Crystal engineering of vapochromic porous crystals composed of Pt(II

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Crystal engineering of vapochromic porous crystals composed of Pt(II)-diimine luminophores for vapor history sensors Yasuhiro Shigeta, Atsushi Kobayashi, Masaki Yoshida, and Masako Kato Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00130 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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Crystal Growth & Design

Crystal engineering of vapochromic porous crystals composed of Pt(II)-diimine luminophores for vapor history sensors Yasuhiro Shigeta, Atsushi Kobayashi*, Masaki Yoshida, Masako Kato* Department of Chemistry, Faculty of Science, Hokkaido University, North-10 West-8, Kita-ku, Sapporo 060-0810, Japan

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ABSTRACT: A novel Pt(II) diimine complex, [Pt(CN)2(H2dpcpbpy)] (1, H2dpcpbpy = 4,4'-di(pcarboxyphenyl)-2,2'-bipyridine) was synthesized and its vapochromic behavior was investigated. The yellow amorphous form of 1, 1-Ya, transformed into the porous orange crystalline form, 1Oc, upon exposure to ethanol vapor. This behavior is similar to that of the previously reported complex, [Pt(CN)2(H2dcphen)] (2, H2dcphen = 4,7-dicarboxy-1,10-phenanthroline). X-ray diffraction study showed that 1-Oc possessed similar but larger porous channels (14.3 × 8.6 Å) compared to the red crystalline form of 2, 2-Rc (6.4 × 6.8 Å). Although the porous structure of 2-Rc was retained after vapor desorption, that of 1-Oc collapsed to form the orange amorphous solid, 1-Oa. However, the orange color was unchanged in this process. The initial color was recovered by grinding 1-Oa and 2-Rc. These vapor-writing and grinding-erasing functions can be applied to both in situ vapor sensing and vapor-history sensing, i.e., sensors that can memorize the existence of previous vapors. A notable difference was observed for humid air sensitivity; the orange emission of 1-Oa was largely unaffected upon exposure to humid air, whereas the red emission of 2-Rc was significantly affected. The lesser sensitivity of 1-Oa toward humidity is important for stable vapor-history sensor applications.


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Introduction Chromic materials showing reversible color change in response to external stimuli such as heat, vapor, and mechanical pressure, have drawn considerable attention in recent years.1-3 This is because such materials enable a visualization of the invisible stimuli as a change in the color of the material. Especially, vapochromism has generated a lot of interest in the field of chemical sensors because it enables us to know the existence of invisible and harmful volatile organic compounds (VOCs).4-37, 41 Many vapochromic materials have been reported to date, and one of the most promising systems is that of Pt(II) complexes with intermolecular metallophilic interactions.18-37 The color and luminescence of these complexes is strongly dependent on the degree of the interactions originating from the intermolecular 5dz2 orbital overlaps of the Pt(II) ions to form the occupied anti-bonding dσ* orbital. In other words, even a slight modification of the crystal or the aggregated structure in this system can induce a significant change in the transition energy from the Pt···Pt dσ* orbital to the π* orbital of the organic ligand, the so-called metal-metal-to-ligand charge-transfer (MMLCT) transition, resulting in a change in the color and luminescence.38,39 For example, Aliprandi et al. reported an interesting and outstanding color change from blue to red of the amphipathic Pt(II) complex.40 Chen and co-workers also reported an interesting halocarbon-selective vapochromic response of a diimine-Pt(II) complex.29 These examples clearly indicate the promising character of Pt(II) complexes as chromophores for the design of vapochromic materials. However, it is still challenging to achieve selective vapochromic response based on the Pt(II) chromophore. One of the challenges is that vapochromic Pt(II) complexes are typically molecular crystals in which weak intermolecular metallophilic, CH⋅⋅⋅π, and π⋅⋅⋅π interactions compete with each other. This prevents us from


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being able to finely and systematically modify the intermolecular metallophilic interaction in the solid state. To overcome this difficulty, we have focused our attention on the intermolecular hydrogenbonding interactions, which may enable us to control the molecular arrangement of the Pt(II) chromophore in the crystal structure. Eisenberg and co-workers found that the nicotinamidefunctionalized Pt(II)-terpyridine complex exhibits single-crystal-to-single-crystal vapochromic transformation induced by the adsorption/desorption of methanol vapor.23 Our group also reported a unique vapochromism of the carboxylic acid-functionalized Pt(II)-diimine complexes, [Pt(CN)2(H2dcphen)] and [Pt(CN)2(H2dcbpy)] (2 and 3, H2dcphen = 4,7-dicaboxy-1,10phenanthroline, H2dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) with “shape-memory” behavior; the porous red-luminescent crystalline forms (2-Rc and 3-Rc) obtained from the purple amorphous solids (2-Pa and 3-Pa) by vapor adsorption-induced structural transformation were stable enough to retain the porous character without the adsorbed vapor molecules, but reverted to the original purple amorphous solids by simple manual grinding.41 This unique behavior may be a promising feature for vapor-history sensors that can memorize the past existence of vapor. Therefore, we synthesized a Pt(II) analogue complex, [Pt(CN)2(H2dpcpbpy)] (1, H2dpcpbpy = 4,4′-di(pcarboxyphenyl)-2,2′-bipyridine) with the phenyl spacer between the bipyridine moiety and the terminal carboxy group. The phenyl spacers enabled us to expand the pore diameter and examine the stability of the porous structure supported by the intermolecular -COOH⋅⋅⋅NC-type hydrogenbonded network, which could be the key for the shape-memory behaviors of complexes 2 and 3. Herein, we demonstrate the vapor-history sensing function of 1 in comparison with that of 2 which have the smallest pore channel structure among the three complexes. The less stable but larger porous structure of 1 as compared to 2 allowed an effective suppression of water vapor


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responsiveness and was thus a key determinant of the “water-resistant” vapor-history sensing property.

Scheme 1. Molecular structures of complexes 1 and 2.


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Experimental Section General procedures All commercially available starting materials were used without any purification. The precursor materials, namely, 4,4′-dibromo-2,2′-bipyridine,42-44 4-carboxyphenylboronic acid,45 Pd(PPh3)4,46 and Pt(CN)247 were prepared according to previously reported methods. The 1H NMR spectrum of each sample was measured using a JEOL EX-270 NMR spectrometer at room temperature. Elemental analyses were conducted at the Hokkaido University. Synthesis of H2dpcpbpy ligand Na2CO3 (1.68 g, 16.0 mmol) was added to a solvent mixture of H2O (80 mL) and methanol (96 mL). Next, 4,4′-dibromo-2,2′-bipyridine (1.20 g, 3.83 mmol) was added to the mixture and N2 was bubbled through the resulting suspension for 30 min. After purging the mixture with N2, 4carboxyphenylboronic acid (1.26 g, 9.70 mmol) and Pd(PPh3)4 (480 mg, 0.4 mmol) were added. The resulting mixture was left to stir at reflux for 3 days under N2. Subsequently, the reaction mixture was filtered and the filtrate was evaporated to dryness. The residue was suspended in H2O (50 mL) and dilute aq. HCl was added. The resulting white suspension was centrifuged, washed with H2O, MeOH, and acetone. After drying under reduced pressure, the target ligand, H2dpcpbpy was obtained as a white powder. Yield: 1.08 g, 71%. This compound was used for subsequent syntheses without further purification. 1H NMR (270 MHz, NaOD in CD3OD/D2O): δ = 8.80 (d, 2H), 8.58 (s, 2H), 8.11 (d, 4H), 7.91 (d, 4H), 7.88 (d, 2H). Synthesis of yellow amorphous [Pt(CN)2(H2dpcpbpy)] (1-Ya) 1-Ya was synthesized by a slightly modified synthetic procedure of the complex 2. Pt(CN)2 (480 mg, 1.94 mmol) was suspended in conc. aq. NH3 (98 mL). H2dpcpbpy (385 mg, 0.97 mmol) was added to the mixture and the resulting suspension was heated at 100 °C for 2 days. After the


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reaction, the obtained suspension was filtered off. dilute aq. HNO3 was added to the filtrate and a yellow precipitate emerged. The precipitate was collected by filtration, washed with H2O, and dried under vacuum to afford the target compound as a yellow powder (1-Ya). Yield: 460 mg, 73%. 1H NMR (270 MHz, DMSO-d6) δ = 9.28 (s, 2H), 9.21 (d, 2H), 8.35 (d, 2H), 8.25 (d, 4H), 8.19 (d, 4H); Elemental analysis calcd. for C26H16N4O4Pt·9H2O: C 38.76, H 4.25, N 6.95; found: C 38.85, H 3.97, N 7.25. Synthesis of orange crystals [Pt(CN)2(H2dpcpbpy)]·4H2O (1-Oc) To a suspension of [Pt(CN)2(H2dpcpbpy)] (0.25 mg, 4 µmol) in methanol (7 mL), the methanolic solution of lithium methoxide (10% solution in methanol, 25 µL) was added. Diffusion of acetic acid vapors into the methanol solution at 30 °C for 2 days yielded 1-Oc as orange crystals suitable for single crystal X-ray diffraction. Powder X-ray diffraction Powder X-ray diffraction studies were conducted using a Rigaku SPD diffractometer at beamline BL-8B of the Photon Factory, KEK, Japan or a Bruker D8 Advance diffractometer equipped with a graphite monochromator using CuKα radiation and one-dimensional LinxEye detector. The wavelength of the synchrotron X-ray was 1.5385(1) Å. Luminescence properties The luminescence spectra of all complexes were measured using a JASCO FP-6600 spectrofluorometer at room temperature. Typical slit width of the excitation and emission light were 5 and 6 nm, respectively. UV-Vis spectroscopy The UV-Vis diffuse reflectance spectra of the prepared complexes were recorded on a Shimadzu UV-2500PC spectrophotometer.


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Vapor adsorption isotherms. The vapor adsorption isotherms of each complex were measured with the BELSORP-max vapor adsorption isotherm measurement equipment at 298 K. Single crystal X-ray diffraction measurement The XRD study of 1-Oc was conducted using a Rigaku Mercury CCD diffractometer with monochromated MoKα radiation (λ = 0.71069 Å) and a sealed X-ray tube generator. The single crystal was mounted on a MicroMount coated with paraffin oil. The crystal was then cooled by using an N2 flow-type temperature controller. The diffraction data were collected and processed with Crystal Clear software.48 The structures were solved with the direct method using SHELXS2013.49 Structural refinements were conducted by the full-matrix least-squares method with SHELXL2013.49 Non-hydrogen atoms were refined anisotropically and all hydrogen atoms were refined with the riding model. All calculations were conducted with the Crystal Structure crystallographic software package.50 Full crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (CCDC 1817798)


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Results and discussion Synthesis and Characterization Although 1 was synthesized by following almost the same synthetic procedure as that of 2, it was obtained in a yellow form (1-Ya) with a completely different color to that of the purple form of 2-Pa. To elucidate the difference between 1-Ya and 2-Pa, PXRD, UV-Vis absorption, and diffuse reflectance spectral measurements were conducted. The PXRD pattern for 1-Ya was featureless and without any sharp diffraction peaks, indicating that 1-Ya is an amorphous solid, same as 2-Pa (see Figure S1 in the supporting information, SI). However, the UV-Vis absorption spectra of these complexes (Figure 1) were clearly different. The absorption spectrum of complex 2-Pa in solution exhibited a band up to 400 nm and the broad absorption band extended to 700 nm in the UV-Vis diffuse reflectance spectrum of the amorphous solid 2-Pa. This result clearly indicated that the intermolecular Pt···Pt interactions contributed to the electronic transition of 2-Pa in the solid state. Similarly, complex 1-Ya exhibited an obvious absorption band at around 340 nm in the solution state, which extended to longer wavelengths up to 500 nm in the solid state. This observation also indicated the contribution of intermolecular interactions to the electronic transition of 1-Ya. The higher molar absorption coefficient of 1-Ya as compared to that of 2-Pa in the solution state could be explained by the more extensive π conjugation in the H2dpcpbpy ligand in comparison with that in the H2dcphen ligand. It should be noted that the observed absorption band of 1-Ya in the solid state was located at a shorter wavelength than that of 2-Pa, which suggested that the intermolecular interactions of the former were weaker than that of the latter. This difference can probably be attributed to the steric effect of the additional phenyl ring that can rotate along the C–C bond to weaken the intermolecular interactions.


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Figure 1. (a) Solution-state UV-Vis absorption and (b) solid-state UV-Vis diffuse reflectance spectra of 1-Ya (red) and 2-Pa (black). The absorption spectra of 1-Ya and 2-Pa were obtained in DMSO and aq. NH3 (2.8%), respectively, because of the low solubility of 1 in dilute aq. NH3.

Crystal structure of [Pt(CN)2(H2dpcpbpy)]･･4H2O (1-Oc) Figure 2 displays the crystal structure of the orange crystalline form of 1 (1-Oc) in comparison with that of 2 (2-Rc). Crystallographic data and selected bond length angles of 1-Oc are listed in Tables 1 and 2, respectively. It was found that 1-Oc crystallized in the same space group (Cmcm) as 2-Rc. In this structure, half of one molecule was crystallographically independent because of the mirror plane of this space group. The Pt(II) center of 1-Oc was coordinated to two N atoms from the H2dpcpbpy ligand and two C atoms from the two cyanide ligands to form the typical


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square-planar coordination geometry. The bond lengths around the Pt(II) center in 1-Oc were almost comparable to those in 2-Rc, suggesting that the introduction of a phenyl group between the bpy ligand and the carboxyl groups had a negligible effect on the structure of the complexes. The short distance between the O atom of the carboxylic acid group and the nitrogen atom of the cyanide ligand of the adjacent molecule (O-H⋅⋅⋅N: 2.67(3) Å) suggested that there was a strong hydrogen bond between them. As the result, a two-dimensional hydrogen bonding network structure was constructed in the ab plane, same as that present in 2-Rc (Figure 2(b)). As shown in Figure 2(a), this square planar molecule was stacked one-dimensionally along the c axis with an anti-parallel orientation of the phenyl carboxyl groups attached to the bpy moiety. The intermolecular Pt···Pt distance (3.353(2) Å) was shorter than twice of the van der Waals radius of Pt atom (3.5 Å), clearly indicating an effective Pt···Pt interaction in this stacked structure. Notably, the intermolecular Pt···Pt distance in 1-Oc (3.353(2) Å) was longer than that of 2-Rc (3.2668(7) Å) despite the almost linear stacking manner, i.e., the Pt···Pt···Pt stacking angle in 1Ya was closer to 180° than that in 2-Rc. Thus, the intermolecular Pt···Pt interaction in 1-Oc was weaker than that in 2-Rc, probably as a result of the steric effect of the phenyl ring attached on the 4- and 4′- positions of the bpy moiety. Since these phenyl groups between bpy and carboxyl groups were also stacked along the c axis at a relatively short distance (3.350 Å), the intermolecular π–π stacking interactions were also effective in this structure. As a result of the 2D hydrogen bonding network in the ab plane and the intermolecular Pt···Pt and π–π stacking interactions along the c axis, a large porous channel structure was formed in 1-Oc and 2-Rc (Figure 2(c)). As expected from the molecular structure of 1, the pore size (14.3 × 8.6 Å) and void fraction (48.7%) of 1-Oc were larger than those of 2-Rc (6.4 × 6.8 Å and 35.2%). In this


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large void space, four crystal water molecules were found and two of them were directly hydrogen bonded to the O atom of the carboxyl group of 1 at a distance of 2.84(3) Å.

Figure 2. Crystal structures of (top) 1-Oc and (bottom) 2-Rc. (a) 1-D Pt···Pt interaction along the c axis, (b) 2-D hydrogen bonding crystal structure in the ab plane, and (c) the porous structure viewed along the c axis. The blue, brown, light-blue, and red ellipsoids represent Pt, C, N, and O atoms, respectively. Solvent molecules and hydrogen atoms are omitted for clarity. Displacement parameters are drawn at a 40% probability level. These pictures were drawn by VESTA computer program.51


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Table 1. Crystal parameters and refinement results of 1-Oc in comparison with those of 2-Rc. Complex

1-Oc

2-Rca

T/K

200(1)

200(1)

Formula

PtC26H16N4O8

PtC16H8N4O7

Formula weight

707.5

563.35

Crystal system

Orthorhombic

Orthorhombic

Space group

Cmcm

Cmcm

a/Å

21.22(1)

17.587(4)

b/Å

26.43(1)

18.288(5)

c/Å

6.699(4)

6.4903(15)

V / Å3

3756(3)

2087.5(9)

Z

4

4

Dcal/ g･cm–3

1.247

1.812

Reflections collected

14364

12292

Unique reflections

2394

1445

GOF

1.096

1.082

Rint

0.1099

0.0828

R (I > 2.00σ (I))

0.0811

0.0326

RW b

0.2123

0.0805

Pore diameter / Å2

14.3 × 8.6

6.4 × 6.8

Void fraction c / %

48.7

35.2

a

Ref.41. bRW = [Σ(w(Fo2 − Fc2)2)/Σw(Fo2)2]1/2. cCalculated by SQUEEZE program.


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Table 2. Selected bond lengths and angles of complexes 1-Oc and 2-Rc. Complex

1-Oc

2-Rca

T/K

200(1)

200(1)

Pt1–N1

2.06(1)

2.047(5)

Pt1–C7

1.95(1)

1.962(5)

O1–H⋅⋅⋅N3

2.67(3)

2.665(8)

Pt1⋅⋅⋅Pt1

3.353(2)

3.2668(7)

Pt1⋅⋅⋅Pt1⋅⋅⋅Pt1

174.92(2)

166.80(1)

a

Ref. 41.

Vapor-adsorption-triggered structure transformation As mentioned in the Introduction, the change in color of complex 2 from purple to red originated from the transformation of the amorphous state to the crystalline form, triggered by the adsorption of EtOH vapors. Since 1-Oc has a very similar porous structure to 2-Rc, we investigated the vapochromic behavior of complex 1. Figure 3(a) shows the PXRD pattern changes of the yellow amorphous solid 1-Ya upon exposure to EtOH and the subsequent heat and grinding treatments. The initial broad XRD peak of 1-Ya changed drastically to a diffraction pattern with many sharp peaks when it was exposed to EtOH vapors. The observed PXRD pattern was qualitatively consistent with the simulation pattern of 1-Oc. Thus, the amorphous complex 1-Ya was transformed into the EtOH-adsorbed crystalline porous phase, 1-Oc⊃ ⊃EtOH, upon exposure to EtOH vapors, same as observed for 2-Pa (see Figure 3(b)). When 1-Oc⊃ ⊃ EtOH was heated at 120 °C for 2 h, most of the diffraction peaks in the XRD pattern disappeared, with the exception of a few peaks in the low angle region. Upon heating, most of the adsorbed EtOH molecules in 1-Oc ⊃ EtOH were removed, as confirmed by


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thermogravimetric analysis (see Figure S4 in the supporting information). This result indicates that the porous crystal structure of 1-Oc collapsed upon the desorption of EtOH vapors, whereas the porous structure of 2-Rc was retained even after the removal of EtOH by heating. Thus, the porous structure of 1-Oc was not stable without the adsorbed EtOH molecules, and the lesser stability of 1-Oc compared to 2-Rc was likely because of the expanded porous structure with a larger void fraction and weaker intermolecular hydrogen-bonding interactions in the former.

Figure 3. Changes in the powder X-ray diffraction patterns of (a) 1-Ya and (b) 2-Pa. Each amorphous solid (black lines) was exposed to EtOH vapor for five days at 303 K. The EtOHadsorbed forms (red lines) were heated to 120 °C for 2 h to remove the adsorbed EtOH molecules. Finally, the heated samples (blue lines) were ground manually using mortar (green lines). Each orange line at the bottom shows the simulated XRD patterns of 1-Oc and 2-Rc.


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In our previous study, we showed that 2 exhibited a unique vapochromism effect, with its ‘Shape-memory’ behavior; the amorphous form of 2, 2-Pa, was transformed into a porous crystalline form upon adsorption of EtOH vapor, and its crystal structure was retained even after the removal of adsorbed EtOH molecules. On the other hand, the PXRD measurement of 1 suggested that the porous crystal structure of 1-Oc was less stable than that of 2-Rc. To elucidate the vapor adsorption process in detail, EtOH vapor adsorption isotherms were measured at 298 K. Figure 4 compares the EtOH adsorption isotherms of 1 and 2. The amorphous solids 1-Ya and 2Pa were first heated for 24 h at 120 °C to remove all adsorbed solvent molecules. Although 1-Ya hardly adsorbed any EtOH vapor at low relative pressures (P/P0 < 0.6), large number of EtOH molecules were adsorbed at high relative pressure (~1.9 mol/mol at P/P0 = 0.99). In the desorption process, most of the adsorbed EtOH molecules were retained at P/P0 = 0.4, and only started to be desorbed at lower pressures. This large hysteresis in the adsorption/desorption cycle is characteristic behavior for the vapor adsorption/desorption involving a significant structural transformation of the adsorbent, i.e., 1-Ya to 1-Oc⊃ ⊃ EtOH. Although 2-Pa adsorbed EtOH vapor even at low pressures, a similar hysteresis to that of 1-Ya was observed. In addition, the residual amounts of EtOH in the desorption process for 1 (0.5 mol/mol) was smaller than that of 2 (0.9 mol/mol). These differences may be attributed to the destabilized porous structure because of the expanded pore size and the increased hydrophobicity of the pore surface as a result of the phenyl rings of the H2dpcpbpy ligand. Both complexes adsorbed almost the same amount of EtOH vapor (~2 mol/mol at the saturated vapor pressure) even though they had pores of different sizes. This indicated that the EtOH molecules were adsorbed at specific sites in the pores. Considering that the crystallization water molecules were hydrogen bonded to the O atoms of the


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carboxyl groups in the H2dpcpbpy ligand in 1-Oc (see Figure S2), EtOH molecules are thought to be adsorbed at the similar positions via hydrogen bond interactions.

Figure 4. EtOH vapor adsorption isotherms of (a) 1 and (b) 2 at 298 K. The same measurement was conducted twice for each complex. For the first measurements (black circles), the amorphous forms of 1-Ya and 2-Pa were used. Before the second measurement (red circles), the samples were heated again at 120 °C under vacuum to remove all the adsorbed vapor molecules in the first measurement.

Next, we conducted the second measurement (see the red circles in Figure 4) by using the same samples which used in the first measurement. The samples were heated at 120 °C for 24 h to remove all of the EtOH molecules adsorbed in the first measurement. As reported in our previous paper, 2 showed a completely different isotherm to the first cycle; a large amount of EtOH molecules were adsorbed at a very low relative pressure (P/P0 = 0.01). An isotherm with a steep increase in vapor uptake at low pressures is referred to as a Type-I isotherm, and is a characteristic feature of microporous materials. In contrast, 1 did not exhibit a similar Type-I


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isotherm, although it did show an almost identical vapor adsorption isotherm to that observed in the first measurement. Therefore, as suggested by the PXRD results (Figure 3), the porous structure of 1-Oc was collapsed by the desorption of EtOH molecules.

Vapochromic behavior As mentioned in the Introduction, the amorphous form of 2-Pa exhibited ‘shape-memory’ vapochromism because of its porous structure that adsorbed the alcoholic vapors and collapsed by manual grinding. Although the porous crystalline structure of 1-Oc was quite similar to that of 2-Rc, the former complex could not retain its form without the adsorbed solvent molecules. Consequently, a different chromic behavior than that shown by 2 was expected for complex 1. To investigate this difference, solid-state UV-Vis diffuse reflectance and emission spectral changes of 1-Ya upon vapor ad/desorption were measured. As shown in Figure 5, 1-Ya showed a broad orange emission centered at 626 nm, which was over 100 nm shorter wavelength than that of 2-Pa (733 nm). This blue-shift of the emission band is probably due to the destabilized MMLCT energy resulting from the weaker intermolecular interaction in 1-Ya in comparison to that of 2-Pa, as suggested by the single crystal X-ray analyses (Table 2) and diffuse reflectance spectra (Figure 5) of these complexes. The absorption band corresponding to the 1MMLCT transition in 1-Ya was observed at a shorter wavelength by about 200 nm as compared to that of 2-Pa. The emission band maximum of 1-Ya was blue-shifted from 626 nm to 603 nm upon exposure to EtOH vapor, which is similar to that observed in the case of 1-Oc. PXRD analyses (Figure 3) also indicated the EtOH vapor adsorption-induced structural transformation from 1Ya to 1-Oc⊃ ⊃EtOH. On the other hand, the absorption band corresponding to the 1MMLCT transition of 1-Ya shifted drastically to a longer wavelength from 500 nm to 600 nm upon


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exposure to EtOH vapor. A similar red shift was also observed in the excitation spectrum; the excitation band of 1MMLCT transition shifted by ~60 nm (Figure S5) when 1-Ya was exposed to EtOH vapor. These red-shifted absorption and excitation bands suggest that the intermolecular Pt···Pt interactions in 1-Oc⊃ ⊃EtOH are basically stronger than those in the yellow amorphous powder, 1-Ya. The inconsistency of these results with the blue-shifted 3MMLCT transition observed in the emission spectrum of 1-Oc⊃ ⊃EtOH may be attributed to the emission from the impurity sites with shorter Pt···Pt distance in the amorphous 1-Ya state. The amorphous powder 1-Ya should have a larger number of structural defects as compared to the orange crystalline form 1-Oc. Such defects in the amorphous state may enable to form small domain with the shorter Pt···Pt distance than that in the highly ordered crystalline form 1-Oc, resulting in the lower energy emission via energy transfer from the other domains with longer Pt···Pt distance.


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Figure 5. Changes in the luminescence and diffuse reflectance spectra of (a, c) 1 and (b, d) 2. The amorphous form (black line) was exposed to EtOH vapor for 5 days at 303 K (red line). Subsequently, the sample was heated to remove all adsorbed EtOH (blue line). The sample was then ground manually by using a mortar (green line). The orange lines show the luminescence spectra of the crystal.

⊃EtOH exhibited a blue-shift of ~20 As reported previously, the 3MMLCT emission of 2-Rc⊃ nm upon the removal of adsorbed EtOH vapor by heating. The 3MMLCT emission wavelength was further shifted to a longer wavelength to ~700 nm after manual grinding. Similar changes were observed in the UV-Vis diffuse reflectance spectrum (Figure 5) and the PXRD measurements indicated that this two-step change originated from the slight structural deformation by EtOH desorption and transformation to the amorphous powdery form by


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grinding (Figure 3). On the other hand, there was hardly any change in the diffuse reflectance spectrum of 1-Oc⊃ ⊃ EtOH after the removal of the adsorbed EtOH by heating whereas the emission band maximum was significantly shifted to longer wavelengths by ~50 nm. As shown in Figure 3, this behavior corresponds to the collapse of the porous crystal structure upon EtOH desorption and the transformation to the orange-colored amorphous solid 1-Oa. Considering that the 1MMLCT absorption band was hardly changed (red and blue lines in Figure 5c), intermolecular Pt···Pt interactions in the ground state of 1-Oa would be comparable to the crystalline form, 1-Oc, which has the same orange color. Therefore, the red-shift of the 3

MMLCT presumably corresponds to the energy transfer to the impurity sites with shorter Pt···Pt

distances, as discussed above for 1-Ya. That is, the amorphous solid of 1-Oa exhibited a larger Stokes shift (see red lines in Figures 5a and 5c) than the crystalline form 1-Oc (see blue lines in Figures 5a and 5c) because of the larger number of structural defects that may allow to form shorter Pt···Pt distance in the stacking structure. This assumption is supported by the IR spectra of these forms (Figure S6 in SI); the C≡N stretching mode of 1-Oa (2134 cm–1) was observed at a very similar wavenumber to that of 1-Ya (2135 cm–1) and also at a lower frequency compared to that of 1-Oc (2159 cm–1). This lower-energy C≡N stretching mode of both the amorphous forms, 1-Ya and 1-Oa, suggests that the rigidity of the intermolecular hydrogen-bonded network structure of the amorphous phases 1-Ya and 1-Oa is weaker than that of the crystalline form 1Oc. The large structural difference between the EtOH-desorbed 1-Oa and 2-Rc certainly affects the sensitivity of the 3MMLCT emission energy for H2O vapor in air. As shown in a previous report,41 the 3MMLCT emission of 2-Rc was blue-shifted from 665 nm to 633 nm with the increase in relative humidity (from 11 to 100% RH), probably as a result of the physisorption of


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H2O molecules into the porous channel of 2-Rc. In contrast, the 3MMLCT emission energy of the EtOH-desorbed complex 1-Oa changed only slightly from 652 nm to 658 nm upon increasing RH in this abovementioned range (Figure S7). The lesser sensitivity of 1-Oa in comparison with that of 2-Rc is probably due to the lack of porous channels in 1-Oa. Interestingly, the emission and diffuse reflectance spectra of the EtOH-desorbed amorphous orange form, 1-Oa, were remarkably blue shifted and similar to that of 1-Ya after manual grinding. Notably, some peaks in the PXRD pattern of 1-Oa completely disappeared after manual grinding (Figure 3). This result suggests that the short-range ordered domains with effective intermolecular Pt···Pt interactions in 1-Oa could be collapsed easily and completely to form 1-Ya by grinding. In addition, the emission spectrum of 1-Oa was changed to almost the identical one of 1-Oc⊃ ⊃EtOH under exposure to EtOH vapor (Figure S8), indicating the reverse transformation from 1-Oa to 1-Oc⊃ ⊃EtOH. We also confirmed that this vapochromic behavior between 1-Oa and 1-Oc⊃ ⊃EtOH was reversibly occurred (Figure S9). The plausible mechanism of the vapochromic behavior of complex 1 is summarized in Scheme 2 in comparison with that of 2. As reported previously, the purple amorphous powder 2-Pa exhibited the 1MMLCT absorption band at lower energy than that of the red crystalline form 2Rc. On the other hand, the as-prepared yellow-colored amorphous powder of 1, 1-Ya, exhibited a higher energy 1MMLCT absorption band compared to the other orange-colored forms because of the weaker intermolecular Pt···Pt interactions. This difference is ascribed to the steric effect of the expanded π conjugated H2dpcpbpy ligand as discussed above. Upon exposure of the amorphous 1-Ya and 2-Pa to EtOH vapor, the vapor adsorption-induced structural transformation to the porous crystalline form, 1-Oc⊃ ⊃EtOH and red-colored 2-Rc⊃ ⊃EtOH took place. In the transformation from 2-Pa to 2Rc⊃ ⊃EtOH, both of the 1MMLCT absorption band


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and 3MMLCT emission band shifted to a shorter wavelength. In contrast, in the transformation from 1-Ya to 1-Oc ⊃ EtOH, the 1MMLCT absorption band clearly shifted to a longer wavelength (from 500 nm to 600 nm) while the 3MMLCT emission band inversely shifted to a shorter wavelength (from 626 nm to 603 nm) as shown in Figures 5a and 5c. It may be concluded from these inverted shifts in the two spectra that there is a contribution from the impurity sites with shorter Pt···Pt distances. In the amorphous powder 1-Ya, a lot of structural defects may allow to form shorter Pt···Pt distances microscopically, leading to the lower energy 3

MMLCT emission in comparison to the highly ordered crystalline form 1-Oc ⊃ EtOH.

Furthermore, 2-Rc⊃ ⊃EtOH can retain its porous crystal structure after the desorption of EtOH vapor, and the resulting 2-Rc showed similar 1MMLCT band and slightly red-shifted 3MMLCT band to that of 2-Rc⊃ ⊃EtOH. On the other hand, 1-Oc⊃ ⊃EtOH can be easily transformed to the orange amorphous solid 1-Oa by desorption of the EtOH molecules assisted by heating. During this structural change to 1-Oa, only a negligible shift was observed in the diffuse reflectance spectrum whereas an obvious red-shift was observed in the emission spectrum. Thus, the intermolecular Pt···Pt interactions are presumably still partly effective in the amorphous form 1Oa. The amorphous nature of 1-Oa and 1-Ya is also responsible for the large Stokes shift in these complexes, i.e., the contribution of the impurity levels derived from the microscopicallyformed domains with the shorter Pt···Pt distances. This can explain why only the emission spectrum was changed upon EtOH desorption.


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Scheme 2. Schematic diagram of the photoexcited 1MMLCT and 3MMLCT levels for the three forms of (a) 1 and (b) 2. Dotted lines of 3MMLCT states of 1-Ya and 1-Oa schematically show the contribution of various strengths of intermolecular Pt⋅⋅⋅Pt distances in amorphous states. Conclusions A new luminescent Pt(II) complex, [Pt(CN)2(H2dpcpbpy)] (1; H2dpcpbpy = 4,4'-pdicarboxyphenyl-2,2'-bipyridine) was successfully synthesized. The orange crystal of 1-Oc was identified to be the isomorphous porous form of the red crystalline form 2-Rc, [Pt(CN)2(H2dcphen)] (2, H2dcphen = 4,7-dicarboxy-1,10-phenanthroline). The porous structure was constructed by the 2-D hydrogen bonding network in the ab plane and the effective intermolecular Pt···Pt interaction along the c axis. As expected from the molecular structure of 1, 1-Oc had a larger pore size and void fraction than that of 2-Rc. The as-synthesized yellow amorphous solid 1-Ya exhibited a completely different color from the as-synthesized purple amorphous form 2-Pa. This is because of the weaker intermolecular Pt···Pt interaction competing with the π–π stacking interactions of the expanded π plane of the ligand. 1-Ya exhibited vapochromic behavior as a result of the reversible EtOH vapor adsorption and desorption, which induced structural transformation between the amorphous 1-Ya and the porous crystalline 1-Oc states. Although the vapochromic behavior of 1 was similar to that of 2,


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the vapor adsorption isotherms and PXRD measurements revealed that the porous crystal structure of 1-Oc could not be retained after the removal of EtOH vapor by heating. In contrast, 2-Rc could retain its porous structure without the adsorbed EtOH. This difference is probably due to the larger void fraction and weaker intermolecular hydrogen bonding interactions in 1-Oc than 2-Rc, as suggested by their crystal structures. On the other hand, after the desorption of EtOH, 1-Oa exhibited a very similar diffuse reflectance spectrum to that of 1-Oc. This behavior could be explained by the small crystal domains with the effective Pt···Pt interactions that still remained in 1-Oa. Moreover, upon these processes, a larger Stokes shift was observed for the amorphous form 1-Oa as compared to the crystalline form 1-Oc. This difference may be a result of the impurity sites with shorter Pt···Pt distances. In the amorphous form, loosely packed molecules could form the small domains with shorter Pt···Pt distances than in the dense packed crystalline form 1-Oc. The emission spectrum of 1-Oa was hardly affected by water vapor in air whereas the 3MMLCT emission band of 2-Rc was moderately shifted by the physisorbed water vapor. In other words, the existence of the porous structure of 2-Rc is a key factor in the sensitivity toward water. Furthermore, 1-Oa could be transformed to the initial 1-Ya form by manual grinding. This was probably caused by the collapse of the small crystalline domains with the effective Pt···Pt interaction upon grinding. Therefore, 1 exhibited vapor history sensing ability because of its partially retained Pt···Pt interactions after the collapse of the porous structure. 1-Oa also showed negligible humiditydependent luminescence because it lacked the porous structure. Thus, 1 would be a more promising vapor history sensor than complex 2 because of its lack of sensitivity for water vapor in air.


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ASSOCIATED CONTENT Supporting Information. Thermogravimetric analysis, PXRD patterns, excitation and IR spectra of 1; Luminescence spectrum changes of 1-Oa upon exposure to H2O or EtOH vapor. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected](A. K.); [email protected](M. K.)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This study was supported by the Shimadzu Science Foundation, Shorai Science and Technology Foundation, Murata Science Foundation, and Grants-in-Aid for Scientific Research (C) (No.26410063), Artificial Photosynthesis (area No. 2406, No.15H00858), Soft Crystals (area No. 2903, No. JP17H06367) and Grant-in-Aid for JSPS Fellow (17J01139) from MEXT, Japan.


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For Table of Contents Use Only

Crystal engineering of vapochromic porous crystals composed of Pt(II)-diimine luminophores for vapor history sensors Yasuhiro Shigeta, Atsushi Kobayashi*, Masaki Yoshida, Masako Kato*

New Pt(II) diimine complex, [Pt(CN)2(H2dpcpbpy)] (H2dpcpbpy = 4,4’-di(p-carboxyphenyl)2,2’-bipyridine) was synthesized and its vapochromic behavior was examined. The yellow amorphous form was transformed to porous orange crystals upon exposure to ethanol vapor. Although the porous structure could not be maintained after desorption, the orange color was retained. The original yellow form was recovered by grinding of the orange colored forms.


35


Figure 1. (a) Solution-state UV-Vis absorption and (b) solid-state UV-Vis diffuse reflectance spectra of 1-Ya (red) and 2-Pa (black). The absorption spectra of 1-Ya and 2-Pa were obtained in DMSO and aq. NH3 (2.8%), respectively, because of the low solubility of 1 in dilute aq. NH3. 100x119mm (300 x 300 DPI)


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Figure 3. Changes in the powder X-ray diffraction patterns of (a) 1-Ya and (b) 2-Pa. Each amorphous solid (black lines) was exposed to EtOH vapor for five days at 303 K. The EtOH-adsorbed forms (red lines) were heated to 120 °C for 2 h to remove the adsorbed EtOH molecules. Finally, the heated samples (blue lines) were ground manually using mortar (green lines). Each orange line at the bottom shows the simulated XRD patterns of 1-Oc and 2-Rc. 89x49mm (300 x 300 DPI)



Figure 5. Changes in the luminescence and diffuse reflectance spectra of (a, c) 1 and (b, d) 2. The amorphous form (black line) was exposed to EtOH vapor for 5 days at 303 K (red line). Subsequently, the sample was heated to remove all adsorbed EtOH (blue line). The sample was then ground manually by using a mortar (green line). The orange lines show the luminescence spectra of the crystal. 100x63mm (300 x 300 DPI)


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Scheme 1. Molecular structures of complexes 1 and 2. 59x42mm (600 x 600 DPI)



Scheme 2. Schematic diagram of the photoexcited 1MMLCT and 3MMLCT levels for the three forms of (a) 1 and (b) 2. Dotted lines of 3MMLCT states of 1-Ya and 1-Oa schematically show the contribution of various strengths of intermolecular Pt⋅⋅⋅Pt distances in amorphous states. 59x21mm (300 x 300 DPI)


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Crystal engineering of vapochromic porous crystals composed of Pt(II

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