Polymer-Chain-Induced Tunable Luminescence Properties

Article pubs.acs.org/Macromolecules

Polymer-Chain-Induced Tunable Luminescence Properties: Amphiphilic Poly(2-oxazoline)s Possessing a N,N-Dialkylpyrene-1carboxamide Chromophore in the Side Chain Yosuke Niko† and Gen-ichi Konishi*,†,‡ †

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Tokyo 152-8552, Japan PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan

‡

S Supporting Information *

ABSTRACT: We report a polymer-chain-induced tunable luminescence system based on a newly synthesized fluorescent polymer, N,N-dialkylpyrene-1-carboxamide-modified poly(2methyl-2-oxazoline) (POZO-py) at various ratios of pyrene induction. Photoluminescence and thermal properties of the POZO-py were analyzed in detail. Differential scanning calorimetry (DSC) measurements revealed that the glasstransition temperature (Tg) of POZO-py was above room temperature, which implies that POZO-py is in the glassy state at room temperature. POZO-py-(0.73) (i.e., pyrene content of 0.73%, Mn = 2270, Tg = 40 °C), having the lowest pyrene induction ratio, showed higher fluorescence quantum yields in several solvents, e.g., water, or in the thin film state than did N,N-diethylpyrene-1-carboxamide (a model compound, PA), because the polymer chain prevented the molecular mobilities of the pyrene moieties. In particular, POZO-py showed strong emission (Φfl = 0.52) in glycerin, which is a highly viscous solvent. Highly modified POZO-py-(2.9) (i.e., with a pyrene content of 2.9%, Mn = 2450, Tg = 48 °C) and POZO-py-(9.1) (Mn = 2640, Tg = 57 °C) exhibited both monomer and excimer emissions. These excimers consisted of a static excimer and a dynamic excimer. These results were not obtained for the low-molecular PA. Thus, POZO can induce improved and tunable photoluminescence properties of N,N-dialkylpyrene-1-carboxamide. The synthesis and photoluminescence properties of N,N-dialkylpyrene-1-carboxamide-modified poly(2-ethyl-2-oxazoline)-gel (PEtOZO-py-gel) were also determined.

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INTRODUCTION Pyrene is used widely as a chromophore because of its high fluorescence quantum yield, long fluorescence lifetime, ability to form excimers, and sensitive vibrational structure.1 Because such characteristics are very useful in the analyses of local structures and molecular mobilities, pyrene has been utilized as a fluorescence probe in multimolecular systems.2 Moreover, in recent years, pyrene has also been used as a fluorescence label for structural analyses and to study the electron transfer physics of polypeptides, DNA oligomers, and duplexes. Specifically, its use as a fluorescence label permitted these molecules to be examined in terms of their distances,3 electric field effects,4 and the conformational dependence of their electron transfer rates.5 Similarly, in the field of polymer science, the use of pyrene, especially its excimer emission, is very popular for carrying out structural analyses. This is because when designing a new material using a polymer, it is extremely important to understand the relationship between the structure and the properties of the polymer. Pyrene is especially useful for studying water-soluble polymers.2d Therefore, the photophysics and structures of water-soluble polymers in water and other organic solvents have been investigated in detail.6 Almost all © 2012 American Chemical Society

pyrene chromophores that have been used in the investigation of pyrene-containing polymers were introduced into the main or side chain of the polymers through a nonconjugated alkyl spacer, π-extended spacer, or donor-group substituents such as triphenylamine or pyridine to develop analytical probes or photonic devices.6,7 It is important to apply the properties of low-molecular pyrene derivatives to polymer materials in such studies.8 As a result, very few investigations have been conducted on polymers modified by using pyrene derivatives that have lost the original pyrene characteristics owing to being functionalized by adding moieties such as carbonyl groups.9 This is despite the fact that many chromophores that have been functionalized by carbonyl groups exhibit unique fluorescence phenomena in response to their external environment.9,10 Consequently, fundamental investigations that have evaluated how polymer chains affect the properties of pyrene dyes are few and far between. Received: January 17, 2012 Revised: February 13, 2012 Published: February 27, 2012 2327

dx.doi.org/10.1021/ma3001252 | Macromolecules 2012, 45, 2327−2337

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nitrogen atmosphere three times, respectively. All photophysical measurements performed in solutions were carried out using dilute solutions with optical density (O.D.) around 0.1 at the maximum absorption wavelength in 1 cm path length quarz cells. Thin films were spin coated from solution of PMMA and 1,2-dichloroethane on quarz plates using MIKASA SPIN COATER 1H-D7. Weight ratios of synthesized N,N-dialkylpyrene-1-carboxamide-modified poly(2-methyl-2-oxazoline) (POZO-py) for PMMA were less than 2%. The conditions of prepared films were with O.D. around 0.1 and their thickness were 300−400 nm. UV−vis spectra were recorded with a Beckman Coulter DU800 UV−vis Spectrophotometer. Fluorescence spectra were recorded on a JASCO FP-6500 Spectrofluorometer. Absolute Quantum Yields were measured by a Hamamatsu Photonics Quantaurus QY. Fluorescence lifetimes were measured using a Hamamatsu Photonics Quantaurus Tau. In situation where POZOpy-(2.9) and POZO-py-(9.1) were monitored at 500 nm, measurements at short-range were performed to investigate short-lived components in addition to those at long-range. Polymerization of 2-Methyl-2-oxazoline. A mixture of 2methyl-2-oxazoline (33.2 g, 390 mmol), benzyl bromide (2.66 g, 16 mmol) and acetonitrile (50 mL) was placed in a 200 mL two necked flask and sealed under nitrogen, which was then heated at 120 °C for 24 h. The resulting poly(N-acetylethylenimine) (POZO) was isolated by dissolving in MeOH and precipitation into diethyl ether. It was purified by fractionation with MeOH and diethyl ether and drying in vacuo. Yield was 29.8 g (90%, Mn = 2220). Partial Hydrolysis of POZO. POZO was dissolved in aqueous NaOH solution (15 mL). The resulting solution was refluxed for 12 h. Water was removed under reduced pressure. Poly(ethylenimine/Nacetylethylenimine)random copolymer (POZO-NH) was extracted with dichloromethane and dried in vacuo. The degree of hydrolysis was determined by the 1H NMR spectrum. The amounts of reagents, yields and number-average molecular weight of the obtained POZONH are summarized in Table S1 (Supporting Information). Amidation of POZO-NH (POZO-py). A benzene suspension of POZO-NH and pyrene-1-carboxylic acid was freeze-dried. After argon was introduced, the mixture was dissolved in a mixture of dry 1,2dichloroethane and distilled DMF under nitrogen atmosphere and cooled at 0 °C. To this solution was added diisopropylcarbodiimide. After the mixture was stirred overnight, the resulting urea was removed by filtration, and the filtrate was concentrated to yield a yellowish polymer, which was purified by using Sephadex LH-20 (Pharmacia, size-exclusion chromatography). Furthermore, resulting polymer was redissolved in dichloromethane and reprecipitated into diethyl ether (five times). Pyrene modified polymer POZO-py was obtained after vacuum-dry oven. The amounts of reagents, yields and numberaverage molecular weight of the obtained POZO-py are summarized in Table S2 (Supporting Information). Partial hydrolysis of PEtOZO. PEtOZO (6.0 g) was dissolved in aqueous NaOH (1.4 mmol) solution. The refluxed for 12 h. Water was removed under reduced pressure. Poly(ethylenimine/N-ethanoylethylenimine) random copolymer (PEtOZO-NH) was extracted with dichloromethane and obtained after vacuum-dry oven. Yield was 3.1 g (67%). The degree of hydrolysis was determined by the 1H NMR spectrum. Partial amidation of PEtOZO-NH. A benzene (6 mL) suspension of a mixture of PEtOZO-NH (2.6 g, including 0.59 mmol amine unit) and pyrene-1-carboxylic acid (80 mg, 0.32 mmol) was freeze-dried. After argon was introduced, the mixture was dissolved in a mixture of dry 1,2-dichloroethane (6 mL) and distilled DMF (5 mL) under nitrogen atmosphere and cooled at 0 °C. To this solution was added diisopropylcarbodiimide (44 mg, 0.030 mmol). After the mixture was stirred overnight, the resulting urea was removed by filtration, and the filtrate was concentrated to yield a yellowish solid, which was purified by using Sephadex LH-20. Furthermore, resulting polymer was redissolved in dichloromethane and reprecipetated into diethyl ether (five times). Pyrene-modified polymer POZO-NH-py was obtained after vacuum-dry oven. Yield was 1.8 g (69%). The degrees of hydrolysis and amidation were determined by the 1H NMR spectroscopy.

Recently, we reported a new pyrene chromophore that possesses a tertiary N,N-dialkyl carboxamide group. The chromophore exhibits weak fluorescence in several organic solvents but shows relatively strong emissions in viscous solvents.9 On the basis of these phenomena, we considered that the photophysical behavior of the chromophore is dominated by molecular motion that results in internal conversion.11 Our latest endeavor has been to discover new photoluminescence properties of N,N-dialkylpyrene-1-carboxamide when it is attached to a polymer chain. More precisely, we think that it might be possible to not only exploit the properties of lowmolecular pyrene derivatives but also improve them using chromophores such as N,N-dialkylpyrene-1-carboxamide. The polymer chain could efficiently suppress the molecular motion that leads to internal conversion during photophysical processes, which is similar to the behavior exhibited by glycerin.11 In addition, it may be possible to observe polymer chain scaffold-mediated excimer emissions by controlling the pyrene content ratio. In this paper, we report on poly(2-alkyl-2-oxazoline)s with an N,N-dialkylpyrene-1-carboxamide moiety in its side chain as a novel pyrene-containing fluorescent polymer. We also analyze the photoluminescence properties of poly(2-alkyl-2-oxazoline)s in detail to determine the influence that a polymer chain can have on a pyrene dye. Our target polymers were synthesized easily by cationic ring-opening polymerization, subsequently hydrolyzed, and then modified with carboxamide.12,13 In addition, since poly(2-methyl-2-oxazoline) is amphiphilic,13 we were able to study its photoluminescent behavior in various organic solvents and water. In recent years, many researchers have investigated poly(2-oxazoline)s because of its potential applications.14 However, to the best of our knowledge, investigations on chromophore-modified poly(2-oxazoline)s have been limited to photogelation,15 the analysis of local structures,16 and temperature sensors using solvatochromic dyes.17 Therefore, the photoluminescence properties of chromophore-modified poly(2-oxazoline)s and their applications have rarely been explored in detail. On the basis of what has been mentioned above, we believe that it is important to evaluate the photoluminescence properties of the tertiary N,Ndialkylpyrene-1-carboxamide chromophore in poly(2oxazoline)s and photofunctional poly(2-oxazoline)s.

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EXPERIMENTAL SECTION

Materials. Unless otherwise noted, all reagents and chemicals were used without further purification. Benzyl bromide and pyrene-1carboxylic acid were obtained from TCI (Tokyo, Japan). Diisopropylcarbodiimide were purchased from Wako (Tokyo, Japan). 2Methyl-2-oxazoline and poly(2-ethyl-2-oxazoline) (Mw = 50 000) were purchased from Sigma-Aldrich and 2-methyl-2-oxazoline was used after purification by distillation. Poly(methyl methacrylate) (PMMA: Mw = 100 000 atactic beads) was purchased from Polyscience, Inc. (Warrington PA). As a model compound, N,N-diethylpyrene-1carboxamide (PA) was prepared along with previous report.9 Spectrograde dichloromethane, ethanol, glycerin and special grade ethylene glycol were purchased from Nacalai Tasque (Kyoto, Japan). Distilled water was prepared from EYELA STILL ACE SA-2100A. Instrumentals. All the 1H NMR spectra were recorded on a 400 MHz JEOL LMN-EX400 instrument with tetramethylsilane (TMS) as the internal standard. FT-IR spectra were recorded on a JASCO FT-IR 469 plus spectrometer. Thermogravimetric analysis (TGA) was performed using a SII TG/DTA 6200 (SEIKO) with a heating rate of 10 °C/min under a nitrogen atmosphere. Differential scanning calorimetry (DSC) was carried out on a SII DSC 220C (SEIKO) at a heating rate of 20 °C/min and cooling rate 20 °C/min under a 2328


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Scheme 1. Synthesis of POZO-py and Structure of Model Compound PA

Table 1. 1H NMR Analysis and Thermal Properties of POZO, POZO-NH, and POZO-py. (* Estimated by 1H NMR)

a

entry

n−m

m

hydrolysisa m/n

amidationa m/n

POZO POZO-NH-(0.73) POZO-py-(0.73) POZO-NH-(2.9) POZO-py-(2.9) POZO-NH-(9.1) POZO-py-(9.1)

26 28 26 25 26 24 23

− 0.22 0.19 1.5 0.79 4.6 2.3

− 0.0078 − 0.057 − 0.16 −

− − 0.0073 − 0.029 − 0.091

Tg [°C]

Td5 [°C]

Td50 [°C]

40

332

383

48

330

384

57

322

390

Mn 2220 2400 2270 2230 2450 2370 2640

Estimated by 1H NMR.

Figure 1. Normalized UV−vis absorption and fluorescence spectra of POZO-py and PA in ethanol solution (λex = λabs, room temperature). Cross-Linking Reaction of PEtOZO-NH-py. PEtOZO-NH-py (1.0 g, including 0.12 mmol amine unit), hexamethylene diisocyanate (50 mg, 0.30 mmol), DBU (84 mg, 0.50 mmol), and DMF (3 mL) were placed in a 50 mL flask and stirred at room temperature for 72 h under nitrogen atmosphere. The isolation of the gel (PEtOZO-py-gel) was purified by Soxhlet extraction with methanol, tetrahydrofuran, and dichloromethane. Yield was 0.86 g (85%).

induction were estimated by using the integral values obtained from 1H NMR spectra. POZO-py was purified using reprecipitation and gel filtration (Sephadex LH-20). The results of these preparations are summarized in Table 1. We obtained three samples, which contained 0.73%, 2.9%, and 9.1% pyrene units, and named them POZO-py- (0.73), POZO-py-(2.9), and POZO-py-(9.1), respectively. As shown in Table 1, purified POZO-py had a lower pyrene content than the ratio of hydrolysis of corresponding POZO-NH. From the comparison between POZO-py modified using large and small amounts of pyrene, we inferred that this lower ratio was the result of either the removal of low-molecular-weight components possessing pyrene moieties or the difference in the solubility of the reprecipitation solvents. Because of the disappearance of the peak derived from the hydrogen atoms on the carbon atoms adjacent to the amine sites, we concluded that the reaction between pyrene-1-carboxylic acid and POZONH proceeded completely. In addition to these POZO-py, we

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RESULTS AND DISCUSSION Synthesis and Characterization of Pyrene-Modified Poly(2-methyl-2-oxazoline) (POZO-py). POZO-NH, a poly(N-acetylethylenimine)/poly(ethylenimine) random copolymer, was synthesized through the partial hydrolysis of poly(2-methyl-2-oxazoline) (POZO). 15 Pyrene-modified POZO (POZO-py) was obtained subsequently by introducing pyrene-1-carboxylic acid to the resulting N−H group by using N,N-diisopropylcarbodiimide (DIC) as a condensing agent. (Scheme 1) In all samples, the number-average molecular weight (Mn), ratio of hydrolysis, and the ratio of pyrene 2329


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prepared low-molecular N,N-dialkylpyrene-1-carboxamide PA, which has already been reported in ref 9, as a model compound. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were carried out to determine the thermal transition and degradation behavior of POZO-py (Table 1, Figure S3 and Figure S4 in Supporting Information). DSC measurements revealed that all POZO-py at various pyrene content ratios showed a glass-transition temperature (Tg) higher than room temperature and that Tg increased with increasing pyrene content. POZO-py tends to form dimers in the ground state with increasing pyrene content. Therefore, we expected that π−π interactions were strong in POZO-py-(9.1). Owing to the limited mobility of the chain around pyrene, Tg increased for POZO-py-(9.1). The TGA curves of POZO-py revealed a single thermal decomposition temperature. Although a clear correlation between the decomposition temperature and the ratio of pyrene induction was not found, the values of Td5 and Td50 were almost the same for all the samples. Although the molecular weights of POZO-py were around 2000, they exhibited the sufficiently high glass transition temperature, and film-forming properties as mentioned below. Therefore, we considered that it is enough to expect the polymer-chain effects. Photoluminescence Properties of POZO-py in Ethanol Solution. UV−Vis Spectra. The UV−vis absorption spectra of POZO-py and the model compound PA in a deaerated ethanol solution were obtained. As shown in Figure 1, the UV− vis spectra of POZO-py were red-shifted slightly and broadened with increasing pyrene content compared to the UV−vis spectra of PA. On the basis of a previous report from Todesco,18 we inferred that these changes in the spectra were the result of a strong hypochromic effect caused by the aggregation of pyrene in the ground state, often called preassociation. In our previous work,9 we assigned the peak of N,N-dialkylpyrene-1-carboxamide derivatives at around 340 nm to the S0 → S1 transition by means of time-dependent density functional theory (TD-DFT) calculations. In this work, we obtained similar results from the absorption spectra. It is widely known that the peak-to-valley ratio of this absorption (Pa) indicates the degree of preassociation.2d In general, a decrease in the value of Pa corresponds to an increase in the degree of preassociation. In this study, we measured the UV−vis spectra under conditions in which the optical density (O.D.) of all the samples was 0.1. Therefore, it can be said that they possessed the same amount of pyrene but that the pyrene density of each sample was clearly different. As can be seen from Table 2, the value of Pa for POZO-py tended to decrease with increasing pyrene content. In addition, the Pa values for all the samples were less than those for PA. These results support the claim that POZO-py formed preassociations in the ground state and that the red-shifted absorption spectra were derived from the hypochromic effect caused by such associations. Fluorescence and Excitation Spectra. The fluorescence spectra of POZO-py and the model compound PA in a deaerated ethanol solution were obtained. We noticed that within the fluorescence spectra (Figure 1), POZO-py-(0.73) had a spectrum similar to that of PA, whereas POZO-py-(2.9) showed a slightly broadened spectrum. In addition, a clear excimer emission was observed at around 480 nm within the spectrum for POZO-py-(9.1). As previously reported, PA does not exhibit an excimer emission because Debye’s diffusion equation indicates that the collision rate of PA under this

Table 2. Spectroscopic Properties of POZO-py and PA in Ethanol Solutions entry POZO-py(0.73) POZO-py(2.9) POZO-py(9.1) model PA

λabs [nm]

Paa

λem [nm]

Φflb

PMc

PEd

338

2.24

378, 398, 420

0.17

1.59

1.48

339

1.90

379, 398, 420

0.22

1.56

1.45

339

1.86

379, 398, 475

0.17

1.53

1.50

337

2.75

377, 387, 397, 419

0.061

1.78

1.79

Pa: Peak-to-valley ratio for the S0 → S1 transition. bΦfl: Measurement for calibrating the integration sphere. cPM: Peak-to-valley ratio for the S0 → S1 transition in the excitation spectrum viewed at a monomer emission (ca. 380 nm). dPE: Peak-to-valley ratio for the S0 → S1 transition in the excitation spectrum viewed at the excimer emission (ca. 480 nm). a

condition is slower than that of radiative and nonradiative processes.9,19 Therefore, the excimer species of POZO-py, which were measured under the same conditions as those for PA, were induced by the aggregation of hydrophobic pyrene moieties in the hydrophilic POZO chain matrix similar to that for aqueous pyrene-modified polymer.2d We continuously measured the excitation spectra of POZO-py and PA and obtained two parameters, PM and PE, which we calculated from observations made at 380 nm (monomer emission) and 480 nm (excimer emission), respectively. In general, the excitation spectra provided compelling evidence for the ground-state interactions of pyrene. For cases in which pyrene preassociation occurs, PE is always less than PM and the spectra monitored in the excimer regions are slightly red-shifted relative to those in the monomer regions. As shown in Figure 2 and Table 3, the results of the excitation spectra support the claim that preassociation of POZO-py occurs, which we also noted based on our analysis of the UV−vis spectra. Thus, we considered that similar spectra were obtained because the values of all of the parameters were relatively close for both PA and POZO-py-(0.73). On the other hand, POZO-py-(2.9) and POZO-py-(9.1) were estimated to form relatively large associative ground-state pyrene dimers, resulting in broadened or excimer emissions. As is common among pyrene-labeled aqueous polymers, polymers with partially overlapping pyrene moieties in an excited state often emit a broad and structureless emission at 420 nm, which is called a static excimer (ref 2d, see Figure 3). In our present work, a static excimer was generated from the preassociation state. Therefore, POZO-py-(2.9), which possesses more pyrene moieties than POZO-py(0.73), showed an emission that was a mixture of a monomer emission and a static excimer emission. In POZO-py-(9.1), clear excimer and monomer emissions were observed at around 475 and 380 nm, respectively. This excimer emission was not due to a static excimer. Rather, it was due to a dynamic excimer, as defined by Birks.20 This was generated by further pyrene modification and the subsequent encounter of an electronically excited pyrene with a second pyrene in its ground electronic state. Considering the shape of the emission spectra, we also inferred that POZO-py-(2.9) and POZO-py-(9.1) contained slightly dynamic and static excimers, respectively. Fluorescence Quantum Yield. As shown in Table 2, POZOpy-(0.73), which has an emission similar to that of PA, exhibited a higher fluorescence quantum yield (Φfl = 0.17) than PA (Φfl = 0.061). Slightly red-shifted POZO-py-(2.9) also 2330


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Figure 2. Excitation spectra of POZO-py (purple line, λex = 380 nm; orange line, λex = 480 nm).

Table 3. Fluorescence Lifetime Decay of POZO-py in Ethanol Solution (λex = 340 nm, Room Temperature) entry

τ1

τ2

τ3

POZO-py-(0.73) (380 nm) POZO-py-(0.73) (420 nm) POZO-py-(2.9) (380 nm) POZO-py-(2.9) (420 nm) POZO-py-(2.9) (500 nm) POZO-py-(9.1) (380 nm) POZO-py-(9.1) (420 nm) POZO-py-(9.1) (500 nm)

1.6 1.06 4.2 3.11 0.05521 3.39 3.39 0.01321

12.4 7.59 17.1 7.94 2.42 13.9 13.2 2.7

36.3 35.1 39 34 44 33.3 35.0 41.2

τ4

0.44

A1

A2

A3

0.22 0.43 0.47 0.59 −0.65 0.45 0.56 −0.41

0.30 0.33 0.32 0.33 0.07 0.38 0.29 0.15

0.48 0.24 0.21 0.08 0.08 0.17 0.15 0.44

A4

0.20

χ2 1.10 1.06 1.11 1.13 1.05 1.1 1.02 1.09

Figure 3. Two proposed mechanisms for excimer formation (left) and schematic potential energy diagrams for pyrene excimer formation in the absence of ground-state and with pyrene ground-state association (right).2d,20

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Figure 4. UV−vis absorption and fluorescence spectra of POZO-py and PA in dichloromethane, ethanol, and water solutions (λex = λabs, room temperature).

chain (Figure S5, Supporting Information). In a previous study,9 we demonstrated that fluorescence quantum yields of tertiary N,N-dialkylpyrene-1-carboxamide derivatives were enhanced significantly with increasing viscosity of the media owing to the suppression of molecular motions, which sometimes cause the occurrence of internal conversion.11 Therefore, the mobilities of the pyrene-1-carboxamide dyes were probably suppressed by the polymer chain, just as would occur in viscous media. The POZO chain has a beneficial structure whereby pyrene moieties can be directly introduced onto the side chain almost like substituents to inhibit the

showed a high quantum yield (Φfl = 0.22), similar to POZOpy-(0.73). Although POZO-py-(9.1) had a higher quantum yield (Φfl = 0.17) than PA, the intensity of the monomer emission was reduced because of the large contribution made by the excimer emission. Considering that increases in the fluorescence intensity and excimer emission were not observed for PA-doped POZO. Moreover, the fluorescence properties of PA are almost not influenced by the polarities of solvents as mentioned our previous work.9 It can be considered that the above-mentioned improved fluorescence was derived from the addition of N,N-dialkylpyrene-1-carboxamide to the polymer 2332


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Table 4. Spectroscopic Parameters of POZO-py in Various Solvents entry

solvent

λabs [nm]

POZO-py-(0.73)

dichloromethane water ethanol ethylene glycol glycerin dichloromethane water ethanol ethylene glycol glycerin dichloromethane water ethanol ethylene glycol glycerin

341 339 338 340 341 340 340 339 341 342 340 341 339 341 342

POZO-py-(2.9)

POZO-py-(9.1)

λem [nm] 378, 379, 378, 379, 379, 379, 380, 379, 379, 379, 379, 380, 379, 379, 379,

398, 398, 398, 399, 399, 397, 399, 398, 399, 399, 399, 399, 398, 399, 399,

420 420 420 420 420 418 421 420 422 420 474 488 475 422 420

Pa

Φfl

PM

PE

2.26 1.78 2.24 1.94 2.06 1.91 1.60 1.90 1.82 1.82 1.90 1.45 1.86 1.76 1.71

0.15 0.27 0.17 0.26 0.52 0.41 0.25 0.21 0.30 0.54 0.21 0.15 0.17 0.21 0.41

1.60 1.46 1.59 1.59 1.59 1.59 1.43 1.56 1.56 1.52 1.60 1.42 1.53 1.54 1.52

1.57 1.41 1.48 1.43 1.44 1.51 1.28 1.45 1.46 1.30 1.50 1.23 1.50 1.34 1.30

luminescent properties of POZO-py on solvent viscosity or polarization, UV−vis absorption (Figure 4), fluorescence spectra, and other photophysical parameters of POZO-py were measured in several solvents. For all measurements, enhancements in fluorescence quantum yields of POZO-py were observed with increasing solvent viscosity. Furthermore, in the most viscous medium (glycerin), the dynamic excimer emission of POZO-py-(9.1) at around 480 nm almost disappeared. We believed that a sandwich-shaped excimer failed to form because the glycerin medium suppressed the arrangement of excited species. With respect to POZO-py-(9.1), the enhanced fluorescence quantum yields were derived not only from the suppression of internal conversion by viscous media but also from the suppression of deactivation brought about by energy transfer from high-energy excited species to low-energy excimer species. For the measurement of POZO-py-(0.73), a highly enhanced monomer emission (Φfl = 0.52), which was much higher than that of PA (Φfl = 0.23), was observed for the glycerin solvent.9 This highly luminescent performance was not obtained from low-molecular N,N-dialkylpyrene-1-carboxamide derivatives. Therefore, we considered that this phenomenon was caused by the interaction between the polymer matrix and the glycerin solvent. Considering that POZO-py consists of polarized amide units, we inferred that a strong hydrogen-bond network was formed between POZO-py and the glycerin medium. As a result, in addition to suppressing vibration of the pyrene moieties, the motion of the POZO-py chain was frozen and highly luminescent performance was achieved. In other words, the poly-2-methyl-2-oxazoline chain induced dramatic improvement in the fluorescence emission of the N,N-dialkylpyrene-1carboxamide derivatives. With respect to the dependence of the luminescent properties of POZO-py on polarization, changes in POZOpy-(2.9) and POZO-py-(9.1), which showed relatively large pyrene aggregation in the ground state, were observed. As can be seen in Table 4, red-shifted and broadened spectra were observed in most polarized water solvents. It is known generally that an excited complex, just like an excimer, is stabilized in polar solvents, and a stabilized excited complex tends to be inactivated by energy transfer. In the nonpolar dichloromethane solvent, such deactivation was suppressed. Therefore, a relatively strong emission was observed.

molecular mobilities of pyrene-1-carboxamides. We think that the enhanced fluorescence emission of POZO-py-(0.73) resulted from the suppression of pyrene-1-carboxamide dye mobility caused by POZO. Additionally, the fluorescence of tertiary N,N-dialkylpyrene-1-carboxamide was enhanced without modification by changing the electronic state of the dye. Thus, a polymer-chain-induced emission was observed for POZO-py-(0.73). Fluorescence Lifetime Decay. For the fluorescence lifetime measurements, significant trends were observed as described below. 1. Roughly three components were observed for almost all of the measurements. 2. As observed in the long-wavelength region, the contribution of long-lived components decreased and that of short-lived components increased. 3. The tendencies described in point 2 became more pronounced as the pyrene content increased. 4. For the results of POZO-py-(2.9) and POZO-py-(9.1) at 500 nm, clear rising components were observed. Although PA showed only a single component (τ = 30.4 ns, observed at 380 nm in ethanol solution) in a previous study,9 in this study, almost all samples of POZO-py exhibited three components in ethanol solution. However, at a short wavelength (380 nm) for POZO-py-(0.73), which showed behavior most similar to PA, the most dominant component (τ = 36.3 ns) was relatively close to that of PA. Therefore, it can be inferred that this component was derived from monomer emission. Given the above results, we considered that the other short-lived components, which were observed predominantly for measurements of highly pyrene modified POZO-py or for measurements at long wavelengths, were derived from preassociated dimers or other geometries induced by the polymer chain. As shown in Table 3, this indicates that POZOpy-(0.73) possesses several excited species. For the measurements of POZO-py-(2.9) and POZO-py-(9.1), in addition to monomer and other short-lived species components, clear rising components were observed at 500 nm. These results are evidence for the formation of dynamic excimer species and support the fact that emissions obtained from POZO-py-(2.9) and POZO-py-(9.1) were a mixture of monomer, static excimer, and dynamic excimer emissions, as mentioned above. Solvent Dependence of Luminescent Properties of POZO-py. In order to investigate the dependence of the 2333


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Figure 5. Normalized UV−vis and fluorescence spectra of POZO-py in the thin film state (λex = λabs, room temperature).

Photoluminescent Properties of Thin-Film POZO-py. Thin films of POZO-py and PA were prepared by spin-coating of a 1,2-dichloroethane solution containing POZO-py or PA and PMMA. The films that we obtained had an O.D. of around 0.1 and a film thickness of approximately 300 to 400 nm. In order to investigate the photophysical behavior of thin-film POZO-py, its UV−vis and fluorescence spectra and other photophysical parameters were measured. As shown in Figure 5 and Table 5, the UV−vis spectra of POZO-py-(0.73) and PA were red-shifted slightly and Pa was

stronger monomer emission was expected than for that under any other conditions, owing to the suppression of molecular motions. Although PA in the film state showed a slightly stronger emission than that it did in the glycerin solvent, POZO-py-(0.73) in the film state exhibited slightly weaker emissions than it did in the glycerin solvent. From the values of Pa, we considered that the preassociated dimer formed more tightly in the thin film than in the other solution states. Therefore, the fluorescence quantum yield of thin-film POZOpy-(0.73) decreased owing to the energy transfer from a monomer excited state to a low-energy dimer excited state. For the fluorescence lifetime measurements (Table 6), POZO-py-(0.73) showed results similar to those of PA. Both POZO-py-(0.73) and PA exhibited long-lived components (τ > 100 ns) and one component derived from monomeric excited species (τ = 32.2 and 30.8 ns, respectively). Furthermore, POZO-py-(0.73) showed short-lived excited species, and we considered that this component was derived from preassociation species, similar to what occurred in the ethanol solution. Similar to the results for the glycerin solutions, POZO-py(2.9) and POZO-py-(9.1) did not exhibit a clear dynamic excimer, showing a broad fluorescence emission that extended to 600 nm. As reported by Sluch,22 the molecular motion of the pyrene-forming dimer in the ground state was frozen. Therefore, it was difficult to form a parallel- or sandwichshaped dimer in the excited state. As a result, static excimer emissions were primarily observed. The value of Pa, which was less than that in any other state without water solution, is one piece of evidence that supports this conclusion. For the fluorescence lifetime measurements, minor components, which were expected from monomer excited species, and dominant components, which were expected to be due to preassociated

Table 5. Spectroscopic Parameters of POZO-py in the Thin Film State entry POZO-py(0.73) POZO-py(2.9) POZO-py(9.1) model PA

λabs [nm]

Pa

λem [nm]

Φfl

PM

PE

342

1.67

0.36

1.72

1.68

343

1.67

378, 388, 398, 420 388, 396, 407

0.24

1.46

1.34

343

1.59

0.11

1.48

1.22

339

2.59

0.28

1.79

1.68

378, 388, 398, 420 377, 388, 397, 420

less than that for the ethanol solution. These results indicated that thin-film POZO-py-(0.73) and PA each formed a preassociation. From the excitation spectra measurements, the values of PM and PE also provided evidence for such aggregation. For the fluorescence spectra, both POZO-py(0.73) and PA showed clear and strong monomer emissions. From the DSC results, we considered that the POZO-py films were in a glassy state at room temperature. Therefore, a

Table 6. Fluorescence Lifetime Decay of POZO-py in the Film State (λex = 340 nm, Room Temperature) entry

τ1

τ2

τ3

A1

A2

A3

χ2

POZO-py-(0.73) (380 nm) POZO-py-(0.73) (420 nm) POZO-py-(2.9) (380 nm) POZO-py-(2.9) (420 nm) POZO-py-(2.9) (500 nm) POZO-py-(9.1) (380 nm) POZO-py-(9.1) (420 nm) POZO-py-(9.1) (500 nm) model PA1 (380 nm) model PA1 (420 nm)

7.09 10.1 2.32 3.41 0.4 1.56 1.9 0.18 30.8 28.6

32.2 43.9 9.93 9.86 10.8 8.39 6.34 4.31 127.8 129.3

100.9 106.6 41.5 41.6 43 62 28 21.1

0.27 0.50 0.54 0.69 −0.32 0.82 0.72 −0.41 0.41 0.40

0.41 0.30 0.44 0.28 0.47 0.16 0.25 0.15 0.59 0.60

0.32 0.20 0.02 0.03 0.21 0.02 0.03 0.44

1.07 1.07 1.38 1.07 1.31 1.4 1.11 1.27 1.28 1.36

2334


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Scheme 2. Synthesis of PEtOZO-py-gela)

a

An asterisk indicates data estimated by 1H NMR.

NH-py. As shown in Figure 6, except for chloroform, the swollen and dried PEtOZO-py-gel exhibited similar fluores-

dimer excited species, were observed for both POZO-py-(2.9) and POZO-py-(9.1). In addition, clear rising components were also observed. Thus, it can be said that these results were quite similar to those for the ethanol solution. Pyrene-Modified Poly(2-ethyl-2-oxazoline) Gel (PEtOZO-py-gel). In order to prepare pyrene-modified poly(2-ethyl2-oxazoline) gel (POZO-py-gel), we used commercially available poly(2-ethyl-2-oxazoline). We expected POZO-pygel to possess a structure that can suppress the molecular motion of the pyrene dye on account of cross-linking. PEtOZO-NH was synthesized by the partial hydrolysis of PEtOZO and PEtOZO-NH-py was prepared subsequently by introducing pyrene-1-carboxylic acid into half of the amine sites of PEtOZO-NH. PEtOZO-py-gel was obtained by the reaction between the remaining amine sites of PEtOZO-NH-py and hexamethylene diisocyanate (Scheme 2).12c The swelling and photoluminescent properties of the PEtOZO-py-gel were determined; the values of the associated parameters are listed in Table 7. The results of the swelling behavior revealed that the degree of swelling was relatively high for good solvents of PEtOZO-

Figure 6. Fluorescence spectra of PEtOZO-py-gel swollen by various solvents (λex = 350 nm, room temperature).

cence spectra. Given that chloroform is the best swelling solvent, we expected that the mobility of pyrene moieties would increase. Consequently, pyrene might form a dimer and show broad red-shifted emission. For all the solvents and the dried gel, the same fluorescence quantum yields were obtained. Contrary to our expectation, the improvement in the fluorescence quantum yields for the model compound was small. This small degree of improvement might be attributed to the fact that the method for measuring the fluorescence quantum yields was not very accurate. This was because of the difficulty associated with removing oxygen and controlling optical density during the measurement of gel compounds.

Table 7. Swelling and Photophysical Properties of PEtOZOpy-gel

a

solvent

(W′ − W)/Wa

− chloroform ethanol DMF H2O

− 3.85 1.11 1.38 0.4

λem [nm] 378, 379, 379, 378, 379,

398, 398, 399, 397, 398,

419 419 421 419 419

Φfl 0.089 0.11 0.12 0.10 0.11

W: Weight of the dried gel. W′: Weight of the swollen gel. 2335


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S6−S15). This material is available free of charge via the Internet at http://pubs.acs.org.

Therefore, quenching caused by a high concentration in a local area around the chromophore or triplet oxygen might have prevented the fluorescence from contribution to the extent of the expected improvement. The swelling solvents, degree of swelling, and absorbance coefficient of N,N-dialkylpyrene-1-carboxamide moiety could be changed by adjusting the pyrene content, types of 2oxazoline, types of cross-linking reagents, and their ratios. Therefore, we believe that the swelling and photoluminescence properties of PEtOZO-py-gel can be controlled as desired.

■

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

■

CONCLUSIONS A novel fluorescent polymer, tertiary N,N-dialkylpyrene-1carboxamide modified-poly(2-methyl-2-oxazoline) (POZOpy), was successfully prepared by the condensation of partially hydrolyzed POZO-NH with pyrenecarboxylic acid. The photophysical properties of POZO-py (in both solution and film states) for various pyrene content ratios were analyzed. The absorption and fluorescence spectra of POZO-py-(0.73), which was the polymer with the lowest pyrene induction ratio, were very similar to those of the model compound PA. However, the fluorescence quantum yields of POZO-py-(0.73) were considerably higher than those of PA and improved with increasing viscosity of the surrounding media, especially for the most viscous solvent, glycerin (Φfl = 0.52). We believe that these enhancements in quantum yield were derived from suppression of molecular motion leading to internal conversion by introducing pyrene moieties onto the polymer chain and by interactions between polarized POZO units and the surrounding media. Therefore, the POZO chain induced a dramatic improvement in the fluorescence emission of the chromophore such that the emission was the same as that of the model compound. The photophysical behaviors of POZO-py-(2.9) and POZO-py-(9.1), which were more strongly pyrenemodified, depended on not only the viscosity but also the polarization of the surrounding media because of the occurrence of relatively large preassociation interactions in the ground state. The static and dynamic excimer emissions of POZO-py-(2.9) and POZO-py-(9.1) could be controlled by changing the conditions under which they were synthesized. In other words, it can be said that POZO, or other poly(2oxazoline), is one of beneficial substituent that can improve the photoluminescence properties of N,N-dialkylpyrene-1-carboxamide. Fluorescent POZO-py is easy to prepare, is amphiphilic, and should have the potential to be blended with several other polymers similar to POZO. Furthermore, POZO itself has been investigated widely, so we expect that these photofunctionalized POZO-py polymers will have many applications. We are now investigating the compatibility of POZO-py with other polymers and the luminescence properties of the resulting polymer blends.

■

AUTHOR INFORMATION

REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental section for N,N-dialkylpyrene-1carboxamide modified poly(2-oxazoline)s, synthesis conditions (Tables S1 and S2), 1H NMR spectra of all prepared samples without PEtOZO-py-gel (Figures S1 and S2), DSC and TGA spectra of POZO-py (Figures S3 and S4), fluorescence spectra of PA and PA-doped POZO (Figure S5), and fluorescence decay profiles of POZO-py in ethanol or thin flim state (Figure 2336


Macromolecules

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Polymer-Chain-Induced Tunable Luminescence Properties

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