2.1 Fabrication and Structural Characterizations

The reduction and dissolution treatment of 2NH₂-PBI was performed via the hydrothermal method with an excess hydrazine hydrate at 120°C for 12 h. After cooling, a purple-red suspension was obtained. Subsequently, the solvent was heated and evaporated in a glove box (H₂O < 1 ppm and O₂ < 4 ppm) to prepare r-2NH₂-PBI powder (Figure 1A). The powder exhibited a closely packed morphology of nanorods (Figure 1B). The prepared film exhibited peaks at 810 and 965 nm, facilitating the identification of the generation of r-2NH₂-PBI radical anions (Figure 1C) [31]. EPR spectroscopy was utilized to investigate the formation of radical anions in the r-2NH₂-PBI powder. A strong resonance signal was observed near g = 2.003, originating from unpaired electrons, indicating a sharp increase in radical anions (Figure 1D). The X-ray diffraction (XRD) pattern of the prepared r-2NH₂-PBI powder is presented in Figure 1E. The peak at 27.7° corresponds to π–π stacking with a calculated interplanar distance of 3.22 Å (shorter than van der Waals contact), while the 11.6° peak aligns with edge-to-face stacking [32, 33]. Compared to the pristine sample, the slightly increased π–π interlayer spacing after reduction results from the insertion of cations and electrostatic repulsion arising from electronic interactions between functional groups and ions. Given the repulsive forces between anions, the ability to maintain a closely packed structure in the ionic state indicates the presence of strong π–π interactions between molecules. The intense interaction between polarized π-electrons favors magnetic coupling. After reduction treatment, the relative intensity ratio of these peaks (I_27.7°/I_11.6°) increases from 0.3 to 1.0, accompanied by a reduction in full width at half maximum (FWHM) for the 27.7° peak. These observations collectively suggest that the reduction process significantly enhances the π–π stacking, establishing it as the dominant structural motif with improved long-range ordering.

2.2 Magnetic and Semiconducting Properties

We investigated the semiconductor properties and magnetic behaviors of r-2NH₂-PBI using the physical property measurement system. The magnetization-magnetic field (M-H) curve at 300 K exhibits a representative ferromagnetic hysteresis loop (Figure 2A). The magnetization reaches saturation at 2 kOe with a coercive field of 170 Oe. The extracted M_s at 300 K is 0.8 emu g⁻¹ (336.3 emu mol⁻¹), which is notably large among organic magnets, significantly exceeding that of weak ferromagnets in porous organic radical frameworks linked by 1,3,5-triazine [34], and comparable to previously reported PBI ferromagnetism [17]. In contrast, the linear M-H curve of pristine 2NH₂-PBI shows diamagnetic characteristics, strongly suggesting that the ferromagnetic signal arises from the reduction process. Furthermore, hysteresis loops were measured within a temperature range of 10–300 K (Supporting Information S1: Figure S1). The temperature dependences of M_s and the coercivity field indicate ferromagnetic behavior above room temperature (Figure 2B). The temperature dependences of magnetization under zero-field-cooled (ZFC) and field-cooled (FC) conditions at a magnetic field of 100 Oe are shown in Figure 2C. The ZFC magnetization curve exhibits a maximum near 100 K. Furthermore, the splitting of ZFC and FC curves below about 300 K, indicating a T_c value above 300 K for the ferromagnetic r-2NH₂-PBI powder. We further observed that after approximately 1 month of continuous exposure to air (Figure 2D), the ferromagnetic property of r-2NH₂-PBI gradually transformed into diamagnetism. Due to the natural chemical activity of free radicals, the previously reported organic ferromagnetic crystals are generally unstable. For instance, tetrakis(dimethylamino)ethylene-C₆₀ will ultimately lose its magnetic properties after exposure to air for just a few seconds [35]. Compared with such organic ferromagnets with low T_c, our ferromagnetic powder exhibits more excellent stability in the air. The total content of magnetic metallic impurities is 28 ppm (Figure 2E), and the magnetization intensity calculated based on this content would not exceed 6 × 10⁻⁵emu g⁻¹ (Fe = 2.2 μ_B, 218 emu g⁻¹; Co = 1.7 μ_B, 161 emu g⁻¹). This value is four orders of magnitude smaller than the measured saturation magnetization, conclusively excluding extrinsic ferromagnetism from impurities. Figure 2F depicts the temperature-dependent resistance curve exhibiting typical semiconductor characteristics. With the increased air exposure time, the electrical conductivity (σ) increased first and then decreased. The maximum is 0.26 S cm⁻¹ (Supporting Information S1: Figure S2).

2.3 Origin of Magnetism

To elucidate the elemental contributions to the observed ferromagnetism, we employed the XMCD technique. This powerful technique, illustrated schematically in Figure 3A, involves a two-step process. First, circularly polarized X-rays with their helicity vector aligned parallel (or antiparallel) to the orbital moment excite electrons with a preferred spin orientation. Subsequently, these excited electrons occupy available states in the unoccupied valence band, resulting in a net positive or negative peak in the XMCD spectrum, depending on the relative abundance of spin-up and spin-down holes [20, 36]. Figure 3B depicts the room-temperature X-ray absorption spectroscopy (XAS) and XMCD spectra at the C K-edge, with the X-ray incident on the film at a 90° angle and an applied magnetic field strength of 3 kOe. The resonance peak located in the 280–290 eV region corresponds to the π* resonance of the perylene core, while the resonance peak in the 290–300 eV region is primarily attributed to the σ* resonance [37, 38]. The first peak at 284 eV is associated with transitions from the C 1s level of carbon atoms in the perylene core to the lowest unoccupied molecular orbital (LUMO, L0). The peak at 285.4 eV belongs to transitions from core levels to higher excited energy orbitals, L1–L3. The peaks near 287.2 and 288 eV can be assigned to transitions from carbon atoms on the imide moiety to L0 and L1–L3, respectively. The XMCD signals were acquired by the difference in the normalized X-ray absorption spectra recorded with right- and left-circularly polarized beams. A nonnegligible C XMCD signal was observed. The strength of XMCD reached ~8.7% by the division between amplitudes of XMCD (0.075) and XAS (0.86). The signal intensity is comparable to the Co L-edge XMCD signal (5%) observed in Co(Cp)₂/SnS₂ [39], underscoring the significance of our findings. Crucially, XMCD signals are exclusively observed in the π* region (280–290 eV) with no detectable magnetic dichroism above 290 eV (σ* region), confirming that the magnetism originates from the polarized π-conjugated electrons within the r-2NH₂-PBI perylene core.

To visualize the spin density of the monomer that constructs this OFM semiconductor, a NH₄-doped r-2NH₂-PBI radical model (r-2NH₂-PBI-NH₄) is calculated at M05-2X/def2-TZVP theoretical level. The spin density distributions and Mulliken spin population of r-2NH₂-PBI-NH₄ are displayed in Figure 3C. It can be seen from the spin density distributions that the oxygen atoms carry an excess of α-electron density, and the total magnetic moment on these atoms is 0.20 μ_B. The spin density distributions on the perylene core follow the spin alternation rule, which means that the spins on neighboring atoms are preferred to be successively antiparallel due to spin polarization. It can be concluded that the unpaired electron of r-2NH₂-PBI-NH₄ is distributed over the π-orbital according to the shape of the isosurface. It is delocalized over the whole radical, and the most significant magnetic moment of the carbon atoms is 0.13 μ_B. Moreover, the total magnetic moment on carbon atoms is 0.83 μ_B, indicating that the spin density contains a significant contribution from the atomic orbitals of carbon atoms compared to that of oxygen atoms. These computational results are in excellent agreement with our XMCD experimental findings, providing a theoretical foundation for understanding the origin of ferromagnetism in r-2NH₂-PBI. A π-dimer (pimer) model consisting of two r-2NH₂-PBI-NH₄ radicals was established to theoretically investigate the origin of ferromagnetism observed in ferromagnetic powders without anionic radical crystal structure characterization. Various π-stacking patterns of r-2NH₂-PBI-NH₄ pimers were generated and optimized, yielding six distinct pimers. The definitions of the structural indicators for r-2NH₂-PBI-NH₄ radical pimers are illustrated in Supporting Information S1: Figure S3. The most representative pimers are summarized in Figure 3D, with the other structures shown in Supporting Information S1: Figure S4. By comparing the relative total energies of the pimers, it is found that Pimer1 has an energy 0.4 kcal mol⁻¹ lower than Pimer2, which may be attributed to the charged hydrogen bonds between the ammonium and the oxygen of the carbonyl group in Pimer1. The relative total energies of the other pimers are at least 3 kcal mol⁻¹ higher than these two, leading to low Boltzmann populations at room temperature. Pimer3 and Pimer6 exhibit similar stacking structures but have significant energy differences due to different cationic positions. Pimer4 and Pimer5 possess distinct twist angles corresponding to different spin states. The superposition of slipping and twisting enhances the transition of magnetic coupling interaction from antiferromagnetic to ferromagnetic. The calculation results indicate that the relative positions between r-2NH₂-PBI-NH₄ radicals significantly influence the magnetic coupling constant, with the interaction between radicals in Pimer1 being ferromagnetic. This could be due to the slipping and twisting that reduce the Pauli repulsion between electrons, making the ferromagnetic state's electron energy (+28.8 cm⁻¹) lower than that of the antiferromagnetic state. However, in Pimer2, the parallel stacking of r-2NH₂-PBI-NH₄ radicals increases the orbital overlap area, leading to enhanced singly occupied molecular orbital (SOMO)-SOMO interactions and resulting in antiferromagnetic coupling interactions between radicals (−104.1 cm⁻¹). It should be noted that due to the simplification of interactions between adjacent radicals and pimer, some quantitative discrepancies may exist compared to experimental results regarding the obtained magnetic coupling interaction.

2.4 LFMA

A typical EPR spectrum of r-2NH₂-PBI powder at room temperature is presented in Figure 4A. Compared to the quiescent background, three distinct resonance signals are observed for the ferromagnetic r-2NH₂-PBI powder: (1) the primary resonance with g = 2.003, (2) a broad near-zero-field resonance, and (3) a narrow zero-field non-resonance. The appearance of LFMA is particularly significant, as it is associated with the onset of magnetic ordering and serves as a sensitive indicator of such ordering [25, 26]. Figure 4B illustrates an apparent hysteresis of this signal on cycling the field. By comparing the M-H hysteresis loop, we observe that the field range exhibiting absorption hysteresis corresponds directly to that associated with the magnetization reversal process. The hysteretic nature of the LFMA signal, typically encountered in domain structures subjected to DC fields below the saturation field, implies a strong connection to the spin responses of various irreversible domain configurations as the DC field is cycled [40]. The hysteresis effect of the LFMA signal appears to be due to nonuniform surface magnetization processes. A ferromagnetic conducting system's absorption of electromagnetic radiation is influenced by domain structure, anisotropy, radiation orientation, conductivity, frequency, and amplitude. This absorption can be adjusted using H_DC, which modifies magnetic properties, domain structures, and spin dynamics, resulting in hysteresis effects observed in domain configurations under sub-saturation DC fields [27]. Interestingly, we observed that rotating the sample results in a diminution of the LFMA signal (Figure 4C). This angular dependence implies the presence of shape anisotropy in r-2NH₂-PBI samples, a characteristic that could be harnessed in future device designs. Figure 4D illustrates the EPR spectra of ferromagnetic r-2NH₂-PBI powder as a function of temperature within the range of 295–115 K. It can be observed that the peak intensity gradually decreases as the temperature decreases. In contrast to the angle-dependent EPR spectra, no significant changes in the shape and position of the EPR spectra are evident across the entire temperature range. This is markedly different from the broadening of linewidth and the shift of resonance field observed in inorganic ferromagnetic materials such as zinc-cobalt oxide [26] and AgMn_xSb_y [29]. As shown in Supporting Information S1: Figure S5, the LFMA intensity of r-2NH₂-PBI exhibits a non-monotonic temperature dependence (295–455 K): the initial increase (295–335 K) may arise from thermally enhanced spin dynamics, while the subsequent decrease (335–455 K) could reflect progressive domain wall relaxation, potentially linked to reduced exchange coupling at higher temperatures [41]. These trends suggest a competition between thermal activation and magnetic ordering decay, which aligns with prior observations of magnon-driven dynamics in systems with Dzyaloshinskii–Moriya interactions [42].

From the perspective of the fundamental principles of magnetism in physics, the prerequisites for designing organic molecular materials with bulk magnetic properties are (i) persistence of spin units. Perylene diimide derivatives possess an extensive planar π-conjugated system, which favors the stabilization of radicals and promotes the delocalization of spin density [18]. Utilizing large π-conjugated structures to stabilize radicals provides a stable source for magnetism. (ii) Establishment of magnetic coupling pathways between adjacent spin units. π-conjugated radicals with face-to-face geometry and short π–π distances for atomic-atomic interactions can form strong SOMO-SOMO interactions comparable to covalent bonds [43-45]. The r-2NH₂-PBI radicals form nanorod aggregates with a π–π distance of just 3.22 Å, shorter than typical van der Waals distances. The synergistic effect of strong intermolecular π–π stacking and hydrogen bonding enhances the spin electron exchange between SOMOs, achieving interaction strengths comparable to those of chemical bonds, thereby significantly enhancing the intermolecular ferromagnetic coupling. (iii) Effective transmission of magnetic interactions within the material is crucial. Magnetic domains arise when intrinsic magnetic moments are confined within minute crystalline domains and undergo spontaneous magnetization through charge exchange between spin electrons. The interactions between these magnetic domains determine the overall magnetic properties of the material. The r-2NH₂-PBI radical aggregates exhibit long-range π–π interactions, enabling the maintenance of magnetic interactions along the material, forming a long-range magnetically ordered structure. Through π–π stacking, the coupled radicals established long-range magnetic order via the spin of unpaired electrons. This discovery deepens our understanding of the mechanism of intermolecular magnetic interaction and the design of magnetic organic semiconductor materials.