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Smart Network Polymers with Bis(piperidyl)naphthalene CrossLinkers: Selective Fluorescence Quenching and Photodegradation in the Presence of Trichloromethyl-Containing Chloroalkanes Shunsuke Sasaki,† Yoshiyuki Sugita,† Masatoshi Tokita,† Tomoyoshi Suenobu,§ Osamu Ishitani,‡ and Gen-ichi Konishi*,† †

Department of Chemical Science and Engineering and ‡Department of Chemistry, Tokyo Institute of Technology, Tokyo 152-8552, Japan § Department of Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan S Supporting Information *

ABSTRACT: 1,4-Bis(piperidyl)naphthalene (1,4-BPN) was employed as a cross-linker for poly(n-butyl methacrylate) gels to generate soft materials that exhibit selective fluorescence quenching and photodegradation in 1,1,1-trichloromethylcontaining chloroalkanes. In chloroform and 1,1,1-trichloroethane, 1,4-BPN is subject to efficient fluorescence quenching via the formation of exciplexes, which results in rapid photodegradation. Similarly, poly(n-butyl methacrylate) gels cross-linked with 1,4-BPN were highly fluorescent and stable in various organic solvents and chloroalkanes but exhibited less intense fluorescence and rapid photodegradation in 1,1,1trichloromethyl-containing chloroalkanes. Upon swelling in 1,1,1-trichloroethane, these gels exhibited a steep drop in the storage modulus during first 10 s of photoirradiation (λ = 365 nm). Further photoirradiation for 10−30 min led to the decomposition of the gels. Thus, we propose that the 1,4-BPN-based cross-linker can be used for the facile preparation of diverse polymeric materials with synergistic fluorescence, detection, and photodecomposition properties toward unreactive and neutral analytes.

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INTRODUCTION Polymeric materials that exhibit quick decomposition in response to specific chemical species, irradiation, electric fields, or temperature are promising tools for plastic recycling,1−14 controlling chemical reactions in microfluidic systems,15,16 transient electronics,17 guided tissue generation,18 and drug delivery systems.19,20 Since irradiation can be controlled spatiotemporally and often induces quick decomposition, photodegradable gels/polymers have been the subject of extensive investigation.1,14,19−31 The adaptation of photodegradable gels/polymers in various applications requires multiple means of characterization, e.g., visualization of their location, the determination of the microenvironments within or around the material, and the presence of specific chemical species. Loading polymeric materials with dyes32 and the development of photolabile monomers and cross-linkers endowed with fluorescence properties33−35 represent approaches that could potentially fulfill these requirements. Recent reports from Zhu and co-workers37,38 have proposed the exploitation of photoinduced electron transfer (PET)36 between a photolabile fluorophore and a detector moiety as a means to realize “target-activated phototriggers” (fluorescence and photodecomposition properties manifest only in the © XXXX American Chemical Society

presence of specific chemical species). The synergistic effects between fluorescence, detection, and photodecomposition properties render target-activated phototriggers highly useful. However, their chemical structures are usually large and complicated and require laborious synthetic procedures, given that these systems consist of at least two building blocks, i.e., a detector moiety and a photolabile fluorophore. Moreover, many fluorescent sensors are driven by either complexation with ionic species36,39 or the conversion of functional groups by reactive analytes,40 which illustrates the difficulties associated with detecting neutral and/or unreactive molecules. To circumvent these obstacles, we have focused on exciplexmediated fluorescence quenching by chloroalkanesa phenomenon that is well-documented for benzene, naphthalene, and other aromatic hydrocarbons.41 The potential applicability of excited-state reactions toward photodegradable gels has been supported by reports on the photodegradation of dyes in the presence of chloroalkanes.42−46 We have recently reported that the HOMO level of an aromatic hydrocarbon rises drastically Received: January 31, 2017 Revised: April 11, 2017

A

DOI: 10.1021/acs.macromol.7b00213 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of Photodegradable (1,4-BHPN-bisMA)n-(PBMA)m-Gel and Model Polymer (1,4-BHPN-MA)n-(PBMA)m

monomer feed ratios x (BMA):y (1,4-BHPN-MA or 1,4BHPN-bisMA) of 333:1, 167:1, and 111:1.

during excited-state relaxation upon introduction of dialkylamino groups at its para-position.47 The enhanced HOMO levels of para-substituted bis(N,N-dialkylamino)arenes suggest that chloroalkane−fluorophore exciplexes are stable. Furthermore, we have recently reported the strong solution- and solidstate fluorescence of 1,4-bis(N,N-dimethylamino)naphthalene.48 This study examines the performance of 1,4-bis(piperidyl)naphthalene (1,4-BPN) as a photodegradable cross-linker that combines fluorescence and detection functionalities in structural simplicity. As a result, a poly(n-butyl methacrylate) gel cross-linked by 1,4-BPN revealed strong fluorescence in addition to selective fluorescence quenching and photodegradation in 1,1,1-trichloromethyl-containing chloroalkanes. Herein, we propose a structurally simple cross-linker based on 1,4-BPN as a practical approach to the generation of soft materials that can detect and respond to the presence of 1,1,1trichloromethyl compounds, i.e., neutral and unreactive species that are known environmental pollutants.49−51

Chart 1. Chemical Structures of 1,4-BPN, 1,5-BPN, and 2,6BPN

The weight-average molecular weight (Mw = 120 000− 156 000 g mol−1) and polydispersity (Mw/Mn = 1.8−2.1) of the copolymers (1,4-BHPN-MA)n-(PBMA)m were determined by size exclusion chromatography (SEC, Figures S30−S32), and the results are summarized in Table 1. The copolymer composition m:n (see Scheme 1) was calculated from UV− vis spectra based on the molar absorption coefficient of 1,4BPN (ε = 7140 M−1 cm−1 at 332 nm). The m:n ratios based on the UV−vis spectra are consistent with the corresponding monomer feed ratios x:y. However, the copolymer composition m:n derived from the 1H NMR spectra (Table 1) differs largely from the monomer feed ratios x:y, probably due to the poor S/ N ratio (Figures S12−S14). In order to employ 1H NMR spectroscopy for the quantitative analysis of trace components, these must possess a functional group with an intense singlet signal at a spectral region that is usually NMR silent, e.g., that of the trimethylsilyl group.53 As described in Scheme 1, (1,4-BHPN-bisMA)n-(PBMA)mgel was cross-linked by copolymerization of n-butyl meth-

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthesis of the photodegradable gel (1,4-BHPN-bisMA)n-(PBMA)m-gel and its model copolymer (1,4-BHPN-MA)n-(PBMA)m is shown in Scheme 1. Although palladium-catalyzed C−N coupling reactions with alcoholamines are usually not successful, Buchwald and co-workers reported that 4-hydroxypiperidine as an exception that is susceptible to this coupling reaction.52 Thus, piperidyl groups were used as linkers between the fluorophore and the poly(n-butyl methacrylate) moiety. Treating methacryloyl chloride with 2 in different ratios afforded 1,4-BHPN-MA and 1,4-BHPN-bisMA in 34% and 60% yield, respectively. Finally, 1,4-BHPN-MA and 1,4-BHPNbisMA were copolymerized with n-butyl methacrylate at B


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Macromolecules Table 1. Characterization of Copolymers (1,4-BHPN-MA)n-(PBMA)m x:ya

m:nb (1H NMR)

m:nc (UV−vis)

Mw [g mol−1] (t = 0 s)d,e

Mw/Mn (t = 0 s)d,e

Mw [g mol−1] (t = 600 s)d,f

Mw/Mn (t = 600 s)d,f

333:1 167:1 111:1

345:1 202:1 123:1

308:1 161:1 101:1

120000 130000 156000

1.77 1.95 2.06

93900 71400 80900

1.93 2.00 1.89

a

Monomer feed ratio. bCopolymer composition calculated based on 1H NMR spectra. cCopolymer composition based on UV−vis spectra. Molecular weight and PDI determined by SEC in CHCl3 based on calibrations with polystyrene standards. eData taken before irradiation with an LED lamp (λ = 365 nm). fData recorded after irradiation with an LED lamp (λ = 365 nm) for t = 600 s. d

Table 2. Swelling Ratio (Rw) and Mass per Volume of Solvent (c in g mL−1 of Solvent) of (1,4-BHPN-BisMA)n(PBMA)m-Gel (Monomer Feed Ratio x:y = 333:1, 167:1, and 111:1) Immersed in Various Solvents x:y = 333:1

x:y = 167:1

Table 3. Fluorescence Maxima (λfl) and Quantum Yields (Φfl) of (1,4-BHPN-BisMA)n-(PBMA)m-Gel (Monomer Feed Ratios x:y = 333:1, 167:1, and 111:1) Swollen in Different Solventsa

x:y = 111:1

x:y = 333:1

x:y = 167:1

x:y = 111:1

solvent

Rwa

c

Rwa

c

Rwa

c

solvent

λflb

Φfl

λflb

Φfl

λflb

Φfl

THF toluene CHCl3 1,1,1-trichloroethane

14.8 16.5 23.1 24.0

0.060 0.053 0.064 0.055

7.9 10.0 14.1 13.0

0.11 0.087 0.11 0.10

5.8 6.5 10.6 9.5

0.15 0.13 0.14 0.14

hexane toluene THF ethyl acetate acetone acetonitrile ethanol dichloromethane chloroform tetrachloromethane

428 438 445 444 456 441 441 454

0.67 0.61 0.61 0.58 0.57 0.73 0.72 0.60 0.01 ≤0.01

430 435 448 444 446 450 449 460

0.58 0.54 0.49 0.48 0.45 0.45 0.39 0.51 0.02 ≤0.01

433 442 444 441 451 444 448 451

0.26 0.33 0.24 0.14 0.23 0.15 0.15 0.20 ≤0.01 ≤0.01

Rw = (Ws − Wd)/Wd, where Ws and Wd refer to the weight of the swollen and dried gel, respectively.

a

acrylate (BMA) with 1,4-BHPN-bisMA, an analogue of 1,4BPN functionalized with two methacryloyl groups. Table 2 summarizes the mass swelling ratio (Rw) and mass per volume of solvent c (in g mL−1 of solvent) for (1,4-BHPN-bisMA)n(PBMA)m-gel immersed for 24 h in THF, toluene, chloroform, or 1,1,1-trichloroethane. The mass swelling ratio Rw also increased with the monomer feed ratio of the BMA unit (x) relative to the 1,4-BHPN-bisMA unit (y). This result confirms that the 1,4-BHPN-bisMA molecules act as cross-linkers in the (1,4-BHPN-bisMA)n-(PBMA)m-gel in accordance with the monomer feed ratio x:y. As model dyes for a detailed photophysical characterization, 1,4-bis(piperidyl)naphthalene (1,4-BPN), 1,5-bis(piperidyl)naphthalene (1,5-BPN), and 2,6-bis(piperidyl)naphthalene (2,6-BPN) were prepared from dibromonaphthalenes with the appropriate regioisomerism using the Buchwald−Hartwig coupling reaction.54,55 1,4-BPN, 1,5-BPN, 2,6-BPN, 1,4-BHPNMA, and 1,4-BHPN-bisMA were characterized by 1H NMR, 13 C NMR, and FT-IR spectroscopy as well as high-resolution mass spectrometry. In addition, melting points were determined for the thermally stable compounds, i.e., 1,4-BPN, 1,5-BPN, and 2,6-BPN. Fluorescence Properties of (1,4-BHPN-BisMA) n (PBMA)m-Gel. Table 3 lists the fluorescence maxima and quantum yields of (1,4-BHPN-bisMA)n-(PBMA)m-gel swollen in common organic solvents. Sparsely cross-linked (1,4-BHPNbisMA)n-(PBMA)m-gel (x:y = 333:1) is highly fluorescent (Φfl ≈ 0.6−0.7), but its fluorescence decreases with the increasing cross-linker density. Intriguingly, swelling in dichloromethane led to similar or even higher fluorescence quantum yields than in other organic solvents, while gels swollen in chloroform are nonfluorescent. The reasons for the lack of a significant difference in the Φfl values between the dichloromethane solution and other solutions are discussed in a later section (vide inf ra). Figure 1 illustrates the fluorescence and quantum yields (Φfl) of (1,4-BHPN-bisMA)n-(PBMA)m-gel (x:y = 333:1) swollen in various chloroalkanes. While (1,4-BHPN-bisMA)n-(PBMA)mgel samples swollen in 1,2-dichloroethane, 1,1,2-trichloro-

Gels immersed for 24 h in the solvent. bMaximum of the fluorescence spectrum measured in a Quantaurus-QY Absolute PL quantum yields measurement system C11347-11. a

Figure 1. Photographic images of (1,4-BHPN-bisMA)n-(PBMA)m-gel (monomer feed ratio x:y = 333:1) swollen in various chloroalkanes under irradiation from an LED lamp (λ = 365 nm; 0.51 mW cm−1). The fluorescence quantum yields (Φfl) of the swollen gels are also shown.

ethane, 1,2-dichloropropane, 1,3-dichloropropane, or 1,2,3trichloropropane exhibited moderate to strong fluorescence (Φfl ≈ 0.2−0.7), the fluorescence of (1,4-BHPN-bisMA)n(PBMA)m-gel was almost inexistent in chloroform, tetrachloromethane, and 1,1,1-trichloroethane. As will be discussed later, the (1,4-BHPN-bisMA)n-(PBMA)m-gel functionality that distinguishes 1,1,1-trichloromethyl-containing chloroalkanes from other chloroalkanes is more pronounced than that of model dye 1,4-BPN. This feature demonstrates the efficacy of (1,4-BHPNbisMA)n-(PBMA)m-gel as a soft material that detects with unreactive and uncharged analytes. Photophysical Properties of 1,4-BPN, 1,5-BPN, and 2,6-BPN. To understand the fluorescence properties of (1,4C


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Table 4. Absorption Maxima (λabs), Fluorescence Maxima (λfl), Absolute Fluorescence Quantum Yields (Φfl), Fluorescence Lifetimes (τfl), and Radiative (kr) and Nonradiative Transition Rate Constants (knr) of 1,4-BPN, 1,5-BPN, and 2,6-BPN Measured in Various Solvents entry

solvent

λabs [nm]

λfla [nm]

Φfl

1,4-BPN

THF dichloromethane chloroform THF dichloromethane

332 335 335 326 329

458 478 418 395 403

0.83 0.57 ≤0.01 0.18 0.21

chloroform

331

448

0.13

THF dichloromethane chloroform

364 367 364

448 460 424

0.82 0.48 0.04

1,5-BPN

2,6-BPN

τflb [ns] 10.0 8.82 1.40 1.40 1.94 1.88 7.63 15.0 11.7

kr [107 s−1]

knr [107 s−1]

8.3 6.4

1.7 4.9

13

58

(0.51) (0.49) (0.02) (0.98) 5.5 4.1

1.2 4.4

a

The excitation wavelengths correspond to their absorption maxima. bValues in parentheses indicate the relative contribution of each lifetime component.

Figure 2. Calculated HOMO and LUMO energy levels for the equilibrium structures of the ground (S0) and excited state (S1) as well as the corresponding wavelengths (λex) and oscillator strength (f) for the S0−S1 transitions. In addition, the Kohn−Sham orbitals for the HOMO of each structure are shown. All calculations were carried out at the ωB97X-D/6-311G(d,p) level of theory.

(for details, see section S1) exhibited strong fluorescence (Φfl = 0.85). While 2,6-BPN also exhibited efficient fluorescence in common organic solvents, low fluorescence quantum yields were observed for 1,5-BPN, a feature that has also been reported for 1,5- and 1,8-diaminonaphthalenes.59 These Φfl differences were ascribed to the 10-fold higher nonradiative transition rate constant (knr) of 1,5-BPN relative to those of 1,4-BPN and 2,6-BPN. 1-Naphthylamine and its alkylated derivatives undergo fast internal conversion due to their small S1−S2 gap,60,61 and an electron-donating group needs to be introduced at the 4-position of the naphthalene chromophore in order to achieve strong fluorescence.62 Our computational calculations suggested that 1,5-BPN follows the mechanism; the absence of para-substituted N,N-dialkylamino groups keeps the HOMO−LUMO gap (Figure 2) widely split, which results in a relatively narrow S1−S2 gap (Table S12, section S6-3). Similar to the bis(piperidyl)anthracene systems of our previous study,47 1,4-BPN exhibits much larger Stokes shifts than 1,5-BPN and 2,6-BPN (Table 4). Considering the intercepts of the Lippert−Mataga plots64,65 (Figure S16) for 1,4-BPN (7800 cm−1), 1,5-BPN (4900 cm−1), and 2,6-BPN (4900 cm−1), the prominent Stokes shifts of 1,4-BPN can be explained by intramolecular processes rather than by solvent−

BHPN-bisMA)n-(PBMA)m-gel, we examined the effect of the regioisomerism on the photophysical properties of bis(piperidyl)naphthalenes (BPNs). Following our previous studies, we expected 1,4-BPN to be highly fluorescent48 and to exhibit substantially larger Stokes shifts compared to the other regioisomers due to the drastic enhancement of its HOMO level during the excited-state relaxation process.47 The UV−vis and fluorescence spectra as well as the fluorescence quantum yields and lifetimes of the model dyes 1,4-BPN, 1,5BPN, and 2,6-BPN were measured in nine common organic solvents (for details, see Table S1 and Figure S15). Table 4 shows the representative examples measured in THF, dichloromethane, and chloroform. For further insight into these photophysical properties, DFT and TD-DFT calculations were performed at the ωB97X-D/6-311G(d,p) level of theory.81 Figure 2 summarizes the HOMO and LUMO energy levels, S0 → S1 transition wavelengths, and the oscillator strength calculated for the optimized structures of S0 and S1 (for details, see section S6). As expected,48 1,4-BPN displayed excellent fluorescence quantum yields in all common organic solvents except for dichloromethane and chloroform (Table 4). Also, a colloidal suspension of 1,4-BPN dispersed in THF:water = 1:9 (v/v) D


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to be dynamic, i.e., to occur through collisions between the excited-state fluorophore and the ground-state chloroalkane.66 In other words, fluorescence quenching of 1,4-BPN is mediated by the formation of exciplexes with chloroalkanes. Except for a few examples, such as the quenching of 1-naphthol by tetrachloromethane in polar solvents,67 fluorescence quenching between a fluorophore and a chloroalkane occurs by dynamic quenching via the formation of exciplexes.41,68−70 The quenching rate constants (kq) were thus calculated under the assumption of a dynamic quenching. Figure 3 shows that kq for the 1,1,1-trichloromethyl-containing chloroalkanes (chloroform and 1,1,1-trichloroethane) is of the same order of magnitude as the diffusion-controlled rate constant (kdiff; toluene, 25 °C).63 In comparison, the kq values of other chloroalkanes are several orders of magnitude smaller than those of chloroform and 1,1,1-trichloroethane. The large differences in kq are ascribed to the proportional relationship of log kq with the electron affinity41,69 and the reduction potential70 of chloroalkanes. 1,4-BPN exhibited clear differences in its fluorescence quantum yields even when chloroalkanes were used as solvents (Table S2). While 1,4-BPN exhibited low fluorescence (Φfl ≤ 0.01) in chloroform and 1,1,1-trichloroethane, solutions in other chloroalkanes presented higher fluorescence quantum yields (Φfl = 0.08−0.57). It is noteworthy that the Φfl of (1,4BHPN-bisMA)n-(PBMA)m-gel (Figure 1) is approximately double as much as that of 1,4-BPN in 1,2-dichloroethane, 1,1,2-trichloroethane, 1,3-dichloropropane, and 1,2,3-trichloropropane. Because of the enhancement of its Φfl values in these chloroalkanes, (1,4-BHPN-bisMA)n-(PBMA)m-gel presents a more pronounced contrast in its Φfl values between 1,1,1trichloromethyl-containing chloroalkanes and the others solvents. At this stage, we have not yet been able to elucidate the reasons behind this enhancement, which will be the subject of future mechanistic studies. Fluorescence Properties of (1,4-BHPN-MA)n-(PBMA)m. In order to understand the particular fluorescence behavior of (1,4-BHPN-bisMA)n-(PBMA)m-gel that was not observed for 1,4-BPN, the fluorescence properties of (1,4-BHPN-MA)n(PBMA)m were examined, and the results are summarized in Table 5. The fluorescence quantum yield of (1,4-BHPNbisMA)n-(PBMA)m-gel decreased with increasing cross-linker concentration (Table 3). A possible reason for this result could be the decomposition of 1,4-BPN moieties during the freeradical polymerization reaction. However, the copolymers (1,4BHPN-MA)n-(PBMA)m prepared under identical conditions displayed no variation in their fluorescence quantum yields (Table 5), shape of their UV−vis spectra, or shape of their fluorescence spectra (Figures S19−S21) as a function of the copolymer composition, m:n (Table 1). Consequently, a potential degradation of the fluorophore during the polymerization process can be discarded; instead, the suppressed fluorescence of the densely cross-linked (1,4-BHPN-bisMA)n(PBMA)m-gel should be attributed to intrinsic properties of the gel. It should also be noted that the Φfl values of (1,4-BHPNbisMA)n-(PBMA)m-gel do not significantly differ in dichloromethane or other solvents (Table 3), in contrast to the behavior of 1,4-BPN (Table 4). One reason for the absence of quenching effects in dichloromethane could be the presence of oxygen. The Φfl (deaerated) and Φfl (not deaerated) values in Table 5 denote the fluorescence quantum yields of (1,4-BHPNMA)n-(PBMA)m solutions after or without argon sparging, respectively. The deaerated THF solutions exhibited signifi-

solute interactions. On the basis of our computational calculations, the Stokes shifts of 1,4-BPN should be undoubtedly attributed to a drastic increase of the HOMO level (Figure 2). According to our previous study,47 the enhancement stems from the electronic repulsion between confronting donors. Our computational calculations also suggest that 2,6-BPN features a high HOMO level in both the S0 and S1 minima (Figure 2), which is corroborated by the bathochromic shifts of the absorption and fluorescence maxima (Table 4 and Table S1). The high HOMO level of 2,6-BPN can be explained by the 1Lb state, which is originally the lowestlying excited state (S1).57 Though the 1Lb−S0 transition is an optically forbidden in unsubstituted naphthalenes,58 2,6substituents induce symmetry-allowed mixing between the charge-transfer (CT) and 1Lb states,56 providing the transition with a substantial oscillator strength. In summary, 1,4-BPN and 2,6-BPN feature strong fluorescence and high HOMO levels at the S1 minima, whereas 1,5-BPN exhibits weaker fluorescence and a low HOMO level. Thus, 1,4-BPN and 2,6-BPN are more likely to form stable exciplexes with chloroalkanes. In accordance with these characteristics, 1,4-BPN and 2,6-BPN exhibit drastic quenching in chloroform, while the fluorescence quantum yield of 1,5BPN is not significantly affected by the nature of the solvent (Table 4). The quenching rate caused by trace amounts of chloroform in toluene was, however, barely affected by the regioisomerism of the piperidyl groups. The quenching rate constants of the BPNs derived from Stern−Volmer plots (Figure S18 and Table S3; kq = 4.5 × 109 to 11 × 109 M−1 s−1) are comparable to the diffusion-controlled rate constant in toluene (kdiff = 11 × 109 M−1 s−1 at 25 °C).63 As the quenching rate constants with chloroalkanes are closely related not only to the HOMO level of the fluorophore and but also to its S0−S1 excitation energy,41 the high HOMO levels of 1,4-BPN and 2,6-BPN should be counterbalanced by their narrow HOMO−LUMO gaps. Thus, the fluorescence sensitivity of BPNs toward trace amounts of chloroform should depend on their intrinsic fluorescence intensity rather than on their HOMO levels. Fluorescence Quenching of 1,4-BPN by Chloroalkanes. The ability of (1,4-BHPN-bisMA)n-(PBMA)m-gel to discriminate 1,1,1-trichlotomethyl-containing chloroalkanes from other chloroalkanes arises from the intrinsic properties of the 1,4-BPN moiety. Figure 3 shows the Stern−Volmer plots obtained from the addition of chloroalkanes to a toluene solution of 1,4-BPN. The plots show good linearity, which is reflected in a determination coefficient close to unity (R2 > 0.99). Thus, the mechanism of fluorescence quenching seems

Figure 3. Stern−Volmer plot of a toluene solution of 1,4-BPN quenched by various chloroalkanes. The kq [M−1 s−1] values represent the fluorescence-quenching rate constants derived from these plots. E


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Table 5. Absorption Maxima (λabs), Fluorescence Maxima (λfl), Absolute Fluorescence Quantum Yields (Φfl), and Fluorescence Lifetimes (τfl) of Copolymer (1,4-BHPN-MA)n-(PBMA)m Measured in Various Solvents x:y

solvent

λabs [nm]

λfla [nm]

Φfl (deaerated)

Φfl (not deaerated)

τfl [ns]

333:1

THF dichloromethane chloroform THF dichloromethane chloroform THF dichloromethane chloroform

331 332 332 332 331 333 331 331 332

459 460 412 459 460 409 457 460 409

0.81 0.69 ≤0.01 0.83 0.72 ≤0.01 0.83 0.67 ≤0.01

0.59 0.57 ≤0.01 0.61 0.52 ≤0.01 0.59 0.46 ≤0.01

10.4 10.0

167:1

111:1

a

10.4 9.89 10.3 9.71

Excitation wavelengths correspond to the absorption maxima.

cantly higher Φfl values than the dichloromethane solutions. Without deaeration, the fluorescence quantum yields of the THF solutions decreased significantly, while those of the dichloromethane solutions were less affected by oxygen. These results indicate that the relatively large quenching rate constant of oxygen governs the Φfl values both in dichloromethane and other organic solvents, rather than the small quenching rate constant by dichloromethane. The same reasoning is applicable to (1,4-BHPN-bisMA)n-(PBMA)m-gel swollen in other chloroalkanes. Nevertheless, the reason why the Φfl values of (1,4BHPN-bisMA)n-(PBMA)m-gel swollen in several chloroalkanes such as 1,2-dichloroethane are double that of 1,4-BPN cannot be explained solely by the presence of oxygen. Selective Photodegradation of (1,4-BHPN-bisMA)n(PBMA)m-Gel. In addition to the fluorescence quenching, a selective photodegradation behavior was observed for (1,4BHPN-bisMA)n-(PBMA)m-gel. Figure 4 depicts the photo-

gel (monomer feed ratio x:y = 333:1) swollen in chloroform was entirely illuminated with the LED lamp and completely immersed in chloroform with agitation during irradiation, its photodegradation was completed within 10 min, and it dissolved in chloroform as a polymer. The residual polymers formed by the photodecomposition of (1,4-BHPN-bisMA)n-(PBMA)m-gel (monomer feed ratio x:y = 333:1) in chloroform were collected through reprecipitation from methanol. Figure 5 shows the 1H NMR spectrum of the collected polymer (black line) in comparison with that of poly(n-butyl methacrylate) synthesized under the same conditions as (1,4-BHPN-bisMA)n-(PBMA)m-gel, except for the absence of the cross-linker (red line). Figure 5 demonstrates that at least at an 1H NMR resolution of 300 MHz the chemical structure of poly(n-butyl methacrylate) was completely preserved in the collected polymer, even after the photodecomposition of the (1,4-BHPN-bisMA)n-(PBMA)mgel. The collected polymer was characterized by SEC (Mw = 108 000 g mol−1; PDI = 2.14), and the obtained values were comparable to those of the model copolymer, (1,4-BHPNMA)n-(PBMA)m (Table 1). Thus, the photodegradation of (1,4-BHPN-bisMA)n-(PBMA)m-gel proceeds mainly through de-cross-linking (Figure 5), rather than main-chain scission. For a further quantitative analysis, the storage (G′) and loss (G″) moduli were determined with a rheometer (for experimental details see section S1.3). The detailed results are summarized in the Supporting Information (Figures S33 and S34). In these experiments, nonvolatile organic solvents, i.e., toluene and 1,1,1-trichloroethane, were employed as swelling solvents to avoid fluctuation of the solute concentration. The G′ values of (1,4-BHPN-bisMA)n-(PBMA)m-gel (monomer feed ratio x:y = 333:1) at a frequency of 10 Hz are plotted against the irradiation time with a UV lamp (λ = 365 nm) in Figure 6. (1,4-BHPN-bisMA)n-(PBMA)m-gel swollen in 1,1,1-trichloroethane displayed a steep drop in G′ during the first 10 s of irradiation, while that of the gel swollen in toluene decreased slowly but maintained a significant elasticity even after 60 s of irradiation. The contrasting behavior of G′ in response to UV irradiation time confirms that the photodegradation of (1,4-BHPN-bisMA)n-(PBMA)m-gel occurs preferentially after swelling in 1,1,1-trichloroethane. As shown later, the steep drop in G′ of (1,4-BHPN-bisMA)n(PBMA)m-gel upon photoirradiation proceeds on a similar time scale (∼10 s) with the photodegradation of 1,4-BPN (Figure 7 and Figure S26). When a 1,1,1-trichloromethyl-containing chloroalkane is employed as the solvent, both the photodegradation of 1,4-BPN and the drop in the G′ of (1,4-BHPNbisMA)n-(PBMA)m-gel were observed within the first 10−20 s

Figure 4. Photographic images of the gradual decomposition of chloroform-swollen (1,4-BHPN-bisMA)n-(PBMA)m-gel (monomer feed ratio x:y = 333:1) upon irradiation with UV light (λ = 365 nm). To illustrate the loss of elasticity of the gel, constant stress was applied by a weight (18 g).

degradation of (1,4-BHPN-bisMA)n-(PBMA)m-gel (monomer feed ratio x:y = 333:1) swollen in chloroform. Irradiation at 365 nm gradually decreased the elasticity of (1,4-BHPN-bisMA)n(PBMA)m-gel and initiated its deformation. Finally, (1,4BHPN-bisMA)n-(PBMA)m-gel became unable to withstand the stress exerted by the weight, resulting in its collapse. Similar photodegradation was also observed for the gel swollen in 1,1,1-trichloroethane. In contrast, the gels swollen in THF and toluene did not degrade, even after 30 min of irradiation. The irradiation time required to decompose (1,4-BHPN-bisMA)n(PBMA)m-gel varied also with sample size, extent of immersion (i.e., complete, partial, or not immersed in the solvent), and agitation of the gel. When the (1,4-BHPN-bisMA)n-(PBMA)mF


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Figure 5. 1H NMR spectra (300 MHz, CDCl3) of poly(n-butyl methacrylate) synthesized under identical conditions as (1,4-BHPN-bisMA)n(PBMA)m-gel, except for the absence of a cross-linker (top, red line), and of the mixture obtained from the photodecomposition (see inset scheme) of chloroform-immersed (1,4-BHPN-bisMA)n-(PBMA)m-gel (monomer feed ratio x:y = 333:1) (bottom, black line).

weight-averaged molecular weight (Mw) and the root-meansquare radius of gyration ⟨S2⟩01/2 according to76 c* =

3M w 1/2 3

4πNA(⟨S2⟩0

)

(1)

where NA is the Avogadro number. According to the literature,77 poly(n-butyl methacrylate) in DMF exhibits a value of ⟨S2⟩01/2/Mw1/2 = 3.09 × 10−9 cm mol1/2 g−1/2 at 23.6 °C. Using Mw = 108 000 g mol−1 for the polymer collected from the photodecomposition residue of (1,4-BHPN-bisMA)n(PBMA)m-gel, an overlap concentration of c* = 0.041 g mL−1 is obtained. If the density of the DMF solution at the overlap concentration is approximated to the density of pure DMF (0.944 g mL−1), the overlap concentration is obtained as c* ≈ 0.043 g mL−1 of DMF. Comparing this value with the mass per volume of solvent for the swollen (1,4-BHPN-bisMA)n(PBMA)m-gel listed in Table 2, one could speculate that the polymer solution immediately after photodegradation is as concentrated as the overlap of poly(n-butyl methacrylate) chains is possible. Although the above calculation is extremely crude, it is possible that some overlap and entanglement of the polymer chains are involved in the time delay between de-crosslinking and the complete degradation of (1,4-BHPN-bisMA)n(PBMA)m-gel. Effect of Photoirradiation on (1,4-BHPN-MA) n (PBMA)m. As shown in Figure 5, the chemical structure of poly(n-butyl methacrylate) was completely preserved in the collected polymer after photodecomposition of the 1,4-BHPNPBMA-gel. However, as shown in Table 1, the molecular weight of (1,4-BHPN-MA)n-(PBMA)m decreased after 10 min of irradiation at λ = 365 nm (for the corresponding SEC chromatograms, see Figures S30−S32). This result implies that photoinduced de-cross-linking of (1,4-BHPN-bisMA) n (PBMA)m-gel is accompanied by side reactions that are difficult to detect by 1H NMR spectroscopy. The photodegradation of aromatic hydrocarbons in chloroalkanes is known to involve various radical species.42−46 Since main-chain scission of

Figure 6. Storage moduli (G′) of (1,4-BHPN-bisMA)n-(PBMA)m-gel (monomer feed ratio x:y = 333:1) swollen in toluene and 1,1,1trichloroethane, plotted as a function of the irradiation time with a UV lamp (λ = 365 nm) (for experimental details see section S1).

of irradiation. This result implies that de-cross-linking of the (1,4-BHPN-bisMA)n-(PBMA)m-gel is completed within 10−20 s. Nevertheless, complete degradation of the (1,4-BHPNbisMA)n-(PBMA)m-gel requires 10−30 min of UV irradiation (Figure 4). Additionally, the G′ of (1,4-BHPN-bisMA)n(PBMA)m-gel swollen by 1,1,1-trichloroethane remained constant at ∼22 Pa, even after photodegradation (Figure 6). Moreover, the shear modulus of each decomposed gel did in general not depend on the frequency of the oscillatory shear (Figure S33). Though further detailed studies are necessary to reveal the detailed mechanism of the gel degradation, the remaining elasticity of the decomposed (1,4-BHPN-bisMA)n(PBMA)m-gel indicates overlap and entanglement of the polymer chains in concentrated solutions. As a rough estimation, the mass per volume of solvent, c (in g mL−1 of solvent), of swollen (1,4-BHPN-bisMA)n-(PBMA)m-gel and the overlap concentration c* (in g mL−1) of poly(n-butyl methacrylate) were compared. c* can be derived from the G


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Figure 7. Time-dependent evolution of the UV−vis spectra of 1,4-BPN in (a) chloroform and (b) THF irradiated with UV light (λ = 365 nm) for t seconds. (c, d) Logarithmic plots of the total absorbance A(t) as a function of the irradiation time t at the specified wavelengths (see legend). Fitting to eq 4 is indicated with solid lines (for details see section S1.2).

isosbestic points during the first 12−40 s of irradiation at λ = 365 nm, followed by a transition to two other isosbestic points (Figure S23). In contrast, the depletion of the longer wavelength band of 1,5-BPN does not coincide with the intensification of a shorter wavelength band (Figure S24). 2,6BPN does not exhibit any absorption band arising during irradiation (Figure S25). The irregular evolution of these UV− vis spectra thus renders a comparison of the photodegradation reaction rates between 1,5-BPN and 2,6-BPN difficult. The photodegradation of 1,4-BPN involves several isosbestic points, implying that its reaction mechanism is complicated and generates various products. Therefore, it is difficult to characterize its kinetics accurately. Since it would be interesting to roughly compare the rates of photodecomposition in different solvent systems, we have herein assumed the following simple pseudo-first-order reaction for convenience:

methacrylate polymers is prompted by radical species,78 it seems inevitable that the average molecular weight of (1,4BHPN-MA)n-(PBMA)m decreases during the irradiation process. Furthermore, it has been reported that dyes (e.g., βcatotene)79 and triplet sensitizers (e.g., thioxanthone)80 are able to initiate the main-chain scission of methacrylate polymers. As long alkyl side chains accelerate main-chain scission,80 the use of different monomers such as methyl methacrylate should suppress any reduction in the molecular weight. In any case, main-chain scission represents merely a minor contribution to the photodegradation of (1,4-BHPN-bisMA)n-(PBMA)m-gel; the main contribution arises from the de-cross-linking of the 1,4-BPN moieties. This notion is also supported by the large Mw of the polymer collected after photodecomposition of the (1,4-BHPN-bisMA)n-(PBMA)m-gel. Photodegradation of Model Dyes. To assess the mechanism and selectivity of the photodegradation of (1,4BHPN-bisMA)n-(PBMA)m-gel, the photodegradation behavior of the model dyes 1,4-BPN, 1.5-BPN, and 2,6-BPN was examined. Figure 7 shows the temporal evolution of UV−vis spectra of 1,4-BPN obtained in THF and chloroform upon irradiation at λ = 365 nm (for details see section S1.2). The evolution of the UV−vis spectra is much faster in chloroform (Figure 7a) than in THF (Figure 7b). Similarly, fast photodegradation is observed for 1,5-BPN (Figure S27) and 2,6-BPN (Figure S28) in chloroform. The depletion of the S0 → 1CT absorption band at λ = ∼335 nm in 1,4-BPN coincides with the appearance of an absorption band at λ = ∼291 nm. 1,4-BPN in toluene presents isosbestic points (Figure 7b), which suggest a one-to-one relationship between the depletion at λ = 335 nm and the rise at λ = 291 nm. Under lower light intensity (0.51 mW cm−1), 1,4-BPN in chloroform exhibits two

hν

1,4‐BPN + solvent (excess) → products

(2)

Based on this assumption, the temporal evolution of the UV− vis spectra can be formulated, and the following equation describes the first-order photoreaction of the photolabile protecting group:71 n

∑i = 1 εi th product(λ) A (λ , t ) = A(λ , t = 0) ε1,4‐BPN(λ) n ⎛ ∑i = 1 εi th product(λ) ⎞ ⎟⎟ exp( −kt ) + ⎜⎜1 − ε1,4‐BPN(λ) ⎠ ⎝

(3)

wherein A(λ,t) represents the total absorbance at irradiation time t and wavelength λ, and ε1,4‑BPN(λ) and εith product(λ) are the H


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Figure 8. 1H NMR spectrum of 1,4-BPN (top, red line) and that of the reaction mixture obtained after irradiation of 1,4-BPN (λ = 365 nm; t = 30 s) (bottom, black line). Irradiation was conducted in chloroform with a solute concentration of 1.0 × 10−5 M in a 1 cm × 1 cm quartz cell. The height of the solution (i.e., the optical path) was fixed at 3 cm.

Figure 9. (a) Plot of Φdegradation as a function of the concentration of chloroform [CHCl3] and (b) plot of 1/Φdegradation as a function of 1/[CHCl3] for the photodegradation of 1,4-BPN by chloroform in deaerated toluene solution ([1,4-BPN]0 = 2.1 × 10−4 M). These plots were fitted using eq 5, which afforded Kobs = 5.1 × 10 [M−1] and Φ∞ = 0.11 (red dashed lines).

(λ = 365 nm; t = 30 s) in comparison with that of 1,4-BPN. After irradiation, the 1H NMR signals assigned to 1,4-BPN (highlighted in red) disappear, and numerous peaks emerge in the aromatic region. Although it is difficult to assign all these peaks to the various components of the reaction mixture, several characteristic peaks can be discerned: a singlet at 6.98 ppm and two doublets of doublets at 7.76 and 8.08 ppm (highlighted in blue) can be assigned to 1,4-naphthoquinone.72 In addition, the mass chromatograms of the mixture (Figure S29) indicated that several chlorinated derivatives of 1,4naphthoquinone are formed as byproducts of the photodegradation. In fact, it is known that cations of p-diaminoacene derivatives are subject to aryl−N bond cleavage through imination73 and hydrolysis.74,75 Considering the fact that the radical cation of 1,4-BPN should be formed in the initial step of the photodegradation,42 it is reasonable to assume that the aryl−N bonds of 1,4-BPN are photolabile as illustrated in Scheme S3. As the cleavage of the aryl−N bonds of the 1,4BPN moiety results in the de-cross-linking of (1,4-BHPNbisMA)n-(PBMA)m-gel, the formation of 1,4-naphthoquinone must be one of the key reactions in the photodegradation of (1,4-BHPN-bisMA)n-(PBMA)m-gel. Finally, quantum yields of the photodegradation of 1,4-BPN in chloroform were evaluated. Given that the initial step of the photodegradation is fluorescence quenching of 1,4-BPN by chloroform, the quantum yield of photodegradation

molar absorption coefficients of 1,4-BPN and an ith product of n

photodegradation, respectively. When

∑i = 1 εi th product(λ) ε1,4‐BPN(λ)

is replaced

by an arbitrary constant c, the following equation is obtained: A (λ , t ) = c + (1 − c) exp(−kt ) A(λ , t = 0)

(4)

The temporal evolution of the absorbance of 1,4-BPN at λ = 335 nm and λ = 291 nm (Figures 7c,d) was fitted to eq 4. The absorbance of 1,4-BPN in THF fits eq 4 well (Figure 7c), and so does the absorbance in chloroform at t = 0−10 s, even though deviations are observed especially at longer irradiation times. As a result, the derived photodegradation rates of 1,4BPN are k = 6.5 × 10−1 s−1 (λ = 335 nm) and k = 7.0 × 10−1 s−1 (λ = 291 nm) in chloroform and k = 1.1 × 10−3 s−1 (λ = 335 nm) and k = 1.9 × 10−3 s−1 (λ = 291 nm) in THF. The same procedure was applied to solutions of 1,4-BPN in 1,1,1trichloroethane and toluene (Figure S26), which afforded k = 6.6 × 10−1 s−1 (λ = 335 nm) and k = 3.3 × 10−3 s−1 (λ = 335 nm), respectively. It can therefore be concluded that the photodegradation of 1,4-BPN occurs at least 2 orders of magnitude faster in 1,1,1-trichloromethyl-containing solvents than in other organic solvents. Figure 8 shows the 1H NMR spectrum of the products obtained after photodegradation of 1,4-BPN upon irradiation I


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The selectivity toward 1,1,1-trichloromethyl-containing chloroalkanes was demonstrated not only through fluorescence quenching but also by the photodegradation of (1,4-BHPNbisMA)n-(PBMA)m-gel. Once chloroform- or 1,1,1-trichloromethane-swollen (1,4-BHPN-bisMA)n-(PBMA)m-gel was illuminated with UV light (λ = 365 nm), the gels quickly dissolved in the solvent. In contrast, gels swollen in toluene or THF were stable under the same conditions. The chemical structure of poly(n-butyl methacrylate) remained intact even after photodegradation of (1,4-BHPN-bisMA)n-(PBMA)m-gel. In addition, the photodegradation of 1,4-BPN afforded 1,4-naphthoquinone. All these results suggest that photodegradation of (1,4BHPN-bisMA)n-(PBMA)m-gel proceeds mainly via de-crosslinking. Photodegradation of 1,4-BPN with 1,1,1-trichloromethyl-containing chloroalkanes was fast and efficient, and quantum yields were comparable to those of reported photoremovable protecting groups. As well as the photodegradation of 1,4-BPN, a steep drop in the storage modulus of (1,4-BHPN-bisMA)n(PBMA)m-gel was observed within 10 s when swollen in 1,1,1trichloroethane. Therefore, (1,4-BHPN-bisMA)n-(PBMA)m-gel represents a unique soft material that integrates fluorescence, detection, and photodegradation properties within a simple fluorophore, which is able to distinguish specific chloroalkanes despite their nonionic and uncharged nature. Since the hydroxyl group of 1,4-bis(4′-hydroxypiperidyl)naphthalene can be converted into other functional groups, it should be possible to incorporate the cross-linker proposed in this study into diverse polymeric materials. The development of other, more sophisticated soft materials that employ 1,4-BPN derivatives as cross-linkers is currently in progress in our laboratories.

(Φdegradation) should be a function of the concentration of chloroform [CHCl3] according to82 Φdegradation = Φ∞

Kobs[CHCl3] 1 + Kobs[CHCl3]

(5)

wherein Φ∞ is the quantum yield at an infinite concentration of chloroform ([CHCl3] → ∞) and Kobs represents the quenching constant (Kq). Therefore, an extrapolation of 1/Φ∞ by the plot of 1/Φdegradation as a function of 1/[CHCl3] delivered Kobs = 5.1 × 10 M−1 and Φ∞ = 0.11 (for experimental details see section S1). The quenching rate constant derived from the photodegradation experiment (kobs = Kobs/τfl = 5.7 × 109 M−1 s−1) is comparable to that obtained from the Stern−Volmer experiment (kq = KSV/τfl = 4.5 × 109 M−1 s−1) (Figure 3), indicating the validity of the assumption that the photodegradation proceeds via a reaction between the singlet excited state of 1,4BPN (11,4-BPN*) and chloroform. The quenching rate constants, kobs as well as kq, were approximately one-half of that of the diffusion-limited rate constant (kdiff = 1.2 × 1010 M−1 s−1 in toluene at 25 °C).63 It is well-known that the fluorescence quenching of aromatic compounds by chloroalkanes is less efficient in solvents of low polarity, as such quenching reactions are mediated by the formation of polar exciplexes.68,83 Considering that the photodegradation and Stern−Volmer experiments were conducted in relatively apolar toluene, it is reasonable to expect that kobs and kq should be lower than kdiff. The Φ∞ value of 0.11 thus obtained is typical for photoremovable protecting groups,33 indicating that 1,4BPN derivatives could find practical applications as photodegradable cross-linkers. Nevertheless, this Φ∞ value is significantly lower than typical Φ∞ values (0.17−1.0) for the photodegradation of aromatic hydrocarbons with carbon tetrachloride instead of chloroform.82 The difference was attributed to the lower electron affinity of the latter with respect to the former.69,70

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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/acs.macromol.7b00213. Detailed synthetic procedures, NMR spectra, photophysical data, and quantum-chemical calculations (PDF)

■

CONCLUSIONS In this study, we present a 1,4-bis(piperidyl)naphthalene (1,4BPN) cross-linker for the generation of network polymers with synergistic fluorescence, detection, and photodegradation properties. Copolymerization of n-butyl methacrylate with methacrylate-functionalized 1,4-bis(4′-hydroxypiperidyl)naphthalene, specifically 1,4-BHPN-bisMA and 1,4-BHPNMA, afforded (1,4-BHPN-bisMA)n-(PBMA)m-gel and its model copolymer (1,4-BHPN-MA)n-(PBMA)m, respectively. While (1,4-BHPN-bisMA)n-(PBMA)m-gel is highly fluorescent in various common organic solvents and chloroalkanes, swelling in chloroform, tetrachloromethane, and 1,1,1-trichloroethane completely quenched its fluorescence. Photophysical analyses employing 1,4-BPN and its regioisomers 1,5-BPN and 2,6-BPN revealed that the fluorescence of 1,4-BPN and 2,6-BPN was, in contrast to that of 1,5-BPN, strong and highly sensitive to chloroform. Further detailed studies on the fluorescence of 1,4BPN and model copolymer (1,4-BHPN-MA)n-(PBMA)m allowed the elucidation of the relation between the photophysical properties of 1,4-BPN and (1,4-BHPN-bisMA)n(PBMA)m-gel. Nevertheless, several issues still remain: (1) the fluorescence quantum yields of (1,4-BHPN-bisMA)n(PBMA)m-gel are approximately twice those of 1,4-BPN in several chloroalkanes, and (2) the fluorescence of the densely cross-linked (1,4-BHPN-bisMA)n-(PBMA)m-gel is relatively weak.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.K.). ORCID

Masatoshi Tokita: 0000-0002-4534-7337 Osamu Ishitani: 0000-0001-9557-7854 Gen-ichi Konishi: 0000-0002-6322-0364 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Suzukakedai Materials Analysis Division, Technical Department, Tokyo institute of Technology, for measurements of mass chromatograms. This work was partially supported by the Grant-in-Aid from the Ministry of Education, Science, and Culture of Japan.

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