Enhanced brightness emission-tuned nanoparticles from heterodifunctional polyfluorene building blocks

Article abstract of DOI:10.1021/ja311497e

Three-coordinate complexes (bromo)[4-(2,2-dimethyl-1,3-dioxolan-4-yl)- phenyl](tri-tert-butyl-phosphine)palladium (1) and (bromo){4-[(tetrahydro-2H- pyran-2-yloxy)methyl]phenyl}(tri-tert-butyl-phosphine)palladium (2) were used to initiate Suzuki-Miyaura chain growth polymerization of 7′-bromo-9′, 9′-dioctyl-fluoren-2′-yl-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane (3). The polymerization was optionally terminated by end-capping with red-emitting N-(2-ethylhexyl)-1,6-bis(4-tert-octylphenoxy)-9-(4,4,5,5- tetramethyl-[1,3,2]dioxaborolan-2-yl)-perylene-3,4-dicarboximide. Heterodisubstituted polyfluorenes of adjustable molecular weights between 5 × 10³ and 1.0 × 10⁴ g mol^-1 and narrow molecular weight distribution (M_w/M_n < 1.2), bearing precisely one or two hydroxyl groups on one chain end and optionally a dye-label on the opposite end, were obtained virtually devoid of any side-products. Covalent attachment of polyethylene glycol (M_n = 2 × 10³ g mol^-1) to the reactive end groups yielded amphiphilic block copolymer, which afforded stable nanoparticles with diameters in the range of 25-50 nm when dispersed in water. These particles exhibited a bright fluorescence emission with quantum yields as high as Φ = 84%, which could optionally be tuned to longer wavelengths by energy transfer to the perylene monoimide dye. The heterodifunctional nature of these polyfluorenes is crucial for a bright and enduring fluorescence brightness as revealed by comparison to nanoparticles containing physically mixed dye. Further addition of terrylene diimide dye to the nanoparticles of perylene-end-capped polyfluorene block copolymers allows for an energy cascade resulting in emission exclusively in the deep red and near-infrared regime.

Full text of DOI:10.1021/ja311497e

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Article
Enhanced brightness emission-tuned nanoparticles
from heterodifunctional polyfluorene building blocks
Christoph Siegfried Fischer, Moritz C Baier, and Stefan Mecking
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja311497e • Publication Date (Web): 31 Dec 2012
Downloaded from http://pubs.acs.org on January 11, 2013
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Enhanced brightness emission-tuned nanoparticles
from heterodifunctional polyfluorene building
blocks
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Christoph S. Fischer, Moritz C. Baier, and Stefan Mecking
Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, 78464
Konstanz, Germany
KEYWORDS
conjugated polymer nanoparticles, polyfluorene, end-functionalization, Suzuki-Miyaura coupling
polymerization, emission tuning.
ABSTRACT
Three-coordinate complexes (bromo)[4-(2,2-dimethyl-1,3-dioxolan-4-yl)-phenyl](tri-tert-butyl-
phosphine)palladium (1) and (bromo){4-[(tetrahydro-2H-pyran-2-yloxy)methyl]phenyl}(tri-tert-
butyl-phosphine)palladium (2) were used to initiate Suzuki-Miyaura chain growth
polymerization of 7’-bromo-9’,9’-dioctyl-fluoren-2’-yl-4,4,5,5,-tetramethyl-[1,3,2]dioxaborolane
(3). The polymerization was optionally terminated by end-capping with red-emitting N-(2-
ethylhexyl)-1,6-bis(4-tert-octylphenoxy)-9-(4,4,5,5,-tetramethyl-[1,3,2]dioxaborolan-2-yl)-
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perylene-3,4-dicarboximide. Heterodisubstituted polyfluorenes of adjustable molecular weights
between 5 × 103 and 1.0 × 104 g mol-1 and narrow molecular weight distribution (Mw/Mn < 1.2),
bearing precisely one or two hydroxyl groups on one chain end and optional a dye-label on the
opposite end were obtained virtually devoid of any side-products. Covalent attachment of
polyethylene glycol (Mn = 2 × 103 g mol-1) to the reactive endgroups yielded amphiphilic block
copolymer, which afforded stable nanoparticles with diameters in the range of 25 to 50 nm when
dispersed in water. These particles exhibited a bright fluorescence emission with quantum yields
as high as = 84 %, which could optionally be tuned to longer wavelengths by energy transfer
to the perylene monoimide dye. The heterodifunctional nature of these polyfluorenes is crucial
for a bright and enduring fluorescence brightness as revealed by comparison to nanoparticles
containing physically mixed dye. Further addition of terrylene diimide dye to the nanoparticles
of perylene-endcapped polyfluorene block copolymers allows for an energy cascade resulting in
emission exclusively in the deep red and near infrared regime.
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Introduction
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Luminescent nanoparticles are of strong interest related to their applications in
optoelectronics,1,2 live cell imaging3-5 and biosensing. Among the different classes of materials
studied,6-9 conjugated polymer nanoparticles10 favorably combine high absorption coefficients,
fluorescence quantum yields, and extraordinarily high nonlinear optical absorption cross
sections.9d,11 By comparison to inorganic quantum dots, the absence of toxic metals can be
advantageous in terms of toxicity.9a,e Most frequently, studies of such luminescent particles have
employed polyfluorene as the conjugated polymer.1,12 Commonly, these nanoparticles employ
physisorbed surfactants as the stabilizing agent necessary to provide colloidal stability and
prevent aggregation. This can be problematic in fundamental studies as well as applications of
such nanoparticles as possible desorption of surfactant adds an unknown parameter affecting e.g.
the particle stability over time or in different environments, e.g. physiological environments with
their variable ion concentration. An elegant alternative approach to nanoparticles in general is
their generation from amphiphilic block copolymers. These may form stable core/shell-type
particles by self-aggregation in water without the need for further addition of stabilizers.
Block copolymers are synthetically accessible by controlled chain-growth polymerization
reactions with sequential addition of various monomers or by linkage of two end-functionalized
blocks. However, polyfluorenes are usually prepared by Suzuki-Miyaura polycondensation in a
classical step-growth fashion, not amenable to the preparation of block copolymers. The
pioneering work of Yokozawa et al. in 200713 enabled the synthesis of well-defined
polyfluorenes by a controlled chain-growth Suzuki-Miyaura type polymerization reaction of
aromatic AB monomers. Chain propagation is initiated by arylpalladium complexes of the
general structure [tBu3P(Br)Pd-Ar], with the aromatic ligand Ar representing the starting end of
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the growing polymer chain. As a result of the control over polymerization behavior the terminal
end can be modified by addition of a different boronic ester compound after consumption of the
AB type monomer.14,15 However, the type of chain-end functionality originating from the
initiator has been limited to unreactive aromatic hydrocarbon aryls unsuitable for further
modifications. The ability of introducing such reactive polymer endgroups via the initiator would
very much enhance the general scope and power of this synthetic methodology, and provide
access to novel narrow distributed polyfluorene materials like block copolymers.
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Here we report the synthesis of heterodifunctional polyfluorenes by controlled chain-growth
Suzuki-Miyaura type polymerization. A chemically reactive functional end-group is introduced
by a functionalized initiator and the second chain-end is modified by end-capping with a
fluorescent dye, to which an energy transfer can occur. Subsequent conjugation of this polymer
chain to a hydrophilic block facilitates the preparation of self-stabilizing emission-tuned
conjugated polymer nanoparticles (Scheme 1).
Scheme 1. Synthetic approach towards size-controlled diblock copolymers with end group
labeling suitable for nanoprecipitation (PF = polyfluorene, PEG = polyethylene glycol, PMI =
perylene monoimide dye).
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Results and Discussion
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The controlled Suzuki-Miyaura chain-growth polymerization is characterized by incorporation
of the aromatic ligand of the Pd initiator at one polymer chain end. In order to introduce a chain-
end functionality into the polymer, tri-coordinated Pd complexes with para- substituted phenyl
ligands containing reactive functional groups were synthesized and isolated. The
functionalization on the aryl moiety consisted of an acetal protected dihydroxy group in case of
initiator 1 and a tetrahydropyran (THP) protected hydroxy group for initiator 2 (Scheme 2).
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Scheme 2. Synthesis of initiators 1 and 2 with protected functional groups.
These protecting groups were chosen to remain intact during initiator synthesis and
polymerization, however allowing facile removal under acidic conditions. The key step of the
initiator synthesis was the oxidative addition of the phenyl bromide derivatives to the Pd(0)
precursor. Due to the formation of side products at elevated reactant conversions, product yields
and purities are crucially dependent on the choice of reaction time and temperature. Optimal
reaction conditions were found at 70 °C and 140 minutes reaction time with three equivalents of
aryl halogide in butanone as a solvent. Both compounds were isolated in high purity by
precipitation from pentane in satisfactory yields of 48 % and 38 % for complex 1 and 2,
respectively. Proton NMR spectroscopy corroborates a 1:1 stoichiometry of tri-t-butylphosphine
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to the para-substituted phenyl ligands for both complexes (Figures S1 and S3, Supporting
Information). The purity of the compounds is further evidenced by a single 31P-NMR resonance.
Elemental analyses are in good agreement with theoretical values and the high resolution ESI
mass spectrum of 1 exhibited signals of Na and K adducts of the desired structure (Figure S2,
Supporting Information).
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Complexes 1 and 2 smoothly initiated the polymerization of fluorene monomer 3 (Scheme 3) to
yield perfectly end-functionalized polymers PF1 and PF2. The remarkably controlled nature of
this chain-growth polymerization is reflected by a strictly linear increase in molecular weights
versus monomer conversion at a constantly low molecular weight distribution (Figure S11,
Supporting Information). As a result, molecular weights can be adjusted by variation of the
monomer to initiator ratio (Table 1). Molecular weights ranging from 5 × 103 to about 1.0 × 104
g mol-1 corresponding to a degree of polymerization of 10 to 20 could be obtained with Mw/Mn
as low as 1.2. Even higher molecular weights were accessible (e.g. 1.7 × 104 g mol-1 by NMR
for a monomer to initiator ration of 40:1), but at the expense of a broader distribution (Mw/Mn =
1.4). MALDI-TOF spectra of PF1 and PF2 are characterized by a series of signals, separated by
the weight difference of one fluorene repeat unit, corresponding to polymer chains with an
initiator-derived endgroup, with side products being virtually absent (Figure 1). This completely
chain-end functionalized polymer structure allowed the determination of the number average
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molecular weights by H-NMR spectroscopy. Within experimental error of the method,
theoretically expected and experimentally found degrees of polymerization (calculated from
monomer to initiator ratio and degree of conversion) coincided (Table 1). The combined data
thus confirm that narrowly distributed polyfluorenes of adjustable molecular weight with one
functional end group on every chain were obtained.
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Scheme 3. Synthesis of end-functionalized polyfluorenes PF1, PF2 and PMI end-capped FG-
PF-PMI (FG = functional group).
Figure 1. MALDI-TOF mass spectra of PF1 and PF2.
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Table 1. Polymerization of fluorene 3 with initiators 1 and 2.
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monomer :
initiator
Mn (theo) a
Mn (NMR) b
Mn (GPC) c
c
entry initiator
Mw/Mn
(g · mol-1)
3.5 × 103 d
6.2 × 103 e
3.5 × 103 d
6.2 × 103 e
(g · mol-1)
4.7 × 103
8.2 × 103
4.5 × 103
6.6 × 103
(g · mol-1)
6.8 × 103
1.5 × 104
7.1 × 103
9.9 × 103
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1
10 : 1
20 : 1
10 : 1
20 : 1
1.15
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Reaction conditions: 0 °C; 2 to 3 h; solvent: THF : H2O = 25 : 1; 4 eq. of CsF and 8 eq. of 18-
crown-6 with respect to amount of monomer. a Mn theo = (monomer : initiator ratio) x M repeat unit
x monomer conversion b determined by 1H-NMR spectroscopy. c determined by GPC in THF at
40 °C vs polystyrene standards. 80 % monomer conversion. 90 % monomer conversion (for
details see Supporting Information).
e
f
Endcapping of the chain by a suitable dye (vide infra) was achieved by addition of an excess of
a boronic ester compound to the polymerization mixture before quenching the reaction with HCl.
A boronic ester substituted perylene monoimide dye (PMI)16 was added to the reaction upon
almost complete conversion of the fluorene monomer. With a 25-fold excess of this end-capping
reagent the hetero-disubstituted structure FG-PF-PMI (Scheme 3, FG: functional group) could be
obtained cleanly. The formation of this structure was verified by MALDI-TOF mass
spectrometry, which shows exclusively masses of the desired product with only trace amounts at
the lower limit of detection of the MALDI-TOF method of non-endcapped polyfluorene present
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(Figure 2). H-NMR spectra exhibit initiator end and end-capper resonances in a 1 to 1 ratio
(Figure S6, Supporting Information). This confirms that all polyfluorene chains bear exclusively
the two desired different end-groups within experimental errors (estimated to be ca. 5 % or
lower). Molecular weight as determined by 1H-NMR spectroscopy (Mn = 6.9 × 103 g mol-1) was
not significantly affected by the introduction of the end-caps, compared to similar
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polymerizations without end-capping (cf. Table 1, entry 2 and 4). Likewise, the molecular
weight distribution remained low at Mw/Mn = 1.2.
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Figure 2. MALDI-TOF mass spectrum of perylene monoimide (PMI) substituted polyfluorene.
Closed and open circles indicate end-capped and non-end-capped chains, respectively. Degrees
of polymerization are represented by the numbers above the closed circles.
Towards the aim of stable, aqueous nanoparticle dispersions, a hydrophilic segment was
attached to the polymer chains to render them amphiphilic (Scheme 1). A hydrophilic
polyethylene glycol (PEG) block was covalently linked to the polyfluorene block via amide
formation. In a first step the terminal alcohol of the polyfluorene was converted into a carboxylic
acid using succinic anhydride. Subsequently, the polyfluorene acid end-group was activated with
carbonyldiimidazole (CDI) and reacted with a primary PEG amine (Scheme 4).
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Scheme 4. Synthesis of amphiphilic block copolymers starting from PF1 (including grey branch)
or PF2 (excludes grey branch)
The DMAP-assisted carboxylation of the OH-endfunctionalized polymers was conducted in
presence of large excess of succinic anhydride (25 eq.). Complete conversion was confirmed by
1H-NMR spectroscopy as well as by MALDI-TOF spectroscopy (Figures S7, S8 and S12,
Supporting Information). Subsequently, the carboxylic acids were activated with an excess of
carbonyldiimidazole which was removed by precipitation of the polymer in acetone. The
imidazole amide-terminated PF was further reacted with mono amino-functionalized
polyethylene glycole, CH3O(CH2CH2O)nCH2CH2NH2 (2 × 103 g mol-1). The formation of the
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desired block copolymer was confirmed by H-NMR spectroscopy (Figures S9 and S10,
Supporting Information). The integrals of the H3CO-PEG-endgroup and the succinylene protons
indicate a 1:1 stoichiometry, underlining a complete PEGylation of the polyfluorene and
successful removal of unreacted excess PEG-amine starting material. The PMI-substituted PF
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was reacted in an identical manner to yield the amphiphilic PMI labeled block copolymer PMI-
PF-PEG.
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For the preparation of aqueous nanoparticle dispersions from these amphiphilic block
copolymers, the nanoprecipitation technique was applied.9a,17 A dilute solution of polymer in a
water miscible solvent is quickly added to rapidly stirred water. This results in polymer
nanoparticles, which are stabilized by the hydrophilic block. For the formation of polyfluorene
nanoparticles, 0.05 wt. % THF solutions of PF-PEG or PMI-PF-PEG were nanoprecipitated in
Milli-Q water to yield aqueous dispersion with 0.003 wt. % polymer content. Particles exhibited
an excellent stability over time as evidenced by unaltered particle sizes measured by DLS even
after weeks and months. These nanoparticles possess a spherical shape and diameters of 25 to 50
nm as evidenced by TEM imaging (Figure 3). A general tendency that increasing PF block
lengths lead to an increase in particle diameters was indicated by DLS measurements. For
example, nanoparticle preparation from a block copolymer with a 4.7 × 103 g mol-1 PF block and
a 2.0 × 103 g mol-1 PEG block resulted in a number average particle diameter of 25 nm, whereas
the fourfold PF block length lead to a particle diameter of 50 nm. Amphiphilic block copolymers
derived from PF1 and PF2 distinguished by covalent attachment of two vs. one PEG block,
respectively, did not exhibit any significant differences in particle size at equal PF block
molecular weights.
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Figure 3. TEM images (top and center) and DLS measurements (bottom) of PF-PEG
nanoparticles. (A) Mn(PF-block) = 4.7 × 103 g mol-1 (B) Mn(PF-block) = 1.7 × 104 g mol-1.
One of the most important characteristics of polyfluorenes and conjugated polymers in general
are their optical properties distinguished by a bright fluorescence emission. The polyfluorene
nanoparticles possess an intenese blue to green fluorescence with fluorescence quantum yields as
high as = 84 %.18 The emission wavelength of these dispersions can be adjusted by furnishing
PF-PEG particles with a red emitting perylene monoimide (PMI) dye, to which an energy
transfer can occur.19 As illustrated in Figure 4A, this can be performed either by blending PMI-
end functionalized PF-PEG block copolymer into PF-PEG particles (System A), or by `doping´
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PF-PEG particles with free PMI molecules (System B). With increasing PMI content in the
polymer the emission band of the perylene imide dye at 600 nm grows in intensity along with a
simultaneous decrease in the PF emission at 450 nm. This energy transfer is significantly more
efficient for the covalently bound PMI system than for the PF-PEG nanoparticle dispersions with
free dye molecules physically mixed with the polymer matrix (Figures 4 D and E). Remarkably,
at a dye content of only 5 wt. % PMI relative to PF (that is for system A a 1:1 mixture of dye-
labeled and unlabeled PF block copolymer) this energy transfer from the PF matrix to the dye is
virtually complete for the nanoparticles from block copolymer with covalently attached dye,
while there is still notable polyfluorene emission for the physical mixture (Figure 4 E). At the
same time, fluorescence quantum yields are higher with the polyfluorene block copolymers with
covalently bound dye endgroups than with free dye (Figure 4 B). With increasing dye content
both systems tend to lower quantum yields, but the decrease is substantially more pronounced for
the comparative system with only physically mixed free PMI dye molecules. This effect becomes
more dramatic over time, as quantum yields of PMI-PF-PEG particles remain virtually
unchanged for weeks, whereas those for the system with free dye molecules drop to values
between 10 to 20 % within days (Figure 4 C). This behavior is attributed to an eventual
aggregation of free dye molecules in the PF matrix. A certain degree of mobility within the
polyfluorene matrix may lead to π-stacking of the dye molecules and subsequent non-radiative
energy dissipation pathways. This clearly indicates the merits of well-defined hetero difunctional
polyfluorene dye-labelled block copolymer as building blocks for nanoparticles, in which the
emitter is covalently bound to the light harvesting polymer matrix.
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Figure 4. Emission properties of perylene monoimide (PMI)-functionalized polyfluorene-block-
polyethylene glycol (PF-PEG) copolymer nanoparticle dispersions. (A) Scheme illustrating the
two different preparation pathways for incorporation of PMI in PF-PEG nanoparticles. PMI
content in System A is adjusted by addition of non-dye labeled PF-PEG copolymer. (B)
Fluorescence quantum yields at various dye concentrations for polymer-bound PMI molecules
(red) and physical mixtures of free PMI molecules and the polyfluorene matrix (blue). (C)
Comparison of the development of fluorescence quantum yields over time of nanoparticle
dispersions at 5 wt. % PMI content of system A (red) vs. system B (blue). (D) and (E) emission
spectra of (PMI-)PF-PEG nanoparticle dispersions at 0.5 wt. % and 5.0 wt. % PMI
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concentration, respectively. Dispersions were excited at the PF absorption maximum at λexc
=
388 nm. (F) Photograph of PMI-PF-PEG dispersions with increasing perylene imide content
from 0.1 wt. % (left) to 10 wt. % (right) under UV irradiation.
An emission in the deep red and near infrared red (NIR) regime is advantageous e.g. in
biological imaging due to the deeper penetrating nature by comparison to visible light of shorter
wavelength. However, the energy transfer efficiency from polyfluorene to most NIR dyes is
negligible, due to the lack of a spectral overlap. To this end, the heterodifunctional polyfluorenes
studied here offer themselves for an energy transfer cascade. The red-emitting PMI-PF-PEG
block copolymer nanoparticles and common NIR dyes exhibit a good spectral overlap of the
particle emission and NIR dye absorption spectra (Figure 5A).
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Figure 5. (A) Absorption (dashed lines) and emission (solid lines) spectra of the PMI-PF-PEG
dispersions (grey) and THF-solution spectra of the terrylene diimide dye (TDI, black). (B)
Emission spectra of TDI doped PMI-PF-PEG nanoparticle dispersion at various dopant
concentrations. Dispersions were excited at the PF absorption maximum at λexc = 388 nm.
Nanoparticles of the PMI functionalized block copolymers thus were `doped´ with terrylene
diimide (TDI), a NIR dye that emits above 660 nm. This system indeed acts as an energy transfer
cascade from the polyfluorene to PMI, and from the latter to TDI, as anticipated. With increasing
NIR-dye content the TDI emission band at 720 nm grows with a simultaneous decrease in the
PMI emission at 600 nm (Figure 5B). As observed with physical mixtures of PMI (vide supra),
increasing the `dopant´ content again lowers the quantum yields. Above 2 wt. % `dopant´
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concentration however, the nanoparticles still show satisfactory quantum yields of 10 to 20 %. In
extension of the concept reported here, quantum yields can likely be improved by covalent
binding of TDI to polyfluorene chain ends. Without PMI as `cascade intermediate´, energy
transfer from polyfluorene to TDI occurs to a notably smaller extent and the polyfluorene
emission is not efficiently quenched (Figure S13, Supporting Information).
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Summary and Conclusion
In conclusion, novel three-coordinate palladium(II) aryl complexes with functional-group
substituted phenyl groups allow for a controlled Suzuki-Miyaura chain-growth type
polymerization to yield polyfluorenes with reactive endgroups. The reaction occurs with high
precision, to yield polyfluorenes fully functionalized with a single end group, amenable to
further reactions after deprotection. This approach strongly enhances the general utility of this
elegant controlled polymerization approach to polyfluorenes, and opens up routes to interesting
and useful polyfluorene architectures. This is illustrated here by the synthesis of amphiphilic
block copolymers via post-polymerization coupling of the functionalized reactive end groups. As
the second endgroup, these precise heterodisubstituted polyfluorenes contain a perylene
monoimide dye moiety. These well-defined block copolymers allow for the synthesis of self-
stabilized very stable polyfluorene nanoparticles as small as 25 nm, without the need for any
additional stabilizers. The heterodifunctional nature of these block copolymers, with a perylene
dye endgroup, was found to be crucial for an efficient and reliable tuning of the particles'
emission wavelength. By comparison, physical mixtures with the free dye featured lower
quantum yields, which further decreased strongly with time, likely due to aggregation of the
mobile free dye molecules. Furthermore, this concept allows for shifting the nanoparticles'
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emission into the deep red and near infrared regime via an energy cascade enabled by the
appropriate spectral overlap of the polyfluorene block copolymer dye endgroup with suitable
NIR dyes, as demonstrated for terrylene diimide 'doped' nanoparticles.
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In summary, this enhanced synthetic approach provides precise heterodifunctional
polyfluorenes and block copolymers which can function as a future toolbox for the generation of
bright luminescent nanoparticles with a well-defined surface chemistry and variable emission.
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ASSOCIATED CONTENT
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Supporting Information.
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All experimental procedures as well as H-NMR and additional MALDI-TOF spectra. This
material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Fax: +49 7531 88 -5152; Tel: +49 7531 88 -2593 and -5151; email:
stefan.mecking@uni-konstanz.de
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
ACKNOWLEDGMENT
Financial support by the DFG (Me1388/7-1) is gratefully acknowledged. The authors thank Lars
Bolk for GPC and Andreas Marquardt for MALDI-TOF measurements.
ABBREVIATIONS
PF, polyfluorene; PEG, polyethylene glycol; PMI, perylene monoimide; TDI, terrylene diimide;
NIR, near infrared.
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blended with additional PMI dye molecules) were ca. half of the quantum yields reported in
Figure 4, thus more resembling the bulk material. It may therefore be assumed that PF-PEG
nanoparticles are more loosely aggregated than their purely hydrophobic counterparts.
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