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Insights Into Magneto-Optics of Helical Conjugated Polymers Insights into Magneto-Optics of Helical Conjugated Polymers The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Wang, Pan et al. "Insights into Magneto-Optics of Helical Conjugated Polymers." Journal of the American Chemical Society 140, 20 (May 2018): 6501–6508 © 2018 American Chemical Society As Published http://dx.doi.org/10.1021/jacs.8b03777 Publisher American Chemical Society (ACS) Version Author's final manuscript Citable link https://hdl.handle.net/1721.1/128127 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. Page 1 of 9 Journal of the American Chemical Society 1 2 3 4 5 6 7 Insights into Magneto-Optics of Helical Conjugated Polymers 8 Pan Wang,1,3 Intak Jeon,2,3 Zhou Lin,1 Martin D. Peeks,1,3 Suchol Savagatrup,1,3 Steven E. Kooi,3 Troy 9 1 1,3 10 Van Voorhis, and Timothy M. Swager * 11 1 Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, United States. 12 2 13 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, United 14 States. 15 3 Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, MA 02139, United States 16 17 ABSTRACT: Materials with magneto-optic (MO) properties have enabled critical fiber-optic applications and highly sensitive mag- 18 netic field sensors. While traditional MO materials are inorganic in nature, new generations of MO materials based on organic semi- 19 conducting polymers could allow increased versatility for device architectures, manufacturing options, and flexible mechanics. How- 20 ever, the origin of MO activity in semiconducting polymers is far from understood. In this paper, we report high MO activity observed 21 in a chiral helical poly-3-(alkylsulfone)thiophene (P3AST), which confirms a new design for the creation of giant Faraday effect with 22 Verdet constants up to (7.63±0.78)´104 deg T-1 m-1 at 532 nm. We have determined that the sign of the Verdet constant and its 23 magnitude are related to the helicity of the polymer at the measured wavelength. The Faraday rotation and the helical conformation 24 of P3AST are modulated by thermal annealing, which is further supported by DFT and MD simulations. Our results demonstrate that 25 helical polymers exhibit enhanced Verdet constants, and expand the previous design space for polythiophene MO materials that was 26 thought to be limited to highly regular lamellar structures. The structure property studies herein provide insights for the design of 27 next generation MO materials based upon semiconducting organic polymers. 28 29 30 1. Introduction the order of 104-105 deg T-1 m-1, that exceed those for TGG (V= 31 4 -1 -1 4 -1 -1 Magneto-optic (MO) materials have broad utility in photonic –1.0´10 deg T m at 532 nm and –0.3´10 deg T m at 980 12c, 12e 32 devices including quantum memory,1 optical isolators,2 optical nm) and other inorganic materials. The large Faraday ro- 33 circulators,3 and magnetic insulators.4 They also find use in bi- tation of these polymers is thought to be dependent on their 34 omedical applications to create highly sensitive magnetic field crystallinity, and has been correlated to their high regioregular- 35 sensors (MFS),5 which are able to detect extremely weak mag- ity and their organization into lamellae-type structures. How- 36 netic field fluctuations associated with real-time human brain ever, polyalkythiophenes have limited oxidative stability and 37 activity. The Faraday effect, discovered by Michael Faraday the crystalline lamellar structures, if not uniformly organized, are highly optically scattering.13 Electronically delocalized or- 38 two centuries ago, is a ubiquitous MO effect, describing the ro- tation of the plane polarized light traveling through a material ganic materials were further confirmed to be a general class of 39 6 MO materials with the measurement of large Faraday rotations 40 along the axis of an applied magnetic field. This effect is quan- tified by a Verdet constant (V), which is wavelength dependent, in poly(arylene ethynylene) and mesogenic organic molecules, 41 and linearly correlated to the applied field and path length which suggested that highly-ordered face-to-face π-stacking 42 through the material. The most common materials used for MFS and the triplet excitation resonances were responsible for the 12b, 12d 43 applications and other photonic devices are terbium gallium observed Verdet constants. Very recently, Prasad and 44 garnet (TGG)7 and other ferromagnetic, rare-earth metal based coworkers reported a strategy for tuning the magneto-optic ac- 45 crystals.8 However, the utility of these paramagnetic materials tivity by using a chiral helical polymer doped with organic bi- 46 is limited by their modest Verdet constants, which exhibit mag- radicals, and found that the Faraday rotation behavior differed 9 netic field saturation and appreciable temperature dependence. substantially between the parent polymer and the blended ma- 47 5a, 12a 48 Moreover, they are rigid crystals that present difficulties for de- terial. These previous investigations highlight the role of vice integration and fabrication. Conjugated polymeric materi- polymer conformation and supramolecular organization to af- 49 fect the electronic delocalization and electromagnetic interac- 50 als have been demonstrated to possess increased flexibility in fabrication, form-factor, and real-world applications.10 Further- tions in the polymer backbone and thereby modulate the Fara- 51 more, understanding the underlying mechanism of MO activity day effect. However, aside from these earlier studies, mecha- 52 and developing new generations of polymeric transducers is a nisms to enhance MO activity in polymeric materials is still far 53 pressing demand and will enable the development of technolo- from understood. 54 gies. 11 Polythiophenes with lamellar crystalline structures are typi- 55 The potential of semiconducting polymers to display extraor- cally highly scattering optical materials and are limited in po- 56 dinary Faraday effects is a relatively recent and unexpected dis- tential applications. As a result, we have been interested in the 57 covery that has precipitated a renewed interest in the develop- performance of alternative polythiophene morphologies as MO 58 ment of new Faraday rotators.12 Polythiophenes and polyalkox- materials. Chiral helical structures are particularly attractive 59 ythiophenes were found to possess large Verdet constants, on 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 2 of 9 and also have prospects in molecular recognition and sens- 3. Results and Discussion 14,15 1 ing. Herein, we report helical poly-3-(alkylsulfone)thio- Scheme 1. Synthetic Route for Head to Tail Regioregular 2 phenes (P3ASTs) with large Verdet constants and that the sign P3ATTs and P3ASTs 3 of Faraday rotation and the helical structure of polymers can be 4 modulated by thermal treatment. Our results provide a basis for Monomer Synthesis: the design of new materials, shedding light on understanding Br SH S R S R 5 S, n-BuLi R-Br NBS, DMF the mechanism of MO activity in polymeric materials. t Br 6 S Et2O, -70 °C S BuOK, EtOH S rt, Ultrasound S 0 °C to rt 7 Polymerization: 2. Material Design O 8 S R S R O S R 9 TMPMgCl•LiCl 1) 0.5 mol % NiCl2(dppe) m-CPBA rt S Br 2) 2 M HCl, MeOH S n Oxidation 10 S n Highly Regioregular Head to Tail C4H9 11 Poly(3-substituted thiophene)s R = S-P1 SO2-P1 12 C6H13 13 14 (S) (S)-S-P2 (S)-SO2-P2 15 thiolate with bromo-alkanes provided facile access to 3-(al- 16 kylthio)thiophenes. Selective mono-bromination using NBS 17 and ultrasound17b afforded the mono-brominated products, 18 which were used for a Kumada polymerization18 as the next step 19 to produce regioregular polythiophenes in high yields. 20 Knochel’s base was used to deprotonate the monomer, and 0.5 21 mol% NiCl2(dppe) was chosen as the catalyst to promote the 22 polymerization. With this method, polymers of P3ATTs with 23 racemic side chains (S-P1) and with chiral side chains ((S)-S- Figure 1. One-dimensional potential energy surface (PES) scan P2) were all synthesized for comparison in 64% yield and 44% 24 with an increment of 2.5 ° along the S-C-C’-S’ dihedral angle of 25 yield, respectively. The physical data of these polymers are dimers D1 and D2, evaluated using DFT at the B3LYP/6-31G* summarized in Table 1. S-P1 was obtained with a M of 54.6 26 n level. The global minima at –37.4° for D1 and –46.2° for D2 are kDa and head to tail (HT-HT) regioregularity >96%, which is also included. 19 27 an improvement from previously published results. Different 28 16 Non-resonant optical phenomena are generally enhanced by from the previous reported P3ATTs with n-alkyl side chains 29 more polarizable heavy atoms and hence we targeted polythio- that are only soluble in CS2, S-P1 with branched side chain 30 phenes with sulfur containing substituents.17 The larger size of shows extraordinary solubility in most common organic sol- 31 sulfide/sulfone groups were also attractive as we suppose that it vents including hexane and silicon oil, which is desirable. (S)- 32 will promote non-planar polymer backbones and promote tight S-P2 is less soluble but can be completely dissolved in hot chlo- 33 helical structures. Our expectation was that high chirality will roform or ortho-dichlorobenzene (o-DCB). Its molecular 34 enhance the electronic delocalization along the polymer back- weight based on the THF soluble portion at room temperature 35 bone and afford a higher sensitivity to magnetic fields.
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