Photoinduced π-Bond breakage causing dynamic closing-opening shell transition of Z-type Diphenylmaleonitriles molecules

Photo-responsive materials offer fast, spatially and temporally precise responses to light and are used in molecular machines, drug delivery, logic operations, information processing, and optoelectronics. Many organic photochromic systems rely on isomerization or rearrangement in closed-shell molecules (e.g., azobenzenes, diarylethenes, spiropyrans), but achieving sensitive, reversible photochromism in the solid state is challenging due to restricted molecular motion. Dynamic covalent systems featuring closed–open shell transformation (COST) leverage reversible bond cleavage/formation and associated electronic-state changes to deliver high color contrast, yet existing COST platforms (e.g., reversible σ-bond scission or quinone/radical interconversion) are often insensitive, slow, and difficult to monitor in aggregates. π-Bonds, with lower bond energies than σ-bonds, could enable dynamic regulation via π-bond cleavage to diradicals, but such species are typically unstable and fail to give robust, visible responses. This work asks whether cis-2,3-diphenylmaleonitriles (Z-DPMNs) can undergo photoinduced π-bond splitting to generate sufficiently stabilized diradicals to realize reversible COST and visible photochromism in both solution and solid states. By exploiting cyano-group electron-withdrawing effects and aryl conjugation to stabilize diradicals, the study targets dual-channel (UV and visible) photochromism, tunability via substituents and packing, and applications in rewritable photopatterning and encrypted inks.

Prior photochromic materials span organic, inorganic, and hybrid classes, with organics attractive for biocompatibility and flexible electronics. Traditional mechanisms include isomerizations of azobenzenes, diarylethenes, and spiropyrans, which often struggle in the solid state. COST systems involving reversible σ-bond (C–C) scission or quinone/radical interconversion provide large optical contrasts but are frequently slow or insensitive, especially in aggregates. Examples include swelling-induced COST in difluorenylsuccinonitrile gels (colorless to pink) and quinone-based small molecules responsive to reactive species in vivo. Despite the potential of π-bond scission—energetically easier than σ-bond cleavage—reports are scarce due to the instability of resulting diradicals and the difficulty of achieving persistent, observable signals. Stabilization strategies via electron-withdrawing groups and conjugation have been explored in related radical systems to enhance lifetimes, motivating the present design of dicyano-substituted aryl ethylenes for photoinduced π-bond cleavage.

Synthesis: Z-DPMNs were obtained from phenylacetonitrile derivatives via dimerization to E-DPMNs and subsequent UV-induced E→Z isomerization in CHCl3 (~3 h), followed by chromatographic purification. Multiple derivatives were prepared: Z-H, Z-F, Z-Cl, Z-TFMe, Z-Me, Z-OMe, Z-2-OMe, Z-3-OMe, Z-3,5-OMe, and Z-3-F (typical yields 30–45%). Characterization: Structures were confirmed by HRMS, 1H NMR, and 13C NMR. Photophysics were studied by UV–Vis absorption (solid: PUXI TU-1901; solution: Shimadzu UV-2600) and steady-state fluorescence (Edinburgh FS5). Photoactivation used 365 nm UV lamps (commonly 5 W) and 450 nm blue lamps (1 W), monitoring coloration/fading kinetics and cycling stability. PXRD (PANalytical X'Pert3 Powder) and ATR-IR (Nicolet IS50) excluded phase and covalent structural changes during photochromism. EPR spectroscopy (Magnettech ESR5000X at room temperature; CIQTEK EPR200M at 100 K) probed radical formation and triplet signatures; spin trapping employed DMPO to capture carbon-centered radicals. Single-crystal X-ray diffraction: Structures of Z-H and E-H were solved (Agilent Technologies SuperNova, Mo-Kα radiation) to assess packing, π–π interactions, hydrogen bonding, twist angles, and C=C bond lengths. Computations: DFT and TDDFT (Gaussian 09) examined diradical Z-H-P. Triplet diradical geometries were optimized using UDFT with B3LYP/6-31g(d), confirmed by frequency analysis (no imaginary frequencies). Single-point energies used def2-TZVP. TDDFT (uB3LYP/6-31g(d)) provided vertical excitations. Spin densities were analyzed with Multiwfn and visualized in VMD. Mechanistic probes: Time-dependent UV–Vis under short (∼2 s) vs long (>30 s) UV irradiation distinguished π-bond splitting (diradical formation) from competing photocyclization to dihydrophenanthrene and downstream products (DPCN, PDCN). Low-temperature EPR examined triplet characteristics (half-field signals). Parametric studies: Substituent effects (–OMe, –Me, –F, –Cl, –CF3; positional variants 2-, 3-, 3,5-) were screened in solid and solution states. External factors included temperature (0–80 °C; 77 K) and solvent polarity (toluene, CH2Cl2, CHCl3 vs EA, dioxane, THF), and oxygen vs anaerobic conditions. Applications: Rewritable photopatterning was demonstrated by impregnating A4 paper with Z-3-F and photoprinting with masks using UV/blue light; erasure via natural decay or heat. Security inks combined photochromic Z-isomers with fluorescent E-isomers (1:1 mixtures) and were deployed to print encrypted patterns and QR codes readable by smartphone within defined time windows.

• Z-H exhibits rapid and reversible photochromism in both solid and solution: solid changes from white to purple in ∼2 s under 365 nm UV; color persists for ~11 min after light-off and is repeatable for >30 cycles without significant loss. In CH2Cl2 solution, coloration occurs in 2 s and fades within ~6 min. • Upon UV irradiation, Z-H shows a new absorption band at 524 nm (solid); E-H shows no spectral change or photochromism. The photochromic state (Z-H-P) shows strong EPR signals (g ≈ 2.0004), consistent with organic carbon-centered radicals; no EPR signal in E-H before/after irradiation. • Spin trapping with DMPO quenches the 480 nm solution absorption and yields an EPR sextet (g ≈ 2.0063), confirming carbon-centered radical adducts. • PXRD and ATR-IR indicate no phase transitions or covalent structural changes during photochromism; 1H NMR shows no impurities, supporting a reversible electronic-state transformation (closed- to open-shell) rather than chemical reaction. • Crystal structures explain isomer-dependent behavior: Z-H has looser packing and a twisted geometry (benzene ring twist 7.15°, C=C 1.360 Å), while E-H is more tightly packed with π-stacking (3.947 Å) and hydrogen bonding (3.646 Å) and coplanar rings (0°), favoring photoresponse only in Z-H. • TDDFT reproduces the new absorption (calc. λmax ≈ 447 nm vs exp. ≈ 480 nm) for the diradical Z-H-P; spin density is centered on the ethylenic carbons and delocalized onto cyano and phenyl groups. DFT indicates a large ΔE S,T ≈ 14 kcal/mol, suggesting triplet-ground-state diradical character. • Low-temperature EPR shows enhanced radical signals and a half-field (ΔmI = ±2) transition near 170 mT, supporting triplet species. The EPR decay matches the 524 nm band decay, linking color change to triplet diradicals. • Competing pathway: under prolonged UV (>30 s), Z-H undergoes photocyclization to dihydrophenanthrene intermediates, yielding DPCN (isomerization) and PDCN (oxidation). The photochromism observed under short irradiation is not due to 2H-DPCN (calc. λmax ≈ 570 nm), implicating diradicals instead. • Substituent effects: In solids, Z-F, Z-Cl, and Z-TFMe (electron-withdrawing) display photochromism with weak luminescence; activation durations ∼20 s (Z-F), 150 s (Z-Cl), 180 s (Z-TFMe). Z-OMe and Z-Me (electron-donating) do not photochromize in solids and are fluorescent; all E-isomers are fluorescent without photochromism. Only photochromic Z-derivatives show EPR radical signals (g ≈ 2.0008–2.0014) after UV. • Positional effects: Z-3-OMe shows photochromism under UV and 450 nm blue light, while Z-2-OMe does not. Z-3,5-OMe lacks solid-state photochromism (due to strong intermolecular interactions) but shows solution photochromism. Z-3-F exhibits longer fading time (∼40 min) than Z-F; stronger EW effects at 3-position favor persistent photochromism. • External factors: Higher temperature accelerates fading (Z-3-F: ~120 min at 0 °C → 15 min at 40 °C → ~1 min at 80 °C). At 77 K, Z-H-P solution remains colored for over a week. Increased solvent polarity shortens radical lifetime; photochromism is prominent in low/nonpolar solvents at room temperature and emerges in more polar solvents at lower temperature. Anaerobic conditions prolong fading times by avoiding oxygen quenching. • Dual-channel activation: Some Z-DPMNs (e.g., Z-F, Z-Cl) absorb up to ~430 nm, enabling visible-light-activated photochromism; fading times depend on light power. • Applications: Rewritable photopatterning on Z-3-F-coated paper enables multiple images erased by time or heat; high-resolution QR codes readable (e.g., FJNU) within ~35 min. Encrypted inks combining Z- and E-isomers (e.g., Z-H:E-H and Z-3-OMe:E-3-OMe at 1:1) provide multi-modal readouts: selective photochromism under blue/UV and strong fluorescence under UV to display hidden information (e.g., characters FJNU and numbers 1907).

The study demonstrates that cis-2,3-diphenylmaleonitriles can undergo photoinduced π-bond cleavage to form relatively stable, triplet diradicals, enabling a reversible closed–open shell transformation (COST) that manifests as strong, repeatable photochromism in both solution and solid states. This directly addresses a central challenge in photochromic dynamic covalent materials: achieving sensitive, solid-state responses without relying on significant molecular motion. The combination of dicyano stabilization, aryl conjugation, and a twisted Z-geometry reduces p-orbital overlap and weakens the C=C π-bond, facilitating diradical formation. Packing that minimizes strong intermolecular interactions further promotes the response in the solid state. Tunability via substituent electronics and positional effects allows control over activation wavelength (UV/visible), coloration kinetics, and lifetime of the open-shell state. External factors—temperature, solvent polarity, and oxygen—modulate diradical stability, aligning with a triplet-radical mechanism. The discovery of a competing photocyclization pathway under prolonged irradiation clarifies operational windows to maintain reversible diradical photochromism. Collectively, the findings establish π-bond splitting as a viable dynamic covalent strategy for responsive materials and translate into practical functions such as rewritable photopatterning and complex, multi-mode encrypted inks.

This work introduces photoinduced π-bond splitting in Z-type 2,3-diphenylmaleonitriles as an effective route to generate stabilized triplet diradicals and realize reversible closed–open shell transformation with conspicuous photochromism in solid and solution. The mechanism is validated by spectroscopy, crystallography, EPR (including triplet signatures), spin trapping, and DFT/TDDFT calculations. Electron-withdrawing substituents, favorable molecular packing, and lower polarity/temperature enhance diradical generation and lifetime, while select derivatives support visible-light activation. The materials enable rewritable photopatterning and sophisticated anti-counterfeiting/encryption through combined photochromic and fluorescent behaviors. Future research could focus on red-shifting activation wavelengths (e.g., into the green–red or NIR), improving radical lifetimes at ambient conditions, mitigating competing photocyclization under prolonged exposure, integrating into device platforms, and expanding the molecular design space (other acceptor/aryl frameworks) for broader functionality and stability.

• Diradical stability is sensitive to environmental factors: higher temperature, higher solvent polarity, and oxygen shorten lifetimes and accelerate fading, limiting operation windows at ambient conditions. • Visible-light activation is limited to certain derivatives (e.g., Z-F, Z-Cl) and wavelengths up to ~430 nm; many materials still require UV excitation. • Prolonged irradiation promotes competing photocyclization to phenanthrene-type products (DPCN/PDCN), potentially reducing reversibility under long exposure. • Solid-state responses can be suppressed by strong intermolecular interactions and tight packing (e.g., Z-3,5-OMe), indicating substrate and morphology dependence. • Not all substitution patterns yield photochromism (e.g., Z-2-OMe, Z-OMe), narrowing the structural scope without careful design.