3.1.1 Introduction
Poly(p-phenylenevinylene) (PPV) and its derivatives have attracted a great deal of attention in recent years because of their interesting electroluminescent properties and their potential application as the active emitting layer in light-emitting diodes (LED).1 Recently, considerable research has focused on an anionic PPV (sulfonated PPV, i.e., MPS-PPV), a type of polyelectrolytes which consist of both polyanions and fluorescent conjugated backbones, and shown that such ionic conjugated polymers (ICPs) are the good candidates as chemo or biosensors which exhibit rapid and collective response to relatively small perturbations in local environment.2 Such amplified fluorescence quenching sensitivity is achieved through electron transfer or energy transfer due to the facile energy migration along the conjugated backbone3 and relatively strong electrostatic binding of the oppositely charged quenchers with ICPs.2 Although the optical and electronic properties of MPS-PPV are remarkable, and MPS-PPV was reported on detecting proteins for biological target through electron transfer2a, this polymer suffers from its intrinsic shortcomings such as a relatively low photoluminescence quantum efficiency and their single anionic charges which can not be used to detect anionic DNA chain through electrostatic attraction.
Most recently, the effect of cationic fluorescent conjugated polymer on anionic quencher4 and DNA sensing5 is gaining high interesting, in which, especially, a cationic polyfluorene was successfully developed to identify anionic DNA through energy transfer.5 To obtain high energy transfer and therefore improve the sensitivity, new types of cationic conjugated polymers with suitable emission wavelength and high photoluminescence quantum efficiency are required. PPV is very appropriate for the color tunability over the full visible range by the control of its HOMO-LUMO band gap and for enhancement of the PL quantum efficiency through introduction of bulky group into the side chain or incorporation of unit with high PL efficiency into the conjugated main chain. Therefore, it is crucial to develop cationic PPVs with higher PL quantum efficiency and different emission wavelength to evaluate a variety of polymer compositions for obtaining optimized sensory materials.
In this paper, we report on the syntheses and photoluminescence properties of cationic phenyl-substituted poly(p-phenylenevinylene) related copolymers containing thiophene, fluorene or alkoxy- or phenyl-substituted phenylene groups. We attempted to gain highly PL quantum efficiency by incorporating the bulky substitutent and the fluorene unit. Phenyl-substituted PPV derivatives are desired as they have proven to exhibit highly efficient fluorescence and enhanced photostability due to the steric hindrance of the bulky phenyl groups which minimize the interchain or intrachain interactions.6-8 The fluorene unit is well known as a highly PL efficient material and has been used to enhance the PL efficiency of PPV derivatives.9 In addition, the emission wavelength was changed by introduction of those units with different
electronic properties.6-9 On the basis of these considerations, we successfully synthesized new cationic PPVs with relative high PL quantum efficiency and tunable emission wavelength.
Figure 3.1.1 The designed neutral polymers for the cationic polymers
3.1.2 Molecular Design
To obtain phenyl-substituted PPV copolymers with different emission wavelength and higher fluorescence efficiency, several moieties, such as thiophene, benzene and fluorene were selected as the comonomers for the copolymerization. One alkyl chain were introduced to the thiophene moieties to adjust the light emission, since thiophene
) (
n
O
O
N
N
OC8H17
H17C8O
O
O
N
N
OC10H21
H21C10O
) (
n )
(
n
O
O
N
N
S
H17C8
) (
n
O
O
N
N
C6H13 H13C6
P4 P5
P6 P2
as an electron rich moiety, will cause a spectral red shift. Alkoxylated benzene and phenyl-substituted benzene were specially chosen for their different steric properties and expected that the optical properties could be tuned via such modification. Alklated fluorene was chosen to cause the light emission blue shift.
In order to synthesize cationic copolymers, tertiary amine as the functional group was also designed for the neutral polymer, which is similar as what we referred in chapter 2.
Meanwhile, long alkoxy chains or alkyl chains were introduced into the side chain of those four copolymers to enhance their corresponding solubility which will be conducive to their post-quaternization in the next step. All molecular structures of those PPV-copolymers are shown in Figure 3.1.1.
3.1.3 Materials and Characterization Methods 3.1.3.1 Materials
All chemical reagents used were purchased from Aldrich Chemical Co. THF was purified by distillation from sodium in the presence of benzophenone. Other organic solvents were used without any further purification. Thionyl chloride was distilled prior to use.
3.1.3.2 Characterization Methods
The NMR spectra were collected on a Bruker Advance 400 spectrometer with tetramethylsilane as the internal standard. FT-IR spectra were recorded on a Bio-Rad
FTS 165 spectrometer by dispersing samples in KBr. Mass spectra (MS) were obtained using a micromass VG 7035E mass spectrometer at an ionizing voltage of 70 eV.
UV-vis spectra were recorded on a Shimadzu 3101 PC spectrometer. The concentrations of copolymer solutions were adjusted to about 0.01 mg/mL or less.
Fluorescence measurement was carried out on a Perkin-Elmer LS 50B photoluminescence spectrometer with a xenon lamp as a light source. TGA measurements were performed on a TA Instruments Hi-Res TGA 2950 Thermogravimetric Analyzer at a heating rate of 10 °C/min under N2. Elemental microanalyses were carried out by the Microanalysis Laboratory of the National University of Singapore. Gel permeation chromatography (GPC) analysis was conducted with a Waters 2690 separation module equipped with a Waters 2410 differential refractometer HPLC system and three 5 àm Waters Styragel columns (pore size: 103, 104 and 105 Å) in series, using polystyrenes as the standard and tetrahydrofuran (THF) as the eluant at a flow rate of 1.0 mL/min and 35 °C.
3.1.4 Results and Discussion
3.1.4.1 Synthesis of Monomers and Polymers
Monomer 5 was directly synthesized from 3-octylthiophene via reaction with n-butyllithium and TMEDA in hexane and then with DMF and HCl solution.
3-octylthioohene was synthesized from 3-bromothiophene with 1-bromooctane via Grignard reaction using [1,3-bis (diphenylphosphino)propane]dichloronickel(II) as the catalyst. Monomer 6 was prepared from 1,4-dioctylbenzene via dibromomethylation,
esterification, hydrolysis and hydroformylation. The 1,4-dioctylbenzene was obtained from 1,4-hydroquinone by reaction with 1-bromooctane in refluxing ethanol in the presence of NaOC2H5. Monomer 7 was prepared from 9,9-di-n-hexylfluorene via dibromomethylation, esterification, hydrolysis and hydroformylation.
9,9-di-n-hexylfluorene was synthesized from fluorene by reaction with 1-bromohexane in the presence of n-butyllithium at –78oC. Monomer 2 and Monomer 3 has been reported in chapter 2. All the synthetic routes except Monomer 2 and 3 are shown in Scheme 3.1.1.
Scheme 3.1.1 The synthetic routes for the monomers
C8H17MgBr, Ni(dppp)2Cl2
Monom er 5 S
Br
S C8H17
S C8H17
OHC CHO
i) n-BuLi, TMEDA, Hexane ii)DMF, HCl 16
C8H17Br, Na, EtOH
OH
OH
OC8H17
OC8H17
OC8H17
OC8H17 CH2Br BrH2C
OC8H17
OC8H17
CH2OAc AcOH2C
OC8H17
OC8H17
CH2OH HOH2C
OC8H17
OC8H17 CHO OHC
Monomer 6
HBr, (CH2O)n, CH3COOH
AcONa, AcOH
NaOH H2O/EtOH (1:1)
PCC CH2Cl2
17 18
19
20 C6H13
H13C6 H13C6 C6H13
CH2OAc AcOH2C
C6H13 H13C6
CH2OH HOH2C
C6H13 H13C6
OHC CHO
Monomer 7 C6H13Br,
n-BuLi, THF
i) HBr, (CH2O)n, CH3COOH ii) AcONa, AcOH
NaOH H2O/EtOH (1:1)
PCC CH2Cl2
21 22
23
Scheme 3.1.2 The synthetic routes for those neutral and quaternized polymers
Ph3PCH2 CH2PPh3 O
O
N
NH Cl- +
Cl- H+
Cl- Cl-
+ +