Influence of a hydrodynamic environment on chain rigidity, liquid crystallinity, absorptivity, and photoluminescence of hydrogen-bonding-assisted helical poly(phenylacetylene)

Abstract

The chain rigidity, liquid crystallinity, absorptivity, and photoluminescence of a helical poly(phenylacetylene) derivative varied significantly depending on the solvent employed, owing to the conformational changes caused by the formation or destruction of intramolecular hydrogen bonds. The polymer chains were rigid and liquid crystalline when dissolved in such non-solvents as toluene and THF to show lower absorptivity and intramolecular excimer emission while the same polymer chains became flexible in piperidine having a high hydrogen-bonding acceptor strength to exhibit strong absorption in the visible region and the emission was significantly quenched.

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Introduction

Conjugated polymers are key components of the active layer in various optoelectronic thin film device applications.¹ Chain conformation is very important because it is reflected in the hierarchical structure and significantly affects physical properties, particularly, the photophysical properties.² Thus, it is important to predict the polymer chain conformation and control it precisely in the initial stages of the wet process of thin film manufacturing. In principle, an ideal solution may be a base of such a starting state because the isolated polymer chains in the solution predominantly reveal the intrinsic physical properties due to the chain conformation. As a matter of course, therefore, the environmental medium, i.e., solvent should be one of the most important factors that must not be overlooked in the wet process because the solution-state chain conformation can be transferred into the final products via self-assembly and significantly affect every aspect of their properties.³ In particular, if a certain target polymer has an intrinsically conformation-variable structure in response to the polarity, solubility, and viscosity of the surrounding medium, the influence of the solvent on the final polymer becomes even more important.⁴

To address this issue, we examined a hydrogen-bonding-assisted helical poly(phenylacetylene) derivative, which was obtained through asymmetric polymerization using a chiral cocatalyst,⁵ as a model polymer for investigating the solvent effect on the physical and photophysical properties of the polymer. The hydrogen bonding ability and the chemical affinity of the solvent for the polymer significantly affected the absorptivity, photoluminescence (PL), chain rigidity, and liquid crystallinity (optical anisotropy) of the polymer. The intramolecular hydrogen bonds of the polymer were retained or destroyed depending on the solvent used, leading to notable changes in chain conformation. This paper describes the details based on various spectroscopic analyses and microscopic observation and suggests that the physical and photophysical properties of the polymer can be precisely controlled by the solvent chosen.

Results and discussion

A poly(phenylacetylene) derivative containing two hydroxyethyl groups at the meta-position of the side phenyl ring was synthesized according to a reported method (Fig. 1a).⁵ This polymer adopts cis-cisoidal conformation in a one-handed helical structure that is stably retained via the intramolecular hydrogen bonding between the side hydroxyethyl groups. The poly(phenylacetylene) derivative was soluble in a wide range of solvents owing to its amphoteric nature, which stemmed from the hydrophobic backbone and hydrophilic side groups. Notably, this polymer dissolved instantly in piperidine, but dissolved very slowly in toluene and hardly dissolved in the polar solvent DMSO. Piperidine has a much higher hydrogen-bonding acceptor strength (pK_HB = 2.35) relative to that of toluene (−0.44),⁶ as shown in Table S1, ESI.† The solubility parameter (δ) of the polymer was determined to be 19.9 according to the groups contribution method reviewed by Van Krevelen, Fedors, and Barton.⁷ This value was closer to that of piperidine (17.8) rather than that of DMSO (26.4) (Table S1, ESI†). Such immediate dissolution of this polymer in piperidine, despite the high molecular weight of the polymer (M_w = 7.82 × 10⁶ g mol⁻¹; determined by GPC using THF as an eluent with polystyrene calibration) might be attributed not only to the high pK_HB of piperidine, but also to the δ value of piperidine being similar to that of the polymer, resulting in effective breaking of the intramolecular hydrogen bonds of the polymer and sufficient relaxation of the molecular structure. Furthermore, the polymer was readily soluble in tetrahydrofuran (THF) despite the much lower pK_HB of THF relative to that of piperidine (Table S1, ESI†), which could be attributed to the higher compatibility of the polymer with THF (δ ∼ 19.4) rather than piperidine. Consequently, the degree of dissolution of the polymer is strongly dependent on the pK_HB or δ of the solvent employed. In addition, this polymer had a relatively higher density (d) of 1.07 g cm⁻³, and the fractional free volume (FFV) was determined to be 0.11 from the van der Waals volume (V_w) and d.⁸

Fig. 1 (a) Chemical structure of the poly(phenylacetylene) derivative. (b) Photograph of upside-down vials containing toluene and piperidine solutions (c = 1 wt%).

Solutions of the polymer in toluene and piperidine (at 0.5 wt%, 25 °C) exhibited significantly different viscosities (η = ∼200 cP and ∼2.0 cP, respectively). Furthermore, at a polymer concentration of 1 wt%, the toluene solution behaved like a gel, while the piperidine solution was flowable (Fig. 1b). This discrepancy could be attributed to the different rigidities of the polymer chain in these two solvents. Namely, the polymer chain in toluene exhibits a rigid rod-like helical structure due to the intramolecular hydrogen bonding, while it may exist in a randomly coiled state in piperidine, wherein the hydrogen bonds are destroyed. The circular dichroism (CD) spectra supported this conclusion. The polymer in toluene solution showed a large Cotton effect at wavelengths longer than 350 nm due to the backbone absorption, while the piperidine solution showed no CD (Fig. 2). This observation indicates a hydrodynamically variable chain conformation related to the intramolecular hydrogen bonding.

Fig. 2 CD spectra of dilute solutions of poly(phenylacetylene) derivative (c = 1.0 × 10⁻⁴ M).

The chain rigidity of the polymer resulted in liquid crystallinity. A remarkable optical anisotropy texture was observed in the polarized optical microscopy (POM) image of the polymer film cast from the toluene solution (Fig. 3). According to Flory's theory,⁹ our polymer is expected to show lyotropic liquid crystallinity in toluene owing to its extremely high molecular weight and chain rigidity. From this viewpoint, the optical anisotropy of the cast film may be attributed to polymer chains reaching the critical concentration during slow solvent evaporation, which results in the formation of a highly ordered hierarchical structure in the bulk solid film. Similarly, the lyotropic liquid crystallinity has been observed previously in other rigid helical poly(phenylacetylene)¹⁰ or poly(diphenylacetylene) derivatives.¹¹ The X-ray diffraction (XRD) pattern of the polymer film cast from the toluene solution showed a sharp peak (full width at half maximum, FWHM ∼ 1.2°) in the small-angle region of 4.3° (intermolecular distance ∼ 20.5 Å according to the Bragg equation), probably due to an interchain packing structure (Fig. 4).¹² In addition, the HR-TEM image of the polymer film cast from the toluene solution clearly showed a lattice fringe (lattice distance ∼ 3.4 Å) due to the intramolecular phenyl–phenyl stack structure, unlike the case of film cast from piperidine solution (Fig. S1, ESI†). This confirmed the ordered anisotropic structure observed in the POM images. Owing to the liquid crystallinity, a uniaxially oriented polymer film was obtained when the highly concentrated solution of the polymer (∼5 wt%) in toluene was sheared manually (Fig. 5a). In the polarized UV-vis spectra, the absorption band at wavelengths greater than 350 nm (which is due to the backbone) gradually decreased as the polarizer angle approached the direction perpendicular to the shear direction (Fig. 5b). This indicates that the polymer chain exists in direction parallel to the shear direction. The dichroic ratio (D_abs = A_∥/A_⊥) at 450 nm and the optical order parameter [S = (D_abs − 2)/(D_abs + 1)] were estimated to be 3.12 and 0.27, respectively. Notably, the sharp absorption band at 300 nm gradually increased as the polarizer angle approached 90° to the shear direction, indicating electron delocalization in the direction perpendicular to the main chain axis. The shorter wavelength absorption was most probably caused by the side phenyl rings that are densely stacked in an efficient distance (intramolecular stack distance ∼ 3.1 Å)⁵ for π–π interaction. In addition, a distinct isosbestic point was observed at 330 nm, indicating that an optically isotropic resonant structure exists between the rigid backbone and the stacked side phenyl rings. On the other hand, the polymer film cast or sprayed from the piperidine solution never showed optical anisotropy (Fig. 3) and the XRD signal appeared in a smaller-angle region (4.07°, ∼21.7 Å, Fig. 4), which was relatively weak and broad (FWHM ∼ 2.5°), indicating an entirely isotropic, highly disordered structure in the amorphous phase.

Fig. 3 POM images of the polymer films cast from toluene and piperidine solutions.

Fig. 4 XRD patterns of the polymer films cast from toluene and piperidine solutions.

Fig. 5 (a) POM images and (b) polarized UV-vis absorption spectra of the uniaxially oriented film from highly concentrated toluene solution (c = 5 wt%). The arrows in spectra indicate the increase of polarizer angle against shearing direction from 0° to 90°, the script ‘A’, ‘S’, ‘P’ in POM images indicates analyzer direction, shearing direction, and polarizer direction, respectively.

Another notable observation was the significant difference in the absorptivity and PL emission spectra between the toluene and piperidine solutions. The polymer was almost colorless in the dilute toluene or THF solution but dark yellow in piperidine solution (Fig. 6a). Solutions of the polymer in toluene or THF did not show any intense absorption at wavelengths greater than 350 nm in the UV-vis spectra, while the polymer–piperidine solution exhibited a very intense absorption band (absorption coefficient, ε = 5.27 × 10³ L cm mol⁻¹) at 450 nm. This significant difference in absorptivity between these solutions may be attributed to the conformational differences caused by the presence or absence of hydrogen bonding within the polymer chain. The present polymer has a cis-cisoidal conformation and strong intramolecular hydrogen bonding in a nonpolar solvent such as toluene (or THF), which results in a highly twisted, rigid rod-like helical structure.⁵ On the other hand, the intramolecular hydrogen bonding is significantly disrupted in the piperidine solution, as described previously, leading to a more conjugation-extended structure. Similarly, other poly(phenylacetylene) derivatives exhibiting strong absorption at longer wavelengths had predominantly trans conformation rather than cis counterpart.¹³ This suggests that the intense absorption at longer wavelengths for the polymer in piperidine solution could be ascribed to a hydrodynamically induced cis-to-trans transition. That is, the conjugation-extended structure in piperidine solution should be mainly based on the trans conformation. In addition, it should be noted that the sharp absorption band at a shorter wavelength of 300 nm, which was observed in toluene (or THF) solution and attributed to the through-space interaction of side phenyl rings, almost disappeared in the piperidine solution along with the hydrodynamic helix-to-coil transition. This is an indicator for the fact that the intramolecular stack structure of the side phenyl rings is based on the well-constructed helical backbone. Moreover, DMSO added at 20% v/v to the THF–polymer solution induced a slightly higher absorption intensity (ε = 5.61 × 10³ L cm mol⁻¹ at 451nm) than that in piperidine (ε = 4.86 × 10³ L cm mol⁻¹ at 441 nm) (Fig. 6b). This can be ascribed to the higher pK_HB of DMSO than that of piperidine, although the chemical affinity of DMSO for the polymer is not as strong as that of piperidine. Thus, we conclude that in the case of our polymer, pK_HB of the solvent is a key factor that governs the hydrodynamic chain conformation. Raman spectra further clarified the conformational differences in solid state caused by the solvent employed. Fig. 7 shows the Raman spectra of the polymer films cast from the toluene and piperidine solutions. Peaks at 1593, 1316, 937 cm⁻¹, due to the cis C=C and C–C bonds, appeared in the spectrum of the film from toluene solution; however, these peaks relatively weakened in the spectrum of the film from piperidine solution, and new peaks appeared at 1520, 1185 cm⁻¹ due to the trans structure.

Fig. 6 UV-vis absorption spectra of (a) each polymer solution and (b) THF solution before and after addition of 20 vol% other solvents (insets: photographs of each solution under day light).

Fig. 7 Raman spectra of films cast from toluene and piperidine solutions (light source: 532 nm laser).

The differences in chain conformation induced by the solvents were clearly reflected in the PL emission spectra as well (Fig. 8). When excited at 280 nm, both toluene and THF solutions showed PL maxima at 310 nm due to the intramolecular stack structure of the side phenyl rings, while the piperidine solution was almost non-emissive owing to the absence of the corresponding excited species at the same excitation wavelength. Notably, the PL emission intensity of the toluene solution was approximately ten times lower than that of the THF solution. This is probably because the side hydroxyethyl groups more strongly interacts to each other via the intramolecular hydrogen bonding in toluene solution than in THF solution so that the polymer chain is more tightly twisted in helical manner and the side phenyl rings become closer to each other, leading to higher probability of existence of intramolecular excimer species in toluene. This indicates that the hydrodynamic environment significantly influences the electronic structure of the polymer in excited state. The differences in chain conformation induced by the solvents were also clearly reflected in differential scanning calorimetry (DSC) thermograms. The polymer film cast from toluene solution showed a transition peak due to the transformation from cis-cisoid to trans-transoid at a higher temperature (∼197 °C) as compared to the polymer film cast from piperidine solution (∼177 °C), indicating that the former film has cis-cisoid sequences with a more extended length (Fig. S2, ESI†).

Fig. 8 PL emission spectra of each polymer solution (excited at 280 nm).

Conclusions

Solvent effects on the hydrodynamic behavior and photophysical properties of a hydrogen-bonding-assisted helical poly(phenylacetylene) derivative in solution were investigated in detail. The chain rigidity, liquid crystallinity, optical anisotropy, absorptivity, and PL emission of the polymer were significantly influenced by the hydrogen bonding ability and chemical affinity of the solvent used. The various solvents employed influenced the intramolecular hydrogen bonds of the polymer by either constructing or breaking them, to cause significant changes in chain conformation. The results of this work could be useful for predicting the hydrodynamic structure of poly(phenylacetylene) derivatives in various solvents and appropriately tuning their properties for practical applications.

Experimental

Materials

The poly(phenylacetylene) derivative used in this study was synthesized using the methodology reported elsewhere.⁵ The polymer has a weight-average molecular weight (M_w) of 7.82 × 10⁶ g mol⁻¹ and a polydispersity index (PDI = M_w/M_n) of 2.06. All chemical reagents were purchased from Aldrich, Tokyo Chemical Industry, and used without further purification.

Measurements

The M_w and M_n values of the polymer were determined using gel permeation chromatography [JASCO Liquid Chromatograph with PU 2080, DG 2080 53, CO 2060, UV 2070, CD 2095, and two polystyrene gel columns (Shodex KF 807L), calibrated with polystyrene standards and THF as the eluent. The CD, UV-vis absorption, and PL emission spectra of the solution were measured on JASCO J-815, JASCO V-650, and JASCO FP-6500 spectrometers, respectively. Raman spectra were recorded on a Thermo Almega X Raman spectrometer (Thermo Fisher scientific Inc., USA) using a 532 nm laser. Photographs were taken using a digital camera (Sony Alpha 6000) equipped with a macro lens (Sony SEL30M35). The POM images were recorded on a Nikon Eclipse E600 optical microscope equipped with a Nikon DS-Fi1 digital camera. XRD analysis (PANalytical X'Pert PRO-MPD) was performed at room temperature at the Korea Basic Science Institute (Daegu). The samples were mounted directly in the diffractometer, and the experiment was carried out using Cu Kα (1.54 Å) radiation, at 40 kV and 25 mA. High resolution transmission electron microscopy (HR-TEM) imaging was performed with a field emission-transmission electron microscope (JEOL JEM-2100F, 200 kV) at the Korea Basic Science Institute (Daegu). Differential scanning calorimetry (DSC, TA Instruments Q2000) was performed at a heating rate of 10 °C min⁻¹ under a N₂ flow. The density was determined using a gas pycnometer (PYC-G100A-1, Porous Materials, Inc., USA) and the viscosity of the polymer solutions was measured using a Brookfield viscometer (DV-II+ Pro, Brookfield Engineering Laboratories, USA).

Acknowledgements

This work was supported by the Basic Science Research Program through National Research Foundation of Korea (NRF) grants, funded by the Korean government (MEST) (No. 2014R1A2A1A11052446).

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Footnote

† Electronic supplementary information (ESI) available: Properties of the solvents. See DOI: 10.1039/c6ra01940d

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