Conjugated Polymers Based on 4,10-Bis(thiophen-2-yl)anthanthrone

Article pubs.acs.org/Macromolecules

Conjugated Polymers Based on 4,10-Bis(thiophen-2-yl)anthanthrone: Synthesis, Characterization, and Fluoride-Promoted Photoinduced Electron Transfer Antoine Lafleur-Lambert, Jean-Benoît Giguère, and Jean-François Morin* Département de chimie and Centre de Recherche sur les Matériaux Avancés (CERMA), Université Laval, 1045 Ave de la Médecine, Québec City, QC Canada G1V 0A6 S Supporting Information *

ABSTRACT: Five new copolymers based on the anthanthrone moiety were synthesized using Stille coupling. The anthanthrone unit was coupled with two electron-rich units (thiophene and carbazole) and three electron-poor units (thienopyrroledione, diketopyrrolopyrrole, and benzothiadiazole). The optoelectronics properties of the polymers were investigated and suggest that the anthanthrone moiety dictates the electronic properties of the materials with little regards to the nature of the comonomers. Fluoride-promoted photoinduced electron transfer on anthanthrone derivatives yielded to radical anion species that can be reversibly oxidized to the neutral form by exposure to air.

■

INTRODUCTION Commercial dyes and pigment derivatives such as diketopyrrolopyrrole (DPP)1,2 and isoindigo3,4 are becoming increasingly popular as building blocks for the preparation of organic semiconductors.5,6 Their tunable electronic and optical properties over the entire visible spectrum provide them with many advantages for organic electronics applications such as organic solar cells (OSCs),7,8 field-effect transistors (OFETs),9 and light-emitting diodes (OLEDs).10 Moreover, their low cost make them very attractive, as high materials cost is still an issue for organic electronics commercialization. Recently, our group undertook the synthesis, characterization, and device testing of π-conjugated small molecules based on a commercially available, low-cost dye named vat orange 3 (Scheme 1).11−13 Also called 4,10-dibromoanthanthrone, this dye possesses a rigid and extended πconjugated backbone, providing useful optical properties in the visible range. Moreover, the anthanthrone bears two ketones and two bromines atoms, allowing the introduction of functional groups and, consequently, the modulation of its electronic and optical properties. Several anthanthrene (reduced form of anthanthrone) derivatives were tested as ptype semiconductors in OFETs and OSCs prototypes, and promising results were obtained for both types of devices with charge mobilities up to 0.078 cm2 V−1 s−1 and solar power conversion efficiencies (PCE) up to 2.4%, respectively.14,15 Although the synthesis of π-conjugated small molecules from vat orange 3 is relatively straightforward, the preparation of π© 2015 American Chemical Society

conjugated polymers remains more challenging. Indeed, the rigidity of vat orange 3 combined with its large and flat π surface precludes good solubility in common organic solvents. In a previous article, we showed that the reduction of the anthanthrone unit to its bis-alkylated fully aromatic anthanthrene analogue is an efficient strategy to prepare soluble πconjugated polymers from vat orange 3.16 However, the lost of the ketone groups on the anthanthrone unit impacted significantly the redox properties of the resulting polymers as the quionidal form provide a π-electron-deficient character to the conjugated structure. DPP,2 isoindigo4 and naphthalenediimide (NDI)17,18 are examples of ketone-containing moieties that have been used as base units for the preparation of efficient n-type materials. The NDI is particularly interesting as Saha and co-workers recently reported that its low-lying LUMO energy level allows its one-electron reduction through anionpromoted photoinduced electron transfer.19−21 To the best of our knowledge, this anion-mediated type of photoinduced electron transfer has never been observed with other πconjugated molecules. According to Saha, such electron transfer from an anion to a π-conjugated molecule is possible when the HOMO energy level of the anion is positioned between the HOMO and the LUMO energy levels of the π-conjugated molecule. Because anthanthrone also possess strong electronReceived: July 1, 2015 Revised: October 26, 2015 Published: November 4, 2015 8376

DOI: 10.1021/acs.macromol.5b01449 Macromolecules 2015, 48, 8376−8381

Article

Macromolecules Scheme 1. Synthesis of 4,10-Bis(4-octylthiophene-2-yl)anthanthrone (TANT) and the Corresponding Polymers

the presence of two octyl chains on the thiophene moieties. The two thiophene units of TANT were then brominated at the 5-position of thiophenes using NBS as a brominating agent in a polar solvents mixture to afford compound 2 in a good yield (80%). Afterward, the latter compound was reacted with hexamethylditin23 under Stille coupling conditions to afford the bis(trimethyl)tin derivative of TANT (compound 3) in 40% yield. The polymerization reactions were conducted using the same conditions for all the polymers: compound 3 and the dibromo comonomers (thiophene,24 carbazole,25 thienopyrroledione,26 diketopyrrolopyrrole,27 and benzothiadiazole28 derivatives) were polymerized through a Stille coupling in toluene with Pd2(dba)3 and P(o-tolyl)3 as the catalytic charge. The resulting mixture was heated to 110 °C until precipitation of the polymer or up to 48 h. Following precipitation of the reaction mixture in a MeOH/H2O solution (9:1), the polymers were purified with a Soxhlet apparatus using acetone to remove oligomers and then extracted with chloroform to recover the soluble fraction of the polymers. The resulting polymers were further purified using diethylphenylazothioformamide as a scavenger to remove catalyst residues.29

withdrawing ketone moieties with similar electron affinity, we hypothesized that anthanthrone could undergo similar anioninduced electron transfer upon irradiation. Herein, we report the synthesis and characterization of the first series of soluble conjugated polymers based on the 4,10bis(thienophen-2-yl)anthanthrone (TANT) unit. The TANT unit was copolymerized with five comonomers, either electronrich or electron-poor, with the intent to obtain insights on the role of anthanthrone as an electron-deficient unit. While we recently probed the 6,12 conjugation axis of anthanthrene,13,16 we explore herein the 4,10 conjugation axis of the anthanthrone core. Fluoride-promoted photoinduced electron transfer with one of the polymers (PTANTT) and its monomeric analogue (TANT) is also presented.

■

RESULTS AND DISCUSSION The synthesis of monomers and polymers are shown in Scheme 1. Because of its inherent poor solubility, the anthanthrone moiety was first reacted, using a Stille coupling reaction, with 4n-octyl-2-thienyltributyltin22 to afford the 4,10-bis(4-octylthiophene-2-yl)anthanthrone (TANT, compound 1) in 71% yield. TANT is soluble in common organic solvents owing to 8377


Article

Macromolecules

Because of the presence of ketones in a quinoidal fashion as electron-withdrawing groups at the 6- and 12-positions, one can expect the polymers to show n-type behavior. In order to investigate its electronic properties as a comonomer, TANT was polymerized with comonomers classified as either electronrich or electron-poor, resulting in alternating donor−acceptor copolymers (PTANTT and PTANTC) and more exclusively ntype copolymers (PTANTTPD, PTANTBT, and PTANTDPP). The optical and electrochemical properties of these polymers are presented in Table 2. The LUMO energy level values of all polymers, ranging from −3.40 eV for PTANTDPP to −3.45 eV for PTANTTPD, are notably similar, suggesting that the LUMO orbitals are mostly localized on the anthanthrone unit as expected because of two electron-withdrawing ketone groups. However, the HOMO energy level values of the polymers do vary in function of the comonomer employed, going from −4.99 to −5.35 eV for PTANTTPD and PTANTBT, respectively. The influence of the comonomers on the HOMO energy level is both modest and unusual. For instance, the second deepest HOMO was measured at −5.22 eV for PTANTC, which possess an electron-rich carbazole unit, while the highest HOMO was measured at −4.99 eV for PTANTTPD, which possess electron-poor TPD units. Although the relationship between electronic density and HOMO energy level seems counterintuitive, it can be rationalized by the higher torsion angle between the carbazole and the thiophene unit (PTANTC) than that between two thiophene units (PTANTTPD), resulting in a lower effective conjugation length. Thus, no trend can be established regarding the influence of the comonomers’ electronic nature on the modulation of the frontier orbitals energy levels. More importantly, these results point to the conclusion that the 4,10 conjugation axis might not be efficient enough to provide low-bandgap polymers. Reinforcing this hypothesis is the fact that on a series of small molecules the 4,10 conjugation axis of the anthanthrene core proved to be less effective for the delocalization of πelectrons than its 6,12 counterparts.13 The same trend is thus observed for polymers. Nonetheless, the polymers have interesting optical properties as they all exhibit light absorption in the visible range in both solid state and chloroform solution. All the polymers exhibit red-shifted λmax values compared to TANT compound 1 (λmax = 525 nm in chloroform solution; see Figure S14), suggesting that polymerization of compound 3 with all the comonomers, including compound 2, resulted in an extension of the conjugation length. The λmax in the solid state ranged from 545 nm for PTANTBT to 674 nm for PTANTDPP (Figure 1) with relatively low optical bandgap values, ranging from 1.80 to 1.39 eV. These optical characteristics are favorable for OSCs applications, especially in the case of PTANTDPP that exhibits

The polymers were characterized by size-exclusion chromatography (SEC) employing polystyrene standards and chloroform as the eluent. The results are summarized in Table 1 along Table 1. Characterization of Polymers polymer

yield (%)

Mn (g/mol)

Mw (g/mol)

PDIa

Tdb (°C)

PTANTDPP PTANTBT PTANTC PTANTT PTANTPD

87 24 94 75 60

11600 1700 5300 4700 4600

33700 2400 12900 14700 17200

2.9 1.4 2.4 3.1 3.7

314 286 319 412 397

a

Determined from SEC analysis. bDetermined using thermogravimetric analysis under nitrogen at 5% weight loss.

with the yields of recovered polymers and the decomposition temperature (Td). The yields of polymerization were calculated by taking into account only the soluble fraction of the polymers after purification through Soxhlet extraction. Thus, the higher molecular weight fractions that were insoluble in chloroform were discarded and were not characterized. The low yield reported for PTANTBT (24%) is attributed to the formation of a significant amount of insoluble material during the reaction. The lack of solubilizing chain on the benzothiadiazole moiety is likely to be responsible for the low solubility of this polymer. PTANTC, PTANTT, and PTANTTPD of relatively low Mn values of 5300, 4700, and 4600 g/mol, respectively, were isolated. Again, the poor solubility of higher molecular weight fractions in the solvent of reaction (toluene) was responsible for the relatively low Mn values measured for all these polymers, despite the presence of solubilizing chains on the comonomers moieties. Interestingly, the presence of two branched alkyl chains per monomeric unit (PTANTDPP) allowed for improved polymer solubility in the reaction media as a higher Mn value (11 600 g/mol) was obtained. It is noteworthy that polymerization was also attempted with bis(ethylhexyl)benzodithiophene (BDT)30 as a comonomer; however, no soluble fraction could be recovered from chloroform. The Td values were measured using thermogravimetric analysis (TGA), and the results are summarized in Table 1. All the polymers decomposed at temperatures higher than 300 °C, except for PTANTBT oligomers which decomposed at 286 °C. This low Td value can be attributed to the low Mn value, resulting in significant amount of thermally fragile end groups such as tributyltin. Differential scanning calorimetry (DSC) measurements were also performed to further explore the thermal properties of the polymers. However, practical information could not be recovered from this investigation, as no phase transition was observed. All the polymers are amorphous, regardless of the nature of the alkyl chains or the comonomers. Table 2. Optical and Electrochemical Properties of Polymersa

a

polymer

λfilm max (nm)

Efilm (eV) g

Vox (V)

Vred (V)

EHOMO (eV)

ELUMO (eV)

Eelectro (eV) g

PTANTDPP PTANTBT PTANTC PTANTT PTANTTPD

674 545 569 578 574

1.39 1.79 1.68 1.59 1.80

0.81 1.09 0.96 0.76 0.73

−0.86 −0.85 −0.85 −0.86 −0.82

−5.07 −5.35 −5.22 −5.02 −4.99

−3.40 −3.41 −3.41 −3.41 −3.45

1.67 1.94 1.81 1.62 1.54

Eg film values were measured at the onset. V: onset vs Ag/AgCl, scan rate of 100 mV s−1, Fc/Fc+ E1/2 measured at 0.44 vs Ag/AgCl. 8378


Article

Macromolecules

since Saha and co-workers showed that it was the most efficient for anion-mediated PET.21 Upon addition of fluoride to the PTANTT solution at room temperature, no color change was observed. However, upon irradiation of the solution with a 500 W white light for 60 s, an immediate color change was observed as the solution went from purple to blue (Figure 2a). This change was monitored by UV−vis spectroscopy, and the results are shown in inset of Figure 2a. Upon irradiation, two new peaks attributed to the formation of PTANTT•− at 883 and 1019 nm appeared. In the titration experiment (Figure 2b), an isosbestic point appeared at 679 nm, indicating the transisiton from a neutral to reduced form. For the sake of comparison, the same experiment was conducted on compound 2 and very similar results were observed (see Figure S15). The fact that these new bands did not appear upon addition of TBAF in the dark at room temperature suggests that thermal electron transfer does not occur and that the photoinduced electron transfer is the only possible electron transfer mechanism involved. It is worth noticing that longer irradiation time in the presence of a large excess of TBAF does not lead to the formation of PTANTT2−. In order to confirm that the newly formed bands in the UV− vis spectrum were attributed to the formation of PTANTT•−, we performed standard spectroelectrochemical analysis (Figure 2a, green trace). As expected, the first reduction process is accompanied by the appearance of two new bands at 874 and 1008 nm, which are at very similar positions than those observed upon irradiation of the solution of PTANTT in the presence of fluoride. This result rules out the scenario of the fluoride ion nucleophilic attack on PTANTT leading to a new chemical species. Unfortunately, we were unable to obtain the dianion species, PTANTT2−, using spectroelectrochemical analysis in the conditions used. In order to form PTANTT2−, we added a large excess of a strong aqueous reducing agent, sodium dithionite (Na2S2O4), to a solution of PTANTT in o-dichlorobenzene in the presence of TBAOH. Shortly after the addition (30 s), PTANTT•− was formed as new bands appeared at 871 and 990 nm, followed by

Figure 1. UV−vis absorption spectra of PTANTBT and PTANTDPP in chloroform (solid lines) and thin film (dashed lines).

a full width at half-maximum (fwhm) value of more than 300 nm in the solid state. Because anthanthrone possesses two conjugated ketone groups, one can expect redox behavior comparable to benzoquinone derivatives. Benzoquinone derivatives are known to undergo two reversible one-electron reductions, leading to stable radical anion (•−) and dianion (2−), when reduced either electrochemically or with reducing agents.31 This reversible redox property of benzoquinone derivatives is very useful for applications such as energy storage32 and electrochromism.33 Thus, we decided to investigate the redox properties of anthanthrone-based polymers to assess their potential for such applications. As a first experiment, we performed the fluoride-promoted photoinduced reduction of PTANT in a degassed THF solution by adding tetrabutylammonium fluoride (TBAF, 1.0 M in THF).21 The fluoride anion was chosen as the Lewis base

Figure 2. (a) UV−vis spectra of PTANTT in THF (red line), after the addition of an excess of TBAF and irradiation under 500 W white light to yield PTANTT•− (blue line) and after electrochemical reduction at −750 mV in 1 M TBAPF6 in o-dichlorobenzene (green line). (b) UV−vis spectra of PTANTT in THF upon successive addition of TBAF. After each addition of TBAF, the solution was irradiated for 30 s under a 500 W white light. 8379


Macromolecules

■

the slow appearance of PTANTT2− (Figure 3). The UV−vis spectrum of PTANTT•− formed upon reduction with Na2S2O4

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.-F.M.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the National Science and Engineering Council of Canada (NSERC) for financial support. Jean-Benoit̂ Giguère thanks the NSERC for a PhD scholarship. We are also grateful for professional support from CQMF and CERMA.

■

Figure 3. UV−vis spectra of PTANTT in o-dichlorobenzene solution in neutral state (red solid line), after 30 s following the addition of Na2S2O4 under nitrogen (blue solid line), after 300 s under nitrogen (green solid line), after exposure to air for 10 s (burgundy dashed line), and after exposure to air for 60 s (purple dashed line).

is almost identical than that obtained by anion-induced electron transfer from fluoride anion to the anthanthrone unit. Interestingly, all the processes leading to PTANTT•− and PTANTT2− from PTANTT are reversible upon exposure to air. As shown in Figure 3, PTANTT2− can be gradually reoxidized by bubbling air through the solution to form PTANTT•− (dashed burgundy trace, Figure 3) and eventually, PTANTT (dashed purple trace, Figure 3). The resulting solution can be degassed again with nitrogen to form the reduced species PTANTT•− and PTANTT2− back without significant spectral changes. This excellent reversibility for both reduction processes makes anthanthrone-based conjugated polymers promising candidates as chromic materials.

■

CONCLUSION To summarize, five new conjugated copolymers based on the 4,10-bis(thienophen-2-yl)anthanthrone (TANT) moiety have been synthesized, and their optical and electronics properties have been studied. The TANT unit was copolymerized with units of various electronic natures. We found that the electronic nature of the copolymers employed offered sparse influence on the resulting electronic properties of the PTANT series. Fluoride-mediated photoinduced electron transfer, spectroelectrochemistry, and chemical reduction experiments conducted demonstrated that the anthanthrone moiety is an attractive candidate for the preparation of low-cost chromic materials.

■

REFERENCES

(1) Farnum, D. G.; Mehta, G.; Moore, G. G. I.; Siegal, F. P. Tetrahedron Lett. 1974, 15, 2549. (2) Nielsen, C. B.; Turbiez, M.; McCulloch, I. Adv. Mater. 2013, 25, 1859. (3) Mei, J.; Graham, K. R.; Stalder, R.; Reynolds, J. R. Org. Lett. 2010, 12, 660. (4) Wang, E.; Mammo, W.; Andersson, M. R. Adv. Mater. 2014, 26, 1801. (5) Ouhib, F.; Tomassetti, M.; Dierckx, W.; Verstappen, P.; Wislez, A.; Duwez, A.-S.; Lemaur, V.; Lazzaroni, R.; Manca, J.; Maes, W.; Jérôme, C.; Detrembleur, C. Org. Electron. 2015, 20, 76. (6) Jaroch, T.; Maranda-Niedbata, A.; Góra, M.; Mieczkowski, J.; Zagórska, M.; Salamonczyk, M.; Górecka, E.; Nowakowski, R. Synth. Met. 2015, 204, 133. (7) Yin, Q.-R.; Miao, J.-S.; Wu, Z.; Chang, Z.-F.; Wang, J.-L.; Wu, H.B.; Cao, Y. J. Mater. Chem. A 2015, 3, 11575. (8) Qu, S.; Tian, H. Chem. Commun. 2012, 48, 3039. (9) Grenier, F.; Aïch, B. R.; Lai, Y.-Y.; Guérette, M.; Holmes, A. B.; Tao, Y.; Wong, W. W. H.; Leclerc, M. Chem. Mater. 2015, 27, 2137. (10) Fenwick, O.; Fusco, S.; Baig, T. N.; Di Stasio, F.; Steckler, T. T.; Henriksson, P.; Flechon, C.; Andersson, M. R.; Cacialli, F. APL Mater. 2013, 1, 032108. (11) Giguère, J.-B.; Verolet, Q.; Morin, J.-F. Chem. - Eur. J. 2013, 19, 372. (12) Giguère, J.-B.; Morin, J.-F. J. Org. Chem. 2013, 78, 12769. (13) Giguère, J.-B.; Boismenu-Lavoie, J.; Morin, J.-F. J. Org. Chem. 2014, 79, 2404. (14) Giguère, J.-B.; Sariciftci, N. S.; Morin, J.-F. J. Mater. Chem. C 2015, 3, 601. (15) Zhang, L.; Walker, B.; Liu, F.; Colella, N. S.; Mannsfeld, S. C. B.; Watkins, J. J.; Nguyen, T.-Q.; Briseno, A. L. J. Mater. Chem. 2012, 22, 4266. (16) Lafleur-Lambert, A.; Giguère, J.-B.; Morin, J.-F. Polym. Chem. 2015, 6, 4859. (17) Kim, Y.; Lim, E. Polymers 2014, 6, 382. (18) Suraru, S.-L.; Würthner, F. Angew. Chem., Int. Ed. 2014, 53, 7428. (19) Guha, S.; Saha, S. J. Am. Chem. Soc. 2010, 132, 17674. (20) Guha, S.; Goodson, F. S.; Roy, S.; Corson, L. J.; Gravenmier, C. A.; Saha, S. J. Am. Chem. Soc. 2011, 133, 15256. (21) Guha, S.; Goodson, F. S.; Corson, L. J.; Saha, S. J. Am. Chem. Soc. 2012, 134, 13679. (22) Cai, T.; Zhou, Y.; Wang, E.; Hellstrom, S.; Zhang, F.; Xu, S.; Inganas, O.; Andersson, M. R. Sol. Energy Mater. Sol. Cells 2010, 94, 1275. (23) Strueben, J.; Gates, P. J.; Staubitz, A. J. Org. Chem. 2014, 79, 1719. (24) Diliën, H.; Palmaerts, A.; Lenes, M.; de Boer, B.; Blom, P.; Cleij, T. J.; Lutsen, L.; Vanderzande, D. Macromolecules 2010, 43, 10231. (25) Dierschke, F.; Grimsdale, A. C.; Müllen, K. Synthesis 2003, 16, 2470. (26) Najari, A.; Berrouard, P.; Ottone, C.; Boivin, M.; Zou, Y.; Gendron, D.; Caron, W.-O.; Legros, P.; Allen, C. N.; Sadki, S.; Leclerc, M. Macromolecules 2012, 45, 1833.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01449. Experimental procedures and characterization data for all the new compounds, cyclovoltamograms, and UV−vis spectra for all the polymers (PDF) 8380


Article

Macromolecules (27) Chen, S.; Sun, B.; Hong, W.; Aziz, H.; Meng, Y.; Li, Y. J. Mater. Chem. C 2014, 2, 2183. (28) Jo, S.; Kim, J.; Noh, J.; Kim, D.; Jang, G.; Lee, N.; Lee, E.; Lee, T. S. ACS Appl. Mater. Interfaces 2014, 6, 22884. (29) Nielsen, K. T.; Bechgaard, K.; Krebs, F. C. Macromolecules 2005, 38, 658. (30) Li, G.; Zhao, B.; Kang, C.; Lu, Z.; Li, C.; Dong, H.; Hu, W.; Wu, H.; Bo, Z. ACS Appl. Mater. Interfaces 2015, 7, 10710. (31) René, A.; Evans, D. H. J. Phys. Chem. C 2012, 116, 14454. (32) Häupler, B.; Wild, A.; Schubert, U. S. Adv. Energy Mater. 2015, 5, 1402034. (33) Yen, H.-J.; Lin, K.-Y.; Liou, G. S. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 61.

8381


Conjugated Polymers Based on 4,10-Bis(thiophen-2-yl)anthanthrone

Recommend Documents