Design of highly efficient deep-blue organic afterglow through guest sensitization and matrices rigidification

The study addresses a central challenge in organic optoelectronics: achieving efficient blue/deep-blue organic ultralong room-temperature phosphorescence (OURTP). Blue emission is essential for solid-state lighting and full-color displays, but most organic afterglow materials emit in the green-red region (500–600 nm) due to low-lying triplet state energies and aggregation-induced red-shifts in the solid state. Constructing blue OURTP is especially difficult because it requires both efficient population and stabilization of high-energy triplet states. Prior strategies such as crystallization/H-aggregation and exciplex formation often cause bathochromic shifts, hindering blue emission. Host-guest doping can mitigate aggregation and concentration quenching, but rigid hosts are hard to process and dopant dispersion is limited by host–guest compatibility, resulting in weak luminance. The purpose of this work is to propose and validate a general strategy that simultaneously enhances triplet population and suppresses non-radiative decay to realize efficient deep-blue OURTP with long lifetimes and high quantum yields under ambient conditions.

Previous approaches for room-temperature phosphorescence include crystallization/H-aggregation and exciplex formation, which often introduce red-shifts relative to monomer fluorescence due to intermolecular coupling in the solid state. Host-guest systems can prevent bathochromic shifts and reduce quenching at low dopant concentrations, but typically require rigid hosts to achieve high phosphorescent quantum yield; such hosts are difficult to process, and poor compatibility can hinder uniform dispersion of emitters. Reports have achieved OURTP using amorphous materials, intramolecular energy transfer, and halogen bonding, with notable advances in lifetime and quantum efficiency. Nonetheless, achieving long-lived (>1 s), high-efficiency (>40% PhQY) blue/deep-blue OURTP remains rare due to challenges in triplet exciton population and stabilization without heavy atoms and without red-shifting the emission.

• Materials and design: Cyanuric acid (CA) selected as an active host due to high S1 and T1 energies, efficient ISC (facilitated by lone pairs on N and carbonyls), optical inertness, and multiple hydrogen-bond donor/acceptor sites to form cross-linked networks. Trimesic acid (TMA) used as guest; also tested isophthalic acid (IPA), terephthalic acid (TPA), and phthalic acid (PA) for generality.
• Preparation of host–guest composites: Aqueous mixtures of CA and guest (e.g., TMA) at low guest loading were ultrasonicated at room temperature, then solvent removed under vacuum at 40 °C for 24 h to obtain powders. Compositions are denoted CTx-y where x is guest wt% (e.g., 5 wt% for CT5) and y is water content (wt%) implemented in the composite (e.g., 0 or 20).
• Water-implemented matrices rigidification: Water was introduced (0–70 wt%) to form hydrogen-bonded networks among CA, guest, and water, increasing matrix rigidity and suppressing non-radiative decays and oxygen quenching.
• Photophysical characterization: Steady-state PL and time-resolved phosphorescence spectra measured across excitation wavelengths (210–300 nm). Lifetimes and phosphorescent quantum yields (PhQY) determined. Temperature-dependent PL (78–278 K) assessed mechanisms. Phosphorescence excitation spectra compared for CA, TMA, and composites.
• Structural and interaction analyses: Differential scanning calorimetry (DSC) to probe bound/free water transitions; solid-state 13C NMR to evaluate hydrogen-bonding changes; powder X-ray diffraction (XRD) to detect lattice expansion upon water addition; Raman spectroscopy to identify shifts indicative of H-bond formation (e.g., C=O interactions).
• Universality tests: Constructed CA–guest composites with IPA (CI), TPA (CTP), and PA (CP) at 5 wt% guest and varied water contents (optimal ~10 wt% for these), measuring lifetimes, wavelengths, and PhQYs.
• Application demonstration: Prepared rewritable encryption paper by coating CT5 in DMSO (100 mg mL−1) onto filter paper, drying, then water-jet printing patterns using pure water ink. Erasure performed via DMSO vapor fuming at 120 °C for 15 min. Durability (multiple write/erase cycles) and ambient stability (>1 month) assessed.
• Mechanistic interpretation: Compared phosphorescence under 248 nm (host excitation) vs 288–293 nm (guest excitation) to infer energy transfer routes. Proposed host-sensitized triplet population via Dexter energy transfer from CA T1 to guest T1, and possible Förster transfer to guest S1 followed by ISC, with water-induced H-bonds enhancing ISC and ET and rigidifying the matrix.

• Efficient deep-blue OURTP achieved: emission wavelengths 405–428 nm, lifetimes up to 1.67 s, and phosphorescent quantum yields up to 46.1% at room temperature.
• CA–TMA composite performance: TMA crystal alone shows green OURTP at 524 nm (τ ≈ 0.15 s, PhQY ≈ 2.7%). Doped into CA at 5 wt% (CT5-0), deep-blue phosphorescence at 406 nm with τ ≈ 1.13 s and PhQY ≈ 9.3%. With 20 wt% water (CT5-20), τ increases to 1.67 s and PhQY to 46.1%.
• Excitation dependence: Two excitation bands (~248 nm for CA, ~288–293 nm for TMA). Under 248 nm, CA acts as active host; stronger OURTP intensity via host sensitization. Under 288–293 nm, direct guest excitation yields higher PhQY (peak 46.1% at 293 nm) but lower afterglow intensity.
• Water-induced rigidification: Water implements form H-bond networks that rigidify the matrix, suppress non-radiative decays and oxygen quenching, and enhance ISC/energy transfer. Evidence: DSC shows bound/free water features (melting ~7 °C; broad evaporation 45–110 °C at higher water content); solid-state 13C NMR splitting in CA carbonyl disappears after water addition; XRD shows reduced 2θ (lattice expansion); Raman shows shifts at ~1725 and 701 cm−1 indicating C=O–water interactions.
• Thermal effects: Lowering temperature increases phosphorescence intensity and lifetime (e.g., CT5-0 lifetime from ~1.1 s at 278 K to ~2.0 s at 78 K), consistent with suppressed non-radiative decay. Water-containing samples show smaller additional gains upon cooling, indicating water already rigidifies the matrix.
• Doping/water trends: Afterglow intensity depends on guest loading and water content; lifetime is comparatively insensitive, especially for water content ≥20 wt%. Optimal for TMA: 5 wt% guest, 20 wt% water.
• Universality with other guests: CA host with IPA (CI), TPA (CTP), and PA (CP) yields deep-blue OURTP at 405, 428, and 425 nm, respectively, with lifetimes up to 1.36 s and PhQY up to 11.4%. Optimal water content ~10 wt% for these guests; performance correlates with ability to form strong H-bond networks (IPA > TPA > PA).
• Application: Water-jet rewritable lifetime-encryption paper prepared using CT5 in DMSO (100 mg mL−1). Patterns printed with water are invisible under daylight/UV on but reveal long-lived afterglow upon UV off (254 nm excitation); erasable by DMSO vapor at 120 °C in 15 min; high reversibility over many cycles; patterns retain for >1 month under ambient conditions.
• Mechanism: Active-host triplet sensitization via Dexter energy transfer from CA T1 to guest T1 combined with water-induced matrix rigidification underpins the high-efficiency deep-blue OURTP.

The work directly addresses the longstanding challenge of realizing efficient blue/deep-blue organic afterglow by simultaneously enhancing triplet exciton population and suppressing non-radiative decay. Using cyanuric acid as an active host enables efficient intersystem crossing and energy transfer to guest molecules, populating high-energy triplet states of the guest without invoking aggregation-induced red-shifts. The addition of water forms an extended hydrogen-bonding network among CA, guest, and water, rigidifying the matrix, suppressing vibrational relaxation and oxygen diffusion, and thereby extending lifetime and boosting quantum yield. Spectroscopic and structural evidence (temperature-dependent PL, DSC, solid-state NMR, XRD, Raman) supports this mechanism. The excitation-wavelength dependence further corroborates host-sensitized triplet generation under 248 nm excitation (stronger afterglow) and direct guest excitation near 288–293 nm (higher PhQY). Generalization to other benzoic acid derivatives demonstrates that the strategy is not limited to TMA, though performance depends on the guest’s ability to form robust H-bond networks. The demonstration of rewritable, lifetime-encrypted paper highlights practical utility in secure information storage and anti-counterfeiting, leveraging water responsiveness and erase-by-solvent vapor processes.

This study introduces a general, heavy-atom-free strategy for highly efficient deep-blue OURTP by combining active-host triplet sensitization (cyanuric acid) with water-induced matrix rigidification. The approach yields deep-blue emissions (405–428 nm) with record-level performance for organic afterglow (lifetime up to 1.67 s; PhQY up to 46.1%) at room temperature. Mechanistic investigations attribute the efficiency gains to host-to-guest Dexter energy transfer populating guest triplet states and hydrogen-bond networks that suppress non-radiative decay and oxygen quenching. The concept extends to other benzoic acid derivatives (IPA, TPA, PA), and enables practical rewritable, lifetime-encryption paper that is printable with water and erasable by DMSO vapor, retaining patterns for over a month and supporting multiple write/erase cycles. Future work could optimize host–guest combinations and hydrogen-bond architectures to further enhance blue OURTP efficiency, expand excitation to longer wavelengths, and integrate the materials into device platforms for displays, sensing, and secure information technologies.

• Excitation requirements: Strongest OURTP intensity relies on short-wavelength UV excitation (~248 nm), and highest PhQY occurs near 288–293 nm, which may limit practical illumination sources and safety considerations.
• Water dependence: Performance is sensitive to water content (e.g., optimal 20 wt% for TMA, ~10 wt% for other guests); deviations reduce intensity, indicating processing and environmental humidity control are important.
• Guest specificity: While the strategy generalizes to IPA, TPA, and PA, their efficiencies are lower than TMA, suggesting dependence on guest structure and H-bonding capability.
• Erasure conditions: Rewritable paper erasure requires DMSO vapor at elevated temperature (120 °C) for ~15 min, which may constrain some applications.
• pH sensitivity: OURTP intensity varies with pH (strongest near pH ~2 in the reported conditions), indicating environmental conditions can affect performance.