Two-step changes in luminescence color of Pt(II) complex bearing an amide moiety by mechano- and vapochromism

Article abstract of DOI:10.1246/cl.2012.65

A pincer Pt(II) complex with amide groups was investigated with respect to the change in their luminescence color in solid state. The Pt(II) complex exhibited mechanochromic behavior, changing from green to orange upon grinding. In addition, the orange lu

Full text of DOI:10.1246/cl.2012.65

doi:10.1246/cl.2012.65
Published on the web December 24, 2011
65
Two-step Changes in Luminescence Color of Pt(II) Complex Bearing an Amide Moiety
by Mechano- and Vapochromism
Seong Jib Choi,1,2 Junpei Kuwabara,1,2 Yoshinobu Nishimura,1,3 Tatsuo Arai,1,3 and Takaki Kanbara*1,2
1Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), Graduate School of Pure and Applied Sciences,
University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573
2Department of Material Science, Graduate School of Pure and Applied Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573
3Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573
(Received October 15, 2011; CL-111013; E-mail: kanbara@ims.tsukuba.ac.jp)
A pincer Pt(II) complex with amide groups was investigated
with respect to the change in their luminescence color in solid
state. The Pt(II) complex exhibited mechanochromic behavior,
changing from green to orange upon grinding. In addition, the
orange luminescence changed to yellow via vapochromism.
Luminescent transition-metal complexes in solid state have
attracted the attention of scientists because of their application in
material sciences, such as in organic light-emitting diodes.1 In
solution state, luminescence properties depend on the chemical
structure of the metal complex,2 while in solid state, they also
depend on the molecular orientation.3 External stimuli such as
mechanical pressure and exposure to volatile molecules affect
the molecular orientation; therefore, they can change lumines-
cence in solid state. For example, transition-metal complexes
exhibit dramatic luminescence color changes through mechano-
chromism and vapochromism.4 Since the chromisms are caused
by changes in intermolecular interactions such as ³-³ stacking
and metal-metal interactions, planar structures of pincer com-
plexes are suitable for demonstrating interesting luminescence
color changes. In previous work, we investigated luminescence
properties of pincer complexes bearing hydrogen-bonding
moieties.5 The hydrogen-bonding capability of the N-H groups
of the complexes with solvent molecules or a polymer matrix
affected the intensity and color of the photoluminescence in
solid state, suggesting that hydrogen bonding can control the
molecular orientation and luminescence properties in solid state.
Therefore, the purpose of this work was to investigate the strong
luminescent pincer Pt(II) complexes with amide groups, with the
aim of obtaining a novel mechano- and vapochromic complex
via hydrogen bonding. Herein, we report the two-step changes in
luminescence color of the Pt(II) complex bearing amide moieties
induced by external stimuli.
Pincer Pt(II) complexes (Figure 1) were synthesized accord-
ing to a method described in the literature;5c their photophysical
properties are summarized in Table 1. The absorption spectra
of the complexes in N,N-dimethylformamide (DMF) exhibited
maxima at 398 (1) and 381 nm (2 and 3) (Figure S1).6 The
complexes 2 and 3 exhibited strong luminescence, with a
maximum at 480 nm in DMF (Table 1). According to the
literature,7 the luminescence at about 480 nm is assigned to the
phosphorescence from ligand-centered ³-³* transitions. The
quantum yields of complexes 2 and 3 were 0.39 and 0.40,
respectively. The luminescence decay time (¸) of complexes 2
and 3 was 1.59 ¯s, indicating phosphorescence. On the other
Figure 1. Structures of complexes 1, 2, and 3.
Table 1. Photophysical properties of Pt(II) complexes
Complex
-ex/nm
-em/nm
Φem
1a
400
378
378
472
428
434
518
480
480
512
635
574
<0.01
0.39
2a
3a
0.40
3¢DMFb
3¢Powderc
3¢MeOHd
aThe photoluminescence spectra were measured in 1.0 ©
10¹5 M solution in DMF at room temperature. The quantum
yields were measured in degassed DMF at room temperature.
bCrystals obtained by recrystallization from DMF. cPowder
sample of complex 3 obtained by grinding. dExposure of
3¢Powder to MeOH vapors.
hand, complex 1 exhibited a weak luminescence at 518 nm.
Since an iridium complex with carbamoyl groups has weak
luminescence,8 weak luminescence in complex 1 is attributed to
the the carbamoyl group. The complexes 2 and 3 exhibited
strong luminescence in solid state.
Remarkably, complex 3 exhibited interesting luminescence
properties in solid state, which is two-step changes in the
photoluminescence by external stimuli (Figure 2). Thus, green
luminescence was observed from the crystals obtained by
recrystallization from DMF (3¢DMF in Figure 2a) under UV
light. Then, the green luminescent crystals changed to orange
(3¢Powder) when grinding in a ceramic mortar (Figure 2b). In
addition, exposure of orange luminescent 3¢Powder to methanol
vapors induced a change to yellow luminescence (3¢MeOH in
Figure 2c). The orange luminescence could be recovered from
yellow luminescence by mechanical grinding or by heating at
200 °C (Figure 2). Figure 3 shows photoluminescence spectra of
complex 3 in solution and in each of the three solid states. The
Chem. Lett. 2012, 41, 65-67
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66
Figure 2. Two-step changes in the photoluminescence of
complex 3 by external stimuli. The photographs were taken
during irradiation with UV light (365 nm).
Figure 3. Photoluminescence spectra of complex 3 in solution
state (1 © 10¹5 M solution in DMF, -ex = 378 nm, -em = 480
nm, solid line), 3¢DMF (-ex = 472 nm, -em = 512 nm, broken
line), 3¢MeOH (-ex = 434 nm, -em = 574 nm, dashed/dotted
line), and solid after mechanical grinding (-ex = 428 nm,
Figure 4. Packing diagrams of complex 3¢DMF; a) top view,
b) side view and 3¢MeOH, c) side view, and d) top view.
Element colors are as follows: Pt: violet, Cl: green, N: blue, C:
dark gray, and H: white gray.
-
em = 635 nm, dotted line).
maximum wavelength of green luminescent crystals (3¢DMF)
was 512 nm, whereas those of orange luminescent powder
(3¢Powder) and yellow luminescent solid (3¢MeOH) were 635
and 574 nm, respectively.
square-planar Pt(II) complex in 3¢DMF and 3¢MeOH are
similar (Figure 4 and Table S2),6,9,10 whereas their molecular
orientations and hydrogen-bonding networks are different. The
packing diagram of 3¢DMF is shown in Figure 4a. One of the
N-H moieties in the amide groups interacts with the chloro
ligand in the neighboring Pt(II) complex. Another N-H moiety
has hydrogen-bonding interaction with the solvated DMF
molecule (Figure 4a). In the case of 3¢MeOH, three types of
hydrogen bonds are observed (Figure 4d). In addition to N-
H£Cl and N-H£O (MeOH), one of the N-H moieties shows
hydrogen-bonding interaction with the carbonyl group in the
neighboring complex. The hydrogen-bonding network affects
the Pt-Pt distance in the crystal. The packing diagram of 3¢DMF
reveals a Pt-Pt distance of 4.854 ¡ (Figure 4b). On the other
hand, the packing diagram of 3¢MeOH reveals a shorter Pt-Pt
distance, of 3.385 ¡ (Figure 4c), than that of 3¢DMF. The
shorter Pt-Pt distance indicates metal-metal interaction that
induces excimer luminescence. Since the solvent molecules
affect the distances between Pt and Pt in the solid state via
To explore the differences in molecular orientation among
3¢DMF, 3¢Powder, and 3¢MeOH, these samples of complex 3
were further investigated by powder X-ray diffraction (XRD).
The green luminescent crystals of 3¢DMF exhibited sharp
diffraction at 5.90° (001) (Figure S2b).6 In contrast, the ground
3¢Powder did not exhibit clear diffraction. This result indicates
that the mechanical grinding scatters the regular arrangements of
3¢DMF. Interestingly, new sharp diffraction appeared at 4.94°
(001) upon exposure of 3¢Powder to methanol vapors for several
minutes. Since the diffraction pattern was similar to that of
crystals of 3 possessing methanol molecules in the crystal lattice
(Figure S2e),6 vapors of methanol are likely to induce crystal-
lization of 3 in powder state. Since the XRD pattern of 3¢DMF
differs from that of 3¢MeOH, the arrangement of complex 3 in
the solid state is assumed to be different. To elucidate this
difference, 3¢DMF and 3¢MeOH were subjected to single-
crystal X-ray diffraction studies.9 Molecular structures of
Chem. Lett. 2012, 41, 65-67
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67
hydrogen bonding, the hydrogen-bonding interaction is likely to
play an important role in determining luminescence color.
To confirm the effect of Pt-Pt interaction on luminescence
properties, luminescence spectra were measured under various
concentrations in DMF (Figure S3).6 Although green lumines-
cence (-em = 480 nm) was observed in a dilute solution of
complex 3, luminescence at a long wavelength (-em = 651 nm)
was observed at a high concentration (10 mM). Since a high
concentration promotes metal-metal interactions, luminescence
at 651 nm is presumably caused by excimer formation through
metal-metal interactions.11 These results support the assumption
that the luminescence at a long wavelength in 3¢Powder is
associated with the metal-metal interactions. In addition to
mechanical grinding, heating induced a change in the lumines-
cence color of 3¢DMF to orange. Thermogravimetric analysis of
3¢DMF at temperatures ranging from 50 to 500 °C showed a
decrease in weight at around 150 °C (Figure S4).6 From the
amount of weight loss, it was estimated that one DMF molecule
was eliminated from 3¢DMF for each Pt complex through
heating. The molar ratio was consistent with the crystal
structure. The loss of the solvent molecules upon heating caused
the same effect as grinding, which induced formation of an
amorphous and orange luminescent solid. The formation of
the amorphous solid by loss of DMF indicates that the
hydrogen-bonding capacity of the amide groups with DMF
plays a crucial role in keeping the solid crystalline and
maintaining its green luminescence. In contrast to complex 3,
complex 2 exhibits similar luminescence color in 2¢Powder and
2¢MeOH (Figure S5).3,4,6
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3
4
5
6
7
In conclusion, Pt(II) complex 3 bearing the hexanoylamide
group exhibited interesting two-step luminescence color changes
from green to yellow and orange, in the solid state. This is a rare
example of the combination of vapochromism and mechano-
chromism. The luminescence color was strongly affected by the
Pt-Pt distance in molecular packing, depending on the hydro-
gen-bonding network. Therefore, the hydrogen-bonding capa-
bility of luminescent material is expected to be a key factor for
tuning luminescence color in solid state.
8
9
K. Y. Zhang, K. K.-W. Lo, Inorg. Chem. 2009, 48, 6011.
Crystal data for 3¢DMF: 2(C28H33N4O2PtCl¢DMF), Mr =
ꢀ
1552.53, triclinic, P1 (#2), a = 14.409(3), b = 14.824(3),
c = 15.720(3) ¡, ¡ = 68.155(2), ¢ = 83.331(2), £ =
84.179(2)°, V = 3089.4(10) ¡3, Z = 2,
Dcalcd = 1.669
¹3
g cm
, observed reflections 13496 (I > 2·(I)), R1 =
0.0202, wR2 = 0.0538, S = 1.042. For 3¢MeOH:
ꢀ
2(C28H33N4O2PtCl¢MeOH), Mr = 1464.38, triclinic, P1
(#2), a = 12.726(2), b = 14.062(2), c = 18.434(3) ¡,
¡ = 71.007(2), ¢ = 87.466(2), £ = 65.605(2)°, V =
2825.1(8) ¡3, Z = 2, Dcalcd = 1.721 g cm¹3, observed reflec-
tions 12318 (I > 2·(I)), R1 = 0.0270, wR2 = 0.0626, S =
1.026. Crystallographic data reported in this manuscript have
been deposited with Cambridge Crystallographic Data
Centre as supplementary publication Nos. CCDC-842935
(3¢2DMF) and CCDC-842936 (3¢2MeOH). Copies of the
data can be obtained free of charge via www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12, Union Road, Cambridge, CB2 1EZ,
U.K.; fax: +44 1223 336033; or deposit@ccdc.cam.ac.uk).
The authors kindly acknowledge the Chemical Analysis
Centre of the University of Tsukuba for permitting the measure-
ments of X-ray diffraction and NMR spectroscopy. Prof. T.
Nabeshima and Dr. M. Yamamura are grateful for the support
provided during quantum yield measurements. The authors
thank Prof. Y. Ootuka for the support on the digital photo-
micrographing work. J. Kuwabara kindly acknowledges the
financial support provided by the Kurata Memorial Hitachi
Science and Technology Foundation.
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Chem. Lett. 2012, 41, 65-67
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/