Article pubs.acs.org/Macromolecules
Semiconducting Nanotubes by Intrachain Folding Following Macroscopic Assembly of a Naphthalene−Diimide (NDI) Appended Polyurethane Tathagata Mondal,† Tsuneaki Sakurai,‡ Satoru Yoneda,‡ Shu Seki,*,‡ and Suhrit Ghosh*,† †
Polymer Science Unit, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Kolkata, India-700032; Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Osaka, Japan
‡
S Supporting Information *
ABSTRACT: This article describes a well-designed supramolecular assembly of a classical polyurethane scaffold containing pendant n-type semiconducting naphthalene− diimide (NDI) chromophores and consequences on excited state dynamics and charge carrier mobilities. A polycondensation reaction between hexamethylene−diisocyanate and a NDI-containing diol in the presence of a chiral “monofunctional impurity” produced the desired polymer (P1) with a predictable degree of polymerization and end-capping by chiral units. In aliphatic hydrocarbons, such as methylcyclohexane (MCH), P1 adopts a folded conformation with appreciably high thermal stability by intrachain H-bonding among the urethane groups as established by solvent, concentration and temperature-dependent FT-IR and 1H NMR spectroscopy and small angle XRD studies. Folded structure can be further ascertained by the pronounced Cotton effect in MCH owing to the chiral induction by the so-called “sergeant and soldiers” principle from the asymmetric units located only at the chain ends. Intrachain folding facilitates spatial organization of the pendant groups leading to π−π interaction among the neighboring NDI chromophores attached to the same polymer chain resulting in intense green emission in MCH in sharp contrast to the blue-emitting unfolded polymer in benign solvents such as CHCl3 or THF. P1 in the folded state resembles the organization of classical bolaamphiphile and thus adopts a polymersome-like spherical structure. Upon aging macroscopic gelation can be observed owing to the fusion of these discrete spherical assemblies generating micrometer long multiwall nanotubes as noticed in HRTEM, AFM and fluorescence microscopy images. Transient absorption spectroscopy studies indicate formation of NDI radical anions in the excited state both in unfolded and folded conformation which contribute to their intrinsic electron transporting (n-type) property, as revealed by flash-photolysis timeresolved microwave conductivity (FP-TRMC). Significantly larger electron mobility and longer lifetime of charge carriers were observed for the folded tubular assembly than those for unfolded polymer, likely due to a better delocalization of the chargecarriers in the integrated tubular assembly consisting of stacked NDI arrays inside the multilayer wall.
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INTRODUCTION In the recent past, there has been immense interest in the field of designing foldamers1 from abiotic units to mimic the elegant structure and function of biomacromolecules. However, major activities in this area are still restricted to understanding structural nuances on folding propensity of mostly oligomer species,1,8a−c except limited examples on folding of polymers.2 Furthermore, little is known on macroscopic assembly of foldamers2d,3 (oligomers or polymers) or their functional utilities in applied fields such as catalysis, organic optoelectronics and so on4 which possibly are the logical next steps forward considering the strong dependence of the biological functions of proteins not only on their secondary structure but also on tertiary and quaternary structures. Even though several complex and elegant molecules have been studied for foldamer design, classical polyurethane (PU) scaffolds5 surprisingly have not been used to precisely demonstrate intrachain folding likely © XXXX American Chemical Society
due to lack of a suitable strategy for preventing strong interchain interactions by multiple H-bonding. We envisaged appropriately designed PUs might provide ample opportunities to design functional foldamers owing to (i) the availability of well-established and straightforward synthetic methods for preparation of structurally diverse PUs (ii) possibility of enthalpy driven folding by intrachain H-bonding among the urethane groups, inherently embedded in the polymer backbone and (iii) assorted scopes of introducing functional groups as pendants and/or in the main chain by structural engineering of the monomers. Scheme 1a depicts a PUderivative (P1) containing pendant naphthalene-diimide (NDI) chromophore and branched alkyl chains to provide steric Received: November 29, 2014 Revised: February 1, 2015
A
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Scheme 1. (a) Synthetic Scheme and (b) 1H NMR Spectrum of P1 in CDCl3/ TFA (98:2)a
a
The asterisk indicates peaks from residual CHCl3 of the solvent.
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RESULTS AND DISCUSSION Synthesis. P1 was synthesized (Scheme 1a) by a stepgrowth polymerization between hexamethylene-diisocyanate (M1, BB) and a NDI-containing diol (M2, AA) in the presence of small amount of (s)-(−)-2-methyl 1-butanol (A*) as a “mono-functional impurity” (MFI) with a specific ratio satisfying the relationship 2NBB = 2NAA + NA*, (where N stands for the mole fraction of each component), to ensure capping of all the chain-ends by the chiral alcohol.16 The chiral monofunctional impurity was used particularly to examine whether it would induce helicity in the folded structure by the so-called “sergeant and soldiers” principle17,18 and thus enable one to probe the folding process by circular dichroism. With this strategy, the poly condensation was carried out in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) catalyst in THF to produce the desired PU (P1) as a light yellow powder in 75% yield. All the peaks in the 1H NMR spectrum of P1 could be assigned unambiguously as indicated in Scheme 1b. The end group analysis using the integration of He (from the chain end, indicated by arrow) and Hd (from the repeat unit, indicated by arrow) estimated the molecular weight of P1 to be 8000 Da which matched reasonably well with the number average degree of polymerization =13, estimated from the Carothers’s equation.16a GPC chromatogram of P1 in THF (Figure S1, Supporting Information) showed a unimodal distribution with weight-average molecular weight (Mw) = 7600 Da (PDI = 1.72) that corroborated well with the value obtained from end group analysis.
insulation against interchain H-bonding. Notably different NDI-derivatives have been extensively studied in the recent past by many groups including ours6f−h,7,8f−g for diverse range of supramolecular assemblies owing to their well-known ability for π-stacking,6 coassembly with electron rich chromophores by charge-transfer (CT) interaction,7,8 ion−π interaction,9 organocatalysis,10 constructing photosystems,11 and n-type semiconductivity12 among others.13 In most of these events and also in large number of examples on supramolecular assembly of other π-systems,14 although well-defined macroscopic ordering and through space communication among the chromophores have been achieved, they often lack physical properties like macromolecules desired for practical utility. We envisioned that chromophore-attached foldable polymeric scaffolds15 such as P1 may unravel an unexplored generalized strategy for well-defined spacial organization and concentration-independent through space communication of the πsystem with long-range order as a consequence of intrachain Hbonding driven folding of the PU backbone which in turn would impact their photophysical properties and functional utilities. To test such possibilities, we have studied self-assembly of P1 (Scheme 1a), and herein, we have shown its intrachain folding followed by multistep macroscopic-assembly pathway to produce chiral nanotubes and the strong impact on π−π interaction, photoluminescence, excited state dynamics of the NDI-radical anion, and charge-carrier mobility. B
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Nanotubular Assembly. Self-assembly of P1 was tested in a nonpolar solvent methylcyclohexane (MCH). At 5.0 mM concentration (based on the repeat unit) the polymer was soluble at high temperature and upon allowing to settle at room temperature the solution became gradually more viscous and eventually produced a nearly transparent green emitting gel (Figure 1a)19 after 72 h with a critical gelation concentration
solution (Figure S2) showed hollow spherical objects corroborating with the fluorescence microscopy images. Spectroscopic Studies. To correlate the macroscopic structure with molecular interaction, we conducted UV/vis, 1H NMR, fluorescence, FT-IR, and CD spectroscopy experiments. UV/vis spectra of P1 in a “good” solvent such as CHCl3 (Figure 2a) showed sharp absorption bands in the range of 300−400 nm with vibronic features indicating presence of monomeric NDI−chromophores. Nevertheless, in a nonpolar medium MCH, a significant lowering of intensity with concomitant reversal of the intensities of the peaks at 382 and 361 nm were noticed (Figure 2a) owing to π−π interaction.6 Notably the absorption bands in MCH did not exhibit appreciable red-shift as commonly observed for offset πstacking of NDI chromophores,13 suggesting interchromophoric interaction without longitudinal displacement between the adjacent NDI units6b,c,24 in the present case of selfassembly. Concentration dependent UV/vis experiments showed no change in the spectral nature (Figure S3) in the entire tested range (10−3−10−5 M) with the absorption intensity varying linearly (Figure 2b) with concentration indicating intrachain NDI-NDI stacking. Furthermore, variable temperature UV/vis studies showed no signature of disassembly (Figure 2c) even at 85 °C, suggesting appreciable stability of the assembled structure. To examine the impact of π−π interaction on the emission properties, photoluminescence spectra were compared in CHCl3 and MCH (Figure 2d) which showed appearance of a broad emission bands at ∼500 nm corresponding to NDIexcimer and thus justified the green emissive nature of the gel (Figure 1a) produced by stacking of NDI-chromophores.25 Noteworthy that similar excimer band, although not as prominent as in MCH, was also noticed in CHCl3 possibly due to an interchain NDI−NDI interaction. This proposition is supported by the fact that at higher concentration (1.0 mM) the spectral nature in CHCl3 changed significantly with more intense excimer emission compared to monomeric bands (380−450 nm), while the spectral nature in MCH remained mostly unchanged (Figure S4). Solvent-dependent 1H NMR studies (Figure 3a) provided additional evidence supporting self-assembly. In MCH, the NDI ring proton exhibited a pronounced upfield shift (1.97 ppm in 500 MHz spectrometer) compared to CDCl3, due to the shielding effect as a consequence of π−π interactions. Interestingly, none of the PU backbone protons were detectable (6.00−2.00 ppm region) in the NMR spectrum of P1 recorded in MCH suggesting their very long relaxation time as a result of H-bonded self-assembled structure formation.26 Variable temperature 1H NMR experiments showed (Figure S5) no spectral shift of the aromatic proton or appearance of the PU backbone protons even at elevated temperature and thus supplements the results obtained from UV/Vis studies on high thermal stability of folded structure. FT-IR spectra of P1 in MCH (Figure 3b) showed distinct peaks at 3360, 1665, and 1537 cm−1 for N−H (stretching), carbonyl (stretching) and N−H (bending), respectively, as also observed for the polymer in the solid state (Figure S1) further confirming H-bonding among the urethane groups.27 Even a 100-fold dilution had virtually no effect (Figure 3b) on the position of these peaks substantiating intrachain H-bonding. Identical spectral pattern at 85 °C (Figure S6) indicated thermal stability of the folded conformation as also concluded earlier from UV/vis studies. Interestingly by gentle heating in the presence of a trace
Figure 1. (a) Images of P1 gel (c = 5.0 mM) in MCH in day light (left) and under UV light (right) (λex = 365 nm). (b) Variation of gelation time with concentration of P1. (c,d) Fluorescence microscopic images of freshly prepared solution and aged (48 h, room temperature) gel of P1 in MCH. (e−g) HRTEM images of P1 gel in MCH showing tubular structures.
(CGC) = 3.0 mM and Tg = 52 °C at CGC. Self-assembly was also tested in a few other organic solvents while gelation was noticed only in MCH and trans-decalin (Table S1). Interestingly the gelation time was found to be significantly faster with increasing concentration (Figure 1b) suggesting involvement of interchain interaction. Fluorescence microscopy images of freshly prepared solution and aged gel revealed contrasting morphologies. Initially formed green emitting spherical objects (Figure 1c) were converted to entangled 1D network structure (Figure 1d) upon aging and thus explained the observed slow gelation.20 To have a closer look at the morphology, high resolution transmission electron microscopy (HRTEM)-images were recorded which showed nanotubular structures (Figure 1e−g, indicated by arrow)21−23 for the aged sample. The inner diameter and wall thickness of the tubes were measured (Figure 1f) to be 85 ± 5 nm and 26 ± 2 nm, respectively, while the length extended over 5−9 μm. In contrast, the HRTEM images of a freshly prepared C
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Figure 2. Optical spectroscopic studies with P1 (c = 1.0 mM and 0.01 mM for UV/vis and PL experiments). (a) Solvent-dependent UV/vis spectra at room temperature. (b) Variation of absorption intensity as a function of concentration. (c) Variable temperature UV/vis spectra in MCH and comparison with that in CHCl3 at room temperature. (d) Solvent-dependent emission spectra at room temperature (λex = 340 nm).
conformation in MCH by intrachain H-bonding among the urethane groups, leading to stacking of the pendant NDI units. Scheme 2. Proposed Model for Self-Assembly of P1
The folded structure, having similarity in its topological structure with array of bolaamphiphiles, possibly adopted a lamellar packing and eventually polymersome-like structure to avoid the unfavorable edge interaction with the medium28 Colloidal instability possibly due to the strain in the curved assembly imposed by the rigid H-bonding initiated vesicle− vesicle fusion29 producing nanotubes upon aging. At higher concentration (>CGC), entangled tubes produced macroscopic gel while below CGC, aging resulted in increased solution viscosity. The proposed folded structure of P1 could be supported by solvent dependent CD experiments which showed (Figure 4) an intense Cotton effect in MCH while no such band was noticed in CHCl3, confirming helical assembly in MCH. The bisignated features of the CD spectra originate from exciton-coupling of NDI-chromophores that are located in an asymmetric environment. In presence of a trace amount of TFA which interrupts the H-bonding and thus destroys the folded conformation, the CD
Figure 3. a) 1H NMR spectra of P1 in CDCl3 and MCH/C6D6 (90/ 10). * indicates solvent peaks; b) Concentration dependent FT-IR spectra (selected region) of P1 in MCH; c) Effect of MeOH addition on UV/vis absorption spectra (recorded after incubating the sample at ∼45 °C for 5 min) of P1 in MCH.
amount of H-bonding competing protic solvent MeOH, the absorption spectrum of P1 in MCH showed significant change and resembled the monomeric features (Figure 3c) similar to that in CHCl3. This indicates decisive role of H-bonding among the backbone urethane groups on self-assembly and consequent π−π interaction among the pendant NDI chromophores. Model for Nanotube Formation and Evidence. Corroborating these experimental observations, it was proposed (Scheme 2) that P1 spontaneously adopted a folded D
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Figure 4. Solvent-dependent CD spectra of P1 at 25 °C.
bands almost diminished (Figure 4) confirming that indeed they appeared as a result of H-bonding driven folding of the polymer chain. As neither the polymer backbone nor the pendant NDI units contain any chiral group, the observed Cotton effect is attributed to helicity induction to the folded structure from the chiral end groups by a “sergeant and soldiers” principle.17,18 While several reports exist on such chiral induction in helical polymers17,18a−d with rigid backbone and also in supramolecular assemblies18e−g of small molecules, to our knowledge such a pronounced chirality induction from a single asymmetric center present only at the chain end for a macromolecular foldamer with flexible backbone is unprecedented. In principle this phenomenon should be of wide interest for future studies to utilize such induced helicity in chiral recognition and separation, asymmetric catalysis, and so on. Existence of the folded structure was also verified by small angle powder X-ray diffraction (XRD) measurements which showed (Figure 5) a sharp peak at 2θ = 1.11°, corresponding to
Figure 5. XRD of a drop-casted film of P1 in MCH.
Figure 6. Tapping mode AFM images of drop-casted P1 solution in MCH (a, 0.5 mM; b, 0.001 mM) on mica after different aging times. Graphs in the inset show height-width profile from parts a and b in the corresponding image.
d = 78 Å, matching closely with the estimated width of P1 in folded state (For an energy minimized structure, see Figure S7). Furthermore, presence of subsequent peaks at periodic distance of 38 Å (d/2) and 20 Å (d/3) supported proposed lamellar packing (Scheme 2) with a growth direction along the width of the folded structure likely by van der Waals interaction among the peripheral alkyl chains. While the CD and XRD results supported the folded conformation, AFM studies (Figure 6) provided further insights on the macroscopic tubular assembly of the folded P1. AFM images of the freshly prepared solution (0.5 mM) revealed discrete spherical assemblies (width∼ 140−150 nm; height ∼9
± 2 nm), corroborating well with TEM and fluorescence microscopy images discussed before. Upon aging the solution for ∼30 h these discrete particles began to collide with each other leading to extended structure formation after 150 h. Extremely low height (1.8 ± 0.2 nm) of these elongated structures compared to their width (160 nm) can be attributed to flattening of the objects on the surface which is more likely to happen for the hollow structures and thus further supports tubular morphology as observed in TEM. Similar studies at a relatively lower concentration (0.01 mM) showed significant slowing down in rate of this morphology transition as shown in Figure 6b. Initially almost identical spherical particles are E
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Figure 7. (a, b) Conductivity transients of films of P1 dropcast from MCH gel (red), MCH solution (blue), and CHCl3/MeOH solution (green) upon photoexcitation at 355 nm.
Figure 8. (a) Kinetic traces at 490 nm and (b) snapshot at 0−0.3 μs of transient absorption spectra of dropcast films of P1 from MCH gel (red), MCH solution (blue), and CHCl3/MeOH solution (green) upon photoexcitation at 355 nm. (c) Kinetic traces at 490 nm and (d) snapshot at 0−0.3 μs of transient absorption spectra of MCH solution of P1 (black) and P1/TCNQ (10/1 molar ratio; gray) upon photoexcitation at 355 nm.
tectures of P1 by flash-photolysis time-resolved microwave conductivity (FP-TRMC).31 FP-TRMC technique applies electric field of microwave that probes the local-scale charge carrier motions and thus enables the device-less evaluation of charge carrier transport.32 Upon photoexcitation by 355 nm laser pulses, a dried-gel film of P1 on a quartz plate, prepared by drop-casting of MCH gel, showed a prompt rise followed by a slow decay in the conductivity transient (Figure 7a), indicating that π-stacked NDIs in folded P1 indeed provide charge transport pathways. In fact, a drop-cast film of P1 from CHCl3/MeOH solution, consisting of unfolded state of P1, displayed much smaller conductivity. The observed (Figure 7a) maximum φΣμ (φ and Σμ denote charge carrier generation efficiency and sum of charge carrier mobility, respectively) values (3.5 × 10−5 cm2 V−1 s−1 and ∼0.7 × 10−5 cm2 V−1 s−1 for gel in MCH and solution in CHCl3/ MCH, respectively) indicated five times larger conductivity for P1 in folded state compared to unfolded form. A drop-cast film from freshly prepared MCH solution, including vesicle-shaped assembly, also showed a transient conductivity whose maximum value was quite similar to MCH gel state. In transient absorption spectroscopy (TAS), all the three films exhibited a characteristic absorption band at around 490 nm (Figure 8b) indicative of NDI radical anion species,33 where their kinetic profiles were similar to those of transient
observed (height 16 ± 3 nm, width ∼ 150 nm) which upon aging for 150 h produce larger particles (width and height of 600−800 and 12 ± 2 nm, respectively) resembling the shape of maple leafs unlike the tubes observed in the case of concentrated samples after the same aging time. Although at the moment, we are unable to provide a reasonable argument on the mechanism of such unusual nanostructure formation, it is conceivable that the shape will depend on the relative rate of growth at various directions during fusion which is likely to be affected by concentration. However, upon aging for significantly longer time (240 h), tube formation was also observed (Figure 6b) in this dilute sample. Thus, it was evident the folded structure spontaneously formed polymersomes even at dilute concentration which then bumped onto each other and eventually manufactured nanotubes which above CGC showed macroscopic gelation. Notably UV/vis, NMR and FT-IR spectra of P1 in MCH virtually remained unchanged (Figures S8−S10) upon aging and thus confirmed no disruption of the folded conformation or π-stacking during the morphology transition and nanotube formation. Electron-Transporting Property. Since NDI is known as a representative electron- accepting and transporting core,12,30 its π-stacked architectures should serve as n-type organic semiconducting materials. We evaluated the electron-transporting properties of the well-defined supramolecular archiF
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conductivity (Figure 8a). MCH solution of P1 (1.25 × 10−4 M) also showed the TAS signal in the same wavelength region (Figure 8c and 8d), while the absorption decreased upon addition of TCNQ,34 a strong electron acceptor (Figures 8d and S11). We have also attempted chemical reduction experiments of P1 in CH2Cl2 solution upon addition of cobaltocene(II) (CoCp2).35 As a result, characteristic absorption bands including the peak at 480 nm appeared (Figure S12), which ascertains the peak observed in Figure 8, parts b and d, is indeed due to formation of NDI radical anions in P1. Judging from these experimental evidence, we concluded that the observed conductivity in TRMC dominantly relied on the electron-transporting event. After comparing the absorption coefficient of NDI•− at 490 nm with the observed maximum TAS intensity, we evaluated the φ values as 1.5 × 10−4 and 1.8 × 10−4 for the dropcast films from MCH gel and CHCl3/MeOH solution, respectively. These φ values are similar to that of reported organic crystals and thus indicate similar carrier generation mechanism.36 Finally, the electron mobility (μe) was calculated by combining the above φ and φΣμ values. The calculated electron mobility values were 2 × 10−3 cm2 V−1 s−1 and 4 × 10−4 cm2 V−1 s−1 for the dropcast films from MCH gel and CHCl3/MeOH solution, respectively, indicating strong impact of folded structure on charge carrier mobility. Of further interest, although the films from MCH gel and freshly prepared MCH solution showed the similar electron mobility, the former allowed much longer halflifetime of charge carriers (τ1/2) than that from the latter (τ1/2 of ca. 40 and 5 μs, respectively) which was more obvious in the kinetic traces in a wider time region (Figure 7b). In contrast, for the unfolded state, although the value could not be estimated due to poor signal intensity (green line, Figure 7a), the nature of the transient undoubtedly showed much faster decay. Taking into account this observation along with the fact that TRMC/TAS measurements were carried out under air, it is conceivable that confinement of NDI arrays within the folded polymer wall in nanotubularly assembled P1 reduced the quenching probability by oxygen in one hand and provided long-range electron transporting pathways on the other hand,37 leading to the observed remarkable prolongation of negative charge carrier’s lifetime. Overall, combination of FP-TRMC and TAS measurements revealed the strong impact of folded/ unfolded structural difference of NDI-based foldamer P1 on the intrinsic electron mobility and carrier lifetime of the materials. Tuning of the structure of the appended π-conjugated cores would be a subject worthy of investigations for further developing the supramolecular electronics.
impact is clearly visible in remarkably longer lifetime of the NDI radical anions in the nanotubular assembly which is of enormous importance in contemporary research areas such as photocatalysis and light harvesting. It is shown that encapsulation of the electron transporting pathways in the integrated wall of the nanotubes consisting of folded polymers stabilizes the negative charges (electrons) away from the interference by electron quenchers such as oxygen and consequently their lifetime is prolonged by an order of magnitude in the folded state compared to unfolded polymer. Likewise FP-TRMC measurements reveal strong impact of such an integrated nanotubular assembly on n-type conductivity, showing an appreciable value of 2 × 10−3 cm2 V−1 s−1 that is comparable with those reported 31 in structurally better studied materials like liquid crystals,38 conducting polymers39 and organogelators40 and expected to only get better in future considering the present example is just the beginning for any foldamer based materials for optoelectronic applications. Even after significant progress in research related to abiotic foldamers, reports on their macroscopic assembly (similar to quaternary structures of proteins) or functional utilities are still scarce. The current results are certainly encouraging for future attempts to unfold many exciting possibilities by studying assembly of custom-designed macromolecular foldamers based on PU scaffold. As the structure formation in this design primarily depends on intrachain H-bonding among the urethane groups, in principle it is possible to explore this polymeric scaffold with different attached chromophores or functional groups of choice for diverse range of applications utilizing intrachain folding induced 1D organization of the pendant groups and subsequent macroscopic assembly as described in the manuscript. Currently our efforts are underway in these directions.
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EXPERIMENTAL SECTION
Materials and Methods. Solvents and reagents were purchased from commercials sources and purified by prescribed protocols.41 Hexamethylene diisocyanate, Serinol, 2-octyl dodecanol, (S)-(−)-2methyl butanol, 1,4-diazabicyclo[2.2.2]octane (DABCO), and naphthalene dianhydride were purchased from Sigma-Aldrich chemical company. Hexamethylene-diisocyanate was purified by distillation under reduced pressure using Kugelrohr equipment and the other reagents were used as received. Spectroscopic grade solvents were used for all physical studies. 1H NMR spectra were recorded in a Bruker DPX-500 MHz spectrometer the peak positions were calibrated using TMS as an internal standard. UV/vis experiments were done in a PerkinElmer Lambda 25 spectrometer equipped with a Peltier for variable temperature experiments. FT-IR spectra were recorded in a PerkinElmer Spectrum 100FT-IR spectrometer. Emission spectral studies were carried out in a FluoroMax-3 spectrophotometer from HORIBA Jobin Yvon. Transmission electron microscopy images were captured in a JEOL-2010EX machine operating at an accelerating voltage of 200 kV. Fluorescence microscopic images were taken by an Olympus (1 × 2-KSP, 6 M 24413) machine, Japan. Atomic force microscopy (AFM) was done in tapping mode in an Innova instrument from Bruker. Molecular weight of the polymer was estimated in THF (1.0 mg/mL) solvent at 30 °C with respect to poly(methyl methacrylate) (PMMA) standards in a Waters GPC machine equipped with a 515 HPLC pump, Waters 2414 RI detector, and HSPgel HT 4.0/HSPgel HT 2.5 columns connected in a series. The flow rate of the eluent was maintained as 0.6 mL/min. X-ray diffraction (XRD) was measured on a Seifert XRD3000P diffractometer with Cu Kα radiation (a = 0.154 06 nm) and operating voltage and current of 40 kV and 30 mA, respectively. Synthesis of P1. A solution of compound M242 (100 mg, 0.160 mmol) and (S)-(−)-2-methyl 1-butanol (MF1) (0.057 mmol) in 0.3
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SUMMARY In summary, we have shown controlled synthesis and selfassembly of a novel polyurethane attached with a n-type semiconducting NDI chromophores as pendant functionality. It adopts a folded conformation in aliphatic hydrocarbon solvents by intrachain H-bonding among the urethane groups resulting in helical organization of the NDI units with close proximity for π−π interaction leading to intense green emission in contrast to the blue emitting monomeric NDI. The helicity is induced from the chiral chain ends by the “soldier and sergeants” effect. Such NDI encased folded PU owing to the similarity of its topological structure with lipid organization, shows spontaneous polymersome like assembly which gradually converts to micrometer long nanotubes as a result of fusion resulting in macroscopic gelation beyond a critical concentration. The G
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Article
mL of dry tetrahydrofuran (THF) was taken in a polymerization vessel and purged with dry argon gas for 10 min. To this solution were added a solution of hexamethylene diisocyanate (M1) (32 mg, 0.189 mmol) in 0.1 mL of dry THF and 1,4-diazabicyclo [2.2.2] octane (DABCO) (0.8 mg, 0.006 mmol) in 0.1 mL of dry THF, and the mixture was further degassed for another 15 min. Subsequently the homogeneous solution was stirred at 65 °C for 8 h under argon atmosphere while it became highly viscous. Heating was stopped and the contents were diluted with 2 mL of THF and the resulting solution was precipitated from excess MeOH to obtain the solid polymer. It was redissolved in THF and precipitated out from MeOH and the solid obtained was dried under reduced pressure to get the desire P1 as a light yellow solid in 75% yield. Mw = 7600 g/mol (PDI = 1.72). 1HNMR (CDCl3, 2% TFA was added for better solubility; 400 MHz, TMS): δ (ppm) = 8.71 (4H, S); 5.53 (1H, broad peak); 4.6 (4H, broad peak); 4.1 (2H, broad peak); 3.63 (4H, broad peak); 3.04 (4H, broad peak); 1.95 (1H, broad peak); 1.62 and 1.22 (40 H, broad multiple peaks), 0.86 (6H, broad peak). FT-IR (KBR pellet, wavenumber/cm−1): 3355 cm−1 (N− H urethane stretching); 1706 cm−1 (carbonyl of NDI); 1665 cm−1 (carbonyl of urethane); 1534 cm−1 (N−H urethane bending). Gelation Studies. A stock solution of P1 (10.0 mM) in a good solvent CHCl3 was prepared. A measured amount of this stock was transferred to a gelation vial; the solvent was evaporated by heating and to the resulting film was added the bad solvent MCH quantitatively so that the concentration of P1 was 10.0 mM. The contents were heated under closed condition with a hot air gun while a homogeneous solution was obtained which was allowed to stand at room temperature. It gradually became viscous and transformed to a gel (as confirmed by stable-to-inversion of vial method) after 24 h. Similarly the gelation was tested in a few other organic solvents including THF, TCE, decalin, octane, and dodecane. For systemically studying effect of concentration on gelation time, solutions of P1 in MCH were prepared following similar protocol at three different concentrations (12.0, 6.0, and 3.0 mM) and were monitored for next several hours to note the gel formation time. To determine the critical gelation concentration in MCH, solutions of P1 were prepared with varying concentrations (10.0, 5.0, 3.0, 2.0, 1.0, and 0.5.0 mM) and allowed to stand at room temperature for 7 days to ensure complete aging. Subsequently it was noticed gel formation until 3.0 mM and thus that is noted as the CGC of P1 in MCH. The gel-to-sol conversion temperature (Tg) of P1 in MCH (10.0 mM) was determined by gradually heating a gel (7 days aged sample) from 25 °C to upper values in a water bath equipped with a temperature controller. Existence of gel phase was checked at 2 °C interval by stable-to-inversion method and that indicated a Tg value of 52 °C. Sample Preparation and Self-Assembly Studies in Solution State by Various Spectroscopic Methods. Solution of P1 in MCH was prepared at 1.0 mM concentration following the procedure described before for gelation. However, at this concentration, the solution did not transform to gel even after several days. All spectroscopic measurements were carried out with this sample after 2−3 h of equilibration time. For comparison of spectral features with the disassembled state, spectra were recorded in CHCl3 in the presence of trace amount of trifluoroacetic acid under identical conditions. UV/Vis, PL, and CD Spectroscopic Studies. UV/vis spectra of P1 in MCH and CHCl3 were recorded at 1.0 mM concentration at 25 °C using a 1.0 mm quartz cuvette. For concentration dependent studies, a concentrated solution (0.4 mM) was gradually diluted with MCH and the spectra were recorded after each dilution until the concentration reached 0.003 mM. For variable temperature studies, temperature was increased from 25 to 85 °C with 5 °C interval and before each spectral run an equilibration time of 10 min was provided after a desired temperature was reached. Photoluminescence spectra were recorded (λex = 340 nm) for the same samples in CHCl3 and MCH at 1.0 mM and also for diluted samples at 0.001 mM concentrations. CD experiments were performed at 25 °C at 5.0 mM concentration using 1.0 mm path length cuvette. Transmission Electron Microscopy (TEM) and Fluorescence Microscopy. A freshly prepared solution of P1 in MCH (c = 2.0 mM)
were drop-casted on a carbon-coated copper grid and dried at air for overnight before TEM images were captured. The same solution was aged for 48 h while it became visibly more viscous and then was dropcasted again on another TEM-grid for recording TEM-images. Similarly samples were prepared on glass slides for recording fluorescence microscopic images of freshly prepared and aged samples. Atomic Force Microscopy (AFM). Solutions of P1 in MCH were prepared at two different concentrations (10−5 M and O.5 mM) following the method described before and were allowed to stand at room temperature. A portion of each solution was drop-casted on mica at different time intervals, air-dried, and AFM images were recorded. XRD Study. A solution of P1 in MCH (c = 10.0 mM) after aging for 24 h was drop-casted on a glass slide to make a relatively thick film and air-dried for 24 h before the data was recorded from 1° to 30° with sampling interval of 0.02 Å per state. Transient Absorption Spectroscopy (TAS). Transient absorption spectroscopy (TAS) measurements were carried out at room temperature in air. The identical film samples used for FP-TRMC measurement and freshly prepared MCH solution (1.25 × 10−4 M) of P1 were used for TAS measurements. The sample was photoexcited using a third harmonic generation (λ = 355 nm) of a Spectra Physics model INDI-HG Nd:YAG laser with a pulse duration of 5−8 ns, where the photon density of a 355 nm pulse was 9.1 × 1015 photons cm−2. A white light continuum from a Xe lamp was used as a probe light source for transient absorption spectroscopy. The monochromated probe light was guided into a Hamamatsu model C7700 wide-dynamic-range streak camera system, which collected a two-dimensional image of the spectral and temporal profiles of light intensity. Flash-Photolysis Time-Resolved Microwave Conductivity (FP-TRMC). The charge carrier mobility was measured by FPTRMC technique at room temperature in air. Film samples on a quartz plate were prepared by drop-casting of 5 mM MCH gel (5 days aging from solution), MCH solution, or CHCl3/MeOH solution of P1. Charge carriers were photochemically generated using a third harmonic generation (λ = 355 nm) of a Spectra Physics model INDIHG Nd:YAG laser with a pulse duration of 5−8 ns. The photon density of a 355 nm pulse was 9.1 × 1015 photons cm−2. The microwave frequency and power were set at ∼9.1 GHz and 3 mW, respectively. The TRMC signal, picked up by a diode (rise time
