Controlled emission of platinum(II) terpyridyl complexes with poly-l-glutamic acid

Article abstract of DOI:10.1016/j.jorganchem.2010.07.024

Poly-l-glutamic acid, P(Glu), bearing multiple negatively charged side chains served as a polymeric spatially aligned scaffold for the aggregation of positively charged platinum(II) complexes [Pt(trpy)CCR](OTf) (trpy = 2,2′,6′,2″-terpyridine; R = Ph (PtH), PhC₁₂H ₂₅-p (PtC₁₂)) through electrostatic interaction, resulting in tunable emission properties. PtC₁₂ was found to exhibit gradual increase in the emission intensity based on the triplet metal-metal-to-ligand charge transfer (³MMLCT) in a tris/HCl buffer (pH 7.6)/MeOH (v/v = 1/14) solution with concomitant decrease in the emission intensity based on the triplet metal-to-ligand charge transfer (³MLCT)/the triplet ligand-to-ligand charge transfer (³LLCT) as the amount of P(Glu) was increased. Such synergistic effect was not observed in the case of PtH, wherein the emission intensity based on ³MLCT/³LLCT was increased by the increase in the amount of P(Glu), indicating that alkyl long chain of PtC₁₂ is considered to play an important role in the aggregation of the platinum(II) terpyridyl moieties to show Pt(II)-Pt(II) and π-π interactions.

Full text of DOI:10.1016/j.jorganchem.2010.07.024

Journal of Organometallic Chemistry 695 (2010) 2562e2566
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Controlled emission of platinum(II) terpyridyl complexes with
poly-
L
-glutamic acid
*
*
Toshiyuki Moriuchi , Masahiro Yamada, Kazuki Yoshii, Toshikazu Hirao
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Poly-L-glutamic acid, P(Glu), bearing multiple negatively charged side chains served as a polymeric
Received 9 June 2010
Received in revised form
21 July 2010
Accepted 27 July 2010
Available online 11 August 2010
spatially aligned scaffold for the aggregation of positively charged platinum(II) complexes [Pt(trpy)
C^CR](OTf) (trpy ¼ 2,20,60,200-terpyridine; R ¼ Ph (PtH), PhC12H25-p (PtC12)) through electrostatic
interaction, resulting in tunable emission properties. PtC12 was found to exhibit gradual increase in
the emission intensity based on the triplet metal-metal-to-ligand charge transfer (3MMLCT) in a tris/
HCl buffer (pH 7.6)/MeOH (v/v ¼ 1/14) solution with concomitant decrease in the emission intensity
based on the triplet metal-to-ligand charge transfer (3MLCT)/the triplet ligand-to-ligand charge
transfer (3LLCT) as the amount of P(Glu) was increased. Such synergistic effect was not observed in
the case of PtH, wherein the emission intensity based on 3MLCT/3LLCT was increased by the increase
in the amount of P(Glu), indicating that alkyl long chain of PtC12 is considered to play an important
Keywords:
Bioorganometallic chemistry
Poly-L-glutamic acid
Platinum(II) complex
Emission properties
role in the aggregation of the platinum(II) terpyridyl moieties to show Pt(II)ePt(II) and
pep
Metalemetal interaction
interactions.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
charged functional groups along polyelectrolytes through electro-
static interaction [1m,1o,1r,4]. In a previous paper, redox-active
ferrocenes bearing a long alkylene chain have been aggregated
along the backbone of anionic double helical DNA, presenting
a redox-active (outer) and hydrophobic (inner) spheres around the
double helical core [5]. We have also demonstrated that poly-
Square-planar d8 transition metal complexes possess the
intriguing photophysical and photochemical properties. In partic-
ular, luminescent platinum(II) complexes with oligopyridine and
cyclometallating ligands have attracted much attention because of
their interesting luminescence properties based on metallophilic
(allylamine hydrochloride) and poly-L-lysine hydrobromide
2
interaction through dZ2/dZ and/or
p
ep
interactions [1]. On the
bearing positively charged side chains along the polymer chain
serve as a polymeric spatially aligned scaffold for aggregation and
self-association of negatively charged [Au(CN)2]ꢀ through electro-
static interaction to afford the luminescent [Au(CN)2]ꢀ aggregates
other hand, highly-ordered molecular assemblies are constructed
in bio-systems to fulfill unique functions as observed in enzymes,
receptors, etc. Introduction of functional complexes into highly-
ordered biomolecules is considered to be a convenient approach to
novel biomaterials, bio-inspired systems, etc. Recently, the field of
bioorganometallic chemistry has drawn much attention and
undergone rapid development [2]. Conjuction of organometallic
compounds with biomolecules such as peptides and nucleobases is
envisioned to afford such bioconjugates. Architectural control of
molecular aggregation is of importance for the development of
functional materials [3]. The utilization of polyelectrolytes has been
recognized to be a reliable strategy for the assembly of opposite-
[6]. Poly-L-glutamic acid (P(Glu)) is known to exist in an a-helix
form at around pH 4.3 and a random coil conformation at a neutral
pH due to repulsion between negatively charged side chains. P(Glu)
bearing multiple negatively charged side chains is envisioned to
serve as a polymeric spatially aligned scaffold for the aggregation of
positively charged organometallic compounds through electro-
static interaction. Anionic polyelectrolytes such as single-stranded
nucleic acids, P(Glu), and poly(acrylic acid) have been demon-
strated to induce the aggregation of the positively charged alky-
nylplatinum(II) complexes, resulting in luminescence based on the
metalemetal and
pep interactions [1m,1o,1r]. We have also
embarked upon the controlled aggregation of cationic platinum(II)
terpyridyl complexes with arylacetylide ligands spatially along
* Corresponding authors. Fax: þ81 6 6879 7415.
E-mail addresses: moriuchi@chem.eng.osaka-u.ac.jp (T. Moriuchi), hirao@chem.
eng.osaka-u.ac.jp (T. Hirao).
the anionic
a-helical P(Glu) to regulate the formation of the
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.07.024

T. Moriuchi et al. / Journal of Organometallic Chemistry 695 (2010) 2562e2566
2563
+
+
20
10
0
R
P(Glu)
)
P(Glu)-PtH (1:1
N
)
P(Glu)-PtH (3:1
N
N
Pt
P(Glu)-PtH (10:1)
(unit ratio)
−
OTf
Pt
N
N
N
R
10
20
30
−
COO
PtH: R = H
PtC12 : R = C12H25
O
O
H
N
H
N
N
Tris-HCl buffer (pH 7.6)/MeOH
n
n-x
x
H
+
O
−
+
−
COO Na
COO Na
200
250
300
350
400
Fig. 1. The introduction of cationic platinum(II) terpyridyl complexes with arylacety-
Wavelength / nm
lide ligands [Pt(trpy)C^CR](OTf) (trpy ¼ 2,20,60,200-terpyridine; R ¼ Ph (PtH), PhC12H25
p (PtC12)) into P(Glu).
-
Fig. 3. CD spectra of P(Glu) (5.0 ꢁ 10ꢀ5 M) and PtH (5.0 ꢁ 10ꢀ5 M) in a tris/HCl buffer
(pH 7.6)/MeOH (v/v ¼ 1/14) containing various amounts of P(Glu) (0.5, 1.5, and
5.0 ꢁ 10ꢀ4 M Glu unit, respectively) at 298 K.
luminescent platinum(II) aggregates (Fig. 1). Herein, we report
the effect of the alkyl chain of platinum(II) terpyridyl complexes
with arylacetylide ligands in the formation of the platinum(II)
aggregates accommodated in P(Glu) in a tris/HCl buffer (pH 7.6)/
MeOH (v/v ¼ 1/14) solution to control emission properties.
dichroism (ICD) at around 250e350 nm based on the absorbance
region of PtH was observed in the CD spectra of each mixture of
P(Glu) and PtH (Fig. 3). These results suggest the aggregation of
the cationic complex PtH spatially around the backbone of anionic
P(Glu). Also, negative double minima at 209 and 221 nm indicate
the preservation of an a-helical structure. The electrostatic inter-
actions between the cationic platinum(II) complexes and nega-
tively charged side chains of P(Glu) are thought to stabilize the
2. Results and discussion
The aggregation of the cationic platinum(II) terpyridyl
complexes with arylacetylide ligands [Pt(trpy)C^CR](OTf)
(trpy ¼ 2,20,60,200-terpyridine; R ¼ Ph (PtH), PhC12H25-p (PtC12))
a
-helical structure by suppressing repulsion between negatively
charged side chains.
PtH (5.0 ꢁ 10ꢀ5 M) showed weak emission based on the
triplet metal-to-ligand charge transfer (3MLCT)/the triplet
ligand-to-ligand charge transfer (3LLCT) at around 600 nm in
a tris/HCl buffer (pH 7.6)/MeOH (v/v ¼ 1/14) solution probably
due to solvent-induced quenching (Fig. 4). It is noteworthy
mentioned that the addition of P(Glu) to the 5.0 ꢁ 10ꢀ5 M
solution of PtH caused the increase in emission intensity.
Increasing the ratio of the Glu unit to PtH led to gradual
increase in the emission intensity as shown in Fig. 4. PtH is
considered to be accommodated in a hydrophobic sphere of P
(Glu) to avoid the solvent effect. Furthermore, a new emission
shoulder band was detected with an emission maximum at
around 820 nm. The cationic platinum(II) terpyridyl complexes
with arylacetylide ligands [Pt(trpy)C^CR]þ have been reported
to show the triplet metal-metal-to-ligand charge transfer
(3MMLCT) emission band at around 800 nm resulting from Pt
spatially along the anionic
a-helical P(Glu) was investigated by
UV/vis spectroscopy (Fig. 2). The platinum(II) complex PtH
showed an absorption band at around 450 nm, which probably
originates from a mixed d
transfer (MLCT) and
p
p
(Pt) / p*(trpy) metal-to-ligand charge
(alkynyl) / p*(trpy) ligand-to-ligand
charge transfer (LLCT) transitions. The addition of 100 mol%
amount (based on the Glu unit) of poly- -glutamic acid sodium
L
salt (P(Glu)) to a tris/HCl buffer (pH 7.6)/MeOH (v/v ¼ 1/14) solu-
tion of PtH led to the appearance of a new shoulder band at
around 500 nm, and increase in the ratio of the Glu unit to PtH
resulted in gradual increase in the peak in the UV/vis spectra
(5.0 ꢁ 10ꢀ5 M PtH unit) as shown in Fig. 2. By reference to
previous works [1m,1q,1r,1s,1u], this new shoulder band might be
assignable as a metal-metal-to-ligand charge transfer (MMLCT)
transition. Circular dichroism (CD) spectrometry is a useful tool to
determine an ordered structure in solution. An induced circular
(II)ePt(II) and
p
ep
interactions based on the aggregation
0.2
PtH
PtH
P(Glu)-PtH (1:1)
P(Glu)-PtH (3:1)
P(Glu)-PtH (1:1)
P(Glu)-PtH (3:1)
P(Glu)-PtH (10:1)
(unit ratio)
P(Glu)-PtH (10:1)
(unit ratio)
0.1
0
400
500
600
700
600
700
800
900
Wavelength / nm
Wavelength / nm
Fig. 2. UV/vis spectra of PtH (5.0 ꢁ 10ꢀ5 M) in a tris/HCl buffer (pH 7.6)/MeOH
(v/v ¼ 1/14) containing various amounts of P(Glu) (0.0, 0.5, 1.5, and 5.0 ꢁ 10ꢀ4 M
Glu unit, respectively) at 298 K.
Fig. 4. Emission spectra (lex ¼ 350 nm) of PtH (5.0 ꢁ 10ꢀ5 M) in a tris/HCl buffer
(pH 7.6)/MeOH (v/v ¼ 1/14) containing various amounts of P(Glu) (0.0, 0.5, 1.5, and
5.0 ꢁ 10ꢀ4 M Glu unit, respectively) at 298 K.

2564
T. Moriuchi et al. / Journal of Organometallic Chemistry 695 (2010) 2562e2566
Fig. 5. Schematic representation of P(Glu)-induced aggregation for (a) PtH and (b) PtC12
.
[1m,1q,1r,1s,1u]. The new emission band at around 820 nm
might be derived from the 3MMLCT excited state resulting
platinum(II) terpyridyl moieties to induce Pt(II)ePt(II) and pep
interactions. Furthermore, increase in the ratio of P(Glu) caused
gradual increase in the 3MMLCT emission intensity with concomi-
tant decrease in the 3MLCT/3LLCT emission intensity. High loading
of P(Glu) per PtC12 would lead to the accommodation and aggre-
gation of cationic PtC12 in a hydrophobic sphere of P(Glu). The
from Pt(II)ePt(II) and
pep interactions based on the aggre-
gation of PtH. P(Glu) bearing multiple negatively charged side
chains was found to serve as a polymeric spatially aligned
scaffold for the accommodation and aggregation of cationic
PtH (Fig. 5a).
aggregation of PtC12 around the backbone of a-helical P(Glu) was
In comparison to the emission of PtH, the platinum(II) complex
PtC12 having a dodecyl chain (5.0 ꢁ 10ꢀ5 M) exhibited a strong
emission based on 3MLCT/3LLCT at around 650 nm even in the
absence of the P(Glu) as shown in Fig. 6. The platinum(II) complex
PtC12 could assemble easily in a polar environment. This result
suggests the self-aggregation through hydrophobic interaction
between dodecyl chains to avoid the solvent effect. Interestingly,
the 3MMLCT emission at around 820 nm resulting from Pt(II)ePt(II)
also confirmed by UV/vis and CD spectra. The appearance of a new
absorption band at around 500 nm with a shoulder band at around
600 nm was observed by the addition of 100 mol% amount (based
on the Glu unit) of P(Glu) to a tris/HCl buffer (pH 7.6)/MeOH
(v/v ¼ 1/14) solution of PtC12 in the UV/vis spectra (5.0 ꢁ 10ꢀ5 M
PtC12 unit) as shown in Fig. 7. The absorption at >500 nm is
probably assignable to the MMLCT transition based on the P(Glu)
induced aggregation. Furthermore, increase in the ratio of the
Glu unit to PtC12 resulted in gradual increase in the peak. The
absorption spectrum was changed largely between 1:1 and 3:1 of
P(Glu)ePtC12. High loading of P(Glu) per PtC12 would lead to the
accommodation and aggregation of cationic PtC12 in a hydrophobic
sphere of P(Glu), resulting in the change of the absorption spec-
trum. The accommodation of PtC12 in a hydrophobic sphere of
P(Glu) was supported by the appearance of an ICD at around
250e350 nm based on the absorbance region of PtC12 in the CD
spectra of PtC12 complex in the presence of P(Glu) as shown in
Fig. 8.
and
pep interactions based on the aggregation of PtC12 was
observed with concomitant disappearance of the 3MLCT/3LLCT
emission by the addition of 100 mol% amount of P(Glu) to a tris/HCl
buffer (pH 7.6)/MeOH (v/v ¼ 1/14) solution of PtC12 (Fig. 6). This
result indicates hydrophobic interaction between dodecyl chains
and electrostatic interaction between the cationic platinum(II)
complexes and negatively charged side chains of P(Glu) to arrange
the platinum(II) terpyridyl moieties regularly around the backbone
of P(Glu) (Fig. 5b). The dodecyl moiety of the platinum(II) complex
was found to play an important role in the aggregation of the
0.5
PtC12
PtC12
P(Glu)-PtC12 (1:1)
P(Glu)-PtC12 (1:1)
P(Glu)-PtC12 (3:1)
P(Glu)-PtC12 (10:1)
(unit ratio)
0.4
P(Glu)-PtC12 (3:1)
P(Glu)-PtC12 (10:1)
(unit ratio)
0.3
0.2
0.1
0
600
700
800
900
400
500
600
700
Wavelength / nm
Wavelength / nm
Fig. 6. Emission spectra (lex ¼ 350 nm) of PtC12 (5.0 ꢁ 10ꢀ5 M) in a tris/HCl buffer (pH
7.6)/MeOH (v/v ¼ 1/14) containing various amounts of P(Glu) (0.0, 0.5, 1.5, and
5.0 ꢁ 10ꢀ4 M Glu unit, respectively) at 298 K.
Fig. 7. UV/vis spectra of PtC12 (5.0 ꢁ 10ꢀ5 M) in a tris/HCl buffer (pH 7.6)/MeOH (v/
v ¼ 1/14) containing various amounts of P(Glu) (0.0, 0.5, 1.5, and 5.0 ꢁ 10ꢀ4 M Glu unit,
respectively) at 298 K.

T. Moriuchi et al. / Journal of Organometallic Chemistry 695 (2010) 2562e2566
2565
20
10
0
2H, J ¼ 7.8 Hz), 7.58e7.54 (m, 2H), 7.21 (d, 2H, J ¼ 8.4 Hz), 7.17 (d, 2H,
J ¼ 8.4 Hz), 2.66 (t, 2H, J ¼ 7.8 Hz), 1.69e1.62 (m, 2H), 1.39e1.25 (m,
18H), 0.89 (t, 3H, J ¼ 6.9 Hz); HRMS (FAB) m/z calcd for C35H40N3Pt
([M ꢀ OTf]þ), 697.2870; found, 697.2880; Anal. Calcd. for
C36H40N3O3F3SPt: C, 51.06; H, 4.76; N, 4.96. Found: C, 50.79; H, 4.69;
N, 5.09.
P(Glu)
P(Glu)-PtC12 (1:1
P(Glu)-PtC12 (3:1
)
)
P(Glu)-PtC12 (10:1)
(unit ratio)
4.3. General procedure of UV/vis measurement
10
20
30
UV/vis spectra were obtained using a Hitachi U-3500 spectro-
photometerinadeaeratedtris/HClbuffer(pH7.6)/MeOH(v/v ¼ 1/14)
solution with the concentration 5.0 ꢁ 10ꢀ5 M for the platinum(II)
complex under argon at 298 K. UV/vis spectra were measured using
1-cm pathlength quartz cuvettes.
200
250
300
350
400
Wavelength / nm
Fig. 8. CD spectra of P(Glu) (5.0 ꢁ 10ꢀ5 M) and PtC12 (5.0 ꢁ 10ꢀ5 M) in a tris/HCl buffer
(pH 7.6)/MeOH (v/v ¼ 1/14) containing various amounts of P(Glu) (0.5, 1.5, and
5.0 ꢁ 10ꢀ4 M Glu unit, respectively) at 298 K.
4.4. General procedure of CD measurement
CD spectra were recorded using a JASCO J-720 spectropolarimeter
in a deaerated tris/HCl buffer (pH 7.6)/MeOH (v/v ¼ 1/14) solution
with the concentration 5.0 ꢁ 10ꢀ5 M for the platinum(II) complex
under argon at 298 K. CD spectra were measured using 1-cm path-
length quartz cuvettes.
3. Conclusion
Tunable emission properties of positively charged platinum(II)
terpyridyl complexes with arylacetylide ligands were demon-
strated by use of poly-L-glutamic acid, P(Glu), bearing multiple
4.5. General procedure of emission measurement
negatively charged side chains as a polymeric spatially aligned
scaffold for the aggregation of the platinum(II) complexes through
the electrostatic and hydrophobic interactions. The dodecyl
moiety of the platinum(II) complex was found to play an impor-
tant role in the aggregation of the platinum(II) terpyridyl moieties
Emission spectra were measured using a Shimadzu RF-5300PC
spectrofluorophotometer in a deaerated tris/HCl buffer (pH 7.6)/
MeOH (v/v ¼ 1/14) solution with the concentration 5.0 ꢁ 10ꢀ5 M for
the platinum(II) complex under argon at 298 K. Emission spectra
were measured using 1-cm pathlength quartz cuvettes.
to induce Pt(II)ePt(II) and
pep interactions. The architectural
control of molecular assemblies utilizing biomolecules is envi-
sioned to be a useful approach to artificial highly-ordered bio-
organometallic systems. Studies on the application of polypeptide
induced metal ion aggregates including functional materials and
catalysts are now in progress.
Acknowledgement
Thiswork wassupportedbyGrant-in-AidsforScienceResearch on
Priority Areas (No. 20036034) and Innovative Areas (No. 21111512)
from the Ministry of Education, Culture, Sports, Science and Tech-
nology, Japan. Thanks are also due to the Analytical Center, Graduate
School of Engineering, Osaka University, for the use of the CD
instrument.
4. Experimental
4.1. General materials and experimental procedures
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sources and were further purified by the standard methods, if
necessary. Poly-L-glutamic acid sodium salt with mol. wt. >8000
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