Boosting Red Room‐Temperature Phosphorescence by Host–Guest Confinement: Acrylonitrile Derivatives in PMMA Toward Long‐Lived Afterglow and Anti‐Counterfeiting

ABSTRACT

Organic room‐temperature phosphorescence (RTP) materials have attracted extensive interest in optoelectronics, information encryption, and biosensing due to their unique long‐lived luminescence. However, most state‐of‐the‐art organic RTP systems suffer from short lifetimes, low quantum efficiency, and severe nonradiative quenching under ambient conditions, greatly limiting practical applications. Herein, we develop a robust host–guest confinement strategy to achieve efficient ultra‐long‐lived red RTP by incorporating three newly designed acrylonitrile‐based luminophores (Cz‐Cz, Cz‐Py, and Cz‐DiPy) into a rigid PMMA matrix. All doped films show intense and stable red RTP emission; the Cz‐DiPy@PMMA composite affords a remarkable lifetime of 260.32 ms, nearly three orders of magnitude higher than pristine powder. Mechanistic studies combined with theoretical calculations reveal that the rigid PMMA framework restricts intramolecular motions, suppresses nonradiative relaxation, and shields triplet excitons from oxygen and collisional quenching, synergistically improving RTP performance. Enabled by excellent long‐afterglow properties, these films are successfully applied to time‐resolved multi‐level information encryption and advanced anti‐counterfeiting. This work provides a universal route to high‐performance red‐emissive organic RTP materials and offers valuable mechanistic insights to advance their deployment in next‐generation information security and optoelectronic technologies.

A host–guest confinement strategy embeds acrylonitrile‐based luminophores in PMMA to realize efficient ultra‐long red organic room‐temperature phosphorescence for information encryption and anti‐counterfeiting.

1. Introduction

Organic room‐temperature phosphorescence (RTP) materials represent a highly attractive research frontier in modern functional optoelectronic materials, distinguished by unique triplet exciton‐derived photophysical properties, including ultralong emission lifetimes, large Stokes shifts, and characteristic time‐resolved luminescence behavior. These exceptional merits render pure organic RTP materials highly competitive for next‐generation applications such as high‐resolution bioimaging [1, 2], secure information encryption [3, 4], advanced anti‐counterfeiting [5, 6], and high‐sensitivity chemical sensing [7, 8]. Distinct from conventional phosphors based on noble metal complexes, metal‐free organic luminophores overcome the constraints of high cost and potential biotoxicity, while offering outstanding advantages including structural tunability, facile synthesis, excellent biocompatibility, and superior solution processability, thereby opening a sustainable and versatile pathway for advanced luminescent materials.

Nevertheless, a persistent scientific challenge plagues this promising field: achieving efficient and long‐lived red‐emissive organic RTP under ambient conditions remains extremely difficult [9, 10]. In sharp contrast to blue and green‐emissive analogs, red RTP systems are inherently trapped in a contradictory dilemma. Extending emission into the red region requires a narrowed molecular band gap, which inevitably accelerates nonradiative energy dissipation and thermal deactivation of triplet excitons [9, 10, 11, 12]. Meanwhile, unconstrained intramolecular vibrations, rotations, and disordered intermolecular interactions in the solid state further induce severe triplet quenching, making it highly challenging to simultaneously realize efficient triplet generation and stable red phosphorescence output within a single molecular framework [7, 9, 10, 11, 12, 13, 14]. Despite these inherent drawbacks, tremendous advances have been achieved in recent years via rational molecular design and supramolecular assembly engineering. Of particular note, growing research efforts have been dedicated to addressing the key bottlenecks hindering the clinical translation of red and near‐infrared (NIR) RTP luminogens. Such materials exhibit unique superiority for deep‐tissue biological imaging, benefiting from negligible biological autofluorescence interference and superior tissue penetration capability [15, 16, 17, 18]. For example, Fan et al. regulated molecular geometry and electronic structures to fabricate efficient red RTP with a lifetime of 344 ms. The obtained materials can be safely excited by sunlight and mobile phone flashlight and realize afterglow imaging of mouse lymph nodes with high signal‐to‐noise ratio and deep tissue penetration, overcoming the drawbacks of UV excitation and poor tissue penetration of traditional green and yellow RTP materials [18].

To surmount this long‐standing obstacle, rational molecular engineering has emerged as the critical key to unlocking the full potential of triplet luminescence. Among diverse structural design strategies, acrylonitrile‐based frameworks incorporating cyano and carbonyl groups stand out as a uniquely powerful dual‐functional platform for constructing red‐emissive RTP molecules. On one hand, the lone‐pair electrons of cyano and carbonyl moieties effectively activate n–π* transitions, significantly enhancing spin‐orbit coupling (SOC) and promoting intersystem crossing (ISC), thus establishing a robust foundation for sufficient triplet exciton population [19, 20]. On the other hand, as strong electron‐withdrawing units, cyano and carbonyl groups readily form a typical donor‐acceptor (D‐A) electronic structure when combined with electron‐donating carbazole segments, triggering strong intramolecular charge transfer (ICT), narrowing the molecular band gap, and intrinsically shifting emission toward the red region [1, 21]. This dual‐functional design ingeniously integrates triplet generation capability and red luminescence potential, providing an unprecedented molecular blueprint for high‐performance red RTP materials.

Inspired by this innovative design principle, we strategically developed three novel acrylonitrile‐based organic luminophores (Cz‐Cz, Cz‐Py, and Cz‐DiPy) featuring a rigid 3‐acetyl‐2‐butenenitrile core, with carbazole and pyrene units covalently linked via phenyl bridges through concise Suzuki–Miyaura coupling and Knoevenagel condensation reactions. The introduction of aromatic segments with adjustable sizes and steric configurations further optimizes molecular conformations and exciton dynamics: carbazole rigidifies the conjugated backbone and stabilizes the frontier molecular orbital profiles, whereas the propeller‐shaped pyrene motif mitigates triplet‐triplet annihilation and concentration quenching in the solid state [1, 12, 22]. Benefiting from the synergistic effect of SOC‐enhanced triplet generation and D‐A‐induced band gap narrowing, these molecules theoretically possess outstanding intrinsic red phosphorescence potential. However, despite elaborate molecular design, the intrinsic red RTP of these acrylonitrile‐based molecules remains severely quenched in their neat powder state [19, 20]. Unrestricted molecular motions and disordered intermolecular packing dominate excited‐state deactivation pathways, completely obscuring the latent triplet luminescence properties. To fully unlock their intrinsic RTP potential, we adopt a universal and effective regulation strategy: host‐guest doping within rigid polymer matrices. The rigid and ordered microenvironment constructed by polymer hosts physically restricts molecular motions, eliminates vibrational energy loss, and blocks nonradiative decay channels, thereby activating and amplifying the inherent RTP performance of guest molecules [23, 24, 25, 26, 27].

In this work, poly (methyl methacrylate) (PMMA) is selected as an ideal rigid matrix to fabricate uniform doped composite films, successfully activating stable and efficient red RTP from the acrylonitrile derivatives. Notably, the phosphorescence lifetime of the guest molecules is dramatically increased by three orders of magnitude relative to neat powders, reaching up to 260.32 ms. Systematic photophysical characterizations and structural analyses clearly elucidate the dual contributions of acrylonitrile backbone engineering (imparting red emission and triplet generation capability) and polymer rigidification (activating and optimizing RTP performance). This study not only establishes a feasible integrated strategy for constructing high‐performance long‐lived red organic RTP materials, but also provides a new paradigm for balancing triplet generation and nonradiative decay, paving the way for the practical application of organic RTP materials in advanced optoelectronics and information security fields.

2. Results and Discussion

2.1. Structures and Synthesis

Herein, three novel organic small‐molecule luminophores shown in Figure 1A (Cz‐Cz, Cz‐Py, and Cz‐DiPy) are rationally designed and synthesized, centered on a rigid 3‐acetyl‐2‐butenenitrile core functionalized with carbazole (Cz) and pyrene (Py) units symmetrically appended via phenylene bridges. All target compounds are efficiently constructed through sequential Suzuki–Miyaura cross‐coupling and Knoevenagel condensation reactions. Detailed synthetic procedures, including full reaction schemes, NMR spectra, and HPLC chromatogram, are provided in the Supporting Information (SI, Schemes S1–S3 and Figures S1–S12). Within the molecular architecture, carbazole and pyrene exhibit complementary structural characteristics: (1) the nitrogen atom in carbazole serves as an intrinsic SOC enhancer, facilitating molecular (n, π*) transitions while simultaneously boosting phosphorescence efficiency; (2) the rigid planar conjugated structure of pyrene prevents triplet exciton annihilation and promotes (π, π*) transitions, which is conducive to prolonging the phosphorescence lifetime, its planar backbone facilitates tight π–π stacking in aggregates or crystals, rigidifying the surrounding environment, which synergistically induces a red‐shift into the red/NIR region. Concurrently, the carbonyl moiety along the conjugated skeleton and the lone‐pair electrons residing on the cyano nitrogen within the acrylonitrile segment synergistically activate (n, π*) electronic transitions, which markedly strengthens SOC efficiency and promotes efficient ISC for abundant triplet exciton generation, establishing a critical prerequisite for robust RTP.

FIGURE 1.

(A) Molecular design of Cz‐Cz, Cz‐Py, and Cz‐DiPy. (B) Target molecules with lower efficient phosphorescence under powder state. (C) Target molecules doped into PMMA polymers with highly efficient phosphorescence.

At ambient conditions, neat powders of Cz‐Cz exhibit intense yellow fluorescence, while Cz‐Py and Cz‐DiPy display yellowish‐orange fluorescence, with corresponding transient RTP lifetimes of 29.55, 19.27, and 23.21 μs, respectively. Upon dilute doping of these phosphors into a rigid PMMA host matrix, all composite films exhibit distinct and stable red‐shifted RTP emission, with characteristic peaks centered at 585, 620, and 610 nm, alongside dramatically prolonged RTP lifetimes of 12.61, 133.07, and 260.32 ms, respectively. These values represent an extraordinary 3–4 orders of magnitude elongation in RTP lifetime relative to the neat powders, accompanied by drastically enhanced phosphorescence intensity. Remarkably, the 1 wt% Cz‐DiPy@PMMA composite film sustains visible red afterglow for over 3 s following cessation of UV excitation, and its absolute photoluminescence quantum yield (PLQY) is elevated substantially from 4.6% in dilute solution to 21.7% in the solid composite film. Comprehensive photophysical characterizations and theoretical simulations collectively verify that the rigid, amorphous PMMA matrix effectively restricts intramolecular vibrational and rotational motions of the phosphor molecules in the aggregated state, shields triplet excitons from oxygen‐ and moisture‐mediated quenching, and maximally suppresses nonradiative decay pathways. This synergistic confinement effect stabilizes triplet excitons, extends triplet state population lifetimes, and ultimately unlocks exceptional red RTP performance. Collectively, these solution‐processable composite films demonstrate exceptional promise for optical information storage and visualization, and their superior, long‐lived red RTP characteristics render them highly attractive for cutting‐edge applications in advanced anti‐counterfeiting and high‐security information encryption technologies.

To gain preliminary insights into the intrinsic photophysical characteristics of Cz‐Cz, Cz‐Py, and Cz‐DiPy, their UV–vis absorption and steady‐state PL spectra are first recorded in solvents of varying polarity (Figures S13–S15). The absorption profiles of all three compounds remained nearly unaffected by solvent polarity, with characteristic absorption maxima located at approximately 400 nm for Cz‐Cz, 380 nm for Cz‐Py, and 350 nm for Cz‐DiPy. In sharp contrast, their PL emission spectra displayed pronounced sensitivity to solvation effects. Notably, Cz‐DiPy exhibited dual fluorescence emission in CHCl₃, 1,4‐dioxane, and THF, which arises from a solvent‐dependent equilibrium between the locally excited (LE) state and the twisted ICT (TICT) state (Figure 2C) [21, 22, 28]. In comparison, Cz‐Cz and Cz‐Py displayed no dual‐emission behavior, owing to their much smaller conformational flexibility relative to Cz‐DiPy, as further verified by aggregation‐induced emission (AIE) experiments (Figures S16 and S17) [29, 30].

FIGURE 2.

(A–C) PL spectra of Cz‐Cz, Cz‐Py, and Cz‐DiPy in different solutions, respectively. (D) Doping concentration‐dependent PL spectra of Cz‐Cz‐doped PMMA film. (E) Plot of the relative PL intensity (I/I ₀) versus water content; inset: the PL spectrum of Cz‐Py (10⁻⁵ M) in THF with varying water fractions upon excitation at 380 nm. (F) Delayed emission spectra of Cz‐DiPy in DCM and under anhydrous and oxygen‐free conditions.

In AIE measurements, Cz‐Cz and Cz‐Py exhibit a typical “decrease‐then‐increase” PL intensity evolution. This behavior originates primarily from their relatively weak steric hindrance. At low fractions of water (a poor solvent), enhanced solvation facilitates molecular twisting and populates nonradiative TICT states, leading to the initial reduction in PL intensity [31, 32, 33]. As the water volume fraction further increases beyond the critical aggregation concentration, intermolecular packing becomes dominant, and the restriction of intramolecular rotation and vibration (RIR/RIV) effect effectively suppresses nonradiative decay, resulting in recovered and intensified luminescence. In contrast, the highly sterically congested structure of Cz‐DiPy prevents extensive conformational twisting even with increasing poor solvent content [34, 35, 36]. Its strong intermolecular association propensity allows it to rapidly bypass the dissolution‐disaggregation stage and reach the aggregation regime. When the water fraction reached 90 vol%, the PL intensity of Cz‐DiPy increased drastically relative to that in pure organic solvent (Figure 2E). This enhancement arises from the formation of highly rigid molecular aggregates, which efficiently suppress nonradiative relaxation pathways and thereby boost fluorescence emission.

To examine the intrinsic phosphorescence characteristics of the target compounds in monodispersed solutions, low‐temperature phosphorescence spectra are first recorded in dilute dichloromethane (DCM), followed by repeated measurements under rigorously anhydrous and oxygen‐free conditions at identical concentrations. A measurable increase in the intensity of the delayed emission spectra for both Cz‐Cz and Cz‐Py in DCM solution follows rigorous removal of water and oxygen procedures. For Cz‐DiPy, in addition to a similar intensity enhancement, a distinct new emission band emerged at approximately 555 nm after degassing and dehydration, confirming that water and molecular oxygen exert a substantial quenching effect on the triplet‐involved phosphorescence emission of these luminophores (Figures 2F and S18–S21).

2.2. Phosphorescence Emission Performance

Efficient RTP from organic luminophores is readily realized within spatially confined, rigid microenvironments. Poly (methyl methacrylate) (PMMA), with its inherent rigid, cage‐like matrix structure, can effectively restrict intramolecular rotation and vibration of guest emitters while shielding triplet excitons from oxygen‐ and collisional quenching pathways, thereby affording a highly favorable microenvironment for activating and stabilizing RTP emission. Accordingly, Cz‐Cz, Cz‐Py, and Cz‐DiPy are physically doped into a PMMA host matrix, and the influence of doping concentration on the photophysical properties of the resultant composite films is systematically examined. For Cz‐Cz, the most intense fluorescence emission is achieved at a doping concentration of 2 wt% (Figure 2D), whereas the maximum phosphorescence intensity is attained at 1 wt% (Figure S22). Cz‐Py similarly exhibited optimal phosphorescence emission at a doping level of 1 wt% (Figure S23). Therefore, a fixed doping concentration of 1 wt% is employed for all subsequent measurements, and the resulting composite films are denoted as Cz‐Cz@PMMA, Cz‐Py@PMMA, and Cz‐DiPy@PMMA in the following discussion.

Fluorescence and phosphorescence emission spectra recorded under ambient conditions reveal that the emission intensities of Cz‐Cz@PMMA, Cz‐Py@PMMA, and Cz‐DiPy@PMMA at 77 K are uniformly enhanced relative to those at 298 K (Figures S24–S26). This observation confirms that low temperature effectively suppresses intramolecular motions and favors radiative decay pathways during the relaxation of excited states to the ground state. Variable‐temperature phosphorescence spectra of the neat molecular powders and the corresponding PMMA‐doped composite films are further acquired under vacuum (Figures 3A–C). For all three neat powders, the overall phosphorescence intensity increases monotonically as temperature decreases. In marked contrast, the PMMA‐doped films exhibit distinctly different temperature‐dependent emission behaviors.

FIGURE 3.

(A–C) Temperature‐dependent phosphorescence spectra for Cz‐Cz powder/Cz‐Cz@PMMA, Cz‐Py powder/Cz‐Py@PMMA, and Cz‐DiPy powder/Cz‐DiPy@PMMA film under vacuum, respectively. (D–F) Temperature‐dependent lifetime decay profiles of phosphorescence emissions for Cz‐Cz (580 nm), Cz‐Py (580 nm), and Cz‐DiPy (610 nm) powder under vacuum, respectively. (G–I) Temperature‐dependent lifetime decay profiles of phosphorescence emissions for Cz‐Cz@PMMA (585 nm), Cz‐Py@PMMA (620 nm), and Cz‐DiPy@PMMA film (610 nm) under vacuum, respectively.

For Cz‐Cz@PMMA, the phosphorescence maximum exhibits a gradual red shift from 560 nm at 300 K to 585 nm upon cooling. This distinctive behavior is ascribed to the formation of triplet excimers upon photoexcitation. Triplet excimers are characterized by a repulsive ground state (S₀), where stable association of two ground‐state molecules is energetically unfavorable. In stark contrast, their excited triplet state (T₁) is attractive, with the excited molecule and its neighboring ground‐state counterpart stabilized by strong intermolecular π–π interactions. Consequently, the T₁ state energy of triplet excimers is substantially lower than that of isolated monomers, giving rise to a significantly red‐shifted emission relative to monomeric phosphorescence. In comparison, Cz‐Py@PMMA displays a broad, structureless emission band at 300 K (Figure 3B), which resolves into two well‐defined peaks as the temperature is lowered to 100 K: the first peak blue‐shifts from 543 to 522 nm, whereas the second peak centered at 620 nm shows negligible spectral shift but a dramatically intensified emission. Compared to Cz‐Cz, the incorporation of a pyrene moiety imparts Cz‐Py with enhanced planar rigidity, increased steric bulk, and more efficient ISC for triplet state population. Both emission bands exhibit a monotonic increase in intensity upon cooling. The shorter‐wavelength band is assigned to phosphorescence, which undergoes a subtle blue shift with decreasing temperature, whereas the longer‐wavelength band corresponds to phosphorescence, whose intensity increases progressively upon cooling. The observed delayed fluorescence is unambiguously attributed to triplet‐triplet annihilation (TTA), proceeding via the bimolecular reaction: (T₁ + T₁ → S₁ + S₀). TTA‐generated DF originates from the radiative decay of the resulting S₁ state to S₀. Owing to the intrinsically short lifetime of the S₁ state, radiative decay occurs prior to the completion of conformational relaxation and PMMA matrix polarization relaxation, even at elevated temperatures. Upon cooling, the suppression of these relaxation processes raises the energy of the emitting S₁ state, thereby accounting for the observed subtle blue shift.

Cz‐DiPy@PMMA manifest a similar trend, featuring two dominant emission bands at 470 and 610 nm, while the peak at 680 nm is attributed to the harmonic peak of the 340 nm excitation source (Figure 3C). This remarkable wavelength stability arises from the exceptionally large steric hindrance of Cz‐DiPy, which locks the molecular conformation such that the S₀ ground state and S₁ excited state adopt nearly identical geometries. Consequently, temperature variations cannot induce any significant conformational relaxation in this rigidified molecular scaffold. Nevertheless, cooling still effectively suppresses non‐radiative decay pathways of triplet states, leading to enhanced emission intensities for both bands while preserving their original spectral positions.

Phosphorescence lifetime measurements were additionally carried out for the short‐wavelength phosphorescence emission peaks, as illustrated in Figures S27 and S28. In both Cz‐Py@PMMA and Cz‐DiPy@PMMA, the lifetimes of the short‐wavelength phosphorescence emissions are drastically shorter than those of the corresponding long‐wavelength emission peaks. Phosphorescence lifetimes increase progressively with decreasing temperature (Figures 3D–F). The average lifetime of neat Cz‐DiPy powder is 23.21 μs at 300 K and increases to 201.54 μs at 100 K (Figure 3F). In sharp contrast, Cz‐DiPy@PMMA achieves an exceptional average phosphorescence lifetime of 260.32 ms at 300 K (Figure 3I), accompanied by a 3‐s naked‐eye‐observable red afterglow at room temperature; by comparison, the neat powder exhibits no discernible room‐temperature RTP under identical excitation conditions. The average lifetime of Cz‐DiPy@PMMA further rose to 401.55 ms at 100 K, representing a nearly one‐order‐of‐magnitude improvement over that of the neat powder.

2.3. DFT/TD‐DFT Analysis of Electronic and Structural Changes

To further establish the relationship between molecular electronic structure and photophysical behavior, we perform density functional theory (DFT) and time‐dependent density functional theory (TDDFT) calculations on Cz‐Cz, Cz‐Py, and Cz‐DiPy to uncover the mechanism underlying their RTP. This computational approach is particularly appropriate because the phosphorescence of PMMA‐doped films arises primarily from the monomeric state of the three guest emitters. Frontier molecular orbitals (FMOs) and SOC matrix elements are computed in the gas phase (Figures 4 and S29–S32). The three phosphors exhibit similar electrostatic potential (ESP) distributions, with negative charge concentrated on the cyano nitrogen and carbonyl oxygen atoms, reaching −34, −33, and −34 kcal mol⁻¹ for Cz‐Cz, Cz‐Py, and Cz‐DiPy, respectively. By contrast, positive charge is predominantly localized on the carbazole and pyrene units at both termini of the molecular backbone (Figure S33).

FIGURE 4.

(A) Jablonski diagram and SOC matrix elements for Cz‐Cz, Cz‐Py, and Cz‐DiPy, respectively. (B) Electron–hole pairs for Cz‐Cz, Cz‐Py, and Cz‐DiPy, respectively. (C) The RMSD for Cz‐Cz, Cz‐Py, and Cz‐DiPy, respectively.

Frontier molecular orbital analyses are performed for all three compounds in both the ground and excited states. The energy gap between the excited and ground states (ΔE_ES‐GS) increases sequentially in the order Cz‐Cz < Cz‐Py < Cz‐DiPy, with a ΔE_ES‐GS of 0.707 eV for Cz‐DiPy, considerably larger than those of Cz‐Cz (0.478 eV) and Cz‐Py (0.578 eV). A larger ΔE_ES‐GS corresponds to more substantial electronic rearrangement in the excited state, as well as greater structural relaxation and charge transfer character during the transition from the ground to the excited state. The monotonically increasing root‐mean‐square deviation (RMSD) values presented in Figure 4C further confirm progressively enhanced structural variation between the ground and excited states along the series Cz‐Cz, Cz‐Py, and Cz‐DiPy. A higher excitation energy required for the transition corresponds to a shorter absorption wavelength, consistent with the solution absorption maximum of Cz‐DiPy at approximately 350 nm, which is significantly blue‐shifted relative to those of Cz‐Cz (∼400 nm) and Cz‐Py (∼380 nm).

TDDFT calculations are further conducted to evaluate the SOC characteristics of the three molecules (Figures S34–S37). For Cz‐Cz, only the T₁ and T₂ states lie energetically below the S₁ state, whereas for Cz‐Py, three triplet states (T₁–T₃) are lower in energy than S₁. In striking contrast, Cz‐DiPy possesses five triplet states (T₁‐T₅) below S₁, revealing multiple accessible ISC channels available to all three guest emitters. For Cz‐Cz, the D index of the S₁ state lies well above 2 Å, indicating strong charge‐transfer (CT) character. The t indices of the T₁ and T₂ states are significantly greater than 0, confirming distinct net charge transfer for these two triplet states (Table S1). For Cz‐Py, the D index of the S₁ state is 1.687 Å, intermediate between the typical ranges for LE and CT states, whereas the T₁ and T₂ states exhibit small negative t indices and complete hole–electron overlap (Figure 4B and Table S2), corresponding to pure LE states.

In sharp contrast to the other two molecules, the S₁ state of Cz‐DiPy exhibits a D index of 6.512 Å, well above 3 Å, indicative of pronounced long‐range CT character with substantial charge separation (Figure 4B, Table S3). This pronounced long‐range CT character significantly reduces ΔE_ST between the S₁ state and the nearby T_n states. According to the El‐Sayed rule, smaller ΔE_ST values enhance the SOC between these singlet and triplet manifolds, which facilitates efficient ISC and thus promotes triplet exciton population [37, 38]. Notably, the T₄ and T₅ states yield D indices of 5.599 and 5.066 Å, respectively, both far exceeding 3 Å, characteristic of strong long‐range CT states, while positive t indices confirm distinct net charge transfer character.

As shown in Figure 4B, the electronic excitation of the Cz‐Cz molecule is dominated by the n–π* transition, whereas the π–π* transition serves as the primary excitation mode for the Cz‐DiPy molecule. For Cz‐Cz, the electron density of the electron component in both the S₁ and T₁ excited states is mainly localized on the 3‐acetyl‐2‐butenenitrile moiety, while the hole density is predominantly distributed on the carbazole segment adjacent to the alkenyl nitrile group. Benefiting from the introduction of pyrene, the S₁ state of Cz‐Py still exhibits a dominant n–π* transition similar to that of Cz‐Cz, whereas the T₁ state is governed by the π–π* transition. This distinction is more prominent in Cz‐DiPy, where both S₁ and T₁ states are dominated by π–π* transitions. The completely different excited‐state characteristics of Cz‐Cz, Cz‐Py, and Cz‐DiPy originate from the structural modification of substituents on the alkenyl nitrile side. The nitrogen atom in carbazole acts as an effective SOC promoter and facilitates the occurrence of n–π* transitions. As a large planar conjugated group, pyrene possesses excellent torsional resistance and tends to generate π–π* transitions. Such transition characteristics intrinsically narrow the singlet triplet energy gap (ΔE_ST) and generally contribute to prolonged phosphorescence lifetime. Consequently, Cz‐Py achieves intense red phosphorescence emission with a moderate lifetime, while Cz‐DiPy containing two pyrene units exhibits the longest phosphorescence lifetime in PMMA (260.32 ms) among the three molecules.

These theoretical findings are fully consistent with experimental AIE observations: Cz‐Cz and Cz‐Py undergo an initial decrease in PL intensity upon gradual addition of the poor solvent (water), whereas Cz‐DiPy does not. The inherently strong long‐range CT character of the S₁ state in Cz‐DiPy allows significantly faster population of the TICT state than Cz‐Cz and Cz‐Py as the poor solvent fraction increases, followed by rapid attainment of the critical aggregation concentration. This rationalizes its remarkably enhanced PL intensity at high water content relative to the other two molecules. The SOC values for the T₁ → S₀ transition of Cz‐Cz, Cz‐Py, and Cz‐DiPy are calculated to be 4.772, 1.956, and 0.394 cm⁻¹, respectively. A smaller SOC magnitude for the T₁ → S₀ transition favors efficient triplet exciton generation via multiple parallel ISC channels [37, 38, 39, 40]. Meanwhile, the reduced SOC suppresses the phosphorescence radiative decay rate, which is intrinsically favorable for prolonging the phosphorescence lifetime.

Upon doping into a PMMA matrix, all three molecules achieve an extraordinary enhancement in average phosphorescence lifetime of at least three orders of magnitude. The neat powders of Cz‐Cz, Cz‐Py, and Cz‐DiPy exhibit RTP lifetimes of approximately 20 μs, whereas the corresponding composite films (Cz‐Cz@PMMA, Cz‐Py@PMMA, and Cz‐DiPy@PMMA) deliver dramatically prolonged lifetimes of 12.61, 133.07, and 260.32 ms, respectively. Concurrently, the absolute PL quantum yields (PLQYs) are substantially improved (Table 1): the solution‐state PLQYs of Cz‐Cz, Cz‐Py, and Cz‐DiPy are 9.5%, 2.4%, and 4.6%, respectively, which increased to 13.5%, 23.8%, and 21.7% upon doping into PMMA.

TABLE 1.

Corresponding absolute photoluminescence quantum yield in solution/PMMA film for Cz‐Cz, Cz‐Py, and Cz‐DiPy, respectively.

PLQY/%	Cz‐Cz	Cz‐Py	CZ‐DiPy
Solution	9.5	2.4	4.6
PMMA film	13.5	23.8	21.7

The remarkable RTP enhancement can be rationalized by the dual stabilizing functions of the PMMA host matrix. First, the cage‐like polymeric network of PMMA establishes a rigid microenvironment that effectively restricts intramolecular motions, including stretching, rotation, and vibration of the guest emitters. Following photoexcitation, excited‐state molecules typically undergo structural relaxation to reach conformational and electronic equilibrium; such rigid confinement strongly suppresses nonradiative decay channels, redirecting excited electrons toward radiative relaxation pathways and promoting efficient ISC from singlet to triplet excitons. Second, the PMMA matrix dilutes the guest emitters to a low local concentration, minimizing triplet‐triplet annihilation and collisional quenching between triplet excitons and the surrounding lattice. Moreover, the polymeric matrix effectively blocks external triplet quenchers such as molecular oxygen and moisture, providing further stabilization of triplet excitons.

All phosphorescence emission and lifetime measurements are performed under identical experimental conditions, including excitation wavelength, slit width, and chopper delay time, for both neat powders and PMMA‐doped films. Pronounced differences are observed between the phosphorescence spectra of the two sample types. First, the phosphorescence emission intensities of the PMMA composite films are drastically higher than those of the corresponding neat powders. Notably, Cz‐Py@PMMA and Cz‐DiPy@PMMA exhibit two additional emission bands near 470 and 550 nm in their phosphorescence spectra (Figures 3B,C), which are assigned to fluorescence based on solution‐state fluorescence characterizations [39, 40, 41, 42]. The intensities of these peaks increased gradually with decreasing temperature, a hallmark of phosphorescence [43, 44, 45, 46].

This unique spectral behavior arises from the efficient stabilization of guest excited states by the PMMA host matrix. The rigid PMMA environment concurrently stabilizes both singlet and triplet excitons and decouples their respective radiative relaxation pathways to the ground state. By contrast, the ultrashort phosphorescence lifetimes of the neat powders result in the simultaneous overlap of phosphorescence emission [43, 44, 47, 48, 49]. For Cz‐Cz@PMMA, the gradual red shift of the phosphorescence maximum with decreasing temperature is attributed to enhanced triplet exciton stabilization at cryogenic conditions, which elevates the relative contribution of phosphorescence emission. Collectively, these results unambiguously elucidate the remarkable enhancement of phosphorescence lifetime and RTP emission intensity achieved for Cz‐Cz@PMMA, Cz‐Py@PMMA, and Cz‐DiPy@PMMA upon embedding within the rigid PMMA matrix.

2.4. Information Encryption Applications

Benefiting from their intense afterglow and long‐lived RTP characteristics, these host‐guest composite films are further explored for applications in multi‐level information encryption and advanced anti‐counterfeiting (Figure 5). Capitalizing on the nearly identical fluorescence emission profiles of Cz‐Cz@PMMA and Cz‐Py@PMMA under both ambient white light and UV excitation, a time‐resolved information encryption strategy based on ASCII coding is devised. As illustrated in Figure 5, a 4 × 8 pixel array is constructed using individual 0.5 × 0.5‐cm PMMA film units, in which Cz‐Cz@PMMA and Cz‐Py@PMMA are arranged in a predefined encrypted pattern (Figure S38). The resulting encryption film appeared transparent yellow under ambient light and exhibited uniform greenish‐yellow luminescence under UV irradiation, yielding no decipherable information upon ASCII conversion. Following 30 s of UV exposure and subsequent switching off of the excitation source, readable information emerged within 1 s: Dark pixels are designated as binary 0, while pixels displaying bright red afterglow are assigned as binary 1, which successfully decoded to the explicit message “GLUT” via ASCII conversion. The integration of distinct RTP materials enables the encryption of extensive information sets, establishing a versatile and robust platform for cutting‐edge RTP‐based anti‐counterfeiting technologies. Notably, this time‐resolved information storage device is fully reusable, with encrypted information repeatedly readable simply by toggling the UV excitation source on and off.

FIGURE 5.

(A) Luminescence photographs taken with and without UV irradiation (365 nm) under and after removing the UV irradiation. (B) Schematic diagram of time‐resolved information encryption based on the afterglow characteristics of Cz@PMMA and Cz‐Py@PMMA combined with ASCII codes (𝜆ex = 365 nm).

3. Conclusion

In summary, we report a facile and effective strategy to dramatically enhance the phosphorescence intensity and lifetime of purely organic RTP luminophores. Three novel acrylonitrile‐based organic emitters, namely, Cz‐Cz, Cz‐Py, and Cz‐DiPy, are rationally designed and successfully synthesized. Although none of these compounds show visible RTP in their neat solid state, physical doping into rigid poly (methyl methacrylate) (PMMA) matrices affords extraordinary enhancement of PL intensity and unprecedented prolongation of phosphorescence lifetimes. Impressively, the RTP lifetime of Cz‐DiPy is drastically elevated from 23.21 μs in the neat powder to 260.32 ms in the Cz‐DiPy@PMMA composite film, representing an increase of more than three orders of magnitude. Meanwhile, the absolute PL quantum yield is substantially improved from 4.6% in dilute solution to 21.7% in the doped film. Mechanistic investigations combined with theoretical calculations reveal that the rigid, cage‐like PMMA framework exerts dual beneficial functions: it effectively immobilizes the guest phosphors to suppress intramolecular rotations and vibrations, and shields triplet excitons from oxygen‐ and collision‐mediated quenching. These effects synergistically inhibit nonradiative decay pathways, facilitate efficient ISC, and stabilize triplet excited states, thereby activating and boosting intense red RTP emission. Moreover, the long‐lived afterglow features of these composite films enable successful applications in time‐resolved multi‐level information encryption and advanced anti‐counterfeiting. Overall, this work establishes a universal and reliable approach for developing high‐performance pure organic RTP materials, deepens mechanistic understanding of structure–property relationships in donor‐acceptor type red‐emissive phosphors, and further expands the practical application scope of organic RTP systems in advanced information security and optoelectronic technologies.

Author Contributions

Hong Chen: conceptualization, methodology, investigation, writing – original draft. Jian Li: investigation, validation, data curation. Yingxiu Chen: investigation, formal analysis. Shiqi Guo: investigation, visualization. Meijing Li: supervision, writing – review and editing, project administration. Shijie Zhen: supervision, methodology, formal analysis, writing – review and editing, funding acquisition.

Funding

This work was supported by the National Natural Science Foundation of China (22265006).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Schemes S1: Synthetic route of Cz‐Cz.

Schemes S2: Synthetic route of Cz‐Py.

Figure S1: bio70580‐sup‐0001‐Supplementary_Material.docx. ¹H NMR spectrum of Cz‐Cz in DMSO‐d ₆ .

Figure S2: bio70580‐sup‐0001‐Supplementary_Material.docx. ¹³C NMR spectrum of Cz‐Cz in CDCl₃.

Figure S3: bio70580‐sup‐0001‐Supplementary_Material.docx. ¹H NMR spectrum of Cz‐Py in CDCl₃.

Figure S4: bio70580‐sup‐0001‐Supplementary_Material.docx. ¹³C NMR spectrum of Cz‐Py in CDCl₃.

Figure S5: bio70580‐sup‐0001‐Supplementary_Material.docx. ¹H NMR spectrum of Cz‐DiPy in CDCl₃.

Figure S6: bio70580‐sup‐0001‐Supplementary_Material.docx. ¹³C NMR spectrum of Cz‐DiPy in CDCl₃.

Figure S7: bio70580‐sup‐0001‐Supplementary_Material.docx. ¹H NMR spectrum of compound 3 in CDCl₃.

Figure S8: bio70580‐sup‐0001‐Supplementary_Material.docx. ¹H NMR spectrum of compound 7 in CDCl₃.

Figure S9: bio70580‐sup‐0001‐Supplementary_Material.docx. ¹H NMR spectrum of compound 9 in DMSO‐d ₆ .

Figure S10: The HPLC chromatogram of compound Cz‐Cz (mobile phase: acetonitrile/water = 7:3, v/v).

Figure S11: The HPLC chromatogram of compound Cz‐Py (mobile phase: acetonitrile/water = 9:1, v/v).

Figure S12: The HPLC chromatogram of compound Cz‐DiPy (mobile phase: acetonitrile/water = 7:3, v/v).

Figure S13: Absorption spectra of Cz‐Cz in different solutions.

Figure S14: Absorption spectra of Cz‐Py in different solutions.

Figure S15: Absorption spectra of Cz‐DiPy in different solutions.

Figure S16: The PL spectrum (A) of Cz‐Cz (10⁻⁵ M) in THF with varying water fractions upon excitation at 380 nm; plot of the relative PL intensity (I/I₀) versus water content (B).

Figure S17: The PL spectrum (A) of Cz‐Py (10⁻⁵ M) in THF with varying water fractions upon excitation at 380 nm; plot of the relative PL intensity (I/I₀) versus water content (B).

Figure S18: Photoluminescence spectra of Cz‐Cz in DCM and under anhydrous and oxygen‐free conditions.

Figure S19: Delayed emission spectra of Cz‐Cz in DCM and under anhydrous and oxygen‐free conditions.

Figure S20: Photoluminescence spectra of Cz‐Py in DCM and under anhydrous and oxygen‐free conditions.

Figure S21: Delayed emission spectra of Cz‐Py in DCM and under anhydrous and oxygen‐free conditions.

Figure S22: Doping concentration‐dependent delayed emission spectra of Cz‐Cz‐doped PMMA film.

Figure S23: Doping concentration‐dependent delayed emission spectra of Cz‐Py‐doped PMMA film.

Figure S24: Temperature‐dependent fluorescence spectra (A) and phosphorescence spectra (B) of Cz‐Cz in 298 K and 77 K.

Figure S25: Temperature‐dependent fluorescence spectra (A) and phosphorescence spectra (B) of Cz‐Py in 298 K and 77 K.

Figure S26: Temperature‐dependent fluorescence spectra (A) and phosphorescence spectra (B) of Cz‐DiPy in 298 K and 77 K.

Figure S27: Temperature‐dependent lifetime decay profiles of phosphorescence emissions for Cz‐Py@PMMA (520 nm) under vacuum.

Figure S28: Temperature‐dependent lifetime decay profiles of phosphorescence emissions for Cz‐DiPy@PMMA (470 nm) under vacuum.

Figure S29: Frontier Molecular Orbital Distributions of Cz‐Cz in the Ground State and Excited States.

Figure S30: Frontier Molecular Orbital Distributions of Cz‐Py in the Ground State and Excited States.

Figure S31: Frontier Molecular Orbital Distributions of Cz‐DiPy in the Ground State and Excited States.

Figure S32: SOC matrix element heat maps for Cz‐Cz、Cz‐Py and Cz‐DiPy, respectively.

Figure S33: ESP for Cz‐Cz、Cz‐Py and Cz‐DiPy, respectively.

Figure S34: Theoretical parameters related to the transition of the Cz‐Cz molecule from the S₁ state to triplet states (T₂, T₁).

Figure S35: Theoretical parameters related to the transition of the Cz‐Py molecule from the S₁ state to triplet states (T₃, T₂).

Figure S36: Theoretical parameters related to the transition of the Cz‐DiPy molecule from the S₁ state to triplet states (T₅, T₄).

Figure S37: Theoretical parameters of the Cz‐DiPy molecule triplet states (T₂, T₃).

Figure S38: The flow chart of the PMMA films doped with different compounds is made.

Table S1: ISC transition process and energy levels of the Cz‐Cz.

Table S2: ISC transition process and energy levels of the Cz‐Py.

Table S3: ISC transition process and energy levels of the Cz‐DiPy.

Data Availability Statement

The authors declare that all the data supporting the findings of this manuscript are available within the manuscript and Supporting Information files and available from the corresponding authors upon request.