Abstract

The formation of a cylinder-like helical conformation induced by chloroform was observed from a conjugated polymer, which was prepared by Knoevenagel condensation using N-octyl-3, 6-diformylcarbazole and p-phenylene diacetonitrile as the monomers. The helical conformation by solvent induction was further proved by the measurements of CD spectrum and specific rotatory power. Additionally, a helical aggregation of the conjugated polymer was obtained by volatilizing the solvent in the polymer solution and observed by polarized optical microscopy. The helical aggregation took the form of a right-handed helix. Computer simulation revealed that the polymer could form into a hollow tubular nanostructure with a cavity less than 1.5 nm in diameter by folding of its strand.
Keywords

conjugated polymer, helical conformation, helical aggregation, solvent-induction, tubular nanostructure
Introduction

In the past two decades, conjugated polymers have successively been used as organic conducting materials, light-emitting diodes (LEDs)[1], photovoltaic cells (PVs)[2, 3], solid-state laser materials[4] and biosensors[5]. It is currently noticeable that helical conjugated polymers have been applied to second-order nonlinear optics[6, 7], circularly polarized luminescence[8, 9] and organic nanotube[10, 11], which may be used for optical information processing, display and data storage. In addition, nano polymer materials with a cylinder-like helical architecture at the molecular scale have been prepared by solvophobically driven folding[12], intramolecular cross-linking of helical folds[11], etc. Therefore, macromolecular design related to helical architecture is an interesting subject, especially for helical conjugated polymers. On the other hand, helical macromolecules are indispensable in biological systems such as proteins. Consequently, research on helical conjugated polymers is propitious not only to capacity improvement of optical information storage for artificial polymers but also to a better understanding on the stereochemistry of biological polymers.

Helical architecture is the necessory factor to result in chiroptical property[13], such as helicenes and atropisomers. Generally, the helical architecture of synthetic polymers is shaped through the introduction of a chiral substituent into the polymeric chain[14-17], direct synthesis using a chiral catalysis[18, 19], synthesis in a chiral field[13], or chiral molecular induction through supramolecular effect,[20, 21] etc. However, the achievement of helical architecture is rarely reported directly from the folding of a synthetic polymer[12]. According to Moore’s “shape-persistent” approach to nanoscale architectures[22], the formation of a helical conformation can result from a geometrical condition in the polymeric main chain, such as the introduction of a turn unit like m-phenylene[12] or 2, 5-thiophene[23], and an external condition, such as an appropriate solvent,[12] to stabilize the helical conformation by folding of the linear synthetic polymer under solvophobically-driven packing. In other words, the helical conformation is formed and stabilized by spontaneous folding of a linear molecular strand under a certain driving force and a proper external condition when there is a turn angle in the polymeric main chain. This kind of synthetic linear molecular strand, which can be folded into a compact and defined molecular shape, is termed a “foldamer” [24, 25].

On the basis of the understanding of Moore’s “shape-persistent” approach to nanoscale architectures, we previously reported a conjugated polymer, which was folded into a helical conformation by solvent induction, employing McMurry condensation polymerization using the alkylcarbazolyl group as a turn angle (approximately 60o, an optimal bond angle)[26]. The helical conformation formed by solvent induction was further proved by the measurements of CD spectrum, specific rotatory power and FL spectrum. However, the mechanism of McMurry condensation reaction, consisting of coupling and elimination reactions, made it difficult to obtain a polymer with a regular molecular structure, in which the alkylcarbazolyl and the vinylene units were strictly alternated in the polymer main chain. Therefore, a helical aggregation is difficult to observe from the polymer. In order to obtain a polymer having regular molecular structure, we herein reported a conjugated polymer employing Knoevenagel condensation reaction. As the monomers, N-octyl-3, 6-diformylcarbazole and p-phenylene diacetonitrile, were used and the alkylcarbazolyl group functioned as a turn angle; this polymer could be expected to form a helical conformation induced by solvent. Computer simulation suggested that the polymer folded a cylinder-like conformation with a cavity about 1.6 nm in diameter. In addition, helical aggregation was acquired by volatilizing the solvent in the solution of the synthesized conjugated polymer and was observed by polarized optical microscopy.
Experiments
Instruments

1H-NMR (400 MHz) spectra were recorded on a Bruker DPX-400 spectrometer. IR spectra (KBr tablets) were measured on a Nicolet Protégé 460 infrared spectrophotometer. Relative molecular weights were determined by gel permeation chromatography (GPC) on a Waters M515 instrument with a Waters 410 differential refractometer at 40oC calibrated with polystyrene standards using tetrahydrofuran as eluent. Melting points were determined using an X-5A melting point measurement instrument. Fluorescent spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer. UV-vis spectra were measured on a Shimadzu UV-3010 instrument. Circular dichrosm (CD) spectra were recorded on JASCO J-810 spectropolarimeter. The observation of helical aggregation was performed on a Leica DML polarized optical microscope (POM).
Reagent

p-phenylene diacetonitrile, 1-bromooctane and carbazole were purchased from ACROS. Chemical reagents were obtained with analytical or chemical purity and used as received. Tetrahydrofuran (THF) was dried with anhydrous calcium dichloride overnight, distilled over calcium hydride, and then distilled over sodium prior to use.

AZoJoMo - AZoM Journal of Materials Online - Synthetic routes of the polymer and the model compound.

Scheme 1. Synthetic routes of the polymer and the model compound.
Synthesis

The intermediates, the model compound and the conjugated polymer were synthesized following the procedure in the Reference [27] according to the synthetic route shown in Scheme 1.
Synthesis of N-octylcarbazole (Compound 1)

Compound 1 was synthesized by the reaction of carbazole with 1-bromooctane. 1-Bromooctane (20.3 g, 105 mmol) was added dropwise to a mixture of carbazole (15 g, 30 mmol) and sodium hydride (NaH, 6 g, 250 mmol) in N, N-dimethylformamide (DMF, 60 ml), followed by refluxing for 2 h. The mixture was poured into distilled water (600 ml), extracted with n-hexane for three times (400 ml each) and dried with anhydrous magnesium sulfate overnight. The amaranthine residue was purified by silica-gel column chromatography using n-hexane as eluent (Rf=0.15). A transparent viscous liquid was obtained with 81% yield. 1H-NMR, (CDCl3, TMS, d): 0.86 (t, 3H, J=7.2Hz), 1.23-1.40 (m, 10H), 1.85 (m, 2H), 4.28 (t, 2H, J=7.2Hz), 7.22 (t, 2H, J=7.2Hz), 7.40 (d, 2H, J=8.0Hz), 7.46 (t, 2H, J=7.2Hz), 8.09 (d, 2H, J=7.6Hz). IR, umax (KBr / cm-1): 3050 (Ar-H), 2926 (uas, CH), 2869 (us, CH), 1598, 1489, 1466 (benzene ring), 1453 (CH2), 1380 (CH3), 1229 (Ar-N), 1065 (C-N), 722 (CH2).
Synthesis of N-octyl-3, 6-diformylcarbazole (Compound 2)

Compound 2 was synthesized by Vilsmeier reaction. Phosphoryl chloride (43.9 g, 290 mmol) was added dropwise to a mixture of DMF (31.9 g, 340 mmol) and 1, 2-dichloroethane (26 ml) cooled to 0oC. Then, N-octylcarbazole (4.0 g, 14 mmol) was added to the vigorously stirred mixture, and then heated to 90oC. The reaction was maintained for 2 days, and then the mixture was poured into distilled water (300ml), extracted with chloroform and dried over anhydrous magnesium sulfate. The solvent was removed under reduced pressure. The residue was dissolved in a minimal amount of methylene dichloride and then purified by silica-gel column chromatography using methylene dichloride as eluent (Rf=0.06). The product was obtained as a white powder with 51% yield. Mp = 128-129oC. 1H-NMR, (CDCl3, TMS, d): 0.86 (t, 3H, J=6.4Hz), 1.24-1.42 (m, 10H), 1.92 (m, 2H), 4.39 (t, 2H, J=7.2Hz), 8.11 (d, 2H, J=7.2Hz), 8.68 (s, 2H), 10.09 (s, 2H). IR, umax (KBr / cm-1): 2922 (uas, CH), 2847 (us, CH), 1686 (C=O), 1593, 1489, 1473 (benzene ring), 1383 (CH3), 1350 (Ar-N), 725 (CH2).
Synthesis of N-octyl-3-formylcarbazole (Compound 3)

Compound 3 was prepared by the same procedure as compound 2 except that the amount of phosphoryl chloride was reduced to 140 mmol (21.2 g). The amounts of other reagents including N-octylcarbazole were equal to those in the procedure of compound 2. The product was obtained with 63% yield. Mp = 57-58oC. 1H-NMR, (CDCl3, TMS, d): 0.86 (t, 3H, J=6.4Hz), 1.23-1.40 (m, 10H), 1.88 (m, 2H), 4.32 (t, 2H, J=7.2Hz), 7.33 (t, 1H, J=7.2Hz), 7.47 (t, 2H, J=5.6Hz), 7.54 (t, 1H, J=7.2Hz), 8.00 (d, 1H, J=8.4Hz), 8.15 (d, 1H, J=8.0Hz), 8.61 (s, 1H), 10.09 (s, 1H). IR, umax (KBr / cm-1): 2921 (uas, CH), 2849 (us, CH), 1691 (C=O), 1594, 1567, 1498, 1473 (benzene ring), 1470 (CH2), 1382 (CH3), 1355 (Ar-N), 720 (CH2).
Synthesis of Model Compound 4

(n-Bu)4NOH (10%, 0.5 ml) was dropwise added to a stirred solution of N-octyl-3-diformylcarbazole (2 mmol) and p-phenylene diacetonitrile (1 mmol) in THF (5 ml). Then, the mixture was heated to 60oC under the protection of a nitrogen atmosphere. After the reaction was carried out for 4 hours, the mixture was cooled to room temperature and precipitated with distilled water (300 ml). The precipitate was filtered and dried in vacuo. The crude product was dissolved in a small quantity of chloroform and purified by silica-gel column chromatography using chloroform as eluent. The yellow powder was obtained in 71% yield. Mp = 172-174oC. 1H-NMR, (CDCl3, TMS, d): 0.869 (t, 6H, J=7.2 Hz), 1.25-1.41 (m, 20H), 1.89 (m, 4H), 4.31 (t, 4H, J=7.2 Hz), 7.31 (t, 2H, J=7.2Hz), 7.45 (d, 4H, J=8.0Hz), 7.52 (t, 2H, J=8.0Hz), 7.77 (s, 2H), 7.78 (s, 4H), 8.16 (d, 4H, 8.4Hz), 8.66 (s, 2H). IR, umax (KBr / cm-1): 3134 (=CH), 2925 (uas, CH), 2853 (us, CH), 2208 (CN), 1585, 1495, 1471 (benzene ring), 1351 (Ar-N).
Synthesis of polymer

(n-Bu)4NOH (10%, 1ml) was dropwise added to a stirred solution of N-octyl-3, 6-diformylcarbazole (5 mmol) and p-phenylene diacetonitrile (5 mmol) in THF (10 ml). Then, the mixture was heated to 60oC under the protection of a nitrogen atmosphere. After the reaction was carried out for 4 hours, the mixture was cooled to room temperature and precipitated with methanol (100ml) for four times. The precipitate was filtered and dried in vacuo. The yellow powder was obtained in 54% yield. 1H-NMR, (CDCl3, TMS, d in ppm): 0.78 (CH3), 1.19 (CH2), 1.49 (CH2), 1.77 (CH2), 4.24 (NCH2), 6.7-8.5 (the protons on the benzene ring and the double bond). IR, umax (KBr / cm-1): 2924 (uas, CH), 2853 (us, CH), 2210 (CN), 1584, 1483 (benzene ring), 1352 (Ar-N).
Results and Discussion
Synthesis

The conjugated polymer was synthesized by an ordinary Knoevenagel condensation between 1,4-phenylene diacetonitrile and N-octyl-3, 6-diformylcarbazole in THF at 60oC under the protection of nitrogen following the synthetic route in Scheme 1. Mn, Mw and polydispersity were respectively 111000, 482000 and 4.19 as measured by GPC using polystyrene as a standard. Therefore, the polymer had approximately 243 repeating units. Spectral analysis through 1H-NMR (in CDCl3) and IR (KBr tablet) was basically in agreement with the chemical structure shown in Scheme 1. In addition, the synthesized polymer can be dissolved in common organic solvents such as tetrahydrofuran, chloroform, dichloromethane, acetone, etc. Surprisingly, a signal at d 3.46 appeared in the Figure 1. This signal could not be ascribed to the protons on the polymeric backbone according to the general rule of nuclear magnetic resonance. In order to understand this signal, we synthesized a model compound 4 following the same procedure as the conjugated polymer. The result of 1H-NMR spectrum (in CDCl3) in Figure 2 showed that no signal at d 3.46 appeared. A similar phenomenon was also found in the Reference [28]. We assume that the unexpected peak is probably due to the special conformation of the resulting polymer.

AZoJoMo - AZoM Journal of Materials Online - 1H-NMR spectrum of the conjugated polymer in CDCl3.

Figure 1. 1H-NMR spectrum of the conjugated polymer in CDCl3.

AZoJoMo - AZoM Journal of Materials Online - 1H-NMR spectrum of model compound 4 in CDCl3.

Figure 2. 1H-NMR spectrum of model compound 4 in CDCl3.
Computer Simulation

Computer simulation after minimizing molecular energy by MM2 (molecular mechanics) force field in Chem3D Ultra 8.0 suggested that the model compound 4 took on an arched conformation as illustrated in Figure 3. It was natural that the polymer should take on a helical conformation with an increase in the polymeric repeating unit. The result by computer simulation showed that the synthesized polymer had exactly a cylinder-like helical conformation folded into a hollow tubular nanostructure with a cavity about 1.6 nanometer in diameter (Figure 4). The structure was similar to that previously reported by McMurry condensation polymerization[26]. However, the presently prepared polymer had a somewhat larger cavity than the previously reported polymer (0.7 nm). This indicated that the cavity formed by individual macromolecules can be tuned by introduction of different group between alkylcarbazoles in the main chain. The formation of helical conformation was further proved by measurements of CD, specific rotatory power and POM. In addition, one pitch of helical conformation consisted of approximately 4 repeating units. The outside diameter of the helical conformation was about 3.2 nm and the inside diameter was about 1.6 nm on average. The length of one pitch of helical conformation was about 5.6 nm.

AZoJoMo - AZoM Journal of Materials Online - 3-D ball & stick model of model compound after minimizing molecular energy by MM2.

Figure 3. 3-D ball & stick model of model compound after minimizing molecular energy by MM2.

AZoJoMo - AZoM Journal of Materials Online - 3-D space-filling model of the p-polymer after minimizing molecular energy by MM2 (a, top view) and the helical conformation model (b, side view).

Figure 4. 3-D space-filling model of the p-polymer after minimizing molecular energy by MM2 (a, top view) and the helical conformation model (b, side view).
Photophysical Property

The photophysical properties of the intermediates, the model compound 4 and the synthesized conjugated polymer were characterized by UV-vis and fluorescence (FL) spectra. The data of UV-vis and FL spectra are listed in Table 1. From the UV-Vis spectra of intermediates and model compound 4 in THF in Figure 5a, the absorption maximum (405.5 nm) of model compound 4 was red-shifted about 68.5 nm compared with that (337 nm) of compound 2. Additionally, the absorption maximum (406.5 nm) of the polymer in THF was approximately equal to that of model compound 4 from the UV-vis spectra of the polymer in Figure 5b, which indicated that the effective conjugated length of the polymer was approximately equivalent to the molecular length of model compound 4. However, the band gap (2.55 eV) of the polymer, which was calculated using the formula Eg =1240/ λonsetabs eV from the onset absorption in THF, was less 0.13 eV than that (2.68 eV) of model compound 4.

From UV-vis spectra of the dilute solutions of the polymer in different organic solvents (Figure 5b) such as benzene, dioxane, chloroform and tetrahydrofuran, whose dipole moments are 0, 1.50, 3.84 and 5.70 (x10-30 C·m), respectively, it was found that the absorption maxima were respectively 386.5, 393, 385, 406.5 nm and gradually red-shifted (Table 1) with increase in dipole moment except in chloroform. The band gap (Eg) of the polymer also decreased with increase in dipole moment except in chloroform. All the data of UV-vis spectra are listed in Table 1.

AZoJoMo - AZoM Journal of Materials Online - Normalized UV-vis spectra of intermediates, model compound in THF (a) and the polymer in different solvents (b).

Figure 5. Normalized UV-vis spectra of intermediates, model compound in THF (a) and the polymer in different solvents (b).

AZoJoMo - AZoM Journal of Materials Online - Normalized excitation spectra (a) and emission spectra excitated by the excitation maxima (b) of the polymer in different solvents.

Figure 6. Normalized excitation spectra (a) and emission spectra excitated by the excitation maxima (b) of the polymer in different solvents.

The polymer was characterized by FL spectra including excitation spectra and emission spectra (Figure 6). From the excitation spectra, the wavelength of the excitation maxima was observed at longer wavelength compared with that of the absorption maxima, as shown in Table 1. The emission spectra are excitated by the wavelengths of the excitation maxima of 410, 404, 422 and 409 nm for benzene, dioxane, chloroform and tetrahydrofuran, respectively. As a result, the emission maxima were observed at 521, 502, 486 and 492 nm. Therefore, Stokes’ shifts were 111, 98, 64 and 83 nm respectively and gradually decreased with increase in dipole moment, except in chloroform. The data are listed in Table 1.

Table 1. Data from UV-vis and FL data of the polymer*

Solvents


Benzene


Dioxane


Chloroform


Tetrahydrofurane

Dipole
Moments/10-31C.m


0


1.50


3.84


5.70

λmaxabs/nm


386.5


393


385


406.5

λonsetabs/nm


461


473


453


487

Eg/eV


2.69


2.62


2.74


2.55

λmaxex/nm


410


404


422


409

λmaxem/nm


521


502


486


492

Stokes Shift/nm


111


98


64


83

* λmaxabs: the absorption maximum; λonsetabs : the onset on the absorption spectrum; Eg: the band gap calculated from λonsetabs ; λmaxex : the excitation maximum; λmaxem : the emission maximum.
Helical Conformation and Helical Aggregation

From the circular dichroism (CD) spectrum of the polymer solution in chloroform as shown in Figure 7, it was indicated that the polymer was CD-active and showed a positive Cotton effect. Since the chiroptical property does not necessarily result from a chiral center but from a helical architecture[13], such as helicenes and atropisomers, the CD result indicated that a right-handed helical conformation was shaped in the chloroform solution of the polymer. Unfortunately, the polymer was CD-silent in hexane solution, as shown in Figure 7. The CD spectra in benzene, dioxane and THF were the same as in hexane and were CD-silent. This indicated that the helical conformation of the polymer was induced by chloroform.

AZoJoMo - AZoM Journal of Materials Online - CD spectra of the polymer in chloroform and in hexane.

Figure 7. CD spectra of the polymer in chloroform and in hexane.

The formation of helical conformation was also explained by the measurement of specific rotatory power in spite of a vibrational range from -1774o to -1041o in chloroform. The vibrational specific rotatory power was caused by the tertiary amine in the carbazolyl group flipping inside out very rapidly, as showed in the inset of Figure 8. Although no rapid flip permits the chiral tertiary amines to be separated into two enantiomers, the chirality of the tertiary amine in the carbazolyl group may be stabilized and measurable to a certain extent. Here, despite the vibration of specific rotatory power as a result of the increase in steric hindrance, the helical conformation formed under solvent induction. However, the specific rotatory power changed from a negative-value range (-1774o, -1041o) in chloroform to near zero range (-6o,+13o) in hexane, as the hexane content increased in the solvent component. This implied the disappearance of helical conformation, as shown in Figure 9. This is proved by the above CD spectrum in hexane as illustrated in Figure 7. Figure 8 shows the change in specific rotatory power observed with different hexane chloroform ratios. Additionally, the vibration of specific rotatory power was mainly induced by the vibration of helical radius and helical pitch of the assembled helix because the molar rotatory power ([M]) was connected with the helical radius and the helical pitch[26]: [M]=kr2s/l2 where k was constant, r the helical radius, and l the length of helical line[29]. Finally, a great deal of helical aggregation was observed by POM, as shown in Figure 10. These data were obtained by volatilizing the solvent in the polymer solution. It was notable that the helical aggregation became a right-handed helix, as shown in the inset of Figure 10. In accordance with the CD result, the formation of helical aggregation suggested the formation of helical conformation from the conjugated polymer.

AZoJoMo - AZoM Journal of Materials Online - Change of specific rotatory power (20oC, 365 nm, 0.1mg/ml) for the polymer in component solvent with different ratios of chloroform and hexane; configuration of alkylcarbazole (inset).

Figure 8. Change of specific rotatory power (20oC, 365 nm, 0.1mg/ml) for the polymer in component solvent with different ratios of chloroform and hexane; configuration of alkylcarbazole (inset).

AZoJoMo - AZoM Journal of Materials Online - Disappearance of helical conformation with the increase of the hexane content in the component solvent.

Figure 9. Disappearance of helical conformation with the increase of the hexane content in the component solvent.

AZoJoMo - AZoM Journal of Materials Online - POM photo (x1000) of the helical aggregation from the polymer solution in chloroform; different forms of helical aggregation (a, b in the inset and c in the photo).

Figure 10. POM photo (x1000) of the helical aggregation from the polymer solution in chloroform; different forms of helical aggregation (a, b in the inset and c in the photo).
Conclusion

A conjugated polymer was prepared by Knoevenagel condensation using N-octyl-3, 6-diformylcarbazole and p-phenylene diacetonitrile as the monomers. The polymer can be dissolved in ordinary solvents such as THF and chloroform. The alkylcarbazolyl group functioned as a turn angle. Computer simulation after minimizing molecular energy by MM2 indicated that the polymer could form into a hollow tubular nanostructure with a cavity less than 1.5nm in diameter by folding of its strand. The photophysical property of the polymer was affected by the dipole moment of the solvents. The measurements of specific rotatory power and CD indicated the formation of helical conformation in chloroform and the helical conformation takes a right-handed helical architecture. The formation of a helical aggregation from the conjugated polymer by volatilizing the solvent in the polymer solution was observed by POM, where the helical aggregation forms a right-handed helix, which is in accordance with the CD result.