DOI: 10.1002/chem.201301285

Modulating Dye Aggregation by Incorporation into 1D-MgAPONanochannels**

Virginia Mart�nez-Mart�nez,*[a] Raquel Garc�a,*[b] Luis G�mez-Hortig�ela,[b]

Joaqu�n P�rez-Pariente,[b] and IÇigo L�pez-Arbeloa[a]

Introduction

The incorporation of photoactive molecules into orderednanostructured systems for the development of new func-tional optical materials is a growing field.[1] In particular,crystalline systems with 1D and 2D nanosized pores affordsupramolecular organization of embedded molecules andenhanced properties (e.g., better photo- and thermostabili-ties and higher luminescent efficiencies), due to the protec-tion and rigidity imposed by the matrix host.[2] Such orderedhybrid materials usually show a highly anisotropic responseto light, a vital property for their use in NLO (non-linearoptics) applications such as second harmonic generation, di-chroic filters and light waveguides.[3] The control of thephysical and chemical properties of these hybrid systems is achallenging goal for the future design and engineering ofnew optical materials. Because high optical density materialsare required for optical applications, the use of many fluo-

rescent dyes with a molecular structure built up by threefused aromatic rings containing heteroatoms (Figure 1), suchas rhodamines, pyronines, thionines, oxazines and others, isusually limited. The main reason is that these dyes have astrong tendency to aggregate in aqueous solution at relative-ly low concentrations or when they are adsorbed or chemi-cally linked to the surface of solid particles, due to highlocal concentrations.[4]

Dye aggregation drastically affects the photophysicalproperties of the dye monomers due to the coherent cou-pling of molecular excitons (Davydov theory).[5] Two typesof aggregates exist (Scheme S1 and Figure S1 in the Sup-porting Information): H-aggregates (monomers in parallel

Abstract: The fluorescing dye Pyro-ACHTUNGTRENNUNGnine Y has been incorporated by crys-tallization inclusion into three differentone-dimensional microporous alumino-phosphate host materials. A computer-aided rational choice of the frameworkof the host material made it possible tomodulate the aggregation state of theguest dye molecules. Undesirable H-type dimers of Pyronine Y are includedwithin the large channels of the AFIstructure, which allow the inclusion ofany of the aggregated species of thedye. Density functional theory (DFT)

calculations show that H-type aggre-gate formation is suppressed within theATS framework. Experimental resultsindicate that red-emissive J-type aggre-gates are formed instead, offering aone-directional, organized, multicolouremission system that is interesting forenergy transport. Complete suppres-

sion of aggregation is achieved by theinclusion of Pyronine Y within theAEL-type structure, due to its particu-lar topology and channel dimensionsThis results in a highly fluorescenthybrid system with extraordinarilypreferential alignment of the chromo-phores. Here, we report experimentalevidence and modelling insights forhow the “cage effect” of the nanochan-nels can tune the optical properties ofthe hybrid composite material by influ-encing the aggregation state of the dye.

Keywords: aggregation · antennasystem · computational modeling ·fluorescence · high dichroic ratios ·high emission efficiency

[a] Dr. V. Mart�nez-Mart�nez, Prof. I. L�pez-ArbeloaDepartamento de Qu�mica F�sicaUniversidad del Pa�s Vasco, UPV/EHUApartado 644, 48080 Bilbao (Spain)E-mail : virginia.martinez@ehu.es

[b] Dr. R. Garc�a, Dr. L. G�mez-Hortig�ela, Prof. J. P�rez-ParienteInstituto de Cat�lisis y Petroleoqu�mica (CSIC)Marie Curie 2, 28049, Cantoblanco, Madrid (Spain)E-mail : rgs@icp.csic.es

[**] MgAPO = magnesium aluminophosphates

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201301285.

Figure 1. a) General formula for the dyes. For PY, R1 = R2 =N ACHTUNGTRENNUNG(CH3)2;R3 =R4 =H; Y =O; X=H; n=1. b) Molecular structure and dimensionsof PY.

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planes, in a sandwich-like configuration) are undesirable be-cause they are usually non-fluorescent and quench themono ACHTUNGTRENNUNGmer fluorescence, decreasing the quantum yield andfluorescence lifetimes significantly; in contrast, coplanar orJ-dimers (head-to-tail monomers), with in-line transition di-poles, may be fluorescent according to exciton theory.[5]

Many unidirectional molecular sieve systems such as alu-minosilicates (zeolites) or aluminophosphates (AlPO) haveproven to be very versatile hosts for the encapsulation ofseveral dyes.[6] It is well known that the photophysical prop-erties, aggregation state and orientation of dyes can bemodulated by different characteristic features of the 1D-host system, such as chemical composition (i.e., Si/Al atomicratio) or the shape and size of the pores. In addition, themethod of dye inclusion can dramatically affect the opticalproperties.[7] Molecular adsorption into a pre-formed porousmatrix from a solution or gas phase is size restricted by dif-fusion. An alternative approach is the crystallization inclu-sion method, the bottle-around-the-ship approach, wheredyes are incorporated in situ during the crystallization of thenanoporous material. This approach is driven by a specificstructure-directing agent (SDA) and allows for the inclusionof molecules whose size is either a tight fit for the poreopenings of the zeolitic host, imposing a high energy barrierfor intracrystalline diffusion, or even molecules that exceedthe size of the pore openings and are instead accommodatedin large inner cavities.

The aggregation process can in principle be controlled byadjusting the cavity dimensions to the size of the dye, fol-lowing the rigid “lock-and-key” model. In other words, onecan envisage � la carte design of appropriate porous cagesfor different photoactive molecules by rationally choosingthe host.[6,8] In the last decades, numerous investigations ofthe new photophysical properties of several organic dyes in-corporated into various porous materials of different naturesand pore sizes, such as ZSM-5, Zeolite L, MCM-41 orAlPO-5, have been performed.[6,8]

In this context, the present work uses computer-aided ra-tional choice of 1D-nanoporous hosts to prepare highly fluo-rescent hybrid materials in which aggregation is prevented.H-aggregation can be avoided by reducing the pore diame-ter to below the H-sandwich dimensions. However, preven-tion of J-aggregation is more difficult to envisage; in princi-ple, it would require a particular channel topology so thatdyes are incorporated as isolated species.

Pyronine Y (PY), a xanthene-type dye (Figure 1) was se-lected as a representative of the group of dyes with a molec-ular structure built by three fused rings, since it is a goodcandidate for sensing the aggregation state: PY monomerunits present a characteristic green emission while J-type ag-gregates show a distinctive band in the red region, whichhas been detected when it is encapsulated within narrownanochannels (i.e. , Zeolite L, Sepiolite clay).[9]

PY was incorporated in situ into three crystalline, nanopo-rous magnesium aluminophosphates (MgAPO) possessing1D channel systems with similar chemical compositions thatwere prepared under similar synthesis conditions. We select-

ed these frameworks based on their 1D-channel systems,their different pore dimensions (from 7.3 to 4 ) and dis-tinct pore topologies (see Figures S2 and S3 in the Support-ing Information). Due to the dimensions of PY (13.7 6.2 3.2 ), the molecule is expected to be incorporated withinthe channels of the host materials with its longer axis rough-ly aligned with the channel direction, offering a hybrid ma-terial with an anisotropic response.

A computational study based on density functional theory(DFT) calculations was performed in an attempt to aid inthe rational selection of the frameworks and in the interpre-tation of the spectroscopic experimental data for the posi-tion, geometry and stability of the dye aggregates, as well asthe confinement effect and preferential dye orientationinside the different MgAPO structures.

Results and Discussion

In order to study the cage effect on the photophysical prop-erties of the occluding dyes, three different nanoporousstructures were chosen (Figure 2): 1) MgAPO-5 (AFI struc-ture), with a 12-ring system of cylindrical channels and a di-ameter of 7.3 ; 2) MgAPO-36 (ATS structure), which pos-sesses a 12 ring elliptical channel system with slightly small-er dimensions, 6.7 7.5 ; and 3) MgAPO-11 (AEL struc-ture) with a 10 ring channel system of even smaller dimen-sions, 4 6.5 .[10] To our knowledge, this is the first timethat the photophysical properties of a fluorescing dye incor-porated in MgAPO-36 (ATS) and in MgAPO-11 (AEL) arereported.

Figure 2. a) Transmission and b) fluorescence images of PY dye in differ-ent MgAlPO structures. c) View along the straight channels of theMAPO structures, together with d) the representation of some of the PYspecies in the structures of: MgAPO-5, PY/AFI-H (left); MgAPO-36,sample PY/ATS-H (centre); and MgAPO-11, PY/AEL-H (right).

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The dye-containing materials were prepared by the addi-tion of the cationic PY dye as its chloride salt to the syn-thesised gel, together with the specific structure-directingagent for each framework: triethylamine, tripropylamineand ethylbutylamine for AFI, ATS and AEL, respectively.For MgAPO-5 and MgAPO-36, a high (y= 0.024, denotedas H) and a low (y=0.008, denoted as L) amount of dyewere tested, in order to determine if the fluorescence prop-erties were dependent on the amount of dye contained inthe materials (Table 1).

Figure 3 contains the XRD diffraction patterns of thethree PY/MgAPO solids prepared in this work, which showthat these three structures were obtained as the major prod-uct in these preparations. An impurity of trydimite appearsin the diffraction pattern of the samples of AFI and ATSprepared with a higher amount of PY in the gel; however,being a dense phase, the PY dye molecules cannot end upoccluding this material. The solids possess a Mg/ ACHTUNGTRENNUNG(Mg+

Al+P) molar ratio of 0.05 (measured by inductively coupledplasma-atomic emission spectroscopy, ICP-AES), due to the

isomorphic substitution of framework aluminum by magne-sium (MgAPO), resulting in a negatively charged structure.The amount of dye incorporated in the solids was estimatedphotometrically after dissolving the composite materials(Table 1). All the samples uptake a different amount of dye,depending on the crystal structure and on the initial amountof dye in the synthesised gel.

Transmission and fluorescence images of PY incorporatedin crystals of the three different MgAPO structures pre-pared with a high amount of dye are shown in Figure 2; asummary of the main fluorescence properties is reported inTable 2. As recently noted,[11] the pore size of MgAPO-5 is

large enough to host PY in a sandwich-like geometry (H-ag-gregates), considerably reducing the fluorescence of PY(Table 2). This was evidenced by the low value of the quan-tum yield obtained for the sample prepared with a highamount of PY, PY/AFI-H. This experimental result was con-firmed by simulations, which predict a relatively low H-ag-gregation energy (ca. 4 kcal mol�1 per dimer), showing thatthe AFI channels enable a comfortable fit for PY H-dimers(Table 3, Figure S4 in the Supporting Information) and thatAFI does not impose the required constraints to prevent ag-gregation. Indeed, aggregation in this structure occurs evenat very low loadings of PY, as in PY/AFI (0.008 or -L),though to a much lower extent.[11] Thereby, the quantum

Table 1. Main parameters of the synthesis and characterization of thePY/MgAlPO hybrid materials prepared in this work.[a]

Material SDA x y T [h] Mg/P [g] PY in100 g

ACHTUNGTRENNUNG[mol] PY in1000

PY/AFI-H TEA 0.75 0.024 24 0.10 0.200 1.26PY/AFI-L TEA 0.75 0.008 24 0.12 0.002 0.01PY/ATS-H TPA 0.75 0.024 12 0.11 0.156 0.83PY/ATS-L TPA 0.75 0.008 12 0.12 0.006 0.03PY/AEL-H BEA 1 0.024 18 0.11 0.008 0.04

[a] Gel composition was 0.2MgO/1 P2O5/0.9 Al2O3/x SDA/yPY/305 H2O,where SDA stands for the structure directing agent used for the synthesisof each structure (TEA: triethylamine; TPA: tripropylamine; BEA: bu-tylethylamine). Time refers to the crystallization time, Mg/P is the molarratio measured in the samples by ICP-AES. The amount of dye loaded inthe samples is expressed as g of dye per 100 g of solid product. The densi-ty of PY molecules incorporated in the materials is expressed in mols ofPY per channel in 1000 length.

Table 2. The main photophysical parameters of PY/MgAPO materials atdifferent dye loadings; PY dye in diluted aqueous solution is also inclu-ded.[a]

MgAPO ACHTUNGTRENNUNG[mol] PY/1000

lexc[b]

[nm]lfl

[c]

[nm]tfl

[c]

[ns](%)Ffl

[b] D[c]

PY/AFI-H[11] 1.26 534 5551.3(35)

0.4 10–153.1(65)

PY/AFI-L[11] 0.01 536 555 2.9[d] 7.5 9–12

PY/ATS-H 0.83 539560 0.6(40)

1 3–5610 1.9(50)660 4.1(10)

PY/ATS-L 0.03 539 5581.2(30)

3.5 10–152.8(70)

PY/AEL-H 0.04 523 536 2.95 21 40Py (aq) – 548 568 2 22 1

[a] The density of PY molecules incorporated in the materials is givenper 1000 along the channel length; lexc : excitation maxima wavelengthin nm; lfl : fluorescence maxima wavelength in nm; tfl : lifetimes in ns (%of the statistical weights of each exponential); Ffl : quantum yield; D : flu-orescence dichroic ratio. [b] Powder and [c] single particle measurements.[d] Two different types of crystals were observed in this sample (see ref-erence [11]).

Table 3. Aggregation energy per dimer [kcal mol�1].

Structure Monomer A: H-Sandwich C: Head-to-tail D2: zigzagss op ss op ss op

AFI 0.0 3.6 4.2 �2.5 �0.5 �2.0 �0.1ATS 0.0 13.6 14.5 5.6 6.4 1.7 2.6AEL 0.0 >150 214.7 20.9 15.9 * *

* Indicates that these systems are not local minima in the PES, andgeometry optimization reverts into other configurations.

Figure 3. XRD patterns of PY/MgAPO materials prepared in this work:a) PY/AFI-L, b) PY/AFI-H, c) PY/ATS-L, d) PY/ATS-H and e) PY/AEL-H (* indicates diffraction from trydimite).

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FULL PAPERModulating Dye Aggregation

yield of this sample, though being higher than that of PY/AFI-H, is low due to the presence of aggregates.[11]

A slight reduction of the pore dimensions, as in MgAPO-36 (6.7 7.5 ), brings a suppression of H-aggregation, asexplained by the computational study, which predicts amuch higher H-aggregation energy (ca. 14 kcal mol�1 perdimer, Table 3, Figure S5 in the Supporting Information).Then, in contrast to previous findings for the AFI structure,the smaller dimensions of the ATS channels mean that thebulkier H-dimers cannot fit comfortably, and indeed, the in-clusion of H-dimers brings about a partial distortion of theATS framework (Figure S5 in the Supporting Information).

The sample with a high dye loading, PY/ATS-H, is com-posed of crystals in a bouquet arrangement, which show amulticolour emission under blue excitation light (480/40 nm)in the true colour images (Figure 2). This corresponds to agradual change from green to yellow to red emission alongthe needles in the fluorescence images, which correlateswith a gradual reduction of the average lifetime values fromaround 3 to 1 ns in the lifetime image (Figure 4 a). This ob-

servation points to the existence of PY monomers (with adistinctive green emission and lifetimes around 2–3 ns) atone end, and J-type aggregates (red fluorescence and life-times around 1–2 ns) at the other end.[9] Indeed, the fluores-cence colour change from red to green corresponds in thetransmission image to a gradual decrease in the characteris-tic pink colour of PY dye from the joint part of the bouquettowards the other end of the needles (Figure 2). Thus, appa-rently the dye is incorporated mainly in a J-type associationat the initial steps of crystallization and in monomeric unitsat the last steps of the crystal growth, as crystallization pro-ceeds from the joined centre of the bouquet to the ends.

DFT calculations for the ATS structure confirm that bothJ-type dimers (in a zigzag configuration, with an aggregationenergy of ca. 2 kcal mol�1) and monomers fit nicely within

the ATS channels (Table 3, Figure S5 in the Supporting In-formation).

Three emission spectra taken at the middle and both endsof a single PY/ATS-H particle are displayed in Figure 4 b.The green end of the particle shows only one emission bandcentred at 556 nm, ascribed to the monomer fluorescenceband (Figure 4 b). The middle part of the particle, withyellow emission, shows a main band at around 560 nm to-gether with two shoulders at around 600 and 650 nm, indica-tive of the coexistence of monomer and J-aggregate species.In the red-emissive part of the crystal, those shoulders areresolved into well-defined fluorescence bands, slightly red-shifted, likely due to reabsorption effects, recording fluores-cence bands centred at 560, 610 and 660 nm, with relativeareas under the curves of 14:10:65, respectively. The newbands, redshifted with respect to the emission band of themonomers, are ascribed to different J-type Davydov cou-plings.[5] The higher splitting of the most important band atthe red emission edge at 660 nm, with respect to the emis-sion at 610 nm, could be a consequence of the greater dyeincorporation, which leads to a more dense packing of themolecules, increasing the J-coupling and/or the number ofmonomer units forming the aggregate, resulting in high-order J-aggregates. Indeed, fluorescence images collectedwith two-colour detection (one detector collects only thered-emission band with a 650–720 nm band pass filter and si-multaneously a second detector collects the green emissionof the monomer with a 500–550 nm band pass filter) showthat the maximum fluorescence intensities for the red andgreen channels are at opposite edges of the particle andalmost negligible at the other respective end of the particle(Figure S7 in the Supporting Information).

It is worth remarking that the spatially resolved emissionspectrum along the PY/ATS-H crystals has allowed for theexperimental recording of well-resolved J-aggregate fluores-cence bands, since in bulk measurements they are usuallymasked under the more strongly fluorescent monomer emis-sion, and are observed, at most, as a redshift together with abroadening or shoulder of the main band (Figure S9 in theSupporting Information).

The PY/ATS-H system, with a high incorporation of dye,is an interesting material for light transport. Generally, thedesign of an artificial photonic antenna system is based on aone-dimensional energy-transfer process (FRET, Fçrsterresonance energy transfer) between different chromophoresincorporated within ordered nanoporous materials, in whichchromophores are close enough to enable coupling of theirelectronic transition dipole moments. In our PY/MgAPO-36system, the excitation energy can be transported along theparticle in one direction, achieving a similar antenna effect,but with a unique chromophore, thanks to the particular ar-rangement of PY species (monomers and J-aggregates)within the nanochannels; the green PY monomers can act asenergy transfer donors to the redshifted PY aggregates,which behave as acceptors at the other end of the crystalneedle. This is because an efficient excitation energy migra-tion process takes place between identical chromophores

Figure 4. a) Fluorescence lifetime image (FLIM) and b) emission spectraof different areas of PY/ATS-H particles. The colours of the spectra cor-respond with the colours of the area recorded in image a). Deconvolutedcurves are shown in black.

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V. Mart�nez-Mart�nez, R. Garc�a et al.

that have suffered an exciton splitting of their excited states,due to the same type of dipole–dipole interactions, as descri-bed by Davydov.[5] Similar antenna systems have been re-ported by de Cola et al.[9,12] in zeolite L crystals, though inthat system, the energy was transported from the centre toboth ends of the crystals rather than from one end to theother end, as in the PY/ATS-H system reported here.

As for MgAPO-5, loading less PY in the ATS structure,sample PY/ATS-L, results in an increase in the quantumyield with respect to that of PY/ATS-H, probably due to thereduction in the amount of aggregated species. Nevertheless,the global fluorescence efficiency of these samples (Table 2)is still low due to the presence of J-aggregates. These resultssuggest that in order to improve the overall fluorescent effi-ciency of the composite materials, not only does the poresize need to be reduced, but the host channels should alsopossess a particular topology in order to avoid J-aggrega-tion.

In this context, we selected a different structure, MgAPO-11 (AEL-type structure), with a pore size of 6.5 4 , closeto the cross section of the PY molecule measured perpen-dicular to its main axis and with a particular channel topolo-gy that contains pockets along the channel direction (Fig-ure S3 in the Supporting Information). It is important tonote that incorporation of PY within this structure can onlybe achieved by crystallization inclusion, as the molecular di-mensions of the dye do not allow its diffusion through thepores.

DFT calculations confirm that the formation of H-aggre-gates or J-aggregates of PY molecules inside the AEL chan-nel is energetically very unfavoured: H-aggregates are ex-tremely unstable; D2 (zigzag) dimers are not minima on thepotential energy surface and geometry optimisation revertsthese aggregates into monomers; and C-type dimers are alsovery unstable (Table 3, Figure S6 in the Supporting Informa-tion). In this structure, H-aggregation is prevented by thepore size and J-aggregation is avoided by the reduced poresize together with the particular channel topology and di-mensions of the AEL channels.

Figure 5 shows the geometry-optimized AEL channelswith PY arranged as monomers. AEL channels containpockets along the channel direction, where the methyl

groups of the PY molecule sit vertically oriented in thosechannel pockets. The particular channel topology and di-mensions mean that at least one pocket has to remainempty between two consecutive PY molecules to avoidsteric intermolecular repulsions. Therefore, to preserve thesitting of methyl groups in the pockets, PY molecules are lo-cated a certain distance from each other (at least half a unitcell width along the channel direction is between them),thus only allowing the incorporation of PY monomers inthis structure.

The fluorescence image of the PY/AEL-H sample(Figure 2) shows rectangular-like particles with an intensegreen colour, indicative of the monomer emission of PY.The unique inclusion of PY monomers inside MgAPO-11 isconfirmed by confocal fluorescence microscopy (Figure S8in the Supporting Information), by the monoexponentialdecay curves registered in single PY/AEL-H particles. Thelifetime value of 2.95 ns derived for the PY/AEL-H sampleis longer than that for PY in diluted solution, 2 ns (Table 2),as a consequence of the rigidity imposed on the dye by thematrix, and a decrease of the non-radiative process is ex-pected. Indeed, excitation and emission spectra of PY/AEL-H (powder and single particle) show a greater blue shiftwith respect to the other MgAPO structures described hereand PY in solution (Figures S9 and S10 in the Supporting In-formation), likely due to the high degree of confinement.Note that the absolute quantum yield derived for PY incor-porated into MgAPO-11 is similar to that recorded in a di-luted solution of PY and is one order of magnitude higherthan those obtained for MgAPO-5 and MgAPO-36, regard-less of the amount of dye contained in these structures(Table 2, Figure S11 in the Supporting Information). Inother words, we succeeded in preserving the high fluores-cence efficiency of PY in diluted solutions when it was oc-cluded as monomers within the AEL framework. PY/AEL-H material is not only highly fluorescent but also has an ex-traordinarily preferential alignment of the chromophoresalong the channels. As the principal transition dipolemoment of the molecule coincides with its main axis, an ani-sotropic response to the linear polarized radiation will indi-cate a preferential orientation of the incorporated chromo-phores within the channels, and the fluorescence intensityshould be maximized in images registered with a polariza-tion orientation parallel to the alignment of the molecules.This information is qualitatively provided by the fluores-cence dichroic ratio, D. Large D values indicate that a highdegree of dye molecules are in the preferential order.[13]

Indeed, a huge anisotropic response to linearly polarizedlight is derived in this material (Figure S8 in the SupportingInformation), with dichroic ratios �40, which represent, toour knowledge, the highest value found so far.[13]

Conclusion

We have demonstrated that a computer-aided rationalchoice of the framework to host dyes enables fine tuning of

Figure 5. Detail of the topology of geometry-optimised AEL channelswith PY arranged as monomers, showing the free volume calculatedusing a probe radius of 1 . The structure possesses pockets along thechannel direction that contain the methyl groups of the PY molecule. Atleast one pocket has to remain empty between two consecutive PY mole-cules to avoid steric repulsions between these groups.

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FULL PAPERModulating Dye Aggregation

their aggregation state. In this example, Pyronine Y mole-cules are forced to rearrange from undesirable H-typedimers in AFI structure to the red-emissive J-type aggre-gates in ATS. In the latter hybrid system, PY/ATS, crystalswith an organized multicolour emission are formed (greento red from one edge of the needles to the other), offeringan interesting material for energy transport. Total suppres-sion of aggregation is achieved in a tighter 10 ring channelaluminophosphate structure, PY in an AEL-type framework,achieving a highly fluorescent hybrid system with an extra-ordinarily preferential alignment of the chromophores andpotential applications in optics.

Experimental Section

Synthesis : The microporous magnesium aluminophosphate materialswere prepared using phosphoric acid (Aldrich, 85 wt %), magnesium ace-tate tetrahydrate (99 wt %, Aldrich), aluminium hydroxide (Aldrich),triethylamine (TEA, Aldrich), tripropylamine (TPA, Aldrich), ethylbu-tylamine (EBA, Aldrich) and Pyronine Y chloride (PY) (>75% purity,Acros Organics) from gels with molar compositions of 0.2 MgO/1 P2O5/0.9Al2O3/xSDA/yPY/305 H2O, where the SDA was TEA, TPA or EBAand x and y were varied as stated in Table 1. The gels were heated stati-cally at 180 8C under autogenous pressure for the crystallization timespecified in Table 1. The solid products were recovered by filtration, ex-haustively washed with ethanol and water and dried at room temperatureovernight.

Characterization : X-ray diffraction (XRD) patterns were collected with aPanalytical X’Pro diffractometer using CuKa radiation. Chemical analyseson selected samples were obtained by ICP-AES (Fluxy-30, Claisse). Thedye content within the solid product was determined photometricallyusing a UV/Vis Shimadzu Spectrophotometer 2101/3101PC by dissolvingthe composite material in hydrochloric acid (5 m) followed by neutraliza-tion with NaOH (5 m) to reach a neutral pH (around 6–7). The solid wascentrifuged and an aliquot of the supernatant liquid was taken and subse-quently diluted to measure its UV/Vis spectrum. The final concentrationof the aliquot was determined by comparison with standard solutions pre-pared from known concentrations of the dye. Fluorescent images wererecorded with an optical inverted microscope with an epi configuration(Olympus BX51) equipped with a colour CCD (DP72). Samples were ex-cited with blue light provided by a Chroma band pass filter (470/40) andemission was collected with a Chroma cut-off filter (E515 LPv2) from515 nm. For polarized emission experiments, a polarizer (U-AN-360–3)was incorporated before the registration of the image in the CCDcamera. Fluorescence single-particle measurements were performed in atime-resolved fluorescence confocal microscope (model Micro Time 200,PicoQuant). The excitation was performed at 470 nm with a picosecondpulsed diode laser with 70 ps pulses at a 20 MHz repetition rate. The flu-orescence signal was collected by the same objective and focused(through a 50 mm pinhole) onto avalanche photodiode detectors (Micro-Photon-Devices MPD-APD). Fluorescence lifetime images were process-ed with ShymPhotime software (Picoquant) by sorting all photons of onepixel into a histogram and then fitted to an exponential decay function toextract lifetime information, a procedure repeated for every pixel in theimage. The decay curves were adjusted normally to a sum of bi-exponen-tial decays (i.e., as multi-exponentials) by means of Equation (1).

I fl ðtÞ ¼ A1 expð-t=t1Þ þA2 expð-t=t2Þ ð1Þ

Ai are the pre-exponential factors related to the statistical weights ofeach exponential and ti are the lifetimes of each exponential decay. Thegoodness of the deconvolution process was controlled by the chi-squared(c2) and Durbin–Watson (D.W.) statistical parameters and the residualanalysis. Unpolarized excitation light was obtained by means of a l/4

filter (Thorlabs WPQ05M-488). For polarization measurements, the emis-sion signal collected was divided by a polarizer beam splitter into twomutually perpendicular polarization orientation beams, which are simul-taneously detected by two detector channels. We analysed the dichroicratio (D), defined as the relation between the emission intensity countscollected for two perpendicularly polarized radiations, previously correct-ed by an isotropic factor G to minimize the effect of the instrumental re-sponse to the different light polarization (different sensitivity of detectorsto the plane of polarization and other artefacts produced by the opticsystem) by means of Dcor = (Ik/I? ) G. The correction factor, G= (I?/Ik)iso, was obtained by recording point measurements in both orthogonalemission channels of a diluted isotropic solution of the dye under thesame conditions as the dye/particle samples (power laser, alignment, etc.)with the laser focused deep into the dye solution. Single-particle fluores-cence spectra were recorded by directing the emission beam to an exitport, where a spectrograph (model Shamrock 300 mm) coupled to aCCD camera (Newton EMCCD 1600 200, Andor) was mounted. Exci-tation and emission spectra of the powder were recorded in a SPEXspectrofluorimeter (model Fluorolog 3-22) in front-face configuration.The absolute photoluminescence quantum yields of the PY/MgAPOpowders were measured in an integrated sphere (Hamamatsu, model9920-20). For each sample, the measurement was repeated at least threetimes and the quantum yield shown is obtained after averaging. Reflec-tance from Spectralon� was used as reference. The main results of thefluorescence measurements are summarized in Table 2.

Computational study : The different AlPO framework structures with in-corporated PY molecules were studied by density functional theorymethodology, using atomic orbitals as a numerical basis set, as imple-mented in DMol3 Module[14, 15] in Material Studio software package.[16]

Calculations were performed under periodic boundary conditions (PBC),using a DNP basis set and the PBE generalized gradient approximationas an exchange-correlation functional.[17] Dispersion interactions were ac-counted for through the Grimme dispersion method (DFT +D).[18] Super-cells of 1 1 4, 1 1 8 and 1 1 8 unit cells along the channel directionwere built for AFI, ATS and AEL systems in order to enable the accom-modation of two PY molecules in different orientations and aggregationstates. Apart from the molecules arranged as monomers, three types ofaggregations were studied, case A (sandwich H-type dimers), case C(head-to-tail J-type dimers) and case D2 (zigzag J-type dimers). Each ofthese dimers was studied with two different relative orientations of themolecules composing the dimers: in the same orientation (with NMe2

substituents in the same side, ss) or one rotated 1808 with respect to theother (with NMe2 substituents in opposite sides, op; see Figure S1 in theSupporting Information). Mg was not implicitly included in the frame-work, due to the extremely large number of different possibilities (interms of Mg spatial distribution). The aggregation energies were calculat-ed by subtracting the energy of the system loaded with separated PYmolecules (monomers), and normalized per dimer (i.e., per two PY mol-ecules); all energies are given in kcal mol�1 per dimer (Table 3).

Acknowledgements

This work was funded by the Spanish Ministerio de Economia y Compet-itividad, MICINN (MAT 2010–20646-C04–04), the Basque Government(IT339–10) and the European Research Council, under the Marie CurieCareer Integration Grant program (FP7-PEOPLE-2011-CIG), GrantAgreement PCIG09-GA-2011–291877. VMM and LGH acknowledgeMinisterio de Econom�a y Competitividad for Ram�n y Cajal (RYC-2011–09505) and Juan de la Cierva contracts. Accelrys is acknowledgedfor providing software and Centro T�cnico de Inform�tica-CSIC for run-ning the calculations.

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Received: April 5, 2013Published online: June 18, 2013

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FULL PAPERModulating Dye Aggregation