The synthesis and photophysical studies of cyclometalated Pt(ii) complexes with C,N,N-ligands containing imidazolyl donors

Article abstract of DOI:10.1039/c1dt11037c

Two new C,N,N-type ligands (HL₂ and HL₃), containing a C_phenyl, a N_pyridyl, and a N_imidazolyl donor, and their cycloplatinated complexes, [Pt(L₂)Cl] (1), [Pt(L ₃)Cl] (2), [Pt(L₂)(PPh₃)]⁺ (3) and [Pt(L₃)(PPh₃)]⁺ (4), have been successfully synthesized and characterized. Spectroscopic and ³MLCT luminescent properties of these Pt(ii) cyclometalated complexes were found to be pH dependent. This was attributed to the protonation/deprotonation of the acidic 1-imidazolyl-NH moieties on the ligands. All the cycloplatinated complexes (both protonated and deprotonated forms) possessed two-photon excitability with two-photon absorption cross-sections ranging from 6.0 to 30.0 GM (protonated forms) and from 16.2 to 24.9 GM (deprotonated forms).

Full text of DOI:10.1039/c1dt11037c

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PAPER
The synthesis and photophysical studies of cyclometalated Pt(II) complexes
with C,N,N-ligands containing imidazolyl donors†
Yu-Man Ho,a Chi-Kin Koo,a Ka-Leung Wong,b Hoi-Kuan Kong,a Chris Tsz-Leung Chan,c Wai-Ming Kwok,c
Cheuk-Fai Chow,a Michael Hon-Wah Lam*a and Wai-Yeung Wongb
Received 3rd June 2011, Accepted 4th November 2011
DOI: 10.1039/c1dt11037c
Two new C,N,N-type ligands (HL2 and HL3), containing a Cphenyl, a Npyridyl, and a Nimidazolyl donor, and
their cycloplatinated complexes, [Pt(L2)Cl] (1), [Pt(L3)Cl] (2), [Pt(L2)(PPh3)]+ (3) and [Pt(L3)(PPh3)]+
(4), have been successfully synthesized and characterized. Spectroscopic and 3MLCT luminescent
properties of these Pt(II) cyclometalated complexes were found to be pH dependent. This was attributed
to the protonation/deprotonation of the acidic 1-imidazolyl-NH moieties on the ligands. All the
cycloplatinated complexes (both protonated and deprotonated forms) possessed two-photon
excitability with two-photon absorption cross-sections ranging from 6.0 to 30.0 GM (protonated forms)
and from 16.2 to 24.9 GM (deprotonated forms).
a Cphenyl, a Npyridyl, and a Npyrazolyl donor (HL1, Fig. 1).7 The 1-
pyrazolyl-NH of the 1,2-azole (pyrazole) donors in those ligands
Introduction
Cyclometalation of a d8 Pt(II) centre by conjugated tridentate
phenyl-substituted pyridyl and bipyridyl ligands with low-lying
are free for further chemical interactions. The room temperature
3
luminescent MLCT excited states of their corresponding Pt(II)
p*-orbitals is a well-established strategy to achieve novel photo-
complexes were found to be accessible via two-photon excitation.
This, together with their low cytotoxicity, renders them to be useful
physical properties.1 These cycloplatinated luminophores display
3
long-lived MLCT photoluminescence at room temperature and
are potentially useful in fields of optoelectronics, chemosensing
and bio-labeling.2 The strong s-donating deprotonated C-donor
of these cyclometalating ligands separates the luminescent MLCT
excited states of the organometallic complexes from their ligand-
field excited states, which are generally non-emissive at room
temperature in solutions.3 The coordination geometry of these
tridentate ligands can also fix the d8 Pt(II) centre in a square-
planar configuration and minimizes radiationless decay due to
D
2d distortion.4 The usual approach to fine-tune the photophysical
properties of this class of cyclometalated Pt(II) luminophores is via
the modification of the electronic properties of the cyclometalating
and ancillary ligands, commonly through the incorporation of
electron-withdrawing and electron-donating substituents.5 In re-
cent years, another strategy which involves the substitution of one
of the pyridyl N-donor of the cyclometalating ligands by other 5-
membered aromatic N-heterocyclic donors has received increasing
attention.5d,6 We have developed a series of p-conjugated cyclomet-
alated ligands with the C,N,N coordination motif composed of
aDepartment of Biology & Chemistry, City University of Hong Kong, 83 Tat
Chee Avenue, Hong Kong SAR, China. E-mail: bhmhwlam@cityu.edu.hk
bDepartment of Chemistry, Hong Kong Baptist University, Waterloo Road,
Kowloon Tong, Hong Kong SAR, China
cDepartment of Applied Biology & Chemical technology, Hong Kong
Polytechnic University, Hung Hom, Hong Kong SAR, China
† Electronic supplementary information (ESI) available: Additional data.
CCDC reference numbers 828775–828777. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1dt11037c
Fig. 1 Chemical structures of the 1,2-/1,3-azole-containing C,N,N
ligands and their cycloplatinated complexes.
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two-photon live cell imaging fluorophores.8 Besides, luminescent
properties of this class of new cyclometalated Pt(II) complexes are
also pH dependent. The pKa of [Pt(L1)(PPh3)]+ was found to be
ca. 4.0.7c
69% yield). ESI-MS: 277 (M + 1)+. 1H NMR (400 MHz, CDCl3):
d 8.09–8.07 (dt, 2H), 8.05–8.02 (dd, 1H), 7.99–7.97 (dd, 1H), 7.95–
7.92 (m, 1H), 7.54–7.46 (m, 3H), 4.99 (s, 2H).
These interesting properties of HL1 have prompted us to explore
other analogous ligand designs that contain the more basic 1,3-
azole donors, HL2 and HL3 (Fig. 1). Spectroscopic, lumines-
cent and photophysical properties of their cyclometalated Pt(II)
complexes were studied to comprehend the effects of electronic
properties of the imidazolyl ring on these C,N,N coordination
motifs.
2-(1H-Imidazol-4-yl)-6-phenylpyridine (HL3)
2-(Bromoacetyl)-6-phenyl-pyridine (2.76 g, 10 mmol) was dis-
solved in formamide (16 ml, 0.4 mol) and heated at 155 ◦C
overnight. After that, 30 ml of water was added and the solution
was extracted three times with chloroform. The organic layer
was combined, dried over anhydrous magnesium sulfate and
concentrated in vacuo. The resulting dark oil was purified by a
silica column (ethyl acetate : n-hexane = 2 : 1) and recrystallized
with ethyl acetate to give the desired product as a yellowish solid
(0.57 g, 26% yield). ESI-MS: 222 (M + 1)+. 1H NMR (400 MHz,
CDCl3): d 8.03–8.01 (d, 2H), 7.89 (s, 1H), 7.78–7.74 (t, 1H), 7.67
(s, 1H), 7.66–7.64 (d, 1H), 7.59–7.57 (d, 1H), 7.49–7.42 (m, 3H),
7.00–6.75 (s, 1H). Anal. Calcd for C14H11N3: C, 76.00; H, 5.01; N,
18.99. Found: C, 75.24; H, 4.95; N, 18.68.
Experimental
Materials and general procedures
All starting materials, 2,6-dibromopyridine, n-butyl lithium (2.5
M in hexane), N,N-dimethylformamide, N,N-dimethylacetamide,
phenylboronic acid, aqueous glyoxal (40%), aqueous NH3 (30%),
bromine, hydrobromic acid in acetic acid (33%), formamide, tri-
ethylamine and K2PtCl4 were purchased from commercial sources
and used as received unless stated otherwise. Solvents used for
synthesis were of analytical grade. Diethyl ether was distilled from
sodium-benzophenone and acetonitrile was distilled from anhy-
drous calcium hydride prior to use. Acetonitrile for photophysical
measurements was distilled over potassium permanganate and
calcium hydride. 2-Acetyl-6-phenyl-pyridine, 2-carboxyaldehyde-
6-phenyl-pyridine and tetrakis-(trisphenylphosphine)palladium
were prepared according to literature methods.9,10
[Pt(L2)Cl] (1)
A mixture of the free ligand HL2 (0.20 g, 0.9 mmol) and K2PtCl4
(0.38 g, 0.9 mmol) in degassed glacial acetic acid (25 ml) was
refluxed for 12 h to give a yellow precipitate. The precipitate
was collected by filtration and washed with 10 ml of water,
methanol and diethyl ether respectively. Crystals suitable for
X-ray crystallography was grown by slow evaporation of an
acetone/MeOH solution (0.3 g, 76% yield). ESI-MS: 450 (M -
1)-. 1H NMR (400 MHz, CDCl3): d 8.18–8.14 (dt, 1H), 7.97–7.90
(dd, 1H), 7.79–7.69 (m, 3H), 7.54–7.46 (m, 2H), 7.18(s, 1H), 7.12–
7.02 (dt, 1H). Anal. Calcd for C14H10N3PtCl·H2O: C, 35.87; H,
2.58; N, 8.96. Found: C, 36.29; H, 2.56; N, 8.99.
2-(1H-Imidazol-2-yl)-6-phenylpyridine (HL2)
To a solution of 40% aqueous glyoxal in cold ethanol, a solution
of 2-carboxyaldehyde-6-phenylpyridine (3 g, 16 mmol) in cold
ethanol (10 ml) was added and stirred in an ice bath. Cold aqueous
NH3 (30%, 5.5 ml) was added while maintaining the temperature of
the entire mixture below 5 ◦C. Stirring continued in an ice bath for
2 h and the reaction mixture was allowed to warm up gradually to
room temperature overnight. The volume was reduced by gentle
warming under vacuum. The remaining solution was extracted
several times with ethyl acetate. The organic layer was collected,
dried over anhydrous magnesium sulfate, concentrated in vacuo
and recrystallized from ethyl acetate to give the desired product
(1.1 g, 31% yield) as a light yellow solid. ESI-MS: 222 (M + 1)+.
1H NMR (400 MHz, CDCl3): d 8.12–8.10 (d, 1H), 8.04–8.02 (d,
2H), 7.85–7.81 (t, 1H), 7.68–7.66 (d, 1H), 7.49–7.44 (m, 3H), 7.22
(s, 2H). Anal. Calcd for C14H11N3: C, 76.00; H, 5.01; N, 18.99.
Found: C, 75.77; H, 5.01; N, 18.62.
[Pt(L2)(PPh3)](ClO4) (3·ClO4)
To an acetonitrile solution (50 ml) of 1 (0.1 g, 0.22 mmol) was
added PPh3 (0.087 g, 0.33 mmol). The mixture was stirred at room
temperature for 12 h. A greenish-yellow suspension was obtained
and a methanolic solution of LiClO4 (0.23 g, 2.2 mmol) was added.
The mixture was stirred at room temperature for another 12 h and
the resulting clear yellow solution was filtered and evaporated
to a minimum amount. The desired product was precipitated by
the addition of diethyl ether as a bright greenish-yellow solid.
The product was collected by filtration, washed with diethyl ether
and recrystallized by slow vapor diffusion of diethyl ether into an
acetonitrile solution (0.12 g, 70% yield). ESI-MS: 677 (M)+. H
1
NMR (400 MHz, CDCl3): d 8.28–8.24 (t, 1H), 8.12–8.10 (d, 1H),
7.89–7.84 (m, 7H), 7.77–7.74 (dd, 1H), 7.69–7.64 (m, 3H), 7.62–
7.57 (m, 6H), 7.46–7.46 (d, 1H), 7.07–7.03 (dt, 1H), 6.708–6.667
(dt, 1H), 6.423–6.401 (m, 1H), 4.505–4.501 (s, 1H). Anal. Calcd
for C32H25N3O4PPtCl: C, 49.46; H, 3.24; N, 5.41. Found: C, 49.28;
H, 3.22; N, 5.44.
2-(Bromoacetyl)-6-phenyl-pyridine
To a mixture of 2-acetyl-6-phenylpyridine (3.00 g, 16 mmol) and
hydrobromic acid (10% solution in acetic acid, 10 ml), a solution
of bromine (0.82 ml, 16.8 mmol) in acetic acid (3 ml) was added
dropwise over an ice bath. The mixture was stirred at 70 ◦C
overnight, basified with saturated aqueous hydrogen bicarbonate
and extracted with ethyl acetate. The organic layer was collected,
dried over anhydrous magnesium sulfate, concentrated in vacuo
and the product was purified by a silica column (CH2Cl2 : n-
hexane/1 : 4) to give the desired product as a yellowish oil (2.9 g,
[Pt(L3)Cl] (2)
Complex 2 was synthesized using the same method as for complex
1, except the use of HL3 as the ligand. Yield 71%. ESI-MS: 450
(M - 1)-. 1H NMR (400 MHz, CDCl3): d 8.43–8.36 (d, 1H), 8.23–
7.99 (m, 2H), 7.96–7.88 (dd, 1H), 7.77–7.58 (m, 1H), 7.72–7.70
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(dd, 1H), 7.56–7.523 (m, 1H), 7.24–7.21 (m, 1H), 7.13–7.06 (m,
1H). Anal. Calcd for C14H10N3PtCl·H2O: C, 35.87; H, 2.58; N,
8.96. Found: C, 36.39; H, 2.59; N, 8.98.
optical parametric amplifier (Coherent) pumped by the SHG of
the 800 nm femtosecond pulses. The lasers were focused to a spot
size of ca. 50 mm via a f = 10 cm lens onto the sample. The
emitting light was collected with a backscattering configuration
into a 0.5 m spectrograph and detected by a liquid nitrogen-cooled
CCD detector. A power meter was used to monitor the uniform
excitation.
[Pt(L3)(PPh3)](ClO4) (4·ClO4)
Complex 4 was synthesized using the same method as for complex
3. Yield 79%. ESI-MS: 677 (M)+. 1H NMR (400 MHz, CDCl3): d
8.24–8.20 (m, 2H), 8.03–8.01 (d, 1H), 7.95–7.93 (d, 1H), 7.90–7.85
(m, 6H), 7.77–7.75 (dd, 1H), 7.68–7.64 (m, 3H), 7.62–7.58 (m,
6H), 7.07–7.03 (dt, 1H), 6.70–6.66 (dt, 1H), 6.47–6.44 (m, 1H),
5.01–5.01 (s, 1H). Anal. Calcd for C32H25N3O4PPtCl: C, 49.33; H,
3.24; N, 5.41. Found: C, 49.28; H, 3.22; N, 5.43.
Crystal structure determination†
Crystallographic data for complexes 1, 3·ClO4 and 4·ClO4 are
tabulated in the ESI (Table S1).† All intensity data were collected
at 293 K on a Bruker Axs SMART 1000 CCD area detector
using graphite-monochromated Mo-Ka radiation (l = 0.71073
13
˚
A). All collected frames were processed with the software SAINT
Physical measurements and instrumentation
and absorption correct was applied (SADABS14) to the collected
reflections. The structure of the complex was solved by direct
methods (SHELXTL15) in conjunction with standard difference
Fourier syntheses. All non-hydrogen atoms were assigned with
anisotropic displacement parameters. The hydrogen atoms were
generated in their idealized positions and allowed to ride on the
respective carbon atoms.
Infrared spectra in the range 500–4000 cm-1 in KBr plates were
recorded on a Perkin Elmer Model FTIR-1600 spectrometer. UV-
Vis spectra were measured on a Hewlett Packard 8452A ultra-
violet visible diode array spectrophotometer. Emission spectra
were recorded using a Horiba FluoroMax-3 spectrofluorimetric
with 5 nm slit width and 0.5 s integration time. Acid–base
properties of the cycloplatinated complexes were measured in
2 : 1 DMF : aqueous buffer media. Buffer solutions with similar
ionic strength, I = 0.01, but different pH were prepared according
to Perrin and Dempsey.11 Chloroacetic acid/chloroacetate buffer
system was adopted for pH < 3.4, while succinic acid/succinate
buffer system was adopted for pH > 3.4. Acid dissociation
constants, pKa, of the cycloplatinated complexes were estimated
from the absorbances or luminescent intensities of these complexes
at their selected wavelengths of maximum spectral changes
in a series of organic aqueous buffer solutions of different
pH. The best fitted sigmoid curve (by the non-linear curve
fitting algorithm of the Microcal Origin 5.0 software) of the
absorbance/luminescent intensity vs. pH plot of each complex
was used to determine the media pH values where maximum
and minimum absorbance/luminescent intensity occurred. The
corresponding pKa of the complex was taken as the mid-pH value
between the maximum and minimum pH values. Low-temperature
(77 K) emission spectra were collected in 5-mm diameter quartz
tubes which were placed in a liquid nitrogen Dewar equipped with
Results and discussion
Synthesis and characterization
The two new C,N,Nimidazolyl ligands, HL2 and HL3, are structurely
related to each other, with the only difference in the orientation
of the imidazolyl moiety relative to the central pyridyl ring.
HL2 involves a 2-(6-phenylpyridyl) substitutent on the imidazole
at the C-2 position while HL3 has the substitution at the C-4
position. This causes a significant difference in the conjugation
of p-electrons over the tridentate ligands. There is an unbranched
system of conjugated p-bonds from the central pyridine to the
imidazole (head-to-tail type of conjugation) in HL2, but not for
HL3.16 Theoretical studies on analogous systems of 2- and 4-
phenylimidazole have revealed that due to the relatively longer
conjugate chain of p-electrons, 2-phenylimidazole showed a higher
interaction energy between the phenyl and the heterocyclic rings,
and a lower p–p* energy gap, than 4-phenylimidazole.17 Such
differences in electronic properties are also expected in HL2 and
HL3.
These new C,N,N-ligands were prepared by modified literature
procedures outlined in Scheme 1. 2-Bromo-6-carboxyaldehyde-
pyridine and 2-acetyl-6-bromopyridine were obtained from 2,6-
dibromopyridine, via lithiation with n-butyl lithium and N,N-
dimethylformamide or N,N-dimethylacetamide as substrate re-
spectively. They were then converted to 2-carboxyaldehyde-6-
phenylpyridine and 2-acetyl-6-phenylpyridine, respectively, by
palladium catalyzed Suzuki cross-coupling reaction with phenyl-
boronic acid. By treating 2-carboxyaldehyde-6-phenylpyridine
with 40% aqueous glyoxal and 30% aqueous ammonia, pure
HL2 was obtained after recrystallization with ethyl acetate.
The intermediate of HL3 was obtained by treating 2-acetyl-6-
phenylpyridine with bromine in acetic acid. The resulting 2-
(bromoacetyl)-6-phenyl-pyridine was refluxed with formamide
and HL3 was obtained after purification with column chromatog-
raphy and recrystallization.
1
quartz windows. H NMR spectra were recorded using a Varian
YH300 300 MHz NMR spectrometer. Electrospray (ESI) mass
spectra were measured by a PE SCIEX API 365 LC/MS/MS
system.
Emission quantum yields were measured by the method of
Demas and Crosby12 with [Ru(bpy)3](PF6)2 in degassed acetoni-
trile as the standard (fr = 0.062). Lifetime measurements were
performed by a nitrogen laser (Spectra Physics) at 337 nm with
the maximum power of 15 mW. The luminescence was dispersed
by a monochromator and was detected using a cooled R636-
10 Hamamastu photon-multiplier (PMT) in combination with a
lock-in amplifier system. The decay spectra were monitored by a
HP54522A 500 MHz oscilloscope. Sample and standard solutions
were degassed with at least three freeze–pump–thaw cycles.
Multi-photon excitation experiments were carried out using
a femtosecond mode-locked Ti:Sapphire laser system (output
beam ~ 150 fs duration and 1 kHz repetition rate). The 700–
900 nm pump wavelengths were generated from a commercial
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Scheme
1
Synthesis of the new C,N,N-ligands: (i) n-BuLi,
N,N-dimethylformamide, Et2O, -60 ◦C; (ii) phenylboronic acid in toluene,
Pd(PPh3)4 (5 mol%), K2CO3 in H2O, reflux; (iii) 40% aqueous glyoxal, 30%
NH3, EtOH, 0 ◦C; (iv) n-BuLi, N,N-dimethylacetamide, Et2O, -60 ◦C; (v)
phenylboronic acid in toluene, Pd(PPh3)4 (5 mol%), K2CO3 in H2O, reflux;
(vi) Br2, HBr in acetic acid, 70 ◦C; (vii) formamide, 155 ◦C.
The neutral cycloplatinated complexes, [Pt(L2)Cl] (1) and
[Pt(L3)Cl] (2), were obtained in ca. 70% yield by refluxing the
corresponding ligands with K2PtCl4 in glacial acetic acid for
12 h.18 Substitution of the chloride ligand by triphenylphos-
phine in acetonitrile generated the cationic cycloplatinated com-
plexes [Pt(L2)(PPh3)]+ (3) and [Pt(L3)(PPh3)]+ (4). All these Pt(II)
C,N,Nimidazoyl cyclometalated complexes were characterized by 1H
NMR, mass spectrometry and elementary analysis. Structures
of complexes 1, 3·ClO4 and 4·ClO4 were revealed by X-ray
crystallography.
Fig. 2 Molecular structure and crystal packing mode of [Pt(L2)Cl] (1)
with the numbering scheme adopted. Thermal ellipsoids are shown at the
50% probability level.
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) of complex 3·ClO4
Pt(1)–C(1)
Pt(1)–N(2)
C(1)–Pt(1)–N(1)
N(1)–Pt(1)–N(2)
N(1)–Pt(1)–P(1)
2.019(3)
2.124(2)
80.64(10)
78.21(9)
177.56(6)
Pt(1)–N(1)
Pt(1)–P(1)
C(1)–Pt(1)–P(1)
N(2)–Pt(1)–P(1)
C(1)–Pt(1)–N(2)
2.022(2)
2.2426(6)
97.94(8)
103.26(6)
158.77(10)
Crystal structures
X-Ray quality single crystals of 1 were grown by slow evaporation
of its acetone/methanol solution, and those of 3·ClO4 and 4·ClO4
were grown by slow diffusion of diethyl ether into their acetonitrile
solutions. Fig. 2 shows the perspective view and crystal packing
of 1. Selected bond lengths and angles are listed in Table 1. The
coordinate geometry of the Pt centre is a distorted square planar
configuration with a C(1)–Pt(1)–N(2) angle of 160.8(2)◦. Bond
distances of Pt(1)–C(1), Pt(1)–N(1) and Pt(1)–N(2) are 1.990(5),
Fig. 3 shows the perspective view and crystal packing of the
cationic complex 3. Selected bond lengths and angles are listed in
Table 2. Similar to the analogous [Pt(L1)(PPh3)]+, 3 has a slightly
distorted square pl◦anar configuration. It has a C(1)–Pt(1)–N(2)
both comparable to that of [Pt(L1)(PPh3)]+. No Pt–Pt interaction
is observed among platinum centres in the crystal packing and the
˚
angle of 158.77(10) and a Pt(1)–P(1) bond length of 2.2426(6) A,
˚
1.953(4) and 2.114(5) A respectively, which are all comparable to
the analogous [Pt(L1)Cl].7b The crystal lattice of 1 contains two
planar [Pt(L2)Cl] units that are stacked in a head-to-tail fashion
˚
shortest Pt–Pt distance measured is 6.420(5) A. This is probably
due to the presence of the bulky PPh3 ligand which hinders
the approach of the two Pt(II) centres. The two [Pt(L2)(PPh3)]+
units are paired in a head-to-tail fashion with an interplanar
˚
with a Pt–Pt distance of 4.89 A and parallel to each other. No
intermolecular Pt–Pt interaction is evident. On the other hand,
˚
the interplanar separation was measured to be 3.848 A indicating
˚
separation between the C,N,Nimidazolyl ligands of 3.630 A indicating
the presence of weak p–p interaction between the ligands of the
two [Pt(L2)Cl] units. Continuous chains with alternating long and
the presence of weak p–p interactions. The phenylpyridyl part of
the ligand is mainly involved in the stacking.
˚
˚
short Pt–Pt distances of 4.89 A and 6.437 A resulted from the
packing of these dimeric units.
While no X-ray quality crystal of the neutral [Pt(L3)Cl] (2) could
be successfully grown, its conversion into cationic [Pt(L3)(PPh3)]+
(4) via the substitution of the chloride ancillary ligand by a
neutral triphenylphosphine afforded X-ray quality crystals that
enabled the crystallographic characterization of the ligand L3
with its special orientation of the imidazole-ring. Fig. 4 shows
the perspective view and crystal packing of the cationic complex
4, with selected bond lengths and angles listed in Table 3. Closely
resembling 3, complex 4 has a slightly distorted square planar
◦
˚
Table 1 Selected bond lengths (A) and angles ( ) of complex 1
Pt(1)–C(1)
Pt(1)–N(2)
C(1)–Pt(1)–N(1)
N(1)–Pt(1)–N(2)
N(1)–Pt(1)–Cl(1)
1.990(5)
2.114(5)
81.09(19)
79.79(16)
179.09(12)
Pt(1)–N(1)
Pt(1)–Cl(1)
C(1)–Pt(1)–Cl(1)
N(2)–Pt(1)–Cl(1)
C(1)–Pt(1)–N(2)
1.953(4)
2.3129(14)
98.07(16)
101.06(12)
160.8(2)
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Fig. 4 Molecular structure and crystal packing mode of [Pt(L3)(PPh3)]+
(4) with the numbering scheme adopted. Thermal ellipsoids are shown at
the 50% probability level.
Fig. 3 Molecular structure and crystal packing mode of [Pt(L2)(PPh3)]+
(3) with the numbering scheme adopted. Thermal ellipsoids are shown at
the 50% probability level.
reference). Both the neutral cycloplatinated complexes 1 and 2
display intensive absorption bands at ca. 240–375 nm with extinc-
tion coefficients (e) of the order of 104 dm3 mol-1 cm-1 and a less
intense band at ca. 375–410 nm with e of the order of 103 dm3 mol-1
cm-1 (Fig. 5). By comparing with [Pt(L1)Cl], those higher energy
absorption bands are attributed to the intraligand (p(L) → p*(L))
transitions, while the lower energy absorption bands are assigned
the spin-allowed singlet metal-to-ligand charge-transfer (1MLCT)
transition. The solvent dependency of transition energy of this
absorption band (lmax shifts from 376–400 nm in acetonitrile
to 385–413 nm in dichloromethane) supports this assignment.
Both complexes show an absorption tail at 410–475 nm that are
attributable to the spin-forbidden 3MLCT transition.
◦
˚
Table 3 Selected bond lengths (A) and angles ( ) of complex 4·ClO4
Pt(1)–C(1)
Pt(1)–N(2)
C(1)–Pt(1)–N(1)
N(1)–Pt(1)–N(2)
N(1)–Pt(1)–P(1)
2.005(4)
2.107(3)
81.11(15)
78.56(14)
175.78(9)
Pt(1)–N(1)
Pt(1)–P(1)
C(1)–Pt(1)–P(1)
N(2)–Pt(1)–P(1)
C(1)–Pt(1)–N(2)
2.025(3)
2.2272(9)
97.59(11)
103.02(9)
159.13(15)
configuration. The C(1)–Pt(1)–N(2) angle is 159.13(15)◦ and the
˚
Pt(1)–P(1) bond length is 2.2272(9) A. No Pt–Pt interaction is
observed in the crystal packing of 4 (shortest Pt–Pt distance
measured is 6.993 (3)). Two cyclometalated complex units were
stacked in a head-to-tail configuration. However, unlike 3, p–
p stacking was observed not between the phenylpyridyl part of
the ligand, but between the pyridylimidazolyl part. The distance
For the cationic complexes 3 and 4, their intraligand transition
1
3
bands, as well as their MLCT and MLCT transitions were all
blue-shifted ca. 40 nm compared to their precursor complexes 1
and 2.
˚
between the two stacking rings was found to be 3.732 A. This
All four new cycloplatinated complexes show intense photolu-
minescence in solutions at room temperature. Fig. 6 shows the
normalized room temperature excitation and emission spectra of
the neutral complexes 1 and 2 in acetonitrile. Complex 1 gives a
poorly-resolved emission profile with lmax at 521 nm (to = 1.28 ms;
f = 0.18), while complex 2 shows its emission maxima at 503 nm
(to = 2.36 ms; f = 0.10), with a well-resolved vibronic structured
emission profile at 490–580 nm with spacing of ca. 1300 cm-1 which
correlates to the skeletal vibrational frequency of the C,N,Nimidazolyl
ligand (Fig. 6). The observed large Stokes shifts and relatively long
may be accounted by the observed partial overlapping between
imidazole rings of neighbouring stacking unit, with a separation
˚
of 3.805 A.
Spectroscopic and luminescent properties
Detailed spectroscopic and luminescent properties of the new
cyclometalated platinum(II) complexes are summerized in the ESI
(Table S2)† (where spectroscopic and luminescent data of the
previously reported analogous [Pt(L1)Cl] are also tabulated for
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Fig. 5 Spectroscopic properties of [Pt(L2)Cl] (1) (upper) and [Pt(L3)Cl] (2)
(lower) in acetonitrile at 298 K, with solvent dependence of the absorption
band at 375–420 nm shown in the insets.
Fig. 6 Normalized excitation and emission spectra of 1 (upper) and 2
(lower) in acetonitrile at room temperature (concentration = 5 ¥ 10-5 M).
emission lifetimes of both complexes suggest that their emissions
originate from a spin-forbidden triplet excited state. With reference
to [Pt(L1)Cl], the luminescence of 1 and 2 can be assigned the
3MLCT transition.
of the triphenylphosphine ancillary ligand nor the coordinated
cyclometalating ligands by DMF was observed (Fig. S1, ESI†).
Spectroscopic and luminescent properties of 3 and 4 vary with
media pH (Fig. 7 and 8). These phenomena are attributable to the
protonation/deprotonation of the 1-imidazolyl-NH moiety on L2
It is interesting to compare the spectroscopic and luminescent
properties of both cyclometalated Pt(II) complexes 1 and 2 with
1,3-azole donors in their cyclometalating ligands to those of
[Pt(L1)Cl] that contains a 1,2-azole (pyrazolyl) donor. Both the
electronic and excited-state MLCT transitions of 1 are red-shifted
compared to those of [Pt(L1)Cl]. Although imidazole is more basic
than pyrazole, numerous studies have shown that the imidazolyl-
N is only slightly stronger in terms of s-donor strength.19 On
the other hand, imidazole is a relatively poorer p-acceptor than
pyrazole.20 This causes 1 to possess relatively higher energy dp(Pt)
orbitals and a smaller MLCT energy gap compared to [Pt(L1)Cl].
Contrary to 1, energies of both the electronic and excited-state
MLCT transitions of complex 2 are higher than those of [Pt(L1)Cl].
This is attributable to the cross-conjugate system of L3 resulting in
a relatively shorter p-conjugate chain and a higher energy p*(L).
1
and L3. H NMR titration of d-DMSO solutions of complexes
3 and 4 by triethylamine revealed the disappearance of their 1-
imidazolyl-NH bands at ca. 13.3–14.3 ppm (Fig. S2 and S3, ESI†).
A gradual increase of media pH from 3.5 onward results in a
1
continuous red-shift of the intraligand and MLCT absorption
bands with a series of well defined isosbestic points, indicating a
clean conversion between the protonated and deprotonated forms
of 3 and 4. The lmax of the 3MLCT emissions of 3 and 4 at pH <
3.6 are observed at 510 and 491 nm respectively. Their luminescent
intensities are reduced, concomitant with slightly red-shifts in lmax
to 516 and 498 nm respectively, upon further increase in media
pH.
These spectroscopic and luminescent responses of 3 and 4
are similar to other coordination systems in the literature that
display spectroscopic or electrochemical pH responses due to the
protonation/deprotonation of the imidazolyl functionality.21 The
red-shift of the 1MLCT bands may be the result of a decrease in the
bond distance between the anionic deprotonated imidazolyl ligand
and the Pt(II) centre that destablizes the d-orbitals and reduces the
MLCT gap.20a
Acid–base responses of the spectroscopic and luminescent
properties of 3 and 4
The cationic 3 and 4 possess better aqueous solubility than
their neutral precursors. They were used to investigate the acid–
base properties of the 1-imidazolyl-NH functionality on the
L2 and L3 in aqueous organic media via pH titrations in 2 : 1
DMF : aqueous buffer (with complex concentration of 5 ¥ 10-5
M). Integrity of the cycloplatinated complexes in DMF was
checked by ESI-MS in 2 : 1 DMF : acetonitrile. No displacement
The pKa values of 3 and 4 at room temperature are estimated
from their spectroscopic responses at selected wavelengths (336
nm in complex 3 and 345 nm in complex 4) (insets of Fig. 7(a)
and 8(a)). They are found to be ca. 4.33 (for complex 3) and 5.08
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Fig. 8 Spectroscopic (a) and luminescent (b) responses of 4·ClO4 in 2 : 1
(v/v) DMF : aqueous buffer (5 ¥ 10-5 M) to changes in media pH at 298 K.
(Excitation l = 345 nm.)
Fig. 7 Spectroscopic (a) and luminescent (b) responses of 3·ClO4 in 2 : 1
(v/v) DMF : aqueous buffer (5 ¥ 10-5 M) to changes in media pH at 298
K. (Excitation l = 345 nm.)
(for complex 4), which are considerably higher than that of our
previously reported [Pt(L1)(PPh3)]+ (pKa = 4.0). This is probably
due to the presence of the more basic imidazolyl donors.
Changes in the luminescent properties of 3 and 4 with media
pH also enable the estimation of their excited state pKa* values.
From the insets of Fig. 7(b) and 8(b), the pKa* values of the two
complexes are found to be 4.26 (complex 3) and 5.00 (complex 4).
Such small differences between ground state and excited state acid
dissociation constants suggest that the electron promoted to the
ligand p*-orbital in the MLCT excited-states of 3 and 4 may not be
distributed on the imidazolyl ring of the ligands L2 and L3, where
the deprotonation process takes place.22 Similar observations
were also obtained by Deverlay et al.6b in their asymmetric
N,C,N cyclometalated Pt(II) systems that contained N-linked
1-arylpyrazoles where the LUMOs were located largely on the
six-membered aromatic rings rather than the N-heterocycles. Of
course, these are just preliminary assessments which have to be
further substantiated by time-resolved photophysical measure-
ments. Work towards the further understanding of the excited state
photophysics of these cycloplatinated complexes are in progress.
Fig. 9 Two-photon induced luminescent spectra of 1–4 in DMF (con-
centration = 1 ¥ 10-4 M) at 298 K (lex = 750 nm). The log–log plots of the
power dependence of luminescent intensities of the complexes shown in
the inset confirm the two-photon excitation processes.
3
these complexes. The MLCT emission induced by two-photon
absorption processes were confirmed by the evolution of the
intensities of the peak that are plotted as a function of the incident
power at 750 nm. The slopes of the log–log plots (1.98–2.09, lex
=
750 nm) for the cyclometalated Pt(II) complexes confirm the non-
linear absorption processes at 750 nm.
Two-photon induced photophysical properties
Fig. 9 shows the two-photon induced emission spectra and
emission-laser power relationship of complexes 1–4 in DMF
(concentration = 1 ¥ 10-4 M) by near-IR (lexcitation = 750 nm)
femto-second excitation. These NIR induced emission spectra
The efficiency of the two-photon processes was evaluated by
two-photon absorption cross-section measurements with Rho-
damine 6G in DMF as the reference standard.23 All four
complexes (1–4) possess modest two-photon absorption cross-
sections ranging from 6 to 30 GM (GM = 10-50 cm4 s photon-1
3
closely resembles the single-photon MLCT emission spectra of
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Table 4 The two-photon absorption cross-section (GM) of complexes
1–4 in neutral, protonated and deprotonated forms
Scheme (AoE/P-03/08) of the University Grants Committee of
Hong Kong SAR, China.
Complex
Neutrala
Deprotonatedb,c
1
2
3
4
12
9
30
6
22.9 (0.025)
24.9 (0.058)
16.2 (0.012)
23.2 (0.009)
Notes and references
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a Measured in DMF (concentration = 1 ¥ 10-4 M). b Measured in DMF with
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molecule-1) (Table 4). To examine whether the deprotonated form
of these cycloplatinated luminophores also possess multi-photon
excitation and emission properties, we have also measured the
two-photon absorption cross-section of the four cycloplatinated
complexes in their deprotonated states in DMF (concentration =
1 ¥ 10-4 M) (Table 4 and Fig. S4, ESI†). These deprotonated
luminophores also possess modest two-photon absorption cross-
section ranging from 16.2 to 24.9 GM.
Conclusions
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To conclude, we have developed two new C,N,N-type ligands that
contain the more basic imidazolyl donors for the cyclometalation
of d8 Pt(II) centres. The two structural related ligands, HL2
and HL3, possess different electronic properties because of the
difference in the substitution position of the phenylpyridyl moiety
on the imidazole. All the resultant cycloplatinated complexes,
[Pt(L2)Cl] (1) and [Pt(L3)Cl] (2), as well as their cationic derviatives,
[Pt(L2)(PPh3)]+ (3) and [Pt(L3)(PPh3)]+ (4), are luminescent at
room temperature in solutions. The acid–base properties of the
1-imidazolyl-NH functionality on both L2 and L3 are able to
bring about spectroscopic and luminescent responses in the cyclo-
platinated luminophores. The measured ground state and excited
state pKa values are 4.33 and 4.26, respectively, for Pt(L2)(PPh3)]+
(3), and 5.08 and 5.00, respectively, for [Pt(L3)(PPh3)]+ (4). These
pKa values, especially that of 4, are physiological relevant. While
normal cytoplasmic pH of live cells is in the range of 7.0–7.5,
pH in selected intracellular organelles of the endosomal and
secretory compartments is much lower due to the action of
vacuolar-type proton pumps.24 For example, pH of lysosomes can
be as low as 4.7–4.8.25 The specific pH luminescent responses
of 3 and 4 can be further explored to bring about “switch-
on” luminescent imaging of these more acidic organelles in live
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Acknowledgements
This work is jointly funded by a CERG grant (CityU 101208)
from the Research Grant Council and the Areas of Excellence
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