Artificial Light-Harvesting System with Energy Migration Functionality

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Artificial Light-Harvesting System with Energy Migration Functionality in a Cationic Dye/Inorganic Nanosheet Complex Yuta Ohtani,†,‡ Tetsuya Shimada,† and Shinsuke Takagi*,† †

Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, Tokyo Metropolitan University, Minami-ohsawa 1-1, Hachioji, Tokyo 192-0397, Japan ‡ Research Fellow of Japan Society for the Promotion of Science (DC2), Kojimachi 5-3-1, Chiyoda, Tokyo 102-0083, Japan S Supporting Information *

ABSTRACT: We investigated a reaction involving photochemical energy transfer between a cationic xanthene derivative (Flu(D)) and a cationic porphyrin (Por(A)) with an energy migration functionality, which is crucial for efficient lightharvesting on an inorganic nanosheet. Efficient energy transfer from excited Flu(D) to Por(A) took place, and the maximum energy transfer efficiency was 99%. Even under light-harvesting conditions, Por(A) concentration was much less than Flu(D) concentration (Flu(D)/Por(A) concentration ratio = 15), and the energy transfer efficiency was still 80%. Steady-state, timeresolved, anisotropic fluorescence measurements indicate energy migration between Flu(D) molecules. This system has the functionality of a light-harvesting system using a dye and having a large overlap between its absorption and fluorescence spectra.

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INTRODUCTION A natural photosynthetic system is mainly composed of a lightharvesting system (LHS) and photosystems I and II (PS I and PS II, respectively).1−8 The LHS has several important functions in the photosynthetic system, including (i) increasing the range of absorption wavelength, (ii) dissipating excess excitation energy, and (iii) concentrating absorbed photons to PS I and PS II. The LHS of purple bacteria is composed of dye molecules such as carotenoids and bacteriochlorophylls.9−13 Carotenoids in the LHS absorb energy (450−540 nm) higher than that absorbed by bacteriochlorophylls. The excitation energy of carotenoids is transferred to bacteriochlorophylls. Carotenoids increase the utilizable sunlight energy in the LHS of purple bacteria. Approximately 50% of the energy absorbed by carotenoids dissipates through an internal conversion process, depending on the strength of the light. This reaction is one of the protective mechanisms against excess excitation energy. In purple bacteria, approximately 200 dye molecules that comprise the LHS surround the reaction centers,1 and the dye molecules in the LHS form a circular array. The excitation energy migrates efficiently in the ring and between rings; thus, the LHS can transfer light energy efficiently and frequently to the PS I and PS II. One of the most important roles of the LHS is concentration of dilute photon flux to enable the photochemical reaction with multielectron conversion. In an artificial multielectron conversion system, electrons or photons must be frequently supplied to catalysts within the lifetime of one of their electron-oxidized or reduced species, because they become unstable and have a short lifetime. This problem is known as the photon-flux-density problem.4 Thus, construction © XXXX American Chemical Society

of artificial LHSs is crucial for the realization of an artificial photosynthetic system that resembles natural photosynthetic systems. To realize an efficient artificial LHS, regularly arranged structures of dyes for transferring excited energy smoothly are necessary. Artificial LHSs using covalently linked systems and dendrimer systems have been reported, and efficient energy transfer reactions have been achieved in such systems.14−18 Although these systems provide interesting insight from a theoretical viewpoint, synthesis and further modifications in such systems is difficult. Use of supramolecular assemblies of dyes in strategies for structuring artificial LHS have been reported recently.19−24 Kobuke et al. found that a supramolecular assembly of zinc−porphyrin derivatives could function as an efficient artificial LHS.19 In their system, fast excitation-energy hopping (5.3 ± 0.6 ps) proceeded similarly to a natural system. Inagaki et al. reported an artificial LHS that was constructed by using periodic mesoporous organosilicas (PMOs), which are stable and practical systems.21,22 They introduced chromophores into the PMO framework and constructed a regularly arranged chromophore assembly. In their system, a small amount of energy acceptor (0.5−1% (mol/mol) relative to the amount of energy donor in the structures) quenched the fluorescence of the donor completely. The above studies indicate that energy migration between Received: May 12, 2015 Revised: July 6, 2015

A

DOI: 10.1021/acs.jpcc.5b04578 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

heated to 80 °C for 70 h. The mixture yielded p-2,4,5,7(pyridinyl)-6-potassiumoxy-3-fluorone. Afterward, synthesized p-2,4,5,7(pyridinyl)-6-potassiumoxy-3-fluorone and iodomethane were added to degassed dimethyl sulfoxide. The mixture was stirred for 2 h at room temperature to yield Flu(D). Water was deionized through an Organo BB-5A system (PF filter ×2 + G-10 column). The saponite clay in this experiment, Sumecton SA (SSA) having a cationic exchange capacity (CEC) of 0.997 mmol g−1 (Kunimine Industries Co. Ltd.), was used after purification. Purification of Clay.27 SSA (2.1 g) was dispersed in water (200 mL), and the colloidal solution was allowed to stand for 9 days. The supernatant liquid (150 mL) was then decanted, and SSA was separated by centrifugation (38900 × g, 10 h, 10 °C). The supernatant was removed, and deionized water was added to it. After its colloidal solution was allowed to stand for 1 day, SSA was separated again by centrifugation (38900 × g, 5 h, 10 °C). The supernatant was removed, and SSA was collected by filtration (polytetrafluoroethylene, 0.45 μm, Millipore; 1.0 g, 48% yield). Analysis. Absorption spectra were obtained on a Shimadzu UV-3150 spectrophotometer, and corrected fluorescence spectra were obtained on a Jasco FP-6600 spectrofluorometer. A quartz cell and a plastic cell poly(methyl methacrylate) were, respectively, used for absorption measurements and fluorescence measurements on the aqueous clay/dye solution. Thermogravimetry-differential thermal analysis measurements were carried out on a Shimadzu DTG-60H analyzer to determine the water content of the dyes and clay. Timeresolved fluorescence measurements were conducted with an EKSPLA PG-432 optical parametric generator (430 nm, 25 ps fwhm, 20 μJ, 1 kHz) under photon-counting conditions (Hamamatsu Photonics, C4334 streak scope connected to a Chromex 250IS polychrometer). The generator was pumped by using third harmonic radiation from a Nd3+-YAG laser (355 nm, 25 ps fwhm, 300 μJ, 1 kHz; EKSPLA PL2210JE). The laser flux was reduced by using neutral-density filters to avoid multiphoton absorption processes and nonlinear effects. The time-resolved fluorescence spectra were not corrected; thus, the obtained spectral shape was not the same as that obtained by steady-state fluorescence spectroscopy even under the same condition. Methods for the Preparation of the Dye/Clay Complexes. Preparation of Flu(D)/Clay Complex for Fluorescence Measurements. Fluorescence spectra of Flu(D)/clay complexes were observed as described below. The Flu(D)/clay complex was typically prepared by mixing the aqueous clay solution and the respective aqueous Flu(D) solution under stirring. The dye loadings (vs CEC of the clay) were changed by changing the concentration of the clay. The dye concentration was always kept at 1.0 × 10−1 μmol L−1. Preparation of Flu(D)/Por(A)/Clay Complexes for Measurements on the Energy Transfer Reaction. The typical procedure used to prepare energy-transfer samples was as follows. Flu(D) and Por(A) were used as an energy donor and energy acceptor, respectively. Aqueous solutions of Flu(D) and Por(A) were mixed, and the resulting solutions were mixed with an aqueous clay solution (9.9 mg L−1) under vigorous stirring. When the Flu(D)/Por(A) molar ratio was 1.0, the total concentrations of Flu(D) and Por(A) were set at 1.0 × 10−7 M, and the clay loadings were varied by changing the clay concentration. In experiments in which the D/A ratio was changed, the Por(A) concentration was set at 5.0 × 10−8 M,

chromophores plays a very important role in efficient lightharvesting energy transfer. Recently, we studied a reaction involving transfer of excited energy between adsorbed porphyrins on the surface of clay nanosheets.25−27 Clay nanosheet has an array of negative charges on its surface on which various cationic dyes can adsorb. In this system, the maximum quantum yield for energy transfer between two types of porphyrins on the clay surface is ∼100%. Their nonaggregation adsorption behavior was rationalized by size matching of distances between altered sites in the porphyrin molecule and those between anionic sites on the clay surface. We named this effect as the “size-matching effect”. These dye−clay systems have high chemical stability and superior extensibility. For example, addition of dyes used as energy donors or acceptors and photocatalyst is simple. However, the energy transfer efficiency in these clay−dye systems decreases as the donor/acceptor ratio increases.26 Excited energy could be transferred to the adjacent molecules only. When the donor/acceptor ratio increases, the number of donor molecules adjacent to the acceptor molecules on the clay surface decreases, thus, decreasing the energy transfer efficiency. In this paper, we aim to develop an energy transfer system using a dye. Energy migration reaction is possible in the dye− clay system. Here, the energy migration reaction means energy transfer between same species. We investigated the photochemical behavior of a tetracationic xanthene derivative (p2,4,5,7-tetrakis(N-methylpyridinium-4-yl)-6-pottasiumoxy-3fluorone tetraiodide, Flu(D)) on a clay surface and a reaction involving energy transfer from Flu(D) to p-tetrakis(1methylpyridinium-4-yl) porphyrin tetrachloride (Por(A)). Conditions for efficient reaction with energy migration are (1) a large overlap (J) between the fluorescence and absorption spectra of the energy donor and (2) a distance between energy donor molecules that is sufficiently smaller than the Förster radius, R0. R0 is the distance at which the energy transfer rate and deactivation rate are equal. To fulfill these conditions, we recently designed and synthesized a novel Flu(D) dye.27 Flu(D) produces a large spectral overlap (J = 2.9 × 10−13 M−1 cm3 and R0 = 6.1 nm), and the typical distance between Flu(D) molecules is 2.4 nm. Thus, efficient energy migration between Flu(D) molecules is expected.

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EXPERIMENTAL SECTION Materials. Structures of Por(A) and Flu(D) were shown in Figure 1. Por(A) was purchased from Frontier Scientific. Flu(D) was synthesized according to the literature.27 First, 2,4,5,7-tetrabromo-6-hydroxy-3-fluorone, 4-(4,4,5,5-tetramethyl-1,3,2- dioxaborolan-2-yl) pyridine, tetrakis(triphenylphosphine) palladium, and potassium carbonic acid were added to degassed water with N2, and the mixture was

Figure 1. Structures of p-2,4,5,7-tetrakis(N-methylpyridinium-4-yl)-6potassiumoxy-3-fluorone tetraiodide (Flu(D), left) and p-tetrakis(1methylpyridinium-4-yl) porphyrin tetrachloride (Por(A), right). B

DOI: 10.1021/acs.jpcc.5b04578 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C and that of Flu(D) was set at 5.0, 15, 25, 40, 55, 65, or 75 × 10−8 M. The total dye loading was set at 100% versus CEC by varying the clay concentration. Under these conditions, the clay sheets existed in the form of individually exfoliated sheets, and the obtained solution was substantially transparent.

sufficiently larger than kq. The kq value is assumed to be the maximum value in the absence of an acceptor.

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RESULTS AND DISCUSSION Fluorescence Self-Quenching Behavior of Flu(D) in Flu(D)/Clay Complexes. The fluorescence self-quenching behavior of Flu(D) on the clay surface was examined by measuring the fluorescence quantum yields (ϕf) at various loading levels of Flu(D) before the energy transfer experiment. Self-quenching decreases the excited-singlet lifetime of the dye and thus decreases the energy transfer efficiency. To construct efficient energy transfer systems, an understanding of selfquenching behavior is essential. As reported before, selfquenching of Por(A) was not observed on the clay surface even at a high-density-adsorption conditions (not shown).25,26 On the other hand, significant self-quenching was observed with typical dyes on the clay surface.25 Such self-quenching was induced by a collisional reaction such as electron transfer from an excited molecule to an adjacent identical molecule.25 Under our experimental conditions, the Flu(D) concentration was always kept at 1.0 × 10−7 M, and the loading level of adsorption was varied by controlling the clay concentration. The observed ϕf values for the Flu/clay complexes when their loading levels were varied from 0.025 to 100% versus CEC are shown in Figure 2. Fluorescence below 0.06% adsorption could

φD =

kf 1/τD

(1)

φD′ =

kf kq + 1/τD

(2)

The energy transfer rate constant kET may be theoretically calculated by using the Förster eq 3, kET =

9000 ln 10κ 2φD 128π 5n 4NAτDR6

J (3)

where κ is the orientation factor (κ = 5/4 for in-plane random orientation), ϕD is the fluorescence quantum yield of the donor (0.49), n is the refractive index of the bulk medium (n = 1.33 for water), NA is the Avogadro constant, τD is the lifetime of the excited singlet of the donor on the clay surface (2.9 ns), R is the center-to-center distance between adsorbed dye molecules (2.4 nm at 100% vs CEC), and J is the integral of spectral overlap between the fluorescence spectrum of the donor and the absorption of an acceptor according to eq 4. According to the analysis of the overlap between the fluorescence of Flu(D) and the absorption of Por(A) based on eq 4, J was found be 1.6 × 10−13 M−1 cm−1 cm4. 2

J=

∑ FD(λ)εA(λ)λ 4Δλ

(4) −1

where λ is the wavenumber in cm , εA(λ) is the extinction coefficient of the acceptor at wavelength λ, and FD(λ) is a fraction of the total fluorescence intensity of the donor. Under the present conditions, the calculated kET value is 5.0 × 1010 s−1. This value is sufficiently larger than kq for an efficient energy transfer reaction. Energy Transfer Reaction of the Flu(D)/Por(A)/Clay Complex. The absorption spectrum of the Flu(D)/Por(A)/ clay complex with loading levels at 20% was obtained. We found that the spectrum was completely identical to the sum of individual absorption spectra of Flu(D)/clay and Por(A)/clay complexes, as shown in Figure 3. Thus, aggregation was completely suppressed, and the dye molecules existed as single molecules when two types of dyes coexisted on the clay surface. Energy transfer from the excited singlet state of Flu(D) to the ground state of Por(A) on the clay surface was examined by obtaining steady-state fluorescence spectra. Individual fluo-

Figure 2. Fluorescence quantum yields (ϕf) of Flu(D)/clay complexes at various clay concentrations in water. The Flu(D) concentration was 1.0 × 10−7 M. Inset: 0−0.1% vs CEC. A 430 nm long-pass filter was installed in front of the detector. The excitation wavelength was 420 nm. ϕf values under aerated conditions were determined by using rhodamine 6G (ϕf = 0.95) as a standard.28

be used as the intrinsic fluorescence intensity because almost the same ϕf value at 0.025% and 0.6% loading indicates that self-quenching was negligible under these conditions (Figure 2, inset). As the Flu loading increases, the ϕf decreases markedly, as shown in Figure 2. ϕf of the Flu(D)/clay complex (ϕD) was 0.49 at 0.06% versus CEC. At 100% loading, ϕf of the Flu(D)/clay complex (ϕD′ ) was 0.027 times as large as ϕD (ϕD′ = 0.013). The lifetime of the excited Flu/clay complex was 2.9 ns (τD), as reported in a previous paper.27 The calculated rate constant of self-quenching between Flu(D) molecules (kq) at 100% loading was thus 1.2 × 1010 s−1 according to eqs 1 and 2. For an efficient energy transfer reaction, the energy transfer rate (kET) should be

Figure 3. Absorption spectra of Flu(D)/clay ([Flu(D)] = 5.0 × 10−7 M; 20% vs CEC), Por(A)/clay ([Por(A)] = 5.0 × 10−7 M; 20% vs CEC) complexes, as well as the sum of the individual absorption spectra and the spectrum of a coadsorption sample ([Flu(D)] = 5.0 × 10−7 M, [Por(A)] = 5.0 × 10−7 M; 20% vs CEC). C

DOI: 10.1021/acs.jpcc.5b04578 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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the energy transfer sample at 630 nm (not shown), it was expected that quenching between the excited acceptor and the ground-state donor did not occur. Thus, self-quenching of the donor was induced by the quenching reaction in the present system. On the basis of eq 5, the fluorescence spectrum FET(ν), was simulated with the respective reference fluorescence spectra, F0D(ν) and F0A(ν). Thus, the parameters ηET and ϕq could be obtained by spectral simulation. Energy transfer experiments were conducted at various loadings. The Flu(D)/Por(A) molar ratio for the Flu(D)/ Por(A)/clay complex was 1:1. The dye loading of the clay was modulated to 0.12, 0.2, 1, 2, 5, 10, 20, 40, 60, 80, and 100% versus CEC. The obtained ηET and ϕq values are plotted versus the dye loading in Figure 5. As the loading level increased, ηET

rescence spectra of Flu(D)/clay and Por(A)/clay complexes and energy transfer samples of Flu(D)/Por(A)/clay complex are shown in Figure 4. Dye loadings of 0.06% versus CEC for

Figure 4. Fluorescence spectra of Flu(D)/clay ([Flu(D)] = 5.0 × 10−8 M; 0.06% loading), Por(A)/clay ([Por(A)] = 5.0 × 10−8 M; 20% loading), and Flu(D)/Por(A)/clay ([Flu(D)] = [Por(A)] = 5.0 × 10−8 M; 100% loading) complexes in water. The excitation wavelength was 510 nm. The intensity of Flu(D)/clay fluorescence is indicated on the left vertical axis, and that of Flu(D)/Por(A)/clay and Por(A)/clay fluorescence is indicated on the right vertical axis.

individual dyes and 100% versus CEC for the Flu(D)/Por(A)/ clay complex were 5.0 × 10−8 M. The Flu(D)/Por(A) molar ratio for all Flu(D)/Por(A)/clay complexes was 1:1. The excitation wavelength was set at 510 nm. The shape of fluorescence spectra of the energy transfer sample show that the intensity of the acceptor fluorescence increased and that the donor fluorescence was quenched. Using the fluorescence spectra of Flu(D), Por(A), and Flu(D)/Por(A)/clay complexes, we calculated the energy transfer efficiency (ηET) and the self-quenching efficiency (ϕq) as follows.25,26 The total fluorescence of the Flu(D)/ Por(A)/clay complex (FET(ν)) may be expressed through eq 5:

Figure 5. Energy transfer efficiency (ηET) and quenching efficiency (ϕq) at various loadings of Flu(D) and Por(A) on the clay surface in water ([Flu(D)] = [Por(A)] = 5.0 × 10−8 M). The excitation wavelength was 510 nm.

increased. ηET values were larger than 90% at 20−100% versus CEC condition, and the ϕq value was almost constant (nearly 0% at 0.12−100% vs CEC). According to Figure 5, we can presume the adsorption structure of the dyes on the clay surface. When the adsorption distribution of the adsorbed dyes was uniform and hexagonal, the average intermolecular distance was 5.3 nm at 20% versus CEC. The kET value calculated from the Förster equation and 5.3 nm intermolecular distance is 4.1 × 108 s−1; thus, ηET was 54% (ηET = 4.1 × 108 s−1/(4.1 × 108 s−1 + 3.5 × 108 s−1 (equal to τD−1))) without kq, while the actual ηET was 93%. Accordingly, dyes segregated on the clay surface and the actual intermolecular distance between molecules was smaller than that expected from the uniform distribution. Details of the adsorption structure of dyes are discussed in the next chapter. Energy Transfer Reaction at Various Flu(D)/Por(A) Ratios. The energy-transfer efficiency was evaluated at [Flu(D)]/[Por(A)] ratios of 1−15. A large [Flu(D)]/[Por(A)] ratio is preferable for light harvesting. However, we expected that, as the ratio of Flu(D) increases, the adjacent probability between Flu(D) molecules increases, and that between Flu(D) and Por(A) decreases; thus, self-quenching of Flu(D) occurs, and the energy transfer efficiency decreases. The observed ηET and ϕq values are shown in Figure 6. ηET values were larger than 80% under all conditions. For example, when [Flu(D)]/ [Por(A)] = 15, which is disadvantageous for energy transfer because of the lower amounts of energy acceptors, the obtained ηET was still 80%. This efficiency is much higher than expected. The obtained ηET value was compared with the theoretical ηET value to examine the details of the energy transfer pathway.

FET(ν) = (1 − ηET − φq )FD0(ν) ⎧ (1 − 10−AD) ⎫ 0 + ⎨1 + η ⎬FA(ν) (1 − 10−AA ) ET⎭ ⎩

(5)

where FET(ν), and are the fluorescence spectra of the Flu(D)/Por(A)/clay, Flu(D)/clay, and Por(A)/clay complexes, respectively. AD and AA are, respectively, the absorbances of the Flu(D)/clay and Por(A)/clay complexes at 510 nm excitation wavelength. ηET is the energy transfer efficiency, as defined in eq 6, and ϕq is the quenching efficiency due to electron transfer and enhanced thermal deactivation due to collision between neighboring guests, as defined in eq 7. F0D(ν),

ηET =

φq =

F0A(ν)

kET kET +

kdD

+

k fD

+ kq

=

kET

kET + 1/τD + kq

(6)

kq kET +

kdD

+ k fD + kq

(7)

kDd

where is the sum of the nonradiative deactivation rate constant and intersystem crossing rate constant of the Flu(D)/ clay complex, kDf is the radiative deactivation rate constant of the Flu(D)/clay complex, kq is the rate constant for quenching from the excited state of Flu(D) to the ground state of Flu(D) or Por(A), and kET is the energy transfer rate constant. Selfquenching behavior was not observed with Por(A). As no quenching was observed with excitation only of the acceptor in D

DOI: 10.1021/acs.jpcc.5b04578 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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distance between Flu(D) and Por(A) is longer than 4.8 nm, the theoretical ηET is lower than 10%. Thus, energy transfer reaction at an intermolecular distance longer than 4.8 nm was neglected. On the basis of the ideal adsorption structure shown on the left in Figure 7, the ratios of the primary, secondary, and third neighboring Flu(D) in the unit lattice are 40% (black circle: 2.4 nm intermolecular distance), 40% (gray circle: 4.2 nm intermolecular distance), and 20% (open circle: 4.8 nm intermolecular distance), respectively. Thus, the theoretical ηET value when [Flu(D)]/[Por(A)] = 15 is 39% (ηET = 0.4 × 80% + 0.4 × 12% + 0.2 × 11%). The observed ηET value (80%) when Flu(D)/Por(A) = 15 is apparently higher than the theoretical ηET value (39%). The experimentally obtained ηET value of this system (80%) is nearly equal to the theoretical efficiency of energy transfer from primary neighboring Flu(D) molecules to adjacent Por(A) molecules (80%). To explain why the observed ηET value was higher than the theoretical value, the possibility of energy migration between donor molecules is examined. The rate constant for energy migration between Flu(D) molecules calculated from the Förster equation and the J value of 2.9 × 10−13 M−1 cm3 is 9.2 × 1010 s−1. This rate constant is much higher than rate 8 −1 constants of the deactivation (τ−1 D = 3.5 × 10 s ), energy transfer (kET = 1.7 × 109 and 1.6 × 109 s−1), and the selfquenching reaction (kq = 1.2 × 1010 s−1). If energy migration occurs between Flu(D) molecules, then the anisotropy of Flu(D) fluorescence decreases as the frequency of energy migration increases. Measurement of the fluorescence anisotropy was shown in Supporting Information. Fluorescence spectra of Flu(D) adsorbed on the clay surface and its anisotropy were obtained at 0.1, 2.5, and 40% versus CEC adsorption. The anisotropy decreased as the adsorption density of Flu(D) increased on the clay surface, as shown in Table S1. This result indicates energy migration between Flu(D) molecules on the clay surface. Thus, energy migration due to the large J value between donor molecules enhances the energy transfer efficiency in the present system. Time-Resolved Fluorescence Measurements on the Flu(D)/Por(A)/Clay Complex. To determine the energy transfer rate, time-resolved fluorescence spectra for Flu(D)/ Por(A)/clay complexes were obtained. In this experiment, the Flu(D)/Por(A) molar ratio was set at 1:1, and the dye concentration of the clay was set at 100% loading versus CEC. The wavelength at which Flu(D) was almost selectively excited was 510 nm. Time-resolved fluorescence spectra (normalized at 650 nm) for the Flu(D)/Por(A)/clay complex are shown in Figure 8. Fluorescence decay profiles of Flu(D)/Por(A)/clay complexes in the regions of 550−590 and 740−795 nm, which correspond to fluorescence of Flu(D) and Por(A), are shown in Figure 9. The long time-range decay curve is shown in Figure S1. The time-resolved fluorescence spectrum just after excitation (0−50 ps) was identical to the Flu(D) fluorescence spectrum (Figure 8). Fluorescence from excited Por(A) appeared and then increased at 650−800 nm for 50−450 and 450−950 ps. The decay curve at 550−590 nm may be analyzed by fitting three components (Figure 9). The calculated lifetimes were