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Acoustic Alignment of a Supramolecular Nanofiber in Harmony with the Sound of Music [ChemPlusChem]
[June 21, 2014]

Acoustic Alignment of a Supramolecular Nanofiber in Harmony with the Sound of Music [ChemPlusChem]


(ChemPlusChem Via Acquire Media NewsEdge) Audible sound with a low-frequency vibration brings about hydrodynamic alignment of a supramolecular nanofiber in solution. Design of the nanoscale molecules and molecular assemblies, which can sense a wide range of frequencies of the audible sound wave with high sensitivity, develops sound-driven molecular machines and sound-responsive nanomaterials, and is also interesting for investigation of unknown physical interactions between the molecules and audible sound vibrations. In this study, it was found that a supramolecular nanofiber, composed of an anthracene derivative AN,inann-hexane solution aligned upon exposure to an audible sound wave at frequencies up to 1000 Hz, with quick responses to the sound and silence, and to amplitude and frequency changes of the sound wave. These properties are of great advantage to sense dynamic changes of fluid flows, such as those induced by the sound of music. Music is composed of multiple complex sounds and silence, which characteristically change in the course of its playing time. When classical music was playing, the AN nanofiber aligned itself in harmony with the sound of the music. Time course linear dichroism spectroscopy revealed the dynamic acoustic alignments of the AN nanofiber in the solution upon playing the music. The sound vibrations of music, which generate acoustic streaming flows in liquid media, allowed shear-induced alignments of the nanofiber.



Keywords : acoustic alignment · linear dichroism · nanofibers · sound · supramolecular chemistry Introduction Sound is vibration of matter, having a frequency,[1] in which physical interactions occur between acoustically vibrating media and solute molecules or molecular assemblies. Control of dynamic molecular orientation in solution with sound waves has been an important research subject in sonochemistry, materials science, and nanotechnology.[2-10] It is known that solu- tions containing anisotropic macroscopic objects, such as poly- mers and colloidal particles, provide ultrasonically induced bi- refringence.[7-10] The timescale of ultrasonic vibrations with > 1 MHz frequency allows the formation of a microscopic ve- locity gradient or radiation pressure for the macromolecules in solution, which induces them to become aligned. However, it is difficult to see such a phenomenon with audible sound having a lower frequency in the range of 20-20 000 Hz, at which the wavelength is substantially longer than molecular length scales.[11] In line with this we recently reported that a de- signed supramolecular nanofiber, composed of a zinc porphyrin de- rivative in solution, showed linear dichroism (LD) upon exposure to an audible sound wave with si- nusoidal frequency.[12] The nano- fibers align in the solution along the direction of the sound wave. Although the detailed mechanism of the observed acoustic align- ment has not been clarified, the velocity gradient generated in the acoustically vibrating medium may allow shear-induced align- ment of the nanofiber.[13-16] With the added advantage of using audible sound for controlling the orientation of the nanofiber, we had the idea of develop- ing a supramolecular nanofiber capable of sensing the sound of music in its acoustic alignment. Music is an art form consist- ing of sound and silence expressed through time, and charac- terized by rhythm, harmony, and melody.[17] The question of whether music can cause any kind of molecular or macromo- lecular event is controversial,[18, 19] and the physical interaction between the molecules and the sound of music has never been reported. Herein, we show that our designed supra- molecular nanofiber in solution, when classical music was play- ing, brought about dynamic alignments in harmony with the sound of the music.

To design a supramolecular nanofiber that can dynamically align in response to the sound of music, we conceived the fol- lowing requirements : 1) a molecular structure that senses weak fluid motions of audible sounds leading to certain molec- ular actions beyond Brownian motions of solvent mole- cules ;[20-23] 2) capability of sensing and screening the complex sound mixtures of music over a wide range of audible frequen- cies ; and 3) showing quick responses to dynamic changes of the sound waves. The previous zinc porphyrin nanofiber was, however, inadequate for requirement (2), because it could only sense the lower-frequency sounds below 300 Hz, which covers only a limited range of the audible sound used in music. Fur- thermore, the solutions containing the nanofibers, which have large needle-shaped structures, scatter light and give notable noise in spectral measurements (Figure S1 in the Supporting Information). The hydrodynamic behavior of nanofibers may be modified by the length, thickness, and stiffness of the formed nanofibers, as well as their affinity with solvent mole- cules.[24, 25] Herein, we found that a supramolecular nanofiber, composed of an anthracene derivative AN having benzamide substituents bearing long chiral alkyl chains (Figure 1), can align by sensing higher-frequency sounds up to 1000 Hz with quick responses, and further, the solution dramatically reduces the light scattering property.


Results and Discussion Formation of supramolecular AN nanofiber Synthesis of AN nanofiber with hydrodynamic behavior in vortex flows has been reported previously by our group.[23] The AN molecules in n-hexane self-assemble by hydrogen bonding and p-p stacking interactions to form linear supramolecular polymers, which can further assemble to form one-handedly twisted helical bundles (Figure 1 b and Figures S2 and S3).[26-28] In this study, the sample n-hexane solutions were prepared by dilution of a CHCl3 solution of AN, allowing it to stand at 25 8C for 2 hours, and then stirring for 1 hour. Scanning electron mi- croscopy (SEM) of the sample prepared from a solution with a concentration of [AN]= 4.2^ 10^5 m shows that nanofibers formed under these conditions have linear fibrous structures, with approximate length distribution of 10-1000 nm (Fig- ure 2 a). They have, however, an almost uniform diameter of approximately 8 nm, which may form through bundling of a couple of single nanofibers with a diameter of approximately 3 nm (Figure 1 b). The nanofibers prepared at double the con- centration of AN (8.3 ^ 10^5 m) showed elongations of the linear structures without notable change of their diameter, with some entanglements (Figure 2 b). The attached chiral alkyl chains of AN at the 3-, 4-, and 5-positions of the phenyl groups may allow formation of such shape-consistent supra- molecular polymers, because an AN analogue with achiral do- decyl chains, AN_C12, provided larger nanofibers with nonuni- form diameters, which were hardly soluble in n-hexane solvent (Figure S4). Van der Waals interactions as well as steric repul- sions between the attached alkyl chains affect significantly the bundling of the single supramolecular polymers.

Acoustic alignment of the AN nanofiber with sinusoidal au- dible sound waves The orientations of AN nanofibers in solution can be character- ized by LD spectroscopy.[29-31] The LD spectrometer in this study was equipped with a 12 ^ 12 ^ 44 mm quartz optical cuv- ette with a f10 ^ 10 mm cylindrical neck (outer diameter), com- posed of 1 mm-thick quartz glass, which was filled with an n-hexane solution of AN (Figure 3 a). An n-hexane solution con- taining AN nanofibers with a concentration of 4.2 ^ 10^5 m at 25 8C displayed no LD response without irradiation by audible sound. Strong LD responses were obtained upon irradiation with sound in the frequency range of 100-1000 Hz (Figure 3 b). On exposure of the solution to 120 Hz sound with a sound pressure level of 31.6 Pa, intense LD bands were observed in the characteristic vibronic absorption bands of anthracene [402 (Do.d. = ^0.011; o.d. represents LD intensity), 380 (^0.007), and 362 nm (^0.004)] , which originated mainly from 1La, 1Lb transitions and polarization along the shorter in-plane axis.[32, 33] As the dip-coated thin film, in which nanofibers are oriented preferentially along the vertical direction, showed an analogous LD pattern (Figure S5), the nanofibers may also ver- tically align dominantly in the solution. However, no LD re- sponses were observed after attachment of a glass cap at the top of the cuvette, which prevents sound transmission into the solution, or if using a CHCl3 solution of AN, in which AN mole- cules cannot form self-assemblies. Further, the observed LD in- tensity decreased nonlinearly with increasing distance between the sound speaker and the optical cuvette (Figure S6). The LD intensity was also highly dependent on the frequency and am- plitude of the sound (Figure 3 c). The frequency up to 1000 Hz allowed LD induction ; the maximum Do.d. of ^0.013 was ach- ieved at 100 Hz (30 Pa).[34] Upon monitoring the LD response of the sample solution at 403 nm, and turning the sound on and off, the nanofibers quickly responded to the rise and fall of the fluid flows with an induction time of 8 seconds, includ- ing a leading time of 6 seconds (see below) and a relaxation time with a half-life of 10 seconds (Figure 3 d, blue curve). In this experiment, the induction time is defined as the time needed to reach the maximum LD intensity at the plateau of the LD profile, after turning the sound on. With the higher concentration of AN (8.3 ^10^5 m), which allows formation of longer fibers (Figure 2 b), both the induction and relaxation times for the acoustic alignment of the nanofibers increased to 15 seconds, including the leading time of 7-8 seconds, and 32 seconds, respectively (Figure 3 d, red curve). The LD intensi- ty is increased monotonically with increasing sound pressure level in the low concentration of AN, and this property allows quantitative evaluation of the sound intensity (Figure S9). In control experiments, no acoustic LD responses were observed for solutions of conjugated polymers, such as polystyrene and polycarbonate, which allow ultrasonically induced birefrin- gence.[10] The observed alignments of the supramolecular nanofibers with audible sound may, thus, occur through a mechanism different from those observed in the ultrasonic wave.

Mechanism of the acoustic alignment The wavelength of an audible sound with frequency of 120 Hz in n-hexane is 9.0 meters at 25 8C.[35] There is a significant size mismatch between the sound wave and nanoscale objects, so direct physical interaction, as observed in the ultrasonic wave and the polymers,[10] is hardly expected for nanoscale objects and the audible sound waves. From this and the above experi- mental result of the LD induction for sound irradiation, which showed a leading time of 6-8 seconds after turning the sound on (Figure 3 d, inset), we might assume that the nanofiber in- teracts predominantly with an acoustic streaming owing to the sound vibration. The acoustic streaming is known as a secon- dary steady flow generated from the primary oscillatory flow.[36, 37] Nonzero time averages of fluid mass transport occur because of attenuation and nonlinearity of the fundamental equations. We actually found that the diffusion rate of ink dropped into the solution was accelerated under exposure to audible sound (Figure 4 and Movies S1 and S2). In the case without sound irradiation, a little downward dispersion of the ink deposited on the surface of the solution was observed. However, in the case with sound irradiation, the ink, under ex- posure to a 120 Hz sound wave, flowed rapidly toward the bottom of the cuvette through the center of the sample solu- tion, and then it reversed flowing upward along the wall side of the cuvette. The overall flow in the glass cuvette was more accurately visualized by using a suspension containing small solid particles, in which both the downward and upward flows occurred synchronously at the center and the wall of the cuv- ette (Movie S3).

Hence, the observed leading time in the LD induction after turning the sound on, described above, may be the time before a strong acoustic streaming generates at the light pass of the LD spectroscopy. As the acoustic streaming occurs toward the sound propagation direction, and the flows dimin- ish slowly after turning the sound off, we then investigated the induction response of LD against the acoustic streaming of the fluid by generating short irradiation of the sinusoidal sound wave to the sample solution (Figure 5). When a sample solution with a concentration of 4.2 ^ 10^5 m was exposed to 120 Hz sound for 1.0, 3.0, or 5.0 seconds, the LD response oc- curred with induction times of 11.9, 10.8, or 8.9 seconds with LD intensities of ^0.008, ^0.009, or ^0.018, respectively (Fig- ure 5 a). The observed induction time was shortened with in- creased LD intensity by longer sound irradiation, which most likely accelerates the generation of the acoustic streaming. These results indicate that the observed acoustic streaming, owing to sound vibration, can bring about alignment of the supramolecular nanofiber. As lower-frequency sound, which causes larger vibration, could generate stronger acoustic streaming than higher-frequency sound in this system (Movie S4), the nanofiber might provide a higher LD response to lower-frequency sound. Here, the nanofiber aligns by sens- ing velocity gradients of the medium occurring in the crossing of the downward and upward streaming flows, and by the laminar flows generated around the glass surfaces of the vessel as a result of the acoustic streaming (Figure 6). In sup- port of this proposed mechanism, a larger LD intensity was ac- tually observed in pointwise LD spectroscopy around the wall side of the cuvette, including both the glass-surface boundary layer and the crossing area of the downward and upward flows (Figure S10). Further, no LD response was observed for acoustic LD spectroscopy with a larger 32 ^ 32 ^45 mm (outer diameter) quartz optical cuvette, which allows generation of more complex fluid mixtures in the solution under exposure to the audible sound. With this proposed mechanism, the notable changes in increase and decrease of the induced LD intensity while the sample solution was exposed to the sinusoidal sound wave, as observed in Figure 3 d, can be explained by complex fluid fluctuations owing to mixing of the local acous- tic streaming flows that occurred through the continuous sound irradiation of the sample solution (Figure 4 b and Movie S2).

We further found that AN nanofiber can respond to dynamic changes of the acoustic streaming, caused by alternate switch- ing of sound and silence in seconds. For example, if the sample solution containing AN nanofiber was exposed to 120 Hz sound for 3.0 seconds with a 3.0 second interval of si- lence, and subsequent 3.0 seconds of sound, the LD profile showed two clear peaks at 11.2 and 14.5 seconds with LD in- tensities of ^0.009 and ^0.015, respectively ; nevertheless, both the responses also showed time lags after the sound irra- diation (Figure 5 b). The observed peak interval corresponds to the applied period of silence, so the results indicate that dy- namic alignments of AN nanofiber occurred in response to the changes in acoustic streaming of fluid, generated by the dis- continuous irradiation of the sound wave, at light pass of the LD spectroscopy.

Dynamic alignment of a supramolecular nanofiber with the sound of classical music With the above LD responses of AN nanofibers to the sinusoi- dal sound waves in mind, we conducted LD spectral measure- ments of an n-hexane solution of the nanofiber while playing classical music. We initially chose Symphony No. 5 in C minor, first movement : allegro con brio, written by Ludwig van Beet- hoven, which is one of the most well-known symphonies in the world.[38] The first movement of this symphony has a typical sonata form and can be divided into four parts : exposition with a repeat, development, recapitulation, and coda. It begins by stating a distinctive four-note short-short-short-long motif twice (eight-note motif) to compose the introductory part of the music, which is known to represent "fate knocking at the door" (Figure 7 a, musical score). When we conducted a time course LD spectral measurement, monitoring at 403 nm, for the n-hexane solution of AN nanofiber while playing the music, a characteristic LD profile for the melody was obtained with high reproducibility (Figure 7 a, black curve). Representa- tively, the first and second four-note motifs in the introductory part gave strong LD peaks at 11 and 14 seconds, respectively. The sample solution then provided two peaks at 25 and 31 seconds, and multiple peaks at 48-60 seconds when the first theme of the exposition was played, and the LD signal became silent at around 60-75 seconds in the quiet part of the second theme. As the symphony continued, LD induction also occurred in response to the sound of music with the char- acteristic LD profile. The intense changes of LD intensity in the coda reflected the liveliest part of the music at around 390- 500 seconds. To clarify the mechanism of the observed phe- nomenon, the temporal waveform of the music was trans- formed into a band-filtered short-time root-mean-square (RMS) value corresponding to the acoustic power. The values were calculated in the frequency range of 100-1000 Hz with a mean running time of 1.0 seconds, and plotted with respect to the playing time of the music (Figure 7 a, red curve).[39] Quite inter- estingly, this profile fitted well with the observed LD profile, thus indicating that AN nanofibers in the solution clearly align and relax dynamically in response to the sound of the music. Some deviations of the peaks and gradient in the LD profile from the RMS profile were observed, but may be explained by such factors as induction and relaxation times of the LD re- sponses for the dynamically changing complex sound waves. The higher concentration of the AN molecule allows formation of the longer nanofibers, which have slower responses of rising and relaxation in their alignment in solution (Figure 3 d), and brought about a broadening of the observed peaks giving low-resolution LD profiles (Figure 7 a and Figure S11). For ex- ample, the characteristic two peaks, corresponding to the first and second four-note motifs, are unified in a single peak with the maximum at 16 seconds in the double concentration of AN. Hence, we conclude that the observed harmonization, the dynamic alignment of nanofiber to the sound of music, charac- terizes short-scaled AN nanofibers capable of moving quickly as well as reducing entanglements of the fibers in the solution.

As expected from these experimental results, the solution of AN nanofiber also showed the characteristic acoustic LD pro- files for a variety of music. A representative example of the LD profile obtained from "Symphony No. 40 in G minor, K. 550, first movement", written by Wolfgang Amadeus Mozart, is shown in Figure 7 b. In this music, the AN nanofiber also aligned in response to the changes of the acoustic streaming flows in the solution, generated by the sound and silence of the music to give the characteristic profile.

Conclusion A supramolecular nanofiber composed of anthracene deriva- tive (AN) molecules, dissolved in an n-hexane solution, is capa- ble of sensing weak fluid flows generated by audible sound waves through its hydrodynamic alignment. The sample solu- tion provided characteristic linear dichroism profiles under playing of classical music with high reproducibility, in which the nanofiber aligned in harmony with the sound of the music. The sound vibrations of the music, which generate acoustic streaming flows in liquid media, most likely allowed shear-in- duced alignments of the AN nanofiber. This study encourages investigation of the newly explored dynamics of molecules and macromolecules in sounds, and we hope this will develop into new acoustic nanotechnologies.

Experimental Section Materials Unless otherwise noted, reagents and solvents were used as re- ceived from Kishida Chemical Co., Ltd. (CHCl3 > 99 % and hexane > 96 %). For column chromatography, Wakogel C-300HG (particle size 40-60 mm, silica), C-400HG (particle size 20-40 mm, silica), and standardized aluminum oxide 90 (Merck) were used. Anthracene de- rivative AN was prepared by pro- cedures analogous to those re- ported in our previous study,[23] and was characterized unambigu- ously by means of 1H and 13C NMR spectroscopy, IR spectroscopy, and fast atom bombardment (FAB) mass spectrometry.

Structural characterization 1H and 13C NMR spectra were re- corded on a Varian INOVA 400 spectrometer (400 MHz for 1H) or a Bruker AVANCE 500 spectrome- ter (500 MHz for 1H and 125 MHz for 13C), for which chemical shifts (d in ppm) were determined with respect to tetramethylsilane as the internal standard. Infrared absorp- tion spectra were recorded on a JASCO FTIR-4200 Fourier trans- form infrared spectrometer. FAB mass spectrometry was performed on a JEOL JMS-BU30 LC Mate spectrometer with 3-nitrobenzyl alcohol as the matrix. Dynamic light scattering (DLS) measure- ments were performed using an Otsuka model ELS-Z2 instrument. Scanning electron microscopy (SEM) was performed using a JEOL JSM-7600F FE-SEM instrument op- erating at 30 kV. DLS measure- ments were performed using an Otsuka model ELS-Z2 instrument.

Synthesis of AN_C12 3,4,5-Tris(dodecyloxy)benzoic acid (196 mg, 0.29 mmol) was dissolved in thionyl chloride (5 mL), and the reaction mixture was heated at reflux for 4 h and then evaporated to dryness. The residue was mixed with 9,10-bis(aminomethyl)anthra- cene (0.02 g, 0.08 mmol) and N,N'- dimethyl-4-aminopyridine (0.07 g, 0.57 mmol) in dry CH2Cl2 (10 mL), and the solution was heated at reflux under Ar for 24 h and then evaporated to dryness. The resi- due was dissolved in CHCl3 and washed with aqueous solutions of 1 n HCl, saturated NaHCO3, and saturated NaCl. The organic layer was then extracted, dried over anhydrous Na2SO4, and evaporated to dryness. The residue was washed with CH2Cl2/CH3OH to leave AN as an orange oil in 98 % yield. 1H NMR (500 MHz, CDCl3,208C): d=8.45 (dd, J=3.1, 7.0 Hz, 4H; anthracene), 7.63 (dd, J = 3.1, 7.0 Hz, 4 H; anthracene), 6.90 (s, 4H; phenyl), 6.19 (t, J =4.5 Hz, 2 H; amide), 5.64 (d, J =4.5 Hz, 4 H; methylene), 3.95 (t, J=6.5Hz, 4H;^OCH2^), 3,91 (t, J=6.3Hz, 8H; ^OCH2^), 1.76-1.66 (m, 12 H; alkyl), 1.46-1.36 (m, 12 H; alkyl), 1.33- 1.21 (br, 96H; alkyl), 0.89-0.86 ppm (m, 18H; methyl); 13C NMR (125 MHz, CDCl3,208C): d= 167.27 (C=O), 153.30 (aromatic), 141.81 (aromatic), 130.58 (aromatic), 130.26 (aromatic), 129.03 (aromatic), 126.85 (aromatic), 124.95 (aromatic), 106.23 (aromatic), 73.66 (^ OCH2^), 69.70 (^OCH2^), 37.14, 32.09, 30.46, 29.9-29.7 (m), 29.54, 26.22, 14.25 ppm ; FAB-MS : m/z calcd for C102H168N2O8 : 1549 [M +]; found : 1550 (M+H+ ) ; elemental analysis calcd (%) for C102H168N2O8 : C 79.02, H 10.92, N 1.81; found : C 79.35, H 11.10, N 1.73.

Acoustic LD spectral measurements Prior to spectral measurements, sample n-hexane solutions ([AN] = 4.2^ 10^5 or 8.3^ 10^5m) were prepared by dilution of a CHCl3 so- lution of AN ([AN]=5.0^ 10^2m), allowed to stand in the dark at 25 8C for 2 h, and then stirred at 1350 rpm in both clockwise and counterclockwise directions for 1 h. LD spectra were recorded using a JASCO type J-820 spectropolarimeter equipped with a JASCO type PTC-423L temperature/stirring controller and a custom-made sound generator ; the latter comprised a digital function generator (NF model DF1906), a compact disk (CD) player (DENON model DCD-755SE), an integral amplifier (DENON model PMA-390AE), and a sound speaker (AURA Sound model NS3-193- 4A). The LD spectrometer was equipped with a 12 ^12 ^44 mm quartz optical cuvette having a f10^ 10 mm cylindrical neck (outer diameter), composed of 1 mm-thick quartz glass, which was filled with an n-hexane solution of AN. LD intensity is defined as DLDA = Ak^A ? (DLDA represents the magnitude of LD, and Ak and A ? denote horizontal and perpendicular absorbances, respectively). The cuvette containing the sample solution was fixed in the steel holder of the spectrometer. For the acoustic LD spectroscopy per- formed in this study, LD responses were monitored at a position 39 mm below the top of the optical cuvette by using an 8 mm-di- ameter beam of linearly polarized light. Sound waves, produced by a function generator or a CD player, were intensified by an amplifi- er to give sinusoidal audible sound with variable frequency or the sound of the music generated from a speaker, respectively. A full- range speaker, with a frequency response of approximately 15 kHz, was located 20 mm above the cuvette. The sound pressure varied with the frequency (100-1000 Hz) and covered the range 18.9- 31.6 Pa (Figure S7 b). The received sound pressure is proportional to the applied voltage of the function generator (Figure S7 c). Time course LD spectral measurements were demonstrated by plotting LD intensities at 0.1 s intervals with 0.032 s response. The acoustic LD spectral measurements were taken when Symphony No. 5 in C minor, first movement : allegro con brio, recorded in "Great Beet- hoven Songs" (KPTC-3009, 2010, Snapper Bay's Music Company) or Symphony No. 40 in G minor, K. 550, first movement : molto allegro, recorded in "KUBELIK MOZART" (SICC 258, 1981, Sony Music Japan International Inc.) was playing.

Calculation of a short-time RMS profile The temporal waveform of the music was band-filtered with a band-pass frequency range of 100-1000 Hz, and was evaluated from the observed frequency-dependent LD intensity of an n- hexane solution of AN nanofiber. Amplitudes of the sound waves were calibrated by measuring the sound pressures [Pa] of our ex- perimental setup for the acoustic LD spectroscopy. The sound waves were received and recorded with a microphone system (Br^el & Kjær model types 4939 and 2690) and an oscilloscope (Tektronix model DPO2024), respectively. The band-filtered wave was transformed into a short-time RMS value with a mean running time of 1.0 s, for which neither the relaxation factor nor possible nonlinear response was employed.

Acknowledgements We acknowledge Dr. Y. Nakajima and Dr. N. Kikuchi of JEOL Ltd. for FE-SEM measurements. This study was sponsored by a Grant- in-Aid for Scientific Research (B) (No. 25286017) and Challenging Exploratory Research (No. 24655125) from the Ministry of Educa- tion, Science, Sports, and Culture, Japan, and a Toray Science and Technology Grant.

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Received : November 27, 2013 Published online on January 7, 2014 Ryosuke Miura,[a] Yasunari Ando,[a] Yasuhisa Hotta,[a] Yoshiki Nagatani,*[b] and Akihiko Tsuda*[a] [a] R. Miura,+ Y. Ando,+ Y. Hotta, Prof. Dr. A. Tsuda Department of Chemistry, Graduate School of Science Kobe University 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501 (Japan) Fax: (+81) 78-803-5671 E-mail: [email protected] [b] Prof. Dr. Y. Nagatani++ Department of Electronics Kobe City College of Technology 8-3 Gakuen-Higashi-machi, Nishi-ku, Kobe 651-2194 (Japan) E-mail: [email protected] [+] These authors contributed equally to this work.

[++] Carried out acoustic measurements and characterizations of music.

Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201300400.

(c) 2014 Blackwell Publishing Ltd.

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