Article pubs.acs.org/JPCC
One-Dimensional Antenna Systems by Crystallization Inclusion of Dyes (One-Pot Synthesis) within Zeolitic MgAPO-36 Nanochannels Raquel García,*,† Virginia Martínez-Martínez,*,‡ Rebeca Sola Llano,‡ Iñigo López-Arbeloa,‡ and Joaquín Pérez-Pariente† †
Instituto de Catálisis y Petroleoquímica (CSIC), C/Marie Curie 2, Cantoblanco, 28049 Madrid, Spain Departamento de Química Física, Universidad del País Vasco, UPV/EHU, Apartado 644, 48080 Bilbao, Spain
‡
S Supporting Information *
ABSTRACT: The nanopores of magnesium aluminophosphate MgAPO-36 (ATS structure) favor the formation of J-type aggregates in dyes with a general molecular skeleton of three fused aromatic rings. The particular distribution of these J-aggregate species and monomers, in a practically collinear disposition along the 1D-channels, allows an efficient transfer of electronic energy by dipole−dipole interaction. In order to achieve a material susceptible to be used as an artificial photonic antenna, covering a broader range of the UV−vis light spectrum and available by one-pot synthesis, the fluorescing dye acridine (AC) is occluded for the first time within MgAPO-36, both alone and with different ratios of the fluorescing dye Pyronine Y (PY). The one-dimensional energytransfer process between the different chromophores and between the monomers and their red-shifted J-aggregates is controlled by the total concentration of dyes and their relative AC:PY ratio in the matrix. In the optimized AC:PY concentrations, the wavelength range antenna action is extended from near-UV to about 700 nm, transporting the light energy through the whole visible spectrum (blue to green to red) and spatially, from end to the end of the crystals.
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INTRODUCTION Zeolitic materials found their main applications as catalysts, adsorbents, and ion exchangers.1 The framework of these materials is made by the spatial arrangement of corner sharing tetrahedra creating three-dimensional structures with channels and voids of molecular dimensions. The properties of these solids are related to their framework structure and to the variety of compositions that can be prepared by varying the isomorphic substitution of the elements in tetrahedral positions. Zeolitic materials are also finding increasing applications as host materials for certain functional molecules (homogeneous catalysts, fluorescing dyes, chemical sensors, etc.)2−5 which can result in certain practical advantages for the use of the guest molecules, as for example, it can ease their handling and/or provide them additional stability. One of the main characteristics of certain fluorescing dye molecules is their relatively high tendency to self-associate as their concentration is increased. Formation of these aggregated species causes a decrease in the fluorescence capacity of the dye.5−10 Particularly, sandwich-like or H-type aggregates are not desirable because they are not emissive. Instead, they are effective quenchers of the monomer fluorescence. However, in the cases in which molecular association cannot be suppressed in the solid state, the so-called J-type dimers are preferred, since they are potentially fluorescent with emission bands at longer wavelengths compared to the monomer.11 The encapsulation of fluorescing dyes within a zeolitic matrix may not only decrease the formation of aggregates but it can also provide an additional supramolecular ordering to the guest molecules © 2013 American Chemical Society
organizing them in specific patterns. This can provide the composite material with very interesting physical properties for nonlinear optics applications. Materials with challenging properties such as light harvesting, generation of the second harmonics, microlasers, and other optical applications12−16 have been obtained in this way. Among these applications, the possibility of building materials that mimic natural photosynthesis is particularly interesting.17,18 The group of G. Calzaferri has designed a series of materials (ZEOFRET) based on highly organized dye−zeolite L composites with photonic antenna function to be used for solar energy conversion devices or as fluorescent concentrators.19−21 Generally, such photonic artificial antenna systems are based on an energy-transfer process (FRET = Förster resonance energy transfer) between different chromophores incorporated within ordered nanoporous materials, where chromophores are close enough for enabling coupling of their electronic transition dipole moments; excitation of the donor molecules can be transferred to the acceptor molecules by means of Förster-type energy transfer (FRET). FRET represents a mechanism for the harvesting and transportation of light, and therefore, it is of importance to control the energy-transfer path at a nanoscale level.22,23 In our previous work,24 we have demonstrated that Pyronine Y dye, characterized by a bright pink color and green fluorescence emission, was incorporated within MgAPO-36 Received: September 6, 2013 Revised: October 10, 2013 Published: October 17, 2013 24063
dx.doi.org/10.1021/jp408939r | J. Phys. Chem. C 2013, 117, 24063−24070
The Journal of Physical Chemistry C
Article
(ATS structure type) at high concentrations forming Jaggregated species. MgAPO-36 is a magnesium aluminophosphate material with a one-dimensional 12-member ring channel system with oval pores of dimensions 6.7 × 7.5 Å.25 PY/ATS hybrid system has offered crystals with an organized multicolored emission being the energy of light transported from green in one end of the crystal, indicative of the predominance of the monomeric units of PY, to the red at the other end where J-aggregates were located.24 This material is interesting for one-directional antenna function as the excitation energy can be transported in one direction (from end to end) along the crystal: blue light excites PY monomers that act as energytransfer donors to the PY aggregates which behave as acceptor at the other end of the crystal needle emitting red light. The one-dimensional energy-transfer process between the monomers and their red-shifted J-aggregates (Homo-FRET) is nearly complete thanks to the practically collinear disposition of molecules adopted along the 1D-channel, as this is the optimal for dipole−dipole interactions. In this work, we have explored the use of an additional chromophore together with PY dye to broaden the range of light harvesting and transporting. In this sense, the dye acridine (AC), with a closely related molecular structure but with a characteristic UV absorption and blue emission which overlaps with the absorption of PY, is a good candidate to extend the antenna system action to the UV range.
Fluorescence single-particle measurements were performed in a time-resolved fluorescence confocal microscope (model Micro Time 200, PicoQuant). The excitation was performed at 410 and 470 nm with picosecond pulsed diode laser with 100 ps pulses at 5 MHz repetition rate. The fluorescence signal was collected by the same objective and focused (via a 50 μm pinhole) onto avalanche photodiode detectors (Micro-PhotonDevices MPD-APD). For polarization measurements, the emission signal collected was divided by a polarizer beam splitter into two mutually perpendicular polarization orientation beams, which are simultaneously detected by two detector channels. We analyzed the dichroic ratio (D = I∥/I⊥), defined as the relation between the emission intensity counts collected for two perpendicularly polarized radiations (parallel to the main c-axis of the crystal over perpendicular to it). Single-particle fluorescence spectra were recorded by directing the emission beam to an exit port, where a spectrograph (model Shamrock 300 mm) coupled to a CCD camera (Newton EMCCD 1600 × 200, Andor) was mounted. The absorption spectra of the dye-MgAPO samples were registered in powder samples in a Varian spectrophotometer (model Cary 4E) detecting the reflected light, by means of integrating sphere. The respective spectra of MgAlPO crystals synthesized in identical conditions but without dye were recorded and subtracted from the sample signal to eliminate the scattering contribution of particles to the absorption spectra. The excitation and emission spectra of the powder were recorded in a SPEX spectrofluorimether (model Fluorolog 322) in front-face configuration. Radiative decay curves were registered with the timecorrelated single-photon-counting technique (Edinburgh Instruments, model FL920) using a microchannel plate detector (Hamamatsu C4878), with picosecond time resolution (∼20 ps). Fluorescence emission was monitored at the maximum emission wavelength after excitation at 370 nm by means of a diode laser (PicoQuant, model LDH) with 150 ps fwhm pulses. The decay curves were adjusted normally to a sum of biexponential decays (i.e., as multiexponentials) by means of
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EXPERIMENTAL SECTION Synthesis of MgAPO Materials. The microporous magnesium aluminophosphates were prepared using phosphoric acid (Aldrich, 85 wt %), magnesium acetate tetrahydrate (99 wt %, Aldrich), aluminum hydroxide (Aldrich), tripropylamine (TPA, Aldrich), Pyronine Y chloride (PY) (>75% purity, Across Organics), and acridine (Scharlau, pure) from gels with molar composition of 0.2 MgO:1 P2O5:0.9 Al2O3:0.75 TPA:xPY:(0.024 − x) AC:300 H2O, where x was varied between 0 and 0.024 as given in Table 1. In a typical preparation, 1.61 g of phosphoric acid was mixed with 36.49 g of water and 0.30 g of magnesium acetate tetrahydrate. Then 0.99 g of aluminum hydroxide and 0.76 g of the SDA tripropylamine (TPA) were added over this mixture, together with 0.0462 g of Pyronine Y and 0.0026 g of acridine, to yield a gel of composition 0.2 MgO:1 P2O5:0.9 Al2O3:0.75 TPA:0.022 PY:0.002 AC:300 H2O. After stirring for 1 h, the resulting gel (pH ∼ 3.72) was introduced into 100 mL Teflonlined stainless steel autoclaves, which were heated statically at 180 °C under autogenous pressure for 12 h. The solid products were recovered by filtration, exhaustively washed with ethanol and water, and dried at room temperature overnight. Characterization. X-ray powder diffraction (XRD) patterns were collected with a Panalytical X’Pro diffractometer using Cu Kα radiation. The dye contents within the solid products were determined photometrically using a UV−vis Shimadzu spectrophotometer 2101/3101PC, after the composite material was dissolved in 5 M hydrochloric acid and by comparison with standard solutions prepared from known concentrations of the dyes employed in this work. Fluorescence images were recorded with an optical inverted microscope with epi configuration (Olympus BX51) equipped with a color CCD (DP72). Samples were excited with UV by Chroma band-pass filters (350/50) and emission was collected with a Chroma cutoff filter 400.
Ifl(t ) = A1 exp( −t /τ1) + A 2 exp(−t /τ2)
where Ai are the preexponential factors related to the statistical weights of each exponential and τi are the lifetimes of each exponential decay. The goodness of the deconvolution process was controlled by the chi-squared (χ2) and Durbin−Watson (DW) statistical parameters and the residual analysis.
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RESULTS AND DISCUSSION It has been previously shown that the composite material formed by the incorporation of Pyronine Y within MgAPO-36 by crystallization inclusion yielded a material with interesting properties for light transport.24 The incorporation of an additional cromophore within the MgAPO-36 channels, together with PY, was required in order to broaden the range of light harvesting and transportation of the system and to extend its antenna action to the UV range. The dye acridine (AC) was selected as a good candidate for this purpose due the following reasons: (a) its molecular structure is similar to Pyronine Y (three fused aromatic rings though with one nitrogen instead oxygen in the central ring but no lateral pendant groups, see Figure 1); (b) it presents a high solubility in the acidic medium of synthesis (pH ∼ 3−4); (c) it shows a relatively high fluorescence efficiency and relatively long 24064
dx.doi.org/10.1021/jp408939r | J. Phys. Chem. C 2013, 117, 24063−24070
The Journal of Physical Chemistry C
Article
Figure 1. Molecular structures of the dyes used in this work: protonated form of acridine (ACH+) and Pyronine Y (PY).
lifetimes (see photophysical characterization in aqueous solution in the Supporing Information (SI), Figure S1, Table S1); and (d) its blue emission band overlaps with the absorption spectra of PY dye (Figure S2 in the SI). The two first points are favorable properties for the incorporation of AC within MgAPO-36 channels and the two latter points are strategic factors to get an effective energy transfer between AC and PY for typical Förster distances (20−60 Å).26 For the aforementioned reasons, the dye AC was considered a good donor candidate for an efficient FRET which would collect UV light that would be transferred to PY. Thus, the wavelength range antenna action could be extended from near-UV to about 700 nm. The incorporation of AC as the only dye and at different PY:AC molar ratios in the synthesis gels was studied (Table 1),
Figure 2. XRD patterns of samples shown in Table 1: (a) AC-PY/1:0; (b) AC-PY/1:1; (c) AC-PY/1:3; (d) AC-PY/1:1. * indicates diffraction of MgAPO-5(AFI) as impurity phase.
Incorporation of Acridine. Transmission images of AC dye occluded into MgAPO-36 crystals in a relatively high extension (sample AC-PY/1:0) show a slight pale yellow color (Figure 3A) and in the corresponding fluorescence image (Figure 3B), under UV illumination, the characteristic cyan emission color of AC together with a green fluorescence at the intersection of the particles. According to the FLIM image (Figure 3C), the cyan emission part corresponds to fluorescence lifetimes of around 30 ns that gradually decrease up to 5 ns at the green edge (Figure 3D). To elucidate the AC species incorporated within the ATS channels, a previous photophysical study of the dye in diluted aqueous solution at different pH was performed (see Figure S1 in the SI). As it has been also previously described,27,28 AC in acidic aqueous solution (pH < pKa = 5.6) shows a fluorescence band centered at around 475 nm (cyan emission) with a characteristic long lifetime of around 31 ns, mainly attributed to its protonated form ACH+, whereas the neutral AC species is predominant at pKa < pH and shows a blue-shifted emission band at 430 nm (dark blue emission near to the UV region) and shorter lifetime around 7 ns. Concerning the absorption spectra, although both species AC and ACH+ show a band centered at 354 nm, the ACH+ spectrum (for pH < pKa) has also a broaden shoulder at around 400 nm, probing the groundstate equilibrium of aqueous acridine. In this respect, the absorption and emission spectra registered for the sample ACPY/1:0 in powder (Figure S4A in the SI) shows a more intense absorbance at 410 nm and similar fluorescence band with respect to AC dye in acidic conditions, which will indicate that the dye is incorporated within the pores mainly in its protonated monomeric form. Formation of the protonated form of acridine would probably be favored by the low pH of the synthesis gel (pH ∼ 3−4) and its incorporation within the solid will be favored as it will contribute to the stabilization of the MgAPO structure by balancing the negative charge of the framework caused by the isomorphic substitution of Al by Mg. On the other hand, the long lifetimes of around 30 ns recorded at one edge of a single crystal (Figure 3C,D) indicates the precise distribution of the ACH+ species. However, the lifetimes of around 5 ns registered at the other end of the crystals, though are closer to the ones registered for the neutral form of AC dye (Table S1 in the SI), in this case should be attributed to another species. This is because, contrary to
Table 1. Synthesis Parameters of Samples Discussed in This Worka expt AC-PY/ 1:0 AC-PY/ 1:1 AC-PY/ 1:3 AC-PY/ 1:11 AC-PY/ 0:1b
PY (x)
AC (0.024 − x)
0
0.024
0.012
0.012
0.018
0.006
0.022
0.002
0.024
0
product MgAPO36 MgAPO36 MgAPO36 MgAPO36 MgAPO36
mmol AC/100 g of solid
mmol PY/100 g of solid
4.095
0
2.092
0.323
1.092
0.511
0.360
0.377
0
0.514
a
Molar ratio of AC and PY for samples with a gel composition of 0.2 MgO: 1 P2O5: 0.9 Al2O3: 0.75 TPA: x PY: (0.024−x) AC: 300 H2O. Amount of dye loaded in the samples expressed as mmol of dye per 100 g of product. bTaken from ref 24 (PY/ATS-H in that reference).
keeping constant the total ratio (AC + PY)/1 P2O5 = 0.024. These preparations yielded MgAPO-36 as the major phase, though with the presence of a minor amount of MgAPO-5 as impurity phase in some preparations (Figure 2). The crystals of AC-PY/1:0, with AC as the only fluorescing dye, were yellow in color, while the other samples showed different pink hues, characteristic of the presence of Pyronine Y. Remarkably, the incorporation of AC (Table 1) in the composite materials linearly correlates with the molar ratio of dye in the initial gel (Figure S3 in the SI), achieving a high incorporation even for a very low concentration of dye in the initial gel (around 35 wt % of the added AC). However, for PY, the dye uptake is much lower (∼6 wt % of the added PY) and the only way to achieve a roughly equimolar uptake of incorporated PY and AC dyes in the material is to highly decrease the AC/PY ratio (Table 1). This might be due to the smaller size of AC and/or to more favored interactions between AC and the MgAPO matrix. All the samples showed crystal morphology in a bouquet arrangement, similar to that previously described for the PY/ ATS system.24 24065
dx.doi.org/10.1021/jp408939r | J. Phys. Chem. C 2013, 117, 24063−24070
The Journal of Physical Chemistry C
Article
Figure 3. (A) Transmission, (B) fluorescence (under UV illumination), and (C) FLIM (under 410 excitation) images of sample AC-PY/1:0 sample. (D) Lifetime profile along the single particle of image C, and (E) emission spectra of different areas of particle in image C together with the spectra in aqueous solution (gray). The color of the spectra corresponds to the color of the area recorded.
lifetime at around 6 ns (30%) could be ascribed to the Jaggregated species of AC, as commented above, and the shortest lifetime of 1 ns (50%) would be assigned to the quenching of the donor monomer AC due to the FRET process to the acceptor J-aggregate and/or to scattering effect in such big crystals. Corresponding to previous results obtained for PY occluded within MgAPO-36 system,24 the excitation energy is transported one-directionally from the AC monomers, absorbing in the UV, to the red-shifted AC aggregates (emitting in the green Vis region) located at the other end of the crystal needle. Considering the amount of AC dye molecules incorporated in the solid (4.09 mmol AC/100 g of solid, Table 1) and the unit cell dimensions of MgAPO-36 along the channel direction (c = 5.26 Å), there is 1 AC molecule per 180 Å along the channel direction. Therefore, as sample AC-PY/1:0 shows the highest loading of dye (AC + PY) of all the samples studied in this work, this suggests that the FRET process in these materials does not occur between dye molecules located along the same channel (typical FRET distances
