Photocured room temperature phosphorescent materials from lignosulfonate

The study addresses the challenge of creating practical, photocured organic room temperature phosphorescent (RTP) materials. Conventional RTP processing relies on solvent-based coating and drying, which is time-consuming and may alter the matrix; moreover, photocured RTP formulations typically require separate photosensitizers and chromophores, increasing complexity and risking quenching of RTP. The work leverages two design principles for RTP—efficient intersystem crossing and radiative triplet emission—and proposes using lignosulfonate, a lignin derivative with intrinsic optical activity, as both photoinitiator and RTP chromophore in an ionic liquid/acrylamide system. The goal is to realize fast, ambient photocuring to confine lignosulfonate within a crosslinked matrix, thus suppressing nonradiative decay and achieving efficient RTP, while simplifying formulation and enabling applications such as coatings, printing, and 3D fabrication.

Organic RTP materials have been developed across supramolecular systems, molecular crystals, MOFs, polymer composites, and carbon dots, guided by promoting intersystem crossing and stabilizing triplet emissions. Practical deployment often uses solvent processing, with associated drawbacks. Photocuring offers rapid, non-contact processing but few photocured RTP systems exist; using separate photoinitiators can quench chromophore triplets. Lignin, an abundant aromatic biopolymer, shows UV-blocking, fluorescence, photothermal, photocatalytic, and RTP behaviors, and has been explored as macromolecular photoinitiators for polymerizations and for afterglow materials in wood and hydrogels. Recent works demonstrate heavy-atom effects, rigidity-mediated afterglow tuning, and phosphorescence energy transfer strategies for delayed fluorescence. These inform the present approach: exploiting lignosulfonate’s radical generation and chromophore roles, confinement in a polymer matrix, and external heavy-atom effects from ionic liquids to enhance RTP.

Formulation and photocuring: Mixtures comprised acrylamide monomer (typically 500 mg, 7.03 mmol), lignosulfonate (1 mg; optimized at 0.1–0.15% w/w relative to ionic liquid), and 1-ethyl-3-methylimidazolium bromide (EMIM-Br; 400 mg, 2.09 mmol). Precursors were heated at 80 °C to homogeneity, then irradiated with 365 nm UV LED (170 mW cm⁻²) for 20 min to yield bulk, transparent photocured P-Lig. A Rhodamine B (RhB)-doped variant (P-Lig/RhB) was prepared identically with RhB (0.7 mg, 0.0015 mmol). Control samples included: (i) ammonium persulfate-initiated curing of acrylamide in EMIM-Br; (ii) systems using Ir2959 or benzophenone as external photoinitiators with lignosulfonate or phenylboronic acid chromophores; (iii) ionic liquids with different anions (Br⁻, Cl⁻, CH₃COO⁻); (iv) varying lignin sources (kraft, alkali, enzymatic hydrolysis lignin); and (v) monomer scope (acrylic acid, methyl acrylate, methyl methacrylate, styrene, N-isopropylacrylamide). Monitoring and characterization: Electron spin resonance (ESR, with DMPO spin trap) tracked radical generation from lignosulfonate under 365 nm irradiation and guided optimal lignosulfonate loading. FT-IR and in situ FT-IR quantified double-bond conversion and probed hydrogen bonding (carbonyl shifts). 1H NMR and in situ FT-IR evidenced covalent attachment of lignosulfonate to polyacrylamide via phenolic group reaction with double bonds (C–O–C formation). UV–Vis assessed lignosulfonate absorption (300–400 nm). Photoluminescence (standard and delayed) and lifetime measurements (e.g., exc. 320 nm; collection at 510 nm; delay 10 ms) determined RTP intensity, wavelength, quantum yield, and lifetime as a function of curing time, humidity, temperature, solvent exposure, and formulation variables. Energy transfer analysis for P-Lig/RhB included overlap assessment, donor quenching studies (intensity and lifetime vs RhB concentration), direct vs indirect excitation tests, and FRET efficiency calculation across RhB loadings (0.01–0.1%). Mechanical properties (hardness, Young’s modulus) of cured samples were compared among initiator/chromophore conditions. Application demonstrations: Photocured coatings on cotton yarns (UV 365 nm, 170 mW cm⁻², 30 min) to create green (P-Lig) and red (P-Lig/RhB) afterglow yarns; screen printing on diverse substrates (plastic, glass, paper, PVA) with P-Lig/RhB as printable RTP coatings; layer-by-layer photocuring to build 3D bulk materials with multicolor RTP; water-assisted reprocessing of P-Lig 3D materials into new shapes via dispersion and evaporation-induced drying; preparation of portable solid inks by dispersing P-Lig/RhB in water for information encryption applications. Recycling: Recovery of ~96% of EMIM-Br ionic liquid, verified by 1H NMR to match virgin IL; performance of P-Lig prepared using recycled IL was benchmarked by RTP lifetime.

• Lignosulfonate acts as both photoinitiator and RTP chromophore: under 365 nm UV, ESR confirmed formation of quinone and hydroxyl radicals in EMIM-Br; optimal lignosulfonate loading for radical generation was 0.1–0.15% w/w in ionic liquid.
• Efficient photocuring: FT-IR showed ~96% double bond conversion in 20 min. Radical scavenger (DMPO) suppressed polymerization, confirming light-mediated radical formation.
• Covalent integration: 1H NMR and in situ FT-IR indicated lignosulfonate covalently attaches to polyacrylamide via phenolic group reaction forming C–O–C, confining chromophores in a crosslinked matrix.
• Strong RTP from P-Lig: No RTP before curing; after curing, green RTP at 510 nm with phosphorescence quantum yield 11.04% and lifetime increasing with curing time to ~110 ms after 20 min. Reproducibility confirmed over five preparations (~110 ms lifetimes).
• Environmental response: Humidity reduced lifetime to 24 ms; immersion in water decreased lifetime progressively (13.7 ms at 30 min; complete quenching at 40 min), fully recoverable by drying at 80 °C for 60 min and over multiple humidity-drying cycles. Exposure to various organic solvents for three months did not quench RTP, with negligible lifetime changes.
• Temperature effect: Higher temperatures decreased RTP intensity and lifetime due to enhanced nonradiative processes.
• Photoinitiator quenching effect: Systems initiated by Ir2959 or benzophenone with lignosulfonate produced shorter lifetimes (42.63 ms and 36.97 ms) than P-Lig, and with phenylboronic acid chromophore, lifetimes were shorter than APS-initiated controls, evidencing external photoinitiator-induced quenching. Lignosulfonate-initiated samples also showed superior mechanical properties (hardness, Young’s modulus) compared to Ir2959-initiated controls.
• Generality: Other monomers (acrylic acid, methyl acrylate, methyl methacrylate, styrene, N-isopropylacrylamide) photocured in 20 min and exhibited RTP lifetimes of 76.96, 62.95, 57.95, 67.19, and 85.56 ms, respectively. Other lignin sources (kraft, alkali, enzymatic hydrolysis) served as photoinitiators, achieving double bond conversions of 89.27%, 97.24%, and 73.89%, with RTP emission.
• Energy transfer to RhB: Spectral overlap enabled triplet-to-singlet FRET from P-Lig to RhB in P-Lig/RhB, yielding red afterglow (~600 nm) with delayed lifetime ~39 ms and combined fluorescence/delayed fluorescence quantum yield of 57.62%. No delayed emission upon direct 550 nm excitation of RhB, but strong delayed emission via donor excitation at 320 nm. FRET efficiencies at RhB loadings were 16.5%, 19.4%, 23.9%, 34.0%, 45.9%, 64.6%, and 79.7% for 0.01%, 0.03%, 0.05%, 0.07%, 0.08%, 0.09%, and 0.1%, respectively.
• Mechanistic factors: In situ FT-IR showed carbonyl shift (1677→1670 cm⁻1), indicating enhanced hydrogen bonding upon curing. Calculations showed increased binding energies between lignosulfonate and polymerized acrylamide (−9.29 eV monomer; −11.93 eV dimer; −17.14 eV for 6-mer; vs −13.82 eV for lignosulfonate with six monomers), supporting vibration restriction and RTP enhancement. External heavy-atom effect from ionic liquid anions was critical: lifetimes for Br⁻, Cl⁻, and CH₃COO⁻ systems were 109.72 ± 0.52 ms, 89.27 ± 0.43 ms, and 38.67 ± 0.41 ms, respectively.
• Applications: Successful fabrication of RTP-coated yarns (green/red), screen-printed RTP patterns on diverse substrates, multilayer 3D RTP structures via layer-by-layer curing, water-assisted reprocessing of 3D P-Lig, and P-Lig/RhB-based portable solid inks for information encryption. ~96% of ionic liquid was recyclable; P-Lig made with recycled IL retained ~110 ms lifetime.

Using lignosulfonate as a dual-function component simplifies photocured RTP formulations and avoids photoinitiator-induced quenching, yielding long-lived RTP (~110 ms) and robust mechanical performance. Photocuring confines lignosulfonate within a crosslinked matrix, enhancing hydrogen bonding and restricting molecular motion, which stabilizes triplet states. The ionic liquid’s halide anions impart an external heavy-atom effect that further promotes intersystem crossing and RTP efficiency. The system’s modularity (broad monomer and lignin source compatibility) and stability in organic solvents support versatility, while humidity sensitivity is reversible via drying. Triplet-to-singlet energy transfer from P-Lig to RhB enables red afterglow in P-Lig/RhB, expanding color tunability without direct excitation of the acceptor. Collectively, these results address the need for fast, ambient, and scalable RTP fabrication, enabling practical applications in coatings, anti-counterfeiting, printing, 3D manufacturing, and inks with recyclability of the ionic liquid reducing waste.

The work demonstrates a simple, efficient approach to photocured organic RTP by employing lignosulfonate as both photoinitiator and chromophore in an acrylamide/ionic liquid system, achieving strong, long-lived RTP (~110 ms) and eliminating external photoinitiators that can quench emission. Incorporation of RhB yields red afterglow via triplet-to-singlet FRET, broadening functionality. The materials are readily processed into coatings, printed patterns, 3D structures, and inks, with robust mechanical properties and recyclable ionic liquid. Future directions include integrating lignin with sustainable deep eutectic solvents and diverse monomers to create additional scalable, high-performance, and environmentally benign RTP materials with tunable emissions and improved moisture tolerance.

• Moisture sensitivity: RTP lifetime decreases significantly with humidity and water immersion (down to 13.7 ms at 30 min, quenched by 40 min), though recoverable by drying.
• Temperature dependence: Elevated temperatures reduce RTP intensity and lifetime due to increased nonradiative decay.
• Aqueous processing limitation: Lignosulfonate does not effectively initiate curing in aqueous media (only ~10% conversion), requiring ionic liquid environments for efficient radical generation and curing.
• Color-shifted system lifetime: The red afterglow in P-Lig/RhB exhibits a shorter delayed lifetime (~39 ms) compared to green P-Lig (~110 ms).
• Dependence on heavy halide anions: Optimal RTP relies on ionic liquids with halides (Br⁻, Cl⁻); acetate anions yield much lower lifetimes.