Photophysical Properties and Adsorption Behaviors of Novel Tri

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Photophysical Properties and Adsorption Behaviors of Novel Tri-Cationic Boron(III) Subporphyrin on Anionic Clay Surface Takamasa Tsukamoto, Tetsuya Shimada, and Shinsuke Takagi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11988 • Publication Date (Web): 29 Feb 2016 Downloaded from http://pubs.acs.org on March 3, 2016

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ACS Applied Materials & Interfaces

Photophysical Properties and Adsorption Behaviors of Novel Tri-Cationic Boron(III) Subporphyrin on Anionic Clay Surface Takamasa Tsukamoto a, b, Tetsuya Shimada c and Shinsuke Takagi* c a

Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

b

Japan Society for the Promotion of Science (JSPS / PD), Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan c

Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1, Minami-ohsawa, Hachiohji-shi, Tokyo 192-0397, Japan

*Corresponding author E-mail address: [email protected] (Shinsuke Takagi)

ABSTRACT Two types of +3-charged subporphyrin derivatives with m- and p-methylpyridinium as the mesoaryl substituents were designed and synthesized. Their photophysical properties with and without anionic saponite clay were investigated. These cationic subporphyrins were suitably designed for adsorption on the saponite nano-sheet surface with their photoactivity. Absorption and emission spectra of these subporphyrin-saponite complexes exhibited strong bathochromic shifts due to the flattening of the molecules on the nano-sheet. This behavior was observed as drastic visual changes in their luminescence colors. Additionally, aggregation behaviors were not observed in the saponite complexes even at high dye loading levels for both subporphyrins. Moreover, under such condition, their fluorescence properties on the saponite surface were not only maintained but also enhanced without unexpected deactivations despite the dye molecules are densely introduced on the solid surface. These findings are beneficial for applications of the dye-clay complexes to photo-functional materials such as strongly luminescent materials, highly sensitive clay sensors and artificial photosynthesis systems.

ACS Paragon Plus Environment

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KEYWORDS Subporphyrin, Clay minerals, Organic/Inorganic complex, Fluorescence, Size-Matching Effect

INTRODUCTION Dye-clay complexes have been investigated as one of organic-inorganic hybrid compounds by numerous researchers1-8. In general, clay minerals have anionic layered structures with atomically flat surfaces and cation exchange capacities (CEC). Therefore, various cationic dye molecules can be introduced on the surface or within the layers mainly by an electrostatic interaction between cationic guest and anionic clay. Some clay minerals such as saponite exfoliate in water as a single nano-sheet. Especially, synthetic saponites not containing transition metal ions such as iron are beneficial as host materials for photo-functional applications because the clay dispersion is substantially transparent in the UV-visible range and the clay itself is not photo or redox active9. Energy transfer between dye molecules or photo-induced electron transfer reaction on / in the clay layers have been known as photophysical applications of the dye-clay complexes10-15. However, excited energy of dye molecules is often lost by unexpected aggregation behaviors originated with guest-guest interactions16-18. Recently, we have reported that particular multi cationic and planar dye molecules such as +4charged porphyrins could be strongly adsorbed on the clay surface by a strong electrostatic interaction. For such molecules, photophysical properties such as fluorescence quantum yields or lifetimes have been enhanced due to fixation of the molecular structures onto the clay surface and / or suppression of aggregation and self-fluorescence quenching19-24. Especially, in the case that the distance between positive charges in the guest molecules and an average negative charge distance on the clay surface


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coincide well, the guest molecules can be adsorbed up to high density without aggregation because Coulomb’s force is strengthened especially in such conditions (Size-matching Rule)23-25. However, only a few examples of such the beneficial guest dye molecules exhibiting adsorption behaviors with suitable photoactivity have been discovered. Therefore, we designed, synthesized and investigated new fluorescent guest molecules with a view to applications of dye-clay complexes for photo-functional materials in this paper. Novel two types of +3-charged subporphyrin derivatives with different cationic substituents were investigated in the clay complex and without clay in water. The subporphyrin

derivatives

are

hydroxo[5,10,15-tris(N-methyl-pyridinium-3-

yl)subporphyrinato]boron(III) trichloride ([BIII(m-TMPySp)(OH)]Cl3) and hydroxo[5,10,15-tris(Nmethyl-pyridinium-4-yl)subporphyrinato]boron(III) trichloride ([BIII(p-TMPySp)(OH)]Cl3) (Figure 1). meso-aryl substituted subporphyrin derivatives are recently reported new compound group26,27 and these +3-charged subporphyrins are novel water soluble and cationic subporphyrins. It can be expected that these subporphyrins show high photoactivity on the clay without deactivation due to the unexpected guest-guest interactions because they have equilateral-triangular molecular structures with planar pyridinium substituents, multi intramolecular positive charges (+3), intramolecular charge arrays fulfilled the “Size-matching Rule” and similar molecular structure to those of porphyrin ones. As described later, the distance between the positive charges in their structures and the average negative charge distance on the clay are in conforming. Subporphyrin derivatives exhibit higher fluorescence quantum yields than porphyrin analogues and various fluorescence colors by the introduction of the functional groups, whereas those of porphyrins are generally up to 10% and their colors are basically red. Therefore, the subporphyrin derivatives are more applicable for photo-functional materials. Two types of methylpyridinium substituents were introduced mainly for modification of absorption and fluorescence wavelengths. The photophysical properties and adsorption behaviors of the subporphyrins


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on the clay surface were investigated by UV-visible absorption and steady and time resolved fluorescence spectroscopy.

[BIII(m-TMPySp)(OH)]Cl3

[BIII(p-TMPySp)(OH)]Cl3

Figure 1. Structures of hydroxo[5,10,15-tris(N-methyl-pyridinium-3-yl)subporphyrinato] boron(III) trichloride

([BIII(m-TMPySp)(OH)]Cl3)

and

hydroxo[5,10,15-tris(N-methyl-pyridinium-4-

yl)subporphyrinato] boron(III) trichloride ([BIII(p-TMPySp)(OH)]Cl3).

EXPERIMENTAL METHODS Materials and Measurement. Clay Minerals (saponite): Sumecton SA was received from Kunimine Industries Co., Ltd. The structure and stoichiometric formula of the saponite are shown in Figure S1 (Supporting Information). The cation exchange capacity (CEC) of the saponite is ca. 1.0 × 10–3 equiv. g– 1 25

. The average distance between the anionic points on the saponite surface is estimated to be 1.2 nm,

on the basis of the assumption of a hexagonal array25. The aqueous dispersion of saponite nano-sheet whose particle size is small (< ~100 nm) is substantially transparent in the UV-visible range. Hydroxo[5,10,15-tris(N-methyl-pyridinium-3-yl)subporphyrinato]boron(III)

trichloride

([BIII(m-

TMPySp)(OH)]Cl3 denoted as m-BIIITMPySp) and hydroxo[5,10,15-tris(N-methyl-pyridinium-4yl)subporphyrinato] boron(III) trichloride ([BIII(p-TMPySp)(OH)]Cl3 denoted as p-BIIITMPySp) were synthesized as described in the Supporting Information. The distances of the intramolecular cationic points of m-BIIITMPySp and p-BIIITMPySp were estimated to be 1.1 and 1.2 nm from DFT calculations28 as shown in Figure S2. UV-visible absorption spectra were obtained on Shimadzu UV-


4

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3150 spectrophotometer. Fluorescence spectra were recorded on JASCO FP-6600 spectrofluorometer. Time-resolved fluorescence signals were measured by Hamamatsu Photonics C4780 detector system based on a streak camera. An Nd3+ YAG laser with an OPG (EKSPLA PL2210JE + PG-432, FWHM 25 ps, 1 kHz) was used for excitation.

Sample Preparation for Absorption Measurement. Subporphyrin-saponite complexes were prepared by mixing aqueous saponite dispersion and subporphyrin aqueous solutions under stirring. Concentration of saponite for Lambert-Beer plot was 3.0 × 10–3 g L–1. The loading levels of cationic subporphyrins versus the CEC of the saponite in the complexes were controlled by varying the concentrations of the subporphyrins.

Sample Preparation for Fluorescence Measurement. The preparation method for the subporphyrinsaponite complex is same to that for absorption spectra. The concentrations of both subporphyrins were 1.0 × 10-7 M. The loading levels of cationic subporphyrins versus the CEC of the saponite in the complexes were controlled by varying the concentration of the saponite. The fluorescence quantum yields of the subporphyrins were determined using dihydroxo(5,10,15,20-tetraphenylporphyrinato) antimony(V) chloride as the standard (Φf = 0.052)21.

Sample Preparation for Time Resolved Fluorescence Measurement. The preparation method for the subporphyrin-saponite complexes is the same as that for the absorption spectra. The subporphyrin loadings were set at 0.2% versus the CEC of the saponite.

RESULTS & DISCUSSION


5


Absorption Properties of Subporphyrins with and without Saponite The aqueous saponite dispersion was substantially transparent in the UV-visible region under the present experimental conditions. UV-visible absorption spectra of the subporphyrins in water and that adsorbed on the saponite clay surface were observed. The absorption spectra of both the subporphyrins with and without the saponite are shown in Figure 2 and the absorption properties are summarized in Table 1. In water, the differences of absorption spectral peaks (λabs) at the B-band and Q-band between m-BIIITMPySp and p-BIIITMPySp were 28 and 25 nm (0.24 and 0.14 eV) originated from the electronic properties of the m- and p-methylpyridinium rings. This result is supported by the fact that the energy levels of the LUMO and LUMO+1 of the subporphyrin with m-methylpyridinium are estimated to be higher than those having p-methylpyridinium in the DFT calculations (Figure S3).

370 (5.03)

Without clay 401 (5.01)

With clay

100 B-band

80 60

Q-band

40

458 (4.07)

20

487 (4.09)

300

400 500 Wavelength / nm

600

Extinction Coefficient / 103 M-1 cm-1

120

[BIII(p-TMPySp)(OH)]Cl3

140

[BIII(m-TMPySp)(OH)]Cl3

140

Extinction Coefficient / 103 M-1 cm-1

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426 (5.10)

120

398 (5.02)

Without clay

100

With clay

80

B-band

60 Q-band

40

483 (4.09) 515 (4.19)

20

300

400

500

600

Wavelength / nm

Figure 2. Absorption spectra of m-BIIITMPySp and p-BIIITMPySp with and without the saponite in water. [saponite] = 3.0 × 10–2 g L–1 (Loading levels of porphyrins are 10% versus CEC of the saponite).

Table 1. Values of Absorption Maxima (λabs) and Integrals of the Extinction Coefficients (∫εd̅ ) of Subporphyrins with and without Saponite in Water


6

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Compound III

m-B TMPySp p-BIIITMPySp

Absorption maxima values / nm λabsW (log εW) 370 (5.03), 458 (4.07) 398 (5.02), 483 (4.09)

λabsC (log εC) 401 (5.01), 487 (4.09) 426 (5.10), 515 (4.19)

∆λabs 31, 29 28, 32

Integral of the extinction coefficient / ×107 M–1 cm–2 ∫εd̅ W ∫εd̅ C ∫εd̅ C / ∫εd̅ W 3.7 3.9 1.05 3.7 4.2 1.14

λabsW (log εabsW) and λabsC (log εC) are the λabs (ε : extinction coefficient) of subporphyrins without and with the saponite. ∆λabs = λabsC – λabsW. ∫εd̅ W and ∫εd̅ C are the ∫εd̅ of the subporphyrins without and with the saponite. The integral range is 16700 to 33300 cm–1 (300 to 600 nm).

Both the subporphyrins exhibited spectral shifts to longer wavelengths upon the complex formation with the saponite, whereas their spectral shapes were nearly unchanged. The values of the spectral shifts in the presence and absence of saponite (∆λabs) at the B-band and Q-band were 31 and 29 nm (0.26 and 0.16 eV) and 28 and 32 nm (0.20 and 0.16 eV) for m-BIIITMPySp and p-BIIITMPySp, respectively. There was almost no difference between the ∆λabs of both the subporphyrins. These red shifts of the subporphyrins upon adsorption on the saponite surface would be mainly induced by the co-planarization of the meso-aryl substituents with respect to the subporphyrin rings along with porphyrin derivatives29,30. The meso-substituents can rotate freely compared with other dye molecules such as porphyrins due to smaller steric hindrance between the meso-aryl substituents and the β-pyrrole protons of the subporphyrin macrocycle26,27. This speculation is also supported by the DFT calculations. Both dihedral angles of pyridinium substituents against the macrocycles at the optimized structures are estimated to be 45° although that of porphyrin is 67° (Figure S2). The values of the integrals of the extinction coefficients in the B-band and Q-band wavenumber ranges (∫εd ̅ ) slightly increased upon the complex formation with the saponite for both the subporphyrins (Table 1). For many dye molecules, an increase in the transition probability on the saponite surface has been observed. This effect would be induced by an increase in the Franck-Condon factor due to resembling of the molecular structures of the ground and excited states21,22. In this study,


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the ∫εd̅ values of m-BIIITMPySp and p-BIIITMPySp with the saponite increased about 1.05 and 1.14 times compared to those in the bulk solution, respectively (Table 1).

Adsorption Behaviors of Subporphyrins on the Saponite Surface Concentration effects of the subporphyrins on the absorption spectra were examined as shown in Figure 3. The subporphyrin derivatives are expected to accomplish high-density adsorption without aggregation because they fulfill the “Size-matching Rule”23-25. As mentioned in the experimental section, their intramolecular positive charge distances estimated from the DFT calculation (1.1 and 1.2 nm for mBIIITMPySp and p-BIIITMPySp) and the average negative charge distances on the saponite clay surface (1.2 nm) coincide well each other. As a result, with an increase in concentration of dye molecules, spectra of the adsorption species increased without any shape change up to certain loading levels as shown in Figure 3 (More detailed spectra were shown in Figure S4). Judging from the Lambert-Beer plots, maximum adsorption amounts of both the subporphyrins were determined to be 85 and 100% versus CEC for m-BIIITMPySp and p-BIIITMPySp, respectively (Figure 4). Above these loading levels, those of non-adsorbed species in the bulk solution were newly superimposed on the spectra (Figure 3). The subporphyrins did not aggregate on the saponite surface according to spectral shapes of the adsorption species. The normalized spectra in concentrations up to maximum adsorption densities completely conformed. The linearities of the Lambert-Beer plots also indicate that aggregation is completely suppressed (Figure 4). The average distances between the subporphyrin molecules under the maximum non-aggregated adsorption condition were estimated to be 2.3 and 2.1 nm, respectively (center to center) (Table 2). These densities were the highest of the dye molecules previously investigated in our group (maximum ca. 0.20 molecules nm–2)21-25. This high-density adsorption would be achieved by the equilateral-triangular arrangement of the intramolecular cation points. Namely both


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the subporphyrins can fulfill the “Size-matching Rule” from the aspect of not only distance matching but also arrangement matching of the charges, on the basis of the assumption of a hexagonal array of anionic points on the saponite surface. Additionally, the high-density adsorption would be also due to enhanced electrostatic interactions between the dyes and the saponite because pyridinium can co-planarize easily to allow the subporphyrin to approach the saponite surface.

0.14

0.12 [m-BIIITMPySp]3+ = 140% ↑ 20% vs. CEC of the clay

Solution II

0.1

II 0.1

0.08 I

0.06

[p-BIIITMPySp]3+ = 180% ↑ 20% vs. CEC of the clay

Solution

0.12

Absorbance

Absorbance

100% vs. CEC

On the clay surface 0.04

0.08

I On the clay surface

0.06

100% vs. CEC

0.04

0.02

0.02 0

0 300

400

500 Wavelength / nm

600

300

700

400

500 Wavelength / nm

600

700

Figure 3. Absorption spectra of m-BIIITMPySp-saponite complex (left) and p-BIIITMPySp-saponite complexes (right) at various subporphyrin concentrations at up to 140% and 180% versus CEC in aqueous transparent solution ([saponite] = 3.0 × 10–3 g L–1).

0.14

0.12 On the clay surface

0.1

Solution II

0.12

On the clay surface I

I

Solution II

0.1

Absorbance

0.08

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.06

0.08 100% vs. CEC

0.06

0.04

0.04 370 nm

0.02 85% vs. CEC

396 nm

0.02

401 nm

426 nm

0

0 0%

50%

100% vs. CEC of clay

150%

200%

0%

50%

100% vs. CEC of clay

150%

200%


9


Figure 4. Lambert-Beer plots of m-BIIITMPySp-saponite complex (left) and p-BIIITMPySp-saponite complexes (right) in aqueous transparent solution.

Table 2. Loading levels, Adsorption Densities and Average Distances between Subporphyrin Molecules at Maximum Adsorption Conditions without Aggregation

Compound

Loading level / % vs. CEC

m-BIIITMPySp p-BIIITMPySp

85 100

Aggregation over maximum adsorption No No

Adsorption density / molecules nm–2

Average intermolecular distance / nm

0.27 0.23

2.3 2.1

The average intermolecular distance is center to center of the subporphyrin molecules.

Fluorescence Properties of Subporphyrins with and without Saponite Fluorescence spectra of the subporphyrins with and without the saponite in water were observed as shown in Figure 5. The loading levels of the subporphyrins were set at 2.0% versus CEC of the saponite, where self-fluorescence quenching behavior was not observed.

90

50 [m-BIII(TMPySp)(OH)]Cl3 Without clay

70 540

60 50

584

With clay (2% vs. CEC)

515 (sh)

40 30

550 (sh)

20

[p-BIII(TMPySp)(OH)]Cl3 Fluorescence intensity / a.u.

80 Fluorescence intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 20

10 0

40

600

30

Without clay 630

With clay (2% vs. CEC)

20 10 0

450


750

500


800

Figure 5. Fluorescence spectra normalized by absorbance at excitation wavelength. The excitation wavelengths are 370 and 400 nm for m-BIIITMPySp and 398 and 426 nm for p-BIIITMPySp without and


10

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with the saponite in water, respectively. Fluorescence maxima values (λfl / nm) are shown in spectra. [Subporphyrin] = 1.0 × 10–7 M (Loading levels of the subporphyrins were 2.0% versus CEC of the saponite.).

Figure 6. Changes in fluorescence color of m-BIIITMPySp and p-BIIITMPySp without and with the saponite excited at 365 nm in water. [Subporphyrin] = 2.7 × 10–6 M (0.4% versus CEC of the saponite).

For both the subporphyrins, spectral shifts of fluorescence maxima (λfl) to longer wavelengths were observed upon the complex formation with the saponite along with the absorption spectra. Their spectral shapes were rarely different between before and after the complexation with saponite. The values of the spectral shifts (∆λfl) were 44 and 30 nm (0.17 and 0.10 eV) for m-BIIITMPySp and p-BIIITMPySp, respectively (Table 3). Upon complexation of both the subporphyrins and the saponite, obvious visual luminescence color changes were observed (Figure 6).

Table 3. Values of Fluorescence Maxima (λfl) of Subporphyrins with and without Saponite in Water Compound III

m-B TMPySp p-BIIITMPySp

Fluorescence maxima values / nm λflW 515sh, 540 600

λflC 550sh, 584 630

∆λfl 35, 44 30

λflW and λflC are the λfl of subporphyrins without and with the saponite. ∆λfl = λflC – λflW.


11


Fluorescence quantum yields (Φf) of the subporphyrins are summarized in Table 4. The value of Φf for m-BIIITMPySp with the saponite (ΦfC) was 1.3 times larger than that without the saponite (ΦfW). On the other hand, that of p-BIIITMPySp decreased slightly by the complex formation with the saponite. To discuss the details of the photophysical behaviors of the subporphyrins on the saponite surface, timeresolved fluorescence spectra for both the subporphyrins with and without the saponite were measured to obtain excited lifetimes by using a picoseconds fluorescence measurement system. As shown in Figure 7, all decay curves for the subporphyrins can be interpreted as a single exponential decay, and fluorescence lifetimes with and without the saponite (τfC and τfW) were obtained. Such simple fluorescence decay behaviors are rarely observed for dye molecules on inorganic surface and indicate that the dye molecules do not aggregate on the saponite. The values of τfW, τfC and τfC / τfW are summarized in Table 4. Although the fluorescence lifetime of m-BIIITMPySp with the saponite was longer than that without the saponite, that for p-BIIITMPySp did not change between with and without

1

Log fluorescence intensity / a.u.

the saponite. Log fluorescence intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 20

[m-BIII(TMPySp)(OH)]Cl3 with clay τ = 9.0 ns without clay τ = 6.9 ns

0.1

0.01 0

5

10 15 Time / ns

20

25

[p-BIII(TMPySp)(OH)]Cl3

1

with clay τ = 7.7 ns without clay τ = 7.6 ns

0.1

0.01 0

5

10 15 Time / ns

20

25

Figure 7. Fluorescence decay profiles and fluorescence lifetimes (τf) for m-BIIITMPySp and pBIIITMPySp with and without the saponite in water. Subporphyrin loadings on the saponite surface were 0.2% versus CEC of the saponite. [Subporphyrin] = 8.0 × 10–8 M.


12

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The radiative deactivation rate constants for fluorescence (kf) and non-radiative deactivation rate constants (knr) were calculated from the values of fluorescence quantum yields (Φf) and fluorescence 31

lifetimes (τf) according to equations 1 and 2

, and are shown in Table 4. The ratios of kC and kW are

shown in Table 4 to discuss effects of the saponite in detail. =

(1)

=

(2)

Table 4. Fluorescence Quantum Yields (Φf), Fluorescence Lifetimes (τf) and Radiative (kf) and NonRadiative (knr) Deactivation Rate Constants of Subporphyrins with and without Saponite in Water Fluorescence quantum yield

Compound m-BIIITMPySp p-BIIITMPySp

Fluorescence lifetime / 10–9 s

Radiative deactivation rate constant / 107 s–1

Non-radiative deactivation rate constant / 107 s–1

ΦfW

ΦfC

ΦfC / ΦfW

τfW

τfC

τfC / τfW

kfW

kfC

kfC / kfW

knrW

knrC

knrC / knrW

0.51 0.45

0.64 0.37

1.3 0.82

6.9 7.6

9.0 7.7

1.3 1.0

7.4 5.9

7.1 4.8

0.95 0.82

7.1 7.2

4.0 8.2

0.56 1.1

ΦfW and ΦfC are Φf of subporphyrin without and with the saponite (2% vs. CEC). τfW and τfC are τf of subporphyrin without and with the saponite (0.2% vs. CEC). kW and kC are rate constants of subporphyrin without and with the saponite.

For both the subporphyrins, the kf values slightly decreased by the complex formation with the saponite. On the other hand, the knr value apparently decreased only for m-BIIITMPySp. Considering this result, it is expected that rotation movements of the m-methylpyridinium substituent is more suppressed than that of the p-methylpyridinium one. It is suggested that the rotation of the m-methylpyridinium ring can be suppressed by the fixation of the aromatic ring because a cationic quaternized nitrogen atom of the m-methylpyridinium is not located in the same axis as the rotation direction of the meso-substituted aromatic ring against the subporphyrin chromophore, leading to the decrease in the non-radiative deactivation processes such as internal conversion. For the p-methylpyridinium substituent, the rotation


13


of the aromatic ring cannot be suppressed because the quaternized nitrogen atom is located in the same axis as the rotation direction of the meso-substituted aromatic ring. Thus, it would be newly clarified that the m-methylpyridinium substituent of subporphyrins can suppress its own rotation movement differently from the p-methylpyridinium one due to the location of the quatenized nitrogen atom in the aromatic ring.

Self-fluorescence Quenching Behaviors of Subporphyrins on the Saponite Surface Self-fluorescence quenching behaviors of the subporphyrin-saponite complexes were examined. In the present system, aggregation behaviors are completely suppressed and the decomposition due to light irradiation was not observed, judging from the absorption spectra. Fluorescence spectra of the subporphyrins at different loading levels on the saponite are shown in Figure 8. The concentration of the subporphyrins was kept constant to be 1.0 × 10–7 M to retain the same absorbance in all cases, and the loading level was controlled by change in the concentration of the saponite. Under these conditions, the subporphyrins are not desorbed from the saponite nano-sheet. 90

50 [m-BIIITMPySp]3+ = 1%, 2%, 3%, 5% 10% 20% 30% 40% 70% 50% 60% 70% 80% vs. CEC

80 70 60 50 40 30

Fluorescence intensity / a.u.

Fluorescence intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20 10 0

[p-BIIITMPySp]3+ = 1%, 2%, 3%, 5% 10% 10% 20% 30% 40% 50% 60% 70% 80% vs. CEC

40 30 20 10 0

450


750

500


800

Figure 8. Fluorescence spectra of m-BIIITMPySp-saponite and p-BIIITMPySp-saponite complexes excited at 400 and 426 nm in water at various subporphyrin loadings up to 80% versus CEC of the saponite. [Subporphyrin] = 1.0 × 10–7 M.


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The fluorescence intensities of both subporphyrins on the saponite surface were maintained enough even under high-density adsorption condition compared to other molecules20,23,24. It was expected that the subporphyrins remain their own well-ordered arrangement on the nano-sheet independently of adsorption density as the evolution of the absorption spectra with the relative dye/saponite concentration suggests. This result would be due to the “Size-matching” of not only the distance but also the arrangement of the charges between the dyes and saponite. For m-BIIITMPySp, fluorescence quenching was less efficient than that of p-BIIITMPySp. From this result, it is expected that the adsorption force due to electrostatic interactions between the m-methylpyridinium substituent and the saponite are stronger and suppresses the mobility and collision frequency of molecules more compared with the pmethylpyridinium one. It is suggested that m-methylpyridinium can enhance electrostatic interactions by rotation due to decrease in the distance between its own cation and anion on the saponite surface. In other words, the cation points of m-BIIITMPySp may respond flexibly to anion points on the saponite compared with p-BIIITMPySp.

CONCLUSIONS Using two types of meso-substituted +3-charged subporphyrin derivatives having mmethylpyridinium and p-methylpyridinium (m-BIIITMPySp and p-BIIITMPySp), their photophysical properties with and without anionic saponite were investigated. The absorption and fluorescence spectra of subporphyrin-saponite complexes were shifted to longer wavelengths due to the flattening of the molecules on the saponite surface. By these bathochromic shifts, the hybridization of dyes with the saponite can be observed as the obvious luminescence color changes. Both the subporphyrins could be adsorbed up to extremely high density and the aggregation behavior was not observed even under such a


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condition, judging from the Lambert-Beer plots. Additionally, the self-fluorescence quenching behavior was also almost maintained at high dye loadings. These results suggested that the “Size-matching” of not only distance matching but also arrangement matching of the charges between dye and saponite are an important factor for high-density adsorption without aggregation. These findings are beneficial for applications such as the dye-clay complexes to photo-functional materials.

ACKNOWLEDGEMENT This work was partly supported by a Grant-in-Aid for Scientific Research on Innovative Areas “All Nippon Artificial Photosynthesis Project for Living Earth (AnApple)” grant (No. 25107521), a Grant-in-Aid for Scientific Research (B) (No. 24350100) from the JSPS and a Grant-in-Aid for JSPS Fellows (No. 2603441). We also thank Dr. Tetsuro Kusamoto (The University of Tokyo) for high resolution mass spectrometry measurements.

SUPPORTING INFORMATION AVAILABLE The synthesis methods, the structure of synthetic saponite (Figure S1) and the optimized molecular structures and the estimated energy levels of the subporphyrins by DFT calculation (Figure S2 and S3) are available free of charge via the Internet at http://pubs.acs.org.

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Photophysical Properties and Adsorption Behaviors of Novel Tri

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