Phosphorescent organogels via "metallophilic" interactions for reversible RGB-color switching

Article abstract of DOI:10.1021/ja0441007

A trinuclear Au(I) pyrazolate complex bearing long alkyl chains (1) in hexane self-assembles via a Au(I)-Au(I) metallophilic interaction, to form a red-luminescent organogel (λ_em = 640 nm, λ _ext = 284 nm). Scanning electron microscop

Full text of DOI:10.1021/ja0441007

Published on Web 12/14/2004
Phosphorescent Organogels via “Metallophilic” Interactions
for Reversible RGB-Color Switching
Akihiro Kishimura,† Takashi Yamashita,‡ and Takuzo Aida*,§
Contribution from Department of Chemistry and Biotechnology, School of Engineering,
The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan; Department of Pure
and Applied Chemistry, Faculty of Science and Technology, Tokyo UniVersity of Science,
2641 Yamazaki, Noda, Chiba 278-8510, Japan; and Aida Nanospace Project, Exploratory
Research for AdVanced Technology, Japan Science and Technology Agency (JST),
2-41 Aomi, Koto-ku, Tokyo 135-0064, Japan
Received September 28, 2004; E-mail: aida@macro.t.u-tokyo.ac.jp
Abstract: A trinuclear Au(I) pyrazolate complex bearing long alkyl chains (1) in hexane self-assembles via
a Au(I)-Au(I) metallophilic interaction, to form a red-luminescent organogel (λem ) 640 nm, λext ) 284
nm). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis of an air-dried gel with 1
show the presence of heavily entangled fibers, each consisting of a rectangularly packed columnar assembly
of 1. Doping of the organogel with a small amount of Ag+ results in a blue luminescence (λem ) 458 nm,
λext ) 370 nm) without disruption of the gel, while removal of doped Ag+ with cetyltrimethylammonium
chloride results in complete recovery of the original red-luminescent gel. Upon heating, these organogels
undergo gel-to-sol transition due to the destabilization of the metallophilic interactions, where the red
luminescence of the nondoped system becomes hardly visible, while the blue luminescence of the Ag+-
doped system turns green (λem ) 501 nm, λext ) 370 nm). On cooling, these solutions undergo gelation
and synchronously recover the original luminescences. The observed RGB (red-green-blue) luminescences
are all long-lived (3-6 µs) and assigned to electronic transitions from triplet-excited states.
Scheme 1. Schematic Representation of Synthesis of Au(I)
Complex 1
Introduction
In general, phosphorescence is characterized by its longer
lifetime and higher luminescence quantum yield than fluores-
cence. Therefore, phosphorescent materials that are capable of
reversibly changing their luminescence colors are interesting
in application for sensors1 and displays.2 Although there have
been reported a variety color-tunable fluorescent materials in
response to physical and chemical stimuli, only a few examples
of phosphorescent materials with such color-tuning capabilities
have been reported,3 since compounds that can phosphoresce
at ambient temperatures are limited.4 Here we report a novel
phosphorescent organogel formed with trinuclear Au(I) pyra-
zolate complex 1 (Scheme 1) via a Au(I)-Au(I) metallophilic
interaction, where the luminescence color can be changed
† The University of Tokyo.
‡ Tokyo University of Science.
§ Japan Science and Technology Agency.
(1) Demas, J. N.; DeGraff, B. A. Coord. Chem. ReV. 2001, 211, 317-351.
(2) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151-154.
(3) Recent examples: (a) Dias, H. V. R.; Diyabalanage, H. V. K.; Rawashdeh-
Omary, M. A.; Franzman, M. A.; Omary, M. A. J. Am. Chem. Soc. 2003,
125, 12072-12073. (b) White-Morris, R. L.; Olmstead, M. M.; Balch, A.
L. J. Am. Chem. Soc. 2003, 125, 1033-1040.
(4) (a) Tsuzuki, T.; Shirakawa, N.; Suzuki, T.; Tokito, S. AdV. Mater. 2003,
15, 1455-1458. (b) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide,
T.; Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada,
S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971-12979.
(c) Coppo, P.; Plummer, A. P.; De Cola, L. Chem. Commun. 2004, 1774-
1775. (d) Wang, Q.-M.; Lee, Y.-A.; Crespo, O.; Deaton, J.; Tang, C.;
Gysling, H. J.; Gimeno, M. C.; Larraz, C.; Villacampa, M. D.; Laguna,
A.; Eisenberg, R. J. Am. Chem. Soc. 2004, 126, 9488-9489.
synchronously to thermoreversible sol-gel transition and dop-
ing/dedoping of Ag+. Metallophilic interactions are known to
operate among closed-shell group 11 metal ions such as Au(I),
Ag(I), and Cu(I), where the ∆G values with Au(I), for example,
are as large as those of hydrogen-bonding interactions.5 Another
important aspect of metallophilic interactions is that they can
provide phosphorescent solids or crystalline metal complexes,3,6
(5) Pyykko¨, P. Chem. ReV. 1997, 97, 597-636.
9
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J. AM. CHEM. SOC. 2005, 127, 179-183
179

A R T I C L E S
Kishimura et al.
some of which serve as potential EL materials.7 We have
reported that trinuclear dendritic pyrazolate complexes of group
11 metal ions self-assemble in paraffinic solvents via metallo-
philic interactions to give luminescent superhelical fibers.8 More
recently, Omary and co-workers have reported that a Cu(I) bis-
(trifluoromethyl)pyrazolate complex in frozen solvents at 77 K
self-assembles via a metallophilic interaction and becomes
phosphorescent,3a where the phosphorescence color can be tuned
by changing the solvent and concentration. Balch and co-
workers have reported similar phenomena for a Au(I) isocyanide
complex in several frozen solvents at 77 K.3b In the present
work, we synthesized a novel Au(I) pyrazolate complex carrying
long octadecyloxy chains (1), with an expectation that a strong
self-assembling nature of the paraffinic side chains could
promote the metallophilic interaction even at room temperature
in appropriate solvents. The present paper reports detailed
accounts of a study along this conception, with an emphasis on
the luminescence color switching in response to external
physical and chemical stimuli.
Figure 1. Field-emission scanning electron micrographs (FE-SEM) of an
air-dried gel (xerogel) with 1, spattered with Pt under an electric current of
15 mA at 10 Pa for 10 s.
Results and Discussion
Gelation with a Trinuclear Au(I) Pyrazolate Complex (1).
4-(3,5-Dioctadecyloxybenzyl)-3,5-dimethylpyrazole ([C18]pzH)
was synthesized by the reaction of 4-(3,5-dioctadecyloxybenzyl)-
2,4-pentanedione ([C18]acac) and hydrazine according to
Scheme 18 and unambiguously characterized by 1H NMR,
MALDI-TOF-MS spectrometry, and elemental analysis. [C18]-
pzH was allowed to react with [Au(Me2S)]Cl in THF/MeOH
containing KOH, and the resulting trinuclear Au(I) pyrazolate
complex (1) was isolated by preparative size-exclusion chro-
Figure 2. X-ray diffraction (XRD) patterns of an air-dried gel (xerogel)
with 1.
Table 1. Indexation of an Air-dried Gel with 1a
1
matography and characterized by H NMR, MALDI-TOF-MS
spectrometry, and elemental analysis.
c
c
c
c
peak no.
hkb
θobsd
θcalcd
dobsd
dcalcd
1
2
3
4
5
6
7
8
20
11
40
22
51
60
71
91
1.38
1.81
2.75
3.60
3.80
4.13
5.11
6.39
1.38
1.81
2.76
3.62
3.84
4.13
5.12
6.44
32.0
24.4
16.1
12.3
11.6
10.7
32.0
24.4
16.0
12.2
11.5
10.7
When a hexane (0.3 mL) suspension of Au(I) complex 1 (10.4
mg, 3.8 × 10-3 mmol) was once heated to 40-50 °C and
allowed to cool to room temperature, the resultant solution
turned to an opaque gel (critical gel concentration; 5 wt %).
This sol-gel transition took place thermoreversibly at around
35-40 °C. Scanning electron microscopy (FE-SEM) of an air-
dried gel (xerogel) with 1 showed the presence of heavily
entangled fibrous assemblies (Figure 1). X-ray diffraction (XRD)
analysis of the material indicated a rectangular columnar packing
with a symmetry group C2mm (Figure 2, Table 1). The lattice
parameters a and b were evaluated as 64.0 and 26.4 Å,
respectively, which are similar to those reported for crystalline
Au(I) pyrazolate complexes.9
8.65
6.92
8.64
6.87
a Rectangular C2mm symmetry, lattice parameters a ) 64.0 Å, b ) 26.4
Å. b Indexation of two-dimensional lattice. c Observed and calculated values
for diffraction angle (deg) and corresponding spacing (Å).
to room temperature, it again became a gel and recovered the
red luminescence (Figure 3b). Thus, the red luminescence can
be switched “on” (sol-to-gel) and “off” (gel-to-sol) synchro-
nously to the phase transition. This synchronous luminescence
color switching could be repeated many times without any
decay.
A trinuclear Au(I) complex with benzylimidazolate ligands,
when crystallized in the presence of Ag+, has been reported to
give a green-luminescent solid material, where Ag+ is interca-
lated into a columnar assembly of the Au(I) complex.10 In
relation to this observation, upon addition of AgOTf to a hot
hexane solution of 1, a green-luminescent, clear yellow solution
resulted, which however did not gel even on cooling, when the
ratio [AgOTf]/[1] was as high as unity. In contrast, on addition
of 0.01 equiv of AgOTf with respect to 1 (Figure 3c; [AgOTf]/
[1] ) 0.01/1), the hot solution gelled when allowed to cool to
room temperature. While the gel thus formed emitted an intense
Switching of Luminescence Colors. Upon excitation at 254
nm with a hand-held UV lamp, the organogel with Au(I)
complex 1 emitted a red luminescence (Figure 3b). On heating,
the gel turned to a clear solution and lost the red luminescence
(Figure 3a). On the other hand, when the solution was cooled
(6) (a) Yam, V. W.-W.; Lo, K. K.-W. In Multimetallic and Macromolecular
Inorganic Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Molec-
ular and Supramolecular Photochemistry; Marcel Dekker: New York, 1999;
pp 31-112, Vol. 4. (b) Forward, J. M.; Fackler, J. P., Jr.; Assefa, Z.
Photophysical and Photochemical Properties of Au(I) Complexes. In
Optoelectronic Properties of Inorganic Compounds; Roundhill, D. M.,
Fackler, J. P., Jr., Eds.; Plenum Press: New York, 1999; pp 195-229.
(7) Ma, Y.; Che, C.-M.; Chao, H.-Y.; Zhou, X.; Chan, W.-H.; Shen, J. AdV.
Mater. 1999, 11, 852-857.
(8) Enomoto, M.; Kishimura, A.; Aida, T. J. Am. Chem. Soc. 2001, 123, 5608-
5609.
(9) (a) Kim, S. J.; Kang S. H.; Park, K.-M.; Kim, H.; Zin, W.-C.; Choi, M.-
G.; Kim, K. Chem. Mater. 1998, 10, 1889-1893. (b) Barbera´, J.; Elduque,
A.; Gime´nez, R.; Lahoz F. J.; Lo´pez, J. A.; Oro, L. A.; Serrano, J. L. Inorg.
Chem. 1998, 37, 2960-2967.
(10) Burini, A.; Bravi, R.; Fackler, J. P., Jr.; Galassi, R.; Grant, T. A.; Omary,
M. A.; Pietroni, B. R.; Staples, R. J. Inorg. Chem. 2000, 39, 3158-3165.
9
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Phosphorescent Organogels via “Metallophilic” Interactions
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Figure 3. Luminescence profiles of Au(I) pyrazolate complex 1 in hexane. Pictures and schematic self-assembling structures; (a) sol, (b) gel, (c) sol
containing AgOTf (0.01 equiv), and (d) gel containing AgOTf (0.01 equiv).
Figure 4. Selected pictures (illuminated at 365 nm), taken over a period
of 24 h after the addition of (a) one drop of a THF solution of AgOTf (10
mM) onto a hexane gel prepared with 1 ([AgOTf]/[1] ) 0.01/1) and (b)
one drop of a CHCl3 solution of CTAC (10 mM) onto the resulting Ag+-
doped organogel ([AgOTf]/[CTAC] ) 1/1).
blue luminescence when excited at 365 nm (Figure 3d), a bluish-
red luminescence resulted when excited at 254 nm, due to the
presence of the original red-luminescent species in the gel. Such
Figure 5. MALDI-TOF-MS spectra (dithranol as a matrix) of (a) 1 alone
and (b) an equimolar mixture of 1 and AgOTf.
a luminescence color change accompanying the sol-gel transi-
quently, the gel became nonluminescent when excited at 365
tion was thermoreversible. We also found that removal of Ag+
nm (Figure 4b), while it recovered the original red luminescence
from the green-luminescent hot solution using cetyltrimethy-
upon excitation at 254 nm.12
lammonium chloride (CTAC) (Figure 3a), followed by cooling,
When a hexane gel prepared at [AgOTf]/[1] ) 0.01/1 (Figure
3d) was air-dried, a blue-luminescent xerogel resulted, which
adopted, as observed by XRD, a rectangular packing geometry,
identical to that formed without AgOTf (Figure 2). We also
found that 1 does not undergo metal ion scrambling with Ag+
resulted in the recovery of the red-luminescent gel (Figure 3b).11
Interestingly, when one drop of a THF solution of AgOTf
(10 mM; [AgOTf]/[1] ) 0.01/1) was put on the hexane gel
prepared with 1, the luminescence color of the upper part of
the gel changed to blue when excited at 365 nm (Figure 4a),
even in the presence of a large amount of AgOTf (e.g., [AgOTf]/
while the lower part remained nonluminescent (red-luminescent
[1] ) 1/1), where only intact 1 and its Ag+ adducts (1/Ag+ and
when λext [excitation wavelength] ) 254 nm).12 The blue-
12/Ag+) were detected by MALDI-TOF-MS spectrometry
luminescent part then spread downward without disruption of
(Figure 5). Furthermore, 1 was recovered almost quantitatively
the gel, as the result of permeation and intercalation of Ag+,
when the mixture was treated with an equimolar amount of
and finally the whole gel turned blue-luminescent after 1 day.
CTAC with respect to AgOTf.13 Owing to such a chemical
On the other hand, when one drop of a CHCl3 solution of CTAC
robustness of the trinuclear Au(I) pyrazolate complex (1), the
(10 mM; [CTAC]/[AgOTf] ) 1/1) was put on the resulting blue-
blue and green luminescence colors could be switched reversibly
luminescent gel, CTAC permeated the gel without its disruption
in response to repeated sol-gel transitions in the presence of
and converted the intercalating Ag+ ions into AgCl. Conse-
Ag+.
(11) See Supporting Information.
(12) Addition of one drop of THF or CHCl3 alone did not result in any change
of the luminescence color.
(13) Preparative TLC on a silica gel plate allowed quantitative recovery of 1,
which was identified by MALDI-TOF-MS and 1H NMR analyses.
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Kishimura et al.
Figure 7. Luminescence decay profiles at 25 °C of (a) a hexane gel with
1 (λext ) 266 nm), (b) a hexane gel with 1 containing Ag+ ([AgOTf]/[1] )
0.01/1), and (c) its solution (λext ) 355 nm) measured immediately after
the gel-to-sol transition.
while the green-luminescent solution emitted at 501 nm (green
curve, Figure 6b).16 When the hot solution was cooled and
thermostated at 25 °C, a continuous blue shift of the lumines-
cence center took place from 501 to 458 nm (black curves,
Figure 6b) as the gelation proceeded. When monitored at 458
nm, the gel displayed an excitation spectrum with a major peak
at 369 nm and a broad shoulder centered at 300 nm,11 again
indicating a large Stokes shift. Upon heating to induce the gel-
to-sol transition, the major peak in the excitation spectrum was
red-shifted to 396 nm.11 As shown by the decay curves in Figure
7b and c, the blue and green luminescences are both long-lived
with decay lifetimes of 3 and 5 µs, respectively, again indicating
that these emissions are phosphorescence.
Figure 6. (a) Emission spectra (λext ) 284 nm) of a hexane gel with 1
(red) and its solution (gray) and time-dependent spectral change at 25 °C
over a period of 30 min immediately after the sol-to-gel transition. (b)
Emission spectra (λext ) 370 nm) of a hexane gel with AgOTf/1 (0.01/1)
(blue) and its solution (green) and time-dependent spectral change at 25
°C over a period of 120 min immediately after the sol-to-gel transition.
The spectra were taken every 3 min.
Spectroscopy and Lifetime Analysis. The red light-emitting
gel formed with 1, upon excitation at 284 nm, showed a
luminescence centered at 640 nm (red curve, Figure 6a).14 On
heating to induce the gel-to-sol transition, the emission was red-
shifted to 704 nm and became weak (gray curve, Figure 6a), so
that it was hardly visible for human eyes (Figure 3a). On the
other hand, when the hot solution was cooled to 25 °C and held
at the same temperature, the gelation progressed gradually and
a blue shift of the luminescence center continuously from 704
to 640 nm followed (black curves, Figure 6a). The gel finally
formed displayed an excitation band centered at 286 nm,11 which
is far apart from the luminescence band at 640 nm (Stokes shift
∆λ ) 354 nm). Analogous large Stokes shifts have been
reported for solid materials involving a Au(I)-Au(I) metallo-
philic interaction, such as [Au(dmpz)]3 (dmpz ) 3,5-dimeth-
ylpyrazolate) (299/680 nm, ∆λ ) 381 nm) and [Au(4-Me-pz)]3
(4-Me-pz ) 4-methylpyrazolate) (230/631 nm, ∆λ ) 401 nm).15
Analysis of the emission decay profile of the gel (Figure 7a)
gave a lifetime of 6 µs, indicating that this emission is
phosphorescence assignable to a triplet metal-centered excited-
state modified with a Au(I)-Au(I) metallophilic interaction.6
We also investigated the luminescence properties of the Ag+-
containing system ([AgOTf]/[1] ) 0.01/1) before and after the
gel-to-sol transition and found that the blue-luminescent gel
emitted at 458 nm (blue curve, Figure 6b; λext ) 370 nm),14
Due to the opacity, the physical gels prepared with 1 alone
and 1/AgOTf (1/0.01) gave absorption spectra that were
contaminated with scattered lights. On the other hand, we
successfully obtained a qualified absorption spectrum of the
hexane solution of 1 by a quick measurement before the gelation
started. As shown in Figure 8a, the spectrum showed only an
intense absorption band at λ < 300 nm, which is mostly due to
the aromatic parts of 1. Likewise, we could measure the
absorption spectrum of the green-luminescent solution of a
mixture of 1 and AgOTf (1/0.01) before the gelation (Figure
8b), which showed a weak absorption band at around 400 nm
in addition to the intense absorption band due to 1. Such a weak
absorption band, assignable to a Au(I)-Ag(I) heterometallic
species, was pronounced when an equimolar amount of AgOTf
with respect to 1 was added (Figure 8c). While the system at
this stage lost the ability of forming a gel (vide ante), the
absorption spectrum was essentially identical to the excitation
spectrum, indicating that the heterometallic Au(I)-Ag(I) species
is the origin of the 500-nm green phosphorescence.
Secondary Effects of Self-Assembly on Luminescences. As
described in the above section, the gelation processes with 1
alone and 1/Ag+ (1/0.01) are both accompanied by a continuous
blue shift in their luminescence spectra. This trend appears to
be incompatible with a general consideration that growth of the
(14) The emission spectrum did not change when the gel was air-dried.
(15) Yang, G.; Raptis, R. G. Inorg. Chem. 2003, 42, 261-263.
(16) Without Ag+ doping, the hexane gel with 1 alone did not emit at 458 nm,
irrespective of whether it was excited at 254 or 365 nm.
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Phosphorescent Organogels via “Metallophilic” Interactions
A R T I C L E S
geometry15 possibly due to a secondary interaction among the
long alkyl chains. Consequently, the gelation is likely ac-
companied by an increase in the HOMO-LUMO gap.
Conclusions
We have reported the first phosphorescent organogels via
metallophilic interactions, using a trinuclear Au(I) pyrazolate
complex (1) carrying long C18 alkyl chains, where the sol-
gel transition, coupled with Ag+ doping/dedoping, allows
reversible switching of the RGB luminescence color. While
related systems so far reported are operative under frozen or
solid-state conditions to stabilize metal-metal bonds, our
strategy makes use of the strong self-assembling nature of the
ligand, which enables metallophilic interactions even at ambient
temperatures. Although several fluorescent materials that can
switch their luminescence colors in response to phase transitions
have been reported,18-20 they are mostly utilizing changes in
orientation or association of organic dye molecules for lumi-
nescence color switching. Thus, our system is essentially
different from these fluorescent systems in terms of the
switching mechanism of the luminescence color. Considering
also the easy processability of soft materials, application of our
phosphorescent system to imaging media is one of the interesting
subjects worthy of further investigation.
Figure 8. Electronic absorption spectra at 25 °C of hexane solutions of
(a) 1 (5 wt %) and mixtures of 1 with AgOTf at [AgOTf]/[1] ) (b) 0.01/1
and (c) 1/1. Samples a and b were measured immediately after the gel-to-
sol transition. The spectra were not normalized.
self-assembly via metallophilic interactions should result in
lowering the HOMO-LUMO gap.17 In fact, the luminescence
of a nongelling Cu(I) pyrazolate complex has been reported to
be red-shifted when the solution is concentrated to allow
propagation of the metallophilic interaction.3a We also observed
that a reference complex without long alkyl chains such as
[Au(MeO-pz)]3 (MeO-pz ) 4-(3,5-dimethoxybenzyl)-3,5-dim-
ethylpyrazolate), in the presence of an equimolar amount of
AgOTf, shows a red shift of the luminescence from 485.5 to
500.5 nm when it is solidified from a solution.11 Thus, the blue
shift of the luminescence upon growth of the self-assembled
structure is unique to our sol-gel systems. We consider that,
as the fibers grow and form a bundle with one another, the
luminescent species would be forced to adopt a distorted
Acknowledgment. A.K. thanks the JSPS Young Scientist
Fellowship.
Supporting Information Available: Full experimental details,
an emission spectrum of an organogel with 1/Ag+ (1/0.01) after
treatment with CTAC, excitation spectra of organogels and a
solution of 1/Ag+ (1/0.01), emission spectra of [Au(MeO-pz)]3
in CHCl3/THF and its solid. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA0441007
(18) Recent examples of color-tunable fluorescent materials: (a) Davis, R.; Rath,
N. P.; Das, S. Chem. Commun. 2004, 74-75. (b) Gime´nez, R.; Pin˜ol, M.;
Serrano, J. S. Chem. Mater. 2004, 16, 1377-1383.
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S. J.; Praveen, V. K. Angew. Chem., Int. Ed. 2003, 42, 332-335. (b)
Sugiyasu, K.; Fujita, N.; Takeuchi, M.; Yamada, S.; Shinkai, S. Org.
Biomol. Chem. 2003, 1, 895-899. (c) Ryu, S. Y.; Kim, S.; Seo, J.; Kim,
Y.-W.; Kwon, O.-H.; Jang, D.-J.; Park, S. Y. Chem. Commun. 2004, 70-
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