Communication pubs.acs.org/IC
A Crystalline Mesolamellar Gallium Phosphate with Zwitterionic-type Templates Exhibiting Green Afterglow Property Hui-Lin Huang, Yu-Ting Huang, and Sue-Lein Wang* Department of Chemistry, Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *
might be effective in enhancing the crystallinity of mesolamellar structures as substantiated by a series of aluminum phosphates10 with crystalline inorganic sheets. Still, the information on template arrangement was missing in that long-chain carbon atoms were severely disordered and only nitrogen atoms could be located. To keep the skeleton carbon atoms of the bolaamphiphilic template in place, we speculated that the incorporation of an additional rigid counter-charged carboxylate to form zwitterionic-type templates might fortify the interlayer structure. By adopting 1,12-diaminododecane (DADD), which contains the longest carbon chain commercially available, and 4,4′biphenyldicarboxylate (BPDC) as cotemplates, we synthesized the first mesolamellar gallium phosphate (HDADD)2(BPDC)0.5[Ga3(OH)2(HPO4)4] (1) with a clearly resolved structure of templates. As shown in Figure 1, the
ABSTRACT: We synthesized a unique layer structure of gallium phosphates containing zwitterionic-type templates under mild hydrothermal reactions. The zwitterionic-type templates, formed of long-alkyl-chain diamine cations and biphenyldicarboxylate anions, resided upright between adjacent layers, propping the interlayer distance up to 2.2 nm. For the first time, the mesoscale interlayer templates were sufficiently well-ordered to afford elucidat io n t o t h e at o m i c - l e v e l . T h e m e so l a m e l l ar (HDADD)2(BPDC)0.5[Ga3(OH)2(HPO4)4] (1; DADD = 1,12-diaminododecane, BPDC = 4,4′-biphenyldicarboxylate) was composed of inorganic layers built up exclusively with a unique type of heptameric unit which featured an unprecedented trimeric cluster of [Ga3(OH)2O12]. Unexpectedly, compound 1 possessed an unusual green afterglow. To interpret the interesting photoluminescence (PL) property, three other lowdimensional structures related to 1 were prepared as well. The data from PL and electron paramagnetic resonance indicated that the afterglow was mainly attributed to lattice defects and the orientations of BPDC.
C
rystalline porous materials developed from silicates, germanates, and phosphates have attracted intensive research owing to their rich structure chemistry and versatile applications such as catalysis, gas adsorption, and, more recently, their intrinsic photoluminescence (PL) property.1−5 In the case of microporous materials with a periodic ordering nature, the structures can always be clearly defined on the atomic scale. For mesoporous materials, however, they are usually observed with channel topology6 from powder samples. They are not seen as mesoporous single crystals until the bimetal phosphite NTHU13 system has been prepared.7 This system sets a new stage for using surfactant-type templates to produce complete crystalline mesoporous phases which have not previously been seen. While crystalline 3D phases have now been extended to the mesoporous regime, 2D materials showing unambiguous information in the mesolamellar structure, particularly the periodical arrangement of templates, have not yet been realized.8 Many layer topologies with an interlayer gap 12 h) and thermally stable up to 200 °C (Figure S2) as the thermogravimetric analysis (TGA) showed that it only began to lose weight at higher temperatures; i.e., the organic components were decomposing in temperatures from 200 to 800 °C. The total observed weight loss up to 800 °C was 50.35%, which is in good agreement with the calculated value of 50.86%. The inorganic layer of 1 contains two unique Ga centers: one is square pyramidal Ga(1)(OH)O4, and the other is octahedral Ga(2)(OH) 2 O 4 . Two Ga(OH)O 4 and one Ga(OH) 2 O 4 polyhedra are connected to form an unprecedented trimeric cluster of {Ga3(OH)2O12} (Figure 2a). Each trimer is linked with
Figure 3. PL and EPR spectra: (a) Emission curves under excitation with 325 nm UV light measured at different delay times showing that the afterglow intensities were decreasing with increasing delay time. The top-right photos showing the crystals before (left) and after (right) exposure to portable UV-light (365 nm). (b) EPR spectra for 1, DBP-1, and DBP-2 showing significant peaks at about giso = 2.003 likely originated from lattice defects.
maxima can be observed in the visible-light region of the PL emission spectra of 1, which are located at ca. 470, 510, and 540 nm. The two bands resulted in the emission of the green light (CIE coordinates: (0.27, 0.44) with a quantum efficiency (QE) of 24.6%). Interestingly, we found that 1 exhibited an afterglow property. The green emission was observed even after turning off the portable UV light source, which ultimately decayed in 5 s. The phenomenon was then examined by measuring emission spectra at various delay times: 500, 1000, 1500, 2000, and 3000 ms. The intensities of afterglow decreased with increases in the delay time (Figure 3a); this observation was consistent with that under the use of a portable UV light. Incidentally, we found that the byproduct of 1, which contained no gallium, i.e., (H2DADD)4(BPDC)(H2PO4)2(HPO4)2·H2O (DBP-1), also exhibited afterglow (Figure S4). As depicted in Scheme 1, the crystal structure of DBP-1 is nearly a replicate of 1 in that it possesses a similar arrangement of zwitterionic-type templates (Figure S5). DBP-1 contains a supramolecular phosphate layer instead of a gallium phosphate layer. Like 1, there are hydrogen bonds between BPDC/DADD and phosphate groups in DBP-1. Accordingly, the afterglow might be related to the zwitterionoic templates. To get deeper insight into the afterglow phenomenon, we designed another two supramolecular structures. One is (H2DADD)(H2PO4)2·0.6(BPDA) (DBP-2),which is compositionally related to DBP-1, whereas the intrapacking mode of the templates is different. The orientations of the carboxylic groups are parallel to the inorganic phosphate layer without hydrogen bonds between the two. The other one is (H2DADD)(BPDC)· 2H2O (DB), which possesses only zwitterionic content (Scheme
Figure 2. Structure plots of 1: (a) section of the anionic gallium phosphate sheet showing 10R windows and the heptameric SBU and (b) the layer topology showing hydrogen bonds (in dotted lines) between the inorganic sheets and the templates. No forms of interaction among the templates.
four HPO4 tetrahedra into a new 7-heteropolyhedral secondary building unit (SBU) [Ga3(OH)2(HPO4)4]− (Figure 2a). Heptameric units are rarely discovered in the gallium phosphate/phosphite system, and only two structures under this category have been reported.13 However, the previous structures were solely composed of octahedral gallium centers. The inorganic layer of 1 is built up exclusively with the SBUs, and through the symmetry operations of a 2-fold rotation axis along the b axis and a c glide plane, the SBUs are interconnected via Ga−O−P bonds into 2D inorganic sheets parallel to the bc plane B
DOI: 10.1021/acs.inorgchem.6b00941 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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Scheme 1. Schematic Drawing of the Four Structures Encapsulating Zwitterionic Speciesa
Communication
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to Ministry of Science and Technology of Taiwan (MOST 103-2113-M-007-003-MY3) and Frontier Research Center on Fundamental and Applied Sciences of Matters of National Tsing Hua University for financial support.
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a
GaPO sheets in pink-and-yellow blocks, phosphate groups in yellow, DADD in cyan, and BPDC/BPDA in brown. Hydrogen bonds are shown in green dotted lines.
1). Unlike 1 and DBP-1, DBP-2 and DB did not show afterglow, and the PL emission of DB was barely observable. The difference in afterglow between DBP-1 and DBP-2 apparently was related to the acid molecules, which resulted in the different hydrogen bonding network. BPDC should be responsible for absorbing energy as indicated by UV−vis spectral measurements (Figure S6). Because of the lack of an appropriate pass (hydrogen bond), DBP-2 did not show afterglow like DBP-1. Remarkably, all these structures exhibited EPR signals which were likely originated from lattice defects of phosphates except DB. The afterglow property should also be related to lattice defects. In this system, therefore, BPDC (absorption center), hydrogen bonds (energy path), and lattice defects (trapping site) were all required for the afterglow property. In conclusion, we have successfully demonstrated that the use of zwitterionic-type templates formed of bolaamphiphilic diamine and rigid carboxylic acid is effective in creating a unique layered topology of gallium phosphate with layer spacing of up to 2.2 nm. For the first time, the mesoscale interlayer templates have been organized into a well-ordered arrangement to afford elucidation to the atomic level. The information on hydrogen bonding and orientations of templates, which was inaccessible before, now has become available. The unique structure of 1 was observed with its unforeseen intrinsic green afterglow. Based on the detailed structure information, the intriguing afterglow property could be successfully interpreted as being related to the exact orientations of the cotemplate acid molecules and the lattice defects. Further investigation on related systems is underway.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00941. Tables of crystal data and hydrogen bonds, ORTEP figure and TGA curve of 1, structure plots, and UV−vis and PL spectra (PDF) Crystallographic information file (CIF) C
DOI: 10.1021/acs.inorgchem.6b00941 Inorg. Chem. XXXX, XXX, XXX−XXX
