Amphiphilic ??-conjugated poly(m-phenylene) photosensitizer for the Eu3+ ion: The role of macromolecular chain aggregation on the color tunability of lanthanides

Article abstract of DOI:10.1021/jp204388m

Here, we report new carboxylic functionalized poly(phenylene)s and their oligomers as selective and efficient photosensitizers for Eu³⁺ ions. Palladium-catalyzed Suzuki polycondensation was developed for the synthesis of carboxylic functionaliz

Full text of DOI:10.1021/jp204388m

ARTICLE
pubs.acs.org/JPCB
Amphiphilic π-Conjugated Poly(m-phenylene) Photosensitizer for the
Eu3+ Ion: The Role of Macromolecular Chain Aggregation on the Color
Tunability of Lanthanides
A. Balamurugan,† M. L. P. Reddy,† and M. Jayakannan*,‡
†Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology,
Thiruvananthapuram-695019, Kerala, India
‡Department of Chemistry, Indian Institute of Science Education and Research (IISER), Dr. Homi Bhabha Road,
Pune 411008, India
S Supporting Information
b
ABSTRACT: Here, we report new carboxylic functionalized poly-
(phenylene)s and their oligomers as selective and efficient photosensitizers
for Eu3+ ions. Palladium-catalyzed Suzuki polycondensation was developed
for the synthesis of carboxylic functionalized π-conjugated materials. The
chemical structures of the polymer skeleton were varied using two anchor-
ing groups consisting of ethylhexyloxy and methoxy substitution in the chain
backbone. The molecular weights of the polymer samples were obtained in
the range of 4000ꢀ8000 containing 20 aromatic units in the chain.
Photoexcitation of the oligomer-Eu3+ complexes resulted in magenta color
emission as a result of the combination of partial blue self-emission from the chromophores along with the red color from the metal
center. The ethylhexyl substituted polymer-Eu3+ complex showed complete excitation energy transfer from the macromolecular
chains to the metal center and produced bright and sharp red emission. The polymer-containing methoxy unit was found to show
largely self-emission rather than photoexcitation to the metal center. Singlet and triplet energy levels of the complexes and
chromophores revealed that both oligomers and polymers have almost identical energy levels for photosensitizing Eu3+ ions. The
polymers possessed typical amphiphilic structures via a rigid aromatic hydrophobic core and hydrophilic anionic periphery for self-
organization in water. Both dynamic light scattering and atomic force microscope analysis confirmed the existence of the spherical
shape nanometer size aggregates of the polymer chains in water. The branched ethylhexyl polymer showed the formation of loosely
packed 500 nm aggregates whereas tightly packed 200 nm particles are produced by the methoxy substituted rigid polymer. These
molecular aggregates behaved as templates for complexation as well as photosensitizing of the Eu3+ ions. The loosely packed
nanoaggregates (ethylhexyl polymer) contain Eu3+ ions in the entire scaffold and showed efficient and complete energy transfer
from the conjugated chain to metal ions. The tightly packed rigid-chains in methoxy polymer restricted the complete energy transfer
to metal center. The molecular self-organization of the polymers played a crucial role on the efficient energy transfer from the
polymer chain to metal center, more specifically Eu3+ ion-based red emission.
’ INTRODUCTION
blending technique has been successfully explored for poly-
(p-phenylene) and poly(fluorene) along with M3+ ions (M = Eu,
Er, and Yb), the phase separation of metal ions from the organic
polymeric matrix is being noticed as an inherent limitation.14,15
The chemical binding of M3+ ions with π-conjugated polymer
chains were achieved via either functionalization of conjugated
backbones or anchoring groups with appropriate units such as
carboxylate,16 bipyridyl,17 or diketone.18 Similarly functionalized
π-conjugated oligomers of fluorene, thiophene, and phenylene
moieties were also explored for Eu3+ complexes.19ꢀ21 However,
one of the most important unanswered fundamental questions is:
what is the advantage of the π-conjugated polymeric ligands over
Aromatic π-conjugated polymer-lanthanide hybrid materials
are emerging as important candidates for the application in the
field of light-emitting diodes.1,2 These hybrid materials possess
unique features of both polymers and lanthanide complexes, for
example, mechanical and thermal stability, flexibility, and a film-
forming tendency of polymers along with unique optical proper-
ties of the lanthanides such as sharp emission, long lifetime, and
high quantum yield.3ꢀ6 Additionally, the low absorption coeffi-
cients of the f-orbitals forbidden transitions in the lanthanides are
also significantly improved by the strong absorbance character-
istics of π-conjugated chains.7 Typically, two approaches are
employed to make conjugated polymerꢀlanthanide hybrids:
(a) blending of lanthanide complexes within π-conjugated poly-
mers8ꢀ10 and (b) anchoring the lanthanide ions via chemical
linkage at the polymer backbone.11ꢀ13 Although the physical
Received: May 11, 2011
Revised:
July 31, 2011
Published: August 11, 2011
r
2011 American Chemical Society
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oligomers on the photosensitization of lanthanide ions? Though
the polymers bring unique features such as processability and
mechanical stability in the hybrid materials, it is very important
and necessary to address the superiority of the photosensitizing
ability of π-conjugated polymers over small molecular ligands.
One of the obvious reasons routinely cited is the mis-match of
the energy levels between the excited states (T1) of the sensitizer
(oligomer or polymer chain) to the excited state of the Eu3+
ions.22 For example, long conjugated polymer chains possessed a
much lower HOMOꢀLUMO energy gap compared to that of its
corresponding short oligomer. As a consequence for a particular
chemical structure, only either a long chain polymer or oligomer
was found as a suitable photoexcitation source. The lack of the
availability of suitable chemical structures with identical energy
levels in both oligomers and polymers hampered the study on the
role of the macromolecular chain effect in the lanthanide ions.
Therefore, a new molecular design with appropriate energy levels
in both oligomer and polymer structures for lanthanide ions are
high in demand. Recently, we have reported carboxylic function-
alized poly(m-phenylenevinylene)s (m-PPV), their random co-
polymers, and structurally similar oligophenylenevinylene (OPV)
as photosensitizers for Eu3+ and Tb3+ ions.23 The co-complexa-
tion of the m-PPV polymer chain (blue) with both Eu3+
(red) and Tb3+ ions (green) resulted in the new generation of
single polymer chain photosensitizers for white light emission.23
However, the m-PPV polymer was found to be a good sensitizer
for Eu3+ ions, whereas the OPV counterpart was not found
suitable. This had limited our attempts to trace the effect of
macromolecular chains over oligomers in lanthanide complexes.
In our continual efforts to search for a new material design for the
above purpose, here, we report new carboxylic functionalized
poly(m-phenylene)s and oligophenylenes (both oligomer and
polymer) as effective and selective photosensitizers for Eu3+ ions.
This provides us with the opportunity to probe the importance of
the π-conjugated macromolecular chain over the small molecular
photosensitizer in the lanthanide ions.
The present work emphasizes the design and development of
new carboxylic functionalized amphiphilic π-conjugated poly-
(m-phenylene)s and their oligomers. The chemical structures of the
conjugated materials were designed based on phenylpropionic
acid via tailor-made synthetic approaches. Palladium-mediated
Suzuki polycondensation was adopted to make the polymer and
oligomer sensitizers. The nature of the polymer was found as
typical amphiphilic polyanion in water, and they exist as nan-
ometer-sized self-organized aggregates. To probe the effect of the
polymer structure on the molecular self-organization as well as
their photosensitizing ability for lanthanide ions, the anchoring
group in the chain backbone was varied using 2-ethylhexyloxy
(branching units) or methoxy units. The structure of the
oligomers, polymers, and their complexes were characterized
by 1D-NMR, 2D-NMR, Fourier transform infrared (FT-IR),
matrix-assisted laser desorption ionization time-of-flight mass
spectrometry (MALDI-TOF-TOF), thermogravimetric analysis
(TGA), and so forth, and their molecular weights were deter-
mined by gel permeation chromatography (GPC). Absorption,
emission, and time-resolved fluorescence decay techniques were
employed to study the photosensitizing characteristics of the
polymers and oligomers. Both oligomers and polymers showed
selective photosensitization to Eu3+ ions among all of the
lanthanides. The excitation energy transfer from the oligomer
to Eu3+ ions resulted in magenta color emission from their
complexes. The polymer showed complete energy transfer from
the macromolecular chain to Eu3+ ions, and pure red emission
was obtained. Further the intrinsic quantum yield determined
from time-resolved experiments revealed that the polymer com-
plex was found highly luminescent compared to that of their
oligomer complexes. Interestingly, the unique photosensitizing
ability of the polymer chains was found highly dependent on the
anchoring groups attached on the chain backbone and its
influence on the molecular self-organization. Detailed studies
by dynamic light scattering (DLS) and atomic force microscopy
(AFM) revealed that the alkyl chain substitution on the polymer
backbone influences their molecular self-organization. The size
and nature of the nanoaggregates played a crucial role on the
Eu3+-polymer complex formation as well as their photosensitizing
tendencies. In a nut shell, the present investigation provides
direct evidence for the superior photosensitizing ability of the
π-conjugated polymers over small molecules in the lanthanide
ions, more specifically Eu3+ ions for red emission.
’ EXPERIMENTAL METHODS
Materials. 3-(4-Hydroxyphenyl)propionic acid, 2-ethylhexyl-
bromide, 1,4-benzene bisboronic acid, tetrakistriphosphenylpho-
sphine palladium(0), acetylacetone, Gd(NO3)3 6H2O,Tb(NO3)3
3
3
6H2O, and Eu(NO3)3 6H2O were purchased from Aldrich
3
Chemicals. K2CO3, iodine, KI, methylamine, and all other reagents
and solvents were purchased locally and purified following
standard procedures.
General Procedures. 1H NMR and 13C NMR spectra of the
monomers, oligomers, and the polymers were recorded using
400 MHz Joel NMR spectrophotometer in CDCl3 containing a
small amount of tetramethylsilane (TMS) as an internal stan-
dard. Infrared spectra of the oligomers, polymers, and their
complexes were recorded using a Thermo-Scientific Nicolet
6700 FT-IR spectrometer with the solid state in KBr. The mass
of all the oligomer Eu3+ complexes was confirmed by using the
Applied Biosystems 4800 PLUS MALDI TOF/TOF analyzer.
The molecular weight of the polymer and purity of the oligomers
was determined by GPC using a Viscotek VE 1122 pump,
Viscotek VE 3580 RI detector, and Viscotek VE 3210 UV/vis
detector in tetrahydrofuran (THF) using polystyrene as a
standard. The thermal stability of the polymeric complexes was
determined using Pelkin Elmer thermal analyzer STA 600 model
at a heating rate of 10 °C/min in nitrogen atmosphere. The
absorption and emission studies were done by a Perkin-Elmer
Lambda 45 UVꢀvisible spectrophotometer and SPEX Fluorolog
HORIBA JOBIN VYON fluorescence spectrophotometer with a
double-grating 0.22 m Spex1680 monochromator and a 450 W
Xe lamp as the excitation source at room temperature. The
excitation spectra are collected at 612 nm (Eu3+ ion emission
wavelength), and the emission spectra are recorded by excitation
at the excitation maxima. The size determination of the polymer
solution is carried out by DLS, using a Nano ZS-90 apparatus
utilizing 633 nm red laser (at 90° angle) from Malvern instru-
ments. The reproducibility of the data was checked for at least
three independent polymer solutions. The solution spectra of the
sodium salt oligomers and polymers were recorded in water. The
concentrations of the polymer and standard solution were
adjusted in such a way to obtain the absorbance equal to 0.1
for the determination of quantum yield. The photoluminescence
lifetime measurements were carried out at room temperature
using a SPEX Fluorolog HORIBA JOBIN VYON 1934 D
phosphorimeter. AFM images were recorded for drop caste
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ARTICLE
samples using JPK instruments attached with Nanowizard-II
setup. AFM is also attached with a Zeiss inverted optical
microscope.
Synthesis of Methyl-3-(3,5-diiodo-4-methoxyphenyl)pro-
panoate (4). Compound 2 (5.0 g, 11.6 mmol) and KOH (0.97 g,
17.4 mmol) were dissolved in dry methanol (10 mL) and stirred
at ice-cold conditions. To the cold solution, dimethyl sulfate (2.2
g, 17.4 mmol) was slowly added. After the addition, the ice bath
was removed, and the reaction mixture was heated to 70 °C for
3 h. The solid precipitate was poured into water and filtered, and
the precipitate was washed with water until the filtrate became
neutral. The product was further purified by crystallization from
hot ethanol. Yield = 4.9 g (96%). 1H NMR (400 MHz, CDCl3) δ:
7.57 (s, 2H, ArꢀH), 3.80 (s, 3H, ArꢀOCH3), 3.65 (s, 3H,
OCH3), 2.80 (t, 2H, ArꢀCH2CH2), 2.56 (t, 2H, ꢀCH2CH2ꢀ
COOCH3). 13C NMR (100 MHz, CDCl3) δ: 171.67, 157.27,
140.37, 139.60, 90.39, 60.67, 51.77, 32.22, and 20.02. FT-IR
(KBr, cmꢀ1): 2926, 2890, 2371, 2330, 1729, 1574, 1531, 1447,
1407, 1358, 1300, 1167, 1055, 988, 858, 761, 703, 582, 577,
and 443. MALDI-TOF-TOF: MW = 446.03 and m/z = 484.84
(M+ + K+).
Synthesis of 4,4,5,5-Tetramethyl-2-(4-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)phenyl)-1,3,2-dioxaborolane (5).
1,4-Benzenediboronic acid (3.0 g, 18.1 mmol) and pinacol (4.3 g,
36.2 mmol) were dissolved in dry dichloromethane (60 mL) and
then refluxed for 12 h under N2 using a DeanꢀStark apparatus.
The organic layer was dried over anhydrous Na2SO4, and the
solvent was evaporated to get white solid product. The product
was further purified by recrystallized from petroleum ether/
chloroform mixture (v/v = 90:10). Yield = 4.7 g (80%). 1H NMR
(400 MHz, CDCl3) δ: 7.78 (s, 4H, ArꢀH), 1.33 (s, 24H,
aliphatic H). 13C NMR (100 MHz, CDCl3) δ: 133.86, 83.83,
24.84. FT-IR (KBr, cmꢀ1): 2990, 2930, 1530, 1360, 1290, 1140,
1100, 1020, 957, 849, and 663. MALDI-TOF-TOF: MW = 330
and m/z = 331.35 (M+).
Synthesis of 3-(4-Hydroxy-3,5-diiodophenyl)propanoic
Acid (1). 3-(4-Hydroxyphenyl)propanoic acid (10.0 g, 60.2
mmol) was dissolved in 20% methylamine (100 mL) and stirred
at room for about 15 min. KI (28.8 g, 173.6 mmol) and iodine
(30.4 g, 120.3 mmol) in water (60 mL) were slowly added into
the solution, and the stirring was continued for 5 h. The mixture
was neutralized with 2 N concentrated HCl (100 mL), and the
white precipitate was filtered and washed with water until the
filtrate became neutral. The white solid was further purified by
recrystallization from hot ethanol to obtain needle-like crystals as
a product. Yield = 23.0 g (91%). 1H NMR (400 MHz, CD3OD)
δ: 7.34 (s, 2H, ArꢀH), 2.73 (t, 2H, ArꢀCH2CH2), 2.51 (t, 2H,
CH2ꢀCH2COOCH3). 13C NMR (100 MHz, CD3OD) δ:
178.92, 157.74, 143.12 (4C), 87.99, 39.16, and 32.52 ppm. FT-
IR (KBr, cmꢀ1): 3460, 3060, 2920, 2850, 2550, 1710, 1540, 1450,
1400, 1340, 1300, 1230, 1130, 1030, 926, 879, 756, and 694.
MALDI-TOF-TOF: MW = 417.97 and m/z = 440.9 (M+ + K+).
Synthesis of Methyl-3-(4-hydroxy-3,5-diiodophenyl)pro-
panoate (2). Compound 1 (10.0 g, 120.0 mmol) was dissolved
in dry methanol (50 mL) in 100 mL RB flask. Concentrated
H2SO4 (6 mL) was slowly added into the methanol solution
while stirring, and the mixture was refluxed for 6 h. It was
concentrated and poured into water, and the precipitate was
filtered and washed with water until the filtrate became neutral.
The solid was redissolved in dichloromethane, washed with 5%
NaHCO3, and dried over anhydrous Na2SO4, and the solvent
was evaporated to get a pale yellow solid as a product. It was
further purified by crystallization from hot ethanol. Yield = 14.8 g
1
(96%). H NMR (400 MHz, CDCl3) δ: 7.48 (s, 2H, ArꢀH),
5.65 (s, 1H, ArꢀOH), 3.64 (s, 2H, OCH3), 2.78 (t, 2H,
ArꢀCH2CH2), 2.54 (t, 2H, CH2ꢀCH2COOCH3). 13C NMR
(100 MHz, CDCl3) δ: 172.77, 152.04, 139.00 (4C), 136.56,
82.15, 51.74, 35.74, and 28.79. FT-IR (KBr, cmꢀ1): 3390, 2930,
2850, 1730, 1540, 1450, 1300, 1190, 1140, 972, 864, 787,
and 694. MALDI-TOF-TOF: MW = 432.0 and m/z = 470.89
(M+ + K+).
Synthesis of Methyl-3-(3,5-diphenyl-4-methoxyphenyl)-
propanoate (OP-OM). Compound 4 (1.0 g, 2.3 mmol), 4-phe-
nylboronic acid (0.7 g, 5.8 mmol), and Pd(PPh3)4 (0.04 g, 0.035
mmol) were dissolved in of dry THF (5 mL) under nitrogen
purge. K2CO3 (1.3 g, 9.2 mmol) in water (1 mL, degassed with
nitrogen) was added, and the mixture was stirred at 90 °C for 24
h. It was poured into water, extracted with ethyl acetate, and dried
over anhydrous Na2SO4. The liquid product was further purified
by column chromatography using 5% ethyl acetate/petroleum
Synthesis of Methyl-3-(4-(2-ethylhexyloxy)-3,5-diiodo-
phenyl)propanoate (3). Compound 2 (5.0 g, 11.5 mmol) and
anhydrous K2CO3 (34.5 g, 11.5 mmol) were dissolved in dry
acetonitrile (60 mL) and then heated to 80 °C under nitrogen
atmosphere for 1 h. After the addition of a catalytic amount of KI,
2-ethylhexyl bromide (17.7 g, (16.3 mL), 91.7 mmol) was slowly
added into the reaction mixture for 15 min. The reaction was
continued for 24 h at 80 °C under nitrogen atmosphere. The
reaction mixture was poured into the water and extracted with
ethyl acetate. The organic layer was washed with 5% NaOH,
brine, and water and then dried over anhydrous Na2SO4. The
solvent was evaporated to get yellow liquid as product. It was
purified by passing through silica gel column using hexane and
10% ethyl acetate as eluent. Yield = 16.9 g (70%). 1H NMR (400
MHz, CDCl3) δ: 7.59 (s, 2H, ArꢀH), 3.80 (d, 2H, ArꢀOCH2),
3.68 (s, 3H, OCH3), 2.82 (t, 2H, ArꢀCH2CH2), 2.57 (t, 2H,
1
ether as eluent. Yield = 0.68 g (88%). H NMR (400 MHz,
CDCl3) δ: 7.59ꢀ7.57 (d, 4H, ArꢀH), 7.42ꢀ7.40 (m, 4H,
ArꢀH), 7.34ꢀ7.30 (m, 2H, ArꢀH), 7.17 (s, 2H, ArꢀH),
7.59ꢀ7.17 (m, 12H, ArꢀH), 3.68 (s, 3H, ArꢀOCH3), 3.13 (s,
3H, OCH3), 2.99 (t, 2H, ArꢀCH2CH2), 2.68 (t, 2H,
ꢀCH2CH2ꢀCOOCH3). 13C NMR (100 MHz, CDCl3) δ:
173.30, 153.28, 138.58 (2C), 136.21, 135.64 (2C), 130.11
(2C), 129.24 (4C), 128.15 (4C), 127.11 (2C), 60.47, 51.69,
35.70, and 30.35. FT-IR (NaCl, liquid state, cmꢀ1): 3040, 2930,
2850, 2380, 1740, 1600, 1430, 1370, 1230, 1170, 1020, 879,
756, and 694. MALDI-TOF-TOF: MW = 346.46 and m/z = 385
(M + K+).
Synthesis of Methyl-3-(3,5-diphenyl-4-(2-ethylhexylox-
y)phenyl)propanoate (OP-OE). Compound 3 (1.0 g, 1.8 mmol)
was reacted with 4-phenylboronic acid (0.6 g, 4.6 mmol) and
K2CO3 (1.0 g, 7.4 mmol) using Pd(PPh3)4 (0.04 g, 0.03 mmol)
as a catalyst THF + water mixture (5 + 1 mL) following the
procedure described for OP-OM. The product was purified by
column chromatography using 5% ethyl acetate/petroleum ether
as eluent. Yield = 0.61 g (75%). 1H NMR (400 MHz, CDCl3) δ:
7.56ꢀ7.55 (d, 4H, ArꢀH), 7.38ꢀ7.36 (m, 4H, ArꢀH),
ꢀCH2CH2ꢀCOOCH3), 1.93ꢀ0.91 (m, 15H, aliphatic H). 13
C
NMR (100 MHz, CDCl3) δ: 172.70, 156.51, 139.98, 139.68
(4C), 90.83,76.10, 50.75, 40.26, 35.25, 30.08, 29.10, 29.01, 23.57,
23.08, 14.14, and 11.28. FT-IR (NaCl, liquid state, cmꢀ1): 2960,
2930, 2860, 1740, 1580, 1530, 1440, 1380, 1250, 1200, 1160,
1040, 989, 872, 831, 775, and 706. MALDI-TOF-TOF: MW =
544.21 and m/z = 582.90 (M+ + K+).
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ARTICLE
7.32ꢀ7.30 (m, 2H, ArꢀH), 7.14 (s, 2H, ArꢀH), 3.66 (s, 3H,
ArꢀOCH3), 3.01 (d, 2H, OCH2), 2.97 (t, 2H, ArꢀCH2CH2),
2.66 (t, 2H, ꢀCH2CH2ꢀCOOCH3), 1.23ꢀ0.37 (m, 15H,
aliphatic H). 13C NMR (100 MHz, CDCl3) δ: 173.30, 153.28,
138.58 (2C), 136.21, 135.64 (2C), 130.11 (2C), 129.24 (4C),
128.15 (4C), 127.11 (2C), 60.47, 51.69, 35.70, and 30.35. FT-IR
(NaCl, liquid state, cmꢀ1): 3040, 2930, 2870, 2380, 1740, 1600,
1430, 1370, 1220, 1170, 1020, 879, 756, and 694. MALDI-TOF
TOF: MW = 444.62 and m/z = 483.22 (M + K+).
1020, 895, 756, and 694. MALDI-TOF TOF: MW = 965.31 and
m/z = 965.53.
Eu(acac)3(PP-OM). Yield = 0.08 g, (62%). FT-IR (KBr, cmꢀ1):
3400, 2950, 2930, 2870, 1540, 1450, 1390, 1250, 1000, 870, 781,
and 698.
Eu(acac)3(PP-OE). Yield = 0.10 g, (45%). FT-IR (KBr, cmꢀ1):
3360, 2930, 1600, 1520, 1460, 1400, 1260, 1190, 1020, 916, 766,
and 658.
Synthesis of Poly[methyl-3-(4-(2-ethylhexyloxy)phenyl)-
alt-(1,4-phenylene)propanoate] (PP-OE). Compound 3 (0.54
g, 1.0 mmol), compound 5 (0.33 g, 1.0 mmol), and Pd(PPh3)4
(0.04 g, 0.03 mmol) were dissolved in dry THF (4 mL). K2CO3
(0.55 g, 4.0 mmol) in water (1 mL, degassed with nitrogen) was
added, and the polymerization was proceeded by stirring at 90 °C
for 24 h. It was poured into methanol, filtered, and purified by
dissolved in THF and reprecipitated into methanol. The polymer
’ RESULTS AND DISCUSSION
Synthesis and Structural Characterization. Carboxylic acid
functionalized monomers, oligomers, and polymers were synthe-
sized as shown in Scheme 1. 3-(4-Hydroxyphenyl)propionic acid
was used as starting material and iodinated with KI/I2 in the
presence of 20% methylamine in water to get the compound
3-(4-hydroxy-3,5-diiodophenyl)propanoic acid (1). The car-
boxylic acid of compound 1 was protected as methyl ester by
acid catalyzed esterification reaction in the presence of concen-
trated H2SO4 in methanol to yield methyl 3-(4-hydroxy-3,
5-diiodophenyl)propionate (2). 2 was further reacted with 2-ethyl-
hexyl bromide or dimethyl sulfate to yield 3-[3,5-diiodo-4-(2-
ethylhexyl)] propionate (3) and 3-(3,5-diiodo-4-methoxyphe-
nyl)propionate (4), respectively. 1,4-Benzenebisboronic acid
was converted into pinacol ester (5). π-Conjugated poly(m-
phenylene)s and oligophenylenes were synthesized by palladium
mediated Suzuki coupling polymerization method as shown in
Scheme 1. Equimolar amounts of monomer 3 were polymerized
with pinacol ester of benzenebisboronic acid (5) in the presence
of K2CO3 in THF/H2O using Pd(PPh3)4 as a catalyst to obtain
the polymer poly[methyl-3-(4-(2-ethylhexyloxy)-phenyl)-alt-
(1,4-phenylene)propanoate] (PP-OE). Similarly 4 was reacted
with 5 to obtain the polymer poly[methyl 3-(4-methoxyphenyl)-
alt-(1,4-phenylene)propanoate] (PP-OM). Structurally identical
two oligophenylene derivatives methyl-3-(3,5-diphenyl-4-meth-
oxyphenyl)propanoate (OP-OM) and 3-(3,5-diphenyl-4-(2-
ethylhexyloxy)phenyl)propanoate (OP-OE) were obtained by
reacting 3 and 4 with two equivalents of phenylboronic acid. The
polymers and oligomers are referred as PP-OX and OP-OX,
respectively, where PP = polyphenylene, OP = oligophenylene,
and X = E and M (E = ethylhexyl and M = methyl unit). The
structures of monomers, oligomers, and polymers were con-
1
was dried in vacuum oven at 45 °C. Yield = 0.23 g (63%). H
NMR (400 MHz, CDCl3) δ: 7.76ꢀ7.67 (m, 4H, ArꢀH), 7.24
(broad s, 2H, ArꢀH), 3.69 (s, 3H, ArꢀOCH3), 3.15 (d, 2H,
OCH2), 3.02 (t, 2H, ArꢀCH2CH2), 2.72 (t, 2H, ꢀCH2CH2ꢀ
COOCH3), 1.3ꢀ0.40 (m, 15H, aliphatic H). 13C NMR (100
MHz, CDCl3) δ: 173.38, 153.01, 138.58, 135.97,135.95, 130.03
(2C), 129.20 (2C), 127.44, 127.39, 126.58, 75.21, 51.70, 40.12,
35.78, 30.43, 30.21, 29.68, 28.94, 23.43, 23.38, 23.32, 22.98,
22.83, 14.08 (2C), 11.05, 10.88, and 10.83. FT-IR (KBr, cmꢀ1):
3390, 2960, 2920, 2870, 1740, 1580, 1450, 1380, 1220, 1180,
1010, 918, 837, and 741. GPC: Mn = 4000, Mw = 7900, and
Mw/Mn = 1.9.
Synthesis of Poly[methyl 3-(4-methoxyphenyl)-alt-(1,4-
phenylene)propanoate] (PP-OM). Compound 4 (0.50 g, 1.1
mmol), compound 5 (0.37 g, 1.1 mmol), and Pd(PPh3)4 (0.04 g,
0.03 mmol) were dissolved in dry THF (4 mL). K2CO3 (0.62 g,
4.50 mmol) in water (1 mL, degassed with nitrogen) was added,
and the rest of the polymerization procedure is the same as
described for PP-OE. Yield = 0.12 g (40%). 1H NMR (400 MHz,
CDCl3) δ: 7.66ꢀ7.60 (m, 4H, ArꢀH), 7.24 (broad s, 2H,
ArꢀH), 3.67 (s, 3H, ArꢀOCH3), 3.23 (s, 3H, OCH3), 3.01 (t,
2H, ArꢀCH2CH2), 2.71 (t, 2H, ꢀCH2CH2ꢀCOOCH3). 13C
NMR (100 MHz, CDCl3) δ: 173.33, 153.52, 137.44, 136.32,
135.37, 130.16, 129.25, 129.07, 60.58, 51.71, 35.75, and 30.40.
FT-IR (KBr, cmꢀ1): 3020, 2930, 2870, 1740, 1600, 1450, 1370,
1230, 1170, 1020, 849, 756, and 694. GPC: Mn = 2300, Mw =
4100, and Mw/Mn = 1.4.
1
firmed by H NMR, 13C NMR, and HSQC-DEPT 135 NMR
spectroscopic analysis. 1H NMR and 13C NMR spectra of
oligophenylene OP-OE and its corresponding polymer PP-OE
are shown in Figure 1. In Figure 1a, the doublet at 7.56 and two
multiplets at 7.38 and 7.30 ppm were assigned to terminal phenyl
rings. The aromatic proton in the central ring appeared as singlet
at 7.13 ppm. To further confirm the formation of the expected
Synthesis of Eu3+ Complexes. The polymers or oligomers
were dissolved hydrolyzed into their corresponding sodium salts
in THF/MeOH. NaOH (2 mol equiv) in water was added to the
solution, and the mixture was refluxed for 12 h at 70 °C.
Acetylacetone (0.07 g, 0.67 mmol) was dissolved in ethanol
(5 mL), and NaOH (0.03 g, 0.67 mmol) in water was added into
the in situ prepared sodium salt polymer or oligomer (0.23
mmol). The whole mixture was stirred well for about 30 min, and
structure, the oligomer OP-OE was subjected to 1Hꢀ C
13
coupled 2D NMR (HSQC-DEPT 135 technique) (see Figure
S8 in the Supporting Information). The 13C NMR spectra on the
right-hand side of the 2D plot possessed features of the DEPT
135 for the carbons atoms. For example, the carbon attached with
odd number of protons appeared in the upward direction,
whereas these attached with an even number of protons appeared
in a downward direction. On the basis of the 2D NMR analysis,
carbon atoms in Figure 1c were assigned in the OP-OE oligomer.
Carbon atoms in Ar-CH2CH2COOCH3 appeared at 30.5,
35.8, and 51.7 ppm, respectively. Other carbon atoms with
respect to ethylhexyl units appeared well at below 30 ppm.
The peaks for aryl carbon atoms appeared above 120 ppm.
Eu(NO3)3 6H2O (0.1 g, 0.23 mmol) in ethanol (1 mL) was
3
slowly added into the mixture. The complex was precipitated in
water, filtered and washed with water and hot ethanol, and dried
in a vacuum oven.
Eu(acac)3(OP-OM). EtOH: Yield = 0.14 g, (61%). FT-IR
(KBr, cmꢀ1): 3430, 2930, 2870, 1570, 1430, 1220, 1020, 870,
756, and 694. MALDI-TOF TOF: MW = 850.53 and m/z =
851.51.
Eu(acac)3(OP-OE). EtOH: Yield = 0.15 g, (66%). FT-IR
(KBr, cmꢀ1): 3430, 3040, 2930, 2870, 2350, 1570, 1430, 1220,
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Scheme 1. Synthesis of Monomers, Oligomers, and Polymers
On the basis of the 2D experiment (see expanded plots in the
Supporting Information, Figure S8), the carbon atoms for 9, 10,
11, and 6 were assigned at 129.6, 128.0, 127.0, and 130.1 ppm,
respectively. The carbonyl carbon atom in the CdO unit
appeared at 172.2 ppm (see Figure 1c). The remaining four
peaks at 152.8, 138.8, 136.3, and 135.9 were assigned to other
methoxy substituted polymer PP-OM and its oligomer OP-OM
were also confirmed by the NMR spectroscopy, and the details
are provided in the Supporting Information (see Figure S6). The
detailed 1H, 13C, and 2D NMR techniques confirmed the
formation of the expected structure both in the polymers and
structurally identical oligomers.
Molecular Weight of the Polymers. The molecular weights
of the polymers were determined by GPC in THF solvent, and
their Mn, Mw, and polydispersity are given along with their GPC
chromatograms in Figure 2. The GPC chromatograms appeared
as a single peak indicating the formation of uniformly dispersed
polymers for Pd(PPh3)4 mediated Suzuki polycondensation.
The molecular weights of the polymers were found as moderate
with the average number of repeating units of n = 9ꢀ12 in the
conjugated backbone. To study the role of the catalyst on the
molecular weight of the chains, the Suzuki polycondensation was
also carried out using Pd(OAc)2 as a catalyst. However, the
four aryl carbon atoms 5, 7, 8, and 12, respectively. 1H and 13
C
NMR spectra of polymer PP-OE are shown along with the
oligomer in the figure 1. In the polymer structure, the aryl
protons in the 1,4-phenyl ring are not identical because of the
unsymmetrical substitution in the neighboring m-phenylene ring.
As a consequence, the aromatic protons 10 and 100 in Figure 1b
appeared as close doublets at 7.76ꢀ7.67 ppm. The aromatic
proton in the 1,3-phenylene ring appeared as board peak along
with solvent at 7.20 ppm. 13C NMR spectrum of the polymer
possessed required number of peaks as similar to their structu-
rally identical oligomer OP-OE. Similarly, the structure of the
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Figure 3. Absorption (a) and emission (b) spectra of sodium salt of
polymers and oligomers at 1 ꢁ 10ꢀ6 Mꢀ1 in water.
The molecular weight of the ethylhexyloxy polymer (PP-OE)
was found to be much higher (Mw = 7900) compared to that of
the methoxy polymer (PP-OM, Mw = 4100). The reason for the
formation of the high molecular weight in the PP-OE is
attributed to the high solubility induced by the branched
ethylhexyl units compared to that of the ꢀOMe units in the
backbone. The photophysical properties of the π-conjugated
polymers are known to attain the maxima at oligomers with 6ꢀ8
aryl units.24 In the present system, the average chain length of n =
9ꢀ12 carries almost 18ꢀ22 aryl units in the polyphenylene
chains which is sufficient enough for the studying their photo-
physical characteristics. The GPC chromatograms of the oligo-
mers were also shown along with the polymers in Figure 2. The
oligomers showed a single peak indicating the high purity of the
samples, and their mass was also confirmed by MALDI-TOF-
TOF analysis (see Supporting Information, Figure S7).
Figure 1. 1H NMR (expanded) and 13C NMR of OP-OE (a and c) and
PP-OE (b and d) in CDCl3.
Photophysical Properties of the Conjugated Materials.
π-Conjugated polymers are typically water-insoluble; however,
the carboxylic functional group attached in the polymer increases
their water solubility. Since the complexation of lanthanide ions
with the polymer chromophores was carried out in aqueous
conditions, it is more appropriate to study their photophysical
characteristics in water rather than other solvents. Therefore, the
ester groups in oligomers and polymers were hydrolyzed into
their corresponding carboxylic acid sodium salts. The solution
absorbance and emission spectra of the sodium salts of polymer
and oligomers in water are shown in Figure 3. Both OP-OE and
OP-OM oligomers showed broad absorbance at 270ꢀ285 nm.
The methoxy substituted polymer PP-OM showed almost
identical absorbance with the oligomers; however, its ethylhexyl
counterpart PP-OE showed 20 nm red shifts. Emission maxima
of the polymers and oligomers were found to follow the order
OP-OM < OP-OE < PP-OM < PP-OE. The polymers showed
Figure 2. GPC chromatogram of the polymers and oligomers in THF at
30 °C.
molecular weight was found at much lower values for the
Pd(OAc)2 assisted process compared to that of the Pd(PPh3)4
(Mn = 1600, Mw = 2100, see GPC data for Supporting Informa-
tion, Figure S9). The incorporation of a 1,3-unit in the con-
jugated polymeric chains may inherently limit the formation of
high molecular weight polymers due to steric hindrance.
A similar trend was earlier observed by us as well as others in the
polycondensation of meta-linkage π-conjugated polymers.23,24
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Eu(acac)3(OP-OE)(EtOH) showed peaks at 949.56 and 965.53
amu with respect to its sodium and potassium ion peaks.
Similarly, the methoxy substituted oligomer complex, Eu(acac)3-
(OP-OM)(EtOH) showed a molecular ion peak at 851.53 with
respect to its sodium ion peak. Both complexes exhibit the
presence of ethanol molecule in the coordination sphere which
satisfies the 9-coordination of Eu3+ ions in the complex. Our
attempt to grow single crystals for the complex was not successful
so far. Nevertheless, the mass spectra analyses of the complex
proved the formation of Eu3+complex with the newly synthesized
oligophenylenes. The thermal stability of the polymer, oligomer,
and their complexes were characterized by thermogravimetric
analysis (see Supporting Information, Figure S11). The poly-
mers, oligomers, and their Eu3+ complexes were stable up to
290ꢀ350 °C.
Excitation and emission spectra of Eu3+ complexes were
recorded at room temperature (298 K) in the powder state, and
the spectra for the oligomer complexes are shown in Figure 5.
Excitation spectra showed broad maxima corresponding to aro-
matic π-conjugated core and weak sharp metal absorption peaks at
380, 393, and 417 nm as a resultant of transition between ground
state (7F5) to higher energy states. The emission spectra were
recorded by exciting the samples at excitation maxima. Upon
excitation, there are two types of emission are possible in com-
plexes: (i) broad self-emission of conjugated chromophore in the
blue region and (ii) excitation energy transfer from conjugated
chromophore to metal ions and subsequent Eu3+ metal centered
sharp emission in the range of 570ꢀ720 nm.26 The emission
spectra of the Eu(acac)3(OP-OE) complex showed very strong
5D0 state emissions centered at 612 nm with very small self-
emission of the OP-OE in the range of 350ꢀ450 nm. The sharp
metal emission peaks at 579, 590, 615, 649, and 699 nm were
attributed to 5D0f7F0, 5D0f7F1, 5D0f7F2, 5D0f7F3, and
5D0f7F4 transitions, respectively.26 The emission spectra of
methoxy oligomer complex Eu(acac)3(OP-OM) were found al-
most similar to that of its ethylhexyl counterpart. The photographs
of the emission for the complex (solid powder and dispersed in
DMSO solution) were taken by excitation with a hand-held UV
lamp (excitation = 365 nm light). The photographs for the
oligomer complexes are shown along with their emission spectra
in Figure 5. The weak partial blue self-emission of the oligomer
chromophores resulted in magenta color emission from the Eu3+-
oligomer complex. The emission spectra of the polymer com-
plexes are shown in Figure 6. The ethylhexyl substituted polymer
complex Eu(acac)3(PP-OE) showed complete energy transfer
from the macromolecular chain chromophore to Eu3+ metal
centered, and sharp and intense red emission at 613 nm was
observed. On the other hand, the emission spectrum of the
methoxy polymer complex Eu(acac)3(PP-OM) was found to
show very strong self-emission in the range of 350ꢀ450 nm along
with relatively weak emission from the metal center at 612 nm.
The comparisons of the emission spectra of polymer complexes
suggest that ethylhexyl polymer (PP-OE) is a very good sensitizer
compared to its methoxy counterpart. To check the selectivity of
the chromophores among lanthanide metals, the samples were
checked for sensitizing Tb3+, Sm3+, and Dy3+ ions. The samples
showed selectivity only for Eu3+ ions, and no energy transfer was
observed in other lanthanide complexes (see Figure S14, Support-
ing Information). The CIE chromaticity diagram for the Eu3+
complexes generated by feeding their emission spectra and the
color coordinates were determined. The ethylhexyl polymer PP-
OE produced x = 0.61 and y = 0.31 which are almost close to the
Figure 4. MALDI-TOF spectra of Eu3+ complexes of oligomers.
more red-shifted compared to that of their oligomers, and among
the polymers the ethylhexyl polymer PP-OE showed a larger red
shift. The large difference in the emission properties of the
samples suggest that, despite the polymer and oligomers posses-
sing identical 1,3-phenylene linkage, their emission characteris-
tics are different with respect to their structural features in the
chain length or anchoring groups. For instance, the branched
anchoring ethylhexyl polymer PP-OE has a strong tendency for
the aggregation which is clearly evident from the appearance of a
shoulder in the absorbance spectrum at 310 nm as well a 60 nm
red-shift in the emission spectrum. On the other hand the
photophysical characteristics of methoxy substituted rigid poly-
mer PP-OM are almost similar to that of the oligomer OP-OM
(also OP-OE). The absorbance and emission spectra of the
polymers and oligomers in their carboxylic acid form in the THF
were found almost identical (see Figure S17, Supporting Infor-
mation) to that of their trend in Figure 3. This suggests that newly
synthesized polymers were very stable in both acid form (in organic
good solvent, like THF) as well as sodium salt form in water.
Synthesis and Characterization of Eu 3+ Complexes. Car-
boxylic acid functionalized polyphenylene and its oligomers were
employed as chromophores for Eu3+complexes using acetylace-
tone as coligands. Three mole equivalents of acetyl acetone (as
coligand) and one mole equivalent of Eu(NO3)3 6H2O in
3
ethanol were complexed with sodium salts of polymers and
oligomers. The resultant complexes were precipitated and pur-
ified by washing with hot ethanol and water. Oligomeric Eu3+
complexes were purified by recrystallization from dichloro-
methane. The formation of Eu3+ complexes was confirmed by
FT-IR spectroscopic techniques (see Supporting Information,
Figure S10). The peak corresponding to the CdO bond
stretching frequency at 1740 cmꢀ1 in the polymer (also
oligomer) completely vanished, and new peaks at 1600 and
1520 cmꢀ1 with respect to symmetric and antisymmetric vibra-
tions of Eu3+ metal coordinated CdO carboxylate group
appeared.25 To confirm the structure and molecular weight of
oligomeric complexes, the solid samples were subjected to high
resolution MALDI-TOF-TOF analysis using 2,4-dihydroxyben-
zoic acid as a matrix (see Figure 4). The mass spectrum of
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Figure 5. Excitation (left) and emission spectra (right) of oligomer-Eu3+ complexes. Emission spectra are recorded using excitation wavelengths of 360
and 340 nm for OP-OE and OP-OM complexes, respectively. The photographs showed their emission both in powder form and dispersed in
dimethylsulfoxide.
Figure 6. Excitation (left) and emission spectra (right) of polymer-Eu3+ complexes. Emission spectra are recorded using excitation wavelengths of 360
and 340 nm for PP-OE and PP-OM complexes, respectively. The photographs showed their emission both in powder form and dispersed in
dimethylsulfoxide.
perfect red-color emission (x = 0.65 and y = 0.35).2,27 The
oligomer complexes OP-OE (x = 0.47 and y = 0.26) and OP-
OM (x = 0.53 and y = 0.28) showed coordinates similar to the
magenta color. On the other hand, the methoxy polymer complex
showed predominantly blue emission (x = 0.29 and y = 0.25) from
the polymer backbone rather than the metal center.
Photoluminescence Lifetime. Luminescent decay lifetime
measurements of the complexes were carried out by a pulsed
tungsten lamp at room temperature (298 K). The luminescent
decay profiles are shown in the Supporting Information (see
Figure S16), and lifetime values are given in Table 1. The decay
profile curves of Eu3+ complexes are fitted with first-order
exponential decay. These results indicate that each Eu3+ complex
has a single chemical environment around the Eu3+ ion. The
methoxy polymer showed relatively fast decay compared to other
three complexes (see Figure S16). Both oligomers and PP-OE
polymer complexes showed lifetime values in the range of
800 μs.28 The methoxy polymer PPꢀOM showed the lifetime
as 170 μs, which is almost four times lower compared to other
complexes. This low lifetime of the methoxy polymer complex is
attributed to the partial excitation energy loss via self-emission
rather than transfer to metal center. To find out the role of the
temperature on the luminescent lifetimes, the decay data were
also recorded at ꢀ77 °C. The lifetime values were found to be
almost similar to that of those measured at 25 °C. The intrinsic
emission quantum yield (ϕLn) of the 5D0 excited state (Eu3+ ion)
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Table 1. Energy Levels, Fluorescent Life Time Rate Constants, and Intrinsic Quantum Yields of the Complexes
τ (μs)
5D0 at 298 K (τobs
)
5D0 calcd. (τrad
)
complexes
ARAD (sꢀ1
)
ANR (sꢀ1
)
ϕLn (%)
singlet state (cmꢀ1
)
triplet state (cmꢀ1
)
Eu(acac)3 PP-OE
Eu(acac)3 PP-OM
Eu(acac)3 OP-OE
Eu(acac)3 OP-OM
740
290
420
490
510
5400
940
800 ( 1
170 ( 1
730 ( 3
805 ( 3
1350
3400
2380
2050
59.0
5.0
28090
30200
30760
31740
20040
20200
21368
21140
30.0
39.0
750
was calculated from the emission spectra and lifetime measure-
ments of 5D0 emitting level. On the basis of the lifetime data, the
radiative decay (ARAD) and nonradiative decay rates were
calculated by following the literature procedure (see the Sup-
porting Information for more detail). The calculated values were
given in Table 1. Among the Eu3+ complexes, Eu(acac)3(PP-OE)
showed the highest intrinsic quantum yield (ϕLn) of 59%. It
suggests that PP-OE chains completely transferred energy from
conjugated backbone to Eu3+ ion for enhanced red emission. The
ϕLn of the oligomer complexes are just half of the PP-OE
complexes, whereas the value is almost 10 times lower for the
methoxy polymer PP-OM complex. To further clarify the energy
transfer mechanism, singlet and triplet energy levels of poly-
meric, oligomeric, and monomeric ligands were calculated, and
schematic energy transfer diagrams for polymer and oligomers
are shown (see Figure S13, Supporting Information). Singlet
(1πꢀπ*) and energy levels were obtained from absorption
spectra (lower energy absorption edges in Figure 3) of the
chromophores. The difference in S1 energy levels (HOMO)
among the polymer and oligomer are very small (only
∼1600 cmꢀ1), which suggests that all of the new chromospheres
have an almost identical energy state for photoexcitation toward
the Eu3+ ions. The triplet energy state (3πꢀπ*) was determined
from phosphorescence spectra of Gd3+ complexes (higher
energy emission edges in Figure S12).19,29 Calculated values of
singlet and triplet state energy levels were given in the Table 1
(see Figure S13 in the Supporting Information for calculation).
According to Latva's empirical rule, energy transfer from the
polymers, the sodium salts of the polymers PP-OE and PP-
OM were subjected to DLS and AFM image analysis. DLS
histograms of sodium salts of polymers and their AFM images
at 1 ꢁ 10ꢀ6 M are shown in Figure 7. The DLS plots of the
polymer samples showed monomodel distribution with the
formation of nanometer size aggregates in water. The formation
of the nanoaggregates may be explained based on the unique
polyanionic nature of the polymers. The hydrophobicity of the
polymers arises from the rigid aromatic m-phenylene backbones,
and carboxylate anions provide hydrophilicity in the structure.32
As the consequence, the entire polymer gains an amphiphilic
nature and self-organizes to produce nanometer size aggregates
in water. To further confirm the nature and shape of the
aggregates, the AFM images of the samples were recorded for
drop caste solutions (1 ꢁ 10ꢀ6 M). AFM images of the PP-OM
showed the formation of tiny 200 nm size particles, whereas the
ethylhexyl substituted polymers (PP-OE) produced mixtures of
500 nm particles with few micrometer size larger aggregates.
Both DLS and AFM studies clearly evidence that the size of the
aggregates in the ethylhexyl polymer (PP-OE-Na+) is almost
double compared to that of its methoxy substituted polymer
counterpart (PP-OM-Na+). The size-dependent self-organiza-
tions of the polymers are schematically shown in Figure 8. The
formation of the large size aggregates in PP-OE polymer
correlated to the loose packing of the aryl backbone due to the
steric hindrance introduced by the ethylhexyl unit. Further, the
branched ethylhexyl units are hydrophobic and prefer to occupy
the central core of the aggregates rather than project outward in
the aqueous medium. On the other hand, the carboxylate anion is
hydrophilic and preferentially occupies the periphery of the
aggregates. In the case of the methoxy substituted polymer, the
hydrophobic volume (or steric hindrance) provided by the
anchoring unit is almost negligible, and PP-OM chains are highly
rigid and tightly packed to produce tiny nanoaggregates of
200 nm. The roles of the methoxy and ethylhexyl units on the
packing of the polymer chains were further evident from the
photophysical characteristics of the polymer samples (see
Figure 3). The loosely packed ethylhexyl polymer possessed a
high possibility for interchain interactions; as a result, both the
absorbance and the emission spectra were red-shifted compared
to that of the methoxy polymer. Hence, all three independent
studies such as DLS, AFM, and emission confirmed the variation
in the molecular self-organization among the methoxy and
ethylhexyl polymers in water.
ligand to excited state of Eu3+ metal ion is more effective when
5
ΔE (3ππ* ꢀ D0) is equal to 2500ꢀ4000 cm
.
ꢀ1 30 As shown in
5
Table 1, the energy gap ΔE (3ππ* ꢀ D0) between the triplet
excited state of the ligand and excited state of Eu3+ ion is obtained
as 2780, 2700, 4000, and 3640 cmꢀ1 for PP-OE, PP-OM, OP-
OE, and OP-OM, respectively. It is clearly evident that all of the
chromophores followed Latva's rule and possessed required energy
levels for being as very good sensitizer of the Eu3+ metal ion. Despite
following Latva's rule, the methoxy substituted rigid polymer did not
show complete photoexcitation to the metal centers. Both methoxy
(PP-OM) and ethylhexyl (PP-OE) substituted polymers have
identical π-conjugated structures in the backbone, and therefore,
their difference in the photoexcitation capabilities based on variation
in the energy levels is ruled out. The difference in their photo-
sensitizing capabilities may arise from the other features of con-
jugated polymer such as aromatic π-stacking, and so forth.
Chain Aggregation of Polymer Photosensitizers. In gen-
eral, π-conjugated polymers possess a high tendency for mole-
cular aggregation via aromatic π-stack interactions.31 Thus,
studying the aggregation behaviors of the ethylhexyl and meth-
oxy substituted polymers (as sodium salts) could provide more
insight into their difference in the photosensitizing capabilities
toward Eu3+ ions. To trace the aggregation states of the
Mechanism of Photosensitization Process. On the basis of
the above studies, the mechanisms for the variation in the
photosensitizing behavior of methoxy and ethylhexyl polymer
are proposed in Figure 8. During the complex formation, the
Eu(acac)3 metal center interacts with the sodium carboxylate
functionality in water to produce the polymer-Eu(acac)3 com-
plexes. In the present case, the metal ions could interact with the
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Figure 7. DLS histograms and AFM images of PP-OM-Na+ and PP-OE-Na+.
Figure 8. Schematic representation of the mechanism of photosensitizing of PP-OM-Na+ and PP-OE-Na+.
carboxylic groups both at peripheries as well as the interior of the
nanoaggregates. The loosely packed sodium salt of the ethylhexyl
polymers provides a larger cavity at the center of the aggregates
which facilitate the penetration of the metal ions into both center
as well as periphery. In the subsequent photoexcitation, the energy
transfer from the polymer backbone to the Eu3+ ion occurred at all
of the sites of the nanoaggregates both interior as well as in the
periphery. As a result, the excitation energy is completely trans-
ferred from the polymeric ligand to metal center, and bright red
emission was obtained (see Figure 6) with a very high intrinsic
quantum yield of 59%. In the case of the methoxy polymer, the
rigid tightly packed polymer backbone resists the entry of the Eu3+
ion to the core of the nanoaggregates. Hence, the Eu3+ ion could
only bind to the periphery of the tiny nanoaggregates. During the
photoexcitation, the excitation energy transfer from the polymer
chain at the exterior to the metal ion results in the red emission.
However, the absence of metal ions in the tightly packed interior
portion of the aggregates promotes self-emission of the polymer
chains (blue color) rather than sensitizing the Eu3+ ions. As a
consequence the emission spectrum of the methoxy polymer-Eu3+
ion complex showed weak metal center emission with a very low
intrinsic quantum yield (5%). The variation in the photosensitiz-
ing abilities among the polymer chromophores (methoxy versus
ethylhexyl substituted chains) are directly correlated to the
nanoaggregates of the polymers in the solution. Loosely packed
larger nanoaggregates are better photosensitizers compared to that
of the tightly packed tiny particles. Both oligomer complexes were
found to be almost identical with a relatively low intrinsic quantum
yield of 31%. The present studies prove that the macromolecular
chromophores are superior photosensitizers for Eu3+ ions com-
pared to their oligomers. It is very important to design an
appropriate polymer backbone with suitable anchoring groups
for efficient and successful emission based on conjugated polymer-
lanthanide hybrid materials.
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’ CONCLUSIONS
and SR/S1/OC-54/2009 for financial support. A.B. thanks
CSIR-New Delhi, India for a senior research fellowship.
In conclusion, we have designed and synthesized new classes
of π-conjugated oligomers and polymers based on poly-
(phenylene)s having carboxylic acid functional groups and
utilized them successfully as efficient photosensitizers for Eu3+
ions. The importance of the present investigation is summarized
as follows: (i) new oligo-phenylenes and poly(phenylene)s were
synthesized based on phenylpropionic acid monomers via a
palladium mediated Suzuki polycondensation route, (ii) the
polymer backbone was anchored with methoxy and ethylhexyl
units to study their effect on the molecular weight formation and
photosensitization, (iii) the ethylhexyl units enhance the molec-
ular weight of the polymers in the Suzuki polycondensation
reaction, (iv) the photophysical studies revealed that the ethyl-
hexyl polymer possessed strong interchain interactions and the
spectra were red-shifted compared to other derivatives, (v) both
oligomer and polymer showed high selectivity toward the photo-
sensitization toward Eu3+ ions compared to other lanthanides, (iv)
the oligomer complexes showed magenta color emission due to the
combination of weak self-emission from the chromophores and
strong red emission from the metal center, (vii) the photosensitiz-
ing capabilities of the polymers were found highly selective toward
their structure: the ethylhexyl substituted polymer showed com-
plete energy transfer from chromophores and produce sharp red
emission, whereas its methoxy counterpart was found not suitable,
(viii) photoluminescent lifetime measurements and the energy level
calculation based on Latva's rule revealed that the reason for the
difference in the photosensitization of polymers reside on their
molecular self-organization, (ix) DLS and AFM studies confirmed
the existence of nanoaggregates for the polymer structure, (x) the
ethylhexyl polymer produced loosely packed aggregates for efficient
binding of the metal ions, (xi) the methoxy polymer produced
tightly packed nanoaggregates and showed largely self-emission,
and (xii) the size and shape of the polymer chain aggregates in water
are found to play a crucial role on the photosensitization of
polymeric chromophores. The carboxylic functionalized π-conju-
gated poly(phenylene) design provides a new opportunity for
tuning the color of the lanthanide complexes which may find
potential applications in opto-electronics. In a nut shell, in the
present investigation, for the first time, new carboxylic acid func-
tionalized oligo and poly(phenylene)s were synthesized to trace the
superiority of the photosensitizing properties of the macromolecu-
lar chain over the small derivatives in lanthanide complexes.
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’ ASSOCIATED CONTENT
S
Supporting Information. Structural characterization
b
data of monomers by H NMR, 13C NMR, IR, and MALDI-
1
TOF-TOF spectra, 2D-NMR spectra of IR, TGA thermograms,
energy level diagram, and phosphorescence spectra of oligomers
and polymers. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Corresponding Author
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’ ACKNOWLEDGMENT
We thank Department of Science of Technology, New Delhi,
India under Scheme: NSTI Programme-SR/NM/NS-42/2009
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