Gold(I) and platinum(II) tetracenes and tetracenyldiacetylides: Structural and fluorescence color changes induced by σ-metalation

Article abstract of DOI:10.1021/om100046h

σ-Metalation of tetracene can change the emission color of and introduce new structural dimensionality to the organic chromophore. Two synthetic entries to σ-metallotetracenes have been explored, leading to emissive mononuclear (Ph₃PAu^I)-tetracene (1) and cis-[Br(Et₃P)₂Pt^II]-tetracene (2) and binuclear (R₃PAu^I)₂-tetracenyldiacetylide (R = Ph (4), Me (5)) and trans-[I(Et₃P)₂Pt^II] ₂-tetracenyldiacetylide (6). Metalation can lower the emission energy of tetracene up to 0.53 eV. The X(Et₃P)₂Pt^II group (X = Br, I) has stronger perturbations on the tetracenyl ring than the R₃PAu^I group. σ-Metalation also leads to different crystal packing of the complexes and thus arrangements of the tetracenyl rings. Aurophilic attraction operates in 5, leading to self-assembly of the molecules into a novel honeycomb structure composed of helical Au^I chains. In contrast to the other complexes, the crystal of 5 is not emissive, possibly due to efficient exciton delocalization.

Full text of DOI:10.1021/om100046h

2422 Organometallics 2010, 29, 2422–2429
DOI: 10.1021/om100046h
Gold(I) and Platinum(II) Tetracenes and Tetracenyldiacetylides:
Structural and Fluorescence Color Changes Induced by σ-Metalation
Minh-Hai Nguyen and John H. K. Yip*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
Received January 21, 2010
σ-Metalation of tetracene can change the emission color of and introduce new structural
dimensionality to the organic chromophore. Two synthetic entries to σ-metallotetracenes have been
explored, leading to emissive mononuclear (Ph3PAuI)-tetracene (1) and cis-[Br(Et3P)2PtII]-tetracene
(2) and binuclear (R3PAuI)2-tetracenyldiacetylide (R = Ph (4), Me (5)) and trans-[I(Et3P)2PtII]2-
tetracenyldiacetylide (6). Metalation can lower the emission energy of tetracene up to 0.53 eV. The
X(Et3P)2PtII group (X=Br, I) has stronger perturbations on the tetracenyl ring than the R3PAuI
group. σ-Metalation also leads to different crystal packing of the complexes and thus arrangements
of the tetracenyl rings. Aurophilic attraction operates in 5, leading to self-assembly of the molecules
into a novel honeycomb structure composed of helical AuI chains. In contrast to the other complexes,
the crystal of 5 is not emissive, possibly due to efficient exciton delocalization.
Introduction
complex has ever been reported. It has been demonstrated
that attaching metal ions such as AuI and PtII to the rims of
anthracene and pyrene has significant effects on the spec-
troscopy or the reactivity of the acenes.5 It is therefore
possible that metalation of LAs and their derivatives can
be a mechanism for tuning the ground-state and excited-state
Larger acenes (LA), especially tetracene and pentacene,
have important applications in advanced materials because
of their strong visible π-π* absorptions, intense lumines-
cence, and high charge mobility in the solid state.1a-c The
electronic structure, photophysics, and solid-state assembly
of LA, which are related to the performance of LA-based
devices, can be changed by functionalization of the rings.2
For example, the recent work of Anthony showed that the
emission of tetracene can be red-shifted up to 5310 cm-1
(0.66 eV) by attaching substituted ethynyl groups at the C5
and C12 positions.3 While many organic derivatives of LA
have been reported,1d-g the coordination chemistry of LA
remains to be fully explored. Notable examples of metal-LA
complexes are sandwich cobalt4a and palladium tetracene4b
π-complexes. To our knowledge, no σ-metallotetracene
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nus.edu.sg.
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pubs.acs.org/Organometallics
Published on Web 05/07/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 11, 2010 2423
Chart 1
Figure 1. (a) ORTEP plotof 1 C3H6O (thermal ellipsoids drawn
3
at the 50% level). The solvent molecule and H atoms are omitted
for clarity. (b) Herringbone structure in the crystal packing of 1.
properties of the organic molecules. Apart from their electronic
effects, metals can increase the structural dimensionality of
LA.8-12,20a,34 A herringbone structure is a common packing
motif for tetracene and some of its derivatives, but in some
cases, π-πstacking is observed.6 It is possible that new patterns
will emerge in tetracene modified with metals which have
geometry and secondary interactions (e.g., metallophilicity)
different from those of organic substituents.
˚
Table 1. Selected Bond Lengths (A) and Angles (deg) of
1 C3H6O and 2
3
1 C3H6O
3
Au(1)-C(5)
Au(1)-P(1)
2.075(6)
2.2956(16)
C(5)-Au-P(1)
169.82(16)
As part of our ongoing effort to develop heavy metal alternant
hydrocarbon σ-complexes into a new class of luminescent
materials and near-infrared emitters,5a-e we embarked on a
study of the synthesis and spectroscopy of σ-metallotetracenes.
Reported in this paper are two synthetic entries into metalated
tetracene and its derivatives. The first approach is direct metala-
tion of tetracene, which resulted in the two mononuclear com-
plexes Ph3PAuI-tetracene (1) and cis-Br(Et3P)2PtII-tetracene (2)
(Chart 1). 5,12-Bis((triisopropylsilyl)ethynyl)tetracene (3), first
synthesized by Anthony,3 was used in the second approach as a
precursor for the dianionic ligand 5,12-tetracenyldiacetylide,
with which the three binuclear complexes [(R3P)AuI2]-tetrace-
nyldiacetylide (R = Ph, 4; R = Me, 5) and trans-[I(Et3P)2Pt]2-
tetracenyldiacetylide (6) were synthesized. The complexes are
new additions to PtII and AuI arylacetylides, which have been
shown to have rich photophysics and optoelectronics.7 Auro-
philic interactions, “super van der Waals bonding”,8 are respon-
sible for the formation of intricate supramolecular structures
such polymers,9 metallacycles,10 helicates,11 and catenanes.12
Interestingly, our results showed that changing the auxiliary
phosphine from Ph3P to Me3P induced aurophilic interactions,
leading to self-assembly of an intriguing honeycomb network.
2
Pt(1)-C(5)
Pt(1)-P(1)
Pt(1)-P(2)
Pt(1)-Br(1)
2.066(7)
2.355(2)
2.2354(13)
2.4968(8)
C(5)-Pt(1)-P(1)
C(5)-Pt(1)-P(2)
P(1)-Pt(1)-P(2)
C(5)-Pt(1)-Br(1)
89.63(19)
94.53(3)
99.67(7)
84.10(19)
from oxidative addition of Pt(PEt3)4 to 5-bromotetracene.
Both compounds are unstable and slowly decompose in
organic solvents after 2 days in air to give tetracene and
metal colloids. The structure of 1 shows a Ph3PAuI group
attached to a planar tetracene ring at its C5 carbon atom
˚
(Figure 1 and Table 1). The Au-Cipso (2.075(6) A) and
˚
Au-P (2.2956(16) A) bond lengths are similar to those
observed in other arylgold(I) phosphine complexes.13 The
P-Au-C angle (169.81ꢀ) is distorted from linearity, and as a
result, the Au and P atoms deviate from the mean plane of
˚
the tetracene by 0.56 and 1.32 A. The distortion could be due
to steric repulsion between one of the phenyl rings and the
tetracenyl ring in the neighboring molecule (Figure 1b).
Similar to the case for tetracene and its derivatives,6a,14 the
molecules of 1 are assembled into a herringbone structure in
the crystal via edge-to-face interactions between adjacent
tetracenyl rings (Figure 1b). The dihedral angle (55.39ꢀ)
between two interacting rings and the calculated (edge)H-
C(face) distance (2.801 A) are close to those of tetracene
(51.40ꢀ, 2.761 A).14 While the herringbones in the crystal of
tetracene interlock and extend two-dimensionally, each her-
ringbone in 1 is isolated from the adjacent ones by AuPPh3
groups.
Results and Discussion
˚
Structures of AuI and PtII Tetracene. The complex 1 was
prepared from lithiation of 5-bromotetracene followed by
addition of Ph3PAuCl, while the complex 2 was prepared
˚
(8) Schmidbaur, H.; Schier, A. Chem. Soc. Rev. 2008, 37, 1931.
(9) (a) Mohr, F.; Jennings, M. C.; Puddephatt, R. J. Angew. Chem.,
The crystal structure of 2 (Figure 2 and Table 1) shows a
PtII ion coordinated to two PEt3 ligands and a Br- ion and
ꢀ
ꢀ
ꢀ
Int. Ed. 2004, 43, 969. (b) Deak, A.; Tunyogi, T.; Palinkas, G. J. Am. Chem.
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Y.-Z.; Yam, V. W.-W. Angew. Chem., Int. Ed. 2008, 47, 4551. (b) Tang,
H.-S.; Zhu, N.; Yam, V. W.-W. Organometallics 2007, 26, 22. (c) Kui,
S. C. F.; Huang, J.-S.; Sun, R. W.-Y.; Zhu, N.; Che, C.-M. Angew. Chem., Int.
(12) (a) McArdle, C. P.; Irwin, M. J.; Jennings, M. C.; Puddephatt,
R. J. Angew. Chem., Int. Ed. 1999, 38, 3376. (b) Mingos, D. M. P.; Yau, J.;
Menzer, S.; Williams, D. J. Angew. Chem., Int. Ed. 1995, 34, 1894.
(13) (a) Croix, C.; Balland-Longeau, A.; Allouchi, H.; Giorgi, M.;
ꢀ
ꢀ
ꢀ
ꢀ
^
Ed. 2006, 45, 4663. (d) Deak, A.; Megyes, T.; Tarkanyi, G.; Kiraly, P.;
Duchene, A.; Thibonnet, J. J. Organomet. Chem. 2005, 690, 4835.
ꢀ
ꢀ
ꢀ
Biczok, L.; Palinkas, G.; Stang, P. J. J. Am. Chem. Soc. 2006, 128, 12668.
(11) (a) Schuh, W.; Kopacka, H.; Wurst, K.; Peringer, P. Eur. J. Inorg.
Chem. 2002, 2002, 2202. (b) Angermaier, K.; Zeller, E.; Schmidbaur, H.
J. Organomet. Chem. 1994, 472, 371.
(b) Yam, V. W.-W.; Cheung, K.-L.; Yip, S.-K.; Zhu, N. Photochem.
Photobiol. Sci. 2005, 4, 149.
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K. P. C. Chem. Eur. J. 1999, 5, 3399.

2424 Organometallics, Vol. 29, No. 11, 2010
Nguyen and Yip
Figure 2. (a) ORTEP plot of 2 (thermal ellipsoids drawn at the
50% level). (b) Crystal packing of 2. Color scheme: blue, Pt;
orange, P; red, Br; gray, C.
Figure 4. ORTEP plot of 5 (thermal ellipsoids drawn at the
50% probability level). The Me3P is disordered over two posi-
tions, P(1) and P(1A), with occupancies of 64% and 36%. Color
scheme: yellow, Au; orange, P; gray, C.
˚
Table 2. Selected Bond Lengths (A) and Angles (deg) for
4 3.5CHCl3 and 5
3
4 3.5CHCl3
3
Au(1)-P(1)
Au(1)-C(1)
2.277(3)
1.9829(14)
P(1)-Au(1)-C(1)
Au(1)-C(1)-C(2)
175.8(4)
177.3(12)
Figure 3. Ball-and-stick diagram of 4 3.5CHCl3 showing edge-
3
to-face interactions (dotted lines). Solvent molecules and H
atoms are omitted for clarity. Color scheme: yellow, Au; orange,
P; gray, C.
5
Au(1)-P(1)
Au(1)-P(1A)
Au(1)-C(1)
2.287(4)
2.327(8)
1.968(8)
P(1)-Au(1)-C(1)
P(1A)-Au(1)-C(1)
C(2)-C(1)-Au(1)
C(1)-C(2)-C(6)
171.6(3)
164.1(5)
173.9(8)
166.1(10)
the C5 atom of the tetracene, showing a distorted-square-
planar geometry.
The two PEt3 are in a cis configuration, and in accord with
the solid-state structure, the 31P NMR spectrum shows two
and 4 (see Table 2 for selected bond lengths and angles),
respectively.
doublets at δ 8.96 (1JP-Pt = 1580 Hz) and 2.94 (1JP-Pt
=
4070 Hz) with a 2JP-P value of 18 Hz, which are assigned to
the P atoms trans and cis to the metalated carbon atom,
The coordination of AuI ions in 4 and 5 is nearly linear and
the Au-P and Au-C bond lengths are similar to those
observed for phosphinegold(I) arylacetylides.17 The Me3P
ligands in 5 are disordered over two positions, and the
corresponding P-Au-C angles are significantly bent
(171.6(3) and 164.1(5)ꢀ for P(1) and P(1A)). The tetracenyl
rings in the complexes are slightly curved. The C-CtC
linkage between the tetracenyl ring and the acetylide group
is nearly linear in 4 (—C-CtC = 176.3(1)ꢀ) but is sig-
nificantly bent in 5 (166.1(1)ꢀ). As a result of the distortion,
the two acetylide groups and AuI ions are tilted slightly
toward the longer end of the molecule. The 31P{1H} NMR
spectra of 4 and 5 display singlets at δ 42.9 and 1.58,
respectively.
1
respectively, according to their different JP-Pt values and
Pt-P distances.15 The coordination plane of the Pt ion and
the plane of the tetracene ring make a dihedral angle of
˚
89.62ꢀ. The Pt-Cipso bond length is 2.066(7) A, typical for
platinated polycyclic aromatic hydrocarbons.16 Because of
the strong trans influence of the carbanion, the Pt(1)-P(1)
˚
bond (2.355(2) A) is notably longer than the Pt(1)-P(2) bond
˚
(2.2354(13) A). Similarly, the P1-Pt-P2 angle is distorted
(—P1-Pt-P2 = 99.67ꢀ), possibly due to steric repulsion
between the ethyl groups in the phosphines. The molecules of
2 are aligned along the c axis (Figure 2b) with their tetracenyl
˚
rings in parallel and separated by ∼6.8 A. There is no overlap
The crystal structure of 4 shows neither intermolecular
Au-Au interactions (shortest intermolecular Au-Au dis-
tance 7.53(9) A) nor π-π stacking. The tetracenyl rings are
parallel to each other, forming a staircaselike array (Figure 3).
between the tetracene rings in adjacent columns. Tetracene
and some of its derivatives such as 1 aggregate into a
herringbone structure in the solid state. Possibly the struc-
ture is disfavored in 2 because of the bulky PEt3 ligands,
especially the ligand close to the ring.
˚
Two adjacent molecules are associated through a C-H
π
3 3 3
interaction involving the phenyl ring of PPh3 and the tetracenyl
ring, which are nearly perpendicular to each other (dihedral
angle 88.8(7)ꢀ). The distance between the centroids of
Structures of Au and Pt Tetracenyldiacetylides. The com-
plexes 4 and 5 were synthesized by reacting 2 mol equiv of
Ph3PAuCl and Me3PAuCl, respectively, with 5,12-diethy-
nyltetracene in CH2Cl2 in the presence of NaOMe. 5,12-
Diethynyltetracene was generated in situ by reacting 5,12-
bis((triisopropylsilyl)ethynyl)tetracene (3) with n-Bu4NF.
The crystal structures of 4 and 5 are depicted in Figures 3
˚
the rings is 4.92(7) A, which is typical for edge-to-face inter-
actions.18
It has been demonstrated that intermolecular aurophilic
interactions between AuI and phosphine can be induced by
(15) Hartley, F. R. The Chemistry of Platinum and Palladium; Applied
Science: London, 1973; p 40.
(16) (a) Singh, A.; Sharp, P. R. J. Am. Chem. Soc. 2006, 128, 5998.
(b) Lucassen, A. C. B.; Shimon, L. J. W.; van der Boom, M. E. Organome-
tallics 2006, 25, 3308. (c) Manna, J.; Kuehl, C. J.; Whiteford, J. A.; Stang,
P. J. Organometallics 1997, 16, 1897.
(17) (a) Hunks, W. J.; MacDonald, M.-A.; Jennings, M. C.; Puddephatt,
R. J. Organometallics 2000, 19, 5063. (b) Bardají, M.; Laguna, A.; Jones, P. G.
Organometallics 2001, 20, 3906.
(18) (a) Paliwal, S.; Geib, S.; Wilcox, C. S. J. Am. Chem. Soc. 1994,
116, 4497. (b) McNelis, B. J.; Nathan, L. C.; Clark, C. J. J. Chem. Soc.,
Dalton Trans. 1999, 11, 1831.

Article
Organometallics, Vol. 29, No. 11, 2010 2425
Figure 5. (a) Honeycomb-like framework of 5 viewed along the
c axis showing the open channels. (b) Hexagonal pore showing
stacking of molecules at the nodes.
Chart 2
Figure 6. (a) Right-handed and (b) left-handed Au helices.
Au-Au bond distances.20 The present compound is a rare
example of an undistorted helical gold chain.
A gold atom in the helix is separated from the CtC bond
coordinated to its adjacent gold atom by a distance of 3.42(9)
I
˚
A, which is too long for any significant Au -acetylide
interactions.21 The network is therefore sustained mainly if
not solely by aurophilic interactions. Au-Au attraction
accounts for many novel solid-state assemblies of AuI com-
plexes, but to our knowledge there is no example of a
honeycomb structure arising from the metallophilic interac-
tion. Metal-organic-based honeycomb frameworks are
known,22 and they are usually assembled by stacking of
hexagonal sheets which are conjoined by metal-ligand
coordination or intermolecular association (i.e., π-π
stacking)22b of individual molecules. The honeycomb net-
work of 5 is distinctly different, as it arises from the Au-Au
bonding between layers, but the molecules in each layer are
not connected by covalent or any secondary bonding. The
tetracenyl rings stacking along the central axis of the helix are
reducing the bulkiness of the phosphine.8 Indeed, aurophilic
attraction operates in the crystal of 5, in which the molecules
are assembled into a honeycomb network with the tetracenyl
rings and the Au ions forming the edges and vertices of
hexagons (Figure 5).
The compound 5 was crystallized in the rhombohedral
R3m space group. The crystal is highly porous with open
channels extending along the c axis. No solvent molecule is
found in the channels, which are lined with the edges of the
˚
tetracenyl rings and have an estimated diameter of ∼8 A. The
honeycomb pores can be constructed by iterative stacking of
three layers of molecules (pink, blue, and green), as shown in
Chart 2.
˚
widely separated by 7.6 A. Each tetracenyl ring is surrounded
by four neighboring rings with an angular separation of 60ꢀ.
The complex 6 was synthesized by Sonogashira coupling
of trans-PtII(PEt3)2I2 and 5,12-diethynyltetracene. Although
The molecules in each layer are related by 3-fold rotation
and reflection, and those in different layers are connected
through their Au ions at the nodes of the network (or the
“vertices” of the hexagonal pore). Parallel to the c axis and
passing through vertices alternately are the 31 and 32 screw
axes. The gold atoms form right-handed (Δ or P) and left-
handed (Λ or M) helices that wind around the 31 and 32 axes,
respectively (Figure 6). The Au atoms in the helix are equally
attempts to obtain the crystal structure of 3 failed, 1H and 31
P
NMR, 1H-1H COSY, high-resolution ESI-MS, and ele-
mental analysis results clearly showed that the complex has
a C2v-symmetric structure whereby the two I(Et3P)2PtII
groups are coordinated to the two acetylide groups and the
phosphines are in the trans configuration (Chart 3). The
31P{1H} NMR spectrum of the complex shows a singlet at δ
9.57 (1JPt-P = 2323 Hz), and the 1H NMR spectrum shows
˚
spaced with a Au-Au distance of 3.26(8) A, which falls into
the range for aurophilic contacts.19 The Au-Au-Au angle is
134.2(3)ꢀ. Zigzag gold chains sustained by aurophilic inter-
actions are known, but most of them display alternate
aromatic signals including two double doublets (H1,4, H7,10
)
and one singlet (H6,11) and a multiplet arising from the
overlap of the two double doublets for H2,3 and H8,9. The
data are consistent with the proposed structure of C2v
symmetry, which is further confirmed by the high-resolution
(19) (a) Leznoff, D. B.; Xue, B.-Y.; Batchelor, R. J.; Einstein, F. W.
B.; Patrick, B. O. Inorg. Chem. 2001, 40, 6026. (b) Crespo, O.; Gimeno, M.
C.; Laguna, A.; Kulcsar, M.; Silvestru, C. Inorg. Chem. 2009, 48, 4134.
ꢀ
ꢀ
(c) Fernandez, E. J.; Laguna, A.; Lopez-de-Luzuriaga, J. M.; Monge, M.;
(21) Shapiro, N. D.; Toste, F. D. Proc. Natl. Acad. Sci. U.S.A. 2008,
105, 2779.
ꢀ
Montiel, M.; Olmos, M. E.; Puelles, R. C.; Sanchez-Forcada, E. Eur.
J. Inorg. Chem. 2007, 2007, 4001. (d) Leznoff, D. B.; Xue, B.-Y.; Patrick,
B. O.; Sanchez, V.; Thompson, R. C. Chem. Commun. 2001, 259. (e) Liau,
R.-Y.; Schier, A.; Schmidbaur, H. Organometallics 2003, 22, 3199.
(20) (a) Katz, M. J.; Sakai, K.; Leznoff, D. B. Chem. Soc. Rev. 2008,
37, 1884. (b) Coker, N. L.; Krause Bauer, J. A.; Elder, R. C. J. Am. Chem.
Soc. 2003, 126, 12.
(22) (a) Wang, Q.-M.; Mak, T. C. W. J. Am. Chem. Soc. 2000, 122,
7608. (b) Kammer, S.; M€uller, H.; Grunwald, N.; Bellin, A.; Kelling, A.;
Schilde, U.; Mickler, W.; Dosche, C.; Holdt, H.-J. Eur. J. Inorg. Chem. 2006,
2006, 1547. (c) Choi, H. J.; Suh, M. P. J. Am. Chem. Soc. 1998, 120, 10622.
ꢀ
ꢀ
(d) Galan-Mascaros, J. R.; Dunbar, K. R. Angew. Chem., Int. Ed. 2003, 42,
2289.

2426 Organometallics, Vol. 29, No. 11, 2010
Nguyen and Yip
Chart 3
ESI-MS mass spectrum, which shows a cluster peak at m/z
1389.2 corresponding to [6]þ ion (Figure 7).
Absorption and Emission Spectroscopy. The UV-visible
and emisison spectra of the complexes are shown in Figure 8,
and the spectroscopic data are summarized in Table 3. The
absorption spectra display a moderately intense vibronic
band at 400-550 nm for 1 (λmax 481 nm, εmax=7.70ꢀ103
M-1 cm-1) and 2 (λmax 467 nm, εmax=1.05ꢀ104 M-1 cm-1
)
and at 450-620 nm for 4 (λmax 553 nm, εmax=3.11ꢀ104 M-1
cm-1), 5 (λmax 552 nm, εmax=2.23ꢀ104 M-1 cm-1), and 6
(λmax=572 nm, εmax=2.62ꢀ104 M-1 cm-1). The vibronic
spacings are ∼1300-1400 cm-1. Vibronic bands are com-
monly observed for the π f π* transitions of aromatic
molecules,23 and in fact, absorption bands with similar
intensities, vibronic spacings, and shapes are found in the
spectra of the parental tetracene and 3. Accordingly, the
vibronic band is assigned to the lowest energy singlet π f π*
(1Ag f 1B1u) transition in the tetracenyl ring, which is also
Figure 7. (a) Simulated isotopic distribution for [6]þ. (b) ESI-
MS cluster peak for [6]þ.
1
known as the La band in Platt’s notation.24 Because of
substituents. Notably, the compounds 4-6 exhibit a larger red
shift than 3, indicating that the metal ions have stronger
perturbation than the Si(iPr)3 group. Unlike the Si(iPr)3 group,
which can only act as a π-acceptor, the metal ions can be both
π-donating and π-accepting. The stronger perturbations ob-
served for the metal ions suggest the importance of the metal-
mixing of orbitals of tetracene and the metal ions (and
ethynyl groups in 4-6), the transitions in the metalated
tetracene are largely but not entirely tetracene-based. How-
ever, for the sake of clarity, the absorption band of the
complexes is still labeled 1La in the following discussion.
Apart from the 1La band, the spectra of 3-6 display
another intense vibronic band at 320-400 nm, which is too
intense (εmax = (1.38-2.32) ꢀ 104 M-1 cm-1) to be a triplet
component of a high-energy singlet excited state and is
1
to-tetracene π-donation in influencing the energy of the La
band. Notably, the 1La bands of the PtII complexes 2 and 6 are
lower in energy than those of the corresponding AuI complexes
1, 4, and 5. Our previous study5c also showed that the energy of
the 1La band of trans-Br(PEt3)2PtII-pyrene is lower than that
of Ph3PAuI-pyrene. The X(PEt3)2PtII group (X = Br, I) has
stronger perturbations than the R3PAuI group (R = Ph, Me),
possibly because it is a stronger π-donor with the electron-
donating PEt3 and Br-/I- ion. Anthony3 first demonstrated
that attaching substituted ethynyl groups X0-CtC- (X0 =
substituent, i.e. Si(iPr)3) to the C5 and C12 positions of
tetracene can red-shift its absorption and fluorescence. It is
noted that the substituent that causes the largest red shift is X0 =
4-(N,N0-diethylamino)phenyl (emission maximum 643 nm),
which is mainly π-donating.
1
tentatively assigned to the Lb band, which arises from the
1
1
-
pseudo-parity-forbidden Ag f B2u transition.23,25 Our
recent work on the spectroscopy of metalated anthracene
and pyrene showed that perturbations of metal ions can lead
to the intensification of the 1Lb band.5a-c As no distinct 1Lb
band is observed in the spectra of 1 and 2, it is reasonable to
assume that the metalated ethynyl groups in 4-6 have
stronger perturbations on the electronic structure of tetra-
cene than the metal ions alone in 1 and 2.
The most notable feature of the spectra of the complexes is
the significant red shift of the 1La bands from that of tetra-
cene, which follows the order 1(1100cm-1) <2(1700 cm-1) <
4 ≈ 5 (3000 cm-1) < 6 (3700 cm-1). The red shift is due to the
perturbation of the substituents on the tetracenyl ring. The
emissions of 4-6 are lower in energy than those of 1 and 2
because two metalated ethynyl groups can exert stronger
perturbation than a single metal center. In a first-order approxi-
All the complexes are emissive in solution (Figure 8 and
Table 3). The emission bands show shoulders with a vibronic
spacing of ∼1200 cm-1. The metalated tetracenyldiacetylides
4, 5 and 6 are g10 times more emissive than the metalated
1
tetracenes 1 and 2 (Table 3). Similar to the case for the La
bands, the emissions are red-shifted from that of tetracene. The
emission energy follows the order tetracene > 1 >2 >3> 4≈
5 > 6, and the fluorescence color changes from blue (tetracene)
to yellow (1-3), orange (4, 5), and red (6) (Figure 9). The small
Stokes shift (350-770 cm-1) and short emission lifetimes
(0.5-8.6 ns) suggest that the emissions are fluorescence arising
1
mation, the La band arises from HOMO f LUMO of the
tetracene, the energy of which can be lowered by destabilization
of the HOMO and/or stabilization of the LUMO.1d,e As both
the HOMO and the LUMO are π-symmetric, they should be
more susceptible to the influence of π-interactions with the
1
from the La excited state. The emissions show very small
(23) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London,
1970.
(24) Platt, J. R. J. Chem. Phys. 1949, 17, 484.
(25) Clar, E. Spectrochim. Acta 1950, 4, 116.
solvent dependence (Supporting Information). For example,
the emission of 4 is red-shifted by 300 cm-1 by changing the
solvent from diethyl ether to DMSO. The complex 6 displays

Article
Organometallics, Vol. 29, No. 11, 2010 2427
Table 3. Absorption and Emission Spectroscopic Data of the Complexes
emission
quantum
yield Φ
solid-state
emission max/
nm
soln emission
max/nm
emission
lifetime τ/ns
complex
1La band/nm (ε, 104 M-1 cm-1
)
1Lb band/nm (ε, 104 M-1 cm-1
)
1
2
4
5
6
498 (1.10), 467 (1.05), 441 (0.56), 417 (0.22) (s)
512 (0.70), 481 (0.77), 455 (0.48)
a
a
509
532
563
562
596
0.5
3.9
5.4
5.5
8.6
0.05
0.06
0.85
0.73
0.60
573
582
614
b
553 (3.11), 515 (2.01), 481 (0.74), 453 (0.20) (s)
552 (2.23), 514 (1.56), 480 (0.59), 453 (0.19) (s)
572 (2.62), 532 (1.78), 498 (0.67) (s)
370 (0.86), 352 (2.32), 335 (1.43)
368 (0.62), 348 (1.90), 332 (1.29)
354 (1.38), 336 (1.20)
642
a No 1Lb band is observed. b Not emissive in the solid state.
Figure 9. Photograph showing the red shift of emission colors
of the metalated tetracenes and tetracenyldiacetylides.
is not certain, but it is probably due to the special orientation
of the tetracenyl rings in the crystal of 5. As mentioned, the
1
fluorescence of the complexes comes from the La excited
state arising from the 1Ag f 1Bu transition, which is polar-
ized along the long axis of the tetracene ring. Although the
perturbations of the substituents would introduce a short-axis
component to the overall transition moment, the major compo-
nent should still lie along the long axis.26 The crystal packings of
1, 2, and 4 show that the long axes of the tetracenyl rings are
parallel: i.e., the dihedral angle between the transition moments
is 0ꢀ, which does not lead to exciton coupling. On the other
hand, the angle between the transition moments of two adjacent
tetracenyl rings is ∼60ꢀ in the crystal of 5, which is close to 70ꢀ
for maximum exciton coupling between two neighboring
chromophores.27 In addition, each tetracenyl ring in 5 is sur-
rounded by four tetracenyl rings. It is therefore possible that the
exciton migration is efficient in the crystal of 5, leading to rapid
transfer excitation energy to nonradiative traps present in the
crystal. Exciton migration has been suggested to be the reason
for the quenching of the solid-state emission of one crystal form
of NO2-4-C6H4-CtC-Au-PCy3 (Cy = cyclohexyl), in which
the dihedral angle between the transition moments of two
neighboring molecules is 79ꢀ.7e
Figure 8. (a) Absorption (solid line) and emission (broken line)
spectra of tetracene (purple), 1 (orange), and 2 (purple) in THF
at room temperature. The excitation wavelength is 420 nm.
(b) Absorption (solid line) and emission (broken line) spectra of
3 (red), 4 (blue), 5 (green), and 6 (black) in CH2Cl2 at room
temperature. The excitation wavelength is 490 nm.
Concluding Remarks
In this study, two synthetic entries to σ-metallotetracenes
were explored. It has been demonstrated that auration and
platination of tetracene and tetracenylacetylide can change
the photophysical properties of the organic chromophore;
particularly, metalation leads to a significant red shift of the
1La absorption band and the corresponding fluorescence.
The X(Et3P)2PtII group has greater perturbations on the
electronic structure of tetracene than the R3PAuI group,
possibly due to its stronger π-donation. The σ-metallotetra-
cenes also exhibit different assemblies of tetracenyl rings in
the largest red shift of 4260 cm-1 (0.53 eV) from that of tetracene.
In addition to the perturbations of the π-donating I(Et3P)2PtII
groups, the red shift could be caused by mixing of the 1La ππ*
excited state and ligand(π-orbital of iodide)-to-ligand(π*-orbital
of tetracene)-charge-transfer excited state.
Solid-State Emission. Crystals of 1, 2, and 4 and powders
of 6 are emissive at room temperature (Figure S1 in the
Supporting Information). The solid emission energies of the
complexes are all slightly red-shifted from the correspond-
ing solution emissions by 1200-2200 cm-1. Surprisingly, the
crystal of 5 is virtually nonemissive, despite the strong
solution luminescence. The cause of the emission quenching
(26) Dewey, H. J.; Deger, H.; Froelich, W.; Dick, B.; Klingensmith,
K. A.; Hohlneicher, G.; Vogel, E.; Michl, J. J. Am. Chem. Soc. 1980, 102,
6412.
(27) Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy:
Exciton Coupling in Organic Stereochemistry; University Science Books:
Mill Valley, CA, 1983.

2428 Organometallics, Vol. 29, No. 11, 2010
Nguyen and Yip
their crystals. The typical herringbone structure is only ob-
served in the crystal of Ph3PAuI-tetracene. An interesting
result is that the crystal of (Me3PAuI)2-tetracenyldiacetylide
exhibits a honeycomb network which is mainly supported by
aurophilic interactions. The finding highlights the potential of
secondary interactions such as hydrogen bonding and metal-
lophilicity in controlling the patterning of tetracenyl rings in
the solid state.
PCH2CH3, PCH2CH3), 0.89-0.79 (m, 9H, PCH2CH3). 31P{1H}
=
NMR (121.5 MHz, CDCl3): δ 8.96 (d, 2JP-P = 17.8 Hz, 1JPt-P
1580 Hz, Ptrans), 2.94 (d, 2JP-P = 17.8 Hz, 1JPt-P = 4070 Hz, Pcis).
ESI-MS: m/z 739 [Mþ þ H].
Synthesis of (Ph3PAuI)2-Tetracenyldiacetylide (4). To a solu-
tion of 5,12-bis((triisopropylsilyl)ethynyl)tetracene (111 mg,
0.188 mmol) in THF (10 mL) was added 2 mL of EtOH and a
solution of Bu4NF (238 mg, 0.754 mmol) in THF (10 mL) with
stirring at room temperature over 1 h in the absence of light. The
mixture was diluted with 50 mL of Et2O, washed with water, and
dried over MgSO4. The solvent was removed under reduced
pressure. The resulting red solid was immediately used in the
next step without further characterizations. A solution of 5,12-
diethynyltetracene in CH2Cl2 (15 mL) was treated with 5 mL of
a methanolic solution of MeONa (122 mg, 2.261 mmol) and a
solution of Ph3PAuCl in CH2Cl2/MeOH (1/3, 20 mL). The
mixture was stirred at room temperature overnight. Upon
evaporation to dryness, the solid residue was extracted with
CH2Cl2. The addition of Et2O afforded a purple solid. Yield:
120 mg, 53%. X-ray-quality crystals of 4 were obtained from
CHCl3/isooctane at -20 ꢀC. Anal. Calcd for 4 (C58H40Au2P2):
C, 58.40; H, 3.38. Found: C, 58.01; H, 3.47. 1H NMR (300 MHz,
CDCl3): δ 9.50 (s, 2H, H6,11), 8.88 (dd, J = 3.3, 6.9 Hz, 2H,
Experimental Section
General Methods. All syntheses were carried out under a
N2 atmosphere. All the solvents used for synthesis and spec-
troscopic measurements were purified according to the lite-
rature procedures. Pt(PEt3)4,28 trans-Pt(PEt3)2I2,29 Ph3PAuCl,30
Me3PAuCl,11b 5-bromotetracene,31 and 5,12-bis((triisopropyl-
silyl)ethynyl)tetracene3 were prepared according to reported
procedures.
Physical Methods. The UV/vis absorption and emission
spectra of the complexes were recorded on a Hewlett-Packard
HP8452A diode array spectrophotometer and a Perkin-Elmer
LS-50D fluorescence spectrophotometer, respectively. Rhod-
amine 640 (also known as rhodamine 101)32 was used as a stan-
dard in measuring the emission quantum yields. Emission
lifetimes were recorded on a Horiba Jobin-Yvon Fluorolog
FL-1057 fluorometer. 1H and 31P{1H} NMR spectra were
H
H
1,4), 8.11 (dd, J = 3.3, 6.7 Hz, 2H, H7,10), 7.70-7.44 (m, 32H,
2,3, Ph), 7.37 (dd, J = 3.3, 6.7 Hz, 2H, H8,9). 31P{1H} NMR
(121.5 MHz, CDCl3): δ 42.94 (s). FAB-MS: m/z 1192.3 [Mþ].
(Me3PAuI)2-Tetracenyldiacetylide (5). The compound was
prepared by following the procedure described above using
Me3PAuCl. Yield: 46%. Crystals of 5 were grown by slow
diffusion of Et2O into a CH2Cl2/CH3CN solution at room
temperature. Anal. Calcd for 5 (C28H28Au2P2): C, 40.99; H,
3.44. Found: C, 40.56; H, 3.50. 1H NMR (300 MHz, CDCl3): δ
9.44 (s, 2H, H6,11), 8.80 (dd, J = 3.1, 6.7 Hz, 2H, H1,4), 8.09 (dd,
J = 3.1, 6.6 Hz, 2H, H7,10), 7.43 (dd, J = 3.1, 6.7 Hz, 2H, H2,3),
7.36 (dd, J = 3.1, 6.6 Hz, 2H, H8,9), 1.62 (d, 2JHP = 10 Hz, 18H,
CH3). 31P{1H} NMR (121.5 MHz, CDCl3): δ 1.58 (s). FAB-MS:
m/z 820.2 [Mþ].
1
recorded on a Bruker ACF 300 spectrometer. H-1H COSY
spectra were recorded on a Bruker DRX500 NMR spectro-
meter with a 5 mm Cryo TXI Probe. All chemical shifts are
quoted relative to SiMe4 (1H) or H3PO4 (31P). Elemental
analyses of the complexes were carried out at the microanalysis
laboratory of the Department of Chemistry at the National
University of Singapore.
Synthesis of Ph3PAuI-Tetracene (1). To a stirred solution
of 5-bromotetracene (143 mg, 0.46 mmol) in freshly distilled
THF (25 mL) was added 0.32 mL (0.5 mmol) of 1.6 M n-BuLi at
-78 ꢀC. After the mixture was stirred for 2 h at -78 ꢀC, 0.23 g
(0.46 mmol) of Ph3PAuCl was added to the resulting deep red
solution and the mixture was warmed to room temperature and
stirred for 12 h. After the solvent was reduced, excess hexane was
added and the precipitate was filtered off. Needlelike crystals
were obtained from the filtrate upon evaporation of the sol-
vents. Yield: 31 mg, 10%. X-ray-quality crystals of 1 were grown
by slow evaporation of an acetone solutions Anal. Calcd for 1
trans-[I(Et3P)2PtII]2-Tetracenyldiacetylide (6). trans-Pt(PEt3)I2
(2 g, 2.9 mmol) was dissolved in toluene (50 mL) containing CuI
(10 mg, 0.05 mmol) and HNEt2 (10 mL). To this mixture was
added a solution of 5,12-diethylnyltetracene (140 mg, 0.51 mmol)
in 100 mL of toluene. The reaction mixture was stirred overnight at
room temperature, and the solvent was then removed under
reduced pressure. The purple product was isolated from column
chromatography (silica gel, 20 cm ꢀ 4 cm column, hexane/
dichromethane 8/3, v/v). Yield: 400 mg, 56%. Anal. Calcd for 6
(C46H70I2P4Pt2): C, 39.72; H, 5.07. Found: C, 40.15; H, 5.17. 1H
NMR (300 MHz, CDCl3): δ 9.34 (s, 2H, H6,11), 8.69 (dd, J = 3.1,
6.8 Hz, 2H, H1,4), 7.96 (dd, J = 3.3, 6.6 Hz, 2H, H7,10), 7.42-7.37
(m, 4H, H2,3,8,9), 2.27-2.22 (m, 24H, PCH2CH3), 1.33-1.18 (m,
36H, PCH2CH3). 31P{1H} NMR (121.5 MHz, CDCl3): δ 9.57 (s,
1JPt-P = 2323 Hz). ESI-MS: m/z 1389.2 [M]þ.
1
(C36H26AuP): C, 62.98; H, 3.82. Found: C, 63.20; H, 3.77. H
NMR (300 MHz, CDCl3): δ 9.51 (s, 1H, H6), 8.81 (m, 1H, H4),
8.63 (s, 1H, H12), 8.49 (s, 1H, H11), 7.99-7.82 (m, 3H, H1,7,10),
7.80-7.75 (m, 6H, Ph), 7.58-7.55 (m, 9H, Ph), 7.34-7.30 (m,
4H, H2,3,8,9). 31P{1H} NMR (121.5 MHz, CDCl3): δ 45.62 (s).
FAB-MS: m/z 686.1 [Mþ].
Synthesis of cis-[Br(Et3P)2PtII]-Tetracene (2). To a toluene
solution (20 mL) of Pt(PEt3)4 (172 mg, 0.26 mmol) was added
5-bromotetracene (49 mg, 0.16 mmol). The resulting solution
was stirred overnight, and an orange precipitate was produced.
The solid was filtered and washed with cold toluene and ether.
Yield: 60 mg, 51%. Slow evaporation of an acetone solution of 2
afforded orange-red crystals suitable for an X-ray crystallogra-
phy study. Anal. Calcd for 2 (C30H41PtP2Br): C, 48.79; H, 5.60.
Found: C, 48.66; H, 5.36. 1H NMR (300 MHz, CDCl3): δ 9.63
(s, 1H, H6), 8.85 (d, J = 8.2 Hz, 1H, H4), 8.55 (s, 1H, H12), 8.33
(s, 1H, H11), 7.94-7.84 (m, 3H, H1,7,10), 7.28-7.23 (m, 4H,
X-ray Crystallography. The diffraction experiments were
carried out on a Bruker AXS SMART CCD three-circle
diffractometer with a sealed tube at 223 K using graphite-
˚
monochromated Mo KR radiation (λ = 0.710 73 A). The
software used was as follows: SMART33a for collecting frames
of data, indexing reflections, and determining lattice para-
meters; SAINT33a for integration of intensity of reflections and
scaling; SADABS33b for empirical absorption correction;
(33) (a) SMART & SAINT Software Reference Manuals, version 4.0;
Siemens Energy and Automation, Inc., Analytical Instrumentation: Madison,
WI, 1996, (b) Sheldrick, G. M. SADABS: Software for Empirical
H
2,3,8,9), 2.28-2.23 (m, 6H, PCH2CH3), 1.47-1.34 (m, 15H,
€
€
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(29) Hannebohn, O.; Klemm, W. Z. Anorg. Allg. Chem. 1936, 229,
Absorption Correction; University of Gottingen, Gottingen, Germany,
1996. (c) SHELXTL Reference Manual, version 5.03; Siemens Energy
and Automation, Inc., Analytical Instrumentation: Madison, WI, 1996.
(34) (a) Burini, A.; Fackler, J. P.; Galassi, R.; Grant, T. A.; Omary,
M. A.; Rawashdeh-Omary, M. A.; Pietroni, B. R.; Staples, R. J. J. Am.
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Article
Organometallics, Vol. 29, No. 11, 2010 2429
SHELXTL33c for space group determination, structure solu-
tion, and least-squares refinements on |F|2. Anisotropic ther-
mal parameters were refined for the rest of the non-hydrogen
atoms. The hydrogen atoms were placed in their ideal posi-
tions. Some of the ethyl groups in 2 have large thermal motion,
resulting in the observed shorter C-C bonds. The Me3P ligand
in 5 is disordered into two positions with 0.64 and 0.36
occupancies. There are some residual peaks in the voids of
the crystal, but the peak heights are too low and do not fit any
definite solvent. The crystal data and details of data collection
and refinement are summarized in Table S1 in the Supporting
Information.
Acknowledgment. We are grateful to Prof. Koh Lip
Lin and Ms. Tan Geok Kheng for determining the X-ray
structures. The Ministry of Education (Grant No. R-143-
000-331-112) of Singapore and the National University of
Singapore are thanked for financial support.
Supporting Information Available: CIF files giving crystal
data for 1, 2, 4, and 5 and tables and figures giving solid state
emission spectra, solution emission spectra in different solvents,
and crystal data. This material is available free of charge via the
Internet at http://pubs.acs.org.