Kinetically and Thermodynamically Controlled Nanostructures of

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C: Physical Processes in Nanomaterials and Nanostructures

Kinetically and Thermodynamically Controlled Nanostructures of Perylene-Substituted Lophine Derivatives Ryosuke Usui, Mitsuaki Yamauchi, Yukihide Ishibashi, Osamu Tsutsumi, Tsuyoshi Asahi, Sadahiro Masuo, Naoto Tamai, and Yoichi Kobayashi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01391 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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The Journal of Physical Chemistry

Kinetically and Thermodynamically Controlled Nanostructures of Perylene-Substituted Lophine Derivatives Ryosuke Usui,† Mitsuaki Yamauchi, ‡ Yukihide Ishibashi,§ Osamu Tsutsumi,† Tsuyoshi Asahi,§ Sadahiro Masuo,‡ Naoto Tamai,‡ and Yoichi Kobayashi*,† †

Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University,

Kusatsu, Shiga 525-8577, Japan. ‡

Department of Chemistry, School of Science and Technology, Kwansei Gakuin University,

Sanda, Hyogo 669-1337, Japan §

Department of Materials Science and Biotechnology, Graduate School of Science and

Engineering, Ehime University, Matsuyama, Ehime 790-8577, Japan

Corresponding Author *[email protected]

ACS Paragon Plus Environment

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ABSTRACT Supramolecular assemblies have been extensively studied because of their fundamental insights into intermolecular interactions and their potential applications to optoelectronic devices. Among various molecular building units, 2,4,5-triphenylimidazole (lophine) has been extensively studied because the molecule itself gives chemiluminescence and bright fluorescence, and because it forms various anisotropic nanostructures. While various potential applications such as to optoelectronic and lasing devices have been reported in lophine-based nanostructures, the relationship among molecular structures, nanostructures, and their optical properties remains elusive. This study reveals that the hydrogen bonding of the imidazole ring plays a crucial role for one-dimensional anisotropic nanostructures and their optical properties. The hydrogen bonding leads to thermodynamically stable nanofibers and gives the monomeric sharp emission, while it also induces the ultrafast nonradiative relaxation. On the other hand, the π-π interaction leads to kinetically favorable nanoparticles and gives the broad emission due to the excimer of the perylene moiety. The insight into the relationship among molecular structures, nanostructures, and optical properties are important for on-demand optical and electrical properties of lophine-based nanostructures.


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1. Introduction Supramolecular assemblies have been extensively studied not only because of their fundamental insights into intermolecular interactions and excitonic coupling1–3 but also their potential applications such as to luminescent materials, field effect transistor (FET), artificial photosynthesis, and lasing devices.4–11 Various supramolecular nanostructures have been reported by using different molecular units as building blocks such as cyanine, porphyrin, chlorophyll, perylene bisimide. Among them, 2,4,5-triphenylimidazole (lophine) has been received considerable attention because the molecule itself has several fascinating properties such as chemiluminescence and highly bright fluorescence in solution and because it is used as a precursor for fading-speed-tunable photochromic molecules12–16 and initiators for radical polymerizations.17 The lophine framework forms various types of anisotropic nanostructures by the combination of the hydrogen bonding of the imidazole ring and π-π interactions.18,19 These microcrystals have been applied to optical waveguides, FET, and lasing devices.20–23 Moreover, crystals of lophine derivatives show ferroelectricity24,25 and these microcrystals are promising for flexible ferroelectric materials.25 Lophine contains the N−H bond at the imidazole ring, which forms the hydrogen bond in solid states. The hydrogen bonds are supposed to play crucial roles in the molecular packing and their optical and electrical properties. However, the relationship among molecular structures, formed nanostructures, and their optical properties remain elusive. Recently, hydrogen-bond assisted self-assemblies have been applied to lasing devices and room temperature persistent luminescence.6,26,27 The investigations of these structure-dependent optical properties are particularly important to realize the on-demand optical and electrical properties of lophine-based nanostructures.

In this study, we synthesized perylene-substituted lophine derivatives as shown in Figure 1 to investigate their nanostructure-dependent optical properties. The N−H bond of the imidazole


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ring of 1 is replaced with the phenyl ring in 2. The comparison of nanostructures and optical properties of these two compounds reveals the effect of the hydrogen bonding on the shape of nanostructures and optical properties. The perylene moiety acts as a molecular indicator for the aggregation because it gives characteristic emissions depending on the aggregation. We analyzed the relationship among molecular structures, shapes of nanostructures, and optical properties in detail by using femtosecond transient absorption spectroscopy and picosecond fluorescence decay measurements of solutions, nanostructures, and crystalline solids.

Figure 1. Molecular structures of 1 and 2.

2. EXPERIMENTAL SECTION 2.1 Materials All reagents were purchased from Tokyo Chemical Industry (TCI) and Wako Co. Ltd. and were used without further purification. All reactions were shielded from light. Column chromatography was performed on alumina gel (200 mesh, Wakogel®). 2.2 Syntheses and Preparations of Nanostructures 4,5-Diphenyl-2-perylene-1-yl-1H-imidazole (1) 3-Perylenecarboxaldehyde (224.6 mg, 0.801 mmol), benzil (235.3 mg, 1.12 mmol), and ammonium acetate (940.3 mg, 12.2 mmol) were stirred at 80 °C in acetic acid (15 mL) for over 12 h. An orange precipitate was neutralized by adding ammonia water and was collected by filtration (Kiriyama No.5B). The precipitate was washed with water and then ethanol. The


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residue was dried in vacuo to give an orange powder. The powder was concentrated and purified by column chromatography on alumina gel using hexane/ethyl aetate (3/1) as eluent. Compound 1 was obtained as a yellow powder and the yield was 69 %. 1H NMR (DMSO-d6, 400 MHz): δ 12.9 (s, 1H), 9.2 (d, J = 8.4 Hz, 1H), 8.50 (dd, J = 10.0, 8.0 Hz, 3H), 8.44 (d, J = 8.0 Hz, 1H), 8.02 (d, J = 8.0 Hz, 1H), 7.84 (dd, J = 8.40, 5.60 Hz, 2H), 7.70-7.57 (m, J, 8H), 7.47 (t, J = 7.2 Hz, 2H), 7.41-7.34 (m, 3H) 7.27 (t, J = 7.6 Hz, 1H), MALDI-TOFMS: m/z 471 [M + H]+ 4,5-Diphenyl-2-perylene-1-yl-1-phenyl-imidazole (2) 3-Perylenecarboxaldehyde (224.3 mg, 0.800 mmol), benzil (253.0 mg, 1.20 mmol), aniline (111 mg, 1.19 mmol) and ammonium acetate (385.0 mg, 4.99 mmol) were stirred at 110 °C in acetic acid (15 mL) for over 12 h. An orange precipitate was neutralized by adding ammonia water. The precipitate was collected by filtration and then washed with water and ethanol. The residue was dried in vacuo to give an orange powder (60 %). 1H NMR (DMSO-d6, 400 MHz): δ 8.41 (dd, J = 7.2, 4.8 Hz, 2H), 8.36 (d, J = 7.2 Hz, 1H), 8.25 (d, J = 8.0 Hz, 1H), 8.04 (d, J = 8.4 Hz, 1H), 7.83 (dd, J = 8.8, 3.2 Hz, 2H), 7.59-7.52 (m, 5H), 7.45 (d, J = 8.0 Hz, 1H), 7.33 (s, 5H), 7.31-7.26 (m, 2H), 7.22-7.11 (m, 6H), MALDI-TOFMS: m/z 546 [M + H]+ Nanostructure solutions were prepared with reprecipitation methods.28,29 Namely, the saturated tetrahydrofuran (THF) solution (200 μL) of the compounds was rapidly injected to 4 mL water and the solution was stirred vigorously at 298 or 338 K. By changing the heating temperature and durations during and after the reprecipitation method, several batches of different nanostructure solutions were prepared. Four types of nanostructure samples (#1-#4) in each compound are introduced in this study. Nanostructures of #1 were prepared at 298 K, while those of #2 and #3 were prepared at 338 K and heated for 10 min at the same temperature. Nanostructures of #4 were prepared at 338 K and heated for 70 min at the same temperature. The effect of the amount of the THF solution on the size of nanostructures was shown in the


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SI. 2.3 APPARATUS Proton nuclear magnetic resonance (1H NMR) spectra were measured at 400 MHz on a JNM-ECS 400 MHz (JEOL). Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was performed with Axima-CFRplus (Shimadzu). The Xray diffraction data of the crystalline solids were collected on the Ultima IV (Rigaku). Steadystate absorption and emission spectra were measured with UV-3600 (Shimadzu) and FP-6500 (JASCO). Transmission electron microscope (TEM) and field emission scanning electron microscope (FE-SEM) images were collected with JEM-3100FEF (JEOL) and SU9000 (HITACHI), respectively at Nara Institute of Science and Technology (NAIST). Fourier transform infrared (FTIR) absorption spectra were measured with FT/IR-6100 (JASCO) with the spectral resolution of 0.48 cm−1. The relative emission quantum yields of solutions were measured by using coumarin 153 in ethanol as a reference.30 The absolute emission quantum yield of crystalline solids were measured by F-7000 (Hitachi) with a 415-nm excitation light. Emission decay measurements were conducted with the setup reported previously except the excitation light source.31 A cavity-dumped Ti:sapphire laser (KM labs, 830 nm, 4 MHz) was passed through a frequency-doubled β-BaB2O4 (BBO) crystal to generate 415 nm laser and the laser was used for the excitation pulse. The emission behaviors of the single aggregate shown in Figures 4 and S9-S11 were measured using a sample-scanning confocal microscope in combination with picosecondpulsed laser excitation at 470 nm (10.0 MHz, 90 ps full width at half-maximum). Briefly, the excitation laser was focused on the isolated aggregate by an objective lens (NA 1.4; Olympus). The photons emitted from the sample were collected by a same objective lens and passed through a confocal pinhole and long-pass filter (LP02-488-RU-25; Semrock) and short-pass filter (FF01-650-SP-25; Semrock). Subsequently, half of the photons were detected with a


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spectrograph (SpectraPro2358; Acton Research Corporation) with a cooled CCD camera (PIXIS400B Princeton Instruments). The remaining half of the photons were detected by an avalanche photodiode (APD) single-photon-counting module (SPCM-AQR-14; PerkinElmer). The signal from the APD were connected via a router to a time-correlated single-photon counting board (SPC-630; Becker & Hickl) for lifetime measurements. All measurements were performed at room temperature in a dark room. The emission decays were analyzed with least-square analyses with multiple exponential decay functions. The instrumental response function (IRF) was convolved with the decay functions for the analyses of solutions and crystalline solids, while simple multiexponential decay functions were used for emission decays of single nanostructure because the IRF of these experiments is too broad to analyze the fast decay component. Thus, the fast decay component of the emission decays of nanostructures was analyzed in detail by the emission decays of the solutions and crystalline solids. The goodness of the fitting was confirmed by the deviation (weighted residual) as shown below.30 ‫ܦ‬௞ =

ܰሺ‫ݐ‬௞ ሻ − ܰ௖ ሺ‫ݐ‬௞ ሻ ඥܰሺ‫ݐ‬௞ ሻ

where, ‫ܦ‬௞ , ܰሺ‫ݐ‬௞ ሻ, ܰ௖ ሺ‫ݐ‬௞ ሻ indicate the deviation, the measured data, and calculated decay data using assumed parameter values of k-th data points. Steady-state absorption and emission spectra for single nanostructures shown in Figure S12 were measured by using another microspectroscopic setup. For the steady-state absorption measurements of single nanostructure, a femtosecond white-light continuum was used as a probe light and focused with a 60x objective lens (NA 0.70). The white-light continuum was generated by focusing a 800-nm fundamental light of the amplified femtosecond Ti:sapphire laser pulse (Spectra-physics, Spitfire-ACE) into a 3-mm CaF2 glass plate,. The estimated beam diameter was 1 m. The transmitted light was collimated with 20x objective lens. The intensity


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was detected with a fiber-coupled high-sensitive CCD camera (Andor, Newton DU970P) with a polychromator (Andor, Shamrock163). For steady-state emission measurement, a 450-nm laser diode (Thorlabs, CPS450) was employed as an excitation light. The excitation light was focused into the sample with the 60x objective lens. The beam diameter at the focal point was estimated to be 3 m. The emission from a single nanostructure was collected with the same objective lens and passed through a confocal pinhole. The intensity was detected with the same CCD camera with the polychromator. Transient absorption measurements were conducted by using the setup reported previously except the slight modifications of the excitation light source.32 An optical parametric amplifier (OPA, TOPAS, Light conversion) was used to generate the 630 nm excitation pulse. The polarization of the pump beam is set at the magic angle (54.7°) with respect to that of the probe to eliminate polarization and photoselection effects.


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Figure 2. (a) TEM image of nanostructures of 1 prepared at 338 K and heated at the same temperature for 10 min (the condition is the same as #2 and #3 in Figure 2b). (b) Steady-state absorption and emission spectra of 1 in ethanol and nanostructures of 1 in water (#1-#4, see the experimental section for the detail of the sample preparations) at room temperature. The excitation wavelength is 450 nm. The emission spectra around 670 nm are omitted because of a noise by the instrument.

3. RESULTS AND DISCUSSION


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Figure 2a shows the transmission electron microscope (TEM) image of nanostructures of 1 prepared at 338 K during the reprecipitation method and heated at the same temperature for 10 min (the condition is the same as #2 and #3 in Figure 2b as explained later). Wide-field scanning electron microscope (SEM) images of nanostructures of 1 are shown in Figure S5. The TEM image shows that the thin and rigid nanofibers are formed by the reprecipitation methods. Wide field SEM images show that two types of nanostructures are mainly observed in the nanostructure solution, namely rectangular or cubic-like nanoparticles and nanofibers. The average diameter of thin nanofibers is approximately 15-40 nm, but thick nanofibers with the diameter of > 50 nm are also observed. It appears that the thick nanofibers are formed by the bundle of multiple nanofibers. The average size of the nanoparticles are tens-of-nanometers. The length of nanofibers are micrometer scales. The nano beam electron diffraction (NBD) measurements show that several parts of nanostructures have crystallinity, although most parts of nanostructures are amorphous (Figure S5). Figure 2b shows the steady-state absorption and emission spectra of 1 in ethanol and the nanostructure solution of 1 prepared in different conditions. In 1 dissolved in ethanol, the sharp absorption bands are observed at 420 and 450 nm, which are attributable to the π-π* transition of the perylene moiety and the imidazole ring. The emission bands are observed at 503 and 523 nm upon excitation with 450 nm. The relative emission quantum yield is 0.70 by using coumarin 153 in ethanol as a reference. Nanostructures of #1 are prepared at 298 K by reprecipitation methods, while those of #2 and #3 are prepared at 338 K and heated for 10 min. Nanostructures of #4 are prepared at 338 K and heated for 70 min. It should be mentioned that the sample preparation conditions of #2 and #3 are the same but the obtained spectra are slightly different depending on batches. However, the clear tendency is observed in absorption and emission spectra depending on the heating condition. The absorption spectrum of #1, which is prepared at room temperature, is broadened and shifts to the red as compared to that in ethanol.


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The emission spectrum is also broadened and shifts to the red, and the Stokes shift largely increases (~100 nm). The broad and largely red-shifted emission spectrum is a characteristic feature of the excimer emission of α-perylene crystals.33–35 Therefore, we assign the broad emission band to the excimer emission of 1. This assignment is consistent with the emission decay measurements, where the longer emission lifetime is observed in the broad and redshifted emission of nanostructures than that in ethanol as shown later. The longer emission lifetime than that of the monomer is a typical characteristic for the excimer emission. In #2 and #3, where the solutions are heated to 338 K for 10 min during the reprecipitation method, the additional absorption and emission bands appear at approximately 500 and 525 nm, respectively. Furthermore, #4, where the nanostructure solution is prepared at 338 K and heated for 70 min, has the sharper absorption and emission bands at 505 and 521 nm, respectively. The sharper absorption and emission spectra and slight red-shifted absorption spectra than those of monomer suggests that the well-aligned nanostructures are formed by heating. The crystalline solids of 1 formed by the recrystallization are needle-like microcrystals and give the similar sharp emission to that of #4. FTIR absorption spectrum of crystalline solids of 1 in a KBr plate has a broad absorption at 2600-3200 cm−1, while that of 2 does not (Figure S7). The broad absorption can be assigned to the N−H⋯N stretching mode of the imidazole ring following to the previous experimental and theoretical studies of imidazole.36 It indicates that the solid state of 1 has the hydrogen bonding network. Moreover, the TEM measurement of the heated sample shows that the nanofibers grow radially from the aggregated nanoparticles as shown in Figure 3. The growth of nanofibers from nanoparticles by heating indicates that nanofibers are thermodynamically stable nanostructures and that nanoparticles are kinetically formed nanostructures. This result also indicates that the broad excimer emission, which is mainly observed in the sample without heating (#1) of 1, is originated from kinetically formed nanoparticles. It also indicates that the sharp absorption and emission spectra, which is mainly


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observed heated nanostructures of 1 (#4), is originated from thermodynamically formed nanofibers. Moreover, nanostructures of 2 do not give any monomer-like sharp emission, and give only the excimer-like broad emission irrespective of the heating duration as shown in the SI (Figure S8). 2 forms wrapped nanosheets by the reprecipitation method and the aspect ratio of the nanostructure is much smaller than that of the nanostructures of 1 (Figure S6). This result strongly suggests that the hydrogen bonding of the imidazole ring is the key interaction for anisotropic nanofibers and the monomeric sharp emission. The reason why the excimer molecular arrangement is kinetically preferable than the hydrogen bonding is most probably that the π-π interaction of 1 is easily formed as compared to the hydrogen bonding. That is, various molecular arrangements can form the π-π stacking in 1 (including metastable arrangements). On the other hands, the molecular arrangements which can form the hydrogen bond is restricted to the direction of the N−H bond of the imidazole ring. In the case of 1, because the hydrogen bond is somehow stronger than the π-π interaction, the heating process gradually converts the π-stacked molecular arrangements to the hydrogen-bonded molecular arrangement. The monomeric emission feature is observed in nanofibers most probably because the hydrogen bonding network inhibits the π-stacked arrangement. Moreover, since the molecular arrangement induced by the hydrogen bonding is restricted, it is expected that molecules in the hydrogen bonding network are well aligned. This is probably the reason why nanofibers formed by the hydrogen bonding network has the sharp absorption and emission spectra. The small Stokes-shift and sharp emission features are similar to that of β-perylene crystal, which is one of the metastable crystal phases of perylene.33,37 In β-perylene crystal, the monomer-like sharp emission is observed because of the herringbone molecular alignment.33 The shape-dependent optical properties of 1 are further investigated in single nanostructure emission spectroscopy.


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Figure 3. TEM image of the nanostructure of 1 prepared at 338 K and heated at the same temperature for 70 min.

TEM measurements of nanostructures of 2 show that 2 forms wrapped nanosheets (Figure S6) as explained before. The lattice fringes are observed in TEM, and NBD measurements show the many parts of wrapped nanosheets have crystallinity. The absorption spectra of nanostructures of 2 do not change irrespective of heating duration once they are heated. This result suggests that wrapped nanosheets of 2 are thermodynamically stable nanostructures. All nanostructures of 2 give broad and red-shifted emissions assigned to the excimer emission irrespective of the heating conditions, although the emission peak is slightly different depending on the sample preparations (Figure S8). Since 2 does not have the hydrogen bonding, it shows that the π−π interaction is the origin for the excimer emission of nanostructures of 2. The details of 2 are shown in SI. The emission quantum yields of all nanostructures of 1 drastically decrease to < 0.01 as compared to that in ethanol solution (0.70). The emission quantum yields of 2 also decreases to 0.02-0.03 as compared to that in ethanol solution (0.92). However, the emission quantum yield of nanostructures of 2 is higher than those of 1. The absolute emission quantum yields of the crystalline solids of 1 and 2 are 0.03 and 0.09, respectively. Emission decay measurements


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reveal that the drastic decrease in the emission quantum yields of nanostructures is due to the intrinsic nonradiative relaxation pathways induced by the hydrogen bonding and carrier trapping as discussed later.

Figure 4. (a) Emission spectra and (b) decays of a single nanofiber and a nanoparticle excited at 470 nm (the probe wavelength is 488-650 nm). (c) Emission decays of the nanostructure solution of 1 (#2), 1 in ethanol, and the crystalline solid of 1. Values in bracket indicate the probe wavelength. The bottom figures of b and c indicate the deviations.

To investigate the detail of shape dependent optical properties of nanostructures, single nanostructure emission measurements are conducted. The sample is prepared by spin coating the nanostructure solution to the glass substrate (see the supporting information for detail). Emission microscope images are shown in Figure S9-S12. As was observed in SEM and TEM measurements, two types of nanostructures, namely nanoparticles and nanofibers, are observed in 1. Figure 4a shows single particle emission spectra of a nanofiber and a nanoparticle of 1. The spectra are averaged by several nanostructures. The emission spectra of each nanostructure are shown in the SI. As was expected in the solution experiments, the nanofiber of 1 gives the sharp emission spectrum and the peak is located at 532 nm, which is similar to the emission spectrum of #4 (Figure 2b). This emission spectrum is also similar to that of needle-like


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crystalline solids of 1 formed by recrystallizations (Figure S13), although the emission peak of the crystalline solid is shifted to the red. The detail of the crystalline solids is shown in the SI. It also indicates that the nanofibers, which is formed by the hydrogen bond interaction, is the thermodynamically stable nanostructures. The emission spectrum of nanoparticles is broad and shifted to the red, which is similar to the excimer emission of the solution of #1.These single nanostructure experiments also show that the spectral change of the emission spectra from #1 to #4 upon heating at 338 K (Figure 2b) is due to the transforms nanoparticles to thermodynamically stable nanofibers as shown in Figure 3. It should be mentioned that the emission spectra of single nanoparticle are different depending on the nanoparticles as shown in Figures S9 and S10. Since some spectra have a shoulder at around 525 nm, one of the reasons for the difference is probably due to the superposition of the small amount of the sharp nanofiber emission in some nanoparticles. Other possibilities may be due to the different molecular arrangement and the different local environments in different nanoparticles. The emission spectra of nanostructure solutions of #2 and #3 are most probably composed of the superposition of individual emission spectra of nanoparticles and nanofibers. Figure 4b show the emission decays of a single nanoparticle and a nanofiber excited at 465 nm. The emission decay of a single nanofiber of 1 has a large amplitude of the fast decay component and a small amplitude of the slow decay components. The emission decay of the single nanofiber is fitted with the three-exponential decay function. The lifetimes of the fast decay are within the instrumental response function (IRF, ~98%) and that of the slow decays are 1.2 ns (~1.5%) and 4.2 ns (

Kinetically and Thermodynamically Controlled Nanostructures of

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