Luminescence tribochromism and bright emission in gold(I) thiouracilate complexes

Article abstract of DOI:10.1021/ja034560k

New dinuclear Au(I) complexes containing bridging thiouracilate and bis(diphenylphosphino)methane ligands have been synthesized and characterized structurally and spectroscopically. The compounds exhibit a unique behavior of solid-state luminescence tribochromism in which photoemission turns on upon gentle grinding of the sample and a sensitivity to pH in fluid solution. The emissive form in the solid state exhibits a bright blue or cyan emission upon irradiating at 375 nm. Structural studies show that the nonemissive form of the complexes has an extended helical ···Au···Au···Au··· structure in the solid with weak aurophilic interactions, whereas the blue emissive form has a strong intermolecular aurophilic interaction in the solid that leads to an arrangement of dimers of dinuclear (Au₂) complexes. Interconversion between the two forms can be carried out by either recrystallization for solid-state samples or by exposure to vapors of volatile acid or base for fluid solutions of the complexes. Copyright

Full text of DOI:10.1021/ja034560k

Published on Web 06/05/2003
Luminescence Tribochromism and Bright Emission in Gold(I) Thiouracilate
Complexes
Young-A Lee and Richard Eisenberg*
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627
Received February 7, 2003; E-mail: eisenberg@chem.rochester.edu
Over the past two decades, interest in gold(I) complexes has been
stimulated by their tendency to aggregate through closed shell
“aurophilic” interactions and their consequent luminescence
properties.1-7 Aurophilic interactions also play a key role in
determining the solid-state structures that many gold(I) compounds
exhibit.4,8-13 The emissive properties of certain Au(I) compounds
have been found to be sensitive to environmental factors such as
volatile organic vapors,14 making them of interest in possible sensor
applications. The present study describes a new set of compounds
that exhibit a unique property among metal complexes in which
luminescence can be switched on under gentle grinding of a solid
sample. This phenomenon, termed luminescence tribochromism,15,16
leads to bright blue or cyan photoemission from the sample. In
Figure 1. (a) View of cationic 1a (phenyl rings omitted) and (b) helical
contrast with triboluminescence which represents transient emission
upon sample grinding or crushing,17,18 luminescence tribochromism
is a sustained change in the photoemission spectrum upon the initial
application of pressure. The process can be reversed through re-
crystallization, and the luminescence can be turned on or off through
control of acidity as well. The compounds reported here are Au(I)
thiouracilate complexes containing bis(diphenylphosphino)methane
(dppm), and their luminescence relates to the different types and
degrees of aurophilic interaction that the solid-state structures reveal.
The title complexes are prepared by the reaction of (µ-dppm)-
Au2(CF3COO)2 with 2-thiouracil (TU) and 6-methyl-2-thiouracil
(Me-TU) to give 1a and 1b, respectively. Exchange of CF3COO-
for other simple anions is accomplished readily by metathesis with
arrangement of Au(I) ions with ligands omitted for clarity.
-
NaY where Y- ) NO3-, ClO4-, Au(CN)2 yielding colorless
crystals exhibiting IR bands characteristic of Y- as well as the dppm
and thiouracilate ligands. The crystal structures of 1a and 1b have
been determined and the cation of 1a is shown in Figure 1a with
selected bond distances and angles tabulated in Supporting Informa-
tion.19 In these structures, the thiouracilate and dppm ligands bridge
two Au(I) ions, each having a linear coordination geometry and a
Au‚‚‚Au intramolecular distance of 2.8797 (4) Å. In 1a, the
CF3COO- anion refines satisfactorily but with large temperature
factors and is H-bonded to the TU ligand protonated at the
uncoordinated N atom. Most striking is the extended arrangement
of the dinuclear cationic complexes that forms an aurophilic helix
(Figure 1b) with hydrophilic thiouracilates stacked in the center of
the helix and hydrophobic dppm ligands on the helix exterior.
Neighboring dinuclear (i.e., Au2) cations link in a head-to-tail
arrangement based on the TU ligand with an intermolecular
Au‚‚‚Au distance of 3.3321 (5) Å. The basic structural arrangement
for 1b is similar but the aurophilic helix is discontinuous with one
Au‚‚‚Au separation of 4.344 Å for a head-to-head connection (the
others are head-to-tail and average 3.236-3.354 Å).
Figure 2. Emission spectra of nonemissive (black line) and crushed crystals
(blue line) of 1b, CH2Cl2 solution (- - -) of 1b and crystals (‚‚‚‚‚‚) of 2b
(blue form) at room temperature (at λex ) 375 nm).
bright blue or cyan photoluminescence at ambient temperature. The
emission spectrum of this form, shown in Figure 2 for the R ) Me
derivative, exhibits a single band at 483 nm and a corresponding
lowest-energy excitation maximum at 375 nm. However, powder
X-ray diffraction patterns of polycrystalline and crushed samples
of 1 reveal that no gross phase change transpires upon crushing
(see Supporting Information). Derivatives with different Y-anions
exhibit similar emission behavior upon crushing. For solid 1, gentle
heating (37 °C) for 5-6 h or sonication in a closed vial also gives
conversion to the emissive form (2) with release of acid as detected
using moist pH paper above the solid.
Brightly emissive 2 can be prepared directly by stirring CH2-
Cl2/MeOH solutions of 1 over Na2CO3 followed by filtration and
crystallization to yield pale yellow crystals of X-ray quality.
Analytical and spectroscopic data support the formulation of 2 as
the neutral dinuclear complex in which the thiouracil is doubly
deprotonated. The structure of 2b (Figure 3) shows the same
µ-thiouracilate, µ-dppm pattern as that for 1a and 1b but a vastly
different packing arrangement. The intermolecular arrangement is
that of dimers held together by a strong aurophilic interaction of
The complexes [Au2(µ-TU)(µ-dppm)]Y and the Me-TU ana-
logues are either nonemissive or weakly photoluminescent. For
1a, the weak emission is white in color indicative of multiple
emissions. When solid samples of 1 are gently crushed under a
spatula, a dramatic conversion occurs to give samples exhibiting
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C O M M U N I C A T I O N S
with its dimeric structure, the intermolecular interaction is strong,
and the Au‚‚‚Au‚‚‚Au‚‚‚Au arrangement, more linear. Application
of pressure to 1 appears to induce cleavage of the weakest links of
the Aun helix, rearrangement to dimers, and release of volatile acid.
The systems reported here thus represent an interesting and exquisite
balance of aurophilic interactions and acid-base properties that
confer on these systems their unique luminescence tribochromism.
Other related systems exhibiting this property will be reported
separately.
Acknowledgment. We thank the donors of the Petroleum
Research Fund, the National Science Foundation (CHE 020718),
and the Eastman Kodak Company for support of this work, Dr.
Rene Lachicotte for experimental help and Drs. Henry Gysling,
Joe Deaton, and Ching Tang of the Eastman Kodak Research
Laboratories, and Professors John Fackler (Texas A&M) and
Mohammad Omary (University of North Texas) for helpful
discussions.
Figure 3. Perspective view of 2b. The phenyl rings on phosphorus atoms
are omitted for clarity.
2.9235 (4) Å and a head-to-head orientation of the thiouracil groups.
The thiouracil carbonyl groups serve to block further aggregation
along the Au‚‚‚Au direction. The emission spectra of 2a and 2b
Supporting Information Available: X-ray crystallographic data
in CIF format for 1a, 1b and 2b. Tables of crystallographic parameters
and selected distances and angles for 1a, 1b and 2b; characterization
and photophysical data for 1 and 2 (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
max
obtained as above show single bands at the same λem as those
seen for the crushed samples of 1, albeit with slightly narrower
band profiles (fwhm ) 58 and 73 nm for the Me derivative).
Excited-state lifetimes for 2a and 2b, synthesized directly or by
crushing 1, are 2-4 µs at 298 and 77 K. Conversion back to 1 is
achieved by recrystallization of 2 from solutions to which trifluo-
roacetic acid is added, while exposure of solid 2 to CF3COOH vapor
leads very slowly to loss of luminescence. The latter process is
reversed upon exposure to NEt3 vapor.
References
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Emission measurements in room-temperature fluid solution
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max
1 are blue emissive (λem
) 489 nm) whereas corresponding
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solutions of 2 are nonemissive. Upon exposure to NEt3 vapor, the
blue-emissive solutions of 1 become nonemissive as acidity
decreases, but a further decrease yields a modest return of emission
at the same λemmax. The process is reversed when CF3COOH is
used and can be carried out repetitively with no loss of emission
intensity. The probable site of protonation/deprotonation in 1 and
2 is the uncoordinated pyrimidine nitrogen atom as evidenced by
H-bonding in the structures and similar thiouracil carbonyl bond
lengths in 1 and 2. Variation of the complex concentration from 1
× 10-2 to 1 × 10-5 M in CH2Cl2/toluene (v/v ) 1/1) shows that
emission intensity does not increase linearly with concentration,
suggestive of a possible equilibrium between emissive and non-
emissive forms.
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In CH2Cl2/toluene frozen glass at 77 K, complex 1b exhibits
dual emission bands at λemmax at 440 and 486 nm. The lower-energy
band is more sensitive to concentration than the higher-energy band
and is attenuated in dilute frozen solution (see Supporting Informa-
tion). Despite differences in the fluid solution emission, qualitatively
similar emission results are obtained for 2b in frozen glass at 77 K
(19) X-ray data were collected at 193 K on a standard Siemens SMART CCD
diffractormeter with Mo KR radiation. Crystal data for 1a (colorless
form): tetragonal I4(1)/a, a ) 33.3063(15) Å, b ) 33.3063(15) Å, c )
13.8485(9) Å, V ) 15362.3(14) Å3, Z ) 16, d(calcd) ) 1.891 Mg/m3,
no. of independent data ) 9162, no. of variables ) 442, R1 ) 0.0516 [I
> 2σ(I)], wR2 ) 0.1441. 1b (colorless form): triclinic P1h, a ) 13.8976-
(12) Å, b ) 21.5548(19) Å, c ) 26.928(2) Å, R ) 84.383(2), â ) 77.259-
(2) °, γ ) 83.480(2), V ) 7794.9(12) Å3, Z ) 2, d(calcd) ) 1.822 Mg/
m3, no. of independent data ) 31312, no. of variables ) 1720, R1 )
0.0556 [I > 2σ(I)], wR2 ) 0.1472. 2b (blue form): monoclinic P2(1)/c,
a ) 12.3737(6) Å, b ) 29.1208(14) Å, c ) 17.4858(8) Å, â ) 93.6570-
(10) °, V ) 6287.9(5) Å3, Z ) 4, d(calcd) ) 2.031 Mg/m3, no. of
independent data ) 14818, no. of variables ) 729, R1 ) 0.0424 [I >
2σ(I)], wR2 ) 0.0795.
max
with emission bands at λem ) 450 and 485 nm.
The emission properties of 1 and 2 in the solid state are clearly
affected by the interplay of thiouracilate protonation and the
different aurophilic interactions exhibited by these systems. We
propose that the latter leads to the unique luminescence tribo-
chromism reported here. In nonemissive 1 with its helical structure,
the intermolecular Au‚‚‚Au interactions are weak, and the extended
Au‚‚‚Au interactions are severely kinked, whereas in emissive 2
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