Luminescent pincer-type cyclometalated platinum(II) complexes with auxiliary isocyanide ligands: Phase-transfer preparation, solvatomorphism, and self-aggregation

Article abstract of DOI:10.1021/om300965b

A series of luminescent pincer-type cyclometalated platinum(II) complexes, namely [4-R-(N^aC^aN) PtCi-≡N(C₆H₃-2,6-Me₂)]PF ₆ (2-4; 4-R-(N^aCH ^aN) represents substituted 1,3-bis(2′- pyridyl)benzene ligands), were prepared under phase-transfer conditions. Complex 2 shows two solvatomorphic crystal structures and exhibits vapoluminescent response toward acetonitrile vapor. The photophysical properties of and supramolecular nanowires formed from these complexes were also investigated.

Full text of DOI:10.1021/om300965b

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pubs.acs.org/Organometallics
Luminescent Pincer-Type Cyclometalated Platinum(II) Complexes
with Auxiliary Isocyanide Ligands: Phase-Transfer Preparation,
Solvatomorphism, and Self-Aggregation
Yong Chen,†,‡ Wei Lu,†,§ and Chi-Ming Che*,†
†State Key Laboratory of Synthetic Chemistry, Institute of Molecular Functional Materials, HKU-CAS Joint Laboratory on New
Materials, and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, People's Republic of China
‡Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese
Academy of Sciences, Beijing 100190, People's Republic of China
§Department of Chemistry, South University of Science and Technology of China, 1088 Xueyuan Boulevard, Shenzhen, Guangdong
518055, People's Republic of China
S
* Supporting Information
ABSTRACT: A series of luminescent pincer-type cyclo-
metalated platinum(II) complexes, namely [4-R-(N∧C∧N)-
PtCN(C6H3-2,6-Me2)]PF6 (2−4; 4-R-(N∧CH∧N) repre-
sents substituted 1,3-bis(2′-pyridyl)benzene ligands), were
prepared under phase-transfer conditions. Complex 2 shows
two solvatomorphic crystal structures and exhibits vapolumi-
nescent response toward acetonitrile vapor. The photophysical
properties of and supramolecular nanowires formed from these
complexes were also investigated.
Chart 1. Chemical Structures for Complexes 1 (Previous
Work) and 2−4 (Present Work)
here has been considerable interest in cyclometalated
∧
∧
TPt(II) complexes supported by the pincer-type (N C N)
ligands (N∧CH∧N = 1,3-bis(2′-pyridyl)benzene) and its
analogues, since Williams and co-workers reported the
3
intriguing intense ππ* emission of this class of complexes in
fluid solutions with emission quantum yields up to 0.68.1 The
dramatic increase in both emission quantum yield and lifetime,
in comparison to their 6-phenyl-2,2′-bipyridine (CH∧N∧N)
isoelectronic congeners,2 have been ascribed to the shorter Pt−
C bond distances of this class of Pt(II) complexes that raise the
energy of the d−d state, thereby diminishing the nonradiative
decay pathway(s).3 The intense phosphorescence enables
charge-neutral cyclometalated Pt(II) complexes of this type
to be attractive candidates for applications in chemosensing and
in optoelectronic devices.4 Despite these advances, only limited
examples of cationic pincer-type (N∧C∧N)Pt complexes have
been reported in the literature. We report herein new members
of cationic cyclometalated Pt(II) complexes of [4-R-(N∧C∧N)-
PtCN(C6H3-2,6-Me2)]PF6 type that display intriguing
spectroscopic and self-assembling properties,
[(N∧C∧N)PtCl] and the desired product, were identified on
the basis of ESI-MS and H NMR data, most probably as a
1
result of reversible ligand exchange reactions. To shift the
reaction equilibrium to allow for the isolation of pure N∧C∧N-
coordinated platinum(II) isocyanides 2−4 (Chart 1), we
designed an aqueous/organic phase-transfer reaction to enrich
the targeted cationic products in the aqueous solution phase.
The two-phase reaction system for the synthesis of
organoplatinum(II) salts in the present work, namely [4-R-
(N∧C∧N)PtCN(C6H3-2,6-Me2)]PF6 (R = H for 2, CF3 for
3, and Me for 4), consisted of equivalent volumes of water and
dichloromethane. When an excess amount of 2,6-dimethyl-
phenyl isocyanide was added to the lower-layer dichloro-
In previous works, we attempted to prepare [(N∧C∧N)-
PtCN(C6H3-2,6-Me2)]Cl from a [(N∧C∧N)PtCl] precursor
and 2,6-dimethylphenyl isocyanide by a simple ligand-
substitution reaction in homogeneous solution,5 as this method
is useful for the synthesis of the C∧N∧N congeners, such as
complex 1 in Chart 1. However, no pure product was obtained
through this approach, even though a large excess of isocyanide
ligand was used. Instead, two species, the starting material
Received: October 17, 2012
Published: December 18, 2012
© 2012 American Chemical Society
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Note
methane solution containing [4-R-(N∧C∧N)PtCl], the cationic
[4-R-(N∧C∧N)PtCN(C6H3-2,6-Me2)]Cl salt thus formed
was quickly transferred to the aqueous phase. The luminescent
color change in the course of the reaction could be monitored
by human eyes under a 365 nm lamp (Figure 1). Generally,
Figure 1. Changes in solution color and luminescence of the two-
phase system upon the addition of isocyanide or chloride ions to
induce the switching of the substitution reaction.
upon the addition of isocyanide ligand under vigorous stirring
conditions, the pale yellow color of the organic layer faded, and
this was accompanied by the disappearance of the green
emission. Meanwhile, the aqueous phase changed from
colorless to pink with a concomitant development of an
intense red emission. The red emission from the aqueous
solution is a signature of molecular aggregation of monomeric
Pt(II) complexes through intermolecular Pt···Pt and/or
ligand−ligand interactions.5 When the pink aqueous phase
containing cationic salts was separated and mixed with a
saturated aqueous NH4PF6 solution, the corresponding salts 2−
4 precipitated out and were isolated as red, yellow, and orange-
red solids, respectively. Interestingly, the colors and lumines-
cence of the organic phase could be restored by adding an
excess of chloride ions into the aqueous layer. The charge-
neutral precursor [(N∧C∧N)PtCl] was regenerated and was
extracted back into the organic phase.
Figure 2. (a) Two solvatomorphic crystals grown for complex 2 and
the crystal-packing diagrams of the red and yellow crystals with
hydrogen atoms, counterions, and solvated molecules omitted for
clarity. (b) Powder X-ray diffraction patterns of 2 in various forms.
alternating intermetal separations of 3.74 and 5.46 Å are much
longer than the sum of van der Waals radii. Efforts to obtain
crystals of 3 and 4 suitable for X-ray crystallographic studies
were not successful. However, the yellow color of 3 and the
orange-red color of 4 in the solid state are indicative that Pt···Pt
interactions are absent in the former but could be present in the
latter.
The two solvatomorphs of crystal 2, the red form and the
yellow form (acetonitrile phase), can be converted into each
other through sorption or desorption of the solvated
acetonitrile molecules. When the needle-shaped crystal of 2
was immersed into a mixture of acetonitrile and diethyl ether, it
changed from red to yellow within several minutes.9 The
powder X-ray diffraction (PXRD) pattern of the yellow solid
thus obtained showed peaks at 2θ = 6.28, 7.24, 9.22, and 9.96°,
which is identical with the pattern simulated from the crystal
data of the yellow crystals obtained from the acetonitrile phase,
indicative of a similar packing of molecules. The changes from
the red form to the yellow form could be ascribed to structural
reorganization induced by acetonitrile solvent molecules on the
basis of X-ray crystallographic analysis. However, the red crystal
form could not transform directly to the yellow form upon
exposure to acetonitrile vapor. Instead, only an orange solid was
obtained, and this solid was envisioned to be an intermediate
state between the red and yellow forms. PXRD measurements
on the resulting orange sample supported this assumption; the
diffraction pattern of the orange powder was different from that
of either the red or the yellow forms (Figure 2b). On the other
hand, the yellow crystal was unstable and swiftly transformed to
its red form upon aging in air (Figure 2a), leaving some colorful
crystals with alternating red and yellow fragments. Several
recent reports attributed the color and emission response of
Pt(II) complexes toward volatile organic compounds (VOCs)
to the vapochromic behavior arising from alterations in
intermolecular packing.10
Two crystal forms of 2 were obtained, depending on the
recrystallization conditions. Slow diffusion of diethyl ether
vapor into a dichloromethane solution of 2 at ambient
temperature afforded red needle-shaped crystals with an Ima2
space group, while yellow prism crystals with a P1 space group
̅
were isolated from an acetonitrile solution. There have been
many reports on the polymorphic behavior of square-planar
Pt(II) complexes in the literature, all of which were found to be
associated with the molecular arrangement and Pt···Pt distances
in the crystal structures.6 The crystal structure of the red form
features infinite Pt···Pt chains along the a axis. Every two
neighboring molecules are in a staggered arrangement with a
torsion angle of 117° around the Pt−Pt axis. The uniform
intermetal distance of 3.43 Å (slightly shorter than the sum of
van der Waals radii of 3.44 Å) and Pt−Pt−Pt angle of 170° are
indicative of extended d8···d8 interactions along the near-linear
Pt···Pt backbone (Figure 2a). The phenyl ring of the isocyanide
ligand and the [(N∧C∧N)Pt] plane are coplanar, which is
similar to the case for its C∧N∧N counterpart7 but different
from that for the [(N∧C∧N)PtCCPh] crystal,8 where the
acetylenic phenyl ring is perpendicular to the plane of the
cyclometalated ligand. There is a solvated acetonitrile molecule
in the unit cell of the yellow form of 2. The cations in the
yellow form are stacked into chains, with every two adjacent
molecules in a head-to-tail fashion with an interplanar distance
of ca. 3.5 Å. Close Pt···Pt interactions are excluded, as the
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The absorption and emission spectra of complexes 2−4 were
recorded under various conditions, with the results depicted in
Figures 3 and Figures S2 and S3 (Supporting Information) and
μs), respectively. When a film of the red sample 2 drop-casted
from a dichloromethane solution onto a quartz plate was
exposed to acetonitrile vapor, a blue shift in emission energy
and an enhancement of the emission intensity were observed
(Figures 4), accompanied by a change in film color from red to
Figure 3. Absorption and emission spectra of complex 2.
Figure 4. Emission spectral traces of the red form of 2 drop-casted on
a quartz plate in response to acetonitrile vapors. The excitation
wavelength was 365 nm. The emission spectra were recorded at
intervals of approximately 1 min.
the photophysical data summarized in Table S1 (Supporting
Information). All three complexes in CH3CN exhibit similar
absorption spectral profiles. The intense bands in the region
248−329 nm with large extinction coefficients (∼104 mol−1
dm3 cm−1) originate from intraligand π−π* transitions
localized on the cyclometalated 1,3-bis(2′-pyridyl)benzene
ligand. The low-energy absorptions at 350−450 nm are
tentatively assigned to transitions having mixed charge-
transfer/intraligand characters, with reference to previous
works on the [(N∧C∧N)PtCC(C6H4-4-R)] system.8,11 As
the 4-substituent on the phenyl ring of the cyclometalated
ligand becomes more electron donating from 3 via 2 to 4, the
low-energy band red-shifts in energy.
orange. The emission of the yellow form of 2 with λmax at 565
nm is further blue-shifted from that of the red and orange forms
(Figure S1, Supporting Information). In contrast to 2, both 3
and 4 show a broad structureless emission at room temperature
and an intense vibronic structured band at 77 K (Figures S2
and S3, Supporting Information), suggesting that the emissive
excited states are temperature dependent.
Recently, we and others have reported the usage of Pt(II)
complexes to construct quasi-one-dimensional nanowires,13
quasi-two-dimensional nanosheets,8 and nanowheels.14 We
examined the self-assembly of complexes 2−4 and found that
the molecular aggregates thus formed are affected by the extent
of Pt···Pt interactions and/or π−π stacking of cyclometalated
ligands. Superstructures of all of the complexes in this work
were obtained by a reprecipitation method without the aid of
surfactants. As an example, a CH3CN solution of 2 at a
concentration of 1000 μM was prepared. The stock solution
was diluted to 100 μM with water to give dispersions in
CH3CN/H2O (1/9 v/v). Scanning electron microscopy (SEM)
and transmission electron microscopy (TEM) images of a
dispersion of 2 (Figure 5) in CH3CN/H2O revealed well-
defined nanowires with width less than 50 nm and length up to
2 μm. Changing the solvent from water to diethyl ether in the
course of reprecipitation could increase the lateral dimensions
while keeping the morphology of nanowires unchanged. Under
similar fabrication conditions, complexes 3 and 4 assembled
into nanorods and nanowires, respectively, both with lower
aspect ratios and ill-defined morphologies. We propose that the
intermolecular Pt···Pt and/or π−π interactions would contrib-
ute to directing the anisotropic growth and self-aggregation to
one-dimensional nanostructures.
Complexes 2−4 emit brightly in the green spectral region in
degassed CH3CN at a concentration of 2.0 × 10−5 mol dm−3 at
298 K, with λmax (lifetime, quantum yield) values of 478 nm
(5.6 μs, 12%), 472 nm (3.5 μs, 8%), and 497 nm (4.60 μs,
14%), respectively. The emission maxima are red-shifted with
the substituent attached to the phenyl unit of the cyclo-
metalated ligand in the order CF3 < H < CH3, and the
excitation spectra monitored at λmax match well with the
corresponding absorption spectra. The vibronically structured
band reveals a spacing of about 1400 cm−1 in accordance with
the CC and CN vibrational modes of the 1,3-bis(2′-
pyridyl)benzene moiety, suggesting that the emission originates
from a state of primarily 3LC character. Lowering the
temperature from 298 to 77 K resulted in a highly structured
emission in the high-energy region, together with a lower
energy broad band. For example, the emission spectrum of 2 in
a glassy butyronitrile solution at 77 K is characterized by a
vibronically resolved emission band with peak λmax at 474 nm (τ
= 25.4 μs) and two broad emission bands with peak λmax at 560
nm (τ = 3.6 μs) and 683 nm (τ = 3.6 μs), respectively. The
high-energy emissions are comparable in energy with those
recorded in solution, whereas the low-energy bands with peaks
λmax 560 and 683 nm could be tentatively ascribed to be triplet
ππ* excimeric and triplet metal−metal-to-ligand charge-transfer
(3MMLCT) excited states, respectively.
In conclusion, three new pincer-type cyclometalated Pt(II)
isocyanides were prepared under phase-transfer conditions.
These intensely phosphorescent complexes can be assembled
into one-dimensional nanowires or nanorods driven by
interamolecular Pt···Pt and/or π−π stacking interactions. The
solvatomorphism and vapoluminescent behaviors of 2 are
rationalized by structural rearrangements induced by the
Solid-state emission of oligopyridyl/cyclometated Pt(II)
complexes could be significantly affected by the packing of
molecules in their crystal structures.12 The red form of 2 shows
a broad and structureless emission at both room temperature
and 77 K, with λmax at 667 (τ = 0.2 μs) and 723 nm (τ = 1.0
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ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files, text, tables, and figures giving crystallographic data for
both the red and yellow forms of 2, experimental details,
structure characterization data, supplementary absorption and
emission spectra, and spectroscopic and photophysical data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: cmche@hku.hk.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Key Basic Research
Program of China (973 Program 2013CB834802 and
2013CB834803) and the Hong Kong Research Grants Council
(HKU 7011/07P and 7008/09P). We thank the Natural
Science Foundation of China (NSFC Grant No. 21001110),
the NSFC/RGC Joint Research Scheme, and CAS-Croucher
Funding Scheme for Joint Laboratories. Y.C. acknowledges the
Technical Institute of Physics and Chemistry, Chinese
Academy of Sciences (“100 Talent” Program), for funding
support.
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