Supramolecular Surface Photochemistry: Cascade Energy Transfer

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Supramolecular-Surface Photochemistry: Cascade Energy Transfer between Encapsulated Dyes Aligned on Clay Nano-sheet Surface Takamasa Tsukamoto, Elamparuthi Ramasamy , Tetsuya Shimada, Shinsuke Takagi, and Vaidhyanathan Ramamurthy Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03962 • Publication Date (Web): 10 Mar 2016 Downloaded from http://pubs.acs.org on March 10, 2016

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Supramolecular-Surface Photochemistry: Cascade Energy Transfer between Encapsulated Dyes Aligned on Clay Nano-sheet Surface Takamasa Tsukamoto a, b, Elamparuthi Ramasamy c, Tetsuya Shimada d, Shinsuke Takagi* d and V. Ramamurthy* 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 1028471, Japan c

Department of Chemistry, University of Miami, Coral Gables, Florida 33146-0431, United States of America

d

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

For contact: Shinsuke Takagi, [email protected]

ABSTRACT Three coumarin derivatives (7-propoxy coumarin, coumarin-480 and coumarin-540a, 2, 3 and 4 respectively) having different absorption and emission spectra were encapsulated within a water-soluble organic capsule formed by the two positively charged ammonium functionalized cavitand octaamine (OAm 1). Guests 2, 3 and 4 absorb in UV, violet and blue regions and emit in violet, blue and green region, respectively.

Energy transfer between the above three

coumarin@(OAm)2 complexes assembled on the surface of a saponite clay nano-sheet was investigated by steady state and time resolved emission techniques. Judging from their emission and excitation spectra we concluded that the singlet-singlet energy transfer proceeded from 2 to 3, 1 ACS Paragon Plus Environment

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from 2 to 4 and from 3 to 4 when OAm encapsulated 2, 3 and 4 were aligned on a clay surface as two-components systems. Under such conditions the energy transfer efficiencies for the paths 2* to 3, 2* to 4 and 3* to 4 were calculated to be 33%, 36% and 50% in two-component. When all three coumarins were assembled on the surface and 2 was excited the energy transfer efficiencies for the paths 2* to 3, 2* to 4 and 3* to 4 were estimated to be 32%, 34% and 33%. A comparison of energy transfer efficiencies of the two components and the three components systems revealed that excitation of 2 leads to emission from 4. Successful merging of supramolecular chemistry and surface chemistry by demonstrating novel multi-step energy transfer in a three components dye encapsulated system on a clay surface opens up newer opportunities for exploring such systems in artificial light harvesting phenomenon.

KEYWORDS Energy transfer, Organic cavitand, Coumarins, Clay minerals, Organic/inorganic hybrid compounds, Artificial light harvesting system

2 ACS Paragon Plus Environment

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INTRODUCTION As part of functional materials neutral and ionic organic dye molecules are used both as donors and acceptors to effect energy and electron transfer processes1. In general to construct photofunctional materials, the dye molecules are either adsorbed or included within solid surfaces/interfaces.2,3 However, dye molecules placed on solid surfaces tend to aggregate at high concentrations making them lose their photoactivities.4–6 Takagi group has recently reported that aggregation of cationic dye molecules could be avoided by adsorbing them on anionic clay nanosheets.7,8 Clay minerals are multilayered inorganic materials that can serve as hosts for cationic organic adsorbents. Negatively charged clay minerals such as saponite have cation exchange capacity (CEC), and they can be completely exfoliated into a single nano-sized sheet in aqueous solution.9 Under such conditions since clay particles are < ~100 nm in size the aqueous dispersion containing clay nano-sheets is substantially transparent in the UV-visible range. This allows the adsorbed molecules to retain their photophysical properties. For example energy transfer between two cationic dye molecules adsorbed on a clay surface even at high loading levels has recently been demonstrated.10-12 The above approach although enables adsorption of multi cationic organic molecules on a clay surface it is not helpful to adsorb neutral molecules which have no tendency to adsorb on anionic surface.

We have recently overcome this problem by enclosing neutral organic

molecules within a cationic capsule and adsorbing the entire complex on anionic clay surface.13 The dye molecules that are enclosed within another host molecule can’t directly π-overlap and therefore would not be expected to aggregate on a surface. In this context capsules made of anionic (octa acid in basic medium) and cationic (octa amine in acidic medium) cavitands have recently been explored as hosts to transport neutral molecules onto the surface of various


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solids.14-16 We have established that OA or OAm encapsulated dye molecules can be adsorbed on the surfaces SiO2, Zr(HPO4)2, TiO2, gold nanoparticles and clay.17–20 Occurrence of energy transfer between OAm encapsulated aromatic molecules adsorbed on anionic clay was also established.13 In continuation of this study, it was expected that the cascade type energy transfer system can be also analyzed in detail because unexpected aggregations of dyes and electron transfer reaction between dyes that typically hinder energy transfer can be suppressed for all dyes in the system by the capsulation even under concentrated condition. Therefore, we have examined the viability of cascade type singlet-singlet energy transfer between three dye molecules adsorbed on a solid surface that has been difficult to be analyzed in detail due to these unexpected behaviors of dyes. Although energy transfer in a two component system is well known, there are only a few reports of multi-step energy transfer in three component systems adsorbed on surfaces of MOF, zeolite and clay nano-sheet21–30. Understanding the possibility of cascade type energy transfer between molecules adsorbed on solid surfaces we believed would help one to construct energy harvesting part of an artificial photosynthetic system. With this in mind we have identified a novel three-component system that has the spectral characteristics to undergo multi-step sequential energy transfer and could be adsorbed on a clay surface without aggregation. The positively charged ammonium cavitand OAm8+ (1) was used as host molecule to transport the dye molecules to the surface of a clay sheet. Coumarin derivatives 7-propoxycoumarin31,32, coumarin-48019,33 and coumarin-540a15,19,33 (denoted as 2, 3 and 4) having similar molecular structures but different absorption and fluorescence spectra were used as donor/acceptor molecules. They are suitable for multi-step energy transfer due to their broad charge-transfer (CT) absorption and emission spectra, high fluorescence quantum yields and large Stoke’s


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shifts.33 These absorption and emission characteristics allow significant spectral overlap between fluorescence of donor and the absorption of acceptor that is necessary for efficient energy transfer.1 Results presented below establish that multi-step cascade type energy transfer between 2@(OAm)216+, 3@(OAm)216+ and 4@(OAm)216+ complexes adsorbed on exfoliated Sumecton SA clay surface (denoted as 5) does occur. The five-component system comprising of three dyes, host OAm and Sumecton SA clay formed a transparent suspension in water. Structures of the host, dye and the clay are provided in Figure 1. Results of this study are presented in this article.

CH3 O

0.8 nm

O O

O O

2

O

O Et2N O

O

NEt2

NEt2

Et2N O

OO

N

O

O

O

N

O

O

O

3 Coumarin dyes

Et2N

Et2N

O

NEt2

4

O2-

Tetrahedral Layer

Si4+ or Al3+

1.4 nm

O

CH3

1.2 nm O O

O

CF3

NEt2

Mg2+

Octahedral Layer

1.1 nm

Tetrahedral Layer

OAm, 1 Clay nano-sheet, 5

Figure 1. Host and guest molecules investigated in this study.

EXPERIMENTAL SECTION Materials The cavitand octa-amino group (OAm, 1) was synthesized according to the literature.15 7propoxycoumarin (2) was prepared from 7-hydroxy coumarin by a reported procedure.31 Coumarin 480 (3) and Coumarin 540a (4) were purchased from Exciton. Saponite: Sumecton SA 5 ACS Paragon Plus Environment

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(Clay Minerals, 5) was received from Kunimine Industries Co., Ltd.. The stoichiometric formula of saponite used in this study is [(Si7.2Al0.8)(Mg5.97Al0.03)O20(OH)4]0.77–(Na0.49Mg0.14)0.77+. The theoretical surface area is 750 m2 g–1 and the CEC is 99.7 mequiv. / 100 g.8 D2O and DMSO-d6 were purchased from Cambridge Isotope Laboratories Inc.. Deuterium chloride (35w%) solution in D2O was purchased from Sigma-Aldrich.

Analysis UV-visible absorption spectra were recorded on Shimadzu UV-2600 spectrophotometer. Emission spectra were recorded on Edinburgh FS920CDT fluorometer equipped with a 300W xenon lamp. Quartz cell was used for optical measurements. 1H-NMR spectra were recorded on a Bruker B-500.

Sample preparation Preparation for the stock solution of OAm8+ and coumarin@(OAm)216+ complex To 5.10 mg of OAm taken in a 10 mL of round-bottomed flask. 2.5 mL methanol was added and the cloudy suspension was sonicated. To this 2-3 drops of deuterium chloride (35% weight in D2O) were added to get a clear transparent solution. Methanol was removed by rotary evaporation and the remaining wet solid was dried under high vacuum for 2-3 hours. Then 2.49 mL of D2O was added to form 1.0 mM OAm8+ stock solution. Stock solutions of 0.5 mM coumarin@(OAm)216+ were made by adding 5 µL of 60 mM solution of coumarin (in DMSO-d6) to 600 µL of 1 mM OAm8+ in D2O for 2:1 (H:G) capsular assembly.


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Preparation of coumarin@(OAm)216+-clay complex for optical measurements A stock solution (5.0 µM) of coumarin@(OAm)216+ complex was prepared in acidic aqueous solution. For absorption and emission studies 0.50 µM (0.10 M and 1.0 M HCl respectively) of coumarin@(OAm)216+ complex was used. The clay aqueous suspension was added to sample solution under stirring. The loading level of coumarin@(OAm)216+ complex versus CEC (cation exchange capacity) of the clay was 300%. However, the effective loading level is 75% because the capsule molecules adsorb on the clay surface vertically and four cationic groups at the bottom of capsule work as adsorption points.13 It was confirmed that all of capsule molecules in sample solution at 300% were adsorbed onto the clay surface, judging from the fact that any absorption of capsule were not observed in solution after filtration with membrane filter (diameter of the pore size = 0.1 µm) with the same technique reported previously.13 The concentration of clay was set at 2.7 equiv. L–1.

Preparation of samples of [2@(OAm)216+ + 3@(OAm)216+]-clay or [2@(OAm)216+ + 3@(OAm)216+ + 4@(OAm)216+]-clay complex for energy transfer studies Two or three types of coumarin@(OAm)216+ complexes were added to the acidic solution (1.0 M HCl) in the molar ratio 1 : 1 or 1 : 1 : 1 and stirred. The concentration of each coumarin@(OAm)216+ complex was 0.50 µM and total guest concentrations were 1.0 µM or 1.5 µM for two and three component systems. Then, the clay aqueous suspension was added to the sample solution under stirring. The loading level of coumarin@OAm216+ complex versus CEC of the clay was kept 300% by controlling of concentration of clay. The concentration of clay was set at 5.3 and 8.0 equiv. L–1 for two and three compounds mixture, respectively.


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RESULTS AND DISCUSSIONS Spectroscopic properties of the three coumarin@(OAm)216+ complexes adsorbed on the surface of clay nano-sheets 7-Propoxycoumarin (2) was encapsulated within a capsule made up of two molecules of OAm8+. We confirmed the inclusion from 1H-NMR spectra and by comparing the fluorescence of 2 in presence and absence of OAm8+. Encapsulation of guests 3 and 4 within cavitand 1 was also confirmed by recording 1H-NMR and emission spectra of free and encapsulated dye molecules (Figure S1-S3 in SI). The normalized absorption spectra of all three coumarin@(OAm)216+ (5.0 µM in 0.10 M HCl aqueous solution) are shown in Figure 2 (dash line). Spectra recorded before normalization are shown in Figure S4 in Supporting Information (SI). Complexes 2@(OAm)216+, 3@(OAm)216+ and 4@(OAm)216+ had absorption peaks at 320, 380 and 420 nm respectively. The absorption spectra of coumarin@(OAm)216+ complexes did not change on the clay surface under the condition where adsorption densities were 300% versus cation exchange capacity (CEC) of clay (the effective loading level at this CEC is 75%) (Figure S5). At this loading level (300% versus CEC of clay) the average intercapsular distance between two guest@(OAm)216+ molecules is expected to be 2.8 nm. This distance is within that expected for Fӧrster resonance energy transfer in these molecules. The normalized fluorescence spectrum of 0.50 µM 4@(OAm)216+ as a terminal energy acceptor in 1.0 M HCl aqueous solution is also shown in Figure 2 (solid line). Complexes 2@(OAm)216+, 3@(OAm)216+ and 4@(OAm)216+ had fluorescence peaks at 395, 420 and 480 nm in 1.0 M HCl aqueous solution, respectively (Figure S6, solid line).


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1.4

Normalized Absorbance and Fluorescence Intensity

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1.2

2@(OAm)216+ 3@(OAm)216+ 4@(OAm)216+

325

480

420

380

1 Absorption Emission

0.8

0.6 0.4 0.2

0 300

350

400

450 500 Wavelength / nm

550

600

650

Figure 2. Normalized absorption spectra for 5.0 µM 2@(OAm)216+, 3@(OAm)216+ and 4@(OAm)216+ in 0.10 M HCl aqueous solution (The absorption spectra of 10 µM free OAm8+ (1) were deducted from those of coumarin@(OAm)216+) and normalized fluorescence spectra for 0.50 µM 4@(OAm)216+ in 1.0 M HCl aqueous solution.

Two-component energy transfer on the surface of clay nano-sheets To establish cascade energy transfer form coumarin 2 to 3 to 4 it was essential first to probe the occurrence of energy transfer between individual pairs (2 to 3, 2 to 4 and 3 to 4). The first set of experiments consisted of establishing conditions and extent of energy transfer between the three pairs. These are described below.

Two-component energy transfer from 2@(OAm)216+ to 3@(OAm)216+ First, the singlet-singlet energy transfer from 2@(OAm)216+ to 3@(OAm)216+ on the clay surface was investigated. Concentrations of both coumarins were maintained at 0.50 µM and the molar ratio between energy donor and acceptor was kept at 1:1. Clay concentration was 5.3 µequiv. L–1 and the total loading level of coumarin@(OAm)216+ was set at 300% versus CEC of the clay. The fluorescence spectrum of [2@(OAm)216+ + 3@(OAm)216+]-clay was recorded by 9 ACS Paragon Plus Environment

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exciting at 320 nm (absorption peak of 2@(OAm)216+). The fluorescence spectrum was fitted as the sum of reference spectra of only [2@(OAm)216+]-clay and only [3@(OAm)216+]-clay excited at 320 nm (Figure 3). Such an analysis indicated a decrease in the fluorescence intensity of 2@(OAm)216+ and an increase in that of 3@(OAm)216+ for [2@(OAm)216+ + 3@(OAm)216+]-clay with respect to individual samples. This suggested that Fӧrster resonance energy transfer from 2@(OAm)216+ to 3@(OAm)216+ has occurred. This energy transfer was “singlet-singlet” type one because the fluorescence spectra that are one of deactivation processes between singletsinglet states (from S1 to S0) changed. The energy transfer process would not be Dexter type because the collision between dye molecules were suppressed due to the isolation of donor and acceptor molecules by the capsule. The energy transfer process also would not be trivial type because the concentrations of coumarins were dilute enough and the donor emission intensity decreased without any changes in spectral shapes depending on that of acceptor absorption. The energy transfer efficiency (ηET) was calculated from the coefficients of fluorescence intensity changes in energy donor and acceptor (α and β) to their spectra of individual sample [2@(OAm)216+]-clay and [3@(OAm)216+]-clay (FD and FA), the spectra of [2@(OAm)216+ + 3@(OAm)216+]-clay (FD+A), and absorbance of them (ID and IA) at the excitation wavelength according to equations 1–3.12,13 If energy transfer takes place from energy donor to acceptor, it is expected that α value for donor emission decreases due to a decrease in fluorescence radiative deactivation process and β value for acceptor emission increases due to an increase in excited acceptor molecules. 𝐹 !!! = 𝛼𝐹 ! + 𝛽𝐹 !

(1)

𝛼 = (1 − 𝜂!" − 𝜙!! )

(2)


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𝛽 = 1+

!!!"!!

!

!!!"!!

!

𝜂!" (1 − 𝜙!! )

(3)

where ΦqD and ΦqA are the energy loss efficiencies in the co-adsorption system due to quenching of excited donor and acceptor that may be induced by an electron transfer reaction between coadsorbed molecules. First, the ΦqA value was estimated based on the fluorescence intensities of 3@(OAm)216+ in the [3@(OAm)216+]-clay alone and [2@(OAm)216+ + 3@(OAm)216+]-clay by selective excitation of 3@OAm216+ at 380 nm. Next, the ηET and ΦqD values were calculated using equations 1–3 with the α, β and ΦqA values. The α and β values were set at 0.67 and 2.2 based on the fact that the fluorescence intensity of 2@(OAm)216+ in [2@(OAm)216+ + 3@(OAm)216+]-clay decreased 0.67 times to that only for [2@(OAm)216+]-clay and that of 3@(OAm)216+ increased 2.2 times only for [3@(OAm)216+]-clay (Figure 3). As a result, the ηET value was calculated to be 33% for [2@(OAm)216+ + 3@(OAm)216+]-clay system. The excitation spectrum set at an emission wavelength of 460 nm (due to 3@(OAm)216+) was also fit as the sum of the excitation spectra of individual [2@(OAm)216+]-clay and [3@(OAm)216+]-clay spectra (Figure S7 in SI).

Obtained results suggest that the energy transfer was not affected by

excitation wavelengths.


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1600 Fluorescence Intensity / a.u.

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Fitting 0.67*([2@(OAm)216+]-clay) + 2.2*([3@(OAm)216+]-clay)

1400

[2@(OAm)216+]-clay

1200

[2@(OAm)216+ + 3@(OAm)216+]-clay

1000 800 600 400

[3@(OAm)216+]-clay

200 330

380

430 Wavelength / nm

480

Figure 3. Emission spectra for [2@(OAm)216+ + 3@(OAm)216+]-clay, [2@(OAm)216+]-clay, [3@(OAm)(OAm)216+]-clay complexes and fitting spectra. (The excitation wavelength was 320 nm. The loading levels were 300% versus the CEC of the clay for all samples. [2@(OAm)216+] = [3@OAm216+] = 0.50 µM. [clay] = 5.3 µequiv. L–1.) Raman scattering is at 360 nm on the spectra.

Two-component energy transfer from 2@(OAm)216+ to 4@(OAm)216+ Next, the energy transfer from 2@(OAm)216+ to 4@(OAm)216+ on the clay surface was investigated along with the previous experiment. The excitation wavelength was 320 nm. As a result, the fluorescence intensity of 2@(OAm)216+ and 4@(OAm)216+ for [2@(OAm)216+ + 4@(OAm)216+]-clay were 0.65 and 4.9 times as those for only [2@(OAm)216+]-clay and [4@(OAm)216+]-clay, respectively (Figure 4). The ηET was calculated to be 36%. The result of fitting of excitation spectrum is shown in Figure S8 in SI.


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Fitting spectra 0.65*([2@OAm216+]-clay) + 4.9*([4@(OAm)216+]-clay)

1200

[2@(OAm)216+]-clay

1000

[2@(OAm)216+ + 4@(OAm)216+]-clay

800 600 400

[4@(OAm)216+]-clay

200 330

380


530

Figure 4. Emission spectra for [2@(OAm)216+ + 4@(OAm)216+]-clay, [2@(OAm)216+]-clay, [4@(OAm)216+]-clay complexes and fitting spectra. (The excitation wavelength was 320 nm. The loading levels were 300% versus the CEC of the clay for all samples. [2@(OAm)216+] = [4@(OAm)216+] = 0.50 µM. [clay] = 5.3 µequiv. L–1.) Raman scattering is at 360 nm on the spectra.

Two-component energy transfer from 3@(OAm)216+ to 4@(OAm)216+ Finally, the energy transfer from 3@(OAm)216+ to 4@(OAm)216+ on the clay surface was investigated. The excitation wavelength was 380 nm. As a result, the fluorescence intensity of 3@(OAm)216+ and 4@(OAm)216+ for [3@(OAm)216+ + 4@(OAm)216+]-clay were 0.30 and 1.9 times as those for only [3@(OAm)216+]-clay and [4@(OAm)216+]-clay, respectively (Figure 5). The ηET was calculated to be 50%. The result of fitting of excitation spectrum is shown in Figure S9 in SI. The parameters for calculations of energy transfer efficiency for three types of paths in these two-component samples were summarized in Table 1.


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[3@(OAm)216+ + 4@(OAm)216+]-clay

1400 1200

Fitting 0.30*([3@(OAm)216+]-clay) + 1.9*([4@(OAm)216+]-clay)

[3@(OAm)216+]-clay

1000

800 [4@(OAm)216+]-clay

600 400 200 390

440


590

640

Figure 5. Emission spectra for [3@(OAm)216+ + 4@(OAm)216+]-clay, [3@(OAm)216+]-clay, [4@(OAm)216+]-clay complexes and fitting spectra. (The excitation wavelength was 380 nm. The loading levels were 300% versus the CEC of the clay for all samples. [3@(OAm)216+] = [4@(OAm)216+] = 0.50 µM. [clay] = 5.3 µequiv. L–1.) Raman scattering is at 440 nm on the spectra. Table 1. Parameters for Calculations of Energy Transfer Efficiency for Three Types of Paths in Two-component Samples

Energy transfer paths (λex / nm)

α

β

ID / 10–3

IA / 10–3

ηET

Φq D

Φq A

2 to 3 (320)

0.67

2.2

4.5

1.2

33%

0%

0%

2 to 4 (320)

0.65

4.9

4.5

0.41

36%

0%

0%

3 to 4 (380)

0.30

1.9

3.6

2.0

50%

20%

0%

λex are excitation wavelengths. α and β are coefficients of fluorescence intensity changes in energy donor and acceptor to their spectra of individual sample. ID and IA are absorbance of them at excitation wavelengths. ηET is energy transfer efficiency. ΦqD and ΦqA are energy loss efficiencies from them at excited states in the coadsorption system.


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Theoretically, Fӧrster resonance energy transfer rate constant (kET) is determined according to equation 4. !""" !" !"! ! ! !

𝑘!" = !"#!! !! !!! !!! 𝐽

(4)

!

where κ is the orientation parameter, ΦFD is the fluorescence quantum yield of donor, n is the refractive index of the bulk medium, N is the Avogadro constant, τFD is the fluorescence lifetime of donor, R is the center-to-center distance between donor and acceptor, and J is the spectral overlap integral between the fluorescence spectrum of donor and the absorption spectrum of acceptor. J value which is an important factor that determines the energy transfer rate constant was estimated to be 5.3, 8.7 and 11 × 10–13 M–1 cm3 for energy transfer from 2 to 3, from 2 to 4 and from 3 to 4, respectively, according to equation 5 below.

𝐽=

!!!! !!

𝑑𝜈

(5)

where 𝜈 is the wavenumber, FD is the fraction of the total fluorescence intensity of donor and εA is the extinction coefficient of acceptor. The orientation parameter is determined by the angle between the transition moments of energy donor and acceptor. In this system, the transition moment of capsulated coumarin would be arranged vertically to the clay surface and parallel to each other because the capsule molecules adsorb on the clay surface at the bottom of capsule vertically (κ = 1). The fluorescence quantum yields (ΦFD) and fluorescence lifetimes (τFD) of 2 and 3 are reported to be 0.57 and 0.95, and 2.7 ns and 4.5 ns, respectively.32,32 The refractive index of water (n) is 1.33. The average center-to-center distance between donor and acceptor (R) is estimated to be 2.8 ×10–9 m from the adsorption 15 ACS Paragon Plus Environment

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density of the capsule molecules on the clay surface. Based on these values the theoretical energy transfer rate constants (kETtheo) for the three energy transfer paths were calculated. Additionally, the experimental energy transfer rate constants (kETexp) were estimated from the actual energy transfer efficiencies according to equation 6 where 𝜂!" is the energy transfer efficiency. The kETtheo and kETexp values are summarized in Table 2.

𝑘!" = !! !

!!"

(6)

!!!!"

Table 2. Theoretical and Experimental Energy Transfer Rate Constants for Three Types of Paths in Two-component Samples

Energy transfer paths 2 to 3 2 to 4 3 to 4

Φf D

τfD / 10–9 s

0.57

2.7

0.95

4.5

J / 10–13 M–1 cm3

kETtheo / 108 s–1

kETexp / 108 s–1

5.3

0.70

1.8

8.7

1.1

2.1

11

1.4

2.2

ΦfD are fluorescence quantum yields of energy donor and τfD are fluorescence lifetimes of energy donor.32,33 J is the spectral overlap integral between the fluorescence spectrum of donor and the absorption spectrum of acceptor. kETtheo and kETexp are theoretical and experimental energy transfer rate constants, respectively.

Judging from the fact that the kETexp is about twice that of kETtheo we believe that the average number of energy acceptor molecules adjacent to donor molecules may be about 2. If the capsule molecules adsorb on clay randomly in a close-hexagonal fashion (Figure 6 (i)), this


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number should be 4. Therefore, it is suggested that two capsule molecules adsorbed on the clay surface with segregation somewhat as shown in Figure 6 (ii).

A D A D A D A D D A D A D A D A A D A D A D A D D A D A D A D A A D A D A D A D D A D A D A D A

A D D D A A A D D A A A D D D A A A A D D D A A A A A D D D A A D D A A A D D D D D A A A D D D

(i)

(ii)

Figure 6. (i) Theoretically integrated and (ii) experimentally speculated arrangement pattern of capsule molecules in two-component samples. (The average numbers of adjacent energy acceptor capsule to excited energy donor capsule is 4 and 2.2 for (i) and (ii), respectively.)

Three-component singlet-singlet energy transfer on the surface of clay nano-sheets Three-component energy transfer excited at 2@(OAm)216+ and 3@(OAm)216+absorptions Encouraged by the above results on energy transfer in two-component systems we initiated a study on energy transfer in a three-component system comprising of 2@(OAm)216+, 3@(OAm)216+ and 4@(OAm)216+ aligned on the clay surface. For this study concentrations of all three coumarins were kept at 0.50 µM and the molar ratio between the three coumarins was maintained at 1:1:1. Clay concentration was 8.0 µequiv. L–1 and total loading level was set at 300% versus CEC of clay. The fluorescence spectra of [2@(OAm)216+ +3@(OAm)216+ + 4@(OAm)216+]-clay were recorded by exciting at the main absorption peak of 2@(OAm)216+ (320 nm). The recorded fluorescence spectra could be fitted as the sum of the reference spectra 17 ACS Paragon Plus Environment

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of

the

three

individual

components

Page 18 of 30

[2@(OAm)216+]-clay,

[3@(OAm)216+]-clay

and

[4@(OAm)216+]-clay excited at 320 nm. Based on this calculation the fluorescence of the three component system [2@(OAm)216+ + 3@(OAm)216+ + 4@(OAm)216+]-clay could be thought to consist of a mixture of emissions from [2@(OAm)216+]-clay, [3@(OAm)216+]-clay and [4@(OAm)216+]-clay at 0.34, 0.70 and 6.0 times the intensity of the individual emissions. Since both fluorescence intensities of 2@(OAm)216+ and 3@(OAm)216+ decreased and that of 4@(OAm)216+ increased, we suggest that both 2@(OAm)216+ and 3@(OAm)216+ were serving as energy donors for 4@(OAm)216+ (Figure 7, left). The excitation spectra at 550 nm that is emitted by 4@(OAm)216+ agrees with the sum of excitation spectra of [2@(OAm)216+]-clay, [3@(OAm)216+]-clay and [4@(OAm)216+]-clay (Figure S10 in SI). Additionally,

the

fluorescence

spectra

of

[2@(OAm)216+

+3@(OAm)216+

+

4@(OAm)216+]-clay were observed by exciting at 380 nm, the main absorption peak of 3@(OAm)216+ and not absorbed by 2@(OAm)216+. Calculation of energy transfer efficiencies was made easier by this apparent two-component system without the influence of 2@(OAm)216+. The fluorescence spectra could be fitted as the sum of only two reference spectra of 0.33 times [3@(OAm)216+]-clay and 1.6 times [4@(OAm)216+]-clay excited at 380 nm (Figure 7, right).


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1600

1600 Fitting 0.35*([2@(OAm)216+]-clay) + 0.73*([3@(OAm)216+]-clay) + 6.0*([4@(OAm)216+]-clay)

1400 1200

[2@(OAm)216+]-clay

1000 800

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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[2@(OAm)216+ + 3@(OAm)216+ + 4@(OAm)216+]-clay

600 [3@(OAm)216+]-clay

400

[4@(OAm)216+]-clay

200 340

390


540

590

1400 [3@(OAm)216+]-clay

1200

Fitting 0.33*([3@(OAm)216+]-clay) + 1.6*([4@(OAm)216+]-clay) [2@(OAm)216+ + 3@(OAm)216+ + 4@(OAm)216+]-clay

1000

800

[4@(OAm)216+]-clay

600 400 200 390

440


590

Figure 7. Emission spectra for [2@(OAm)216+ + 3@(OAm)216+ + 4@(OAm)216+]-clay, [2@(OAm)216+]-clay, [3@(OAm)216+]-clay, [4@(OAm)216+]-clay complexes and fitting spectra excited at 320 nm (left) and 380 nm (right). (The loading levels were 300% versus the CEC of the clay for all samples. [2@(OAm)216+] = [3@(OAm)216+] = [4@(OAm)216+] = 0.50 µM. [clay] = 8.0 µequiv. L–1.) Raman scattering are at 360 and 440 nm on the left and right spectra, respectively.

Energy transfer efficiency in the three-component system The energy transfer efficiency (ηET) values in the above three-component system for the three possible modes of transfer namely, 2 to 3, 2 to 4 and 3 to 4 (ηETAB, ηETAC and ηETBC), were estimated. Upon excitation at 320 nm, the energy transfer efficiencies were calculated from the coefficients of fluorescence intensity changes in 2, 3 and 4 (α, β and γ) between the spectra of individual samples [2@(OAm)216+]-clay, [3@(OAm)216+]-clay and [4@(OAm)216+]-clay (FA, FB and FC) and the sample containing all three ([2@(OAm)216+ + 3@(OAm)216+ + 4@(OAm)216+]clay) (FA+B+C) using the absorbance at 320 nm of 2, 3 and 4 (IA, IB and IC) and at 380 nm of 3 and 4 (I'B and I'C) and equations 7–10.


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𝐹 !!!!! = 𝛼𝐹 ! + 𝛽𝐹 ! + 𝛾𝐹 !

(7)

!" !" 𝛼 = 1 − 𝜂!" − 𝜂!" − 𝜙!!

(8)

𝛽 = (1 +

𝛾=

!!!"!!

!

!!!"!!

!!!"!! !!!"

!

!

!" !" 𝜂!" )(1 − 𝜂!" − 𝜙!! )

!" 𝜂!" ∙ 𝜂!" + !!! !"

!!!"!! !!!"

!

𝜂!" + !!! !"

(9)

!!!"!!

!

!!!"!!

!

!" 𝜂!" + 1 (1 − 𝜙!! )

(10)

where ΦqA, ΦqB and ΦqC are the energy loss efficiencies from 2, 3 and 4 in the co-adsorption system. First, the ΦqC value was estimated according to the fluorescence intensities of 4 in the individual sample [4@(OAm)216+]-clay and the sample containing all three [2@(OAm)216+ + 3@(OAm)216+ + 4@(OAm)216+]-clay upon excitation at 420 nm. As a result, the ΦqC value was 0%. Additionally, in the case excited at 380 nm, the ηETBC and ΦqB values were calculated from the coefficients of fluorescence intensity changes in 3 and 4 (β' and γ') to spectra of individual samples [3@(OAm)216+]-clay and [4@(OAm)216+]-clay (F'B and F'C) and the sample containing both two [2@(OAm)216+ + 3@(OAm)216+ + 4@(OAm)216+]-clay (F'B+C) using the absorbance at 380 nm of 3 and 4 (I'B and I'C) and equations 1–3 with the ΦqC value. Using the obtained ηETBC, ΦqB and ΦqC values, the ηETAB, ηETAC and ΦqA values could be calculated according to equations 7–10. As a result, the ηETAB, ηETAC and ηETBC values for energy transfer from 2 to 3, from 2 to 4 and from 3 to 4 for three-component sample were calculated to be 32%, 34% and 33%, respectively (Figure 8). The energy transfer efficiencies in two- and three-component systems are shown in Table 3 and the parameters for calculations of energy transfer efficiency for three types of paths in these two-component samples were summarized in Table S1 in SI.


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hν 34% O

O

O

ηET

CH3

2@(OAm)216+

32%

N

33%

hν'

hν''

CF3 O

O

4@(OAm)216+

CH3 N

O

O

3@(OAm)216+

Figure 8. Energy transfer processes and efficiencies (ηET) in the three-component system.

Table 3. Experimental Energy Transfer Efficiencies for Three Types of Paths in Two- and Three-component Samples

Energy transfer paths

Energy transfer efficiency ηET Two-component

Three-component

2 to 3

33%

32%

2 to 4

36%

34%

3 to 4

50%

33%

In spite of differences in the distribution of dye molecules in the three-component and two-component systems, the ηET values for energy transfer paths from 2 to 3 and 2 to 4 for the two systems were almost the same. On the other hand, the ηET for the path from 3 to 4 in threecomponent sample decreased 0.66 times as that in two-component one. If the capsule molecules adsorb on clay randomly in a close-hexagonal fashion in Figure 6 (i) and 9 (i), the average number of adjacent acceptors to an excited donor in the three-component system is 0.75 (3/4) times as two-component one for all guests and the energy transfer efficiencies are also 0.75 times for all paths. Judging from these changes in the experimental ηET values in three-component 21 ACS Paragon Plus Environment

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Page 22 of 30

system, it is suggested that the segregation behaviors of the coumarin@(OAm)216+ molecules and the number of adjacent complexes are different somewhat depending on guest coumarins as shown in Figure 9 (ii). This result is expected to be due to the little difference of capsulated conditions of three types of coumarins. In the case of Figure 9 (ii), The average acceptor numbers are 0.91, 0.91 and 0.68 times as those in two-component samples and the theoretical ηETAB, ηETAC and ηETBC values can be estimated to be 30%, 33% and 34%, respectively, using the experimental ηET values of two-component samples considering the arrangement pattern in Figure 6 (ii). These theoretical values agree rather well with the experimental ones in Table 3.

A B C A B C A B B C A B C A B C A B C A B C A B B C A B C A B C A B C A B C A B B C A B C A B C

A A B B C C A A C C A A C C B B C B B A A B B C A B B C C A A C A A C C B B A A B B A A B B C C (ii)

(i)

Figure 9. (i) Theoretically integrated and (ii) experimentally expected arrangement pattern of capsule molecules in three-component sample.

These unique energy transfers proceeded in water only when the donor/acceptor molecules were encapsulated and adsorbed on clay surfaces.

In the absence of clay and

cavitands to encapsulate the donor/acceptor molecules the energy transfer proceeded inefficiently due to aggregation and sedimentation of dyes. When the three capsules were solubilized in water without clay the energy transfer efficiency was very low and the excitation spectra of [2@(OAm)216+ + 3@(OAm)216+ + 4@(OAm)216+]-clay at 550 nm resulted from only [4@(OAm)216+]-clay without participation of the other two coumarins in the emission of 4 22 ACS Paragon Plus Environment

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(Figure S11 in SI). Especially the efficiency for energy transfer from 3@(OAm)216+ to 4@(OAm)216+ was estimated to be 0% because the excitation spectrum of 3@(OAm)216+ hardly appeared on the spectra. Control experiments have established that for efficient cascade energy transfer in the three-component system both clay and cavitands are absolutely essential. In presence of both, excitation of 2 results in the emission of 4 at least part of the energy flowing via 3.

CONCLUSION Three coumarin derivatives (2, 3 and 4) having different absorption and emission spectra were successfully encapsulated within a capsule made up of two molecules of a water-soluble cationic organic cavitand (OAm8+, 1) and characterized by 1H NMR. None of the donor/acceptor molecules adsorbed on clay without being included within a cationic capsule. Energy transfer between the three coumarin@(OAm)216+ complexes aligned on a saponite clay surface was investigated by recording their absorption, emission and excitation spectra. Probing the viability of cascade energy transfer between three components was initiated by establishing the occurrence of energy transfer within the three two-component systems (2 to 3, 2 to 4, and 3 to 4; all included within (OAm)2 capsule) adsorbed on clay surface independently. The energy transfer efficiencies in these three two-component systems were estimated to be 33%, 36% and 50% respectively. Cascade energy transfer from coumarin 2 to coumarin 3 to coumarin 4 adsorbed on saponite clay as OAm capsules in 1 : 1 : 1 ratio was established by monitoring the fluorescence spectra upon excitation of coumarin. Exciting mainly 2 or 3, fluorescence enhancement of 4 due to energy transfer from 2 to 4, from 3 to 4 and from 2 to 3 to 4 was observed. The energy transfer efficiencies for the paths from 2 to 3, from 2 to 4 and from 3 to 4 were estimated to be 32%, 34%


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and 33%. In this study we have established the value of combining supramolecular chemistry and surface chemistry to achieve a goal that is normally not achievable without the help of both. Clay surface helps to align donor/acceptor molecules while cationic organic cavitand helps to transport neutral donors/acceptors to the surface of anionic clay. By carefully selecting three substituted coumarin molecules as probes we have established the value of merging supramolecular, surface and photochemistry to possibly build artificial solar collectors.

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).

VR is grateful to the National Science

Foundation, USA (CHE-1411458) for financial support.

SUPPORTING INFORMATION AVAILABLE The

1

H-NMR spectra for guest@(OAm)216+ complexes, absorption spectra for

guest@(OAm)216+ complexes and host, excitation spectra for energy transfer for two- and threecomponent samples on the clay surface and without clay are available free of charge via the Internet at http://pubs.acs.org.

REFERENCE


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Table of Contents Multi-step energy transfer reaction system

hν

ET

ET

hν'

ET Dye@Cavitand

Normalized Absorbance and Fluorescence Intensity

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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Clay-nanosheet

Absorption Emission

1 0.8 0.6 0.4 0.2 0 300

350


400 450 500 550 Wavelength / nm

600

650

Supramolecular Surface Photochemistry: Cascade Energy Transfer

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