Different Molecular Arrangement of Perylene in Metallic and

Article Cite This: J. Phys. Chem. C 2018, 122, 5805−5812

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Different Molecular Arrangement of Perylene in Metallic and Semiconducting Carbon Nanotubes: Impact of van der Waals Interaction Takeshi Koyama,*,† Kazuma Fujiki,† Yuya Nagasawa,‡ Susumu Okada,‡ Koji Asaka,† Yahachi Saito,† and Hideo Kishida† †

Department of Applied Physics, Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan

‡

S Supporting Information *

ABSTRACT: The arrangement of molecules on material surfaces has been a central focus in the fields of chemistry and physics. The molecular arrangement on two-dimensional surfaces has been extensively studied, and the electronic properties of substrates are one of the crucial factors for the arrangement. Recently, the arrangement of molecules in tubular materials can be investigated by using carbon nanotubes. In this study, the preferential molecular arrangement of perylene in single-walled carbon nanotubes (SWNTs), which is determined by their electronic properties, is reported. A combination of transmission electron microscopy observations, optical measurements, and first-principles calculations revealed differences in the molecular arrangement of perylene in metallic and semiconducting SWNTs. Perylene molecules in metallic SWNTs formed H-type molecular dimers, while those in semiconducting SWNTs were one-dimensionally stacked with their stacking axis directed along the nanotube axis. The difference in the molecular arrangement was discussed in terms of London dispersion force (van der Waals interaction) between the encapsulated molecules and the SWNTs. Our findings provide an insight into the application of SWNTs for encapsulating molecules as nanohybrid materials and nanoscale reaction chambers that possess effective functionalities.

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first is off-center positioning, which is observed in the encapsulation of β-carotene,28 squarylium dye,10 oligothiophene,12,29 and quaterrylene molecules.17 These molecules are positioned close to the nanotube wall (i.e., stacked with the nanotube wall). The other is centered positioning, which is observed for the encapsulation of phthalocyanine,11,30 coronene,14,16 perchlorocoronene,31 and octathio[8]circulene molecules.31 These molecules are stacked with each other such that their stacking axis coincides with the nanotube axis. Recent experiments on the encapsulation of perylene molecules into carbon nanotubes32 have indicated that the pattern of arrangement of the molecules, i.e., whether they are stacked with the nanotube wall or with each other, is dependent on the tube diameter. Theoretical calculations have indicated that the arrangement of encapsulated polycyclic aromatic hydrocarbon molecules such as coronene, sumanene, and corannulene depends on the molecular structure and tube diameter.33 Besides the tube diameter, as in the case of molecular adsorption on substrate surfaces, the electronic properties of carbon nanotubes are expected to influence the preferential arrangement of π-electron molecules in carbon

INTRODUCTION The molecular arrangement is crucial for developing cooperative functionality by utilizing molecular properties such as the conductivity of thin films of organic molecules and polymers1 and the magnetism-related phenomena of metallo−organic compounds.2 The electronic properties of substrates are one of determination factors for the arrangement of molecules on their surfaces. On some metals, π-conjugated molecules are aligned with their molecular face in contact with the surface (face-on configuration) such that the π-stacking of molecules occurs perpendicularly to the surface.3−5 On some semiconductors and insulators, on the other hand, molecules are aligned with their edge or end in contact with the surface (edge- or end-on configuration), resulting in π-stacking of molecules parallel to the surface. Recently, the arrangement of molecules in tubular materials can be investigated by using carbon nanotubes. Molecular encapsulation into carbon nanotubes has been studied over the past decades. These encapsulation techniques enhance the functions of encapsulated molecules6−18 and their polymerized materials,16,19−27 facilitating the development of novel functional nanohybrid materials. For the effective emergence of cooperative functionality and polymerization, control of the arrangement of encapsulated molecules is desirable. For π-conjugated molecules inside carbon nanotubes, two different molecular arrangements have been reported. The © 2018 American Chemical Society

Received: January 25, 2018 Revised: February 25, 2018 Published: March 6, 2018 5805

DOI: 10.1021/acs.jpcc.8b00860 J. Phys. Chem. C 2018, 122, 5805−5812

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The Journal of Physical Chemistry C

mL) with 2% weight sodium cholate by ultrasonication for 20 h (Branson, Sonifier 450, ∼19 W). Vigorous sonication separated individual nanotubes from nanotube bundles and made entrance holes for molecules on the nanotube walls. The dispersion was ultracentrifuged at 140 000g for 4 h (Hitachi Koki, 70P-72), and the upper 80% of the supernatant was collected. Gel column chromatography40,41 was then used to separate m-SWNT- and s-SWNT-enhanced dispersants. The SWNTs were collected by vacuum filtration on membrane filters (Millipore, MF-Millipore, 0.22 μm pore) by adding a large amount of water to remove the surfactants from the SWNTs. The typical diameter and thickness of the obtained SWNT films on the membrane filters were ∼20 mm and ∼300 nm, respectively. The SWNT films on the membrane filters were cut into several pieces. Three or four pieces were used as perylene/SWNT samples, while the others were used as reference samples. The SWNT films were transferred onto quartz substrates, and the membrane filters were then removed by acetone, leaving SWNT films on the quartz substrates. The encapsulation of perylene molecules inside SWNTs was performed by heat treatment.42 The SWNT film on the quartz substrate was sealed with perylene powder (∼1 mg quantity) at ∼3 × 10−5 Torr in a glass ampule. The sealed ampule was heated in an electric furnace (Isuzu, ETR-11K) at 200 °C for 72 h. After the heat treatment, the SWNT film was washed in toluene. Reference samples were prepared with the same heat treatment process in the absence of perylene powder. For comparison, perylene in hexane (solution-phase sample) and in the film form (solid-phase sample) was prepared. Another reference sample with perylene molecules attaching to the outside of SWNTs was also fabricated. Experimental Equipment and Setup. TEM images were taken using JEM-2010 operated at 120 keV.17 Absorption and Raman scattering spectra were measured using a Hitachi U3500 spectrophotometer and a Renishaw spectrometer with a cw diode laser at 488 nm (corresponding photon energy, 2.54 eV), respectively. Measurements of PL spectra were carried out with a home-built measurement system employing a singlegrating spectrometer with a liquid-nitrogen-cooled CCD camera.43 The excitation light source was a cw diode laser at 402 nm (3.08 eV). The sensitivity of the measurement system was measured by observing the spectrum of a standard tungsten lamp. It is noted that the PL measurement system can detect a Raman signal from doped polythiophene.43 In general, the Raman scattering light is orders of magnitude weaker than PL. Hence, PL from perylene molecules in the samples can be observed, even if the lifetime of the excited state of perylene decreases because of excitation energy transfer (EET), which often occurs between the encapsulated molecule and SWNT.10,12,14,17,28,29 Femtosecond transient absorption measurements were conducted based on the pump−probe technique (femtosecond pump−probe measurements).17,44 The pump photon energy and the pump fluence were 4.65 eV and 3.4 × 1014 photons cm−2 per pulse (0.25 mJ cm−2 per pulse), respectively. The ΔA spectrum was calculated by subtracting the absorption spectrum of the sample without pump-pulse irradiation from that with pump-pulse irradiation, measured at each delay time between the pump and probe pulses. All optical measurements in this study were conducted at room temperature in air. Calculation Procedure. Theoretical calculations were performed using density functional theory45,46 implemented in the STATE package.47 For the calculation of the exchange-

nanotubes. Although the difference of adsorption of rare gas atoms34,35 and a linear molecule (n-heptane)35 between metallic single-walled carbon nanotubes (m-SWNTs) and semiconducting SWNTs (s-SWNTs) has been studied, the dependence of arrangement of planar molecules on the electronic properties of carbon nanotubes has not been revealed to date. In this study, perylene molecules (Figure 1a) were encapsulated inside m- and s-SWNTs. The perylene (and its

Figure 1. Depiction of (a) perylene molecule, (b) isolated perylene (monomer), (c) H-type molecular dimer, and (d) one-dimensionally stacked column inside a SWNT. These images were drawn using VESTA (URL: http://jp-minerals.org/vesta/en/).62 (e, f) Typical TEM images for perylene/m-SWNT (e) and perylene/s-SWNT (f) samples.

derivative) is one of the representative planar π-electron molecules that have been long studied for adsorption on materials surfaces36−39 and a suitable trial molecule for the aim of our study. Transmission electron microscopy (TEM) observations for the fabricated samples showed the encapsulation of molecules. The optical properties of the samples were studied using optical absorption, Raman scattering, photoluminescence (PL), and femtosecond transient absorption measurements, and the results suggested the difference in the molecular arrangement of perylene inside m- and s-SWNTs (although some fractions of the molecules are present as monomers in both the SWNTs (Figure 1b)). In the m-SWNTs, perylene molecules formed H-type molecular dimers (Figure 1c), while in the s-SWNTs, the molecules stacked onedimensionally in a slanted orientation and face-to-face manner (Figure 1d). First-principles calculations supported the molecular arrangements. The difference in the arrangement was discussed in terms of the interaction of induced polarization, i.e., London dispersion force (van der Waals interaction) between a SWNT and a perylene molecule, which is stronger for m-SWNTs than for s-SWNTs.

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METHODS Sample Preparation. Samples were prepared using a procedure reported in our previous work.17 The SWNT materials were synthesized using an arc-discharge process (Meijo Nano Carbon, SO) with tube diameters of 1.4 ± 0.1 nm. The SWNT materials (25 mg) were dispersed in water (50 5806


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The Journal of Physical Chemistry C correlation energy among the interacting electrons, the generalized gradient approximation was used with the functional forms of Perdew−Burke−Ernzerhof.48 To describe the weak dispersive interaction between perylene and SWNTs, we considered the van deer Waals interaction by treating vdW-DF2 with the C09 exchange-correlation functional.49,50 An ultrasoft pseudopotential generated with the Vanderbilt scheme was used to describe electron−ion interactions.51 The valence wave functions and deficit charge density were expanded using planewave basis sets with cutoff energies of 25 and 225 Ry, respectively, which give sufficient total energy convergence for carbon materials.52 Structural optimization was performed until the remaining forces on each atom were less than 5 mRy/ Angstrom.

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RESULTS AND DISCUSSION TEM Observations. TEM observations give direct evidence for the molecular encapsulation in SWNTs and have been widely carried out in early studies.6−27 Figures 1e and 1f show typical TEM images for the perylene/m-SWNT and perylene/ s-SWNT samples, respectively. In the two images, two long dark lines running parallel to each other with a separation of 1.4 nm are the nanotube walls. The area between them is an inner space of the SWNT. In this area, some shorter dark lines are observed, showing the presence of encapsulated materials in SWNTs.6−27 Optical Absorption Measurements. Absorption measurements are useful for the investigation of the presence and form of molecules in a sample. The presence of molecules can be easily verified by their own absorption bands. Furthermore, the spectra are sensitive to molecular arrangement; for example, molecular aggregates show a change in the absorption band energy compared to those of isolated molecules (monomers). Absorption spectra for the perylene/m-(s-)SWNT and reference m-(s-)SWNT samples are shown in Figure S1 in the Supporting Information. To investigate the absorption bands of perylene molecules, the differential absorption spectrum was determined by subtracting the absorption spectrum of the reference m-(s-)SWNT sample from that of the perylene/m-(s-)SWNT sample. Figure 2a shows the differential absorption spectra for the perylene/m-SWNT (red curve) and perylene/s-SWNT (blue curve) samples. Both spectra show two principal bands at ∼2.7 and ∼4.8 eV. By referring to the absorption spectrum of perylene in hexane (green curve in Figure 2b), where two absorption bands are observed at 2.85 and 4.88 eV with vibrational progressions, the bands at ∼2.7 and ∼4.8 eV for the m-(s-)SWNT sample are assigned to the S0 → S1 and S0 → S2 transitions of perylene,53 respectively. The redshift and broadening of the bands are probably due to molecule− SWNT interactions and changes in the dielectric constant of the materials surrounding the molecules.10,17 The two principal bands indicate the presence of perylene molecules as monomers in both samples. In addition to the two principal bands, a large absorbance in the range of ∼3.0−3.5 eV and a large shoulder at 4.42 eV are observed in the spectrum for the perylene/s-SWNT sample. These additional structures show similar shapes to those of perylene in the film form (purple curve in Figure 2b), suggesting the presence of perylene aggregates. By assuming that the additional structures at ∼3.0−3.5 and 4.42 eV are caused by a blueshift and redshift of the principal bands at 2.77 eV (S0 → S1) and 4.84 eV (S0 → S2) due to the

Figure 2. (a) Differential absorption spectra for perylene/m-SWNT (red curve) and perylene/s-SWNT (blue) samples. (b) Absorption spectra for perylene in hexane (green) and in the film form (purple).

molecular aggregation, respectively, the molecular arrangement in the aggregates can be discussed. Since the transition dipole moments of S0 → S1 and S0 → S2 transitions are parallel to the x- and y-axis of perylene in Figure 1a, respectively,53 the blueshift and redshift of the S0 → S1 and S0 → S2 transitions for the aggregates are obtained in a one-dimensionally stacked column (Figure 1d and Figure S2) with an angle θ between the stacking axis and the molecular x-axis being above 54.5° and an angle ϕ between the stacking axis and the molecular y-axis being below 54.5°. Details of the calculations based on the extended dipole model54 are described in the Supporting Information. Raman Scattering Measurements. A one-dimensionally stacked column of perylene in the perylene/s-SWNT sample was suggested by the absorption spectra. Raman scattering measurements enable a more thorough understanding of molecular arrangements through the change in Raman peaks. For many types of encapsulated molecule/SWNT composites, the Raman peak intensity for the radial breathing mode of the SWNT is suppressed by a factor of 1/2.7 A similar suppression of this mode was observed in this work (see Figure S3 in the Supporting Information). In encapsulated molecule/SWNT systems, vibration modes for the encapsulated molecules are similarly expected to be suppressed because atomic displacements are prevented by the nanotube wall. This prevention of displacement should also be related to the arrangement of molecules in SWNTs. Figure 3 shows Raman scattering spectra for the perylene/m(s-)SWNT (red (blue) curves) and reference m-(s-)SWNT (black curves) samples with excitation photon energy of 2.54 eV. The PL components of perylene were subtracted from the spectra, and the intensities were normalized at 1593 cm−1 (squares in Figures 3e and 3f), which is the peak position of the G band of SWNTs. The contribution of perylene is negligibly small at 1593 cm−1.55 The excitation photon energy is resonant with the third and/or forth exciton transitions of s-SWNTs and not with the exciton transitions of m-SWNTs. Since the intensity of light emitted by the resonant Raman scattering is several orders of magnitude higher than that by the nonresonant scattering, the Raman signals for SWNTs in the 5807


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Figure 3. Raman scattering spectra for perylene/m-SWNT (red curve), perylene/s-SWNT (blue), and their reference (black) samples in the ranges of (a, b) 320−400 cm−1, (c, d) 510−590 cm−1, and (e, f) 1550−1630 cm−1. The excitation photon energy was 2.54 eV. The photoluminescence components were subtracted from the spectra, and the intensities were normalized at 1593 cm−1. (g) Peak height ratio among three Raman peaks at 352, 360, and 548 cm−1. The ratio was normalized at 352 cm−1.

perylene/m-SWNT sample are attributed to the residual sSWNTs. In the spectra for the perylene/m-SWNT and perylene/sSWNT samples, three typical Raman peaks of perylene are observed at 352 (circle), 360 (diamond), and 548 cm−1 (triangle). These peaks are attributed to the C−C−C bending mode with atomic displacement mainly in the x-direction (Figure 1a), that in the xy-direction (diagonal direction), and the C−C stretching mode with atomic displacement mainly in the y-direction, respectively.56 For the perylene/m-SWNT sample (Figure 3a), the peak height at 360 cm−1 from the background component of the reference m-SWNT spectrum is 1.23 times higher than that at 352 cm−1. In other words, the ratio of peak height at 360 cm−1 to that at 352 cm−1 is 1.23 (red diamond in Figure 3g). In contrast, for the perylene/s-SWNT sample (Figure 3b), the ratio of peak height at 360 cm−1 to that at 352 cm−1 is 0.92 (blue diamond in Figure 3g). The ratio of peak height at 548 cm−1 to 352 cm−1 is 0.72 for both samples (triangles in Figure 3g). Consequently, among the three ratios shown in Figure 3g, only the ratio at 360 cm−1 is lower in the perylene/s-SWNT sample than that in the perylene/m-SWNT sample, implying that, in comparison with the x- and ydirections, displacement in the xy-direction is prevented more effectively in the perylene/s-SWNT sample than in the perylene/m-SWNT sample. When the molecular shape of perylene is assumed to be rectangular, its apexes are closest to the nanotube wall for the molecular arrangement shown in Figure 1d. For this arrangement, extensions along the diagonal direction (xy-direction) can be prevented by the nanotube wall more effectively than those along the x- and y-directions. PL Measurements. PL spectra provide an additional insight into the arrangement of molecules. Figure 4a shows the PL spectrum of perylene in the perylene/m-SWNT sample with excitation photon energy of 3.08 eV. By referring to the PL spectra of perylene in hexane (green curve) and in the film form (purple curve) shown in Figure 4b, the spectrum for the perylene/m-SWNT sample is decomposed into two components, which consist of (1) a broad band at 1.95 eV (dashed curve), superimposed by fine structures, and (2) a weak shoulder above ∼2.3 eV with fine structures. These components with the redshift of ∼0.1 eV observed in the absorption spectrum correspond well to the PL spectra of perylene in hexane (perylene monomers) and in the film form, respectively.

Figure 4. Photoluminescence spectra of perylene in (a) perylene/mSWNT (red curve), (b) hexane (green) and the film form (purple), and (c) perylene/s-SWNT (blue) samples. Dashed curves are guides for the eye. The excitation photon energy was 3.08 eV.

The PL band in the perylene film in Figure 4b is called the E emission band57,58 and results from the fully relaxed excimer state in α-perylene crystals at room temperature, where coplanar perylene dimers (sandwich-like pairs, H-type dimers) are packed. Thus, some fractions of perylene molecules in the perylene/m-SWNT sample formed H-type dimers, which can approach each other in the photoexcited state to form the fully relaxed excimer. For the molecular arrangement in Figure 1c, excimer formation is free from the structural influence of the nanotube wall, and molecular separation was short enough to form a fully relaxed state. The fine structures on the broad band at 1.95 eV might be assigned to hot luminescence emitted during vibrational relaxation in excimers, which can be observed because of a short lifetime of the excited state (see below in the part of femtosecond pump−probe measurements); otherwise, 5808


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The Journal of Physical Chemistry C it is obscured by strong ordinary luminescence emitted from the relaxed excited state. Figure 4c shows the PL spectrum of perylene in the perylene/s-SWNT sample. Similar to the perylene/m-SWNT sample, the spectrum is decomposed into (1) a broad band at 2.25 eV (dashed curve) and (2) superimposing fine structures. The fine structures correspond to the spectrum of perylene in hexane in Figure 4b with the slight redshift, and they are attributed to perylene monomers. The peak energy of the broad band is not as low as that of the E emission band in Figure 4b (film) but consistent with the Y emission band, which has been observed for an α-perylene crystal at low temperatures57,58 and recently observed for perylene molecules in SWNTs at room temperature.32 The initial state of Y emission is the partially relaxed state, where the separation between the two perylene molecules is larger than that in the fully relaxed state.57,58 The Y-like emission in the perylene/s-SWNT sample can be explained by the molecular arrangement in Figure 1d. For the formation of the fully relaxed excimer59 in this arrangement, molecular displacement along the stacking axis and in its perpendicular direction is required. However, displacement in the perpendicular direction was structurally prevented by the nanotube wall, and perylene molecules could not form the fully relaxed excimer state but formed a partially relaxed state, which was the origin of Y-like emission. It should be noted that the PL spectrum for the perylene/mSWNT sample displayed stronger excimer PL than monomer PL, while the differential absorption spectrum shown in Figure 2a had no clear dimer band and had almost the same shape as that of the solution-phase sample. This is probably because the separation between molecules in the H-type dimer in the ground state is not short enough for the emergence of a clear dimer band (but short enough for the excimer formation in the excited state), resulting from a strong perylene−SWNT interaction compared with an intermolecular interaction in the perylene/m-SWNT composites as shown in a calculation part below. It is also noted that the PL spectrum of the reference sample with perylene molecules attaching to the outside of SWNTs shows a different spectral shape from those observed for the perylene/m-(s-)SWNT sample, which is seen as a superposition of the spectra of perylene in solution and solid phases (see Figure S4 in the Supporting Information). This result indicates that perylene molecules outside SWNTs show different optical response from those in SWNTs. Femtosecond Pump−Probe Measurements. For further investigation, femtosecond pump−probe measurements (for details, see Figure S5 in the Supporting Information) were carried out to compare the rate of EET between encapsulated perylene molecules and SWNTs, which is relevant to the arrangement of molecules in SWNTs. Figure 5 shows the time evolution of Δ(ΔA) for the perylene/m-SWNT (red curve) and perylene/s-SWNT (blue curve) samples probed at 2.80 eV, where the pump energy was 4.65 eV and resonant with the S0 → S2 transition of perylene. The energy of 2.80 eV approximately corresponds to the lower principal band of perylene molecules. Parameter Δ(ΔA) is the change in the absorption of perylene molecules in the samples caused by the pump pulses. For the perylene/m-SWNT sample, the Δ(ΔA) signal completely decays after ∼2 ps, indicating that the photoexcited state of perylene is relaxed by that time. In contrast, the Δ(ΔA) signal for the perylene/s-SWNT sample

Figure 5. Time evolutions of Δ(ΔA) for perylene/m-SWNT (red curve) and perylene/s-SWNT (blue) samples.

persists over ∼5 ps and displays longer decay behavior than that of the perylene/m-SWNT sample. These results are explained by the proposed molecular arrangements of perylene in SWNTs as follows. In previous studies, the lifetime of the photoexcited state of encapsulated molecules in SWNT was shown to be governed by EET to surrounding SWNT,10,12,14,17,28,29 and EET due to the Coulomb interaction between electric dipoles was shown to be much faster than that via electron exchange.17 For the two proposed molecular arrangements (i.e., the H-type dimers in the perylene/m-SWNT sample (Figure 1c) and the stacked columns (H-type aggregates) in the perylene/s-SWNT sample (Figure 1d)), the lowest excited states of the encapsulated molecules have even parity. Since the ground states of both the encapsulated molecules and the SWNT have even parity, the Coulomb interaction between the dipoles was not effective (dipole forbidden). The overlap of p orbitals between the molecule in the stacked column and the nanotube wall in perylene/s-SWNT was smaller than that between the H-type dimer (excimer) and the nanotube wall in perylene/m-SWNT. Therefore, the EET rate for the perylene/s-SWNT sample was expected to be lower than that for the perylene/m-SWNT sample, leading to the longer decay in the perylene/s-SWNT sample. Origin of the Different Molecular Arrangements. Here, we discuss the origin of the different molecular arrangements between the perylene/m-SWNT and perylene/s-SWNT samples. The difference of polarizability between m- and sSWNTs is considered. The π-electron molecules in a SWNT can stack with each other, as well as with the internal surface of the nanotube wall via van der Waals interactions. When the energy per molecule decreased by stacking with the nanotube wall is larger than that decreased by the one-dimensional stacking of many molecules, stacking with the nanotube wall is preferable. The polarizability along the nanotube axis in mSWNTs is orders of magnitude larger than that in sSWNTs.60,61 Thus, if the separation between the molecular plane of perylene and the nanotube wall is sufficiently short, the interaction of induced polarization (London dispersion force) between a perylene molecule and a SWNT is stronger in mSWNTs than in s-SWNTs, and stacking with the nanotube wall is preferred in m-SWNTs. Indeed, first-principles total-energy calculations based on the density functional theory with the van der Waals interactions support these molecular arrangements. Table 1 shows energy gain ΔE upon the perylene encapsulation into (11,11) and (19,0) SWNTs with its molecular orientation angle θ with respect to the SWNT axis. These two SWNTs have the almost same diameter of 1.49 nm. Table 1 indicates the encapsulated perylenes prefer their angles θ of 0° and 70° for both the 5809


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Table 1. Encapsulation Energy ΔE of Perylene in (11,11) and (19,0) SWNTs with Its Mutual Angle θ with Respect to the SWNT Axis θ

0°

10°

20°

30°

40°

50°

60°

70°

80°

90°

−ΔE for (11,11) (eV) −ΔE for (19,0) (eV)

1.346 1.307

1.209 1.193

1.174 1.185

1.017 1.020

1.020 1.037

1.188 1.200

1.268 1.275

1.337 1.339

1.265 1.273

1.242 1.255

SWNTs as their stable molecular conformations in SWNT. For (11,11) SWNT, ΔE at θ of 0° is about 9 meV lower than that of 70°, while for (19,0) SWNT, ΔE at θ of 70° is about 32 meV lower than that of 0°. Therefore, the perylene prefers the parallel arrangement with respect to the tube wall in mSWNTs, while they form a stacking columnar arrangement in sSWNTs. Since the calculation indicates that the electronic structure of perylene-encapsulating SWNTs retains the electronic structure of each constituent, the van der Waals interactions mainly contribute to the energetics of the perylene/SWNT. It is noted that the calculations were done for (11,11) and (19,0) SWNTs, which have slightly larger diameters than the average diameter of SWNTs in our experiments. For the molecular encapsulation in SWNTs with the smaller diameter, a finite value of ϕ might be required as suggested in the absorption spectrum. The details of the calculations will appear elsewhere.

CONCLUSION In this study, the molecular arrangement of perylene inside SWNTs with an average diameter of 1.4 nm was investigated. A one-dimensionally stacked column was found to form in sSWNTs, while an H-type dimer was formed in m-SWNTs. This difference in the molecular arrangement was explained by the polarizability along the nanotube axis in m-SWNTs being larger than that in s-SWNTs. These findings provide a key to preparation of one-dimensionally aligned materials that possess effective functionalities. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b00860. Details of absorption spectra, estimation of transition energies for a one-dimensionally stacked column of perylene molecules, Raman scattering spectra in the range of 130−210 cm−1, further PL measurement, and femtosecond pump−probe measurements (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-52-7894450. ORCID

Takeshi Koyama: 0000-0003-4468-3707 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank T. Tsunekawa for cooperation in sample preparation. This work was supported by JSPS KAKENHI Grant Numbers JP26107520, JP16H00908, The Tatematsu Foundation, The Ogasawara Foundation for the Promotion of Science and Engineering, and Research Foundation for the Electrotechnology of Chubu. 5810


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Different Molecular Arrangement of Perylene in Metallic and

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