Visualization of Ion Conductivity: Vapochromic Luminescence of an

Nov 24, 2015 - Synopsis. A new luminescent and ion-conducting porous coordination polymer composed of a luminescent ruthenium(II) metalloligand and La...
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Visualization of Ion Conductivity: Vapochromic Luminescence of an Ion-Conductive Ruthenium(II) Metalloligand-Based Porous Coordination Polymer Ayako Watanabe,† Atsushi Kobayashi,*,†,‡ Erika Saitoh,† Yuki Nagao,§ Masaki Yoshida,† and Masako Kato*,† †

Department of Chemistry, Faculty of Science, Hokkaido University, North-10 West-8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan § School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan ‡

S Supporting Information *

The proton conductivity is normally evaluated by the alternatingcurrent (ac) impedance method. In this method, at least two electrodes need to be attached on the solid surface of the material. This makes in situ evaluation of the conductivity in a complicated system such as electrochemical cells difficult. Therefore, recently we have focused on the development of a new cooperative phenomenon of proton conduction and luminescence, by introducing a luminescent metalloligand to MOFs that exhibit proton conductivity. In this paper, we report on the crystal structure, vapochromic luminescence, and ion conductivity of a newly synthesized PCP, La 7 -[4Ru] 4 ({La1.75(OH)1.25[4Ru]·16H2O}, where [4Ru] = [Ru(4,4′dcbpy)3]4− and 4,4′-H2dcbpy = 4,4′-dicarboxy-2,2′-bipyridine), and demonstrate that La7-[4Ru]4 shows an interesting vapochromic luminescence coupled with a significant change of the ion conductivity. La7-[4Ru]4 was obtained by the simple reaction between LaCl3 and the ruthenium(II) metalloligand in a basic aqueous solution.22 Two [4Ru] metalloligands and four LaIII cations were found to be crystallographically independent (Figure 1). Although the two enantiomers Λ- and Δ-[4Ru] were completely disordered in both the independent [4Ru] sites, all six carboxyl groups of [4Ru] in both sites were commonly coordinated to LaIII cations (Figure S1). There are four crystallographically independent LaIII sites (Figure S2); the trinuclear clusters composed of the central La1 and two La4 cations were surrounded by six [4Ru]. The La2 cation was also bound by six [4Ru], whereas the La3 site was coordinated by the three carboxy groups. Each of the trans positions was occupied by O atoms probably belonging to water molecules. As a result, a twodimensional coordination sheet was formed in the ab plane. Interestingly, the two types of crystallographically independent void spaces (denoted as A and B in Figure 1) found along the a + b axis, had apertures of almost the same size (3 × 5 Å). The void fraction was calculated by Platon SQUEEZE to be 25.5%. The trivalent La3+ ion and tetravalent anionic [4Ru]4− in La7-[4Ru]4 were crystallized with a molar ratio of 7:4. Elemental analysis indicates that no Cl− counterions were contained in La7-

ABSTRACT: We synthesized a new porous coordination polymer {La 1.75 (OH) 1.25 [Ru(dcbpy) 3 ]·16H 2O} (La7[4Ru]4; H2dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) composed of a luminescent ruthenium(II) metalloligand [Ru(4,4′-dcbpy)3]4− and La3+ cations. X-ray analysis for La7-[4Ru]4 revealed that the La3+ cations and [4Ru] metalloligands are crystallized in a molar ratio of 7:4 with OH− counteranions and a void fraction of ∼25.5%. Interestingly, La7-[4Ru]4 shows a reversible structural transition triggered by water ad/desorption, which affects not only the triplet metal-to-ligand charge-transfer (3MLCT) emission energy but also the ion conductivity in the solid state. This correlation suggests that La7-[4Ru]4 is an interesting material that enables visualization of the ion conductivity via the 3MLCT emission energy.

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orous coordination polymers (PCPs) and metal−organic frameworks (MOFs) have attracted much attention as new functional porous materials not only for gas storage but also for heterogeneous catalysis and solid-state electrolytes because functionalization of the PCPs/MOFs can be achieved using functional molecular building blocks.1−5 One typical example is the MOF-based chemical sensor composed of luminescent linkers as the chromophores and/or luminophores.6−8 Kitagawa et al. reported an intelligent chemosensor that can interact effectively with the adsorbed guest molecules, built from the luminescent naphthalenediimide moiety.6 We also reported on the solid-state solvatochromic behavior of the coordination polymer (CP) composed of a phosphorescent platinum(II) metalloligand with alkaline-earth metal ions.7,8 These luminescent sensing materials are useful for visualizing the existence of harmful chemical vapors by changes in their luminescence.9−12 On the other hand, only a few materials show luminescence associated with the change of a macroscopic phenomenon like ion conduction. CPs have been known as proton conductors for several decades.13,14 Recently, a high proton conductivity comparable to the well-known proton-conducting film Nafion15,16 has been achieved with several MOFs, by introducing highly acidic sulfonic or phosphoric groups in the porous channels.17−21 © XXXX American Chemical Society

Received: September 9, 2015

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DOI: 10.1021/acs.inorgchem.5b02077 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

transfer (3MLCT) emission of [4Ru] in aqueous solution (Figure S5). This difference suggests the influence of coordination polymerization. We previously reported a similar red-shifted 3MLCT for the PCP Mg2[4Ru] and Sr2[4Ru],23 but these red shifts were smaller, by about 10 nm, than those of La7[4Ru]4. Consequently, the π* orbital of the 4,4′-dcbpy ligand could be more stabilized electrostatically by coordination with trivalent La3+ cations than with divalent alkaline-earth metal ions.24 As expected from the crystal structure of La7-[4Ru]4, the emission behavior strongly depends on the RH. The emission band shifted to a longer wavelength (by about 14 nm) after drying at 110 °C under a N2 atmosphere. In addition, the wavelength of the emission maximum gradually shifted to a shorter wavelength with increasing RH. At RH of 100%, almost the same spectrum as that of the as-synthesized one was clearly recovered. Thus, La7-[4Ru]4 shows luminescence vapochromism originating from the reversible ad/desorption of water vapor.25 A water-vapor adsorption isotherm and the change of the powder X-ray diffraction (PXRD) patterns in various RHs were measured to directly clarify the origin of luminescence vapochromism (Figure 3). The amount of water-vapor

Figure 1. Crystal structure of La7-[4Ru]4 viewed along the a + b axis. The two kinds of pores are denoted by A and B. Coordination spheres of RuII and LaIII ions are shown as blue and green polyhedra, respectively. Brown, light-blue, and red ellipsoids represent C, N, and O atoms, respectively. Noncoordinated water and H atoms are omitted for clarity.

[4Ru]4,22 suggesting the existence of OH− ions to maintain the charge neutrality. In fact, the OH vibration was still observed in the IR spectrum above 423 K, the temperature at which all water molecules were removed (Figures S3 and S4). It was difficult to determine the positions of the OH− ions because of the highly disordered structures around [4Ru]. However, the presence of OH− ions and the large number of water molecules in the porous channels (16 per one [4Ru]) may contribute to the ion conductivity. Because La7-[4Ru]4 contains 16 water molecules per one [4Ru], vapochromic luminescence may occur by water vapor ad/ desorption. Figure 2 shows the relative humidity (RH) dependence of the emission spectrum. As-synthesized La7[4Ru]4 exhibits a dark-red broad emission centered at 691 nm without any vibronic progressions, which is largely shifted to a longer wavelength than that of the triplet metal-to-ligand chargeFigure 3. (a) Water-vapor adsorption isotherm (298 K, black squares) and RH dependence of the ion conductivity (313 K, red triangles). (b) PXRD pattern of La7-[4Ru]4. Closed and open symbols in panel a show the adsorption and desorption processes, respectively. The red arrow in panel b indicates the peak characteristic of the intermediate phase.

adsorption sharply increased (up to 4 mol mol−1 per one [4Ru]) at relative pressures lower than P/P0 = 0.1 and then increased monotonically at higher-pressure regions. This effective adsorption in the low-pressure region should be due to the chemisorption derived from the coordinatively unsaturated La3+ cations. In fact, the number of the coordinated water molecules of La7-[4Ru]4 (4.5 per one [4Ru] estimated from the crystal structure) corresponds to the vapor uptake at P/P0 = 0.1. The saturated amount was 16.0 mol mol−1. This is consistent with the hydration number of La7-[4Ru]4 confirmed by elemental analysis. In the desorption process, the adsorbed water molecules were gradually released with large hysteresis, suggesting hydrogen bonds between the adsorbed water and PCP pore walls composed of hydrophilic carboxylates of [4Ru] and OH− anions. The observed PXRD pattern for the dried La7[4Ru]4 was different from that of as-synthesized La7-[4Ru]4,

Figure 2. RH dependence of the luminescence spectra of La7-[4Ru]4 (λex = 573 nm). The inset graph shows the RH dependence of the wavelength of the emission maximum (λem). The dotted lines in both graphs are guides showing the position at 699 nm. B

DOI: 10.1021/acs.inorgchem.5b02077 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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suggesting that the porous structure of La7-[4Ru]4 is not stable enough to retain the structure without crystal water molecules. Except for the appearance of a new diffraction peak at 8.3° (indicated by the red arrow in Figures 3b and S6), patterns similar to that of the dried sample were observed from an RH region of 11−43%. The characteristic peak weakened gradually at RH regions above 54% and completely disappeared above 85%. Simultaneously, the peaks assignable to the original porous phase of La7-[4Ru]4 were gradually recovered, and the pattern at 100% RH agreed qualitatively with the simulation of La7-[4Ru]4. Thus, the luminescence vapochromism of La7-[4Ru]4 is due to the structural transformation triggered by water-vapor ad/desorption and the humidity-insensitive region between 11% and 43% RH in the emission spectrum. This should be due to the intermediate phase, which can adsorb water vapor without any large structural transformations. As mentioned above, La7-[4Ru]4 contains a significant amount of water molecules and several OH− anions in the pores, suggesting that La7-[4Ru]4 may be an ion-conducting (H+ or OH−) PCP. Thus, we measured the ion conductivity (σi) by a quasi-four-probe ac impedance method (Figure 3a). Although the conductivity was found to be low at a low RH region (3.3 × 10−9 S cm−1 at RH 40%), it increased drastically by 2 orders of magnitude with increasing RH and finally reached a value of 5.5 × 10−7 S cm−1 at 95% RH. Interestingly, the rate of increase in the ion conductivity seems to be linear at all measured RH regions (RH > 40%), implying that the adsorbed water molecules in La7[4Ru]4 are important for ion conduction. In other words, the main ion-conduction pathway would be in the highly hydrophilic porous channels with the basic OH− anions of La7-[4Ru]4. Our preliminary result on the ion conductivity of Sr2[4Ru] (2.3 × 10−10 S cm−1 at RH 40%) implies that the existence of the OH− ion in La7-[4Ru]4 may enhance the ion conductivity at the low RH region. It is worth noting that the ion conductivity of La7[4Ru]4 largely changed in the RH region (>40%), where vapochromic luminescence was observed (Figure S7). This correlation may enable us to evaluate the ion conductivity from the emission wavelength without any directly attached electrodes. In summary, we newly synthesized a luminescent PCP from the ruthenium(II) metalloligand [4Ru] and La3+ and found that it is a unique luminescent material in which the ion conductivity and 3MLCT emission energy are correlated with each other via water ad/desorption. This correlation suggests that La7-[4Ru]4 is an interesting luminescent material that can help us visualize the ion conductivity as a luminescent wavelength. Because the La3+ ion in La7-[4Ru]4 plays an important role at the water adsorption and OH− binding sites, further development by replacing the La3+ ion is now in progress.

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Communication

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

All authors approve the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. S. Takeda and Dr. S. Noro (Hokkaido University) for their kind help. This study was supported by JSTPRESTO, Grant-in-Aid for Scientific Research(C) (26410063), and Artificial Photosynthesis (No. 2406) from MEXT, Japan.

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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.5b02077. X-ray crystallographic data in CIF format (CIF) Experimental details, coordination structures of Ru and La sites, TG-DTA, IR, UV−vis spectra, RH dependences of PXRD, emission maximum, and ion conductivity (PDF) C

DOI: 10.1021/acs.inorgchem.5b02077 Inorg. Chem. XXXX, XXX, XXX−XXX