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Langmuir 2007, 23, 7760-7767
Conjugated Polyelectrolytes with pH-Dependent Conformations and Optical Properties Yuan Gao,† Chun-Chih Wang,† Leeyih Wang,*,‡ and Hsing-Lin Wang*,† Physical Chemistry and Spectroscopy, Chemistry DiVision, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and Center for Condensed Matter Science, National Taiwan UniVersity, Taipei, Taiwan 10167 ReceiVed December 6, 2006. In Final Form: April 16, 2007 We report here the synthesis and characterization of two new conjugated polymers: poly{2,5-bis[3-(N,Ndiethylammonium acetate)-1-oxapropyl]-1,4-phenylenevinylene} (P1′) and poly{2,5-bis[3-(N,N,N-triethylammonium bromide)-1-oxapropyl]-1,4-phenylenevinylene} (P2). Both polymers exhibit unique pH-dependent optical properties in aqueous solution. These pH-dependent optical properties are attributed to the mutual electrostatic repulsions of positive charges pendent on the benzene rings. This electrostatic repulsion leads to an increased or decreased torsional angle in the conjugated backbone, thus affecting the effective conjugation length of these polymers. The UV-vis spectra of P1 in various pH solutions exhibit a near-isosbestic point, which indicates changes in the composition of the two distinct conformations (the charged and the neutral forms). The transition between the highly charged state and the neutral state was clearly observed in the UV-vis and photoluminescence studies on both P1 and P2. This transition is particularly sensitive in the pH range from 6.2 to 7.0, a range that would allow the detection of minor environmental changes. P2 has a quantum efficiency of 14% in water, which is considered to be relatively high among water-soluble PPVs.
Introduction Recently, there has been increasing interest in the syntheses of conjugated polymers that are soluble in water because of their potential applications in the construction of light-emitting diodes (LED) via self-assembly1-3 and as highly sensitive fluorescent sensory materials in living bodies.4-7 Among conjugated polymers, water-soluble poly(p-phenylenevinylene) (PPV) derivatives are of particular interest because of their thermal stability and easily tunable optical and electronic properties.8-13 The water solubility of the PPV is generally achieved by attaching hydrophilic charged units, such as carboxylate, phosphonate, and sulfonate groups (anionic) or tertiary ammonium groups (cationic) onto the polymer backbone.14 One of the most commonly used anionic, water-soluble PPVs is poly(methoxypropyloxy sulfonate phenylene vinylene) (MPS-PPV).15 MPS-PPV has attracted a lot of attention as a result of its demonstrated potential in the fabrication of highly * Corresponding authors. E-mail:
[email protected],
[email protected]. † Los Alamos National Laboratory. ‡ National Taiwan University. (1) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 7107. (2) Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl. Phys. 1996, 79, 7501. (3) Baur, J. W.; Kim, S.; Balanda, P. B.; Reynolds, J. R.; Rubner, M. F. AdV. Mater. 1998, 10, 1452. (4) Garnier, F.; Korri-Youssoufi, H.; Srivastava, P.; Mandrand, B.; Delair, T. Synth. Met. 1999, 100, 89. (5) Faid, K.; Leclerc, M. Chem. Commun. 1996, 2761. (6) Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 10954. (7) Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2003, 125, 896. (8) Zheng, M.; Sarker, A. M.; Gurel, E. E.; Lahti, P. M.; Karasz, F. E. Macromolecules 2000, 33, 7426. (9) Braun, D.; Heeger, A. J. Appl. Phys. Lett. 1991, 58, 1982. (10) Ahn, T.; Jang, M. S.; Shim, H. K.; Hwang, D. H.; Zyung, T. Macromolecules 1999, 32, 3279. (11) Pang, Y.; Li, J.; Hu, B.; Karasz, F. E. Macromolecules 1999, 32, 3946. (12) Yang, Z.; Sokolik, I.; Karasz, F. E. Macromolecules 1993, 26, 1186. (13) Hay, M.; Klavetter, F. L. J. Am. Chem. Soc. 1995, 117, 7112. (14) Pinto, M. R.; Schanze, K. S. Synthesis 2002, 1293. (15) Shi, S. Q.; Wudl, F. Macromolecules 1990, 23, 2119.
sensitive biological and chemical sensors.16,17 A more recent study using MPS-PPV to fabricate multilayer structures through the polyelectrolyte self-assembly method exhibits unidirectional energy18 and charge-transfer19 properties that further exemplify the potential of this important class of materials. Despite the fact that MPS-PPV has excellent solubility in water, it suffers from low fluorescence quantum efficiency (∼1%)20 and long-term instability. Wagaman et al. have shown the synthesis of an anionic, water-soluble PPV with carboxylate functional groups attached to the 2 and 3 positions of the benzene ring using ring-opening metathesis polymerization (ROMP). This polymer has good polydispersity (∼1.1-1.5) and a high quantum efficiency (1020%) with a number-average molecular weight (Mn) range from 16 000 to 40 000.21 Interestingly, the cationic water-soluble PPVsscounterparts of the anionic PPVs15,21-23sare rarely discussed in the literature. The as-synthesized cationic, conjugated polyelectrolytes were generally prepared by either Wittig-Horner or Heck reactions.24,25 The preparation of monomers for such types of coupling reactions generally involves a complex multistep synthetic pathway. As a result, the desired optical and electronic properties are often compromised in order to achieve the synthesis of the desired polymers. The water-soluble, cationic poly(phenylene ethynylene) (16) Chen, L. H.; Mcbranch, D. W.; Wang, H. L.; Helgeson, R.; Wudl, F.; Whitten, D. G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12287. (17) Liaohai, C.; Mcbranch, D.; Rong, W.; Whitten, D. Chem. Phys. Lett. 2000, 330, 27. (18) Wang, H. L.; Mcbranch, D. W.; Klimov, V. I.; Helgeson, R.; Wudl, F. Chem. Phys. Lett. 1999, 315, 173. (19) Wang, H.-L.; Mcbranch, D. W.; Donohoe, R. J.; Xu, S.; Kraabel, B.; Liaohai, C.; Whitten, D.; Helgeson, R.; Wudl, F. Synth. Met. 2001, 121, 1367. (20) Abe, S.; Chen, L. H. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 1676. (21) Wagaman, M. W.; Grubbs, R. H. Macromolecules 1997, 30, 3978. (22) Fujii, A.; Sonoda, T.; Yoshino, K. Jpn. J. Appl. Phys. Part 2 2000, 39, L249. (23) Fujii, A.; Sonoda, T.; Fujisawa, T.; Ootake, R.; Yoshino, K. Synth. Met. 2001, 119, 189. (24) Li, H. M.; Xiang, C. H.; Li, Y. L.; Xiao, S. Q.; Fang, H. J.; Zhu, D. B. Synth. Met. 2003, 135, 483. (25) Fan, Q. L.; Lu, S.; Lai, Y. H.; Hou, X. Y.; Huang, W. Macromolecules 2003, 36, 6976.
10.1021/la063536s CCC: $37.00 © 2007 American Chemical Society Published on Web 06/09/2007
Properties of Conjugated Polyelectrolytes
(PPE) derivatives represent another class of conjugated polyelectrolytes that demonstrate optical properties that are pH- and ionic strength-dependent. However, the extent of tunable optical properties is somewhat restricted by their rigid backbone structures such that λmax of the emission spectrum changes by a relatively small amount (85%) by using the Williamson ether reaction with 2-chlorotriethylamine hydrochloride in refluxing acetone following similar procedures for analogous compounds (Scheme 1).3,25 Direct chloromethylation of 2 to yield 3 is straightforward. Compound 3 was obtained by removing the solvents, washing with ethyl acetate, and drying under dynamic vacuum. White product 3 was used directly for the next step of the reaction without further purification. Poly{2,5-bis[3-(N,N-diethylamino)-1-oxapropyl]-1,4-phenylenevinylene} (P1) was then prepared from 3 through the Gilch reaction with t-BuOK in THF at room temperature. It is important to note that the P1 used for this study is the portion that is soluble in CHCl3. A large amount of red material (insoluble in all common organic solvents), presumably the polymer with a much higher molecular weight, was also produced in this experiment. This material was not fully characterized because of its poor solubility in organic solvents. The conversion of P1 to the corresponding cationic polymer (P2) was achieved by stirring bromoethane in chloroform at 48 °C for 5 days. The post-quaternization approach has the advantage because the neutral polymer can be purified and characterized easily.3,25 The polymers were characterized by 1H NMR and FTIR. The neutral polymer (P1) exhibited two broad peaks at 2.7 and 3.0 ppm in its 1H NMR spectrum (Figure 1B). These two peaks correspond to the methylene groups adjacent to the nitrogen (-CH2NCH2-) atoms. The methylene protons adjacent to the oxygen atoms were observed as a broad singlet at 4.1 ppm, and the phenyl protons appeared as a broad hump at around 7.2 ppm in the spectrum. The broad peak at 7.4 ppm in the 1H NMR spectrum was determined to arise from the trans vinyl protons.35 The resonance signals for the cis vinyl protons at 6.5 ppm35 are very weak (less than 0.7% compared to the signal for the trans
vinyl protons) and clearly indicate that the trans-CHdCH configuration dominates the structure in P1. Such a conclusion was further substantiated by the FTIR spectrum in which only the absorption peak corresponding to the vibrational frequency of trans-CHdCH (∼975 cm-1)25 was observed. The molecular weight of P1 was measured by means of gel permeation chromatography (GPC) using polystyrene as the standard. The number-average molecular weight and polydispersity of the PPV sample were found to be 10 700 g mol-1 and 1.53, respectively. We can estimate that this as-synthesized polymer has a chain length of 32 repeat units, which is comparable to that of the known water-soluble cationic PPVs in the literature.24,25 The quaternization of P1 using bromoethane resulted in an incompletely quarternized polymer, P2. A careful examination of the 1H NMR spectrum of P2 suggests the formation of a polymer with quarternized and unquarternized groups, which is manifested by the NMR spectrum (Figure 1B-D). P2 has two proton peaks at 1.40 and 1.55 pm corresponding to CH3- in the ethyl groups connected to nitrogen. We believe that these two peaks may represent the CH3- in the ethyl group connected to the protonated nitrogens and unprotonated amine, respectively. In addition, all of these peaks were shifted downfield compared to those of the neutral polymer and the monomer (2) (Figure 1). Should that be the case, we can then determine the level of quarternization to be ∼50%, which is a rough estimate obtained by comparing the integration of these two peaks at 1.40 and 1.55 ppm. This structural inhomogeneity resulting from incomplete quaternization is likely due to the low boiling point of ethyl bromide (36 °C), which prevents the quaternization reaction from (35) Liao, L.; Pang, Y.; Ding, L. M.; Karasz, F. E. Macromolecules 2001, 34, 7300.
Properties of Conjugated Polyelectrolytes
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Figure 2. UV-vis and photoluminescence spectra of P1 in chloroform.
Figure 1. (A) 1H NMR spectrum of 2 in CDCl3. (B) 1H NMR spectrum of P1 in CDCl3. (C) 1H NMR spectrum of P1 in CDCl3 in the presence of CD3COOD. (D) 1H NMR spectrum of P2 in CD3OD. *CH3OH.
being carried out at elevated temperature. This hypothesis is validated by using a high-boiling-point bromoethanol (149 °C) to quarternize the tertiary imine and results in a nearly completely quarternized polymer, P2. Despite the observed formation of the mixture of polymers with different degrees of quaternization, the IR spectrum of P2 is quite similar to that of P1 (i.e., only the absorption peak corresponding to the vibrational frequency of trans-CHdCH (∼975 cm-1) was observed).
Thermal Stability The thermal stability of the polymers was studied by thermogravimetric analysis (TGA) under an inert (argon) atmosphere. The thermograms show that neutral polymers P1 and P2 exhibit good thermal stability with a weight loss of 2.3% at 200 °C. This 2.3% weight loss is likely due to residual moisture or molecular impurities encapsulated within the polymers when they were precipitated from the reaction mixture. However, thermograms suggest that the quaternized polymer (P2) is not as thermally stable as P1. P2 has a degradation onset at 200 °C with the initial weight loss derived from associated water. P2 shows a weight loss of ∼40% as the temperature reaches 255 °C. In contrast, P1 has a weight loss of less than 10% at this temperature. This variation can be explained by the lower stability of ethyl bromide in the quaternized salt structure. The enhanced water solubility may have compromised the polymer’s thermal stability. In either case, the relatively high thermostability renders these polymers good candidates for fabricating light-emitting diodes and fluorescence-based sensors using spin casting or polyelectrolyte self-assembly methods.
Figure 3. Titration of P1′ using KOH.
Solubility and General Optical Properties of Polymers. The neutral polymer P1 is soluble in less polar solvents such as chloroform. The UV-vis and photoluminescence spectra of P1 in chloroform were shown in Figure 2. In chloroform, P1 exhibited a strong, broad absorption peak centered at 486 nm in the UVvis spectrum, and an emission maximum appeared at 555 nm, which was significantly red-shifted compared to that of the underivatized PPV. The electron-donating nature of the attached side chains is believed to be responsible for such a large bathochromic shift. In strong polar solvents such as water and methanol, P1 is insoluble. However, it becomes readily soluble in these solvents if a small amount of an organic acid, such as formic acid or acetic acid, is present. The 1H NMR spectrum of P1 in water in the presence of acetic acid (CD3COOD) is shown in Figure 1C. The resonance peaks for -CH2NCH2- (δ 3.28 and 3.63) and for -OCH2- (δ 4.5) were shifted downfield compared to those for neutral polymer P1. We believe that the formation of the cationic polymer (P1′) occurred under the acidic condition. (See Discussion in next section.) The pKa of the cationic polymer was determined to be -8.4 from the standard titration curve (Figure 3). It should be noted that the degree of quarternization of P1 using the organic acid at low pH (
