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Cascade Polyannulation of Diyne and Benzoylacetonitrile: A New Strategy for Synthesizing Functional Substituted Poly(naphthopyran)s Yajing Liu,†,‡ Zheng Zhao,‡ Jacky W. Y. Lam,†,‡ Yueyue Zhao,‡ Yuncong Chen,‡ Yong Liu,§ and Ben Zhong Tang*,†,‡,§ †

HKUST-Shenzhen Research Institute, No. 9 Yuexing first RD, South Area, Hi-tech Park, Nanshan, Shenzhen 518057, China Department of Chemistry, Division of Life Science, State Key Laboratory of Molecular Neuroscience, Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China § Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ‡

S Supporting Information *

ABSTRACT: A new strategy for synthesizing multisubstituted poly(naphthopyran)s (PNPs) with novel functionalities was described. The cascade oxidative polyannulation of benzoylacetonitrile and internal diynes are catalyzed by [RhCp*Cl 2 ] 2 and Cu(II) acetate in dimethylformamide at 90 °C, generating PNPs with high molecular weight of up to 19 300 in excellent yields (isolation yield up to 96.4%). This polymerization method enjoys the remarkable advantages of high reaction rate, high efficiency, and atom-economy. All of the polymers show good solubility in common organic solvents and high thermal stability (degradation temperature up to 431 °C). The thin films of PNPs display high refractive indices (1.5137−1.7524) in a wide wavelength range of 450−1600 nm. PNP containing tetraphenylethene units can be utilized to generate a well-resolved nanoscale photopattern by UV irradiation of its film through a copper mask. The PNPs exhibit the phenomena of mechanochromism and vapochromism: their emission is turned on upon scratching and solvent fuming their solid powders. Such attributes allow the polymers to be used as security materials or fluorescent indicators in various fields.



INTRODUCTION The development of novel polymerization routes affording polymers with heteroaromatic rings has claimed an area of phenomenal potential and diversity. Because of the unique electronic and photophysical properties of these polymers,1 a variety of high-tech applications in, for example, polymer lightemitting diodes,2 polymer field effect transistors,3 nonlinear optics4 and polymer solar cells,5 have been explored. Among them, polymers with fused heterocycles, such as benzylcarbazole,2a thieonopyran,2b and benzothiadiazole,5a compose a vital family that provides a high versatility in polymer structure and application. The preparation of fused heterocyclic polymers are often realized by transition metal-catalyzed coupling reactions from limited fused aromatic substrates and readily available heteroatom-containing compounds.6 Introduction of appropriate substituents can not only improve the processability of the polymers but also enrich the polymer structure and functionality. Thus, developing new synthetic methods to fused heteroaromatic polymers with multisubstituents from commercial available substrates with advanced functionalities is of both academic and application values. © 2015 American Chemical Society

In 2012, Wang et al. reported a facile strategy for synthesizing naphthopyrans through rhodium-catalyzed oxidative annulation of benzoylacetonitrile derivatives with alkynes.7 The reaction involves sequential activation of both C(sp2)−H and C(sp3)−H, which leads to instant in situ generation of fused heterocycles with multisubstituents (Scheme 1). Many naphthopyran derivatives have been discovered to be photoScheme 1. Rhodium-Catalyzed Oxidative Annulation of Benzoylacetonitrile and Internal Alkyne

Received: April 25, 2015 Revised: June 19, 2015 Published: July 2, 2015 4241

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Polymer Synthesis. All the polymerization reactions were performed under nitrogen atmosphere using a standard Schlenk technique. A typical procedure for the polymerization of 1a and 2 is given below as an example. Into a 25 mL Schlenk tube with a magnetic stirrer were placed 0.1 mmol of 1a, 0.1 mmol of 2, 20 mol % of [Cp*RhCl2]2, and 0.4 mmol of Cu(OAc)·H2O in 5 mL of distilled DMF. The solution was stirred under nitrogen at 90 °C for 2 h. The polymerization was terminated by pouring the mixture into a large quantity of hexane through a dropper filled with neutral Al2O3 to remove the residue catalyst and any insoluble substrates if formed. The precipitates were collected by filtration through a sand-core funnel. The crude product was then dissolved in THF, reprecipitated in hexane and washed repeatedly with hexane before being dried in vacuum to a constant weight. A yellow powder of polymer P1a/2 was obtained in 74.8% yield (Table 7, entry 1). Mw = 15 800; Mw/Mn = 1.51. IR (film), ν (cm−1): 3057, 2940, 2866, 2217, 1775, 1630, 1606, 1572, 1473, 1245, 1176. 1H NMR (CD2Cl2, 400 MHz), δ (ppm): 7.45−6.65 (21H, aromatic protons), 4.01 (4H, CH2 protons), 1.79 (4H, CH2 protons), 1.55 (4H, CH2 protons). 13C NMR (CD2Cl2, 100 MHz), δ (ppm): 159.06, 156.40, 149.16, 140.20, 138.31, 137.77, 135.52, 132.94, 131.84, 130.30, 129.43, 128.88, 127.95, 126.94, 115.32, 114.08, 113.85, 67.96, 29.21, 25.96. P1b/2. Yellow powder; yield 91.4% (Table 7, entry 2). Mw = 12 100; Mw/Mn = 1.39. IR (film), ν (cm−1): 3058, 2935, 2858, 2250, 2217, 1741, 1632, 1574, 1512, 1388, 1246, 1176. 1H NMR (CD2Cl2, 400 MHz), δ (ppm): 7.47, 7.34, 7.27, 7.19, 7.10, 6.99, 6.78, 6.76 (aromatic protons), 4.00, 3.93, 1.77, 1.63, 1.42 (CH2 protons). 13C NMR (CD2Cl2, 100 MHz), δ (ppm): 159.59, 158.83, 157.95, 156.21, 140.08, 137.56, 135.73, 132.78, 131.85, 130.86, 130.08, 129.98, 128.67, 127.55, 126.07, 124.68, 120.70, 117.04, 113.68, 68.01, 29.07, 25.79. P1c/2. Yellow powder; yield 96.4% (Table 7, entry 3). Mw = 19 300; Mw/Mn = 1.46. IR (film), ν (cm−1): 3057, 2926, 2854, 2216, 1629, 1606, 1573, 1367, 1244, 1175. 1H NMR (CD2Cl2, 400 MHz), δ (ppm): 7.44, 7.26, 7.10, 6.97, 6.74 (aromatic protons), 3.98, 3.90, 1.73, 1.57, 1.42, 1.31 (CH2 protons). 13C NMR (CD2Cl2, 100 MHz), δ (ppm): 160.33, 149.70, 136.07, 132.62, 131.51, 131.38, 130.08, 129.98, 128.74, 128.25, 127.50, 115.85, 114.38, 114.07, 68.66, 29.97, 26.59. P1d/2. Yellow powder; yield 50.5% (Table 7, entry 4). Mw = 6700; Mw/Mn = 1.49. IR (film), ν (cm−1): 3056, 3025, 2962, 2926, 2854, 2217, 1716, 1632, 1570, 1494, 1442, 1385, 1354. 1H NMR (CD2Cl2, 400 MHz), δ (ppm): 7.44, 7.31, 7.14, 6.98, 6.87, 6.69 (aromatic protons). 13C NMR (CD2Cl2, 100 MHz), δ (ppm): 160.00, 149.94, 143.95, 140.53, 136.25, 131.75, 128.50, 124.06, 121.49, 118.28, 116.15, 93.26. Model Reaction. Model compound 3 was synthesized by the oxidative annulation of diphenylacetylene and benzoylacetonitrile. The experimental procedure was similar to that for preparing P1a/2. Orange solid; yield 49.6% (after purification by column chromatography). 1H NMR (400 MHz, CD2Cl2), δ (ppm): 7.47 (m, 5H), 7.28 (m, 14H), 7.15 (m, 3H) and 6.72 (d, 1H, aromatic protons). HRMS (MALDI−TOF): m/z 497.1790 (M+, calcd 497.1780) (Figure S1 in the Supporting Information). Photopatterning. The photo-oxidation of the polymer film was conducted using 365 nm light obtained from a Spectroline ENF280C/F UV lamp. The procedures were similar to those described in our previous publication.11

chromic because of their switchable structures after photocatalyzed ring-opening/closing reactions.8 Thus, polymers consisting of naphthopyran moieties are anticipated to exhibit interesting properties as well. With such regards, in this paper, we tried to develop such anorganic reaction into a useful tool for the preparation of functional poly(naphthopyran)s (PNPs). Making use of commercial available and handy substrates, we have successfully developed a one-pot cascade polyannulation of internal diynes and benzoylacetonitrile (Scheme 2) toward Scheme 2. Rhodium-Catalyzed Oxidative Polyannulation of Internal Diynes and Benzoylacetonitrile

multisubstituted PNPs. This polymerization can be carried out in mild conditions within only 2 h, producing PNPs with good solubility and high thermal stability in excellent yields. The tetraphenylethene (TPE)-containing PNP is photosensitive and exhibits a turn-on emission response to shear force and vapor stimulus. Taking advantage of these properties, PNPs may serve as security materials or fluorescent sensors used in various fields.



EXPERIMENTAL SECTION

Materials and Instrumentation. Benzoylacetonitrile, diphenylacetylene and other reagents were purchased from Aldrich and used as received without further purification. The catalyst, named 1,2,3,4tetramethylcyclopentadienylrhodium(III) chloride dimer [RhCp*Cl2]2, was prepared according to the literature method.9 All the organic solvents such as tetrahydrofuran (THF) and dimethyformamide (DMF) were distilled prior to use. Monomers 1a−1d were prepared according to reported procedures.10 Gel permeation chromatography (GPC) of the polymers was performed in THF at 40 °C at an elution rate of 1.0 mL min−1 on a Waters GPC system equipped with a Waters 515 HPLC pump, a Waters 486 UV−vis detector, a column temperature controller, and a set of Styragel columns (HT3, HT4 and HT6; molecular weight range 102−107). The polymers were dissolved in THF (about 2 mg mL−1) and filtered through a 0.45 μm PTFE filter before being injected into the GPC system. IR spectra were recorded on a PerkinElmer 16 PC FTIR spectrophotometer. 1H and 13C NMR spectra were recorded on a Bruker AV 400 spectrometer in deuterated chloroform or dichloromethane using tetramethylsilane (TMS; δ = 0) as internal reference. High-resolution mass spectra (HRMS) were recorded on a GCT premier CAB048 mass spectrometer operating in MALDI-TOF mode. UV spectra were measured on a Milton Ray Spectronic 3000 Array spectrophotometer. Photoluminescence (PL) spectra were recorded on a PerkinElmer LS 55 spectrophotometer. Thermogravimetric analysis (TGA) was performed on a TA TGA Q5000 under nitrogen at a heating rate of 10 °C min−1.



RESULTS AND DISCUSSION Polymerization. To develop the oxidative annulation of benzoylacetonitrile and alkyne into a new methodology for preparing high molecular weight PNPs with functional properties, we first optimized the reaction conditions for the polymerization using 1a and 2 as model monomers. We first examined the loading effect of copper acetate, an oxidant, on the polymerization. The polymerization of 1a and 2 in the absence of oxidant yields no polymeric product, which is demonstrative of its important role in the polymerization (Table 1, entry 1).12 A polymer with moderate yield (65.4%)

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Macromolecules Table 1. Effect of Oxidant Loading on the Polymerization of 1a and 2a entry

equiv of oxidant

yield (%)

1 2 3 4

0 1 2 4

0 17.3 43.3 65.4

Mw 5900 7100 9100

Table 4. Temperature Effect on the Polymerization of 1a and 2a

Mw/Mn

entry

temp (°C)

yield (%)

Mw

Mw/Mn

1.30 1.96 1.47

1 2b 3c 4

50 70 90 110

63.4 85.6 ∼99 ∼99

7800 7100 12600 10200

1.52 1.36 1.65 1.54

Polymerization at 90 °C in DMF under nitrogen for 24 h in the presence of 10% equiv of [RhCp*Cl2]2. [1a]:[2] = 0.026 M:0.02 M.

a

a

and molecular weight (Mw = 9100) was afforded when four equivalent of oxidant to the concentration of 2 was added (Table 1, entry 4). Increasing the amount of oxidant results in poorer results. Afterward, the variation in the catalyst amount was investigated. As shown in Table 2, satisfactory results were

ratio of [1a]:[2] = 1.3:1, the yield and molecular weight are gradually enhanced with the prolongation of reaction time (Table 5, entries 3−5). However, at a stoichiometric monomer

Polymerization in DMF under nitrogen for 24 h in the presence of 20% equiv of [RhCp*Cl2]2 and 4 equiv of Cu(OAc) 2·H2O. [1a]:[2] = 0.026 M:0.02 M. bData taken from Table 3, entry 5. cData taken from Table 2, entry 3.

Table 5. Time Course on the Polymerization of 1a and 2a

Table 2. Effect of Catalyst Loading on the Polymerization of 1a and 2a entry

equiv of catalyst (%)

yield (%)

Mw

Mw/Mn

1 2b 3

5 10 20

15.7 65.4 ∼99

3400 9100 12600

1.31 1.47 1.65

Polymerization at 90 °C in DMF under nitrogen for 24 h in the presence of 4 equiv of Cu(OAc)2·H2O. [1a]:[2] = 0.026 M:0.02 M. b Data taken from Table 1, entry 4. a

solvent

yield (%)

Mw

Mw/Mn

THF toluene o-xylene MeCN DMF

10.8 4.9 7.8 33.3 85.6

2700 3000 2800 9800 7100

1.23 1.25 1.23 1.84 1.36

yield (%)

Mw

Mw/Mn

1 2 3 6 24

65.7 74.8 67.7 99.0 99.0

9500 15800 9300 8200 12600

1.34 1.51 1.45 1.45 1.65

Polymerization at 90 °C in DMF under nitrogen in the presence of 20% equiv of [RhCp*Cl2]2 and 4 equiv of Cu(OAc)2·H2O. [1a]:[2] = 0.02 M:0.02 M (entries 1 and 2) and 0.026 M:0.02 M (entries 3−5). b Data taken from Table 2, entry 3.

feeding ratio, a polymer with Mw of 15800 was obtained in a satisfactory yield of 74.8% within 2 h (Table 5, entry 2). This is indicative of the fast reaction rate of the polymerization. We then shifted our attention to the effect of monomer feeding ratio on the polymerization. As shown in Table 6, an excess Table 6. Effect of Monomer Feeding Ratio on the Polymerization of 1a and 2a

Table 3. Solvent Effect on the Polymerization of 1a and 2a 1 2 3 4 5

time (h)

1 2 3 4 5b a

obtained when the polymerization was conducted with 20% equivalent of catalyst (Table 2, entry 3). Further increment of the catalyst loading may lead to a further improvement in the molecular weight but such possibility was not examined from the economical point of view. The effect of solvent on the polymerization is shown in Table 3. Results showed that PNPs with the highest molecular

entry

entry

entry

[1a] (M)

[2] (M)

yield (%)

Mw

Mw/Mn

1b 2 3 4

0.026 0.02 0.02 0.02

0.02 0.02 0.024 0.030

67.7 52.3 62.7 85.0

9300 9800 6200 3400

1.45 1.32 1.16 1.12

Polymerization at 70 °C under nitrogen for 24 h in the presence of 20% equiv of [RhCp*Cl2]2 and 4 equiv of Cu(OAc)2·H2O. [1a]:[2] = 0.026 M:0.02 M.

Polymerization at 90 °C in DMF under nitrogen for 3 h in the presence of 20% equiv of [RhCp*Cl2]2 and 4 equiv of Cu(OAc)2· H2O. [M] = 0.02 M. bData taken from Table 5, entry 3.

weights were generated in acetonitrile, while the highest polymer yield was obtained in DMF (Table 3, entries 4 and 5). In contrast, polymers with lower molecular weights are given in THF, toluene and o-xylene. Because of the satisfactory results obtained in DMF, it was selected as solvent for further investigation. Temperature also exerts a strong influence on the polymerization. The polymerization performed at 90 °C gives PNPs in a nearly quantitative yield (Table 4, entry 3). Lowering the temperature results in polymers with comparatively lower molecular weights (Table 4, entries 1 and 2), whereas raising the temperature to 110 °C does not help improve the polymer result. Clearly, the polymerization is likely to take place at 90 °C. A series of experiments tracking the time course on the polymerization are carried thereafter. At a monomer feeding

amount of either 1a or 2 will result in a decrease in the molecular weight, while polymers prepared from equimolar concentration of monomers enjoy a high molecular weight (Table 6, entry 2). This result suggests the requirement of stoichiometric balance for this polymerization. On the basis of the investigation above, we adopted optimized reaction conditions and explored the polymerization scope of a series of internal diynes. Monomers 1a−c are internal diynes with different alkyl chain lengths, and monomer 1d is a TPE-containing diyne. All the polymerization all proceed smoothly under the mentioned conditions. Notably, P1a−c/2 are all produced in excellent yields (74.8−96.4%) with high molecular weights ranging from 12 100 to 19 300 (Table 7, entries 1−3). However, the polymerization results are inferior in P1d/2 in both aspects (Table 7, entry 4). This may be due to the bulky and rigid structure of 1d, which imposes a

a

a

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Macromolecules Table 7. Polymerization of Different Monomersa entry

monomer

yield (%)

Mw

Mw/Mn

1b 2 3 4

1a/2 1b/2 1c/2 1d/2

74.8 91.4 96.4 50.5

15800 12100 19300 6700

1.51 1.39 1.46 1.49

Polymerization at 90 °C in DMF under nitrogen for 2 h in the presence of 20% equiv of [RhCp*Cl2]2 and 4 equiv of Cu(OAc)·H2O. [1] = [2] = 0.03 M. bData taken from Table 5, entry 2. a

steric effect for the propagation of the polymer chain. Despite its whole conjugated molecular structure, P1d/2 was well soluble in common organic solvents. Thus, the results given in Table 7 demonstrate the huge potential of this polymerization with a wide monomer scope. Model Reaction. To gain an insight into the structures of PNPs, we prepared a model compound from diphenylacetylene and benzoylacetonitrile under the same synthetic conditions for the polymers (Scheme 3). The obtained product 3 was

Figure 2. IR spectra of (A) 1a, (B) 2, and (C) P1a/2.

absorption band in monomer 2 at 1692 cm−1 was absent in the spectrum of P1a/2, denoting the complete consumption of the carbonyl group during the polymerization. The 1H NMR analysis provides a more detailed structural information. Figure 3 gives the 1H NMR spectra of monomers

Scheme 3. Synthesis of Model Compound 3

characterized by standard spectroscopic techniques with satisfactory results. The needle-shaped single crystals of 3 were obtained by slow evaporation of its methanol/dichloromethane (5:1 by volume) mixture at room temperature, which emitted green light under UV irradiation (Figure 1). Considering the apparent more intense emission at their both ends, the crystals may find potential application in optical waveguide.13

Figure 1. Micrographs of single crystals of 3 taken under (A) bright field and (B) UV irradiation. Excitation wavelength: 330−385 nm.

Figure 3. 1H NMR spectra of (A) 1a, (B) 2, (C) 3, and (D) P1a/2 in (A) CDCl3 and (B, C, and D) CD2Cl2. The solvent peaks were marked with asterisks.

Structural Characterization. All the polymers were fully characterized by IR and NMR spectroscopies and gave good results corresponding to their molecular structures (see the Experimental Section). Here we presented the IR and NMR spectra of P1a/2 as examples. Figure 2 shows the IR spectra of monomers 1a and 2 and their corresponding polymer P1a/2. Obviously, the absorption band for the methylene C−H stretching at 2922 cm−1 in the spectrum of P1a/2 was inherited majority from 1a. Likewise, the absorption band located at 2257 cm−1 in Figure 2B and 2C confirms the presence of CN group in both 2 and P1a/2. Meanwhile, the characteristic CO

1a and 2, model compound 3 and polymer P1a/2. The resonance peaks for the methylene protons of 1a at δ 4.00, 1.83, and 1.55 all remain but become broader in the spectrum of P1a/2 (Figure 3, parts A and D), proving the participation of 1a in the polymerization. More convincingly, the peaks for the aryl and methylene protons in benzoylacetonitrile 2 resonate at δ 7.92 and 4.15, respectively, which all disappear completely in the spectra of 3 and P1a/2. This shows the position of the 4244

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Macromolecules activated C(sp2)−H and C(sp3)−H bonds accordingly. Moreover, the spectrum of P1a/2 resembles that of 3 in profile, further confirming the occurrence of the polymerization and the expected polymer structure. Similar information was provided by the 13C NMR measurement (Figure S2). The IR and NMR data of P1a-d/2 are summarized in the Experimental Section and all solidly confirm their respective structures. Thermal Stability. Polymers with high thermal stability are anticipated to be good candidates as heat-resistant materials. Generally, polymers with conjugated or cross-linked structures tend to be thermally stable. The thermal stability of P1a−d/2 was evaluated by TGA analysis. Figure 4 shows the thermal

Figure 5. Wavelength-dependent refractive indices of thin films of P1a−d/2.

Table 8. Refractive Indices and Dispersion of Films of P1a− d/2a polymer

n632.8

νD

D

P1a/2 P1b/2 P1c/2 P1d/2

1.6433 1.6071 1.6381 1.6563

10.9520 24.4517 12.6863 6.8757

0.0913 0.0409 0.0788 0.1454

Abbreviation: n = refractive index, νD = Abbé number = (nD − 1)/(nF − nC), where nD, nF, and nC are the RI values at wavelengths of Fraunhofer D, F and C spectral lines of 589.2, 486.1, and 656.3 nm, respectively, D = 1/νD. a

6.8757−24.4517 and 0.0409−0.14, respectively. Evidently, the low optical dispersion of our polymers enriches greatly their application potentials. Photopatterning. P1d/2 is solid-state emissive and photosensitive. Such characteristics enable the facile fabrication of luminescent patterns at the nanoscale. We spin-coated the dichloroethane solution of P1d/2 on silica wafer. The formed polymer film was then irradiated with UV light in air through a copper mask, which generated a well-resolved fluorescent pattern (Figure 6). The exposed parts (lines) undergo photooxidation in the presence of UV irradiation. This has quenched their light emission and they thus appears black in color. The double bonds in P1a/2 may undergo photodecomposition and photochemical reaction with oxygen upon UV irradiation. This breaks the electronic conjugation and hence leads to emission quenching.19 The unexposed areas (squares), on the other

Figure 4. TGA thermograms of P1a−d/2 recorded under nitrogen at a heating rate of 10 °C/min.

degradation process of the four PNPs when being heated from 50 to 800 °C. The degradation temperature (Td), which is defined as the temperature for 5% weight loss, is in the range of 330−431 °C. These values are much higher than many commercial polymers, such as poly(methyl methacrylate) (∼255 °C), high density polyethylene (∼233 °C) and acrylonitrile butadiene styrene (∼167 °C).14 Refractivity. Polymers with high refractive indices (RI) are highly demanded for antireflective coating and photonic devices such as image sensors, optical lens and light emitting diodes.15 Basically, polymers with regular oriented backbone, large conjugation and high degree of polarization are supposed to possess high RI values.16 The large polarity of the skeletons of PNPs may bestow the polymers with considerable RI values. Thanks to their good film-forming property, uniform films of P1a−d/2 can be fabricated on silica wafers. Their RI values are measured and shown in Figure 5. In a wavelength region of 450−1600 nm, the RI values of P1a−d/2 are 1.7524−1.6083, 1.7182−1.5137, 1.7446−1.5927 and 1.7879−1.6251, respectively. Among all the polymers, P1d/2 possesses the highest RI values, which are in some sense anticipated due to its higher conjugation. The RI values of PNPs are comparable to many reported materials with real-world applications in the aspect of refractivity.17 The chromatic dispersion (D) is a vital factor in assessing the performance of an optical material. Polymer films with small D values are promising for imaging materials with high resolution. As shown in Table 8, the Abbe numbers (νD)18 and the corresponding D values of P1a−d/2 fall within the scope of

Figure 6. Photopatterns generated by photolithography of a film of P1d/2 through a copper mask taken under UV irradiation. Excitation wavelength: 330−385 nm. 4245

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Figure 7. (A) Absorption spectrum of P1d/2 in THF solution. Solution concentration: 20 μM. (B) Normalized PL spectra of P1d/2 in different states. Excitation wavelength: 410 nm. Inset: Photographs of (a) film and (b) powder of P1d/2 taken under UV irradiation.

Figure 8. (A) PL spectra of P1d/2 in THF/H2O mixtures with different water fractions (f w). Concentration: 20 μM; excitation wavelength: 435 nm. (B) Plot of relative PL intensity (I/I0) versus the composition of the THF/H2O mixture of P1d/2.

aggregated in the presence of a large amount of water. On the other hand, all the aqueous solutions are homogeneous with no precipitates even at 98% water content. Definitely, the emission quenching is associated with the aggregate formation. At 98% water content, the PL intensity was only 10% of that in pure THF solution (Figure 8B). The PL spectra of model compound 3 in different states provide a more clear vision into such aggregation-caused emission quenching effect (Figure S3A). Similarly, the maximum emission in the solution state (482 nm) was located at a shorter wavelength than that in film (492 nm) and powder (528 nm). Different from P1d/2, the aggregates suspended in methanol/H2O mixture of 3 are virtually nonemissive (Figure S3B). The molecules of 3 may form tightly packed excimers and/or exciplexes in the presence of a large amount of poor solvent. On the other hand, the steric effect posed by the twisted TPE units in P1d/2 may relieve the strong intermolecular interaction, thus enabling its suspended aggregates to show some sort of light emission.

hand, remain intact and emit light when irradiated. The discernible luminescent patterns at nanoscale proves such technique highly efficient and applicable in the construction of various electronic and photonic devices.20 Optical Property. More detailed study on the optical properties of P1d/2 was carried out. P1d/2 absorbs at 370 and 440 nm in dilute THF solution (Figure 7A). When excited at 410 nm, the polymer emits at 509 nm in the solution state, which shifts to 523 and 539 nm in the solid film and powder states, respectively (Figure 7B and the photos inside). Such bathochromic shift may result from the close packing of the naphthopyran units, which form excimers and/or exciplexes in the solid state that leads to red-shift in the emission.21 Figure 8 shows the PL spectra of P1d/2 in THF and THF/H2O mixtures, which demonstrates that except red-shift in the emission maximum, the emission is quenched in the aggregated state. When an increasing amount of water was added into the THF solution of P1d/2, the emission was declined gradually. Since P1d/2 is not soluble in water, its chains must have been 4246

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Figure 9. (A) PL spectra of P1d/2 powder before and after scratching. Excitation wavelength: 400 nm. (B) Photographs of powder of P1d/2 on filter paper taken under UV irradiation before and after scratching with a spatula. (C) Photographs of powder of P1d/2 on quartz taken under (a) daylight and (b) UV irradiation before and after scratching with a spatula.

Mechanochromism. The detection of mechanical stimuli in polymeric materials through optical variations has attracted much attention over the past few decades.22 These mechanical stimuli-responsive materials can be used as smart mechanosensors, luminescence switches, security, data storage media, etc.23 Generally, the common methods of fabricating mechanochromic polymers are physical dispersion of specific dyes into pristine polymers24 or covalent bonding of chromophoric units to macromolecules.25 Yet, dyes physically dispersed into polymer substrates may suffer from migrationcaused aggregation, and consequently lower the thermal stability and processability of the polymer. On the other hand, introduction of mechanophores to polymers via chemical bonds are strongly relied on existing chromophores,26 while the synthetic methods for the in situ generation of mechanochromic chromophores in the polymer backbone are still hardly attempted.27 In view of the optical properties of P1d/2, we explored the responsive behavior of the polymer toward diverse external stimuli. Interestingly, we discovered that P1d/2 exhibits a “turn-on” response to both shear force and organic vapor. As shown in Figure 9B, the P1d/2 powder placed on filter paper was barely emissive under UV illumination. However, after being scratched with a spatula, the emission become stronger, as proved by the PL analysis shown in Figure 9A. We believe that the “turn-on” phenomenon is closely associated with the alteration of the packing mode of the naphthopyran units in P1d/2. The strong intermolecular interaction is partially destroyed when a shear force such as scratching (Figure 9B) or writing (Figure 10) is applied. This destroys the excimer and/or exciplex formation, leading to the recovery of emission. Although such process is irreversible, the polymer can still be utilized as a sensitive disposable material for security or fluorescent indicator for mechanical defect in a variety of domains. Vapochromism. Volatile organic compounds (VOCs), especially the anthropogenic ones, can be harmful to longterm human health and the environment. Thus, detection of VOCs is of great importance in environmental, industrial and medical fields.28 Luminescent indicators can implement realtime monitoring of VOCs in a highly sensitive manner. Taking

Figure 10. Photographs of powder of P1d/2 on filter paper taken under UV irradiation (left) before and after writing with a spatula (middle) and THF fumigation (right).

advantage of the solid-state emission of P1d/2, we explored its vapor-induced emission characteristic and potential of being a fluorescent VOC indicator. As shown in Figure 10, after fumigation by THF for only 5 s, the fumed part of the filter paper (round circle in Figure 10) emits a bright green light with a high contrast to its background. The organic vapor may diffuse into the cavities of P1d/2. This swells the polymer and partially segregate the chromophoric units, making them less likely to form species that are detrimental to the light emission and thus leading to the observed emission change. The PL spectra of powder of P1d/2 before and after THF fumigation are shown in Figure 11. The emission maximum was initially located at 532 nm, which gradually shifted to 516 nm after solvent treatment. However, even though the fuming time was prolonged to 1 h, it was hard to achieve the solution value (507 nm). The interchain interaction of the polymer is so strong that the discrete polymer chain achieved in the solution state is hardly be realized by swelling. The plot of relative PL intensity (I/I0 − 1) versus the fuming time shows an upward bending curve, indicating that the emission is enhanced along with the fuming time (Figure 11B). Accounting for its good solubility in many organic solvents, P1d/2 can be adopted as a fast and sensitive detector for many types of organic solvents or VOCs (chloroform, dichloromethane, o-xylene, acetonitrile, etc.) for environmental monitoring or indoor respiratory safety inspection. 4247

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Macromolecules

Figure 11. (A) PL spectra of powder of P1d/2 before and after THF fumigation for different times. Excitation wavelength: 400 nm. (B) Plot of relative intensity (I/I0 − 1) versus the fumigation time. I0 = PL intensity before solvent fumigation. Inset: Photographs of powder of P1d/2 on quartz taken under UV irradiation (a) before and (b) after THF fumigation.



CONCLUSION In this paper, we succeeded in developing an efficient polymerization route to multisubstituted poly(naphthopyran)s by rhodium-catalyzed oxidative polyannulation of diyne and benzoyl acetonitrile. This polymerization route enjoys the advantages of atom-economy, high efficiency and wide monomer scope. Most of the synthesized PNPs show high RI values. P1d/2 is photosensitive and can generate a wellresolved fluorescent pattern by photolithography. Because of the strong intermolecular interaction between the naphthopyran units, the emission of ̀ P1d/2 changes in response to external stimuli such as shear force and organic vapor fumigation in the solid state. Coupled with their high thermal stability, the PNPs are potential to be used as heat-resistant, photopatterning or security materials as well as prompt detectors for VOCs. Further research will be conducted to explore the functionalities of the polymers by modifying their structure and potential real-world applications.



(16305014, 604913 and 602212), the Nissan Chemical Industries, Ltd. and the Innovation and Technology Commission (ITCPD117-9). B.Z.T. expresses thanks for the support of the Guangdong Innovative Research Team Program (201101C0105067115).



ASSOCIATED CONTENT

S Supporting Information *

HRMS spectrum of 3, 13C NMR spectrum of 1a, 2, and P1a/2, and PL spectra of 3. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00860.



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AUTHOR INFORMATION

Corresponding Author

*(B.Z.T.) E-mail: [email protected]. Telephone: +852-23587375. Fax: +852-2358-1594. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was partially supported by the National Basic Research Program of China (973 Program; 2013CB834701), the National Science Foundation of China (21490570 and 21490574), the University Grants Committee of Hong Kong (AoE/P-03/08), the Research Grants Council of Hong Kong 4248

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