Highly Emissive, Optically Active Poly(diphenylacetylene) Having a

May 3, 2016 - Annealing-Induced Circular Dichroism Enhancement in Luminescent Conjugated Polymers with an Intramolecular Stack Structure. Young-Jae Ji...
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Highly Emissive, Optically Active Poly(diphenylacetylene) Having a Bulky Chiral Side Group Hyojin Kim,†,‡ Kyo-Un Seo,† Young-Jae Jin,† Chang-Lyoul Lee,§ Masahiro Teraguchi,∥ Takashi Kaneko,∥ Toshiki Aoki,*,∥ and Giseop Kwak*,† †

School of Applied Chemical Engineering, Major in Polymer Science and Engineering, Kyungpook National University, 1370 Sankyuk-dong, Buk-ku, Daegu 702−701, Korea ‡ Daegu Technopark Nano Convergence Practical Application Center, 891−5 Daecheon-dong, Dalseo-ku, Daegu 704−801, Korea § Advanced Photonics Research Institute (APRI), Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Korea ∥ Graduate School of Science and Technology, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata, Japan S Supporting Information *

ABSTRACT: A highly emissive, optically active poly(diphenylacetylene) derivative, NpSi*-PDPA, was synthesized by introducing a bulky chiral pendant group into the polymer chain. NpSi*-PDPA exists in a glassy state having a highly disordered and amorphous structure. Hence, the fractional free volume is quite high, i.e., 0.29. NpSi*-PDPA emits a green light in solution and a yellow light in film. This polymer is quite emissive, as the photoluminescence (PL) quantum yield is 56.1% in solution and 6.2% in film, and the PL lifetime is relatively long, 1.71 ns in solution and 1.05 ns in film. NpSi*PDPA shows the strongest circular dichroism (CD) signal at 384 nm, with a magnitude of circular polarization (gCD) of 0.90 × 10−3 and an optical rotation of 110° (4.30 × 10−3 g mL−1, in CHCl3). The CD intensity is significantly increased by annealing at 80 °C, reaching an equilibrium at gCD of 7.10 × 10−3 after 72 h.

C

the PDPA. Herein we describe the details and suggest a definite molecular design for highly emissive, optically active CPs. We conducted synthetic reactions similar to those of a literature method7 by using a chiral silane compound (1 in Scheme 1) as a starting material to obtain the corresponding diphenylacetylene derivative (NpSi*-DPA in Scheme 1) as a chiral monomer at a yield of 54%. In the side group, NpSi*DPA consists of a chiral silicon center, an extremely bulky naphthyl ring, and two phenyl rings. NpSi*-DPA has an optical rotation ([α]D) of 2° [concentration (c) = 1.0 g dL−1, in cyclohexane] and appears as white crystals (mp 112−113 °C). NpSi*-DPA was polymerized in toluene using TaCl5−n-Bu4Sn as a catalyst to produce a bulk solid polymer (NpSi*-PDPA in Scheme 1) at a yield of 66%. NpSi*-PDPA had a high weightaverage molecular weight (Mw) of 1.6 × 106 g mol−1 [evaluated by gel permeation chromatography (GPC) using tetrahydrofuran (THF) as an eluent solvent, with a polystyrene standard]. Various properties of NpSi*-PDPA were measured (Table 1) and compared with those of other PDPA derivatives (C1-, C18-,4b,c and 2MB-PDPA5a in Scheme 1). NpSi*-PDPA

onjugated polymers (CPs) having circular dichroism (CD) have potential for highly advanced optoelectronic device applications.1 On the other hand, conventional CPs exhibit significant photoluminescence (PL) quenching in the solid state owing to the intrinsic planar geometry of the backbone and very strong intermolecular π−π interaction.2 Either highly emissive or optically active CPs may also have circularly polarized luminescence, but this is still very rare. Poly(diphenylacetylene)s (PDPAs) are known to be highly emissive even in the solid state owing to the nonplanar geometry based on the highly twisted backbone.3 The chain conformation and intramolecular stack structure (IaSS) of the side phenyl rings are significantly affected by the length, bulkiness, and substitution position of the pendant groups on the side phenyl rings. Therefore, PDPAs can be designed precisely as desired for high PL quantum efficiency in the early stage of molecular design of the corresponding monomers.4 Moreover, PDPAs can readily be made optically active by introducing chiral side groups5 or annealing the achiral polymers in chiral solvents.6 Despite these advantages, however, there has been no attempt to realize high PL efficiency and large optical dissymmetry simultaneously. In this study, we achieved the goal of obtaining these two properties at the same time by introducing an extremely bulky chiral side group into © XXXX American Chemical Society

Received: March 4, 2016 Accepted: May 2, 2016

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DOI: 10.1021/acsmacrolett.6b00184 ACS Macro Lett. 2016, 5, 622−625

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ACS Macro Letters Scheme 1. Synthesis of NpSi*-PDPA and Chemical Structures of NpSi*-PDPA and Other PDPAs

Figure 1. (a) Photographs of PDPAs under room light and UV light (excited at >365 nm). (b) DSC thermogram of NpSi*-PDPA (heat flow rate 10 °C min−1, under nitrogen gas). (c) XRD patterns of NpSi*-PDPA in drop-cast film.

appeared yellow; this is very similar to C18-PDPA, which has a long alkyl chain in the side group, but quite different from the dark orange color of C1-PDPA, which has a short alkyl chain (Figure 1a). The reason why NpSi*-PDPA looks yellow unlike the dark orange color of C1-PDPA may be due to the fact that NpSi*-PDPA absorbs very weak UV light even under the ambient environment but emits a relatively strong yellow light. This fluorescence emission can camouflage the polymer in itself to give the appearance of being pale yellow. That is, the difference in color between NpSi*-PDPA and C1-PDPA comes from the significant difference in UV−vis absorption and PL emission properties between the two polymers. Details will be described later in Figure 2. NpSi*-PDPA was more brittle than the other PDPAs. This polymer easily crumbled to powder when pressed, indicating a glassy state at room temperature. The brittleness is attributed to the higher ratio of aromaticity compared to the other PDPAs. NpSi*-PDPA showed a unique solubility in common organic solvents such as toluene, THF, and CHCl3. For example, this polymer dissolved in toluene slowly to become transparent in dilute solution, whereas the toluene solution became slightly translucent as the concentration increased up to several wt %. This concentration-

Figure 2. UV−vis absorption (solid line) and PL emission (dot line) spectra (excited at 420 nm) of NpSi*-PDPA (red line) in (a) solution (c = 1.0 × 10−5 mol L−1, in toluene) and (b) film (thickness ∼200 nm, spin-cast from toluene solution) compared with C1- (green line) and C18- (blue line) PDPAs.

dependent solubility may be ascribed to the high ratio of aromaticity and high molecular weight. Namely, the translucent appearance at higher concentration may be due to liquid crystallinity. Many PDPA derivatives are known to be lyotropic liquid crystals owing to the relatively high molecular weights and intrinsically rigid backbones.4 In a differential scanning

Table 1. Physical, PL Emission, and Chiroptical Properties of NpSi*-PDPA Compared with Other PDPAs PL emission properites physical properties polymer (PDPAs) NpSi* C1 C18 2MBb

Mw, × 106 g mol−1 1.6 5.2a 4.2a 4.2

Mw/Mn 1.7 3.2a 2.5a 4.9

color yellow orange yellow orange

in film

in toluene d, gm−3 0.92 0.91a 0.96a N/A

FFV 0.29 0.26a 0.16a N/A

λmax, nm 510 506 505 505

QYPL, % 56.1 (51.8) 19.9 36.6 N/A

c

chiroptical properties

τave, nsd

λmax, nm

QYPL, %

τave, nsd

gCD, × 10−3

[α]D, °

1.71 0.64 1.03 0.73

537 530 510 524

6.2 0.9 18.0 N/A

1.05 0.46 1.25 1.01

0.90 (7.10)c 0.69

110 (295)c N/A

Data from ref 4d. bData from ref 5a. cValue after annealing at 80 °C for 72 h. dMonitor wavelength of all PDPA derivatives is 530 nm except 2 MB in solution (Monitor wavelength of 2 MB solution is 500 nm), and average lifetime (τavr) is intensity-weighted average lifetime.

a

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Figure 3 shows CD spectra of NpSi*-PDPA in solution. Like most other chiral PDPAs, NpSi*-PDPA showed the biggest CD

calorimetry (DSC) analysis, this polymer did not show any thermodynamic transition in a wide temperature range from −50 to 200 °C, indicating a glassy state over the entire temperature range (Figure 1b). The X-ray diffraction (XRD) pattern exhibited a very broad halo signal in a wide angle region, indicating that this polymer is basically amorphous (Figure 1c). The signal seen in a small angle of 6.0° (corresponding to a distance of 15.4 Å according to Bragg’s equation) is due to an intermolecular chain packing structure. This small-angle signal was much weaker and broader than those of other PDPAs.4 This means that NpSi*-PDPA has more disordered structure than other PDPAs. Surprisingly, NpSi*-PDPA had an extremely low density (d) of 0.92, and the fractional free volume (FFV) was determined to be 0.29 from the van der Waals volume (Vw) and d,8 indicating the existence of many microvoids in the solid state (Table 1). This FFV value is comparable to that (0.26) of C1-PDPA and much higher than that (0.16) of C18-PDPA.4d Actually, C1-PDPA is known to have the second greatest FFV value among the gaspermeable polymers reported to date.9 From the results of DSC, XRD, and FFV measurements, NpSi*-PDPA is thought to have many free spaces in the rigid, coarsened, semipermanent frameworks that exist in an amorphous glassy state. Figure 2 shows the UV−vis absorption and PL emission spectra of NpSi*-PDPA in solution and film compared with those of C1- and C18-PDPA. NpSi*-PDPA showed an absorption maximum at around 420 nm. The molar extinction coefficient (ε) was 2.4 × 10−3 L cm−1 mol−1, which was smaller than that (5.4 × 10−3 L cm−1 mol−1) of C1-PDPA. This is in good agreement with the fact that NpSi*-PDPA appeared yellow, unlike the dark orange color of C1-PDPA. NpSi*PDPA emitted a green light with a maximum intensity at 510 nm and a shoulder at 475 nm in toluene solution, whereas it emitted a yellow light with a maximum intensity at 537 nm in film. The wavelengths of PL maximum (λmax) of NpSi*-PDPA in both solution and film were longer than those of other polymers. Although the polyene backbone will be highly twisted owing to the steric hindrance of the bulky pendant groups, leading to reduced conjugation length, the aromatic side groups may further resonate with the backbone via the silylene σ linkage. This should be responsible for the redder emission of NpSi*-PDPA compared to the other polymers. This polymer was quite emissive both in solution (a PL quantum yield, QYPL, of ∼56.1% in toluene solution, Table 1) and in film (QYPL ∼ 6.2% in film, Table 1). The QYPL values of NpSi*-PDPA were much larger than those (19.9% in solution, 0.9% in film, Table 1) of C1-PDPA and comparable to those (36.6% in solution, 18% in film, Table 1) of C18-PDPA, which is known as the most emissive polymer among the PDPA derivatives.4b The high QYPL is attributed to the fact that the IaSS of the side phenyl rings is highly relaxed owing to efficient steric hindrance of the bulky aromatic pendant groups to degenerate nonradiative emission decay channels in the electronic transition. Time-correlated single-photon counting analysis (TCSPC) showed similar results: NpSi*-PDPA has a much longer PL lifetime (τave ∼ 1.71 ns in toluene solution and 1.05 ns in film, Table 1) than C1-PDPA (0.64 and 0.46 ns, respectively, Table 1) and a similar PL lifetime to that of C18PDPA (1.03 and 1.25 ns, respectively, Table 1). The longer PL lifetime of NpSi*-PDPA indicates highly efficient exciton confinement between the polyene backbone and the resonant side aromatic rings in the excited state.

Figure 3. CD and UV−vis spectra of NpSi*-PDPA solution (c = 5.0 × 10−4 mol L−1, in toluene) after different annealing times at 80 °C (inset: plot of gCD according to annealing time).

signal at 384 nm as the first Cotton band. The second, third, and fourth Cotton bands also appeared at 362, 339, and 324 nm, respectively. These Cotton bands did not appear in the CD spectra of the monomer, indicating that the CD signals originate from the asymmetric structure of the polymer (Figure S1). The first Cotton band well matched the absorption band due to the resonant isotropic structure between the backbone and side aromatic rings, indicating that an axial chirality occurred in the polymer chain.6 The magnitude of circular polarization (gCD) at 384 nm was ∼0.90 × 10−3, which is slightly larger than that (0.69 × 10−3)5a of 2MB-PDPA, which has a smaller chiral side group. In addition, NpSi*-PDPA showed a large [α]D of 110° (c = 4.30 × 10−3 g mL−1, in CHCl3). This indicates that the bulkier side group is effective for achieving a larger optical dissymmetry. According to our previous study, the IaSS of side phenyl rings in PDPAs was variable in solution or a swollen state; thus, the side phenyl rings underwent an asymmetric arrangement under a chiral environment through solvent annealing on heating.6 To examine the same solvent annealing effect on the chiral structure of NpSi*-PDPA in solution, annealing time-dependent CD spectra were also measured with the solutions heated at 80 °C for different times (Figure 3). Surprisingly, this polymer showed a significant increase in CD intensity through the annealing process, reaching an equilibrium at gCD ∼ 7.10 × 10−3 for the sample annealed for 72 h, which was approximately 8 times larger than that of the solution before annealing. Similarly, the [α]D value of NpSi*-PDPA considerably increased up to 295° after the annealing at 80 °C for 72 h (Table 1). The asymmetric IaSS of NpSi*-PDPA was probably not optimized for the optically dissymmetric, energy-minimized form just after polymerization owing to the extremely large steric hindrance of the bulky side group, while it overcame a certain energy barrier for conformational transition through the solvent annealing process to undergo asymmetric change under the presence of the chiral side group, leading to a more definite axial chirality. We also measured the CD spectra of NpSi*PDPA film before and after annealing at 80 °C for 48 h. However, the film did not showed any significant CD enhancement unlike the case of in the solution (Figure S2). It should be due to the fact that the polymer exists in the glassy 624

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(3) (a) Jin, Y.-J.; Kwak, G. Polym. Rev. 2016, DOI: 10.1080/ 15583724.2015.1125919. (b) Lam, J. W. Y.; Dong, Y.; Law, C. C.W.; Dong, Y.; Cheuk, K. K. L.; Lai, L. M.; Li, Z.; Sun, J.; Chen, H.; Zheng, Q.; Kwok, H. S.; Wang, M.; Feng, X.; Shen, J.; Tang, B. Z. Macromolecules 2005, 38, 3290−3300. (c) Shukla, A. Chem. Phys. 2004, 300, 177−188. (d) Hidayat, R.; Fujii, A.; Ozaki, M.; Teraguchi, M.; Masuda, T.; Yoshino, K. Synth. Met. 2001, 119, 597−598. (e) Fujii, A.; Hidayat, R.; Sonoda, T.; Fujisawa, T.; Ozaki, M.; Vardeny, Z. V.; Teraguchi, M.; Masuda, T.; Yoshino, K. Synth. Met. 2001, 116, 95−99. (f) Hidayat, R.; Tatsuhara, S.; Kim, D. W.; Ozaki, M.; Yoshino, K.; Teraguchi, M.; Masuda, T. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 10167−10173. (g) Ghosh, H.; Shukla, A.; Mazumdar, S. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, 12763−12774. (h) Shukla, A.; Ghosh, H.; Mazumdar, S. Synth. Met. 2001, 116, 87− 90. (4) (a) Jin, Y.-J.; Bae, J.-E.; Cho, K.-S.; Lee, W.-E.; Hwang, D.-Y.; Kwak, G. Adv. Funct. Mater. 2014, 24, 1928−1937. (b) Lee, W.-E.; Kim, J.-W.; Oh, C.-J.; Sakaguchi, T.; Fujiki, M.; Kwak, G. Angew. Chem., Int. Ed. 2010, 49, 1406−1409. (c) Lee, W.-E.; Oh, C.-J.; Park, G.-T.; Kim, J.-W.; Choi, H.-J.; Sakaguchi, T.; Fujiki, M.; Nakao, A.; Shinohara, K.; Kwak, G. Chem. Commun. 2010, 46, 6491−6493. (d) Kwak, G.; Minakuchi, M.; Sakaguchi, T.; Masuda, T.; Fujiki, M. Macromolecules 2008, 41, 2743−2746. (e) Kwak, G.; Minakuchi, M.; Sakaguchi, T.; Masuda, T.; Fujiki, M. Chem. Mater. 2007, 19, 3654− 3661. (5) (a) Lee, D.; Kim, H.; Suzuki, N.; Fujiki, M.; Lee, C.-L.; Lee, W.E.; Kwak, G. Chem. Commun. 2012, 48, 9275−9277. (b) San Jose, B. A.; Matsushita, S.; Akagi, K. J. Am. Chem. Soc. 2012, 134, 19795− 19807. (c) Jim, C. K. W.; Lam, J. W. Y.; Leung, C. W. T.; Qin, A.; Mahtab, F.; Tang, B. Z. Macromolecules 2011, 44, 2427−2437. (d) Aoki, T.; Kobayashi, Y.; Kaneko, T.; Oikawa, E.; Yamamura, Y.; Fujita, Y.; Teraguchi, M.; Nomura, R.; Masuda, T. Macromolecules 1999, 32, 79−85. (6) (a) Kim, H.; Jin, Y.-J.; Kim, B. S.-I.; Aoki, T.; Kwak, G. Macromolecules 2015, 48, 4754−4757. (b) Kim, H.; Lee, D.; Lee, S.; Suzuki, N.; Fujiki, M.; Lee, C.-L.; Kwak, G. Macromol. Rapid Commun. 2013, 34, 1471−1479. (c) Lee, D.; Jin, Y.-J.; Kim, H.; Suzuki, N.; Fujiki, M.; Sakaguchi, T.; Kim, S. K.; Lee, W.-E.; Kwak, G. Macromolecules 2012, 45, 5379−5386. (7) Kwak, G.; Masuda, T. Macromolecules 2000, 33, 6633−6635. (8) (a) van Krevelen, D. W. In Properties of Polymers: Their Correlation with Chemical Structure; Their Numerical Estimation and Prediction from Additive Group Contributions, 3rd ed.; Elsevier Science: Amsterdam, 1990; pp 71−107. (b) Bondi, A. In Physical Properties of Molecular Crystals, Liquids, and Glasses; John Wiley and Sons: New York, 1968; pp 25−52, 53−97. (9) (a) Raharjo, R. D.; Lee, H. J.; Freeman, B. D.; Sakaguchi, T.; Masuda, T. Polymer 2005, 46, 6316−6324. (b) Toy, L. G.; Nagai, K.; Freeman, B. D.; Pinnau, I.; He, Z.; Masuda, T.; Teraguchi, M.; Yampolskii, Y. P. Macromolecules 2000, 33, 2516−2524.

state at the same temperature and hence hardly undergoes conformational transition thermodynamically in the solid state. Notably, the QYPL value of NpSi*-PDPA in the same solution did not change much during the annealing process. It decreased slightly by 4.3% to show a QYPL of 51.8% for the sample annealed for 72 h (Table 1). This indicates that the asymmetric change of chain conformation through the solvent annealing does not affect the degree of stacking of side phenyl rings. In summary, a highly emissive, optically active PDPA derivative, NpSi*-PDPA, was successfully developed by introducing a bulky chiral side group. This polymer appeared yellow and existed in a glassy state with an amorphous, highly coarsened structure in a wide temperature range; hence, it exhibited many microvoids in the solid state. Owing to these structural features, NpSi*-PDPA was quite emissive in both solution and film. Moreover, this polymer was optically active and, when sufficiently annealed in solution, showed an extremely large optical dissymmetry at the CD-enhancing equilibrium, while keeping the high QYPL value. Our molecular design for either emissive or optically active conjugated polymers will be helpful for development of highly advanced optoelectronic materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00184. Materials, synthesis of monomer and polymer, annealing treatment for CD and optical rotation measurement, measurement details, and chiroptical analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.A.). *E-mail: [email protected] (G.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) grants funded by the Korea government (MEST) (2014R1A2A1A11052446).

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REFERENCES

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DOI: 10.1021/acsmacrolett.6b00184 ACS Macro Lett. 2016, 5, 622−625