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Hybrid Systems Based on Layered Silicate and Organic Dyes for Cascade Energy Transfer Silvia Belušáková, Kamil Lang, and Juraj Bujdak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04982 • Publication Date (Web): 31 Aug 2015 Downloaded from http://pubs.acs.org on September 8, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Hybrid Systems Based on Layered Silicate and Organic Dyes for Cascade Energy Transfer.

Silvia Belušáková,a Kamil Lang,b Juraj Bujdáka,c

a

Comenius University in Bratislava, Department of Physical and Theoretical Chemistry, Faculty of

Natural Sciences, 842 15 Bratislava, Slovakia b

Institute of Inorganic Chemistry of the Czech Academy of Sciences, v. v. i., Husinec-Řež 1001, 250

68 Řež, Czech Republic c

Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 845 36 Bratislava,

Slovakia

Corresponding author: Juraj Bujdák e-mail: [email protected] Phone: +241 2 60296 602 Fax: +241 2 59410 444

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ABSTRACT: This work describes multistep Förster resonance energy transfer (FRET) among laser dyes adsorbed onto the nanoparticles of a layered silicate, saponite (Sap). Six cationic laser dyes with appropriate spectral properties to fit the resonance conditions were selected. Fluorescence spectroscopy proved cascade FRET in the experiments based on hybrid colloidal systems. The emission from preferentially excited energy donor molecules was reduced in favor of the emission from energy acceptors. The colloidal systems were used for the preparation of thin solid films with embedded dye cations. The films were characterized by X-ray diffraction, absorption spectroscopy and linearly-polarized absorption spectroscopy. Results from steady-state fluorescence, fluorescence anisotropy measurements and time-resolved fluorescence spectroscopy confirmed the occurrence of a cascade FRET in the films. The effect of the concentration of embedded dye molecules on FRET efficiency was proved. The multicomponent films based on Sap and laser dyes are affordable materials useful for the manipulation of light energy at a molecular level, thus mimicking a cascade process occurring in photosynthesis systems in green plants.

Keywords: saponite, rhodamines, fluorescence, laser dyes, FRET


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INTRODUCTION Hybrid materials are complex, multi-component systems composed of nanoparticles, molecules or their assemblies. The properties of hybrid materials can be altered by structural modifications, or by varying the type or amount of the respective components. The development of methods for the synthesis of hybrid materials provides efficient tools for controlling their structure at a molecular level. The boom in materials science is providing new possibilities for the construction of novel materials and adjusting their properties for applications in modern industries.1-2 A large group of hybrid materials is composed of inorganic layered particles with dimensions of tenths to hundreds of nanometres. An inorganic component often plays the role of a carrier or host material for adsorbed or intercalated guest molecules, which impose specific functionalities.3 Combining organic photoactive dyes as guest molecules embedded in an inorganic template often leads to materials with interesting photonic properties2,

4-5

. The properties and structure of photo-inactive

inorganic hosts may significantly alter the properties of embedded dye molecules.6 This work is focused on the preparation and characterization of hybrid systems based on a saponite synthetic expandable layered silicate with adsorbed or intercalated laser dyes. Saponite is a trioctahedral smectite whose particles carry a net negative charge.7-8 Concentrating dye cations on the silicate surface leads to physical interactions which are absent in homogeneous systems. For example, Förster resonance energy transfer (FRET) has been repeatedly observed in such hybrid materials.9-10 FRET represents a phenomenon which plays an important role in natural photosynthetic systems. Full control over this phenomenon in artificial materials may enable the efficient manipulation of light energy, applicable in future solar cells.11-12 Several parameters need to be satisfied for efficient FRET to occur.13-14 The resonance condition can be easily achieved by an appropriate selection of photoactive components13, 15, and it is limited to a distance of a few nanometres.16 Appropriate intermolecular distances between interacting molecules, as well as suitable molecular orientation, are further factors influencing FRET 3 ACS Paragon Plus Environment


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efficiency. Low intermolecular distances require sufficiently high fluorophore concentrations.14 However molecular aggregation often occurs, which causes a significant loss of photoactivity and fluorescence quenching.6 Molecular aggregation for systems based on layered silicates and organic dyes have been extensively studied6, 17-18 and it was found that selecting a silicate with a low surface charge leads to the suppression of dye aggregation.19-20 The inclusion of surfactants based on alkylammonium cations is another way to improve the material's photoactivity21-23, because surfactant molecules compete with dye cations for adsorption sites, separating dye molecules and suppressing their aggregation. In this paper we focus on the photophysical properties of complex systems based on saponite containing a series of six laser dyes, which participate in multi-step FRET. The dye molecules were concentrated on the surface of the saponite particles. The ability of the materials to act in cascade-type FRET was investigated by fluorescence spectroscopy.

MATERIALS AND METHODS Synthetic expandable saponite, Sumecton SA (Sap),24 was purchased from Kunimine Industries Co., Ltd., Japan. Its cation exchange capacity is 0.71 mmol g-1.

25

Laser dyes, rhodamine 123 (Dye1),

rhodamine 6G (Dye2), pyronine Y (Dye3), rhodamine 3B (Dye4), oxazine 4 (Dye5) and oxazine 1 (Dye6), were purchased from Exciton, Inc. (USA) or Sigma Aldrich (Germany) and used as received. Structural formulas of dye cations are shown in Figure 1. More information on the spectral properties of the dyes is listed in Table 1. The cationic alkylammonium surfactant, hexadecyltrimethylammonium chloride (AA), was purchased from Sigma-Aldrich. Isopropanol and ethanol (EtOH) of spectroscopic grade purity, hydrogen peroxide and sulphuric acid of analytical grade were purchased from Slavus s.r.o. (Slovakia). Quartz slides used as substrates for the preparation of thin films were transparent in the UV-vis range. The hydrophilization of slide surfaces with peroxosulphuric acid solution (mixture of 4 ACS Paragon Plus Environment

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96% sulphuric acid and 35% hydrogen peroxide, 1:1 (v/v)) was performed at room temperature. After the treatment, the slides were washed with deionised water and used as substrates for Sap films. All solutions and aqueous colloids were prepared using high-purity deionised water. Aqueous Sap colloids were prepared by the immersion of silicate powder in water and stirring for 24 h. Stock aqueous solutions of each dye were prepared in high-purity water containing 5 % (v/v) isopropanol. The concentrations were above 10-5 mol dm-3 and were determined using the Lambert-Beer law and molar absorption coefficients listed in Table 1. Dye solutions of the required concentrations were prepared from the stock solutions by dilution with deionized water.

Figure 1. Structural formulas of dye cations.

Table 1. Laser dyes, their abbreviations and spectral parameters. 5 ACS Paragon Plus Environment


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Dye

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λAbs / nm

ε / L mol-1 cm-1

λfl / nm

φfl

Rhodamine 123

Dye1

511

85 200 (EtOH)26

530

0.90 (0.1 M H2SO4)27

Rhodamine 6G

Dye2

530

116 000 (EtOH)26

560

0.95 (0.1 M H2SO4)27

Pyronine Y

Dye3

547

69 200 (H2O)28

568

0.35 (H2O)29

Rhodamine 3B

Dye4

556

115 00030

574

0.45 (0.1 M H2SO4)27

Oxazine 4

Dye5

616

109 000 (EtOH)31

625

0.63 (EtOH)29

Oxazine 1

Dye6

645

123 000 (Methanol)26

680

0.14 (EtOH)32

The labels Dye1-Dye6 reflect the positions of the dyes in cascade FRET, starting from the energy donor, Dye1, to the last energy acceptor, Dye6, i.e. increasing wavelength of both the absorption and emission maxima. The molar absorption coefficients (ε), absorption maxima (λAbs) and fluorescence quantum yields (φfl) of the dyes were obtained from the Exciton (Dayton, Ohio) or indicated references.

First, colloidal dispersions were investigated (Table 2). Two solutions and two colloids contained the same concentration of the dyes, 1.2 ×10-5 mol dm-3 (2 × 10-6 mol dm-3 for each dye). The concentrations of AA and Sap were 7.2 × 10-5 mol dm-3 and 0.72 g dm-3, respectively. The Dyes/Sap and AA/Sap ratios were 1.6 × 10-5 and 1 × 10-4 mol g-1, respectively. Saponite surface was highly under-saturated with respect to the amount of added dye cations. In other words, there was much less dye cations than the capacity of saponite. Considering a large affinity of saponite surface to organic cations, all the dye cations were adsorbed and none remained in the solution.

Table 2. Composition of colloids and solutions. Specimen

Dye

Sap

AA 6 ACS Paragon Plus Environment

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Colloid1

+

+

-

Colloid2

+

+

+

Solution1

+

-

-

Solution2

+

-

+

Film1 was prepared by mixing 2.5 ml of the Sap colloid (0.2 g dm-3) with 2.5 ml of solution containing a total dye concentration of 1.2 × 10-6 mol dm-3 (2.0 × 10-7 mol dm-3 of each dye). Thus, the dye loading in saponite was 6 × 10-6 mol g-1 (10-6 mol g-1 each dye). The films containing one dye (FilmDye1 up to FilmDye6) or both Dye1 and Dye2 (FilmDye12) were also prepared via this procedure. The colloid used for the preparation of Film2 was prepared in the same way as for Film1, however, the total concentration of dyes was ten times lower (1.2 × 10-7 mol dm-3, 2.0 × 10-8 mol dm-3 each dye ). Thin films were prepared by the deposition of the colloids using a vacuum filtration method (filtration membrane with pore size < 0.1 µm, radius of 12.5 mm). The wet films obtained were transferred from membranes on quartz slides. As a result, the Sap coverage in the film was approximately ~10-4 g cm-2 with a dye density of approximately 6 × 10-10 or 6 × 10-11 mol cm-2 in Film1 or Film2, respectively. Based on the amount and density of Sap (2.3 g cm-3), thickness value of the films was estimated to be about 2 µm. Optimal conditions were tested to get Film3 containing AA cations with similar spectral profile to that of Film1. Film3 was prepared in several steps. The Sap colloid (2.5 cm3, 0.2 g dm-3) was mixed with the same volume of deionised water to get the same Sap concentration, used for Film1, and was deposited on a slide in the same way as described above. The Sap film was then treated with AA solution (1.44 × 10-5 mol dm-3, 1 mmol g-1 AA/Sap ratio) and after approximately 24 h, the film was washed three times with isopropanol to remove the excess AA. The final amount of intercalated AA was 0.67 mmol g-1. The AA-saturated film was then treated with the dye solution (1.2 × 10-5 mol dm-3 7 ACS Paragon Plus Environment


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in total) for 60 min, washed with isopropanol and dried in air at room temperature. Based on absorption spectra (see below) very similar composition and concentrations of dyes are expected for both Film1 and Film3, the differences would be below ±10 %. The structural characterization of the films was investigated by X-ray diffraction (XRD). Samples were measured with an EMPYREAN system (PANalytical) using CuKα radiation. A generator was set to 40 mA and 45 kV and the sampling was 0.039 °/95 s per step for a 2θ range between 2.5 and 15 °. Absorption spectroscopy in the UV-Vis region and linearly-polarized absorption spectroscopy were measured in a Cary 100 UV/VIS Spectrometer (Varian). A reversed optics system using an Agilent 8453 UV-Visible Spectrophotometer was used to determine the concentrations of dye solutions. Linearly-polarized UV-Vis spectroscopy was used to estimate the mean molecular orientation of the chromophores with respect to the plane of the oriented film. Preferential parallel orientation of silicate layers on quartz substrate results from high aspect ratio (the ratio of diameter to the thickness dimension of layers), an exfoliated form of the colloids used for film preparation and external force applied by vacuum filtration technique. The spectra, polarized in the direction of the x- (vertical, V) or y-axis (horizontal, H), were recorded in the range 250-800 nm using polarized radiation propagated along the z-axis. Absorbance values AV or AH were recorded using vertically- or horizontally-polarized light. The angle α between the light beam and the normal of the plane of the film surface was changed by rotating the film around the x-axis to get values of 0, 40, 50 and 60 °. The angle between the intercalated dye molecule and surface normal (γ) was estimated using the following equation:

(

)

AH 2 sin α − 3 sin 2 α − 1 sin 2 γ = AV sin 2 γ

(1)

Steady-state fluorescence measurements were performed using a Fluorolog-3 spectrometer 8 ACS Paragon Plus Environment

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(Horiba Jobin-Yvon). The excitation spectra were mostly measured at 710 nm to selectively record the emission spectra of the last FRET member. The emission spectra were recorded upon excitation at 480 nm to predominantly excite Dye1, unless otherwise stated. Linearly-polarized fluorescence spectra were measured using V-polarized excitation with angles between the excitation light beam and film surface plane normal to 0, 30, 45 and 60° using a front-face sampling setup. Automated polarizers FL1044 (Horiba Jobin-Yvon) were used in the L-format measurement configuration. The emission was measured for both the V- and H- polarized light. The symbols of the recorded fluorescence intensities IVV and IVH reflect the adjustments of the excitation and emission polarizers to either V- or Hpolarizations. In order to correct an instrumental response to polarized light, the isotropic factor GV was evaluated by the measurement of an isotropic system based on a dye mixture solution. GV represented the I’VH/I’VV ratio, where the I’ symbols represented the fluorescence intensities from isotropic systems. The different film tilt angles led to very similar values of fluorescence anisotropy, which were calculated using eq. (2):

I VV r=

I VV

I VH I VH

(GV − 1)

(2)

(GV + 2)

The overlap integral reflects the energy transfer efficiency between the energy donor (ED) / energy acceptor (EA) pairs and it is controled by the corresponding absorption and emission spectra of EA and ED, respectively. The absorption and emission spectra of FilmDye1- FilmDye6 were used for the calculations. Relative overlap integrals (Jrel) between the normalized emission of ED and absorption spectrum of EA (Soverlap) were normalized to the area of the ED emission spectrum (SFl):

Jrel =

Soverlap

(3)

SFl 9

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Film1, FilmDye1 up to FilmDye6 contain the same amounts of respective dyes, hence, their respective fluorescence spectra FFilm1, FDye1, FDye2 ….FDye6 can be directly used to estimate FRET efficiencies. This model does not consider possible differences of dye molecular aggregation between Film1 and single dye component films (FilmDye1 - FilmDye6). The profile of the emission spectrum of Film1 (FFilm1) was decomposed to the respective spectra of the components (FDye1, FDye2 ….FDye6), which were identical to those of the films containing single dyes, i.e., FilmDye1 up to FilmDye6. The deconvolution was performed using multiple linear regression method (OriginPro 8.0, OriginLab Corp.). The model defined the spectrum of Film1 as a dependent variable and those of FilmDye1 up to FilmDye6 as independent variables: FFilm1=A0+A1 FDye1+ A2 FDye2 +A3 FDye3 +A4 FDye4+ A5 FDye5 +A6 FDye6 + E

(4a)

A0 and E represent an intercept value and spectral error residual, respectively. Coefficients (A1-A6) quantitatively express the ratio of fluorescence between Film1 and FilmDye1 - FilmDye6 for each respective dye. They directly express photophysical interaction between the dyes. The coefficients were bellow 1 for Dye1, Dye2, Dye3 and Dye4 reflecting the ratio of the signals in the presence and absence of EAs. Thus, the FRET efficiency E defined by fluorescence signal from ED in presence and absence of EA (E=1-FDA/FD ) can be expressed for each k-th ED by following equation: Ek = 1-Ak

(4b)

Where Ak represents the coefficient obtained from the regression for the respective energy donor. Fluorescence lifetime measurements were performed with a Fluorolog 3 spectrometer using a laser-diode excitation and a cooled TBX-05-C photon detection module in a time-correlated singlephoton counting regime. The following excitation wavelengths were used: 494 nm (NanoLED-495, pulse width 1.4ns, 1 MHz), 667 nm (NanoLED-670L, pulse width < 200 ps, 1 MHz), or 442 nm (NanoLED-450, pulse width 1.2 ns, 1 MHz). The decay curves were fitted to exponential functions 10 ACS Paragon Plus Environment

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using the iterative reconvolution procedure in DAS6 software (v. 6.4, Horiba Jobin Yvon, 2009). The average fluorescence lifetime was calculated using the formula recommended for FRET analysis in multiple fluorophore systems:33 τa = (α1τ1 + α2τ2)/(α1 + α2),

(5)

where αi represents the fractional intensity of each decay time (in %).

RESULTS AND DISCUSSION Solutions and colloids. Dye solutions and Sap colloids were investigated to test the possibility of FRET. Solution1 and Solution2 are quite diluted (1.2 × 10-6 mol dm-3), giving estimated average intermolecular distances of approximately 100 nm. Since FRET is limited to distances of a few nanometres between an acceptor and donor, the estimated distances are far beyond the limit for FRET to occur. In contrast, colloid saponite particles may represent suitable carriers. The occurrence of FRET in Colloid1 is much more probable than that in Solution1, due to the high local concentrations of the adsorbed fluorophores on the colloid particles surface. The intermolecular distance can be roughly estimated by considering two adjacent basal surfaces of an individual saponite layer as a single plane and a theoretical saponite surface area (750 m2 g-1).34 Under the conditions of negligible inter-particle association achieved at a dye/Sap loading of 1.6 × 10-5 mol g-1, the estimated intermolecular distance is approximately 8 nm. The fluorescence spectra of the colloids and solutions are compared in Figure 2. The fluorescence intensity of Solution1 was higher than that of Colloid1, although the dye bulk concentration was the same. Emission maxima summarized in Table 1 (λem) were considered in order to assign emission bands to the respective dyes (see arrows in Figure 2a, c). A broad band of Solution1 (Figure 2a) corresponds to the emission of Dye1 and Dye2, overlapped with much weaker bands of Dye3 and Dye4. No apparent fluorescence emissions of Dye5 or Dye6 were observed, as these dyes are not 11 ACS Paragon Plus Environment


excited at 480 nm and no FRET occurs.

Figure 2. Emission (a, c) and excitation spectra (b) of dye mixture solution and colloids. The arrows show the wavelength for the maxima of the emission (a,c) and absorption (b) of the dyes adopted from Table 1 (from left to right in the order Dye1-Dye6).

a.

λexc= 480 nm

IF / a.u.

Solution1 Colloid1

500

600

700

800

λ / nm

b. λem= 700 nm Solution1 Colloid2 Colloid1

IF / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

400

450

500

550

600

650

λ / nm 12 ACS Paragon Plus Environment

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c.

λexc= 480 nm Colloid1 Colloid2

IF / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

600

700

800

λ / nm

The low fluorescence intensity of Colloid1 indicates fluorescence quenching (Figure 2a). New emission bands at longer wavelengths of about 575 and 675 nm can be assigned to the fluorescence emissions of Dye4 and Dye6. This indicates that the energy absorbed by Dye1 or Dye2 was rapidly transferred to the dyes that only absorb at longer wavelengths. The φfl values of Dye1 or Dye2 are close to 1; however, the other dyes in the series have significantly lower φfl values (Table 1). From this comparison, it follows that the emission intensity of Dye3 up to Dye6 will be low, even under conditions of very efficient FRET (Figure SI1 in the Supporting Information). Fluorescence excitation spectroscopy provided further evidence of FRET in Colloid1 (Figure 2b). With Solution1, the excitation spectrum only showed the absorption bands of Dye5 and Dye6. In contrast, Colloid1 exhibited a broad envelope of bands in the 480 - 560 nm region, indicating the participation of Dye1 up to Dye4 in the FRET, resulting in the emission from Dye5 and Dye6. Besides FRET, dye aggregation and subsequent fluorescence quenching were also observed. Both rhodamine and oxazine dyes have a tendency to form H-aggregates onto silicate colloid 13 ACS Paragon Plus Environment


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nanoparticles.35,

36

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The H-aggregates are non-fluorescent. One of the strategies to reduce the H-

aggregation in dye/smectite systems is based on the inclusion of alkylammonium cations to modify the silicate surface. Thus, the presence of AA has the potential to improve the material’s photoactivity and to increase φfl.21-22,

36

The amounts of dyes in compared colloids were kept same. As a result, the

presence of AA in Colloid2 partially suppressed the dye aggregation, leading to an increase in fluorescence intensities in both the emission and excitation spectra when compared with those of Colloid1 (Figure 2b, c). A new emission band at 630 nm, assigned to the emission of Dye5, indicates that Dye5 was highly aggregated in Colloid1 (Figure 2c). No spectral differences were observed between Solution1 and Solution2, where the latter solution contained AA (not shown). In summary, the adsorption of the mixture of dyes on the surface of saponite particles leads to a considerable increase in dye concentrations. As a consequence, multistep FRET can be achieved in the colloids, whereas the phenomenon does not occur in solutions of the same concentrations. No segregation of dyes was observed, which could otherwise reduce FRET efficiency significantly.37-38

Characterization of hybrid films. The properties of Film1, Film2 and Film3 containing all dyes were compared with the films containing only one dye (FilmDye1-FilmDye6) or a mixture of Dye1 and Dye2 (FilmDye12). Brief characterization of XRDs of the films is summarized in SI2. XRD patterns of the hybrid films were almost the same as that of pure saponite reflecting a relatively low amount of intercalated dye cations (d001=1.27-1.39 nm). The larger value of the basal spacing was determined for Film3 (1.47 nm), which can be assigned to the presence of AA in the interlayer space.

Absorption spectra. Absorption spectra were measured to characterize the basic spectral properties of the films (Figure 3). FilmDye1-FilmDye4 contained predominantly monomer species. Light scattering from the films contributed to the spectral band broadening and baseline increase. Molecular 14 ACS Paragon Plus Environment

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aggregation was more apparent in FilmDye5 and FilmDye6, which led to the substantial changes of the absorption spectra of Dye5 and Dye6, respectively (Figure 3). The shift to longer wavelengths (621 and 668 nm) was evidence of J-aggregation, and the H-bands at 576 and 617 nm indicate that J-aggregates have an oblique structure. These assemblies have been observed in hybrid systems of layered silicates with oxazines.39 The absorption spectra of selected multi-component films are shown in Figure 4. Equivalent amounts of each dye in Film1 are guaranteed by the preparation procedure. Film3 seems to have similar features to Film1. The most apparent absorption bands at 553, 622 and 669 nm can be assigned to Dye3 (Dye4), Dye5 and Dye6, respectively. The assignment of the other dyes is obscured due to overlapping absorption bands at shorter wavelengths. Similarly to colloids, the formation of Jaggregates is apparent with Dye5 and Dye6. The absorption bands are significantly shifted to longer wavelengths relative to the values indicated by the arrows (Figure 4). A slight shift of the absorption bands to longer wavelengths was also observed for Film3. Evidently, pre-saturation with AA before the adsorption of the dyes suppresses aggregation in Film3 similarly to colloids.

Figure 3. Absorption spectra of the films with one and two dyes. The arrows show the peak positions of the respective dyes in solution (values taken from Table 1)



FilmDye1

0.3

FilmDye2

Dye3

Dye2

FilmDye3

Dye4

Dye1

FilmDye4

A

0.2

0.1

0.0 500

600

λ / nm

0.6

FilmDye5 Dye6

FilmDye6

668

Dye5

FilmDye12

621

0.4

Dye1

A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

644

Dye2 600

0.2

0.0 400

576

500

617

600

700

λ / nm


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Figure 4. Absorption spectra of films containing six dyes. The assignment of the arrows is the same as in Figure 3.

0.12

Film1 Film3

622 553 669

0.08

A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.04 Dye1-Dye4

0.00 400

500

Dye5

600

Dye6

700

800

λ / nm

Spectral anisotropy of hybrid films. H-aggregates are photo-inactive and non-fluorescent structures, and their presence leads to efficient fluorescence quenching. Linearly-polarized spectroscopy has a high sensitivity for detecting H-aggregates in anisotropic hybrid films based on layered silicates.40-41 The transition dipole moments of rhodamine dye molecules within H-aggregates are oriented nearly perpendicularly relative to the silicate surface. As a result, these aggregates are not detectable when the film surface is perpendicular to the incident light. H-aggregates can, however, be detected by a dichroism induced by the variation of light polarity and film orientation.41-44 As shown below, the formation of H-aggregates occurred mainly in Film3: Absorption at lower wavelengths increased when H-polarized light was used and the film was rotated by 60° around the vertical spatial axis (Figure 5). The rotation selectively enhanced light absorption by the fraction of the dyes with their transition moment oriented perpendicularly to the plane of the film. A molecular 17 ACS Paragon Plus Environment


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orientation angle of the dyes was estimated using eq. (1) and expressed as 90°-γ.41 The species absorbing at lower wavelengths and exhibiting a positive dichroism are most likely the H-aggregates of Dye1 and/or Dye2 (absorption at < 550 nm) contributing to the slightly higher value of the average molecular orientation angle of approximately 40° (Figure SI3 in the Supporting Information). An opposite trend was observed for the dyes absorbing at longer wavelengths, which are characterized by transition moments more parallel to the film plane (25-30°) (Figure SI3). The presence of H-aggregates was not detected in Film1. Polarized absorption spectroscopy proved the presence of H-aggregates of Dye1 and Dye2 in Film3. These aggregates absorb light at higher energies than the corresponding monomers, and their presence does not affect FRET from the monomers of Dye1 and Dye2 to acceptor molecules, Dye5 and Dye6, which absorb light at longer wavelengths. However, aggregation decreases the contribution of the fluorescent monomers of Dye1 and Dye2, and as a result, it leads to lower fluorescence intensities of the EAs observed for Film3 than Film1 (see discussion below). Interestingly, polarized spectra did not indicate the presence of H-aggregates of Dye5 and Dye6 in any film. This observation confirms the identification of the molecular assemblies of Dye5 and Dye6 to be rather oblique J-type aggregates. Fluorescence spectroscopy results support this assumption (see discussion below).

Figure 5. Linearly-polarized absorption spectra of Film1 and Film3 recorded using horizontally- (H) and vertically-polarized (V) light. The films were rotated along a vertical axis to a 60° angle.


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0.08 V H 0.06

A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Film1

α = 60o

0.04

0.02

0.00 450

500

550

600

650

700

λ / nm

0.10

Film3

α = 60° 0.08

A

H V

547 554

0.06 0.04 0.02 0.00 450

500

550

600

650

700

λ / nm

Fluorescence spectra of hybrid films. The overlap integral expresses the degree of spectral overlap between the absorption of EA and the emission of ED, and is directly proportional to the Förster’s distance R0, representing conditions for 50 % FRET efficiency. A large overlap integral is one of most relevant conditions for efficient FRET to occur. Relative overlap integrals Jrel can be used for comparing FRET efficiencies of ED/EA pairs in the films (Table 3). 19 ACS Paragon Plus Environment


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In order to transfer absorbed energy to the end-member, Dye6, the most effective way is by FRET from Dye5 (Jrel = 0.67). The other dyes have significantly less efficient FRET to Dye6 (Jrel=0.26-0.12). Small overlaps between Dye5 and EDs Dye1-Dye4 suggest low efficiencies of FRET to Dye5. In this case, the highest overlap is for the Dye4/Dye5 pair (Jrel = 0.48). Relatively high FRET efficiency can be expected for the pair of Dye4 with any of the relevant EDs, Dye1-Dye3 (Jrel = 0.68-0.89). The similar spectral properties of Dye1 and Dye2 are reflected in their relatively high Jrel and high FRET efficiencies from these dyes to Dye3. Summing up, FRET in a multi-step pathway would be the most efficient mechanism for relaxing the excited states of EDs. The presence of dyes with varying spectral properties enables much more efficient FRET than systems limited to the interaction of two dyes with a low spectral overlap.

Table 3. Relative overlap integrals, Jrel, between the couples of energy donors (left column) and energy acceptors (top line). ED / EA

Dye2

Dye3

Dye4

Dye5

Dye6

Dye1

0.86

0.63

0.68

0.26

0.12

0.63

0.81

0.32

0.15

0.89

0.42

0.21

0.48

0.26

Dye2 Dye3 Dye4 Dye5

0.67

Emission spectra of hybrid films. The emission spectrum of Film1 was compared with the spectra of FilmDye1-FilmDye6 containing single dyes of the same concentration as in Film1 (Figure 6). The excitation wavelength of 480 nm enabled the efficient excitation of Dye1 (Table 1), which resulted 20 ACS Paragon Plus Environment

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in the strong emission of FilmDye1. The emission intensity was approximately three times higher than that of Dye1 from Film1 (Figure 6). This florescence quenching and the appearance of new emission bands at 625 and 670 nm demonstrate FRET occurring in Film1. The high efficiency of FRET follows from the comparison of Film1, FilmDye5 and FilmDye6 emission spectra. The excitation of Dye6 in FilmDye6 using the excitation wavelength of 480 nm was inefficient and led to negligible emission. A similar trend was observed in FilmDye5, indicating that in Film1 Dye5 and Dye6 are energy acceptors. The emission intensity of Dye6 in Film1 was higher than that of Dye5 (Figure 6), although Dye6 has a much lower φfl than Dye5 (Table 1). This indicates efficient transfer of energy from Dye5 to Dye6 in the film. The contribution of J-aggregates to this process is rather unclear. The emission intensities of Dye2 and Dye3 were also quenched in Film1 when compared with the intensities of FilmDye2 FilmDye3 (Figure 6). This clearly indicates that Dye2 and Dye3 act dominantly as EDs. Dye4 evidently acts as an intermediate in the overall FRET process: It may function both as an ED and EA. Evidently, the emission spectra of Film1 directly proved efficient multi-step FRET, although not all of the excitation energy is quantitatively transferred to the last member of multi-step FRET, i.e., Dye6.

Figure 6. Emission spectra of multicomponent Film1 and FilmDye1-FilmDye6, containing single dye components.



530

λex= 480 nm

IF / a.u.

Film1 FilmDye1 FilmDye6 FilmDye5

675 558 625

500

600

700

λ / nm

λex= 480 nm

FilmDye2 FilmDye3 FilmDye4 Film1

IF / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500

600

700

λ / nm

Excitation spectra of hybrid films. The excitation spectra of Film1, FilmDye4, FilmDye5 and FilmDye6 are shown in Figure 7. The spectra reflect the fact that the films exhibit emission at 710 nm after excitation at various wavelengths. As expected, Dye6 in FilmDye6 exhibited the strongest fluorescence emission. This spectrum reproduces the corresponding absorption spectrum well (Figure 22 ACS Paragon Plus Environment

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3). The bands at approximately 666 and 610 nm confirm the presence of J-aggregates with an oblique structure (Figure 7). The same spectral features of J-type assemblies of this dye were observed recently.39 A shoulder at approximately 640 nm can be assigned to the coexisting monomer (Table 1). The excitation spectra of FilmDye5 and FilmDye4 also copy the absorption spectra of the corresponding dyes; however, their intensities are much lower, since their emission bands are at lower wavelengths than Dye6. The emission spectra of FilmDye1 - FilmDye3 were hardly detectable (Figure SI4).

Figure 7. Excitation spectra of selected films measured at 710 nm. F1, FD4-FD6 denote Film1, FilmDye4-FilmDye6.

666

λem= 710 nm

FD6

620

IF / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


550

F1

FD5 FD4

450

500

550

600

650

700

λ / nm

Film1 exhibited a high intensity of 710 nm emission after excitation in the measured visible region (Figure 7). Its spectral features were identical to the absorption spectrum (Figure 4) and can be explained in terms of FRET starting from directly excited dye molecules in the series Dye1-Dye223 ACS Paragon Plus Environment


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Dye3-Dye4 to the EAs, Dye5 and predominantly Dye6. The films with a single dye did not exhibit such features. Comparison of the multicomponent films. The emission spectra of multicomponent Film1Film3 are shown in Figure 8. Film1 exhibited the highest fluorescence intensity. This result is consistent with the finding that all dyes do not form H-aggregates in this film (see above). The low charge density and the charge location in the tetrahedral sheets result in relatively strong electrostatic interactions which may reduce the mobility of adsorbed dyes, thus preventing their association and the formation of H-aggregates. In contrast, Film 3 with a similar absorption spectrum (Figure 4) exhibited reduced emission intensity and less effective FRET. The only difference is the presence of Haggregates in Film3, as follows from the analyses of linearly-polarized absorption spectra (Figure 5). It is known that the formation of H-aggregates is connected with fluorescence quenching. Evidently, the quenching is the main reason for low FRET efficiency in this film. Thus, the presence of AA cations supports molecular aggregation in films, but has the opposite effect in colloids. Film2 exhibited the lowest fluorescence and the lowest FRET efficiency. The results provide evidence for the influence of dye concentration, i.e., the effect of intermolecular distances among dye molecules. Film1 and Film2 were prepared via the same procedure, but Film2 contained a tenfold lower concentration of the dyes. Although the emission from Dye1 and Dye2 embedded in Film2 achieved relatively high fluorescence intensities (half of those of Film1), the emission from the EAs, Dye5 and Dye6, was hardly detectable. The distance between the dye cations in Film2 was evidently much larger than the Förster radius. Excitation spectra (SI5) supported conclusions based on emission spectra. Fluorescence anisotropy of the films (Figure 9) was in accordance with the trends found for emission spectra (Figure 8). The spectra were measured using a tilt angle of 45°. For both samples, there were relatively high anisotropy values observed for the range of emission wavelengths < 530 nm 24 ACS Paragon Plus Environment

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(r = 0.3-0.4), mainly assigned to the emission from Dye1 and Dye2 (Figure 9). Fluorescence anisotropy decreased with increasing wavelength, which can be explained by FRET. The effect of light depolarisation was more remarkable for Film1. Film2, with lower dye concentration, also maintained a relatively high anisotropy at wavelengths > 625 nm (~0.2). This difference proves that the depolarization that occurred in Film1 was due to FRET.

Figure 8. Emission spectra of multicomponent films Film1-Film3.

λex= 480 nm Film1 Film2 Film3

IF / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

550

600

650

700

750

λ / nm

Figure 9. Fluorescence anisotropy of Film1 and Film2.



0.5 Film1 Film2

0.4 0.3

r

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 38

0.2 0.1 0.0 520

550

580

610

640

λ / nm

Time-resolved fluorescence spectroscopy. In order to confirm the occurrence of FRET in the transparent saponite films, the time-resolved fluorescence behavior of pertinent dyes was investigated in detail (FilmDye1-FilmDye6). In all cases, the fluorescence decays were best analyzed by two exponential functions. As follows from Table 4, all the dyes behave similarly, with an average fluorescence lifetime of approximately 3 ns. The fluorescence emission of Dye6 in FilmDye6 is characterized by nearly monoexponential decay kinetics, which confirms the presence of one type of emissive species. The absorption spectrum of FilmDye6 (Figure 3) indicates that these species are photoactive J-aggregates. The other films have a more balanced contribution of two lifetimes, which can be attributed to the effects of saponite-dye interactions. The parameters of the fluorescence decays of FilmDye1 and FilmDye2 are very similar to those of FilmDye12, and therefore this film does not enable FRET analysis. In contrast, the analysis of the fluorescence decays of Film1, Film2 and Film3 confirms FRET in the cascade of dyes used. The excitation wavelengths of 442 or 494 nm are predominantly absorbed by Dye1, as follows 26 ACS Paragon Plus Environment

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from the absorption spectra (Figure 3). The fluorescence decay curves recorded near the maxima of the fluorescence bands of the respective dyes are shown in Figure 10, and the corresponding fluorescence lifetimes are given in Table 4. In accordance with the results of the fluorescence emission spectroscopy, FRET is indicated by the behavior of the ending energy acceptor, Dye6 (Figure 10, SI6-7). The same lifetime of Dye6 is observed in FilmDye6, and also in the mixture of all dyes in Film1-Film3 whether Dye1 or Dye6 were predominantly excited. Clearly, there are no quenching interactions of Dye6 with other dyes. The difference between the behavior of FilmDye6 and Film1-Film3 lies in the appearance of a rise component of Dye6 with a lifetime of about 1.0 ns. This component is not present in the spectrum of FilmDye6 when only Dye6 is excited (Figure 10, curve d). The contribution of this rising component is reflected by a negative amplitude value (Table 4). After FRET from Dye1 to Dye6 occurring with a rate constant of 1×109 s-1, the fluorescence emission of Dye6 in complex films decays with the same kinetics as Dye6 in FilmDye6 (Figure SI6-7). The average lifetimes of Dye4 and Dye5 in Film1 (Table 4) are reduced from 3.3 to 1.5 ns (Figure SI8) and from 3.0 to 1.8 ns (Figure 11), respectively. This indicates that the FRET process is occurring from Dye4 to further members of the series and from Dye5 to Dye6. The results observed for Film1 reflect the roles of dye molecules in FRET. Energy donors Dye1 to Dye3 were efficiently quenched, transferring their excitation energy to any of the EAs. Dye4 exhibited intermediate properties, playing the role of both ED and EA. It was excited via FRET from Dye1-Dye3, but was also efficiently quenched by both Dye5 and Dye6.

Table 4. Fluorescence lifetimes of dyes in saponite transparent films.

Film

λexc, λem

τ1, τ2

τa Comment 27

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(nm)

(ns)

(ns)

Page 28 of 38

FilmDye1

494, 580

2.4 (83%), 5.7 (17%)

3.0

-

FilmDye2

494, 580

2.3 (84%), 5.4 (16%)

2.8

-

FilmDye3

494, 580

2.1 (48%), 3.3 (42%)

2.4

-

FilmDye4

494, 580

2.6 (73%), 5.3 (27%)

3.3

-

FilmDye5

494, 625

1.4 (9%), 3.2 (91%)

3.0

-

FilmDye6

494, 675

1.4 (1%), 3.2 (99%)

3.2

-

FilmDye12

494, 580

2.3 (87%), 6.3 (13%)

2.8

-

494, 625

2.4 (87%), 7.0 (13%)

3.0

-

494, 580

1.2 (81%), 2.9 (19%)

1.5

exc. Dye1, em. Dye4

494, 625

1.1 (48%), 2.5 (52%)

1.8

exc. Dye1, em. Dye5

494, 675

1.0(-15%), 3.3(115%)

3.3

exc. Dye1, em. Dye6

494, 710

1.0(-15%), 3.3(115%)

3.3

exc. Dye1, em. Dye6

442, 675

0.9(-11%), 3.3(111%)

3.3

exc. Dye1, em. Dye6

667, 710

3.1 (100%)

3.1

exc. Dye6, em. Dye6

Film1

The average fluorescence lifetime was calculated using eq. (5); τ1, τ2 represent decay times from fits of a two-component exponential decay. αi representing the fractional intensity of each decay time (eq. (5)) is given in parentheses in %. λexc is the excitation wavelength, λem is the wavelength of measured fluorescence emission, exc. means predominantly excited dye, and em. means the emission of a given dye. The rising component has a negative value of αi.

Figure 10. Fluorescence decay curves of Film1 excited at 494 nm and recorded at 580 nm (Emission of Dye4) (a), recorded at 625 nm (Emission of Dye5) (b), recorded at 710 nm (Emission of Dye6) (c), excited at 667 nm and recorded at 710 nm (Excitation of Dye6, Emission of Dye6) (d), and the corresponding laser diode profiles (e – 494 nm, f – 667 nm).


Page 29 of 38

10000

I / counts

1000 f e

a

100

10

b

d

60

c

80 t / ns

100

Figure 11. Fluorescence decay curves of FilmDye5 (a), Film1 (b), and the corresponding laser diode profile (c). The films were excited at 494 nm and the emission of Dye 5 was recorded at 625 nm. 10000

I / counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

c

b

a

100

10

60

80 t / ns


100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Resonance energy transfer. Selective excitation of the respective dyes enabled the occurrence of FRET in multicomponent systems to be proved and qualitatively described based on six laser dyes adsorbed onto saponite nanoparticles. Both emission and excitation fluorescence spectra proved the presence of multi-step energy transfer (Figures 6, 7). Scheme 1 shows possible relaxation pathways in Film1-Film3, containing the mixture of dyes in a series from Dye1 to Dye6. Besides fluorescence and non-radiative internal conversion in the respective monomeric dyes, other relaxation pathways include quenching within H-aggregates and FRET. In general, the quenching by forming H-aggregates reduces the fluorescence quantum yields of co-existing photoactive species.

Scheme 1. Possible relaxation pathways in dye multi-component systems. A: Consecutive FRET from Dye1 via all members of a series to Dye6. B, C: Absorbed energy by Dye1 and Dye 4 is transferred to other dyes. B: Excitation energy from Dye1 can be transferred to all dyes, but the probability of the energy transfer to Dye2 is highest. C: Energy from Dye4 can be transferred only to Dye5 and Dye6. Finally the energy is accumulated in Dye6 (the most probable scenario).


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FRET can proceed under specific conditions, including large spectral overlap (eq. 3, Table 3), suitable molecular orientation and short distances between interacting molecules. The spectral properties of Dye1 to Dye6 were chosen to uninterruptedly cover the spectral region from 500-750 nm and create the cascade for multistep FRET starting from Dye1 and leading to Dye6. Using the simplest approach, FRET from Dye1 to Dye6 would represent five consecutive steps, where the excitation energy is transferred in the cascade Dye1-Dye2-Dye3-Dye4-Dye5-Dye6 (Scheme 1). The presence of H- or J-aggregates makes the relaxation pathways more complex. (The mechanism of FRET based on electronic dipole-dipole coupling makes the process much faster than a radiative relaxation.) The overall rate of the consecutive processes is determined by the rate of the slowest process, which represents the fluorescence relaxation of Dye6 (τ ~ 3 ns). Therefore, energy is accumulated in Dye6 by being transferred to its excited state. Since Dye6 does not further act in rapid FRET processes, Dye6 31 ACS Paragon Plus Environment


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relaxes to its ground state via non-radiative internal conversion and fluorescence. The effect of intermolecular distance (i.e. dye concentration) on the efficiency of FRET follows from a comparison of Film1 and Film2 (Figure 8, 9). Considering solid saponite with an apparent density of approximately 2.8 g cm-3 (ref.45) and the dyes/Sap ratio, the average intermolecular distances of the respective molecules are approximately 5 and 10 nm in Film1 and Film2, respectively. These estimations do not reflect the heterogeneity in a given dye’s distribution and do not consider molecular aggregation and saponite swelling. The intermolecular distance for Film1 is very close to common Förster radii.46 On the other hand, Film2 with less concentrated fluorophores exhibited an intermolecular distance which is inefficient for FRET. An even more remarkable effect of dye concentration was described for colloids and a solution containing the mixture of dyes (Figure 2). FRET is a partially anisotropic phenomenon, and proceeds preferentially between coupled dipoles with parallel orientation. Some preferential orientation of dye molecules with respect to the silicate surface plane was observed as negative, as well as positive dichroism by linearly-polarized absorption spectroscopy (Figure 5, SI3). However, the films based on oriented two-dimensional Sap particles do not represent perfectly anisotropic systems. The orientation of the dye molecules with respect to the two spatial axes in the plane of the Sap surface is random, and, therefore, perfectly anisotropic energy transfer could not be expected. Similar to isotropic systems, FRET in the films led to light depolarization, as was observed by polarized fluorescence (Figure 9). The extent of light depolarization with increasing wavelength of emitted light was in agreement with the FRET efficiency, as qualitatively proven by the comparison between Film1 and Film2. Since FRET in Film1 containing six dyes is a multistep process (Scheme 1), the analysis of FRET efficiencies can be performed under specific prerequisites (neglecting the influence of molecular aggregation, considering only direct activation of EDs). The decomposition of complex emission spectrum of Film1 allowed for identifying the contributions of the respective dye spectra. The results 32 ACS Paragon Plus Environment

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and statistical data of the regression are shown in Supporting Information (Figure SI9 in the Supporting Information). The analysis indicated FRET efficiency above 80 % for Dye1, Dye2, Dye3 and Dye4 (Table 5). These values were expressed with respect to the emission from the films containing single dye components (FilmDye1-FilmDye6) and therefore, represent the fraction of the energy transferred from directly excited Dye1-Dye4 molecules. Time-resolved fluorescence spectroscopy corroborated the results of steady-state fluorescence spectroscopy that the FRET process occurs from Dye1 to further members of the series (Table 4). Estimation of energy transfer efficiencies from Dye4 or Dye5 can be made using the following equation:

φfl = 1 - τDA/τD,

(6)

where τDA and τD are the average lifetimes of Dye4 or Dye5 in the presence and absence of the following dyes in a series (Table 5). The FRET efficiencies from Dye4 or Dye5 were approximately 55 and 40%, respectively. Both dyes played a role of both ED and EA. Thus, Dye4 was excited via FRET from Dye1-Dye3 and was efficiently quenched by both Dye5 and Dye6. The FRET efficiency from Dye4 is significantly smaller than that obtained from steady-state experiment (E = 86 %). This discrepancy can be explained by a contribution of preceding dyes to the fluorescence emission due to spectral overlaps.

Table 5 FRET efficiencies from energy donors to subsequent members of the series.

Dye

E (%) (SS)

E (%) (TRS)

Dye1

85

-

Dye2

83



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Dye3

85

-

Dye4

86

55

Dye5

-

40

Page 34 of 38

(SS), (TRS) - values determined from steady-state or time-resolved spectroscopy measurements, respectively.

CONCLUSIONS Saponite with a low layer charge density represents a suitable inorganic component for the design of hybrid materials preserving the photoactivity and fluorescence of the intercalated dyes. Dye molecular aggregation occurring in colloidal dispersions was efficiently suppressed by the coadsorption of alkylammonium cations. On the other hand, the hybrid films prepared by deposition from the colloids exhibited optimal properties without co-adsorbed alkylammonium cations. The concentration of the dye molecules in the films can be easily controlled by adjusting the ratio between the components in the colloid. The photoactive forms such as monomers and J-aggregates were dominant in the prepared films. Although the loading of saponite amounted to less than 0.6 % of the cation-exchange capacity, the dye concentration was sufficient for efficient FRET to take place. The utilization of a tenfold lower dye concentration led to a significant decrease in FRET. In summary, the hybrid films of saponite with the mixture of dyes represent suitable systems for the transfer of light energy.

ASSOCIATED CONTENT Supporting Information. Fluorescence spectra of aqueous solution and colloid containing the mixture of dyes measured in synchronous mode, X-ray diffraction patterns of selected films, average 34 ACS Paragon Plus Environment

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orientation angles calculated from measurements based on linearly-polarized absorption spectroscopy, fluorescence excitation spectra, decay curves of selected films and deconvolution of fluorescence spectrum of Film1. The Supporting Information is available free of charge on the ACS Publications website.

ACKNOWLEDGEMENTS This work was supported by the Slovak Research and Development Agency under the contract No. APVV-0291-11 and the Grant Agency VEGA (2/0107/13, 1/0943/13).

REFERENCES 1. Sanchez, C.; Belleville, P.; Popall, M.; Nicole, L. Applications of Advanced Hybrid OrganicInorganic Nanomaterials: From Laboratory to Market. Chem. Soc. Rev. 2011, 40, 696-753. 2. Shibata, S.; Yano, T.; Segawa, H. Organic-Inorganic Hybrid Materials for Photonic Applications. IEEE J. Sel. Top. Quantum Electron. 2008, 14, 1361-1369. 3. Zhou, C. H.; Shen, Z. F.; Liu, L. H.; Liu, S. M. Preparation and Functionality of Clay-Containing Films. J. Mater. Chem. 2011, 21, 15132-15153. 4. Innocenzi, P.; Lebeau, B. Organic-Inorganic Hybrid Materials for Non-Linear Optics. J. Mater. Chem. 2005, 15, 3821-3831. 5. Felheck, T.; Hoffmann, K.; Lezhnina, M. M.; Kynast, U. H.; Resch-Genger, U. Fluorescent Nanoclays: Covalent Functionalization with Amine Reactive Dyes from Different Fluorophore Classes and Surface Group Quantification. J. Phys. Chem. C 2015, 119, 12978-12987. 6. Bujdák, J. Effect of the Layer Charge of Clay Minerals on Optical Properties of Organic Dyes. A Review. Appl. Clay Sci. 2006, 34, 58-73. 7. Iwasaki, M.; Kita, M.; Ito, K.; Kohno, A.; Fukunishi, K. Intercalation Characteristics of 1,1 'Diethyl-2,2 '-Cyanine and Other Cationic Dyes in Synthetic Saponite: Orientation in the Interlayer. Clays Clay Miner. 2000, 48, 392-399. 8. Takagi, S.; Eguchi, M.; Shimada, T.; Hamatani, S.; Inoue, H. Energy Transfer Reaction of Cationic Porphyrin Complexes on the Clay Surface: Effect of Sample Preparation Method. Res. Chem. Intermed. 2007, 33, 177-189. 9. Bujdák, J.; Czímerová, A.; Arbeloa, F. L. Two-Step Resonance Energy Transfer between Dyes in Layered Silicate Films. J. Colloid Interface Sci. 2011, 364, 497-504. 10. Dey, D.; Bhattacharjee, D.; Chakraborty, S.; Hussain, S. A. Effect of Nanoclay Laponite and Ph on the Energy Transfer between Fluorescent Dyes. J. Photochem. Photobiol. A 2013, 252, 174-182. 11. Li, H. R.; Devaux, A.; Ruiz, A. Z.; Calzaferri, G. Electronic Excitation Energy Transfer from DyeLoaded Zeolite L Monolayers to a Semiconductor - Art. No. 61951g. Nanophotonics 2006, 6195, G1951-G1951. 12. Siegers, C.; Wurfel, U.; Zistler, M.; Gores, H.; Hohl-Ebinger, J.; Hinsch, A.; Haag, R. Overcoming Kinetic Limitations of Electron Injection in the Dye Solar Cell Via Coadsorption and Fret. 35 ACS Paragon Plus Environment


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ChemPhysChem 2008, 9, 793-798. 13. Sapsford, K. E.; Berti, L.; Medintz, I. L. Fluorescence Resonance Energy Transfer - Concepts, Applications and Advances. Minerva Biotecnologica 2004, 16, 247-273. 14. Singh, H.; Bagchi, B. Non-Forster Distance and Orientation Dependence of Energy Transfer and Applications of Fluorescence Resonance Energy Transfer to Polymers and Nanoparticles: How Accurate Is the Spectroscopic Ruler with 1/R-6 Rule? Curr. Sci. 2005, 89, 1710-1719. 15. Liu, L. Z.; Liu, Z. H.; He, Z. K.; Cai, R. X. Quantum Dots: The New Development of Fret. Prog. Chem. 2006, 18, 337-343. 16. Peralta, S.; Habib-Jiwan, J. L.; Jonas, A. M. Ordered Polyelectrolyte Multilayers: Unidirectional Fret Cascade in Nanocompartmentalized Polyelectrolyte Multilayers. ChemPhysChem 2009, 10, 137143. 17. Bergmann, K.; O'Konski, C. T. A Spectroscopic Study of Methylene Blue Monomer, Dimer, and Complexes with Montmorillonite. J. Phys. Chem. 1963, 67, 2169-2177. 18. Bujdák, J.; Komadel, P. Interaction of Methylene Blue with Reduced Charge Montmorillonite. J. Phys. Chem. B 1997, 101, 9065-9068. 19. Bujdák, J.; Iyi, N.; Sasai, R. Spectral Properties, Formation of Dye Molecular Aggregates, and Reactions in Rhodamine 6g/Layered Silicate Dispersions. J. Phys. Chem. B 2004, 108, 4470-4477. 20. Das, S.; Kamat, P. V. Can H-Aggregates Serve as Eight-Harvesting Antennae? Triplet-Triplet Energy Transfer between Excited Aggregates and Monomer Thionine in Aersol-Ot Solutions. J. Phys. Chem. B 1999, 103, 209-215. 21. Sasai, R.; Iyi, N.; Fujita, T. H.; Takagi, K.; Itoh, H. Synthesis of Rhodamine 6g/Cationic Surfactant/Clay Hybrid Materials and Its Luminescent Characterization. Chem. Lett. 2003, 32, 550551. 22. Sasai, R.; Iyi, N.; Fujita, T.; Arbeloa, F. L.; Martinez, V. M.; Takagi, K.; Itoh, H. Luminescence Properties of Rhodamine 6g Intercalated in Surfactant/Clay Hybrid Thin Solid Films. Langmuir 2004, 20, 4715-4719. 23. Salleres, S.; Arbeloa, F. L.; Martinez, V.; Arbeloa, T.; Arbeloa, I. L. Photophysics of Rhodamine 6g Laser Dye in Ordered Surfactant (C12tma)/Clay (Laponite) Hybrid Films. J. Phys. Chem. C 2009, 113, 965-970. 24. He, H.; Li, T.; Tao, Q.; Chen, T.; Zhang, D.; Zhu, J.; Yuan, P.; Zhu, R. Aluminum Ion Occupancy in the Structure of Synthetic Saponites: Effect on Crystallinity. Am. Mineral. 2014, 99, 109-116. 25. Ogawa, M.; Sohmiya, M.; Watase, Y. Stabilization of Photosensitizing Dyes by Complexation with Clay. Chem. Commun. 2011, 47, 8602-8604. 26. Scott, P. Photochemcad Spectra (by Lindsey S. Jonathan). 27. Kubin, R. F.; Fletcher, A. N. Fluorescence Quantum Yields of Some Rhodamine Dyes. J. Lumin. 1982, 27, 455-462. 28. Arik, M.; Onganer, Y. Molecular Excitons of Pyronin B and Pyronin Y in Colloidal Silica Suspension. Chem. Phys. Lett. 2003, 375, 126-133. 29. Çelebi, N.; Arik, M.; Onganer, Y. Analysis of Fluorescence Quenching of Pyronin B and Pyronin Y by Molecular Oxygen in Aqueous Solution. J. Lumin. 2007, 126, 103-108. 30. López Arbeloa, F.; López Arbeloa, T.; Tapia Estévez, M. J.; López Arbeloa, I. Photophysics of Rhodamines. Molecular Structure and Solvent Effects. J. Phys. Chem. 1991, 95, 2203-2208. 31. Brackmann, U., Lambdachrome Laser Dyes. 3rd ed.; Lambda Physik GmbH: D-37079 Goettingen, · Germany, 1986. 32. Rurack, K.; Spieles, M. Fluorescence Quantum Yields of a Series of Red and near-Infrared Dyes Emitting at 600-1000 Nm. Anal. Chem. 2011, 83, 1232-1242. 33. Sillen, A.; Engelborghs, Y. The Correct Use of "Average" Fluorescence Parameters. Photochem. Photobiol. 1998, 67, 475-486. 36 ACS Paragon Plus Environment

Page 37 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


34. Laird, D. A., Interactions between Atrazine and Smectite Surfaces. In Herbicide Metabolites in Surface Water and Groundwater, Meyer, M. T.; Thurman, E. M., Eds. Amer Chemical Soc: Washington, 1996; Vol. 630, pp 86-100. 35. Bujdák, J.; Iyi, N. Molecular Aggregation of Rhodamine Dyes in Dispersions of Layered Silicates: Influence of Dye Molecular Structure and Silicate Properties. J. Phys. Chem. B 2006, 110, 2180-2186. 36. Bujdák, J.; Iyi, N. Spectral and Structural Characteristics of Oxazine 4/Hexadecyltrimethylammonium Montmorillonite Films. Chem. Mater. 2006, 18, 2618-2624. 37. Konno, S.; Fujimura, T.; Otani, Y.; Shimada, T.; Inoue, H.; Takagi, S. Microstructures of the Porphyrin/Viologen Mono Layer on the Clay Surface: Segregation or Integration? J. Phys. Chem. C 2014, 118, 20504-20510. 38. Ishida, Y.; Kulasekharan, R.; Shimada, T.; Ramamurthy, V.; Takagi, S. Supramolecular-Surface Photochemistry: Supramolecular Assembly Organized on a Clay Surface Facilitates Energy Transfer between an Encapsulated Donor and a Free Acceptor. J. Phys. Chem. C 2014, 118, 10198-10203. 39. Bujdák, J. Layer-by-Layer Assemblies Composed of Polycationic Electrolyte, Organic Dyes, and Layered Silicates. J. Phys. Chem. C 2014. 40. Bujdák, J.; Iyi, N.; Kaneko, Y.; Czímerová, A.; Sasai, R. Molecular Arrangement of Rhodamine 6g Cations in the Films of Layered Silicates: The Effect of the Layer Charge. Phys. Chem. Chem. Phys. 2003, 5, 4680-4685. 41. Bujdák, J.; Iyi, N. Molecular Orientation of Rhodamine Dyes on Surfaces of Layered Silicates. J. Phys. Chem. B 2005, 109, 4608-4615. 42. Sasai, R.; Fujita, T.; Iyi, N.; Itoh, H.; Takagi, K. Aggregated Structures of Rhodamine 6g Intercalated in a Fluor-Taeniolite Thin Film. Langmuir 2002, 18, 6578-6583. 43. Iyi, N.; Sasai, R.; Fujita, T.; Deguchi, T.; Sota, T.; Arbeloa, F. L.; Kitamura, K. Orientation and Aggregation of Cationic Laser Dyes in a Fluoromica: Polarized Spectrometry Studies. Appl. Clay Sci. 2002, 22, 125-136. 44. Salleres, S.; Arbeloa, F. L.; Martinez, V. M.; Arbeloa, T.; Arbeloa, I. L. On the Arrangements of R6g Molecules in Organophilic C12tma/Lap Clay Films for Low Dye Loadings. Langmuir 2010, 26, 930-937. 45. Post, J. L. Saponite from near Ballarat, California. Clays Clay Miner. 1984, 32, 147-153. 46. Muller, S. M.; Galliardt, H.; Schneider, J.; Barisas, B. G.; Seidel, T. Quantification of Forster Resonance Energy Transfer by Monitoring Sensitized Emission in Living Plant Cells. Front Plant Sci 2013, 4, 413.



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Hybrid Systems Based on Layered Silicate and ... - ACS Publications

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