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Keywords:

  • Luminescence;
  • Amorphous–crystalline transformation;
  • Platinum;
  • ­Vapochromism;
  • Sensors

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

A novel luminescent platinum(II)–diimine complex, Na2[Pt(CN)2(dcbpy)]·nH2O (H2dcbpy = 4,4′-dicarboxy-2,2′-bipyridine, n = 2 or 5) was synthesized and investigated by using X-ray diffraction measurements and spectroscopy. Complex Na2[Pt(CN)2(dcbpy)]·2H2O was first obtained as a red amorphous solid (1A) that exhibits bright luminescence at 623 nm. After recrystallization, two crystalline forms, the dihydrate form, 1C, and the pentahydrate form, Na2[Pt(CN)2(dcbpy)]·5H2O (2), were obtained and analyzed by the single-crystal X-ray diffraction method. Upon exposure of the amorphous form 1A to hydrophilic vapours such as those of MeOH and acetone, 1A irreversibly changed to a yellow crystalline solid 1C that showed luminescence at 608 nm with a shoulder at 568 nm. This vapour-induced amorphous–crystalline transformation has never been observed either by exposing 1A to hydrophobic vapours nor by increasing the temperature up to 410 K. IR spectroscopy revealed that the amorphous–crystalline transformation did not involve the adsorption of the vapour. This phenomenon of irreversible structural transformation accompanied by change of luminescence is possibly applicable to vapour history sensors.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

The design and synthesis of stable and reversible chemical sensors have received considerable attention in the past decade.1 Vapochromic or vapoluminescent materials showing drastic colour or luminescence change upon exposure to vapours of volatile organic compounds (VOCs) are some of the most promising materials for sensing applications. Mann and co-workers reported on the vapochromic behaviour of the [Pt(aryl isocyanide)4][Pd(CN)4] complex and suggested that the intermolecular metallophilic interaction between d8 metal ions plays a critical role in the chromic behaviour.2 By taking advantage of the intermolecular metallophilic interaction mentioned in their report, much effort has been focused on developing new vapochromic materials that can be applied as chemical sensors.310 One of the best-developed systems is a PtII–diimine system.25 The vapochromic behaviour in this series usually originates from the structural change induced by adsorption of vapour molecules. The adsorption of vapour molecules changes the intermolecular metallophilic and/or π–π interactions, resulting in the perturbation of the transition energy in the visible region. For example, [Pt(bpy)(CN)2] (bpy: 2,2′-bipyridine) is a simple vapochromic material showing bright luminescence from the metal-metal-to-ligand charge transfer (3MMLCT) transition state and undergoing a reversible colour change upon exposure to water vapour.5 This chromic behaviour is due to the water-vapour-induced structural transformation between nonhydrated [Pt(bpy)(CN)2] and monohydrate [Pt(bpy)(CN)2]·H2O. The adsorption of water molecules changes the distance between the Pt ions, which mainly contributes to the energy of the HOMO, resulting in colour and luminescence changes. Similarly, vapochromic materials with metallophilic interaction among AuI ions have also been reported.6,7 Some of the AuI complexes show not only vapochromism but also mechanochromism, which is the reversible colour change induced by the application of mechanical pressure.7 Another approach is to incorporate well-established solvatochromic molecules into solid-state materials.8 Incorporation of Tb3+ ions into the framework of a microporous solid permits shape- and size-selective sensing.8b Recently, some groups have reported that complexes of nonprecious metals can show vapochromic behaviour that originates from the vapour-induced transformation of the coordination environment of the metallic centre.9 Most of these vapour-sensing abilities are basically derived from a reversible structural transformation induced by adsorption of vapour molecules. However, there are few examples showing a sensing ability based on an irreversible structural transformation. This type of transformation may be promising for vapour history sensors.

We also designed a vapochromic PtII complex, [Pt(CN)2(H2dcbpy)] (H2dcbpy: 4,4′-dicarboxy-2,2′-bipyridine), and demonstrated that the flexible two-dimensional hydrogen bond networks formed between the cyano and carboxyl groups play an important role in the vapochromic behaviour.10 Taking advantage of the coordination ability of the carboxyl and cyano groups of this complex, we focused our attention on the development of new sensing materials. In this work, we have synthesized a cation-substituted complex, Na2[Pt(CN)2(dcbpy)]·nH2O (n = 2 and 5), and have found that the complex exhibits a vapour-induced amorphous–crystalline structural transformation accompanied by remarkable changes in colour and luminescence.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Vapour-Induced Amorphous–Crystalline Transformation

Complex Na2[Pt(CN)2(dcbpy)]·2H2O (1) was first obtained as a red amorphous solid (1A). Elemental analysis of 1A clearly shows that the complex contains two water molecules per Na2[Pt(CN)2(dcbpy)] unit. This red amorphous form is stable in dry air, but the colour gradually changes from red to yellow in humid air. In order to clarify the origin of this colour change, powder X-ray diffraction (PXRD) measurements were conducted on the amorphous red solid 1A upon exposure to various vapours. Figure 1 shows the PXRD patterns of 1A exposed to several kinds of vapours over a period of two weeks. Before exposure, the PXRD pattern was very broad, indicating that 1A is an amorphous solid. After exposure to hydrophilic vapours such as MeOH and acetone, the pattern dramatically changed to one with many sharp peaks, which is characteristic of crystalline solids. Surprisingly, all the patterns obtained for the samples exposed to hydrophilic vapours were almost identical to each other and they essentially corresponded to those obtained for the crystal of the dihydrate form, 1C. The transformation was almost completed within 1 d in a saturated MeOH vapour atmosphere (Figure S1). These results suggest that the amorphous solid 1A changed to the crystalline phase 1C (the structure of 1C is discussed below) upon exposure to the hydrophilic vapours. It should be noted that the amorphous solid 1A was never converted into the crystalline 1C by raising the temperature up to 410 K (Figure S2). In the 1H NMR spectra of the samples exposed to the vapour for two weeks, vapour molecules were hardly detected (Figure S3). In addition, IR spectra measured for samples exposed to acetone vapour suggested that the acetone vapour was not included in the solid (Figure S4). Thus, this structural transformation was triggered by exposure to the vapours, but the inclusion of vapour molecules into the solid did not occur. On the other hand, upon exposure to hydrophobic vapours such as Et2O and CHCl3, no change was observed, suggesting that the amorphous–crystalline transformation from 1A to 1C is probably related to the hydrophilic nature of the vapour. It should be emphasized that the amorphous solid 1A has very poor solubility not only for hydrophobic solvents but also for hydrophilic ones, except for water. When exposed to water vapour, the PXRD pattern of 1A was also converted to a pattern similar to that calculated for the crystal structure of 1C, but other peaks that could not be indexed were also observed. Since the PXRD pattern of 1A immersed in a small amount of water was almost identical to the pattern calculated from the structure of pentahydrate form 2 (Figure S5), the pattern may be due to other transformations, for example 1C to 2. The obtained crystalline solid of 1C is thermally stable and 1C could not be converted back to 1A even under vacuum at a high temperature around 373 K. Thus, this vapour-induced amorphous–crystalline transformation is irreversible in the solid state. As described in the introduction, many vapochromic PtII–diimine complexes have been reported so far, and their vapochromic behaviour originates primarily from the adsorption of vapour molecules, resulting in the reversible colour change. In the case of the transformation from red amorphous 1A to yellow crystalline 1C, the structure was transformed irreversibly from 1A to 1C by exposure to hydrophilic vapour, but the adsorption of vapour did not occur except for water vapour. To the best of our knowledge, such ordering induced by vapour molecules has not been reported for the system based on the PtII–diimine complex so far.

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Figure 1. Changes in the powder X-ray diffraction pattern of 1A upon exposure to several organic vapours. The two patterns labelled as 1C and 2 are simulations based on the crystal structures of 1C and 2 at 150 K. The exposure time for each vapour was two weeks at room temperature.

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Crystal Structures

Dihydrate Form: Na2[Pt(CN)2(dcbpy)]·2H2O

As discussed above, complex Na2[Pt(CN)2(dcbpy)]·2H2O (1) shows an interesting vapour-induced amorphous–crystalline transformation upon exposure to hydrophilic vapour. To determine the crystal structure of 1C and 2, we tried to obtain single crystals by using several recrystallization methods and succeeded for the pentahydrate form, Na2[Pt(CN)2(dcbpy)]·5H2O (2). From the single crystal of 2, we successfully obtained a single crystal of 1C by the single-crystal-to-single-crystal transformation (see Experimental Section). The transformation from 2 to 1C was also confirmed by the thermogravimetric analysis of 2 by slow drying at room temperature (Figure S6). Although the quality of the single crystal became worse, it was still possible to determine the framework of the structure except for temperature factors. Figure 2a shows the crystal structure of the dihydrate form, 1C. There are two chemically independent Na+ ions and one [Pt(CN)2(dcbpy)]2– anion in this crystal, indicating that the two carboxyl groups should be completely deprotonated. Although the molecular structure of the [Pt(CN)2(dcbpy)]2– anion was similar to that of [Pt(CN)2(H2dcbpy)]·4H2O,10 the bond lengths around the carboxyl group in 1C are significantly different from those in [Pt(CN)2(H2dcbpy)]·4H2O. The four C–O bond lengths of the carboxyl groups are close to the typical value of the C–O bond length of carboxylate, indicating that the C–O bond order in 1C is 1.5. In the IR spectra of 1C, the stretching mode of the C–O bond was observed around 1650 cm–1 (see Figure S7). This result indicates that the two carboxyl groups were deprotonated and bound to Na ions. The two Na ions have a five-coordinate distorted square-pyramidal geometry. The Na1 ion is surrounded by one cyano nitrogen, three carboxyl oxygen atoms and one water molecule, while the Na2 ion is bound to two cyano nitrogen atoms and three carboxyl oxygen atoms. These two Na ions are μ-bridged by one carboxyl and one cyano group. Two independent water molecules are included in this crystal. One water molecule (labelled as O5) bridges two adjacent Na1 ions and another is weakly bound to the Na ion, with a distance of about 2.86(2) Å. As shown in Figure 2b, the bipyridine rings are stacked along the a axis. In this column, the intermolecular Pt–Pt distances [4.928(1) and 5.927(1) Å] are longer than twice the Van der Waals radius of the platinum atom (3.50 Å), indicating that the metallophilic interaction in this complex is negligibly weak. On the other hand, the relatively short distance between two adjacent bipyridine ligands (3.28 and 3.33 Å) suggests that the π–π interaction is effective in this column. This stacking manner of 1C is quite different from that of the protonated complex [Pt(CN)2(H2dcbpy)]·4H2O, whose crystal structure is supported by a two-dimensional hydrogen-bonding network and a metallophilic interaction between Pt ions,10 implying that the structure of 1C is mainly dominated by electrostatic potential rather than by hydrogen bonding or metallophilic interaction.

Pentahydrate Form: Na2[Pt(CN)2(dcbpy)]·5H2O

As mentioned in the Experimental Section, the recrystallization of amorphous solid 1A produced the pentahydrate form, Na2[Pt(CN)2(dcbpy)]·5H2O (2). Pentahydrate form 2 easily releases water of crystallization in air at room temperature. Thermogravimetric analysis indicates that 2 transformed into 1C in dry N2 atmosphere (Figure S6). To prevent the water of crystallization from being released during X-ray diffraction analysis of 2, the single crystal was coated with paraffin oil and the diffraction measurement was conducted at low temperature (150 K). Figure 3 shows the crystal structure of pentahydrate form 2. The observed bond lengths and angles around the [Pt(CN)2(dcbpy)]2– unit were very similar to those of 1C. The remarkable difference between 1C and 2 is the coordination environment of the two Na ions. For complex 2, two Na ions have octahedral coordination geometry. The Na1 ion is bound to two μ-bridged water molecules, one oxygen atom of carboxyl group and two water molecules, while the Na2 ion is bound to two cyano groups and two μ-bridged and two nonbridged water molecules. It was noteworthy that only one of the two carboxyl groups is directly bound to a Na ion in 2. The [Pt(CN)2(dcbpy)]2– unit is also stacked to form an almost uniform 1D column, which is similar to that in 1C. The long intermolecular distances between two adjacent Pt ions [4.372(1) and 4.995(1) Å] are indicative of no Pt–Pt interaction. Judging from the distance between two adjacent bipyridine ligands (3.33 and 3.39 Å), the π–π interaction is moderately effective in 2 but weaker than that in 1C.

Luminescence Properties

Since complex 1A exhibited the irreversible amorphous–crystalline transformation induced by exposure to hydrophilic vapours, a change in luminescent properties after the transformation were expected. Figure 4 shows changes in the luminescence spectrum of 1A upon exposure to MeOH vapour. Before exposure to MeOH, the amorphous solid 1A showed a luminescence band at 623 nm, with a relatively high luminescence quantum yield (Φem = 0.27). The luminescence band shifted to a higher energy upon exposure to MeOH vapour. Simultaneously, a new emission band gradually appeared around 560 nm. As described above, red amorphous solid 1A changed gradually to yellow crystalline solid 1C (Figure 5). Thus, this change in the luminescence spectrum may be attributed to this transformation. Actually, the crystalline solid of 1C exhibits an emission band at 608 nm, with a shoulder around 562 nm (Φem = 0.42), which is almost the same as that of the spectrum of 1A exposed to MeOH vapour. Slight disagreement of the emission spectra of 1C and 1A under exposure to MeOH vapour may be due to the difference of the surface state of the solid. It is well known that the emission energy of the solid can be affected by the surface state. Under exposure to MeOH vapour, surface adsorption may contribute to the shift of the emission energy. It is well known that the emission energy of the platinum(II)–diimine complex tends to depend on both the degree of metallophilic interaction between Pt ions and the π–π interaction between diimine ligands.11 Considering the crystal structure of 1C, this change in the emission spectrum is due to the modification of these intermolecular interactions. We believe that this vapour-induced luminescence colour change from red to yellow is a unique phenomenon that could be applicable to vapour history sensors.

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Figure 2. (a) Molecular structure of 1C. (b) Stacked structure of [Pt(CN)2(dcbpy)]2– anions. (c) Perspective view of the crystal structure along the a axis of 1C at 150 K. Blue planes and yellow polyhedrons show the coordination sphere of Pt and Na ions, respectively. Hydrogen atoms are omitted for clarity.

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Figure 3. (a) Molecular structure of 2. (b) Stacked structure of [Pt(CN)2(dcbpy)]2– anions. (c) Perspective view of crystal structure along the a axis at 150 K. Blue planes and yellow polyhedrons show the coordination sphere of Pt and Na ions, respectively. Hydrogen atoms are omitted for clarity.

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Figure 4. Exposure-time-dependence of the luminescence spectrum of 1A under MeOH vapour. The grey dotted line shows the luminescence spectrum of 1C; λex. = 400 nm.

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Figure 5. Bright-field images of 1A (a) and 1C (b). Luminescence images of 1A (c) and 1C (d). The sample of 1C was obtained from the exposure of 1A to MeOH vapour.

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Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

We synthesized and investigated a novel luminescent PtII–diimine complex, Na2[Pt(CN)2(dcbpy)]·nH2O (n: 2 or 5). The complex was synthesized as an amorphous dihydrate solid, 1A, which exhibits bright luminescence at 623 nm. Amorphous solid 1A is stable in dry air and at high temperatures up to 410 K. An irreversible amorphous–crystalline transformation from 1A to dihydrate crystalline 1C was not triggered by hydrophobic vapours but by hydrophilic vapours such as MeOH, acetone, dmf, and so on. After the transformation, the colour changed from red to yellow, and the luminescence band shifted to 600 nm with a new peak at 560 nm; this corresponds to the spectrum of 1C. Most vapochromism observed for a series of PtII–diimine complexes is derived from the structural transformation induced by the adsorption of the vapour. However, it is interesting that the vapours could not be adsorbed by the amorphous solid 1A. Thus, this transformation may be derived from a new type of structural modification induced by vapours without adsorption of the vapours. This irreversible colour change is a promising phenomenon applicable to new types of vapour history sensors.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Syntheses: Starting material [Pt(CN)2(H2dcbpy)]·2H2O was synthesized according to the published method.10

Amorphous Solid Na2[Pt(CN)2(dcbpy)]·2H2O (1A): The purple powder of [Pt(CN)2(H2dcbpy)]·2H2O (200 mg, 0.379 mmol) was suspended in MeOH (5 mL). To this reddish-pink suspension was added a 28 % MeOH solution of sodium methoxide (100 μL, 1.61 mmol). The colour of the suspension turned orange immediately. The product was collected by filtration, washed with MeOH and dried in vacuo to yield 125.4 mg {57.9 % based on [Pt(CN)2(H2dcbpy)]·2H2O} of Na2[Pt(CN)2(dcbpy)]·2H2O (1A) as a red amorphous powder. This synthesis of 1A was conducted in air before drying. C14H6N4Na2O4Pt·2H2O (571.32): calcd. C 29.43, H 1.76, N 9.81; found C 29.14, H 2.06, N 9.64. Since this amorphous solid gradually changed to the dihydrated crystalline 1C in humid air, the solid was stored in a dried Ar atmosphere.

Crystalline Solid Pentahydrated Complex Na2[Pt(CN)2(dcbpy)]·5H2O (2): The red powder of 1A (50 mg) was dissolved in water (0.6 mL). To this yellow solution was added MeOH (0.8 mL). Diffusion of ethyl ether into this water–MeOH mixed solution gave the pale yellow crystalline solid of 2 (yield: 8.5 mg, 15.5 %). C14H6N4Na2O4Pt·5H2O (625.36): calcd. C 26.89, H 2.58, N 8.96; found C 26.96, H 2.64, N 8.93.

Crystalline Solid Dihydrated Complex Na2[Pt(CN)2(dcbpy)]·2H2O (1C): The crystalline solid of dihydrated complex 1C has not been obtained from the recrystallization of 1A. The crystalline solid of 1C was obtained by slow drying of pentahydrate complex 2. C14H6N4Na2O4Pt·2H2O (571.32): calcd. C 29.43, H 1.76, N 9.81; found C 29.50, H 1.83, N 9.75.

Single-Crystal X-ray Diffraction Measurement: A single crystal was mounted on a glass fibre with silicon grease. Since the single crystal of 2 was not stable in air at room temperature, the crystal was coated with paraffin oil and quickly cooled down to 150 K. All measurements were made with a Rigaku AFC-7R diffractometer with a Mercury CCD area detector, graphite monochromated Mo-Kα radiation (λ = 0.71069 Å) and a rotating anode generator. The data were collected at a temperature of 150 ± 1 K to a maximum 2θ value of 59.2°. The Pt atom was refined anisotropically and the other atoms were refined isotropically for 1C. The non-hydrogen atoms were refined anisotropically and the hydrogen atoms were refined isotropically for 2. The structures were solved by direct methods (SIR92)12 and expanded by using Fourier techniques (DIRDIF99).13 All calculations were performed with the CrystalStructure crystallographic software package,14 except for refinement, which was performed by using SHELXL-97.15 A summary of the crystallographic data for dihydrated form 1C and pentahydrated form 2 is given in Table 1. CCDC-CCDC-769439 and -CCDC-769440 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Table 1. Crystallographic data for 1C and 2.
Complex1C2
  • [a]

    RW = [Σ(w(Fo2Fc2)2)/Σw(Fo2)2]1/2.

Chemical formulaC14H6N4Na2O4Pt·2H2OC14H6N4Na2O4Pt·5H2O
Temperature /K150150
Crystal systemtriclinictricilinic
Space groupPequation imagePequation image
a7.137(4)6.969(2)
b9.981(5)11.618(3)
c11.292(6)12.600(4)
α83.25(2)80.152(9)
β83.46(2)82.203(9)
γ82.73(2)74.394(8)
V3788.3(7)963.7(5)
Z22
Dcalcd. /Mg m–32.3902.120
Collected reflections51607895
Unique reflections27184363
No. of parameters245272
R0.04730.0338
Rw[a]0.07450.0902
Goodness-of-fit1.1631.066

Powder X-ray Diffraction: Powder X-ray diffraction patterns [λ = 1.200(1) Å] of the samples were measured at the BL-8B beamline in a photon factory at KEK, Japan.

Luminescence Properties: The luminescence spectra of the samples were measured by using a JASCO FR-6600 spectrofluorometer at room temperature. About 1 mg of the sample was placed in a glass capillary of 0.5 mm in diameter. Slit widths of excitation and emission light were 5 and 6 nm, respectively. Luminescence quantum efficiency was recorded with a HAMAMATSU C9920-02 absolute photoluminescence quantum yield measurement system.

Thermogravimetric Analysis: Thermogravimetry and differential thermal analysis were performed by using a Rigaku ThermoEvo TG8120 analyzer.

Supporting Information (see footnote on the first page of this article): Exposure time and temperature dependences of PXRD patterns of 1A, IR spectra, 1H NMR spectra and thermogravimetric analysis are presented.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

This work is supported by a Grant-in-Aid for Scientific Research, Photochromism (No.471), Young Scientists (B) (19750050) and the Global COE Program (Project No. B01: Catalysis as the Basis for Innovation in Materials Science) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors.

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