Stepwise taming of triplet excitons via multiple confinements in intrinsic polymers for long-lived room-temperature phosphorescence

Long-lived organic RTP materials are attractive for illumination displays, bioimaging, data encryption, and anti-counterfeiting due to biocompatibility, large Stokes shifts, and low cost. Their performance is limited by spin-forbidden S→T intersystem crossing (ISC) and rapid nonradiative decay of triplet excitons. Prior strategies enhance RTP by (i) maximizing ISC via heteroatoms/heavy atoms to increase spin–orbit coupling and (ii) suppressing nonradiative processes via rigid environments (crystals, aggregation, host–guest doping). Polymer-based RTP, commonly achieved by doping phosphors into polymer matrices such as PVA, can suffer from phase separation, poor compatibility, and insufficient control of ISC and nonradiative decay. Copolymerization of phosphor groups with matrix monomers (e.g., acrylic acid, acrylamide, styrene sulfonate, vinyl pyridine) introduces covalent bonding to improve stability and reduce phase separation but often lacks extensive covalent crosslinking and thus shows limited suppression of nonradiative deactivation. Fast click reactions between boric acid and PVA can form borate ester RTP, but aryl boronic acid frameworks can increase segmental mobility, undermining confinement. The study proposes a stepwise confinement strategy—primary (copolymerization), secondary (hydrogen bonding after alcoholysis), and tertiary (boric acid crosslinking)—to simultaneously promote ISC and suppress nonradiative decay, thereby taming triplet excitons and enabling intrinsically polymeric RTP with long lifetimes, higher quantum yields, and multicolor emission.

- Doped polymer RTP systems (polymer@PVA, small molecule@PVA, carbon dot@PVA) leverage hydrogen-bonding networks but face phase separation and compatibility issues. - Copolymer approaches with monomers like AA, AM, styrene sulfonate/sodium, and vinyl pyridine yield ambient RTP copolymers with improved stability via covalent bonding; however, limited covalent crosslinking can leave nonradiative decay insufficiently suppressed. - Boric acid–PVA click crosslinking can generate RTP but high mobility in aryl boronic acid-based frameworks may not effectively confine local motions, potentially increasing nonradiative pathways. - Overall, prior art often improves only one aspect (either increasing ISC or restricting nonradiative decay) rather than simultaneously addressing both; the need is for structural confinement combining covalent crosslinking and hydrogen bonding to tame triplet excitons.

- Stepwise structural confinement design: 1) Primary confinement (P): Copolymerize vinyl acetate (VAc) with phosphor units to form PVAc copolymers. Main model system uses 2-vinyl naphthalene (2VN) as phosphor. Four feed ratios explored (VAc:2VN = 300:1, 500:1, 700:1, 1000:1; denoted P1–P4). Polymerization with AIBN in methanol at 65 °C for 48 h, workup by precipitation and drying. 2) Secondary confinement (H): Alcoholysis of PVAc copolymers to PVA-based copolymers using NaOH in methanol at 45–65 °C to introduce hydroxyls and hydrogen-bonding networks, followed by neutralization, washing, and drying, yielding H1–H4. 3) Tertiary confinement (B): Crosslink H-series with boric acid (0.5 M aqueous) at elevated temperature (~100 °C) to form B–O–C crosslinked networks (B1–B4), then wash and dry. - Extension to other phosphors: analogous P/H/B series prepared for 1-vinyl naphthalene (1VN), 9-vinyl anthracene (9VA), vinyl imidazole (MZ), and N-vinyl phthalimide (NVP) to test generality. - Characterization: - Photophysics: Steady-state and delayed phosphorescence spectra, decay kinetics, quantum yields; excitation-dependent emission, time-resolved excitation spectra (TRES); temperature-dependent phosphorescence and decays (80 K to room temperature); CIE coordinates. - Kinetic analysis: Extract fluorescence lifetimes (τF), triplet lifetimes, ISC rate constants (kISC), and nonradiative decay constants (knr) from spectral data. - Structural analysis: FTIR (C=O disappearance post-alcoholysis; O–H H-bond band sharpening; B–O stretch ~1287 cm−1), XPS (C 1s, O 1s shifts; B–O ~192.2 eV), powder XRD (amorphous), TG-IR (crosslink collapse signature 300–400 °C), DSC/Tg evolution (P3: 32.6 °C; H3: 82.9 °C; B3: 110.9 °C), 1H NMR (loss of ester/vinyl signals, appearance of –OH and aromatic signals) where soluble. - Theoretical calculations: Simplified molecular models of P/H/B; interaction region indicator (IRI) and electrostatic potential (ESP) analyses via Multiwfn to visualize noncovalent interactions and bonding; evaluation of vertical excitation energies of singlet/triplet states. - Applications: - Microcrack detection: Encapsulate crosslinked RTP polymers in epoxy; introduce artificial microcracks (<2 mm); expose to water vapor (fumigation ~14 h), then assess RTP quenching versus controls; reactivation by drying (65 °C). - Information encoding: Morse code encryption using samples with different RTP lifetimes (e.g., long-lived H3 as dash, short-lived 9VA-H as dot, NVP-H as interference symbol), observe on/off UV transitions for readout.

- Stepwise confinement substantially enhances RTP: - In 2VN series, afterglow advances from barely visible in P3 to bright, long-lived in B3; B3 afterglow persists >10 s after 254 nm excitation. - RTP intensity increases from near-zero (P3) to ~1.68×10^5 a.u. (B3) at 520 nm; emission is excitation-independent and centered ~520 nm; CIE for B3/H3 ~ (0.33, 0.53)/(0.33, 0.52). - Triplet lifetimes increase markedly: representative P3→H3→B3 lifetimes from 4.70 ms to 1263.60 ms; separately, overall stepwise confinement reported to boost lifetime from 14.3 µs to 256.5 ms, and achieve up to 1.26 s lifetime at room temperature depending on system. - Phosphorescence quantum yield rises from 0.06% (P3) to 10.17% (B3); across systems, maximum ΦPhos reaches 16.04% (NVP-B, halogen-free). - Nonradiative decay suppressed while ISC promoted: for P3/H3/B3, knr drops from ~2.1×10^2 s−1 to 7.1×10^1 s−1 (and as low as 3.9 s−1 in crosslinked systems per overall analysis); kISC increases during alcoholysis (e.g., P3 τF 50.8 ns, kISC 1.2×10^7 s−1; H3 τF 38.8 ns, kISC 6.4×10^7 s−1; B3 τF 42.0 ns, kISC 2.4×10^7 s−1). ΦF trends mirror τF, consistent with greater triplet population and restricted nonradiative pathways as rigidity increases. - Temperature dependence: heating reduces RTP intensity and lifetime; P3 lifetime drops to 9.8 ms at 230 K due to lack of H-bonds/crosslinks, indicating dominance of nonradiative decay without confinement. - Structural and theoretical corroboration: - FTIR: C=O (1727 cm−1) vanishes post-alcoholysis; O–H H-bond band (~3276 cm−1) sharpens (H3) and remains in B3; B–O stretch at ~1287 cm−1 confirms crosslinking. - XPS: shifts consistent with loss of ester and formation of C–O; B–O peak at ~192.2 eV in B3. - Tg increases with confinement: 32.6 °C (P3) → 82.9 °C (H3) → 110.9 °C (B3), indicating increased rigidity. - IRI/ESP analyses show emergence and strengthening of weak interactions (H-bonds, vdW) and covalent crosslinking in B relative to P and H; crosslinking enriches vertical excitation energy of triplet states, aiding RTP. - Generality across phosphors (1VN, 9VA, MZ, NVP): - RTP becomes visible after stepwise confinement; emission colors span blue (NVP-B 482 nm; MZ-B 486 nm) to green (1VN-B 520 nm) to yellowish (9VA-B 558 nm); lifetimes increase by 1–4 orders of magnitude from P to B; amorphous XRD maintained; presence of B–O (~1284–1288 cm−1; XPS B–O 192.3–193.4 eV) and stronger H-bonds (~3281–3285 cm−1) confirmed. - Applications: - Microcrack detection: Water vapor selectively quenches RTP in cracked epoxy-encapsulated samples; cracked samples show near-complete quenching and lifetimes too short to fit versus ~690 ms in controls; RTP recovers after drying (e.g., crack lifetime ~682.5 ms after drying). - Information encryption: Morse code encoded via lifetime contrasts; upon UV off, short-lifetime elements vanish first, enabling decryption (e.g., CQUT and 1940); scheme flexible to flipped encoding (e.g., phrase “I MISS YOU”).

The results substantiate that a hierarchical confinement strategy can simultaneously enhance ISC and suppress nonradiative decay to effectively tame triplet excitons in intrinsic polymer systems. Primary confinement via copolymerization alone is insufficient due to weak interactions and low Tg, leading to rapid nonradiative losses. Introducing secondary confinement by alcoholysis to PVA forms hydrogen-bonding networks that increase rigidity, elevate Tg, and reduce nonradiative pathways, thereby enabling RTP. Tertiary confinement through boric acid crosslinking establishes a B–O–C covalent network while retaining hydrogen bonds, further compressing nonradiative channels (lower knr), enriching triplet state excitation energy, and delivering long-lived, bright RTP with higher quantum yields. The strategy is versatile across multiple phosphor chemistries and provides practical utility in humidity-responsive microcrack detection and lifetime-encoded information storage. These findings address the need for intrinsically polymeric, halogen-free RTP materials with robust performance and tunable color without relying on dopant dispersion or heavy atoms.

An intrinsically polymeric RTP platform was developed based on stepwise structural confinement: copolymerization (primary), hydrogen bonding via alcoholysis (secondary), and boric acid crosslinking (tertiary). This approach concurrently promotes ISC and suppresses nonradiative decay, yielding substantial gains in lifetime (up to ~1.26 s; increases of up to four orders of magnitude in some systems), intensity, and quantum yield (up to 16.04% in halogen-free NVP-B), with stable, excitation-independent emission spanning blue–green–yellow. Structural characterizations (FTIR, XPS, Tg), along with IRI/ESP analyses and vertical excitation energy calculations, corroborate the confinement mechanism. Demonstrations in microcrack detection under humid environments and lifetime-based Morse code encryption illustrate application potential. Future work may expand the library of intrinsic phosphors, optimize crosslink density and network architecture to further balance rigidity/solubility, and explore stability and sensing in diverse environmental conditions.

- Post-crosslinking samples exhibit poor solubility, which constrained some solution-phase characterizations (e.g., detailed NMR performed on P and H; B analyzed via FTIR/XPS/TG-IR). - The RTP is sensitive to moisture due to PVA hydroxyl groups; water vapor effectively quenches emission (leveraged for sensing), indicating environmental humidity can impact performance and may require encapsulation or conditioning for stable operation. - Microcrack detection method relies on vapor ingress through cracks under humid conditions; efficacy depends on crack connectivity and exposure parameters (e.g., fumigation time).