Synthesis and spectroscopic properties of modular platinum(II) terpyridine phosphorescent pockets

Article abstract of DOI:10.1039/b900030e

A modular design approach has been utilized to develop molecular 'pockets' featuring three integrated components, namely a phosphorescent Pt(II)-π(organic) moiety, a suitable receptor group, and a rigid, conjugated connecting backbone, and the cavities therein are examined by NMR spectroscopy and an X-ray crystal structure. Their photophysical properties have been investigated; unusually for Pt-terpyridine derivatives, the complexes are luminescent (near-red ³MLCT emission) in solution at room temperature. It has been shown that their emissive characteristics are affected by the nature of the backbone component but not the receptor group. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b900030e

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www.rsc.org/dalton | Dalton Transactions
Synthesis and spectroscopic properties of modular platinum(II) terpyridine
phosphorescent pockets†
Wah-Leung Tong,a Michael C. W. Chan,*a Nianyong Zhub and Sze-Kar Leunga
Received 6th January 2009, Accepted 27th March 2009
First published as an Advance Article on the web 29th April 2009
DOI: 10.1039/b900030e
A modular design approach has been utilized to develop molecular ‘pockets’ featuring three integrated
components, namely a phosphorescent Pt(II)-p(organic) moiety, a suitable receptor group, and a rigid,
conjugated connecting backbone, and the cavities therein are examined by NMR spectroscopy and an
X-ray crystal structure. Their photophysical properties have been investigated; unusually for
Pt-terpyridine derivatives, the complexes are luminescent (near-red 3MLCT emission) in solution at
room temperature. It has been shown that their emissive characteristics are affected by the nature of the
backbone component but not the receptor group.
displacement assay approach pioneered by Anslyn et al.3 has
Introduction
been highly successful.4 We deemed that the pursuit of integrated
The design of fluorescent and colorimetric sensors for anionic
analytes has employed approaches based on molecular recognition
and supramolecular chemistry.1 While organic hosts have been
investigated intensively, the development of transition metal-
organic systems can offer distinct advantages including visible-
light signalling, inherent electrostatic attraction, good tunability,
and well-defined structures and coordination geometries with
potential binding sites.2 For example, the versatile indicator
phosphorescent molecular hosts which reversibly utilize non-
covalent interactions would be desirable, and proceeded to devise a
modular-based design strategy encompassing the following chem-
ical and structural features: 1) a coordinatively unsaturated tran-
sition metal-(p-organic) phosphorescent signalling/sensing unit,
enabling axial substrate-binding interactions and visible-light
reporting without interference from organic/background fluores-
cence; 2) a receptor component displaying complementarity with
intended guest(s), linked to the luminophore by a rigid backbone
to maintain structural integrity; 3) conjugated units for improved
communication between modules; 4) a robust framework that is
easy to functionalize (Scheme 1).
Pt(II)-oligopyridine moieties are ideally suited as luminophores
in this strategy because of their ability to exhibit diverse and
tunable excited states that are exceptionally sensitive to pertur-
bations in the surrounding micro-environment.5,6 In this study,
the cationic Pt-terpyridine (tpy) moiety is expected to facilitate
anion-binding, although the charge should not be localized,
aDepartment of Biology and Chemistry and Centre for Marine Environmental
Research and Innovative Technology, City University of Hong Kong, Tat
Chee Avenue, Kowloon, Hong Kong, China. E-mail: mcwchan@cityu.edu.hk
bDepartment of Chemistry, The University of Hong Kong, Pokfulam Road,
Hong Kong, China
† Electronic supplementary information (ESI) available: Experimental
details and characterization data for functionalized terpyridine ligands and
1–4; 77 K glass and solid-state emission data for 1–4; absorption and solu-
tion emission spectra of 1 and 2 in CH2Cl2 and CH3CN at 298 K. CCDC
reference number 716277 for 2[ClO4]. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/b900030e
Scheme 1 Modular design of phosphorescent pockets.
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thus reducing unfavourable solvation effects. In addition, the
associated near-red emission can help to minimize background
interference in sensing applications. This would cumulate in the
creation of a ditopic phosphorescent pocket, which may display
interesting binding properties and selectivity. Unusually, the Pt-
terpyridine complexes in this work are emissive in solution at room
temperature.
titrations to investigate a [Pt(urea-isoquinoline)4]2+ receptor, which
employs urea hydrogen-bond donors to encapsulate anions in
solution,9 while analogous organic systems have been reported.10
It is generally known that conventional ion-pairing contacts in
solution can alter the structure/conformation and generally cause
1H NMR signals to shift. Here, since only specific protons in the
cavity (Hd–f) are greatly affected and the majority are undisturbed,
we propose that the unusual NMR perturbations for different
anions may also be attributed to weak C–H ◊ ◊ ◊ X- or p-anion11
interactions with the said aryl protons (see next section).
Results and discussion
Synthesis and NMR characterization
Crystal structure
A modular synthetic methodology has been developed for this
series of Pt(II) complexes. Starting from a rigid backbone unit
such as xanthene7 or dibenzofuran8 (Scheme 2), the attachment
and integration of any chosen luminophore and receptor group
can be readily achieved by Pd-coupling reactions, and has been
demonstrated for Pt-tpy and benzoic acid/benzoate/diamino-
triazine respectively. Hence, new Pt(II) molecular ‘pockets’ bearing
these modules have been prepared in order to examine the impact
of cavity size and receptor group upon luminescent properties and
ultimately, sensing capabilities (1–4, Scheme 1). Bulky substituents
on the backbone are incorporated to impede intermolecular
p-stacking and related aggregation processes, which may compli-
cate future sensing studies. As may be anticipated, the solubility
of 1 and 4 is poor in non-polar organic solvents but improved
in polar media, rendering them suitable for polar and possibly
aqueous applications.
The molecular structure of 2[ClO4] has been determined by
X-ray crystallography (Fig. 2).12 The xanthene backbone is slightly
puckered as expected, and dihedral angles of ca. 58 and 38◦ with
the central pyridine and benzoate rings respectively are observed.
Importantly, a dihedral angle of 0.6◦ is evident between the Pt-
tpy and benzoate planes, demonstrating that these fragments are
rotated to approach coplanarity. This conformation enables them
to be sufficiently close to maximize intramolecular edge-to-edge
˚
p–p interactions (interplanar separations ca. 3.5 A; e.g.
˚
C(6) ◊ ◊ ◊ C(33) and C(7) ◊ ◊ ◊ C(34) of 3.54 and 3.52 A respectively),
and changing the afore-mentioned dihedral angles would pre-
sumably reduce the p-overlap. While Pt ◊ ◊ ◊ O(ester) contacts are
not detected, weak C–H ◊ ◊ ◊ anion interactions are found at the
-
˚
cavity/tpy unit. The shortest C–H ◊ ◊ ◊ OClO3 separation of 2.7 A
(C ◊ ◊ ◊ O 3.4 A; C–H ◊ ◊ ◊ O 132 ◦) is observed at H(4), which
˚
corresponds to the aryl hydrogen Hd in the cavity (see above).
It is intriguing to note that during the synthesis of the Cl-,
-
Nevertheless, the C–H ◊ ◊ ◊ ClO4 interactions in this structure are
-
-
1
ClO4 and PF6 salts of 2, substantial H NMR perturbations
were observed for the aryl hydrogens Hd, He and Hf in the cavity
(Fig. 1a: relative to Cl- in CD2Cl2, Dppm = -0.54, -0.52 and
relatively weak, and this may be attributed to crystal packing
effects.
-
-
-0.24 for ClO4 ; -0.67, -0.61 and -0.33 for PF6 respectively).
Furthermore, we have shown that by using 2[Cl] as a prototypical
anion sensor, the qualitative NMR discrimination of anions is
feasible (Fig. 1b). Loeb and co-workers have used 1H NMR
Absorption and emission spectroscopy
The photophysical properties of 1–4 [ClO4] have been investigated
(Table 1). The UV-vis absorption spectra display similar patterns,
Scheme 2 Synthesis of dibenzofuran-linked complex 4.
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Fig. 1 (a) Aromatic region in 1H NMR spectra (300 MHz, CD2Cl2, 298 K)
of 2[X]. (b) Aromatic region in 1H NMR spectra (300 MHz, CDCl3, 298 K;
* = solvent satellites) of 2[Cl] (1 ¥ 10-3 M) upon addition of 0.25 equivalents
of anions (nBu4N+ salts).
Fig. 2 Perspective views of 2[ClO4] (hydrogen atoms are omitted for
clarity; top: 30% probability ellipsoids; bottom: coplanarity of tpy and
benzoate groups, and position of perchlorate anion).
with intense bands at <350 nm (e > 1 ¥ 104 dm3 mol-1 cm-1)
which closely resemble those of the respective non-metallated
ligand in terms of the structured nature and large extinction
coefficients, plus a low-energy band at lmax ~ 410 nm of moderate
intensity (e > 4000 dm3 mol-1 cm-1) that is absent from the ligand
precursor. As reported in spectroscopic studies of related Pt(II)-
tpy complexes,13,14 the high-energy bands at < 350 nm are assigned
as the intraligand pp* transitions of the aryl and tpy groups,
whereas the absorption at lmax ~ 410 nm is assigned as spin-allowed
MLCT 1[(5d)Pt → p*(tpy)]. The similarity between the absorption
spectra of 1–3 suggests that the change from benzoate to acid and
triazine results in minimal differences in the charge transfer nature
of the complexes.
Table 1 Photophysical data at 298 K a
UV-vis lmax/nm
Fluid emission
lmax/nm (t/ns; U)
Complex
1[ClO4]
2[ClO4]
3[ClO4]
4[ClO4]
(e ¥ 104/dm3 mol-1 cm-1)
257 (4.44), 306 (2.06), 333 (1.31),
415 (0.422)
257 (5.67), 281 (3.60), 308 (2.34),
331 (1.81), 415 (0.557)
257 (3.40), 283 (2.36), 337 (1.13),
412 (0.463)
612 (30; 2.5 ¥ 10-3)
617 (39; 2.5 ¥ 10-3)
607 (28; 2.2 ¥ 10-3)
b,c
263 (5.18), 340 (1.51), 359 (1.14),
389 (0.833), 406 (0.748)b
a In CH3CN (10-5 M) unless specified. b In DMSO. c Extremely weak.
The parent terpyridine-Pt(II) complex is non-emissive in solu-
tion at room temperature,13 but judicious substitution, especially
at the tpy moiety, can yield appreciably emissive derivatives.14
In this work, the xanthene complexes 1–3 are luminescent in
solution at room temperature (Table 1 and Fig. 3 for 1; see ESI†
for 77 K glassy and solid-state emissive data). The excitation
spectra obtained closely resemble the corresponding absorption
spectral bands, especially around lmax 410 nm. The respective
orange emission, lifetime and Stokes shift for 1–3 are entirely
consistent with a 3MLCT excited state, as has been assigned
for analogous Pt-tpy complexes.14 Conventional solvatochromic
shifts for the absorption and solution emission bands of 1 and
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Fig. 3 Absorption (298 K, CH2Cl2) and 298 K (CH2Cl2: blue) and 77 K
(MeOH:EtOH [1:4]: green) fluid emission (lex 410 nm) spectra of 1[ClO4]
(10-5 M).
Fig. 4 Normalized 77-K glassy emission spectra of 2[ClO4] in butyroni-
trile at different concentrations (lex 450 nm).
2 in CH2Cl2 and CH3CN at 298 K have been observed (see
ESI†). Estimated radiative decay rate constants (kr = Uo/to,
assuming Uisc = 1) of ca. 83000, 64000 and 76000 are calculated
for 1–3 respectively. Values of 103–105 for kr indicate the heavy
metal effect and are consistent with the forbidden nature of the
triplet excited state. Like the absorption data, the comparable
emission energies, lifetime and quantum yields signify that the
receptor group does not significantly alter the excited state in this
1
series. Unlike the H NMR and solid-state emission data, the
Cl-, ClO4 and PF6 salts of 2 afford identical MLCT emission
at lmax 580 nm in CH2Cl2 at 298 K (see ESI†), indicating that
the impact of counterions upon the excited state in solution is
negligible. In contrast to 1–3, the emission of the dibenzofuran
analogue 4, which is reasonably soluble in polar solvents such as
methanol and DMSO, is extremely weak possibly due to enhanced
solvent quenching15 in the enlarged cavity. This would impose
limitations on subsequent sensing studies, and demonstrates that
the nature/size of the cavity in these complexes can affect their
emissive properties.
-
-
3
Fig. 5 Normalized solid-state emission spectra for Cl- (red), ClO4- (black)
and PF6 (green) salts of 2 at 298 K.
-
These complexes generally exhibit 77-K glassy emissions that
are blue-shifted from the respective solution emission at 298 K. For
example, the glassy emission of 2[ClO4] in butyronitrile at 77 K
features a structured band at lmax 520 nm with a vibronic pro-
gression of ~1200 cm-1 (assigned as metal-perturbed intraligand),
plus an additional low-energy band at lmax 610 nm (Fig. 4)
which begins to appear at 10-4 M and becomes more intense
at higher concentrations. The energy and broad bandshape
of this feature do not correspond to an additional vibronic
structure. Moreover, while the vibronic emission band displays
a lifetime of 35 ms at 520 nm, monitoring of the emission at
610 nm yielded a second-order exponential decay (t1 = 3.4, t2 =
41 ms), indicating the presence of two emitting species. Hence the
concentration-dependent 610 nm band is tentatively assigned to
emissive properties of [Pt(tpy)Cl]+ salts and their relatives are
known to be highly dependent on the counterion and solvent used
for precipitation.13,17 As expected, the observed fluid and solid-
state emission lifetimes are generally longer at low temperatures
(see ESI†) as non-radiative processes become restricted.
Conclusion
Strategies to create modular Pt(II) terpyridine phospho-
rescent complexes, and tune the cavity size and receptor
group/recognition capabilities in a rational manner, have been
developed. Information regarding the nature of the cavity has
been obtained by 1H NMR and X-ray crystallography. The
photophysical properties of the complexes have been examined
using absorption and fluid emission spectra, and the modularity of
the system has allowed us to illustrate that luminescent behaviour
is affected by the size and nature of the cavity/backbone, but not
the receptor component. Applications in phosphorescent sensing
are under development.
excimeric emission.13,14d,16 The recrystallization of the Cl-, ClO4
-
-
and PF6 salts of 2 by diffusion of diethyl ether into CH3CN give
yellow, yellow-orange and orange crystalline solids respectively.
A comparison of their solid-state emission spectra at 298 K
(Fig. 5) revealed red-shifted emission maxima for larger anions,
which is attributed to differences in intermolecular p-stacking and
aggregation rather than C–H ◊ ◊ ◊ anion contacts. The solid-state
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yellow crystals (yield: 68%), one of which was used for X-ray
crystal analysis. Found: C, 53.17; H, 4.53; N, 4.28. Calc. for
C46H45N3O7Cl2Pt·H2O (1035.99): C, 53.34; H, 4.57; N, 4.06. IR
(KBr, cm-1): 1718, 1609. 1H NMR (300 MHz, CD2Cl2): d 9.11 (d,
J = 5.6 Hz, 2H), 8.32-8.26 (m, 2H), 8.14 (s, 2H), 8.00 (d, J =
7.6 Hz, 2H), 7.81 (t, J = 6.2 Hz, 2H), 7.72 (d, J = 2.2 Hz, 1H),
7.55 (d, J = 2.3 Hz, 1H), 7.52 (d, J = 2.2 Hz, 1H), 7.44 (d, J =
8.3 Hz, 2H), 7.32 (d, J = 8.3 Hz, 2H), 7.19 (d, J = 2.3 Hz, 1H),
3.91 (s, 3H), 1.81 (s, 6H), 1.44 (s, 9H), 1.36 (s, 9H). ES-MS (+ve
mode): m/z 918 [M+].
Experimental
General considerations
All reactions were performed under a nitrogen atmosphere and
solvents for syntheses (analytical grade) were used without further
purification. Solvents for photophysical measurements were puri-
1
fied according to conventional methods. H NMR spectra were
obtained on Bruker DRX 300 and 400 FT-NMR spectrometers
(ppm) using Me4Si as internal standard. ESI mass spectra were
measured on a Perkin-Elmer Sciex API 365 mass spectrometer.
IR spectra were recorded on a Perkin-Elmer 1600 series FT-IR
spectrophotometer. Elemental analyses were performed on an
Elementar Analysensysteme GmbH Vario EL elemental analyzer.
UV-vis absorption spectra were obtained on an Agilent 8453 diode
array spectrophotometer.
3[ClO4]. The procedure for 1[ClO4] was adopted using the
terpyridine-xanthene-diaminotriazine ligand. Recrystallization by
vapour diffusion of diethyl ether into a CH3CN solution afforded
yellow crystals (yield: 63%). Found: C, 49.76; H, 4.41; N, 11.02.
Calc. for C41H42N8O5Cl2Pt (992.81): C, 49.60; H, 4.26; N, 11.29.
IR (KBr, cm-1): 1638, 1618. 1H NMR (CD2Cl2 and CD3OD,
300 MHz) d 9.17 (d, 2H), 8.38–8.30 (m, 2H), 8.25 (s, 2H), 8.10–
8.06 (d, 2H), 7.86–7.78 (m, 2H), 7.72 (s, 1H), 7.63 (s, 1H), 7.51 (s,
1H), 7.44 (s, 1H), 1.78 (s, 6H), 1.43 (s, 9H), 1.37 (s, 9H). ES-MS
(+ve mode): m/z 893 [M+].
Steady-state emission spectra were recorded on a SPEX Fluo-
roLog 3-TCSPC spectrophotometer equipped with a Hamamatsu
R928 PMT detector, and emission lifetime measurements were
conducted using NanoLed sources in the fast MCS mode and
checked using the TCSPC mode. Sample and standard solutions
were degassed with at least three freeze–pump–thaw cycles. Low-
temperature (77 K) emission spectra for glasses and solid-state
samples were recorded in 5-mm diameter quartz tubes which were
placed in a liquid nitrogen Dewar equipped with quartz windows.
The emission quantum yield was measured18 by using
[Ru(bpy)3](PF6)2 in degassed acetonitrile as the standard (Ur =
0.062) and calculated by: Us = Ur(Br/Bs)(ns/nr)2(Ds/Dr), where
the subscripts s and r refer to sample and reference standard
solution respectively, n is the refractive index of the solvents, D
is the integrated intensity, and U is the luminescence quantum
4[ClO4]. A mixture of K2PtCl4 (0.011 g, 0.0258 mmol) in
DI water (2 mL) and the terpyridine-dibenzofuran-benzoic acid
ligand (0.017 g, 0.0258 mmol) in CH3CN/CH2Cl2 (4 mL) was
stirred under reflux for 24 h to give a yellow suspension.
After the removal of volatiles, the yellow solid was dissolved in
CH3CN/CH2Cl2, filtered into saturated LiClO4 in MeOH, and
the yellow solution was washed with DI water. After removal of
solvents, the product was obtained by washing copiously with
diethyl ether to yield an orange solid (0.010 g, 40%). Found: C,
48.61; H, 3.97; N, 4.14. Calc. for C40H37N3O7Cl2Si2Pt (993.89):
C, 48.34; H, 3.75; N, 4.23. IR (KBr, cm-1): 1701, 1609. 1H NMR
(300 MHz, d6-DMSO): d 9.17 (s, 2H), 8.76 (s, 1H), 8.72 (d, J =
4.7 Hz, 2H), 8.65 (d, J = 7.9 Hz, 2H), 8.61 (s, 1H), 8.47 (t, J =
7.9 Hz, 1H), 8.20 (d, J = 8.5 Hz, 2H), 8.17 (s, 1H), 7.94 (d, J =
8.2 Hz, 2H), 7.91 (s, 1H), 7.87–7.83 (m, 2H), 7.71–7.69 (m, 1H),
0.50 (s, 9H), 0.46 (s, 9H). ES-MS (+ve mode): m/z 894 [M+].
yield. The quantity B is calculated by the equation: B = 1 - 10-AL
;
where A is the absorbance at the excitation wavelength and L is
the optical path length. Errors for l ( 1 nm), t ( 10%), and U
( 10%) are estimated.
Syntheses
Please see ESI† for full synthetic details.
1[ClO4]. A mixture of K2PtCl4 (0.017 g, 0.041 mmol) in
water (0.5 mL) and the terpyridine-xanthene-benzoic acid ligand
(0.043 g, 0.063 mmol) in CH3CN/CH2Cl2 (3:1; 8 mL) was stirred
under reflux for 48 h to give a yellow solution containing 1[Cl].
After cooling, the solution was mixed with saturated NaClO4 in
MeOH. After removal of volatiles, the mixture was re-suspended in
CH2Cl2, then filtered, and the filtrate was dried in vacuo to afford
1[ClO4] as a yellow solid. Recrystallization by vapour diffusion
of diethyl ether into a DMF/DMSO mixture afforded a yellow
crystalline solid (yield: 65%). Found: C, 53.65; H, 4.61; N, 4.39.
Calc. for C45H43N3O7Cl2Pt (1003.83): C, 53.84; H, 4.32; N, 4.19.
IR (KBr, cm-1): 1701, 1609. 1H NMR (300 MHz, CD3OD): d 8.96
(d, J = 5.2 Hz, 2H), 8.40 (s, 2H), 8.29 (t, J = 7.5 Hz, 2H), 8.19 (d,
J = 7.5 Hz, 2H), 7.84–7.80 (m, 2H), 7.80 (d, J = 2.3 Hz, 1H), 7.62
(d, J = 2.3 Hz, 1H), 7.61 (d, J = 2.3 Hz, 1H), 7.44 (d, J = 8.2 Hz,
2H), 7.33 (d, J = 8.5 Hz, 2H), 7.21 (d, J = 2.3 Hz, 1H), 1.81 (s,
6H), 1.46 (s, 9H), 1.36 (s, 9H). ES-MS (+ve mode): m/z 904 [M+].
Acknowledgements
The work described in this paper was fully supported by grants
from the Research Grants Council of the Hong Kong Special
Administrative Region, China (CityU 100405) and City University
of Hong Kong (7001787).
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12 X-ray diffraction data were collected on a MAR diffractometer with a
300 mm image plate detector using graphite monochromatised Mo-
˚
Ka radiation (l = 0.71073 A). Selected crystal data for 2[ClO4]:
C46H45N3O7Cl2Pt, Mw = 1017.84, monoclinic, P21/c, a = 21.261(7), b =
◦
3
˚
˚
12.008(4), c = 21.963(7) A, b = 113.71(3) , V = 5134(3) A , Z = 4, T =
293(2) K, Dc = 1.317 g cm-3, m(Mo Ka) = 2.883 mm-1, F(000) = 2040,
2qmax= 51.4◦, 9693 independent reflections (Rint = 0.1083), 477 variable
parameters, R1 = 0.081 [I > 2s(I)], wR2 = 0.189. During refinement,
the methyl groups on xanthene are found to be disordered. In the
final stage of least-squares refinement, the disordered C atoms, plus
the N atoms around Pt atom were refined isotropically, and other non-
hydrogen atoms were refined anisotropically. Due to the marginal data
quality, only general conclusions regarding the structure are drawn.
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4746 | Dalton Trans., 2009, 4741–4746
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