DOI: 10.1002/chem.201403393

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& Chromophores

Light-Harvesting Three-Chromophore Systems Based on BiphenylBridged Periodic Mesoporous Organosilica Lyubov Grçsch, Young Joo Lee, Frank Hoffmann, and Michael Frçba*[a]

amine G). High energy-transfer efficiencies ranging from 70 to 80 % are obtained for two-step-FRET, indicating that the mesochannel structure with one-dimensional ordering provides spatial arrangement of chromophore pairs for an efficient direct energy transfer. The emission wavelength can be tuned by a choice of acceptor dye: 477 nm (diethylaminocoumarin), 519 nm (aminoacridone), 567 nm (sulforhodamine G), 630 nm (dibenzothiacarbocyanine), and 692 nm (indodicarbocyanine).

Abstract: Three-chromophore systems with light-harvesting behavior were prepared, which are based on periodic mesoporous organosilica (PMO) with crystal-like ordered structure. The organic bridges of biphenyl-PMO in the pore walls act as donors and two types of dye are incorporated in the one-dimensional channels. Consecutive two-step-Fçrster resonance energy transfer is observed from the biphenyl moieties to mediators (diethyl-aminocoumarin or aminoacridone), followed by energy transfer from mediators to acceptors (dibenzothiacarbocyanine, indodicarbocyanine, sulforhod-

Introduction

Traditional optoelectronic materials are based on inorganic semiconductors. The biomimetic materials including organic polymers,[11, 12] dendrimers,[13–15] clays,[16–18] hybrid organic–inorganic materials[19, 20] and organic–inorganic nanostructured semiconductor materials[21, 22] with light-harvesting behaviors have great potential to find an application in the optoelectronics. The photoactive porous materials, such as mesoporous silica materials, periodic mesoporous organosilicas (PMOs),[23–33] metal–organic frameworks (MOFs),[34–36] and zeolites[37–39] provide promising approaches as efficient light-harvesting antenna due to their versatile macroscopic organization. In particular, PMOs[40] exhibit several advantages, which make these materials especially attractive. The silica framework of PMOs provides thermal, mechanical, and chemical stability. A steric separation between donor and acceptor pigments found in the nature can be achieved by designing the structural organization.[18] Donors, which absorb the light energy, can be placed in the pore walls as organic bridges, whereas acceptors, which collect the energy from donors, can be placed in the pores. The high surface area of these materials offers a possibility to insert a large amount of chromophores inside the pores without aggregation. The pore size of PMOs (typically 2.0– 8.0 nm) are in the range of the optimal distance for efficient FRET (up to 8.0 nm). In addition, the emission wavelength of the materials can be tuned by incorporating a variety of chromophores into the pores. PMOs with crystal-like pore walls[40] are especially interesting because of the large amount of the densely packed photoactive organic bridges in the pore walls. The alignment of the photoactive moieties enhances the light absorption and the transfer of the excitation energy. The classical two-chromophore–FRET systems, which consist of one type of donor and one type of acceptor, can be extended to the systems containing three or more chromophores,

Light harvesting, the absorption of the incident light by chromophores, has attracted increasing attention for applications as photovoltaic devices, photocatalysts and light-emitting devices. Most of the research up to now has been focused on developing materials that absorb as many photons as possible at the broadest range of wavelength as possible. In recent years, utilization of Fçrster resonance energy transfer (FRET)[1–3] in complex arrays of molecules[4–10] has become a topic of studies because light energy absorbed by donor chromophores can be transferred to acceptor chromophores through FRET, resulting in shift of emission wavelength. FRET plays a decisive role in light harvesting in various photosystems of algae, land plants, and of bacteria with photosynthetic ability. In spite of the tremendous variety of protein structure and diversity of pigments existing in nature, their light-harvesting antenna exhibit high energy-transfer efficiency by adapting various schemes. The association of different pigments, which absorbs light at a broad range of wavelength and channels, collect energy together to one reaction center and enhance the efficiency of light absorption. The incorporation of a large amount of the absorbing pigments in one protein unity enhance the efficiency of energy transfer.[4] Different alignment of devices in the light-harvesting complexes cause an alteration in the absorption wavelength range and consequently in the energy transfer.[4] [a] Dr. L. Grçsch, Dr. Y. J. Lee, Dr. F. Hoffmann, Prof. Dr. M. Frçba University of Hamburg, Institute of Inorganic Chemistry Martin-Luther-King-Platz 6, 20146 Hamburg (Germany) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201403393. Chem. Eur. J. 2015, 21, 331 – 346

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Full Paper which are composed of donors, mediators, and acceptors.[41] Results and Discussion These systems have the following features: The absorption of light in a broader wavelength range, the energy transfer at Two-chromophore systems, in which the donors for the energy longer distances of the donor/acceptor pairs, the enhanced eftransfer are placed in the framework and the acceptors are ficiency of energy transfer from a chromophore 1 to a chromoloaded in the mesopores, and three-chromophore systems, phore 3 in the presence of a mediator-chromophore (chromocomposed of the donors in the framework and two chromophore 2), and the detectability of the chromophore 3 at the phores in the tunnels for the consecutive energy transfer, were broader wavelength range.[40–42] Recently, Zhang et al. prepared prepared as shown in Table 1. a mesoporous, organosilicabased three-chromophore-FRET system by co-condensation of Table 1. Summary of the synthesized materials with the energy transfer efficiency from chromophore 1 to tetraethyl orthosilicate (TEOS) chromophore 2 (E12), from chromophore 2 to chromophore 3 (E23), and of two-step-FRET from chromophore 1 and bis-silylated biphenyl-, anthrough chromophore 2 to chromophore 3 (E123).[a] thracene-, and naphthalimideE12 E23 E123 Chr 1 Chr 2 Chr 3 Emission precursor.[44] To our best knowl[%] [%] [%] maxima[b] [nm] edge, there is, however, no donors acceptors report about three-chromobiphenyl-PMO – – 374 – – – phore-FRET systems based on biphenyl-PMO aminoacridone – 519 95 – – PMOs with crystal-like pore biphenyl-PMO diethylaminocoumarin – 477 91 – – donors mediators Acceptors walls. biphenyl-PMO aminoacridone Dibenzothiacarbocyanine 630 95 84 80 The aim of this study is to inbiphenyl-PMO aminoacridone Indodicarbocyanine 692 95 78 74 vestigate the energy transfer bebiphenyl-PMO diethylaminocoumarin Dibenzothiacarbocyanine 634 91 80 73 havior through multi-step FRET biphenyl-PMO diethylaminocoumarin sulforhodamine G 567 91 76 69 in the PMO-based systems and [a] The wavelength of the emission maxima of acceptors and the values of the energy transfer efficiency are to explore structural and specrepresented for samples with maximal concentration of the acceptors before the onset of emission quenching of the acceptors occurs. Chr = Chromophore. [b] Emission maxima of the last chromophore. troscopic properties that are responsible for the high energytransfer efficiency. Three-chroBiphenyl-bridges of biphenyl-PMO (chromophore 1) with mophore systems based on the biphenyl-bridged PMO (chrocrystal-like periodicity of the pore walls and the average pore mophore 1, donors) with crystal-like pore walls and two-types diameter of 34  (determined from the powder X-ray diffracof chromophores (chromophore 2: Mediators, and chromotion pattern and the N2 physisorption measurements, see in phore 3: Acceptors) loaded in the pores are reported. The twostep-FRET in the systems is investigated, which includes the the Supporting Information) were used as donors for the energy transfer from the chromophore 1 to the chromoloaded dye molecules. phore 3, mediated by the chromophore 2 (Figure 1). First, pores of the extracted biphenyl-bridged PMO (donors) were loaded with one dye (chromophore 2 acting as acceptors) to build two-chromophore systems. Two different acceptors were used: Aminoacridone (compound 1, Figure 1) and diethylaminocoumarin (compound 2, Figure 1), respectively. The amounts of the chromophore 2 were varied to optimize the concentration of dye to achieve the highest energy-transfer efficiency without quenching of fluorescence. Secondly, three-chromophore systems were prepared by doping two different chromophores (chromophores 2 and 3) into biphenyl-bridged PMO to investigate consecutive twostep energy-transfer behaviors from donors to chromophore 2 (mediators), followed by energy transfer from chromophore 2 (mediators) to chromophore 3 (acceptors). In this case, the acceptors of the two-chromophore systems are used as chromophore 2 and will play a role as mediators in three-chromophore Figure 1. Schematic diagram of different chromophore systems, which were systems. The amounts of the chromophore 2 were chosen as built by loading of extracted biphenyl-bridged PMO with different dye molethe optimal concentrations for the highest energy-transfer efficules. The red arrows show the route of the energy transfer from donors to acceptors. ciency without quenching of fluorescence and were constant in all samples. The amounts of the chromophore 3 were varied. Three different dye acceptors were applied as chromophore 3: dibenzothiacarbocyanine (3), indodicarbocyanine (4), and sulforhodamine G (5; Figure 1). Two different threeChem. Eur. J. 2015, 21, 331 – 346

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Full Paper chromophore systems were built with each chromophore 2. Altogether, four different three-chromophore systems were designed (Figure 1).

Two-chromophore system: Biphenyl-PMO and aminoacridone (1) An important requirement for resonance energy transfer from donors to acceptors is the overlap between the emission band of the donors and the absorption band of the acceptors. Spectral characteristics of the biphenyl-bridged PMO and aminoacridone show a good overlap between the emission band of the biphenyl-PMO and the absorption spectra of aminoacridone (shown in the Supporting Information, Figure S3). The Fçrster distance, defined as the distance between donor/acceptor pair for efficient energy transfer, can be deduced by a conventional method using spectral overlap integral (see the Experimental Section), yielding 22 . This distance is larger than the maximum possible distance between the donors and the acceptors in this system, which is the radius of the biphenyl-PMO pores (17 ) because the donors are placed in the pore walls and the acceptors are placed in the pores. The shorter distance of biphenyl-dye pair compared to the Fçrster distance suggests that Fçrster-type direct energy transfer can occur in these systems. Two-dimensional (2D) fluorescence spectroscopy is a method that allows three parameters of fluorescence (excitation and emission wavelength and emission intensity) to be monitored simultaneously within a short measurement time. In 2D-fluorescence experiment, the excitation wavelength is scanned and the wavelength and intensity of the emission are measured, giving an excitation–emission data matrix. The obtained data matrixes are often represented as a contour plot with emission wavelength on the x-axis, excitation wavelength on the y-axis, and the emission intensity as a shading of the peak. 2D fluorescence spectroscopy provides an insight into correlation between excitation and emission characteristics of chromophores, which will be denoted as a (lem, lex) coordinates in the 2D contour plot in this paper. Understanding the characteristics of the chromophores obtained from 2D fluorescence helps identifying FRET features that may be ambiguous due to overlapping or weak signals in 1D spectra. As can be seen in Figure 2 (plot 1), the contour plot of the 2D fluorescence spectrum of the biphenyl-PMO shows a peak at (lem, lex) = (375, 310 nm). The 2D fluorescence spectrum of aminoacridone (Figure 2, plot 2) exhibits a strong peak at (540, 410 nm) and a very weak peak at (540, 320 nm). In comparison to these, for the biphenyl-PMO/aminoacridone sample, the fluorescence peak due to biphenyl moiety at (375, 310 nm) disappears and an additional peak at (520, 310 nm) is observed (denoted as “a” in Figure 2, plot 3). The appearance of a new peak and the extinction of a fluorescence peak related with biphenyl moiety indicate that the excitation energy is transferred from biphenyl-moieties to aminoacridone. Based on the 2D fluorescence spectroscopic results, the 1D emission spectra of the biphenyl-PMO loaded with aminoacriChem. Eur. J. 2015, 21, 331 – 346

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Figure 2. The contour plots of the 2D fluorescence spectra. 1) BiphenylPMO; 2) aminoacridone (4.66  105 mol L1) in water containing OTAC; 3) biphenyl-PMO/aminoacridone with the molar ratio of 100:5.15.

done, excited at the maximum absorption of the biphenylPMO (309 nm), were acquired (Figure 3). As the amount of aminoacridone increases, the emission peak of the biphenyl moieties at 374 nm decreases in intensity gradually, whereas the emission band arising from aminoacridone increases in intensity and shifts to higher wavelength, reaching a plateau value at approximately 520 nm (Figure 4 B). These results are in agreement with the direct fluorescence energy transfer from the biphenyl group to aminoacridone. Above the molar concentration of biphenyl-PMO/aminoacridone = 100:5.15, the emission intensity of the aminoacridone starts to decrease. The maximal energy-transfer efficiency can be determined by the degree of reduction in the emission intensity of the donors before the onset of the strong emission quenching of the acceptors, yielding 95 % (at molar ratio of biphenyl-PMO/aminoacridone = 100:5.15; Figure 4, see the Experimental Section). The reduction in the emission intensity of the donors was calculated in the wavelength range, in which 333

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Figure 3. Emission spectra of biphenyl-PMO loaded with various concentration of aminoacridone, excited at 309 nm. All spectra were acquired with the slit size of 1 nm and the integration time of 0.3 s. Molar ratios of biphenyl-PMO/aminoacridone are depicted in the figure.

the self-absorption of donors emission does not occur or is very weak (for biphenyl in the wavelength range from 350 to 450 nm). There are various mechanisms that are responsible for the decrease of the emission peak of acceptors: 1) concentration quenching, and 2) formation and the growth of aggregates, resulting in increased collision probability with quenching molecules like oxygen.[45] At a low concentration, the emission strength of the fluorescence is proportional to the concentration of dye, following the Equation (1) I ¼ constantff eP0 C

Figure 4. A) Energy transfer efficiency of biphenyl-PMO/aminoacridone samples. The point denoted with the arrow corresponds to the maximal energytransfer efficiency before the appearance of the strong emission quenching of the acceptors (in Figure 3 molar ratio of biphenyl-PMO/aminoacridone = 100:5.15). B) Intensity (red) and wavelength (blue) of the emission maxima from aminoacridone in biphenyl-PMO/aminoacridone samples as a function of the concentration of aminoacridone. The corresponding fluorescence data are shown in Figure 3.

shifted absorption with respect to the absorption band of the monomer. Here, because of exciton splitting, triplet state excitation is enhanced through nonradiative intersystem crossing because radiative transition from the lower exciton level to the ground state is forbidden. Hence, quenching of the fluorescence takes place.[47] There are three types of fluorescent aggregates (J-aggregates): One with transition dipoles aligned inline (head-to-tail J-aggregates, Figure 5 b), second with transition dipoles possessing co-planarly inclined alignment (Figure 5 c), and third with oblique alignment of transition dipoles (Figure 5 d).[48] The head-to-tail and the co-planarly inclined Jaggregates have a redshifted absorption with respect to the absorption band of the monomer. The oblique J-aggregates have two absorption bands: One band is blueshifted and the other redshifted with respect to the absorption band of the monomer. The emission of all types of J-aggregates is redshifted from the emission of the monomer. Recently, Zhang et al. have investigated formation of H- and J-type aggregates of tetra(a-phenoxy) zinc phthalocyanine in different solvent by using TEM, electronic absorption and fluorescence methods. As the content of water increases in DMF/water mixture, a new, blueshifted absorption band appears at the expense of the original absorption band and fluorescence signal decreases in intensity down to zero level, indicating that the formed ag-

ð1Þ

in which I is the emission strength of fluorescence, ff is the quantum efficiency of fluorescence, e is the molar absorptivity of the fluorescent molecules, and P0 is the power of the incident beam on the sample. On the contrary, at high concentration, the fluorescence strength deviates from a concentration dependence because the absorbance becomes significantly large (primary absorption) or the emission is reabsorbed by dye molecules or other species, which often occurs for a system in which the emission and absorption bands overlap (secondary absorption).[46] The second mechanism, aggregation quenching, was observed first time in 1930s, showing that aggregation of a dye provided enhanced photochemical sensitizing effect, accompanied by diminished fluorescence efficiency.[45] To explain the spectral behaviors of these molecular aggregates and composite molecules, the molecular exciton model has been exploited, which describes resonance interaction between excited states of various types of high-ordered molecular aggregates.[47, 48] The molecular aggregates with the parallel-ordered transition dipoles (H-aggregates, Figure 5 a) exhibit a blueChem. Eur. J. 2015, 21, 331 – 346

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Full Paper gregate is a non-fluorescent H-aggregate. On the contrary, tetra(a-phenoxy) zinc phthalocyanine in chloroform, shows narrower, redshifted absorption and emission bands and reduced fluorescence efficiency by 50 % compared with the monomer, which are characteristic of J-aggregates.[49] Optical and photophysical properties of J-aggregates of cyanines, porphyrins, phthalocyanines, and perylenebisimides and their potential applications are summarized in an overview by Wrthner et al.[50]

(475, 420 nm). In a PMO-sample loaded with diethylaminocoumarin, the fluorescence peak resulting from biphenyl moiety is no longer visible. Moreover, a new peak is clearly seen at (475, 310 nm); denoted as “a” in Figure 6, plot 2. These observations indicate energy transfer from the biphenyl moiety to diethylaminocoumarin.

Figure 5. Energy diagrams of the exciton bands for different types of high ordered molecular aggregates;[47] a) H-aggregates; b) head-to-tail J-aggregates; c) co-planarly inclined J-aggregates; d) oblique J-aggregate. The forbidden transitions are depicted in red and the allowed transitions in blue. Transition dipoles are pictured as arrows. G = ground state; E = excited state; M = monomer; A = aggregate.

Figure 6. The contour plots of the 2D fluorescence spectra. 1) Diethylaminocoumarin (3.48  105 mol L1) in water containing OTAC. 2) Biphenyl-PMO/diethylaminocoumarin with the molar ratio of 100:1.63.

Returning to the biphenyl-PMO/aminoacridone system, the emission intensity of the aminoacridone starts to decrease only above the molar concentration of biphenyl-PMO/aminoacridone = 100:5.15 without a complete loss of emission signal, and the emission maxima are gradually redshifted with increasing amount of dye (Figure 4 B), suggesting that J-aggregates are formed in the biphenyl-PMO loaded with aminoacridone. Two new absorption signals, one redshifted band (at approximately 475 nm) and the other a blueshifted shoulder (at approximately 375 nm), were observed in the excitation spectra, confirming the formation of oblique J-aggregates (shown in Figure S5 in the Supporting Information).

1D emission spectra were obtained with an excitation wavelength at 309 nm, corresponding to the maximum absorption of biphenyl-PMO (Figure 7). As the amount of diethylaminocoumarin increases, the emission band of biphenyl group at 375 nm decreases in intensity, whereas the emission peak of diethylaminocoumarin at 475 nm increases in intensity and shifts to a higher wavelength. Again, these 1D emission results confirm the energy transfer from the biphenyl-moieties to diethylaminocoumarin as is observed in 2D fluorescence spectra. Above molar ratio of biphenyl/diethylaminocoumarin = 100:0.744, the emission peak of biphenyl group completely disappears and the emission peak of diethylaminocoumarin starts to decrease in intensity, reaching constant level above molar ratio of 100:1.63 (Figure 8 B).The emission peak of diethylaminocoumarin is gradually redshifted on increasing concentration of diethylaminocoumarin. Similar to the biphenyl-PMO/aminoacridone-system, red- (at approximately 450 nm) and blueshifted (at approximately 375 nm) absorption signals, characteristic for building of J-aggregates, were detected in the excitation spectra of the loaded PMO samples (shown in Figure S13 in the Supporting Information). Thus, it appears that the formation of oblique J-

Two-chromophore system: Biphenyl-PMO and diethylaminocoumarin (2) The absorption band of diethylaminocoumarin overlaps with the emission band of biphenyl-PMO (see the Supporting Information, Figure S11) and the calculated Fçrster distance for these chromophores is 22 . Therefore, spectral characteristics of biphenyl-PMO/diethylaminocoumarin system comply with efficient resonance energy transfer. The contour plot of the 2D fluorescence spectra of diethylaminocoumarin (Figure 6, plot 1) shows a fluorescence band at Chem. Eur. J. 2015, 21, 331 – 346

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Figure 7. Emission spectra of biphenyl-PMO loaded with diethylaminocoumarin, excited at 309 nm. All spectra were acquired with the slit size of 1 nm and the integration time of 0.3 s. Molar ratios of biphenyl-PMO/diethylaminocoumarin are depicted in the figure.

aggregates is associated with the diminished fluorescence signal of acceptors in biphenyl/diethylaminocoumarin system. The maximal energy-transfer efficiency before the onset of the emission quenching of the acceptors is 91 % (at molar ratio of biphenyl/diethylaminocoumarin = 100:0.744; Figure 8 A). Three-chromophore system: Biphenyl-PMO, aminoacridone (1), and dibenzothiacarbocyanine (3)

Figure 8. A) Energy transfer efficiency of biphenyl-PMO/diethylaminocoumarin samples. The point labeled with an arrow corresponds to the maximal energy transfer efficiency before the appearance of the strong emission quenching of the acceptors (in Figure 7 molar ratio of biphenyl PMO/diethylaminocoumarin = 100:0.744). B) Intensity (red) and wavelength (blue) of the emission maxima of diethylaminocoumarin as a function of the concentration of diethylaminocoumarin. The corresponding fluorescence data are shown in in Figure 7.

Spectral characteristics of the donors and the acceptors in this system (see the Supporting Information, Figure S3) show a good overlap between the emission band of aminoacridone (mediator dye) and the absorption band of dibenzothiacarbocyanine (acceptor dye). Moreover, this system features a low overlap between the emission band of the biphenyl-PMO and the absorption band of dibenzothiacarbocyanine. The calculated Fçrster distance for aminoacridone/dibenzothiacarbocyanine pair is 37 , which is slightly larger than the diameter of the pores. The three-chromophore system was prepared with the constant molar ratio of biphenyl-PMO/aminoacridone = 100:4.88 (this concentration was chosen because a weak quenching of the emission of the acceptors was observed at a molar ratio of 100:5.15) and varying amount of dibenzothiacarbocyanine. The contour plot of the 2D fluorescence spectra of dibenzothiacarbocyanine (Figure 9, plot 1) exhibits two fluorescence peaks at (lem, lex) = (620, 600 nm) and (620, 555 nm). For the three-chromophore system with low amount of acceptors (Figure 9, plot 2), additional peaks at (515, 310 nm) and (625, 425 nm) were observed, which result from the energy transfer from biphenyl group to aminoacridone (denoted as “a” in Figure 9, plot 2) and from aminoacridone to dibenzothiacarbocyanine (denoted as “b” in Figure 9, plot 2), respectively. In ad-

dition, a fluorescence peak at (625, 310 nm) is shown, which can be attributed to the consecutive two-step energy transfer from biphenyl group to dibenzothiacarbocyanine, mediated by aminoacridone (“c” in Figure 9, plot 2), in which the emission signal from the biphenyl group disappears. When the amount of dibenzothiacarbocyanine is increased further, the emission band of aminoacridone becomes invisible as well and the fluorescence peaks resulting from energy transfer from aminoacridone to dibenzothiacarbocyanine (b, Figure 9, plot 3) and from biphenyl group to dibenzothiacarbocyanine, mediated by aminoacridone (c, Figure 9, plot 3) are enhanced. Two series of 1D emission spectra were measured with excitation wavelengths at (414 and 325 nm), corresponding to the absorption wavelength of aminoacridone and biphenyl group, respectively. When the loaded samples were excited at the absorption wavelength of aminoacridone (at 414 nm), the emission bands of aminoacridone decrease in intensity with increasing amount of dibenzothiacarbocyanine, whereas the emission peaks of dibenzothiacarbocyanine grow in intensity as the concentration of the dibenzothiacarbocyanine increases up to the molar ratio of biphenyl/aminoacridone/dibenzothiacarbocyanine = 100:4.88:0.339 (Figure 10, top). Above this concentration level, dibenzothiacarbocyanine shows a gradual de-

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Figure 10. Emission spectra of biphenyl-PMO loaded with aminoacridone and various concentration of dibenzothiacarbocyanine, excited at 414 nm (top) and at 325 nm (below). All spectra were acquired with the slit size of 1 nm and the integration time of 0.3 s. Molar ratios of biphenyl-PMO/aminoacridone/dibenzothiacarbocyanine are depicted in the figure.

Figure 9. The contour plots of the 2D fluorescence spectra. 1) Dibenzothiacarbocyanine (6.03  106 mol L1) in water containing OTAC. Biphenyl-PMO/ aminoacridone/dibenzothiacarbocyanine samples with the molar ratio of 2) 100:4.88:0.0733, and 3) 100:4.88:0.706.

crease in emission peak height. These results demonstrate energy transfer from aminoacridone to dibenzothiacarbocyanine. When the samples are excited at 325 nm (the absorption of the biphenyl-PMO), a similar behavior is observed, in which a decrease of the emission peak of aminoacridone is accompanied by an increase of the emission signals of dibenzothiacarbocyanine as a function of the amounts of dibenzothiacarbocyanine (Figure 10, below and Figure 11 B). Again, the emission signals of dibenzothiacarbocyanine start to decrease above a relative concentration of dibenzothiacarbocyanine = 0.238. No signals ascribed to the emission bands of biphenyl moiety were observed. The wavelength of emission peaks of dibenzothiacarbocyanine gradually shifts to higher values as the amount of dibenzothiacarbocyanine increases (Figure 11 B). It is noteworthy that an additional emission peak is observed at higher wavelength from the emission band of dibenzothiacarbocyanine (approximately at 680 nm; Figure 10, top). This redChem. Eur. J. 2015, 21, 331 – 346

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shifted emission band can be assigned to the J-aggregates. Thus, the decrease of emission intensity of acceptors above a certain concentration can be ascribed to the growth of J-aggregates of dibenzothiacarbocyanine. The maximal energy-transfer efficiency from aminoacridone to dibenzothiacarbocyanine before the appearance of the emission quenching of the acceptor dye is 84 % (at molar ratio of biphenyl/aminoacridone/dibenzothiacarbocyanine = 100:4.88:0.23). The energy-transfer efficiency of the two-stepFRET (from biphenyl-PMO to dibenzothiacarbocyanine, mediated by aminoacridone) can be deduced by the multiplication of individual energy-transfer efficiencies for each step (see the Experimental Section), yielding 80 % at this concentration (Figure 11). Separately, the energy transfer behavior from biphenyl-PMO to dibenzothiacarbocyanine without aminoacridone was examined at the molar ratio of dibenzothiacarbocyanine = 0.238, showing a weak emission signal arising from 337

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Full Paper tional peaks at (520, 310 nm) and (685, 415 nm) are present, which arise from the fluorescence energy transfer from biphenyl group to aminoacridone (“a” in Figure 12, plot 2) and from aminoacridone to indodicarbocyanine (“b” in Figure 12, plot 2), respectively. A weak fluorescence peak at (685, 310 nm) is also shown, which is presumably ascribed to the consecutive twostep energy transfer from the biphenyl group to indodicarbocyanine, mediated by aminoacridone (“c” in Figure 12, plot 2). The fluorescence signal of the biphenyl moiety is not observable. For the system with a larger amount of indodicarbocyanine acceptors (Figure 12, plot 3), the emission intensity of a signal “c” at (715, 300 nm) is increased. In addition, the signal “b” at (715, 415 nm) is enhanced, accompanied by the disappearance of the fluorescence peak of aminoacridone at (520, 415 nm) and a peak “a” at (520, 310 nm) originating from FRET from biphenyl group to the aminoacridone group. These behaviors are similar to those with dibenzothiacarbocyanine acceptors, which is discussed above.

Figure 11. A) Energy transfer efficiency of the two-step-FRET of the biphenylPMO/aminoacridone/dibenzothiacarbocyanine samples. The point labeled with an arrow corresponds to the maximal energy-transfer efficiency before the appearance of the strong emission quenching of the acceptors (in Figure 9 molar ratio of biphenyl PMO/aminoacridone/dibenzothiacarbocyanine = 100:4.88:0.238). B) Intensity (red) and wavelength (blue) of the emission maxima from dibenzothiacarbocyanine as a function of the concentration of dibenzothiacarbocyanine. The corresponding fluorescence data are shown in Figure 10 (below).

dibenzothiacarbocyanine with 36 % of the energy-transfer efficiency (see the Supporting Information, Figure S7). Thus, the high energy-transfer efficiency of 80 % in biphenyl/aminoacridone/dibenzothiacarbocyanine agrees well with two-stepFRET phenomena mediated by aminoacridone. Three-chromophore system: Biphenyl-PMO, aminoacridone (1), and indodicarbocyanine (4) The emission maximum of the acceptor dye in the system biphenyl-PMO/aminoacridone/indodicarbocyanine (see the Supporting Information, Figure S8) is approximately 60 nm redshifted compared to the system with dibenzothiacarbocyanine acceptors. Also, this system exhibits a low overlapping between the emission band of PMO and the absorption band of the acceptor dye (indodicarbocyanine). The Fçrster distance between aminoacridone and indodicarbocyanine is deduced as 37 , which is similar to the value obtained for the threechromophore system with dibenzothiacarbocyanine acceptors. The 2D fluorescence spectra of indodicarbocyanine (Figure 12, plot 1) shows two strong fluorescence peaks at (lem, lex) = (690, 675 nm) and (690, 565 nm), and two very weak peaks at (690, 375 nm) and (690, 330 nm). For the three-chromophore system with low concentration of acceptors, addiChem. Eur. J. 2015, 21, 331 – 346

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Figure 12. The contour plots of the 2D fluorescence spectra. 1) Indodicarbocyanine (4.30  106 mol L1) in water containing OTAC. Biphenyl-PMO/aminoacridone/indodicarbocyanine samples with the molar ratio of 2) 100:4.88:0.0612, and 3) 100:4.88:1.21.

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Full Paper which accounts for the emission signal quenching above the concentration of indodicarbocyanine = 0.184. When the samples are excited at 325 nm (the absorption wavelength of the biphenyl-moiety), the emission peak of aminoacridone decreases in intensity as a function of the concentration of indodicarbocyanine and the emission peak of biphenyl group is barely detectable (Figure 13, below). A similar behavior has been observed in another three chromophore systems, biphenyl/ aminoacridone/dibenzothiacarbocyanine as shown above, suggesting consecutive two-step energy transfer from biphenyl group to indodicarbocyanine mediated by aminoacridone. However, due to the instrumental limitation, the wavelength range from biphenyl to indodicarbocyanine emission cannot be scanned at the same time. Thus, the variation of intensity and wavelength as shown in Figure 14 B are obtained from the 1D fluorescence spectra with excitation wavelength at 414 nm for aminoacridone absorption.

Two series of 1D emission spectra were acquired with excitation wavelength at (414 and 325 nm), corresponding to the absorption wavelength of aminoacridone and the biphenyl groups, respectively. The emission spectra of indodicarbocyanine, which were excited at the absorption wavelength of aminoacridone (414 nm), exhibit similar behaviors of the energy transfer from aminoacridone to the acceptor dye as shown in the system with dibenzothiacarbocyanine acceptors (Figure 13, top). As the amount of indodicarbocyanine increases, the emission bands of aminoacridone decrease in intensity, accompanied by growth of emission peaks of indodicarbocyanine (Figure 13, top). Above the concentration of biphenyl/aminoacridone/indodicarbocyanine = 100:4.88:0.184, the emission intensity of the acceptors starts decreasing. The wavelength of emission maxima gradually increases, reaching a steady value of 715 nm (Figure 14). An appearance of an additional emission at 750 nm is probably associated with the formation of J-aggregates of indodicarbocyanine,

Figure 14. A) Energy transfer efficiency of the two-step-FRET of the biphenylPMO/aminoacridone/indodicarbocyanine samples. The point labeled with an arrow corresponds to the maximal energy-transfer efficiency before the appearance of the strong emission quenching of the acceptors (in Figure 11 molar ratio of biphenyl PMO/aminoacridone/indodicarbocyanine = 100:4.88:0.184). B) Intensity (red) and wavelength (blue) of the emission maxima of indodicarbocyanine as a function of the concentration of indodicarbocyanine. The corresponding fluorescence data are shown in Figure 13 (below).

The maximal energy-transfer efficiency from aminoacridone to indodicarbocyanine before the onset of quenching of the acceptors emission is 78 % (at molar ratio of biphenyl/aminoacridone/indodicarbocyanine = 100:4.88:0.184). The energy transfer efficiency of the two-step-FRET (from biphenyl-PMO to indodicarbocyanine, mediated by aminoacridone) for this indodicarbocyanine content is 74 % (Figure 14).The energy-transfer

Figure 13. Emission spectra of biphenyl-PMO loaded with aminoacridone and with various concentration of indodicarbocyanine, excited at 414 nm (top) and at 325 nm (below). All spectra were acquired with the slit size of 1 nm and the integration time of 0.3 s. Molar ratios of biphenyl-PMO/aminoacridone/indodicarbocyanine are depicted in the figure. Chem. Eur. J. 2015, 21, 331 – 346

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Full Paper efficiency from biphenyl-PMO to indodicarbocyanine without aminoacridone for this indodicarbocyanine content is lower, confirming that the consecutive two-step-FRET is responsible for the high energy transfer efficiency of three-chromophore system (see the Supporting Information, Figure S10).

Again, two sets of 1D emission experiments were performed, one with excitation at the absorption wavelength of diethylaminocoumarin (421 nm) and the other with excitation at the absorption wavelength of biphenyl group (325 nm). When the systems are excited at 421 nm, the emission peak of diethylaminocoumarin at 480 nm decreases in intensity and the emission peak of dibenzothiacarbocyanine at 630 nm increases in intensity as the concentration of the dibenzothiacarbocyanine increases (Figure 16, top). The same observation is made with excitation wavelength at 325 nm: As the amount of dibenzothiacarbocyanine increases, the emission signal of diethylaminocoumarin decreases in intensity, whereas the emission peak of dibenzothiacarbocyanine increases in intensity (Figure 16, below). The emission signals from the biphenyl group are hardly visible. These results are consistent with FRET phenomena from diethylaminocoumarin to dibenzothiacarbocyanine and two-step-FRET from biphenyl moiety to dibenzothiacarbocyanine mediated by diethylaminocoumarin. An additional emission peak at higher wavelength (  670 nm) from the emission line of dibenzothiacarbocyanine is again present, suggesting formation of J-aggregates. The maximal energy-transfer efficiency from diethylaminocoumarin to dibenzothiacarbocyanine before appearance of the emission quenching of the acceptors is 80 % (at molar ratio of biphenyl-PMO/diethylaminocoumarin/dibenzothiacarbocyanine = 100:0.744:0.458) and that for the two-step-FRET is 73 % for this dibenzothiacarbocyanine content (Figure 17).

Three-chromophore system: Biphenyl-PMO, diethylaminocoumarin (2), and dibenzothiacarbocyanine (3) The emission band of diethylaminocoumarin overlaps with the absorption band of dibenzothiacarbocyanine (see the Supporting Information, Figure S11) and the calculated Fçrster distance for the pair of diethylaminocoumarin and dibenzothiacarbocyanine is 33 , which is slightly smaller than three chromophore systems containing aminoacridone as a mediator dye. Three chromophore systems were prepared with constant molar ratio of biphenyl-PMO/diethylaminocoumarin = 100:0.744, and varying concentration of dibenzothiacarbocyanine acceptors. As shown in 2D fluorescence spectra (Figure 15, plot 2), the biphenyl-PMO/diethylaminocoumarin/dibenzothiacarbocyanine system yields fluorescence peaks of diethylaminocoumarin at (470 nm, 415 nm) and those of dibenzothiacarbocyanine at (630 nm, 570 nm). An additional strong peak at (lem, lex) = (630, 415 nm) is seen (“b” in Figure 15, plot 2), resulting from the FRET from diethylaminocoumarin to dibenzothiacarbocyanine. A weak fluorescence peak (“c”) at (630, 305 nm) is also observed, which presumably arises from two-step-FRET from the biphenyl moiety to the dibenzothiacarbocyanine.

Three-chromophore system: Biphenyl-PMO, diethylaminocoumarin (2), and sulforhodamine G (5) The emission band of diethylaminocoumarin overlaps with the absorption band of sulforhodamine G (see the Supporting Information, Figure S15). The calculated Fçrster distance for the pair of diethylaminocoumarin and sulforhodamine G is 29 . As shown in Figure 18 (plot 1) the sulforhodamine G dye gives rise to two strong fluorescence peaks at (560, 530 nm) and (560, 505 nm). Three-chromophore systems with diethylaminocoumarin mediators and sulforhodamine G acceptors yield a peak at (560, 415 nm; “b” in Figure 18, plot 2) resulting from FRET from diethylaminocoumarin to sulforhodamine G as well as the fluorescence peaks of each dye. When the concentration of sulforhodamine is increased, the peak (“b”) at (570, 415 nm) becomes more intense, accompanied by decrease in intensity of the fluorescence signal of diethylaminocoumarin at (480, 415 nm) as shown in Figure 18 (plot 3). Furthermore, an additional peak (“c”) at (570, 310 nm) appears, which can be attributed to the two-step-FRET from biphenyl to sulforhodamine mediated by diethylaminocoumarin. 1D fluorescence spectra, which were excited at 421 nm for excitation of diethylaminocoumarin and at 309 nm for excitation of biphenyl group, are shown in Figure 19 (top) and (below), respectively. In both experiments, as the concentration of sulforhodamine G increases, the emission of diethylaminocoumarin decreases in intensity gradually, whereas the emission of sulforhodamine G grows in intensity up to the molar concentration of sulforhodamine G = 0.540, followed by a de-

Figure 15. The contour plots of the 2D fluorescence spectra. 1) Dibenzothiacarbocyanine (6.03  106 mol L1) in water containing OTAC; 2) biphenylPMO/diethylaminocoumarin/dibenzothiacarbocyanine sample with the molar ratio of 100:0.744:0.458. Chem. Eur. J. 2015, 21, 331 – 346

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Figure 17. A) Energy transfer efficiency of the two-step-FRET of the biphenylPMO/diethylaminocoumarin/dibenzothiacarbocyanine samples. The point labeled with an arrow corresponds to the maximal energy-transfer efficiency before the appearance of the strong emission quenching of the acceptors (in Figure 16 molar ratio of biphenyl PMO/diethylaminocoumarin/dibenzothiacarbocyanine = 100:0.744:0.458). B) Intensity (red) and wavelength (blue) of the emission maxima of dibenzothiacarbocyanine as a function of the concentration of dibenzothiacarbocyanine. The corresponding fluorescence data are shown in Figure 16 (below).

rence of J-dimers. Thus, formation of J-aggregate of sulforhodamine G plausibly accounts for the emission quenching in this system. The maximal energy-transfer efficiency is deduced as 76 % for the FRET from diethylaminocoumarin to sulforhodamine G before the emission quenching of the acceptor dye occurs at molar concentration of biphenyl-PMO/diethylaminocoumarin/ sulforhodamine G = 100:0.744:0.540 and 69 % for the two-stepFRET from biphenyl group to sulforhodamine G mediated by diethylaminocoumarin at this molar concentration (Figure 20). The energy-transfer efficiency from biphenyl-PMO to sulforhodamine G without diethylaminocoumarin for this sulforhodamine G content is 25 % (see the Supporting Information, Figure S17).

Figure 16. Emission spectra of biphenyl-PMO loaded with diethylaminocoumarin and various concentration of dibenzothiacarbocyanine, excited at 421 nm (top) and at 325 nm (below). All spectra were acquired with the slit size of 1 nm and the integration time of 0.3 s. Molar ratios of biphenyl-PMO/ diethylaminocoumarin/dibenzothiacarbocyanine are depicted in the figure.

crease in intensity above this concentration. These observations indicate that the consecutive two-step-FRET from biphenyl group to sulforhodamine G through diethylaminocoumarin also occurs in this system. The emission peak of sulforhodamine G shifts gradually to higher wavelength with increasing concentration of sulforhodamine G, reaching a plateau at 572 nm, which amounts to a redshift of 13 nm from the value of dye monomer at 559 nm. Monte et al. have reported that rhodamine 110, rhodamine 6G, and rhodamine 101 in silica gels show redshifts in the emission peaks by 31, 11, and 13 nm, respectively, when J-dimers occur.[51–53] The observed redshift of sulforhodamine G is in the similar range with those for Jdimers of rhodamine 6G and rhodamine 101, indicating occurChem. Eur. J. 2015, 21, 331 – 346

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Comparison between the systems All the systems studied in this work containing either twochromophores or three-chromophores show similar FRET behaviors with high efficiencies, demonstrating that the emission wavelength and the efficiency can be tuned for various lightharvesting purposes by using different chromophores. For both two-chromophore systems, very high energy-transfer effi341

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Figure 18. The contour plots of the 2D fluorescence spectra. 1) Sulforhodamine G (7.92  106 mol L1) in water containing OTAC. Biphenyl-PMO/diethylaminocoumarin/sulforhodamine G samples with the molar ratio of 2) 100:0.744:0.0334, and 3) 100:0.744:0.874.

Figure 19. Emission spectra of the biphenyl-PMO loaded with diethylaminocoumarin and various concentration of sulforhodamine G, excited at 421 nm (top) and at 309 nm (below). All spectra were acquired with the slit size of 1 nm and the integration time of 0.3 s. Molar ratios of biphenyl-PMO/diethylaminocoumarin/sulforhodamine G are depicted in the figure.

ciencies of above 90 % are obtained at low concentration of acceptors (2.49 % for aminoacridone and 0.744 % for diethylaminocoumarin), indicating that the light energy absorbed by the biphenyl groups in the framework is channeled to each dye molecule in the pore. Three-chromophore systems yield high energy-transfer efficiencies for the energy transfer from mediators to acceptors (75 %–85 %) and for two-step-FRET (70– 80 %) at very low concentration of mediators (0.7–2.5 %) and acceptors (< 1 %). Previously, Inagaki et al. have shown that the high energy-transfer efficiency of biphenyl-PMO/coumarin1 system can be attributed to a unique geometry of biphenylPMO, in which donor groups are placed in the pore walls surrounding the acceptor dye and funnel light energy absorbed by surplus amounts of biphenyl groups to a single coumarin1.[29] Moreover, our results of three-chromophore systems suggest that the unique geometry of the mesoporous compounds enables mediators and acceptors to be arranged in a close proximity from each other in one-dimensional channels, leadChem. Eur. J. 2015, 21, 331 – 346

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ing to an efficient energy transfer. However, several differences between systems have been noted. For two-chromophore systems, the emission peak of the acceptors is redshifted by 42 nm in the systems with aminoacridone, compared to the systems with diethylaminocoumarin. The Fçrster distances for the donor/acceptor pairs in these two-chromophore systems are similar (22  for biphenyl-PMO/ aminoacridone and 22  for biphenyl-PMO/diethylaminocoumarin). The maximal energy-transfer efficiency before the onset of the quenching of the emission from the acceptors is slightly higher for the system with aminoacridone (95 %) than that of diethylaminocoumarin (91 %). The reason for this efficiency difference is not clear, however, most likely this results from the fact that larger amount of aminoacridone can be doped into biphenyl-PMO than diethylaminocoumarin without quenching. However, the emission intensity of the acceptors is 342

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Full Paper mediators and acceptors are located either in the same pores or in the neighboring pores. Assuming a random distribution of the dye inside the PMO, the probability of the presence of both dye species in the same pore or in the neighboring pores is higher for the systems with larger amount of dye and thus, the distance between donor/acceptor should be smaller for the aminoacridone–dibenzothiacarbocyanine pair. To investigate the influence of acceptor dye, we investigated two types of three-chromophore systems with aminoacridonemediators. The emission of the acceptor dye is redshifted by 60 nm for the system with indodicarbocyanine acceptors compared with the one with dibenzothiacarbocyanine acceptors. The Fçrster distances for the mediator/acceptor dye are similar (36  for aminoacridone/dibenzothiacarbocyanine pair and 37  for aminoacridone/indodicarbocyanine pair). The energy transfer efficiency before the appearance of the quenching is higher for the system with dibenzothiacarbocyanine (80 %) than the one with indodicarbocyanine (74 %). It is likely that the larger amount of dibenzothiacarbocyanine loaded into the systems than indodicarbocyanine contributes to the difference in the transfer efficiency. Two different three-chromophore systems were synthesized with diethylaminocoumarin acting as mediator dye: One with dibenzothiacarbocyanine and the other with sulforhodamine G acting as acceptor dye. The following differences between the systems were noted: First, the system with dibenzothiacarbocyanine (73 %) yields slightly higher maximal energy-transfer efficiency without quenching than the one with sulforhodamine G (69 %). The amounts of the loaded acceptor dye are similar in both cases (molar ratios of dibenzothiacarbocyanine and of sulforhodamine G are 0.458 and 0.54, respectively). The reason for the higher energy-transfer efficiency in the system with dibenzothiacarbocyanine is probably associated with the larger Fçrster distance between dibenzothiacarbocyanine and diethylaminocoumarin (33 ) than the one between sulforhodamine G and diethylaminocoumarin (29 ). Second, the emission intensity of the acceptor dye is higher for the system with sulforhodamine G. Third, the emission of the acceptor dye is redshifted by 67 nm for the system with dibenzothiacarbocyanine. The most efficient energy transfer of the two-step-FRET is achieved for the biphenyl-PMO/aminoacridone/dibenzothiacarbocyanine system (80 %). On the contrary, the threechromophore system biphenyl-PMO/diethylaminocoumarin/ sulforhodamine G exhibits the most intensive emission peak of the acceptor dye. 1D emission spectra of all the two- and three-chromophore systems revealed that the emission intensity of the acceptors started to decrease above a certain concentration of acceptors, with concomitant gradual redshift of the peak maxima. Moreover, an additional signal at higher wavelength from an emission band of the acceptors was also observed for threechromophore systems containing dibenzothiacarbocyanine, indodicarbocyanine, and sulforhodamine G acceptors. In the excitation spectra of these systems, additional signals of acceptors, one blue- and one redshifted signal with respect to the excitation band of the monomers of acceptors were observed.

Figure 20. A) Energy transfer efficiency of the two-step-FRET of biphenylPMO/diethylaminocoumarin/sulforhodamine G samples. The point labeled with an arrow corresponds to the maximal energy-transfer efficiency before the appearance of the strong emission quenching of the acceptors (in Figure 19 molar ratio of biphenyl-PMO/diethylaminocoumarin/sulforhodamine G = 100:0.744:0.540). B) Intensity (red) and wavelength (blue) of the emission maxima of sulforhodamine G as a function of the concentration of sulforhodamine G. The corresponding fluorescence data are shown in Figure 19 (below).

higher for the system with diethylaminocoumarin than the one with aminoacridone. Two types of three-chromophore systems were prepared with dibenzothiacarbocyanine acting as acceptor dye to study the influence of the mediator dye on the energy-transfer efficiency: One with aminoacridone and the other with diethylaminocoumarin acting as the mediator dye. The following differences between the systems were found: First, Fçrster distance for the mediator/acceptor pair in the system with aminoacridone (37 ) is larger than the one in the system with diethylaminocoumarin (33 ), allowing more efficient energy-transfer for the system with aminoacridone at the same donor/acceptor distance. Second, the maximal energy-transfer efficiency from the mediator dye to the acceptor dye before the onset of the quenching is higher for the system with aminoacridone (84 %) than for the system with diethylaminocoumarin (80 %) at the similar amount of dibenzothiacarbocyanine. This difference is probably due to the larger amount of aminoacridone than diethylaminocoumarin loaded into the system without quenching. Since the size of the pores is 34 , which is similar to the Fçrster distance for the mediator/acceptor pairs, the energy transfer is predicted to be the most efficient when the Chem. Eur. J. 2015, 21, 331 – 346

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Full Paper PMO samples were analyzed as solid with front-face detection geometry, by using a solid sample holder. The obtained spectra were corrected taking into account wavelength dependency of spectrofluorometer and dark current offset.

These confirm that oblique J-aggregates of acceptors are formed, resulting in emission quenching of acceptors. The onset of aggregation resulting in emission quenching occurs at lower concentration of acceptors when indodicarbocyanine and dibenzothiacarbocyanine are applied as acceptors. This is plausibly associated with the molecular structures of the dye: The low freedom of torsional motions in indodicarbocyanine and dibenzothiacarbocyanine, the high polarizability of cyanine group, and consequent strong van der Waals forces, facilitating the formation of aggregates even at low concentrations.[50]

Fluorescence quantum yields were measured on HORIBA Quanta-f F-3029 Integrating Sphere at a specific wavelength range with the increment of 1 nm for emission spectra and of 0.1 nm for excitation spectra. The integration time was 1 s for the emission and for the excitation spectra.

Resonance energy transfer

Conclusion

The Fçrster distances for donor/acceptor pairs were calculated using:[3]

Biphenyl-bridged PMO, which consists of a crystal-like pore wall structure and a hexagonal array of mesopores, was used as a template to build two- and three-chromophore systems with a light-harvesting function. Two-chromophore systems, which contain an additional dye in the pores, show that excitation energy absorbed by biphenyl groups (donors) is transferred to either aminoacridone or diethylaminocoumarin (acceptors) through FRET with high transfer efficiencies of 91– 95 %. In addition, three-chromophore systems with light-harvesting behavior are prepared by incorporation of two types of dye molecules into biphenyl-bridged PMO, in which each dye in the mesochannels acts as a mediator and acceptor. The consecutive two-step-FRET from biphenyl groups to acceptors, conveyed by mediators is observed with high energy-transfer efficiencies from 70 to 80 % at very low concentrations of each dye. Our results suggest that the ordered channel structure of PMO enables mediator/acceptor pairs to be arranged in close mutual proximity for an efficient direct energy transfer. The most efficient energy transfer of the two-step-FRET is achieved for the biphenyl-PMO/aminoacridone/dibenzothiacarbocyanine system (80 %). Several requirements for the high energy-transfer efficiency can be envisaged. Larger amounts of dye need to be incorporated in the channels before the formation of aggregate occurs. It appears that higher efficiency is obtained for the system with larger Fçrster distance between chromophore pairs. The emission wavelength can be tuned by a choice of acceptors: 477 nm (diethylaminocoumarin), 519 nm (aminoacridone), 567 nm (sulforhodamine G), 630 nm (dibenzothiacarbocyanine), and 692 nm (indodicarbocyanine). Our study demonstrates that three-chromophore systems based on PMO present considerable advantages in designing light-harvesting devices, providing high energy-transfer efficiency and wide range of emission wavelength.

R0 ¼ ðJK 2 Q0 n4 Þ1=6  9:7  103 ½

ð1Þ

In which J is the spectral overlap integral between the emission band of donors and the absorption band of acceptors, K2 the orientation factor, Q0 the quantum yield of donors in the absence of acceptors, and n the refractive index of the medium. The calculations were carried out with K2 = 0.476 and with n = 1.59. The value of n was measured by Inagaki et al.[29] for biphenyl-PMO. The spectral overlap integral J between the emission band of donors and the absorption band of acceptors was obtained from Equation (2):[3, 54]

J¼

Z

 Z  F ðlÞeðlÞl4 dl = F ðlÞdl

ð2Þ

In which F(l) is the corrected emission of the donors in the wavelength range from l to l + Dl with the area normalized total intensity, e(l) the extinction coefficient of the acceptors at wavelength l. e(l) is expressed in units of m1 cm1, l in centimeters and J in units of m1 cm3. The values calculated with Equations (1) and (2) overlap integrals and Fçrster distances are shown in Table 2. The energy transfer efficiency of the one-step-FRET, E, was determined from the degree of emission quenching of the donors, using the emission intensity of the donors in the absence (FD) and in the presence (FDA) of the acceptors:[54]

E ¼ 1  ðFDA =FD Þ

ð3Þ

The energy transfer efficiency of the two-step-FRET, E123, was calculated in accordance with the method used by Watrob et al.[41]:

Experimental Section

Table 2. Values calculated with Equations (1) and (2).

Fluorescence measurements

Donors

Mediators

Acceptors

Q0

Fluorescence emission and excitation spectra were recorded on a HORIBA Fluorolog-3 Model FL3– 22 spectrofluorometer. The dye solutions in OTAC/H2O were analyzed in quartz cells with rightangle geometry for detection. The

J(l)[b] [cm3 m1]

R0 []

biphenyl-PMO biphenyl-PMO biphenyl-PMO biphenyl-PMO biphenyl-PMO biphenyl-PMO

– – aminoacridone aminoacridone diethylaminocoumarin diethylaminocoumarin

aminoacridone Diethylaminocoumarin Dibenzothiacarbocyanine indodicarbocyanine Dibenzothiacarbocyanine sulforhodamine G

0.278 0.278 0.136 0.195 0.188 0.134

6.49  1015 7.23  1015 2.75  1013 2.24  1013 1.05  1013 7.30  1014

22 22 36 37 33 29

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Full Paper E12  E23 ¼ E123

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ð4Þ

in which E12 and E23 are the energy transfer efficiencies from chromophore 1 to chromophore 2 and from chromophore 2 to chromophore 3, respectively.

UV/Vis measurements UV/Vis spectra were recorded in OTAC/H2O media on a Varian Cary 5E spectrometer in the wavelength range from 200 to 800 nm in quartz cells with path length of 10 mm.

Syntheses Synthesis of biphenyl-bridged precursor and PMO Synthesis of the precursor 4,4’-bis(triethoxysilyl)biphenyl was carried out from 4,4’-dibrombiphenyl (ABCR, purity 98 %) and tetraethylorthosilicate (TEOS) (Merck, purity 99 %) following the synthetic protocol in reference [55]. The biphenyl-bridged PMO was synthesized by using 4,4’-bis(triethoxysilyl)biphenyl precursor and trimethylstearylammonium chloride (OTAC) surfactant (ABCR, 97 %) in a 130 mL autoclave for 24 h at 95 8C in accordance with the method in literature[56] .The obtained material was extracted twice, in each case with 230 mL ethanol for 24 h at room temperature.

Loading of biphenyl-PMO with dye The PMO-samples were loaded following the modified method of Inagaki et al.[29] with various types of dye molecules: 2-Amino-9-acridon (denoted as aminoacridone, Sigma–Aldrich, purity 98 %); 7(diethylamino)coumarin-3-carboxylic acid-N-succinimidyl ester (denoted as 7-(diethylamino)coumarin, Sigma–Aldrich, purity 96 %); 1ethyl-2-[3-(1-ethylnaphtho[1,2-d]thiazolin-2-ylidene)-2-methylpropenyl]naphtha [1,2-d]thiazolium bromide (known as “stains all”, here denoted as dibenzothiacarbocyanine, Acros Organics, purity 95 %); 1,3,3-trimethyl-2-[5-(1,3,3-trimethyl-1,3-dihydro-indol-2-ylidene)penta-1,3-dienyl]-3H-indolium chloride (denoted as indodicarbocyanine, FEW Chemicals, purity 99.1 %); sulforhodamine G (Sigma– Aldrich, dye content 60 %). All dye substances were applied without additional purification. Specific amounts of dye were added to 50 mL of the aqueous OTAC solution (0.0133 g mL1). The mixture was stirred for 75 min at room temperature. The extracted biphenyl-PMO powder (50 mg) was suspended in this mixture and the suspension was stirred for 15.5 h at 4 8C. The loaded PMO-powder was centrifuged for 15 min at 4000 rpm, washed with water (3 mL) and dried at 25 8C. All steps were carried out in a condition excluding the light. The molar ratios of biphenyl-PMO/dye used in the suspension mixture are depicted in the figures of the emission spectra.

Keywords: chromophores · donor–acceptor systems fluorescence spectroscopy · organic–inorganic hybrid mesoporous materials

· ·

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Received: May 5, 2014 Revised: August 18, 2014 Published online on October 28, 2014

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