Enabling robust blue circularly polarized organic afterglow through self-confining isolated chiral chromophore

Blue circularly polarized luminescence (CPL) is crucial for full-color stereoscopic displays, multicolor data recording, anti-counterfeiting, biological imaging, and optical communications. While blue CPL materials based on fluorescence, metal complexes, and TADF have been developed and applied, circularly polarized organic afterglow (CPOA) has emerged as a promising direction due to long-lived photophysics. However, achieving blue CPOA with prolonged lifetimes and strong chirality remains challenging, especially with color tunability. The authors outline three prerequisites for CPOA: (1) effective chirality in the emissive chromophores; (2) efficient intersystem crossing (ISC) to generate triplet excitons; and (3) suppression of non-radiative decay by stabilizing triplet excitons in a rigid environment. Traditional chiral crystal engineering can suppress non-radiative decay via ordered packing but risks triplet–triplet annihilation and red-shifted afterglow, hindering blue emission. In contrast, confining isolated molecules within a polymer matrix can blue-shift emission and stabilize triplets for ultralong lifetimes. Based on this, the paper proposes self-confinement of isolated chiral chromophores within a rigid, hydrogen-bonded polymer matrix to realize robust blue CPOA and, through synergistic afterglow and chirality energy transfer (SACET), enable full-color CPOA.

The study builds on extensive work in CPL and CPOA materials. Prior blue CPL systems include fluorescent, metal-complex, and TADF emitters applied in optoelectronics. Strategies to realize CPOA span chiral chain engineering, ionic cocrystals, polymerization, and host–guest approaches. Chiral crystal engineering can suppress non-radiative decay but often introduces triplet–triplet annihilation and bathochromic shifts, complicating blue afterglow. Recent insights show that isolating emitters in polymers can stabilize triplets and blue-shift emission. Hydrogen-bonded polymer matrices and multicomponent copolymers have been used to boost persistent room-temperature phosphorescence. Energy transfer mechanisms, including Förster-type pathways for both singlet and triplet sensitization, have been explored to tune color and, in some cases, transfer chirality. The present work leverages these concepts by combining a high triplet-energy chiral carbazole-based monomer with a hydrogen-bond-rich polyacrylamide host to achieve blue CPOA and extends color tunability via SACET to common water-soluble dyes.

Design and synthesis: A pair of enantiomeric, high-triplet-energy chiral monomers (R/S-VCOOCz: 2-((2-(9H-carbazol-9-yl)propanoyl)oxy)ethyl acrylate with central chirality) were synthesized and resolved to high enantiomeric excess (ee: R-VCOOCz 99.9%, S-VCOOCz 98.1%). Chiral copolymers R/S-PAMCOOCzX (X = 1–4) were prepared via radical copolymerization of R/S-VCOOCz with acrylamide (AM) at feed ratios of 1:50 (X=1), 1:100 (X=2), 1:200 (X=3), and 1:400 (X=4). Polymerization employed AIBN initiator in freshly distilled THF under argon: monomers dissolved under ice-water cooling, then heated to 55 °C and stirred for 16 h. Polymers were precipitated in methanol, filtered, sequentially washed (petroleum ether, dichloromethane, acetone), dissolved in water, and dialyzed (MWCO 1000) for 72 h. Characterization: Structures confirmed by NMR, PXRD, and GPC (molecular weights and PDI reported). Chiroptical properties verified by CD, with calculated CD matching experimental spectra to confirm absolute configuration. WAXS assessed polymer packing and chromophore isolation. Photophysics: UV–vis absorption, steady-state PL (SSPL), delayed PL (afterglow), lifetimes (ns and s regimes), time-resolved emission spectra (TRES), and excitation–delayed PL maps were recorded for thin films under ambient conditions. CPL spectra and glum were measured to assess circular polarization. Optimization of chromophore content: Effects of feed ratio on afterglow intensity, lifetime, and CPL were investigated using S-PAMCOOCzX series to balance matrix rigidity, hydrogen bonding, and chromophore concentration. Photoluminescence quantum yields (PLQY) were measured. SACET and multicolor films: Full-color CPOA achieved by physically blending water-soluble fluorescent dyes (fluorescein sodium, Fluc; rhodamine 123, Rh123; sulforhodamine SR101) with R/S-PAMCOOCz2. Films were fabricated by dissolving polymer (0.5 g) and dye (varied wt.%) in deionized water (10 mL), sonication (10 min), stirring at 60 °C (1 h) to transparency, casting into Petri dishes, and drying at 70 °C overnight. Energy transfer (ET) was probed via spectral evolution with dye loading, lifetime quenching at donor emission (364 and 414 nm), growth of delayed emission at acceptor wavelengths, TRES stability, and excitation–delayed PL maps. ET efficiencies were calculated from amplitude-averaged lifetimes. CPL of doped films assessed chirality transfer. Applications: Demonstrations included screen-printed multiplex Morse code anti-counterfeiting patterns leveraging color, lifetime, and CPL; CPOA-functionalized fibers by soaking/drying in aqueous polymer solutions; and 3D printed/formed objects showing blue, yellow-green, and red CPOA.

• Self-confinement strategy: Covalent incorporation of isolated chiral carbazole-based chromophores (R/S-VCOOCz) into a hydrogen-bonded PAM matrix yields robust blue CPOA with suppressed non-radiative decay.
• Blue CPOA performance: R/S-PAMCOOCz2 films exhibit SSPL at 364 nm (fluorescence lifetimes ~12.9 ns (R) and 11.7 ns (S)) and long-lived afterglow peaks at 414, 442, and 470 nm with lifetimes of 3.0–3.1 s (ambient conditions). Maximum glum reaches 1.02 × 10^-2 (shoulder at ~442 nm) with mirror-image CPL for R/S enantiomers.
• Excitation characteristics: Afterglow effectively excited from ~210–360 nm with optimal excitation at ~299 nm. The three delayed emission peaks share similar excitation profiles, indicating a common chromophore origin. Low-temperature spectra support a high triplet energy of ~3.0 eV and attribute afterglow to isolated R/S-VCOOCz chromophores within PAM.
• Composition optimization: Varying R/S-VCOOCz:AM feed ratio shows an optimum at 1:100 (X=2), balancing matrix rigidity and chromophore concentration. PLQY for R- and S-PAMCOOCz2 reached 28.6% and 24.7%. Copolymerization outperforms physical blends for CPOA.
• SACET-enabled color tuning: Doping with water-soluble dyes enables efficient afterglow and chirality energy transfer: • Fluc: New delayed emission at ~555 nm; with 0.1 wt.% Fluc, donor afterglow largely quenched and afterglow color shifts to yellow-green. Donor lifetimes reduced from 12.9 ns to 9.4 ns (364 nm) and from 3.0 s to 2.0 s (414 nm). Sensitized afterglow at 555 nm shows lifetimes >1.8 s. ET efficiencies: fluorescence 27.1%, afterglow 64.3%. CPL transferred with glum ≈ +3.4 × 10^-3 (R) and −5.7 × 10^-3 (S) at 555 nm. • Rh123 and SR101: Orange (574 nm) and red (640 nm) afterglow with lifetimes ~1.9 s (Rh123) and ~2.2 s (SR101) at 0.1 wt.% loading. CPL transferred with glum up to +2.1 × 10^-3/−5.5 × 10^-3 (Rh123) and +2.5 × 10^-3/−3.7 × 10^-3 (SR101). Reduced CPL amplitudes relative to Fluc attributed to lower ET efficiencies.
• Chirality origin and excitation selectivity: CD spectra of doped films mirror those of hosts; direct excitation at dye absorption (e.g., 460–550 nm) does not yield CPL, confirming chirality transfer from chiral host via SACET.
• Applications: Demonstrated multilevel information encryption using color/lifetime/CPL, functional CPOA fibers with blue/green afterglow, and 3D objects emitting blue, yellow-green, and red CPOA.

The work directly addresses the challenge of achieving robust blue CPOA by isolating chiral, high-triplet-energy chromophores within a rigid, hydrogen-bonded PAM matrix. This self-confinement suppresses non-radiative triplet deactivation and avoids aggregation-induced triplet–triplet annihilation and red-shifts typical of crystalline packing, enabling blue afterglow with ultralong lifetimes (~3 s) and distinct CPL (glum ~10^-2). The polymer host not only stabilizes triplets but also enhances ISC through carbonyl and amide functionalities and provides water processability. Systematic variation of the chromophore loading shows that optimizing matrix rigidity and chromophore concentration is crucial: too little chromophore reduces intensity/CPL, whereas too much can diminish rigidity and increase non-radiative pathways. Extending the approach via SACET demonstrates generality: long-lived blue afterglow sensitizes common dyes to yield full-color CPOA with transferred chirality, validating the concept of using an afterglow-chiral donor for both color and CPL control. The integration into inks, fibers, and 3D objects underscores the practical relevance for secure displays, anti-counterfeiting, and wearable photonics.

An efficient strategy is presented for blue circularly polarized organic afterglow by covalently self-confining isolated chiral carbazole chromophores within a hydrogen-bonded, water-soluble PAM matrix. The resulting polymers deliver blue afterglow (414/442/470 nm) with lifetimes up to 3.0 s and glum up to 1.02 × 10^-2, with high PLQY and mirror-image CPL for R/S enantiomers. Through synergistic afterglow and chirality energy transfer to water-soluble dyes (Fluc, Rh123, SR101), full-color CPOA with ultralong lifetimes (~1.8–2.2 s) and measurable CPL is realized, enabling multilevel information encryption, functional fibers, and 3D objects. This approach streamlines the design of blue CPOA materials and provides a versatile platform to modulate emission color and chirality on demand for diverse applications. Potential future directions include optimizing host–guest combinations to further increase ET and CPL dissymmetry, expanding to other polymer hosts and chromophores for device integration, and engineering excitation pathways to reduce reliance on deep-UV excitation.

• Excitation constraints: Efficient afterglow and SACET are driven by UV excitation (optimal ~299 nm). Direct excitation in the visible range (e.g., 400–550 nm) does not produce afterglow/CPL in doped systems, which may limit some applications.
• Trade-off with composition: Increasing AM content enhances rigidity/hydrogen bonding and maintains similar lifetimes, but lowers chromophore concentration, reducing afterglow intensity and CPL signal. Conversely, higher chromophore loading can diminish performance.
• Chirality transfer magnitude: For Rh123 and SR101, CPL signals are weaker than for Fluc, consistent with lower ET efficiencies, indicating sensitivity to spectral overlap and host–guest interactions.
• Moderate glum values: Although up to ~10^-2 (blue) is achieved, glum values for doped colors are in the 10^-3 range, which may be insufficient for some polarization-demanding applications.
• Generality of water-processability: The approach relies on water-soluble hosts/guests for optimal mixing and SACET; extension to hydrophobic systems may require additional engineering.