Reversible mechanochromic luminescence of [(C6F 5Au)2(μ-1,4-diisocyanobenzene)]

Article abstract of DOI:10.1021/ja8019356

Reversible mechanochromic luminescence of [(C₆F₅Au)₂(μ-1,4-diisocyanobenzene)] is reported. Grinding of the complex induced a photoluminescent color change, which was restored by exposure to a solvent. This cycle was repeated 20 times with no color degradation in the emissions. Their optical properties, X-ray crystallographic analysis, IR, and XRD measurements strongly suggested that the change in the molecular arrangement is responsible for this mechanochromic property. Intermolecular aurophilic bondings presumably play a key role in the altered emission. Copyright

Full text of DOI:10.1021/ja8019356

Published on Web 07/10/2008
Reversible Mechanochromic Luminescence of
[(C6F5Au)2(µ-1,4-Diisocyanobenzene)]
Hajime Ito,* Tomohisa Saito, Naoya Oshima, Noboru Kitamura, Shoji Ishizaka, Yukio Hinatsu,
Makoto Wakeshima, Masako Kato, Kiyoshi Tsuge, and Masaya Sawamura*
Department of Chemistry, Faculty of Science, Hokkaido UniVersity, Sapporo 060-0810, Japan
Received March 15, 2008; E-mail: sawamura@sci.hokudai.ac.jp; hajito@sci.hokudai.ac.jp
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This paper contains enhanced objects available on the Internet at http://pubs.acs.org/jacs.
Mechanochromism is the phenomenon of color change due to me-
chanical grinding or pressing of a solid sample, and the subsequent
reversion to its original color through treatment such as heating or
recrystallization.1 Photoluminescent compounds that possess mechano-
chromic properties in their solid-state emissions can provide unique
recording or sensing materials that involve luminescence detection.
Unfortunately, reports that describe such mechanochromic, photolumi-
nescent compounds have been limited.2,3 Herein, we report the mecha-
nochromic luminescence behavior of [(C6F5Au)2(µ-1,4-diisocyanoben-
zene)] (1)sthe photoluminescence of the solid, upon grinding, undergoes
a drastic change and, upon exposure to solvents or its vapor, is restored to
its original color.4–6 Optical properties, powder X-ray diffraction (XRD),
IR measurements, and X-ray crystallographic analysis strongly suggest
that the mechanochromic property is attributable to changes in the
molecular arrangement, rather than molecular structure, of the Au(I)
complexes in the solid state.7 Presumably, intermolecular aurophilic
interactions are responsible for the altered emission of the ground samples.5
Our studies help establish 1 as a notable example of a Au(I) complex
with solid-state photoemission that can be reversibly switched by external
stimului.6,8
Figure 1. Photographs showing Au(I) complex 1 on an agate mortar under UV
irradiation with black light (365 nm), unless otherwise noted: (a) Au(I) complex 1
powder after grinding the right-half with a pestle, (b) a same sample under ambient
light, (c) entirely ground powder of 1, (d) partial reversion to the blue luminescence
by dropwise treatment using dichloromethane onto the center of the ground powder,
(e) powder after treatment with dichloromethane, and (f) repetition of the yellow
emission by scratching the powder with a pestle (see video 1 for a movie of the
grinding process).
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video 1 shows the grinding process.
The synthesis of 1 involved the reaction of C6F5Au(tht) (tht: tetrahy-
drothiophene) with 1,4-diisocyanobenzene in dichloromethane. The result-
ing precipitate was collected by filtration and dried under reduced pressure
to give analytically pure 1 as an air-stable white powder (92% yield), which
is sparingly soluble in common solvents.
Figure 2. (a) Absorption and (b) emission (excited at 260 nm) spectra of 1 in
various states.
To our surprise, upon gentle grinding of 1 using a spatula or a pestle,
the blue luminescence that results from UV irradiation at 365 nm changes
to an intense yellow luminescence (Figure 1a). Under ambient light,
however, color changes were not detected (Figure 1b). Upon treatment
of the ground powder with drops of dichloromethane, the yellow
luminescence reverted to the original blue color (Figure 1c,d). Evaporation
of the solvent afforded the original blue luminescent solid (Figure 1e).
Repetitions of the blue-to-yellow conversion by subsequent regrinding
(Figure 1f) did not exhibit any degradation in the luminescence, even after
20 cycles. Furthermore, reversion from the yellow- to the blue-emitting
solid was achieved by the addition of various solvents such as ethyl acetate,
diethyl ether, and chloroform, as well as exposure to the vapor of
dichloromethane.
gray powder, which exhibited a new red-shifted absorption band ranging
max
from 340 to 450 nm and a broad structureless emission band with λem
at 533 nm. Compared to the unground sample, the emission of the ground
sample exhibited an increase in the quantum yield, along with a shorter
lifetime [Φem ) 0.19, τ1 ) 0.18 µs (A1 ) 0.54), τ2 ) 0.45 µs (A2 )
0.46)]. The solid resulting from treatment with dichloromethane followed
by drying under reduced pressure exhibited an emission band similar to
that of the unground sample. A dilute solution of 1 in dichloromethane
(0.025 mM) exhibited a broad absorption below 320 nm and an emission
band (418, 445, 464 nm, Φem ) 0.01, τ ) 6.1 µs) comparable to those
of the unground sample.
Absorption and emission spectra of 1 in various states are shown in
Figure 2.9,10 The absorption spectrum of the unground sample of 1 was
observed as a broadband, ranging from 250 to 350 nm, whereas the
emission spectra showed a structured profile with a maximum wavelength
(λemmax) at 415 nm, accompanied by weaker vibronic bands at ca. 437,
445, and 459 nm [Φem ) 0.09, lifetime: τ1 ) 9.7 µs (A1 ) 0.32), τ2 )
71.2 µs (A2 ) 0.68)]. Thorough grinding of the sample gave a yellowish
Because elemental analysis of the thoroughly ground solid and the solid
after treatment with dichloromethane gave expected values (based on the
formula of 1), the evaporation step in our process was able to completely
remove all traces of the solvent, which is in contrast to other common
solvato- or vapochromism.11 Both unground and thoroughly ground
samples, after heating at 100 °C for 10 min, exhibited comparable emission
spectra. Differential scanning calorimetery (DSC) measurements, over the
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2008 American Chemical Society

C O M M U N I C A T I O N S
the blue luminescent microcrystalline powder into a metastable amorphous
phase, with aurophilic interactions that are responsible for the lower energy
emission. Next, upon treatment of the ground sample with a solvent, the
amorphous phase rearranges into the more stable crystalline phase via a
partial dissolution and recrystallization process. Within the crystals of 1,
the planarity of the molecular structure and the lack of strong intermolecular
forces (such as C-H· · ·π interaction) would allow slipping of the
molecular stacks by mechanical stimulus.
Figure 3. Partial view of the crystal packing of a blue luminescent crystal of 1.
Thermal ellipsoids were drawn at 50%-probability level. Crystal data: monoclinic,
P21/c (No. 14), a ) 5.190(2) Å, b ) 17.154(6) Å, c ) 10.861(4) Å, ꢀ ) 90.83(3)°,
V ) 966(2) Å3, Z ) 2. T ) 296.2 K. 2θmax ) 55.0+, R ) 0.0355 (I > 2.00σ(I)),
Rw ) 0.0976, GOF ) 1.007.
Detailed studies into the mechanism of the reversible mechanochromic
luminescence along with the optical properties of structural variants of 1
are currently underway.
Acknowledgment. We thank Prof. T. Inabe for the X-ray analysis,
and Prof. K. Konishi and Dr. Y. Tajika for the thermal analysis. This
work was partially supported by a Grant-in-Aid for Scientific Research
on Priority Areas (“Synergistic Effects for Creation of Functional
Molecules”) from the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
Supporting Information Available: Experimental procedures for the
synthesis of 1, compound characterization data, and crystallographic data in
CIF format are available free of charge via the Internet at http://pubs.acs.org.
Figure 4. IR and XRD spectra of 1 in various states.
References
temperature range of 25-200 °C, did not indicate any phase transitions.
Upon heating at 200 °C for 10 min, however, the color of both samples
turned into pale red (under ambient light), indicating thermal decomposi-
tion. The emission spectra of the pale red unground sample remained
unchanged, whereas that of the ground sample showed partial reversion
to the blue emission.
Recrystallization of 1 from chlorobenzene afforded single crystals
suitable for X-ray structure analysis (Figure 3). Because the shortest
Au-Au distance of 5.19 Å was beyond the range of significant aurophilic
bonding (2.7-3.3 Å),5 aurophilic interactions were absent within the
crystal. The emission spectrum of the crystals was nearly identical to that
of the unground powder (Figure 2b).
The blue and yellow emissions are attributable to distinct mechanisms.
Similarity among the emission spectrum of the unground powder, the
single crystal, and the dichloromethane solution of 1 suggests that the solid-
state blue emissions from these samples occurred from single molecules.
On the basis of the long emission lifetime, and the structured profile of
the emission band, this blue luminescence can be attributed to the
phosphorescence from the intraligand-localized π-π* excited state.12 On
the other hand, the broad and red-shifted emission from the ground sample
appears to arise from an amorphous material in which aurophilic
interactions may be responsible for the new emissive state.4,5,8
The IR spectrum of the unground solid exhibited a single absorption
(2220 cm-1) that corresponds to the isocyanide N≡C stretching (Figure
4a). In contrast, two absorbances (2218 and 2208 cm-1) were observed
for the ground solid, which indicate that, upon grinding, considerable
changes occur in the coordination mode of the isocyanide ligand to the
Au(I) atom, presumably due to the formation of aurophilic bonds.13 As
observed in the emission properties, the IR spectrum of the sample, upon
treatment with dichloromethane, reverted to that of the unground solid.
The powder X-ray diffraction (XRD) pattern of the unground sample
showed clear reflection peaks, in good agreement with the simulated pattern
of the single crystals of 1 (Figure 4b). In contrast, the XRD pattern of the
ground sample showed significant decreased peak intensities with increased
peak widths, which indicates that the grinding causes crystal-to-amorphous
phase conversion. Upon treatment of the ground sample with dichlo-
romethane, the reflection peaks were restored,14 which indicates reversion
from the amorphous to the crystalline phase.
(1) For an IUPAC technical report on terminologies related to mechanochromism,
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For a review, which concerns mechanochemistry and mechanochromism,
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2948.
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(b) Assefa, Z.; Omary, M. A.; McBurnett, B. G.; Mohamed, A. A.;
Patterson, H. H.; Staples, R. J.; Fackler, J. P. Inorg. Chem. 2002, 41, 6274–
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H.; Sasaki, T.; Kanesato, M. Chem. Mater. 2005, 17, 50–56. (d) Schneider,
J.; Lee, Y. A.; Perez, J.; Brennessel, W. W.; Flaschenriem, C.; Eisenberg,
R. Inorg. Chem. 2008, 47, 957–968.
(3) For luminescence mechanochromism of organic compounds, see: (a) Sagara,
Y.; Mutai, T.; Yoshikawa, I.; Araki, K. J. Am. Chem. Soc. 2007, 129, 1520–
1521. (b) Mizuguchi, J.; Tanifuji, N.; Kobayashi, K. J. Phys. Chem. B 2003,
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Y.; Tsuji, H.; Sakai, D.; Kikuchi, J. Langmuir 2005, 21, 976–981.
(4) For examples of luminescent Au(I) complexes, see: (a) Yam, V. W. W.;
Lo, K. K. W. Chem. Soc. ReV. 1999, 28, 323–334. (b) King, C.; Wang,
J. C.; Khan, M. N. I.; Fackler, J. P. Inorg. Chem. 1989, 28, 2145–2149. (c)
Gussenhoven, E. M.; Fettinger, J. C.; Pham, D. M.; Malwitz, M. M.; Balch,
A. L. J. Am. Chem. Soc. 2005, 127, 10838–10839.
(5) For reviews of aurophilic interactions, see: (a) Schmidbaur, H. Gold Bull.
2000, 33, 3–10. (b) Pyykko¨, P. Angew. Chem., Int. Ed. 2004, 43, 4412–
4456.
(6) Eisenberg and co-workers reported grinding-induced changes in the
photoluminescence of Au(I) complexes that accompany the change of the
molecular structure through a chemical reaction (see ref 2a). Fackler and
co-workers reported the enhancement of the emission intensity upon
grinding the crystals of a Au(I) complex (see ref 2b).
(7) A design principle that relies on an aromatic core with intermolecular
hydrogen-bonding sites was proposed for the mechanochromic fluorescent
materials (see ref 3a).
(8) For an example in which the emission mode of a Au(I) complex was
switched by external stimulus in the solution, see: Yam, V. W. W.; Li,
C. K.; Chan, C. L. Angew. Chem., Int. Ed. 1998, 37, 2857–2859.
(9) Absorbance spectra of solid samples were recorded using a spectrometer
equipped with an integrating sphere.
(10) Repeated pressing and thorough grinding were required for complete
conversion. The lightly ground, yellow emissive parts in Figure 1 show
very minor changes in the IR spectra and are partially amorphous (see
Supporting Information).
(11) For examples, see: (a) Kato, M. Bull. Chem. Soc. Jpn. 2007, 80, 287–294.
(b) Mansour, M. A.; Connick, W. B.; Lachicotte, R. J.; Gysling, H. J.;
Eisenberg, R. J. Am. Chem. Soc. 1998, 120, 1329–1330.
(12) For related complexes, see: Bayon, R.; Coco, S.; Espinet, P. Chem.-Eur.
J. 2005, 11, 1079–1085.
(13) For an example of red-shift in the Nt C stretching band for coordinated
isocyanides in a Au(I) complex through aurophilic interactions, see: White-
Morris, R. L.; Olmstead, M. M.; Balch, A. L. J. Am. Chem. Soc. 2003,
125, 1033–1040.
(14) The broadening of the peaks indicates that the particle sizes of the restored
solid are smaller than those of the unground sample.
On the basis of our results, the mechanism of the mechanochromic
process of 1 can be summarized as follows: First, the grinding transforms
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