Chapter 2 Study of Supramolecular Side-Chain and Cross-Linking Polymers by
2.4 Conclusions
In conclusion, H-donors (asymmetric mono-functional H-donors and symmetric
450 500 550 600 650
0.0
450 500 550 600 650
0.0
mesomorphic and photoluminescent properties effectively by the concept of supramolecular architecture. The H-acceptor copolymers were composed of different molar ratios of pendent N-vinylcarbazole units and light-emitting H-acceptor groups randomly to increase the glass transition temperatures and to reduce the π-π stacking of the conjugated H-acceptor chromorphores in the copolymers as well as in their H-bonded polymer complexes. The supramolecular architectures of H-bonded side-chain/cross-linking polymers were also confirmed by FTIR and XRD measurements. They have distinct mesomorphism and phase transition temperatures related to their supramolecular structures with different nonlinearities and rigidities.
The mesomorphic properties were changed from the nematic phase to the smectic C phase by the introduction of H-bonds to the supramolecular polymers, and then shifted to the nematic and non-mesogenic phases by various H-donor acids and H-acceptor copolymers with corresponding supramolecular side-chain/cross-linking structures. In addition, the emission color of light-emitting H-acceptor polymers can be tuned by their surrounding non-emitting H-donors. Redder shifts in PL emissions were observed in the H-bonded supramolecules with H-donors having smaller pKa values.
Chapter 3
Supramolecular Assembly of H-Bonded
Copolymers/Complexes/Nanocomposites and
Fluorescence Quenching Effects of Surface-Modified Gold Nanoparticles on Fluorescent Copolymers
Containing Pyridyl H-Acceptors and Acid H-Donors
A series of photoluminescent (PL) and liquid crystalline (LC) self-H-bonded
side-chain copolymers (P1-P3) consisting of pyridyl H-acceptors and isomeric acid
H-donors (i.e., para-, meta-, and ortho-benzoic acids) were synthesized.
Supramolecular H-bonded complexes were also obtained by mixing the
photoluminescent H-acceptor homopolymer PBT1 (containing pyridyl pendants) with
isomeric H-donor homopolymers P7-P9. The formation of H-bonds was confirmed by
FTIR, DSC, and XRD measurements. Moreover, PL and LC properties of the
H-bonded copolymers and complexes were affected not only by the H-bonding effect
of the supramolecular structures but also by the acid-substituted positions of isomeric
H-donors. In combination with different functionalized gold nanoparticles (which
bear acid or acid-free surfactants), the emission intensities of nanocomposites
containing self-H-bonded copolymer P1 (bearing both H-acceptor and H-donor
moieties) and non-self-H-bonded copolymer P4 (bearing acid-protected moieties),
respectively, were quenched to different extents by varying the concentration of gold
nanoparticles. The copolymeric H-acceptors and surface-modified gold nanoparticles
demonstrated diverse morphological and PL quenching effects on the supramolecular
architectures of nanocomposites, which result from competition between the H-donors
from the acid pendants on copolymers and the acid surfactants on gold nanoparticles.
3.1 Introduction
Self-assembed phenomena through molecular recognition between complementary constituents have been explored in various areas, such as biomaterials, liquid crystalline (LC) materials, and materials for electro-optical applications.65c,b,67c,83 Not only innovative LC properties of novel supramolecules consisting of two different components can be generated through intermolecular hetero-hydrogen-bonding interaction, but also directed self-assembly of nano-scaled building blocks using non-covalent interactions (e.g. hydrogen bonding, acid/base proton transfer, and electrostatic forces) has been amplified into macroscopically observable phenomena.50b,84 More recently, there is considerable interest in the research of conjugated polymers as highly sensitive chemosensors due to their potential applications in chemistry and biology,32,85 which are based on the different
fluorescence quenching capabilities caused by the varying degrees of supramolecular interactions with particular chemical or biological systems.30b,86 Poly(p-phenylene)s, poly(p-phenylene ethynylene)s, poly(p-phenylene vinylene)s, polythiophenes, and polyfluorenes with receptor groups, such as crown ethers, pyridine derivatives, and ionic groups, in the side-chains or main-chains have been successfully used for sensing ions and biological species.87 Design and synthesis of fluorescent side-chain conjugated polymers with supramolecular asembly, which are able to exhibit either chromogenic and/or fluorogenic responses due to non-covalent interactions, have particularly gained considerable attentions recently.58b,88
Self-assembly of nanoparticles into nanocomposites provides a direct pathway for incorporating the particles’ unique physical properties into the functional materials.33 Due to the stability and biocompatibility of gold, gold nanoparticles protected by mixed monolayer provide highly attractive models for biological and fluorescent conjugated polymers.89 Many high performance fluorescence assay methods have been developed by taking advantage of this superquenching ability of gold nanoparticles for optically sensing biologically important ions and molecules.
Gold nanoparticles can be functionalized so as to become soluble in water as well as in organic solvents with readily variable monolayer structures. Rotello and co-workers reported the synthesis and self-assembly of gold nanoparticles with
inherent optical properties in the literature.45,48b,53,54 Murray and co-workers have investigated the quenching of fluorophores that are attached to monolayer-protected gold nanoparticles, and also the electron transfer from exited fluorophores to gold nanoparticles.90 Direct binding between a fluorophore and a metal surface often results in the quenches of the fluorophore’s excited states. In this scenario, both energy transfer and electron transfer processes are considered to be major deactivation pathways for excited fluoroprobes on a metal surface.91 Furthermore, in the presence of other metal ions, such as Cu2+, Co2+, Fe3+, Ni2+, Zn2+, Pb2+ and Ag+, the gold nanoparticles of the quenched nanocomposites (containing fluorophores) can be replaced with the metal ions to different extents due to the stronger re-coordination or re-complexation of the metal ions with fluorophores, and therefore recover the fluorescence of the chromophores to behave as chemosensors.58a,b In addition, the gold nanoparticles of the quenched nanocomposites can also be reacted with the reduced glutathione in the presence of glutathione reductase enzyme, and then recover the fluorescence of the chromophores to behave as biosensors.58c The sensor applications of this work can be further developed to detect the metal ions and biomolecules based on the modulation of fluorescence quenching and recovery.
Previously, several series of H-bonded fluorescent complexes/dendrimers and side-chain supramolecular polymers consisting of pyridyl fluorophores (small
molecular H-acceptors) and various non-luminescent acid H-donors (including small molecules, dendrimers, and side-chain polymers) have been generated through intermolecular H-bonded interactions.70c,76,77,78 The purpose of this study was to explore photoluminescent (PL) self-H-bonded copolymers (P1-P3), consisting of para-, meta-, and ortho-benzoic acids (M1-M3) and fluorescent pyridyl (PBT) units
(with an expected molar ratio of 1:1), for their potential applications as proton donors (H-donors) and acceptors (H-acceptors), respectively.In order to evaluate the proton donating (H-donor) capabilities of the benzoic acid moieties, acid-protected monomers M4-M6 (containing para-, meta-, and ortho-benzoic acid methyl ester units) and its successive acid-protected (non-self-H-bonded) copolymers P4-P6 were synthesized.
Subsequently, it is more interesting to develop a two-stage self-assembly process in which the recognition (or sensing) of different surface-functionalized gold nanoparticles (with acid and acid-free surfactants) is proceeded by the light-emitting copolymers, i.e., self-H-bonded copolymer P1 (bearing both H-acceptor and H-donor moieties) and non-self-H-bonded copolymer P4 (bearing acid-protected moieties), correspondingly. Hence, side-chain conjugated copolymers bearing fluorescent pyridyl H-acceptor pendants not only behave as highly selective chemosensors for carboxylic acid H-donors, but also exhibit
distinct fluorescent quenching effects upon the addition of surface-functionalized gold nanoparticles (i.e., AuSCOOH and AuSC10, which contain acid and acid-free surfactants, respectively). To our knowledge, this approach (as shown in the schematic illustration of Figure 3.1) is the first exploration of the supramolecular assembly of nanocomposites via fluorescence quenching and TEM morphological analyses. The competition between H-donors from the acid surfactants on the gold nanoparticles and the acid pendent groups on the copolymers to form H-bonds with the pyridyl pendants (as H-acceptors) of the copolymers will be surveyed.
Figure 3.1 Schematic illustration of acid-functionalized gold nanoparticles (AuSCOOH) blended with self-H-bonded copolymer P1 and non-self-H-bonded copolymer P4.
3.2 Experimental Section
3.2.1 Materials
Chemicals and solvents were reagent grades and purchased from Aldrich, ACROS, TCI, TEDIA, and Lancaster Chemical Co. Dichloromathane and THF were distilled to keep anhydrous before use. The other chemicals were used without further purification.
General Synthetic Procedures for Monomers (M1-M6)
The synthetic route of the H-acceptor monomer PBT is described in Chapter 292 and isomeric H-donor monomers M1-M3 (i.e., para-, meta-, and ortho-benzoic acids) were prepared according to the procedure reported by Portugall et al.93 Monomers
M4-M6, with methyl-ester protecting groups, were also successfully synthesized by
the same synthetic method as the H-donor monomers M1-M3. The chemical structures for all products were confirmed by 1H NMR spectroscopy and elemental analyses.
General Synthetic Procedures for Copolymers (P1-P6)
According to Scheme 3.1, copolymers P1-P6 were synthesized from monomers
M1-M6 (H-donor monomers M1-M3 and acid-donor-protected monomers M4-M6)
and the H-acceptor monomer PBT. Equal ratios of PBT (50%) and monomers
M1-M6 (50%), total amounts ~ 1.2 g, were dissolved in THF (6 mL) with AIBN (3
mol%) added as the initiator. The reaction mixture was flushed with nitrogen for 5 min, and then heated in a water bath at 60°C to initiate polymerization. After 24 h, the reaction was terminated and the polymer was precipitated in a large amount of ether.
The resulting copolymers P1-P6 were re-dissolved several times in THF and re-precipitated in ether/hexane (1:1). The yields of copolymers P1-P6 were in the range of 45 ~ 67%. The molecular weights and polydispersity index (PDI) values, relative to polystyrene standards, of copolymers P1-P6 are presented in Table 3.1.
The output copolymer compositions of copolymers P1-P6, i.e., the molar ratios of monomer PBT to (H-donor or acid-protected) monomers M1-M6, which were estimated by NMR experiments, are also listed in Table 3.1. In the 1H NMR spectra of copolymers P1-P6, the disappearance of proton peaks in the region of vinyl units (chemical shifts at 5.4-6.1 ppm of methacrylate and acrylate groups) indicated that no monomers were present. The copolymer compositions in copolymers P1-P6 were estimated by comparing the relative integration areas of the peak at 8.5 ppm, which belong to the two protons of α-pyridyl groups in PBT. The overlapped peaks from 6.7 to 7.9 ppm are assigned to the other aromatic proton regions in PBT groups as well as benzoic groups in M1-M6, respectively. The copolymer compositions (x/y = 1/y) were calculated by the following equation: x/y = 1/y = 1/[(total integration areas of aromatic protons in copolymer - total integration areas of aromatic protons in
PBT)/(total integration areas of aromatic protons in M1-M6)] = 1/[(total integration areas of aromatic protons in copolymer - 12)/4]. This is based on the assumption that the total integration area of the pyridyl protons nearest the heterocyclic N atom in
each copolymer is equal to 2, i.e., 2 protons.
Scheme 3.1 Synthetic Routes for Copolymers P1-P6.
Scheme 3.2 Synthetic Routes for H-Acceptor Homopolymer PBT1 and H-Donor Homopolymers P7-P9.
Table 3.1 Characterization of Copolymers P1-P6 and H-Donor Homopolymers P7-P9
polymer molar ratio of PBT in feeda
a Molar ratio of monomer PBT in the feed before copolymerization.
b Molar ratio of monomer PBT in the output copolymers determined by NMR.
c Molecular weights were determined by GPC in DMF, based on polystyrene standards.
d Decomposition temperatures (°C) at 5% weight loss were measured by TGA at a heating rate of 20°C/min under nitrogen.
Homopolymers PBT1 and P7-P9
By following Scheme 3.2, the H-acceptor monomer PBT, or H-donor monomer
M1-M3, (1 g) was dissolved in THF (5 mL) with AIBN (3 mol%) added as the initiator. The reaction mixture was flushed with nitrogen for 5 min and then heated in a water bath at 60°C to initiate polymerization. After 24 h, the reaction was terminated and the polymer was precipitated into a large amount of ether. The homopolymers PBT1 and P7-P9 were re-dissolved in THF and re-precipitated in hexane several times. The yields were in the range of 49 ~ 85%. The molecular
(i.e., H-donor homopolymers PBAp, PBAm, and PBAo) are presented in Table 3.1.
3.2.2 Preparation of H-Bonded Complexes (PBT1/P7-P9)
All H-bonded complexes were fabricated from equal molar ratios of H-acceptor homopolymer PBT1 and H-donor homopolymers P7-P9 (PBAp, PBAm, and PBAo).
The complexes were dissolved in THF to make a clear solution, and then subsequently dried for several hours in a vacuum oven at 60°C. During solvent evaporation, the complexation through hydrogen bonding occurred between the H-donor and H-acceptor homopolymers. H-bonded complexes PBT1/P7, PBT1/P8, and PBT1/P9, which possess equal molar amounts of both pyridyl (H-acceptor) and carboxylic acid (H-donor) groups (1:1 molar ratio), were produced.
3.2.3 Preparation of Nanocomposites Consisting of Gold Nanoparticles and Polymers
The surface-functionalized gold nanoparticles. AuSCOOH and AuSC10, which contain acid and acid-free surfactants were synthesized, respectively. Nanocomposites were prepared by mixing solutions (0.5 mg/ml) of surface-functionalized gold nanoparticles AuSCOOH and AuSC10 in THF. This was then mixed with solutions of copolymers P1 and P4 in DMF (2 mg/ml). The gold nanoparticles began to assemble into nanocomposites within 2 min. The mixed solutions became visibly turbid, indicating that an aggregation process had occurred. After 24 h, the solid
precipitates were collected and washed extensively with hexane. The nanocomposites were then dried overnight before being subjected to Transmission Electron Microscopy (TEM) measurements.
3.2.4 Characterizations
1H NMR spectra were recorded on a Varian unity 300M Hz spectrometer using CDCl3, DMSO-d6, d-dioxane, and d-THF as solvents. Elemental analyses were performed on a HERAEUS CHN-OS RAPID elemental analyzer. Fourier transform infrared (FT-IR) spectra were performed on a Nicolet 360 FT-IR spectrometer. The textures of mesophases were characterized by a Leica DMLP polarizing optical microscope (POM) equipped with a hot stage (Linkam LTS350).
Temperatures and enthalpies of phase transitions were determined by differential scanning calorimetry (DSC, model: Perkin Elmer Pyris 7) under N2, at a heating and cooling rate of 10°C/min. Thermogravimetric analyses (TGA) were conducted on a Du Pont Thermal Analyst 2100 system with a TGA 2950 thermogravimetric analyzer under N2, at a heating rate of 20°C/min. Molecular weights and molecular weight distributions were determined through gel permeation chromatography (GPC) using a Waters 510 HPLC, equipped with a 410 differential refractometer, a refractive index (RI) detector, and three Ultrastyragel columns (100, 500, and 103) connected in series in order to increase different pore sizes, with DMF as eluent at
a flow rate of 0.6 mL/min. The molecular weight calibration curve was obtained using polystyrene standards. UV-visible absorption spectra in dilute THF solutions (10-6 M) were recorded on a HP G1103A spectrophotometer, and photoluminescence (PL) spectra in dilute THF solutions (10-6 M) were obtained on a Hitachi F-4500 spectrophotometer. Thin films of UV-vis and PL measurements were spin-coated (3000 rpm) on quartz substrates from THF solutions with a concentration of 1 wt%. The fluorescence quantum yields (ΦPL) of the chromophores and H-bonded complexes were determined relative to a standard film of 9,10-diphenylanthracene dispersed in PMMA (Φ = 0.83).94 Synchrotron powder X-ray diffraction (XRD) measurements were performed at beamline BL17A1 of the National Synchrotron Radiation Research Center (NSRRC), Taiwan, where the X-ray wavelength was 1.32633 Å. The XRD data were collected using imaging plates (IP, of an area = 20 × 40 cm2 and a pixel resolution of 100) curved with a radius equivalent to the sample-to-image plate distance of 280 mm, and the diffraction signals were accumulated for 3 min. The powder samples were packed into a capillary tube and heated by a heat gun, where the temperature controller was programmable by a PC with a PID feed back system.
The scattering angle theta was calibrated by a mixture of silver behenate and silicon. Transmission Electron Microscopy (TEM) analyses were performed using
a JEOL 2011 electron microscope with an acceleration voltage of 200 keV. The samples were prepared from THF solutions with a concentration of 1 wt%, and the aggregates were precipitated on TEM sample grids (200 Cu mesh/carbon films).
3.3 Results and Discussion
3.3.1 Synthesis and Characterization
Proton acceptor monomer PBT containing three-conjugated aromatic rings was successfully synthesized via Sonogashira coupling and Wittig-Horner reactions, whose synthetic route was reported in Chapter 2.92 Two series of H-donor monomers
M1-M3 and acid-protected monomers M4-M6 were synthesized. The molecular
structures of monomers M1-M6 were confirmed by 1H NMR and elemental analyses.
According to the synthetic routes shown in Scheme 3.1, copolymers P1-P6 were synthesized from the H-acceptor monomer PBT along with monomers M1-M6 (H-donor M1-M3 and acid-protected M4-M6), with approximately 1:1 molar ratio, via the conventional synthesis of random free radical copolymerization. The random free radical polymerization of H-donor homopolymers P7-P9 (PBAp-PBAo) and H-acceptor homopolymer PBT1, depicted in Scheme 3.2, were proceeded by the same condition in Scheme 3.1. The H-bonding effects on these two series of analogous copolymers, i.e., self-H-bonded copolymers P1-P3 and non-self-H-bonded copolymers P4-P6 (containing H-donor monomers M1-M3 and acid-protected
monomers M4-M6, respectively), were evaluated for their thermal, mesogenic, and optical properties. The average molecular weights measured by gel permeation chromatography (GPC), relative to polystyrene standards, are displayed in Table 3.1.
The number average molecular weights (Mn) of all polymers are between 5300 and 7100 g/mol, and the polydispersity index (PDI) values are between 1.4 and 2.6. Their thermal stability was measured under nitrogen by thermogravimetric analyses (TGA), and these results are also summarized in Table 3.1. The thermal decomposition temperatures (Td) of 5% weight loss are between 284 and 389 °C, where the Td values of polymers P3 and P9 exhibit the lowest thermal stability (284 and 294 °C, respectively) due to the isomeric effect of steric hindrance in ortho-acid.
3.3.2 FT-IR Spectroscopic Studies
H-bonding effects in mesogenic copolymers P1-P6 along with H-bonded complexes PBT1/P7-P9 (H-acceptor homopolymer PBT1 blended with H-donor homopolymers P7-P9 in equal molar ratios) were confirmed by FT-IR spectroscopy.
The IR spectra of the H-acceptor homopolymer PBT1, H-donor homopolymer P7, and their H-bonded complexes PBT1/P7, shown in Figure 3.2(a), are compared to analyze the formation of H-bonds. In contrast to the O-H band of pure H-donor homopolymer P7 (PBAp) at 2658 and 2543 cm-1, the weaker O-H bands observed at 2528-2491 and 1931-1912 cm-1 in H-bonded complexes PBT1/P7, PBT1/P8, and
PBT1/P9 are indicative of stronger H-bonds formed between pyridyl groups of the
H-acceptor homopolymer PBT1 and acid groups of H-donor homopolymers P7-P9 in their H-bonded complexes PBT1/P7-P9. On the other hand, a C=O stretching vibration appeared at 1730-1727 cm-1 in H-bonded complexes PBT1/P7-P9 (a shoulder appeared at 1690 cm-1 for H-bonded complex PBT1/P7). This shows that the carbonyl group is in a more associated state than that it is in the pure H-donor homopolymer P7, which contains a weaker C=O stretching vibration appeared at 1720 and 1669 cm-1. All results suggest that H-bonds were generated in the solid state of H-bonded complexes PBT1/P7, PBT1/P8, and PBT1/P9. As seen in Figure 3.2(b), self-H-bonded copolymers P1-P3 reveal similar IR spectral profiles to those (Figure 3.2(a)) of H-bonded complexes PBT1/P7-P9, respectively. This suggests that analogous H-bonded structures formed in the self-H-bonded copolymers P1-P3 and in the H-bonded complexes PBT1/P7-P9.17 However, in comparison with self-H-bonded copolymers P1, acid-protected copolymer P4 in Figure 3.2(b) shows a weaker C=O stretching vibration appeared at 1720 cm-1 for lack of H-bonding interaction. The C=O stretching peak is sharper because there is less resonance for C=O stretching in the non-self-H-bonded copolymer P4, and so do acid-protected copolymers P5-P6 for lack of H-bonds. Meanwhile, a shifted peak at 1600 cm-1 from a disturbance of the 1590 cm-1 ring mode was observed in Figure 3.2(a), which indicated a change in the
strength of the acid-pyridine interactions.
(a)
(b)
Figure 3.2 Infrared spectra of (a) H-acceptor homopolymer PBT1, H-donor homopolymer P7, and H-bonded homopolymer complexes PBT1/P7; (b) copolymers P1 and P4 in solid films.
3000 2500 2000 1500
Wavenumber (cm-1)
Transmittance
PBT1
P7
PBT1/P7
3000 2500 2000 1500
Wavenumber (cm-1)
Transmittance
P1
P4
3.3.3 Phase Characterization
The thermal properties, including glass transition temperatures (Tg) and isotropization temperatures (Ti) with corresponding enthalpies, of these copolymers and H-bonded complexes, determined by DSC and POM, are shown in Table 3.2. The glass transition temperatures of copolymers P1-P6 and H-bonded complexes
PBT1/P7, PBT1/P8, and PBT1/P9 (H-acceptor homopolymer PBT1 blended with
H-donor homopolymers P7-P9 in equal molar ratios) are ca. 22 ~ 57 °C. Moreover, all self-H-bonded copolymers P1-P3 containing H-donors have higher Tg and Ti values than their corresponding non-self-H-bonded copolymers P4-P6, which do not contain H-donors. This is because the self-H-bonded cross-linking polymer structures, in copolymers P1-P3, have a longer rigid core than the non-H-bonded side-chain polymer structures in acid-protected copolymers P4-P6. Both Tg and Ti values of H-bonded complexes PBT1/P7, PBT1/P8, and PBT1/P9 have a similar tendency (i.e.,
PBT1/P7 > PBT1/P8 > PBT1/P9) as those of self-H-bonded copolymers P1-P3
(para-acid-substituted copolymer P1 > meta-acid-substituted copolymer P2 >
ortho-acid-substituted copolymer P3). Overall, the para-substituted copolymer P1
and the H-bonded complex PBT1/P7 exhibit the highest Tg and Ti values because they have the most linear H-bonded (cross-linking) structures.
Because of the linear H-bonded structures of self-H-bonded copolymer P1 and
H-bonded complex PBT1/P7 (containing para-acid-substituted homopolymer P7), the smectic phase was only generated in the para-acid-substituted copolymer P1 with a mesomorphic range of 92 °C and in the H-bonded complex PBT1/P7 with a mesomorphic range of 114 °C. The H-bonded complex PBT1/P7 had a wider mesophase range than self-H-bonded copolymer P1, because the H-bonded complex (PBT1/P7) contains highly ordered H-bonded mesogens (from homopolymer blends) in the smectic arrangement. Due to the same reason of highly ordered H-bonds in H-bonded complexes, the nematic phases observed in nonlinear H-bonded structures (with non-para-acid-substituted H-donors) of H-bonded complexes PBT1/P8 and
PBT1/P9 were wider than those of self-H-bonded copolymers P2-P3. This result
PBT1/P9 were wider than those of self-H-bonded copolymers P2-P3. This result