. AFM analysis of the polycapsular network assembly shows a homogeneous and very regular continuous network that covers the entire surface. These results illustrate the response of such a dynamic system to chemical effectors (H+ and [C10]), thus demonstrating the adaptive behavior of these systems by constitutional rearrangement in response to a specific trigger. The understanding of the basic self-assembly dynamics of these novel systems paves the way for the preparation of specific supramolecular polymer networks with potential applications in the field of conductive materials and analyte sensing.

Interestingly, the cyclic additions of 1 equiv of Et3N and 1 equiv of TFA to a solution of PC[5]net (PC[5]/C10 2:1) do not modified significantly the intensity of emission of the polymer network (Figure 5, black line). On the other hand, when PC[5]cap (PC[5]/C10 1:1) is considered, a well-defined reversibility was monitored during the cyclic base/acid additions (Figure 5, red line), demonstrating that this recognition process can be controlled and turned on/off. These different behaviors suggest that the polymer network PC[5]net represents a well-founded construction, in which the guest molecules are strongly embedded into the calixarene cavities, and is able to prevent itself from disassembly under basic conditions. AFM Analysis. As a further confirmation of the assembly behavior of the polymer in solution, AFM measurements (20 μL of 1.4 × 10−5 M solutions of each analytes) were performed on solvent casted samples (CHCl3/CH3OH 4:1 v/v). In Figure 6 height images with the corresponding 3D images of PC[5], PC[5]net, and PC[5]cap are reported. AFM shows that when casting the solution of polymer PC[5], many isolated aggregates with size ranging from few nanometers to few hundreds of nanometers are observed (Figure 6, 1a and 1b). On the contrary, casting of PC[5]net (PC[5]/C10 2:1) leads to the formation of a homogeneous continuous network with almost uniform thickness (4.68 ± 0.18 nm) (Figure 6, 2a and 2b), as measured from the cross section (see Figure S12). Indeed, when PC[5]cap (PC[5]/C10 1:1) sample is considered, a heterogeneous coverage of the surface is obtained with both microscopic aggregates and a few residual network fragments (Figure 6, 3a and 3b) as suggested by the steps in the cross section (see Figure S12). The presence of such residual network, as already revealed by NMR experiments, confirms that the capping process is not able to completely disrupt the network. The morphology of solventcasted films on mica can be reconnected to the polymer modes of aggregation observed in solution.

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ASSOCIATED CONTENT

S Supporting Information *

1

H and 13C NMR, diffusion NMR, GPC analysis, MALDITOF, UV−vis, 1H NMR, and fluorescence titrations. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the University of Catania for financial support. 7554

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