ARTICLE pubs.acs.org/JPCC
Solidification and Simultaneous Dual-Wavelength Emission of Rhodamine 6G and Coumarin 102 Codoped in AlPO4 Mesoporous Glass Rihong Li,† Youyu Fan,† Jiacheng Li,† Bin Tang,† Jintai Fan,† Jin He,† Jinjun Ren,‡ Jun Wang,*,† and Long Zhang*,† †
Key Laboratory of Materials for High Power Lasers, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, 201800, Shanghai, China ‡ Institut f€ur Physikalische Chemie, Westf€alische Wilhelms-Universit€at M€unster, Corrensstr. 30, D-48149 M€unster, Germany ABSTRACT: Transparent AlPO4 mesoporous glasses codoped with different amounts, as well as different molar ratios, of Rhodamine 6G (Rh6G) and Coumarin 102 (Cou102) were fabricated by a dipping method. Solidification of dyes in mesoporous AlPO4 glasses was probed by 27Al and 31P solid state NMR techniques. The interaction of dyes with the glassy AlPO4 network has been characterized by a new six-coordinated aluminum species (Al(6)) environment in 27Al MAS spectra, combined with advanced solid state NMR techniques probing 27Al1H and 27Al31P internuclear dipole couplings. Optical excitation and simultaneous dual-wavelength emission spectra of both series of samples show significant dependences on the Rh6G and Cou102 concentration and Cou102/Rh6G molar ratio. Efficient simultaneous dual-wavelength emission was observed at low dye concentration (103 M), quenching of the fluorescence of Cou102 happened, but the fluorescence intensity was dependent on the Cou102/ Rh6G molar ratio.
1. INTRODUCTION Organic dye-doped inorganic hosts are prospects for numerous applications, such as solid state dye lasers,1,2 dye-sensitive solar cells,3 sensors,4 and nonlinear optics.5,6 Especially, increasing demands for uses as the gain media of multiwavelength emission and wide tunable solid state dye lasers has heightened the importance to develop new efficient hosted dyes,7 which have advantages over liquid-state dye lasers by being nonvolatile, nonflammable, nontoxic, compact, and mechanically stable. Lately, much research has been focused on incorporating single or multilaser dyes into solid matrixes, such as polymers,8,9 silica gel,10,11 silicate glasses,12,13 and molecular sieves,14,15 in order to achieve a simultaneous superwide tunable and multispectral output. However, one of the challenges encountered is control of the dye aggregate states and dye concentration in these materials,16 which adversely affects the fluorescent properties and is mainly responsible for the monomer fluorescence quenching by self-absorption. That is why it is difficult to incorporate two laser dyes in one single host to achieve simultaneous dualwavelength output without the use of another tuning gain media. Therefore, a key question is the following: How does one increase the loading dye concentrations in solid inorganic hosts without serious aggregates and how does one incorporate two r 2011 American Chemical Society
different laser dyes in one single host to obtain simultaneous dual-wavelength output? In this regard, mesoporous glasses with high surface areas and thermal stability offer an alternative host for laser dyes.17 Owing to the separation in the mesoscale and confinement effect of mesostructured pores to the molecule, the dyes exhibited a lower tendency to aggregate after incorporation. In this case, a secondary interaction between the pore surface and dyes could be formed, which can avoid serious aggregates and decrease fluorescence quenching. This approach is the so-called “solidification” of dyes in solid hosts, not a simple encapsulation. Recently, considerable efforts have been taken to improve and exploit effective hosts for laser dyes, yet many researchers are just focusing on modifying PMMA and silicate materials.18,1013 However, to the best of our knowledge, few reports refer to the solidification of dyes in inorganic porous materials as it is either difficult to determine or without chemical absorption of guest species. Amorphous mesoporous aluminum phosphates, synthesized following the concept of supramolecular structure direction, have Received: January 20, 2011 Revised: March 28, 2011 Published: April 18, 2011 9176
dx.doi.org/10.1021/jp200596g | J. Phys. Chem. C 2011, 115, 9176–9181
The Journal of Physical Chemistry C
ARTICLE
Table 1. AlPO4 Mesoporous Glass Doped with Different Cou102 and Rh6G Concentrations (C) and Different Cou102/Rh6G Molar Ratios (R) sample ID glass a
C 5 10
6
R M
1.0
sample ID glass e
C
R 3
Cou102: 2.0 10
M
0.5
Rh6G: 4 103 M glass b
5 105 M
1.0
glass f
Cou102: 4 103 M
1.0
Rh6G: 4 103 M glass c
5 104 M
1.0
glass g
Cou102: 4 103 M Rh6G: 2.0 103 M
2.0
glass d
5 103 M
1.0
glass h
Cou102: 4 103 M
3.0
Rh6G: 1.3 10 glass i
3
M
Cou102: 4 103 M
4.0
Rh6G: 1.0 103 M
recently emerged as promising laser host candidates.15,19 Recently, we reported a simple aqueous solgel route yielding transparent and colorless stoichiometric AlPO4 monolith glass possessing a mesoporous structure and surface areas in excess of 400 m2/g.20 Solid state NMR data have shown that the structure of these materials is based on alternating AlO4 and PO4 tetrahedra. It is remarkable that AlO4 and PO4 units exhibit more chemical activity to benefit the solidification of dyes in contrast to SiO4 units in silicate materials. In this case, AlPO4 mesoporous glass is the promising host for incorporating two dyes simultaneously to achieve a dual wavelength output. In the present work, we have studied the solidification and simultaneous dual-wavelength output properties of Rh6G and Cou102 after incorporation into AlPO4 mesoporous glass. As we all know, Rh6G and Cou102 are two of the most popular fluorescent dyes and are widely used for their efficient lasing ability around 550 and 400 nm, respectively, to fabricate several types of popular luminescent materials.2023 To the best of our knowledge, there is no report on codoping Rh6G and Cou102 in solid state hosts. Their functional optical properties are studied by excitation and emission spectra as well as discussed in relation to the dyes' state on the basis of structural information obtained from solid state NMR spectroscopy.
2. EXPERIMENTAL SECTION AlPO4 mesoporous glasses were fabricated via a solgel method as in our previous work20 and characterized by BET surface area measurements obtained from a Micromeritics ASAP 2010 volumetric adsorption analyzer with N2 as an adsorbate at 77 K. Mesopore size distributions were obtained by the BJH (BarrettJoynerHalenda) method, assuming a cylindrical pore model. The obtained transparent and monolithic AlPO4 mesoporous glass possesses a surface area as high as 464 m2/g and an average pore diameter of about 5.0 nm. For the incorporation of Rh6G and Cou102 laser dyes (99%, Aldrich) simultaneous in AlPO4 mesoporous glass, 200 mg of this glass was inserted for 8.0 h in 10 mL ethanolic solutions with different concentrations (C) of Rh6G and Cou102 ranging from 1.0 103 to 1.0 106 M, as well as different predetermined Cou102/Rh6G molar ratios (R) in the range of 0.54.0 (listed in Table 1), followed by washing with absolutely ethanol to remove the absorbed dyes on the surface of glass, and they were subsequently dried at 120 °C for 48 h and stored in a vacuum desiccator.
Figure 1. 162.4 MHz 31P MAS NMR spectra (left) and 104.3 MHz 27Al MAS NMR spectra (right) of AlPO4 mesoporous glass doped with different Cou102 and Rh6G concentrations at a fixed ratio of Cou102/ Rh6G = 1.
Absorption spectra of the glasses were measured using an ultravioletvisible spectra photometer (Varian Company, Cary50 UVvis) at a 500 nm 3 min1 scan rate. The emission spectra and excitation spectra were performed using a HORIBA JobinYvon 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. Spectroscopic properties were measured by reflection on monolith glass with a typical thickness of 0.5 mm. A front-face holder for samples was used to clip the sample and was oriented at 90° to minimize the specular reflection from the excitation beam. Suitable cutoff filters were employed, and the acquired spectra were corrected for the optical transfer function of the systems. All the spectroscopic measurements were performed at room temperature. All the NMR experiments were conducted at ambient temperature on Bruker DSX-400 and DSX-500 spectrometers, using 4.0 mm MAS NMR probes operated at spinning speeds between 12 and 15 kHz. At the two magnetic flux densities, the resonance frequencies were 104.3 and 130.3 MHz for 27Al and 162.4 and 202.5 MHz for 31P, respectively. The 27Al and 31P 90° pulses were set at 2 and 5 μs, respectively, and recycle delays of 1.0 s (27Al) and 90 s (31P) were used. 27Al and 31P chemical shifts are referenced to 1.0 M aluminum nitrate and 85% H3PO4 aqueous solutions, respectively. The proximity of the aluminum species to nearby protons and phosphorus nuclei was probed by evaluating the 27Al/1H and 27 Al/31P magnetic dipoledipole interactions, using 27Al{1H} and 27 Al{31P} rotational echo double resonance (REDOR) spectroscopies. These experiments were conducted at a magnetic flux density of 11.7 T, using the standard pulse sequence published by Schaefer and Gullion,24 phase cycled according to the XY-4 scheme.25 The following typical experimental conditions were used: MAS frequency of 14 kHz, and 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.
3. RESULTS AND DISCUSSION 3.1. Solid State NMR Studies. To gain insight into the structural evolution after incorporation of dyes, 27Al, 31P, and 1H NMR spectra were recorded at the various processing steps. Figure1 summarizes the 31P (left) and 27Al (right) MAS NMR spectra of AlPO4 mesoporous glass doped with different Rh6G and Cou102 concentrations at a fixed Rh6G/Cou102 ratio of 1.0. The 31P spectra 9177
dx.doi.org/10.1021/jp200596g |J. Phys. Chem. C 2011, 115, 9176–9181
The Journal of Physical Chemistry C
ARTICLE
Figure 2. Fractional contribution of the new Al(6) unit to the 27Al NMR spectrum combined with contributions of the new Q(0)3Al and Q(0)2Al units to the 31P NMR spectrum for the samples loaded with different Cou102 and Rh6G concentrations.
indicated a systematic evolution as a function of dye concentration. The spectra can be deconvoluted into three dominant Gaussian components20,26with their chemical shifts near 26, 16, and 5 ppm, which, respectively, are attributed to the tetrahedral P(OAl)4 (Q(0)4Al) units, depolymerizational P(OAl)3 (Q(0)3Al) units, and tetrahedral P(OAl)2 (Q(0)2Al) units of the glassy framework. It is remarkable that obviously spectroscopic changes present in the 27Al MAS NMR spectra obtained on these glasses doped with dyes (Figure 1, right). For the reference AlPO4 mesoporous glass and the one dipped just in ethanol, the spectra show a dominant signal at 40 ppm attributed to the four-coordinated Al(OP)4 (Al(4)) units. In addition, a small fraction of higher-coordinated Al defect sites (Al(OP)5 and Al(OP)6) always appear to be present at low levels as a low-frequency shoulder. A clear new Al site with a peak maximum around 13 ppm emerges, arising from six-coordinated units20 after being doped with Rh6G and Cou102, which signifies the presence of some high-coordinated Al(6) sites attributed to the interaction of laser dyes and the glassy framework. Furthermore, the concentration of this newly formed Al(6) unit exhibits an increasing tendency as the loaded dye concentration increased until some extent of saturation is reached. The fractional contribution of the new Al(6) unit to the 27Al NMR spectrum combined with contributions of the new Q(0)3Al and Q(0)2Al units to the 31P NMR spectrum is summarized in Figure 2 (calculation by DMFIT software).26 It appears that a fraction of decreased Al(4) units are replaced by increased Al(6) units, which supports a transfer from Al(4) units to Al(6) units, attributed to the interaction between loaded dyes and Al(4) units in the surface of the pore. We, therefore, concluded that the newly formed Al(6) units are mostly resulting from some breakage of AlOP bonds and interaction of dyes. To confirm the composition of Al(6) sites, 27Al {1H} and 27Al 31 { P} REDOR were conducted on these AlPO4 glasses doped with Rh6G and Cou102 dyes. For a short evolution time, NTr, where 0 e ΔS/S0 e 0.20.3, the REDOR curve is found to be independent of specific spin system geometries and can be approximated by a simple parabola:27,28 ΔS 4 ¼ 2 M2SI ðNTrÞ2 S0 3π
ð1Þ
Here, MSI 2 is the heterodipolar second moment characterizing the average magnitude of the heteronuclear IS dipolar interactions. The curvature of this parabola is closely related to the van
Figure 3. 27Al{1H} REDOR curves measured for the Al(4) and Al(6) sites in AlPO4 glass doped with Cou102 and Rh6G at different concentrations.
Figure 4. 27Al{31P} REDOR curves measured for the Al(4) and Al(6) sites in AlPO4 glass doped with Cou102 and Rh6G at different concentrations. Data are included for samples obtained with two different Cou102 and Rh6G concentrations (glass a and glass d), as well as the original glass without dyes.
Vleck second moment, MSI 2 = M2(S{I}), which can be calculated by the van Vleck equation29 M2SI ¼
μ0 4π
2
4 IðI þ 1Þp2 γ2s γ2I 15
∑s γ6 IS
ð2Þ
where γI and γS are the gyromagnetic ratios of the nuclei I and S involved and γIS is the internuclear distance. In the present study, we will use this approach to analyze the dipolar field created by 27 Al at the observed 31P nulei. Figure 3 summarizes the 27Al{1H} REDOR dephasing results. It reveals that the Al(6) units are significantly more strongly dipole-coupled to 1H species in comparison to the Al(4) units. Accordingly, the Al(6) sites are mostly arising from the interaction between hydrogen and Al(4) sites. To further confirm the influence to the connectivity of 27Al and 31P, 27Al{31P} REDOR were conducted on the typical samples loaded with dyes (glass a and glass d) and without dyes. Figure 4 summarizes the 27Al{31P} REDOR dephasing results. The MSI 2 values extracted by fitting eq 2 to the initial 27Al{31P}REDOR data are 4.2 and 3.1 kHz2 for 9178
dx.doi.org/10.1021/jp200596g |J. Phys. Chem. C 2011, 115, 9176–9181
The Journal of Physical Chemistry C
ARTICLE
Figure 5. Excitation spectra of AlPO4 mesoporous glass doped with different Cou102 and Rh6G concentrations (a) and different Cou102/Rh6G molar ratios (b) (λemis = 600 nm; emission and excitated bandwidths were 2 nm).
Figure 6. Emission spectra of AlPO4 mesoporous glass doped with different Cou102 and Rh6G concentrations (λexc = 320 nm; emission and excitated bandwidths were 2 nm).
Al(4) and Al(6) sites, respectively, and are independent of the dye concentrations. The MSI 2 value for Al(6) sites is much smaller 2 compared with the MSI 2 = 6.4 kHz measured for the Al(PO3)3 20 crystal, in which each Al is octahedrally coordinated by six phosphor atoms at a distance of 3.251 Å; therefore, the component of Al(6) sites at 13 ppm is probably not octahedrally coordinated by six phosphor atoms, but instead, by four phosphor atoms and two hydrogen atoms. Considering the fitted MSI 2 values, the distance γIS between 27 Al and 31P calculated based on eq 3 are 3.261 and 3.412 Å for Al(4) units and Al(6) units, respectively. The increasing of the distance from 3.261 to 3.421 Å between 27Al and 31P suggests a strong interaction between 27Al and 1H, which results in the distortion of the Al(OP)4 tetrahedron to be an Al(OP)4H2 polyhedron. We, therefore, concluded that the laser dyes were successfully solidified in the AlPO4 mesoporous structure owing to the strong interaction between the dyes and 27Al, which can be explained in a reaction of type AlðOPÞ4 þ 2R H f AlðOPÞ4 ðR HÞ2
ð3Þ
where RH represents the laser dyes (Rh6G and Cou102). 3.2. Optical Properties. Incorporation of laser dyes into a solid state host is determined by the aggregate effect. Rh6G and
Cou102 show a strong tendency to aggregate as a fluorescent J-Dimer and a nonfluorescent H-dimer10,30,31 after embedding in a solid state matrix. It is important to differentiate that these two aggregating forms 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 and broaden the excitation band through interaction of transition dipoles.30 Solidification of dyes into mesoporous glass offers a higher concentration dye loading approach without serious aggregates. A key point to achieving efficient simultaneous dual-wavelength emission is to control the dyes' state affected by the dyes concentration and dye ratio. Figure 5 presents the normalized excitated spectra of AlPO4 mesoporous glasses doped with increasing concentrations of Rh6G and Cou102 (Figure 5a), as well as increasing Cou102/ Rh6G molar ratios (Figure 5b). The excitated spectra of Rh6G in the monomeric form is characterized by an excitated band around 530 nm, whereas the two excitated bands of Cou102 in the monomeric form are placed around 400 nm for the samples with lower dye loading (5 106 M). In addition, all the benzene laser dyes shows a vibronic excitation band around 340 nm, which, therefore, provide a possibility to excite these two dyes simultaneously. Broadening and splitting of the excitation band around 530 nm were observed in Figure 5a as the dye concentration increased from 5 106 M to 5 104 and 5 103 M due to the formation of J-dimers of Rhodamine 6G aggregates, and as expected, its intensity increases with increasing doping concentration, which can be explained by exciton theory.30 Likewise, it can be seen clearly in the excitation spectra (Figure 5b) that a successive broadening of a band around 530 nm increases with decreasing Cou102/Rh6G ratios from 4.0 to 0.5, concomitant with the broadening and increasing of a band around 490 nm attributed to the increased Rh6G J-dimers at high loaded Rh6G concentration (5 103 M). Figure 6 presents the normalized emission properties observed for the samples with different dye concentrations. For the set of samples with different dye concentrations, it can clearly be seen in the emission spectra two pronounced emission bands with a similar fluorescence efficiency centralized around 400 and 530 nm as the loaded dye concentration is low (5 106 M), 9179
dx.doi.org/10.1021/jp200596g |J. Phys. Chem. C 2011, 115, 9176–9181
The Journal of Physical Chemistry C
Figure 7. Schematic energy level diagrams of Cou102 and Rh6G dyes.
Figure 8. Emission spectra of AlPO4 mesoporous glass doped with different Cou102/Rh6G molar ratios (λexc = 320 nm; emission and excitated bandwidths were 2 nm).
which characterized the fluorescence of Cou102 and Rh6G, respectively. There is a relatively pronounced tendency to decrease the fluorescence intensity of Cou102 placed around 400 nm as the dye concentration increased, which suggests some fluorescence of Cou102 quenched by absorption of dye aggregates, especially Rh6G J-dimers. The schematic energy level diagrams of these two dyes doped in AlPO4 mesoporous glasses are illustrated in Figure 7 based on the excitation spectra (Figure 5). The overlapping energy level between Rh6G and Cou102 signifies a partial quenching of the fluorescence of Cou102 by the absorption of Rh6G dyes, and it rapidly undergoes nonradiative decay, transferring its energy to the lower-lying states of the Rh6G monomer and the J-dimers. Particularly, the broadening of the excitation of Rh6G J-dimers to the blue side of the shoulder band around 490 nm presents a successive increasing tendency as the Rh6G dye concentration increased, whereas the fluorescence of Cou102 suggests a progressive red shift.32,33 Namely, more serious fluorescence quenching of Cou102 occurs for higher loaded dye concentration samples as both dye amounts are equivalent. It, therefore, indicated that both dyes can be simultaneously efficiently fluorescent at lower loaded dye concentrations (below 5.0 105 M) as the dyes remain in a monomeric state without seriously quenching, whereas a fraction of the fluorescence of Cou102 quenches is attributed to
ARTICLE
excitation broadening of Rh6G J-dimers as the loaded dye concentration is up to 5.0 105 M, especially for the sample with a loaded dye concentration of 5.0 103 M. That is to say, it is challenging to achieve efficient comparative dual emission in high loaded concentration as both dye amounts are equivalent. Thus, the normalized emission spectra (Figure 8) of AlPO4 mesoporous glasses loaded with Rh6G and Cou102 with different Cou102/Rh6G molar ratios at higher concentration were investigated to optimize the efficient emission intensity of Cou102. It presents an evident tendency to increase the emission intensity of Cou102 around 400 nm as the molar ratio R increases from 0.5 to 4.0. An efficient emission of both dyes can be achieved as the molar ratio R is 4.0. It signifies an efficient approach to adjusting the fluorescence intensity of Rh6G and Cou102 around 530 and 400 nm, respectively. Therefore, it is concluded that dual-wavelength emission placed at 400 and 530 nm can be achieved by codoping Cou102 and Rh6G into mesoporous AlPO4 glass.
4. CONCLUSIONS In summary, we have developed a simple method to achieve simultaneous dual-wavelength emission around 400 and 530 nm by solidifying Cou102 and Rh6G in AlPO4 mesoporous glass hosts. The 27Al and 31P solid state NMR results, particularly the 27 Al MAS NMR data that reveals the presence of new Al(6) sites after incorporating laser dyes, demonstrate convincingly that a strong interaction between laser dyes and the AlPO4 framework signifies the successful solidification of laser dyes in the mesoporous structure. The excitation and emission spectra indicate that the fluorescence of Cou102 and Rh6G depends strongly on the dye concentration and Cou102/Rh6G ratio. It is remarkable that the AlPO4 mesoporous glass loaded with low dye concentration (5.0 106 M) favors efficient simultaneous dualwavelength emission around 400 and 530 nm, whereas a fraction of the fluorescence of Cou102 around 400 nm quenches at higher loaded dye concentrations, which are responsible for the absorption and energy transferring of Rh6G aggregates. However, improvement in the fluorescence intensity of Cou102 at higher concentration is favored by high Cou102/Rh6G ratios (R > 1); for example, as the loaded Cou102/Rh6G ratio is 4.0, the efficient simultaneous dual-wavelength emission is comparable. Thus, the novel solgel AlPO4 mesoporous glass is suitable to solidify Cou102 and Rh6G in applications as a tunable simultaneous dual-wavelength emission dye host. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (L.Z.),
[email protected] (J.W.). Tel: þ86 2169918793 (L.Z.), þ862169914184 (J.W.). Fax: þ86 2169918932 (L.Z.), þ862169918932 (J.W.).
’ ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant 51072207) and Shanghai Natural Science Foundation (Grant 11ZR1441500). J.W. acknowledges the financial support from the starting grant of the 100-Talent Program of SIOM, Chinese Academy of Sciences (1108221-JR0). We thank Geng Lin for some help in optical measurements. 9180
dx.doi.org/10.1021/jp200596g |J. Phys. Chem. C 2011, 115, 9176–9181
The Journal of Physical Chemistry C
ARTICLE
’ REFERENCES (1) Al-Shamiri, H. A. S.; Abou Kana, M. T. H. Appl. Phys. B: Lasers Opt. 2010, 101, 129–135. (2) Garcia-Moreno, I.; Costela, A.; Martin, V.; Pintado-Sierra, M.; Sastre, R. Adv. Funct. Mater. 2009, 19, 2547–2552. (3) Boucharef, M.; Di Bin, C.; Boumaza, M. S.; et al. Nanotechnology 2010, 21, 205203–205215. (4) Junhai, H.; Yufang, X.; Xuhong, Q. J. Org. Chem. 2009, 74, 2167–2170. (5) Sanchez, C.; Lebeau, B.; Chaput, F.; Boilot, J. P. Adv. Mater. 2003, 15, 1969–1994. (6) Levitus, M.; Aramendia, P. F. J. Phys. Chem. B 1999, 103, 1864–1870. (7) Salcedo, W. J.; Fernandez, F. J. R.; Rubim, J. C. Phys. Status. Solidi C 2010, S1, 26–30. (8) Voss, T. D.; Scheel; Schade, W. Appl. Phys. B 2001, 73, 105–109. (9) Somasundaram, G.; Ramalingam, A. J. Lumin. 2000, 90, 1–5. (10) Monte, F. D.; Mackenzie, J. D.; Levy, D. Langmuir 2000, 16, 7377–7382. (11) Qinyuan, Z.; Que, W.; Buddhudu, S.; Pita, K. J. Phys. Chem. Solids 2002, 63, 1723–1727. (12) Avnir, D.; Levy, D.; Reisfeld, R. J. Phys. Chem. 1984, 88, 5956–5959. (13) Scott, B. J.; Wirnsberger, G.; Mcgehee, M. D.; Chmelka, B. F.; Stucky, D. Adv. Mater. 2001, 13, 1231–1234. (14) Wark, M.; Rohlfing, Y.; Altindag, Y.; Wellmann, H. Phys. Chem. Chem. Phys. 2003, 5, 5188–5194. (15) Jin, Y.; Chon, H. Chem. Commum. 1996, 135–136. (16) Jelvani, S.; Khodadoost, B. Opt. Laser Technol. 2007, 39, 182–188. (17) Sathy, P.; Penzkofer, A. J. Photochem. Photobiol., A 1997, 109, 53–57. (18) Perez-Bueno, J. J.; Vasquez-García, S. R.; et al. J. Phys. Chem. B 2002, 106, 1550–1556. . .; Loerke, J.; W€ustefeld, U.; Marlow, F.; Sch€uth, F. (19) Weiss, O J. Solid State Chem. 2002, 167, 302–309. (20) Long., Z.; B€ogershausen, A.; Eckert, H. J. Am. Ceram. Soc. 2005, 88, 897–902. (21) Costela, A.; Garcia- Moreno, I.; Figuera, J. M.; Amat-Guerri, F.; Sastre, R. Appl. Phys. Lett. 1996, 68, 593–596. (22) Mckiernan, J. M.; Yamanaka, S. A.; Dunn, B.; Zink, J. I. J. Phys. Chem. 1990, 94, 5652–5654. (23) Littman, M. G.; Metcalf, H. J. Appl. Opt. 1978, 17, 2224–2227. (24) Gullion, T.; Schaefer, J. J. Magn. Reson. 1989, 81, 196–200. (25) Gullion, T. Magn. Reson. Rev. 1997, 17, 83–131. (26) Massiot, D.; Fayon, F.; Capron, M.; et al. Magn. Reson. Chem. 2002, 40, 70–76. (27) Bertmer, M.; Eckert, H. Solid State Nucl. Magn. Reson. 1999, 15, 139–152. (28) Chan, J. C. C; Bertmer, M.; Eckert, H. Chem. Phys. Lett. 1998, 292, 154–160. (29) Van Vleck, J. H. Phys. Rev. 1948, 74, 1168–1183. (30) Kasha, M.; Rawls, H. R.; Ashraf El-Bayoumi, M. Pure Appl. Chem. 1965, 11, 371–392. (31) Anedda, A.; Carbonaro, C. M.; Corpino, R.; et al. J. Non-Cryst. Solids 2007, 353, 481–485. (32) Rihong., L.; Youyu, F.; Jintai, F.; Long, Z. Acta Opt. Sin. 2010, 10, 2983–2987. (33) Nibbering, E. T. J.; Tschirschwitz, F.; Chudoba, C.; Elsaesser, T. J. Phys. Chem. A 2000, 104, 4236–4246.
9181
dx.doi.org/10.1021/jp200596g |J. Phys. Chem. C 2011, 115, 9176–9181
