Luminescence enhancement of Tb3+ ion in assemblies of amphiphilic linear-dendritic block copolymers: Antenna and microenvironment effects

Article abstract of DOI:10.1021/jp0014824

The luminescence enhancement of Tb³⁺ ion bound to amphiphilic linear-dendritic block copolymers [poly-(acrylic acid)-dendritic polyether, PAA-Dendr.PE] was investigated. Tb³⁺ ion complexes with a PAA chain of block copolymer to form a supramolecular luminophore. The observed prominent enhancement in luminescence properties can be attributed to two important factors. The antenna effect, energy harvest and transfer from the dendritic polyether subunit to the ground state of the Tb³⁺ ion, leads to a remarkable increase of luminescence intensity of Tb³⁺ ion, which enlarges obviously with increasing generation of dendrimer. Furthermore, the unique microenvironment surrounding the Tb³⁺ ion that derives from multimolecular assembly of copolymer results in a large decrease of the number of water molecules coordinated to Tb³⁺ from 3.5 to 1.3, which is the other factor to greatly enhance the luminescence intensity of Tb³⁺ ion.

Full text of DOI:10.1021/jp0014824

J. Phys. Chem. B 2001, 105, 2461-2464
2461
Luminescence Enhancement of Tb3+ Ion in Assemblies of Amphiphilic Linear-Dendritic
Block Copolymers: Antenna and Microenvironment Effects
Linyong Zhu,* Xiaofeng Tong, Miaozhen Li, and Erjian Wang
Institute of Photographic Chemistry, The Chinese Academy of Sciences,
Beijing 100101, People’s Republic of China
ReceiVed: April 18, 2000; In Final Form: October 26, 2000
The luminescence enhancement of Tb3 ion bound to amphiphilic linear-dendritic block copolymers [poly-
+
3+
(
acrylic acid)-dendritic polyether, PAA-Dendr.PE] was investigated. Tb ion complexes with a PAA chain
of block copolymer to form a supramolecular luminophore. The observed prominent enhancement in
luminescence properties can be attributed to two important factors. The antenna effect, energy harvest and
3+
transfer from the dendritic polyether subunit to the ground state of the Tb ion, leads to a remarkable increase
of luminescence intensity of Tb3 ion, which enlarges obviously with increasing generation of dendrimer.
+
3+
Furthermore, the unique microenvironment surrounding the Tb ion that derives from multimolecular assembly
3
+
of copolymer results in a large decrease of the number of water molecules coordinated to Tb from 3.5 to
1
.3, which is the other factor to greatly enhance the luminescence intensity of Tb3+ ion.
I. Introduction
SCHEME 1
Dendimers are highly branched yet structurally perfect
molecules with a large number of end groups that grow
exponentially with their generation number.1-3 Their unique
structure is expected to provide a broad range of possible
applications; especially the treelike macromolecules are promis-
ing candidates as large single-molecule photonic antenna
systems.4 The authors have recently developed a novel class
of anionic linear-dendritic amphiphiles, PAA-Dendr.PE, with
-10
11
a linear poly(acrylic acid) chain and a dendritic polyether unit
Scheme 1). The versatile copolymers combine a metal cation
(
binding subunit (PAA) and an efficient ultraviolet light-
harvesting dendron. Moreover, the amphiphilic copolymers
derived from linear PAA as a hydrophilic block and polyether
dendron as a hydrophobic block can self-assemble into micelle-
like aggregates in aqueous solution.12 Therefore, the molecular
architecture of the linear-dendritic copolymer (PAA-Den-
dr.PE) provides a possibility to design an unusual supramo-
lecular photoreaction device, where the energy harvested by
appended aryl chromophores of the dendritic subunit permit
migration along with the PAA chain to the reaction center to
which active metal ions are bound.
aqueous solution, Fendler24 and co-workers showed energy
transfer from micellar solubilized naphthalone to the terbium
ion. In addition, a suitable change in coordinative environment
and a decrease in coordinated water molecules serving as
efficient quenchers of the emission are alternative methods to
Lanthanide metal ions such as Tb3 and Eu3+ are known for
their unique luminescence properties such as long, millisecond-
order lifetime, narrow-width emission bond, and hypersensitivity
to coordinative environment. Due to these special characteristics,
+
29,30
these lanthanide elements may be used as lasers, optical
enhance the fluorescence of the lanthanide ions. Okamoto
communication amplifiers, photophysical probes, etc.1
3-16
How-
3+
observed that the emission intensity of Tb ion is enhanced
ever, the fluorescence intensity of the ions is normally quite
weak in aqueous solution. In an effort to obtain stronger
fluorescence, one general approach involves the addition of a
sensitizer chromophore to the solution, usually an organic ligand,
which will absorb energy and transfer it to a rare earth ion. In
this respect, intramolecular energy transfer from the lowest
3+
upon binding to polycarboxylates. The results indicate Tb ion
was strongly bound to these polyanions in aqueous solution and
that some of their inner coordinated water molecules were
expelled upon binding.
Here we report the luminescence enhancement of Tb3 ion
+
triplet state of the ligand molecules to the metal ion in lanthanide
in the assemblies of linear-dendritic block copolymers (PAA-
complexes is well-known.1
7-23
However, supramolecular sen-
3+
Dendr.PE) in which Tb ion was bound to PAA chains. Both
sitization is less studied and is mainly focused on micellar
the microenvironment and antenna effects provided by unique
supramolecular systems have been investigated in detail.
2
4-28
organization.
Using sodium dodecanesulfonate micelles in
1
0.1021/jp0014824 CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/03/2001

2462 J. Phys. Chem. B, Vol. 105, No. 12, 2001
Zhu et al.
3
+
-6
-1
3+
-4
Figure 1. (a) Luminescence of Tb ion in the presence of PAA-Dendr.PE in aqueous solution: [G
3
-101] 9.02 × 10 mol‚L ; [Tb ] 8 × 10
-5 -1
3
-
1
3+
3+
mol‚L ; λex 288 nm. (b) Luminescence of G
ex 288 nm.
3
-101/Tb complex under different Tb concentrations: [G
-101] 4.51 × 10 mol‚L ; λem 545 nm;
λ
-101] 4.51 × 10 mol‚L-1. (b) Excitation spectra of G
-5
-101/Tb3+ complex:
Figure 2. (a) Absorption spectra of G
3
-101 in aqueous solution: [G
3
3
-
5
-1
3+
-3
-1
3
[G -101] 4.51 × 10 mol‚L ; [Tb ] 1 × 10 mol‚L ; λem 490 nm.
TABLE 1: Data of PAA-Dendr.PE with Different
Generations and Chain Lengths
II. Experimental Section
Materials. The block copolymers, PAA-Dendr.PE, were
synthesized as described in ref 11. The molecular weight
characteristics of the polymers used are summarized in Table
PAA- generation deg polymzn
M
n
CAC,
mol‚L
-
1
Dendr.PE
(Gi)
(n)
(NMR) M /Mn
w
G
1
G
2
G
3
G
3
G
3
G
3
-75
-85
-14
-42
-101
-217
1
2
3
3
3
3
75
85
14
42
101
217
7 325
1
. Terbia (Tb2O3) (99.99%) was purchased from Yue Long
-6
8 734 1.21 3.58 × 10
2 908 2.18 6.46 × 10
5 540 1.38
-
7
Chemical Factory, Shanghai, China. TbCl3‚6H2O was prepared
by dissolving Tb2O3 in hydrochloric acid.
-
-
7
6
11 086 1.42 9.02 × 10
Fluorescence Measurement. The given amount of PAA-
Dendr.PE was dissolved in deionized water. Aqueous solutions
were quantitatively neutralized with NaOH to pH˜8.52 and were
then sonicated for 10 min at room temperature before the
addition of Tb3 ion. The mixture was stirred for 1/2 h.
Fluorescence spectra were recorded on a Hitachi MPF-4500
instrument at ambient temperature.
21 990 1.47 4.54 × 10
luminescence intensity obviously increases with increasing
3
+
solution concentration of Tb ion in the presence of PAA-
+
Dendr.PE. These phenomena suggest the formation of the
3
+
supramolecular complex of the trivalent Tb ion with PAA-
Dendr.PE. As shown in Figure 2, the excitation spectra of the
same solution (using 490 nm as emission wavelength) are in
excellent agreement with the observed absorption spectra of the
aqueous solution of PAA-Dendr.PE. The absorption of PAA
III. Results and Discussion
Antenna Effect. The typical emission spectra of Tb3 ion in
+
3
1
the presence of PAA-Dendr.PE are shown in Figure 1a. The
block is so weak in this area that only the strong absorption
(λmax ) 280 nm, ꢀ ) 6735) of aryl chromophore in the dendritic
subunit at the UV region exhibits at the observed absorption
spectra of the aqueous solution of PAA-Dendr.PE (Figure 2a).
Since aryl chromophore in the dendritic subunit does not emit
5
emission peaks at 490 and 545 nm correspond to the D4 f
7
5
7
3+
F6 and D4 f F5 transitions of Tb ion, respectively. The
fluorescence intensity of Tb3 ion is remarkably enhanced in
+
the presence of the polymer. Figure 1b shows that the

Tb3 Ion in Block Copolymers
+
J. Phys. Chem. B, Vol. 105, No. 12, 2001 2463
Figure 3. Highest fluorescence intensity (545 nm) of PAA-Dendr.PE/
Figure 4. Effect of aggregation of copolymer on luminescence intensity
3
+
3+
Tb complexes with different generations: λex 288 nm; [Tb ] 1 ×
of Tb3 ion. Curve 1: luminescence intensity of Tb ion as a function
+
3+
-
3
-1
-4
-1
-4
1
0
mol‚L ; [G
1
-75] 6.83 × 10 mol‚L ; [G -85] 1.14 × 10
2
-101concentration; [Tb3 ] 1 × 10 mol‚L , λex 288 nm. Curve
+
-3
-1
of G
2
3
-
1
-5
-1
mol‚L ; [G
3
-101] 4.51 × 10 mol‚L
.
3 1
: ratio of peak 3 to peak 1 (I /I ) of pyrene probe as a function of
-
6
-1
G
3
-101 concentration; [pyrene] 2 × 10 mol‚L ; λex 335 nm.
at the wavelength at which the excitation spectrum was analyzed
(
490 nm), these results are a clear manifestation of energy
promote Tb3+ ion to preferentially complex with carboxylates
3
+
transfer from the light-harvesting dendritic subunit to the Tb
in the PAA chain. Both further facilitate reduction of the number
ion via the excited state. Furthermore, the generation number
Gi ) 1, 2, 3) effects of PAA-Dendr.PE copolymers on the
3+
of coordinated water molecules for Tb ion. Horrocks and
(
32,33
Sundnick
have developed that the number of coordinated
3+
luminescence enhancement of Tb ion were investigated. The
obtained results (Figure 3) indicate that the luminescence
intensity of Tb3 ion further increases intensively with increasing
dendrimer generation (Gi). Frechet and co-workers also observed
the same phenomena.23 Obviously, owing to the strong absorp-
tion of the aryl groups that increase exponentially with genera-
tion, the light-harvesting “antenna” acting by the dendritic
subunit of copolymer is a significantly important factor for the
great luminescence enhancement of Tb3 ion.
3+
water molecules, q, to Tb ion is given by
+
qτ ) A(1 - τ /τ )
(1)
h
h
d
where A is constant (4.2); τh and τd are the experimental excited-
state lifetimes in H2O and D2O solutions, respectively. As a
23
first approximation, Ih/Id may be used instead of τh/τd, where
Ih and Id are the intensities of the stronger emission peaks in
H2O and D2O solutions, respectively. Thus, eq 1 may be
modified to
+
To further demonstrate the energy transfer process from the
3+
dendritic subunit to Tb ion in the block copolymer PAA-
Dendr.PE/Tb systems, the fluorescence quenching of polyether
3+
qτ ) A(1 - I /I )
(2)
h
h d
3
+
dendrimer by Tb ion was measured. However, dendritic
subunit fluorescence quenching by Tb3 ion was not observed.
Therefore, energy transfer from the sensitizer chromophore to
the terbium cation is a triplet mechanism because of a singlet
spin forbidden process.28
+
Horrocks and Sudnick have also reported an experimental
relationship between the variation in the decay constant, ∆Kobs)
(1/τh - 1/τ0), and the value q which may be expressed by
Microenvironment Effect. The amphiphilic dendritic-linear
block copolymers, PAA-Dendr.PE, which have unique mo-
lecular aggregation behaviors, can self-assemble into spherical
micelle-like aggregates. Undoubtedly, these multimolecular
assemblies will remarkably change the coordinative environment
of Tb3 ion bound to PAA block. Figure 4 shows the
∆Kobs ) 1/τ - 1/τ ) Bq
(3)
h
0
where B is constant (0.237) and τ0 is the experimental excited-
3
+
state lifetime for the nonhydrated Tb ion. The value 1/τ0 was
32,33
-1
3+
reported as 0.83 ms for Tb . Therefore eq 3 can be written
as
+
3+
relationships between the emission intensity of Tb ion and
the concentration of copolymer G3-101. At concentrations below
the critical aggregation concentration (CAC) of PAA-Dendr.PE,
the fluorescence intensity of Tb3 ion is very weak. However,
at concentrations above the CAC, a sharp increase in the
fluorescence intensity of Tb3 ion was observed; even the
luminescence intensity increased to over 150 times as high as
that in the initial state. These results indicate that aggregate
formation leads to a significant change in the microenvironment
surrounding the Tb3 ion. The increment of the fluorescence
intensity ratio I3/I1 (the ratio of the intensity of peak 3 to peak
-
1
q ) qτ (1/τ )(1 - Bqτ )
(4)
h
0
h
+
Finally, using the experimental value Ih/Id obtained from the
luminescence intensity measurements, the hydration number q
may be calculated from eq 4. The measured results show at
PAA-Dendr.PE concentration above the CAC that the average
+
3
+
number of water molecules bound to Tb ion is further
-7
decreased abruptly from 2.8 ([PAA-Dendr.PE] ) 4.51 × 10
+
-1
mol‚L , below the CAC) to 1.3 ([PAA-Dendr.PE] ) 4.51 ×
-
5
-1
10 mol‚L , above the CAC), which is much lower than that
2
9
3+
1
) of pyrene probe as shown in Figure 4 clearly demonstrates
(3.5) in homo-PAA/Tb systems. This result demonstrates
that the hydrophobic dendritic block in the PAA-Dendr.PE can
afford a more hydrophobic microenvironment for the binding
that the molecular aggregation effectively prevents the penetra-
tion of water, thereby resulting in a decrease of environmental
polarity. Additionally, the high packing and favorable orientation
of PAA chain in a micelle-like aggregate provide much a higher
local concentration of carboxylate ligand compared with that
in homo-PAA solution at equal concentration, which would
3
+
of Tb ion. Especially, these microenvironment effects will
be further remarkably strengthened after the amphiphilic block
copolymer aggregates into ordered multimolecular assemblies
due to more water expelled. However, it should be noted that

2464 J. Phys. Chem. B, Vol. 105, No. 12, 2001
Zhu et al.
Tb3 ion, and which obviously enlarges with increasing genera-
tion number of the dendritic block. The second one is the
microenvironment effect of the multimolecular aggregates,
which results in a decrease of the average number of coordinated
+
3
+
water molecules to Tb ion from 3.5 (in homo-PAA solution)
to 1.3. These unique properties of amphiphilic block copolymer
PAA-Dendr.PE may be of general applicability in the design
of energy-harvesting devices and amplifiers for optics.
Acknowledgment. The authors thank the National Key
Fundamental Research & Development Subject for financial
support (Grant No. G1999022503).
References and Notes
(
1) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Angew. Chem.,
Figure 5. Highest fluorescence intensity (545 nm) of PAA-Dendr.PE/
Int. Ed. Engl. 1990, 29, 138.
3
+
Tb complexes with different chain lengths of PAA at equal genera-
(
(
(
2) Frechet, J. M. J. Science 1994, 263, 1710.
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3
+
-3
-1
tions of dendritic block (G
i
) 3); λex 288 nm; [Tb ] 1 × 10 mol‚L
;
-
4
-1
-5
-1
[G
3
-14] 1.72 × 10 mol‚L ; [G
3
-42] 9.02 × 10 mol‚L ; [G
3
-101]
-
5
-1
-5
-1
4
.51 × 10 mol‚L ; [G
3
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.
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(
(
(
(
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3
+
intensity of Tb ion decreases sharply. It is more likely that
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as is required for sensitization.
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+
(
The effect of the chain length of PAA block (degree of
(
3
+
polymerization, n) on the luminescence intensity of Tb ion
was also investigated at given generation number of dendritic
subunit (Gi ) 3) and concentration of Tb3 ion. As Figure 5
shows, the chain length of PAA chain does not afford a
pronounced influence on the luminescence properties in the
extent of n ) 14-101. However, in the case of G3-217, the
luminescence intensity is sharply decreased by 5 times because
no order aggregates were observed in aqueous solution for
(
+
(
Elservier: Amsterdam, 1998; Vol. 25, p 101.
(
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(
1
2
copolymer bearing a too long hydrophilic PAA chain.
Moreover, the length of PAA chain of G3-217 may not permit
1
(
3+
Tb ion bound to the polycarboxylates to be effective, folding
back onto the aryl dendron and sensitizing by the dendritic
subunit. Therefore, these results further demonstrate that the
microenvironment effect provided by PAA-Dendr.PE multi-
(
(
molecular aggregates is alternatively an important factor for the
great enhancement of luminescence intensity of Tb3+ ion.
(
24) Escabi-Perez, J. R.; Name, F.; Fendler, J. H. J. Am. Chem. Soc.
1
977, 99, 7749.
IV. Conclusion
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The amphiphilic hybrid block copolymers PAA-Dendr.PE,
combining a dendritic polyether as the hydrophobic subunit and
a linear PAA as the hydrophilic block, have characteristics of
both dendrimer and linear polymer. These amphiphiles can
easily self-assemble into micelle-like aggregates in very dilute
(26) Gelade, E.; De Schryver, F. C. J. Am. Chem. Soc. 1984, 106, 5871.
(
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1
(
(
1
-6- -7
aqueous solution (10 10 M). The combination of copolymer
(
3
+
(
PAA-Dendr.PE) with Tb ion constructs a supramolecular
Macromolecules 1996, 29, 3511.
(31) Muller, G.; Laine, J. P.; Fenyo, J. C. J. Polym. Sci., Polym. Chem.
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luminophore. Their great enhancement in the luminescence
intensity of Tb3+ ion was observed and can be attributed to two
(
32) Horrocks, W. D.; Sudnick, D. R. J. Am. Chem. Soc. 1979, 101,
34.
(33) Horrocks, W. D.; Sudnick, D. R. Acc. Chem. Res. 1981, 14, 384.
factors. The first one is the antenna effect, the energy harvest
and transfer from the aryl dendritic framework to the bound
3