DOI: 10.1002/chem.201400392

Full Paper

& Materials Chemistry

Tailoring Photoluminescence Properties in Ionic Nanoparticle Networks Martin Kronstein,[a] Johanna Akbarzadeh,[b] Christina Drechsel,[a] Herwig Peterlik,[a] and MarieAlexandra Neouze*[a, c]

Abstract: To investigate the original and promising luminescence properties of ionic nanoparticle networks (INN), various material compositions were investigated. In this work, the linker used to network the silica nanoparticles was varied; numerous substituted or non-substituted imidazolium, pyrazolium and pyridinium linkers are presented. Photoluminescence experiments on the INN hybrid materials revealed strong emission bands over a broad range in the visible region of the light spectrum. Varying the aromatic linker between the imidazolium units induced clear shifts of the emission maxima up to 100 nm, as a consequence of p–p

Introduction Numerous studies on light-emitting hybrid materials have been published in the last years. Some of these materials consist of lanthanide ions[1] or platinum-group metal ions[2] owing to their photophysical properties. Nevertheless, as lanthanides and platinum-group metals are rare and quite expensive, metal-free photoluminescence active materials have been recently developed.[3] Graffionet al. published the synthesis of blue-to-green-emitting bipyridine-bridged silsesquioxanes, which were excited by commercial blue LEDs.[4] In the frame of developing luminescent materials, the community started to focus on some ionic liquids, which were proved to be luminescent.[5, 6] Thus room-temperature ionic liquids (RTIL), for example, 1-butyl-3-methyl-imidazolium-based ionic liquids, were used to improve the electroluminescence properties of lightemitting compounds. R. Martin et al. used a 2-methylimidazolium chloride based ionophilic 9,10-diarylanthracene for the building of OLEDs.[7] Furthermore, R.C. Evans et al. reported the

[a] M. Kronstein, C. Drechsel, Prof. H. Peterlik, Prof. M.-A. Neouze Institute of Materials Chemistry - Applied Inorganic Chemistry Vienna University of Technology; 1060 Vienna (Austria) [b] J. Akbarzadeh Faculty of Physics; Unversity of Vienna 1090 Vienna (Austria) [c] Prof. M.-A. Neouze Physics of Condensed Matter, Ecole Polytechnique 91128 Palaiseau (France) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201400392. Chem. Eur. J. 2014, 20, 10763 – 10774

stacking interactions. Steric hindrance and inductive effects of the substituents, introduced on the aromatic units, also strongly influenced the luminescence properties of the materials by modifying the p–p stacking between the imidazolium rings. Small and wide-angle X-ray scattering (SAXS, WAXS) experiments revealed a clear trend between the obtained structural parameters (short-range order parameter and distance of the aromatic units within the hybrid material) and the luminescence quantum yields of the INN materials.

synthesis of photoluminescent ionogels. In this ionogel1-butyl3-methyl-imidazolium-bis(trifluoromethanesulfonyl)imide [Bmim] + [Tf2N] was entrapped in poly(9,9-dioctylfluorene).[8] Imidazolium salts in general showed interesting fluorescence properties, for example, Bielawskiet al. published the synthesis and application of benzobis(imidazolium) salts and furthermore their use in fluorescent block ionomers and liquid crystals.[9, 6] Recently we published the synthesis of new hybrid materials, ionic nanoparticle networks, which will be hereafter referred to as INN (Scheme 1).[10] The INN materials showed attractive properties, such as catalytic activity,[11] anion exchange ability[12] or thermochromic behavior by complexation of copper dichloride.[13] Moreover, the INN materials showed interesting photoluminescence properties.[14] In the present work, the tailoring of the excitation and emission wavelengths of INN materials was investigated. For this purpose, variations of the substituents on the C2-position of the imidazolium ring and/or varying the organic linker between the two imidazolium units were performed. The photoluminescence properties and small-angle X-ray scattering profiles of mono- and bis(imidazolium) linkers, with or without substituents, as well as pyrazolium- and pyridinium-based INN materials are reported and compared.

Results and Discussion Synthesis of various materials Preparation of the hybrid materials starts with the synthesis of various nitrogen-base-modified trimethoxysilanes. One possi10763

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper Another possibility to get trimethoxysilanes is a one-pot synthesis including a metal-catalyst-free hydrosilylation followed by a methanolysis reaction. This second method was chosen for the pyridine derivatives. In the first step of this reaction, the trichlorosilane reacts with the 2- or 4-vinylpyridine by refluxing in dry acetonitrile to form the respective trichlorosilylethylpyridine (Scheme 3).[15] The second step of this reaction is

Scheme 3. Formation of the 4-(2trimethoxysilylethyl)pyridine (6) in a onepot synthesis.

Scheme 1. Reaction of imidazole-modified silica nanoparticles with chloroalkyl-modified silica nanoparticles to form the ionic nanoparticle networks (INN).

bility to get these trimethoxysilanes is a nucleophilic substitution between different substituted imidazoles or pyrazoles and 3-iodopropyltrimethoxysilane (Scheme 2). Thus, the imidazoles or pyrazoles are allowed to react with sodium hydride to give the respective sodium imidazolide or pyrazolide. This salt then undergoes nucleophilic substitution with 3-iodopropyltrimethoxysilane to give the respective N-(3-propyltrimethoxysilane)imidazole (with yields of 89 % for 1, 78 % for 2 and 76 % for 3) or N-(3-propyltrimethoxysilane)pyrazole (yields of 82 % for 4 and 63 % for compound 5).

the substitution of the chlorine atoms by three methoxy groups. After purification, a colorless transparent liquid was successfully obtained in yields of 33 and 75 % for the 2-(2-trimethoxysilylethyl)pyridine (7) and the 4-(2-trimethoxysilylethyl)pyridine (6), respectively. It turned out that the yield of the 4-(2-trimethoxysilylethyl)pyridine was significantly higher than for the 2-(2-trimethoxysilylethyl)pyridine. One explanation could be the formation of a pentacoordinated silicon species as an intermediate,[16] which lowers the reactivity in connection with the substitution of the chlorine atoms by the methoxy groups. Subsequently, the freshly prepared silica nanoparticles were modified with the trimethoxysilanes. The formation of the hybrid materials consists: either of a nucleophilic substitution between the functional groups of the chloro- and the imidazole-, pyrazole- or pyridine-modified silica nanoparticles (Scheme 4, lines 1 and 5 to 7) or in the nucleophilic substitution between two equivalents of the nitrogen-base-modified silica nanoparticles and one equivalent of a dichloro-substituted linker molecule (Scheme 4, lines 2 to 4). The material suspension could be dip-coated on glass to form transparent thin films, or, after removing the solvent, transparent gels were obtained (see the Supporting Information). Photoluminescence properties

Scheme 2. Synthesis of various N-(3-propyltrimethoxysilane)imidazoles (1–3) and N-(3-propyltrimethoxysilane)pyrazoles (4 and 5). Chem. Eur. J. 2014, 20, 10763 – 10774

www.chemeurj.org

10764

The photoluminescence properties of the various INN materials were investigated in the solid state after drying. The excitation spectra of the substituted and non-substituted mono-imidazolium INN materials, Mat 1 to Mat 3, are characterized by a broad band in the region between 300–400 nm (solid lines in Figure 1). Addition-

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper

Scheme 4. Formation of the various INN hybrid materials by means of nucleophilic substitution reactions (Mat 1–Mat 16).

ally a shift for the absorption maxima could be detected: Mat 1 shows a maximum in absorption at 320 nm, Mat 2 at 365 nm and Mat 3 at 345 nm. By exciting the samples at the wavelength of these maxima, an emission in the range between 350–550 nm could be detected (solid lines in Figure 1), with an emission maxima for Mat 1 to Mat 3 situated at 380, 440 and 400 nm, respectively. These maxima correspond to an emission maximum in the violet area for Mat 1 and Mat 3 and in the blue area of the light spectrum for Mat 2; due to the broadness of these emission bands, the materials seem to all emit in the blue region to the naked eye. When modifying the C2-position of the imidazolium ring, either with a methyl or an isopropyl group, a clear shift for the emission maxima was monitored, Mat 2 and Mat 3 as compared to Mat 1. The largest shift of the excitation and for the emission maxima was observed for the material substituted with a methyl group on the C2-position of the imidazolium ring (Mat 2). For the isopropylsubstituted mono-imidazolium INN material (Mat 3), a slightly smaller value of the shift of excitation and emission maximum, compared to Mat 2, was detected. Chem. Eur. J. 2014, 20, 10763 – 10774

www.chemeurj.org

The study of the photoluminescence properties of bis(imidazolium) INN materials containing the p-dichloroxylene linker (Mat 4–Mat 6) reinforced our previous observations. The excitation spectra of these materials show broad bands in the region between 320 and 380 nm and a second local maximum can be distinguished between 200 and 280 nm. The origin of these two distinct maxima in the spectra could not be assigned yet; however, a similar observation was reported by Paul et al. in a work dedicated to the optical properties of imidazolium ionic liquids.[17] By exciting the materials with the wavelength corresponding to the excitation maxima, emission maxima were observed in a range between 410–450 nm (solid lines in Figure 2), with emission maxima for Mat 4 to Mat 6 at 410, 450 and 440 nm, respectively. By comparing Mat 4 to Mat 6, a shift of the excitation and of the emission maxima could be observed. Here, the same effect of the substituents as for Mat 1 to Mat 3 was monitored. Namely, the largest shift, for the excitation and for the emission maxima, was observed for the material substituted with a methyl group on the C2-position of the imidazolium ring (Mat 5). For the isopropyl-modified bis-

10765

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper

Figure 1. Top) Excitation (excitation Mat 1 (a), Mat 2 (a) and Mat 3 (a)) and emission spectra (emission Mat 1 (c), Mat 2 (c) and Mat 3 (c)) of the substituted and non-substituted mono-imidazolium materials (Mat 1–3). Bottom) Digital photos of the emission of the materials Mat 1–Mat 3 under excitation at 320, 365 and 345 nm, respectively.

(imidazolium) INN material (Mat 6), a slightly smaller shift of excitation and emission maximum, relative to Mat 5, was detected. The explanation for this phenomenon is the same as for Mat 1 to Mat 3: the methyl groups on the imidazolium moieties of Mat 5 cause a positive inductive effect on the imidazolium rings while offering only a weak steric hindrance. For Mat 6, the steric effect dominates the positive inductive effect. In consequence a larger shift in the excitation and emission maxima could be detected for Mat 5 than for Mat 6 compared to Mat 4. In summary it turns out that the general trend of the shift between the different substituted imidazolium moieties in these materials is very similar to the mono-imidazolium INN materials. The comparison between the mono-imidazolium and bis(imidazolium) INN hybrid materials illustrates that the p-dichloroxylene linker containing INN materials (Mat 4 to Mat 6) in general shows higher wavelengths for the excitation maxima and emission maxima. This can be explained by the redshift of aromatic rings.[18] For a higher content of delocalized electrons in the material in combination with free rotatability of the aromatic rings, 4,4’bis(chloromethyl)-1,1’-biphenyl was used as the linking reactant for the nucleophilic substitution. The corresponding INN Chem. Eur. J. 2014, 20, 10763 – 10774

www.chemeurj.org

Figure 2. Top) Excitation (excitation Mat 4 (a), Mat 5 (a) and Mat 6 (a)) and emission spectra (emission Mat 4 (c), Mat 5 (c) and Mat 6 (c)) of the substituted and non-substituted bis(imidazolium) materials containing the xylene linker (Mat 4–Mat 6). Bottom) Digital photos of the emission of the materials Mat 4–Mat 6 under excitation at 350, 370 and 370 nm, respectively.

materials are thus derivatives of a bis(imidazolium) biphenyl (Mat 7 to Mat 9 in Scheme 4). The excitation spectra and emission spectra of these materials (Mat 7 to Mat 9 in Figure 3) are more complex than the materials Mat 1 to Mat 6. Regarding the excitation spectra, it can be noticed that the excitation bands for Mat 7 and Mat 8 look quite similar to each other and show excitation maxima at nearly the same wavelength, namely at 300 nm. Mat 7 shows one global absorption maximum at 305 nm (a in the left spectrum in Figure 3). By exciting the material with this wavelength, an emission maximum was observed at 350 nm (see the Supporting Information). Additionally in a range between 310 and 380 nm three local maxima in absorption could be detected. The most intensive one (375 nm) was chosen for exciting the material and resulted in an emission band with a global maximum at 425 nm (blue area of the light spectrum) and two other local maxima. This suggests that the material could be used for an emission at two different wavelengths depending on the excitation wavelength. For Mat 8, with a similar excitation spectrum as Mat 7, an emission band was observed for an excitation at

10766

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper

Figure 3. Top) Excitation (excitation Mat 7 (a), Mat 8 (a) and Mat 9 (a)) and emission spectra (emission Mat 7 (c), Mat 8 (c) and Mat 9 (c)) of the substituted and non-substituted bis(imidazolium) materials connected by the biphenyl linker (Mat 7–Mat 9). Bottom) Digital photos of the emission of the materials Mat 7–Mat 9 under excitation at 375, 300 and 370 nm, respectively. Bottom left) for comparison, emission spectra of Mat 7 (c) and the pure biphenyl linker (c) under excitation at 375 nm.

300 nm. This emission band shows a broad maximum plateau between 320 and 390 nm. At this stage the origin of this behavior could not be identified. Mat 9 shows a different behavior: the very broad absorption band shows two maxima at 370 and 420 nm (a). By exciting the material at these wavelengths the resulting emission bands look quite similar to each other. In both cases a maximum in emission was detected at 475 nm. It can be noticed that by replacing the hydrogen atom at the C2-position of the imidazolium ring by a methyl group, Mat 8, the color of the emitted light could be changed from blue to purple and by replacing the methyl group with an isopropyl group, Mat 9, the color of the emitted light changes to green (Figure 3, right). Large differences in the luminescence spectra of Mat 7 to Mat 9 compared to Mat 1 to Mat 6 were monitored. This difference lies in the presence of the biphenyl linker, in addition to Chem. Eur. J. 2014, 20, 10763 – 10774

www.chemeurj.org

the imidazolium moiety, which already emits by itself (Figure 3, bottom right). For comparison with these INN materials the luminescence properties of the pure linker molecule, 4,4’-bis(chloromethyl)-1,1’-biphenyl, were investigated. It turned out that the linker molecule shows an emission for an excitation at 375 nm too (c on the right in Figure 3), but quite weak in comparison to Mat 7 (c). The luminescence spectra were recorded by measuring the emission of a cuvette fully filled with the compound to investigate. In consequence, in the bottomright spectra in Figure 3, a compound containing only around 20 wt. % of biphenyl, the INN Mat 7, is compared with 100 wt. % of pure biphenyl. In conclusion of this part, it can be claimed that the luminescence observed for Mat 7 to Mat 9, is due to the organization enforced during the formation of the INN. However, it seems that in this case the luminescent behavior of the materials containing bis(imidazolium)biphenyl linkers is governed by the biphenyl and no more only by the imidazolium units. In addition, the materials combining imidazolium and linker luminescence (Mat 7 to Mat 9) present adjustable luminescence properties. For a further increasing of the content of delocalized electrons, but in this case combined with a rigid adjustment of the aromatic rings in the material, 9,10-bis(chloromethyl)anthracene was used as the linking reactant for the nucleophilic substitution. The corresponding INN materials are thus derivatives of a bis(imidazoliumanthracene) (Mat 10 to Mat 12). Similarly, as for Mat 7 to Mat 9, Mat 10 to Mat 12 showed a complex behavior in the excitation as well as in the emission spectra. Mat 10 shows an excitation maximum at 380 nm. Mat 11 and Mat 12 exhibit an emission maximum in the same area. Additionally two local maxima were detected in the excitation spectra. These relative maxima are typical for anthracene chromophores.[7] By exciting Mat 10 at 380 nm, an emission maximum was observed at 490 nm in the green area of the light spectrum. The emission spectra for Mat 11 and Mat 12 show similar behavior. The emission maxima are centered at 430 nm for an excitation at 375 and 380 nm, respectively. One origin for this behavior could be the self-absorption of the material combined with a partial quenching of the fluorescence in the wavelength area around 500 nm. This suggests that the linker molecule adsorbs the emitted light caused by p–p stacking interactions in the material and is able to emit visible light on its own (Figure 4, bottom). This conversion could be responsible for the numerous different maxima in the excitation and emission spectra for the INN materials Mat 10 to Mat 12. Compared to the reference material (Mat 1) the anthracene-linked bis(imidazolium) INN materials show redshifted excitation and emission maxima, but they have in common that these bands are very broad. In consequence, it seems to the naked eye that they are emitting green light (Figure 4, right). Furthermore the linker molecule itself, the 9,10-bis(chloromethyl)anthracene, shows photoluminescence properties as well (c on the left in Figure 4). This molecule was used for the synthesis of some other compounds and materials bearing interesting photoluminescence properties.[19, 20] Compared to Mat 10 the emission for an excitation at 375 nm of the pure linker is quite weak and observed for the biphenyl containing materials and the pure

10767

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper

Figure 5. Top) Excitation (excitation Mat 1 (a), Mat 13 (a) and Mat 14 (a)) and emission spectra (emission Mat 1 (c), Mat 13 (c) and Mat 14 (c)) of the substituted and non-substituted pyrazolium-based materials (Mat 13 and Mat 14) and, for comparison, of the reference material Mat 1. Bottom) Digital photos of the emission of the respective materials under excitation at 320 nm.

Figure 4. Top) Excitation (excitation Mat 10 (a), Mat 11 (a) and Mat 12 (a)) and emission spectra (emission Mat 10 (c), Mat 11 (c) and Mat 12 (c)) of the substituted and non-substituted bis(imidazolium) materials connected by the anthracene linker (Mat 10–Mat 12). Bottom) Digital photos of the emission of the materials Mat 1–Mat 3 under excitation at 380, 375 and 380 nm, respectively. Bottom left) for comparison, emission spectra of Mat 10 (c) and the pure anthracene linker (c) under excitation at 380 nm.

biphenyl linker. Here, as for the biphenyl described in the precedent paragraph, the difference in the relative intensity is significant. To get a better understanding of the photoluminescence properties of the INN hybrid materials in general, the imidazolium moieties were replaced by a pyrazolium unit (Mat 13) or by 3,5-dimethylpyrazolium moiety (Mat 14). The excitation spectrum of Mat 13 shows a maximum at 310 nm (Figure 5). By exciting the material at this wavelength a broad emission band with a maximum at 375 nm could be observed. In comparison to Mat 1 the bands of the pyrazolium-based material show a shift towards lower wavelengths from 380 to 375 nm. This fact could be explained by the different geometry of the bridging organic part of the INN material (Scheme 4). The two nitrogen atoms of the pyrazolium are localized right next to each other, which makes a linear connection of the nanoparticles impossible, as opposed to the linking in imidazolium INN, Chem. Eur. J. 2014, 20, 10763 – 10774

www.chemeurj.org

such as Mat 1. The excitation spectrum of Mat 14 shows two maxima: at 275 and at 320 nm. By exciting the material at these wavelengths, one emission maxima could be detected at 380 nm. By comparing the emission maxima of Mat 13 and Mat 14 only a small difference was observed. This may be caused by the methyl groups at the C3- and C5-position of the imidazolium moiety by generating slightly repulsive steric interactions to the propyl groups located on the nitrogen atoms of the pyrazolium unit in combination with the positive inductive effect. Both Mat 13 and Mat 14 absorb and emit light at lower wavelengths compared to the imidazolium-based INN reference material (Mat 1). To extend the photoluminescence, investigations into sixatom rings, ortho- or para-linked pyridinium-based INN materials (Mat 15 and Mat 16) were investigated. The excitation spectrum of the para-linked pyridinium-based INN material (Mat 15) shows a maximum at 365 nm (Figure 6). The excitation of the material at this wavelength leads to a broad emission band with a maximum at 420 nm, which is localized in the blue area of the light spectrum. By comparing Mat 15 to Mat 1, it could be observed that the pyridinium-containing material shows a redshifted excitation and emission spectra. This could be explained by the different electronic properties of the pyridinium unit compared to the imidazolium unit in the INN material. Similarly, the excitation spectrum of the ortho-linked pyridinium-based INN material (Mat 16) shows a maximum at 320 nm. By exciting the material at this wavelength, two emission maxima could be detected: one global maximum at

10768

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper

Figure 7. Scattering curves of Mat 1–Mat 3 (*: Mat 1, the respective fits (c).

Figure 6. Top) Excitation (excitation Mat 1 (a), Mat 15 (a) and Mat 16 (a)) and emission spectra (emission Mat 1 (c), Mat 15 (c) and Mat 16 (c)) of the substituted and non-substituted pyrazolium-based materials (Mat 15 and Mat 16) and, for comparison, of the reference material Mat 1. Bottom) Digital photos of the emission of the respective materials under excitation at 365 nm.

430 nm and one local maximum at 490 nm. This may be caused by the different geometry: In Mat 15 the silica nanoparticles could be linearly linked, whereas in Mat 16 the linking angle is more acute (Scheme 4). Compared to the reference imidazolium-based INN material (Mat 1), the materials Mat 15 and Mat 16 absorb and emit at higher wavelengths. For all the photoluminescence experiments the silica nanoparticles are not part of the luminescence mechanism, no quantum yield can be estimated from these emission curves as the intensity is far too low (see the Supporting Information).

!:

Mat 2, &: Mat 3) and

inter-particle distance resulting from the bridging of the linker molecules. Since two structural levels co-exist in the materials (the large silica particles and the smaller bridging ligands), a unified model proposed by Beaucage for hierarchical structures[21] was used for data fitting. The short-range order is described by a hard sphere model,[22] from which the mean distance, d, and hard sphere volume fraction, h, can be deduced. For the silica particles, the set of parameters is given the subscript 1(d1 and h1), whereas the subscript 2 refers to the linkers (d2 and h2). The hard sphere volume fraction h is a measure of the degree of order in the material. The higher h, the higher the agglomeration and the more pronounced the scattering peaks are. Scheme 5 shows a sketch of the INN materials and the parameter describing the nanostructural arrangement. The fit pa-

Small- and wide-angle X-ray scattering experiments SAXS and WAXS measurements, covering a range of the scattering vector from q = 0.1 to 20 nm 1 were carried out on all materials to prove the influence of the different aromatic linker molecules and the different substitutional units (H , Me , iPr ) on the nanostructural organization. In the following, only the q-range from 0.1 to 10 nm 1 is shown, as this range covers the most interesting features: Figure 7 shows exemplary scattering curves for Mat 1–Mat 3 and the respective fits (c). The first shoulder or peak at about q = 0.4 nm 1 (which corresponds to a distance in real space of about 16 nm) corresponds to the size and distance of the silica nanoparticles. The broad peaks in the regime from q = 3 to 6 nm 1 (corresponding to distances of approximately 1–2 nm in real space) were attributed to the Chem. Eur. J. 2014, 20, 10763 – 10774

www.chemeurj.org

Scheme 5. Non-substituted mono-imidazolium-based ionic nanoparticle networks (INN) and the parameters describing the structural arrangement and degree of order extracted from the SAXS fitting. r1 is the radius of the silica nanoparticles, d1 and d2 the distances between the silica particles and between the imidazolium rings. h1 and h2 are the hard sphere volume fractions describing the degree of order of nanoparticles and linkers, respectively.

10769

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper rameters are the radius r1 of the silica particles, their distance d1, and the hard sphere volume fraction h1. Additionally, the degree of order of the imidazolium rings was described by two values: their distance d2 and the hard sphere volume fraction h2 value. Since it can be expected that the luminescence properties of the INN materials are dependent on the degree of short range order of the linkers, we restrict the discussion to the structural parameters, h2 and d2. All other fit results associated to the silica nanoparticles are summarized in the Supporting Information. The numerical values h2 and d2 and the measured emission/ excitation maxima and quantum yields are listed in Table 1.

“conjugation lengths” are generated, causing various HOMO– LUMO energy band gaps in the INN material.[7, 26] From the luminescence measurements the associated quantum yields were determined. The values are reported in Table 1. To compare the structural parameters obtained from the fits of the SAXS curves with the luminescence quantum yields, the mean values of h2, d2 and QY were calculated for the different substitutional units (H , Me , iPr). Although not all of the numerical values for h2 and d2 differ at a level of statistical significance, it is possible to identify an overall trend between the substituted end-groups, the obtained structural parameters, and the luminescence quantum yields (Figure 8).

Table 1. Photoluminescence excitation/emission maxima and quantum yields compared with the hard sphere volume fraction h2 and distance d2 of linkers obtained from the fitting of SAXS-WAXS curves for the various INN materials. Entry Material Excitation max. [nm] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Mat 1 Mat 2 Mat 3 Mat 4 Mat 5 Mat 6 Mat 7 Mat 7 Mat 8 Mat 8 Mat 9 Mat 9 Mat 10 Mat 10 Mat 11 Mat 11 Mat 12 Mat 12 Mat 13 Mat 14 Mat 15 Mat 16

Emission max. [nm]

320 365 345 350 370 370 305 375 275 300 370 420 380 470 375 470 375 490 320 320 365 365

380 440 400 410 450 440 350 425 320 390 475 475 490 550 430 540 430 550 375 380 420 430

Quantum yield [%]

h2  0.02 d2  0.05 [nm]

10 15 4 11 18 9 6 14 12 16 12 15 4 7 5 8 2 4 4 6 4 5

0.31 0.33 0.29 0.25 0.25 0.11 – – 0.27 – 0.26 – – – 0.3 – 0.29 – 0.13 0.29 0.33 0.30

1.37 1.40 1.40 1.70 1.50 1.50 – – 1.20 – 1.60 – – – 1.40 – 1.54 – 0.88 1.34 1.11 1.40

Figure 8. Correlation between h2, d2 and quantum yield.

The scattering curves of the INN materials exhibit a shortrange order peak, caused by the self-organization of the aromatic linkers in the material. This phenomenon was already discussed in the literature.[23] The luminescence feature was interpreted as a consequence of p–p stacking interactions between these aromatic rings. Therefore different substituted imidazolium-based INN materials in combination with various aromatic linker molecules as well as pyridinium- and pyrazoliumbased INN materials were used for a systematic investigation of the photoluminescence properties and their dependence on the structural organization on the nanometer level. The photoluminescence phenomena in this aromatic-containing INN materials is caused by the transition of the p* excited state to the p energy level.[24, 25] The origin of the broad excitation as well as emission bands could be attributed to slight differences in the distances d2 between the aromatic rings. Thus various Chem. Eur. J. 2014, 20, 10763 – 10774

www.chemeurj.org

The highest quantum yields are associated with the highest short-range order and small mean distance, observed for the methyl-substituted materials (Figure 8). A reasonable explanation is that the methyl groups, at the C2-position of the imidazolium ring, induce only a weak steric hindrance, whereas the positive inductive effect is responsible for a slightly higher electron density in the imidazolium ring. These combined effects lower the electrostatic repulsions between the positive charged imidazolium units in the material and thus leads to a better ordering of the imidazolium rings and a high quantum yields. The isopropyl-modified materials show intermediate h2 values but the smallest quantum yields. This can be explained in a way that the steric effect of the isopropyl moieties dominates the positive inductive effect. The lowest short range order and the smallest mean distances are found for the hydrogen-substituted materials due to their small size. As mentioned previously, the shifts of the excitation as well as of the emission maxima of the materials Mat 1 to Mat 6 has the same origin. The biphenyl-linked bis(imidazolium) INN materials Mat 7–Mat 9 showed quantum yields comparable to Mat 1–Mat 6 and also similar h2 values. The anthracene-linked bis(imidazolium) materials (Mat 10–Mat 12) showed lower quantum yields compared to Mat 7–Mat 9. This observation leads to the conclusion that the free rotatability and flexibility of the linking molecules between the imidazolium moieties plays an important role for photoluminescence properties of the bis(imidazolium) INN materials. Additionally the pyrazolium- and pyridinium-based INN materials (Mat 13–Mat 16)

10770

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper showed quite low quantum yields compared to the mono-imidazolium-based INN materials (Mat 1–Mat 3). This observation may have its origin from the pyrazolium-based INN materials (Mat 13 and Mat 14) in the different electronic structure of the pyrazolium ring, in which the two nitrogen atoms are placed right next to each other. For the pyridinium-based INN materials (Mat 15 and Mat 16), the origin of the comparable low quantum yields could be attributed to the different electronic structure, compared to the imidazolium-based INN materials (Mat 1–Mat 3) as well.

Conclusion We reported the synthesis of novel ionic nanoparticle networks. Based on previous works,[13] the imidazolium unit was now modified at the C2-position of the imidazolium ring with methyl and isopropyl groups and new linker molecules, for example, 4,4’-bis(chloromethyl)-1,1’-biphenyl and 9,10-bis(chloromethyl)anthracene, were introduced. Furthermore pyrazoliumand pyridinium-based INN materials were synthesized. For the pyridinium-based INN materials, a novel synthesis route, a onepot synthesis including a metal-catalyst-free hydrosilylation followed by a methanolysis reaction, for the precursor molecule was successfully applied. The photoluminescence investigations of the INN materials showed exciting photoluminescence features. For imidazoliumbased INN, the change of the substituent at the C2-position of the imidazolium ring from hydrogen to methyl or isopropyl group provoked a redshifted maximum in the excitation and emission spectrum. Furthermore, the spectral area of the emission and for the excitation could be changed by varying the linker molecule between the imidazolium moieties for the bis(imidazolium) INN materials. The photoluminescence activity results from the self-organization of the aromatic rings in the material by p–p stacking interactions. All the materials, which were investigated here, have in common that the photoluminescence activity is strongly enhanced by the self-organization in the material relative to the educts of the nucleophilic substitution, to the bare silica nanoparticles and to the neat imidazolium salts. Small and wide-angle X-ray scattering experiments were conducted, revealing the impact of different substituted imidazoliums in combination with various aromatic dichloro-substituted linker molecules on the structure and thus on the optical properties of the INN materials. It could be shown that the quantum yield of the presented materials is affected by structural parameters: The combination of small distance and high degree of short-range order enhances the quantum yields, visible for the methyl substituted imidazoliums. In contrast, isopropyl-substituted imidazoles exhibited the lowest quantum yields due to the large distance of the aromatic units as a consequence of steric hindrance. Additionally, the results from biphenyl and anthracene-linked bis(imidazolium) materials suggests that free rotatability and flexibility of the aromatic linker molecule increases the photoluminescence activity of the INN material. Chem. Eur. J. 2014, 20, 10763 – 10774

www.chemeurj.org

Experimental Section Chemicals All starting chemicals were of reagent grade and used as purchased.

Syntheses Synthesis of the silica nanoparticles: In a 250 mL round-bottomed flask, ammonia solution (32 %, 0.01 mol, 60 mL) and water (1.98 g, 0.11 mol) were added to absolute methanol (100 mL, 10.41 g, 0,05 mol). Tetraethoxysilane was added dropwise with stirring. The final solution was stirred for 3 days at room temperature. The resulting silica nanoparticles have an average hydrodynamic diameter of 16 nm. Synthesis of 3-iodopropyltrimethoxysilane: Synthesis was carried out under an argon atmosphere. Sodium iodide (36.9 g, 0.246 mol) was dissolved in absolute acetone (150 mL). 3-Chloropropyltrimethoxysilane (48.9 g, 0.246 mol) was added dropwise under stirring. The mixture was refluxed whilst stirring overnight. After this time, the precipitated sodium chloride was filtered off under an argon atmosphere. Then, the product was distilled at 52 8C under vacuum conditions (1 mbar) with a yield of 96 %. The resulting 3iodopropyltrimethoxysilane was obtained as a yellowish liquid. 1 H NMR (250 MHz, CDCl3): d = 3.58 (s, 9 H; Si(OCH3)3), 3.22 (t, 2 H; I CH2 ), 1.93 (q, 2 H; I CH2 CH2 CH2 ), 0.75 ppm (t, 2 H; CH2 Si). Synthesis of 2-chloroethyltrimethoxysilane: Synthesis was carried out under an argon atmosphere. 2-Chloroethyltrichlorosilane (9.9 g, 50 mmol) was dissolved in dry THF (150 mL). Then the reaction mixture was cooled to approximately 0 8C with an ice bath and triethylamine (15.2 g, 150 mmol) and dry methanol (30 mL) were added dropwise. The obtained colorless precipitate was filtered off under argon and the solvent was removed under vacuum conditions. For purification, the crude product was distilled at 42 8C under vacuum (1 mbar). 2-Chloroethyltrimethoxysilane was obtained in a yield of 95 % as a colorless and transparent liquid. 1 H NMR (250 MHz, CDCl3): d = 3.77 (t, 2 H; Cl CH2 ), 3.67 (s, 9 H; Si(OCH3)3), 1.41 ppm (t, 2 H; Cl CH2 CH2 ); 13C NMR (250 MHz, CDCl3): d = 50.4 ( Si(OCH3)3), 40.9 (CH2 Cl), 16.1 ppm (Si CH2). Synthesis of N-(3-propyltrimethoxysilane)imidazole (1), N-(3-propyltrimethoxysilane)-2-methylimidazole (2) and N-(3-propyltrimethoxysilane)-2-isopropylimidazole (3): Synthesis was carried out under an argon atmosphere. Sodium hydride (2.9 g, 0.12 mol) is dissolved in absolute THF (150 mL), and the mixture is cooled to approximately 0 8C with an ice bath. The respective imidazole (0.12 mol; for 1: 8.3 g of imidazole; for 2: 9.8 g of 2-methylimidazole; for 3: 13.2 g of 2-isopropylimidazole) was added portionwise whilst stirring. After the respective imidazole had been completely added, the ice bath was removed and the mixture maintained whilst stirring until no more hydrogen gas was evacuated. Then, 3iodopropyltrimethoxysilane (23.12 g, 0.09 mol) was added and the mixture was maintained at reflux overnight. The orange suspension was filtered off and the solvent removed under vacuum conditions. By addition of absolute dichloromethane (150 mL), a colorless precipitate appeared and was filtered off under an argon atmosphere. For purification, the crude product was distilled at 105 8C under vacuum conditions. Compound 1 was distilled at 105 8C under vacuum (1 mbar). N-(3Propyltrimethoxysilane)imidazole was obtained in a yield of 89 % as a colorless and transparent liquid. 1H NMR (250 MHz, CDCl3): d = 7.54 (s, 1 H; N CH N ), 7.01 (s, 1 H; N CH CH N=), 6.88 (s, 1 H; N CH CH N=), 3.88 (t, 2 H; =N CH2 CH2 ); 3.53 (s, 9 H;

10771

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper Si(OCH3)3); 1.83 (q, 2 H; =N CH2 CH2 CH2 Si), 0.54 ppm (t, 2 H; CH2 Si); 13C NMR (250 MHz, CDCl3): d = 136.8 ( N CH N ), 128.1 ( N CH CH N=), 120.7 ( N CH CH N=), 56.2 ( Si(OCH3)3), 55.7 (=N CH2 CH2 ), 25.1 (=N CH2 CH2 CH2 Si), 7.4 ppm ( CH2 Si). Compound 2 was distilled at 107 8C under vacuum (1 mbar). N-(3Propyltrimethoxysilane)-2-methylimidazole was obtained in a 78 % yield as a colorless and transparent liquid. 1

H NMR (250 MHz, CDCl3): d = 6.78 (s, 1 H; N CH CH N=), 6.71 (s, 1 H; N CH CH N=), 3.71 (t, 2 H; =N CH2 CH2 ), 3.46 (s, 9 H; Si(OCH3)3), 2.26 (s, 3 H; N C CH3), 1.72 (q, 2 H; =N CH2 CH2 CH2 Si), 0.49 ppm (t, 2 H, CH2 Si); 13C NMR (250 MHz, CDCl3): d = 144.2 ( N C(CH3) N ), 126.8 ( N CH CH N=), 119.0 ( N CH CH N=), 50.5 ( Si(OCH3)3), 48.0 (=N CH2 CH2 ), 24.1 (=N CH2 CH2 CH2 Si), 12.8 ( CH2 Si), 6.0 ppm ( N C CH3).

Compound 3 was distilled at 138 8C under vacuum (1 mbar). N-(3Propyltrimethoxysilane)-2-isopropylimidazole was obtained in a 76 % yield as a colorless and transparent liquid. 1H NMR (250 MHz, CDCl3): d = 7.04 (d, 1 H; N CH CH N=), 6.88 (d, 1 H, N CH CH N=), 3.95 (t, 2 H; =N CH2 CH2 ), 3.67 (s, 9 H; Si(OCH3)3), 3.09 (m, 1 H; N C CH), 1.95 (q, 2 H; =N CH2 CH2 CH2 Si), 1.44 (d, 6 H; CH3 CH CH3), 0.72 ppm (t, 2 H; CH2 Si); 13C NMR (250 MHz, CDCl3): d = 152.7 ( N C(CH) N ), 126.9 ( N CH CH N=), 118.4 ( N CH CH N=), 50.5 ( Si(OCH3)3), 47.4 (=N CH2 CH2 ), 25.7 (CH (CH3)2), 24.6 (=N CH2 CH2 CH2 Si), 21.8 (CH (CH3)2), 6.1 ppm ( CH2 Si). Synthesis of N-(3-propyltrimethoxysilane)pyrazole (4) and N-(3propyltrimethoxysilane)-3,5-dimethylpyrazole (5): Synthesis is carried out under argon atmosphere. Sodium hydride (2.9 g, 0.12 mol) was dissolved in absolute THF (150 mL) and the mixture was cooled to approximately 0 8C with an ice bath. The respective pyrazole (0.12 mol; for 4: 8.3 g of pyrazole; for 5: 11.5 g of 3,5-dimethylpyrazole) was added portionwise whilst stirring. After the respective pyrazole had been completely added, the ice bath was removed and the mixture was maintained whilst stirring until no more hydrogen gas was evacuated. Then 3-iodopropyltrimethoxysilane (23.12 g, 0.09 mol) was added and the mixture was maintained at reflux overnight. The slightly yellow suspension was filtered off and the solvent removed under vacuum conditions. By addition of absolute dichloromethane (150 mL), a colorless precipitate appeared and was filtered off under an argon atmosphere. For purification, the crude product was distilled under vacuum conditions. Compound 4 was distilled at 91 8C under vacuum (1 mbar). N-(3Propyltrimethoxysilane)pyrazole was obtained with a yield of 82 % as a colorless and transparent liquid. 1H NMR (250 MHz, CDCl3): d = 7.60 (d, 1 H, CH2 N CH CH CH ), 7.48 (d, 1 H; CH2 N CH CH CH ), 6.33 (t, 1 H; CH2 N CH CH CH ), 4.22 (t, 2 H; =N CH2 CH2 ), 3.65 (s, 9 H; Si(OCH3)3), 2.08 (q, 2 H; =N CH2 CH2 CH2 Si), 0.72 ppm (t, 2 H; CH2 Si); 13C NMR (250 MHz, CDCl3): d = 140.0 (CH2 N CH CH CH ); 129.0 (CH2 N CH CH CH ), 105.0 (CH2 N CH CH CH ), 54.1 (=N CH2 CH2 ), 50.4 ( Si(OCH3)3), 23.8 (=N CH2 CH2 ), 6.0 ppm ( CH2 Si). Compound 5 was distilled at 119 8C under vacuum (1 mbar). N-(3Propyltrimethoxysilane)-3,5-dimethylpyrazole was obtained in a 63 % yield as a colorless and transparent liquid. 1H NMR (250 MHz, CDCl3): d = 5.85 (s, 1 H; CH2 N C CH ); 4.02 (t, 2 H; = N CH2 CH2 ); 3.64 (s, 9 H; Si(OCH3)3); 2.30 (d, 6 H; N C CH3), 1.99 (q, 2 H; =N CH2 CH2 CH2 Si), 0.70 ppm (t, 2 H; CH2 Si); 13C NMR (250 MHz, CDCl3): d = 146.9 (CH2 N C CH C ); 138.4 (CH2 N C CH C ), 104.6 (CH2 N C CH C ), 50.7 (=N CH2 CH2 ), 50.4 ( Si(OCH3)3), 23.7 (=N CH2 CH2 ), 13.3 (CH2 N C CH3), 10.8 (CH2 N N C CH3), 6.0 ppm ( CH2 Si). Chem. Eur. J. 2014, 20, 10763 – 10774

www.chemeurj.org

Synthesis of 4-(2-trimethoxysilylethyl)pyridine (6) and 2-(2-trimethoxysilylethyl)pyridine (7): Synthesis was carried out under an argon atmosphere. Trichlorosilane (10.84 g, 80 mmol) was dissolved in dry acetonitrile (250 mL) and 4-vinylpyridine or 2-vinylpyridine (8.41 g, 80 mmol) were added dropwise whilst stirring at room temperature. After the vinylpyridine had been completely added, the reaction mixture was refluxed for 2 h. After cooling to ambient temperature, a yellowish solution was obtained. Then the mixture was cooled to approximately 0 8C and dry triethylamine (24.3 g, 0.24 mol) and dry methanol (25 mL) were added. The obtained colorless precipitate was filtered off under argon and the solvent was removed under vacuum conditions. The crude product was washed with THF (150 mL) and a colorless solid precipitated again. The precipitate was filtered off and the solvent was removed under vacuum conditions. For purification, the crude product was distilled under vacuum conditions. Compound 6 was distilled at 111 8C under vacuum (1 mbar). 4-(2-Trimethoxysilylethyl)pyridine was obtained in a 75 % yield as a colorless and transparent liquid. 1 H NMR (500 MHz, CDCl3): d = 7.97 (d, 2 H; N CH CH ), 6.62 (d, 2 H; N CH CH ), 3.04 (s, 9 H; Si (OCH3)3), 2.20 (t, 2 H; Si CH2 CH2 ), 0.45 ppm (t, 2 H; Si CH2 CH2 ); 13C NMR (500 MHz, CDCl3): d = 152.2 (N CH CH C ), 148.6 (N CH CH ), 122.3 (N CH CH ), 49.3 (Si (OCH3)3), 27.2 (Si CH2 CH2 ), 9.2 ppm (Si CH2 CH2 ); 29Si NMR (IGATED, 500 MHz, CDCl3): d = 44.4 ppm (Si (OCH3)3). Compound 7 was distilled at 106 8C under vacuum (1 mbar). 2-(2Trimethoxysilylethyl)pyridine was obtained in a 33 % yield as a colorless and transparent liquid. 1H NMR (500 MHz, CDCl3): d = 8.06 (d, 1 H; N CH CH ), 7.11 (t, 1 H; N CH CH CH ), 6.70 (d, 1 H; N C CH ), 6.62 (t, 1 H; N CH CH ), 3.10 (s, 9 H; Si (OCH3)3), 2.44 (t, 2 H; Si CH2 CH2 ), 0.68 ppm (t, 2 H; Si CH2 CH2 ). 13C NMR (500 MHz, CDCl3): d = 162.5 (N C CH ), 148.2 (N CH CH CH), 135.3 (N CH CH CH ), 121.2 (N C CH ), 119.8 (N CH CH ), 49.3 (Si (OCH3)3), 30.2 (Si CH2 CH2 ), 8.41 ppm (Si CH2 CH2 ); 29 Si NMR (IGATED, 500 MHz, CDCl3): d = 43.2 ppm (Si (OCH3)3). Synthesis of chloropropyl- or chloroethyl-modified silica nanoparticles: The previously prepared silica nanoparticle solution (32 mL) was degassed under vacuum for several minutes to remove excessive ammonia. Then, the respective 2-chloroalkyltrimethoxysilane (14.29 mmol; 2.84 g of 3-chloropropyltrimethoxysilane, 2.64 g of 2-chloroethyltrimethoxysilane) were added dropwise. The solution was stirred at room temperature for 24 h. Synthesis of N-propylimidazole-modified silica nanoparticles (SiO2ImR with R = H, Me, iPr): The previously prepared silica nanoparticle solution (32 mL) was degassed under vacuum for several minutes to remove excessive ammonia. Then, the respective N-(3propyltrimethoxysilane)imidazole (14.29 mmol, 1–3) was added dropwise. The solution was stirred at room temperature for 24 h. Synthesis of N-propylpyrazole-modified silica nanoparticles (SiO2 PyrazoleR2 with R = H, Me): The previously prepared silica nanoparticle solution (32 mL) was degassed under vacuum for several minutes to remove excessive ammonia. Then, N-(3-propyltrimethoxysilane)pyrazole (4) (3.3 g, 14.29 mmol) or N-(3-propyltrimethoxysilane)-3,5-dimethylpyrazole (5) (3.7 g, 14.29 mmol) was added dropwise. The solution was stirred at room temperature for 24 h. Synthesis of 4-ethylpyridine- and 2-ethylpyridine-modified silica nanoparticles (SiO2 4-Py and SiO2 2-Py, respectively): The previously prepared silica nanoparticle solution (32 mL) was degassed under vacuum for several minutes to remove excessive ammonia. Then, 4-(2-trimethoxysilylethyl)pyridine (3.2 g, 14.29 mmol) or 2-(2trimethoxysilylethyl)pyridine are (3.2 g, 14.29 mmol) were added dropwise. The solution was stirred at room temperature for 24 h.

10772

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper Synthesis of the mono-imidazolium chloride based INN (Mat 1– Mat 3): A solution of silica nanoparticles (16 mL) modified with the respective N-(3-propyltrimethoxysilane)imidazole, SiO2ImR with R = H, Me, iPr, and a solution of silica nanoparticles modified with 3chloropropyltrimethoxysilane (16 mL) were transferred to a 100 mL round-bottomed flask. Then, dry methanol (10 mL) was added. The solution was stirred for 2 days at room temperature and was then dried under vacuum conditions (3 mbar). A transparent gel was obtained. Synthesis of the bis(imidazolium) xylene based INN (Mat 4– Mat 6): A solution of silica nanoparticles (32 mL) modified with the respective N-(3-propyltrimethoxysilane)imidazole, SiO2ImR with R = H, Me, iPr and dry methanol (20 mL) were transferred to a 100 mL round-bottomed flask. Then, a,a’-dichloro-p-xylene (1.25 g, 7.14 mmol) was added. The solution was stirred for 2 days at room temperature and was then dried under vacuum conditions (3 mbar). A transparent gel was obtained. Synthesis of the bis(imidazolium) biphenyl based INN (Mat 7– Mat 9): A solution of silica nanoparticles (32 mL) modified with the respective N-(3-propyltrimethoxysilane)imidazole, SiO2ImR with R = H, Me, iPr, and dry methanol (20 mL) were transferred to a 100 mL round-bottomed flask. Then 4,4’-bis(chloromethyl)-1,1’-biphenyl (1.8 g, 7.14 mmol) was added. The solution was stirred for 2 days at room temperature and was then dried under vacuum conditions (3 mbar). A slightly yellow transparent gel was obtained. Synthesis of the bis(imidazoliumanthracene)-based INN (Mat 10– Mat 12): A solution of silica nanoparticles (32 mL) modified with the respective N-(3-propyltrimethoxysilane)imidazole, SiO2ImR with R = H, Me, iPr, and dry methanol (20 mL) were transferred to a 100 mL round-bottomed flask. Then, 9,10-bis(chloromethyl)anthracene (0.98 g, 3.57 mmol) was added. The solution was stirred for 2 days at room temperature and was then dried under vacuum conditions (3 mbar). Some excessive linker was removed by centrifugation. An orange transparent gel was obtained. Synthesis of the pyrazolium-based INN (Mat 13 and Mat 14): A solution of silica nanoparticles (16 mL) modified with the respective N-(3-propyltrimethoxysilane)pyrazole, SiO2 PyrazoleR2 with R = H, Me, and a solution of silica nanoparticles modified with 3-chloropropyltrimethoxysilane (16 mL) were transferred to a 100 mL round-bottomed flask. Then, dry methanol (10 mL) was added. The solution is stirred for 2 days at room temperature and then finally dried under vacuum conditions (3 mbar). A colorless transparent gel was obtained. Synthesis of the pyridinium-based INN (Mat 15 and Mat 16): A solution of silica nanoparticles (16 mL) modified with the respective 2-trimethoxysilylethylpyridine, SiO2 4-Py or SiO2 2-Py, and solution of silica nanoparticles modified with 2-chloroethyltrimethoxysilane (16 mL) were transferred to a 100 mL round-bottomed flask. Then, dry methanol (10 mL) was added. The solution was stirred for 2 days at room temperature and was then dried under vacuum conditions (3 mbar). A colorless transparent gel was obtained.

a xenon arc lamp and double grating monochromators. The INN materials were measured in the solid state. The excitation and emission spectra were recorded with slit width of 1.0 nm. The dwell time was adjusted to 0.5 s and the recording steps were 0.5 nm. For each spectrum three scans were executed. The quantum yield measurements were executed by using a photomultiplier tube equipped with a barium sulfate coated integration sphere (150 mm internal diameter). All spectra were recorded at room temperature with excitation and emission slit widths of 3.0 nm. The dwell time was of 0.5 s and the recording steps were 0.2 nm. For each spectrum three scans were executed. The quantum yield was calculated by dividing the number of emitted photons by the number of absorbed photons. The number of absorbed photons was determined from the decrease of the scattered excitation light intensity relative to the measured intensity with an empty SUPRASIL glass cuvette. The spectra were corrected for system specific effects, such as the detector sensitivity, monochromator efficiency and the BaSO4 coating. The spectral distribution of the lamp intensity was corrected by using a Si photodiode reference detector. Small and wide-angle X-ray scattering (SAXS/WAXS): Small and wide-angle X-ray scattering experiments were performed at a laboratory X-ray source (Nanostar, Bruker AXS) by using a rotating anode generator. CuKa radiation was monochromatized and collimated by crossed Gçbel mirrors and a pinhole system. The X-ray patterns were recorded with a position sensitive area detector (VNTEC 2000) and radially averaged to obtain the scattering intensity in dependence on the scattering vector q = (4p/l) sinq, with 2q being the scattering angle and l = 0.1542 nm the X-ray wavelength. SAXS and WAXS measurements were carried out at a sample to detector distance of 108 and 13 cm, respectively. The integrated scattering data were merged together in the overlap regime leading to scattering curves ranging from 0.1 to 20 nm 1. Dynamic light scattering (DLS): For the measurement, the solid was suspended in ethanol. The DLS experiments were carried out without previous sonication of the samples. The run time of the measurements is 10 s. Every size distribution curve is obtained by averaging 10 measurements. The apparatus is an ALV/CGS-3 compact goniometer system, equipped with an ALV/LSE-5003 light scattering electronics and multiple t digital correlator, and a 632,8 nm JDSU laser 1145P. Digital photos: Digital photos were made by using a Medion Life P44016 MD 86535.

Acknowledgements J.A. and H.P. thank the Austrian science funds FWF (project number I449) for financial support. Keywords: nanoparticles · networks · photoluminescence · SAXS · silica

Characterizations NMR spectroscopy: 1H and 13C solution NMR spectra were recorded on a Bruker AVANCE 250 (250.13 [1H], 62.86 MHz [13C]) equipped with a 5 mm inverse-broadband probe head and a z-gradient unit. The NMR spectra for the 2- and 4-(2-trimethoxysilylethyl)pyridine were recorded on a Bruker Avance III Ultrashield Plus at TU Bergakademie Freiberg (500.13 [1H], 125.76 [13C] and 99.36 MHz [29Si]). Fluorescence and quantum yield: These measurements were carried out on an Edinburgh Instruments spectrometer (FSP 920) with Chem. Eur. J. 2014, 20, 10763 – 10774

www.chemeurj.org

[1] a) L. D. Carlos, R. A. S. Ferreira, V. de Zea Bermudez, B. Julian-Lopez, P. Escribano, Chem. Soc. Rev. 2011, 40, 536 – 549; b) L. D. Carlos, R. A. S. Ferreira, V. de Zea Bermudez, S. J.L Ribeiro, Adv. Mater. 2009, 21, 509 – 534; c) J. Graffion, A. M. Cojocariu, X. Cattoen, R. A. S. Ferreira, V. R. Fernandes, P. S. Andre, L. D. Carlos, M. Wong Chi Man, J. R. Bartlett, J. Mater. Chem. 2012, 22, 13279 – 13285; d) D. Giaume, M. Poggi, D. Casanova, G. Mialon, K. Lahlil, A. Alexandrou, T. Gacoin, J.-P. Boilot, Langmuir 2008, 24, 11018 – 11026; e) D. Casanova, D. Giaume, T. Gacoin, J.-P. Boilot, A. Alexandrou, J. Phys. Chem. B 2006, 110, 19264 – 19270. [2] a) C. A. Strassert, C. H. Chien, M. D. Galvez Lopez, D. Kourkoulos, D. Hertel, K. Meerholz, L. De Cola, Angew. Chem. 2011, 123, 976 – 980;

10773

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper

[3] [4] [5] [6] [7] [8] [9]

[10]

[11] [12] [13] [14]

Angew. Chem. Int. Ed. 2011, 50, 946 – 950; b) J. M. Fernndez-Hernndez, J. I. Beltran, V. Lemaur, M.-D. Galvez-Lopez, C.-H. Chien, F. Polo, E. Orselli, R. Frçhlich, J. Cornil, L. De Cola, Inorg. Chem. 2013, 52, 1812 – 1824; c) C.-H. Yang, M. Mauro, F. Polo, S. Watanabe, I. Muenster, R. Frçhlich, L. De Cola, Chem. Mater. 2012, 24, 3684 – 3695. P. C. Marr, K. McBride, R. C. Evans, Chem. Commun. 2013, 49, 6155 – 6157. J. Graffion, X. Cattoen, V. T. Freitas, R. A. S. Ferreira, M. Wong Chi Man, L. D. Carlos, J. Mater. Chem. 2012, 22, 6711 – 6715. A. Samanta, J. Phys. Chem. B 2006, 110, 13704 – 13716. A. J. Boydston, C. S. Pecinovsky, S. T. Chao, C. W. Bielawski, J. Am. Chem. Soc. 2007, 129, 14550 – 14551. R. Martn, L. Teruel, C. Aprile, J. F. Cabeza, M. Alvaro, H. Garcia, Tetrahedron 2008, 64, 6270 – 6274. R. C. Evans, P. C. Marr, Chem. Commun. 2012, 48, 3742 – 3744. a) A. J. Boydston, P. D. Vu, O. L. Dykhno, V. Chang, A. R. Wyatt, A. S. Stockett, E. T. Ritschdorff, J. B. Shear, C. W. Bielawski, J. Am. Chem. Soc. 2008, 130, 3143 – 3156; b) T. Tang, D. J. Coady, A. J. Boydston, O. L. Dykhno, C. W. Bielawski, Adv. Mater. 2008, 20, 3096 – 3099; c) K. M. Wiggins, R. L. Kerr, Z. Chen, C. W. Bielawski, J. Mater. Chem. 2010, 20, 5709 – 5714. a) M. Litschauer, M.-A. Neouze, J. Mater. Chem. 2008, 18, 640; b) M.-A. Neouze, M. Litschauer, M. Puchberger, J. Bernardi, Monatsh. Chem. 2012, 143, 519 – 525; c) M.-A. Neouze, J. Mater. Chem. 2010, 20, 9593 – 9607. J. Roeser, M. Kronstein, M. Litschauer, A. Thomas, M.-A. Neouze, Eur. J. Inorg. Chem. 2012, 5305 – 5311. M. Litschauer, M. Puchberger, H. Peterlik, M. A. Neouze, J. Mater. Chem. 2010, 20, 1269 – 1276. M. Kronstein, K. Kriechbaum, J. Akbarzadeh, H. Peterlik, M.-A. Neouze, Phys. Chem. Chem. Phys. 2013, 15, 12717 – 12723. M. Czakler, M. Litschauer, K. Foettinger, H. Peterlik, M. A. Neouze, J. Phys. Chem. C 2010, 114, 21342 – 21347.

Chem. Eur. J. 2014, 20, 10763 – 10774

www.chemeurj.org

[15] a) H. R. Grniger, G. Calzaferri, Helv. Chim. Acta 1979, 8, 2547 – 2550; b) R. J. P. Corriu, E. Lancelle-Beltran, A. Mehdi, C. Reye, S. Brandes, R. Guilard, J. Mater. Chem. 2002, 12, 1355 – 1362. [16] a) G. W. Fester, PhD Thesis, TU Bergakademie Freiberg 2009; b) S. Nozakura, Bull. Acad. Vet. Fr. Bull. Chem. Soc. Japan 1956, 29, 784 – 789. [17] a) A. Paul, P. K. Mandal, A. Samanta, Chem. Phys. Lett. 2005, 402, 375 – 379; b) P. K. Mandal, A. Paul, A. Samanta, J. Photochem. Photobiol. A 2006, 182, 113 – 120. [18] a) D. Zhang, R. Pelton, Langmuir 2012, 28, 3112 – 3119; b) N. K. Al-Rasbi, C. Sabatini, F. Barigelletti, M. Ward, Dalton Trans. 2006, 4769 – 4772. [19] W. Q. Kann, J. Yang, Y. Y. Liu, J. F. Ma, Polyhedron 2011, 30, 2106 – 2113. [20] P. Suresh, S. Radhakrishnan, C. N. Babu, A. Sathyanarayana, N. Sampath, G. Prabusankar, Dalton Trans. 2013, 42, 10838 – 10846. [21] a) G. Beaucage, J. Appl. Crystallogr. 1995, 28, 717 – 728; b) G. Beaucage, J. Appl. Crystallogr. 1996, 29, 134 – 146. [22] a) J. S. Pedersen, Adv. Colloid Interface Sci. 1997, 70, 171 – 210; b) D. J. Kinning, E. L. Thomas, Macromolecules 1984, 17, 1712 – 1718. [23] a) Y. Zhou, J. H. Schattka, M. Antonietti, NanoLett. 2004, 4, 477 – 481; b) R. P. Matthews, T. Welton, P. A. Hunt, Phys. Chem. Chem. Phys. 2014, 16, 3238 – 3253; c) M.-A. Neouze, M. Litschauer, J. Phys. Chem. B 2008, 112, 16721 – 16725. [24] A. Tolkki, E. Vuorimaa, V. Chukharev, H. Lemmetyinen, P. Ihalainen, J. Peltonen, V. Dehm, F. Wurthner, Langmuir 2010, 26, 6630 – 6637. [25] G. P. Yong, C. F. Li, Y. Z. Li, S. W. Luo, Chem. Commun. 2010, 46, 3194 – 3196. [26] T. Singh, A. J. Kumar, J. Phys. Chem. B 2008, 112, 4079 – 4086.

Received: January 1, 2014 Revised: April 4, 2014 Published online on May 26, 2014

10774

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Tailoring photoluminescence properties in ionic nanoparticle networks.

To investigate the original and promising luminescence properties of ionic nanoparticle networks (INN), various material compositions were investigate...
2MB Sizes 0 Downloads 0 Views