Soluble Conjugated One-Dimensional Nanowires Prepared by

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Soluble Conjugated One-Dimensional Nanowires Prepared by Topochemical Polymerization of a Butadiynes-Containing StarShaped Molecule in the Xerogel State Jules Roméo Néabo, Simon Rondeau-Gagné, Cécile Vigier-Carrière, and Jean-François Morin* 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 S Supporting Information *

ABSTRACT: A star-shaped molecule with three butadiyne moieties attached to a central phenyl core was self-assembled via organogel formation in different solvents and subjected to UV irradiation in its xerogels form to give a soluble conjugated 1D nanowire made of three connected polydiacetylene (PDA) chains. The resulting polymer has a slightly lower optical band gap than its linear counterpart and presents no chromism property, indicative of the rigid nature of the polymer thus obtained.

1. INTRODUCTION The preparation of one-dimensional (1D) semiconducting supramolecular assemblies has attracted a lot of attention recently because it represents one of the most promising architectures for organic electronics applications.1−5 Onedimensional assemblies in which conjugated moieties are stacked in a face-to-face configuration allow for improved πorbital overlap between molecules in the solid state, thus increasing the charge mobility within the materials.6 One major drawback of this strategy is the difficulty of predicting the exact mode of assembly and the final structural parameters (length, width, and shape) of the supramolecular architecture for a given building block. Moreover, supramolecular assemblies can be altered upon device operation because of their kinetic instability toward variations of the assembly conditions (temperature, moisture, electric field, etc.).7 A very promising approach to circumventing these issues is to link the building blocks together covalently immediately after the self-assembly process. This strategy has been successfully used to fix the structure of foldamers,8 organic nanotubes,9−11 and, to a lesser extent, 1D supramolecular assemblies.12−14 The greatest challenge to address in order to make this strategy suitable for a wide variety of building blocks is to use a reaction that could allow the creation of covalent bonds between building blocks under the self-assembly conditions. In this context, catalyst-free photoinitiated reactions are particularly interesting because they can be conducted in the solid, gel, and solution states without the addition of chemicals that can be detrimental to the purity of the final material. Among all of the reactions known to date for this purpose, the topochemical polymerization of butadiyne units embedded within the assemblies’ building blocks is probably one of the most advantageous.15,16 Indeed, © 2013 American Chemical Society

this reaction can be accomplished using standard UV light (254 nm) in the crystal state but also in the gel state to form polydiacetylene (PDA), a well-known semiconducting polymer with interesting chromic properties.17−21 The formation of PDA from butadiyne has been used many times to prepare 1D architectures of various shapes and sizes.22−34 To the best of our knowledge, however, all of the examples reported in the literature so far involved the formation of a single PDA chain as the cross-linking unit. Moreover, very few reports have been published on the use of this reaction to create rigid, covalently linked semiconducting nanowires in which the active part is not the PDA itself. In 2005, Shinkai et al. reported the preparation of a stable porphyrin-based 1D assembly by linking the porphyrin units using the topochemical polymerization of butadiyne in the gel state at four different points on the molecules, leading to a stack of porphyrin units surrounded by four individual PDA chains.14 Because the porphyrin units are attached to the PDA chains through alkyl spacers, there is no through-bond electronic communication between the two species. Thus, we hypothesized that connecting semiconducting species (an aromatic core and the PDAs) together could be interesting in increasing the effective conjugation length within the nanowire and, consequently, enhancing the opto-electronic properties of such architectures. Therefore, we have recently demonstrated that it is possible to form PDA from 1,4diarylbutadiyne derivatives in the gel state very efficiently using the proper functions allowing hydrogen bonding and van der Waals interactions.35 Received: December 20, 2012 Revised: February 13, 2013 Published: February 18, 2013 3446

dx.doi.org/10.1021/la305045n | Langmuir 2013, 29, 3446−3452

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Figure 1. General strategy for the preparation of covalently linked, conjugated nanowires and their chemical structure. precoated TLC plates (Silicycle, Québec, Canada). Compounds were visualized using 254 nm and/or 365 nm UV and/or an aqueous sulfuric acid solution of ammonium heptamolybdate tetrahydrate (10 g/100 mL H2SO4 + 900 mL of H2O). Flash column chromatography was performed on 230−400 mesh silica gel R10030B (Silicycle, Québec, Canada). Nuclear magnetic resonance (NMR) spectra were recorded on a Varian Inova AS400 spectrometer (Varian, Palo Alto, CA, USA) operating at 400 MHz (1H) and 100 MHz (13C). Highresolution mass spectra (HRMS) were recorded with an Agilent 6210 time-of-flight (TOF) LC-MS apparatus equipped with an ESI or APPI ion source (Agilent Technologies, Toronto, Canada). FT-IR spectra were recorded in ATR mode (Thermo-Nicolet Magne 850). UV− visible absorption spectra were recorded on a Varian diode-array spectrophotometer (model Cary 500) using 3 mm path length quartz cells. Fluorescence spectroscopy was performed using a fluorescence spectrophotometer (model Cary Eclipse) coupled with a Cary temperature controller. DSC and TGA measurements were made on Mettler Toledo instruments (DSC 823e and TGA/SDTA851e). Scanning electron microscopy (SEM) and transmission electron miscroscopy (TEM) images were taken using a JEOL JSM-6360 LV and JEOL 1230, respectively. X-ray diffraction was recorded on a Siemens X-ray diffractometer (model D5000). Raman spectra were recorded at 22.0 ± 0.5 °C using a LABRAM 800HR Raman spectrometer (Horiba Jobin Yvon, Villeneuve d’Ascq, France) coupled to an Olympus BX 30 fixed-stage microscope. The excitation light source was the 633 nm line of a He−Ne laser (Melles Griot, Carlsbad, CA). The laser beam was focused, generating intensity at the sample of approximately 5−10 mW. The confocal hole and the entrance slit of the monochromator were generally fixed at 200 and 100 μm, respectively. Data were collected with a 1 in. open electrode Peltiercooled CCD detector (1024 pixels × 256 pixels). 2.2. Gelation Test. To test the gelation properties of 3 in organic solvents, we proceeded as follows. In a vial, 3 was dissolved in a solvent, and the vial was sealed and heated until a clear solution was

Herein, we report the synthesis and characterization of a 1D organic nanowire with a phenyl ring as the aromatic core and three PDA chains directly attached to it in order to provide a nanowire in which the core is conjugated to the PDA chains. The 1D configuration was obtained by the formation of organogels from a carefully designed star-shaped lowmolecular-weight building block. The butadiyne-containing building block is represented in Figure 1. As the core of the nanowire, a single phenyl group was chosen for ease of synthesis. Obviously, once our strategy proved to be successful in preparing aryl-based PDA nanowires, other more complex πconjugated units will be tested. For the side groups containing the butadiyne units, we attached a phenyl group bearing amide moieties and a short ethylene oxide chain. We previously showed that this particular unit is very effective at driving organogel formation, particularly in aromatic solvents.35,36 The nanowires were characterized using vibrational and optical spectroscopy and electronic microscopy.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Chemical reagents were purchased from Sigma-Aldrich Co. Canada, Alfa Aesar Co., TCI America Co., and Oakwood Products Inc. and were used as received. Solvents used for organic synthesis were obtained from Fisher Scientific (except THF from Sigma-Aldrich Co. Canada) and purified with a solvent purifier system (SPS) (Vacuum Atmospheres Company, Hawthorne, CA, USA). Other solvents were obtained from Fisher Scientific and were used as received. Tetrahydrofuran (THF) and triethylamine (Et3N) used for Sonogashira reactions were degassed 30 min prior to use. All anhydrous and air-sensitive reactions were performed in ovendried glassware under positive argon pressure. Analytical thin-layer chromatography was performed with silica gel 60 F254, 0.25 mm 3447


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Scheme 1. Synthesis of 3

obtained. The clear solution was allowed to cool to room temperature, and the stability of the gel was confirmed by the tube inversion method. 2.3. Synthesis of 2. In a 50 mL round-bottomed flask equipped with a stir bar, 1 (1.00 g, 1.72 mmol) was dissolved in a mixture 1:1 THF/MeOH (17 mL) and K2CO3 (0.054 g, 0.39 mmol) was added. After 15 min of stirring, the mixture was quenched with aqueous NH4Cl and extracted with benzene. The organic layer was dried over Na2SO4 and filtered under vacuum. The mixture was evaporated under reduce pressure until 10% of the original volume remained. In the meantime, in a 50 mL oven-dried round-bottomed flask equipped with a stir bar, 1,3,5-triiodobenzene (0.195 g, 0.428 mmol) was dissolved in NEt3 (17 mL). The previous solution was added, and a stream of argon was continuously bubbled through the mixture for 30 min. Then, PdCl2(PPh3)2 (0.036 g, 0.051 mmol) and CuI (0,010 g, 0,051 mmol) were added. The flask was purged four times with argon, and the mixture was stirred overnight at room temperature. Upon completion of the reaction as determined by TLC analysis, aqueous NH4Cl was added and the mixture was extracted with ethyl acetate. The organic layers were combined and dried over Na2SO4. The mixture was filtered under vacuum, and the solvent was removed under reduced pressure. The crude material was purified by column chromatography (silica gel, hexanes/ethyl acetate 7:3 as the eluant) to provide 2 as a gray solid in 55% yield. 1H NMR (500 MHz, CDCl3, ppm): 7.86 (d, J = 1.8 Hz, 3H), 7.81 (dd, J = 2.0 Hz, J = 8.6 Hz, 3H), 7.6 (s, 3H), 6.96 (d, J = 8.7 Hz, 3H), 6.16 (t, J = 5.4 Hz, 3H), 4.17 (t, J = 4.8 Hz, 6H), 4.05 (t, J = 4.8 Hz, 6H), 3.43 (m, 6H), 1.61 (m, 6H), 1.26 (m, 54H), 0.92 (m, 27H), 0.87 (m, 9H), 0.15 (s, 18H). 13C NMR (500 MHz, CDCl3, ppm): 165.97, 163.16, 136.07, 132.88, 132.86, 130.33, 127.19, 123.22, 111.75, 110.88, 79.67, 78.58, 77.81, 75.94, 70.38, 61.76, 40.20, 31.92, 31.64, 29.71, 29.69, 29.66, 29.64, 29.61, 29.57, 29.36, 27.03, 25.91, 22.70, 18.40, 14.14. HRMS: M*+ calcd for C99H147N3O9Si3, 1606.04; found, 1606.044. 2.4. Synthesis of 3. In a round-bottomed flask equipped with a magnetic stir bar, 2 (0.381 g, 0.235 mmol) was dissolved in THF (5 mL) and tetrabutylammonium fluoride (TBAF, 1.40 mL, 1.41 mmol) was added. After being stirred for 2 h at room temperature, the mixture was precipitated into methanol. The solid was filtered under vacuum and dried under vacuum to provide 3 as a gray solid in 81% yield. 1H NMR (400 MHz, (CD3)2SO, ppm): 8.20 (t, J = 5.3 Hz, 3H), 8.01 (d, 2.1 Hz, 7.86 (m, 6H), 7.15 (d, J = 8.9 Hz, 3H), 4.74 (t, J = 5.2 Hz, 3H), 4.15 (t, J = 5.0 Hz, 6H), 3.73 (m, 6H), 1.47 (m, 6H), 1.2 (m, 54H), 0.8 (m, 9H). HRMS: [M + H]+ calcd for C81H105N3O9, 1263.79; found, 1264.79.

yield. Then, the alcohol groups were deprotected using tetrabutylammonium fluoride (TBAF) in 81% yield. It is worth mentioning that the protection of the alcohol was necessary to obtain a soluble intermediate (2) that can be easily purified by standard column chromatography. The gelation properties of 3 were studied in different solvents, and the results are presented in Table 1. In all cases, 3 Table 1. Gelation Properties of 3 solvent

observationsa

CGC (mg/mL)b

Tc (°C)c

Tgel (°C)d

toluene o-DCB cyclohexane chlorobenzene ethyl acetate MeOH chloroform THF

G G I G P S V S

1 5

−126 −40

93 125

2

−69

135

a

Observations for organogel at 5 mg/mL. bCritical gelation concentration. cCrystallization temperature of the solvent trapped in an organogel at 10 mg/mL. dMelting temperature for the organogel at 10 mg/mL for chlorobenzene and o-DCB and 5 mg/mL for toluene using the drop-ball method.37

was added to the desired solvent in a capped glass tube at a concentration of 10 mg/mL and sonicated to break the large aggregates. The above mixture was then heated until a clear solution was obtained. This solution was slowly cooled to room temperature, allowing the formation of organogels. 3 gave stable gels in DMSO and all aromatic solvents tested. Gel formation was confirmed by the tube inversion method. In nonaromatic solvents, viscous solutions or suspensions were observed. The gels obtained from aromatic solvents can be heated and cooled repeatedly without a significant change in the final gel morphology. Surprisingly, the melting temperatures of the organogels in aromatic solvent are very high, indicative of a very strong supramolecular network. To get a better understanding of the supramolecular organization in the gel state, a small amount of the gel (10 mg/mL in toluene) was dried under ambient conditions on a metallic substrate and subjected to scanning electron microscopy (SEM) imaging. As shown in Figure 2, the xerogel thus formed is made of a very dense array of 1D-entangled fibrils with diameters of a few tens of nanometers. This rather dense packing of fibrils is indicative of strong interfiber

3. RESULTS AND DISCUSSION 3 was synthesized in two steps as shown in Scheme 1. Threefold Sonogashira coupling between 136 and 1,3,5triiodobenzene using Pd/Cu as catalysts afforded 2 in 58% 3448


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order between the alkyl chains and the π−π distance between the molecules, respectively, can be observed. This diffractogram suggests that the molecules stacked on top of each other to create long 1D supramolecular arrays of molecules that assembled into larger fibrils owing to the alkyl chain interdigitation. To assess the importance of the gelation process on the molecular assembly, PXRD analysis was performed on 3 deposited onto a glass slide from THF at a concentration of 10 mg/mL. As expected, no particular arrangement was observed. The molecular packing mode has also been investigated by a variable-temperature 1H NMR (VT-1H NMR) experiment. In fact, VT-NMR experiments have been widely used to study, among others, the role of intermolecular H-bonding in the gelation process.41 The importance of H-bonding in the gelation process can be determined by measuring the NH chemical shift upon heating. The results obtained for such a study in DMSO-d6 are shown in Figure 3. It is worth mentioning that DMSO-d6 was the only solvent in which a clear 1H NMR spectrum was obtained. As expected, the NH signal was shifted upfield upon heating the solution, meaning that H bonding became weaker as the temperature increased. The shift (ca. 0.5 ppm) observed from 298 to 368 K is relatively large considering that the study was conducted in DMSO, which is known to disrupt H bonding because of its very polar nature. Interestingly, increasing the temperature of the solution also induced an upfield shift of the aromatic protons of the core (Hb in Figure 3). This shift indicates that these protons are

Figure 2. SEM image of dried organogel from 3 in toluene (10 mg/ mL). The scale bars are (a) 10 and (b) 1 μm.

interactions responsible for the robust macroscopic properties of the gel.38 Powder X-ray diffraction (PXRD) analysis was performed to study the packing configuration and the distances within the supramolecular assembly. The gel was deposited on a glass substrate and slowly dried, and a diffractogram was recorded between angles of 2θ = 0 and 40°. The PXRD pattern of 3 is characteristic of a disordered columnar phase39 (Figure S2 in the SI section). The diffraction pattern is characterized by a particularly sharp and intense peak at 2θ = 2.4° (36.7 Å) and another small one at 2θ = 4.2° (21.0 Å), which are attributed to the (10) and (11) reflections, respectively, with a lattice constant of a = 42.4 Å. This intercolumnar distance is very close to the expected 47.6 Å calculated for noninterdigitating extended alkyl chains, meaning that the columns are slightly interdigitated.40 Moreover, two broad bands peaking at 2θ = 21.5 (4.21 Å) and 28.0° (3.29 Å), attributed to the liquidlike

Figure 3. Aromatic and amide regions of the 1H NMR spectra of 3 in DMSO-d6 at different temperatures: 298, 323, 338, 353, and 368 K. Inset: The molecular structure of 3 with peak assignments. 3449


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core would shift the λmax to lower energy, giving access to lowband-gap materials. The difference in the band gap of these two materials is unlikely to be attributed to the difference in molecular weight. In fact, the molecular weight values of both NW-PDA and L-PDA are much higher than the lower limit (about 10 monomeric units) necessary to obtain saturation of the electronic properties. Although we have not been able to determine the molecular weight of NW-PDA by size-exclusion chromatography because of its high degree of stiffness (beyond the exclusion limit), one can assume that the degree of polymerization is relatively high on the basis of TEM images showing nanowires that are a few tens of nanometers long (vide infra). Unlike L-PDA that exhibits thermochromism properties in both the solution and the solid state,19,35 NW-PDA does not exhibit thermochromic properties in the solution or in the solid state because the dihedral angle between the alkyne and alkene moieties within the main chain is locked. This expected lack of chromic properties for NW-PDA is indicative of its rigid nature. The lack of thermochromic properties was also recently observed for rigid nanorods based on PDA.11 To assess whether all of the butadiyne units have reacted during the irradiation step, the Raman spectrum of the purified blue material was recorded using a low-energy 633 nm laser, and the result is shown in Figure 5. The proof of the formation

involved in π stacking at low temperature and confirms the columnar assembly found by PXRD. The sample for topochemical polymerization was prepared by depositing a gel of 3 (toluene, 10 mg/mL, Figure S3 in the SI section) on a glass substrate followed by drying for 1 h under ambient conditions to yield a xerogel. Then, the substrate was placed under UV light for 72 h, and a dark-blue film was obtained, indicative of the formation of high-molecular-weight nanowire PDA (NW-PDA) with a planar backbone configuration. To remove unreacted 3, we dissolved the irradiated material in THF and subjected the resulting solution to purification by semipreparative size exclusion chromatography (SEC) in order to isolate the high-molecular-weight fraction corresponding to the desired NW-PDA (Figure S3 in the SI section). Surprisingly, 50 wt % of the 3 that was irradiated under the specified conditions was transformed into polymeric material, which is quite high for a reaction performed in the solid state. However, the soluble fraction corresponds to approximately one-third of the blue material formed upon irradiation. It is worth mentioning that the topochemical reaction has been attempted directly on the gel by irradiating a hermetically closed quartz UV cell containing the gel. However, after only a few minutes of irradiation, the gel state was rapidly lost and no blue material was formed. This phenomenon was previously observed by us35 and others42 and can be attributed to a significant conformational change in the side groups within the gel when the topochemical reaction is initiated, thus disturbing the hydrogen bond network. Consequently, we pursued all further characterization of the material obtained from the irradiation of the xerogel. The formation of NW-PDA was confirmed first by UV− visible spectroscopy, and the result is shown in Figure 4. The

Figure 5. Raman spectra of 3 (black) and purified NW-PDA (blue).

of PDA was confirmed by the presence of two bands at 1466 and 2106 cm−1 attributed to the stretching vibrational modes of newly formed alkene and alkyne groups, respectively. Interestingly, no band at 2218 cm−1 corresponding to the butadiyne unit of 3 was observed, meaning that all of the butadiyne units have reacted to form PDA and that all of unreacted 3 was removed in the purification step. Considering the columnar arrangement observed by PXRD for the dried gel and by VT-1H NMR analysis of a solution of 3, it is very unlikely that the resulting blue PDA material could be made of architectures other than that of the expected nanowire. In fact, intercolumnar topochemical reactions are highly unlikely because the formation of PDA is very dependent on the distance between monomers (∼4.9 Å) and the angle (45°) between reacting butadiyne units.15,16,43 For the same reasons, cross-linking reactions occurring in amorphous phases are highly improbable, leaving the formation of nanowires as the only probable event. To study the nanoscale morphology of the nanowires in the solid state, TEM analysis was performed and the results are

Figure 4. UV−visible spectra of linear phenyl-substituted PDA (LPDA, red curve) and purified NW-PDA (blue curve) in THF. Inset: structure of L-PDA, whose UV−visible spectrum is shown.

absorption maximum (λmax) is centered at 650 nm with a shoulder at 590 nm, corresponding to a red shift of ca. 35 nm compared to its linear analog, L-PDA, whose structure is shown in the inset of Figure 4. The measured optical bandgap for NWPDA is 1.80 eV, which is ca. 0.1 eV lower than that measured for L-PDA. This indicates that the nanowires present a slightly higher effective conjugation length even though the central phenyl is linked to other PDA chains through a meta linkage. One can argue that a different linkage or a more π-extended 3450


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shown in Figure 6. A sample of the nanowires was dispersed in MeOH, in which the nanowires formed a suspension rather

than a solution, and deposited on a TEM grid prior to imaging. Interestingly, nanowire-like structures with a diameter of 10 to 15 nm were observed, meaning that the morphology of the initial gel was somehow retained during the photochemical process. Moreover, nanowires with diameters of between 5 and 6 nm were also observed, although to a lesser extent, which probably corresponds to individualized nanowires (based on PXRD analysis obtained for the columnar arrangement and on theoretical calculations). Unfortunately, we have not been able to obtain a clear image with sufficient contrast from these nanowires from a THF solution in which the nanowires form a translucent solution. Thus, we cannot assess with certainty yet that the nanowires could be individualized, as demonstrated by Shinkai and co-workers for porphyrin-based nanowires.14

4. CONCLUSIONS Polydiacetylene-based nanowires were synthesized by the topochemical polymerization of a butadiyne-containing starshaped molecule in the xerogel state. Soluble nanowires with improved intramolecular conjugation were obtained by attaching the butadiyne units directly to the aromatic core. Star-shaped molecules with different, more extended πconjugated cores will be synthesized in the near future to evaluate the potential of our strategy for preparing low-bandgap nanowires. ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures and characterization data for all new compounds, PXRD pattern of the xerogel of 3, and DCS traces of the organogels. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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Figure 6. TEM images of PDA dispersed in MeOH. Scale bars are (a) 200 and (b) 100 nm.

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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 This work was supported by NSERC through a Discovery Grant. We thank Richard Janvier (U. Laval) for his help with the SEM and TEM experiments and Philippe Dufour (U. Laval) for the HRMS experiments. S.R.-G. thanks the NSERC for a Ph.D. scholarship. 3451


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