Emission behavior of secondary thioamide-based cationic pincer platinum(ii) complexes in the aggregate state

Article abstract of DOI:10.1002/ejic.201301404

Cationic pincer platinum(II) complexes bearing secondary thioamide units showed aggregation-induced emission (AIE) activity in chloroform/hexane and methanol/water mixtures. Hydrogen bonding of the thioamide units and interionic interactions contributed t

Full text of DOI:10.1002/ejic.201301404

SHORT COMMUNICATION
DOI:10.1002/ejic.201301404
Emission Behavior of Secondary Thioamide-Based
Cationic Pincer Platinum(II) Complexes in the Aggregate
State
Hiroya Honda,[a] Yasuyuki Ogawa,[a] Junpei Kuwabara,[a] and
Takaki Kanbara*[a]
Keywords: Photoluminescence / Hydrogen bonds / Interionic interactions / Aggregation / Platinum
Cationic pincer platinum(II) complexes bearing secondary
thioamide units showed aggregation-induced emission (AIE)
activity in chloroform/hexane and methanol/water mixtures.
Hydrogen bonding of the thioamide units and interionic in-
teractions contributed to the AIE activities.
chloride anion. This result suggested that hydrogen-bond-
ing interactions of thioamide contribute to AIE activity. In
addition to hydrogen bonding, the introduction of ionic in-
teractions into thioamide-based complexes should effec-
tively suppress the molecular motion. Therefore, it is ex-
pected that secondary thioamide-based cationic PtII com-
plexes should exhibit AIE activity because of the restriction
of the molecular motion owing to intermolecular hydrogen-
bonding and ionic interactions with the counteranions.
Herein, we report AIE-active cationic PtII complexes having
secondary thioamide units and the effects of hydrogen-
bonding and ionic interactions on the AIE activities.
Introduction
The emission of conventional luminophores such as per-
ylene and rhodamine is often quenched if their molecules
are aggregated (aggregation-caused quenching, ACQ).[1]
However, recently, Tang and co-workers and other research-
ers reported luminophores exhibiting efficient emission in
the aggregate and solid states.[1–4] This unusual luminescent
phenomenon is termed aggregation-induced emission
(AIE). AIE-active compounds have received considerable
attention because of their potential applications in organic
light-emitting diodes and sensors.[5] Studies have revealed
that the main cause of AIE activity is the restriction of mo-
lecular motion in the aggregate state.[2,3a] In some AIE-
active compounds, hydrogen-bonding interactions also in-
duce suppression of the molecular motion.[3b,3c] Yam and
Che reported AIE-active platinum(II) complexes.[6] The
emission color and intensity of the PtII complexes depend
on the extent of the Pt···Pt and π–π interactions, that is,
metal–metal-to-ligand charge transfer (MMLCT) in the ag-
gregate and the solid states.[6,7]
In previous work, we found that the emission intensity
of a neutral PtII complex bearing secondary thioamide units
Bn
[Pt(BnS C S)Cl] (1,
S C S
=
N,NЈ-dibenzyl-1,3-
∧ ∧
∧ ∧
benzenedicarbothioamide, Figure 1) increased upon adding
tetra-n-butylammonium chloride to the solution.[8] Efficient
emission was caused by the suppression of molecular mo-
tion resulting from hydrogen-bonding interactions between
the N–H moiety in the secondary thioamide group and the
Figure 1. Structures of complexes 1–3.
[a] Tsukuba Research Center for Interdisciplinary Materials
Science (TIMS), Graduate School of Pure and Applied
Sciences, University of Tsukuba,
Results and Discussion
[Pt(BnS C S)(PPh3)]Cl (2-Cl, Figure 1) was synthesized
∧ ∧
1-1-1, Tennodai, Tsukuba, Japan
E-mail: kanbara@ims.tsukuba.ac.jp
to investigate the effects of hydrogen bonding of the second-
ary thioamide sites and the ionic interactions on AIE. In
addition, complex 1[8] (Figure 1), which is neutral, and
www.ims.tsukuba.ac.jp/~kanbara_lab/index.htm
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301404.
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SHORT COMMUNICATION
Pi
Pi
∧ ∧
∧ ∧
[Pt( S C S)(PPh3)]Cl {3-Cl, S C S = 1,3-bis(1-piperid- respectively. These results indicate that 2-Cl and 3-Cl are
inothiocarbonyl)benzene} with tertiary thioamide units AIE active in the chloroform/hexane mixtures. In addition,
were prepared[9] as control samples to compare the emission the emission wavelength of 2-Cl in the chloroform/hexane
behavior in the aggregate states.
(90 vol.-%) shifted to higher energy (12 nm) relative to that
Complexes 2-Cl and 3-Cl exhibited good solubility in in the chloroform solution.
chloroform. To examine the emission behavior of 2-Cl and
To examine the effects of solvents on the AIE properties
3-Cl in the aggregate state, aggregates were prepared by of the complexes, water was used as a poor solvent in com-
adding hexane (a poor solvent) into the respective chloro- bination with dimethylformamide (DMF) for 1 or methanol
form solutions. Because aggregate states are usually pre- for 2-Cl and 3-Cl as good solvents. These good solvents
pared by use of a poor solvent,[1a,2a] hexane was used to were selected by considering the solubility of each complex
induce the formation of nanoparticles of 2-Cl and 3-Cl. Ag- and their compatibility with water. DLS measurements in
gregation was confirmed by dynamic light scattering (DLS). 90 vol.-% water showed the formation of aggregates with
DLS measurements in 90 vol.-% hexane showed the forma- particle sizes of 91, 63, and 73 nm for 1, 2-Cl, and 3-Cl,
tion of aggregates with particle sizes of 518 and 573 nm for respectively (Figure S3, Supporting Information). As shown
2-Cl and 3-Cl, respectively (Figure S1, Supporting Infor- in Figure 3 (a), the addition of water (Ͼ50 vol.-%) into a
mation). The absorption and emission spectra were ob- methanol solution of 2-Cl led to a redshift in the absorption
tained immediately after preparing the aggregates because bands. The emission spectrum of 2-Cl exhibited an increase
the nanoparticles began to precipitate within 15 min.
in the emission intensity accompanied by a redshift (from
The absorption spectra of 2-Cl and 3-Cl in chloroform 585 to 640 nm) upon increasing the proportion of water in
solution and as chloroform/hexane mixtures are shown in the methanol/water mixtures (Figure 3, b). The emission in-
Figure S2. Tailing was apparent in the long-wavelength re- tensity of the mixture with 70% water fraction was approxi-
gion of the absorption spectra of the chloroform/hexane mately 11 times higher than that in the methanol solution,
mixtures. These tails were attributed to light scattering by whereas negligible AIE activity of 1 and 3-Cl was observed
the nanoparticles of the complex molecules.[2b] As shown in in the aggregate states with water (Figures S4 and S5, Sup-
Table S1 and Figure 2, 2-Cl (λem = 589 nm) and 3-Cl (λem porting Information). The absorption and emission wave-
= 663 nm) exhibited weak luminescence in the chloroform length shift of 3-Cl were small in comparison with those of
solutions. However, a significant enhancement in the emis- 2-Cl. Notably, the AIE activities of the complexes depended
sion of 2-Cl and 3-Cl was observed if their hexane fractions on the solvent systems; complexes 2-Cl and 3-Cl were AIE
were above 80 and 85%, respectively, in the chloroform/ active in the chloroform/hexane systems, whereas only com-
hexane mixtures. If the volume fraction of hexane was 90%, plex 2-Cl showed AIE activity in the methanol/water sys-
the corresponding emission intensities of 2-Cl (λem
=
tem. The luminescence decay time (τ) of 2-Cl in the solution
577 nm) and 3-Cl (λem = 660 nm) were approximately 46 (τchloroform = 3.1 μs, τmethanol = 3.1 μs) and aggregate states
and 9 times higher than those in the chloroform solution, (τchloroform/hexane = 6.3 μs, τmethanol/water = 3.3 μs) were exam-
Figure 2. Emission spectra of (a) 2-Cl (λex = 384 nm) and (b) 3-Cl Figure 3. (a) Absorption and (b) emission spectra of 2-Cl in meth-
(λex = 404 nm) in chloroform/hexane mixtures (5ϫ10–5 m).
anol/water mixtures (5ϫ10–5 m, λex = 400 nm).
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ined (Table S2 and Figure S6, Supporting Information).
The long emission life time indicates that 2-Cl exhibited
phosphorescent emissions.
One of the factors of AIE-active PtII complexes is
MMLCT[6] and their emission shift to lower energies upon
cooling (77 K) because of the enhancement in the metal–
metal interactions.[6b] To determine the main cause of the
AIE activities of 2-Cl and 3-Cl, the photoluminescence
spectra of 2-Cl and 3-Cl were obtained in CH2Cl2/THF
(3:2 v/v) solution at room temperature and at 77 K. Because
CH2Cl2 and THF are good solvents for 2-Cl and 3-Cl, the
formation of aggregates was not observed. The emission
bands of 2-Cl and 3-Cl in solution shifted to higher energies
at 77 K (glass matrix) relative to those obtained at room
temperature (Figure S7, Supporting Information); emission
originating from MMLCT was not observed. X-ray crystal-
lographic analysis of 2-Cl[10] was consistent with the photo-
luminescence data, which indicated the absence of d8–d8
metal–metal interactions because the Pt–Pt distance be-
tween adjacent complexes was 9.66 Å. Thus, it is postulated
that the AIE activities of 2-Cl and 3-Cl originate primarily
from the suppression of molecular motion in the aggregate
state, as proposed by Tang and other researchers.[1–3] Elec-
trostatic interactions promote the tendency to aggregate
and enhance the AIE activity.[11] X-ray crystallographic
analysis of 2-Cl revealed that the thioamide group of 2-Cl
forms hydrogen-bonding interactions with the counteranion
(N–H···Cl–, 2.45 Å, Figure 4, a). The results suggest that
the AIE activity of 2-Cl is induced by these hydrogen-bond-
ing interactions.
Figure 4. Hydrogen-bonding networks of (a) 2-Cl·CHCl3 and (b) 2-
BF4·H2O·MeOH. Chloroform, methanol, and hydrogen atoms ex-
cept for N–H are omitted for clarity.
As mentioned above, in the water system only complex
2-Cl showed AIE activity. These results indicate that water In addition, the origin of emission of 2-Cl in the aggregate
molecules interrupt the ionic interactions but that the hy- state was also supposed to be MLCT, because the decay
drogen-bonding interactions in 2-Cl remain. The hydrogen- lifetime curves of 2-Cl in the solution state (methanol) were
bonding interactions in 2-Cl in the presence of water were similar to those in the aggregate state (methanol/water; see
evaluated from a crystal structure of 2-BF4 instead of 2- Figure S6, Supporting Information). It was reported that
Cl, because the crystals of the latter could not be obtained the wavelength of emission through MLCT excited states
(Figure 4, b).[12] The thioamide groups of 2-BF4 have depends on solvent polarity.[14] Because the emission wave-
hydrogen-bonding interactions with the counteranion length of 2-Cl shifted to lower energy with an increase in
(N–H···F, 1.95 Å) and the water molecule (N–H···O, the solvent polarity (positive solvatochromism; Figure S10,
2.06 Å). Because 2-BF4 also exhibited AIE activity in the Supporting Information),[14] a blueshift or redshift in the
methanol/water system (Figure S8, Supporting Infor- emission spectra of 2-Cl in the aggregate states could be
mation), it was deduced that the hydrogen-bonding interac- induced by changes in the solvent polarities around the
tions of the secondary thioamide contribute to the AIE ac- complex molecules. To confirm the effect of the hydrogen-
tivities of 2-Cl.
bonding interactions on the emission wavelength, the emis-
To consider the blue- and redshifts in the emission spec- sion behavior of 2-BF4 in the crystal state was examined,
tra of 2-Cl in the chloroform/hexane and methanol/water because the crystal structure exhibited the presence of hy-
mixtures relative to the pure solutions, we focused on the drogen-bonding interactions with water molecules (Fig-
origin of absorption in the long-wavelength region and on ure 4, b). The emission wavelength of 2-BF4 in the crystal
the emission of 2-Cl. Time-dependent DFT calculations of state was much shorter (588 nm) than that of 2-Cl in the
2+, which was used as a model complex for 2-Cl, revealed aggregate state in methanol/water (640 nm) and was almost
that the absorption band in the long-wavelength region the same as that of 2-Cl in chloroform solution (589 nm;
(380–500 nm) was assigned as metal-to-ligand charge trans- see Figure S11, Supporting Information). Therefore, the ef-
fer (MLCT) and ligand-centered (LC) transitions (Fig- fect of hydrogen bonding on the emission wavelength is neg-
ure S9 and Table S3, Supporting Information). This assign- ligible. These results suggested that the redshift in the aggre-
ment is consistent with the thioamide-based pincer com- gate state in methanol/water was due to positive solvato-
plexes reported previously.[8,13] These results suggested that chromism induced by the solvent polarity around the com-
MLCT excited states were involved in the emission of 2-Cl. plex.
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SHORT COMMUNICATION
mal parameters method by the SHELXL-97 program. Hydrogen
atoms except for H1 and H2 were placed at the calculated positions
Conclusions
In summary, cationic complex 2-Cl bearing secondary and were included in the structure calculation without further re-
finement of the parameters. H1 and H2 of 2-Cl and 2-BF4 were
determined by difference Fourier map and refined isotropically.
thioamide units exhibited AIE activity induced by hydro-
gen-bonding and interionic interactions. The origin of the
AIE in the pincer platinum(II) complexes differs from that
of conventional AIE-active platinum(II) complexes, which
occurs by metal–metal interactions. These results should
CCDC-957173 (for 2-Cl) and -957174 (for 2-BF4) contain the sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
provide valuable information for the design of AIE-active Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
complexes by exploiting hydrogen-bonding and ionic inter-
actions with counteranions.
Computational Details: The geometrical structures were optimized
at the B3LYP level for 2+ with the LANL2DZ basis set im-
plemented in the Gaussian 09 program suite.[15] By using the opti-
mized geometries, time-dependent DFT calculations were per-
formed at the B3LYP level for 2+ to predict their absorptions.
Experimental Section
Supporting Information (see footnote on the first page of this arti-
cle): Size distribution, absorption spectra, and emission spectra of
1–3; luminescence decay time of 2-Cl; and DFT calculations for
2+.
General Procedures: All NMR spectra were obtained with a Bruker
Avance-400S with tetramethylsilane or [D3]phosphoric acid solu-
tion as an internal standard. MALDI-MS spectra were recorded
with a Kratos-Shimadzu AXIMA-CFR plus MALDI-TOF MS.
Elemental analyses were performed with a Perkin–Elmer 2400
CHN Elemental Analyzer. Average particle sizes of the aggregates
were measured by dynamic light scattering (FDLS3000, Otsuka
Electronics). Absorption spectra were recorded with a JASCO V-
630 spectrometer. The emission spectra at room temperature were
measured with a JASCO FP-6200 spectrophotometer. The emission
spectra at room temperature and 77 K were measured with a Hita-
chi F-2700 spectrophotometer. The temporal profiles of the lumi-
nescence decay were recorded by using a microchannel plate photo-
multiplier (Hamamatsu, R3809U) equipped with a TCSPC com-
puter board module (Becker and Hickl, SPC630).
Acknowledgments
The authors are grateful to Prof. T. Arai and Dr. Y. Nishimura
for measuring the luminescence decay times. This work was partly
supported by the Japan Science Society (Sasakawa Scientific Re-
search Grant). The Chemical Analysis Center of the University of
Tsukuba is thanked for X-ray diffraction studies and elemental
analyses.
Synthetic Methods: Complex 1[8] and 1,3-bis(1-piperidinothio-
carbonyl)phenyl{C2,S,SЈ}chloroplatinum(II)[9] were synthesized ac-
cording to methods reported in the literature.
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Synthesis of Complex 2-Cl: A mixture of complex 1 (151.5 mg,
0.25 mmol) and triphenylphosphine (78.7 mg, 0.30 mmol) was
stirred by ultrasound in acetone (100 mL) for 1 h at 40 °C. The
solvent was evaporated under reduced pressure. The residue was
washed with hexane and extracted with chloroform. Recrystall-
ization from chloroform/hexane gave complex 2-Cl (204.7 mg,
1
94%). H NMR (400 MHz, [D6]DMSO): δ = 12.17 (s, 2 H), 8.32
(dd, J = 8.0, 2.0 Hz, 2 H), 7.62–7.53 (m, 9 H), 7.49–7.44 (m, 7 H),
7.39–7.29 (m, 10 H), 4.89 (s, 4 H) ppm. 31P{1H} NMR (162 MHz,
[D6]DMSO): δ = 19.64 [J(Pt,P) = 2227.9 Hz] ppm. MALDI-TOF-
MS: calcd. for C40H34N2PPtS2 [M – Cl + H]2+ 833.2; found 833.1.
C40H34ClN2PPtS2 (868.4): calcd. C 55.33, H 3.95, N 3.23; found C
54.94, H 4.04, N 3.27.
Synthesis of Complex 3-Cl: Synthesized in the same manner as
complex 2-Cl, except that 3-bis(1-piperidinothiocarbonyl)phen-
yl{C2,S,SЈ}chloroplatinum(II) was used instead of complex 1, yield
1
31.6 mg, 70%. H NMR (400 MHz, [D6]DMSO): δ = 7.65 (dd, J
= 8.0, 2.0 Hz, 2 H), 7.56–7.58 (m, 9 H), 7.45–7.50 (m, 6 H), 7.34
(t, J = 8.0 Hz, 1 H), 4.24 (br. s, 4 H), 4.06 (br. s, 4 H), 1.79 (br. m,
12 H) ppm. 31P{1H} NMR (162 MHz, [D6]DMSO): δ = 18.95
[J(Pt,P)
= 1135.8 Hz] ppm. MALDI-TOF-MS: calcd. for
C36H38N2PPtS2 [M – Cl]+ 788.2; found 788.1. C36H38ClN2PPtS2·
1.5H2O (851.4): calcd. C 50.79, H 4.85, N 3.29; found C 50.73, H
4.82, N 3.19.
Crystal Structure Determination: Intensity data were collected with
a Rigaku R-AXIS RAPID and a Bruker APEX-II CCD dif-
fractometer with Mo-Kα radiation. A full-matrix least-squares re-
finement was used for non-hydrogen atoms with anisotropic ther-
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[8] K. Okamoto, T. Yamamoto, M. Akita, A. Wada, T. Kanbara,
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84.3080(10)°, V = 1976.0(3) Å3, Z = 2, Dcalcd. = 1.630 gcm–3,
22323 measured reflections, 8612 independent reflections
[Rint = 0.0139], R1[IϾ2σ(I)] = 0.0208, wR2 (all data) = 0.0569,
GOF = 1.063.
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[10] A single crystal of 2-Cl·CHCl3 was obtained from a concen-
trated chloroform solution of 2-Cl. Crystallographic details for
2-Cl·CHCl3: C41H35N2PPtS2Cl4, M = 987.74, Rigaku R-AXIS
RAPID (Mo-Kα radiation), T = 90 K, monoclinic space group
P21, a = 11. 4766(4) Å, b = 9.6619(3) Å, c = 18.1480(6) Å, β =
96.764(1)°, V = 1998.3(1) Å3, Z = 2, Dcalcd. = 1.641 gcm–3,
32342 measured reflections, 8820 independent reflections
[Rint = 0.0769], R1[IϾ2σ(I)] = 0.0479, wR2 (all data) = 0.1284,
GOF = 1.059.
[13]
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[12] A single crystal of 2-Cl could not be obtained from a methanol/
water mixture, whereas a single crystal of 2-BF4·H2O·CH3OH
was obtained by the slow evaporation of methanol into a meth-
anol/water solution of 2-BF4. Crystallographic details for 2-
BF4·H2O·CH3OH: C41H40BF4N2O2PPtS2, M = 969.77, Bruker
APEX-II CCD (Mo-Kα radiation), T = 90 K, triclinic space
Received: November 2, 2013
Published Online: March 14, 2014
¯
group P1,
a = 10.0406(7) Å, b = 14.1142(10) Å, c =
14.7104(10) Å, α = 74.4260(10)°, β = 80.2570(10)°, γ =
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