Introduction

Photo-responsive materials are a kind of intelligent ones showing fast and sensitive response to light stimulus1,2,3,4, and are widely used in molecular machine5, drug delivery6, logic operation7, information processing8, and photoelectric devices9, due to the higher spatiotemporal accuracy of light stimulation compared to other stimuli10,11. Among them, photochromic materials, which undergo reversible changes in appearance color under specific wavelength light irradiation, have attracted significant interest12,13,14,15. Photochromic phenomena have been observed in organic16, inorganic17,18, and hybrid materials19,20, among which organic materials have good biocompatibility and are suitable for next-generation flexible electronic devices, biosensing, and detection21,22,23. Most reported photochromic mechanisms rely on the isomerization or rearrangement of closed-shell molecules (such as derivatives of azobenzene24,25, diarylethenes26,27, spiropyrans28,29 and fulgide30, etc.). However, it is difficult for these molecules to achieve sensitive photochromism in the solid state because strong intermolecular forces suppress molecular movement or rotation.

A type of stimulus-response system with closed-open shell transformation (COST) has garnered attention in the field of dynamic covalent chemistry because it involves the reversible breaking/formation of chemical bonds and conversion of electronic properties31,32,33. The system exhibits high color resolution of photochromism due to the significant differences in the optical properties of the closed-open shell structures34. The typical COST system is based on reversible C-C bond (σ-bond) cleavage/formation (Fig. 1a)35,36,37, or the transition between quinone structure and radicals (Fig. 1b)38,39,40. For example, Watabe et al. constructed a gel system of difluorenylsuccinonitrile molecules15. During the slow swelling process of the gel system, the swelling force induced the COST, resulting in the color transformation from colorless to pink in the gel. Ye et al. designed and synthesized a small organic molecule with the quinone structure, which could realize the reversible COST induced by hydroxyl radical (·OH) and hydrogen sulfide (H2S) in vivo and could be applied to the field of biosensing41. However, most of the stimulus-responsive systems based on COST are insensitive, time-consuming, and difficult to monitor in real-time, especially in the aggregated state42. It is still a huge challenge to develop a sensitive stimulus-responsive material with dynamic covalent bonds.

Fig. 1: Schematic diagram of organic radicals, and chemical structures of studied materials.
figure 1

a The schematic diagram of σ-bond breaking to form organic radicals. b The schematic diagram of the transformation of quinone structure into organic diradicals. c The schematic diagram of photo-induced π-bond splitting to form organic diradicals in this work. d The molecular structure of Z-DPMNs.

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Compared with σ-bonds, π-bonds with smaller bond energy are more susceptible to breakage, thus showing greater potential in the dynamic regulation of organic molecular structure. However, there are few studies on π-bond splitting to form diradicals43,44. It is mainly due to the poor stability of the obtained diradicals, which cannot form a visualized stimulus response related to COST. It is very important to design a π-bond cleavage/formation system that can generate relatively stable diradicals for developing stimulus response materials.

In this work, photoinduced π-bond splitting, a strategy of dynamic covalent chemistry, is proposed to form stable diradicals and realize the visualized stimulus-response of COST system based on cis-form 2,3-diphenylmaleonitriles (Z-DPMNs) (Fig. 1c, d). With the help of the electronic effect of cyano groups and the conjugation effect of aromatic rings, ethylene π-bond cleavage of Z-DPMNs is promoted to form stable diradicals, achieving the reversible transformation of closed-open shell structure and optical properties. The photochromic phenomenon of Z-DPMNs can appear in both solid and solution states. Moreover, the dual-channel photochromism of visible light and UV light can be realized by adjusting substituents. The trans-form DPMNs (E-DPMNs) do not show significant photochromism but strong fluorescence. The rich optical performance of DPMNs has been successfully applied in rewritable photopatterning paper and encrypted ink.

Results

Synthesis and photophysical properties

Z-DPMNs were synthesized from phenyl acetonitrile by dimerization and Z/E isomerization (Supplementary Fig. 1). The structures of these Z/E isomers were confirmed by high-resolution mass spectrometry, 1H NMR and 13C NMR (Supplementary Figs. 57–106). The photochromic effect was first observed in Z-H solid with the appearance color changing rapidly from white to purple after UV irradiation for 2 s, and the purple color can last for about 11 min after removing the UV lamp (Fig. 2a, c). Furthermore, the photochromism of Z-H can also occur under sunlight (Supplementary Fig. 2). It indicates that compound Z-H has sensitive photo-responsive characteristics. The photochromic solid of Z-H is named as Z-H-P. However, its E-type isomer E-H has no photochromic phenomenon after UV lamp irradiating for 30 min (Fig. 2b). As shown in Supplementary Fig. 3, Z-H and E-H solids show similar absorption spectra with maximum absorption wavelengths at around 384 nm. However, under 365 nm UV irradiation, a new band at 524 nm appears in the absorption spectrum of only Z-H solid. The intensity of this peak rapidly increases and reaches its maximum after irradiation for 5 s (Fig. 2d). Then, the absorption peak gradually disappears after the light source is removed (Fig. 2e). While the absorption spectrum of E-H is unchanged after irradiation (Supplementary Fig. 4). Furthermore, the photochromic process of Z-H can be repeated for more than 30 times, and the absorption intensity has not changed significantly (Fig. 2f), indicating that the photochromic properties of Z-H have excellent reversibility and durability, which endows it with great practical potential.

Fig. 2: Photophysical properties and EPR spectra of Z-H solid.
figure 2

a The photochromic behavior of Z-H solid. The photographs were taken before and after 365 nm UV irradiation. b The photographs of E-H solid before and after 365 nm UV irradiation. c The fading process of Z-H-P solid. The photographs were taken at different times after turning off the 365 nm UV light. d Time-dependent UV absorption spectra of the Z-H solid under the 365 nm UV irradiation for 10 s. e Time-dependent UV absorption spectra for the fading process of Z-H solid. f The coloration/fading cycles of Z-H solid. g EPR spectra of Z-H solid before and after 5 W UV light irradiation.

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To further explore the photochromic behavior, the 1H NMR of Z-H after illumination is first investigated. As shown in Supplementary Fig. 5, there are no changes in the NMR spectrum compared with Z-H, indicating that no impurities were generated. Moreover, the phase transition and structural transformation of Z-H in the photochromic process were excluded through powder X-ray powder diffraction (PXRD) and the attenuated total reflection Fourier transform infrared (ATR-IR) (Supplementary Figs. 6 and 7). Electron paramagnetic resonance (EPR) was employed to analyze whether there is a radical signal in Z-H-P. As shown in Fig. 2g, no obvious signal is observed in the EPR spectrum of Z-H before UV irradiation, while strong signal peaks ranging from 335 to 338 mT appear after UV irradiation, accompanied by the photochromism. The g value calculated to be 2.0004 is consistent with organic carbon-centered free radicals (g = 1.99–2.01), which indicates that Z-H-P is the relatively stable carbon-centered radicals45,46. In addition, after standing for about 11 min, the photoactivated solid will recover to the initial white color state without an EPR signal. By contrast, there is no significant signal found in the EPR spectrum of E-H solid before and after UV irradiation (Supplementary Fig. 8). Consequently, the generation of photoinduced radicals is the cause of photochromism of Z-H solid, which quenches its fluorescence. E-H exhibits strong blue fluorescence but no photochromism (Supplementary Fig. 9).

Then, the photochromic properties of the Z-H solution are studied. The color of Z-H solution in dichloromethane rapidly changed from colorless to red upon photo-stimulation of 2 s (Supplementary Fig. 10a). After turning off the UV light, the color will fade within 6 min (Supplementary Fig. 10b), and this process also has good repeatability (Supplementary Fig. 11). The absorption spectra, 1H NMR and EPR of Z-H solution confirm that the photochromism originates from the closed-open shell structure transformation of Z-H molecule (Supplementary Figs. 12–14). In addition, 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) was added as the spin-trapping reagent to capture free radicals of Z-H-P. As can be seen from Supplementary Fig. 15, the absorption peak at 480 nm disappears after adding DMPO, indicating that the photoactivated diradicals are captured by DMPO. The corresponding EPR energy spectrum has six broad peaks with a g value of 2.0063 (Supplementary Fig. 16), which is consistent with the signal peak of the adducts of carbon-centered radicals and DMPO46,47. It also proves the mechanism of Z-H photo-induced radical generation.

Mechanistic investigation

Why does Z-H solid exhibit photochromism, but E-H does not? In order to further understand the internal mechanism of photoinduced radicals, the single crystal structures of two isomers were analyzed. As shown in Fig. 3, the molecular stacking of the Z-H crystal is loose and the intermolecular interaction is relatively weak. By contrast, the E-H crystal exhibits strong intermolecular interaction with a π-π stacking distance of 3.947 Å and hydrogen bonding of 3.646 Å. In the E-H crystal structure, two benzene rings are coplanar with a twist angle of 0°. However, due to the steric hindrance between benzene rings in Z-H, the twist angle is 7.15°. It leads to a smaller degree of overlap of the non-hybridized p orbitals of two carbon atoms in ethylene and a longer C = C double bond length of Z-H (1.360 Å) than that of E-H (1.354 Å). Detailed crystal structures of Z-H and E-H are listed in Supplementary Table 1. Therefore, we speculate that the π-bond of the ethylene unit of the Z-H molecule is broken to release the tension of benzene rings and form diradicals, resulting in photochromism.

Fig. 3: Crystal structure and theoretical calculation.
figure 3

a The ethylene double bond length and the twist angle between benzene rings of Z-H (up) and E-H (down). b Single-crystal structure and intermolecular interaction of Z-H (up) and E-H (down). c The calculated absorption wavelength of Z-H-P with uB3LYP functional at the basis set level of 6–31 g(d). d Computed Mulliken atomic spin densities for compound Z-H-P and visualized total spin density.

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Density functional theory (DFT) and time-dependent density functional theory (TDDFT) are performed to further prove the diradical structure. As shown in Fig. 3c, the calculated maximum absorption wavelength (447.1 nm) of Z-H-P matches well with the experimental absorption peak (480 nm). It proves that the new absorption bands of Z-H solution after UV irradiation should belong to diradical species. Moreover, the calculated spin density of Z-H-P is mainly distributed at the central carbon atoms of the ethylene unit, but also partially delocalized to the cyano and phenyl groups (Fig. 3d). Detailed vertical absorption data and spin population analysis are listed in Supplementary Tables 2 and 3, respectively. In order to confirm the spin delocalization of the cyano group contributing to the stability of diradicals, the photo-response performance of tetraphenyl ethylene (TPE), 2,3-dicyanostilbene and cis-stilbene was investigated, none of them have photochromic phenomenon in either solid or solution state (Supplementary Fig. 17). Thus, the dicyano group, as well as the twisted conformation and the conjugation effect of aromatic rings in Z-H, is essential for the formation of diradicals. A def2-TZVP basis set was utilized to calculate the single-point energy of the diradicals. DFT results show the existence of a large ∆ES-T (14 kcal/mol) in Z-H-P diradical, indicating its triplet-ground-state characteristics (Supplementary Table 4). The low-temperature EPR spectrum of Z-H after UV irradiation was performed. As shown in Supplementary Fig. 18, it can be found that the intensity of the radical signal is significantly enhanced, and a half-field splitting signal of the Δms = ± 2 forbidden transition is observed at around 170 mT, suggesting the presence of spin-triplet species. Moreover, the decay rate of the EPR signal is consistent with that of the absorption band at 524 nm, indicating the photochromism of Z-H originated from the triplet diradicals (Supplementary Fig. 19).

The photochromic of aryl ethylene is usually considered to form dihydrophenanthrene derivatives through a photocyclization reaction. Therefore, the maximum absorption peak of the cyclization intermediate dihydrophenanthrene (2H-DPCN) was calculated to be 570 nm (Supplementary Fig. 20), which is completely different from the experimental one. It indicated that the photochromism of Z-H is not caused by 2H-DPCN. However, why didn’t we see the occurrence of the photocyclization reaction? Driven by this issue, we extended the irradiation time of the Z-H solution. Interestingly, when the photoactivation time exceeded 30 s, two photocyclization products (PDCN and DPCN) were obtained (Supplementary Figs. 21–23), both of which originated from dihydrophenanthrene48. As a result, we suppose that photoinduced π-bond splitting and photocyclization reaction of Z-H should be a pair of competitive reactions49 (Supplementary Fig. 24). Under the short time (about 2 s) ultraviolet irradiation, the π-bond splitting reaction mainly occurs in the Z-H solution, which produces diradicals and changes color. When exposed to ultraviolet light for a long time (more than 30 s), the photocyclization reaction will occur to produce dihydrophenanthrene, which will be further isomerized to obtain DPCN, or oxidized to produce PDCN. In summary, it can be inferred that the photochromism of Z-DPMNs is not derived from 2H-PDCNs, but from the triplet-ground-state diradicals, which can survive for a long time50.

Influencing factor of the COST

In order to explore the influence of substituent effect on photo-induced diradical generation, the photochromic behaviors of DPMNs with different substituents (-OMe, -Me, -F, -Cl, and -TFMe) were studied. All E-DPMNs, Z-OMe, and Z-Me solids without photochromism exhibit bright luminescence (Fig. 4a, Supplementary Figs. 25, 26 and Supplementary Tables 5, 6). However, Z-DPMNs with substituents of -F, -Cl, and -TFMe have photochromism and weak luminescence in solids, accompanied by their colors altering from white to purple, light purple, and orange-red after UV irradiation with the duration time is 20 s, 150 s, and 180 s respectively. After photoactivation, Z-Cl and Z-TFMe show the new absorption peaks at 539 nm and 507 nm, respectively, but no change in absorption is observed for Z-F because of the short fading time (20 s) (Supplementary Figs. 27–29). In addition, the absorption bands of Z-F and Z-Cl solids extend to 430 nm, thus exhibiting visible light-activated photochromic properties (Supplementary Fig. 30 and Supplementary Table 7). Supplementary Table 7 clearly indicates that the photochromic fading time resulting from UV lamp activation is longer compared to that of blue lamp activation, which is due to the power of light source (Supplementary Fig. 31). The EPR spectra of these Z-DPMNs and E-DPMNs before and after irradiation show that only molecules (Z-F, Z-Cl, and Z-TFMe) with photochromic properties have the signal of radicals after UV irradiation, and the g factor is in the range of 2.0008-2.0014 (Supplementary Figs. 32–36). It is indicated that Z-DPMNs molecules with the electron-withdrawing substituents on the benzene ring are easy to form diradicals in the aggregated state. It may be that the electron-withdrawing groups lead to the decrease in electron density of carbon atoms on ethylene, thus accelerating the π-bond breakage of the ethylene unit to form diradicals. Besides, the formation of photoactivated radicals has a quenching effect on luminescence (Supplementary Fig. 37), and the change in fluorescence intensity is as reversible as photochromism.

Fig. 4: Influence factors of photochromism.
figure 4

a Photos of Z-OMe, Z-Me, Z-F, Z-Cl, and Z-TFMe solids before and after 5 W UV light or 1 W blue light irradiation. b Photos of Z-2-OMe, Z-3-OMe, Z-3-F, and Z-3,5-OMe solids before and after 5 W UV light or 1 W blue light irradiation. c Bar chart of fading time and temperature of Z-3-F and Z-H solids after 365 nm UV light with power of 5 W irradiation. d The photographs of Z-3-OMe solution (5 × 10−3 M) in different polar solvents before and after 365 nm UV light with power of 5 W irradiation at room temperature.

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Z-2-OMe, Z-3-OMe, Z-3,5-OMe, and Z-3-F were synthesized to further explore the influence of substitution sites on the COST of Z-DPMNs. Among them, Z-2-OMe has no photochromic phenomenon (Fig. 4b, Supplementary Fig. 38 and Supplementary Table 6). However, Z-3-OMe solid has obvious photochromic phenomenon under UV light and 450 nm blue light with power of 1 W, and its appearance color changes from light yellow to dark green after irradiation (Supplementary Fig. 30 and Supplementary Table 7), corresponding to the change of absorption spectrum (Supplementary Fig. 39). It is due to the fact that according to Hammett’s law, the methoxy groups exhibit electron-donating effect at position 2, and electron-withdrawing effect at position 351. Similarly, the fading time (40 min) of Z-3-F solid after UV irradiation is longer than that of Z-F (Supplementary Fig. 40). It is also attributed to the stronger electron-withdrawing ability of F atom substituted at position 3 than 451, resulting in persistent photochromism. Interestingly, the Z-3,5-OMe solid has no photochromic phenomenon, while its solution does (Supplementary Figs. 41 and 42). It should be ascribed to the rich intermolecular interactions in the crystal structure of Z-3,5-OMe (Supplementary Fig. 43 and Supplementary Table 1), which locks the conformation of Z-3,5-OMe molecules and inhibits the formation of diradicals under light stimulation. In the solution state of Z-3,5-OMe, the intermolecular interactions are very weak, which is beneficial to the formation of diradicals. All of the Z-DPMNs in solution have photochromic properties except Z-2-OMe and Z-OMe, also due to the strong electron donating ability of the methoxy group at positions 2 or 4 of benzene rings51. However, the cyclization product of Z-MeO (MeO-PDCN) can be generated after UV irradiation (Supplementary Figs. 44 and 45), which also indirectly indicates that the photochromism of Z-DPMNs is independent of the cyclization product. The optical properties and EPR spectra of the corresponding molecules are detailed in Supplementary Information (Supplementary Figs. 46–52 and Supplementary Tables 5 and 6).

Temperature has a great influence on the stability of radicals52. With the increase in temperature, the fading time of photoactivated Z-3-F solid significantly shortened from 120 min at 0 °C to 15 min at 40 °C, and it can only last for 1 min at 80 °C (Fig. 4c). Z-H solid has a similar photochromic phenomenon at different temperatures. Especially at 77 K, the red color of the Z-H-P solution does not fade beyond one week (Supplementary Fig. 53), indicating that increasing temperature will accelerate the transformation from open shell to closed shell structure, which is unfavorable to the stability of free radicals. The polarity of solvents also has a negative effect on the stability of free radicals. Only nonpolar or low polar solvents (like toluene, chloroform, and dichloromethane) can induce the photochromism of Z-3-OMe at room temperature (Fig. 4d). When the temperature drops to 0 °C, more polar solvents (such as EA, 1,4-dioxane and THF) exhibit significant photochromic phenomena (Supplementary Fig. 54). Moreover, the fading time of Z-3-OMe photochromism in strong polar solvents is significantly shorter than that in low polar solvents. Therefore, it can be inferred that the interaction between polar solvent and triplet radicals may lead to the instability of radicals and reduction in their lifetime. In addition, the quenching effect of oxygen on triplet radicals is avoided in anaerobic environments, thereby resulting in an increase in the fading time of photochromism (Supplementary Fig. 55).

Application

Photopatterning has been widely used in various industries due to its advantages of low cost, bright colors, and good reversibility53. To ensure the security and timeliness of information, it is crucial to be able to automatically clear the information after reading it. Considering the interesting and reversible photochromic characteristics of Z-DPMNs, the rewriting and erasing of pattern information could be realized based on these materials. Take the compound Z-3-F as an example, a A4 paper was immersed in the Z-3-F solution and dried to obtain the rewritable paper. Then, the rewritable paper was irradiated with UV light or blue light through different masks so that a series of pattern information such as numbers, letters, animals, and plants could be obtained. The information can be completely erased by natural decay or heat treatment, and then the next pattern information can be photoprinted (Supplementary Fig. 56). Moreover, a high-resolution Quick Response (QR) code was printed on paper by UV light photopatterning, and the encrypted information could be read as FJNU by a smartphone. It can be read within 35 min. Subsequently, the resolution of the QR code gradually decreases, and the information can also be quickly erased by heat treatment. It has important application potential in the field of real-time security display. Furthermore, Z-DPMNs exhibit prolonged stability at low temperatures (Supplementary Fig. 53). Consequently, they hold significant value for temperature monitoring in specialized materials requiring preservation at low temperatures.

Under the background of products and information security, the demand for encrypted ink is increasing. The majority of previously reported anti-counterfeiting systems are easily identified due to their single stimulation mode. Due to the dual-channel stimulus-response and sensitive photochromic properties of Z-DPMNs, as well as the fluorescent properties of E-DPMNs, DPMNs exhibit enormous potential as safety inks for application in complex anti-counterfeiting systems. Z-H and Z-3-OMe solids have photochromic properties, while E-H and E-3-OMe solids have strong fluorescence. Therefore, the two isomers are mixed in a 1:1 ratio to obtain the inks with both photochromic and fluorescence properties (Fig. 5a). The different optical properties of these inks are summarized in Table 1. The six inks were spotted on the newspaper of Fujian Normal University according to the design diagram and dried in the air (Fig. 5b, c). When irradiated with blue light, only No. 4 and No. 6 inks have photochromic properties, and the dark green character JU appears under daylight. After being irradiated by UV light, No. 1, 3, 4, and 6 inks have photochromism, showing the FJNU in the newspaper under daylight with purple F and N, and dark green J and U. Due to the strong fluorescence of No. 2, 3, 5 and 6 inks, the numbers 1907 (year of establishment of FJNU) is displayed under the UV light with blue emission for 1 and 0, and green emission for 9 and 7. Therefore, the security of the newspaper has been improved by encrypted ink.

Fig. 5: Application for encrypted ink.
figure 5

a The photochromic behavior and luminescent properties of the two mixtures. The power of the ultraviolet lamp is 5 W, and the blue lamp is 1 W. b The schematic procedure of safety ink used in anti-counterfeiting pattern. c The anti-counterfeiting and identification process in school newspaper.

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Table 1 Solid-state photophysical properties of compounds applied in different inks
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Discussion

In summary, we proposed a strategy named photoinduced π-bond splitting to construct organic stimulus-responsive materials based on aryl ethylene derivatives DPMNs. Both experimental data and theoretical calculations have demonstrated that the photochromism of Z-DPMNs derives from the photoinduced π-bond splitting, which produces stable diradicals and realizes a closed-open shell transformation. Moreover, the photochromic properties of Z-DPMNs can be regulated by altering substituent electronic effects and intermolecular interactions. The substituents with electron-withdrawing effects have been found to facilitate the closed-open shell transformation of Z-DPMNs in both solid and solution states. The loose stacking and weak interactions of Z-DPMNs in the solid state are favorable for the generation of diradicals. In addition, the dual-channel stimulus-response of Z-DPMNs in visible light and UV light has been realized. Leveraging the sensitive and reversible photochromic phenomenon of Z-DPMNs, we have successfully developed a type of rewritable photopatterning paper and security ink. This work provides an efficient strategy for the formation and regulation of organic radicals, opening avenues for potential applications in adaptive camouflage, anti-counterfeiting, and information storage.

Methods

Measurements

NMR spectra were measured by a Bruker Ascend 400 FT-NMR at room temperature. UV-visible absorption spectra of the dilute solution were measured by a Shimadzu UV-2600 spectrophotometer, and the solid were measured by a PUXI TU-1901 UV spectrophotometer. Fluorescence spectra of the solid were obtained on an Edinburgh Instruments FS5 Steady State Fluorescence Spectrofluorometer. Powder X-ray diffraction (PXRD) data were collected using a PANalytical X-ray Diffractometer (X’Pert3 Powder) with Cu Kα radiation (λ = 1.54178 Å). Attenuated total reflection Fourier transform infrared (ATR-IR) spectrum was acquired on a Thermo Scientific Nicolet IS50 spectrometer within the wavenumber range 4000–400 cm−1. Electron paramagnetic resonance (EPR) measurements were performed at room temperature using a Magnettech ESR5000X spectrometer. Electron paramagnetic resonance (EPR) measurements were performed at 100 K using a CIQTEK EPR200M spectrometer. High-resolution mass spectra (HRMS) in positive mode were recorded using a G6520B series quadrupole time-of-flight (Q-TOF) mass spectrometer (Agilent) combined with liquid chromatography.

X-ray crystallography

The single crystals of DPMNs were mounted on a glass fiber for the X-ray diffraction analysis. Data sets were collected on an Agilent Technologies SuperNova single crystal diffractometer equipped with a graphite monochromated Mo-Kα radiation (λ = 1.54184 Å) from a rotating anode generator at 298 K. Intensities were corrected for Lorentz-Polarization (LP) factors and empirical absorption using the ω scan technique. The structures were solved by direct methods and refined on F2 with full-matrix least-squares techniques using Siemens SHELXTL version 5 package of crystallographic software.

Theoretical calculation

Gaussian 09 D.01 program package was used to study the diradical nature of Z-H-P. Geometry optimization and frequency calculation of triplet diradical structure was performed using unrestricted Density Functional Theory (UDFT) method with B3LYP functional at the basis set level of 6-31 g(d). The optimized structure was confirmed to be located at its energy minimum without imaginary frequencies before further calculation. A larger basis set def2-TZVP was utilized for further calculation of single-point energy. The time-dependent Density Functional Theory (TDDFT) method was applied to obtain the ultraviolet absorption spectrum of triplet diradical at the level of uB3LYP/6-31 g(d). The spin density population was analyzed using Multiwfn and visualized on VMD software.

Materials

All reagents and solvents obtained from commercial suppliers were directly used without further purification unless otherwise stated. Column chromatography was performed by Qingdao Haiyang Chemical silica gel (200–300 mesh).

Synthesis of compound E-DPMNs

Compound E-DPMNs shown in Supplementary Fig. 1 were prepared. The following is the synthesis method of related compounds54,55,56,57,58. Phenylacetonitrile derivatives (17 mmol) and iodine (4.35 g, 17 mmol) were dissolved in 17 mL of dry diethyl ether at dry ice temperature under a nitrogen atmosphere. Then, to the solution slowly added 10 mL CH3ONa solution (Na, 0.8 g, 34.78 mmol and methanol, 10 mL). The reaction solution was allowed to warm by replacing the dry ice bath with an ice-water bath before the temperature rose to above 0 °C. The reaction solution was stirred for another 3-4 h at room temperature, and then the reaction was quenched with 3-6% hydrochloric acid at less than 10 °C and filtered to isolate the solid, which was rinsed with cold methanol-water solution to wash away ionic substances. The resultant mixture was extracted several times with ethyl acetate, and then the organic layer was washed in a saturated NaCl aqueous solution and dried over anhydrous Na2SO4. After the solvent was removed under reduced pressure, the crude product was purified by column chromatography or recrystallization to give the pure product.

Synthesis of compound Z-DPMNs

To a solution of compound E-DPMNs (50 mg) in CHCl3 (30 mL) was irradiated by a UV lamp and the reaction monitored by TLC. After 3 h, the solvent was evaporated under reduced pressure. Further purification was carried out by column chromatography with the mixture of petroleum ether/ dichloromethane (1: 1, v/v) as eluent to give the pure product.

Z -H (Z-2,3-diphenylmaleonitrile)

E-H (50 mg, 0.22 mmol) was utilized as the raw material to synthesize Z-H. White solid: Isolated mass 22.5 mg, Yield 45%. 1H NMR (400 MHz, Chloroform-d) δ 7.45-7.40 (m, 2H), 7.38-7.30 (m, 8H); 13C NMR (101 MHz, Chloroform-d) δ 131.20, 130.61, 129.42, 129.22, 126.09, 116.85. ESI MASS m/z [M + H]+ calcd 231.0917, found 231.0928.

Z -F (Z-2,3-bis(4-fluorophenyl)maleonitrile)

E-F (50 mg, 0.19 mmol) was utilized as the raw material to synthesize Z-F. White solid: Isolated mass 20 mg, Yield 40%. 1H NMR (400 MHz, Chloroform-d) δ 7.34 (dd, J = 9.0, 5.1 Hz, 4H), 7.13-7.03 (m, 4H); 13C NMR (101 MHz, Chloroform-d) δ 165.31, 162.77, 131.72, 131.63, 126.48, 126.45, 124.89, 116.94, 116.72, 116.53. ESI MASS m/z [M + H]+ calcd 267.0728, found 267.0722.

Z -Cl (Z-2,3-bis(4-chlorophenyl)maleonitrile)

E-Cl (50 mg, 0.17 mmol) was utilized as the raw material to synthesize Z-Cl. White solid: Isolated mass 20 mg, Yield 40%. 1H NMR (400 MHz, Chloroform-d) δ 7.36 (d, J = 8.4 Hz, 4H), 7.27 (d, J = 8.6 Hz, 4H); 13C NMR (101 MHz, Chloroform-d) δ 137.91, 130.69, 129.82, 128.73, 125.22, 116.35. ESI MASS m/z [M + H]+ calcd 299.0137, found 299.0125.

Z -TFMe (Z-2,3-bis(4-(trifluoromethyl)phenyl)maleonitrile)

E-TFMe (50 mg, 0.14 mmol) was utilized as the raw material to synthesize Z-TFMe. White solid: Isolated mass 17.5 mg, Yield 35%. 1H NMR (400 MHz, Chloroform-d) δ 7.67 (d, J = 8.2 Hz, 4H), 7.46 (d, J = 8.1 Hz, 4H); 13C NMR (101 MHz, Chloroform-d) δ 133.46, 129.91, 126.56,126.39, 124.48, 121.77, 115.88. ESI MASS m/z [M + NH4]+ calcd 384.093, found 384.0948.

Z -Me (Z-2,3-di-p-tolylmaleonitrile)

E-Me (50 mg, 0.19 mmol) was utilized as the raw material to synthesize Z-Me. Faint yellow solid: Isolated mass 15 mg, Yield 30%. 1H NMR (400 MHz, Chloroform-d) δ 7.24 (d, J = 8.3 Hz, 4H), 7.14 (d, J = 8.6 Hz, 4H), 2.38 (s, 6H); 13C NMR (101 MHz, Chloroform-d) δ 141.78, 129.88, 129.31, 127.97, 125.16, 117.15, 21.52. ESI MASS m/z [M+Na]+ calcd 281.1049, found 281.1034.

Z -OMe (Z-2,3-bis(4-methoxyphenyl)maleonitrile)

E-OMe (50 mg, 0.17 mmol) was utilized as the raw material to synthesize Z-OMe. Yellow solid: Isolated mass 15 mg, Yield 30%. 1H NMR (400 MHz, Chloroform-d) δ 7.31 (d, J = 8.8 Hz, 4H), 6.85 (d, J = 8.8 Hz, 4H), 3.85 (s, 6H); 13C NMR (101 MHz, Chloroform-d) δ 161.56, 131.10, 123.46, 123.17, 117.36, 114.61, 55.48. ESI MASS m/z [M+Na]+ calcd 313.0947, found 313.0951.

Z -2-OMe (Z-2,3-bis(2-methoxyphenyl)maleonitrile)

E-2-OMe (50 mg, 0.17 mmol) was utilized as the raw material to synthesize Z-2-OMe. White solid: Isolated mass 16 mg, Yield 32%. 1H NMR (400 MHz, Chloroform-d) δ 7.37-7.30 (m, 2H), 6.94 (dd, J = 7.7, 1.5 Hz, 2H), 6.88 (d, J = 8.4 Hz, 2H), 6.80 (t, J = 7.5 Hz, 2H), 3.77 (s, 6H); 13C NMR (101 MHz, Chloroform-d) δ 156.81, 132.23, 130.11, 125.22, 120.73, 120.51, 116.29, 111.44, 55.59. ESI MASS m/z [M + H]+ calcd 291.1128, found 291.1126.

Z -3-OMe (Z-2,3-bis(3-methoxyphenyl)maleonitrile)

E-3-OMe (50 mg, 0.17 mmol) was utilized as the raw material to synthesize Z-3-OMe. Yellow solid: Isolated mass 19 mg, Yield 38%.1H NMR (400 MHz, Chloroform-d) δ 7.27 (d, J = 8.3 Hz, 2H), 6.94 (t, J = 8.2 Hz, 4H), 6.83 (s, 2H), 3.69 (s, 6H); 13C NMR (101 MHz, Chloroform-d) δ 159.84, 131.72, 130.32, 126.09, 121.76, 117.41, 116.74, 114.20, 55.36. ESI MASS m/z [M + H]+ calcd 291.1128, found 291.1125.

Z -3,5-OMe (Z-2,3-bis(3,5-dimethoxyphenyl)maleonitrile)

E-3,5-OMe (50 mg, 0.14 mmol) was utilized as the raw material to synthesize Z-3,5-OMe. Faint yellow solid: Isolated mass 15 mg, Yield 30%. 1H NMR (400 MHz, Chloroform-d) δ 6.48 (t, J = 2.3 Hz, 6H), 3.69 (s, 12H); 13C NMR (101 MHz, Chloroform-d) δ 161.08, 132.12, 126.25, 116.23, 107.16, 103.55, 55.53. ESI MASS m/z [M + K]+ calcd 389.0898, found 389.0918.

Z -3-F (Z-2,3-bis(3-fluorophenyl)maleonitrile)

E-3-F (50 mg, 0.19 mmol) was utilized as the raw material to synthesize Z-3-F. Faint yellow solid: Isolated mass 21 mg, Yield 42%. 1H NMR (400 MHz, Chloroform-d) δ 7.41-7.34 (m, 2H), 7.23-7.09 (m, 4H), 7.02 (d, J = 9.0 Hz, 2H); 13C NMR (101 MHz, Chloroform-d) δ 163.88, 161.40, 132.02, 131.94, 131.31, 131.23, 125.81, 125.78, 125.26, 125.23, 118.89, 118.68, 116.52, 116.29, 116.13. ESI MASS m/z [M+Na]+ calcd 289.0548, found 289.0555.