Chem, Volume 4

Supplemental Information

Organic Ferromagnetism:

Trapping Spins in the Glassy State

of an Organic Network Structure

Javeed Mahmood, Jungmin Park, Dongbin Shin, Hyun-Jung Choi, Jeong-Min Seo, Jung-Woo Yoo, and Jong-Beom Baek

Supplemental Information

Table of Contents

Supplementary Figures (S1-S17)

Supplementary Tables (S1-S3)

Supplementary References

Figure S1. Digital photographs of the reaction and schematic representation of the formation of p-TCNQ. (A) TCNQ dissolved in TFMSA after stirring at room temperature before self-polymerization. (B) Complete gelation of the reaction mixture after self-polymerization at 155 °C overnight. (C) Schematic presentation of the p-TCNQ framework with edge groups after aqueous quenching.

Figure S2. XPS survey spectrum from the p-TCNQ, showing C 1s, N 1s and O 1s peaks. Detailed element composition is summarized in Table S1.

Figure S3. Thermal stability and XRD pattern. (A) TGA thermograms of p-TCNQ with ramping rate of 10 °C min-1 in nitrogen and air atmosphere. (B) Powder XRD pattern of p-TCNQ exhibiting a broad peak

centered at 224.9°, implying local ordering in the amorphous matrix of the material.

Figure S4. SEM images of the p-TCNQ at different magnification showing compact and irregular morphology. (A) Lower magnification. Scale bar is 100 µm. (B) Higher magnification. Scale bar is 50 µm.

Figure S5. Solid-state ESR spectrum of the p-TCNQ measured at −100 °C. Compared with ESR spectrum obtained at ambient condition (Figure 1D), the peak intensity at low-temperature is higher, indicating the stability of free-radicals in p-TCNQ. The generated free radical (i.e., electron) is free to move over all the orientation. The p-TCNQ does not exhibit hyperfine interaction; the absence of hyperfine line in the p-TCNQ could be related to the EPR spectra at X-band and lower frequencies fall near the rapid tumbling limit.1 We should take into account the important factors of spin relaxation induced by tumbling motion of the free radical. The sharp symmetrical isotropic signal without hyperfine structure may be caused by the hopping of free radical centre around the triazine rings. The hopping of radical centre is fast enough because of the conjugated system.2,3

Figure S6. Solid-state NMR spectra of p-TCNQ: As-prepared sample, and after annealing at 370, 400 and 450 °C at the same experimental condition.

Figure S7. ESR study of the sample annealed at different temperatures. (A) ESR spectra after annealing p-TCNQ: room-temperature measurements after annealing at 370, 400 and 450 °C at the same experimental condition. (B) Total integrated ESR intensity after annealing 370, 400 and 450 °C. The results are in good agreement with the solid-state 13C-NMR (see Figure S6) and saturation magnetization shown in Figure 4A.

Figure S8. Magnetization of p-TCNQ as a function of field recorded at 300 K. Dark red line is the raw experimental data of magnetization. Dark blue line is magnetization after subtracting linear diamagnetic background. Inset displays low field hysteresis loop. The diamagnetic background was estimated based on the slope of M(H) at high magnetic field (H > 7T) and 300 K. The obtained diamagnetic susceptibility M/H was around −2.2×10−7 emu/(Oe∙g), which was reasonable value for typical diamagnetism. The mass of sample used was 100 mg. The sample was kept in gelatin capsule. The total mass of diamagnetic background (sample + gelatin capsule) was 140 mg. In order to confirm our procedure of background subtraction, we also measured the diamagnetism of empty sample holder (gelatin capsule). The measured diamagnetism was less than that estimated from high field data at 300 K. This discrepancy is originated from the additional mass of carbon and nitrogen in our polymerized sample. Thus, throughout the manuscript, we performed diamagnetic background subtraction by using high field data at 300 K.

Figure S9. Magnetization of p-TCNQ as a function of field recorded at 300 K. Dark blue line is for data recorded two weeks after initial measurement and dark green line is for data recorded five weeks after initial measurement.

Figure S10. Field-cooled magnetization of p-TCNQ as a function of temperature measured at 10 Oe (dark

red squares). Black line is fit to conventional critical behavior, with recorded data between

350 K and 395 K. The estimated Tc is ~ 495 K. The best fitting parameter for the critical exponent was 0.48, which is close to the value of the mean field theory.

)( TTc

Figure S11. Determination of the angular momentum quantum number J via Brillouin function fit: (A) The estimated Mpara component as a function of field H at 5 K (black symbols). Fitting to Brillouin function with J =1/2, 1, 3/2 are indicated with solid lines, respectively. The g value was assumed to be 2 in fitting procedure. Only J = 1/2 produces excellent Brillouin function fit to the paramagnetic component Mpara. (B) The estimated Mpara component as a function of field H at different temperatures (T = 2, 3, 5, and 10 K). Dotted cyan curves are fits to Brillouin function with J = 1/2 and g = 2.

Figure S12. The temperature dependence of field-cooled (FC) vs. zero-field-cooled (ZFC) magnetizations of p-TCNQ. The measurements were conducted with different applied fields: (A) H = 10 Oe. (B) H = 100 Oe. The magnetization exhibited strong irreversibility, given by the divergence of the FC and ZFC magnetization curves below a bifurcation temperature (Tirr). The ZFC magnetization exhibited a maximum at a field-dependent temperature Tmax < Tirr and continuously decreased below this temperature. The bifurcation was shifted to lower temperature as H was increased. Strong irreversibility and the shift of Tirr to lower T with increasing H are signatures of spin-glass like behavior. However, this behavior is not restricted to the spin-glass, because magnetic anisotropy can also induce a strong bifurcation between FC and ZFC magnetization. To determine if the divergence between FC and ZFC magnetization originated from spin-glass order, we need to obtain field dependent Tmax, which follows Tmax(H) = a – bH2/3 for a canonical spin-glass. Unfortunately, we were not able to justify this relation, because of a high magnetic transition temperature along with a structural modification through the structural-glass transition. The FC and ZFC measurements for defining Tmax at a given magnetic field should be executed with cooling the sample from above the magnetic transition temperature. Due to the instrumental limitation (2-400 K) and the structural modification when heating the sample over 400 K, we were not able to perform proper FC vs ZFC measurements. We performed FC vs ZFC measurements with cooling the sample from 400 K, which was sort of a temporary expedient. In addition to the irreversibility, FC magnetization displays a slight reentrant spin-glass behavior at below 200 K, i.e., the decrease of magnetization as lowering the temperature. This behavior is a transition from ferromagnetic to spin-glass order, following the Sherrington-Kirkpatrick phase diagram.4,5 It is likely that our sample retains randomly distributed spin hosts, leading to the spin-glass-like behavior.

Figure S13. Determination of total saturated magnetization. (A-D) M vs. H plots using parameters obtained from Brillouin function fit to the measured data as shown in the inset. We extracted Ms value using Brillouin function fit. First, we subtracted Ms, ferro components from M(H, 5K) curves resulting in Mpara(H, 5K). Here, Ms, ferro component was determined from M vs. H curve measured at 300 K. Then, we performed Brillouin function fit to Mpara(H, 5K). The obtained fitting parameter allows us to get Ms, para as displayed in (A-D). Finally, we added Ms, ferro and Ms, para to get the total saturation magnetization as displayed in Figure 4A.

Figure S14. Formation energy variation depending on the number of 90-degree rotation of cyclohexadiene (CHD) ring and spin configuration. Optimized geometry of one triazine ring formed p-TCNQ structural unit with 90-degree rotation: (A) 0-rotated, 3-planar CHD rings, (B) 1-rotated, 2-planar CHD rings; (C) 2-rotated, 1-planar CHD rings; (D) 3-rotated, 0-planar CHD rings. To understand the formation of ferromagnetic ordering in p-TCNQ, we compared the formation energy with spin-unpolarized and spin-polarized calculations as shown in (A). In the (B) geometry, there is no formation energy difference between both cases. The result indicates that this non-magnetic spin ordering is more energetically favorable by π-conjugation than ferromagnetic configuration. On the other hand, the presence of 90-degree rotated phenyl rings makes a difference on the spin configurations of electronic structure by the formation of radical carbons. The presence of 90-degree rotated phenyl rings gives more energetically stable spin-ordered electronic structure than non-magnetic spin configuration by 120-degree spin exchange with triazine ring. These results indicate that the rotation of benzene ring generates radical carbon states, which form energetically stable ferromagnetic spin order.

Figure S15. Formation energy of various geometries with respect to the number of triazine rings. (A) Formation energy of various p-TCNQ geometries with neighboring triazine rings. Optimized geometry of p-TCNQ with respect to the number of triazine rings: (B) 1; (C) 2; (D) 3; (E) 4; (F) 5; and (G) 6 triazine rings. To analyze the geometry configuration in p-TCNQ, we evaluated the formation energy of p-TCNQ with geometries of neighboring triazine rings, which are connected by bridging carbon atoms. We found that the formation of p-TCNQ with serially neighboring 4 triazine rings is more energetically stable than the other cases. This result shows that p-TCNQ has energetically more geometric configurations of neighboring 4 triazine rings on its local geometry than other configurations.

Figure S16. Potential energy surface related to the rotation of one cyclohexadiene ring geometries (up to 90-degree). (A) 2 triazine rings; (B) 3 triazine rings; (C) 4 triazine rings and (D) 5 triazine rings. Left and right figures indicate the initial and final geometries, respectively. The red dashed circles indicate the geometry variation sites.

Figure S17. Optimized geometry of two triazine rings of p-TCNQs stacked through a van der Waals type intermolecular interaction. Another possible mechanism of ferromagnetic ordering is intermolecular ferromagnetic spin ordering via van der Waals distance. In the p-TCNQ system, the two p-TCNQ fragmented triazine rings can be facing each other with 3.8Å distance as shown in (A). (B) Ferromagnetic spin ordered spin density. (C) Anti-ferromagnetic spin ordered spin density. Red and yellow isosurfaces indicate spin-up and -down states, respectively. When two triazine rings interact with three radical carbon states around them, their ferromagnetic spin ordering is more stabilized by −22 meV/TCNQ than its anti-ferromagnetic ordering at this geometry configuration. The results reveal that there are two types of plausible mechanism for ferromagnetic ordering in p-TCNQ: one is intramolecular spin ordering by neighboring traizine rings with three radical carbons and the other is intermolecular spin ordering using van der Waals space.

Table S1. Elemental composition of the p-TCNQ from different characterization techniques

Technique C H N O Total

Theoretical (wt.%) 60.57 2.45 13.08 23.91 100

EA (wt.%)a 61.42 3.00 9.57 25.49 99.48

XPS (at.%)b 76.97 ---- 7.46 15.57 100

a EA is most reliable element counts for bulk sample.

b XPS is more sensitive to surface chemical composition.

Table S2. Analysis of metallic impurities in the p-TCNQ determined by ICP-MS

Elements Mg Al V Cr Mn Fe Co

Concentration

(ppm) 0.00306 0.00390 0.00672 0.00401 0.000121 0.00619 0.0242

Elements Ni Cd In Pb B Ti Zr

Concentration

(ppm) 0.000288 0.0237 0.00514 0.00014 0.0894 0.0326 0.00132

Table S3. The classification of sampled fragment geometries

Sample

group

Number of

triazine rings

Number of

phenyl rings

Number of

sampled

geometry

Number of distorted geometries around

triazine ring (Number of sampled

geometry)

A 2 5 4 0(1), 1(2), 2(1)

B 3 7 7 0(1), 1(3), 2(2), 3(1)

C 4 9 12 0(1), 1(4), 2(3), 3(3), 4(1)

D 5 11 11 0(1), 1(3), 2(3), 3(2), 4(1), 5(1)

References

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