Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
pubs.acs.org/Macromolecules
Synthesis and Properties of Chiral Polyazobenzenes with Photoinduced Change in Optical Activity Masashi Otaki,† Reiji Kumai,‡ Hajime Sagayama,‡ and Hiromasa Goto*,† †
Department of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
‡
Macromolecules Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 03/11/19. For personal use only.
S Supporting Information *
ABSTRACT: Azobenzene is one of the most famous photoresponsive chromophores, and it repeatedly photoisomerizes. In this study, main-chain type polyazobenzenes bearing chiral substituents in the side-chain are synthesized by the Suzuki−Miyaura coupling method. The chiral aggregation process of main-chain type polyazobenzene is investigated. Then, intermolecular interaction of these polymers allows the formation of chiral aggregates as higher order structures. Trans-to-cis photoisomerization of the polymers was confirmed with in situ UV−vis absorption spectra. Development of π-conjugation in the backbone can be an important factor in the design of main-chain type azobenzene polymers. In addition, we induced light-driven changes in the chiroptical activity. The microstructure of the polymers is examined with synchrotron radiation X-ray diffraction and a technique to define “chiral photochromism”.
■
ior.11,12 Also, unique applications such as a molecular zipper13 or photomechemical material14 have been investigated due to the direct photoisomerization behavior of a polymer having azobenzene introduced in the main-chain. In addition, studies on polyazobenzene have also been conducted from the viewpoint of conductive polymers,15 and main-chain type polyazobenzene having unique light emission characteristics was synthesized.16 Compared to imparting chirality to polymers having azobenzene in the side-chain,17,18 there are few reported cases of main-chain type conjugated polyazobenzene with chirality.19−21 Introduction of azobenzene into the main-chain has the potential to dynamically change the association state between the polymers. As an approach for imparting chirality to the main-chain, there is a method of introducing a chiral substituent into a side-chain19 and a method of adding a chiral dopant.20 It is reported that addition of a chiral dopant to trans-polyazobenzene forms chiral aggregates but that addition of a chiral dopant to cis-polyazobenzene forms achiral aggregates. In this study, main-chain type polyazobenzenes bearing chiral substituents in the side-chain are synthesized by the Suzuki−Miyaura coupling method. The chiroptical activity of the polyazobenzenes can be tuned by irradiation of light through the change of aggregates. We synthesized polyazobenzenes and induced light-driven changes in their
INTRODUCTION Conductive polymers such as polyaniline, polypyrrole, poly(pphenylenevinylene), and polythiophene have been the subject of research for a long time due to their excellent properties. These materials are used in applications such as organic transistors,1 sensors,2 electrochromism devices,3,4 and organic solar cells.5 Introduction of a chiral substituent on the sidechain of the conductive polymers leads to optical absorption and chiroptical activity in the visible wavelength range and the formation of chiral J-aggregates due to π-stacking from the conjugated structure of the main-chain.6−8 Azobenzene is one of the most famous photoresponsive chromophores, and its molecular structure instantaneously changes from the rod-like trans isomer to the cis isomer upon UV light irradiation. Furthermore, its structure changes from the cis to trans isomer with visible light irradiation or heat. Such photoisomerization occurs repeatedly. Azobenzene has been used in the opto-sciences as a photoresponsive material. The trans isomer of azobenzene has an absorption at around 320 nm derived from the π−π* transition, and the cis isomer has a weak absorption band around 430 nm derived from the n−π* transition. Introduction of side-chains has been performed for azobenzene to give processability. Also, conjugated polymers having an azobenzene moiety, and main-chain type conjugated polymers having azobenzene have been synthesized.9,10 According to previous reports, the molecular weight of main-chain type polyazobenzene and the change in the absorption band due to an increase the conjugation length influence the photoisomerization behav© XXXX American Chemical Society
Received: December 7, 2018 Revised: February 18, 2019
A
DOI: 10.1021/acs.macromol.8b02588 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 1. Synthetic Route for Azobenzene Main-Chain Type Polymersa
a
Pd(PPh3)4 = tetrakis(triphenylphosphine)palladium(0), and THF = tetrahydrofuran.
Table 1. Optical and Electronic Properties of Azobenzene Main-Chain Polymers in solution Mna Poly[Azo-1] Poly[Azo-2] Poly[Azo-3] Poly[Azo-4] Poly[Azo-5]
4200 8300 9200 7900 5900
Mw/Mn f
1.17 1.33 1.50 1.60 1.26
a
b λabs max
(nm)
378 415 368 368 370
λabs onset
oxidation potential c λPL max
(nm)
432 537 420 418 416
(nm)
569 569 569 569 569
d EOx1 1/2
(V)
−1.60 −1.63g −1.64 −1.50g −1.60g g
d EOx2 1/2
(V)
−1.76 −1.74 −1.63
energy levels d EOx3 1/2
(V)
−2.20
Ege (eV) 2.87 2.31 2.95 2.97 2.98
a Evaluated by gel permeation chromatography (GPC) in THF against a polystyrene standard at room temperature. bMeasured in dilute solutions in THF at a concentration of ca. 0.02 mg/mL. cExcited at the absorption maximum. dDetermined by cyclic voltammetry (CV) with polymers dropcasted on ITO glass. 0.1 M TBAP (tetrabutylammonium perchlorate) in CH3CN was used as the electrolyte, Ag/Ag+ as the reference electrode, and Pt wire as the counter electrode. Ferrocene (E1/2 = 0.20 V) was used as an external standard. eOptical bandgap obtained from Eg = 1240/λonset. f Determined from the soluble part of the polymer in THF. gIrreversible signal (Epa).
molecular weight (Mn) of the soluble part of Poly[Azo-1] in THF is 4200 g/mol vs polystyrene standard. Mn of Poly[Azo2] is 8300 g/mol. The molecular weights of thiophene-based copolymers are greater than those of benzene-based polymers because the high activity of the thiophene unit in the polymerization reaction. This is in good accordance with previous research.11,12 Mn of Poly[Azo-3] and Poly[Azo-4] is 9200 and 7900 g/mol, respectively. Introduction of chiral sidechains at the 2- and 6-positions of the benzene units in the main-chain improves the solubility and molecular weights. The branched structure in the side-chain increases solubility, resulting in polymerization activity in the reaction to afford an increase in molecular weight. The molecular structure of the polymer was confirmed from an FT-IR spectrum obtained using the KBr method (Figure 1). The absorption band at 1140 cm−1 can be assigned to ν(C−O−C,−C−N). Vibration bands originating from alkyl groups (νCH2) appear at 2940 cm−1. The IR results confirm the chemical structure of the polymers thus synthesized. 1H NMR (δ form TMS, tetramethylsilane, ppm) further confirmed the molecular structure of the polymers
chiroptical activity. The microstructure of the polymers is examined with synchrotron radiation X-ray diffraction.
■
RESULTS AND DISCUSSION Synthesis of Polymers. A synthetic route for azobenzene main-chain type polymers is shown in Scheme 1. These polymers were obtained by polycondensation of an azobenzene derivative having a boronic acid ester and a dibromoaryl having a chiral substituent using the Suzuki−Miyaura coupling method with the aid of a Pd(0) catalyst. The copolymers thus obtained are abbreviated as Poly[Azo-1], Poly[Azo-2], Poly[Azo-3], Poly[Azo-4], and Poly[Azo-5], which consist of azobenzene and benzene or thiophene units. In addition, Poly[Azo-1] has a chiral substituent at the 2-position of the benzene moiety in the main-chain. Also, Poly[Azo-3] and Poly[Azo-4] have chiral substituents at the 2-position and 6position of the benzene unit in the main-chain. Poly[Azo-5] has an achiral substituent at the 2- and 6-positions of the benzene moiety in the main-chain. The molecular weights of the polymers are summarized in Table 1. The number-average B
DOI: 10.1021/acs.macromol.8b02588 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 3. Cyclic voltammetry (CV) of Poly[Azo-1], Poly[Azo-2], Poly[Azo-3], Poly[Azo-4] (green), and Poly[Azo-5] in 0.1 M TBAP/ CH3CN solution at scan rate of 50 mV/s.
Figure 1. FT-IR spectra of Poly[Azo-1], Poly[Azo-2], Poly[Azo-3], Poly[Azo-4], Poly[Azo-5], and M1.
Supporting Information. 1H NMR data of other polymers are shown in Figures S16−S19. Optical and Electronic Properties of the Polymers. The optical and electronic properties of these polymers are
(Figure 2). Fluorescence of the polymers is summarized in Table 1. Spectral data are shown in Figures S1−S5 of the
Figure 2. 1H NMR of Poly[Azo-4]. C
DOI: 10.1021/acs.macromol.8b02588 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 4. UV−vis absorption spectra and CD spectra of Poly[Azo-1] (a), Poly[Azo-2] (b), Poly[Azo-3] (c), and Poly[Azo-4] (d) in mixtures of THF/CH3OH (0.01 mg/mL) of various ratios.
reversible redox signals were observed in Poly[Azo-1] (E1/2 = −1.76 V), Poly[Azo-2] (E1/2 = −1.74, −2.20 V), and Poly[Azo-4] (E1/2 = −1.63 V). CV results of the polymers are shown in Figure 3. Chiral Aggregation Process of Azobenzene MainChain Type Polymers in Solution. The chiral aggregation process of azobenzene main-chain type polymers is investigated using a mixture of THF (good solvent) and methanol (poor solvent). Intermolecular interaction of polymers allows the formation of chiral aggregates as higher order structures. The CD signals are not observed in THF solution for all of the polymers. Addition of methanol to the polymer in THF solutions shows the CD signals and change in UV−vis
summarized in Table 1. Optical absorption of the polymers is derived from the π−π* transition of the π-conjugated mainchain. Poly[Azo-2] consisting of alternating azobenzene and thiophene units shows an absorption band derived from overlap of the n−π* and π−π* transitions. The optical bandgaps (band-edge bandgap) of the polymers are determined from the onset of the UV−vis absorption in THF solution. Cyclic voltammetry (CV) with these polymers drop-casted on ITO glass was conducted using 0.1 M TBAP (tetrabutylammonium perchlorate) in CH3CN as an electrolyte, Ag/Ag+ as the reference electrode, and Pt wire as the counter electrode. An irreversible first reduction signal was observed from −1.5 to −1.65 V in these polymers. Also, D
DOI: 10.1021/acs.macromol.8b02588 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 5. Photoisomerization of the polymers. Concentration = 0.01 mg/mL in THF solution.
absorption. The absorption of Poly[Azo-1] at 378 nm decreased with the addition of methanol, while the absorption band at long wavelengths increased, as shown in Figure 4a. The change in absorption indicates the formation of highly planar Jaggregates derived from π−π interaction between the mainchains. In the CD, a positive signal of the polymer is observed at 397 nm and a negative signal at 340 nm in THF/methanol (= 7/3) solution. Increasing the amount of methanol in the mixture intensified the signal. These spectra indicate that the chromophores of the polymers are right-handed helical aggregates. Poly[Azo-2] shows no remarkable change in the UV−vis and the CD spectra upon addition of methanol into the polymer solution, as shown in Figure 4b. These results indicate that Poly[Azo-2] forms no aggregates. In the UV−vis absorption spectrum of Poly[Azo-3] (Figure 4c), the absorption peak at 368 nm decreased with the addition of methanol; however, no shift in absorption upon addition of the
poor solvent is observed. In the CD, a positive signal at 392 nm and a negative signal at 339 nm are observed for polymer in the THF/methanol mixture (= 6/4). The polymer in the mixture of THF/methanol (5/5) shows a maximum value in the series of the measurements. These spectra indicate righthanded helical aggregation of the polymers. In the UV−vis absorption of Poly[Azo-4] (Figure 4d), the absorption peak at 368 nm decreased with the addition of methanol, while the absorption band at long wavelengths increased, indicating formation of J-aggregates, which is similar behavior to that of Poly[Azo-1]. The CD signals at 383 nm (negative) and 330 nm (positive) are observed in THF/methanol (= 5/5) mixture. The polymer shows opposite sign of the Cotton effect in THF/methanol (= 4/6) mixture. The polymer shows an intense CD signal at 383 nm (positive) and 330 nm (negative) in THF/methanol (= 2/8) mixture. Helical inversion is observed for Poly[Azo-4] with a change in E
DOI: 10.1021/acs.macromol.8b02588 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 6. Chiral aggregation process of azobenzene main-chain type polymers in solution after irradiation with UV light for 180 s. CD spectra and UV−vis absorption spectra of Poly[Azo-3] (a) and Poly[Azo-4] (b) in THF/CH3OH solution (0.01 mg/mL).
Figure 7. Plausible structures of the trans and cis forms of the polymer.
molecular aggregation, indicating the occurrence of “chiral solvatochromism”. Photoisomerization of Azobenzene Moieties in the Main-Chain. Trans-to-cis photoisomerization of azobenzene units in the main-chain is investigated with in situ UV−vis absorption spectra under UV light irradiation in the range from 300 to 400 nm. In addition, cis-to-trans photoisomerization is examined under visible light irradiation for polymer solution (0.1 mg/mL in THF). An optical absorption due to the π−π* transition of the main-chain is observed at 378 nm. The intensity of the absorption decreases upon UV light irradiation. The absorption decreases to 70% relative to the initial intensity after UV light irradiation for 180 s. An absorption band at ca. 450−500 nm derived from the n−π* transition of cisazobenzene units in the main-chain increases. Under visible light, the absorption band at 378 nm increases, and the weak
absorption band at 450−500 nm decreases. Subsequent irradiation of visible light restores the absorption intensity of the main-chain up to ca. 100%. Similarly, repeating photoisomerization is confirmed for Poly[Azo-3] (Figure 5c), Poly[Azo-4] (Figure 5d), and Poly[Azo-5] (Figure 5e). The absorption band derived from the π−π* transition decreased to 69% in Poly[Azo-3], 74% in Poly[Azo-4], and 64% in Poly[Azo-5] upon UV light irradiation for 180 s. Irradiation of the polymers with visible light restores their original absorption intensity. In the case of M1 as a monomer of a single azobenzene unit (Figure 5f), the absorption band at 359 nm decreased 0.8% in intensity compared with the initial absorption upon irradiation of UV light for 180 s. Poly[Azo2] as a copolymer consisting of azobenzene and thiophene shows no photoisomerization (Figure 5b). The optical absorption due to the n−π* transition of azobenzene overlaps F
DOI: 10.1021/acs.macromol.8b02588 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 10. GIXD diffraction pattern of Poly[Azo-4] thin films dropped before UV light irradiation (black) and after UV light irradiation (purple).
Photoisomerization-Driven Change in Chiral Aggregation. Photoisomerization of the polymer for chiral aggregation in the solution was investigated by addition of methanol to the polymer solution with irradiation of UV light, as shown in Figure 6. After irradiation of UV light for 3 min to Poly[Azo-3] and Poly[Azo-4] solution (concentration = 0.01 mg/mL in THF), methanol was added dropwise to the polymer solution. Chiral aggregation did not proceed with dropwise addition of methanol to photoisomerized Poly[Azo3], as shown in Figure 6a. The trans polymer in the mixture of THF/methanol (= 5/5) shows an intense CD signal, while photoisomerized Poly[Azo-3] with the cis form after irradiation of UV light forms no aggregates. Poly[Azo-4] of the cis form after irradiation of UV light can partly construct chiral aggregates with addition of methanol to the polymer solution, as shown in Figure 6b. However, the intensity of the CD for the cis form (after irradiation of UV) is smaller than that for the trans isomer (before irradiation of UV). This result suggests that the cis form with its bulky structure has difficulty forming chiral aggregates in solution. The trans structure with
Figure 8. Grazing incidence X-ray diffraction (GIXD) pattern of polymer thin films. Poly[Azo-1] (red), Poly[Azo-2] (orange), Poly[Azo-3] (blue), Poly[Azo-4] (green), and Poly[Azo-5] (purple).
the absorption band due to the π−π* transition of the mainchain, resulting in no change because of the occurrence of the equilibrium between the trans and cis formations.11 Polymers having an optical absorption band at >420 nm exhibit no photoisomerization, while polymers having an absorption band at
