Combining aurophilic interactions and halogen bonding to control the luminescence from bimetallic gold-silver clusters

Article abstract of DOI:10.1021/ja909241m

(Figure Presented) The luminescence in a series of new bimetallic gold-silver vapochromic structures can be efficiently switched among different colors simply by exposure to solvent vapors. The emission color in these systems is controlled by both aurophilic interactions and halogen bonding, which affect the emission energy through different orbitals. Copyright

Full text of DOI:10.1021/ja909241m

Published on Web 12/23/2009
Combining Aurophilic Interactions and Halogen Bonding To Control the
Luminescence from Bimetallic Gold-Silver Clusters
Antonio Laguna,† Tania Lasanta,‡ Jose´ M. Lo´pez-de-Luzuriaga,*,‡ Miguel Monge,‡ Pancˇe Naumov,§
and M. Elena Olmos‡
Departamento de Qu´ımica Inorga´nica, Instituto de Ciencia de Materiales de Arago´n, UniVersidad de
Zaragoza-CSIC, 50009 Zaragoza, Spain, Departamento de Qu´ımica, UniVersidad de la Rioja, Grupo de
S´ıntesis Qu´ımica de La Rioja UA-CSIC Complejo Cient´ıfico Tecnolo´gico, 26001 Logron˜o, Spain, and
Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity, 2-1 Yamada-oka,
565-0871 Suita, Osaka, Japan
Received October 30, 2009; E-mail: josemaria.lopez@unirioja.es
Since the introduction of the term aurophilicity by Schmidbaur
in 19881 to explain the short distances between gold(I) atoms
observed in the solid state, the increasing amount of experimental
evidence of the existence of the aurophilic interaction (AI) in gold
complexes has invoked numerous studies that have provided the
basis for a description of important relativistic effects. The concept
was later expanded to include interactions between other metals in
which the metal atoms are separated by less than the van der Waals
Figure 1. (a) Solid-state spectra and (b-e) mechanochromism of 2 (b, d)
limit; typically, these include the combination of gold(I) with other
before and (c, e) after grinding (dashed line, absorption; solid line, emission).
closed-shell metals or between metals with either identical (d10-d10)
(f) Luminescence of excited 2 (before grinding), 3, 4a, and 5.
or different (e.g., s2-d8, d8-d10) configurations.2 In fact, the
theoretical treatments have regarded the AI as a more general
phenomenon that includes a significant contribution from dispersion
forces reinforced by relativistic effects, pointing out the gold-
containing structures as a special case where both of these two
components are at their maximum.3 In addition to the fundamental
relevance of the AI, the related heterometallic complexes are of
interest because of their unconventional photophysical properties,
most notably their very efficient luminescence.4 The halogen bond
(XB),5 on the other hand, has attracted much attention recently as
a relatively unexploited intermolecular interaction. It appears as a
notably strong (5-180 kJ ·mol-1) and directional interaction
D· · · X-Y (Y ) C, N, halogen, etc.) between a halogen atom (X)
and a Lewis base as an electron donor (D). The XB is of critical
relevance to the structure and properties of halogen-containing
systems and has already been successfully applied in the fields of
crystal engineering, superconductors, liquid crystals, and analytical
separation as well as to explain substrate-receptor interactions.5
To provide a qualitative assessment of the relative importance of
AIs and XBs in determining the luminescence from gold-containing
metal clusters, here we combined these interactions in a series of
novel bimetallic Au-Ag structures. We utilized the 4-C6F4I group
as a ligand because of the strong affinity and lack of steric hindrance
of the p-iodine for halogen bonding. Some of the new materials
exhibit remarkable mechanochromic and luminescent properties;
moreover, they are vapochromic, and the luminescence color can
be switched rapidly by short exposure to solvent vapors.
Scheme 1. (left) Vapochromic Reactions of 2-5 and (right)
Dimer-Polymer Equilibrium of the [Au2Ag2(4-C6F4I)4] Cluster
here seems to be different from that described by Lee and Eisenberg,6b
where the luminescence tribochromism was related to a chemical
reaction. Notably, solid 2 is selectively vapochromic toward coordinat-
ing solvents: whereas Et2O and toluene do not react, brief exposure to
Me2CO, THF, or MeCN vapors results in a rapid color change. The
products, which were obtained as pure phases by recrystallization from
the respective solvents and characterized by X-ray diffraction (Figures
S2-S5) are bright-colored: the solvates with Me2CO (3), THF (4),
and MeCN (5) are dark-red, red, and orange, respectively, and they
emit vivid red, dark-red and bright-yellow light (Figure 1e and Table
1). From THF, two structural isomers were obtained: 4a (the
thermodynamic isomer) and 4b (the kinetic isomer). The compositions
of the solvates were established as [Au2Ag2(4-C6F4I)4L2]n ·xL, where
L ) Me2CO (x ) 2; 3), THF (x ) 0; 4), and MeCN (x ) 0; 5). Similar
to 2, compounds 3 and 4 are vapochromic; the luminescence color
can be switched rapidly by exposure to solvent vapors according to
the reactions shown in Scheme 1. In situ single-crystal XRD showed
loss of crystallinity, evidencing large structure perturbations. The XRD
analyses of 3-5 revealed that all of the structures are based upon the
Au2Ag2(4-C6F4I)4 cluster. However, whereas in 3, 4a, and 5 the clusters
are connected into infinite polymeric chains by Au-Au interactions,
in the kinetic isomer 4b, they exist as monomers that are connected to
each other by I · · ·Au interactions (Figure 2).
The reaction of Bu4N[Au(4-C6F4I)] (1) with AgClO4 afforded the
precursor [Au2Ag2(4-C6F4I)4]n (2) as creme-colored solid [details of
the syntheses are described in the Supporting Information (SI)]. When
as-obtained 2 is exposed to air, it starts to emit bright-yellow light
(Figure S1 in the SI). The emission color of 2 can also be switched
by shear-induced stress: partial amorphization by grinding [as indicated
by powder X-ray diffraction (XRD)] results in a remarkable change
of the luminescence color to bright-orange (Figure 1a-d). Fackler and
co-workers6a and Lee and Eisenberg6b have reported on similar
grinding-stimulated luminescence phenomena. The behavior observed
What are the major structural factors driving the drastic vapo-
chromic switching of luminescence? The stability order of single
crystals in air (XRD) is 5 . 4 > 3.7 Thus, the reactivity pathways
shown in Scheme 1 are determined by the relative coordination
ability of the solvent. Moreover, the spectral and structural results
† Universidad de Zaragoza.
‡ Universidad de La Rioja.
§ Osaka University.
9
456 J. AM. CHEM. SOC. 2010, 132, 456–457
10.1021/ja909241m  2010 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Selected Structural (Å) and Spectral (nm) Data for 2-5
d(Au · · · Au)
d(Au · · · Ag)
d(Ag · · · Ag)
d(I · · · A)
λ
(RT)
em
λ
(77 K)
em
2
3
-
-
-
-
577
603
600
680
2.7853(14)
2.6814(16)
2.966(3)
2.837(23)
2.7105(16)
(A ) O)
4a
4b
2.8738(9)-2.8989(12)
2.6607(15)-2.8314(15)
2.8123(18)-2.889(2)
2.8799(7)
3.338(10)-3.494(15)
(A ) F)
3.3420(5)
3.4474(5)
(A ) Au)
3.208(1)
611
577
643
614
-
2.6922(4)
2.7893(4)
5
2.9481(3)
2.7377(4)
2.7580(5)
3.1386(9)
575
584,
648(sh)
(A ) N)
in Table 1 suggest that two factors drive the vapochromic switching
of the luminescence color: (a) the degree of cluster aggregation
(polymeric complexes vs 4b; Figure S6) and (b) the geometry of
the Au2Ag2 cluster (1-3, 4a, and 5). The latter is affected by the
nature of the solvent ligand, and in the absence of other strong
intermolecular interactions, it is dominated by the XBs. The
aggregation-specific effects (Scheme 1) were assessed from the
effect of concentration on the excitation and emission spectra of 3
and 4 in the respective solvents (Me2CO and THF; Figure 3 and
Figure S7). The formation of oligomers, which occurs at high
concentrations, results in decreased emission from the π-π*
transition of the [Au(4-C6F4I)2]- unit at 445 nm and concomitant
evolution and red shift of another band at 520-575 nm for 3 and
502-570 nm for 4. Both the excitation and emission spectra are
devoid of isosbestic points, and the maxima exhibit a linear
dependence on the inverse concentration (Figures S6 and S7). The
interaction. The LUMO displays 5pσ bonding density between the
Au(I) atoms as well as bonding density between the interacting
silver centers. In the case of 4b, the lowest-lying orbitals, including
the HOMO, are also centered on the 4-C6F4I ligands, with only
minor contributions from the metals, but the LUMO is spread over
the interacting Ag(I) and Au(I) centers. This implies that the
emission wavelength is mainly determined by aggregation through
energy modulation of the LUMOs. The intermolecular effects, such
as the XBs, affect the emission energy through the HOMOs, which
arelocatedatthe4-C6F4Iligands.Notably,insteadofaHOMO-LUMO
transition between Au(I)-centered orbitals, as would be expected
for C6H5 ligands,8 the four lowest transitions in the case of the
4-C6F4I ligand occur from orbitals on the [Au(4-C6F4I)2]- units to
a single LUMO centered on the Au2Ag2 core. Apparently, the
perhalogenation plays a critical role in the communication of the
effects of the intermolecular interactions to the emission. Accord-
ingly, whereas the 4-C6F4I ligands in 4a, 4b and 5 utilize their iodine
atom for I · · · F, I · · · Au or I · · · N contacts, one of the ligands in 3
is an I · · · O halogen bonded [2.837(23) Å] to a noncoordinated
acetone molecule. Notably, the Au-Au distance [2.7853(14) Å]
and the Au-Ag distance [2.6814(16) Å] for 3 are the shortest within
the [Au2M2(C6F5)4L2]n complexes [Au-Au, 2.8807(4)-3.1959(3)
Å; Au-Ag, 2.7267(5)-2.7903(9) Å]. The cluster compression in
3 causes a strong red shift in the emission energy to the red region
at 603 nm. The same compound also exhibits the largest red shift
(1877 cm-1) on cooling. On the basis of these observations, we
envisage utilization of XBs and AIs for control of the (vapo)lu-
minescent properties in similar systems in the future.
ex
em
extrapolated values of λ and λ in the solution of 3 (441 and
max
max
588 nm, respectively) compare well with the respective experimental
solid-state maxima (448 and 603 nm). As expected from the gradual
ex
polymerization in solution, for 4, the extrapolated λ at 450 nm
max
and λemmax at 627 nm reproduce the values of the polymeric structure
ex
em
4a (λ
) 503 nm, λ
) 611 nm) better than those of the
max
max
ex
em
monomeric isomer 4b (λ ) 522 nm, λ ) 577 nm). Thus, the
max
max
stepwise self-assembly of the Au2Ag2 clusters into thermodynami-
cally stabilized oligomers lowers the emission energy and causes
a red shift in the luminescence wavelength.
To assess the effects of the cluster geometry and, indirectly, the
intermolecular interactions, the molecular orbitals (Figure S16) and
DFT/TD-DFT excitation spectra (Figures S8-S12) were calculated
on the basis of the experimental geometries of the [Au2Ag2(4-
C6F4I)4(Me2CO)2]2 and [Au2Ag2(4-C6F4I)4(THF)2]2 units as models
for the C2 tetranuclear clusters in polymeric 3 and the Ci clusters
in monomeric 4b, respectively. In the case of 3, the highest occupied
orbitals, including the relevant HOMO-1, are mainly spread over
Acknowledgment. This work was supported by Project D.G-
.I.(MEC)/FEDER (CTQ2007-67273-C02-02). T.L. thanks the M.I-
.C.INN for a grant. This work is dedicated to Prof. John P. Fackler,
Jr., on the occasion of his 75th birthday.
Supporting Information Available: Experimental and theoretical
details, plots of crystal structures (CIF), relevant orbitals, and spectra.
This material is available free of charge via the Internet at http://
pubs.acs.org.
2
the 4-C6F4I ligands, but there is also a contribution with 5dz σ*
character from the Au(I) centers, as expected for the Au-Au
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Figure 3. (left) Excitation and emission profiles of solid 3 and 3 in Me2CO
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JA909241M
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