Chemical stimuli induced phosphorescence modulation of secondary thioamide-based pincer platinum complexes

Article abstract of DOI:10.1021/om800894j

The photochemical properties of pincer platinum complexes bearing secondary thioamide groups haVe been examined. The presence of the coordinated secondary thioamide linkage in the pincer ligand was found to work as a reactiVe site upon addition of bases and anions and to result in modulating absorption and emission properties of the complexes.

Full text of DOI:10.1021/om800894j

Organometallics 2009, 28, 3307–3310
3307
Chemical Stimuli Induced Phosphorescence Modulation of
Secondary Thioamide-Based Pincer Platinum Complexes
Ken Okamoto,†,‡ Takakazu Yamamoto,† Munetaka Akita,† Akihide Wada,§ and
Takaki Kanbara*,‡,|
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama
226-8503, Japan, Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), UniVersity of
Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8573, Japan, Molecular Photoscience Research Center, Kobe
UniVersity, Nada-ku, Kobe 657-8501, Japan, and Graduate School of Pure and Applied Sciences,
UniVersity of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8573, Japan
ReceiVed September 15, 2008
Scheme 1. Equilibrium of Secondary Thioamide Group
under Neutral and Basic Conditions
Summary: The photochemical properties of pincer platinum
complexes bearing secondary thioamide groups haVe been
examined. The presence of the coordinated secondary thioamide
linkage in the pincer ligand was found to work as a reactiVe
site upon addition of bases and anions and to result in
modulating absorption and emission properties of the complexes.
Cyclometalated platinum(II) complexes represent an impor-
tant class of complexes, particularly from the point of view of
their luminescent properties and catalytic function.1-3 The use
of phosphorescent complexes as emitters in light-emitting diodes
(LEDs) has attracted much attention from researchers in both
academic institutions and industry.4 Luminescent cycloplatinated
complexes are also potential candidates for luminescent sensors,
in which the incorporation of various reactive sites into a ligand
framework has been examined.5,6 We previously reported that
κ3SCS pincer Pt(II) and Pd(II) complexes containing thioamide-
based pincer ligands exhibited strong phosphorescent emission
in the glassy frozen state and in the solid state.7 As an extension
of this research, we here demonstrate that the presence of a
secondary thioamide group in the pincer ligand framework
induces modulating the luminescent properties of the complexes.
The secondary thioamide group is involved in equilibrium
with its amino-thione and imino-thiol tautomers, as shown in
Scheme 1, and exhibits stronger acidity than the corresponding
amide group.8 Consequently, the secondary thioamide ligands
are easily deprotonated to produce their thionate anionic form,
which enhances the electron donor ability of the sulfur atom
via the N-to-S backbone. The N-H hydrogen of thioamide also
exhibits strong hydrogen bond donor ability, which is discussed
in recent reports on anion receptors9 and supramolecular
networks.10
This situation motivates us to utilize the secondary thioamide
group as not only a coordination site but also a reactive site on
the ligand for modulating the photoluminescent properties of
the pincer complexes upon exposure to chemical stimuli. We
here report that pincer complexes bearing secondary thioamide
groups exhibit a luminescent modulation upon the addition of
a base and anions. Among the pincer complexes, [2,6-bis(ben-
* To whom correspondence should be addressed at the Tsukuba Research
Center for Interdisciplinary Materials Science (TIMS), University of
Tsukuba. E-mail: kanbara@ims.tsukuba.ac.jp. Fax: +81-29-853-4490. Tel:
+81-29-853-5066.
† Tokyo Institute of Technology.
‡ Tsukuba Research Center for Interdisciplinary Materials Science
(TIMS), University of Tsukuba.
§ Kobe University.
| Graduate School of Pure and Applied Sciences, University of Tsukuba.
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Fukumoto, H.; Yamamoto, T. J. Organomet. Chem. 2005, 690, 4192. (c)
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10.1021/om800894j CCC: $40.75
2009 American Chemical Society
Publication on Web 04/24/2009

3308 Organometallics, Vol. 28, No. 11, 2009
Notes
Figure 1. Changes in (a) absorption (inset: absorbance change at
450 nm) and (b) emission spectra of 1 (5 × 10-5 M in THF at
room temperature) upon addition of DBU.
Figure 2. Changes in (a) absorption and (b) emission spectra of 1
(5 × 10-5 M, in THF at room temperature) upon addition of TBACl.
zylaminothiocarbonyl)-κ2S,S′-phenyl-κC1]chloroplatinum(II) (1)
was selected for initial investigation because it possessed good
solubility in common organic solvents and phosphorescent
emission at room temperature; thus, the spectral features of 1
have mainly been examined.
Chart 1. SCS Pincer Pt(II) Complexes
Results and Discussion
Complex 1 was prepared in accordance with the previous
reports7 and was characterized by NMR and FAB-MS spec-
troscopy and elemental analysis. Complex 1 has absorption
bands in the visible region of 400-500 nm. The absorption
bands exhibit negative solvatochromism (Figure S1 in the
Supporting Information), which has been often observed for
metal-to-ligand charge transfer (MLCT) transitions.11 Complex
1 is light-emitting in solution at room temperature, and the
emission has a decay lifetime (τ) of 8.5 µs in the glassy frozen
state (λem 578 nm), which is indicative of phosphorescent
emission. These observations are consistent with those for
previously reported pincer Pt(II) complexes.7 Related pincer
ligands and their Pt(II) complexes (2-4, Chart 1) were prepared
and characterized analogously (see the Supporting Information).
The effect of deprotonation of the coordinated pincer ligand
on the absorption and emission properties of 1 was examined
by adding a small, controlled amount of THF solution containing
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Upon the addition
of DBU, the MLCT band at 450 nm gradually decreased with
isosbestic points at 366 and 527 nm that were observed up to
a DBU/1 ratio of 1 (as depicted in Figure 1a). When larger
amounts of DBU were added, further changes in absorption
spectra were observed and the change leveled off at a DBU/1
ratio of about 2 (see also Figure 1a inset), indicating that the
deprotonation of the N-H units of 1 consecutively proceeded
to give a dideprotonated form of 1, as shown in Scheme 2. After
treatment with the base, the addition of an excess of methane-
sulfonic acid (MSA) to the solution led to the immediate
recovery of 1. The reversible deprotonation-protonation be-
havior of 1 was also monitored by emission spectroscopy. The
emission peak at 632 nm gradually decreased in intensity upon
the addition of DBU, and the emission almost disappeared upon
the addition of 1 equiv of DBU (Figure 1b), indicating that the
monodeprotonation of 1 caused the quenching. From a change
in emission intensity of 1 with DBU (pKa(DBU-H+) ) 19.1 in
THF12), pKa1 of the secondary thioamide unit of 1 was estimated
Scheme 2. Deprotonation-Protonation Reaction of 1

Notes
Organometallics, Vol. 28, No. 11, 2009 3309
Table 1. Photophysical Data for Complexes 1-4
1
2
3
4
a
a
a
a
+Cl-
+Cl-
+ Cl-
+Cl-
b
λabs /nm (ꢀ, 103 M-1 cm-1
)
257 (34.0)
450 (8.7)
632
578
0.06
0.6
1.4
8.5
256 (31.9)
431 (9.9)
608
563
0.12
0.7
2.0
10.0
254 (29.7)
451 (7.1)
635
583
0.05
0.5
1.4
7.4
254 (29.0)
432 (8.2)
613
563
0.09
0.6
1.9
8.1
248 (25.4)
448 (5.9)
630
570
0.03
0.3
0.7
4.0
245 (23.7)
428 (6.7)
614
562
0.06
0.4
1.5
4.4
245c (23.9)
447c (8.1)
245c (23.9)
447c (8.1)
λemb/nm, room temp
λemd/nm, 77 K
φem,e room temp
rel φem,f 77 Κ
638
0.2g
5.2g
637
0.2g
5.6g
τh/µs, room temp
τi/µs, 77 Κ
a A 600 equiv amount of TBACl was added. b In THF. The values in parentheses are molar absorption coefficients. c In CH2Cl2. d In 2-MeTHF glass
matrix at 77 K. e Absolute quantum yield in THF at room temperature. f In 2-MeTHF glass matrix, the relative quantum yield values at 77 K were
estimated from the ratio of the integrated emission intensity area of each sample to that of fac-tris(2-phenylpyridine)iridium as a standard (regarding the
quantum yield value as 1.0).14 g In a CH2Cl2-THF (3:2) glass matrix at 77 K. h Emission decay lifetime in 2-MeTHF at room temperature. i Emission
decay lifetime in 2-MeTHF glass matrix at 77 K.
to be 17.7 (Figure S2 in the Supporting Information). The
reversible protonation upon adding MSA to give 1 was
confirmed. Similar changes in the absorption and emission
spectra of 1 were observed by treatment with other bases such
as 1,5-diazabicyclo[4.3.0]nonene (DBN) and N,N,N′,N′-tetram-
ethylguanidine (TMG: pKa(TMG-H+) ) 17.8 in THF12),
whereas the addition of excess Et3N (pKa(Et3N-H+) ) 14.9 in
THF12) did not cause a significant spectral change. The result
of 1H NMR spectroscopy also suggests the deprotonation of 1.
Upon the addition of excess DBU, the N-H proton resonance
for the pincer Pt complexes. In contrast to 1-3, 4, a related
complex bearing a tertiary thioamide group, was spectroscopi-
cally unreactive upon the addition of excess base as well as
1
TBACl and essentially exhibited the same absorption and H
NMR spectra. These observations reveal that the formation of-
a hydrogen-bonding interaction between the N-H hydrogen of
the complexes and Cl- ion is essential to the enhancement of
emission properties of the complexes.
These observations suggest that the formation of hydrogen
bonds between the N-H hydrogen of 1 and Cl- ion blocks the
nonradiative decay channels of the excited state of 1 in solution
at room temperature. The interaction of Cl- ion with 1 is
considered to remove the ability of the solvent molecules on 1
to quench the 3MLCT emissive state and to result in the increase
in τ.15 Actually, 1 exhibits weak emission in DMSO solution,
which acts as both a strong hydrogen bond acceptor and a
quencher of the emission of 1;16 the addition of Cl- ion to the
DMSO solution of 1 also led the enhancement of emission
intensity. Alternatively, the hydrogen-bonding interaction of Cl-
ion with the N-H hydrogen of 1 may rigidify the molecules in
solution, rendering the relaxation of the excited state of 1 to
thegroundstatevianonradiativedecaychannelslessefficient.6c,d,17
As shown in Table 1, the emission enhancement upon the
addition of Cl- ion is small at 77 K; the complexes 1-3 seem
to have an energetically stable conformation and no molecular
reorganization is present in the low-temperature glass matrix.18
As described above, the incorporation of a coordinated
secondary thioamide linkage into the pincer ligand framework
allows its Pt complex to take part in chemical reactions and
hydrogen-bonding interactions and to result in a modulation of
its photoluminescent properties. Although the hydrogen-bonding
ability of 1 with anions is poor in the present study, the
luminescent properties of the pincer ligands bearing secondary
thioamide groups are of interest because this will open the way
to manipulating luminescent properties of cycloplatinated
complexes upon exposure to chemical stimuli.
1
of 1 completely disappeared in the H NMR spectrum.
In contrast to the optical response of 1 to the bases, we found
that the addition of tetra-n-butylammonium chloride (TBACl)
induced an enhancement of the emission intensity of 1. Figure
2 shows changes in the absorption and emission spectra of 1
upon the addition of TBACl at room temperature. As shown in
Figure 2a, the addition of TBACl to the THF solution led to a
blue shift of the MLCT absorption band from 450 to 431 nm.
The addition of TBACl also caused a blue shift of the emission
spectrum by 24 nm with an approximately 2.3-fold increase in
emission intensity upon excitation at 441 nm (at which the
change in absorbance was minimum; ∆abs ) (0.01). The change
in the emission spectrum was accompanied by an increase in
the luminescence decay lifetime (τ ) 1.4 to 2.0 µs). It is known
that the N-H hydrogen of secondary thioamide units exhibits
affinity toward anions.9 Among the anions (used as a tetra-n-
butylammonium (TBA) salt) tested, the most significant emis-
sion enhancement of 1 was observed for Cl- ion followed by
-
Br- > BF4 > PF6-, as depicted in Figure S3 (Supporting
Information); this trend reflects the hydrogen bond acceptor
ability of the anions.13 Similar enhancement of emission
properties was also observed upon the addition of tetraphe-
nylphosphonium chloride. The hydrogen-bonding interaction of
1
1 with Cl- ion was also monitored by H NMR spectroscopy;
the addition of excess TBACl led to a downfield shift of the
N-H resonance by 0.40 ppm (Figure S4, Supporting Informa-
tion).
Similar changes in the absorption and emission spectra were
observed for the other pincer Pt(II) complexes 2 and 3 bearing
secondary thioamide groups. Table 1 summarizes optical data
(14) (a) King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc.
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3310 Organometallics, Vol. 28, No. 11, 2009
Notes
d6): δ 11.24 (s, 2H), 7.91 (d, J ) 8.0 Hz, 2H), 7.47-7.31 (m, 11H),
5.00 (s, 2H). 13C{1H} NMR (67 MHz in DMSO-d6): δ 201.40,
162.44, 142.51, 135.52, 128.46, 127.75, 127.55, 127.62, 120.13,
49.55. Anal. Calcd for C22H19ClN2PtS2: C, 43.60; H, 3.16; N, 4.62;
Cl, 5.85; S, 10.58. Found: C, 43.83; H, 3.22; N, 4.67; Cl, 5.34; S,
10.63.
PL Titration of 1 and pKa Determination. The pKa1 of the
secondary thioamide unit of 1 was determined by titrating a solution
of 1 with aliquots of a solution of DBU. Details are given in the
Supporting Information.
Experimental Section
General Procedures. NMR spectra were recorded on a JEOL
JNM-EX-400 and JEOL JNM-EX-270 NMR spectrometers. El-
emental analyses were carried out with a Yanaco CHN Corder MT-5
instrument. Absorption spectra were recorded using a Shimadzu
UV-2550 UV-visible spectrophotometer. The emission spectra
were measured with a Hitachi F-4500 fluorescence spectrophotom-
eter. To measure the PL characteristics of the complexes in solution
and the glass-matrix state, the dissolved oxygen in the solution was
removed by bubbling with N2 for 30 min. Quantum yields at room
temperature were measured using an absolute PL quantum yield
measurement system (C9920-02, Hamamatsu photonics k.k.).
Samples for time-resolved measurements were excited at 337 nm
using an N2 laser, and the luminescence was detected with a
photomultiplier tube (Hamamatsu R928) and recorded using a
digital storage oscilloscope, before transfer to a PC for analysis.
Synthesis of [2,6-Bis(benzylaminothiocarbonyl)-K2S,S′-phenyl-
KC1]chloroplatinum(II) (1). A mixture of N,N′-dibenzyl-1,3-ben-
zenedicarbothioamide (376 mg, 1.0 mmol) and K2PtCl4 (413 mg,
1.0 mmol) in acetic acid (5 mL) was refluxed for 72 h. The resulting
orange precipitate was filtered and washed with methanol and water
to give a bright reddish powder (520 mg, 89% yield). FAB-mass:
m/z 556 [M - Cl]+, 592 [M + H]+. 1H NMR (400 MHz in DMSO-
Acknowledgment. We are grateful to Dr. J. Kuwabara
of our laboratory, Prof. Koizumi of Tokyo Institute of
Technology, Prof. Nabeshima, and Dr. Ikeda of University
of Tsukuba for useful discussion and technical support. We
thank Chemical Analysis Center of the University of Tsukuba
for measurements of NMR and emission spectra.
Supporting Information Available: Text and figures giving
details of the syntheses, characterization data, UV-vis absorption
spectra, PL spectra, and experimental details. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM800894J