Zeolite/dye hybrid composites: organization of photoactive azobenzene
molecules inside AlPO4-5
Michelangelo Polisi, a Rossella Arletti,b,c * Sara Morandi,d,c Marco Fabbiani,e Gianmario Martra,d,c
Simona Quartieri,f Linda Pasterob and Giovanna Vezzalinia
a Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio
Emilia, via Campi 103, I- 41125 Modena
b Dipartimento di Scienze della Terra, Università degli Studi di Torino, Via Valperga Caluso
35, I-10125 Torino
c Interdepartmental Centre “Nanostructure Interfaces and Surfaces NIS”, Via Pietro Giuria
7, I-10125 Torino
d Dipartimento di Chimica, Università di Torino, Via Giuria 7, I-10125 Torino e Dipartimento di Scienza ed alta Tecnologia, Università degli Studi dell’Insubria, Via
Valleggio 11, I-22100 Como
f Dipartimento di Scienze Matematiche e Informatiche, Scienze Fisiche e Scienze della Terra,
Università degli Studi di Messina, viale F. Stagno d’Alcontres 31, I-98166 Messina S. Agata
* Corresponding author: rossella.arletti@unito.it Abstract
Organic/inorganic hybrid materials - like zeolites+dyes - are currently used in strategic areas, from sustainable energy technologies to biomedical sciences. In these systems, photoactive molecules are organized in one-dimensional nanostructures inside the zeolite channels. In this paper we present the results of a study aimed to the synthesis and the structural and spectroscopic characterization of azobenzene/AlPO4-5 hybrid composites, performed by a multi-technique approach based on material
synthesis, thermal gravimetric analysis, synchrotron X-ray powder diffraction, UV-VIS and IR spectroscopies. The results indicate that azobenzene molecules are hosted in the 12-membered ring channel of AFI framework and the maximum loading is 0.9 molecules per unit cell. The combination of spectroscopic and diffractometric methods point out that, being AlPO4-5 starting material affected
by Brönsted acidity, a consistent portion of the azobenzene molecules is protonated and interact with the framework oxygen atoms.
Keywords
Zeolite-dye hybrid, Structural characterization, X ray diffractio, UV-VIS and IR spectroscopies, Brönsted acidity
Introduction
The regular pore system of nanometric openings exhibited by zeolite frameworks make these materials ideal host matrices for photoactive species. Noteworthy, the obtained host-guest systems can be exploited as versatile building blocks for the realization of hierarchically organized multifunctional composite materials [1,2,3].
Organic/inorganic hybrid materials of the zeolites+dyes type are currently used in strategic areas, from sustainable energy technologies to biomedical sciences. The excellent optical properties, chemical stability and biocompatibility of these composites make them key components of artificial antenna systems, sensors, light emitting and bio-nano devices, such as in-vivo markers of tumour cells [4].
It is known that the properties of the dye/zeolite systems depend on the pore dimension/shape and on the packing of the molecules inside the channels, which control at the microscopic level the intermolecular interactions of the dyes with the framework. For instance, the encapsulation of fluorenone molecules in zeolite L framework resulted in a one-dimensional supramolecular organisation of the dye hosted in the linear channels [5].
Also zeolites with AFI framework type [6], in particular AlPO
4-5, have been studied by
many groups with the aim to produce hybrid materials with new optical properties. These studies [7,8,9,10,11] demonstrated the incorporation of dyes into AlPO
4-5 resulting in new
properties, such as laser emission, with wide-range tuneable properties [7].
Moving from the host to the guest, azobenzene have been considered during last years as a promising dye for the development of advanced hybrid materials, especially because of the reversible cis-trans photo-isomerization (well known in isotropic media [12]) exhibited by the
molecule confined in zeolite cavities10. The intrazeolitic isomerization of this molecule causes
changes in the composite optical properties like absorbance and birefringence [13,14].
Notwithstanding the deep interest for these advanced hybrid materials, detailed information on the crystal structure and on the host/guest interactions, fundamental to unravel the relationships among structure and properties, is still lacking.
This work is aimed to contribute to the elucidation of some of these issues, by investigating at the molecular and atomic levels, an azobenzene/AlPO4-5 organic/inorganic hybrid
composite. A multitechique approach was adopted, targeting the quantitation of the dye loading within the zeolite channels and on the external surface of zeolite grains (by TG-DTG), the structural characterization of the organization of dye guest molecules within the zeolite pores (by synchrotron X-ray powder diffraction, XRPD); the chemical states and the interaction with
the host zeolite framework of dye molecules (by diffuse reflectance UV-Vis and IR spectroscopy in controlled atmosphere).
Materials and methods
AlPO4-5: preliminary remarks on the structure
The host chosen for this study is the aluminophosphate AlPO4-5, with AFI framework type
[6]. The topological symmetry of AFI is P6/mcc. The structure is characterized by 12 rings-channels parallel to the [001] direction, with free diameter of 7.3 Å. The rings-channels are connected to each other by “pseudo-cages” confined in the (001) plane by single 6-membered rings of tetrahedra centered on the 3-fold axis (Figure 1).
The ordered distribution of Al3+ and P5+ tetrahedra is responsible of the symmetry
reduction from the topological s.g. P6/mcc to the real symmetry. This real symmetry has been largely debated, and different structural models have been proposed, among which: 1) a symmetry reduction to the orthorhombic Pcc2 subgroup [15]; 2) an average P6cc structure
deriving by the presence of three co-existing structural micro-domains with local symmetry P6; 3) a structure characterized by the presence of different rigid unit modes (RUMs) of structure deformation leading to a dynamical disorder of the framework atoms [16,17]; 4) an
incommensurately modulated structure [18]. In this work, the refinements were performed in the
P6cc space group, using the framework atomic model reported in [19].
AlPO4-5: synthesis
AlPO4-5 synthesis was obtained following the protocol proposed in [19]. Briefly, 4.1 g of
aluminum isopropoxide were hydrolyzed under magnetic stirring in 18 ml of ultrapure water for 4 hours. 3 g of phosphoric acid (85%) were added and the suspension was stirred further for 1 h. Then 0.835 ml of trimethylamine (TEA) template was added under no stirring and finally stirred for 10 minutes. The precursor gel was kept at room temperature for 12 hours and then transferred into a Teflon-lined autoclave and heated at 210 °C for 4 days. The as-synthesized sample was then calcined at 600 °C in order to remove the organic template from the zeolite porosities.
Preparation of the azobenzene/ALPO4-5 hybrid
Azobenzene incorporation was carried out by gas-phase adsorption. AlPO4-5 was firstly
of material were deposited in a Teflon-lined autoclave and mixed with 12 mg of trans-azobenzene (Aldrich), with the molar ratio 1:1.
The mixture was heated at 150° C for 12 h. The resulting sample was then Soxhlet extracted with ethanol for 24 h to remove azobenzene from the crystals surface. The final loaded sample (AFI-azo from now on) has a mustard color.
Thermogravimetric analysis
TG analysis of calcined AlPO4-5 sample, pure trans-azobenzene and AFI-azo composite
were carried out using a Seiko SSC/5200 thermal analyzer. The samples were loaded in a Pt crucible, the experimental conditions were the following: heating rate 10°C/min, air flux 100 µL/min, temperature range RT-950° C for calcined AlPO4-5 and for AFI-azo composite. For
pure trans-azobenzene sample the maximum temperature reached was 250° C.
X-ray powder diffraction
Synchrotron XRPD pattern of the AFI-azo sample was collected at the X-Press beamline of Elettra Sincrotrone Trieste. A monochromatic beam with a fixed wavelength of 0.4957Å was used. The sample was loaded in a 0.3 boron capillary. The bidimensional pattern was recorded with an image plate MAR 345 at a fixed distance of 350 mm from the sample. The one-dimensional diffraction pattern was obtained in a 2θ range 0-20° by integrating the bidimensional image with the software FIT2D [20].
Structural refinements
The structural information of AlPO4-5 before loading used in this paper are taken from
[19]. The structure refinement of AFI-azo composite was performed by full profile Rietveld analysis using the GSAS package [21] with EXPGUI interface [22]. No evidence of
superstructure or symmetry change was detected with respect to AFI. In addition to the AlPO4
-5 diffraction peaks, the presence of few peaks belonging to a second aluminum-phosphate phase, with low-cristobalite structure [23], were observed, thus the phase was added in the
AFI-azo refinement to obtain better according factor. The refined phase fractions indicate that AlPO4-5 was 97 wt. % of the whole sample. The position of the trans-azobenzene molecule
was derived by the inspection of the difference Fourier maps. From the electronic density map it was possible to locate only atoms from C1 to C6 and N1 (Table 1S). The symmetrical portion of the molecule (atoms from C7 to C12 and N2) was built by geometrical calculation keeping into account the theoretical interatomic distances. The molecule was then allowed to
move along the c axis during the refinements. The background curve was fitted by a Chebyshew polynomial with 12 coefficients. The pseudo-Voigt profile function proposed by [24] was applied, and the peak intensity cut-off was set to 0.001 of the peak maximum. The
scale factor, 2θ-zero shift and unit-cell parameters were accurately refined. Soft constraints were imposed on tetrahedral bond lengths P-O (1.54 Å), Al-O (1.68 Å) and on C-C (1.38 Å) and N-N (1.25 Å) distances, with a tolerance value of 0.03 Å. The thermal displacement parameters were constrained in the following way: the same value for framework oxygen atoms and another one for the azobenzene molecule atoms. The unit cell axes and refinements parameters are reported in Table 1. The final atomic positions, thermal parameters and occupancy factors are given in Tables S1, while the framework and extraframework interatomic distances are reported in Table S2. The final observed and calculated patterns are shown in Figure S1.
UV-Vis spectroscopy
Diffuse reflectance (DR) UV-Vis spectra were run at room temperature (RT) in air on powdered samples, pressed from the back toward a Suprasil quartz window. A Varian Cary 5000 spectrophotometer was used, equipped with an integrating sphere with the inner surface coated by Spectralon. The same material was used as reference. Spectra are reported as Kubelka-Munk function [f(R∞) = (1−R∞)2/2R∞, where R∞ is the reflectance of an “infinitely
thick” layer of the sample] in the ordinate scale.
IR spectroscopy
Absorption IR spectra were collected with a Perkin-Elmer FT-IR System 2000 spectrophotometer equipped with a Hg-Cd-Te cryo-detector, working in the range of wavenumbers 7200-580 cm-1 at a resolution of 2 cm-1 (number of scans 64). For IR analysis the
powder samples were compressed in self-supporting discs with an “optical thickness” of about 10 mg cm-2, and placed in a home-made quartz IR cell equipped with KBr windows and
connected to a glass vacuum line (residual pressure: 1x10-4 mbar). The spectra were recorded at
RT during the interaction with increasing pressures of ammonia or after sample deuteration. The deuteration was achieved by admitting doses of D2O (10 mbar) with outgassing at RT
between one dose and another, until invariance of the spectra. Before spectra recording and before the interaction with the selected molecules, the samples underwent prolonged outgassing at RT in order to remove physisorbed water and make the spectra clearer in the region of (O-H) and (C-H) vibration modes.
Results and discussion Thermogravimetric analysis
Figure 2a and b show the TG and DTG curves of calcined AlPO4-5, pure trans-azobenzene
and AFI-azo composite. The weight loss of calcined AlPO4-5 (about 16.8%) occurs mostly in
the range 40-120° C, corresponding to 16.4 water molecules per unit cell (p.u.c). This value is consistent with what reported in the study of the dehydration dynamic of AlPO4-5 performed
by Polisi et al. [19]. This study, in fact, shows that this porous material, characterized by very weekly bonded water molecules, is completely dehydrated below 150°C.
AFI-azo weight loss is approximately the same of the non-loaded sample (16.9% wt.%), but the weight loss occurs in two different steps. The loss below 110° C (11.5%) is associated to water molecules re-adsorbed in the channels after the azobenzene loading and/or physisorbed on crystals surface. The latter option seems to be the most probable, since the weight loss occurs at a temperature even lower than that found for AlPO4-5. However, the
presence of re-adsorbed water in the porosities cannot be ruled out.
The subsequent weight loss (5.4 wt. %) occurs up to 750° C and is related to the azobenzene molecule release (0.5 p.u.c.). Concerning the analysis on pure trans-azobenzene sample, the total loss, corresponding to the 99% of the sample weight, is in the 70-150° C range. The higher release temperature of dye in the hybrid material compared to the degradation temperature of pure trans-azobenzene is a clear indication of the penetration of the dye inside a confined zeolitic environment.
Structural refinements
Figure 3 shows that the peaks intensity of the XRPD pattern of the AlPO4-5 sample [19]
-used for the loading- in the low 2θ range are significantly different from that of AFI-azo. As well known, in zeolites XRD spectra the intensities in this angle region are strongly influenced by the extra-framework contents and distribution. This observation, along with the increase of the cell parameters (strong for a, from 13.7213 Å [19] to 13.7727 Å and slight for c, from 8.4054 [19] to 8.4095, accounting for a total volume variation from 1370.51 Å3 [19] to 1383.1
Å3) indicates the penetration of azobenzene inside the AlPO
4-5 channels, in agreement with the
results of TG analysis (Figure 2).
The penetration of the dye molecule induces slight deformations to the framework. The 12ring-channel, showing an almost regular circular shape in the AlPO4-5 starting material [19],
diameter from 9.82 to 9.75 Å, Table S3). The 6ring-channel becomes tighter with O4-O1 decreasing from 5.14 to 4.96 Å.
The orientation of the dye molecule inside the AFI 12ring-channel obtained with this model is shown in Figure 4. The inspection of the differences Fourier map (DELF) allowed locating only seven sites in the 12ring-channel, representing one aromatic ring and one N atom. The position of the remaining part of the molecule was mathematically calculated considering a planar geometry (atoms represented in grey in Figure 4). In this configuration, the center of the molecule, placed in the middle of the nitrogen-nitrogen bond, is located at the center of the channel, on the 6-fold axis. The molecule is tilted of 36° with respect to the channel axis. A similar tilting (22°) has been observed for 2,2’-bipyridyl-3,3’-diol incorporated into AlPO4-5
crystals [25] and is in agreement with Megelski et al. [26] that described the orientation of
fluorescent dyes, inside a similar mono-dimensional channel system of zeolite L, as a function of their dimensions. Based on the occupancy factors and the multiplicity of the atomic sites, azobenzene content estimated by the refinement (about 0.9 molecules p.u.c.) is higher than that derived by TGA
This inconsistence can be due to: i) overestimation of the occupancy factors in the AFI-azo refinement, as a consequence of the presence, in the same crystallographic sites, of some water molecules (not considered in the refinement due to the complexity of the system); ii) azobenzene underestimation in the TGA, due to residual - not released - carbon present in the porosities after the decomposition of the dye molecules. The lack of DELF peaks in the positions corresponding to the symmetrical portion of azobenzene molecule (C7-C12, N2) could indicate a statistical disorder. Targeting the disclosure of possible origins of these absences, spectroscopic studies were performed.
UV-Vis and IR spectroscopies
DR UV-Vis spectra of AlPO4-5 and AFI-azo samples are reported in Figure 5. From the
spectra comparison, it is clear that absorption bands observed for the AFI-azo sample at 328 and 425 nm are related to the presence of the azobenzene molecules. On the basis of literature data [10,27,28] the band at 328 nm is assigned to the →* transition of trans-azobenzene, while
the band at 425 nm is assigned to the same transition of trans-azobenzene molecules protonated on the azo group. Reasonably, the protonation occurs by means of Brönsted acid sites of AlPO4-5 [10, 28].
The actual presence of these sites was assess by IR spectroscopy of adsorbed NH3. In the
1643, 1616 and 1457 cm-1 are present and increase upon increasing NH
3 pressure up to 2 mbar
(solid curves). The bands at 1643 and 1457 cm-1 are quite stable during the subsequent
outgassing at RT (dashed curve) and are assigned to the sym (N-H) and asym (N-H) modes of
NH4+ species, respectively [29]. The formation of ammonium ions confirms the presence of
Brönsted acid sites able to protonate the azobenzene molecules on the azo groups, supporting the assignment of the band at 425 nm in the UV-Vis spectrum of AFI-azo sample to the protonated form of the dye. The band at 1616 cm-1 is much less stable to the outgassing of
gaseous ammonia and is assigned to the asym (N-H) mode of NH3 bonded through nitrogen lone
pairs to Lewis acid sites, i.e. low-coordinated Al atoms on the AlPO4-5 lattice.
As far as the relative amount of protonated dye molecules is concerned, the UV-Vis spectral pattern in Figure 5 indicates that the transfer of acid protons occurred only to a fraction of the total amount of azobenzene present in the AlPO4-5 channels. The evaluation of such
relative amount is not straightforward. Indeed, the absolute or relative values of absorption coefficients related to the →* transition of the non-protonated and protonated azobenzene
guest molecules should be determined. In principle, starting from a known concentration azobenzene aqueous solution, and taking into account the pKa of azobenzene, it is possible to
determine the ratio between the absorption coefficients of the →* transition for protonated
and non-protonated species by varying the pH of the solution and running UV-Vis spectra However, the intensity of the →* absorption band strongly depends on the solvent; moreover,
also the encapsulation into zeolite channels strongly affects the absorption coefficient [30]. For
these reasons, the evaluation of the amount of protonated azobenzene molecules in AlPO4-5
channels is not feasible.
In order to deepen the investigation of protonated azobenzene molecules, IR spectra of AlPO4-5 and AFI-azo were run after prolonged outgassing at RT (see Fig. S2, grey and black
curves, in the supporting information). To better observe the bands related to the azobenzene molecules, AlPO4-5 contribution was subtracted from AFI-azo spectrum. In Figure 7, the
difference spectrum - in the high (A section), medium (B section) and low (C section) wavenumber regions - is compared to the spectrum of azobenzene dissolved in CCl4 (non-polar
solvent medium establishing weak interaction with the dissolved molecules). In the high wavenumber region, a very broad absorption band centered at about 3280 cm-1 is present,
assignable to the N-H stretching mode of protonated azobenzene molecules.
In the medium wavenumber region, bands related to the (C-H) modes of benzene ring are present in the range 3100-3020 cm-1. It is worth of note the presence of a band centered at
about 2977 cm-1 for AFI-azo sample, not observable for azobenzene alone. At the same time, in
the low wavenumber region, the band centered at about 1390 cm-1 for AFI-azo is not
observable for azobenzene in CCl4.
In order to obtain insights on the nature of the two additional bands at 2977 and 1390 cm-1,
the deuteration of AFI-azo sample was performed (see Fig. S2, red curve). The bands are not affected by the H/D isotopic exchange, indicating that the related vibrations are not due to O-H or N-H groups. On the contrary, the (N-H) band, reported in Figure 7A, disappears upon deuteration, as expected.
Beside these results, it is observed that the deuteration is not effective for all the O-H groups of the zeolite (residual O-H band in the range 3800-2700 cm-1 after deuteration is
observed, Fig. S2, red curve), showing that not all the hydroxyls appear on open channels of the AFI structure. This is well evident also after H/D isotopic exchange of the AlPO4-5 sample
(see Fig. S3).
In search of the origin of the bands at 2977 and 1390 cm-1, it is necessary to take into
account that the protonation of the azo group is the first step of a complex photolysis chemistry that starts with the photoisomerization of trans-azobenzene into the cis-form and leads to the formation of fragments, such as benzene and benzenediazonium cation [31], or photocyclitation
products, such as benzo[c]cinnoline and benzidine [10,32]. Nevertheless, these species should
give much more IR bands than those observed. Moreover, benzenediazonium cation should give a band related to the (NN) mode, not observed in the spectrum of Figure 7.
Douberly et al. studied gas phase benzenium ions with IR photodissociation spectroscopy (IRPD) and the method of rare gas tagging [33]. Their compared IRPD bands of protonated
benzene to the predictions obtained by density functional theory computations. They found the bands related to the (C-H) modes divided into two groups: one group of bands is at wavenumbers characteristic of benzene; the other one is located at wavenumbers lower than that characteristic of benzene.
In light of these literature data and of the spectroscopic results reported above, it is reasonable to hypothesize that the band at 2977 cm-1 could be related to (C-H) modes of
protonated benzene ring. This is coherent with the resonance structures reported in Scheme 1. These resonance structures justify also the band observed at 1390 cm-1, which is consistent with
Scheme 1.
In the light of the spectroscopic results, indicating the N protonation in the azobenzene molecule, the shortage of the symmetrical portion of the molecule in the DELF map of the structural refinement can be justified. Protonated molecule deviates from the planar geometry of the trans-azobenzene as a consequence of the N protonation. This leads to a higher degree of freedom of the two aromatic rings and allows their reciprocal rotation inducing a more spread electronic density in the DELF map.
On the basis of the spectroscopic data, only a portion of the azobenzene molecules adsorbed - enough to prevent the location of the entire molecule from DELF - is protonated. The remaining portion maintains the planar geometry of trans-azobenzene.
Conclusions
In this work the azobenzene/AlPO4-5 organic/inorganic hybrid composite was synthesized
and characterized by the combination of spectroscopic and diffractometric methods.
By means of UV-Vis and IR spectroscopies, it was demonstrated that a fraction of azobenzene molecules inside the AlPO4-5 structure is protonated on the azo group, because of
the presence of Brönsted acid sites in the zeolite channels. As a consequence of the N protonation, the dye molecules deviate from the planar geometry of the trans-azobenzene, since, taking into account the resonance structures, the reciprocal rotation of the aromatic rings is allowed. On the basis of the structural data, the aromatic ring located by the electronic density map (C1-C6) in the 12-membered ring channel of AFI framework, shows short distances with lattice oxygen atoms (see table S3). This could be due to the presence of a portion of protonated benzene ring (Scheme 1) searching for basic oxygens.
These results put in evidence the strong influence of the host material on the guest molecule. The presence of water in the zeolitic channels, not considered in the structural refinement, may play a significant role in the structural modification and the behavior of the composite material. The comprehension of the host/guest interactions and of the modification of the guest is fundamental to understand the relationship among structure and properties. In the specific case, the protonation of azobenzene can lead to a complex photolysis chemistry that starts with the photo-isomerization of the trans- into the cis-form.
X-Press beamline of Elettra Sincrotrone Trieste is acknowledged for the assistance during the experiments. Dr. Simona Bigi of University of Modena and Reggio Emilia is acknowledged for the thermal analyses. This work was supported by the Italian MIUR, (PRIN2015 “ZAPPING” High-pressure nanoconfinement in Zeolites: the Mineral Science know-how APPlied to engineerING of innovative materials for technological and environmental applications” 2015HK93L7 and ImPACT - FIRB 2012 RBFR12CLQD),
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