Topochemical Polymerization of Phenylacetylene Macrocycles Under

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C: Physical Processes in Nanomaterials and Nanostructures

Topochemical Polymerization of Phenylacetylene Macrocycles under Pressure Andrea Lapini, Samuele Fanetti, Margherita Citroni, Roberto Bini, CharlesOlivier Gilbert, Simon Rondeau-Gagné, and Jean-Francois Morin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06724 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 6, 2018

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The Journal of Physical Chemistry

Topochemical Polymerization of Phenylacetylene Macrocycles Under Pressure Andrea Lapini,†,‡ Samuele Fanetti,∗,†,¶ Margherita Citroni,† Roberto Bini,†,¶ Charles-Olivier Gilbert,§ Simon Rondeau-Gagné,k and Jean-Francois Morin§ †European Laboratory for Nonlinear Spectroscopy (LENS), via Nello Carrara 1, 50019 Sesto Fiorentino (FI), Italy ‡Istituto Nazionale di Ottica (INO), Largo Fermi 6, Firenze (FI), Italy ¶Dipartimento di Chimica “Ugo Schiff ”, Università di Firenze, via della Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy §Département de Chimie and Centre de Recherche sur les Matériaux Avancés (CERMA), Université Laval, 1045 Ave de la Médecine, Pavillon Alexandre-Vachon, Québec, QC, Canada G1V 0A6 k Department of Chemistry and Biochemistry, University of Windsor, 401 Sunset Ave. Windsor,Ontario, ON, Canada N9B3P4 E-mail: [email protected] Phone: +39 055 457 2536

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Abstract Self-assembly of organic macrocycles has been exploited as a preliminary step in the synthesis of soluble and tailorable carbon based nanostructures. Functionalised nanotubes have been prepared using, as core building-blocks, nearly planar ring structures containing several alkyne units, exploiting the geometry achieved in the spontaneous preassembling step driven by π interaction. Covalent cross-linking between these units was achieved by thermal or photochemical activation with UV light. Here we apply a moderate pressure in a sapphire anvil cell (1.0 GPa) to facilitate the preassembling, and induce the cross-linking under pressure either with visible light, absorbed by two-photon absorption, or thermally. We observe a high-yield of enhanced quality cross-linked nanotubes in a sample showing, at ambient pressure, only side-chain decomposition. These results show that moderate pressures, easily achievable in large volume cells, are able to selectively favour topochemical reactions in such complex organic systems.

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Introduction Self-assembly of alkyne-rich molecules that can react in the solid state, upon heating or irradiation, to give carbon nanomaterials of different shapes and sizes, has been increasingly popular in recent years. 1,2 Nanorods, 3 nanoparticles, 4 two-dimensional polymers, 5 and nanotubes 6–9 have been prepared using this strategy, and among these examples, nanotubes have been the most studied as a soluble alternative to carbon ones. To prepare such nanotubes, butadiyne-containing macrocycles are self-assembled, through the formation of crystals or organogels, in a columnar fashion in which the butadiyne units face each other with precise geometrical parameters determined by π-stacking and hydrogen bonding interactions. Then, the macrocycles are polymerized using light or gentle heat to form covalently-linked, soluble organic nanotubes with different properties. In this process, a 1,4-addition occurs, leading to a polydiacetylene (PDA) backbone in the direction perpendicular to the molecular planes. One successful examples of this type of synthesis has been obtained by polymerizing PAM2 (a phenylacetylene macrocycle, see Fig. 1). Upon irradiation of a dried gel (xerogel, from ethyl acetate) of PAM2 at 254 nm, PDA-based nanotubes were obtained in 15% yield after purification. 10 Likewise, PBM1 (a phenylene-butadiynylene macrocycle), which contains butadiyne units on each side of the macrocycle, yields similar nanotubes in 10% yield. 11 Although promising, this strategy has not proven efficient yet at providing large quantity of nanotubes in good yield. A possible way to increase the efficiency of a topochemical polymerization can be applying pressure to the preassembled gel. In fact, pressure could favour the self assembling step, a prerequisite for the cross-linking, and reduces the intermolecular distances, thus favoring the breaking of the butadiyne units and their cross-linking. Several simple unsaturated molecules are known to react under pressure, in their liquid or crystal phases, forming polymeric structures. 12 Ideally, a reaction in a condensed phase should follow the path requiring, in each step, the least displacement of the reactants (topochemical principle) due to the high activation barriers to rotations and translations as observed in the polymerization of ethylene, 13 3



acetylene, 14,15 NaCN 16 and acetonitrile. 17 Besides the crystal geometry, anisotropic stress both mechanical, as in benzene, 18,19 or due to the H-bonding, as in aniline, 20 plays an active role in selecting specific reaction paths leading to nanothreads formation. In more complex structures, where different functional groups are in principle able to react, the reactivity is driven by those that are less hindered. For instance, diphenylacetylene 21 reacts at 9 GPa through the phenyl groups, because the acetylene units, which would react at lower pressure, 14 are sterically hindered. This issue is particularly relevant in the present study where the reactivity of the macrocyclic molecules of the class of PAMs and PBMs is studied. Here, the most likely candidate for the reactivity is the butadyine unit if, as in the case of preassembling in gels, the units are facing each other in the appropriate position to cross-link when they are under pressure. Topochemical reactions triggered by pressure involving C-C triple bonds have been reported both in acetylenes 14,15 and diacetylenes. 22,23 An additional efficient tool for lowering the threshold pressure and favoring irreversible reactions under high-pressure is photochemical initiation. 24,25 In particular, optical initiation by population of the lowest excited HOMO-LUMO states through two-photon absorption of visible light was found to be particularly useful to understand and control the reactivity in unsaturated molecular systems under pressure, in the liquid or solid state, because of the very small amounts of excited molecules created. 26,27 More stable molecular compounds such as saturated simple alcohols, 28,29 clathrate hydrates, 30,31 water, 32,34 and alcoholic 33 mixtures, also react at mild pressure conditions (< 1 GPa) exploiting the contribution of dissociative excited states. Photopolymerization induced by UV radiation has also been reported in compressed diacetylenes. 35 Here we show that a pressure of 1 GPa is able to induce the cross-linking in two representative macrocycles, PAM2 and PBM2, with the aid of visible irradiation or moderate heating (100 ◦

C). Remarkably, PBM2 which is stable under irradiation at ambient pressure, 11 reacts

under pressure with a yield much larger than that observed in PAM2 at ambient pressure. 10 Transmission electron microscopy (TEM) and X-ray diffraction (XRD) provided evidence

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of the formation of nanometric rods of 30-100 nm in diameter and hundreds of nm to few microns in length. A very important result of this study, in view of extending the synthesis to large volume apparatuses, is that irradiation can be replaced by gentle heating to obtain a high yield of high quality product. C12H 25 C12H 25 O

O

H N

H N 4

4

C8H17

C8H17

C8H17

PAM2

PBM2

C8H17

C8H17

C8H17

N 4H

C8H17

C8H17

O N 4H

C12H 25

O

C12H 25

Figure 1: Molecular structure of phenylene-butadiynylene macrocycle 2 (PBM2) and phenylacetylene macrocycle 2 (PAM2)

Methods PAM2 and PBM2 were synthesizes according to previous reports. 10,11 PBM2 and PAM2 gels were prepared by dissolving 30 mg of sample in 1 ml of in CHCl3 . 11 The gel was then loaded into a sapphire anvil cell (SAC) equipped with high-purity, low fluorescence sapphire anvils with culet diameter of 1 mm and Cu-Be gasket. A ruby chip was loaded within the sample as pressure gauge. 36 Irradiation of the samples was performed by using the 514 nm visible line and the 350 nm UV multiline of a CW Ar+ laser. The sample was heated inside the SAC using a resistive heater and the temperature was measured by a K-thermocouple touching one anvil. Loaded samples, products and reaction evolutions were monitored by FTIR spectroscopy. The product was also characterised by TEM and XRD using synchrotron light. For each 5



run, a drop of fresh white gel was loaded into a high pressure or room pressure cell (see S.I.). All the samples were monitored by measuring the IR spectrum upon compression and before and after any irradiation cycle. They were also visually observed with an optical microscope, because a change in colour from transparent to dark blue is expected upon polymerization due to the formation of conjugated chains along the nanotube direction. 10

Results & Discussion As a preliminary step, we confirmed that PBM2 gel is non-reactive under UV irradiation at ambient conditions (see S.I.), as previously reported. 11 Irradiation of samples under pressure was performed in SAC at 1.0 GPa, with the 514.5 nm line of an Ar+ laser, focused to a spot size matching with the sample’s diameter with a power on the sample of 50 mW. This wavelength is absorbed through a two-photon absorption process, as the absorption edge of the sample is roughly at 250 nm. 10 As shown in Figure 2, the spectral region of the C≡C stretching absorptions is completely accessible in the SAC, and we can detect a doublet at 2150 and 2225 cm−1 due to the butadyine moieties. The intensity of this doublet slightly decreased after each irradiation step (Figure 2a), indicating that these moieties are consumed due to a chemical reaction. 37 In fact, the sample’s appearance changed due to irradiation to a dark blue colour, becoming black already after the initial 4 irradiation hours. PBM2 thus becomes reactive under pressure. To quantify the amount of reacted sample, we fitted the IR multiplet with six Voigt profiles using for the calculation the areas of the two strongest components which exhibit a consistent evolution of the relevant fitting parameters (frequency, area, lineshape and width) with the reaction time. An example of this deconvolution is reported in Figure 2e. Figure 2c shows that the reacted percentage reaches 24% after 40 h of irradiation with 50 mW at 514.5 nm. This yield is quite higher than that reported for PBM1, the reactive form of the synthesized phenylene-butadyine macrocycles, at room pressure. 11 During the reaction, a very small

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amount of CO2 is formed, revealed by the sharp absorption band at 2335 cm−1 (Figure 2a), which is likely related to the disruption of the carbonyl group. A new gelified sample of PBM2 in CHCl3 was loaded into a SAC, pressurized to 1.0 GPa, and then resistively heated while the sample’s appearance was visually monitored with an optical microscope. The sample’s colour started changing from transparent to blue when the temperature reached 373 K (Figure 2d). The temperature was then maintained at 390 K for 21 hours. Also in this case, the change in colour was accompanied by a decrease in the intensity of the C≡C stretching modes (Figure 2b). In the last spectrum acquired after 21 h the intensities of the residual absorptions of the monomer are apparently inverted. However, a weaker single band is expected in polydiacetylene 38 at slightly lower frequency (2135 cm−1 ) with respect to the monomer doublet. Therefore we assign the broader lower frequency peak to the superposition of the emerging polydiacetylene and the residual monomer peaks. By estimating the reacted percentage using the same procedure as for the irradiated sample, we found a yield of 50% in half of the time (21 h). Therefore at the same pressure of 1 GPa, a temperature increase of 100 K, in the absence of any irradiation, allowed a much faster reaction and a much higher yield than irradiation. Most importantly, almost no CO2 is formed during this reaction, in spite of the much larger fraction of reacted sample. We collected TEM images of the two polymerized samples of PBM2, recovered respectively from the reaction at 1.0 GPa and irradiation with visible light, and from the reaction at 1.0 GPa and 100◦ C (see S.I.). The variable contrast exhibited by the images (Fig.3) is likely originated by the inhomogeneous thickness of the fibers and by their nearly cylindrical shape: thicker is the sample, darker is the area because more electrons are scattered. Both products present nanorods few microns long and with diameters ranging from 30 to 100 nm. These diameter values are 5 - 15 times larger than expected for the single cross-linked nanotube and is due to the assembling of the nanotubes through the lateral alkyl chains. In addition to the variable bundles dimensions, the disordered piling of the nanorods also contributes to have variable thickness, and then variable contrast, across the different regions of the sample.

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(c)

21 h 3h 0h

1 GPa 390 K

50 40

* Absorbance

30 20 1 GPa 514 nm, 50 mW 0

10

20

30

Reacted %

1 GPa 390 K

10 0 40

time (h)

(d)

(b) 2100

2200

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Frequency (cm ) 1 GPa 514 nm, 50 mW

Absorbance

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40 h 27 h 19 h 4h 0h

(e)

*

(a) 2100

2200

2300

2400

2100

2200

2300

-1

Frequency (cm )

Figure 2: Reaction of PBM2 gel at 1.0 GPa using temperature and visible irradiation. Left panels: IR spectra in the region of the C≡C butadyine triple bond stretching modes (bands at 2150 and 2220 cm−1 ), measured as a function of time after each irradiation (a) or heating (b) step. The spectra are labelled according to the total number of hours at high temperature or under visible irradiation. The CO2 stretching band is marked with an asterisk. (c): percentage of reacted sample as a function of heating (orange dots) or irradiation (green dots) time, estimated by the decrease in the integrated area of the C≡C stretching modes; (d): microphotograph of the sample heated at 1 GPa acquired when the temperature reached 373 K; (e) example of deconvolution of the butadyne C≡C stretching region.

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The fibers obtained by high temperature-high pressure treatment appear as more isolated and of higher quality. Synchrotron XRD patterns have been measured on the sample produced in the reaction induced by pressure and temperature (PT) and on the one produced under pressure and photoirradiation (Phν). Representative patterns are reported in Figure 4. The two patterns are quite different although the main features extend almost at the same d-spacing. The pattern of the PT sample is remarkably resolved, consisting of three main narrow intense peaks at d-spacings of 4.13, 4.42 and 4.75 Å. The diffraction rings appearance indicates a powdered sample. Almost indistinguishable patterns are acquired in a mesh (step 20 µm) over the entire sample thus suggesting structural homogeneity across the sample. The dispersion of both patterns nicely agrees with that reported for the 1D nanorods obtained

Figure 3: TEM images of the polymerized samples of PBM2 obtained in this work. A,B,C) images of the product obtained with visible irradiation at 1.0 GPa, at different magnifications. D,E,F) images of the product obtained at 1.0 GPa and 100◦ C, at different magnifications.

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3.77

4.49

Intensity (a.u.)

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4.13 4.42 4.75


2

4

6

8

10

12

d-spacing(Å) Figure 4: XRD patterns measured on the recovered samples of the reaction of PBM2. Black trace from the reaction at 1 GPa and 100◦ C (PT); red tracefrom the photoinduced reaction at 1 GPa (Phν). Both traces are the raw data as obtained by the integration of the 2D pattern without subtracting the Compton scattering of air. The 2D image of the PT sample is also reported. Besides the three strong sample diffraction rings, Bragg spots due to the ruby and inhomogeneous diffraction rings due to the copper-beryllium gasket can be observed at larger angles. by ambient pressure columnar polymerization of PAM2, which was peaked at 4.39 Å and extended from 2.9 to 5.9 Å. 10 Similar results have been also reported for the formation of polydiacetylenes nanotubes. 39 The patterns are consistent with the diffraction between the parallel macrocycles stacks of the columnar structure of the nanorods. Remarkably, the pattern quality is much better in the case of the high temperature synthesis, likely because of the improved relaxation of the stress. The Phν sample produced a pattern of worse quality, but still better than that reported for the polydiacetylene synthesized by PAM2, 10 enlightening the role of pressure in improving the topochemical interaction of the monomeric units. The ideal interplanar distance should be 4.8 Å, in excellent agreement with the lowest 10


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angle diffraction peak of the PT sample which is always clearly separated from the two other peaks. The central peak, 4.42 Å, in the PT pattern closely resembles both the center of the broad diffraction pattern previously reported 10 (4.39 Å) and that of the pattern from the Phν sample, being likely related to a distorted configuration with a decreased angle between the diacetylenic linkers and the macrocycles. The last peak at 4.13 Å, although well within the dispersion of the Phν and literature patterns, presents an intensity comparable to the two other peaks suggesting that the structural modification responsible of the two peaks at lower d-spacing are not ascribable to a kind of structural defects but to a precise motif of the 1D nanorods. The effect of pressure on the topochemical polymerization was also probed on PAM2. Unlike PBM2, gelified preassembled solutions of PAM2 are reactive at room pressure under UV irradiation, 10 producing ordered nanotubes. Here, a gelified sample in CHCl3 was irradiated with the Ar+ near UV multiline centered at 350 nm (100 mW) in a SAC at nearly room pressure (< 0.1 GPa) at room temperature. This experiment was done in order to repeat the photochemical reaction at room pressure already reported, 10 but in a closed environment, provided by the SAC. Thus, we were able to detect the presence of volatile products, such as CO2 , which were not reported before. As expected, the sample became dark blue after 45 minutes of irradiation. The intensity decrease of the C≡C stretching bands indicates a ∼35% of reacted sample. However, the main modification in the IR spectrum concerns the presence of the intense asymmetric stretching band of CO2 , at 2335 cm−1 (see Fig SI-1). A new gelified PAM2 sample was pressurized in the SAC to 3.4 GPa and then decompressed, without any irradiation. Also in this case the samples turned blue. The amount of reacted sample, as estimated by the decrease of the C≡C stretching absorption intensities, in this case is around 14%, but the IR spectrum shows no formation of CO2 .

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Conclusions In this work, we demonstrate that the application of a moderate pressure is able to induce effectively a cross-linking between butadyine-containing macrocycles such as PAM2 and PBM2, with quality and yield superior than the room pressure photochemical methods that have been used so far. In particular, PBM2 does not react under irradiation at ambient pressure, whereas a topochemical reaction is easily induced here with a quite moderate pressure and temperature conditions, which would be easily obtained in large volume devices allowing the attainment of a larger amount of product. Namely, the fact that irradiation is not needed, makes production in large volume cells straightforward to realize. From a fundamental point of view, it is remarkable that the application of a moderate temperature assists the topochemical reaction with higher yield and better quality than the photochemical initiation. The temperature is indeed a key parameter in driving reactions in condensed phases often ruled by diffusion. 20,40,41 The better quality of the final product is proved by the TEM images and X ray diffraction, and by the absence of CO2 revealed by the IR spectra, implying a lower degree of decarboxylation of the macrocycles associated with the reaction conditions. PAM2, which is able to react at room pressure under irradiation, was here polymerized either with irradiation at near-room pressure (

Topochemical Polymerization of Phenylacetylene Macrocycles Under

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