Quasi-heavy atom effect for room-temperature phosphorescence

Abstract

Organic room temperature phosphorescence (RTP) materials are vital for applications from bioimaging to anti-counterfeiting, offering advantages over inorganic materials. A key challenge is enhancing spin–orbit coupling (SOC), intersystem crossing (ISC), and stabilizing triplet excitons, which are prone to environmental quenching. Although molecular and material design strategies have been explored, the RTP mechanism in small molecules doped into polymer matrices, particularly poly(vinyl alcohol) (PVA), remains incompletely elucidated. Conventional explanations attribute the RTP of doped PVA to its oxygen barrier and rigid hydrogen-bonded network. However, our research shows that these alone are insufficient. We demonstrate that typically non-phosphorescent organic small molecules like biphenyl and fluorene exhibit ultralong blue phosphorescence (λ_em = 455 nm, quantum efficiency: 8.72%, lifetime: 4.20 s) only within a PVA matrix. This suggests an unrecognized intrinsic PVA property promoting efficient SOC and ISC. We propose a novel “quasi-heavy atom effect,” where PVA's unique characteristics facilitate SOC similar to heavy atoms, but without their toxicity or cost. This understanding is critical for designing novel RTP materials.

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New concepts

Previous studies have shown that room-temperature phosphorescence (RTP) in polyvinyl alcohol (PVA)-doped systems primarily arises from the rigid matrix environment created by hydrogen bonding of PVA hydroxyl groups and its oxygen-barrier properties. This study challenges this conventional perspective by introducing a novel concept: the quasi-heavy atom effect (QHAE). We propose that, in addition to matrix rigidification and suppression of oxygen quenching, the PVA matrix possesses a property analogous to the heavy atom effect. The QHAE enhances the spin–orbit coupling constant of small molecule compounds doped in the PVA matrix, thereby promoting the RTP emission of PVA-doped films. By introducing the QHAE, this work offers a more comprehensive and broadly applicable explanation for the widespread RTP behavior in PVA-doped systems, significantly advancing the mechanistic understanding of phosphorescence in polymer-based materials.

1. Introduction

In recent years, organic room temperature phosphorescence (RTP) materials have attracted considerable attention due to their diverse applications in fields, such as information encryption, anti-counterfeiting, chemical sensing, bioimaging, therapy, and emergency signage.^1–6 Unlike traditional inorganic long-afterglow materials, organic phosphors offer distinct advantages, including tunable photophysical properties, flexible structural design, excellent biocompatibility, readily available raw materials, and cost-effective preparation.⁷

A central challenge in RTP research involves effectively increasing the spin–orbit coupling (SOC) constant, enhancing the intersystem crossing (ISC) efficiency from singlet to triplet states, and stabilizing the triplet excitons. This is crucial because phosphorescence-generating triplet excitons are inherently spin-forbidden and highly susceptible to quenching by environmental factors like water and oxygen. To overcome these limitations and achieve efficient organic RTP, various strategies have been developed. These include, at the molecular design level, promoting triplet exciton generation through the introduction of heavy atoms or aromatic carbonyl groups, deuteration, or the construction of donor–acceptor structures.⁸ Concurrently, at the material system level, rigid structures are engineered via host–guest doping, crystal engineering, and leveraging intermolecular/intramolecular interactions to stabilize the active triplet excitons and minimize non-radiative transitions.⁹ In practice, a synergistic combination of these approaches is often employed to develop RTP material systems exhibiting both long lifetimes and high quantum efficiencies. Significant progress has been made in controlling phosphorescence lifetime, efficiency, and emission wavelength (color) in organic RTP materials to date.¹⁰

Beyond small-molecule RTP materials, polymer-based RTP systems have garnered substantial interest. This is primarily due to their excellent processability and suitability for large-area flexible applications, which hold immense promise for organic flexible devices.¹¹ Polymer-based RTP materials typically comprise either small molecules doped within a polymer matrix or homopolymer/copolymer systems.¹⁰ The utility of polymers in constructing RTP phosphorescent materials stems from several key attributes: (1) the entanglement of polymer chains creates a rigid environment, thereby restricting molecular motion and reducing excited-state energy dissipation (e.g., from triplet states); (2) intramolecular or intermolecular interactions within the polymer, particularly hydrogen bonds, effectively suppress non-radiative transitions of excited states; (3) the polymer matrix can help isolate oxygen and water molecules, further stabilizing triplet excitons; and (4) polymer-based phosphorescent materials are readily processed and shaped, simplifying their practical implementation. Common polymers utilized in these systems include poly(vinyl alcohol) (PVA),^12–14 poly(methyl methacrylate) (PMMA),^15,16 poly(lactic acid) (PLA),^17–19 cellulose,^20–22 and so on.

Among various polymer-based RTP material systems, water-soluble PVA matrices are particularly noteworthy due to their good biocompatibility and the advantage of not requiring toxic solvents during the doping process. However, a comprehensive understanding of the precise mechanism by which PVA facilitates RTP remains incomplete, and the existing research presents certain limitations. Most literature reports attribute PVA-based RTP to two primary factors: firstly, the excellent oxygen barrier properties of the PVA matrix, which effectively isolates phosphorescent luminogens from oxygen quenching; and secondly, the rigid network framework formed by extensive hydrogen bonds among PVA's numerous hydroxyl groups, which is believed to suppress non-radiative transitions of doped small molecules, thereby promoting RTP emission.^7,23 Additionally, some studies suggest that general intermolecular interactions between the small molecules and PVA contribute to the observed RTP in PVA-doped films.²⁴

However, these conventional explanations alone do not fully account for all observed phenomena. Our in-depth investigations reveal a more profound and previously unrecognized role for the PVA matrix. Specifically, we found that organic small molecules such as biphenyl and fluorene, which typically exhibit negligible phosphorescence in solid states, do not show significant RTP emission even when embedded in other matrices with higher glass transition temperatures (T_g) or under strictly anaerobic conditions. Strikingly, when incorporated into a PVA matrix, these very similar molecules display ultralong blue phosphorescence, characterized by a peak emission wavelength of 455 nm, CIE color coordinates of (0.155, 0.162), a luminescence efficiency of 8.72%, and a long lifetime of 4.20 s. This compelling experimental discrepancy suggests that beyond merely providing a rigid, oxygen-free environment, the PVA matrix possesses an intrinsic property that actively promotes the efficient ISC of doped small molecules to generate triplet excitons, thereby enabling phosphorescence emission. We term this novel phenomenon the “quasi-heavy atom effect (QHAE)”, proposing that PVA's unique structural and electronic characteristics facilitate SOC in a manner analogous to traditional heavy atoms, yet without their inherent toxicity or cost. Notably, in addition to PVA, numerous studies have reported the use of various polyhydroxy matrices—such as filter paper, sucrose, and cyclodextrins—to enhance the RTP of aromatic compounds. The proposed QHAE can also elucidate the phosphorescence emission mechanisms of these systems.^25–28 This deeper understanding is crucial for the rational design of novel high-performance organic RTP materials. Moreover, considering the ultra-long blue phosphorescence lifetime of such systems, which can be used for energy transfer between triplet hosts and singlet guests, advanced anti-counterfeiting applications with multiple colors can be realized by tuning the afterglow color based on color mixing principles.²⁹

2. Results and discussion

2.1. Photophysical properties

The small organic molecules BP and 9HFL were synthesized following the procedure outlined in Scheme S1. Both small molecule compounds (Fig. 1) were subjected to further purification by recrystallization prior to spectroscopic analysis and were thoroughly characterized using nuclear magnetic resonance (NMR), mass spectrometry (MS), and high-performance liquid chromatography (HPLC). The analytical data obtained were satisfactory, indicating the high purity of all compounds (Fig. S1–S11, SI).

Fig. 1 Photoluminescence photographs of a 9HFL@PVA film under 312 nm UV excitation and its afterglow image recorded after the UV lamp was turned off (a); molecular structures of BP and 9HFL (b); prompt and delayed spectra and phosphorescence lifetimes of PVA-doped films containing BP and 9HFL. The delayed spectrum was recorded with a delay time of 1.0 ms (c). The doping concentration of the organic small molecules was 2 wt%.

Efficient blue emission was observed under 312 nm UV excitation for PVA-doped films containing 2.0% of the organic conjugated compounds BP and 9HFL (Fig. S12 and S13, and Videos S1 and S2, SI). Notably, the 9HFL@PVA doped film displayed a visually perceivable afterglow of up to 32 s after the UV light was turned off (Fig. 1). To delve deeper into their optical properties, steady-state and time-resolved spectroscopy were conducted on these doped films. The prompt emission peaks of BP@PVA and 9HFL@PVA doped films were located at 310 and 302 nm, respectively. These values are consistent with their fluorescence emission maxima in THF solution (Fig. S14, SI). However, the delay emission spectra, obtained with a 1.0 ms delay, showed peaks at 470 and 455 nm, respectively (Fig. 1 and Table 1). Compared to the corresponding prompt emission peaks, the delayed emission peaks exhibited red shifts at about 160 and 153 nm, respectively, suggesting that the delayed emissions originated from phosphorescence. These results are in good agreement with previously reported phosphorescence spectra for BP³⁰ and 9HFL,³¹ observed at 468 and 446 nm, respectively, at 77 K. Further measurements revealed phosphorescence lifetimes of 2.41 and 4.20 s, and phosphorescence quantum efficiencies of 4.43 and 8.72%, for BP@PVA and 9HFL@PVA doped films, respectively (Table 1). Remarkably, the 9HFL@PVA film displayed a remarkable phosphorescence lifetime of 4.20 s, which, to our knowledge, ranks as one of the longest reported for a purely organic blue RTP system.³² It is noteworthy that the CIE 1931 (x, y) color coordinates of the phosphorescence spectrum of the 9HFL@PVA doped film reached CIE (0.155, 0.162), with a y-value less than 0.20, indicating a typical deep blue phosphorescence.³³ This is exceedingly rare in RTP material research (Table S1, SI), offering possibilities for constructing multi-color afterglows.

Table 1 Optical properties of PVA films doped with various compounds

Doped film^a	Fluor Em^b [nm]	Phos Em^c [nm]	Δλ^d [nm]	Lifetime [s]	Φ phos [%]	CIE (x, y)^f
a The doping concentration of the organic small molecule is 2 wt%. b Fluor Em = the peak wavelength of the prompt fluorescence from the doped film. c Phos Em = the peak wavelength of the phosphorescence spectrum of the doped film measured with a 1.0 ms delay time. d The difference in peak wavelength between the phosphorescence and fluorescence spectra of the doped film. e The photoluminescence quantum yield of the phosphorescence was determined by integrating sphere measurements. f Chromatic coordinates of the phosphorescence spectrum measured with a 1.0 ms delay.
BP@PVA	310	470	160	2.41	4.43	(0.168, 0.246)
9HFL@PVA	302	455	154	4.20	8.72	(0.155, 0.162)

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Numerous studies on RTP exhibited by small molecules doped in PVA have been widely attributed to two key mechanisms:⁷ the effective suppression of oxygen quenching by the PVA matrix and the creation of a rigid microenvironment by the extensive hydrogen bonding network of abundant hydroxyl groups in PVA. This rigid environment has been shown to restrict the thermal motion of triplet excitons, thereby inhibiting non-radiative decay pathways and facilitating efficient phosphorescence emission. Whether the observed RTP in BP and 9HFL doped films is solely attributable to the two aforementioned factors warrants further investigation.

The T_g serves as a critical parameter for characterizing the physical state transition of amorphous polymers. Below T_g, the polymers exhibit high rigidity and brittleness, with restricted segmental motion of their molecular chains.³⁴ Above T_g, the polymer segments gain significant mobility, and the material transitions into a soft, rubbery state. Therefore, T_g directly reflects the change in polymer rigidity with temperature. A higher T_g is typically associated with greater polymer rigidity at room temperature, which is expected to effectively limit the molecular motion of doped small molecules, subsequently reducing non-radiative triplet exciton decay and enhancing phosphorescence emission. Consequently, T_g can serve as an effective indicator for evaluating the rigidity of a polymer matrix.³⁵ Differential scanning calorimetry (DSC) measurements revealed that the T_g of PVA was 71.6 °C, while that of polystyrene (PS) was 96.0 °C (Fig. 2). The significantly higher T_g of PS compared to PVA led us to further investigate the optical properties of BP and 9HFL within the high glass transition temperature PS matrix. Experimental results revealed that, under ambient atmospheric conditions, doping BP and 9HFL into the high-T_g PS matrix did not result in significant phosphorescence emission, contrary to observations in PVA-doped films. Considering that PS possesses weaker oxygen barrier properties compared to PVA, we initially hypothesized that oxygen quenching of triplet states might be responsible for the suppressed phosphorescence in PS. To eliminate the influence of oxygen, we further tested the optical properties of BP and 9HFL in the high-rigidity PS matrix under anaerobic conditions (Fig. S15, SI). Surprisingly, even in an oxygen-free environment, the high-rigidity PS matrix failed to induce significant phosphorescence from BP and 9HFL. Notably, at room temperature, the fluorescence lifetimes of BP and 9HFL in a PVA matrix were measured to be 10.68 ns and 7.98 ns, respectively, significantly shorter than their corresponding lifetimes in a PS matrix, which were 15.24 ns and 12.72 ns, respectively (Fig. S16, SI). This reduction in fluorescence lifetime in PVA compared to PS suggests that PVA facilitates more efficient ISC, potentially enhancing triplet state population and subsequent phosphorescence emission. To investigate the role of molecular structure in RTP, we examined a heavy atom derivative, 2-bromofluorene (2BFL), which exhibits phosphorescence emission even under ambient atmospheric conditions when doped in either PVA or PS matrices (Fig. S17, SI). This observation indicates that the combination of matrix rigidity, which restricts molecular motion to suppress non-radiative triplet exciton decay, and oxygen barrier properties, which prevent triplet exciton quenching, constitutes a sufficient but not strictly necessary condition for achieving significant RTP. These findings suggest that PVA possesses additional factors, beyond rigidity and oxygen exclusion, that promote ISC for RTP in BP and 9HFL. The nature of these factors, potentially related to specific intermolecular interactions or matrix-specific effects, warrants further investigation to fully elucidate the mechanisms underlying efficient phosphorescence emission in these systems.

Fig. 2 Differential scanning calorimetry thermogram of PS and PVA.

Furthermore, it has been proposed that the phosphorescence emitted by small molecules doped in PVA films is due to specific interactions with PVA, typically involving functional groups capable of forming hydrogen bonds with PVA's hydroxyl groups, such as hydroxyl, carboxyl, aldehyde, pyridine, or ester groups.^24,36–38 Molecular dynamics simulations were utilized to explore the interactions of BP and 9HFL within a PVA matrix. A 10 nm × 10 nm × 10 nm cubic simulation box was constructed, containing 10 PVA molecules (with a polymerization degree of 40) and 3 BP or 9HFL molecules. Following an initial quenching procedure at 0 and 400 K, the system was subjected to a 2.0 ns simulation at 297.15 K.

A hydrogen bond is defined as an attractive interaction between a hydrogen atom covalently bonded to an electronegative donor atom (D) and an electronegative acceptor atom (A) possessing lone pair electrons or high electron density.^39,40 The formation of a hydrogen bond is governed by specific geometric criteria: (1) the distance between the donor and acceptor atoms (D⋯A) is typically ≤3.5 Å, often shorter for strong hydrogen bonds (e.g., 2.5–3.0 Å for O–H⋯O bonds), with the H⋯A distance generally ≤2.5 Å; (2) the D–H⋯A angle is typically ≥150°, with stronger hydrogen bonds approaching linearity (≈180°).⁴¹ Molecular dynamics simulations revealed a high prevalence of hydrogen bonding interactions among PVA molecules. In contrast, no hydrogen bonds were observed between BP and PVA or between 9HFL and PVA throughout the simulation period (see Fig. 3). These results indicate that BP and 9HFL do not form significant non-covalent interactions, such as hydrogen bonds, with PVA in thin-film environments. Consequently, the RTP observed for BP and 9HFL in the PVA matrix likely arises from additional, as-yet-unidentified mechanisms.

Fig. 3 Molecular dynamics simulations at 2000 ps illustrating the spatial distribution of BP and 9HFL in BP@PVA (a) and 9HFL@PVA (b) doped films, respectively. Green denotes PVA, while yellow represents BP and 9HFL. The intra- and intermolecular hydrogen bond counts within PVA, as well as hydrogen bond counts between BP, 9HFL, and PVA in the BP@PVA (c) and 9HFL@PVA (d) doped films.

PVA is characterized by a high density of hydroxyl groups, which are known to exhibit typical n → σ* transitions. Notably, at low temperatures (77 K), common hydroxyl-containing compounds such as methanol display unusually long afterglows, lasting on the order of tens of seconds (Fig. 4a, and Video S3, SI). Spectroscopic analysis of methanol at 77 K reveals prompt emissions at 293 and 397 nm, whereas the delayed emission spectrum (recorded with a 10 ms delay) exhibits an emission peak at 400 nm. This delayed emission peak at 400 nm has a measured lifetime of 5.73 s (Fig. 4b–d), suggesting that methanol itself possesses the intrinsic ability to undergo ISC, enabling phosphorescence emission at 77 K.

Fig. 4 Emission photographs of methanol solution at 77 K under 254 nm UV light excitation, along with afterglow photographs after the UV light was turned off (a); the prompt emission spectrum of methanol at 77 K under 260 nm UV light excitation (b), and the delayed emission spectrum at a 10 ms delay (c); decay profiles of the 400 nm phosphorescence emission peak of methanol solution at 77 K under 260 nm excitation (d). Vertical excitation energy levels of the singlet and triplet states of methanol (e), BP (f), and 9HFL (g) and the SOC constants between these states.

The occurrence of phosphorescence is fundamentally linked to changes in the spin state of electrons. A crucial physical parameter quantifying the strength of the interaction between an electron's spin and its orbital angular momentum is the SOC constant.⁴² This interaction is essential for phosphorescence to occur. In the design of phosphorescent materials, common strategies to enhance the SOC constant, thereby promoting ISC and enabling phosphorescence emission, include incorporating heavy atoms or utilizing n → π* transitions associated with moieties containing lone pair electrons. Theoretical calculations have revealed that methanol molecules possess high SOC constants (48.94 cm⁻¹), a value comparable to the heavy atom effect exerted by bromine or iodine (Fig. 4e). In contrast, molecules exhibiting π → π* transition characteristics, such as BP and 9HFL, typically possess remarkably very small SOC constants (Fig. 4f and g). For instance, BP has a reported SOC value of ξ(S₁–T₁) = 0 cm⁻¹. Therefore, we can hypothesise that the phosphorescence observed from BP and 9HFL within a PVA matrix is primarily due to the QHAE. This phenomenon occurs when the hydroxyl groups in the PVA matrix behave like traditional external heavy atoms, facilitating efficient ISC from singlet to triplet states. The QHAE enhances SOC in π → π* systems, enabling radiative transitions from triplet excited states and generating RTP. This effect provides a robust explanation for the photoluminescence mechanisms that drive phosphorescence in many PVA-doped RTP systems.

Additionally, we further investigated other matrix polymers commonly employed for preparing doped phosphorescent films, including poly(ethylene terephthalate) (PET),⁴³ PMMA,⁴⁴ polylactic acid (PLA),¹⁷ polyacrylamide (PAM),⁴⁵ polyacrylic acid (PAA),¹⁹ and poly(hexamethylene adipamide) (nylon 6,6).⁴⁶ Theoretical calculations reveal that the repeating units of these polymers exhibit substantial SOC constants (Fig. S18, SI). The mechanism underlying RTP from small molecules doped within these polymer matrices may be associated with the intrinsic QHAE of the matrix.

2.3. Tuning afterglow colors

The modulation of afterglow color represents a critical aspect of phosphorescence research.^47–49 While direct tuning of the triplet energy levels of phosphorescent molecules via chemical modifications constitutes the most straightforward approach,^50–52 it often involves complex synthesis processes and alter electronic transition properties, thereby hindering ISC and quenching phosphorescence. As an alternative strategy, Förster resonance energy transfer (FRET) from triplet hosts to singlet guests enables multicolor afterglow by carefully matching hosts and guests with distinct energy levels.^53–55 However, this methodology necessitates the selection of multiple guest molecules exhibiting compatible energy levels. Recently, Yuan et al. proposed an alternative strategy predicated on FRET and color mixing between triplet hosts and singlet guests, wherein the afterglow color can be precisely controlled by simply varying the guest concentration to modulate the intensity ratio of host phosphorescence to guest fluorescence (Fig. 5a). This method offers relative simplicity and efficiency.²⁹

Fig. 5 Schematic illustration of afterglow color regulation through FRET and color-mixing principles (a), blue dots represent long-persistent phosphorescent host materials, while orange dots indicate fluorescent acceptor materials; the afterglow emission of the doped system exhibits progressive red-shift with increasing acceptor content; photographic images of R6G/9HFL@PVA co-doped films containing different weight percentages of R6G under 312 nm UV excitation and at specified time intervals after UV cessation (b); normalized prompt (c) and delayed (d) emission spectra of R6G/9HFL@PVA co-doped composite films with different R6G doping ratios, delay time = 1.0 ms; life time of R6G/9HFL@PVA co-doped composite films at 560 nm (e).

We investigated the tuning of afterglow color in PVA-doped films by varying the concentration of the singlet guest molecule, rhodamine 6G (R6G), within a 9HFL triplet host in a PVA matrix. The choice of R6G was driven by the substantial overlap between its absorption spectrum and the phosphorescence emission of 9HFL, enabling efficient FRET from the triplet to the singlet state (Fig. S19, SI). As illustrated in Fig. 5b, increasing the R6G content in R6G/9HFL@PVA films resulted in a gradual red-shift of the afterglow color. Correspondingly, prompt emission spectra showed an increased fluorescence intensity of R6G at approximately 560 nm. Delayed emission spectra (1.0 ms) displayed a decrease in the 9HFL phosphorescence peak at 455 nm and a simultaneous increase and slight red-shift of the R6G emission peak at 560 nm with increasing R6G concentration (Fig. 5c and d). Interestingly, while R6G exhibited only a nanosecond fluorescence lifetime inPVA-doped film, the emission at 560 nm displayed a lifetime on the order of seconds in R6G/9HFL@PVA, a phenomenon attributed to triplet energy transfer from 9HFL to R6G. The lifetime of the R6G emission at 560 nm decreased from 4.10 s (0.1% R6G) to 2.74 s (5.0% R6G) (Fig. 5e). The calculated FRET efficiencies⁵⁶ between the triplet state of 9HFL and the singlet state of R6G showed a strong correlation with R6G concentration (Table S2 and Fig. S20, SI), ranging from 1.90% to 20.71% for R6G concentrations of 0.1% to 5.0%. This enhanced FRET with increasing R6G concentration led to a gradual shift in the observed afterglow color from green to orange-red. The observed decrease in the acceptor lifetime with increasing R6G concentration highlights the competition between the triplet state radiative decay and the FRET process. These findings demonstrate the effectiveness of this energy transfer mechanism for creating multi-color afterglow materials (Fig. S21, SI).

2.4. Anti-counterfeiting applications

Given the ultra-long blue phosphorescence of 9HFL@PVA and BP@PVA doped films and the ability to efficiently tune the afterglow color by introducing fluorescent guests, this material offers a promising foundation for advanced multi-color anti-counterfeiting applications.^55,57 As shown in Fig. 6a and b, a cartoon image incorporating a flower and the Chinese character ‘’ (fú) was successfully constructed utilizing 9HFL@PVA with 5.0% R6G as a guest fluorophore within the 9HFL@PVA film. This image exhibited distinct afterglow colors both under and after 312 nm UV irradiation, demonstrating its potential for straightforward anti-counterfeiting applications. Beyond simple image-based anti-counterfeiting, digital and textual anti-counterfeiting methods also hold significant value for securing important documents. As illustrated in Fig. 6c, an 8 × 4 matrix fabricated from 9HFL@PVA and BP@PVA displayed effective luminescence under UV excitation. Upon removal of the UV light, owing to the substantial difference in the afterglow lifetimes of the two materials, the matrix points composed of 9HFL@PVA remained discernible after 10 seconds (designated as ‘1’), whereas those comprising BP@PVA became unobservable (designated as ‘0’). By converting the resulting binary code within this matrix into ASCII format, the text “pgos” could be retrieved. Owing to the high light transmittance of 9HFL@PVA and BP@PVA films (Fig. 6d), these materials, when combined with their tunable afterglow colors and time-resolved properties, can be effectively employed to develop versatile encryption and anti-counterfeiting applications.^58–60

Fig. 6 (a) A flower pattern and (b) a Chinese character ‘’ (fú) pattern, both fabricated using a 9HFL@PVA and R6G/9HFL@PVA co-doped film (5.0 wt% R6G). These patterns demonstrate strong emission under 312 nm UV excitation (left) and a sustained afterglow observed at different time points after the UV source is removed. (c) An 8 × 4 luminescent matrix, assembled from 9HFL@PVA and BP@PVA, illustrating both its afterglow under 312 nm UV excitation (and after UV removal) and its application in encryption and anti-counterfeiting. (d) Optical transmittance of 9HFL@PVA and BP@PVA films.

3. Conclusion

This study reexamines conventional explanations for RTP in PVA-based systems, moving beyond the traditional focus on oxygen barrier properties and rigid hydrogen-bonded networks. Our findings demonstrate that non-phosphorescent organic small molecules in solid states, such as BP and 9HFL, exhibit ultralong blue phosphorescence (e.g., a lifetime of 4.20 s for 9HFL@PVA) exclusively within a PVA matrix. This phosphorescence is absent in other high-T_g matrices and under anaerobic conditions, suggesting that matrix rigidity and oxygen exclusion alone are insufficient to explain the observed RTP. Furthermore, the lack of strong functional group interactions between BP/9HFL and PVA rules out specific hydrogen bonding as a dominant factor. Inspired by the intrinsic phosphorescence of methanol, we propose a novel mechanism termed the QHAE. This mechanism suggests that the abundant hydroxyl groups in PVA mimic the role of traditional heavy atoms, promoting efficient ISC and enhancing SOC in π → π* systems, thereby enabling RTP. The QHAE concept also offers a framework to explain enhanced RTP phenomena in other matrix enhanced RTP systems, such as sucrose, filter paper, cyclodextrin matrices, and so on. This understanding of QHAE provides valuable insights for designing effective organic RTP materials. Additionally, the ultralong blue RTP achieved in this study opens possibilities for applications such as multi-color anti-counterfeiting technologies. Our findings highlight a previously unrecognized role of the PVA matrix in facilitating photoluminescence, offering a comprehensive explanation for RTP mechanisms in PVA-doped systems and providing guidance for future developments in this field.

4. Experimental section

The Experimental section is available in the SI.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information.

Supplementary information: Experimental instrumentation, synthesis details, theoretical calculations, characterization data (NMR, mass spectrometry, liquid chromatography, gas chromatography), additional spectral data, and tables. See DOI: https://doi.org/10.1039/d5mh01328c.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (52163017) and the Key Research and Development Program Project of Guangxi (Guike AB24010230).

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Footnote

† Zuoan Liu and Bingli Jiang contributed equally to this paper.

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