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Polyimide and Imide Compound Exhibiting Bright Red Fluorescence with Very Large Stokes Shifts via Excited-State Intramolecular Proton Transfer II. Ultrafast Proton Transfer Dynamics in the Excited State Kenta Kanosue,† Ramunas Augulis,‡ Domantas Peckus,‡,§ Renata Karpicz,‡ Tomas Tamulevičius,§ ̅ § Sigitas Tamulevičius, Vidmantas Gulbinas,*,‡ and Shinji Ando*,† †

Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama 2-12-1-E4-5, Meguro-ku, Tokyo 152-8552, Japan ‡ Center for Physical Sciences and Technology, Savanorių Ave. 231, Vilnius LT-02300, Lithuania § Institute of Materials Science, Kaunas University of Technology, K. Baršausko g. 59, Kaunas LT-51423, Lithuania S Supporting Information *

ABSTRACT: A novel polyimide (PI) and imide compound emitting prominent reddish-orange fluorescence under excitation by UV light were prepared based on 3-hydroxypyromellitic dianhydride (PHDA), and their fluorescence properties were examined. The steady-state fluorescence spectrum of a PI film displayed an emission band at 590 nm with a very large Stokes shift (ν = 10 448 cm−1) via the excited-state intramolecular proton transfer (ESIPT), while the time-resolved fluorescence spectrum showed a rapid decay of the emission band of the enol form at around 400 nm within 15 ps. Transient absorption measurements showed an induced absorption and stimulated emission of the keto form with a time constant of ca. 3.0 ps, implying that ESIPT occurs on this time scale. Consequently, introduction of a hydroxy group into the pyromellitic moiety of PIs and imide compounds led to the long-wavelength ESIPT emission applicable to spectral converters having high thermal, mechanical, and environmental stabilities.

1. INTRODUCTION Polyimides (PIs) are a class of high-performance polymers that are widely known for their high thermal, mechanical, and environmental stabilities originating from their rigid molecular structures and strong intermolecular interactions.1 Moreover, they are characterized by low dielectric constants, low thermal expansion, light weight, good flexibility, facile preparation of films, and high radiation resistance. Therefore, they have been widely utilized in the electric, microelectronics, photonics, and aerospace industries. Recently, fluorescence emission properties of PI films have attracted much interest owing to the films’ applicability to new types of optical materials with high thermal and environmental stabilities.2−18 Highly fluorescent PIs are expected to be used as “wavelength downshift converters” for flat-panel displays, photovoltaic devices, and crop cultivators.19,20 Owing to their high thermal and chemical stability, the physical properties of PIs are tolerable even if they pass through tough device manufacturing processes at elevated temperatures over 350 °C. Optical absorption and fluorescence properties of PI films are generally investigated in the solid state because almost all PIs are insoluble in common organic solvents. However, it is difficult to clarify fluorescence properties of a single PI chain because PI chains undergo various intermolecular interactions in the solid state. Thus, to understand and predict fluorescence properties of © XXXX American Chemical Society

PIs, low-molecular-weight imide model compounds soluble in organic solvents are often used.2−7,17,18 Demeter et al.3 measured fluorescence spectra of several N-phenylnaphthalimide derivatives and investigated substituent and solvent effects on their fluorescence properties. They suggested that structural relaxation of the Franck−Condon state and rearrangement of the solvent molecules yield locally excited (LE) and charge transfer (CT) states, which emit fluorescence at shorter and longer wavelengths, respectively. In our previous work, we proposed a novel molecular design concept for “highly fluorescent PIs” based on the optical investigations of imide compounds and PIs with the aid of time-dependent density functional theory (TD-DFT) calculations.6 The most important strategy in this concept is the application of weak electron-donating alicyclic diamines instead of aromatics to restrain the intra/intermolecular CT interactions from the diamine to the dianhydride moieties. In this way, the electronic transition from the ground state to the lowest excited state (S0 → S1) becomes not a CT transition, but an LE transition. CT transitions in PIs generally have low oscillator strengths (f) due to the spatially separated HOMO and LUMO, which are respectively located at the diamine and dianhydride Received: October 8, 2015 Revised: February 6, 2016

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DOI: 10.1021/acs.macromol.5b02224 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Chart 1. Chemical Structures of 3H-PI and 3H-MC

moieties. For example, a thin film of a wholly aromatic PI prepared from pyromellitic dianhydride and bis(4-aminophenyl) ether (PMDA/ODA) exhibits only a weak fluorescence emission at 400−700 nm with a photoluminescence quantum efficiency (Φ) of 9.7 × 10−7.8,9 Meanwhile, a film of the semiaromatic PI prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and trans-1,4-diaminocyclohexane (s-BPDA/t-CHDA) exhibits an intense emission peak at 385 nm (Φ = 0.05), which is much stronger than that of PMDA/ODA.15 In our previous study, we successfully synthesized a semiaromatic PI using 1,4-bis(3,4dicarboxyphenoxy)benzene dianhydride (HQDEA) and 4,4′diaminocyclohexylmethane (DCHM), which showed a significantly high Φ value of 0.11 in the solid film state.6 This high Φ value was attributable to the flexible dianhydride structure, which can loosen the molecular packing or aggregation of PI chains in the solid film, as well as to the restraint of CT interactions through the use of an alicyclic diamine. In addition, we have reported that semiaromatic PIs with perfluorinated dianhydride structures, such as 1,4-bis(3,4-dicarboxytrifluorophenoxy)tetrafluorobenzene dianhydride (10FEDA) and difluoropyromellitic dianhydride (P2FDA), exhibit intense bluishgreen and red fluorescence, respectively.14 The fluorescent PIs developed thus far still have points to be improved for their application as wavelength converting materials, particularly small energy differences between the excitation and emission; i.e., Stokes shifts shall be enlarged. For example, the spectral sensitivity of crystalline silicon (c-Si) solar cells begins in the near-UV region (∼380 nm) and has its maximum in the near-infrared region (∼800 nm). Owing to the spectral mismatch between solar power and c-Si solar cells, the contribution of UV light to the electric power generation is very limited.20−22 In this case, it is highly preferable to convert UV light into visible light at longer wavelengths or near-infrared light. Therefore, in addition to the enhancement of their fluorescence quantum yields, the enlargement of Stokes shifts is strongly desirable for the fluorescent PIs for spectral converters. Recently, we reported that an imide compound, 3-hydroxy-Ncyclohexylphthalimide (3HNHPI), containing a hydroxy (−OH) group in the phthalic anhydride moiety, which forms a planar intramolecular hydrogen bonding (intra-HB) structure, displayed a very large Stokes shift (ν = 11 394 cm−1) via excitedstate intramolecular proton transfer (ESIPT).7,17,23−25 In the photophysical processes of 3HNHPI, an excited enol form generated by UV irradiation undergoes a rapid ESIPT process to generate an excited keto form. Since the keto structure is the most stable in the excited state, fluorescence is emitted from the keto form. Because of the major structural change in the excited state, the keto form exhibits an extraordinary large Stokes-shifted green emission. Polymers containing ESIPT units in their repeating structure have also been reported. Chu et al.26 clarified that a polymer that has two 2-(2′-hydroxyphenyl)benzoxazole (HBO) moieties in its backbone exhibits an ESIPT emission at 616 nm with a large Stokes shift of 7520 cm−1. Campo et al.27

reported that a benzazole containing poly(methyl methacrylate) (PMMA) exhibits various-color ESIPT emission responding to solvents and chemical structures of the benzazole moieties. Following this idea, we synthesized a novel semiaromatic PI (P2H-DC) by using pyromellitic dianhydride containing two −OH groups and found that P2H-DC exhibited a prominent red emission via ESIPT with a large Stokes shift (ν = 8994 cm−1).17 Moreover, the corresponding monoanion and dianion species were formed under basic conditions with a basic salt (NaOH), which was characterized by highly visible halochromism.17 Fluorescent PIs undergoing ESIPT seem to be effective for applications as wavelength converting materials, but their photoexcitation dynamics have never been investigated in detail. In order to precisely control their fluorescence properties, it is necessary to analyze the excited-state behaviors extensively. For detailed investigations on photoexcitation dynamics, femtosecond or picosecond spectroscopies are often used. Recently, various ultrafast spectroscopy techniques (e.g., transient absorption (TA) measurements, time-resolved fluorescence by means of streak camera or up-conversion method) have been applied for the investigation of ESIPT processes.28−38 AmeerBeg et al.31 examined the solvent dependence of ESIPT rate for 3-hydroxyflavone by measuring femtosecond TA (fs-TA) spectra by means of ultrafast pump−probe spectroscopy. The ESIPT rate in ethanol (60 fs) was slower than that in methylcyclohexane (35 fs) owing to formation of intermolecular hydrogen bonds with ethanol, which weaken the intra-HB. Fita et al.34 analyzed the fs-TA spectra of 2-hydroxynaphthylidene-1′-naphthylamine dissolved in methylcyclohexane and determined the ESIPT rate to be 20−30 fs. Ciuciu et al.37 analyzed time-resolved fluorescence spectra of an imidazole derivative dissolved in toluene by using a streak camera with a 2 ps time resolution and determined the lifetime of the enol form in the excited state to be 20 ps. Ultrafast spectroscopies have also been used for investigation of ESIPT processes of polymer solid films. Kim et al.32 measured TA spectra of a film of 2-[4-(4-methoxyphenyl)quinolin-2-yl]phenol and observed a glow of stimulated emission from the proton-transferred form at a delay time of shorter than 20 ps. Kim et al.35 measured time-resolved fluorescence spectra of a 2-(2′-hydroxyphenyl)benzoxazole derivative in solutions and in polystyrene film. They found that the ESIPT in a solid is faster than in a solution, which is due to the restraint of the twisting motion of the proton-donating −OH group in a solid film. As described above, ultrafast spectroscopies are useful tools for investigation of the ESIPT processes in solid samples, and it is highly expected that they will provide important knowledge about the photophysical dynamics in the excited state of fluorescent PIs. In this study, for the development of fluorescent PIs exhibiting very large Stokes shifts, a semiaromatic PI (3H-PI, Chart 1) that has an −OH group in the dianhydride moiety was newly designed and synthesized. In addition, an imide compound (3HMC, Chart 1) was synthesized as a model of 3H-PI, and optical B


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Macromolecules Chart 2. Enol/Keto Tautomerism of 3H-MC via ESIPT

employed. The oscillator produced 80 fs, 1030 nm light pulses at a repetition rate of 76 MHz, which were frequency tripled to 343 nm (HIRO harmonics generator, Light Conversion Ltd.), attenuated, and focused into an ∼100 μm spot on the sample, resulting in an average excitation power of approximately 1 mW/mm2. The maximum time resolution of the entire system was approximately 3.0 ps. A liquid helium coldfinger cryostat (Janis CCS-100/204) was used for low-temperature fluorescence dynamics measurements at a slow cooling regime. 2.3. Quantum Chemical Calculations. The time-dependent density functional theory (TD-DFT) was adopted for calculation of the electronic structures and spectroscopic properties of the enol and keto forms of 3H-MC. The long-range corrected functional (ωB97X-D) was used, and the geometry optimizations with the 6-311G(d) basis set were performed for the ground (S0) state.39 The basis set of 6-311+ +G(d,p) was used for generating molecular orbitals (MOs) and calculating vertical one-electron excitation wavelengths and oscillator strengths ( f) at the S0 geometries. All calculations were performed with the Gaussian-09C.01 program package, which implements analytical gradients at the TD-DFT level.40 This software package is installed in the Global Scientific Information and Computing Center (GSIC), Tokyo Institute of Technology.

investigations and TD-DFT calculations were performed to better understand the fluorescence properties and electronic structures of 3H-PI. The −OH group in the dianhydride moiety of 3H-PI and 3H-MC is highly expected to form intra-HB and undergoes ESIPT upon photoexcitation (Chart 2). In addition, ultrafast TA spectra and time-resolved fluorescence spectra were measured for both 3H-PI film and 3H-MC solution to investigate the details of the photoexcitation dynamics. The present study provides valuable knowledge for the development of highly fluorescent PIs that are more suitable for wavelength converting materials.

2. EXPERIMENTAL SECTION 2.1. Synthesis. The synthetic procedure of 3H-MC and the preparation method of 3H-PI film are described in the Supporting Information. 2.2. Measurements. 2.2.1. Steady-State Optical Measurements. The concentration of 3H-MC in chloroform (CHCl3) was set in the 10−5−10−4 M range. The solvent, CHCl3 (99.9%, Kanto Chemical Co., Inc., fluorescence grade), was used without further purification. UV−vis absorption and fluorescence excitation/emission spectra of the solution and the PI film were measured with a Hitachi U-3500 spectrophotometer and Hitachi F-4500 fluorescence spectrometer equipped with a Hamamatsu R928 photomultiplier tube, respectively. The fluorescence spectra of the solutions were measured without degassing. The frontface method was adopted for film samples to reduce self-absorption of the emitted fluorescence. Emission spectra were measured with the excitation at the peak wavelength (λex) of the corresponding excitation spectra. In contrast, excitation spectra were measured by monitoring the fluorescence intensity at the peak wavelength (λem) of the emission spectra. The measured spectra were not corrected for the sensitivity of the photomultiplier tubes to the fluorescence wavelength. The photoluminescence quantum efficiencies (Φ) of the solution and film samples were measured through another method by using a calibrated integrating sphere (Hamamatsu C9920) connected to a multichannel analyzer (Hamamatsu C7473) via an optical fiber link. In this measurement, the samples were excited at a controlled λex using a monochromated xenon light source, and the solution sample was degassed with argon prior to measurement. 2.2.2. Ultrafast Optical Measurements. Ultrafast photophysical processes were investigated by means of a transient absorption (TA) technique employing a transient absorption spectrometer (HARPIA, Light Conversion Ltd.) and time-resolved fluorescence spectroscopy with a streak camera (Hamamatsu C5680). The samples for TA measurements were excited using a Pharos ultrafast laser (Light Conversion Ltd.) with a regenerative amplifier generating 1030 nm, 290 fs duration pulses at 200 kHz. The proper wavelengths of 367 and 455 nm were obtained with an Orpheus optical parametric generator and Lyra harmonic generator (Light Conversion Ltd.). The samples were probed with a white light supercontinuum generated in a 2 mm thick sapphire plate excited with the fundamental laser radiation. The spectral supercontinuum range and the detection range were from 480 to 1000 nm. The excitation beam was focused to a spot approximately 500 μm in diameter, while the probe white light supercontinuum beam diameter was approximately 300 μm. For the streak camera measurements, a femtosecond Yb:KGW oscillator (Light Conversion Ltd.) was

3. RESULTS AND DISCUSSION 3.1. Optical Properties of 3H-MC in Steady State. Figure 1 shows the steady-state UV−vis absorption and fluorescence

Figure 1. Steady-state UV−vis absorption and fluorescence spectra of 3H-MC dissolved in CHCl3 (5 × 10−5 M). Emission spectra were monitored at λex of 367 nm (orange) and 446 nm (green). The emission spectrum excited at 446 nm is magnified by 5 times. Photo images of 3HMC CHCl3 solution were taken under irradiation by white light (upper) and UV light (λ = 365 nm) (lower).

emission spectra of 3H-MC dissolved in CHCl3 (5 × 10−5 M). Table 1 lists the peak wavelengths in the absorption (λabs) and excitation/emission (λex, λem) spectra (see Figure S1 in the Supporting Information) with the Stokes shifts (ν) and fluorescence quantum yields (Φ). In addition, Table 2 lists the calculated values of vertical excitation wavelengths, oscillator strengths ( f), dominantly contributing molecular orbitals (MOs), and assignments of one-electron transitions for the enol and keto forms of 3H-MC at the optimized S0 geometry. C


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(Figure 2). When excited at 372 nm, the ESIPT fluorescence (ν = 10 131 cm−1, Φ = 0.136) was observed at 597 nm (Figure

Table 1. Experimental Absorption Wavelengths (λabs), Excitation/Emission Wavelengths (λex, λem), Stokes Shifts (ν), and Photoluminescence Quantum Efficiencies (Φ) of 3H-MC (5 × 10−5 M) in CHCl3 and Solid State and Those of 3H-PI Film compound

λabs (nm)

λex (nm)

λem (nm)

νa (cm−1)

Φ

369 446

367 446

592 517

10356 3079

0.194

372 467

597 531

10131 2581

0.136

365 480

590 546

10448 2518

0.068

3H-MC (CHCl3 solution) 3H-MC (solid powder) 3H-PI (film) a

360 460

ν = 107 (1/λex − 1/λem).

The spatial distributions of the calculated MOs are illustrated in Figure S2. An intense absorption band observed in the UV−vis spectrum at 369 nm is attributable to the LE(π−π*) transition S0 → S1 of the enol form of 3H-MC (Chart 2) with a large f value of 0.1987 (see Table 2). In the emission spectrum when excited at 367 nm, an intense fluorescence band was observed at 592 nm (Φ = 0.194) with a very large Stokes shift (ν = 10 356 cm−1). This large shift strongly suggests that 3H-MC undergoes an efficient ESIPT process in CHCl3, and the fluorescence is essentially emitted from the keto form (Chart 2).23−25,41 According to the TD-DFT calculation of the keto form of 3H-MC, the S0 → S1 LE(π−π*) transition has a very large f value of 0.2462, resulting in the high Φ of the ESIPT emission (see Table 2). In Figure 1, no fluorescence band attributable to the enol form is observed, which should be due to the ultrafast proton transfer reaction from the enol to the keto form in the excited state. Such fast proton transfer is attributable to the high acidity of the −OH group of 3H-MC originated from the two electron-withdrawing imide groups.42,43 A separate weak absorption band is observed at around 440 nm in Figure 1, and a weak fluorescence band is also observed at 517 nm owing to excitation at 446 nm (Figure 1). According to the TD-DFT calculations of the enol form of 3H-MC, no absorption band is expected to appear at longer wavelengths than 400 nm (see Table 2). In order to clarify the origins of these weak absorption and fluorescence bands, we measured the absorption spectra of 3H-MC solutions with varying concentrations (see Figure S3). The intensity of the 440 nm band relative to the enol LE(π−π*) band at 369 nm increases with increasing concentration, suggesting that the former band is attributable to aggregated molecules in solution.44,45 To verify this assumption, solid-state fluorescence of a powdery sample was measured through excitation at two different wavelengths

Figure 2. Steady-state excitation/emission spectra of 3H-MC in solid state. Excitation/emission wavelengths are (a) 372 nm/597 nm and (b) 467 nm/531 nm, respectively.

2a), which is slightly red-shifted relative to that at 592 nm in CHCl3. This bathochromic shift may originate from the enhanced intermolecular π−π interactions in the solid state.46,47 In the emission spectrum with excitation at 467 nm, a strong fluorescence band was observed at 531 nm (Figure 2b) with a relatively small Stokes shift (ν = 2581 cm−1). This fluorescence is attributable to the densely packed state without undergoing ESIPT process. Since these wavelengths are close to the λex and λem of the unassignable bands at 446 and 517 nm in CHCl3, these bands are attributable to the aggregated form of 3H-MC in solution. 3.2. Steady-State Optical Properties of 3H-PI Film. Figure 3 shows the steady-state UV−vis and fluorescence emission spectra of a 3H-PI film. In the UV−vis spectrum, intense absorption bands observed at around 360 and 460 nm are attributable to the enol LE(π−π*) band of the repeating unit of 3H-PI and to the aggregated forms, respectively, because these peak positions are very close to those of 3H-MC in CHCl3 and in the solid state. Compared to the 3H-MC solution, the absorption

Table 2. Calculated Electronic Transitions of 3H-MC at Optimized S0 Geometry

form

state

transition wavelength (nm)

enol

S1 S2 S3 S1 S2 S3

317.8 311.3 302.9 446.9 377.0 332.7

keto

oscillator strength

orbitals

assignment

0.1987 0.0002 0.0046 0.2462 0.0000 0.0017

HOMO → LUMO HOMO−2 → LUMO HOMO−1 → LUMO HOMO → LUMO HOMO−1 → LUMO HOMO−2 → LUMO

π−π* n−π* π−π* π−π* n−π* π−π*

D


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dynamics, we performed time-resolved fluorescence and transient absorption (TA) experiments. Figure 4 shows the time-resolved fluorescence spectra excited at 343 nm and kinetics at three wavelengths for 3H-MC in CHCl3 at room temperature. The emission band observed at around 590 nm corresponds to the keto fluorescence, while the band at 400 nm was absent in the steady-state spectra (see Figure 1). We attribute this emission band to the enol LE(π−π*) fluorescence prior to undergoing ESIPT. At longer times, the enol fluorescence decays, while the intensity of the keto fluorescence increases. The presence of the keto fluorescence in the earliest time spectrum is due to the limited time resolution of the streak camera of approximately 3.0 ps. The fluorescence kinetics reveal that the ESIPT process is completed within ca. 20 ps, which agrees with the values reported for organic molecules in solution undergoing ESIPT phenomena.32 Figure 5 shows the TA spectra and kinetics at three wavelengths for 3H-MC in CHCl3 under excitation at 367 nm with a ca. 0.3 ps time resolution. A negative TA band observed at around 600 nm is attributable to a stimulated emission of the keto form because the band position coincides well with that of the keto fluorescence in the steady state (see Figure 1). In addition, an intense positive band observed at around 490 nm is attributable to the TA band of the keto form because the evolution rate of this band is close to that of the stimulated emission band at 605 nm (see Figure 5c) and also the same order of that of the keto fluorescence obtained by streak camera (see Figure 4b). Consequently, we attributed the growth of the concomitant TA changes below 500 nm to the ESIPT process. As shown in Figure 5c, the TA changes in different spectral regions can be well approximated by exponential processes with a 2.5 ps time constant, which is attributable to the rate of ESIPT. 3.3. Ultrafast Excited-State Dynamics of 3H-PI Film. Figure 6 shows the time-resolved fluorescence spectra and the temporal fluorescence evolutions at three wavelengths for a 3HPI film at room temperature and 15 K measured by a streak camera system. Similar to 3H-MC in CHCl3, the time-resolved fluorescence spectra at room temperature display a rapidly quenched enol fluorescence within ca. 15 ps at around 400 nm, which was not observed in the steady-state spectrum, either (see

Figure 3. Steady-state UV−vis absorption and fluorescence spectra of 3H-PI film. Emission spectra were monitored at λex of 365 nm (orange) and 480 nm (green).

of aggregated forms is much stronger in the 3H-PI film, which is explainable by the highly packed polymer chains in the solid film. In the emission spectrum when excited at 365 nm, a fluorescence band was observed at 590 nm (Φ = 0.068), which displayed a significantly large Stokes shift of 10 448 cm−1. This clearly indicates that ESIPT also takes place in the solid film of 3H-PI. In the emission spectrum when excited at 480 nm, a fluorescence band of the aggregated form was observed at 546 nm. The λex and λem of the aggregated form show slight bathochromic shifts compared to those of 3H-MC in the solid state (see Table 1). These shifts could be due to stronger π−π interactions of aggregated units in the 3H-PI film, which were formed during thermal imidization at 220 °C. The 3H-PI film looks yellow owing to the intense absorption in the blue-green region (400− 560 nm) associated with the aggregated form. The 3H-PI film shows a much larger Stokes shift (ν = 10 448 cm−1) with a high Φ value (Φ = 0.068) compared to the conventional PIs, such as PMDA/ODA. In addition, the thermal decomposition temperature at a 5% weight loss (T5d) was 408 °C for a 3H-PI film (the thermogravimetric analysis (TGA) curve of 3H-PI is shown in Figure S4), which indicates that 3H-PI films have sufficient thermal stability. 3.3. Ultrafast Excited-State Dynamics of 3H-MC Solution. To gain in-depth insight into the photoexcitation

Figure 4. Time-resolved fluorescence dynamics of 3H-MC in CHCl3 at room temperature measured by streak camera when excited at 343 nm: (a) spectrum at each delay time and (b) temporal fluorescence evolution at each wavelength. E


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Figure 5. TA spectra and temporal behaviors of 3H-MC in CHCl3 when pump wavelength was 367 nm: (a) overall view, (b) spectrum at each delay time, and (c) temporal behavior at each wavelength with fitting curves.

Figure 3). Within the instrumental resolution of the streak camera, the intensity-raising process of the keto emission at 500−700 nm was not clearly detected. However, it seems that the emission intensity of the keto form at 580 nm increased slightly within 5.0 ps (see Figure 6c). This very fast generation of the keto form agrees well with a femtosecond transient absorption experiment for other polymer films undergoing ESIPT phenomena.32 The emission band at around 500 nm, attributable to the aggregated form, was also already observed in the initial spectra (see Figure 6c). Because the aggregated form was not expected to absorb at 343 nm (see Figure 3), this fast appearance of its fluorescence band implies that the excitation energy of the enol form was transferred immediately to the aggregated form in the early stage before ESIPT took place. The enol fluorescence band at around 400 nm overlaps partially with the absorption band of the aggregated form ranging from 400 to 560 nm (see Figure 3), which indicates that a Förster resonance energy transfer (FRET) is feasible. At long times, the emission of the keto form becomes dominant, and subsequently, the spectrum coincides well with the steady-state fluorescence spectrum (see Figure 3). It should be noted that the relative emission intensity of the enol form to the keto form for 3H-PI film is significantly lower than that for 3H-MC in CHCl3 at the early times. Hence, we conclude that the FRET process from the enol form to the aggregated form is competitive with the ESIPT process in the 3H-PI film, and this process significantly affects the efficiency of ESIPT. We also performed streak camera measurements at cryogenic temperature of 15 K to gain further insight into the excited-state dynamics of 3H-PI. Figures 6d−f show the dynamics of the fluorescence spectra for a 3H-PI film at 15 K. At 15 K, the keto form emission at around 580 nm became much weaker than that at room temperature. In contrast, the enol form emission at around 400 nm became much stronger in the overall decay time range (0−17.5 ps). The temporal behavior at 400 nm (enol emission) at 15 K shows a much slower decay compared to that at room temperature (Figure 6c,f). At the very low temperature, significantly reduced overall kinetic energy (e.g., vibration, rotation) hinders the probability of ESIPT, resulting in a higher

population of the enol form compared to the keto form in the excited state.48,49 It should be also noted that the keto emission at 580 nm shows a slower increase at 15 K than at room temperature, which also supports the suppressed efficiency of ESIPT at the low temperature. This view is also supported by the steady-state emission spectra at lower temperatures. As shown in Figure S5, the intensity of ESIPT emission was apparently decreased below 173 to 77 K, whereas the emission from the aggregated form at around 520 nm was gradually enhanced. Because of the increased population of the excited enol form below 173 K, the FRET process from the enol to the aggregated form became efficient, resulting in the enhancement of the aggregated emission. Figure 7 presents the TA data obtained for the 3H-PI film. Note that the TA spectra of the 3H-PI film are upshifted in the whole spectral region, which should be caused by the presence of the aggregated form. In the TA spectra when excited at 455 nm, which corresponds to the excitation of the aggregated form, a broad TA band at around 700 nm and a negative stimulated emission band at around 550 nm were observed (see Figure S6). This stimulated emission partly cancels the TA bands below 520 nm, while the TA band above 640 nm contributes to the spectral upshift. The fast appearance of the TA bands of the aggregated form is due to the very fast FRET process in the early stage upon excitation. Similar to 3H-MC solution (Figure 5b), a positive TA band below 520 nm and a relatively negative band at around 600 nm are readily attributable to the keto form. Curve fit of the TA kinetics by biexpeonential function gives two time constants of ca. 3.0 ps and more than hundreds of ps. The slow decay component apparently corresponds to the relaxations of the aggregated form and keto form, while the fast spectral change is very similar to that observed in 3H-MC solution; it takes place with a very similar rate (2.5 ps) and thus should be also attributed to ESIPT. Consequently, TA data show that the ESIPT process in the 3H-PI film occurs with very similar rate as in 3H-MC solution. It should be also noted that the TA kinetics at 590 nm contains some additional subpicosecond processes. This could be caused by the already discussed FRET process from the enol form to the aggregated form; however, it may be also related to F


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Figure 6. Time-resolved fluorescence dynamics of 3H-PI film measured by streak camera (a−c) at room temperature and (d−f) 15 K when excited at 343 nm: (a,d) spectrum at each delay time, (b,e) normalized spectra, and (c,f) temporal fluorescence evolution at each wavelength.

such processes as exciton localizations on low-energy species or

4. CONCLUSIONS

nonlinear exciton annihilations often observed in TA inves-

A novel polyimide (3H-PI) and its model compound (3H-MC) featuring an −OH group in the pyromellitic dianhydride moiety undergoing ESIPT were prepared, and their fluorescence

tigations of molecular solids. G


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Figure 7. TA spectra and temporal behaviors of 3H-PI film when pump wavelength was 367 nm: (a) overall view, (b) spectrum at each delay time, and (c) temporal behavior at each wavelength with fitting curves.

controllable emission wavelengths as well as high thermal, environmental, and radiation stability.

properties were extensively investigated by using the ultrafast spectroscopy techniques. We summarize the entire picture of this study as Scheme 1. The time-resolved fluorescence spectra of

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ASSOCIATED CONTENT

* Supporting Information

Scheme 1. Schematic Representation of Excitation and Emission Mechanism in 3H-PI Film

S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02224. Synthesis part and figures showing the calculated MOs of the enol and keto forms of 3H-MC (TD-DFT method at the ωB97X-D/6-311++G(d,p) level), the concentrationdependent absorption spectra of 3H-MC in CHCl3, TGA curve of the 3H-PI film, the steady-state emission spectra of the 3H-PI film at lower temperatures, and the transient absorption spectra of the 3H-PI film when the pump pulse wavelength was 455 nm (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (S.A.). *E-mail [email protected] (V.G.).

both 3H-MC dissolved in CHCl3 and solid 3H-PI film measured by the streak camera displayed clear evidence of ESIPT, in which the enol emission at around 400 nm showed a rapid decay; in contrast, the keto emission at around 580 nm showed a rapid increase in intensity. For the 3H-PI film, the fluorescence band of the aggregated form also immediately appeared at around 500 nm after excitation, indicating that a FRET process from the enol to the aggregated form took place at an early stage. The transient absorption (TA) spectra of both samples displayed an induced absorption below 500 nm and a stimulated emission at around 600 nm, which are attributable to the keto form. The estimated rates of the ESIPT processes from the TA decays of less than 3.0 ps for both samples coincide well with those of conventional ESIPT molecules. We have revealed a new insight that the FRET process is competitive with the ESIPT process in the early stage upon excitation. We consider that this knowledge is important for understanding and development of ESIPT polymers with

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Program for Leading Graduate Schools “Academy for Co-creative Education of Environment and Energy Science”, MEXT, Japan. This work was partly supported by Grants-in-Aid for Scientific Research, Japan Society for the Promotion of Science (25288096).

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