Formation and optical properties of metal/10-hydroxybenzo[h]quinolone complexes in the interlayer spaces of magadiite by solid–solid reactions

Intercalation and in situ formation of three fluorescent complexes, Al(III)-, Cr(III)- and Cu(II)-10-hydroxybenzo[h]quinolone (M-HBQ, M = Al, Cr and Cu), in the interlayer spaces of magadiite (mag) were studied by solid–solid reactions between metal ions exchanged mags (M-mag, M = Al, Cr and Cu) and HBQ. Results show that the basal spacings of the intercalated composites increase after the intercalation of HBQ into M-mags. The amount of HBQ in the intercalated compounds is different due to the amount of metal ions and the diversification of coordination ability of metal ions, and the order of the coordination ability of these three metal ions is Cu2+ > Cr3+ > Al3+. The amount of the metal cations in the interlayer of mag is enough for the in situ complex formation of M-HBQ complexes. The slight shift of the absorption and luminescence bands of the complexes suggests the different microstructures, including molecular packing of the complexes in the interlayer spaces of mags, resulting that the host–guest interactions are formed. These findings show that the intercalation and in situ formation of M-HBQ complexes (M = Al, Cr and Cu) in the interlayer space of mag are successfully achieved in the current work.


Introduction
In the past decades, intercalation of guest species into layered inorganic solids has attracted great attention from a wide range of scientific and practical viewpoints [1][2][3][4][5][6][7]. Intercalation of photoactive species like organic dyes into layered solids has been investigated to understand the nature of host-guest systems and to prepare novel photofunctional supramolecular systems [8], because the characteristics of the photoprocesses are sensitive to the environment in which the photoactive species are located [9]. It has been found that solid-solid reactions are promising ways to intercalate organic guest species into the interlayer spaces of inorganic solids [2,9,10]. Solid-solid reactions are one of the most suitable techniques for intercalated processing due to the facile operation and the possibility to prepare compounds, which are not accessible from solutions, and so on [11,12]. Owing to the advantages of solid-solid reactions, it is worth investigating for constructing novel low-dimensional nanohybrid materials with novel structures and properties by the solid-state intercalation of metal complexes into layered inorganic solids [9,10,13]. The solid-solid intercalation of both cationic and non-ionic species into inorganic solids, such as layered clay minerals, layered zirconium phosphate and zeolites, has been reported so far [2,9,[14][15][16][17].
Magadiite (Na 2 Si 14 O 29 ·xH 2 O, abbreviated as mag) was first found in the deposits of Lake Magadi in Kenya and described by Eugster in 1967 [18]. The structure of mag is composed of one or multiple negatively charged sheets of SiO 4 tetrahedra with abundant silanol-terminated surfaces. Negative charges in the layers of mag are counterbalanced by hydrated cations (Na + or H + et al. ) in the interlayer spaces [19][20][21][22]. Mag has a high cation exchange capacity (CEC) that can be applied to ion exchange, whereby the sodium ions can be replaced by protons, other cations or large quaternary ammonium ions [23][24][25]. These properties of mag prove it to be a good candidate for the formation of organic-inorganic nanocomposites as the host material. In the previous reports, Makoto Ogawa intercalated tris(2,2 -bipyridine)ruthenium(II) complex [8,26], 1,1 -diethyl-2,2 -cyanine [27], 4-dodecyloxy-4 -(trimethylammoniopentyloxy)azobenzene and 4-(ω-trimethylammoniodecyloxy)-p -(octyloxy)azobenzene bromide [28], p-[2-(2-hydroxyethyldimethylammonio)ethoxy]azobenzene bromide [29] in the interlayer space of mag. However, these works mainly focused on the intercalation, and their properties were not studied. Recently, the intercalation of inorganic nanoparticles (e.g. ZnO, CuO, CdS and ZnS) into mag has been received increasing attention [30][31][32][33][34]. The intercalation of fluorescers into the layer space of mag has been less reported compared with montmorillonite [9,14,15,35]. In our previous reports, we studied the self-assembly of 8-hydroxyquinoline-Li(I), Al(III) and Cu(II) complexes [36] and the intercalation and in situ formation of coordination compounds with ligand 8hydroxyquinoline-5-sulfonic acid [37] into the interlayer surfaces of mag. Herein, we are interested in extending the previous studies to the intercalation of 10-hydroxybenzo[h]quinolone (abbreviated as HBQ, the molecular structure is shown in scheme 1a) into the interlayer space of metal ions exchanged mags (abbreviated as M-mags, M = Al, Cr and Cu). To the best of our knowledge, the intercalation of HBQ into the layered inorganic solids has not been fabricated so far.
HBQ (scheme 1a) has been found potential application as a reagent in the synthesis of optical filter agents in photographic emulsion in the previous decades [38,39]. Metal-HBQ and substituted HBQ complexes have attracted great attention owing to their useful luminescence properties especially in the use for organic light-emitting devices (OLEDs) [40,41]. The design and fabrication of efficient OLEDs based on organic materials have been areas of active research because of their important applications in the large area display technology. However, the formation and properties of metal-HBQ in the confined regions of layered silicates like mag have not been reported until now. In the present work, the intercalation and in situ complex formation of HBQ with the interlayer metal cations (Al 3+ , Cr 3+ and Cu 2+ ) were investigated to form coordination complexes (scheme 1b-d) in the interlayer spaces of mag by the solid-solid reactions.  synthetic process of HBQ-M-mags (M = Al, Cr and Cu) in the interlayer spaces of mags and the detailed process is described in the following sections.

Synthesis of mag and H-mag
Sodium magadiite (abbreviated as mag) was hydrothermally synthesized based on our previous report [36]. In a typical synthesis, mixtures of silica gel and NaOH with a molar ratio  Intercalation of HBQ into the interlayer spaces of M-mags (M = Al, Cr, Cu) was prepared by solid-solid reactions reported by Makoto Ogawa [9,10,14]. HBQ was mixed with M-mags and ground manually using an agate mortar and a pestle at ambient environment for 15 min. Molar ratios of HBQ to interlayer cations were 3 : 1, 3 : 1 and 2 : 1 for Al-, Cr-and Cu-mags, respectively. After solid-solid reactions, intercalated compounds were washed with n-hexane and ethanol several times and dried at 60°C for 24 h. The synthesized products were marked as HBQ-Al-mag, HBQ-Cr-mag and HBQ-Cu-mag, respectively.

Materials characterizations
Powder X-ray diffraction (XRD) was collected on a Panalytical X'Pert Powder diffractometer using monochromatic Cu Kα radiation. The amounts of exchange cations were determined by an energydispersive X-ray spectrometer (EDS) attached to a scanning electron microscope (SEM, QUANTA450). X-ray photoelectron spectroscopy (XPS) was used to investigate the surface composition of the products performed on ESCALAB250Xi, Thermo Fisher Scientific. Fourier-transform infrared (FTIR) spectra of the samples were recorded by KBr disc method on a Niole Avatar 360 FTIR spectrometer (USA) over the spectral region of 400-4000 cm −1 . The morphology and dimensions of the products were observed by field emission scanning electron microscopy (FE-SEM, NOVA NanoSEM 450, FEI). Samples for FE-SEM observation were gold-sputtered in order to get better morphology of the surface. Diffuse reflectance spectra (UV-Vis) of the solid samples were collected on an American HP-8453 scanning spectrophotometer using an integrated sphere. Photoluminescence spectra (PL) were characterized on a standard Jasco FP-6500 spectrofluorophotometer with the excitation at 395 nm. Inverted fluorescence microscope was tested on a Japan Olympus (IX71+DP71) with the excitation at 360 nm.

Cu-mag Al-mag
Cr-mag    Because the interlayer exchangeable cations of mags are surrounded by water molecules at ambient conditions [3,43], the increase in the basal spacings of M-HBQ complexes is caused by the intercalation of HBQ though ligand displacement reactions between the adsorbed H 2 O and HBQ molecules. The gallery heights are determined by subtracting the thickness of the silicate layer (1.12 nm) [44] from the observed basal spacings to be 0.26 nm for HBQ-Al-mag, 0.25 nm for HBQ-Cr-mag and 0.25 nm for HBQ-Cu-mag. Taking the gallery heights into account, it is worthwhile to mention that M-HBQ complexes form a monolayer arrangement in between the silicate layers, which is very consistent with the results reported for coordination complexes-montmorillonite hybrids [9,14,45]. By the way, free M-HBQ complexes (M = Al, Cr, Cu) cannot be directly intercalated in the interlayer spaces of mag because of their molecular sizes and neutral charges [9].    copper is +2. The results are also in a good agreement with the data previously reported [46]. Figure 5g shows the core-level spectra of C 1s in HBQ-M-mags. The binding energies of C 1s locate at 284.8, 284.8 and 284.6 eV in HBQ-Al-mag, HBQ-Cr-mag and HBQ-Cu-mag, respectively, which may be caused by the difference of the constrained geometry of the host, and the arrangements of the complexes in the interlayer spaces as well as the increased intermolecular interaction between adjacent molecules in solid state [9,14,47,48]. Table 1 summarizes chemical content of HBQ-M-mags by XPS and the corresponding calculations. From C/M in the sample and C/M in theory, the amount of the metal cations (Al 3+ , Cr 3+ or Cu 2+ ) in the interlayer of mag is fair enough for the in situ complex formation of M-HBQ complexes; that is, these samples contain residual metal cations after the solid-solid reactions between M-mags and HBQ. The same results can also be provided by M/N in the sample and M/N in theory. The above results further suggest the formation of M-HBQ complexes in the interlayer space of mag. The order of the coordination ability of these three metal ions is Cu 2+ > Cr 3+ > Al 3+ . As depicted in table 1, the ratio of C/N in HBQ-Mmags is close to the corresponding values in HBQ. Therefore, XPS results further indicate the successful intercalation and formation of M-8HqS complexes (M = Al, Ca and Zn) in the interlayer space of mag.

Results and discussion
To calculate the amount of the intercalated HBQ in HBQ-M-mags (M = Al, Cr and Cu), the heat method in the air atmosphere was used to study the amount of intercalated HBQ. M-mags and HBQ-M-mags were calcined at 500°C for 2 h in air, and the corresponding results are shown in table 2. The amounts of HBQ measure 7.12, 11.50 and 9.17 wt% in HBQ-Al-mag, HBQ-Cr-mag and HBQ-Cu-mag, respectively. The difference in the amount of HBQ in the intercalated compounds that is caused by the coordination ability of these three metal ions (Al 3+ , Cr 3+ and Cu 2+ ) is diverse in agreement with XPS results. All the above results confirm the intercalation of HBQ into the interlayer spaces of M-mags.
Morphologies of mag, M-mags and HBQ-M-mags (M = Al, Cr and Cu) were observed by FE-SEM. Figure S3 in the electronic supplementary material and figures 6 and 7 show FE-SEM images of mag, Al-mag, Cr-mag, Cu-mag, HBQ-Al-mag, HBQ-Cr-mag and HBQ-Cu-mag, respectively. As shown in figure 6, most of the platelets in Al-mag, Cr-mag and Cu-mag are well preserved without     severe destruction compared with original mag (figure S3 in the electronic supplementary material). Figure 7 depicts FE-SEM images of HBQ-Al-mag, HBQ-Cr-mag and HBQ-Cu-mag. It is observed that HBQ-Al-mag, HBQ-Cr-mag and HBQ-Cu-mag kept the original platelets. However, the platelets became disordered to some extent. The morphology of platelets of various intercalated mags is slightly destroyed, which is caused by the grind process [36].
The in situ complex formation of Al, Cr and Cu interlayer cations and the ligand HBQ was further determined by FTIR, UV-Vis and PL spectroscopies. Figure S4a-c in the electronic supplementary material shows FTIR spectra of HBQ, mag, H-mag, M-mags and HBQ-M-mags (M = Al, Cr and Cu). Comparing these FTIR spectra, their main difference is in the range from 1700 to 1200 cm −1 due to the overlap between HBQ and mag in the other region. Figure 8 and table 3 summarize the FTIR spectra of HBQ and HBQ-M-mags in 1700-1200 cm −1 , and these FTIR peaks are assigned to HBQ [40,41]. Some FTIR peaks of HBQ are not seen in figure 8 because the amount of HBQ in HBQ-M-mags is relatively less. The bands of intercalated products in this region assigned to HBQ are slightly shifted when compared with those observed for free HBQ molecule, supporting the coordination between HBQ and metal interlayer cations in mag [9,49]. FTIR spectra of the intercalated composites do not show any additional absorption bands due to decomposed species, suggesting no decomposition of HBQ molecule. Therefore, the results suggest the formation of M-HBQ complexes in the interlayer space of mag. Because   the homogeneous mixture of HBQ or M-HBQ complexes in the hybrids may exhibit the shifts in FTIR spectra [9], the further characterizations of UV-Vis and PL spectroscopies were also used to confirm the in situ complex formation of the M-HBQ complexes in the interlayer space of mags. Figure 9 shows UV-Vis spectra of HBQ, HBQ-Al-mag, HBQ-Cr-mag and HBQ-Cu-mag. In the solid state, HBQ exhibits a broad peak at 416 nm in agreement with the previous report [50]. After intercalation HBQ into M-mags (M = Al, Cr and Cu), this absorption band shifts to the low wavelength. Blue shift suggests the formation of M-HBQ complexes in the interlayer spaces of mag [9,15]. PL spectra of HBQ, M-HBQ and HBQ-M-mags (M = Al, Cr and Cu) are shown in figure 10. As shown in figure 10a, the maximum emission band due to the π to π* transition of HBQ in the solid state is located at around 604 nm, which is consistent with previous report [39,50]. After intercalating HBQ into M-mags, as depicted in figure 10b-d, PL bands of HBQ-Al-mag, HBQ-Cr-mag and HBQ-Cu-mag are located at 473, 470 and 481 nm, respectively. The blue-shifted fluorescence further supports the formation of M-HBQ complexes in the interlayer spaces of mag. In neutral water, HBQ exhibits a dominant fluorescence band at 585 nm, which is tentatively ascribed to the proton transfer tautomer emission, which seems to be reasonable due to the proposed ultrafast rate of excited-state proton transfer for HBQ in both aprotic and protic non-aqueous solutions [51]. This fluorescence band can be adjusted by the acidity of solution. The variation of the luminescence bands is thought to be attributed to the host-guest interactions, though   the details are still difficult to illustrate [14]. The PL bands observed in figure 10 for HBQ-Al-mag (473 nm), HBQ-Cr-mag (470 nm) and HBQ-Cu-mag (481 nm) can be attributed to M-HBQ complexes formed in the interlayer spaces of mag. The blue shift of the emission bands of M-HBQ complexes in mag may be potentially due to the constrained geometry of the host, and the arrangements of the complexes in the interlayer spaces as well as the increased intermolecular interaction between adjacent molecules in the solid state [9,14]. These observations uphold the in situ complex formation of M-HBQ complexes in the interlayer spaces of mag. Therefore, the molecular structure and packing of M-HBQ complexes formed in the mag differently to make the tiny difference in the wavelength of PL spectra [36]. Inverted fluorescence microscopic images of HBQ-Al-mag, HBQ-Cr-mag and HBQ-Cu-mag were detected under 360 nm UV light irradiation, as shown in figure S5 in the electronic supplementary material. As clearly demonstrated by fluorescence microscope images, HBQ-Al-mag, HBQ-Cr-mag and HBQ-Cu-mag presented yellow fluorescence. On the basis of the above analyses, the intercalation and in situ formation of M-HBQ complexes (M = Al, Cr and Cu) in the interlayer space of mag were successfully achieved. The mechanism of the solid-state intercalation has not been well established, but the process may be envisaged as resulting from the tendency of the reaction of a neutral organic molecule and hydrated cations in the interlayer space. According to the states of reactants, the mechanism of HBQ intercalated M-mags can be assigned to the ion-dipole intercalation and coordination [36,52], which are