Fluorescent AlPO4 gels and glasses doped with Rhodamine 6G: Preparation, structural and optical characterization

Journal of Non-Crystalline Solids 356 (2010) 2089–2096

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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l

Fluorescent AlPO4 gels and glasses doped with Rhodamine 6G: Preparation, structural and optical characterization Rihong Li a,b, Long Zhang a,⁎, Jinjun Ren a,b, Thiago B. de Queiroz c, Andrea S.S. de Camargo b,c, Hellmut Eckert b,⁎ a b c

Key Laboratory of Materials for High Power Lasers, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, 201800, Shanghai, China Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstr. 30, D-48149 Münster, Germany Instituto de Física de São Carlos, Universidade de São Paulo (USP), C.P. 369, CEP 13560-970, São Carlos, SP, Brazil

a r t i c l e

i n f o

Article history: Received 21 May 2010 Received in revised form 29 July 2010 Available online 7 September 2010 Keywords: Aluminum phosphate; Xerogels; Mesoporous glass; Rhodamine 6G; Solid-state NMR

a b s t r a c t Fluorescent AlPO4 xerogels doped with different amounts of Rhodamine 6G (Rh6G) laser dye were prepared by a one-step sol–gel process. In addition, mesoporous AlPO4 glasses obtained from undoped gels were loaded with different amounts of Rh6G by wet impregnation. Optical excitation and emission spectra of both series of samples show significant dependences on Rh6G concentration, revealing the influence of dye molecular aggregation. At comparable dye concentrations the aggregation effects are found to be significantly stronger in the gels than in the mesoporous glasses. This effect might be attributed to stronger interactions between the dye molecules and the glass matrix, resulting in more efficient dye dispersion in the latter. The interaction of Rh6G with the glassy AlPO4 network has been probed by 27Al and 31P solid-state NMR techniques. New five- and six-coordinated aluminum environments have been observed and characterized by advanced solid-state NMR techniques probing 27Al–1H and 27Al–31P internuclear dipole couplings. The fractional area of these new Al sites is correlated with the combined fractional area of two new Q(0)3Al and Q(0)2Al phosphate species observed in the 31P MAS NMR spectra. Based on this correlation as well as detailed composition dependent studies, we suggest that the new signals arise from the breakage of Al–O–P linkages associated with the insertion process. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Functional dye-doped porous inorganic hosts have attracted significant attention in recent years [1]. Owing to the combination of the optical functionality of the organic dye with the high stability of the inorganic matrices, these materials find important applications in photonics and have been recognized as promising candidates for the next-generation of integrated devices. Among the dye molecules under consideration, Rhodamine 6G (Rh6G) is one of the most popular candidates for future applications in solid-state dye lasers, which have advantages over liquid state dye lasers by being nonvolatile, nonflammable, nontoxic, compact, and more thermally and mechanically stable. Over the years, efficient procedures have been developed for incorporating Rh6G into solid matrices such as PMMA [2,3], silica gel [4–6], silicate glasses [7–9], and molecular sieves [10–14]. One of the challenges encountered in this research is the need to control and/or limit Rh6G molecular aggregation, which adversely affects the optical properties [15]. In this regard, mesoporous glasses with high surface areas appear to be promising host ⁎ Corresponding authors. H. Eckert is to be contacted at Tel.: +49 251 8329161. E-mail addresses: [email protected] (L. Zhang), [email protected] (H. Eckert). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.07.058

matrices, especially for developing stable, highly transparent materials, with larger pore size. Amorphous mesoporous aluminum phosphates, synthesized following the concept of supramolecular structure direction [16,17], have recently emerged as promising laser host candidates. We have recently reported a simple aqueous sol–gel route yielding transparent and colorless stoichiometric AlPO4 monolith glass with mesoporous structure and surface areas in excess of 400 m2/g [18]. In the present contribution, we explore the incorporation of Rhodamine 6G dye molecules into such AlPO4-based gels and glasses as well as their optical excitation and emission spectra. The extent of aggregation indicated by these data is discussed in relation to the structural properties studied by advanced solid-state nuclear magnetic resonance (NMR) spectroscopic techniques. 2. Experimental 2.1. Sample preparation and characterization AlPO4 xerogels and mesoporous glasses doped with Rh6G dye were prepared via the sol–gel route in aqueous solutions, using 1.176 g aluminum lactate (98%, Fluka) and 4 ml H3PO4 (1 M aqueous solutions) as precursors. The Rh6G dye dopant was dissolved in

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absolute ethanol (Sigma-Aldrich) and incorporated into the AlPO4 xerogels and glasses according to the flowchart in Fig. 1. In all the sol– gel preparations the pH of the precursors and sols were adjusted with nitric acid (1 M) or ammonia solution (1 M) and controlled within 0.01 units by a pH meter (WTW pH 320, Germany). Gel annealing was done in a Heraeus muffle furnace. For the preparation of the Rh6G containing AlPO4 xerogels, the ethanol solutions of Rh6G with different concentrations were added to the AlPO4 sol two hours after the aluminum and phosphate precursors had been mixed and clear solutions had been obtained. After Rh6G addition, the pH values of the solutions were adjusted to 3.0. This was followed by gelling at room temperature for several days. Once gelling was complete the samples were dried at 120 °C. Fully transparent monolith xerogels (gels a–d) were obtained, the Rh6G levels of which could be controlled by the concentration of the Rh6G ethanol solution added to the sol (see Table 1). Mesoporous AlPO4 glasses were prepared according to Ref. [18] and characterized by BET surface area measurements using a Micromeritics ASAP 2010 volumetric adsorption analyzer with N2 as adsorbate at 77 K. Prior to the analyses, samples weighing from 0.1 to 0.3 g were outgassed at 150–300 °C, for at least 12 h under vacuum, until a residual pressure of ≤6 μm Hg was reached. The BET specific surface area was calculated according to the BET equation [19], using nitrogen adsorption data in the relative adsorption range from 0.06 to 0.2. The total pore volume, Vp, was obtained from the amount of N2 adsorbed at a p/p0 of about 0.99. Mesopore size distributions were obtained by the BJH (Barrett–Joyner–Halenda) method, assuming a cylindrical pore model [20]. Fig. 2 shows the absorption–desorption isotherms indicating a mesoporous structure with surface area as high as 464 m2/g, and an average pore diameter of about 5 nm. For the topochemical insertion of Rh6G dye into mesoporous AlPO4 glass, 200 mg of the host material was dispersed for 8 h in 10 ml ethanolic solutions with different concentrations of Rh6G ranging from 5 × 10− 6 M to 5 × 10− 3 M. Following the periods of exposure, the samples were briefly washed with ethanol, subsequently dried at 120 °C for 48 h and stored in a desiccator. The dopant levels were calculated from the absorption spectra of the diluted residual Rh6G solutions, and compared with a standard 5 × 10− 6 M Rh6G solution. Table 1 presents the dopant levels in terms of Rh6G/AlPO4 molar ratios. Again, the results indicate that the dopant levels can be controlled to some extent by the concentration of the ethanolic Rh6G solutions used for exposure. For the sample with the highest dopant level (prepared from the 5 × 10− 3 M solution), elemental analysis using a commercial CHN analyzer yielded C contents of 0.73 and

Fig. 1. Flow chart depicting the preparation of AlPO4 xerogels and mesoporous glasses incorporated with Rh6G.

Table 1 AlPO4 xerogel and mesoporous glass doped with different Rh6G/AlPO4 molar ratios. Sample ID

Rh6G/AlPO4 molar ratio

Sample ID

Rh6G/AlPO4 molar ratioa

Gel a

2.4 × 10− 7 (5 × 10− 6 M, 1.2 × 10− 6 (5 × 10− 5 M, 6.0 × 10− 5 (5 × 10− 4 M, 3.0 × 10− 4 (5 × 10− 3 M,

Glass a

2.4 × 10− 6 (5 × 10− 6 M, 8.5 × 10− 5 (5 × 10− 5 M, 9.1 × 10− 4 (5 × 10− 4 M, 8.8 × 10− 3 (5 × 10− 3 M,

Gel b Gel c Gel d

2.0 ml) Glass b 0.9 ml) Glass c 0.5 ml) Glass d 0.24 ml)

10 ml) 10 ml) 10 ml) 10 ml)

a

Calculated from absorption spectra of the residual Rh6G solution, compared with standard Rh6G solution at the concentration 5 × 10− 6 M. Values in parentheses list the concentrations (in mol/L) of the Rh6G solutions and the volumes used, either for the preparation of the gels or for dipping the glass samples.

0.84 wt.% in two replicate analyses, corresponding to Rh6G/AlPO4 ratios of 2.7 × 10− 3 and 3.1 × 10− 3, respectively. 2.2. Solid-state NMR All the NMR experiments were conducted at ambient temperature on Bruker DSX-400 and DSX-500 spectrometers, using 4 mm MAS NMR probes operated at spinning rates between 12 and 15 kHz. At the two magnetic flux densities the resonance frequencies were 104.3 MHz and 130.3 MHz for 27Al, and 162.4 MHz and 202.5 MHz for 31P, respectively. The 27Al and 31P 90° pulses were set at 2 μs and 5 μs, respectively, and recycle delays of 1 s (27Al) and 90 s (31P) were used. NMR chemical shifts were referenced to 1 M aluminum nitrate and 85% H3PO4 aqueous solutions. The aluminum sites were further characterized by 27Al triple-quantum (TQ-) MAS NMR, by means of which the Zeeman interactions can be separated from the quadrupolar interactions, and isotropic chemical shifts are measureable. These TQMAS spectra were conducted at 9.4 T, using the three-pulse, zeroquantum filtering method [21–23]. Typical pulse lengths for the hard initial excitation and reconversion pulses were 3.6 and 1.7 μs, respectively, at a 27Al nutation frequency (liquid sample) of 130 kHz. Typically 200 t1 data sets were acquired, incremented by 16.7 μs, and the single quantum coherence was detected with a soft pulse (nutation frequency of 6 kHz) of 9 μs length. Typically, 360 scans were accumulated. Finally, the proximity of the aluminum species to nearby protons and phosphorus nuclei was probed by evaluating the 27Al/1H and 27Al/31P magnetic dipole–dipole interactions, using 27Al{1H} and 27 Al{31P} rotational echo double resonance (REDOR) spectroscopies. These experiments were conducted at a magnetic flux density of

Fig. 2. Sorption–desorption isotherms of AlPO4 mesoporous glass annealed at 600 °C. The inset indicates the pore size (diameter) distribution of the same glass. Symbol sizes depict experimental errors. Solid lines are guides to the eye.

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11.7 T, using the standard pulse sequence published by Gullion and Schaefer [24], phase cycled according to the XY-4 scheme [25]. The following typical experimental conditions were used: MAS frequency 14 kHz, 180° pulse lengths of 8 μs for both 1H and 31P nuclei, respectively. The 180° pulse lengths were optimized by maximizing the REDOR difference signal at a chosen dephasing time. 2.3. Optical spectroscopy Absorption spectra of the xerogels, glasses and Rh6G solutions were measured in a UV–VIS spectrophotometer (Varian, Cary-50) at 0.5 μm/min scan rate. The emission and excitation spectra were recorded in a HORIBA Jobin-Yvon IBH FL-322 Fluorolog 3 spectrometer equipped with a 450 W xenon arc lamp, double grating excitation and emission monochromators (2.1 nm/mm dispersion; 1200 grooves/ mm) and a Hamamatsu R928 photomultiplier tube or a TBX-4-X single-photon-counting detector. The spectra of monolith samples, with typical thickness of 0.5 mm, were measured on a right angle

Fig. 4. Emission spectra of Rh6G-doped AlPO4 xerogels with different Rh6G/AlPO4 molar ratios, obtained with excitation at 490 nm.

configuration to minimize the specular reflection of the excitation beam. The emission spectra were recorded by exciting the samples at a fixed wavelength of 490 nm and the excitation spectra were recorded by monitoring the emissions at 560 nm and 590 nm for the gels and the glasses, respectively. All the measurements were done at room temperature. 3. Results 3.1. Optical spectroscopy studies

Fig. 3. Absorption (top) and excitation spectra (bottom) of Rh6G-doped AlPO4 xerogels with different Rh6G/AlPO4 molar ratios. The emission was monitored either at 560 nm.

Fig. 3 presents the absorption and normalized excitation spectra of AlPO4 xerogel samples doped with different amounts of Rh6G. The absorption spectra are characterized by a broad band centered at around 530 nm, and their lineshapes are practically independent on Rh6G concentration. As for the excitation, the spectrum of the sample with lowest Rh6G loading (Gel-a), is dominated by this band, which is attributed to the monomeric state of Rh6G, and the shoulder around 492 nm indicates the presence of a small amount of dimeric molecular species. As expected, the intensity of the latter increases with increasing doping concentration. Also, a new feature at around 470 nm becomes visible, which has been previously observed for Rh6G in ethanolic solutions and is ascribed to trimeric and tetrameric aggregates [26]. For the two most concentrated samples (Gel-c and Gel-d), the spectra are increasingly dominated by excitation of such oligomers, resulting in significant blue shift and broadening, while the excitation of the monomer is decreased. In contrast to the excitation behavior, the normalized emission spectra presented in Fig. 4 show a systematic red-shift from 540 nm to 562 nm with increasing Rh6G concentration. A very similar behavior has been reported for Rh6G intercalated in solid thin films of the laponite clay [27] and although several possibilities were considered, the most plausible conclusion is that such shift is due to the increased contribution of aggregate molecular species to the emission spectrum. Besides presenting much lower fluorescence efficiency themselves, the aggregates are efficient quenchers of the monomeric emission, giving rise to a drastic decrease in intensity at 530 nm. Thus, the lasing properties of Rh6G can be critically hindered by such aggregation effects [6–10]. Fig. 5 presents the absorption and excitation spectra of Rh6G-doped AlPO4 glasses with different loading levels. Again, the absorption and excitation spectra are essentially identical at low concentrations, indicating a dominance of monomeric species. At higher concentrations the excitation spectra reveal that the successive red-shift and broadening of a band around 520 nm increases with increasing dye concentration, concomitant with the broadening on the blue side of the

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Fig. 5. Absorption (top) and excitation spectra (bottom) of Rh6G-doped AlPO4 mesoporous glasses with different Rh6G/AlPO4 molar ratios. The emission was monitored at 590 nm.

spectra. The emission lineshapes (Fig. 6) observed for the samples with the two lower concentrations of Rh6G (glasses a and b) remain practically the same around 544 nm, while progressive line narrowing and red-shifting, ultimately to 559 nm, is observed for the Glass d with higher doping concentration (Rh6G/AlPO4 = 8 × 10− 3). Again, this redshift occurs at much higher doping concentration compared to the situation in the gels, suggesting that the aggregation phenomenon is considerably weaker in the AlPO4 glass. This conclusion is particularly evident by comparison of the spectra of “Gel-c” and “Glass b”, with similar Rh6G/AlPO4 ratios.

plexes [Al(lact)2(H2PO4)2]− and [Al(lact)(H2PO4)4]2− species, respectively. In addition, a less intense peak observed near 40 ppm indicates the presence of some tetrahedral aluminum sites, bound to phosphate species. All of the spectra shown are more or less identical to those measured in Rh6G-free samples [28]. The 31P MAS NMR spectra of the Rh6G-doped glasses are summarized in Fig. 8 (left). The spectra can be deconvoluted into three Gaussian components with chemical shifts near − 26, −16, and −5 ppm. The dominant signal near − 26 ppm arises from the tetrahedral P(OAl)4 (Q(0)4Al) units of the glassy framework. The spectral components near − 16 and − 5 ppm are tentatively attributed to Q(0)3Al and Q(0)2Al units, which are formed by some extent of depolymerization of the tetrahedral AlPO4-like structure of the host glass. The fractional contribution of these latter sites appears to increase with increasing Rh6G concentration. Even stronger spectroscopic changes are evident in the 27Al MAS NMR spectra obtained on these dye-doped glasses (Fig. 8, right). The spectrum of the AlPO4 glass (reference sample) dispersed just in the solvent ethanol shows the dominant framework signal at 40 ppm, which arises from fourcoordinated Al(OP)4 units [18]. In addition, the low-frequency shoulder indicates the presence of some higher-coordinated Al defect sites (Al(5) and Al(6)) that always appear to be present at low levels in these glasses. After exposing the undoped glass samples to ethanol solutions of Rh6G, a clear new signal at − 13 ppm emerges, which can be attributed to the six-coordinated units [28]; in addition the spectral intensity changes at resonance shifts near 10 ppm suggest that the concentration of Al(5) environments appears to increase as well. Fig. 8 (right) illustrates that the concentration of these higher-coordinated Al species appears to increase with increasing dopant level until some extent of saturation is reached. Similar spectroscopic trends are observed in time-dependent studies as summarized in Fig. 9 for a series of glass samples exposed to an ethanolic 5 × 10− 3 M Rhodamine 6G solution. The apparent concentration of the Al(6) species in particular increases with increasing exposure time, until a saturation level appears to be reached for samples exposed to the Rh6G solution for 16–24 h; again increasing amounts of the high-frequency signal components are seen in the 31P NMR spectra. Analysis of the 27Al TQMAS-NMR spectrum (Fig. 10) reveals that the new Al(6) environment is characterized by an isotropic chemical shift near −6 to − 9 ppm and a second-order quadrupolar effect (SOQE = CQ(1 + η2/3)1/2) near 2.7 MHz. The values in Table 2 indicate that there also seems to be a small dependence of the 27Al isotropic chemical shifts of both the Al(4) and the Al(6) units on the doping level. Both aluminum sites were further characterized with respect to their dipole–dipole interactions with 31P and 1H using the corresponding REDOR experiments (see Figs. 11 and 12). Following a previously

3.2. Solid-state NMR Fig. 7 summarizes the 31P and 27Al solid-state NMR spectra of the Rh6G-doped gel samples studied at the lowest and the highest concentrations, respectively. The 31P MAS NMR spectra show a broad, structureless, slightly asymmetric band with a maximum near −19 ppm, which can be attributed to Al-bound phosphate species. The 27Al MAS NMR spectra are characterized by two dominant peaks near 8 and −9 ppm, which can be attributed to six-coordinated aluminum environments within the mixed lactate/phosphate com-

Fig. 6. Emission spectra of Rh6G-doped AlPO4 mesoporous glasses with different Rh6G/ AlPO4 molar ratios, obtained with excitation at 490 nm.

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Fig. 7. (Left) 162.4 MHz 31P MAS NMR spectra of xerogel doped with different Rh6G/AlPO4 ratios; (right) 104.3 MHz 27Al MAS NMR spectra of xerogel doped with different Rh6G/ AlPO4 molar ratios.

Fig. 8. 162.4 MHz31P MAS-NMR spectra of Rh6G-doped AlPO4 mesoporous glass with different Rh6G/AlPO4 molar ratios (left) and 104.3 MHz 27Al MAS NMR spectra of Rh6G-doped AlPO4 mesoporous glass with different Rh6G/AlPO4 molar ratios (right).

outlined procedure, the corresponding dipolar second moments M2 (27Al{31P}) characterizing the 31P/27Al multi-spin interactions can be extracted from these REDOR curves, by approximating their shapes at short evolution times (ΔS/So b 0.2) with a parabola, as described in reference [29]. These values are summarized in Table 2. In the case of the 27Al{1H} REDOR studies, such a quantitative analysis is precluded by the rapid dephasing observed at short evolution times. Nevertheless, Fig. 12 illustrates on a qualitative level that the Al(6) species are subject to much stronger dipolar fields from 1H than the Al(4) species are.

4. Discussion 4.1. Optical properties of the gels and glasses As described above, the systematic changes in the excitation and emission spectra observed as a function of dye loading give clear evidence of dye aggregation both in the prepared gels and in the topochemically loaded mesoporous glasses. In order to fully understand the excitation and emission spectral changes associated with these aggregates, it is important to differentiate two main forms of

Fig. 9. Dependence of the 31P (left) and 27Al (right) MAS NMR spectra of mesoporous AlPO4 samples exposed for different lengths of time to 5 × 10− 3 M ethanolic solution of Rh6G.

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Fig. 10. 27Al TQMAS-NMR spectra of mesoporous AlPO4 glass (left) and mesoporous AlPO4 glass doped with Rh6G (right).

molecular pairing (“dimerization”) known as “H-” and “J-dimers”. Such dimers arise from plane-to-plane or end-to-end molecular stacking due to van der Waals interactions, and their spectral features are often explained in terms of the “molecular exciton coupling theory”, based on the coupling of their transition moments. In this approach, the dye molecules are regarded as point dipoles and the excitonic states of the dye aggregates split into two levels through interaction of transition dipoles [30,31]. Fig. 13 illustrates the schematic energy level diagrams of the dimers corresponding to different molecular and transition dipole arrangements, in comparison to the monomer. For the case of the H-dimers, the lower excited state (H′), with antiparallel alignment of dipole moments, corresponds to a more stable molecular configuration than the upper state (H″). However, transitions between the ground state (G) and H′ are forbidden due to the vanishing resulting dipole moment, therefore, the blue feature at 492 nm (Figs. 3 and 5) corresponds mainly to G → H″ transitions, which can also be superimposed with the G → J″ transitions of J-dimers. Similarly, G → J′ transitions on the lower energy side of the monomer excitation are also allowed, though not really perceived in the spectra of Figs. 3 and 5. The spectral broadening on the high energy side is more pronounced than that on the lower energy side because H-dimers, associated to a more stable molecular interaction geometry, are more likely to occur than J-dimers, especially in the gels. The fact that these effects are more pronounced for the glass samples than for the gels indicates that the J-dimers seem to be dominant in the mesoporous glass up to a dopant-to-host ratio 9.1 × 10− 4. This level is significantly higher than that leading to comparable spectra in the xerogel state.

Regarding the emission characteristics (Figs. 4 and 6), note that although H″ can be promptly excited, this state is associated with a less stable parallel molecular configuration. Because of this, it rapidly undergoes non-radiative decay transferring its energy to the lower lying states of the monomer and the J-dimers. That is to say H-dimers are practically non-fluorescent and the broad and red-shifted emission observed in the spectra of Fig. 4 originates from the long living J′ states in addition to the monomer emission. The progressive red-shift with increasing dye concentration is attributed to the increase in J-dimers with a larger angle α defining the relative orientations of the molecular dipole moments as schematized in Fig. 13. 4.2. Structural aspects of dye insertion For the gel samples, both the 27Al and the 31P MAS NMR spectra are found essentially independent of Rh6G/AlPO4 ratios and closely resemble the spectra of the undoped gels [28]. These results give no

Table 2 27 Al isotropic chemical shifts (± 1 ppm) and SOQE values (± 0.5 MHz) determined via TQMAS, and dipolar second moments M2(27Al{31P}) (± 10%) extracted from the REDOR experiments for the AlPO4 glasses dipped into the Rh6G solutions. Rh6G solution Al(4) concentration δiso/ ppm AlPO4 glass without dye 5 × 10− 6 M 5 × 10− 5 M 5 × 10− 4 M 5 × 10− 3 M

Al(6) SOQE/ MHz

MAl–P / 2 6 2

10 rad /s2

δiso/ ppm

SOQE/ MHz

MAl–P / 2 106 rad2/s2

41.5

3.1

4.2

–

–

–

42.1 43.0 43.2 44.2

3.2 2.9 2.9 2.8

4.1 4.3 4.2 4.0

− 9.6 − 8.9 − 8.1 − 6.4

2.7 2.7 2.5 2.6

3.1 3.0 3.2 3.1

Fig. 11. 27Al{31P} REDOR curves measured for the Al(4) and Al(6) sites in AlPO4 glass doped with Rhodamine 6G. Data are included for samples obtained with two different exposure times to a 5 × 10− 3M solution of Rh6G. Fitting the initial curvatures to parabolae yielded the approximate second moment values listed in Table 2. Symbol sizes depict experimental errors.

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45

Al(5)+Al(6) area fraction (%)

40 35 30 25 20 15 10 5 0 0

5

10

15

20

25

30

35

40

45

Q03Al+Q02Al area fraction (%)

Fig. 12. 27Al{1H} REDOR curves measured for the Al(4) and Al(6) sites in AlPO4 glass doped with Rhodamine 6G. Data are included for samples obtained with two different exposure times to a 5 × 10− 3 M solution of Rh6G. Symbol sizes depict experimental errors.

indication of any interactions between the Rh6G molecules and the AlPO4 xerogel network. In contrast, for the glass samples topochemically loaded with the Rh6G molecules the distinct spectroscopic trends observed in the 31P and 27Al MAS NMR spectra warrant further discussion in terms of structural changes in the host lattice resulting from the insertion process. Fig. 14 illustrates that both in the time- and the concentration-dependent measurements the fractional contribution of the new Al(5) and Al(6) units to the 27Al NMR spectrum appears to be correlated to the combined contributions of the new Q(0)3Al and Q(0)2Al units to the 31P NMR spectrum. Thus we conclude that these new signals reflect new sites resulting from the breakage of one or more Al–O–P bonds in the host glass. The time dependence shown in Fig. 9 indicates that this process is a relatively slow one, suggesting that it may be related to the diffusion of the dopant species into the AlPO4 mesopores. The results shown in Table 2 indicate that the dipole–dipole interaction strengths measured for the Al(4) units do not depend on the Rh6G/AlPO4 ratio, and in fact closely match the values previously measured for undoped mesoporous AlPO4 glass [28]. This result indicates that the extent of Al–O–P connectivity for the Al(4) units remains unaltered within this series. Table 2 also suggests strong Al– O–P connectivity for the Al(6) units even though the M2(31P{27Al}) values are significantly smaller compared to those measured for the Al(4) units. Quantitatively, these M2 values would be compatible with

Fig. 14. Fractional area contributions attributed to the new phosphate species plotted versus the fractional area contributions attributed to the new Al[5] and Al[6] units. The solid line is a linear least-squares fit to the data, yielding a correlation coefficient of 0.925. The correlation suggests that these species are formed by the same process, involving depolymerization of the AlPO4 network. Symbol sizes depict experimental errors.

a number of Al–O–P connectivities that has been reduced from four to three at an unchanged average internuclear distance. Alternatively, the reduced M2 value might also arise if the number of Al–O–P linkages remains unchanged and the internuclear distances are increased by a few percent. Additional information is available from the 27Al{1H} REDOR experiments, which indicate that the Al(6) units are significantly more strongly dipole-coupled to 1H species in comparison to the Al(4) units. This result suggests very close spatial proximity between Al(6) and either the dye molecules or some hydrous species, consistent with the idea that the Al(6) sites are formed as a consequence of the rhodamine guest species insertion process. The exact nature of this interaction is not clear at the present time, however. Because of the very small molar ratios of Rh6G/AlPO4 it is clear that the higher-coordinated Al sites cannot simply arise from a coordinative binding of the guest species to the Al(4) units of the host matrix, as this would result in only minimal NMR spectroscopic changes. In this connection it is interesting to note that the concentration of these new Al environments, as apparent from the NMR spectra, does not increase proportionally with increasing Rh6G/AlPO4 ratio. This finding may be related to the fact that the new 27 Al and 31P MAS NMR signals in amorphous mesoporous AlPO4 can also be observed sometimes in samples exposed to moisture, possibly indicating the hydrolysis of Al–O–P linkages. To clarify the detailed mechanism of interaction between the rhodamine guest species and the mesoporous AlPO4 host matrix further studies are currently being conducted in our laboratories. 5. Conclusions

Fig. 13. Schematic energy level diagrams of monomer, H-dimers and J-dimers of Rh6G. In the parallel and head-to-tail molecular configurations, the angle α specifies the relative orientation of the aggregated molecules, and the transition dipole moments are indicated by arrows.

In summary, we have developed a simple method to incorporate the Rh6G dye into AlPO4 xerogel and mesoporous glass hosts. The excitation and emission spectra indicate that in the xerogels the Rh6G dye is seriously aggregated, the effect increasing with increasing dopant concentration. In contrast, the dye dispersion into mesoporous AlPO4 glass results in more efficient suppression of the aggregation phenomenon, and the aggregates exist mainly as fluorescent J-dimer aggregates. Incorporation of Rh6G into the xerogels formed during the sol–gel synthesis leads to materials with guest–host interactions that are so weak that they are essentially not detectable by NMR spectroscopy. Thus, the respective 31P and 27Al spectra are identical both in the presence and the absence of guest species. In contrast, incorporation of rhodamine dopant into mesoporous AlPO4 glasses produces new higher-coordinated Al environments (Al(5) and Al(6)

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sites), as well as new phosphate environments, which were thoroughly characterized by advanced solid-state NMR techniques. While these new environments are clearly formed as a consequence of the insertion process, further experimentation will be required to elucidate the detailed mechanism of the interaction between the guest molecules and the amorphous host matrix. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant 50802103). R. Li thanks the financial support by the Deutscher Akademischer Austauschdienst (DAAD) and the Chinese Academy of Science (Grant A-0713112). A.S.S. de Camargo thanks the Alexander-von-Humboldt Foundation for a personal fellowship. References [1] H.S. Zhou, I. Honma, Adv. Mater. 11 (1999) 683. [2] T. Watanabe, Y. Iketaki, M. Sakai, T. Ohmori, T. Ueda, T. Yamanaka, S.I. Ishiuchi, M. Fujii, Chem. Phys. Lett. 420 (2006) 410. [3] G. Somasundaram, A. Ramalingam, J. Photochem. and Photobiol. A-Chem. 125 (1999) 93. [4] F. Ammer, A. Penzkofer, P. Weidener, Chem. Phys. 192 (1995) 325. [5] F. del Monte, J.D. Mackenzie, D. Levy, Langmuir 16 (2000) 7377. [6] A.P. Rao, A.V. Rao, Sci. Technol. Adv. Mater. 4 (2003) 121. [7] A. Anedda, C.M. Carbonaro, R. Corpino, P.C. Ricci, S. Grandi, P.C. Mustarelli, J. NonCryst. Solids 353 (2007) 481.

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