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).