Divergent luminescence behaviour from differential interactions in dinuclear Pt, Au, and mixed Pt-Au complexes built on a xanthene scaffold

Article abstract of DOI:10.1039/c2cc32218h

A diethynylxanthene unit is used to tether cyclometallated Pt(ii) and Au(iii) complexes in a geometry that favours interfacial interactions. This leads to protection from non-radiative decay pathways in the Au₂ dimer and to low-energy 'aggregate-like' emission in a closely related Pt ₂ dimer, whilst rapid Au-to-Pt energy transfer occurs in the heterodimer.

Full text of DOI:10.1039/c2cc32218h

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COMMUNICATION
Divergent luminescence behaviour from differential interactions in dinuclear
Pt, Au, and mixed Pt–Au complexes built on a xanthene scaffoldw
a
a
b
a
Rebeca Munoz-Rodrıguez, Elena Bunuel, J. A. Gareth Williams* and Diego J. Cardenas*
´
´
Received 27th March 2012, Accepted 24th April 2012
DOI: 10.1039/c2cc32218h
A diethynylxanthene unit is used to tether cyclometallated Pt(II)
and Au(III) complexes in a geometry that favours interfacial
interactions. This leads to protection from non-radiative decay
pathways in the Au2 dimer and to low-energy ‘aggregate-like’
emission in a closely related Pt2 dimer, whilst rapid Au-to-Pt
energy transfer occurs in the heterodimer.
same class of complex linked by bridging diphosphine ligands
of appropriate length to allow interfacial interactions.5
More recently, it was shown that, by linking together two
cyclometallated Pt(N^C^N)Cl complexes through the 4 and 5
positions of a xanthene core, intense luminescence in the red
region of the spectrum could be achieved, apparently due
to the formation of an intramolecular excimer {N^C^N
represents cyclometallated 1,3-di(2-pyridyl)benzene}.6
The photophysical properties of second and third-row d6 and
d8 metal ions have attracted much attention due to diverse
applications, ranging from molecular photovoltaic cells to
light-emitting displays and chemical sensors.1a–c Cyclo-
metallating ligands have proved to be particularly successful
in generating luminescent complexes, owing to their strong
ligand fields.2 Amongst such systems, tridentate ligands may
be superior to their bidentate analogues, due in part to the
greater rigidity they confer on the complex.3
Square-planar complexes of d8 metal ions {e.g. Pt(II) and
Au(III)} offer additional scope for controlling excited states,
not normally open to 6-coordinate d6 metal ions, owing to the
possibility of face-to-face interactions between complexes,
either in the ground state (e.g. aggregation) or in the excited
state (e.g. excimer formation). Numerous examples of aggre-
gation and excimer formation have been reported for Pt(II)
complexes with tridentate cyclometallating ligands. For
example, Che and co-workers have pioneered investigations
into Pt(II) complexes of N^N^C-coordinating ligands based on
6-phenyl-2,20-bipyridine.4 These complexes are typically
luminescent in solution at room temperature, emitting from
3[d - p*] (MLCT) states. In the solid state, on the other hand,
examples were found where the emission was strongly shifted
to the red (lmax B 700 nm) compared to the values in solution
(B565 nm). This effect was interpreted in terms of an emissive
3[ds* - p*] MMLCT state (metal–metal-bond-to-ligand
charge-transfer), arising from the overlap of dz2 metal orbitals
of adjacent molecules in the solid.4 Similar effects were
observed in solution in dinuclear systems comprised of the
In contrast to Pt(II), there are far fewer examples of Au(III)
complexes that display appreciable luminescence, despite the two
metal ions being isoelectronic. The lack of luminescence from
many Au(III) complexes may be due to the presence of higher-
lying but thermally accessible deactivating states (e.g. d–d or
LMCT states), and/or to the lower energy of filled metal orbitals
in Au(III) complexes compared to similar Pt(II) complexes.2b,7 This
latter feature results in lower metal character in the excited state
and hence to less efficient spin-orbit coupling pathways and low
radiative rate constants. The use of strong-field cyclometallating
ligands and/or s-donating alkynyl or N-heterocylic carbenes can
alleviate both issues;8 indeed, quite intense luminescence has
recently been reported for a group of cyclometallated Au(III)
complexes with 2-phenylpyridine ligands and alkynyl co-ligands.8c
The influence of face-to-face interactions on the luminescence of
Au(III) complexes is, however, only rarely addressed,9 nor have
mixed-metal Pt(II)–Au(III) interactions been explored.
In this contribution, we report on the synthesis and spectro-
scopic properties of homo- and heterodinuclear Pt(II) and
Au(III) complexes with tridentate, cyclometallated ligands,
N^N^C- and C^N^C-coordinating ligands for Pt(II) and
Au(III) respectively, tethered by
a 4,5-diethynylxanthene
derivative. The properties are compared with those of their
mononuclear analogues in order to determine the influence of
interfacial interactions. The structures of the compounds
studied are shown in Fig. 1. The use of the diethynylxanthene
unit as an insulating (non-conjugated) scaffold allows the two
complex units to be held in close proximity, in a geometry that
may facilitate interfacial interactions.6,10
a
´
Department of Organic Chemistry, Universidad Autonoma de
Madrid, Cantoblanco, 28049-Madrid, Spain.
The preparation of the compounds was carried out as
shown in Scheme 1. Iodination of 2,7-di-tert-butyl-4,5-dibromo-
9,9-dimethylxanthene, followed by a double Sonogashira
coupling with trimethylsilylacetylene and silyl deprotection, gave
diyne 4. In the presence of CuI and Et3N, treatment of 4 with 1 or
2 equivalents of the Pt(N^N^C)Cl or Au(C^N^C)Cl complexesw
gave the desired mononuclear (Pt and Au) and homodinuclear
E-mail: diego.cardenas@uam.es
b Department of Chemistry, University of Durham, Durham,
DH1 3LE, U.K. E-mail: j.a.g.williams@durham.ac.uk;
Fax: 44 191 384 4737
w Electronic supplementary information (ESI) available: experimental
details; characterisation and 1H NMR spectra of all new compounds;
additional absorption and emission spectra. See DOI:10.1039/c2cc32218h
c
5980 Chem. Commun., 2012, 48, 5980–5982
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Fig. 2 Absorption spectrum of Pt2 (black solid line) and the emission
and excitation spectra of Pt and Pt2 (solid red and dashed green lines
respectively) in CH2Cl2 at 298 K.
400 nm, very similar to that reported previously by Yam
and co-workers for Au(C^N^C)(CRC-Ph), who assigned it
to a metal-perturbed intraligand transition on the C^N^C
ligand.8a Complex Au displays a highly structured emission
band, lmax = 485 nm, assigned to the corresponding triplet
state. The emission is very weak, Flum = 3.4 ꢀ 10–4.
Fig. 1 Molecular structures of the complexes (R = n-Bu, R0 = t-Bu).
Comparing dinuclear complex Au2 with Au, the absorption
spectra have essentially identical profiles, with no evidence of
low-energy bands arising from intramolecular ground-state inter-
actions between the gold centres in Au2. The emission spectra
likewise are very similar, Au2 showing just slightly higher relative
intensity of the v = 1 and v = 2 vibrational bands relative to the
fundamental (Fig. 3). Interestingly, however, the luminescence
lifetime and quantum yield of Au2 are around 50 times higher
than for Au. Inspection of the radiative and non-radiative decay
rate constants in Table 1 confirms that the augmentation is due
to the decreased rate of non-radiative decay pathways in the
dinuclear compound. Given that the spectra scarcely differ from
one another, this unprecedented result is intriguing and suggests
that the presence of a second Au unit partially protects the other
from deactivation pathways, for example, either those involving
the solvent, or simply through increased rigidity. Motion of the
Au(C^N^C) units should clearly be more limited in Au2 than in
Au. We also note that a protective effect of aryl rings positioned
parallel to the plane of a terpyridyl ligand was found to enhance
emission in Ru(II) complexes, apparently through steric protec-
tion against non-radiative deactivation.11
Scheme 1 The synthetic route employed.
(Pt2 and Au2) complexes respectively. The heterodimer AuPt was
similarly prepared by treatment of Au with Pt(N^N^C)Cl.
Due to the lower symmetry of the N^N^C-coordination in
the Pt complexes, two possible stereoisomers for Pt2 (Cs, meso,
and C2, chiral) could be expected, depending on the relative
arrangement of both metal complexes. NMR spectra reveal
that only one of these isomers is formed. Chiral shift reagents
did not lead to signal splitting, though this result does not
allow unequivocal assignment of the stereochemistry.
Photophysical data for the complexes are compiled in
Table 1 and selected absorption and photoluminescence spec-
tra are shown in Fig. 2 and 3. First we consider the mono-
nuclear and homodinuclear Au(III) complexes. The absorption
spectrum of the mononuclear gold complex Au displays a
moderately intense, vibronically structured band around
Fig. 3 Absorption spectra of Au2 (black solid line) and AuPt
(red dashed-dot line) and the emission spectra of Au (blue dotted line),
Au2 (green solid line) and AuPt (dashed red line) in CH2Cl2 at 298 K.
c
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Chem. Commun., 2012, 48, 5980–5982 5981

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Table 1 Photophysical data for the mono- and dinuclear complexes in deoxygenated CH2Cl2 at 298 K except where indicated otherwise
Emission at 77 Kd
Emission
max/nm t/nsa
P
Complex Absorption lmax/nm (e/Mꢁ1 cmꢁ1
)
l
Flum ꢀ 102b kr/103 sꢁ1c
knr/105 sꢁ1c lmax/nm
t/ms
Pt
250 (60 500), 284 (49 700), 340sh (17 100), 626
372 (12 800), 442 (5190), 469sh (4620)
274 (93 500), 320sh (41 100), 342 (28 800), 817
368sh (20 900), 457 (7900)
53 [45]
26 [24]
0.73
0.26
0.034
140
100
3.5
190
380
100
1.9
546, 581
715
6.7
Pt2
Au
1.0
220
275
5.4
248 (66 200), 262sh (60 600), 308 (19 700), 490, 522, 96 [89]
483, 518,
554, 599
478, 516,
550, 596
549, 589,
640sh
375 (5120), 392 (6170), 413 (5100)
255 (113 000), 314 (32 400), 376 (8580),
393 (11 200), 414 (8900)
260 (123 000), 314 (47 100), 380 (16 800),
420 (9520), 465 (4850)
557sh
485, 518, 5100 [1100] 1.0
552, 600
Au2
AuPt
2.0
639
47 [42]
0.76
160
210
a
b
Values in air-equilibrated solution in parentheses; lifetimes were independent of lem across the emission bands, in each case. Quantum yield
measured using Ru(bpy)3Cl2 (aq) as the standard. kr and knr are the radiative and non-radiative decay rate constants, estimated assuming that
P
c
P
d
the emissive state is formed with unitary effiency, such that kr = Flum/t and knr = (1/t)–kr. Data at 77 K recorded in diethyl ether/isopentane/
ethanol (2 : 2 : 1 by volume).
The mononuclear complex Pt displays an absorption
spectrum typical of cyclometallated platinum(II) complexes
of 6-phenyl-bipyridines, with moderately intense bands in
the 400–500 nm region associated with 1MLCT transitions.4,12
Upon photoexcitation, the compound emits in solution at
room temperature, displaying a structureless emission band,
lmax = 626 nm, Flum = 7.3 ꢀ 10–3, t = 53 ns. Compared to
the related N^N^C complex with a simple phenylacetylide
suggest that excitation of the Au unit is followed by very rapid
energy transfer to the Pt unit. Although there is no evidence of a
ground-state interaction between the two metal units, through-
space energy transfer will be facilitated by their close proximity.
At 77 K, a very weak band assignable to Au emission is
detected.w Though this could be indicative of a reduced rate
of energy transfer, it more likely reflects simply the reduction in
other non-radiative decay pathways of the Au centre (e.g. t =
220 ms for Au at 77 K, Table 1).
In summary, the diethynylxanthene unit offers a versatile
platform for the design of homo-and heterodinuclear d8 metal
complexes with interesting photophysical properties. The
unexpected reduced non-radiative decay observed for the
di-gold species points to a potential new strategy to be
explored for improving the performance of gold(III) emitters.
We acknowledge MICINN (projects CTQ2007-60494/BQU
and CTQ2010-15927, and FPU fellowship to R.M.R.) and
CAM (AVANCAT) for financial support.
co-ligand, namely Pt(phbpy)(CRC-Ph), for which lmax
=
571 nm, Flum = 8 ꢀ 10–2 and t = 1.0 ms,12 the emission is red-
shifted in the xanthene system, and non-radiative decay is
surprisingly increased by an order of magnitude.
The emission spectrum of the dinuclear complex Pt2 is very
different from that of Pt (Fig. 2). Its emission is substantially red-
shifted, falling in the near-infrared region of the spectrum,
lmax = 815 nm. This could be due to the formation of an
MMLCT state involving interaction of the two metallic units
within the compound. In this case, an additional low-energy
absorption band would typically be anticipated, as the interaction
is already present in the ground state. Although the absorption
profiles of Pt and Pt2 are very similar to one another, close
inspection of the excitation spectra registered for their respective
emission bands does reveal slightly enhanced absorption to long
wavelength of the lowest-energy absorption band for Pt2, sugges-
tive of some interaction between the two complex centres in the
ground state. An alternative explanation may be that the xanthene
unit provides some conjugation between the two metallic units; the
non-innocence of the xanthene is evident from the difference in
properties of Pt and Pt(phbpy)(CRC-Ph)2 discussed above.
The absorption spectrum of the heterodinuclear complex
AuPt displays features characteristic of both Au and Pt; indeed,
the experimental spectrum is very similar to that simulated by
addition of the spectra of the two mononuclear complexes
(see ESIw). On the other hand, excitation of AuPt at any
wavelength >250 nm at room temperature results in a single
emission band of similar energy to that displayed by Pt (Fig. 3).
The quantum yield and lifetime of this band are likewise similar
to the values for Pt. There is no evidence of emission from the
Au unit. It is not possible to excite the Au unit of AuPt
selectively. However, the fact that a Pt-like emission band is
exclusively observed, irrespective of lex, together with the good
match between excitation and absorption spectra,w strongly
Notes and references
1 (a) A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson,
Chem. Rev., 2010, 110, 6595; (b) Highly Efficient OLEDs with
Phosphorescent Materials, ed. H. Yersin, Wiley-VCH, Berlin, 2007;
(c) Q. Zhao, F. Li and C. Huang, Chem. Soc. Rev., 2010, 39, 3007.
2 (a) J. A. G. Williams, Top. Curr. Chem., 2007, 281, 205;
(b) C. Bronner and O. S. Wenger, Dalton Trans., 2011, 40, 12409.
3 S. Develay, O. Blackburn, A. L. Thompson and J. A. G. Williams,
Inorg. Chem., 2008, 47, 11129.
4 S.-W. Lai, M. C.-W. Chan, T.-C. Cheung, S. M. Peng and
C.-M. Che, Inorg. Chem., 1999, 38, 4046.
5 W. Lu, M. C.-W. Chan, N. Zhu, C.-M. Che, C. Li and Z. Hui,
J. Am. Chem. Soc., 2004, 126, 7639.
6 S. Develay and J. A. G. Williams, Dalton Trans., 2008, 4562.
7 V. W.-W. Yam and E. C.-C. Cheng, Chem. Soc. Rev., 2008, 37, 1806.
8 (a) V. W.-W. Yam, K. M.-C. Wong, L.-L. Hung and N. Zhu,
Angew. Chem., Int. Ed., 2005, 44, 3107; (b) V. K.-M. Au, K. M.-C.
Wong, N. Zhu and V. W.-W. Yam, J. Am. Chem. Soc., 2009,
131, 9076; (c) V. K.-M. Au, K. M.-C. Wong, N. Zhu and
V. W.-W. Yam, Chem.–Eur. J., 2011, 17, 130.
9 K. M.-C. Wong, X. Zhu, L.-L. Hung, N. Zhu, V. W.-W. Yam and
H.-S. Kwok, Chem. Commun., 2005, 2906.
10 R. Okamura, T. Wada, K. Aikawa, T. Nagata and K. Tanaka,
Inorg. Chem., 2004, 43, 7210.
11 F. Barigelletti, B. Ventura, J. P. Collin, R. Kayhanian, P. Gavina
and J. P. Sauvage, Eur. J. Inorg. Chem., 2000, 113.
12 W. Lu, B.-X. Mi, M. C.-W. Chan, Z. Hui, C.-M. Che, N. Zhu and
S.-T. Lee, J. Am. Chem. Soc., 2004, 126, 4958.
c
5982 Chem. Commun., 2012, 48, 5980–5982
This journal is The Royal Society of Chemistry 2012