J. Phys. Chem. C 2010, 114, 983–989
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Ultrafast Energy Transfer in a Multichromophoric Layered Silicate Takashi Kuroda,* Kazuko Fujii, and Kazuaki Sakoda National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan ReceiVed: October 29, 2009; ReVised Manuscript ReceiVed: NoVember 11, 2009
Making use of novel organic/inorganic synthesis, we achieved two-dimensional positioning in chromophores, which were confined to the interlayer of luminous silicate nanofilms synthesized with organoalkoxysilane precursors. Time-resolved photoluminescence demonstrated efficient energy transfer from a donor dye (coumarin) to an acceptor dye (cyanine) with a characteristic time of less than 5 ps. The copresence of slow radiative energy transfer, associated with emission and reabsorption of photons, was also confirmed in the acceptor emission decay. The relative efficiency of the slow and fast energy transfers was quantified, and found to depend on the molecular concentration. Introduction The construction of an ordered photoactive molecular assembly is currently a hot topic from a wide range of scientific and practical viewpoints. Various types of macromolecular and supramolecular architectures, such as dendrimers1 and cyclodextrins,2 have been designed to show efficient light-harvesting properties. Inorganic porous materials, such as zeolites3 and mesoporous silica,4,5 have been used as stable and transparent substrates which could incorporate active molecules in their cavities as well as in the framework. Another versatile system is based on nanoscopic thin films where functional molecules are packed on the surface and/or in the interlayers. The earliest studies on the interlayer resonant energy transfer (RET) focused on Langmuir-Blodgett multilayer films.6,7 Later, the layer-bylayer synthesis was applied to produce ionic organic/inorganic multilayers which utilized a clay mineral,8,9 zirconium phosphate,10 and layered double hydroxide11 as an inorganic polyanion sheet, confirming efficient interlayer RET whose probability depended on spacer thickness. Clay minerals have attracted much attention because they consist of stable silicate films of a few nanometers in thickness and macroscopic lateral dimensions on the order of 1 µm, serving as a well-defined inorganic scaffold for photoreaction systems. Because of their anionic characteristic, cationic organic molecules can be adsorbed and immobilized on the surface, achieving a two-dimensional arrangement in the active molecules.12,13 However, a drawback to this simple preparation approach was the difficulty of avoiding molecular aggregation, especially at high loading conditions.9 Recently, we demonstrated covalent linking of coumarin dyes to a phyllosilicate, to produce a luminescent two-dimensional framework, which was synthesized with organoalkoxysilane precursors.14,15 In this system, each coumarin molecule was covalently bonded to the Si-O-Si surface of the silicate moiety, thus avoiding deintercalation or stratification of the molecules. Using this organic/inorganic hybrid as a photoactive host material, we could incorporate another kind of fluorophore such as rhodamine16 or cyanine,17 where coumarin serves as a photon donor and rhodamine or cyanine serves as an acceptor. We previously reported on the preparation of integrated coumarin/DOC (3,3′-diethyloxacar* To whom correspondence should be addressed. Phone: (+81) 29 860 4194. Fax: (+81) 29 860 4795. E-mail:
[email protected].
bocyanine)/phyllosilicate hybrids, where DOC was a cyanine derivative whose absorption band matched coumarin’s emission band. Their photoluminescence (PL) showed suppression of donor emission with increasing acceptor density, suggesting the occurrence of RET in this system.17 Efficient coupling between a chromophore pair led to significant modification in their steady-state optical response. Thus, fluorescence quenching measurement has commonly been performed to evaluate RET efficiency, which can serve as a spectroscopic ruler for the molecular distance. However, in order to reveal the microscopic mechanism responsible for the relevant energy transfer, it is of great importance to study transient behavior in the photoreaction by quantifying its efficiency and distinguishing between several competing processes, such as Fo¨rster-type fluorescence resonant energy transfer (F-RET) via near-field dipole-dipole interactions,18,19 Dexter-type energy transfer via electron exchange process,20 and radiative transfer associated with emission and reabsorption of photons. This contribution examines time-resolved PL in the coumarin/ DOC/phyllosilicate hybrids. We observed ps dynamics of both donor emission and acceptor emission as a function of acceptor concentration. We analyzed the PL transients in terms of a F-RET model while taking into account the effect of macroscopic reabsorption. Our results confirmed ultrafast energy transfer occurred on a ps time scale in this two-dimensional dye integrated system. Such fast RET was due to the proximity of the donors and acceptors confined to the interlayer of the silicate, and the inhomogenous distribution of these molecules, which would have been enhanced by the two-dimensional confinement. Experimental Details Sample Preparation. The samples examined in this study were the same as those previously described.17 We first synthesized a coumarin/phyllosilicate hybrid (host hybrid, (C14H14NO4)0.01(C5H10O2N)0.2Li0.16(Li0.16Mg2.84)Si4O10(OH)2) by the reaction of organoalkoxysilane, silica sol, and inorganic salts (see the Supporting Information). Note that the host hybrid was not prepared by the intercalation of coumarin dyes to a clay mineral but synthesized with organoalkoxysilane precursors, enabling high loading levels of chromophores. Then, the DOC molecule was intercalated into the interlayer space of the host hybrid, by adding 2 mL of aqueous solutions (2.00 × 10-5,
10.1021/jp910341f 2010 American Chemical Society Published on Web 12/03/2009
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Figure 1. Schematic representation of coumarin/DOC/phyllosilicate hybrids.
TABLE 1: Concentrations of Coumarin and DOC Dyes type donor acceptor x ) 0.10 x ) 1.0 x ) 10
volume density (nm-3) 2.75 × 10
-2
1.0 × 10-3 1.1 × 10-2 1.1 × 10-1
surface densitya (nm-2) 3.8 × 10-2 1.4 × 10-3 1.5 × 10-2 1.5 × 10-1
a We assumed a layer-to-layer distance of 1.4 nm, which was revealed by the X-ray diffraction.
2.00 × 10-4, and 2.00 × 10-3 mol/L) of 3,3′-diethyloxacarbocyanine iodide to 2 mL of an aqueous dispersion of the host hybrid (the host hybrid/H2O ratio was 0.020 g/mL). The ratios of DOC to the host hybrid ([DOC]/[host hybrid]), abbreviated as x, were 0.10, 1.0, and 10 mmol/100 g. After stirring the mixtures at room temperature for a week, yellowish precipitates formed and were filtered, washed with water, and dried, yielding coumarin/DOC/phyllosilicate hybrids (Figure 1). The powdery specimens were inserted in a quartz glass container which enabled careful handling for optical measurement. Concentrations of coumarin and DOC were evaluated by the conventional method, and are given in Table 1. Optical Setup. We used second harmonic output from a mode-locked Ti-sapphire laser (Coherent Mira 900) as an excitation source. It produced femtosecond pulses with 150 fs duration and 76 MHz repetition. The wavelength was tuned to be 350 nm for exciting donor dyes and to 460 nm for exciting acceptor dyes. The excitation intensity was kept at less than 0.1 µW (with a focusing spot of ∼100 µm diameter), eliminating the effect of PL quenching by photoirradiation, during signal integration as long as 30 min. PL signals were fed into a grating spectrometer equipped with a cooled charge coupled device (CCD) detector for spectral characterization, and a microchannel plate photomultiplier tube (MCP-PMT, Hamamatsu R3809U-51) for time-resolved measurement. The spectral window for the MCP-PMT was around 2 nm. Electric output from the MCP-PMT was sent to a timecorrelated single photon counter (Picoquant PicoHarp 300), which measured photon arrival with a time resolution of ∼40 ps in full width at half-maximum (fwhm). A synchronously scanning streak camera (Hamamatsu C5680) was additionally used for resolving fast emission dynamics with a temporal resolution of 6 ps fwhm. All experiments were performed at room temperature. Experimental Results Spectral Characteristics. Emission spectra of a sample series of coumarin/DOC/phyllosilicate hybrids are illustrated in Figure 2. The spectrum of the host hybrid containing coumarins in the absence of DOC is given in Figure 2a, which presents donor emission (coumarin) appearing at a peak wavelength of 415
Figure 2. Emission spectra of (a) coumarin/phyllosilicate host hybrid, (b) DOC intercalated in a phyllosilicate without donor, at a concentration of 0.1 mmol/100 g, and (c, d, and e) coumarin/DOC/phyllosilicate hybrids with various DOC concentrations. They were excited at a wavelength of 350 nm, except in the case shown in part b which was excited at 460 nm. PL excitation in the DOC sample, recorded at a PL wavelength of 510 nm, is shown by the broken line in part b. Relative intensity is plotted for parts a, c, d, and e, while the spectra in part b are normalized to unity.
nm. The emission from acceptors, which was obtained in phyllosilicate in the absence of coumarins but incorporating DOC, is given in Figure 2b. The highest peak at a wavelength of 505 nm is attributable to the E isomers of DOC, while a peak at 530 nm is to the Z isomers.21 This spectral doublet, reflecting molecular conformation, was also confirmed in the absorption spectra of the present samples.17 PL excitation of DOC shows its absorption band at 470 nm, which is overlapped by the low energy side of the coumarin emission spectrum. This enabled coumarin-to-DOC energy transfer. The PL spectrum of coumarin/DOC in a phyllosilicate with a low DOC concentration of x ) 0.10 shows donor emission and acceptor emission with similar intensity (Figure 2c). As the DOC is increased to x ) 1.0, the donor intensity began to decrease, while the acceptor intensity was almost unchanged, implying the occurrence of RET from coumarin to DOC (Figure 2d). Further increase in DOC to x ) 10 resulted in a further decrease in donor emission, together with a significant reduction in acceptor emission due to the reabsorption of photons by dense DOC (Figure 2e). Reabsorption also led to red shift in the acceptor spectra and blue shift in the donor spectra, as seen in the data for high DOC concentrations. Note that the dependence of cyanine’s emission spectra on its concentration was simply ascribed to reabsorption, and not to aggregation of DOC. It has been known that aggregation of cyanines on clay surfaces was not efficient in this concentration range.22 Moreover, the absorption spectra of these samples had shown that doublet peaks due to the E and Z isomers were commonly present independent of concentration, maintaining a constant intensity ratio without showing any signature for the aggregates.17 Thus, the DOC molecules were expected to be in their monomeric forms in the host layer. The following section describes our observations of donor emission at a wavelength of 400 nm, which was not largely affected by reabsorption, and acceptor emission at a wavelength of 520 nm for x ) 0.10 and 540 nm for x ) 1.0 and 10, which gave a peak intensity in each PL spectrum. For excitation, we set the wavelength to 350 nm for exciting the donor and 460
Ultrafast Energy Transfer
Figure 3. Donor emission decay signals in coumarin/DOC/phyllosilicates with various DOC concentrations. The samples were excited at a wavelength of 350 nm, and detected at a wavelength of 400 nm. Each PL curve was normalized, and arbitrarily shifted in the vertical direction.
nm for exciting the acceptor. With these wavelengths, we were able to excite either donor or acceptor in a selective manner. Donor Emission Dynamics. Figure 3 shows nanosecond decays in donor emission. They were resolved by means of MCP-PMT. The sample in the absence of acceptors shows a single exponential decay with a decay time of 2.3 ((0.1) ns. We had confirmed a similar decay time present in a dilute coumarin solution dispersed in methanol (not shown). Thus, the observed decay reflects the radiative probability of coumarin, that was free from nonradiative relaxations. Surprisingly, the decay time was found to be unchanged, even if the sample contained acceptors, and the time-integrated intensity of donor emission had significantly been suppressed (Figure 2c-e). Moreover, the decay curve shows that it was independent of acceptor concentration, except for PL in the highest concentration (x ) 10), which exhibited the presence of a very slow decay signal whose decay time was longer than the repetition of excitation pulses. Note that such slow dynamics might have been emphasized due to their small emission intensity (see Figure 2e). Still, this was a minor component which was less than 5% of the time-integrated intensity. Thus, we will ignore these slow dynamics, and pay attention only to the PL dynamics with time scales of less than 10 ns. The results of the above observations were in stark contrast to a common feature of F-RET, in which the donor lifetime decreases with increasing acceptor concentration. We will consider two possible origins which may have engendered the constant donor lifetime. First, RET in this system may have been a quite fast process, which was deleted in the time-resolved data due to the low resolution of this apparatus (∼40 ps). Second, RET may have been absent, where the donors were perfectly decoupled to the acceptors. In the latter case, the spectral change reported in Figure 2 would have come solely from the reabsorption effect. To determine which origin gave rise to the constant lifetime, we resolved initial transients in the donor emission by means of a fast-scanning streak camera, which allowed ps time resolving. The results are summarized in Figure 4. The sample in the absence of acceptors showed a steplike response, where we may resolve a finite rise which reflects energy relaxation inside a coumarin molecule. In the samples which contained acceptors, we were now able to analyze a fast decay signal
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Figure 4. Initial transients for donor emission signals detected at a wavelength of 400 nm. The bottom curve shows instrumental response, exhibiting a temporal resolution of 5.5 ps fwhm.
Figure 5. Acceptor PL decays for the sample with various acceptor densities. They were excited at a wavelength of 460 nm, which was in the absorption band of DOC; see Figure 2. The detection wavelength was 520 nm for x ) 0.1 and 540 nm for x ) 1.0 and 10.
which was superimposed on the steplike response. This transient component became larger for higher acceptor concentrations. The decay time was similar to our resolution (6 ps), confirming that very rapid RET was achieved in this system. These observations suggest that donor dynamics were affected by at least two kinds of decay paths, one with an inherent radiative process and the other with extremely fast RET with a ps time scale. Such conditions can be understood by the following scenario: We are considering donors and acceptors which are dispersed inside the interlayer, and irradiated by UV light which is resonant to the donor absorption band. When photoinjection is made on a donor which is far from any acceptor, PL decays according to its radiative lifetime. On the other hand, when photoinjection is made on a donor which is adjacent to the acceptor, the energy is rapidly transferred, thus leading to a short decay time. The ratio of the fast component to the slow component must increase with acceptor density, as was experimentally confirmed in Figure 4. The copresence of the first component and the slow component reflects inhomogeneity in the distribution of the dye molecules, as will be discussed later. Acceptor Emission Dynamics. The occurrence of fast RET was also confirmed by the analysis of acceptor emission dynamics. Before starting this discussion, we would like to mention that the decay time of the acceptors heavily depended on their concentration. Figure 5 shows the variation of acceptor PL decay as a function of acceptor concentration. Here, the samples were excited at a wavelength of 460 nm, which was in the acceptor absorption band, but far from the donor absorption
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TABLE 2: Summary of Emission Parameters x 0.10 1.0 10
donor acceptor lifetimea (ns) lifetimeb (ns) 2.2 2.2 2.2
1.6 0.9 0.3
AF/ARc
Ed σd
1.9 ((0.2) 0.4 0.3 3.2 ((0.3) 0.7 0.7 10 ((1) 0.9 0.8
AF/AR ) E/[(1 - E)σ] 2.2 ((0.5) 3.3 ((0.5) 11 ((2)
a Determined with the data in Figure 3. b Determined with the data in Figure 5. c According to the fit with eq 1 to the data in Figure 6. d Determined with the data in Figure 2.
Figure 6. Comparison between the acceptor PL signals after exciting donors (violet circles) and that after exciting acceptors (red diamonds). The excitation wavelength is 350 nm for donor excitation and 460 nm for acceptor excitation. The detection wavelength was 520 nm for x ) 0.1 and 540 nm for x ) 1.0 and 10. Fits of eq 1 to the donor excited PL are shown by broken lines. Each curve was normalized and arbitrarily shifted in the vertical direction.
band. Thus, the signal indicates PL that occurred after direct excitation to the acceptors. We observed dramatic reductions in the decay time by increasing acceptor concentrations. Such lifetime shortening at high concentrations was likely due to selfquenching, homotransfer, or the collective excitation effect such as excitonic formations, but here, we will not examine this feature in detail. The acceptor lifetime for each sample was derived, and is summarized in Table 2. Note that, at every concentration, the decay time of the acceptor was shorter than that of the donor. Figure 6 compares the acceptor PL decays after acceptor excitation with those after donor excitation: The former are equivalent to the data in Figure 5. It should be noted that we could avoid photoinjection of acceptors in the case of donor excitation, and vice versa. Thus, the acceptor PL after donor excitation was purely associated with energy passage from the excited donor to the acceptor. The sample with a low concentration of acceptors (x ) 0.1) shows that the PL decay for donor excitation was slower than that for acceptor excitation (Figure 6a). Moreover, the decay constant for donor excitation agrees with the donor lifetime reported in Figure 3. The sample with x ) 1.0 also shows that the PL decay for donor excitation was slower than that for acceptor excitation (Figure 6b). However, in this sample, we found the onset of a relatively fast decaying component. For the sample with x ) 10, we were able to confirm that the fast component followed the acceptor’s radiative decay, which made the dominant contribution to PL in this sample. However, the donor-decaylike component was additionally analyzed in the long-time region (Figure 6c).
The results of donor/acceptor PL can be summarized as follows: • For the dilute acceptor sample, where the donor PL intensity was not very different from that of the sample without acceptors, the donor PL and the acceptor PL showed the same dynamics, when the donor was photoexcited. • For the dense acceptor sample, whose donor PL intensity was quenched, the donor PL consisted mainly of an extremely rapid component with a ps time scale. Correspondingly, the acceptor PL was determined by the acceptor’s radiative process, even when the donor was photoexcited. These characteristic phenomena can be consistently interpreted in terms of ultrafast RET. Let us assume that the energy of excited donors rapidly moves to the acceptors with a characteristic time much shorter than the acceptor’s lifetime. In this case, the population of the excited acceptors simply followed the acceptor’s radiative decay, as shown in Figure 6c. In the case of the dilute acceptor sample, on the other hand, RET did not effectively occur, so that the excited donors tended to emit photons according to their own radiative decay. Some of these photons were reabsorbed by the acceptors, which emitted photons again. Acceptor PL associated with reabsorption of photons would have developed as the convolution of the donor decay and the acceptor decay. Since the donor decay was slower than the acceptor decay, this component manifested itself in the donor-decay-like component, especially at the long-time region. The broken lines in Figure 6 were derived theoretically, and will be discussed below. Discussion Discrimination between RET and Radiative Energy Transfer. Spectral overlapping of donor emission and acceptor absorption is a fundamental condition for realizing RET. Such a spectral property also induces radiative energy transfer from donor to acceptor due to the emission and reabsorption of photons. Both RET and radiative transfer result in suppression of donor PL; thus, it is sometimes difficult to distinguish which process is responsible for the emission dynamics in the relevant D-A pairs. We confirmed the occurrence of RET by analyzing donor emission decay. Here, we will examine an alternative way to quantify the effect of radiative energy transfer by analyzing acceptor emission dynamics. We will describe the acceptor PL decay after exciting donors in terms of a model
Ia(t) ) AFna(t) + AR
∫0t nd(τ)na(t - τ) dτ
(1)
where AF(R) is a constant which characterizes the efficiency of RET (efficiency of radiative transfer) and na(d)(t) is the normalized population decay of acceptors (donors). The first term on the right-hand side of eq 1 corresponds to PL from the acceptors which were excited via RET. The second term corresponds to PL from the acceptors which were excited via radiative transfer with reabsorption of photons. In the analysis, we set the radiative lifetimes of the donor and acceptor to the values listed in Table 2, which were determined with the data from Figures 3 and 5, respectively. The least-squares fits of eq 1 using AF and AR as fitting parameters are given by broken lines in Figure 6. They show that this model well reproduced the experimental decay curves. Through the fit, we can determine the quantity of AF/AR, which is the ratio of RET and radiative transfer, that contributes to the excitation of the acceptors. The value determined for each
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sample is given in Table 2. As expected, we found that AF/AR was larger for higher acceptor concentrations. The parameters of AF and AR are also related to the RET efficiency E and the reabsorption coefficient σ, such that
AF ) EN0d AR ) (1 -
(2)
E)N0dσ
(3)
where Nd0 is the initial population of excited donors. Thus, we obtain another expression for this ratio
AF E ) AR (1 - E)σ
(4)
Note that we are able to estimate the values of E and σ by comparing donor PL spectra of the sample containing acceptors with those of the sample in the absence of acceptors. For this derivation, we must distinguish between spectral change due to RET and spectral change due to reabsorption. Thus, we evaluated the RET efficiency by looking at the PL intensity at a wavelength of 370 nm, which was almost free from reabsorption. Then, we determined the reabsorption coefficient by looking at the intensity at a wavelength of 440 nm, which roughly corresponded to the maximum in the joint spectrum of the donor PL (Figure 2a) and the acceptor PL excitation (Figure 2b), thus being maximally affected by reabsorption. The values of E and σ, together with the ratio of AF/AR derived according to eq 3, are given in Table 2. Although there was large uncertainty, we found striking agreement between the values of AF/AR determined by the temporal analysis and those determined by the spectral analysis. This suggests the validity of our interpretation of the presence of two kinds of energy transfer channels, one with ultrafast RET and the other with slow radiative transfer. Comparison with Fo¨rster Models for Dispersed Acceptors. It is well-known that the rate of Fo¨rster-type energy transfer ΓT for a donor-acceptor (D-A) pair, which is separated by a distance of r, is given by
( )
ΓT ) Γd
R0 r
6
(5)
where Γd is the radiative rate for the donor in the absence of acceptors and R0 stands for the so-called Fo¨rster distance, which is defined by a D-A distance for which the probability of RET becomes equal to that of photon radiation. The donor emission decay Id(t) is, therefore, proportional to the product of the radiative decay and the energy transfer decay, i.e., Id(t) ∝ exp[-(Γd + ΓT)t]. The above expression is valid for a single and isolated D-A pair. When the donor is surrounded by multiple acceptors, the donor decay is characterized by statistical integration of ΓT taken over the distribution of the D-A distance. In special cases when the acceptors are homogeneously distributed, and the size of the molecules is sufficiently smaller than a characteristic D-A distance, we obtain an analytic form
Id(t) ) I0 exp[-Γdt - C(Γdt)d/6]
(6)
where d is the dimensionality of the relevant system and the constant C is related to acceptor concentration, and roughly given by the mean number of acceptors within a linear distance of R0 of the donor. Exact forms for C, which depend on the dimensionality, are found in ref 19. Here, we will consider the two-dimensional case (2D, d ) 2) and the three-dimensional case (3D, d ) 3). The 2D case
Figure 7. Comparison between the observed donor decays (a and b) and simulations for the 2D cases (c and d) and the 3D cases (e and f) with various values of C in eq 5. The three panels in the left column represent PL dynamics of the ps time scale, spanning 100 ps, and the other three panels in the right column represent those of the ns time scale, spanning 2 ns. In every panel, the vertical axis scales the intensity variation by 3 orders of magnitude on a logarithmic scale. In the simulations, we assumed that Γ-1 d ) 2.2 ns. They were convoluted by the instrumental response given by Gaussian with 5 ps fwhm in parts c and e and 20 ps fwhm in parts d and f.
represents a system with R0 larger than layer-to-layer thickness, and the 3D case is for the opposite condition; thus, RET should occur in every direction with the same probability. We also assume that R0 of our D-A pair is in the range 2-9 nm, the values of representative D-A pairs reported in the literature.19 Accordingly, the C parameter is approximated to be on the order of 100 to 101 (101 to 102) for the 2D (3D) case with x ) 1, and it varies in proportion to x. Note that the Fo¨rster distance for our chromophore pair was roughly evaluated to be 5((1) nm, which is consistent with the above assumption (see the Supporting Information). Comparisons between these models and the observed decays are given in Figure 7. Logarithmic plots of donor emission for the short-time region (
