of Porphyrazine Dye by a Complexation with Inorganic Nanosheets

Article pubs.acs.org/JPCC

“Surface-Fixation Induced Emission” of Porphyrazine Dye by a Complexation with Inorganic Nanosheets Yohei Ishida,† Tetsuya Shimada,‡ and Shinsuke Takagi*,‡,§ †

Division of Material Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan ‡ Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, Tokyo Metropolitan University, Minami-ohsawa 1-1, Hachiohji, Tokyo 192-0397, Japan § Research Center for Artificial Photosynthesis, Tokyo Metropolitan University, Minami-ohsawa 1-1, Hachiohji, Tokyo 192-0397 Japan S Supporting Information *

ABSTRACT: This paper proposes a unique phenomenon of the strong enhancement in the fluorescence quantum yield (φf) and the excited lifetime (τ) of tetra-cationic porphyrazine dye (Pz) upon a complexation with inorganic nanosheets. Although Pz does not strongly fluoresce in a bulk solution (φf = 0.01, τ = 0.1 ns), φf and τ increased up to 19 and 34 times by an intercalation into stacked clay nanosheets. Steady-sate and time-resolved fluorescence measurements revealed that this strong enhancement in φf and τ is derived from the suppression of nonradiative deactivation pathways of Pz by a complexation with clay nanosheets. We here name this phenomenon a “Surface-Fixation Induced Emission (S-FIE)”. S-FIE can be predicted easier than aggregationinduced emission (AIE) due to its clear mechanism depending on the flat solid surface, and we can thus simply design the photophysically enhanced system. Since photophysical characteristics of organic molecules directly influence the efficiency of objective reactions such as energy or electron transfers and photocatalysis, this study is beneficial to propose a novel strategy to create efficient photochemical reaction systems and photodevices.

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INTRODUCTION The fluorescence quantum yield and excited lifetime of organic molecules directly influence the efficiency of objective reactions, such as energy or electron transfers and photocatalysis.1 These characteristics under the excited states can be modulated and enhanced by the combination with other organic or inorganic materials. Thus, the modifications of photophysical properties of organic molecules by adopting a suitable chemical reaction field have been widely investigated. Several techniques for the purpose have been reported including a complexation with inorganic host materials such as inorganic nanosheets2,3 or silica templates,4 a complexation with organic hosts such as DNAs5,6 or cyclodextrins,7,8 a complexation with metal nanoparticles9,10 or metal single layers,11,12 and a formation of aggregation.13−15 The emission enhancement by aggregation formation has been called an “aggregation induced emission (AIE)” and widely investigated since the debut of its concept in 2001. Although the nonregular aggregation drastically decreases the excited lifetime of the molecule and the molecule cannot show emission in general (this phenomenon is called “aggregation caused quenching”13), the regularly formed aggregates enhance the emission properties when the intermolecular packing is favorable. We have investigated a dye complexation with clay minerals for the purpose of efficient photochemical reactions.16−19 One of the clay minerals, saponite, is a negatively charged nanosheet with a diameter of ∼40−100 nm and intercharge distance of ∼1.2 nm.20−25 Additionally, it could be exfoliated to individual © 2014 American Chemical Society

nanosheets having optical transparency in the visible region under low concentration in aqueous solution. By using saponite clay nanosheets, we have successfully prepared unique clay/ porphyrin complexes in which the porphyrin molecules adsorb on the clay surface without aggregation even at high dye loadings.16,25 This simple adsorption state of dyes makes it easy to discuss the photophysical properties in detail. The formation of these unique hybrids was rationalized by a size-matching of distance between the charged sites in the porphyrin molecule and the distance between anionic sites on the clay mineral surface. By using this clay/dye complex, we have demonstrated an almost quantitative energy transfer between two types of dyes and constructed an artificial light-harvesting model.17,18 Through these studies, we have often observed the enhancement of fluorescence upon the complex formation with clay. Fluorescence enhancement by a complexation with clay nanosheets has been previously reported in a methylviologen/ montmorillonite system by Villemure et al.2 and in a triphenylbenzene derivatives/saponite system by our group.3 Both methylviologen and triphenylbenzene have rotatable pyridinium or phenyl substituents, and the suppression of the free-rotation of the substituents upon adsorption to the flat clay surface has been revealed as the driving force for the enhancement in fluorescence and excited lifetime. While Received: July 7, 2014 Revised: August 15, 2014 Published: August 15, 2014 20466

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Figure 1. (a) The structure of tetrapyridino[3,4-b:3′,4′-g:3″,4″-l:3‴,4‴-q]porphyrazine (Pz) and the schematic illustrations of (b) Pz on the exfoliated clay surface (sample b) and (c) Pz in the interlayer space between stacked clay (sample c).

flux was reduced with neutral density filters to avoid multiphoton absorption processes and nonlinear effects. The time-resolved fluorescence spectra were not corrected; thus, the obtained spectral shape was not the same as that of the steady state fluorescence spectroscopy even under the same condition. Sample Preparation. Clay/Pz complexes were prepared as follows. The concentration of Pz was set at 1.0 × 10−6 M for the absorption measurements and 1.0 × 10−7 M for the steadystate and time-resolved fluorescence measurements. The dye loading level vs cation exchange capacity (CEC) of the clay were set at 10% and 0.5% CEC for the absorption measurements and the steady-state and time-resolved fluorescence measurements, respectively. Three types of samples are prepared. Type a Sample. Aqueous Pz solutions were prepared at appropriate concentrations as reference samples for the clay/Pz complex. Type b Sample. Type b samples were prepared by mixing an aqueous clay solution and the respective aqueous Pz solutions (Figure 1b). The final concentration of clay was 8.0 and 0.8 mg L−1 for absorption and fluorescence measurements, respectively. Type c Sample. Type c samples were prepared by repeating six times of a freeze (liquid N2)-thaw cycle with type b complexes (Figure 1c). This procedure can stack the clay nanosheets and form the intercalated complex. The details of structure and characteristics have been reported in our previous paper.16

these unique phenomena have been reported, there are still no reports to apply this phenomenon for the molecules that have no rotatable parts in the molecule and visible−light emitting dyes such as porphyrin or porphyrazine derivatives to photoemitting applications. In this paper, we present a strong enhancement in the fluorescence quantum yield and the excited lifetime of tetra-cationic porphyrazine dye upon a complexation with clay nanosheets. We report the mechanism of this phenomenon investigated by detailed steady-state and timeresolved fluorescence measurements.

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EXPERIMENTAL SECTION Materials. The saponite clay mineral used in this experiment was synthesized by hydrothermal synthesis according to a previous paper.26 The cation-exchange capacity (CEC) was 1.00 mequiv g−1, and the average intercharge distance on the clay surface was calculated to be 1.2 nm on the basis of a hexagonal array. Tetramethyltetrapyridino[3,4-b:3′,4′-g:3″,4″l:3‴,4‴-q]porphyrazine (Pz, Figure 1a) was synthesized by the reaction of 3,4-dicyanopyridine in octan-1-ol in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to the tetrapyridino[3,4-b:3′,4′-g:3″,4″-l:3‴,4‴-q]porphyrazine followed by methylation with dimethylsulfate, according to literature.27−29 The compound was obtained as a mixture of the four possible regioisomers and could not be separated. 1H NMR: (D2O, 500 MHz) σ: 10.9 (m, 4H), 9.9 (m, 4H), 9.6 (m, 4H), 5.0 (s, 12H) ppm. Elemental analysis: Calculated for C32H26Cl4N12·5H2O: C, 47.42; H, 4.48; N, 20.74%; Found: C, 47.63; H, 4.43; N, 20.86%. The counterion of Pz was exchanged for chloride by use of an ion-exchange column (Organo Amberlite IRA400JCL). Water was deionized with an ORGANO BB-5A system (PF filter × 2 + G-10 column). Analysis. UV/vis Absorption spectra were measured with a Shimadzu UV-3150 spectrophotometer. The corrected fluorescence spectra were measured with a Jasco FP-6600 spectrofluorometer. In absorption and fluorescence measurements, a quartz cell was used for the aqueous clay/dye solutions. TG/DTA measurement was carried out with Shimadzu DTG-60H to determine the water contents of dyes and clay. The time-resolved fluorescence measurement was conducted under a photon-counting condition (Hamamatsu Photonics, C4334 streak scope, connected with CHROMEX 250IS polychrometer) with EKSPLA PG-432 optical parametric generator (430 nm, 25 ps fwhm, 20 μJ, 1 kHz) pumped by the third harmonic radiation of Nd3+‑YAG laser, EKSPLA PL2210JE (355 nm, 25 ps fwhm, 300 μJ, 1 kHz). The laser

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RESULTS AND DISCUSSION Absorption Spectra of Pz in Water (a), on the Exfoliated Clay Surface (b), and in the Interlayer Space of Stacked Clay (c). UV−vis absorption spectra of Pz in water (a), on the exfoliated clay surface (b), and in the interlayer space of stacked clay (c) were observed (Figure 2). The preparation methods for three types of samples are outlined in the Experimental Section. The dye loading level vs cation exchange capacity (CEC) of clay was set at 10% vs CEC of the clay for b and c samples. As shown in Figure 2, the absorption maxima (λab) of the Pz Q-band was slightly red-shifted from a (687 nm) to b (698 nm) and c (700 nm). We have already reported that Pz adsorbs on the clay surface without aggregation judging from the detailed Lambert−Beer plots of Pz on the clay surface (Figure S1, Supporting Information).17 In this study, aggregation is defined as the interaction between transition dipole moments of dyes that causes absorption 20467

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enough small in sample c, we can calculate the absorbance from their concentration and absorption coefficient. The fluorescence quantum yield for sample c was determined by using this calculated absorbance without scattering. Pz is not an efficient fluorescence dye in a bulk solution; however, the φf value drastically increased up to 19 times through a complexation with clay. The mechanism of this phenomenon is discussed in the next chapter. Here, we discuss the differences between positions of the bands maxima of the absorption (λab) and fluorescence (λem). The difference (Δλ) corresponds to Stokes shift. Δλ of three types of samples can be calculated as summarized in Table 1. As shown in Table 1, Δλ drastically decreases from a (412 cm−1) to b (163 cm−1) and c (102 cm−1). Basically, Δλ corresponds to the changes between the most stable structures under the ground state and the excited state. Thus, it was found that a complexation with clay nanosheets suppresses the change of the most stable structures by the excitation. The reason why c exhibited a lower Δλ value than b is that the sandwiching by clay nanosheets produces a more restricted field for guest Pz molecules, and it is difficult to change its molecular structure upon the excitation. Time-Resolved Fluorescence Measurement for Pz in Water (a), on the Exfoliated Clay Surface (b), and in the Interlayer Space of Stacked Clay (c). To discuss the detailed mechanism of the strong enhancement in the fluorescence, a time-resolved fluorescence measurement was carried out. Fluorescence decays for a, b, and c excited at 450 nm are shown in Figure 4. As shown in Figure 4, the decay curve for all samples can be analyzed as a single exponential decay. This result also indicates that Pz does not aggregate in b and c samples, as already described in the previous chapter. The excited lifetimes were determined to be 0.1 ns (a), 1.0 ns (b), and 3.4 ns (c), respectively. The drastic enhancement of excited lifetimes was observed upon a complexation with clay, as with the fluorescence quantum yields. Moreover, the fluorescence spectral shape of all samples was completely the same during the decay (see Figure S2, Supporting Information). While photochemical behaviors of dyes on solid surfaces tend to be complicated due to the aggregation formation,25,33−36 simple decay curves were observed in the present system due to the size-matching effect.16 Here, we discuss the mechanism of the enhancement in fluorescence quantum yields (φf) and the excited lifetimes (τ) by using experimental values. The radiative deactivation rate constant for fluorescence (kf) and the sum of the nonradiative deactivation rate constant (knr) (equal to the sum of thermal deactivation rate constant and intersystem crossing rate constant) can be calculated by using values of φf and τ according to eqs 1 and 2.

Figure 2. Absorption spectra for Pz in water (a), on the exfoliated clay surface (b), and in the interlayer space of stacked clay (c). [Pz] = 1.0 × 10−6 M, [clay] = 8.0 mg L−1. The CEC value for b and c was 10%.

spectral shift. The λab shift of Pz upon complexation with clay is probably due to the ruffled structure29−31 of the phthalocyanine ring, as discussed in our previous papers.17,31 It should be noted that the vibrational structure of absorption spectra becomes definite in the order of samples a, b, and c. This would indicate the fixation of molecular structure by a complexation with clay. Steady-State Fluorescence Spectra of Pz in Water (a), on the Exfoliated Clay Surface (b), and in the Interlayer Space of Stacked Clay (c). Steady-state fluorescence spectra of Pz in water (a), on the exfoliated clay surface (b), and in the interlayer space of stacked clay (c) were observed. To measure the intrinsic fluorescence quantum yields without any quenching processes between dyes under the excited state,32 the dye loading level vs CEC of clay was set at 0.5% vs CEC of the clay for b and c samples. The 0.5% CEC condition corresponds to the concentration where only one Pz molecule exists on one clay nanosheet (the calculation has been described in the previous paper).19 Under the 0.5% CEC condition, the absorption spectral shapes of all samples were the same as those of samples under the 10% CEC condition in Figure 2. The noncorrected fluorescence spectra of a, b, and c samples excited at 450 nm are shown in Figure 3. First of all, the fluorescence intensities are different in three samples. By using tetraphenyl porphyrin in toluene as a standard fluorescent compound, fluorescence quantum yields (φf) are determined to be 0.01, 0.10, and 0.19 for a, b, and c samples, respectively. Since the fluorescence spectra were measured under a 10−7 M concentration and the scattering was

kf = k nr =

ϕf (1)

τ (1 − ϕf ) τ

(2)

The calculated kf and knr values are summarized in Table 1. Although the kf values did not change so much for all samples, knr drastically decreases from a to c. A complexation with clay nanosheets decreased not only Δλ (described in the previous chapter) but also knr. According to these results, a plausible potential energy curve for the present system can be depicted (Figure S3, Supporting

Figure 3. Fluorescence spectra for Pz in water (a), on the exfoliated clay surface (b), and in the interlayer space of stacked clay (c). Excited wavelength was 450 nm. [Pz] = 1.0 × 10−7 M, [clay] = 0.8 mg L−1. The CEC value for b and c was 0.5%. 20468

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Table 1. Band Maxima of Absorption (λab) and Fluorescence (λem) in nm and cm−1 at 10% vs CEC of the Clay, Differences between λab and λem (Δλ), Fluorescence Quantum Yields (φf), Excited Lifetimes (τ) at 0.5% CEC, Radiative Deactivation Rate Constants (kf) and Nonradiative Deactivation Rate Constants (knr) for for Pz in Water (a), on the Exfoliated Clay Surface (b), and in the Interlayer Space of Stacked Clay (c)a a b c a

λab/nm (cm−1)

λem/nm (cm−1)

Δλ/cm−1

φf

τ/ns

kf/108 s−1

knr/108 s−1

687 (14556) 698 (14327) 700 (14286)

707 (14144) 706 (14164) 705 (14184)

412 163 102

0.01 0.10 0.19

0.1 1.0 3.4

1.0 1.0 0.56

99 9.0 2.4

The concentration of all samples was set at [Pz] = 1.0 × 10−7 M.

Although we have investigated photophysical properties of more than 15 kinds of meso-substituted porphyrin derivatives,16,35,36 they have not performed such strong enhancement in their φf and τ by a complexation with clay nanosheets. We consider that the difference in the photophysical enhancement is probably derived from the differences of molecular structures of porphyrins and porphyrazines. In the case of tetrakis(1methylpyridinium-4-yl) porphyrin and the derivatives, the bulky 4 meso-substituents can freely rotate in a bulk solution, and are fixed and flattened upon adsorption onto clay due to the flatness of the surface. The effect of structure fixation upon adsorption to the clay surface cannot adequately affect the porphyrin core-fluorophore, and thus, the enhancement in φf and τ is not large. On the other hand, Pz does not have any rotatable substituents and has a flat molecular structure. Thus, the core-fluorophore of Pz can be fixed by the clay surface effectively, and the enhancement in φf and τ for Pz is large compared to porphyrin derivatives. These are the most plausible reasons for the drastic improvement in photophysical properties for Pz. We name this phenomenon a “SurfaceFixation Induced Emission”, which is a similar phenomenon to “aggregation induced emission (AIE)”.

Figure 4. Time resolved fluorescence decays for Pz in water (a), on the exfoliated clay surface (b), and in the interlayer space of stacked clay (c). Excited wavelength was 450 nm. Observed wavelength was 650−780 nm. [Pz] = 1.0 × 10−7 M, [clay] = 0.8 mg L−1. The CEC value for b and c was 0.5%.

Information). We recently reported that the effects of a complexation with clay on photophysical properties of adsorbed molecules can be separated into two mechanisms; effect i (structure resembling effect) is that the most stable structure becomes relatively similar between the ground state and the excited state compared to that without clay in water (sample a in this work), and effect ii (structure fixing effect) is that the potential energy curves become relatively sensitive against the nuclear coordinates (see the qualitative potential curves and the explanation of effects i and ii for the present system with/without clay in Figure S3, Supporting Information).35 Effect ii corresponds to the suppression of vibrational motion of the molecule on the clay surface, leading to the suppression of nonradiative deactivation. Effect i induces the decrease of Δλ values and may induce the increase of kf due to the similarity of molecular structure between the ground and excited states. Effect ii would induce the decrease of the knr value, because the effective crossing between the potential curve of ground and excited states should be suppressed. While knr consists of the sum of the thermal deactivation rate constant (kd) and intersystem crossing rate constant (kisc), effect ii would mainly suppress kd. Judging from the experimental results, effect ii is mainly working, and therefore, the increase of φf and τ was observed in the present system. Effect i can induce the increase of the Franck−Condon factor for S0−S1 electronic transition and radiative deactivation and results in the increase of kf values. On the other hand, effect ii can decrease kf because the sharpened potential curves of the ground and excited states can decrease the Franck−Condon factor between the lowest vibration mode of S1 and the ground state. The total effect by the complexation with clay depends on the balance of effects i and ii, and thus, it is presumed that the almost constant kf values for a, b, and c samples resulted in this study.

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CONCLUSION In this paper, we presented a unique phenomenon of the strong enhancement in the fluorescence quantum yield (φf) and the excited lifetime (τ) of tetra-cationic porphyrazine dye (Pz) upon a complexation with clay nanosheets. Although Pz does not strongly fluoresce in a bulk solution, φf and τ increased up to 19 and 34 times by an intercalation into the stacked clay nanosheets (sample c). By steady-sate and time-resolved fluorescence measurements, it was found that this strong enhancement in φf and τ is derived from the suppression of a nonradiative deactivation process of Pz by a complexation with clay nanosheets, and we named this phenomenon a “SurfaceFixation Induced Emission (S-FIE)”. S-FIE can be predicted easier than aggregation-induced emission (AIE) due to its mechanism depending on the flat solid surface, and we can thus simply design the photophysically enhanced system. Since photophysical characteristics of organic molecules directly influence the efficiency of objective reactions such as energy or electron transfers and photocatalysis, this study is beneficial to propose a novel strategy to create efficient photochemical reaction systems and photodevices.

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ASSOCIATED CONTENT

S Supporting Information *

The absorption spectra and Lambert−Beer plot for b under 20−240% vs CEC of the clay, global time-resolved fluorescence decays and time-resolved fluorescence spectra, and plausible potential energy curves of the ground and excited states of Pz. 20469

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(16) Takagi, S.; Shimada, T.; Ishida, Y.; Fujimura, T.; Masui, D.; Tachibana, H.; Eguchi, M.; Inoue, H. Size-Matching Effect on Inorganic Nanosheets: Control of Distance, Alignment, and Orientation of Molecular Adsorption as a Bottom-Up Methodology for Nanomaterials. Langmuir 2013, 29, 2108−2119. (17) Ishida, Y.; Shimada, T.; Takagi, S. Artificial Light-Harvesting Model in a Self-Assembly Composed of Cationic Dyes and Inorganic Nanosheet. J. Phys. Chem. C 2013, 117, 9154−9163. (18) Ishida, Y.; Shimada, T.; Masui, D.; Tachibana, H.; Inoue, H.; Takagi, S. Efficient Excited Energy Transfer Reaction in Clay/ Porphyrin Complex Toward an Artificial Light-Harvesting System. J. Am. Chem. Soc. 2011, 133, 14280−14286. (19) Ishida, Y.; Kulasekharan, R.; Shimada, T.; Takagi, S.; Ramamurthy, V. Efficient Singlet-Singlet Energy Transfer in a Novel Host-Guest Assembly Composed of an Organic Cavitand, Aromatic Molecules, and a Clay Nanosheet. Langmuir 2013, 29, 1748−1753. (20) Ras, R. H. A.; Umemura, Y.; Johnston, C. T.; Yamagishi, A.; Schoonheydt, R. A. Ultrathin Hybrid Films of Clay Minerals. Phys. Chem. Chem. Phys. 2007, 9, 918−932. (21) Sato, H.; Tamura, K.; Ohara, K.; Nagaoka, S.-I. Multi-Emitting Properties of Hybrid Langmuir−Blodgett Films of Amphiphilic Iridium Complexes and the Exfoliated Nanosheets of Saponite Clay. New J. Chem. 2013, 38, 132−139. (22) Shichi, T.; Takagi, K. Clay Minerals as Photochemical Reaction Fields. J. Photochem. Photobiol., C: Photochem. Rev. 2000, 1, 113−130. (23) López Arbeloa, F.; Martínez Martínez, V.; Arbeloa, T.; López Arbeloa, I. Photoresponse and Anisotropy of Rhodamine Dye Intercalated in Ordered Clay Layered Films. J. Photochem. Photobiol., C: Photochem. Rev. 2007, 8, 85−108. (24) Okada, T.; Ide, Y.; Ogawa, M. Organic-Inorganic Hybrids Based on Ultrathin Oxide Layers: Designed Nanostructures for Molecular Recognition. Chem.Asian J. 2012, 7, 1980−1992. (25) Bujdák, J. Effect of the Layer Charge of Clay Minerals on Optical Properties of Organic Dyes. A Review. Appl. Clay Sci. 2006, 34, 58−73. (26) Egawa, T.; Watanabe, H.; Fujimura, T.; Ishida, Y.; Yamato, M.; Masui, D.; Shimada, T.; Tachibana, H.; Yoshida, H.; Inoue, H.; et al. Novel Methodology to Control the Adsorption Structure of Cationic Porphyrins on the Clay Surface Using the “Size-Matching Rule”. Langmuir 2011, 27, 10722−10729. (27) Ramirez, C.; Antonacci, C.; Ferreira, J.; Sheardy, R. D. The Facile Synthesis and Characterization of Novel Cationic Metallated and Nonmetallated Tetrapyridino Porphyrazines Having Different Metal Centers. Synth. Commun. 2004, 34, 3373−3379. (28) Martí, C.; Nonell, S.; Nicolau, M.; Torres, T. Photophysical Properties of Neutral and Cationic Tetrapyridinoporphyrazines. Photochem. Photobiol. 2000, 71, 53−59. (29) Sobbi, A. K.; Wöhrle, D.; Schlettwein, D. Photochemical Stability of Various Porphyrins in Solution and as Thin Film Electrodes. J. Chem. Soc., Perkin Trans. 2 1993, 481−488. (30) Okada, S.; Segawa, H. Substituent-Control Exciton in JAggregates of Protonated Water-Insoluble Porphyrins. J. Am. Chem. Soc. 2003, 125, 2792−2796. (31) Ishida, Y.; Masui, D.; Shimada, T.; Tachibana, H.; Inoue, H.; Takagi, S. The Mechanism of the Porphyrin Spectral Shift on Inorganic Nanosheets: The Molecular Flattening Induced by the Strong Host−Guest Interaction Due to the “Size-Matching Rule. J. Phys. Chem. C 2012, 116, 7879−7885. (32) Ishida, Y.; Shimada, T.; Tachibana, H.; Inoue, H.; Takagi, S. Regulation of the Collisional Self-Quenching of Fluorescence in Clay/ Porphyrin Complex by Strong Host-Guest Interaction. J. Phys. Chem. A 2012, 116, 12065−12072. (33) Lofaj, M.; Valent, I.; Bujdák, J. Mechanism of Rhodamine 6G Molecular Aggregation in Montmorillonite Colloid. Cent. Eur. J. Chem. 2013, 11, 1606−1619. (34) Sato, H.; Tamura, K.; Ohara, K.; Nagaoka, S.-I.; Yamagishi, A. Hybridization of Clay Minerals with the Floating Film of a Cationic Ir(III) Complex at an Air−Water Interface. New J. Chem. 2011, 35, 394−399.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.I. thanks Japan Society for the Promotion of Science (PD). This work has been partly supported by a Grant-in-Aid for Scientific Research (B), a Grant-in-Aid for Scientific Research on Innovative Areas (No. 25107521).

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