Photophysical studies and excited-state structure of a blue phosphorescent gold-thallium complex

Article abstract of DOI:10.1021/ic062127g

The dinuclear complex {[TI(η⁶-toluene)][Au(C ₆Cl₅)₂]} (1) displays very intense blue phosphorescence and a rigidochromic behavior in the solid state. The photophysical measurements in a glassy solution display an oligomerization process via metal-metal interactions. Density functional theory calculations show a distortion of the aurate-thallium T shape in the lowest triplet excited state, leading to a triplet metal-to-metal charge-transfer state.

Full text of DOI:10.1021/ic062127g

Inorg. Chem. 2007, 46, 2953−2955
Photophysical Studies and Excited-State Structure of a Blue
Phosphorescent Gold Thallium Complex
−
Eduardo J. Ferna´ndez,*,† Antonio Laguna,*,‡ Jose´ M. Lo´pez-de-Luzuriaga,† Miguel Monge,†
Manuel Montiel,† and M. Elena Olmos†
Grupo de S´ıntesis Qu´ımica de La Rioja, Departamento de Qu´ımica, UniVersidad de La Rioja,
UA-CSIC, Complejo Cient´ıfico-Tecnolo´gico, 26004 Logron˜o, Spain, and Departamento de
Qu´ımica Inorga´nica, Instituto de Ciencia de Materiales de Arago´n, UniVersidad de Zaragoza,
CSIC, 50009 Zaragoza, Spain
Received November 9, 2006
The dinuclear complex
very intense blue phosphorescence and a rigidochromic behavior
in the solid state. The photophysical measurements in a glassy
{
[Tl(
η
6-toluene)][Au(C6Cl5)2]
}
(1) displays
observed. The coordination hemisphere of thallium(I) is com-
pleted by an unusual η6-arene contact to a toluene molecule.
Thus, by reaction of a dichloromethane suspension of TlPF6
with NBu4[Au(C6Cl5)2], filtration of the yellow solid, and sub-
sequent treatment of this with toluene, 1 was obtained as an
ocher-colored solid (see the Supporting Information for details).
X-ray-quality crystals of 1 were obtained by slow diffusion
of hexane in a saturated solution of the complex in toluene,
which was allowed to univocally establish its crystal struc-
ture.7 It consists of discrete dinuclear units of 1 in which the
metal centers are joined via an unsupported Au‚‚‚Tl contact
and the toluene molecule interacts with each thallium(I) cen-
ter in a η6 mode (Figure 1and Table 1). The Au-Tl distance,
of 2.9115(2) Å, is one of the shortest described to date,8 be-
ing even shorter than the sum of the Au-Tl covalent radii
(2.929a or 3.08 Å9b), and is on the same order as those found
in the discrete gold/thallium systems [Tl{Au2(P2-phen)3}]-
(ClO4)3 (P2-phen ) 2,9-bis(diphenylphosphino)-1,10-phenan-
throline) (2.911-2.917 Å)10 and [{AuTl(C6Cl5)2(toluene)}2-
solution display an oligomerization process via metal
interactions. Density functional theory calculations show a distortion
of the aurate thallium T shape in the lowest triplet excited state,
−metal
−
leading to a triplet metal-to-metal charge-transfer state.
For main-group metals in their low oxidation states, lone
pairs of electrons play an important role in the geometry
and chemical behavior and often become apparent by a
distortion of regular coordination geometries or by a cation
with ligands only coordinating one hemisphere.1 Thus,
thallium(I) complexes can show several structural arrange-
ments of the ligands around the metal center, as is observed
in the crystal structure of complexes between thallium and
bis(pyrazolyl)borate or tris(pyrazolyl)borate, with planar
trigonal, tetrahedral, or trigonal-bipyramidal distorted ge-
ometry around thallium(I), where the lone pair occupies the
third, fourth, or fifth coordination position, respectively.2,3
In addition, gold(I)/thallium(I) heteronuclear systems
represent an important class of materials with interesting
photophysical properties and a large structural variety, such
as loosely bound butterfly clusters,4 1D linear or zigzag
polymeric chains,5 or 2D and 3D arrays.6
(4) For example, see: Ferna´ndez, E. J.; Lo´pez-de-Luzuriaga, J. M.; Monge,
M.; Olmos, M. E.; Pe´rez, J.; Laguna, A. J. Am. Chem. Soc. 2002,
124, 5942.
(5) For example, see: (a) Ferna´ndez, E. J.; Lo´pez-de-Luzuriaga, J. M.;
Monge, M.; Olmos, M. E.; Pe´rez, J.; Laguna, A.; Mohamed, A. A.;
Fackler, J. P., Jr. J. Am. Chem. Soc. 2003, 125, 2022. (b) Ferna´ndez,
E. J.; Laguna, A.; Lo´pez-de-Luzuriaga, J. M.; Montiel, M.; Olmos,
M. E.; Pe´rez, J. Organometallics 2005, 24, 1631.
(6) For example, see: (a) Ferna´ndez, E. J.; Laguna, A.; Lo´pez-de-
Luzuriaga, J. M.; Olmos, M. E.; Pe´rez, J. Dalton Trans. 2004, 1801.
(b) Ferna´ndez, E. J.; Jones, P. G.; Laguna, A.; Lo´pez-de-Luzuriaga, J.
M.; Monge, M.; Olmos, M. E.; Pe´rez, J. Inorg. Chem. 2002, 41, 1056.
(7) Crystal data of 1: C19H8AuCl10Tl, triclinic, P1h, a ) 8.4051(2) Å, b
) 10.4775(2) Å, c ) 14.7541(3) Å, R ) 76.352(1)°, â ) 77.136(1)°,
γ ) 80.214(1)°, V ) 1221.44(4) Å3, Z ) 2, µ ) 13.689 mm-1, 19 226
reflections, 2θmax ) 56°, 5790 unique (Rint ) 0.0420), R1 ) 0.0299,
Rw ) 0.0690 for 281 parameters, 88 restrictions, S ) 1.027, max ∆F
) 2.401 e Å-3. Crystal data of 1 were measured at -50 °C using a
Nonius Kappa CCD diffractometer, Mo KR radiation, and ω and φ
scans. The structure was solved by direct methods and refined aniso-
tropically on F 2 (Sheldick, G. M. SHELXL-97, University of Go¨ttin-
gen: Go¨ttingen, Germany, 1997). Absorption correction: multiscan.
(8) Ferna´ndez, E. J.; Laguna, A.; Lo´pez-de-Luzuriaga, J. M.; Montiel,
M.; Olmos, M. E.; Pe´rez, J. Organometallics 2006, 25, 1689 and
references cited therein.
In this work, we report the synthesis, luminescent properties,
theoretical calculations, and crystal structure of the complex
{[Tl(η6-toluene)][Au(C6Cl5)2]} (1), in which a discrete mole-
cule with a short gold-thallium unsupported interaction is
* To whom correspondence should be addressed. E-mail:
eduardo.fernandez@unirioja.es (E.J.F.), alaguna@unizar.es (A.L.).
† Universidad de La Rioja.
‡ Universidad de Zaragoza.
(1) Kristiansson, O. Eur. J. Inorg. Chem 2002, 2355 and references cited
therein.
(2) Deacon, G. B.; Delbridge, E. E.; Forsyth, C. M.; Skelton, B. W.; White,
A. H. J. Chem. Soc., Dalton Trans. 2000, 745.
(3) Adams, H.; Batten, S. R.; Davies, G. M.; Duriska, M. B.; Jeffery, J.
C.; Jensen, P.; Lu, J.; Motson, G. R.; Coles, S. J.; Hursthouse, M. B.;
Ward, M. D. Dalton Trans. 2005, 1910.
(9) (a) http://www.webelements.com/, (b) Sheldrick, G. M. SHELXL-97,
Program for Crystal Structure Refinement; University of Go¨ttingen:
Go¨ttingen, Germany, 1997.
10.1021/ic062127g CCC: $37.00
Published on Web 03/21/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 8, 2007 2953

COMMUNICATION
Figure 1. Crystal structure of 1. Selected bond lengths [Å]: Au-Tl
2.9115(3), Au-C1 2.051(5), Au-C11 2.051(5), Tl-Ct 3.068.
Figure 2. Excitation and emission spectra of 1 at room temperature (solid
line) and 77 K (dashed line) in the solid state. Inset: Absorption spectra of
1 in EtOH/MeOH (4:1) at room temperature (solid line) and 77 K (dashed
line).
Table 1. Experimental and Theoretical Structural Parameters (Distance
in Angstroms and Angles in Degrees)
state
Au‚‚‚Tl
Au-C
Tl‚‚‚C
C-Au-C
wavelength. The less energetic zone of the excitation
spectrum is significantly red-shifted from the major absorp-
tion bands seen in solution as well as in glass at 77 K,
indicating that those transitions are forbidden and, therefore,
in accordance with phosphorescent processes. In this regard,
the lifetime in the solid state at room temperature agrees
with this proposal because it fits a biexponential decay with
values of 14.5 ( 0.5 and 2.5 ( 0.2 µs (R2 ) 0.998).
What is usual in extended systems of these two metals is
that they lose the optical properties by rupture of the Au‚‚‚Tl
interactions when dissolved in solvents with donor charac-
teristics,5,6 while in nondonor solvents, they are insoluble.
Nevertheless, in this case, complex 1 is luminescent in
toluene, showing a band at 490 nm with a small shoulder in
the less energetic zone. The value close to that found in the
solid state (470 nm) seems to suggest similar emissive states,
although slightly influenced by the dipole moment of the
solvent. The evaporation of the solvent regenerates 1 in a
quantitative yield.
A different behavior is observed in alcohols at low temper-
ature because the measurement carried out in a glassy EtOH/
MeOH (4:1) solution at 77 K is concentration-dependent.
Thus, when the measurement is carried out in a concentration
of 4 × 10-5 M, an emission at 475 nm appears (maximum
excitation of 325 nm), but increments of the concentration
to 4 × 10-4 M lead to an additional and independent emission
at 515 nm (maximum excitation of 360 nm), increasing the
intensity of the lower-energy band as the concentration
increases. This result is likely due to the presence of
oligomers of different lengths or, alternatively, to complex
1 (470 nm) and to an oligomer responsible for the low-energy
band, respectively. As has been previously reported, as the
number of metal-metal interactions increases, the emission
wavelengths are shifted to lower energies. The spectrum of
the precursor complex [AuTl(C6Cl5)2]n, under similar condi-
tions, displays only one emission at 515 nm. In this case,
the evaporation of the solvents does not lead to complex 1
and instead the gold/thallium precursor appears.
experiment
singlet (S0)
triplet (T1)
2.91
2.86
3.30
2.05
2.09
2.17
3.28-3.46
3.57
3.63
176.9
179.3
121.7
(dioxane)] [2.8935(3) Å].11 Each Tl atom interacts with a tolu-
ene molecule in an unusual η6-like π-arene mode with Tl-C
distances in the range of 3.281(5)-3.458(5) Å (Tl-Ct ) 3.068
Å), similar to those observed in the few examples of η6-like
π-arene thallium complexes described to date.11,12 The gold-
(I) center displays its typical linear coordination, and both
gold(I) and thallium(I) keep additional M‚‚‚Cl contacts in
the ranges of 3.281(1)-3.347(1) and 3.332(1)-3.530(1) Å.
Finally, the shortest Au‚‚‚Au distance, 3.4625(2) Å, rules
out the presence of intermolecular metallophilic contacts.
As expected by our preliminary work in similar systems,
complex 1 shows a very intense luminescence in the solid
state, although it displays an unconventional behavior if com-
pared with other gold/thallium complexes.4-6 It is strongly
luminescent at room temperature (Figure 2) and at 77 K in the
solid state, displaying a broad and unstructured emission in
the blue region at 470 nm (maximum excitation at 370 nm)
and a narrower band at similar emission and excitation ener-
gies at 77 K, respectively, showing an interesting rigidochro-
mic effect.13,14 Although the reason of this phenomenon in
the solid state is not fully understood, some authors suggest14
that the emission energy maximum depends on the environ-
mental rigidity. In this sense, usually at lower temperatures,
the metal-metal distances are expected to decrease, produc-
ing a red shift of the emission bands. The very short
experimental Au-Tl distance found in this case could be
the reason why this observation is prevented in this complex.
The excitation spectrum of the complex shows a compli-
cated profile with an emission independent of the excitation
(10) Catalano, V. J.; Bennett, B. L.; Kar, H. M.; Noll, B. C. J. Am. Chem.
Soc. 1999, 121, 10235.
(11) Ferna´ndez, E. J.; Laguna, A.; Lo´pez-de-Luzuriaga, J. M.; Olmos, M.
E.; Pe´rez, J. Chem. Commun. 2003, 1760.
(12) (a) Janiak, C. Coord. Chem. ReV. 1997, 163, 107. (b) Schebler, P. J.;
Riordan, C. G.; Guzei, I. A.; Rheingold, A. L. Inorg. Chem. 1998, 37,
4754. (c) Wiesbrock, F.; Schmidbaur, H. J. Am. Chem. Soc. 2003, 125,
3622. (d) Kristiansson, O. Eur. J. Inorg. Chem. 2002, 2355. (e) Nakai,
H.; Tang, Y.; Gantzel, P.; Meyer, K. Chem. Commun. 2003, 24.
(13) Wrighton, M.; Morse, D. L. J. Am. Chem. Soc. 1974, 96, 998.
(14) Wang, S.; Garzo´n, G.; King, C.; Wang, J. C.; Fackler, J. P., Jr. Inorg.
Chem. 1989, 28, 4623.
Also, the absorption spectra in degassed EtOH/MeOH (4:
1) (inset in Figure 2), at room temperature, of complex 1
and the precursor derivative display identical profiles with
absorptions at 240 nm (ꢀ ) 6.3 × 103 mol-1 dm3 cm-1),
2954 Inorganic Chemistry, Vol. 46, No. 8, 2007

COMMUNICATION
Figure 3. DFT-optimized structures for the singlet ground state and the
lowest triplet excited state.
Figure 4. Molecular orbital diagrams for the ground state S0 (left) and
the lowest triplet excited state T1 (right).
278 nm (ꢀ ) 5.7 × 103 mol-1 dm3 cm-1), and 301 nm (ꢀ )
3.7 × 103 mol-1 dm3 cm-1), peaks that appear at wavelengths
similar to those in the precursor NBu4[Au(C6Cl5)2] complex
in the same solvents, and, therefore, we assign them
tentatively to ππ* transitions located in the pentachlorophe-
nyl rings. By contrast, the absorption spectrum of the glassy
solution at 77 K shows, in addition, the presence of an
additional band at 325 nm, of small intensity, whose position
matches one of the excitation maxima found under the same
experimental conditions. This fact could indicate that this
absorption is responsible for one of the observed emission
bands (475 nm) and is related to the formation of Au-Tl
interactions, because the precursor gold and thallium com-
plexes do not show this absorption, although it might be
caused by aggregation giving rise to oligomers. Similarly,
complex [{AuTl(C6Cl5)2(toluene)}2(dioxane)] displays an
analogous optical behavior, indicating a similar origin.11 The
photophysical properties of 1 have been measured several
times in samples prepared separately, giving rise in all cases
to similar results. Therefore, this fact seems to rule out the
possibility of a luminescent impurity in the bulk material.
We carried out density functional theory (DFT)/B3LYP
calculations to study the geometries for the ground state and
the lowest triplet excited state for 1 in order to gain insight
about the mechanism responsible for the phosphorescent
behavior observed experimentally.
First, we have fully optimized the model system {[Tl(η6-
benzene)][Au(C6Cl5)2]} (1A; C2V symmetry) in the ground
and first triplet excited states at the DFT level of theory.
The optimized ground state for model 1A displays a T-shaped
disposition for the aurate/thallium moiety in which a Au‚‚‚Tl
interaction of 2.86 Å (exptl 2.91 Å) is achieved (see the
Supporting Information). On the other hand, full optimization
of the first triplet excited state displays a distortion of the
aurate/thallium T shape through an increase of the Au‚‚‚Tl
distance (3.30 Å) together with a bend outward of the
C-Au-C angle from linear 180° to 121.7° (see Figure 3).
This result is similar to the one proposed by Omary et al.
for [Au(PR3)2X] complexes15 in which a distortion beyond
a T shape of the tricoordinated gold(I) center is reached in
the emissive lowest triplet excited state.
is localized on the [Au(C6Cl5)2]- fragment, while the lowest
unoccupied molecular orbital is mainly placed at the Tl+ ion.
On the other hand, upon excitation of one electron to the
lowest triplet excited state (T1), the shape of the SOMO-1
orbital (SOMO ) singly occupied molecular orbital) remains
at the aurate, while the SOMO orbital is located at the metal
centers (see Figure 4). These electronic structures together
with the lowest triplet excited-state distortion can be
interpreted in terms of a charge transfer from the aurate
(doubly occupied HOMO in the ground state) to the metal
centers, as can be observed in the shape of the 95a orbital
for the T1 state. In this way, upon charge transfer, the system
loses the strong ionic interaction between the metals and,
therefore, the intermetallic Au‚‚‚Tl distance increases.
The theoretical results shown here on the charge-transfer
character of the transition responsible for the luminescent be-
havior are in agreement with the reported experimental data of
complex 1. In contrast to already reported polymeric chains,
gold/thallium systems of alternating metals in which the lumi-
nescence is attributed to a fluorescent process with lifetimes
in the nanoseconds range, complex 1 represents an example
of a discrete Au-Tl system in which the luminescence arises
from a T1 excited state, giving rise to a phosphorescent
process. In this regard, the optimization of the lowest triplet
excited state shows how the [Au(C6Cl5)2]- moiety bends
outward. This trend would not be possible in the case of a
polymeric system because the aurate unit is always placed
between two acidic Tl+ ions, which may prevent the bend
of the C-Au-C angle and a Au-Tl distance enlargement.
In conclusion, with regard to the presented data and taking
into account the theoretical calculations, we assign the emis-
sion in the solid state as arising from a triplet metal-to-metal
charge-transfer state, from the electron-rich bis(perchloro-
phenyl)gold(I) anion to the metals gold(I) and thallium(I).
Acknowledgment. The DGI MEC/FEDER (Grant CTQ-
2004-05495) is thanked for financial support. Mi.M. thanks
the MEC-Universidad de La Rioja for his research contract
“Ramo´n y Cajal”. Ma.M. thanks the CAR for a grant.
The analysis of the frontier orbitals for both the ground and
triplet excited states for 1 has been carried out. In the ground
state (S0), the highest occupied molecular orbital (HOMO)
Supporting Information Available: X-ray crystallographic data
in CIF format for 1; experimental procedure, characterization data,
photophysical measurements, and computational details. This mater-
ial is available free of charge via the Internet at http://pubs.acs.org.
(15) Sinha, P.; Wilson, A. K.; Omary, M. A. J. Am. Chem. Soc. 2005,
127, 12488.
IC062127G
Inorganic Chemistry, Vol. 46, No. 8, 2007 2955