Spin Radical Enhanced Magnetocapacitance Effect in Intermolecular

Oct 21, 2013 - Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States. ‡ Institute of ...
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Spin Radical Enhanced Magnetocapacitance Effect in Intermolecular Excited States Huidong Zang,†,∥ Jianguo Wang,‡ Mingxing Li,† Lei He,† Zitong Liu,‡ Deqing Zhang,*,‡ and Bin Hu*,†,§ †

Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China § Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China ‡

S Supporting Information *

ABSTRACT: This article reports the magnetocapacitance effect (MFC) based on both pristine polymer MEH-PPV and its composite system doped with spin radicals (6R-BDTSCSB). We observed that a photoexcitation leads to a significant positive MFC in the pristine MEH-PPV. Moreover, we found that a low doping of spin radicals in polymer MEH-PPV causes a significant change on the MFC signal: an amplitude increase and a line-shape narrowing under light illumination at room temperature. However, no MFC signal was observed under dark conditions in either the pristine MEH-PPV or the radical-doped MEH-PPV. Furthermore, the magnitude increase and line-shape narrowing caused by the doped spin radicals are very similar to the phenomena induced by increasing the photoexcitation intensity. Our studies suggest that the MFC is essentially originated from the intermolecular excited states, namely, intermolecular electron−hole pairs, generated by a photoexcitation in the MEH-PPV. More importantly, by comparing the effects of spin radicals and electrically polar molecules on the MFC magnitude and line shape, we concluded that the doped spin radicals can have the spin interaction with intermolecular excited states and consequently affect the internal spin-exchange interaction within intermolecular excited states in the development of MFC. Clearly, our experimental results indicate that dispersing spin radicals forms a convenient method to enhance the magnetocapacitance effect in organic semiconducting materials.

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INTRODUCTION Organic radicals and conjugated polymers are two typical types of materials with localized and delocalized π electrons, respectively. The localized spin electrons can generate magnetic properties through spin interactions in radicals,1−3 while the delocalized π electrons are accountable for semiconducting properties through charge transport and excited states in conjugated polymers.4,5 However, the delocalized π electrons normally exhibit unappreciable spin−spin interactions and thus lead to negligible magnetic properties. Naturally, combining localized spins and delocalized π electrons would bring an important promise for the development of multifunctions in organic materials. However, experimental studies have found that combining spin radicals with conjugated molecules often experience difficulties to develop multiple functions due to the lack of desirable spin interactions between localized spin electrons and delocalized π electrons. In this work, we explore a new route to investigate the spin interactions between localized spin electrons and delocalized π electrons by using magnetocapacitance studies through intermolecular excited states based on a spin-radical-doped semiconducting polymer composite. Specifically, we mix spin radical molecules (2,6R-BDTSCSB) with a conjugated polymer (MEH-PPV) through composite design and then study the spin−spin interactions between localized spin electrons from radicals and delocalized π electrons through intermolecular © 2013 American Chemical Society

excited states by characterizing the line shape and magnitude of magnetocapacitance response. Our previous studies have shown that the intermolecular excited states can generate a magnetocapacitance signal, a new phenomenon in the family of magnetic field effects, in organic semicodnucting materials.6 Here, we find through magnetocapacitance measurements that the doped radicals can indeed have spin interactions with intermolecular excited states and consequently lead to a tuning on both magnitude and line shape of magnetocapacitance signal in the radical-doped MEH-PPV.

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EXPERIMENTAL SECTION Synthesis of Radical Molecule 2,6R-BDTSCSB. The spin radicals were prepared by Stille coupling of 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)-benzo[1,2-b:4,5-b′]dithiophene with 2-(4-bromophenyl)-1,3-dihydroxy-4,4,5,5-tetramethyl-imidazolidine7 in DMF at 60 °C in 56% yield, as shown in Scheme S1 (Supporting Information). The radicals 2,6R-BDTSCSB were synthesized as follows: Pd(PPh3)4 (8.0 mg, 0.007 mmol), 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)-benzo[1,2-b:4,5-b′]dithiophene (120 mg, 0.155 mmol), and 2-(4-bromophenyl)-1,3-dihydroxy-4,4,5,5-tetramethyl-imiReceived: August 4, 2013 Revised: October 11, 2013 Published: October 21, 2013 14136

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Figure 1. Absolute (a) and normalized (b) magnetocapacitance curves for pristine MEH-PPV (red) and 5% 2,6R-BDTSCSB-doped MEH-PPV (blue) under photoexcitation (4 mW/cm2). The negligible magnetocapacitance under dark conditions is also shown as a flat line for both pristine and radical-doped MEH-PPV. The insert figure in part b shows the chemical structure of 2,6R-BDTSCSB.

exhibit any magnetocapacitance signal under dark conditions. Second, photoexcitation leads to a magnetocapacitance signal in both pristine and radical-doped MEH-PPV. Third, dispersing radical leads to a line-shape narrowing and a magnitude increase (approximately 7 times) on the magnetocapacitance signal in radical-doped MEH-PPV as compared to the pristine MEH-PPV. These three phenomena clearly indicate that (i) magnetocapacitance is from photoexcitation-generated excited states and (ii) spin radicals can interact with intermolecular excited states in the development of magnetocapacitance response. We know that a low magnetic field can cause an appreciable change on singlet and triplet ratios in intermolecular excited states8−13 but a negligible change on the singlet and triplet ratio in intramolecular excited states (namely, Frenkel excitons) in organic semiconducting materials.13,14 In the pristine MEH-PPV, the intermolecular excited states can be referred to interchain polaron pairs generated by photoexcitation. We should note that changing singlet and triplet ratios can lead to a modification on the electric polarization in the MEH-PPV. This is because the singlets and triplets are assumed to have stronger and weaker electrical polarizations, respectively.15,16 This assumption was made on the basis of the consideration that the coherent antiparallel (singlet) and parallel (triplet) spin precessions can exhibit larger and smaller orbital dipole moments in an electron−hole pair.6,15,16 Nevertheless, changing the singlet and triplet ratios in intermolecular excited states can generate a magnetocapacitance response, namely, a magnetic field effect on dielectric property under excited state. We should further note that the singlet and triplet ratios can be essentially modified when a magnetic field perturbs the balance between spin-conserving process from spin-exchange interaction and spin-random process from internal magnetic interaction (hyperfine or spin−orbital coupling). Changing either spin-exchange interaction or internal magnetic interaction can lead to a new balance between spin-conserving and spin-random processes in intermolecular excited states. A new balance requires a different magnetic field strength to change the singlet and triplet ratios, generating a tuning on the line shape of magnetocapacitance response. Therefore, we can suggest from the line-shape narrowing (Figure 1b) that the dispersed radical decreases either spin-exchange energy or internal magnetic interaction (hyperfine or spin−orbital coupling) in intermolecular excited states in the MEH-PPV. On the other hand, the increase of magnetocapacitance magnitude shown in Figure 1a indicates that the doped radicals can increase the coupling between electric and magnetic polarizations when the applied magnetic

dazolidine (97 mg, 0.31 mmol) in DMF (10 mL) were stirred at 60 °C for 2.0 h. After being cooled to 40 °C, dichloromethane (30 mL) and H2O (20 mL) were added. The organic layer was washed with brine (3 × 15 mL) and dried with Na2SO4. Then, solvents were removed under reduced pressure and the residue was purified by flash column chromatography with dichloromethane/petroleum ether/ethyl acetate (v/v/v, 3/1/0.03 to 3/1/0.2) to afford the titled compound as green solids. MALDI-TOF M/Z ([M−2O+2H] +: 878). ESR: nine peaks with αN = 2.0088 G. Anal. Calcd for C48H48N2O2: C, 68.69; H, 7.54; N, 6.16; S, 7.05. Found: C, 68.86; H, 7.67; N, 6.10; S, 7.00. The cyclic voltammograms, MALDI-TOF, and ESR characterization are provided in the Supporting Information. Device Fabrication and Measurement. The organic semiconducting polymer: Poly[2-methoxy-5-(2-ethylhexyloxy)1,4-phenylenevinylene] (MEH-PPV) used in this work was purchased from Sigma-Aldrich. The devices used for magnetocapacitance studies were fabricated by using ITO (indium− tin−oxide) and Al (aluminum) electrodes to sandwich the active layer, MEH-PPV or radical-doped MEH-PPV film, with the architecture of ITO/active layer/Al. The active layers, pristine MEH-PPV and 5% radical-doped MEH-PPV, were spin-cast on precleaned ITO substrates with a thickness of 100 nm from chloroform solution. The Al electrodes were thermally deposited on the active layers under the vacuum of 7 × 10−7 Torr with a thickness of 80 nm. The effective device area is 0.05 cm2. The magnetocapacitance measurements were performed on the fabricated devices located in an electrically controllable magnetic field by using an Agilent E4980A LCR meter with an AC bias of 50 mV at 1 kHz (DC bias = 0). The laser beam of 488 nm from an argon ion laser was used as the photoexcitation perpendicularly through the ITO side to excited radical-doped MEH-PPV in magnetocapacitance measurements. The electrically controllable magnetic field was applied parallel to the device plane. The magnetocapacitance, defined as the magnetic field effect on capacitance change, was given by ΔC/C = (CB − C0)/C0, where CB and C0 are the capacitance with and without magnetic field, respectively. All the measurements were performed at room temperature in a nitrogen atmosphere.

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RESULTS AND DISCUSSION Figure 1 shows the magnetocapacitance characteristics from two different systems under photoexcitation: pristine MEHPPV and radical (2,6R-BDTSCSB)-doped MEH-PPV. The results in Figure 1 indicate three interesting phenomena. First, the pristine and radical-doped MEH-PPV samples do not 14137

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field changes the singlet and triplet ratios in intermolecular excited states in the MEH-PPV. Figure 2 shows the magnetocapacitance curves for pristine MEH-PPV at different photoexcitation intensities. We can see

the interaction between intermolecular excited states can cause the line-shape narrowing by reducing the value of B0 through two possible mechanisms: (i) spin-exchange interaction or (ii) magnetic interaction from hyperfine or spin−orbital coupling. We know that the magnetic interaction within an intermolecular excited state is mainly from hyperfine coupling due to the absence of heavy atomic elements.17−20 Increasing the interaction between intermolecular excited states would have an unappreciable effect on the hyperfine coupling. In addition, the hyperfine related magnetic field effects occur only at low field below 10 mT,21,22 and our magnetoacpacitance effects were measured at higher fields above hyperfine interaction. Thus, we can remove the possibility that internal magnetic interaction can be a reason for the line-shape narrowing. It can be therefore argued that the spin-exchange interaction is accountable for the line-shape narrowing of magnetocapacitance response when the density of intermolecular excited states is increased by photoexcitation. We know that the intermolecular excited states are largely separated electron−hole pairs with extended wave functions. In principle, the interaction between intermolecular excited states can consist of electric and magnetic components. The electric component is from electric dipole−dipole interaction when an intermolecular excited state is considered as an electric dipole. The magnetic component comes from intercharge spin interaction occurring between two adjacent intermolecular excited states. In general, both electric and magnetic components in the interaction between intermolecular excited states can affect the spin-exchange interaction within intermolecular excited states and then change the line shape of magnetocapacitance response. Specifically, the electric component in the interaction between intermolecular excited states can decrease the spin-exchange interaction within intermolecular excited states through Coulomb screening effects or redistribution of individual electrical dipole fields. The magnetic component in the interaction between intermolecular excited states can induce interstate spin scattering or redistribution of individual spin dipole fields and consequently reduces the spin-exchange interaction within intermolecular excited states. Nevertheless, increasing the interaction between intermolecular excited states can lead to a line-shape narrowing on magnetocapacitance response through electric interstate dipole−dipole interaction or intercharge spin−spin interaction. By considering the similarity on line-shape narrowing and magnitude increase caused by doping and photoexcitation intensity, we can see that the radicals experience an interaction with intermolecular excited states in the doped MEH-PPV. Now, we discuss the interaction between spin radicals and intermolecular excited states. It is known that a radical molecule (2,6R-BDTSCSB) carries both electric and magnetic dipoles (Figure 3). When a radical molecule is placed between two intermolecular excited states, the electric dipole from a radical molecule may electrically polarize the wave function of an adjacent intermolecular excited state and consequently enhances the electric component in the interaction between two adjacent intermolecular excited states. On the other hand, the magnetic dipole from a radical molecule may have spin interaction with the electron and hole in an adjacent intermolecular excited state. Here we further explore the effects of electric and magnetic dipoles on the line shape and magnitude of magnetocapacitance response by introducing an electrically polarizable molecule, camphoric anhydride (CA), into the MEH-PPV with a weight ratio of 5 wt

Figure 2. Normalized magnetocapacitance curves for pristine MEHPPV at different photoexcitation intensities. The inset figure shows the real amplitudes.

that increasing photoexcitation intensity causes a line-shape narrowing as well as a magnitude enhancement in the pristine MEH-PPV, very similar to the phenomena caused by doping in the radical-doped MEH-PPV. It is known that increasing photoexcitation intensity can largely increase the density of intermolecular excited states. Therefore, we can suggest that the density of intermolecular excited states is accountable for both line-shape narrowing and magnitude increase in the pristine MEH-PPV upon increasing photoexcitation intensity. The magnitude enhancement can be attributed to two factors: the density of intermolecular excited states and the interaction between intermolecular excited states, namely, density and interaction effects. For the density effect, the dielectric constant can be divided into two parts related to ground and excited states, namely, εg and εe, in a material. The dielectric constants εe and εg are sensitive and in-sensitive to magnetic field, respectively. Increasing the density of intermolecular excited states can inevitably increase the part of excited-states-related dielectric constant εe. As a consequence, increasing the density of intermolecular excited states can lead to an increase of the magnetocapacitance magnitude in the pristine MEH-PPV upon increasing photoexcitation intensity. For the interaction effect, the interaction between intermolecular excited states can contribute to the increase of magnetocapacitance magnitude through the electric−magnetic polarization coupling mechanism when the density of intermolecular excited states is increased by photoexcitation intensity. For line-shape narrowing, we use a non-Lorentzian function (B2/(B2 + B02)) as a convenient model to discuss how the density of intermolecular excited states can change the line shape of magnetocapacitance response. In this non-Lorentzian function, we can see that the value of B0 determines the line shape of magnetocapacitance response. We know that a magnetic field effect can be developed when an applied field changes the singlet/triplet ratio by disturbing the balance between spin-exchange interaction or internal magnetic interaction. The parameter B0 can contain the contribution from both spin-exchange interaction and internal magnetic interaction (from hyperfine or spin−orbital coupling). In particular, the parameter B0 determines the line shape in magnetic field effects. Naturally, 14138

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changing rate of singlet/triplet ratio in magnetic-field-dependent singlet−triplet conversion in intermolecular excited states. Obviously, the electric−magnetic coupling constant c and the changing rate ∂(S/T)/∂B of singlet/triplet ratio determine the magnitude and line shape of magnetocapacitance response, respectively. On the basis of this analysis, the magnitude enhancement in magnetocapacitance response reflects that the electric−magnetic coupling constant c can be increased by the interaction between intermolecular excited states upon dispersing spin radicals. We know that the electric−magnetic coupling constant c is determined by the change in electrical polarization caused by the modification of singlet/triplet ratio upon applying a magnetic field. Therefore, we can suggest that spin radicals can increase the modification of singlet/triplet ratio in intermolecular excited states at a given magnetic field. From the line-shape narrowing in magnetocapacitance response, we can argue that the interaction between intermolecular excited states can increase the changing rate (∂(S/T)/∂B) of singlet/triplet ratio with dispersing spin radicals. Here, we introduce the force constant (∂2U/∂B2) of singlet−triplet intersystem crossing to describe the changing rate (∂(S/T)/∂B) of singlet/triplet ratio, where U is the spinexchange energy. Clearly, our experimental results of line-shape narrowing suggests that the magnetic component in the interaction between intermolecular excited states and spin radicals can weaken the force constant of magnetic-fielddependent singlet−triplet intersystem crossing by redistributing the spin dipole fields within intermolecular excited states. On the other hand, the reduction of the force constant of magneticfield-dependent singlet−triplet intersystem crossing can increase the changing rate of singlet/triplet ratio at a given magnetic field. This can reflect as an increase on the electric− magnetic coupling constant c in the development of magnetocapacitance response. Nevertheless, dispersing spin radicals can lead to a line-shape narrowing and a magnitude increase in the magnetocapacitance response through spin interaction with intermolecular excited states in organic smeiconducting materials under photoexcitation.

Figure 3. Interaction between a spin radical with intermolecular excited state (singlet and triplet) to decrease the spin-exchange interaction within an intermolecular excited state and to increase the electric interaction between two adjacent intermolecular excited states.

%. The CA molecules have a large electric dipole moment around 5 D.23,24 The photoluminescence studies have found that with the large electric dipole the CA molecules can electrically polarize organic molecules without introducing spin polarization.24 Figure 4 shows that the magnetocapacitance curve for CAdoped MEH-PPV as compared to pristine MEH-PPV. We can

Figure 4. Normalized magnetocapacitance curves for CA-doped MEH-PPV and pristine MEH-PPV under photoexcitation (25 mW/ cm2). The inset shows the real magnitudes and the chemical structure of CA molecule.

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CONCLUSIONS In summary, we find that dispersing spin radicals (2,6RBDTSCSB) into semiconducting MEH-PPV causes a tuning on magnetocapacitance signal: line-shape narrowing and magnitude increase in the radical-doped MEH-PPV, very similar to the phenomena caused by increasing photoexcitation intensity in the pristine MEH-PPV. This experimental finding indicates that spin radicals increase the interaction between intermolecular excited states. On the other hand, we observe opposite phenomena upon doping electrically polarizable CA molecules: line-shape broadening and magnitude reduction on magnetocapacitance response in the CA-doped MEH-PPV. The distinct effects caused by spin radicals and electrically polarizable molecules suggest that spin radicals can have spin interaction with intermolecular excited states in the development of magnetocapacitance response. More importantly, the spin interaction between spin radicals and intermolecular excited states can lead to an increase in the electric−magnetic coupling constant and a decrease in the force constant of singlet−triplet conversion. This generates a magnitude increase and a lineshape narrowing on the magnetocapacitace signal. Therefore, our experimental studies indicate that combining spin radicals with intermolecular excited states forms a new method to

see that the electrically polarizable CA molecules cause a lineshape broadening and a magnitude decrease in the magnetocapacitance characteristics. Clearly, spin radicals and electrically polarizable CA molecules generate opposite effects: narrowing and broadening on line shape but increase and decrease on magnitude. Obviously, this difference can be attributed to spin dipoles of radical molecules (2,6R-BDTSCSB). Therefore, the comparison between spin radicals (2,6R-BDTSCSB) and electrically polarizable (CA) molecules confirms that the dispersed radicals can have spin interactions with intermolecular excited states and consequently causes a line-shape narrowing and magnitude enhancement in the magneticapacitance response from intermolecular excited states in the MEH-PPV. When a magnetic field perturbs the singlet/ triplet ratio, the dielectric constant can be approximately written as ε = k·(S/T), where k is a constant and S/T is the singlet/triplet ratio. The magnetocapacitance effect developed by magnetic-field-dependent singlet/triplet ratio can then be given by ∂ε/∂B = C·(∂(S/T)/∂B). We should note that the constant c reflects a coupling between electric and magnetic polarizations when a magnetic field modifies the singlet/triplet ratio in intermolecular excited states. The ∂(S/T)/∂B is the 14139

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develop magnetocapacitance effects in organic semiconducting materials.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic route for compound 2,6R-BDTSCSB and cyclic voltammogram, MALDI-TOF spectrum, and ESR spectrum of 2,6R-BDTSCSB. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address ∥

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, United States. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors (H.Z. and B.H.) acknowledge the funding support from the NSF (ECCS-0644945) and AFOSR (FA9550-11-10082).

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