Artificial Light-Harvesting Model in a Self-Assembly Composed of

Apr 18, 2013 - Photosynthesis, Tokyo Metropolitan University, Minami-ohsawa 1-1, ... guest assembly composed of functional dyes and inorganic nanoshee...
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Artificial Light-Harvesting Model in a Self-Assembly Composed of Cationic Dyes and Inorganic Nanosheet Yohei Ishida,†,‡ Tetsuya Shimada,†,§ and Shinsuke Takagi*,†,§ †

Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, and §Research Center for Artificial Photosynthesis, Tokyo Metropolitan University, Minami-ohsawa 1-1, Hachiohji, Tokyo 192-0397, Japan ‡ Japan Society for the Promotion of Science (PD), Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan S Supporting Information *

ABSTRACT: This paper proposes an efficient artificial light-harvesting system in a host− guest assembly composed of functional dyes and inorganic nanosheet. Although we have already reported an efficient energy transfer between two types of porphyrin molecules on inorganic nanosheets (e.g., J. Am. Chem. Soc. 2011, 133, 14280), the number of photons captured by one acceptor molecule (photon-harvesting efficiency: the donor/acceptor ratio when the total energy transfer efficiency is 50% as defined in the main text) was a few. To overcome this low photon-harvesting efficiency, we designed and investigated a new nanosheet type light-harvesting system including phthalocyanine. As a result from steady-state and time-resolved fluorescence measurements, the energy transfer reaction was highly efficient even under the donor excess conditions. The efficiency was almost 100% even under the ratio of donor/acceptor = 1/1−6/1. The most advanced point of this study is the presence of energy transfer between nonadjacent donor−acceptor, and the photon-harvesting efficiency of this system progressed seven times compared to that of the previous porphyrin−porphyrin system. Additionally, the efficient utilization of visible region of sunlight (visible-lightharvesting efficiency: the percentage of visible region of sunlight (380−780 nm), in which the extinction coefficient of the lightharvesting molecules excesses 104 M−1 cm−1) was realized in the present donor−acceptor combination. The visible-lightharvesting efficiency of the present system reached 86%. Thus, our host−guest system took a step closer to realize an artificial light-harvesting system utilizing the wide-wavelength region of sunlight with high photon-harvesting efficiency, in which a few energy acceptor molecules can harvest the excitation energies from a large number of adjacent and/or nonadjacent donor molecules efficiently.

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INTRODUCTION It is well-recognized that human beings will face an energy crisis due to a shortage of fossil fuels within several decades. Among various candidates for solving this problem, artificial photosynthesis is one of the most probable.1 An artificial photosynthesis can be defined as a light-driven system that transports electron from a water molecule to an appropriate electron-accepting system and would consist of a photoinduced substrate conversion reaction system such as O2 evolution from water and a light-harvesting system.1,2 There are many excellent researches on the artificial photoinduced substrate conversion reaction systems such as O2 evolution via the oxidation of water molecule by ruthenium complex,3 CO evolution via the reduction of CO2 by rhenium complex,4 and so on. One of the bottlenecks for a realization of artificial photosynthesis including multielectron conversion reaction such as O 2 evolution from water splitting via four-electron conversion is to overcome a problem of low photon-flux density of sunlight.1 Because the lifetimes of oxidized or reduced species of catalyst are typically short such as microsecond to millisecond time scales and they will decompose or be deactivated before accepting or donating the next electron, the objective multielectron conversion reaction should be impossible under © 2013 American Chemical Society

the normal sunlight condition without a light-harvesting system as described below. Here, we elucidate the significance of lightharvesting system for the O2 evolution of Mn4CaO5 cluster via four-electron conversion as an example. Kamiya et al. recently reported a detailed crystal structure of Mn4CaO5 cluster at 1.9 Å resolution.5 The mechanism of oxygen evolution in Mn4CaO5 cluster system has been studied intensively, and the reaction cycle is well-known as the “Kok cycle”.6,7 In the Kok cycle, four types of intermediates (S1, S2, S3, and S4) are quasi-stable having about a few seconds−several hundred seconds of lifetimes.6−8 According to a calculation outlined in the literature,1 a stepwise light-absorption of light-harvesting chlorophylls takes about 5 s per 1 photon absorption under sunlight conditions (0.5 mW cm−2 = 1 × 1015 photons cm−2 s−1 at 420 nm).1 The calculation flow is as follows. The extinction coefficient of chlorophyll a is 1.1 × 105 M−1 cm−1 = 1.1 × 108 mol−1 cm2 at 420 nm,9 and the cross-section of the light absorption is 1.9 × 10−16 cm2 (=1.1 × 108 mol−1 cm2 × (6 × 1023)−1). Thus, 5 s (=(1.9 × 10−16 cm2 × 1 × 1015 photons Received: March 5, 2013 Revised: April 9, 2013 Published: April 18, 2013 9154

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cm−2 s−1)−1) is necessary for absorbing one photon. Supposing that the lifetimes of Mn4CaO5 intermediates are 1 s and the energy transfer from light-harvesting chlorophylls is enough faster than the lifetimes of intermediates,6−8 a realization of stepwise four-electron conversion in Mn4CaO5 cluster system requires at least 5 times the photon-flux density of sunlight (=5 s photon−1 × (1 s photon−1)−1). In other words, five chlorophyll molecules as the light-harvesting antennas are leastwise necessary for a realization of stepwise four-electron conversion of Mn4CaO5 cluster. Moreover, the lifetimes of typical oxidized or reduced species are short such as microsecond to millisecond time scales and the extinction coefficients of them are small in general; thus, a realization of an artificial light-harvesting system is essential toward an artificial photosynthesis including multielectron conversion reaction process. In recent years, excellent works have revealed that the lightharvesting system is composed of the amazingly beautiful alignment of chlorophyll molecules, and the regulated alignment of chlorophylls realizes the efficient and selective lightharvesting energy transfer processes in purple bacteria.10 This finding has led us to construct a regularly arranged assembly of functional dyes toward a realization of artificial light-harvesting system. A fair amount of research has been carried out to realize an artificial light-harvesting system such as supramolecular assemblies of organic molecules, covalently linked systems, and dendrimer systems.11−32 In contrast, the focus of our investigations has been on using clay minerals33−41 as a novel host material for guest functional dyes. Saponite clay minerals33−41 are characterized by nanostructured flat sheet structures with the diameter of ∼40−100 nm possessing negatively charged surfaces with intercharge distance of 1.2 nm. Additionally these materials could be exfoliated to individual nanosheets in aqueous solution. The dispersion shows good optical transparency in the visible region. Our previous studies have realized the high-density alignment of cationic porphyrin molecules on the clay surface without aggregation,42−46 while typical guests on solid surfaces tend to be aggregated due to the relatively strong guest−guest interaction.47−49 Such an arrangement without aggregation is resulting from the distance matching due to guest−host Coulombic interaction between positively charged dye molecules and negatively charged clay surface. We termed this a “size-matching effect”.42−46 Because the H-aggregation formation significantly decreases the excited lifetime and the efficiency of photochemical reaction,47,48 a complete suppression of aggregation in our clay/dye system is important. We here propose that the performance of an artificial lightharvesting system should be evaluated from two aspects. In this paper we define them as a photon-harvesting efficiency (ηPH) and a visible-light-harvesting efficiency (ηVLH). ηPH expresses the number of photons captured by one acceptor molecule and is defined experimentally as the donor/acceptor ratio when the total energy transfer efficiency is 50%. ηVLH expresses the availability of the visible region of sunlight and is defined as the percentage of visible region of sunlight (380−780 nm), in which the extinction coefficient of the light-harvesting molecules exceeds 104 M−1 cm−1. A value of 104 M−1 cm−1 of extinction coefficient expresses that 90% of sunlight can be absorbed by the light-harvesting system with 1 μm thickness and 1 M concentration. Both high ηPH and high ηVLH are required for constructing an efficient artificial light-harvesting system. Although we have reported an almost 100% energy

transfer reaction between two types of porphyrin dyes on the clay surface,29 the ηPH and ηVLH are small. The values of ηPH and ηVLH were 3 and 47.5%, respectively. The previous work on the porphyrin−porphyrin system has revealed information about how the 100% efficiency is realized on solid surfaces.29 Aggregation, segregation, and self-fluorescence quenching have been revealed as the main factors lowering the energy transfer efficiency. The suppression of them is important to achieve efficient energy transfers on solid surfaces.29 This paper proposes an efficient artificial light-harvesting model in clay/dye system having high ηPH and ηVLH. Resulting from the novel donor−acceptor combination, ηPH and ηVLH, came to 21 and 86%, respectively. Thus, our clay/dye system took a step closer to realizing an efficient artificial lightharvesting system. The mechanisms to realize an efficient artificial light-harvesting system are presented in this paper.

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EXPERIMENTAL SECTION Materials. The saponite clay used in this experiment was synthesized by hydrothermal synthesis according to a previous paper.45 The synthetic saponite was analyzed with XRD, XRF,27Al-NMR, FT-IR, and TG/DTA. 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. Tetrakis(1methylpyridinium-4-yl)porphyrin (Por) (Figure 1) was pur-

Figure 1. Structures of tetrakis(1-methylpyridinium-4-yl)porphyrin (Por) and tetramethyltetrapyridino[3,4-b:3′,4′-g:3″,4″-l:3‴,4‴-q]porphyrazin (Pc).

chased from Aldrich. The purity of Por was checked by 1H NMR. Tetramethyltetrapyridino[3,4-b:3′,4′-g:3″,4″-l:3‴,4‴-q]porphyrazin (Pc) was synthesized by the reaction of 3,4dicyanopyridine in octan-1-ol in the presence of DBU to the tetrapyridino[3,4-b: 3′,4′-g: 3″,4″-l: 3‴,4‴-q]porphyrazin followed by methylation with dimethylsulfate, according to the literature.50−52 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 counterions of Por and Pc were 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. Absorption spectra were measured with Shimadzu UV-3150 spectrophotometer. The corrected fluorescence spectra were measured with 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 9155

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Figure 2. (a) Absorption spectra of Pc/clay complexes at various dye loadings up to 240% vs CEC in aqueous suspension. The concentration of clay was 8.0 mg L−1. (b) Lambert−Beer plot for Pc/clay complexes at 698 nm.

fluorescence measurement was conducted under a photoncounting 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 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 same to that of the steady-state fluorescence spectroscopy even under the same conditions. Preparation Methods for the Clay/Dye Complexes. Preparation for Pc/Clay Complexes To Measure the Absorption Spectra. Absorption spectra of Pc/clay complexes were observed as described below. Pc/clay complex was typically prepared by mixing of the aqueous clay suspension and the respective aqueous Pc solution under stirring. The dye loadings were changed by changing the concentration of Pc. The concentration of clay was always kept constant at 8.0 mg L−1. Preparation for Energy Transfer Samples To Measure the Fluorescence Spectra. The typical procedure to prepare energy transfer samples was as follows. In this experiment, Por(D) and Pc(A) were used as an energy donor and an energy acceptor, respectively (D and A represent energy donor and acceptor, respectively). Aqueous solutions of Por(D) and Pc(A) were mixed. The obtained solution was then mixed with an aqueous clay suspension (9.9 mg L−1) under vigorous stirring. The total concentrations of Por(D) and Pc(A) were set at 1.0 × 10−7 M, and the dye loading was 90% vs CEC of the clay. The molar ratio of Por(D) to Pc(A) was modulated as Por(D)/Pc(A) = 1/1−31/1. Under these conditions, the clay sheets exist in a form of individually exfoliated sheets, and the obtained solution was substantially transparent. Since no dye absorption was detected in supernatant liquid obtained by a centrifugation of clay/dye mixture (12 000 rpm, 60 min), all dye molecules adsorb on the clay surface.

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loading level (0−100% vs cation exchange capacity (CEC) of the clay).45 We define the concentration of guest vs the CEC of clay as follows: When the number of cationic sites in guest adsorbed on clay surface is the same as the number of anionic sites on the clay surface, the loading level is expressed as 100% vs CEC of the clay. Absorption spectra of Pc on the clay surface were observed at various dye loadings as shown in Figure 2a. Below 100% vs CEC of the clay, the absorption spectra were completely the same shape (see the normalized absorption spectra shown in Figure S1). Above 100% vs CEC, the observed spectra can be expressed by the spectral fitting using two components as described below. The long-wavelength absorption (λmax = 698 nm) was the component of the adsorbed Pc on the clay surface, and the short-wavelength absorption (λmax = 687 nm) was the component of the nonadsorbed Pc in a bulk solution. The λmax shift of Pc upon adsorption on the clay surface is probably due to the ruffled structure of the phthalocyanine ring.19,53 The Lambert−Beer plot for Pc/clay complexes is shown in Figure 2b. As can be seen, the linearity of the plot was observed below 100% vs CEC of the clay. Thus, it turns out that the Pc exists as an independent molecule for 0−100% vs CEC, while typical dye molecules tend to aggregate on the clay even at low-adsorptiondensity condition.39 This unique high-density adsorption of Pc is due to the “size-matching effect”.42−46 This is the first example for the 100% vs CEC adsorption of organic dyes on the clay surface except for porphyrin derivatives.42−46 In the following we widen the choice of dyes generating appropriate intercationic distances in the structure so as to increase the overall conversion efficiency of the system. Self-Fluorescence Quenching Behaviors of Pc and Por on the Clay Surface. The self-fluorescence quenching behaviors of Pc and Por on the clay surface were examined by measuring the steady-state fluorescence spectra. A selfquenching is induced by an electron transfer from an excited molecule to the adjacent identical molecule at ground state in the present system.28,29 The self-quenching decreases the excited singlet lifetime of dyes and, thus, decreases the energy transfer efficiency.28,29 To construct the efficient energy transfer systems, suppression of the self-quenching is essential. Under the present experimental condition, the concentration of dyes is always kept constant to be 1.0 × 10−7 M and the loading level of adsorption is varied by controlling the concentration of clay, indicating that every sample absorbed the same number of

RESULTS AND DISCUSSION

Adsorption Behaviors of Por and Pc on the Clay Surface. Adsorption behaviors of Por and Pc on the clay surface were examined. As we have reported, Por adsorbs on the clay surface as an independent molecule unit at every 9156

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photons under the same absorbance. We can thus discuss the self-quenching by simply comparing the fluorescence intensity. The observed fluorescence spectra for Pc/clay complexes, when their loading was changed from 0.05% to 100% vs CEC, are shown in Figure 3. The fluorescence at 0.05% adsorption can

Figure 4. Absorption spectra for Por(D)/clay, Pc(A)/clay, and Por(D)/Pc(A)/clay (dashed line) complexes in aqueous suspension ([clay] = 9.9 mg L−1, [Por(D)] = [Pc(A)] = 5.0 × 10−7 M, sum of dyes loadings is 40% vs CEC).

system is 86%, which is almost 2 times that of the previous porphyrin−porphyrin system (47.5%).29 The energy transfer from the excited singlet state of Por(D) to the ground state of Pc(A) on the clay surface was examined by measuring steady-state fluorescence spectra. Individual fluorescence spectra of Por(D)/clay and Pc(A)/clay complexes and an energy transfer sample of Por(D)/Pc(A)/clay complex are shown in Figure 5. The fluorescence spectral shape of the

Figure 3. Fluorescence spectra for Pc/clay complexes at 0.05, 0.1, 20, 40, 60, 80, and 100% vs CEC of the clay. [Pc] = 1.0 × 10−7 M. The excitation wavelength was set at 600 nm.

be used as the standard fluorescence intensity which corresponds to the monomer’s fluorescence without any quenching processes, because the self-quenching is negligible under this condition judging from the fact that the fluorescence intensity of 0.05% CEC sample was the same as that of 0.1% CEC sample (Figure 3). As reported before, the self-quenching of Por was not observed on the clay surface even under a high-densityadsorption condition (not shown).29 On the other hand, an extensive self-quenching was observed for Pc/clay complex. The reason why Pc suffers the self-fluorescence quenching more than Por would be due to the naked phthalocyanine ring. Since a self-quenching due to electron transfers requires the efficient collision of chromophores, a naked phthalocyanine ring would enhance the self-quenching compared to Por whose chromophore is surrounded by four pyridinium groups. These results indicate that the acceptor Pc would be quenched by the self-fluorescence quenching process after energy transfers from donors under the energy transfer experiments. Energy Transfer Reaction in Por/Pc/Clay Complex. Absorption spectra of Por(D)/clay and Pc(A)/clay complexes were relatively well differentiated (Figure 4). D and A represent energy donor and acceptor, respectively. The absorption spectra of Por(D)/Pc(A)/clay complexes were completely identical with a sum of individual absorption spectra of Por(D)/clay and Pc(A)/clay complexes. Thus, it was found out that an aggregation, which causes the absorption spectral shift, is completely suppressed, and the dye molecules exist as an independent molecule even when two types of dyes coexist on the clay surface. This independent molecular adsorption of Por and Pc was retained below 100% vs CEC of the clay by the experiments under various CEC conditions. The combination of Por and Pc enables utilization of the wide wavelength region of sunlight. The visible-light-harvesting efficiency (ηVLH) is defined as the percentage of visible region of sunlight (380− 780 nm), in which the extinction efficient of the light-harvesting molecules exceeds 104 M−1cm−1. ηVLH of this Por(D)−Pc(A)

Figure 5. Fluorescence spectra of Por(D)/clay, Pc(A)/clay, and Por(D)/Pc(A)/clay (dashed line) complexes excited at 450 nm. [Por(D)] = [Pc(A)] = 5.0 × 10−8 M. The total dye loadings were set at 0.05% for Por(D)/clay and Pc(A)/clay and 90% for Por(D)/ Pc(A)/clay complex, respectively. The mol ratio of Por(D) to Pc(A) was 1:1 for Por(D)/Pc(A)/clay complex.

energy transfer sample was the same as that of acceptor fluorescence, and the intensity increased. The fluorescence of energy transfer sample did not include the Por(D) component. This indicates that the excited Por(D) molecules were completely deactivated by the energy transfer and/or other quenching processes such as self-quenching. By using the fluorescence spectra (FET(ν)), the energy transfer efficiency and the quenching efficiency can be calculated as follows, based on the previously reported method.29 9157

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reference fluorescence spectra, Fm0(ν) and Fp0(ν). Thus, parameters ηET and ϕAA can be obtained from the spectral fitting analysis. Obtained ηET and ϕAA values are 100% and 55%, respectively. The energy transfer process was highly efficient due to the large spectral overlap between Por(D) and Pc(A) (spectral overlap parameter, J, is 1.1 × 10−12 M−1 cm3 as described later). The 55% ϕAA is due to the self-quenching of Pc(A) molecules on the clay surface. That is, the excitation energy of Por(D) molecules was completely harvested by Pc(A) molecules, and then 55% of the harvested energies was quenched due to the self-quenching between Pc(A) molecules. Thus, to reduce the ϕAA values, we examined the effect of mole ratio of Por(D) to Pc(A) on the clay surface. If the adsorption distribution of two dyes is uniform, an adjacent probability between Pc(A) molecules will decrease as the ratio of Por(D) increases. The schematic representations for possible idealized adsorption structure of Por and Pc under various mole ratios are depicted in Figure 6.

The total fluorescence of Por(D)/Pc(A)/clay complex (FET(ν)) can be expressed by eq 129 FET(v) = (1 − ηET − ϕDD − ϕDA ) × FD0(v) ⎞ ⎛ 1 − 10−AD + ⎜1 + η − ϕAA − ϕAD⎟ × FA0(v) −AA ET ⎠ ⎝ 1 − 10 (1)

where FET(ν) is the fluorescence spectrum of Por(D)/Pc(A)/ clay complex and FD0(ν) and FA0(ν) are the respective fluorescence spectra of Por(D)/clay and Pc(A)/clay complexes at 0.05% vs CEC, respectively, ηET is the energy transfer efficiency, defined in eq 2, ϕDD and ϕAA are the self-quenching efficiencies due to electron transfers from excited Por(D) to the ground state of Por(D) and excited Pc(A) to the ground state of Pc(A), defined in eqs 3 and 4, respectively, ϕDA and ϕAD are the quenching efficiencies due to electron transfers from excited Por(D) to the ground state of Pc(A) and excited Pc(A) to the ground state of Por(D), defined in eqs 5 and 6, respectively, and AD and AA are the absorbance of Por(D)/clay and Pc(A)/ clay complexes at 450 nm, respectively. ηET =

ϕDD =

ϕAA =

ϕDA =

ϕAD = D

kET kET +

kdD

+ k fD + kqDD + kqDA

(2)

kqDD kET + kdD + k fD + kqDD + kqDA

(3)

kqAA kET + kdA + k fA + kqAA + kqAD

(4)

kqDA kET + kdD + k fD + kqDD + kqDA

(5)

kqAD kET + kdA + k fA + kqAA + kqAD

(6)

A

where kd and kd are the sum of nonradiative deactivation rate constant and intersystem-crossing rate constant of Por(D) and Pc(A), respectively, kfD and kfA are the radiative deactivation rate constant of Por(D) and Pc(A), kqDD and kqAA are the selfquenching rate constant due to electron transfers from excited Por(D) to the ground state of Por(D) and excited Pc(A) to the ground state of Pc(A), kqDA and kqAD are the quenching rate constant due to electron transfers from excited Por(D) to the ground state of Pc(A) and excited Pc(A) to the ground state of Por(D), and kET is the energy transfer rate constant. In the present system, ϕDD (kqDD) is negligible judging from the previous experiments for self-quenching. ϕDA and ϕAD (kqDA and kqAD) also can be neglected judging from the experimental results by changing the mol ratio between Por(D) and Pc(A), as described later. Thus, eq 1 can be rewritten as eq 7. ⎛ 1 − 10−AD FET(v) = (1 − ηET) × FD0(v) + ⎜1 + η ⎝ 1 − 10−AA ET ⎞ − ϕAA ⎟ × FA0(v) ⎠

Figure 6. Energy transfer efficiencies (ηET, ■) and self-quenching efficiencies (ϕAA, ○) in Por(D)/Pc(A)/clay complexes under various mole ratio conditions of Por(D) to Pc(A). The mole ratio of Por(D) to Pc(A) was modulated as Por(D)/Pc(A) = 1/1−31/1. The total concentration of Por(D) and Pc(A) was set at 1.0 × 10−7 M. The total dye loading was set at 90% for Por(D)/Pc(A)/clay complex. The mole ratio of Por(D)/Pc(A) is expressed as a/1. The schematic representations for possible idealized adsorption structures of Por(D, ○) and Pc(A, ■) under a = 1, 8, 15 conditions are depicted.

Energy transfer experiments were examined under Por(D)/ Pc(A) = 1/1−31/1. The mole ratio of Por(D)/Pc(A) is expressed as a/1 in Figure 6. The large a value means an efficient light-harvesting type energy transfer in which a few number of acceptor can harvest the excitation energies from many donors. That is, the photon-harvesting efficiency (ηPH, defined in the Introduction) tends to be high under high a conditions. Obtained ηET and ϕAA values are plotted vs a as shown in Figure 6. First of all, when the ratio of Por(D) increased, ϕAA decreased as we expected. This fact indicates that the self-quenching of Pc(A) was suppressed due to the decrease of the adjacent probability between Pc(A) molecules on the clay surface. ϕAA reached to almost 0 under a = 15−31.

(7)

On the basis of eq 7, the fluorescence spectrum, FET(ν), was simulated with the linear combination of the respective 9158

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This indicates that the energy transfer only proceeded and all of other quenching processes were negligible under a = 15−31. Even under the a = 15, Por(D) and Pc(A) molecules are certainly neighboring judging from the schematic image depicted in Figure 6. Thus, it was found that electron transfers between Por(D) and Pc(A), i.e., ϕDA and ϕAD, do not take place in the present dye combination. Under a = 1−5, ηET remained at 100%. The decrease of ηET under a = 6−31 would be due to the decrease of adjacent probability of Por(D) and Pc(A). Since Förster type energy transfer rate constant is inversely proportional to the sixth power of the intermolecular distance,54 an energy transfer from Por(D) to nonadjacent Pc(A) should be inefficient compared to that of the adjacent Por(D)−Pc(A) pair. In general, a distribution of anionic charges on the clay surface is considered to be a hexagonal array.33 Judging from the hexagonal adsorption of two types of dyes on the clay surface, the decrease of ηET is reasonable. We have previously reported 100% energy transfer reaction between two types of porphyrins on the clay surface.29 However, the photon-harvesting efficiency (ηPH) was small (∼3) because of the small value of energy transfer rate constant (kET, 2.4 × 109 s−1) due to the small spectral overlap integral (J, 3.9 × 10−14 M−1cm3).29 That is, the energy transfer was not efficient under the donor excess conditions because the energy transfer between nonadjacent donor−acceptor pair was almost impossible. In the present system, since the spectral overlap between Por(D) fluorescence and Pc(A) absorption is rather large as described below, kET expected is large. The energy transfer rate constant kET in this Por(D)/Pc(A)/clay system was calculated using the Förster eq 854 kET =

9000 ln 10k 2ϕ J 128π 5NτDonorR6

Figure 7. Possible idealized adsorption structure model under Por(D)/Pc(A) = 8/1 condition on the basis of a hexagonal array. □ indicates the Pc(A) molecule, ● indicates the Por(D) molecule which neighbors the Pc(A) molecule, and ○ indicates the Por(D) molecule which does not neighbor the Pc(A) molecule. The unit cell is indicated by broken line.

where R is the center-to-center distance between adjacent molecules on the basis of a hexagonal array, (4 × 100/90 × 1.25) indicates the average area per one molecule (in nm2), 1.25 indicates the average area per one anionic charge of the examined clay (in nm2).39 and kET for adjacent Por(D)−Pc(A) pair (from ● to □ in Figure 7) is calculated to be 1.2 × 1010 s−1. By using this kET value and τDonor (5.6 ns, sum of deactivation rate constants is 1.8 × 108 s−1), the theoretical energy transfer efficiency (ηET′) under this condition is calculated to be 99%, according to eq 11. Thus, the energy transfer from Por(D) to adjacent Pc(A) is expected to be very efficient. ′ = ηET

(8)

J=

∑ Fd(λ)εa(λ)λ

Δλ

kET =

⎛ ⎞ 2 100 × ⎜4 × × 1.25⎟ ⎝ ⎠ 3 90

=

kET kETT + kdD + k fD

(11)

⎛ R 0 ⎞6 ⎜ ⎟ τDonor ⎝ R ⎠ 1

(12)

Judging from Figure 6, we can recognize the presence of energy transfer in the nonadjacent Por(D)−Pc(A) pair as described below. For example, in the case of 8/1 ratio condition, the possible idealized adsorption structure model of Por(D) and Pc(A) on the basis of a hexagonal array can be drawn as shown in Figure 7. Based on this ideal adsorption structure, the ratio of Por(D) which neighbors Pc(A) molecule (●) is 75%, and the ratio of Por(D) which does not neighbor Pc(A) molecule (○) is 25%. The obtained energy transfer efficiency under Por(D)/Pc(A) = 8/1 condition (95%) is apparently higher than the maximum ratio of Por(D) which neighbors Pc(A) molecule (75%). In the same manner, these theoretical discussions were conducted for Por(D)/Pc(A) = 3/ 1, 15/1, and 31/1 conditions. The experimentally obtained energy transfer efficiencies (ηET, ■) and calculated energy transfer efficiencies taking only account of the adjacent

(9)

where λ is the wavelength in cm, εa (λ) is the extinction coefficient of the acceptor at wavelength λ, and Fd (λ) is the fraction of the total fluorescence intensity of the donor. The schematic representation for possible idealized adsorption structure under a = 8 is shown in Figure 7 to discuss the detail of energy transfer pathway. The center-to-center distance between adjacent Por(D) and Pc(A) (from ● to □ in Figure 7) at 90% vs CEC condition is calculated to be 2.53 nm, according to eq 10 R=

kET +

−1 τDonor

The minimum distance between nonadjacent Por(D)−Pc(A) pair (from ○ to □ in Figure 7) is calculated to be 4.38 nm under 90% CEC condition. kET for the nonadjacent Por(D)− Pc(A) pair is calculated to be 4.5 × 108 s−1. The theoretical energy transfer efficiency is calculated to be 71%, according to eq 11. Thus, the energy transfer in the nonadjacent Por(D)− Pc(A) pair is expected to be possible. Actually, the Förster distance (R0) in this Por(D)−Pc(A) system calculated by eq 12 is 5.1 nm, indicating the energy transfer in nonadjacent Por(D)−Pc(A) pair should be possible.

where k is the orientation parameter (k2 = 5/4 for in-plane orientation),55 φ is the fluorescence quantum yield of donor (0.048), J is the spectral overlap integral between the fluorescence spectrum of donor and the absorption of acceptor as defined in eq 9, n is the refractive index of the bulk medium (n = 1.33 in the case of water), N is the Avogadro constant, τDonor is the excited singlet lifetime of donor on the clay surface (5.6 ns), and R is the center-to-center distance between donor and acceptor. According to the analysis of the overlap between the fluorescence of Por(D) and the absorption of Pc(A), based on eq 9,56 the spectral overlap integral (J) is calculated to be 1.1 × 10−12 M−1 cm3 4

kET

(10) 9159

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Por(D)−Pc(A) pair (ηET′, bar chart) are summarized in Figure 8.

9b, the decay curve for Pc(A)/clay complex can be analyzed as a single exponential decay, and the excited lifetime was determined to be 1.0 ns (τPc(A)). While photochemical behaviors of dyes on solid surfaces tend to be complicated due to the aggregation formation, simple decay curves were observed in the present system due to the size-matching effect. The time-resolved fluorescence measurement for energy transfer sample, Por(D)/Pc(A)/clay complexes, was examined. In this experiment, the mole ratios of Por(D)/Pc(A) were set at 1/1 and 7/1. The normalized time-resolved fluorescence spectra for Por(D)/Pc(A)/clay complex under Por(D)/Pc(A) = 1/1 and 7/1 conditions are shown in Figure 10 (raw spectra

Figure 8. Experimentally obtained energy transfer efficiencies (ηET, ■) and the calculated energy transfer efficiencies taking account of only the adjacent Por(D)−Pc(A) pair (ηET′, bar chart). Figure 10. Normalized time-resolved fluorescence spectra for Por(D)/ Pc(A)/clay complexes at 0−20, 100−120, 200−220, and 300−320 ps after excitation. (a) Por(D)/Pc(A) = 1/1; (b) Por(D)/Pc(A) = 7/1. The excitation wavelength was 450 nm. The sum of dyes loading was set at 90% vs CEC of the clay. [Por(D)] + [Pc(A)] = 1.0 × 10−7 M.

Judging from Figure 8, ηET values are higher than ηET′. Thus, it is apparent that there is a certain contribution of the energy transfer from excited Por(D) to nonadjacent ground state of Pc(A). ηPH (defined in the Introduction) of the present Por(D)−Pc(A) system is 21 determined by Figure 6, while that of our previous porphyrin−porphyrin system is about 3.29 Judging from these values, this Por(D)/Pc(A)/clay was much improved in terms of ηPH. Time-Resolved Fluorescence Measurements for Por/ Pc/Clay Complex. Time-resolved fluorescence spectra were measured by using the picoseconds fluorescence measurement system. At first, time-resolved fluorescence measurement for Pc(A)/clay complex was examined to obtain the excited lifetime. The excited lifetime for Por(D)/clay complex is 5.6 ns (τPor(D)) as reported in a previous paper.29 The obtained timeresolved fluorescence spectra and the decay curve for Pc(A)/ clay complex are shown in Figure 9a and b. As shown in Figure 9a, the fluorescence spectral shape of Pc(A)/clay was completely the same during the decay. As shown in Figure

are shown in Figure S2). When Por(D)/Pc(A) = 1/1 (Figure 10a), the time-resolved fluorescence spectrum immediately after excitation (0−20 ps) was completely the same as the Pc(A) fluorescence. This is probably due to the ultrafast kET value. The 30 ps laser pulse used in this experiment is not suitable to detect the rise and decay components due to the energy transfer. In the case of the Por(D)/Pc(A) = 7/1 condition (Figure 10b), kET should be small compared to that under the Por(D)/Pc(A) = 1/1 condition, because small adjacent probability of donor and acceptor molecules should decrease kET. At 0−20 ps, the time-resolved fluorescence spectrum under Por(D)/Pc(A) = 7/1 condition contained a small amount of Por(D) component. After that, the spectrum

Figure 9. (a) Time-resolved fluorescence spectra for Pc(A)/clay complexes at 0.0−0.2, 0.5−0.7, 1.0−1.2, and 2.0−2.2 ns after excitation. (b) Fluorescence decay profile of Pc(A)/clay complex. The excitation wavelength was 450 nm. The dye loadings were set at 0.05% vs CEC of the clay. [Pc(A)] = 1.0 × 10−7 M. The observed wavelength was 655−780 nm. 9160

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became Pc(A) fluorescence. The change of spectral shape indicates the energy transfer from excited Por(D) to Pc(A). The fluorescence decay analysis for the Por(D)/Pc(A) = 7/1 sample was conducted. In Figure 11, the fluorescence decay

in the absence of Pc(A). This kET value is rather large compared to our previous clay/dye system,29 and thus, efficient lightharvesting type energy transfer reaction was successfully achieved in the present system. The only one component of the energy transfer would indicate that the adsorption distribution of Por(D) and Pc(A) is rather uniform, not segregated. It should be noted that the nonsegregated adsorption structure is also important to realize an efficient energy transfer system on the aspect of the adjacent probability.29,57

■

CONCLUSION The efficient light-harvesting type energy transfer reaction in clay/dye system was successfully achieved. Tetra-cationic porphyrin (Por(D)) and tetra-cationic phthalocyanine (Pc(A)) were used as the energy donor and energy acceptor, respectively. In terms of the adsorption properties, Pc(A) molecule adsorbed on the clay surface without aggregation even under high dye loadings. Thus, it was found that our sizematching effect can be adopted for phthalocyanine derivatives which have appropriate intercationic distances, while they have been reported only for a few types of dye including porphyrin derivatives. The energy transfer reaction from Por(D) to Pc(A) on the clay surface was highly efficient judging from the steadystate and picosecond time-resolved fluorescence measurements. The high energy transfer efficiency was achieved by the presence of energy transfer from Por(D) to nonadjacent Pc(A) on the clay surface due to the large spectral overlap. The photon-harvesting efficiency (ηPH) and the visible-light-harvesting efficiency (ηVLH) of the present Por(D)−Pc(A) system came to 21 and 86%, while those of our previous porphyrin− porphyrin system were about 3 and 46.5%.29 Our clay/dye system took a step closer to realize an efficient artificial lightharvesting system having high ηPH and ηVLH, in which a few energy acceptor molecules can harvest the excitation energies from a large number of adjacent and/or nonadjacent donor molecules with the efficient utilization of wide wavelength region of visible sunlight.

Figure 11. Fluorescence decay profiles of Por(D)/Pc(A)/clay complex in the region of (a) 670−690 nm (line) and (b) 700−720 nm (dotted line). The excitation wavelength was 450 nm. The mole ratio of Por(D) and Pc(A) was set at Por(D)/Pc(A) = 7/1. The sum of dye loadings was 90% dyes loadings vs CEC of the clay. [Por(D)] + [Pc(A)] = 1.0 × 10−7 M.

curves in the region of (a) 670−690 nm (line) and (b) 700− 720 nm (dotted line) are shown. The region of 670−690 nm corresponds to the fluorescence maxima of Por(D), and the region of 700−720 nm corresponds to the fluorescence maxima of Pc(A) (individual steady-state fluorescence spectra are shown in Figure 5). As shown in Figure 11a, the decay curve at 670−690 nm can be analyzed as double exponentials decay. The lifetimes are calculated to be