Excited-State Dynamics of Pyrene Incorporated into Poly(substituted

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Excited-State Dynamics of Pyrene Incorporated into Poly(substituted methylene)s: Effects of Dense Packing of Pyrenes on Excimer Formation Tomohisa Takaya,*,† Tatsuya Oda,‡ Yuki Shibazaki,‡ Yumiko Hayashi,‡ Hiroaki Shimomoto,‡ Eiji Ihara,*,‡ Yukihide Ishibashi,‡ Tsuyoshi Asahi,*,‡ and Koichi Iwata*,† †

Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama 790-8577, Japan

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‡

S Supporting Information *

ABSTRACT: The excited-state dynamics of pyrene incorporated into poly(substituted methylene)s is investigated by picosecond time-resolved fluorescence spectroscopy and femtosecond time-resolved near-IR absorption spectroscopy in the 900−1400 nm region. The pyrene rings in poly(substituted methylene)s are photoexcited to the monomer excited state immediately after UV irradiation, followed by prompt excimer formation with time constants of a few picoseconds to a few hundred picoseconds. The excimer formation in poly(substituted methylene)s proceeds with much shorter time constants than that in pyrene-incorporated polyacrylates, vinyl polymer counterparts with the same sidechain structures, indicating the presence of stronger electronic interaction between the pyrene rings in poly(substituted methylene)s. The effects of every methylene substitution hold when each pyrene ring is connected to the polymer backbone with a monomethylene linker, while the effects are observed only weakly when a tetramethylene linker is employed. The results demonstrate the effectiveness of every methylene substitution in the prompt excimer formation of pyrene connected to the polymer backbone either directly or with the monomethylene linker.

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INTRODUCTION Photophysical properties of aromatic vinyl polymers have attracted much attention, primarily because the alignment of chromophores along the polymer main chain would bring about close interaction among them and some unique properties therefrom.1−3 Among a variety of the aromatic chromophores ever employed in related researches, pyrene is a representative one.4 Photoexcited pyrene can form an excited dimer, called an excimer, when a nonexcited pyrene is present in its vicinity. Since the ordinary excimer formation requires a collision between the excited and nonexcited pyrene molecules, its rate constant sensitively reflects the structure and flexibility of the polymer backbone when pyrene is incorporated into a polymer. As a method for the incorporation of pyrene as a vinyl polymer substituent, polymerization of vinylpyrenes is obviously most simple and direct, where pyrenes are directly connected to the main-chain carbon atoms with their aromatic frameworks. Indeed, photophysical properties of poly(1-vinylpyrene) and poly(2vinylpyrene) (Chart 1a) have been intensively investigated by using steady-state and time-resolved fluorescence measurements.5,6 As a result, it was revealed that depending on the substitution pattern on pyrene, full and partial overlap in the excimer formation occurred intramolecularly along the © XXXX American Chemical Society

polymer chain and unstable and stable excimers decayed with a lifetime of ca. 20 and 150 ns, respectively.5,6 Another important approach for pyrene-incorporated vinyl polymers is polymerization of (meth)acrylate monomers with a pyrene-containing ester substituent, where pyrene is connected to the C−C main chain with a linker unit including an ester linkage and a methylene chain with a certain length (i.e., C( O)O(CH2)n) (Chart 1b).7−17 Compared to the abovementioned poly(vinylpyrene)s, the repeating unit structure allows pyrenes much greater molecular motion because of the presence of the flexible linker group, thus leading to the ambiguousness in the interpretation of the photophysical phenomena derived from the aligned pyrenes. Accordingly, the number of reports on the photophysical properties of this type of polymers is limited so far. Instead, the pyrene-containing (meth)acrylates are frequently utilized as fluorescence probes because of the strong excimer emission, which is quite sensitive to the environment around the pyrenes, and their high copolymerizability with a variety of other acrylic monomers.13−17 Received: May 19, 2018 Revised: June 27, 2018

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Macromolecules Chart 1. Pyrene-Incorporated C−C Main-Chain Polymers

the substituent is extremely high as in poly(trityl methacrylate).34 Indeed, evidence for the rigid-rod-type conformation of some poly(substituted methylene)s has started appearing in some literatures.35−40 From a viewpoint of photophysical application, the combination of the dense packing of the chromophores and the rigid-rod conformation is quite attractive because, for example, by appropriate alignment of a series of chromophores with a gradient in absorption and emission wavelength in a regulated sequence by block copolymerization, efficient energy transfer could be realized if the densely packed chromophores allow efficient energy transfer among them. In order to realize such applications, we should first thoroughly investigate the fundamental aspects of photophysical events occurring in such polymer chains. Accordingly, with the future application as new photofunctional polymeric materials in mind, we have decided to investigate photophysical properties of pyrene-incorporated poly(substituted methylene)s in further detail. In particular, it is obvious that dynamics of the excimer formation in the polymers should be clarified because we can expect that the event should be greatly accelerated owing to the dense packing of the chromophores along the polymer main chain. Furthermore, for such investigation of the dynamics, it is reasonably considered that because of the dense packing of pyrenes, the time scale of the measurements should be much shorter, to a range of pico- to femtosecond orders, compared to the above-described example of poly(vinylpyrene)s.5,6 For that purpose, in addition to ultrafast time-resolved fluorescence measurements, we have decided to employ ultrafast timeresolved near-IR absorption spectroscopy, which is one of the powerful tools for the direct observation of the excimer formation because it can record the rise of the characteristic absorption by aromatic excimers regardless of their fluorescence quantum yields.41−44 In particular for pyrene, the decay of the monomer and the formation of the excimer are

Meanwhile, a group among the present authors (Ihara and Shimomoto et al.) and others have succeeded in developing an original method for C−C main-chain polymers [poly(substituted methylene)] using diazoacetates as a monomer.18−33 As shown in Chart 1c, the polymerization constructs the C−C main chain from one carbon unit, derived from N2 elimination of the monomer, thus affording a polymer structure where all the main-chain carbon atoms have an alkoxycarbonyl group (ester) as a substituent. Quite importantly, because of the polymer structure, the ester substituents should be densely packed along the C−C main chain, significantly enhancing the interaction among the ester substituents. From a photophysical perspective, the structural characteristic of the poly(substituted methylene) is quite intriguing because if a chromophore is introduced as an ester substituent, we can reasonably expect much more enhanced interaction among them than that observed in its vinyl polymer counterpart and appearance of unique photophysical properties therefrom. Indeed, in our previous publication, we have prepared polymers from 1-pyrenylmethyl diazoacetate and compared their photophysical properties to those of its vinyl polymer counterpart [poly(1-pyrenylmethyl methacrylate)].23 As a result, we have clearly demonstrated that excimer formation in the former polymer became more efficient to a great extent than in the latter on the basis of the observed ratio of excimer and monomer emission (IE/IM) after UV irradiation. The effect can be ascribed to the dense packing of pyrenes in the poly(substituted methylene). In addition to the dense packing of chromophores along the polymer chain, another crucial advantage of poly(substituted methylene) is that the presence of the substituents on all mainchain carbon atoms inevitably renders the whole polymer chain highly rigid. The characteristic is in sharp contrast to various vinyl polymers, which take random-coil conformation with lower rigidity because of the presence of an unsubstituted CH2 unit in every other main-chain carbon, unless the bulkiness of B

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probe pulse (900−1400 nm) was generated by self-phase modulation on a 3 mm thick sapphire plate. The angle between the pump and probe polarization was set at 54.7° (magic angle) for eliminating the effects of rotational relaxation. Sample solutions were circulated through a 2 mm thick quartz flow cell for avoiding accumulation of photoproducts on the cell window. All the measurements were performed at room temperature. Picosecond Time-Resolved Fluorescence Spectroscopy. The fluorescence time profile for the sample was measured using a timecorrelated single photon counting (TCSPC) system combined with an inverted microscopy (IX71, Olympus). As an excitation laser pulse, the picosecond 377 nm pulse laser (PLP-10 ps Light Pulser, Hamamatsu) was employed with the repetition rate of 5 MHz. An avalanche photodiode (PDM, MPD) and a counting board (SPC-130, Becker & Hickl GmbH) were used for signal detection. A band-pass filter in 10 nm fwhm (Asahi spectra) was placed in front of the avalanche photodiode to select monitoring wavelength and avoid the scattering of excitation light. The instrumental response function (IRF) was 150 ps, which was determined by the light scattering from aqueous silica colloids. The reconstruction of the time-resolved fluorescence spectrum was based on a method similar to that reported by Tamai et al.46 It was assumed that the relative integrated intensity of the fluorescence decay at each wavelength corresponds to that of the steady-state fluorescence spectrum. The number of photons was counted up to delay of 200 ns with a step size of 56 ps, and it was averaged over certain intervals (±200 ps) to obtain a TRF spectrum with a higher S/N ratio.

simultaneously traceable because both the monomer and the excimer exhibit an absorption band in the near-IR region.41,44 In this study, we prepared pyrene-incorporated poly(substituted methylene)s with three types of linkers, with different lengths connecting the pyrene to the ester moiety, and their vinyl polymer counterparts, and record their timeresolved fluorescence and near-IR absorption spectra with pico- to femtosecond time resolution in order to observe the excimer formation dynamics of the pyrene rings in poly(substituted methylene)s and to examine the effects of the dense packing. The detailed photophysical investigation focusing on the comparison between poly(substituted methylene) and its vinyl polymer counterpart would definitely reveal the advantage of the former with respect to the closer interaction among chromophores along the polymer main chain for the first time. Then, the advantage would ensure that poly(substituted methylene) is quite promising as a scaffold for effectively aligning a variety of chromophores in order to create a new type of photofunctional material. Accordingly, we strongly believe that the investigation in this paper is of considerable significance for the entire field of polymer science.

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EXPERIMENTAL SECTION

Synthesis. Synthesis of Poly1, Poly2, and Poly3. As a typical example, the polymerization procedure for poly1 is described as follows. Under a nitrogen atmosphere, a THF (1.0 mL) solution of allylpalladium(II) chloride dimer (0.44 mg, 1.2 × 10−3 mmol) was placed in a Schlenk tube and was cooled to −78 °C. NaBPh4 (0.98 mg, 2.9 × 10−3 mmol) was added to the mixture, and the resulting mixture was stirred at the temperature for 10 min. After a tetrahydrofuran (THF) (2.0 mL) solution of 7-tert-butylpyrenyl diazoacetate (0.15 g. 0.44 mmol) was added using a syringe at −78 °C, the temperature of the mixture was raised to room temperature, and it was stirred for 16 h at the temperature. After the volatiles were removed under reduced pressure, 1 N HCl/MeOH solution (5 mL), 1 N HCl aqueous solution (5 mL), and CHCl3 (10 mL) were added, and the organic layer was separated by using a separatory funnel. The organic layer was washed with 1 N HCl aqueous solution (50 mL) and saturated NaCl aqueous solution (50 mL) and dried over Na2SO4. After the volatiles were removed under reduced pressure from the organic layer, the residue was purified by using preparative size-exclusion chromatography (SEC) with CHCl3 as an eluent to give poly1 (51% yield, Mn = 8100, Mw/Mn = 1.47). Synthesis of Poly1′, Poly2′, and Poly3′. As a typical example, the polymerization procedure for poly1′ is described as follows. A mixture of 7-tert-butylpyrenyl acrylate (0.086 g, 0.26 mmol) and 1,1′azobis(cyclohexanecarbonitrile) (2.56 mg, 1.05 × 10−5 mol) in N,N-dimethylformamide (1.0 mL) was placed in a Schlenk tube, and it was degassed via three cycles of the freeze−pump−thaw procedure. The mixture was heated to 100 °C and stirred at the temperature for 20 h. After it was cooled to room temperature, volatiles were removed under reduced pressure. The crude product was purified by using recycling SEC to give poly1′ (60% yield, Mn = 8500, Mw/Mn = 1.20). Steady-State Absorption and Emission Spectroscopy. Steady-state absorption and emission spectra were measured by a V-570 (JASCO) spectrophotometer and an FP-8300 (JASCO) fluorometer, respectively, by using a fused silica cell with 1.0 cm optical length. Femtosecond Time-Resolved Near-IR Absorption Spectroscopy. Time-resolved near-IR absorption spectra were recorded with a laboratory-built femtosecond time-resolved near-IR spectrometer. The details of the spectrometer have been described elsewhere.45 The amplified output of a Ti:sapphire oscillator (Vitesse/Legend Elite, Coherent, 800 nm, 100 fs, 1 kHz) was used as the source of the pump and probe pulses. The pump pulse (345 nm) was prepared by fourth harmonic generation of signal output (1380 nm) from an optical parametric amplifier (OPerA, Coherent). The broadband

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RESULTS AND DISCUSSION

Polymer Synthesis. Pyrene-incorporated poly(substituted methylene) poly1−3 were prepared by polymerization of diazoacetates with pyrene-containing ester groups initiated by the π-allylPdCl/NaBPh4 system.23 Corresponding vinyl polymer counterparts poly1′−3′ were prepared by conventional radical polymerization of pyrene-containing acrylates using 1,1′-azobis(cyclohexanecarbonitrile) as an initiator. SECestimated Mn and Mw/Mn based on PMMA standards were as follows: poly1: Mn = 8100, Mw/Mn = 1.47; poly2: Mn = 10 900, Mw/Mn = 1.63; poly3: Mn = 12 800, Mw/Mn = 1.69; poly1′: Mn = 8500, Mw/Mn = 1.20; poly2′: Mn = 8300, Mw/Mn = 1.17; poly3′: Mn = 7900, Mw/Mn = 1.31. Steady-State Absorption and Fluorescence Spectra of Pyrene-Incorporated Poly(substituted methylene)s and Polyacrylates. Steady-state UV/vis absorption and fluorescence spectra were recorded for examining how strongly the pyrene rings interact with each other in the ground and Chart 2. Structure of Poly(1pyrenylalkoxycarbonylmethylene) (Poly1−3) and Poly(1pyrenylalkyl methacrylate) (Poly1′−3′) Synthesized in This Study

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Figure 1. Fluorescence spectra of poly1−3 and poly1′−3′ in THF (1.3 μg/mL) (λex = 332−347 nm).

Figure 2. Picosecond time-resolved fluorescence spectra of poly1−3 (a−c), and poly1′−3′ (d−f) with the photoexcitation at 377 nm. Each spectrum is normalized by the fluorescence intensity at its peak wavelength around 450−480 nm.

The peak position becomes shorter as the length of the linker between the pyrene rings and the polymer backbone becomes short. The excimer emission band of poly1 appears at shorter wavelength than that of poly1′ by around 20 nm, while the difference of the peak position becomes small as the linker length becomes longer. It has been reported that pyrene can form an excimer with a different structure from the excimer in ordinary solutions when they are packed in a crystal47,48 or placed in confined environments.4,49 The pyrene excimers in the confined environments show emission bands at shorter wavelength than the excimer in ordinary solutions by around 50 nm. The structure of the excimer may be strongly affected by the dense packing in the order of poly1, poly1′, poly2,

excimer states when they are incorporated into poly(substituted methylene)s and polyacrylates. The results are shown in Figure 1 and Figures S2−S4 of the Supporting Information. Both pyrene-incorporated poly(substituted methylene)s and polyacrylates show an absorption band between 300 and 400 nm with a clear vibrational structure. The positions of the subpeaks are almost identical with those of the pyrene monomer compounds for both polymers (Figures S2−S4). The identical peak positions indicate that the electronic transition energy of pyrene is negligibly affected by the degree of packing in the polymers. The fluorescence spectra of pyrene-incorporated poly(substituted methylene)s and polyacrylates show a strong excimer emission band between 410 and 600 nm (Figure 1). D

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Figure 3. Normalized fluorescence time profiles of poly1−3 (a−c), and poly1′−3′ (d−f) with the photoexcitation at 377 nm. Monitoring wavelengths are 440 nm (black) and 480 nm (460 nm) for poly1 (poly1′) (red), respectively. The gray solid line is the instrumental response function obtained by the light scattering from aqueous silica colloids. The solid traces represent the best fitted curves obtained by a doubleexponential function. The residuals are presented in the Supporting Information.

spectroscopy and was compared to that in the polyacrylates. Figure 2 show the time-resolved fluorescence spectra in the time region of 0−50 ns. The observed broad spectra can be safely assigned to the emission of intrapolymer pyrene excimers, which indicate that excimer formation finishes in the temporal resolution of the time-resolved spectroscopy, i.e., about 100 ps. In the case of polyacrylates poly1′, poly2′, and poly3′, a shoulder or a broad peak around 440 nm is observed immediately after picosecond-laser pulse excitation, and it disappears in a few nanoseconds. The spectral peak wavelength at 5 ns agrees with that of steady-state fluorescence spectrum for each polymer, followed by further peak shift to longer wavelengths with increasing of time. The fluorescence decay curves observed at 440 and 480 nm (460 nm for poly1′) are shown in Figures 3d−f, and the lifetimes are summarized in Table 1. It is clear that the excimer emission at the short wavelength decays faster than that at the long wavelength. These experimental observations are quite similar to the fluorescence dynamics of poly(2-vinylpyrene) reported by Yamamoto et al.6 They examined the monitoring wavelength dependence of the excimer emission decay of poly(2vinylpyrene) and argued that unstable excimers relaxed to the most stable excimer giving the excimer emission in the long wavelength in the time scale of a 10 ns order. Namely, an unstable excimer in which conformation between two pyrene moieties will distort from that in the stable (full-overlap) excimer gives the emission at the short wavelength. The distorted excimers formed in an early time region after photoexcitation change to the stable excimer according to

poly2′, poly3, and poly3′, if the dense packing predominantly determines the peak position of the excimer emission band. In the expanded spectra in the inset of Figure 1, we can compare monomer emission of each polymer normalized with their excimer emission intensities at their maximum emission wavelengths. In a similar manner as reported in our previous publication for the poly(1-pyrenylmethyloxycarbonylmethylene), a polymer with the structure of poly1 without a t-Bu group, and its vinyl polymer counterpart,23 the IE/IM ratio is higher in poly(substituted methylene)s poly2 (IE/IM = 89, λmax,E: 475 nm, λmax,M: 378 nm) and poly3 (IE/IM = 32, λmax,E: 486 nm, λmax,M: 376 nm) than in polyacrylates poly2′ (IE/IM = 20, λmax,E: 475 nm, λmax,M: 378 nm) and poly3′ (IE/IM = 8.0, λmax,E: 485 nm, λmax,M: 376 nm), respectively, indicating that excimer formation is more favorable in the former polymers because of denser packing of pyrenes around polymer main chains. Noteworthy is that the monomer emission of poly2 is remarkably small, suggesting that excimer formation in poly2 is quite favorable, and the characteristic is also supported by the time-resolved fluorescence spectra described in a later section. As for poly1 and poly1′, probably owing to the directly bonded oxygen to pyrene framework, the wavelength of monomer emission red-shifted (poly1: 387 nm; poly1′: 387 nm), resulting in the significant overlapping with other emission peaks which did not allow us to compare the IE/IM ratios for these polymers. Picosecond Time-Resolved Fluorescence Spectra. The excimer formation dynamics of pyrene in poly1, poly2, and poly3 in THF was examined by time-resolved fluorescence E

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spacer between the ester oxygen and pyrene, some of pyrene moieties would be isolated from others and the excimer formation from an isolated pyrene after photoexcitation is expected to be very slow. Femtosecond Time-Resolved Near-IR Absorption Spectra of Poly(1-Pyrenyloxycarbonylmethylene). The excimer formation dynamics of pyrene in poly1 was observed by femtosecond time-resolved near-IR absorption spectroscopy. The monomer excited state of pyrene in poly1 was prepared with the pump pulse at 345 nm. After time delays of 100 fs to 1 ns, absorption of the monomer excited state and/or the excimer state was detected with the near-IR probe pulse in the 900−1400 nm region. The results are shown in Figure 4a.

Table 1. Time Constants and Relative Amplitudes of Poly1, Poly2, and Poly3 Estimated by the Least-Squares Fitting Analysis of Time Profilesa compd poly1 poly1′ poly2 poly2′ poly3 poly3′

monitoring wavelength/nm 440 460 440 460 440 480 440 480 440 480 440 480

τ1/ns 3.4 4.9 3.0 5.1 3.3 5.0 0.17 5.3 2.6 0.30 0.30 1.2

(43%) (37%) (52%) (40%) (46%) (32%) (50%) (51%) (45%) (−46%) (46%) (−54%)

τ2/ns 18 21 15 18 28 31 3.6 25 23 23 3.0 4.5

(57%) (63%) (48%) (60%) (54%) (68%) (35%) (49%) (55%) (146%) (34%) (67%)

τ3/ns

17 (15%)

17 (20%) 21 (87%)

The decays were fitted to f(t) = I1 exp(−t/τ1) + I2 exp(−t/τ2) or f(t) = I1 exp(−t/τ1) + I2 exp(−t/τ2) + I3 exp(−t/τ3). Relative amplitude was calculated by Ii/∑Ii. a

molecular motion of the polymer backbone and the side chain and/or through excitation energy migration into the stable excimer in the polymer chain. By referring to the above results and discussion for the polyacrylates and poly(2-vinylpyrene), we now consider the excimer formation dynamics of pyrene in poly1, poly2, and poly3. In poly1, the fluorescence peak immediately after excitation is 440 nm, and it shifts to the wavelength of 460 nm in a few nanoseconds. As indicated in Figure 3 and Table 1, the excimer formation dynamics is similar to that of poly1′, although the relative amplitude of the short lifetime (3 ns) component of poly1 is slightly smaller than that of poly1′. In the comparison to poly(2-vinylpyrene) without any linker between a main chain carbon atom and pyrene framework, pyrene-incorporated polymers in this study with a series of ester-based linkers obviously have a greater degree of conformational freedom between adjacent pyrenes. In addition, we should consider the steric effect of the presence of a t-Bu group at the 7-position in pyrene, which would render the excimer formation unfavorable. Nonetheless, the formation of the stable excimer is faster than in poly(2-vinylpyrene). On the other hand, it will be notable that the spectral peak of poly2 and poly3 at 0.3 ns after excitation locates at the same wavelength to that of the steady-state fluorescence spectrum. Especially, poly2 exhibits a smaller change in the time-resolved spectra compared to the other polymers examined here, which suggests that the stable-excimer conformation is predominantly formed at the ground state. In other words, adjacent pyrenes in poly2 would be in the most favorable positions for excimer formation among the polymers investigated in this study. The consideration is in agreement with the largest IE/IM ratio of poly2 in the steady-state fluorescence spectra in Figure 1. In the case of poly3, although the dynamic peak shift of the excimer emission is small, a fluorescence peak at 410 nm corresponding to the pyrene monomer emission is observed at 0.3 ps (Figure 3c). This indicates that slow excimer formation in a nanosecond time scale occurs in addition to rapid excimer formation. Thus, a rise component having the time constant of 1.2 ns was observed in the fluorescence time profile at the excimer emission peak wavelength (480 nm), as shown in Figure 3f and Table 1. In poly3 with a tetramethylene (CH2)4

Figure 4. Femtosecond time-resolved near-IR absorption spectra of poly1 (a) and poly1′ (b) and the transient near-IR absorption spectrum of 1 at 1 ns (c) in dichloromethane with photoexcitation at 345 nm. The time evolution of absorbance changes at 1200 nm is plotted in (d) for poly1 (red) and poly1′ (blue). The solid traces in (d) represent the best fitted curves obtained by the least-squares fitting analysis using a linear combination of an exponential function and a constant.

An absorption band is observed immediately after the photoexcitation at 345 nm with an absorbance change of around 1 × 10−3 between 900 and 1100 nm. A similar absorption band is observed for the pyrenol form of the monomer, 1, with more distinct subpeaks at 950 and 1080 nm (Figure 4c). The absorption band of poly1 at 0 ps can be assigned to excited singlet states of the pyrene monomer from the similarity of the band shape to the band of 1, although the subpeaks of the monomer are almost lost. The loss of the subpeaks most probably results from substantial electronic interaction between a pyrene ring and its adjacent ring. The absorption band of poly1 becomes large and shifts from F

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Macromolecules around 1000 to 1200 nm as the time delay increases from 0 to 100 ps. The absorption band at 100 ps and later is assigned to the pyrene excimer according to the results of nanosecond time-resolved absorption spectroscopy.41,44 The absorption band does not show further changes between 100 ps and later delays, indicating that the excimer structure is fully stabilized within 100 ps. Pyrene in poly1 can form the excimer with a much shorter time constant than pyrene in solutions, where the excimer is formed in the nanosecond regime through translational diffusion. The excited-state dynamics of pyrene in poly1′ was observed with the same pump wavelength as the dynamics of pyrene in poly1 for investigating the effects of the introduction of pyrene into every methylene backbone. The results are shown in Figure 4b. A monomer absorption band of pyrene in poly1′ appears immediately after the photoexcitation at 345 nm. The monomer band holds the subpeak structure, although they are less clear than those of 1. The spectral shape indicates that the pyrene rings in poly1′ interact with each other but not as strongly as those in poly1. The weaker interaction is plausible because the pyrene rings in poly1′ have larger average distance between them than those in poly1. The monomer band of poly1′ is converted to the excimer absorption band between 0 and 300 ps. Poly1′ shows a longer rise time constant of the excimer absorption than poly1, as shown in Figure 4d. The slower excimer formation in poly1′ most probably results from the large average distance between the pyrene rings, which requires a large configurational rearrangement for forming the stack structure. Effects of Linker Length between Pyrene Rings and Polymer Backbone. The length of linkers between pyrene rings and the polymer backbone will affect the excimer formation dynamics if the dense packing plays a key role. We recorded femtosecond time-resolved near-IR absorption spectra of poly2 and poly3, in which the pyrene rings are connected to the backbone with an ester bond and a monomethylene or tetramethylene linker, respectively. The results are shown in Figure 5. Poly2 exhibits an absorption band with broad subpeaks at around 950 and 1100 nm immediately after the photoexcitation at 345 nm (Figure 5a). This band is assigned to the excited singlet states of the pyrene monomer from the comparison of the spectral shape with the absorption band of pyrenemethanol, 2 (Figure S6b). The excited pyrene monomer in poly2 shows much broader width of the subpeaks than the monomer in poly2′ (Figure S6a), indicating stronger interaction of the pyrene rings by every methylene substitution with the monomethylene linkers. The monomer band of poly2 is converted to a broad and strong absorption band with a peak at 1380 nm from 0 to 100 ps. The band at 100 ps and later can be assigned to the pyrene excimer, although the peak position is significantly downshifted from the excimer absorption band of poly1. Poly2 shows much faster spectral change than poly2′ (Figure 5c), indicating that the pyrene rings in poly2 can promptly change the configurations to those favorable to the excimer formation in comparison with those in poly2′. The absorbance of the excimer in poly2 slightly decreases between 100 ps and 1 ns while the absorbance in poly2′ approaches a constant value. The excimer in poly2 possibly decays via the singlet annihilation with a high probability, whereas the excimer in poly1 does not exhibit a decay due to the singlet annihilation in our measurements.

Figure 5. Femtosecond time-resolved near-IR absorption spectra of poly2 (a) and poly3 (b) in dichloromethane with photoexcitation at 345 nm. The time evolution of absorbance changes is plotted in (c) for poly2 (red) and poly2′ (blue) at 1380 nm and in (d) for poly3 (red) and poly3′ (blue) at 1200 nm. The solid traces in (c) and (d) represent the best fitted curves obtained by the least-squares fitting analysis using a linear combination of two exponential functions and a constant for poly2′, poly3, and poly3′ and three exponential functions for poly2.

Both the transient absorption bands of poly3 and poly3′ show subpeaks at 0 ps (Figure 5b and Figure S7a). Although the subpeaks of poly3 are broader than those of poly3′, their difference is smaller than the difference between poly1 and poly1′ or between poly2 and poly2′. The excimer absorption of poly3 rises faster than that of poly3′. The difference in the time constant, however, appears smaller than the difference for poly1 and poly2. These results indicate that the effects of the dense substitution are weak when the tetramethylene chain is used as the linker. Evaluation of Time Constants of Excimer Formation. The changes of the femtosecond time-resolved near-IR absorption spectra were analyzed in detail by the global least-squares fitting analysis. The time evolutions of the absorbance changes were simultaneously fitted at every 10 nm from 910 to 1410 nm with the sum of exponential functions with common time constants A (λ , t ) =

∑ Ai(λ)e−t /T

i

i

(1)

where Ai(λ) and Ti indicate the amplitude and time constant of the ith exponential function, respectively. Two-exponential functions are required for satisfactorily reproducing the time evolutions of poly1, poly1′, and poly2′ at all the wavelengths, whereas three-exponential functions are G

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efficiency of energy transfer along the poly(substituted methylene) chain is controllable by means of choosing the appropriate length of methylene linkers and ring substituents.

required for poly2, poly3, and poly3′. The obtained time constants are shown in Table 2. They show that the excimer

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Table 2. Rise Time Constants of Excimer Absorption of Poly(1-pyrenylalkoxycarbonylmethylene)s and Poly(1pyrenylalkyl acrylate)s Estimated by the Least-Squares Fitting Analysis of Time Evolutions at the Wavelength of Excimer Absorption Maximum compd

T1 (r1)a/ps

poly1 poly1′ poly2 poly2′ poly3 poly3′

28 66 2.1 (0.56) 180 27 (0.30) 25 (0.15)

T2 (r2)a/ps

32 (0.44) 250 (0.70) 320 (0.85)

CONCLUSIONS In this study, we have investigated the excited-state dynamics of pyrene incorporated into poly(substituted methylene)s by synthesizing pyrene-incorporated poly(substituted methylene)s with three different lengths of linkers, poly1−3, and their vinyl polymer counterparts, poly1′−3′, and using the timecorrelated single photon counting method and femtosecond time-resolved near-IR absorption spectroscopy. The pyrene rings form the excimer with a single time constant for poly1, poly1′, and poly2′ and with two time constants for poly2, poly3, and poly3′. At a fixed length of linkers, the pyrene rings in pyrene-incorporated poly(substituted methylene)s show faster excimer formation than those in pyrene-incorporated polyacrylates. Pyrene-incorporated poly(substituted methylene) with monomethylene linkers shows significant acceleration of the excimer formation, whereas that with tetramethylene linkers loses significant effects of the dense substitution. The present study suggests a possibility of inducing particularly strong interactions between substituents of poly(substituted methylene)s by optimizing the linker length.

⟨T⟩/psb 28 66 15 180 180 280

a

Calculated from the amplitudes at the absorption maximum wavelength of the excimer using eqs S22 and S23 in the Supporting Information. bThe amplitude-weighted average time constant.

formation in poly1 proceeds with 2.4 times as short time constant as that in poly1′. The difference can be explained well by the strong electronic interaction of the pyrene rings in poly1 as a result of every methylene substitution. The rise kinetics of the pyrene excimer in poly2 are reproduced well with two exponential functions, while the rise kinetics in poly2′ can be reproduced by a single-exponential function. Both the time constants of poly2 are much smaller than the time constant of poly2′. Moreover, the time constants of poly2 are smaller than or similar to the time constant of poly1 while the time constant of poly2′ is larger than that of poly1′. The monomethylene linkers will slow down the excimer formation if the pyrene rings do not interact with each other because it provides the pyrene rings with a number of configurations that prevent them from forming a structure favorable to the excimer. While poly1′ and poly2′ follow this model, poly1 and poly2 indicate the opposite results. The results strongly suggest that the pyrene rings in poly2 hold the strong electronic interaction between them as well as those in poly1 because of the dense packing. On the other hand, the time constants of poly3 are almost identical with those of poly3′ and much larger than those of poly2. The dense substitution is much less effective in the excimer formation of pyrene in poly3 because a large number of configurations provided by the tetramethylene linkers strongly prevent the pyrene rings from interacting with each other. As shown in Table 2, poly2, poly3, and poly3′ exhibit two rise time constants. A detailed analysis of the global leastsquares fitting results shows that the excimer formation proceeds in parallel under two different environments for pyrene in poly(substituted methylene)s, where a pyrene ring can either strongly or weakly interact with its adjacent rings (see Supporting Information for details). If we assume that the extinction coefficient of the excimer does not depend on the processes, the fractions of the monomer involved in the two processes, r1 and r2, can be calculated from the eqs S22 and S23 (see Supporting Information for details). The results are shown in Table 2. In poly2, more than half of the excited pyrene rings form the excimer with the time constant of 2.1 ps. Poly3 shows a smaller fraction of the monomer involved in the fast excimer formation, and poly3′ shows an even smaller fraction. From the results of the femtosecond time-resolved near-IR absorption measurements, we suggest that the

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01060. Monomer synthesis and characterization; steady-state UV−vis absorption spectra of the synthesized compounds in THF solutions; fluorescence lifetime analysis of poly1−3 and poly1′−3′; femtosecond time-resolved near-IR absorption spectra of poly2′ and poly3′; details of global least-squares fitting analysis for time-resolved near-IR absorption spectra; estimation of the ratio of excited pyrene monomers in two different environments (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected] (T.T.). [email protected] (E.I.). [email protected] (T.A.). [email protected] (K.I.).

ORCID

Tomohisa Takaya: 0000-0002-6071-4529 Eiji Ihara: 0000-0002-0279-5105 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant-in-Aids for Scientific Research on Innovative Areas “New Polymeric Materials On the basis of Element-Blocks (No. 2401)” (JSPS KAKENHI Grant JP15H00755), “Dynamical Ordering of Biomolecular Systems for Creation of Integrated Functions (No. 2501)” (JSPS KAKENHI Grants JP26102541 and JP16H00782), “Studying the Function of Soft Molecular Systems by the H

DOI: 10.1021/acs.macromol.8b01060 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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Concerted Use of Theory and Experiment (No. 2503)” (JSPS KAKENHI Grants JP26104525, JP26104534, JP16H00841, and JP16H00850), and “Application of Cooperative Excitation into Innovative Molecular Systems with High-Order Photofunctions (No. 2606)” (JSPS KAKENHI Grant JP26107011), a Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI Grant JP24350012), a Grant-in-Aid for Scientific Research (C) (JSPS KAKENHI Grant JP15K05521), Grant-in-Aids for Young Scientists (B) (JSPS KAKENHI Grants JP24750023 and JP16K17916), and a MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2015− 2019. The authors thank Applied Protein Research Laboratory in Ehime University for its assistance in NMR and Advanced Research Support Center in Ehime University for its assistance in elemental analysis.

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