Optical Properties of Benzotriazole-Based Conjugated

Aug 15, 2016 - A series of conjugated polyelectrolytes (CPEs) based on an electron-deficient polybenzotriazole backbone with various pendant ionic ...
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Optical Properties of Benzotriazole-Based Conjugated Polyelectrolytes Yueqin Shi,†,‡,# Cheng-Kang Mai,*,† Stephanie L. Fronk,† Yiwang Chen,*,‡ and Guillermo C. Bazan*,† †

Center for Polymers and Organic Solids (CPOS), Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106, United States ‡ Institute of Polymers/College of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China # College of Materials & Environmental Engineering, Hangzhou Dianzi University, No. 2 Avenue, Gaojiaoyuan District, Xiasha, Hangzhou 310036, China S Supporting Information *

ABSTRACT: A series of conjugated polyelectrolytes (CPEs) based on an electron-deficient polybenzotriazole backbone with various pendant ionic functionalized side chains were synthesized directly from the corresponding ionic monomers. Of particular interest was to use the different chemical structures to understand how the optical features are influenced by the ionic side chains. We found that interchain aggregation is favored in low dielectric constant solvents for cationic CPEs. Moreover, aggregated species absorb at longer wavelengths and exhibit higher fluorescence quantum yields.

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INTRODUCTION Conjugated polyelectrolytes (CPEs) are defined by a chemical structure containing a π-conjugated backbone with pendant ionic functionalities.1 The π-conjugated backbone provides CPEs with the same type of electronic delocalization found in neutral conjugated polymers, while the pendant charged side chain end groups enable processability from environmentalfriendly, highly polar solvents, such as water and methanol. This feature allows one to fabricate multilayer electronic devices in combination with neutral polymers, which are typically soluble in less polar solvents. CPEs have found useful applications in a variety of organic electronic devices, including solar cells,2 fieldeffect transistors,3 light-emitting diodes,4 and thermoelectric generators.5 Conjugated polymers consisting of electron-deficient heterocyclic units, such as perylene bisimides,6 diketopyrrolopyrrole,7 benzothiadiazole,8 and benzotriazole,9 have received special attention within the context of n-type charge transport in organic electronic devices.10 CPEs with electron-deficient backbones are, however, relatively rare. Nontheless, these materials are finding innovative applications in solar cells, lightemitting diodes, biosensors, and so on.11 Understanding the optical properties of electron-deficient CPEs is a necessary basic knowledge that should enable rational implementation in various technologies. However, optical behavior is anticipated to be dependent on interchain aggregates in media of different polarity,12 which is difficult to predict based only on the consideration of molecular structures. Herein, we report the synthesis of CPEs with electrondeficient benzotriazole (BTz) repeat units by using Nicatalyzed Yamomoto polymerization13 of monomers with © XXXX American Chemical Society

ionic side chains. The benzotriazole-based conjugated backbone is of particular interest14 because polymers with BTz repeat units comprise a class of thermostable, n-type semiconductors with relatively strong electron affinity.15 Dynamic light scattering (DLS), absorption spectroscopy, and photoluminescence (PL) spectroscopy were used to probe how aggregation in different media modulate the optical properties of these polymer chains. There are only a few examples for the preparation of CPEs with electron-deficient conjugated backbones.11 CPE synthesis most commonly involves converting pendant functional groups (i.e., halogens) in nonionic conjugated polymers to ionic functional groups. Pd catalysis16 has been frequently used in the preparations for precursor homopolymers with electron-rich monomers (i.e., thiophene) and D-alt-A polymers consisting of alternating electron-rich and electron-poor units.17 However, this type of synthetic approach is less effective in the preparation of homopolymers containing only electron-poor structural units.9 Additionally, quantitative conversion of functional groups on the precursor polymer chains provides a challenge since the characteristics of the macromolecules change from neutral, and soluble in nonpolar solvents, to ionic, and thus soluble in high dielectric media.

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EXPERIMENTAL SECTION

Typical Procedure for CPE Synthesis. An equimolecular amount of dibromo monomer (1 equiv) was added to the solution of bis(1,5Received: May 8, 2016 Revised: July 24, 2016

A

DOI: 10.1021/acs.macromol.6b00965 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of PBTz-Br, PBTz-PyrBr, and PBTz-TMABr Containing the Same Benzotriazole Conjugated Backbone, but with 6-Bromohexyl, Pyridinium, and Ammonium Side Chains, Respectively (COD = 1,5-Cyclooctadiene, 2,2′-bpy = 2,2′Bipyridyl, Ni(COD)2 = Bis(1,5-cyclooctadiene)nickel(0))

cyclooctadiene)nickel(0) (1 equiv), 2,2-bipyridine (1 equiv), and 1,5cyclooctadiene (1 equiv) at 60 °C under argon. The mixture was vigorously stirred at 60−70 °C under argon for 48 h. After cooling to room temperature, the mixture was poured into chloroform. The precipitate was washed with chloroform, acetone, and hexane, followed by dissolution in Millipore H2O. Finally the solution was purified in a dialysis tube (MWCO = 3500−5000 Da) that was soaked in Millipore water for 5 days. The water was changed every 12 h. After freezedrying, CPEs were obtained as red solids. PBTz-PyrBr. Monomer BTz-PyrBr (600 mg) was used in this homopolymerization. The PBTz-PyrBr is soluble in polar solvents, such as water and methanol. Yield: 360 mg, 88%. 1H NMR (500 MHz, CD3OD) δ 9.5−7.6 (m, 7H), 4.94 (N−CH2−, 2H), 2.5−0.5 (H-alkyl, 10H). GPC: (water, poly(ethylene glycol) standard): Mn = 112.9 kg mol−1, Đ = 3.2. PBTz-TMABr. Monomer BTz-TMABr (500 mg) was used in this homopolymerization. The PBTz-TMABr is soluble in polar solvents, such as water and methanol. Yield: 310 mg, 92%. 1H NMR (500 MHz, CD3OD) δ 9.1−8.5 (benzotriazole ring, 2H), 5.05 (N−CH2−, 2H), 3.42 (N+−CH2−, 2H), 3.15 (−N(CH3)3, 9H), 2.41 (−CH2−, 2H), 1.78 (−CH2−, 2H), 1.51 (−CH2−, 2H), 1.25 (−CH2−, 2H). GPC: (water, poly(ethylene glycol) standard): Mn = 157.5 kg mol−1, Đ = 1.5. PBTz-Na. Monomer BTz-Na (700 mg) was used in this homopolymerization. The PBTz-Na is only soluble in water. Yield: 250 mg, 61%. GPC: (water, poly(ethylene glycol) standard): Mn = 172 kg mol−1, Đ = 3.7. PBTz-Br. Monomer BTz-Br (70 mg) was used in this homopolymerization. The reaction solution was poured into acetone. The precipitate was washed with methanol, diluted HBr, and methanol. The precipitated was further purified by Soxhlet extraction

with methanol and acetone and collected with chloroform. The chloroform fraction was concentrated to provide PBTz-Br. GPC (chloroform, polystyrene standard): Mn = 7.2 kg mol−1, Đ = 2.0.

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RESULTS AND DISCUSSION The preparation of the three polymers PBTz-Br, PBTz-PyrBr, and PBTz-TMABr is outlined in Scheme 1. These polymers have the same benzotriazole conjugated backbone, but with 6bromohexyl, pyridinium, and ammonium side chains, respectively. Monomer BTz-Br for PBTz-Br was synthesized from the commercially available dibromobenzothiadiazole in three steps, including reduction (NaBH4 in EtOH), annulation (NaNO2 in CH3CO2H),6 and alkylation (BrC6H12Br, t-BuOK, MeOH). However, Yamomoto polymerization of BTz-Br provided PBTz-Br that precipitated out of solution as the reaction proceeded. PBTz-Br exhibits low molecular weight (Mn = 7.2 kDa, Đ = 2.0) as indicated by gel permeation chromatography (GPC) in CHCl3 that was calibrated versus polystyrene standards. In addition, PBTz-Br has limited solubility in common organic solvents to carry out subsequent chemical functionalization of the side chains. To circumvent these complications, compound BTz-Br was instead converted to the ionic monomers BTz-PyrBr and BTz-TMABr in nearly quantitative yield by treating BTz-Br with pyridine and trimethylamine, respectively.17−19 Ni-catalyzed Yamamoto polymerizations of BTz-PyrBr and BTz-TMABr independently provide two cationic CPEs with B

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Macromolecules different pendant ionic functionalities in good yields (PBTzPyrBr: 88%; PBTz-TMABr: 92%). After polymerization, the products were precipitated in CHCl3, filtered, and washed with CHCl3, acetone, and hexanes by Soxhlet extraction, followed by further purification via dialysis in water for 1 week. The molecular weight cutoff of the dialysis membrane was 3−5 kDa. The number-average molecular weight (Mn) and dispersity (Đ) were obtained by GPC in water that was calibrated versus poly(ethylene glycol) standards. The GPC studies provided large Mn values (PBTz-PyrBr: Mn = 112 kDa, Đ = 3.2; PBTzTMABr: Mn = 157 kDa, Đ = 1.5). The higher molecular weight of PBTz-TMABr may be due to a stronger tendency of polymer chains to aggregate compared to PBTz-PyrBr chains, a feature that is subsequently discussed within the context of light scattering experiments.20 X-ray photoelectron spectroscopy (XPS) in Figure 1 is a surface-sensitive spectroscopic technique that is used to

Dynamic light scattering (DLS) was used to estimate the particle size through analysis of the hydrodynamic radii (Rhyd) of PBTz-PyrBr and PBTz-TMABr in the same solvent systems used for absorption measurements.24 Both PBTz-PyrBr and PBTz-TMABr show two different peaks in the autocorrelation function which corresponds to two Rhyd for each material. These data are provided in Table 1. Each polymer exhibits Rhyd on the order of tens of nanometers and several hundred nanometers (see Supporting Information). This bimodal distribution suggests the presence of both small and large aggregates in solution. For bimodal distributions, it is necessary to examine both Rhyd and the peak percent by mass. This latter factor describes the relative proportion of the two components based on mass and thereby allows comparisons of similar samples to be made between multimodal distributions. A comparison of the larger aggregates allows us to observe trends with changes in solvent polarity. As the solvent polarity decreases, for both PBTz-PyrBr and PBTz-TMABr, the peak percent by mass of the larger aggregates increases. This is consistent with the absorptions that the absorption maxima red-shift as the solvent polarity decreases. PBTz-TMABr consistently exhibits larger Rhyd with higher peak percent by mass in various solvents than PBTz-PyrBr. This fact is consistent with both the GPC analysis and the absorption spectra. Since these CPEs are well soluble in water, aggregation is likely mediated by the electrostatic pairing of the ionic functionalities in lower dielectric media. Fluorescence spectroscopy can provide an additional handle for understanding polymer chain aggregation. For conjugated polymers, interchain aggregation is known to occur under specific conditions, resulting in decreased fluorescence intensity due to self-quenching.22,25,26 The solution fluorescence spectra of PBTz-PyrBr and PBTz-TMABr (see Supporting Information) show the opposite trend. The emission maxima (λem) of the two CPEs are located at similar values in H2O (583 and 585 nm) and 1:1 H2O:MeOH (560 and 575 nm), but a red-shift in λem, plus an increase of intensity at λem, and better definition of the vibronic features are observed in MeOH. Changes in fluorescence can be quantified by determining the fluorescence quantum yields (Φ). Rhodamine 101 salt in MeOH was chosen as the reference.27 The resulting values of Φ are provided in Table 1, and detailed calculations are provided in the Supporting Information. PBTz-PyrBr and PBTz-TMABr solutions show more intense fluorescence as the polarity of solvent decreases. Specifically, the calculated Φ values of PBTzPyrBr and PBTz-TMABr increase from 11% and 16% in water to 53% and 61% in MeOH, respectively. These results, combined with the absorption and DLS data, suggest that CPEs solutions exhibit enhanced fluorescence upon aggregation.28 Such aggregate-induced emission behavior29 of these CPEs has the potential to be included in the array of materials used in aggregation induced emission enhancement technologies.30 In order to further probe the impact of the choice of ionic functionalities, the anionic, sulfonate-terminated CPE, PBTzNa, was synthesized. The anionic monomer was synthesized in two steps from benzotriazole as shown in Scheme 2.18,31 Yamamoto coupling reaction of BTz-Na provided PBTz-Na in 61% yield. The absorption and fluorescence quantum yield measurements of PBTz-Na in H2O and DMF:H2O (1:1) solutions are provided in Figure 3, Figure 4, and the Supporting Information. Unexpectedly, the anionic-CPE shows the adverse trend of optical features relative to cationic-CPEs. It exhibits a

Figure 1. N 1s peak fitting of XPS spectra of PBTz-Br, PBTz-PyrBr, and PBTz-TMABr.

determine the elemental compositions of materials. The XPS spectra of PBTz-Br, PBTz-PyrBr, and PBTz-TMABr show the N 1s region. The peak fitting curves match well with the chemical structures. Specifically, the spectrum of PBTz-Br shows two peaks at 401.4 and 403.0 eV with an integrated ratio of 2:1, consistent with the two chemically distinct nitrogen atoms on the benzotriazole unit. Both PBTz-PyrBr and PBTzTMABr exhibit an extra peak at higher binding energy, corresponding to the nitrogen atoms on the pendant pyridium and tetraalkylammonium functional groups, respectively. The ratios of the three different nitrogen atoms are 2:1:1 for the two CPEs, which are in good agreement with the expected stoichiometry. Solution absorption spectra of PBTz-PyrBr and PBTzTMABr in three different solvents (H2O, H2O:MeOH (1:1), and MeOH) are provided in Figure 2. CPEs in solution all exhibit a peak characteristic of polymers containing BTz πconjugated backbone.9,21 In the same solvents, the absorption maxima (λmax) of PBTz-TMABr are consistently at higher wavelength than that of PBTz-PyrBr. CPEs reported in the literature typically exhibit a red-shift in absorption with increasing solvent polarity.22 In contrast, both λmax of PBTz-PyrBr and PBTz-TMABr solutions blue-shift as the solvent polarity is increased (polarity: H2O > MeOH). These changes lead to an obvious darkening of the solutions with decreasing solvent polarity (see Supporting Information). Such negative solvatochromic effects indicate increased aggregates of CPE chains in solvents of lower polarity.23 C

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Figure 2. UV/vis absorption and PL intensity of PBTz-PyrBr and PBTz-TMABr solutions in various polar solvents with the fixed concentration of 0.0167 mg/mL. The value of λEx is the corresponding maximum absorption peak.

Table 1. Average Hydrodynamic Radii, the Percentage Peak by Mass, and Fluorescence Quantum Yield (Φ) of PBTzPyrBr and PBTz-TMABr in Various Solvents average hydrodynamic radii (nm)

fluorescence quantum yield (Φ)

solvents

PBTz-PyrBr

PBTzTMABr

PBTzPyrBr

PBTzTMABr

H2O H2O:MeOH (1:1) MeOH

285 (24%)a 370 (68%)a 345 (76%)a

369 (74%)a 409 (78%)a 380 (88%)a

11% 16% 53%

16% 25% 61%

a

Scheme 2. Synthesis of PBTz-Na with Anionic, Sulfonated Side Chains

Percentage of the larger aggregate.

blue-shift of the absorption in DMF:H2O (1:1) solution compared to H2O solution. From DLS results in the Supporting Information, a decrease of aggregation was revealed as the solvent polarity was reduced (polarity: H2O > DMF). However, the calculated Φ value of PBTz-Na in DMF:H2O (1:1) (16%) is higher than the solution in H2O (8%). In order to study the aggregation behaviors, we found that DMF:H2O (1:1), a less polar solvent mixture than H2O, can dissolve all the CPEs, thus allowing us to make a direct comparison. The absorption spectra and the calculated Φ based on fluorescence measurements are provided in Figure 4. The λmax of both the cationic CPEs are located at longer wavelengths, relative to PBTz-Na. CPE-TMABr exhibits the most red-shifted λmax, possibly due to the high degree of aggregation based on DLS results. The larger Rhyd of these CPEs in DMF:H2O (1:1) at [CPE] of 0.0167 mg mL−1 are as follows: PBTz-Na, 103 nm; PBTz-PyrBr, 227 nm; and PBTzTMABr, 145 nm. Unlike many other CPEs reported in the

literature, PBTz-Na, PBTz- PyrBr, and PBTz-TMABr reveal dissimilar optical properties despite containing the same conjugated backbone. Thus, these comparative results indicate that the pendant ionic functionality of these CPEs serves a purpose beyond solubility in polar media. Molecular aggregation induced by the cationic side chains seem to be more significant than for the anionic counterpart. D

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in solution, as observed in absorption and DLS measurements. CPEs with tetraalkylammonium side chains exhibit the most red-shifted absorption maxima, the largest aggregate size, and the highest fluorescence quantum yield (up to 61% in MeOH). The optical properties of these CPEs can also be tuned by the choice of solvent. Negative solvatochromic effects are observed for both cationic CPEs. These studies provide insights into the role of structural modification of pendant ionic functionalities on the optoelectronic properties of the CPEs. These CPEs provide a class of novel materials that may find application in optoelectronic devices.11

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00965. Detailed synthesized information on all monomers, gel permeation chromatography, X-ray photoelectron spectroscopy, fluorescence emission spectra, fluorescence quantum yield, and dynamic light scattering measurements of the polymers (PDF)

Figure 3. UV/vis absorption, fluorescence quantum yield (Φ), and PL intensity of PBTz-Na in H2O.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.K.M.). *E-mail: [email protected] (Y.C.). *E-mail: [email protected] (G.C.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Air Force Office of Scientific Research MURI FA9550-12-1-0002 and Office of Naval Research N00014-14-1-0101. The MRL Shared Experimental Facilities (GPC and DLS), Program of the NSF under Award DMR 1121053; a member of the NSF-funded Materials Research Facilities Network. Y. Shi thanks the State-Sponsored Scholarship for Graduate Students from the China Scholarship Council.

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Figure 4. (a) UV/vis absorbance spectra and (b) fluorescence quantum yield (Φ) and PL intensity of PBTz-Na, PBTz-PyrBr, and PBTz-TMABr in DMF:H2O (1:1) at the same concentration (0.0167 mg mL−1).

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CONCLUSIONS We have synthesized CPEs with polybenzotriazole conjugated backbones but different side chains via Yamamoto coupling polymerization directly from the ionic monomers. The optical properties of this series of polymers were investigated. Different ionic side groups (pyridinium, tetraalkylammonium, and sulfonate-terminated) impact how the polymer chains aggregate E

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DOI: 10.1021/acs.macromol.6b00965 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b00965 Macromolecules XXXX, XXX, XXX−XXX