Design of organic crystals with lone pairs to study spin resonance and spin-lattice interactions

Introduction

In crystals, lone pairs significantly affect phenomena such as local dipoles, generation of ferroelectricity^1,2,3, changes of lattice vibrations, and reduction of thermal conductivity^4,5,6, enabling second-harmonic light generation⁷. Due to the importance of lone pairs, their roles in the electronic structure, phase transitions⁸, optoelectronics^9,10,11, carrier mobility¹², nonlinear optical properties¹³, and life sciences¹⁴ have been recognized and well-studied.

Substitution of a carbon atom by a nitrogen atom is an effective approach for the generation of lone pairs in organic materials due to the similar hybridization of nitrogen and carbon hybridization¹⁵. Nitrogen substitution with nonequivalent sp² hybridization leads to the occupation of the σ orbital by the extra electron to form a lone pair electron. Lone pairs in organic materials can reduce the energy of the π orbitals¹⁶ and affect the charge distribution^17,18 and dipole polarization¹⁹, playing a key role in the optimization of the structure^20,21 and electronic properties^22,23,24. Additionally, lone-pair-driven distortion can also increase the dielectric constant to further modify charge recombination and dissociation^25,26, and this approach has been widely used in the construction of thermally activated delayed fluorescent molecules^27,28,29. In addition, with the mature development of the understanding of the dependence of the optoelectronic properties of organic materials on the lone pairs, spin properties have also gradually attracted the attention of researchers³⁰; however, these studies are still rare. Lone-pair-induced symmetry breaking can lead to spin-orbit coupling (SOC)³¹. Additionally, the lone pair activity also affects the strength of electron-phonon (e-p) coupling³², which may tune the localization of the carrier to change the spin dependence of the properties. Thus, lone-pair-dependent spin behavior is a fascinating field with numerous unexplored issues, such as the lone-pair dependence of spin response time, spin resonance, spin transitions, and spin-lattice interactions. Studies of lone-pair-dependent spin effects will be beneficial for the design of spintronic materials with potential applications in information storage.

Herein, three types of charge transfer crystals containing different ratios of nitrogen substituents were fabricated and utilized to investigate the impacts of lone pairs on spin properties. The introduction of lone pairs in donors can weaken the interactions between donors and acceptors, resulting in a blueshift in photoluminescence and a weaker electron-lattice coupling. Additionally, lone pairs inside charge transfer crystals can modify the spin resonance and spin flipping. Furthermore, the presence of lone pairs can strengthen the spin-lattice interaction of the crystals to facilitate the regulation of the transition between spin singlets and triplets so that the crystal demonstrates a greater variety of magnetic, optical, and spin-related properties.

Results and discussion

1,2,4,5-tetracyanobenzene (TCNB) acceptors and phenanthrene (Phe) donors are used to fabricate charge transfer (CT) cocrystals of phenanthrene-1,2,4,5-tetracyanobenzene. To introduce lone pairs inside crystals, nitrogen substitution in molecular phenanthrene generates benzo(f)quinolone (Phe(N), 1 nitrogen substitution) and 1,10-phenanthroline (Phe(2 N), 2 nitrogen substitutions), and crystals of benzo(f)quinoline-1,2,4,5-tetracyanobenzene and 1,10-phenanthroline-1,2,4,5-tetracyanobenzene with lone pairs are further fabricated. The structures of benzo(f)quinoline and 1,10-phenanthroline are similar to that of phenanthrene, except that the corresponding carbon atoms are replaced by nitrogen atoms (Fig. 1a–c). For the Phe molecule, the negative charge distribution was concentrated mainly in the center of the benzene ring, while for the Phe(N) and Phe(2 N) molecules, the negative charge distribution shifts to the nitrogen atoms which bear the lone pairs. In the following, the three crystals are named Phe-TCNB, Phe(N)-TCNB, and Phe(2 N)-TCNB for convenience. The structures of Phe(N)-TCNB and Phe(2 N)-TCNB were characterized via single-crystal X-ray diffraction (SCXRD) at different temperatures (Supplementary Tables S2 and 3), and all three crystals were further characterized via high-resolution transmission electron microscopy (HRTEM) with element mapping (Figs. 1a–c and Supplementary S2, 3). For these three crystals, the molar ratio between the donor and acceptor is 1:1. The Phe-TCNB and Phe(N)-TCNB crystals are monoclinic and are found in the (P{2}_{1}/c) space group. With increasing nitrogen substitution, the cell volume gradually increased. Each crystal exhibited a mixed stacking structure. In Phe-TCNB and Phe(N)-TCNB, donors and acceptors align face-to-face along the a-axis in a DADADA configuration, forming a one-dimensional molecular chain. By contrast, Phe(2 N)-TCNB presents a more complex arrangement; although the donors and acceptors also exhibit a DADADA face-to-face alignment, they do so along the b-axis and in a zigzag pattern.

Design of organic crystals with lone pairs to study spin resonance and spin-lattice interactions — **Fig. 1: Basic properties of three cocrystals.**

Lone pairs introduced by nitrogen substitution will change the electrostatic potential in donors, which results in not only the movement of the center of the acceptor molecule’s projection on the donor plane but also leads to an increase in the distance between donors and acceptors. As shown in Fig. 1a–c, when TCNB and Phe(N) are combined to form a charge transfer unit, the lone pair in the Phe(N) molecule will strengthen the repulsion with the negative center located at TCNB to increase the distance between the donor and acceptor. When TCNB and Phe(2 N) formed charge transfer crystals, the distance increased. Moreover, it should be noted that the two nitrogen atoms on Phe(2 N) are positioned symmetrically, representing two negatively charged regions. When interacting with TCNB molecules, Phe(2 N)s act as clamps, resulting in a zigzag arrangement of molecules inside Phe(2 N)-TCNB crystals (Supplementary Fig. S4). Thus, for the crystals with nitrogen substitution, the changes induced by lone pairs in both the lattice parameters and the molecular configuration effectively change the lattice vibration-dependent e‒p coupling. In addition, lone pairs enhance the electron-electron interaction inside the charge transfer crystals, which may modulate the electron spin resonance and spin flipping. As shown in Fig. 1d, for the Phe-TCNB, Phe(N)-TCNB, and Phe(2 N)-TCNB units, increasing the density of lone pairs decreases both the magnitude of the g-factor and the electron spin resonance (ESR) linewidth, indicating that electron spin resonance and spin-lattice interactions are strongly dependent on the lone pairs. Furthermore, under the stimulation by a magnetic field, lone pairs suppress the transition from singlet to triplet states to switch the sample M-PL (magnetic field-dependent photoluminescence, M-PL = (PL(B)-PL(B = 0))/PL(B = 0)) from negative to positive (Fig. 1e). In the following, Phe-TCNB-, Phe(N)-TCNB-, and Phe(2 N)-TCNB-dependent spin resonance, spin-lattice interactions and opto-magnetic properties will be studied.

Lone pairs can affect the interaction between electrons and holes to tune carrier recombination. The appearance of lone pairs increases the distance between donors and acceptors (Fig. 1a–c) to weaken the donor-acceptor interaction, leading to a smaller energy level splitting. As illustrated in Fig. 2a, the bandgap of the crystals increases to induce a blueshift in the PL spectrum due to the effect of lone pairs. To further study the tunability of the D-A distance dependence of the PL blueshift, pressure-related PL spectra were studied. Under a pressure of 0.8 GPa, the PL spectra of the three crystals exhibited redshifts of approximately 17 nm and 55 nm for Phe(N)-TCNB and Phe(2 N)-TCNB, respectively (Fig. S6). The phonon effect is suppressed when the interaction between the donors and acceptors inside the crystals decreases (the inset of Fig. 2a). With decreasing e-p coupling, electrons or holes coupled with phonons will induce a more extended state. As a result, extended states of electrons and holes present greater overlap, allowing them to capture each other easily, and thus enhancing the electron-hole recombination to decrease the fluorescence lifetime. In addition, because lone pairs decrease the exchange energy, the increase in reverse intersystem crossing facilitates the rapid conversion of triplet states to singlet states, which participate in light emission to lead to a short fluorescence lifetime. Thus, for Phe(N)-TCNB and Phe(2 N)-TCNB, which have smaller exchange interactions, the fluorescence lifetime becomes shorter than that of Phe-TCNB, which has stronger exchange interactions. As shown in Fig. 2b, c, the fluorescence lifetimes of Phe-TCNB, Phe(N)-TCNB, and Phe(2 N)-TCNB are 14.21 μs, 34.96 ns, and 14.92 ns, respectively. The fluorescence lifetime decreases with increasing nitrogen substitution.

**Fig. 2: Fluorescence lifetime and e-p coupling.**

With increasing temperature, the increased lattice vibration increases e-p coupling in the crystals of Phe-TCNB (see the inset of Fig. 2b), so that the fluorescence lifetime is longer at 300 K than at 100 K. However, the exact opposite is observed for the system with nitrogen substitutions. With nitrogen substitution, lone pairs inside the crystals lead to an apparent spin density distribution between molecules (Fig. 2d), which is much more pronounced than that in the crystals of Phe-TCNB (Fig. 2e). With increasing temperature, more charge transfer will be excited to induce a closed-shell to open-shell transition, where stronger exchange interactions between the neighboring donors and acceptors will suppress the lattice vibration. As a result, e-p coupling decreases with increasing temperature in the system with nitrogen substitution. Thus, increasing the temperature results in more extended states of electrons and holes, allowing them to easily capture each other to decrease the fluorescence lifetime in a system with nitrogen substitutions (Fig. 2c).

As discussed above, nitrogen substitution can increase the distance between donors and acceptors, so that the dipoles cannot respond effectively to an applied alternate electric field, leading to a lower dielectric constant than that of Phe-TCNB (Supplementary Figs. S8, 9). When external light irradiation is applied, the dielectric constants of all three crystals increase owing to the increased generation of electron-hole pairs acting as dipoles inside the crystals (Fig. 3a‒c and Supplementary Fig. S10). However, the response times differ among the three crystals. For Phe-TCNB, when a light field is applied, the dielectric constant immediately increases to a stable value. However, for Phe(N)-TCNB and Phe(2 N)-TCNB with lone pairs, the dielectric constants do not stabilize instantly but rather continue to increase slowly. In a crystal with lone pairs, the fluorescence lifetime is on the order of tens of nanoseconds, giving rise to rapid recombination after excitation by light. Therefore, the density cannot quickly reach a saturation level under light illumination, and the dielectric constant exhibits a long-term response to the applied light in Phe(N)-TCNB and Phe(2 N)-TCNB. In Phe-TCNB, the fluorescence lifetime is so long that electron-hole pairs can reach saturation quickly once external light is applied. Furthermore, increasing the temperature can also enhance electron hopping to increase the density of electron-hole pairs. Increasing the temperature results in similar tunability of the dielectric constant in the crystals (Fig. 3d, e). It is expected that when an external electric field is applied, the recombination will be suppressed, increasing the fluorescence lifetime. Thus, an applied electric field can increase the dielectric constant (Supplementary Fig. S11). Furthermore, the increased fluorescence lifetime of the electric field in the nitrogen-substituted crystals can switch the dependence of the dielectric response on the external stimuli from slow to quick (Supplementary Fig. S12).

**Fig. 3: Light dependence of the dielectric constant.**

According to Langevin’s paramagnetic theory, magnetic susceptibility decreases with increasing temperature. In Phe-TCNB crystals, the ESR signal decreases with increasing temperature (Fig. 4a). However, the opposite is the case for Phe(N)-TCNB with lone pairs. The underlying principle of ESR measurements is that electrons can absorb microwaves and transition to higher energy states in an external magnetic field, indicating that the ESR spectrum is fundamentally an absorption spectrum. Thus, the absorption intensity largely depends on the ability to transfer energy and spin angular momentum to the surrounding environment. Such continuous transfer allows the system to keep absorbing microwaves, thereby generating the ESR signal. In Phe(N)-TCNB, due to the presence of lone pairs generating stronger spin-lattice interactions, the energy absorbed by spin flipping can be dissipated more rapidly through spin-lattice interactions. As a result, increasing the temperature can enhance spin-lattice interactions to give rise to a more pronounced ESR signal in Phe(N)-TCNB (Fig. 4a). However, because the open-shell is different from that generated in Phe(2 N)-TCNB, the ESR signal cannot be observed. However, when the external light illumination is applied, excited states are generated steadily, leading to an open-shell structure in Phe(2 N)-TCNB, where an extremely narrow ESR single can be obtained (Fig. 4b). Additionally, integration of the signal results in a curve with a Lorentzian distribution, indicating that the ESR signal observed under illumination is due mainly to the lone-pair-induced spin-lattice interaction in Phe(2 N)-TCNB. Additionally, in organic donor:acceptor complexes, carriers in the donor present a g-factor of ~2.003, while electrons in the acceptor present a g-factor of 1.999. In the Phe-TCNB, Phe(N)-TCNB and Phe(2 N)-TCNB crystals of, the g factors ranged from 2.0022 to 2.0032, which is very close to the g factor magnitude of organic donor materials. Thus, the ESR signal is expected to be derived mainly from the donors in these crystals. For Phe-TCNB and Phe(N)-TCNB, e-p coupling can induce a localized state, where electron hopping fulfills the requirement of the ESR transition rule to increase the ESR linewidth with a magnetic field ranging from 338 to 340 mT (Figs. 1g and 4b).

**Fig. 4: Light- and temperature-dependent ESR spectra.**

Furthermore, it should be noted that ESR signals are light illumination-dependent in the system with nitrogen substitutions (Fig. 4b, d, e). The application of light did not change the ESR signal in Phe-TCNB (Fig. 4b and Supplementary Fig. S16). Under light illumination, a large density of electrons (or holes) is excited in these three charge transfer crystals. In Phe(N)-TCNB and Phe(2 N)-TCNB crystals, light-generated electrons (or holes) enhance the lone pair dependence of spin-lattice interactions. As a result, the ESR signal becomes stronger under the application of light. However, due to the lack of lone pair dependence of spin-lattice interaction, light cannot tune the intensity of the ESR in Phe-TCNB (Fig. 4b). With constant illumination, the ESR signal needs ~300 s to reach saturation (the inset of Fig. 4e); this is because light-generated electrons and holes can recombine easily (Fig. 2c); thus, ESR signals cannot quickly reach saturation levels under light illumination in Phe(N)-TCNB and Phe(2 N)-TCNB. The density of lone pairs is greater in Phe(2 N)-TCNB crystals than in Phe(N)-TCNB. The ESR signal presented a more pronounced light response (Fig. 4d, e).

In crystals, pure singlet states are initially generated under light illumination. In Phe-TCNB, the magnetic field can increase the ratio of triplet states to decrease the strength of fluorescence, leading to a negative M-PL (Fig. 5a). At 300 K, stronger light illumination leads to the generation of more electron-hole pairs, resulting in greater conversion of singlets into triplets under identical magnetic fields. Consequently, the amplitude of the PL modulated by the magnetic field increases with increasing light intensity. At low temperatures, the singlet states induced by higher light intensity do not have sufficient time to convert into triplet states before recombining and emitting light, resulting in a weaker M-PL (Supplementary Fig. S17). However, the situation is completely different for a system with lone pairs. In crystals of Phe(N)-TCNB and Phe(2 N)-TCNB, for light-generated singlets, spin relaxation leads to the conversion of singlets to triplets due to lone-pair-induced spin-lattice interaction. This means that spin relaxation is enhanced by the effect of lone pairs, which prevents the external magnetic field from tuning the spin states (Supplementary Fig. S18). Moreover, once an external magnetic field is applied, spin relaxation is suppressed to prevent the transition from singlet to triplet, where the magnetic field enhances the PL (Fig. 5b, c). Compared with Phe(N)-TCNB crystals, Phe(2 N)-TCNB crystals have greater lone pair density to enhance spin-lattice interaction, which reduces the control of the PL by the magnetic field. Thus, M-PL is smaller in Phe(2 N)-TCNB than in Phe(N)-TCNB (Fig. 5b, c).

In summary, three charge transfer crystals with different nitrogen substitutions were fabricated to study the effects of lone pairs on spin resonance, spin-lattice interactions, and optical and magnetic properties. In charge transfer crystals, the introduction of lone pairs in donor molecules can weaken the interactions between donors and acceptors, resulting in a blueshift in photoluminescence and a weaker electron-lattice coupling. Lone pairs generated by nitrogen substitution inside charge transfer crystals effectively modified the spin resonance, resulting in decreases in both the magnitude of the g factor and the ESR linewidth. Furthermore, lone pairs will redistribute the spin density, which could affect the exchange interaction to tune the transition between singlet and triplet states. Thus, the introduction of lone pairs through nonequivalent sp² hybridization significantly impacts the spin and dipole properties of crystals, offering valuable insights for the synthesis of functional crystalline materials.

Methods

Materials and crystal growth

1,2,4,5-TCNB and phenanthrene (Phe) were purchased from Sigma‒Aldrich Chemical Company, benzo(f)quinoline (Phe(N)) was purchased from Tokyo Chemical Industry Company, and 1,10-phenanthroline (Phe(2 N)) was purchased from Leeyan Company. All of the materials were used without further purification. Phe and TCNB were dissolved at a 1:1 molar ratio in ethanol. Phe(N) (or Phe(2 N)) and TCNB were dissolved at a 1:1 molar ratio in a mixed solution of acetone and ethanol. The mixed solution was ultrasonicated for 20 min and then placed in a glass bottle. The three crystals were grown by slow evaporation for ~1–2 weeks.

Device structure

Clean ITO substrates were obtained by sequentially sonicating them in deionized water, ethanol, acetone, and isopropanol for 20 min each. A PMMA solution (25 mg/ml) was then coated at 1000 rpm for 60 s. Crystals and Ag were sequentially evaporated in an orderly manner to fabricate the device, enabling dielectric tests. Crystals were evaporated to clean glass substrates to obtain the absorption spectrum.

Characterization

High-quality crystals of Phe(N)-TCNB and Phe(2 N)-TCNB were chosen for single-crystal X-ray diffraction. HRTEM experiments were carried out using a Cs-corrected FEI Titan transmission electron microscope at 300 kV, and HRTEM images were acquired using a direct-detection camera (Gatan K2). The dielectric constants were obtained using a Keysight E4990A impendance analyzer in a frequency range from 20 Hz to 20 MHz. PL spectra were obtained from Nova X, Idea Optics Co. Ltd. The fluorescence lifetime was investigated using an Edinburgh Instruments Model FLS920 spectrometer. The absorption spectra were measured using a UV2600i instrument (Shimadzu Co., Ltd.). The thickness of the crystal layer was measured by AFM. ESR was characterized using an EPR200-Plus instrument working at X-Band, CIQTEK Co. Ltd. Magnetization tests were performed using an 8604 vibrating sample magnetometer (Lake Shore Cryotronics, Inc).

Calculation details

The ab initio calculations described here were implemented in the Quantum ESPRESSO program with a pseudopotential plane-wave scheme. The projected augmented wave method was employed to describe the electron-ion interactions. The exchange-correlation interaction was treated via the generalized gradient approximation (GGA) in the form of the Perdew–Burke–Ernzerhof functional. The cutoff energy was set to 600 eV for all three cocrystals. For the electronic structure calculations, a k-grid of 5 × 4 × 1 for Phe-TCNB and Phe(N)-TCNB and a k-grid of 2 × 4 × 2 for Phe(2 N)-TCNB were employed. The convergence criteria for energy and force were set to 1 × ^10–5 eV and 0.01 eV/Å, respectively.