Two-Step Vapochromic Luminescence of Proton-Conductive

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Two-Step Vapochromic Luminescence of Proton-Conductive Coordination Polymers Composed of Ru(II)-Metalloligands and Lanthanide Cations Atsushi Kobayashi,*,† Kenki Shimizu,† Ayako Watanabe,† Yuki Nagao,‡ Nobutaka Yoshimura,† Masaki Yoshida,† and Masako Kato*,† †

Department of Chemistry, Faculty of Science, Hokkaido University, North-10 West-8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan School of Materials Science, Japan Advanced Institute of Science and Technology, 1−1 Asahidai, Nomi, Ishikawa 923-1292, Japan

‡

Inorg. Chem. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/30/19. For personal use only.

S Supporting Information *

ABSTRACT: Two proton-conductive and phosphorescent porous coordination polymers, La3-[H5.5RuP]2 and Pr3[H5.5RuP]2 (H12RuP = [Ru(H4dpbpy)3]2+, H4dpbpy = 2,2′bipyridine-4,4′-bis(phosphonic acid), composed of Ru(bpy)3type metalloligands (bpy = 2,2′-bipyridine) functionalized with six phosphonate groups and lanthanide cations (Ln3+ = La3+ or Pr3+) were successfully synthesized. X-ray diffraction studies revealed that six to seven protons of the H12RuP metalloligand were removed in the coordination polymerization reaction to form the porous coordination framework (Ln3-[H5.5RuP]2) with Ln3+ cations (Ln3+ = La3+ or Pr3+). Although their porous structures collapsed on the removal of water molecules from the porous channels, the original porous structures were reconstructed by water adsorption. Interestingly, the triplet metal-to-ligand charge-transfer (3MLCT) emission of Ln3[H5.5RuP]2 was blue-shifted on increasing the relative humidity (RH) in the low RH region, whereas the inverse red shift was observed in the high RH region, resulting in the highest-energy 3MLCT emission at medium RH. The origin of this two-step vapochromic luminescence (that is, the blue and red shifts of the 3MLCT emission) is ascribable to the water-adsorptiontriggered reconstruction of the porous structure and the proton release from the H5.5RuP metalloligand to the water filled channels, respectively. The proton conductivity of Ln3-[H5.5RuP]2 is about 1000-times higher at 20% RH and 10-times higher at 95% RH than that of the carboxylate analog, La7-[RuC]4 (H6RuC = [Ru(H2dcbpy)3]2+; H2dcbpy = 2,2′-bipyridine-4,4′bis(carboxylic acid)), probably because of the highly acidic phosphonic acid groups.

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gand.25−34 Lin et al. reported that MOF microcrystals composed of carboxy-functionalized [Ru(bpy)2(CN)2]-type metalloligands and Zn2+ cations exhibited a remarkably amplified triplet metal−ligand charge-transfer (3MLCT) emission quenching derived from the effective intra-MOF energy transfer and subsequent electron transfer at the MOFsolution interface.27 Recently, a MOF thin film comprising a well-known Ru(II) molecular photosensitizer, [Ru(bpy)3], was reported by Morris et al. to act as a porous photosensitizing film in a dye-sensitized solar cell.32 Although achieving cooperativity between the luminescence properties of [Ru(bpy)3]-type molecular luminophores and the macroscopic functions, such as ion conduction derived from the porous nature of PCP/MOF materials, is challenging, it is a worthy subject of study. Ion conduction, especially proton conduction, in PCP/MOF materials is a fascinating macroscopic phenomenon from the

INTRODUCTION Porous coordination polymer (PCP) and metal−organic framework (MOF) materials have attracted considerable attention in recent decades because of their promising functionality, which originates from their well-defined molecular-level porosity.1−9 In addition to this porosity, methods for further functionalization have been recently developed.10−20 The use of functional metal complexes as ligands, the so-called metalloligand technique, is one of the most promising methods, and considerable efforts have been devoted to design PCP/MOF materials with multiple functionality by combining the porosity and metalloligand properties.15−20 From the viewpoint of multifunctionalization using this technique, Ru(II) polypyridine complexes are interesting candidates for the functionalization of PCP/MOF materials because this type of complex can act not only as a chromophore/luminophore but also as a reversible redox reaction center.21−24 Many PCP/MOF materials have been developed so far that take advantage of the versatile properties of the Ru(bpy)3-type (bpy = 2,2′-bipyridine) metalloli© XXXX American Chemical Society

Received: October 15, 2018

A

DOI: 10.1021/acs.inorgchem.8b02928 Inorg. Chem. XXXX, XXX, XXX−XXX

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metalloligand, [Ru(H4dpbpy)3]Cl2, was prepared according to a previously published method.49 Elemental analysis was performed at the analysis center of Hokkaido University. Synthesis of Ln3-[H5.5RuP]2 (Ln3+ = La3+ or Pr3+). A solution of LnCl3·7H2O (10.2 μmol, La and Pr salts were purchased from Wako Pure Chemical Industries, Ltd. and Strem Chemicals, Inc., respectively) in EtOH (1 mL) was carefully layered on top of an aqueous solution of [Ru(H4dpbpy)3]Cl2 (3.8 mg (3.19 μmol) in 3 mL water) with an ethyl acetate buffer (1 mL). Red plate-like crystals began to form after standing at 323 K for several days. After 3 weeks, these crystals were collected by filtration, washed with water, and dried in air. La3-[H5.5RuP]2. Yield: 1.23 mg, 0.76 μmol, 24% based on [Ru(H 4 dpbpy) 3 ]Cl 2 . Elemental analysis (%) calcd for C30H23.5La1.5N6O18P6Ru1·6H2O: C, 26.51; H, 2.63: N, 6.18. Found: C, 26.42; H, 2.34; N, 5.88. Pr3-[H5.5RuP]2. Yield: 1.87 mg, 1.14 μmol, 36% based on [Ru(H 4 dpbpy) 3 ]Cl 2 . Elemental analysis (%) calcd for C30H23.5N6O18P6Pr1.5Ru1·4.5H2O: C, 26.98; H, 2.45: N, 6.29. Found: C, 26.97; H, 2.35; N, 6.00. Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) measurements were conducted using a Rigaku SPD diffractometer at beamline BL-8B at the Photon Factory, KEK, Japan or a Bruker D8 Advance diffractometer equipped with a graphite monochromator using Cu−Kα radiation and a one-dimensional LynxEye detector. The wavelength of the synchrotron X-rays was 1.526 Å. The RH was controlled by using saturated aqueous solutions of various metal salts (see Figure S1).50 Luminescence Properties. The luminescence spectra of the complexes were measured using a JASCO FP-8600 or FP-6600 spectrofluorometer at room temperature. The slit widths of the excitation and emission light were 5 and 6 nm, respectively. The RH was controlled by using saturated aqueous solutions of various metal salts.50 The wavelength of the emission maximum (λem) of each observed spectrum was determined by reading the maximum value of the emission band. The luminescence quantum yield was recorded on a Hamamatsu Photonics C9920-02 absolute photoluminescence quantum yield measurement system equipped with an integrating sphere apparatus and 150 W continuous wave (CW) xenon light source. Ionic Conductivity Measurements. Impedance measurements were performed at RHs of 20−90% at 298 K using a Solartron 1260 impedance/gain-phase analyzer and a Solartron 1296 dielectric interface (Solartron Co., Ltd.) equipped with an SH-221 temperature-humidity controller (ESPEC Corp.). The low RH region below 40% was controlled by a humidity generator (me-40DP-2PW; Micro Equipment Inc.) with a lab made sample cell. The temperature dependence of the ionic conductivity was investigated in the temperature range of 293−333 K. The sample was processed into pellets of 2.5 mm diameter, and SILBEST no. 8560 porous gold paint (Tokuriki Chemical Research Co., Ltd.) was used as the electrode material. Vapor Adsorption Isotherms. The vapor adsorption isotherms of each complex were measured using a BELSORP-Max vapor adsorption isotherm measurement device at 298 K. All samples were dried by heating at 120 °C under vacuum for 12 h to remove all hydrated water molecules before each measurement. Thermogravimetric Analysis. Thermogravimetric analysis (TG) and differential thermal analysis (DTA) were performed in 0.3L/min Ar flow using a Rigaku ThermoEvo TG-8120 analyzer. The heating rate was 1 K/min. Single Crystal X-ray Diffraction. All measurements were conducted with Rigaku Mercury CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71069 Å) and a rotating anode generator. A single crystal was mounted on a MicroMount coated with paraffin oil. The crystal was then cooled using an N2-flowtype temperature controller. Diffraction data were collected and processed using CrysAlisPRO program.51 Structures were solved by direct methods using SHELXT.52 Structure refinement was carried out using the full-matrix least-squares method in SHELXL.52 All non-

viewpoint of various technological applications, for example, fuel cells.35−45 Generally, the evaluation of the ion conductivity in the microscopic region ( 2σ(F2)) wR2b (all data)

C30H23.5N6O18.5P6Pr1.5Ru·21.5H2O 1649.65 monoclinic P21/n 23.783(2) 11.8635(4) 25.362(2) 116.624(8) 6397.1(7) 4 113(1) 1.516 1.900 1.020 0.0932 0.2349

a R1 = ∑||F0| − |Fc|| /∑|F0|. bwR2 = [∑w(F02 − Fc2)/∑w(F0)2]1/2, w = [σc2(F02) + (xP)2 + yP]−1, P = (F02 − 2Fc2)/3.

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RESULTS AND DISCUSSION Crystal Structure. Figure 1 shows the crystal structure of Pr3-[H5.5RuP]2. This complex crystallized in the centrosymmetric monoclinic space group P21/n. Two enantiomers of the Δ- and Λ-RuP metalloligand were located at two symmetry related positions about an inversion center in the unit cell. The Ru−N bond distances ranged from 2.03(1) to 2.07(1) Å, indicating the divalent Ru(II) state. All six phosphonate groups of RuP were coordinated to PrIII cations to form a threedimensional coordination network structure (Figure 1b). There are two different PrIII sites in the unit cell (Figure 1c,d). Three of the six phosphonate groups of RuP are coordinated to the Pr1 cations, and the others are bonded to the Pr2 site. The Pr1 site is coordinated by five O atoms of the phosphonate groups of the RuP metalloligands and three O atoms of the coordinated water molecules to form an octacoordinated square antiprismatic coordination structure. Four of five phosphonate groups on the same bc plane bridge the two adjacent Pr1 sites (Figure 1c). Although the other PrIII site (Pr2) is disordered over two different positions, each with half occupancy (Figure 1d), the PrIII cation is surrounded by six phosphonate groups of the adjacent RuP metalloligands and four water molecules. Four of the six phosphonates and two of the four coordinated water molecules are disordered at two different positions with half occupancy because of the site disorder of the central PrIII cation. The molar ratio of PrIII and RuP was estimated to be 3:2, and no counteranions were found in the unit cell. Thus, the molecular charge of the metalloligand should be [H5.5RuP]4.5−, indicating that six or seven protons of the six phosphonic acid groups were removed in the coordination polymerization reaction. There are two kinds of narrow porous channels (denoted as A and B in Figure 1a) along the b-axis, and the porosity was estimated C

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Figure 1. (a) Crystal structure of Pr3-[H5.5RuP]2 viewed along the b-axis and coordination structures of the (b) Ru, (c) Pr1, and (d) Pr2 sites. The two pore types are denoted A and B. Coordination spheres of Ru(II), Pr(III) ions of Pr1 and Pr2 sites are shown as blue, green, and orange polyhedrons, respectively. Brown, light blue, red, and orange ellipsoids represent C, N, O, and P atoms, respectively. Noncoordinated water and H atoms are omitted for clarity.

but also noncoordinating water molecules are required to recover the porous structure. Notably, the diffraction peak observed at 8.8° for Pr3-[H5.5RuP]2 at 54% and 69% RH was obviously shifted to lower angles on increasing the RH to 75%. Because this peak corresponds to the (200) diffraction peak, lattice expansion along the a-axis is an important step in recovering the original porous structure. The saturated adsorption amounts of La3-[H5.5RuP]2 and Pr3-[H5.5RuP]2 per RuP unit were estimated to be 20.5 and 21.5 mol/mol, respectively. These values are larger by about 30% than that of the carboxyl analog, La7-[RuC]4 (16.5 mol/mol), probably because of the greater porosities of these complexes, as mentioned in our discussion of the crystal structure. Plausible Mechanism of Two-Step Vapochromic Luminescence. As discussed in the previous section, both La3-[H5.5RuP]2 and Pr3-[H5.5RuP]2 exhibited an interesting two-step vapochromic luminescence triggered by water vapor adsorption/desorption. Here, we discuss a plausible mechanism for this phenomenon. Our proposed mechanism is shown schematically in Scheme 2. The blue shift in the 3MLCT

chemisorption, that is, the coordination of adsorbed water molecules to the coordinatively unsaturated Ln3+ ions, because the number of coordinating water molecules (4 or 5 mol per RuP in Pr3-[H5.5RuP]2) almost agrees with the calculated amount of adsorbed water molecules. In contrast, in the higher RH region, the PXRD patterns of La3-[H5.5RuP]2 and Pr3[H5.5RuP]2 significantly changed, and sharp diffraction peaks appeared. It should be noted that these RH values agree with the values where the 3MLCT emission wavelength switched from a blue to red shift. At above 85% and 75% RH for La3[H5.5RuP]2 and Pr3-[H5.5RuP]2, the observed PXRD patterns qualitatively agree with each other and with the simulated pattern calculated from the crystal structure of Pr3-[H5.5RuP]2, indicating that the original porous crystal structures were recovered after water adsorption. At 85% and 75% RH, the adsorbed amount of water vapor was estimated to be 17.8 and 16.2 mol per RuP for La3-[H5.5RuP]2 and Pr3-[H5.5RuP]2, respectively. These values are significantly larger than the number of water molecules coordinating to Pr3+ ions, as discussed above. Thus, not only coordinating water molecules D

DOI: 10.1021/acs.inorgchem.8b02928 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. RH dependence of emission spectra of (a) La3-[H5.5RuP]2 and (b) Pr3-[H5.5RuP]2 in the solid state at 298 K (λex = 400 nm). The blue dotted lines in both panels show the wavelength of the emission maxima of the samples at 0% RH. The top black lines in both panels (a) and (b) show the emission spectra of the as-synthesized samples. (c) RH dependence of the emission maximum wavelength of La3-[H5.5RuP]2 (blue square) and Pr3-[H5.5RuP]2 (red circle) compared to that of La7-[RuC]4 (black triangle).

Figure 3. RH dependence of the PXRD patterns of (a) La3-[H5.5RuP]2 and (b) Pr3-[H5.5RuP]2 and (c) their water vapor adsorption isotherms (blue and red symbols for La3-[H5.5RuP]2 and Pr3-[H5.5RuP]2) compared with that of La7-[RuC]4 (black) at 298 K. The bottom patterns in panels (a) and (b) show the PXRD patterns of the as-synthesized samples. The top black line in panel (b) is the simulated pattern calculated from the crystal structure of Pr3-[H5.5RuP]2. Closed and open symbols in panel (c) represent the adsorption and desorption processes, respectively.

emission observed in the lower RH region is similar to that observed for the carboxy analog, La7-[RuC]4. Because the water vapor adsorption behaviors of Ln3-[H5.5RuP]2 (Ln3+ = La3+ or Pr3+) are like that of La7-[RuC]4, the origin of the blue shift is thought to be the electrostatic changes that occur on

water-adsorption-induced lattice expansion. In other words, the 3MLCT emissive state could be electrostatically stabilized by the more densely packed structure at 0% RH compared to the water-vapor-adsorbed porous structure at higher RHs. In addition, the coordination of water to Ln3+ cations would E

DOI: 10.1021/acs.inorgchem.8b02928 Inorg. Chem. XXXX, XXX, XXX−XXX

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energy of the newly appeared band is closer to the band observed for the RuP metalloligand treated with aqueous NH3 than that treated with aqueous HCl, deprotonation may occur during water vapor adsorption at high RH (Scheme 2c), resulting in the red shift in the 3MLCT emission of Ln3[H5.5RuP]2 because the deprotonation of phosphonic acid groups would reduce the π backdonating ability of the dpbpy ligand. Although the cooperative phenomenon between the 3 MLCT emissive state of RuP and the f−f emissive state of the bridging Pr3+ cations was expected, an almost identical 3MLCT emission to that of La3-[H5.5RuP]2 was observed for Pr3[H5.5RuP]2 even at 77 K (see Figure S7), probably because of the lower excitation energy of the 3MLCT state of RuP than that of the f−f state of the Pr3+ cation. Proton Conductivity. To clarify the effect of replacing the functional groups attached to the [Ru(bpy)3]-moiety from carboxylate to phosphonate, proton conductivity measurements for La3-[H5.5RuP]2 and Pr3-[H5.5RuP]2 were conducted. The obtained results are shown in Figure 4 and

Scheme 2. A Plausible Mechanism of the Two-Step Vapochromic Luminescence of Ln3-[H5.5RuP]2 with Schematic Structures at (a) Low, (b) Medium, and (c) High RH Regionsa

a

Black hexagons, green octagons, and blue ellipsoids show the H5RuP metalloligands with seven protons removed, Ln3+ cations, and water molecules. Red circles and ellipsoids are the protons bound to RuP and protonated water molecules (H3O+), respectively.

suppress the electron-withdrawing nature of the Ln3+ cations, resulting in the destabilization of the 3MLCT emissive state by water adsorption (Scheme 2a,b). On the other hand, a red shift in the 3MLCT emission of Ln3-[H5.5RuP]2 in the high RH region was not observed for the carboxy analog, La7-[RuC]4. As discussed in the crystal structure section, the important difference between Ln3-[H5.5RuP]2 and La7-[RuC]4 is the protonation state of the acid groups, that is, the Ln3[H5.5RuP]2 has removable protons (5.5 protons per RuP on average) on the phosphonate groups, even though all these functional groups are coordinated to Ln3+ cations. In contrast, there are no removable protons at the carboxy groups of La7[RuC]4 because all six carboxy groups are carboxylate anions coordinated to the La3+ cations. As reported previously by Nazeeruddin and Kalyanasundaram, the energies of the 3 MLCT emissions of the RuC metalloligands in aqueous solution depend on the pH.55 Similarly, we found that the 3 MLCT emission energy of the RuP metalloligand in aqueous solution changed from 632 to 652 nm at around pH = 6 (see Figure S4 and Table S2). Since our titration experiments revealed that the first and second pKa values of RuP metalloligand were 1.28 and 6.44 (see Figure S5), the pH dependence of 3MLCT emission energy of the RuP metalloligand clearly indicates that the protonation/deprotonation reaction of the phosphonic acid groups attached to the bpy ligand should influence the 3MLCT emission energy. There are 5.5 protons on average on each RuP metalloligand in Ln3[H5.5RuP]2. In other words, one of the two RuP sites has six protons (H6RuP) and another has five protons (H5RuP) on the phosphonate groups, suggesting that the protonation/ deprotonation reaction easily occurs in the water-filled channels of Ln3-[H5.5RuP]2 to achieve acid−base equilibrium. Thus, the origin of the red shift in the 3MLCT emission in the high RH region could be due to the changes in the protonation state of the RuP metalloligand by water adsorption. In fact, the band positions of both the ν(PO) and ν(P−O) modes (1070 and 920 cm−1) in the IR spectrum of Pr3-[H5.5RuP]2 changed on increasing the RH and a new band at 975 cm−1 gradually appeared (see Figure S6). Considering that the

Figure 4. (a) Log (σp/S cm−1) vs RH profiles at 298 K and (b) Arrhenius plots of proton conductivities in the range of 293−333 K at 40% and 85% RH of La7-[RuC]4 (black triangle) and Pr3-[H5.5RuP]2 (red circle).

compared with the previously reported results for La7-[RuC]4. Pr3-[H5.5RuP]2 exhibited the RH-dependent conductivity, suggesting the water-mediated proton conduction. The proton conductivity of Pr3-[H5.5RuP]2 at 20% RH was found to be 1.1 × 10−7 S cm−1, which is remarkably higher (by about threeorder of magnitudes) than that of La7-[RuC]4 (1.8 × 10−10 S cm−1). This difference clearly indicates that the replacement of the coordinated functional groups from the carboxylate to phosphonate greatly enhanced the proton conductivity, even in the low RH region. The slightly steeper increase in the proton conductivity in the RH region above 80% RH compared to that in the lower RH region could be due to the proton release from the phosphonic acid groups of the H6RuP metalloligand to the water-filled channels, as discussed above (see Scheme 2). The comparable conductivity of La3-[H5.5RuP]2 to that of Pr3-[H5.5RuP]2 indicates that the effect of Ln3+ replacement on the proton conduction is negligible, probably because of the F

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structure. Further studies to develop highly proton-conducting phosphorescent materials to detect microscopic proton conduction are now in progress.

isomorphic structures (see Figure S8). The proton conductivities of La3-[H5.5RuP]2 and Pr3-[H5.5RuP]2 increased to 2.8 × 10−6 and 3.7 × 10−6 S cm−1, respectively, on increasing the RH to 95%, probably because of water vapor adsorption in the porous channels, but the RH dependence of Ln3[H5.5RuP]2 (Ln3+ = La3+ or Pr3+) was significantly lower than that of La7-[RuC]4, despite the larger amount of water vapor adsorption (see Figure 3c). A similar trend was also observed in the temperature dependence of the proton conductivity at constant RH. As shown in Figure 4b, a significant change in the activation energy of La7-[RuC]4 from 0.69 to 0.27 eV, as estimated from the Arrhenius plot, was observed on the RH increase from 40% to 85%, as reported by us previously,48 while an almost constant activation energy (ca. 0.4 eV) was observed for Pr3-[H5.5RuP]2 in this RH region. These different trends for Pr3-[H5.5RuP]2 and La7-[RuC]4 concerning the temperature and RH dependences suggest different proton conducting pathways. As discussed in the crystal structure discussion, all the phosphonate groups of Pr3[H5.5RuP]2 are located at the surface of the narrow porous channels, and most of the phosphonate groups possess one detachable proton that could contribute to proton conduction in the channels. In addition, the distances between the phosphonate groups are small enough to form hydrogen bonds directly. These structural features of Pr3-[H5.5RuP]2 suggest that the contribution of adsorbed water molecules in the porous channels to the proton conduction is smaller than that in La7-[RuC]4. This is consistent with the fact that the activation energy of proton conduction of Pr3-[H5.5RuP]2 was not dependent on the RH, even though the water-adsorptiontriggered structural transformation occurred at around 54% RH (see Figure 3b).

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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.8b02928. X-ray crystallographic data of Pr3-[H5.5RuP]2 in CIF format; photograph of sample preparation in various RH condition; PXRD patterns of Ln3-[H5.5RuP]2 (Ln3+ = La3+ or Pr3+); RH dependence of the emission quantum yield of Pr3-[H5.5RuP]2; TG-DTA curve of Pr3[H5.5RuP]2; pH dependence of the emission spectra of RuP in aqueous solution; titration curve of RuP metalloligand; RH dependence of the IR spectrum of Pr3-[H5.5RuP]2 (PDF) Accession Codes

CCDC 1873435 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

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

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ORCID

CONCLUSION In this study, we synthesized two new proton-conducting phosphorescent porous coordination polymers, Ln 3 [H5.5RuP]2 (Ln3+ = La3+ or Pr3+), composed of Ru(bpy)3type metalloligands functionalized with six phosphonate groups, [H5.5RuP]4.5−, and bridged by lanthanide cations. Xray diffraction study clearly revealed that both La3-[H5.5RuP]2 and Pr3-[H5.5RuP]2 have an isomorphic porous structure with about 35% porosity. The proton conductivities of La3[H5.5RuP]2 and Pr3-[H5.5RuP]2 at 95% RH were estimated to be 2.8 × 10−6 and 3.7 × 10−6 S cm−1, respectively; values remarkably higher than those of the previously reported carboxylate analogs, La7-[RuC]4. Reversible structural transformation between the water-adsorbed porous phase and the water-released amorphous-like phase was achieved for both Ln3-[H5.5RuP]2 (Ln3+ = La3+ or Pr3+) by water vapor adsorption, and their maximum emission wavelength strongly depended on the state of hydration: A blue shift was observed at low RH, and the inverse red shift was observed at high RH. As a result, the 3MLCT emission energy was highest at the medium RH region (at around 80%RH for La3-[H5.5RuP]2 and 60%RH for Pr3-[H5.5RuP]2). This interesting two-step vapochromic luminescence originates from the water-adsorption-triggered structural transformation and the subsequent proton release from the phosphonic acid groups of the H6RuP metalloligands to the water-filled channels. Our present study clearly indicates that not only the structural transformation but also the protonation/deprotonation reaction affects the proton conduction and the luminescence properties of the RuP metalloligand, even in a porous coordination network

Atsushi Kobayashi: 0000-0002-1937-7698 Yuki Nagao: 0000-0003-1249-440X Masako Kato: 0000-0002-6932-9758 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. H. Sato (Rigaku Corp.) for valuable support for single crystal X-ray structure analysis. This study was supported by Murata Science Foundation, ENEOS Hydrogen Trust Fund, and JSPS KAKENHI, grant nos. JP18K19086 and JP17H06367. The PXRD measurements were performed under the approval of the Photon Factory Program Advisory Committee (proposal no. 2017G528).

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DOI: 10.1021/acs.inorgchem.8b02928 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b02928 Inorg. Chem. XXXX, XXX, XXX−XXX