Article pubs.acs.org/Macromolecules
Perylene Diimide-Based Ionene and Zwitterionic Polymers: Synthesis and Solution Photophysical Properties Marcus D. Cole, Madhu Sheri, Chelsea Bielicki, and Todd Emrick* Department of Polymer Science and Engineering, University of Massachusetts Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States S Supporting Information *
ABSTRACT: A series of perylene diimide (PDI)-containing ionene and zwitterionic polymers were prepared by the Menschuktin reaction and by nucleophilic ringopening of a novel bis-sultone monomer. PDI derivatives containing tertiary amine moieties at the imide position, and bromides or phenyl groups within the aromatic core, provided sites for polymerization and imparted solubility during the polymerization, respectively. The solution photophysical behavior of the polymers was studied by UV−vis and photoluminescence spectroscopy as a function of PDI incorporation and cationic/zwitterionic functionality, resulting in the observation of tunable solution spectral features induced by core functionality and/or interzwitterion interactions. This new PDI-based polymer platform affords opportunities to modulate conjugation and charge density within the polymers and examine the effects of cationic vs zwitterionic groups on the resultant optoelectronic properties.
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properties of the resultant aggregates.11 Emerging efforts to prepare hydrophilic PDI-based polymers include water-soluble PDI-containing polyurethanes as fluorescent probes for live cell imaging12 and PDI−terpyridine polymers as metal ion sensors.13 However, in prior reports of hydrophilic, mainchain PDI-containing polymers, the PDI monomers lacked functional group versatility at the aromatic core. Through modification of substituents at the PDI core, materials with improved photoluminescence emission properties and solubility/processability can be realized.14 Here we introduce functionality at the 1- and 7-positions of the PDI aromatic core and utilize a polymerization strategy that allows facile incorporation of either cationic or zwitterionic groups into the polymer backbone synthesized from regioisomerically pure 1,7-dibromo- and 1,7-diphenyl-substituted PDI monomers which exhibit good solubility in polar organic solvents. The photophysical properties of the resultant polymers were investigated spectroscopically following systematic incorporation of PDI derivatives into the polymer backbone to understand interactions between PDI units in the polymer backbone.
INTRODUCTION Ionene polymers, or “polyionenes”, represent a class of polyelectrolytes in which the charged moieties are embedded within the polymer backbone rather than positioned as pendant groups. In general, polyionenes are synthesized by the Menschuktin reaction1 of bis-tertiary amines or bis-diphenylphosphines with electrophilic dihalides in polar organic solvents. Several reports describe syntheses, structure−property relationships, and mechanical properties of water-soluble ammonium- and phosphonium-based polyionenes.2−4 Some commercially available polyionenes, such as polybrene, have proven useful for enhancing viral transfection,5 while other polyionenes have emerged as components of electronically active polymeric and nanocomposite materials.6,7 The covalent integration of optoelectronically active components into polyionenes as pendant or main-chain moieties has also been described by Suzuki, who reported the photophysical properties of functional polyionenes containing pendent anthracene groups for probing intra- and intermolecular chain dynamics.8 Additionally, reinforced polymer folding behavior and self-assembly in aromatic polyionenes via donor−acceptor interactions in the polymer backbone have been examined.9 Such electronically active polyionenes offer promise as active layers or interlayers of organic electronic devices, in which tunable emission and/or charge transport properties are desired. Perylene diimides (PDIs) are of interest for their thermal and chemical stability as well as their tunable electronic and solution assembly properties.10 Among recent reports of fluorescent PDI-containing polymers and supramolecular assemblies, Zhang and co-workers showed that by controlling the morphology PDI assemblies led to tailoring of the optical © XXXX American Chemical Society
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RESULTS AND DISCUSSION Synthesis of PDI-Containing Ionene Polymers. PDIcontaining ionene polymers were synthesized by the reacting the PDI derivatives, PDI-Br2 and PDI-Ph2, with N,N,N′,N′tetramethyl-1,6-hexanediamine (TMHDA) and 1,6-dibromohexane to yield polymers of desired PDI incorporation and Received: June 15, 2017 Revised: September 7, 2017
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DOI: 10.1021/acs.macromol.7b01281 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 1. Preparation of Ionene and Zwitterionic Polymers from Functional PDI Monomers 1 and 2a
a
Reagents and conditions: (i) CHCl3:MeOH, 1,6-dibromohexane, 70 °C; (ii) TFE, 3,3′-(bute-2-ene)bis(1,2-oxathiolane-2,2-dioxide) 3, 70 °C.
Table 1. Molecular Weight and PDI Incorporation of the PDI-Containing Ionene Polymers polymera (4) (5) (6) (7) (8) (9)
PDI-Br2 10 PDI-Br2 50 PDI-Br2 100 PDI-Ph2 10 PDI-Ph2 50 PDI-Ph2 100
monomer feed (mol %)
PDI incorporation in copolymerb (mol %)
Mnc (kDa)
Mwc (kDa)
Đc
yieldd (%)
10 50 100 10 50 100
9 47 100 8 51 100
38.1 33.7 18.5 30.5 30.3 18.1
56.4 47.4 25.5 46.2 39.0 23.0
1.5 1.4 1.4 1.5 1.4 1.3
82 80 43 83 74 48
a
Polymer nomenclature based on theoretical PDI incorporation. bMole percent PDI incorporation determined by 1H NMR spectroscopy. cNumberaverage molecular weight estimated by GPC. dWeight-average molecular weight estimated by GPC.
tailored solubility. The resultant ionene polymers exhibited excellent solubility in water, methanol, and 2,2,2-trifluoroethanol (TFE) at >15 mg/mL in all cases. The bromide and phenyl groups in the perylene core attenuated inter-PDI π−π interactions to the benefit of polymer solubility and appreciable molecular weight growth, while the tertiary amines at the imide positions provided nucleophilic sites for polymerization. Scheme 1 illustrates our step-growth polymerization strategy, in which the PDI monomers were reacted with 1,6dibromohexane, or bis-sultone 3, to afford the desired cationic and zwitterionic polymers. In prior reports of cationic, main-chain PDI polymers, DMF was employed as the reaction solvent.6 Attempted polymerizations of PDI monomers 1 and 2 in DMF resulted in low product yields and poor control over PDI incorporation due to the low solubility of these monomers and polymers in DMF. However, the functional PDI derivatives we describe remained well-solvated during the polymerizations in a mixture of chloroform and methanol. Ionene polymers 4−9 were prepared by polymerization of PDI-Br 2 or PDI-Ph 2 in a 1:1 CHCl3:MeOH mixture at an initial monomer concentration of 0.5 M. After stirring for 48 h at 70 °C, the viscous solutions were precipitated into diethyl ether, and the polymers were isolated by centrifugation, then purified by dialysis against 1:1 methanol:water, followed by pure water, and finally isolated as powders by lyophilization. The purified red or purple polymer products corresponding with incorporation of PDI-Br2 (red) or PDI-Ph2 (purple) into the backbone, contained 10, 50, or 100 mol % PDI in the backbone, allowing for examination of the role of PDI content on solution properties. The nomenclature in Table 1 is based on the selected PDI derivative and its mole percent incorporation into the ionene polymer. Characterization of the molecular weight and solution photophysical properties of these PDI-containing macro-
molecules proved highly solvent dependent, with spectral signatures hinging on solvent-induced aggregation. Previous reports of main-chain PDI-based polyelectrolytes describe gel permeation chromatography (GPC) in water as the mobile phase.6,13 However, in aqueous environments these polymers aggregate, as reflected by the high intensity 0−1 absorbance band and weak vibronic signature observed in their solution UV−vis absorption spectra.6,13 Such aggregates likely skew molecular weight estimation. Here, GPC and spectroscopic analyses were conducted in TFE for both the ionene and zwitterionic polymers. Representative UV−vis spectra of the PDI-containing ionene polymers in TFE, methanol, and water are shown in Figure 1. In TFE, the absorption maxima of the polymers were observed at 528 nm indiciative well-solvated PDI, in which the long-wavelength 0−0 vibronic transition dominated. In water and methanol the absorption maxima
Figure 1. Representative solution UV−vis spectra of 50 mol % PDIBr2 ionene polymer in TFE (green), methanol (blue), and water (red). B
DOI: 10.1021/acs.macromol.7b01281 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Table 2. Molecular Weight and PDI Incorporation of PDI-Containing Polymer Zwitterions polymera (10) (11) (12) (13) (14) (15)
PDI-Br2 10Z PDI-Br2 50Z PDI-Br2 100Z PDI-Ph2 10Z PDI-Ph2 50Z PDI-Ph2 100Z
monomer feed (mol %)
PDI incorporation in copolymerb (mol %)
Mnc (kDa)
Mwc
Đc
yieldd (%)
10 50 100 10 50 100
18 44 100 17 37 100
8.6 7.0 4.5 13.0 8.0 6.0
12.5 9.0 5.0 15.0 10.0 7.0
1.4 1.3 1.3 1.2 1.2 1.1
86 80 40 84 82 40
a Polymer nomenclature based on theoretical PDI incorporation, with “Z” denoting zwitterionic functionality. bMole percent PDI incorporation determined by 1H NMR analysis. cNumber-average molecular weight estimated by GPC. dWeight-average molecular weight estimated by GPC.
Figure 2. UV−vis absorption spectra of (a) PDI-Br2 ionene polymers, (b) PDI-Br2 zwitterionic polymers, (c) PDI-Ph2 ionene polymers, and (d) PDI-Ph2 zwitterionic polymers.
incorporation. Copolymer compositions were determined by integration of imide protons of the PDI (4.40 ppm) against the methylene protons of the THMDA-based ionene block (1.80 and 1.50 ppm). Synthesis of PDI-Containing Polymer Zwitterions. In contrast to the ionene polymers, the PDI polymer zwitterions could not be prepared in a CHCl3:MeOH solvent mixture due to premature precipitation. Instead, the PDI-based sulfobetaine (SB) polymers were synthesized by reacting bis-sultone 3 with the appropriate stoichiometric equivalents of PDI-Br2 or PDIPh2 and TMHDA in TFE. The polymerizations were conducted for 48 h at 70 °C and the products purified as described for the PDI-ionene polymers, with results shown in Table 2. Ring-opening of bis-sultone 3 to form the polymer zwitterions was confirmed by the upfield shift of the methylene protons adjacent to the sulfur, from 4.40 to 3.30 ppm, indicative of the sultone-to-sulfobetaine conversion. These polymer zwitterions differ from conventional versions by integration of sulfobetaine moieties within the plane of the backbone, rather than as pendant groups.15,16 Similar to the
shifted to the shorter wavelength 0−1 vibronic transition at 500 nm (in water) and 491 nm (in methanol), indicative of PDI aggregation. These solution absorption spectra suggest TFE as a preferred solvent for characterization and for suppressing the tendency of PDI units to aggregate. The estimated molecular weights of the PDI ionene polymers proved critically dependent on PDI content. As PDI incorporation increased, polymer molecular weights and yields decreased. This is likely due to the tendency of the charged polymer to aggregate in the CHCl3:MeOH solvent mixture, resulting in termination of the polymerization due to inaccessibility of the reactive chain-ends. The preparation of polymers containing of 50 and 100 mol % PDI was attempted in TFE to increase monomer conversion. However, these polymerizations resulted in low molecular weight oligomers, likely due to protonation of the tertiary amines. The 1H NMR spectra of PDI-ionene polymers displayed chemical shifts corresponding to the polymer structure, as shown in Figures S10−S15. Signals ranging from 7.60 to 9.65 ppm correspond to protons in the PDI core, confirming successful PDI C
DOI: 10.1021/acs.macromol.7b01281 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules ionene polymers, the molecular weights and yields of the SB polymers were influenced by the extent of PDI incorporation. Copolymer composition was calculated based on integration of the imide protons of the PDI (4.40 ppm) and the methylene TMHDA-based zwitterionic block (1.80 and 1.50 ppm). In comparison to the ionene polymers, PDI incorporation into the zwitterionic polymers was not as well-controlled. Independent of PDI incorporation, the zwitterionic polymers exhibited lower molecular weights than the ionene polymers, likely due to the lower reactivity of bis-sultone 3 versus the α,ω-alkyl dihalide and the relative acidity of TFE as solvent. While the PDI-based zwitterionic and ionene polymers exhibited similar solubility in water and TFE, their solubilities in methanol were substantially different. Specifically, the solubility of polymer zwitterions 10− 15 in methanol was significantly lower (
