Crystal Engineering of Vapochromic Porous Crystals Composed of Pt(II)-Diimine Luminophores for Vapor-History Sensors

Article abstract of DOI:10.1021/acs.cgd.8b00130

A novel Pt(II) diimine complex, [Pt(CN)₂(H₂dpcpbpy)] (1, H₂dpcpbpy = 4,4′-di(p-carboxyphenyl)-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, 1-Oc, 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 ?) compared to the red crystalline form of 2, 2-Rc (6.4 × 6.8 ?). 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.

Full text of DOI:10.1021/acs.cgd.8b00130

Subscriber access provided by Kaohsiung Medical University
Article
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
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
Crystal engineering of vapochromic porous crystals
composed of Pt(II)ꢀdiimine luminophores for vapor
history sensors
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
*
*
Yasuhiro Shigeta, Atsushi Kobayashi , Masaki Yoshida, Masako Kato
Department of Chemistry, Faculty of Science, Hokkaido University, Northꢀ10 Westꢀ8, Kitaꢀku,
Sapporo 060ꢀ0810, Japan
ACS Paragon Plus Environment
1

Crystal Growth & Design
Page 2 of 42
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ABSTRACT: A novel Pt(II) diimine complex, [Pt(CN) (H dpcpbpy)] (1, H dpcpbpy = 4,4'ꢀdi(pꢀ
2
2
2
carboxyphenyl)ꢀ2,2'ꢀbipyridine) was synthesized and its vapochromic behavior was investigated.
The yellow amorphous form of 1, 1ꢀYa, transformed into the porous orange crystalline form, 1ꢀ
Oc, upon exposure to ethanol vapor. This behavior is similar to that of the previously reported
complex, [Pt(CN) (H dcphen)] (2, H dcphen = 4,7ꢀdicarboxyꢀ1,10ꢀphenanthroline). Xꢀray
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
2
diffraction study showed that 1ꢀOc possessed similar but larger porous channels (14.3 × 8.6 Å)
compared to the red crystalline form of 2, 2ꢀRc (6.4 × 6.8 Å). Although the porous structure of
2
ꢀRc was retained after vapor desorption, that of 1ꢀOc collapsed to form the orange amorphous
solid, 1ꢀOa. However, the orange color was unchanged in this process. The initial color was
recovered by grinding 1ꢀOa and 2ꢀRc. These vapor-writing and grinding-erasing functions can
be applied to both in situ vapor sensing and vaporꢀhistory 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 1ꢀOa was largely unaffected upon exposure to humid air,
whereas the red emission of 2ꢀRc was significantly affected. The lesser sensitivity of 1ꢀOa
toward humidity is important for stable vaporꢀhistory sensor applications.
ACS Paragon Plus Environment
2

Page 3 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
Introduction
Chromic materials showing reversible color change in response to external stimuli such as
1
ꢀ3
heat, vapor, and mechanical pressure, have drawn considerable attention in recent years. 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
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4
ꢀ37, 41
compounds (VOCs).
Many vapochromic materials have been reported to date, and one of
the most promising systems is that of Pt(II) complexes with intermolecular metallophilic
1
8ꢀ37
interactions.
The color and luminescence of these complexes is strongly dependent on the
2
z
degree of the interactions originating from the intermolecular 5d orbital overlaps of the Pt(II)
ions to form the occupied antiꢀbonding 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 soꢀcalled
metalꢀmetalꢀtoꢀligand chargeꢀtransfer (MMLCT) transition, resulting in a change in the color and
3
8,39
luminescence.
For example, Aliprandi et al. reported an interesting and outstanding color
40
change from blue to red of the amphipathic Pt(II) complex. Chen and coꢀworkers also reported
29
an interesting halocarbonꢀselective vapochromic response of a diimineꢀPt(II) complex. 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
ACS Paragon Plus Environment
3

Crystal Growth & Design
Page 4 of 42
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
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 hydrogenꢀ
bonding interactions, which may enable us to control the molecular arrangement of the Pt(II)
chromophore in the crystal structure. Eisenberg and coꢀworkers found that the nicotinamideꢀ
functionalized Pt(II)ꢀterpyridine complex exhibits singleꢀcrystalꢀtoꢀsingleꢀcrystal vapochromic
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
3
transformation induced by the adsorption/desorption of methanol vapor. Our group also
reported a unique vapochromism of the carboxylic acidꢀfunctionalized Pt(II)ꢀdiimine complexes,
[
Pt(CN) (H dcphen)] and [Pt(CN) (H dcbpy)] (2 and 3, H dcphen = 4,7ꢀdicaboxyꢀ1,10ꢀ
2 2 2 2 2
phenanthroline, H dcbpy = 4,4′ꢀdicarboxyꢀ2,2′ꢀbipyridine) with “shape-memory” behavior; the
2
porous redꢀluminescent crystalline forms (2ꢀRc and 3ꢀRc) obtained from the purple amorphous
solids (2ꢀPa and 3ꢀPa) by vapor adsorptionꢀinduced structural transformation were stable enough
to retain the porous character without the adsorbed vapor molecules, but reverted to the original
41
purple amorphous solids by simple manual grinding. This unique behavior may be a promising
feature for vaporꢀhistory sensors that can memorize the past existence of vapor. Therefore, we
synthesized a Pt(II) analogue complex, [Pt(CN) (H dpcpbpy)] (1, H dpcpbpy = 4,4′ꢀdi(pꢀ
2
2
2
carboxyphenyl)ꢀ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⋅⋅⋅NCꢀtype hydrogenꢀ
bonded network, which could be the key for the shapeꢀmemory behaviors of complexes 2 and 3.
Herein, we demonstrate the vaporꢀhistory 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
ACS Paragon Plus Environment
4

Page 5 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
responsiveness and was thus a key determinant of the “water-resistant” vaporꢀhistory sensing
property.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 1. Molecular structures of complexes 1 and 2.
ACS Paragon Plus Environment
5

Crystal Growth & Design
Page 6 of 42
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Experimental Section
General procedures
All commercially available starting materials were used without any purification. The
4
2ꢀ44
45
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
precursor materials, namely, 4,4′ꢀdibromoꢀ2,2′ꢀbipyridine,
4ꢀcarboxyphenylboronic acid,
4
6
47
1
Pd(PPh ) , and Pt(CN) were prepared according to previously reported methods. The H
3
4
2
NMR spectrum of each sample was measured using a JEOL EXꢀ270 NMR spectrometer at room
temperature. Elemental analyses were conducted at the Hokkaido University.
Synthesis of H dpcpbpy ligand
2
Na CO (1.68 g, 16.0 mmol) was added to a solvent mixture of H O (80 mL) and methanol (96
2
3
2
mL). Next, 4,4′ꢀdibromoꢀ2,2′ꢀ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 N , 4ꢀ
2
carboxyphenylboronic acid (1.26 g, 9.70 mmol) and Pd(PPh ) (480 mg, 0.4 mmol) were added.
3
4
The resulting mixture was left to stir at reflux for 3 days under N . Subsequently, the reaction
2
mixture was filtered and the filtrate was evaporated to dryness. The residue was suspended in
H O (50 mL) and dilute aq. HCl was added. The resulting white suspension was centrifuged,
2
washed with H O, MeOH, and acetone. After drying under reduced pressure, the target ligand,
2
H dpcpbpy was obtained as a white powder. Yield: 1.08 g, 71%. This compound was used for
2
1
subsequent syntheses without further purification. H NMR (270 MHz, NaOD in CD OD/D O):
3
2
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) (H dpcpbpy)] (1ꢀYa)
2
2
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. NH (98 mL). H dpcpbpy (385 mg, 0.97 mmol)
3
2
was added to the mixture and the resulting suspension was heated at 100 °C for 2 days. After the
ACS Paragon Plus Environment
6

Page 7 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
reaction, the obtained suspension was filtered off. dilute aq. HNO was added to the filtrate and a
3
yellow precipitate emerged. The precipitate was collected by filtration, washed with H O, and
2
dried under vacuum to afford the target compound as a yellow powder (1ꢀYa). Yield: 460 mg,
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
7
3%. H NMR (270 MHz, DMSOꢀd ) δ = 9.28 (s, 2H), 9.21 (d, 2H), 8.35 (d, 2H), 8.25 (d, 4H),
6
8
.19 (d, 4H); Elemental analysis calcd. for C H N O Pt·9H O: C 38.76, H 4.25, N 6.95; found:
2
6
16
4
4
2
C 38.85, H 3.97, N 7.25.
Synthesis of orange crystals [Pt(CN) (H dpcpbpy)]·4H O (1ꢀOc)
2
2
2
To a suspension of [Pt(CN) (H dpcpbpy)] (0.25 mg, 4 µmol) in methanol (7 mL), the
2
2
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 1ꢀOc as
orange crystals suitable for single crystal Xꢀray diffraction.
Powder Xꢀray diffraction
Powder Xꢀray diffraction studies were conducted using a Rigaku SPD diffractometer at
beamline BLꢀ8B of the Photon Factory, KEK, Japan or a Bruker D8 Advance diffractometer
equipped with a graphite monochromator using CuKα radiation and oneꢀdimensional LinxEye
detector. The wavelength of the synchrotron Xꢀray was 1.5385(1) Å.
Luminescence properties
The luminescence spectra of all complexes were measured using a JASCO FPꢀ6600
spectrofluorometer at room temperature. Typical slit width of the excitation and emission light
were 5 and 6 nm, respectively.
UVꢀVis spectroscopy
The UVꢀVis diffuse reflectance spectra of the prepared complexes were recorded on a
Shimadzu UVꢀ2500PC spectrophotometer.
ACS Paragon Plus Environment
7

Crystal Growth & Design
Page 8 of 42
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Vapor adsorption isotherms.
The vapor adsorption isotherms of each complex were measured with the BELSORPꢀmax
vapor adsorption isotherm measurement equipment at 298 K.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Single crystal Xꢀray diffraction measurement
The XRD study of 1ꢀOc was conducted using a Rigaku Mercury CCD diffractometer with
monochromated MoKα radiation (λ = 0.71069 Å) and a sealed Xꢀray tube generator. The single
crystal was mounted on a MicroMount coated with paraffin oil. The crystal was then cooled by
using an N flowꢀtype temperature controller. The diffraction data were collected and processed
2
4
8
with Crystal Clear software. The structures were solved with the direct method using
4
9
SHELXS2013. Structural refinements were conducted by the fullꢀmatrix leastꢀsquares method
49
with SHELXL2013. Nonꢀhydrogen atoms were refined anisotropically and all hydrogen atoms
were refined with the riding model. All calculations were conducted with the Crystal Structure
5
0
crystallographic software package. Full crystallographic data have been deposited with the
Cambridge Crystallographic Data Centre (CCDC 1817798)
ACS Paragon Plus Environment
8

Page 9 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
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 (1ꢀYa) with a completely different color to that of the purple form
of 2ꢀPa. To elucidate the difference between 1ꢀYa and 2ꢀPa, PXRD, UVꢀVis absorption, and
diffuse reflectance spectral measurements were conducted. The PXRD pattern for 1ꢀYa was
featureless and without any sharp diffraction peaks, indicating that 1ꢀYa is an amorphous solid,
same as 2ꢀPa (see Figure S1 in the supporting information, SI). However, the UVꢀVis absorption
spectra of these complexes (Figure 1) were clearly different. The absorption spectrum of
complex 2ꢀPa in solution exhibited a band up to 400 nm and the broad absorption band extended
to 700 nm in the UVꢀVis diffuse reflectance spectrum of the amorphous solid 2ꢀPa. This result
clearly indicated that the intermolecular Pt···Pt interactions contributed to the electronic
transition of 2ꢀPa in the solid state. Similarly, complex 1ꢀYa 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 1ꢀYa. The higher molar absorption coefficient of 1ꢀYa as compared
to that of 2ꢀPa in the solution state could be explained by the more extensive π conjugation in the
H dpcpbpy ligand in comparison with that in the H dcphen ligand. It should be noted that the
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
observed absorption band of 1ꢀYa in the solid state was located at a shorter wavelength than that
of 2ꢀPa, 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.
ACS Paragon Plus Environment
9

Crystal Growth & Design
Page 10 of 42
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 1. (a) Solutionꢀstate UVꢀVis absorption and (b) solidꢀstate UVꢀVis diffuse reflectance
spectra of 1ꢀYa (red) and 2ꢀPa (black). The absorption spectra of 1ꢀYa and 2ꢀPa were obtained
in DMSO and aq. NH (2.8%), respectively, because of the low solubility of 1 in dilute aq. NH .
3
3
Crystal structure of [Pt(CN) (H dpcpbpy)]･4H O (1ꢀOc)
2
2
2
Figure 2 displays the crystal structure of the orange crystalline form of 1 (1ꢀOc) in comparison
with that of 2 (2ꢀRc). Crystallographic data and selected bond length angles of 1ꢀOc are listed in
Tables 1 and 2, respectively. It was found that 1ꢀOc crystallized in the same space group (Cmcm)
as 2ꢀRc. In this structure, half of one molecule was crystallographically independent because of
the mirror plane of this space group. The Pt(II) center of 1ꢀOc was coordinated to two N atoms
from the H dpcpbpy ligand and two C atoms from the two cyanide ligands to form the typical
2
ACS Paragon Plus Environment
10

Page 11 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
squareꢀplanar coordination geometry. The bond lengths around the Pt(II) center in 1ꢀOc were
almost comparable to those in 2ꢀRc, 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 (OꢀH⋅⋅⋅N: 2.67(3) Å) suggested that there was a strong
hydrogen bond between them. As the result, a twoꢀdimensional hydrogen bonding network
structure was constructed in the ab plane, same as that present in 2ꢀRc (Figure 2(b)). As shown
in Figure 2(a), this square planar molecule was stacked oneꢀdimensionally along the c axis with
an antiꢀparallel 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 1ꢀOc (3.353(2) Å) was longer than that of 2ꢀRc
(3.2668(7) Å) despite the almost linear stacking manner, i.e., the Pt···Pt···Pt stacking angle in 1ꢀ
Ya was closer to 180° than that in 2ꢀRc. Thus, the intermolecular Pt···Pt interaction in 1ꢀOc was
weaker than that in 2ꢀRc, probably as a result of the steric effect of the phenyl ring attached on
the 4ꢀ 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 2ꢀ
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
D 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 1ꢀOc and 2ꢀRc
(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 1ꢀOc were larger than those of 2ꢀRc (6.4 × 6.8 Å and 35.2%). In this
ACS Paragon Plus Environment
11

Crystal Growth & Design
Page 12 of 42
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
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.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. Crystal structures of (top) 1ꢀOc and (bottom) 2ꢀRc. (a) 1ꢀD Pt···Pt interaction along
the c axis, (b) 2ꢀD hydrogen bonding crystal structure in the ab plane, and (c) the porous
structure viewed along the c axis. The blue, brown, lightꢀblue, 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
51
VESTA computer program.
ACS Paragon Plus Environment
12

Page 13 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
Table 1. Crystal parameters and refinement results of 1ꢀOc in comparison with those of 2ꢀRc.
a
Complex
T / K
1ꢀOc
2ꢀRc
200(1)
200(1)
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Formula
Formula weight
Crystal system
Space group
a / Å
PtC H N O
PtC H N O
2
6
16
4
8
16
8
4
7
707.5
563.35
Orthorhombic Orthorhombic
Cmcm
21.22(1)
26.43(1)
6.699(4)
3756(3)
4
Cmcm
17.587(4)
18.288(5)
6.4903(15)
2087.5(9)
4
b / Å
c / Å
3
V / Å
Z
–3
D / g･cm
cal
1.247
1.812
Reflections collected
Unique reflections
GOF
14364
2394
12292
1445
1.096
1.082
Rint
0.1099
0.0811
0.2123
0.0828
0.0326
0.0805
R (I > 2.00σ (I))
b
RW
2
Pore diameter / Å
14.3 × 8.6
6.4 × 6.8
c
Void fraction / %
48.7
35.2
a
b
2
o
2 2
2 2 1/2 c
Ref.41. R = [Σ(w(F − F ) )/Σw(F ) ] . Calculated by SQUEEZE program.
W
c
o
ACS Paragon Plus Environment
13

Crystal Growth & Design
Page 14 of 42
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 2. Selected bond lengths and angles of complexes 1ꢀOc and 2ꢀRc.
a
Complex
T / K
1ꢀOc
2ꢀRc
200(1)
200(1)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Pt1–N1
2.06(1)
1.95(1)
2.67(3)
3.353(2)
174.92(2)
2.047(5)
1.962(5)
2.665(8)
3.2668(7)
166.80(1)
Pt1–C7
O1–H⋅⋅⋅N3
Pt1⋅⋅⋅Pt1
Pt1⋅⋅⋅Pt1⋅⋅⋅Pt1
a
Ref. 41.
Vaporꢀadsorptionꢀtriggered 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 1ꢀOc has a very similar porous structure to 2ꢀRc, we
investigated the vapochromic behavior of complex 1. Figure 3(a) shows the PXRD pattern
changes of the yellow amorphous solid 1ꢀYa upon exposure to EtOH and the subsequent heat
and grinding treatments. The initial broad XRD peak of 1ꢀYa 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 1ꢀOc. Thus, the amorphous
complex 1ꢀYa was transformed into the EtOHꢀadsorbed crystalline porous phase, 1ꢀOc⊃EtOH,
upon exposure to EtOH vapors, same as observed for 2ꢀPa (see Figure 3(b)). When 1ꢀOc⊃
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 1ꢀOc ⊃ EtOH were removed, as confirmed by
ACS Paragon Plus Environment
14

Page 15 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
thermogravimetric analysis (see Figure S4 in the supporting information). This result indicates
that the porous crystal structure of 1ꢀOc collapsed upon the desorption of EtOH vapors, whereas
the porous structure of 2ꢀRc was retained even after the removal of EtOH by heating. Thus, the
porous structure of 1ꢀOc was not stable without the adsorbed EtOH molecules, and the lesser
stability of 1ꢀOc compared to 2ꢀRc was likely because of the expanded porous structure with a
larger void fraction and weaker intermolecular hydrogenꢀbonding interactions in the former.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 3. Changes in the powder Xꢀray diffraction patterns of (a) 1ꢀYa and (b) 2ꢀPa. Each
amorphous solid (black lines) was exposed to EtOH vapor for five days at 303 K. The EtOHꢀ
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 1ꢀOc and 2ꢀRc.
ACS Paragon Plus Environment
15

Crystal Growth & Design
Page 16 of 42
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
In our previous study, we showed that 2 exhibited a unique vapochromism effect, with its
Shapeꢀmemory’ behavior; the amorphous form of 2, 2ꢀPa, 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 1ꢀOc was less stable than that of 2ꢀRc. 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 1ꢀYa and 2ꢀ
Pa were first heated for 24 h at 120 °C to remove all adsorbed solvent molecules. Although 1ꢀYa
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
hardly adsorbed any EtOH vapor at low relative pressures (P/P < 0.6), large number of EtOH
0
molecules were adsorbed at high relative pressure (~1.9 mol/mol at P/P = 0.99). In the
0
desorption process, most of the adsorbed EtOH molecules were retained at P/P = 0.4, and only
0
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., 1ꢀYa to 1ꢀOc⊃EtOH. Although 2ꢀPa adsorbed EtOH
vapor even at low pressures, a similar hysteresis to that of 1ꢀYa 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 H dpcpbpy ligand. Both complexes adsorbed almost the same amount of
2
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
ACS Paragon Plus Environment
16

Page 17 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
carboxyl groups in the H dpcpbpy ligand in 1ꢀOc (see Figure S2), EtOH molecules are thought
2
to be adsorbed at the similar positions via hydrogen bond interactions.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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 1ꢀYa and 2ꢀPa 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/P = 0.01). An isotherm with a
0
steep increase in vapor uptake at low pressures is referred to as a TypeꢀI isotherm, and is a
characteristic feature of microporous materials. In contrast, 1 did not exhibit a similar TypeꢀI
ACS Paragon Plus Environment
17

Crystal Growth & Design
Page 18 of 42
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
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 1ꢀOc was collapsed by the desorption of EtOH molecules.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Vapochromic behavior
As mentioned in the Introduction, the amorphous form of 2ꢀPa exhibited ‘shapeꢀmemory’
vapochromism because of its porous structure that adsorbed the alcoholic vapors and collapsed
by manual grinding. Although the porous crystalline structure of 1ꢀOc was quite similar to that
of 2ꢀRc, 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, solidꢀstate UVꢀVis diffuse reflectance and emission spectral
changes of 1ꢀYa upon vapor ad/desorption were measured. As shown in Figure 5, 1ꢀYa showed
a broad orange emission centered at 626 nm, which was over 100 nm shorter wavelength than
that of 2ꢀPa (733 nm). This blueꢀshift of the emission band is probably due to the destabilized
MMLCT energy resulting from the weaker intermolecular interaction in 1ꢀYa in comparison to
that of 2ꢀPa, as suggested by the single crystal Xꢀray analyses (Table 2) and diffuse reflectance
1
spectra (Figure 5) of these complexes. The absorption band corresponding to the MMLCT
transition in 1ꢀYa was observed at a shorter wavelength by about 200 nm as compared to that of
2
ꢀPa. The emission band maximum of 1ꢀYa was blueꢀshifted from 626 nm to 603 nm upon
exposure to EtOH vapor, which is similar to that observed in the case of 1ꢀOc. PXRD analyses
(Figure 3) also indicated the EtOH vapor adsorptionꢀinduced structural transformation from 1ꢀ
1
Ya to 1ꢀOc⊃EtOH. On the other hand, the absorption band corresponding to the MMLCT
transition of 1ꢀYa shifted drastically to a longer wavelength from 500 nm to 600 nm upon
ACS Paragon Plus Environment
18

Page 19 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
exposure to EtOH vapor. A similar red shift was also observed in the excitation spectrum; the
1
excitation band of MMLCT transition shifted by ~60 nm (Figure S5) when 1ꢀYa was exposed to
EtOH vapor. These redꢀshifted absorption and excitation bands suggest that the intermolecular
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Pt···Pt interactions in 1ꢀOc⊃EtOH are basically stronger than those in the yellow amorphous
3
powder, 1ꢀYa. The inconsistency of these results with the blueꢀshifted MMLCT transition
observed in the emission spectrum of 1ꢀOc⊃EtOH may be attributed to the emission from the
impurity sites with shorter Pt···Pt distance in the amorphous 1ꢀYa state. The amorphous powder
1ꢀYa should have a larger number of structural defects as compared to the orange crystalline
form 1ꢀOc. 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 1ꢀOc, resulting in the
lower energy emission via energy transfer from the other domains with longer Pt···Pt distance.
ACS Paragon Plus Environment
19

Crystal Growth & Design
Page 20 of 42
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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.
3
As reported previously, the MMLCT emission of 2ꢀRc⊃EtOH exhibited a blueꢀshift of ~20
3
nm upon the removal of adsorbed EtOH vapor by heating. The MMLCT emission wavelength
was further shifted to a longer wavelength to ~700 nm after manual grinding. Similar changes
were observed in the UVꢀVis diffuse reflectance spectrum (Figure 5) and the PXRD
measurements indicated that this twoꢀstep change originated from the slight structural
deformation by EtOH desorption and transformation to the amorphous powdery form by
ACS Paragon Plus Environment
20

Page 21 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
grinding (Figure 3). On the other hand, there was hardly any change in the diffuse reflectance
spectrum of 1ꢀOc⊃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 orangeꢀcolored amorphous solid 1ꢀOa. Considering that
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
the MMLCT absorption band was hardly changed (red and blue lines in Figure 5c),
intermolecular Pt···Pt interactions in the ground state of 1ꢀOa would be comparable to the
crystalline form, 1ꢀOc, which has the same orange color. Therefore, the redꢀshift of the
3
MMLCT presumably corresponds to the energy transfer to the impurity sites with shorter Pt···Pt
distances, as discussed above for 1ꢀYa. That is, the amorphous solid of 1ꢀOa exhibited a larger
Stokes shift (see red lines in Figures 5a and 5c) than the crystalline form 1ꢀOc (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
–
1
of these forms (Figure S6 in SI); the C≡N stretching mode of 1ꢀOa (2134 cm ) was observed at
–
1
a very similar wavenumber to that of 1ꢀYa (2135 cm ) and also at a lower frequency compared
–1
to that of 1ꢀOc (2159 cm ). This lowerꢀenergy C≡N stretching mode of both the amorphous
forms, 1ꢀYa and 1ꢀOa, suggests that the rigidity of the intermolecular hydrogenꢀbonded network
structure of the amorphous phases 1ꢀYa and 1ꢀOa is weaker than that of the crystalline form 1ꢀ
Oc.
The large structural difference between the EtOHꢀdesorbed 1ꢀOa and 2ꢀRc certainly affects the
3
sensitivity of the MMLCT emission energy for H O vapor in air. As shown in a previous
2
41
3
report, the MMLCT emission of 2ꢀRc was blueꢀshifted 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
ACS Paragon Plus Environment
21

Crystal Growth & Design
Page 22 of 42
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
H O molecules into the porous channel of 2ꢀRc. In contrast, the MMLCT emission energy of
2
the EtOHꢀdesorbed complex 1ꢀOa changed only slightly from 652 nm to 658 nm upon
increasing RH in this abovementioned range (Figure S7). The lesser sensitivity of 1ꢀOa in
comparison with that of 2ꢀRc is probably due to the lack of porous channels in 1ꢀOa.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Interestingly, the emission and diffuse reflectance spectra of the EtOHꢀdesorbed amorphous
orange form, 1ꢀOa, were remarkably blue shifted and similar to that of 1ꢀYa after manual
grinding. Notably, some peaks in the PXRD pattern of 1ꢀOa completely disappeared after
manual grinding (Figure 3). This result suggests that the shortꢀrange ordered domains with
effective intermolecular Pt···Pt interactions in 1ꢀOa could be collapsed easily and completely to
form 1ꢀYa by grinding. In addition, the emission spectrum of 1ꢀOa was changed to almost the
identical one of 1ꢀOc⊃EtOH under exposure to EtOH vapor (Figure S8), indicating the reverse
transformation from 1ꢀOa to 1ꢀOc⊃EtOH. We also confirmed that this vapochromic behavior
between 1ꢀOa and 1ꢀOc⊃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 2ꢀPa
1
exhibited the MMLCT absorption band at lower energy than that of the red crystalline form 2ꢀ
Rc. On the other hand, the asꢀprepared yellowꢀcolored amorphous powder of 1, 1ꢀYa, exhibited
1
a higher energy MMLCT absorption band compared to the other orangeꢀcolored forms because
of the weaker intermolecular Pt···Pt interactions. This difference is ascribed to the steric effect of
the expanded π conjugated H dpcpbpy ligand as discussed above. Upon exposure of the
2
amorphous 1ꢀYa and 2ꢀPa to EtOH vapor, the vapor adsorptionꢀinduced structural
transformation to the porous crystalline form, 1ꢀOc⊃EtOH and redꢀcolored 2ꢀRc⊃EtOH took
1
place. In the transformation from 2ꢀPa to 2Rc⊃EtOH, both of the MMLCT absorption band
ACS Paragon Plus Environment
22

Page 23 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
3
and MMLCT emission band shifted to a shorter wavelength. In contrast, in the transformation
1
from 1ꢀYa to 1ꢀOc ⊃EtOH, the MMLCT absorption band clearly shifted to a longer
3
wavelength (from 500 nm to 600 nm) while the MMLCT emission band inversely shifted to a
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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 1ꢀYa, 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 1ꢀOc ⊃EtOH.
Furthermore, 2ꢀRc⊃EtOH can retain its porous crystal structure after the desorption of EtOH
1
3
vapor, and the resulting 2ꢀRc showed similar MMLCT band and slightly redꢀshifted MMLCT
band to that of 2ꢀRc⊃EtOH. On the other hand, 1ꢀOc⊃EtOH can be easily transformed to the
orange amorphous solid 1ꢀOa by desorption of the EtOH molecules assisted by heating. During
this structural change to 1ꢀOa, only a negligible shift was observed in the diffuse reflectance
spectrum whereas an obvious redꢀshift was observed in the emission spectrum. Thus, the
intermolecular Pt···Pt interactions are presumably still partly effective in the amorphous form 1ꢀ
Oa. The amorphous nature of 1ꢀOa and 1ꢀYa is also responsible for the large Stokes shift in
these complexes, i.e., the contribution of the impurity levels derived from the microscopicallyꢀ
formed domains with the shorter Pt···Pt distances. This can explain why only the emission
spectrum was changed upon EtOH desorption.
ACS Paragon Plus Environment
23

Crystal Growth & Design
Page 24 of 42
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
3
Scheme 2. Schematic diagram of the photoexcited MMLCT and MMLCT levels for the three
3
forms of (a) 1 and (b) 2. Dotted lines of MMLCT states of 1ꢀYa and 1ꢀOa schematically show
the contribution of various strengths of intermolecular Pt⋅⋅⋅Pt distances in amorphous states.
Conclusions
A new luminescent Pt(II) complex, [Pt(CN) (H dpcpbpy)] (1; H dpcpbpy = 4,4'ꢀpꢀ
2
2
2
dicarboxyphenylꢀ2,2'ꢀbipyridine) was successfully synthesized. The orange crystal of 1ꢀOc was
identified to be the isomorphous porous form of the red crystalline form 2ꢀRc,
[
Pt(CN) (H dcphen)] (2, H dcphen = 4,7ꢀdicarboxyꢀ1,10ꢀphenanthroline). The porous structure
2 2 2
was constructed by the 2ꢀD 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 2ꢀRc.
The asꢀsynthesized yellow amorphous solid 1ꢀYa exhibited a completely different color from
the asꢀsynthesized purple amorphous form 2ꢀPa. 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. 1ꢀYa exhibited vapochromic behavior as a result of the reversible EtOH vapor adsorption
and desorption, which induced structural transformation between the amorphous 1ꢀYa and the
porous crystalline 1ꢀOc states. Although the vapochromic behavior of 1 was similar to that of 2,
ACS Paragon Plus Environment
24

Page 25 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
the vapor adsorption isotherms and PXRD measurements revealed that the porous crystal
structure of 1ꢀOc could not be retained after the removal of EtOH vapor by heating. In contrast,
2ꢀRc 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 1ꢀOc
than 2ꢀRc, as suggested by their crystal structures.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
On the other hand, after the desorption of EtOH, 1ꢀOa exhibited a very similar diffuse
reflectance spectrum to that of 1ꢀOc. This behavior could be explained by the small crystal
domains with the effective Pt···Pt interactions that still remained in 1ꢀOa. Moreover, upon these
processes, a larger Stokes shift was observed for the amorphous form 1ꢀOa as compared to the
crystalline form 1ꢀOc. 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 1ꢀOc. The emission spectrum
3
of 1ꢀOa was hardly affected by water vapor in air whereas the MMLCT emission band of 2ꢀRc
was moderately shifted by the physisorbed water vapor. In other words, the existence of the
porous structure of 2ꢀRc is a key factor in the sensitivity toward water. Furthermore, 1ꢀOa could
be transformed to the initial 1ꢀYa 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. 1ꢀOa also showed negligible humidityꢀ
dependent 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.
ACS Paragon Plus Environment
25

Crystal Growth & Design
Page 26 of 42
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ASSOCIATED CONTENT
Supporting Information. Thermogravimetric analysis, PXRD patterns, excitation and IR
spectra of 1; Luminescence spectrum changes of 1ꢀOa upon exposure to H O or EtOH vapor.
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*
Eꢀmail: akoba@sci.hokudai.ac.jp(A. K.); mkato@sci.hokudai.ac.jp(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 GrantsꢀinꢀAid for Scientific Research
(C) (No.26410063), Artificial Photosynthesis (area No. 2406, No.15H00858), Soft Crystals (area
No. 2903, No. JP17H06367) and GrantꢀinꢀAid for JSPS Fellow (17J01139) from MEXT, Japan.
ACS Paragon Plus Environment
26

Page 27 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
REFERENCES
1) Keefe M. H.; Benkstein K. D.; Hupp J. T. Luminescent sensor molecules based
(
oncoordinated metals: a review of recent developments, Coord. Chem. Rev, 2000, 205, 201–228.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(2) Linert, W.; Fukuda, Y.; Camard, A Chromotropism of Coordination Compounds and its
Applications in Solution, Coord. Chem. Rev, 2001, 218, 113–152.
(3) Kato, M. Luminescent Platinum Complexes Having Sensing Functionalities, Bull. Chem.
Soc. Jpn, 2007, 80, 287–294.
(4) Lee, E. Y.; Suh, M. P. A Robust Porous Material Constructed of Linear Coordination
Polymer Chains: Reversible Single ‐Crystal to Single ‐Crystal Transformations upon
Dehydration and Rehydration, Angew. Chem. Int. Ed. 2004, 43, 2798–2801.
(5) Yamada, K.; Yagishita, S.; Tanaka, H.; Tohyama, K.; Adachi, K.; Kaizaki, S.; Kumagai,
H.; Inoue, K.; Kitaura, R.; Chang, H.ꢀC.; Kitagawa, S.; Kawata, S. Metal‐Complex Assemblies
Constructed from the Flexible Hinge‐Like Ligand H bhnq: Structural Versatility and Dynamic
2
Behavior in the Solid State, Chem. Eur. J. 2004, 10, 2647ꢀ2660.
(6) Espallargas, G. M.; Brammer, L.; van de Streek, J.; Shankland, K.; Florence, A. J.; Adams,
H. Reversible Extrusion and Uptake of HCl Molecules by Crystalline Solids Involving
Coordination Bond Cleavage and Formation, J. Am. Chem. Soc. 2006, 128, 9584–9585.
(7) Braga, D.; Maini, L.; Mazzeo, P. P.; Ventura, B. Reversible Interconversion between
Luminescent Isomeric Metal –Organic Frameworks of [Cu I (DABCO) ] (DABCO=1,4 ‐
4
4
2
Diazabicyclo[2.2.2]octane), Chem. Eur. J. 2010, 16, 1553–1559.
ACS Paragon Plus Environment
27

Crystal Growth & Design
Page 28 of 42
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(
8) Boonmak, J.; Nakano, M.; Chaichit, N.; Pakawatchai, C.; Youngme, S. Waterꢀinduced
Reversible Structural Phase Transformation with Chromotropism in Metal Supramolecular
Frameworks Containing Aminopyrazine and Sulfate Anions, Dalton Trans. 2010, 39, 8161–8167.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(9) Li, B.; Wei, R.ꢀJ.; Tao, J.; Huang, R.ꢀB.; Zheng, L.ꢀS.; Zheng, Z. SolventꢀInduced
Transformation of Single Crystals of a SpinꢀCrossover (SCO) Compound to Single Crystals with
Two Distinct SCO Centers, J. Am. Chem. Soc. 2010, 132, 1558–1566.
(10) Strasser, C. E.; Catalano, V. J. “On−Off” Au(I)···Cu(I) Interactions in a Au(NHC)2
Luminescent Vapochromic Sensor, J. Am. Chem. Soc. 2010, 132, 10009–10011.
(
11) Lu, Z.ꢀZ.; Zhang, R.; Li, YꢀZ.; Guo, ZꢀJ.; Zheng, H.ꢀG. Solvatochromic Behavior of a
Nanotubular Metal−Organic Framework for Sensing Small Molecules, J. Am. Chem. Soc. 2011,
33, 4172–4174.
1
(12) Nishiuchi, T.; Tanaka, K.; Kuwatani, Y.; Sung, J.; Nishinaga, T.; Kim, D.; Iyoda, M.
Solvent‐Induced Crystalline‐State Emission and Multichromism of a Bent π‐Surface
System Composed of Dibenzocyclooctatetraene Units, Chem. Eur. J. 2013, 19, 4110–4116.
(13) Oliveri, I. P.; Malandrino, G.; Bella S. D. Phase Transition and Vapochromism in
Molecular Assemblies of a Polymorphic Zinc(II) SchiffꢀBase Complex, Inorg, Chem. 2014, 53,
9771–9777.
(14) Sun, H.; Liu, S.; Lin, W.; Zhang, K. Y.; Lv, W.; Huang, X.; Huo, F.; Yang, H.; Jenkins,
G.; Zhao, Q.; Huang, W. Smart Responsive Phosphorescent Materials for Data Recording and
Security Protection, Nature Commun. 2014, 5, 3601.
ACS Paragon Plus Environment
28

Page 29 of 42
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
(
15) Jadhav, T.; Dhokale, B.; Patil, Y.; Mobin, S. M.; Misra, R. MultiꢀStimuli Responsive
Donor–Acceptor Tetraphenylethylene Substituted Benzothiadiazoles, J. Phys. Chem. C, 2016,
120, 24030–24040.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(16) Kar, P.; Yoshida, M.; Shigeta, Y.; Usui, A.; Kobayashi, A.; Minamidate, T.; Matsunaga,
N.; Kato, M. Methanol‐Triggered Vapochromism Coupled with Solid‐State Spin Switching in
a Nickel(II)‐Quinonoid Complex, Angew. Chem. Int. Ed. 2017, 56, 2345–2349.
(17) Yang, N.ꢀN.; Sun, W.; Xi, F.ꢀG.; Sui, Q.; Chen, L.ꢀJ.; Gao, E.ꢀQ. Postsynthetic Nꢀ
methylation Making a Metal–organic Framework Responsive to Alkylamines, Chem. Commun.
2017, 53, 1747–1750.
(18) Daws, C. A.; Exstrom, C. L.; Sowa, J. R. Jr.; Mann, K. R. “Vapochromic” Compounds
as Environmental Sensors. 2. Synthesis and NearꢀInfrared and Infrared Spectroscopy Studies of
[Pt(arylisocyanide) ][Pt(CN) ] upon Exposure to Volatile Organic Compound Vapors. Chem.
4
4
Mater. 1997, 9, 363–368.
(19) Exstrom, C. L.; Pomije, M. K.; Mann, K. R. Infrared Spectroscopy Studies of Platinum
Salts Containing Tetracyanoplatinate(II). Evidence for Strong HydrogenꢀBonding Interactions in
“Vapochromic” Environmental Sensor Materials, Chem. Mater. 1998, 10, 942–945.
(20) Kato, M.; Omura, A.; Toshikawa, A.; Kishi, S.; Sugimoto, Y. VaporꢀInduced
Luminescence Switching in Crystals of the Syn Isomer of a Dinuclear (Bipyridine)platinum(II)
Complex Bridged with Pyridineꢀ2ꢀThiolate Ions, Angew. Chem. Int. Ed. 2002, 41, 3183–3185.
ACS Paragon Plus Environment
29