Magnetic and Magneto Optic properties of Substituted Polythiophenes

Palash Gangopadhyay,a Guy Koeckelberghs,b Alejandra Lopez-Santiago,a Robert A. Norwood,a Nasser Peyghambarian,a and André Persoonsa,b

a College of Optical Sciences, University of Arizona, 1630 E. University Blvd, Tucson, AZ, 85721 USA

b Dept. of Chemistry, University of Leuven, Center for Research in Molecular Electronics and Photonics

Celestijnenlaan 200D, B-3001 Leuven, Belgium.

ABSTRACT

We report on our recent observation of magnetic and magneto optic properties of regioregular substituted polythiophenes. The relevance of existing mechanisms within the classical limit in cognizance of our recent magnetic and MO measurements on a set of π-conjugated polymer is also discussed.

Keywords: Faraday rotation, ferromagnetism, polymer, regioregular substituted polythiophenes

INTRODUCTION Last few decades have witnessed a tremendous growth in studies of conjugated polymers with interesting optical and electrical properties.1 These unusual optoelectronic properties allow these polymers to be used for a large number of applications, including non-linear optical devices, light emitting displays, all-plastic transistors, artificial actuators, sensing devices, terahertz emitters etc and owe their origin largely to the delocalization of electrons in a continuously overlapped π orbital along the polymer backbone.2 The mechanism behind the metal-like properties of these polymers is still a matter of debate and is generally believed to be dependent on the way the charge carriers are stabilized in the polymer chain. A few of these polymers possess intrinsic charge carriers or polarons, which can also be created by p- or n- type doping. Polythiophene presents a unique case because of its semiconducting properties in its native state. Through p-doping (oxidation) it can be made conducting whereas n-doping (reduction) converts it to a complete insulator. Interestingly, polythiophenes also show subtle structure-property relationships which have been exploited to tune its properties. Polyalkylthiophenes also undergo self assembly to form two-dimensional (2D) structures through strong interchain coupling which depends mainly on the length of the side chain and regioregularity of the polymer.3 The regioregular (RR) substituted polythiophenes (P3AT), in which the alkyl side group is attached to the 3rd position of the thiophene rings in a head to tail stereoregular order, form thin films with nanocrystalline lamellae, resulting in high hole mobilities (0.1 cm2V-1s-1)4,5 and strong interchain interaction. On the contrary, P3AT films of regiorandom stereo order do not show supramolecular structures, and the hole mobility is poor (10-5 cm2V-1s-1).4,5 It has been shown very recently that the polarons in RR P3AT thin films become delocalized over several polymer chains, consistent with the high 2D hole mobilities.6 This difference has been cleverly used in a recent report to construct efficient solar cells from this material7 and magnetic field switchable electroluminescent devices.8 Such large dynamic 2D delocalization of the charged carrier may create magnetic ordering in polythiophene derivatives and be easily perturbed by external magnetic fields. The strong interchain coupling also indicates a possible existence of polaron percolation in this material. These factors prompted us to investigate magnetic and magneto optical properties of substituted polyalkylthiophenes. In this report we will discuss our recent experimental findings of magnetic and magneto-optical investigations on polythiophene thin films.

Invited Paper

Linear and Nonlinear Optics of Organic Materials IX, edited by Theodore G. Goodson, III,Proc. of SPIE Vol. 7413, 74130F · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.829331

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EXPERIMENTAL

Materials. Poly(3-dodecylthiophene) with RR > 99% (P3DT) and degree of polymerization of 70 and Poly(3-hexylthiophene) was used for this study. Whereas P3DT was custom synthesized and purified in our laboratory, P3HT with different regioregularity were procured from different commercial sources (Sigma Aldrich, Merck and American Dye Sources) and were used without any further purification. The synthesis, purification and characterization of P3DT were carried out using a procedure developed by this group.9 The samples were characterized in detail for the presence of metal impurities, in particular to relevance of SQUID experiments and are reported here. The solvents used to fabricate thin and thick films were all of HPLC grade and used as received.

Fig. 1. Synthetic scheme for HT coupled highly regioregular poly (3-dodecyl) thiophene using GRIM methodology10.

The polymer was prepared according to the GRIM methodology, starting from 2,5-dibromo-3-dodecylthiophene (10.0 mmol, 4.10 g); 1 mol% (0.1 mmol, 54.2 mg) Ni(dppp)Cl2 was used. After polymerization, the polymer was purified through repeated precipitation using methanol followed by thorough washing with methanol, acetone and hexane using a Soxhlet apparatus. Finally, the polymer was extracted with THF, re-precipitated in methanol, filtered off and dried. Poly(3-dodecyl)thiophene (P3DT) has only 70 monomers (note that the estimation of the number of monomers may be an upper bound estimate), with regioregularity for the chain (without the head-group) > 99% as obtained from NMR. Yield = 1.61 g (64 %) 1H NMR: δ (ppm): 6.99 (1H, s), 2.81 (2H, t), 1.72 (2H, m), 1.1-1.6 (18H, m), 0.88 (3H, t). Mn = 17.5 kg/mol (measured by GPC in THF toward polystryrene standards) D = 1.4 (measured by GPC in THF toward polystryrene standards) Tm = 145°C (measured by DSC at a heating rate of 20 °C/min) Tc = 121°C (measured by DSC at a cooling rate of 20°C/min). The results of ICP (Inductively Coupled Plasma Emission Spectrometry, ICP-ES) analysis on P3DT and the sample holder used for magnetic measurements are shown in Table 1 (weight of the samples used in the ICP analysis is indicated within the bracket). Both these analysis were carried out on the sample and sample holder after the SQUID measurements were performed.

Table 1. ICP-ES data of RRP3DT and non magnetic sample holder used for SQUID measurements

Sample Iron % (w/w)

Cobalt % (w/w)

Nickel % (w/w)

RRP3DT (65 mg) <0.01 <0.005 0.010 Sample holder (72 mg) <0.003 <0.002 <0.002

SQUID / EPR. An MPMS5S SQUID DC/AC magnetometer was used for SQUID (Superconducting Quantum Interference Device) studies at 1.8 to 300 K. Multiple magnet resets (and magnet settings) were employed to attain small residual magnetic fields. Utmost care was taken for sample preparation for SQUID studies to avoid any magnetic impurity and the samples were prepared by solvent evaporation of drop cast dilute solution of P3DT in toluene / 1,2 dichlorobenzene 1:1 mixture on silicon wafer. After drying at room temperature under reduced pressure for 12 hours, the resulting powder were collected and packed in non magnetic SQUID sample holder for magnetization experiments. During a period of 7 months, the identical sequence of measurements was repeated to verify the robustness of the magnetization of P3DT, which was found stable over that period of time. Point-by-point background correction was used to extract the data for the polymers. For CW x-band EPR (Electron Paramagnetic Resonance) studies on P3DT powder a Bruker E580 EPR spectrometer was used. The EPR data were collected at room temperature.

SBr Br

C12H25 CH3MgBr

THF, Reflux

Ni(dppp)Cl2

RefluxS* *

C12H25

n

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Magneto Optics. Magneto optical studies, in particular Faraday rotation measurements were carried out on thin films of P3DT spin-coated on optically polished BK7 substrates using a Faraday rotation geometry and a solenoid as the source of the AC magnetic field. Details of the setup have been reported previously.11

MAGNETIC PROPERTIES Despite intense effort for over two decades aimed at developing alternatives to traditional magnets, robust ferromagnetism in organic polymers has been elusive.12 These materials are of unparalleled technological importance as they can potentially be used as plastic magnets and magneto-optic devices with the advantage of the flexibility of organic chemistry by allowing for further tuning of their properties.13 Physical realization of such a magnetically ordered polymer system involving only π-electrons is a rare occurrence.14 Various strategies for synthetic tailoring are being investigated to establish the optimized supramolecular structure and packing required to maximize magnetic order.[15] Possibilities for polaronic ferromagnetism have been explored in doped π - conjugated polymers.16 Recently the concept of band ferromagnetism in π - conjugated polymers that exploits the itinerant nature of their π - electrons and structural frustration of electron transfer has been proposed theoretically.17 Although unstable at ambient conditions and thereby not viable for large scale applications, ferromagnetic order temperatures on the order of 0.2 to 40 K have been achieved in carefully designed organic polyradicals with long range order.18 An average spin as high as S = 5400 has been achieved in this class of polyradicals.19 In this section we present our observation of low temperature magnetic ordering of a purely organic polymer, RRP3DT in its pristine state. Magnetic studies of various polythiophene derivatives, doped and pristine, have been reported previously.20 Although it is known that regioregularity improves both optical and electronic properties of pristine polythiophene derivatives, it has never been addressed with respect to magnetic properties.21 RR polythiophene chains stack into an organized 3 dimensional structure giving its optical and electronic excitations some interchain character.22 In polymers with a relatively low degree of polymerization flatter thiophene ring configurations and longer π-conjugation length are facilitated, further increasing at lower temperature.23 Our magnetic polymer, RRP3DT is designed to have a length of 70 monomers with regioregularity for the chain (without the head-group) > 99% as obtained from NMR. Regiorandom P3DT of similar molecular weight was used to compare the effects of regioregularity.

The magnetic field (H) dependence of magnetization (M) of RRP3DT measured at several temperatures (T = 1.8 K - 20 K), shows a very fast rise at low H and reaches near saturation as early as 2 Tesla (Fig. 2). The large remnant magnetization and hysteresis in RRP3DT are suggestive of strong ferromagnetic ordering. To study the dependence of the ferromagnetic ordering on regioregularity a regiorandom (RRa) P3DT was procured from Sigma Aldrich and studied under identical condition. RRaP3DT was found to possess weaker susceptibility at low temperatures and could not be measured at temperatures > 25 K. Although the possibility of an ordered phase below 1.5 K (instrumental limit) cannot be ruled out, clearly, magnetically RRaP3DT behaves very much differently from RRP3DT. RRaP3DT was also found to be weekly paramagnetic with zero remnant magnetization even at 1.5K. Detailed ICP-ES analysis was carried out on various batches of samples (result presented in table 1) to identify any possible magnetic metal impurities and their influence on the magnetization measurements. The metal analyses by ICP-ES suggest the presence of trace amounts of Ni (and Fe, Co) in both the sample and the sample holder. However, their magnetic contribution is very small, if any. For example, Ni in RRP3DT is measured as 0.01 % which converts to a total of 6.5 μg of Ni in 65 mg of P3DT. Saturation magnetization of Ni is 58.46, 58.17 and 54.69 emu/G at 4.2, 77 and 300 °K respectively.24 For 48 mg of P3DT used in SQUID measurements, 4.8 μg of Ni would give us saturation magnetization of 2.8 x 10-4 emu. 48 mg of P3DT gives us a saturation magnetization of 1.152 emu which makes the contribution of the nickel as ~ 0.024 %. Note that the magnetization of nickel has little temperature dependence25 where as magnetization of P3DT diminishes rapidly at about 20 °K. The above data preclude any significant interference from magnetic metals on the reported magnetic behavior for RRP3DT. Also note that the zero remnant magnetization and paramagnetic nature of the regiorandom analogue clearly identifies the magnetization as a manifestation of structural ordering in RRP3DT.

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Fig. 2. Magnetization data as a function of applied magnetic field at 1.8, 4.5, 10 and 20 K shows that the large magnetic remnance at low temperature quickly disappears at higher. Magnetization data collected at other temperatures shows similar trend and are not shown here for clarity.

Fig. 3. X-band (9.19 GHz) EPR spectrum of RRP3DT collected at 300 K and 0.5 mW incident power.

Our recent results indicate that the magnetization of RRP3DT strongly depends on the nanostructures formed in the solids and compares closely with ordered spin glass. The exact origin of the magnetic ordering in P3DT remains an open question and is currently under investigation. Electron spin resonance measurements (Fig. 3) show a spin concentration of 5 1013 spins/g in RRP3DT.[26] Such a low spin concentration cannot account for the large remnant magnetization of RRP3DT. We believe that the nano confinement of itinerant π-electrons of RR polythiophene in an anisotropic structure such as a doughnut may be the main origin. With thermal chaos down to a minimum, spontaneous alignment among the toroidal moments generated from the nano doughnuts is a strong possibility for creation of magnetic ordering in P3DT.[27] The role of frustration in spin polarization on sulphur in thiophene rings and an increased dπ-pπ spin orbit interaction[28] in overall magnetic ordering is another intriguing possibility. Another plausible mechanism is the possibility of coexistence of partially flat and dispersive bands in these strongly correlated electronic systems, resulting in the so-called ‘flat band ferromagnetism’, suggested recently.[6] Such a conjecture can be further supported by recent observation of negative magnetoresistance at low temperatures in a shorter chain polythiophene derivative.[8] On the other hand, at low temperature the entire 3D organization typical of RR polythiophene may have collapsed causing self doping and defect induced residual magnetism. We are at present performing a detailed

RR P3DT

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comparative study of structural, magnetic and electronic properties of RRP3DT in order to determine more precisely the origin of the magnetic behaviour reported here. To our knowledge, there is no other purely organic stable polymer system reported to date that displays magnetic ordering in the nano regime.

MAGNETO-OPTIC PROPERTIES To date, polymers have not been attractive as magneto-optical materials, while efforts are ongoing to realize organic molecular magnets and magneto optic materials based on polymers. In most inorganic crystals and glasses Faraday rotation derives from the presence of a significant density of free spins associated with magnetic ions containing triplet energy states, a mechanism that is not applicable to our polymers. In these so-called “paramagnetic transitions” the time average of microscopic magnetic moments is non-zero at room temperature and is not time reversal invariant.29 The basic mechanisms responsible for MO activity observed in diamagnetic materials involving diamagnetic transitions,30 interband effects within spin-split Landau levels,31 positive free carrier rotation32, and dynamic spin organization of magnetic polarons generated due to defects in semiconductors33 are other known processes that are generally not very common in organic systems. In the electric dipole optical transition regime, the classical diamagnetic transition generally addresses the difference in oscillator strength experienced between the right and left circularly polarized light interacting with the excited multiplet caused by Zeeman splitting. The Zeeman energy, given by ΔE = g μB B, where g is the Landé g factor (2 for P3DT, see fig. 3) and μB is the Bohr magneton, is only of the order of 1 μeV at a field of 100 gauss, and therefore much smaller than the thermal energy kT (~ 25000 μeV) at the room temperature. Faraday rotation derived from degenerate Landau levels follows a similar mechanism to the diamagnetic transitions found specifically in semimagnetic semiconductor systems and can be estimated from the balance of thermal and Zeeman energy. Neither of these two traditional mechanisms allows any possible existence of large Faraday rotation organic polymers. Recently, we have reported large magneto-optic activity (Faraday rotation) in several substituted regioregular polythiophene thin films with Verdet constants on the order of 104 ∼ 105 °T−1m−1, as large as those of commercial inorganic magneto- optical materials like terbium gallium garnet (TGG: V = −7.68 × 104 °T−1m−1).11,34,35 There has been another recent report of large Faraday rotation from poly(aryleneethynylene) thin films indicating towards a supramolecular organization controlled mechanism responsible for the Faraday rotation, similar to our observation on substituted polythiophenes.36 In this paper we explore further evidence of dependence of Faraday rotation on the regioregularity of substituted polythiophenes and the relevance of existing mechanisms within the classical limit in cognizance of our recent magnetic and MO measurements on a set of π-conjugated polymers with different regioregularity.

Table 2. V, Verdet constant (/104 ° /Tm) of polymers RRP3DT and RRP3HT with different extent of regioregularity measured at 532 nm from spun coat films. A standard measurement error of ~ 10% is expected.

Faraday rotation37 is the rotation of the plane of polarization of linearly polarized light due to magnetic field induced circular birefringence of a material. In a non-absorbing or weakly absorbing medium a linearly polarized monochromatic light beam passing through the material along the direction of the applied magnetic field experiences circular birefringence, resulting in rotation of the plane of polarization. The angle of rotation θ is expressed as θ = VBL = π L Δn/λ where Δn is the magnitude of circular birefringence (the difference between the refractive indices for left and right circularly polarized light in the medium), λ is the wavelength of light, B is the magnetic flux density applied to the material parallel to the propagation direction of light and L is the length of the medium. The constant V, the Verdet constant (generally expressed in °Tesla-1meter-1) is a materials property and a figure of merit for the Faraday rotation ability of the material. The Verdet constant is strongly wavelength dependent, decreasing dramatically away from resonance and in the case of a paramagnetic material V also depends on the frequency of the magnetic field used.38 Faraday rotation measurements were carried out on multiple spun coat films of RRP3DT, RRP3HT and their regiorandom counter parts. The Verdet constants calculated from the measured Faraday rotation are shown in Table 2. The regiorandom derivatives are either Faraday inactive or the rotation is below the detection limit of our system and

RRP3DT RRP3HT % Regioregularity >99.9 50 67 72 84 90 95 99

V + 2.1 ~ 0 ~ 0 + 0.64 + 0.79 + 3.5 + 4.7 + 8.8

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within the measurement errors both P3DT and P3HTs with similar regioregularity show quite similar Faraday activity. However, there appears that the Faraday rotation and hence the Verdet constant of P3HTs is a strong function of regioregularity, although a weaker trend compared to that reported in ref. 36. Figure 4 shows the dependence of Verdet constant of P3HTs on regioregularity. The empirical fit indicates that the Verdet constant depends exponentially to the regioregularity. Note that such dependence is valid based on the assumption that the reported value of the regioregularity from the vendors are accurate within the measurement errors and the assumption that all samples had identical processing conditions. These observations further confirm our previous observation that the Faraday rotation in this class of conjugated polymers depends strongly on the supramolecular organization in their thin films.

Fig. 4. Dependence of Verdet constants on regioregularity in RRP3HT thin films. Inset shows the structure of P3HT.

Although the exact mechanism behind such large MO activity from these polymers in thin film remains a topic of active investigation for this group, direct photo generation of an optical exciton or polaron pair presents an interesting possibility.39 These excitons / polaron pairs are known to be strongly dependent on the interchain interactions present in the thin films of π-conjugated polymers, such as the regioregular alkyl substituted polythiophenes. The energy equivalence of the magnetic dipole fields between polaron or exciton pairs are expected to be of the order of 100 gauss which also suggests the possibility of forming spin containing units under such low magnetic field excitations. Possibilities of stronger spin-orbit coupling arising from such spin containing units not only supports our observations of Faraday rotation in these thin films under low magnetic fields and continuous wave excitation, but also explains their dependence on the lamellae type organization and strong interchain interactions. To conclude we have presented our recent observations of magnetic and magneto optic properties of substituted polythiophenes that appears to be correlated on the level of supramolecular organization of the polymer in solid thin films. We plan on further investigating the magnetic and magneto optic properties of the regioregular polythiophenes while also pursuing femtosecond time-resolved photoluminescence studies to ultimately elucidate the origin of the large Faraday effect in these materials.

ACKNOWLEDGEMENTS

We are grateful to the AFOSR under grant FA9550-06-1-0039, the Air force Office of Scientific Research, and to EOARD, The European Office of Aerospace Research & Development under grant FA8655-07-1-3004. We also thank Dr. Charles Lee, AFOSR, for his continuing support and interest in this work.

S

C6H13

n

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