A biomimetic phosphor that can build a rigid microenvironment for its long-lived afterglow in aqueous medium

Abstract

Organic phosphorescent materials have great prospects for application, whose performance particularly depends on the preparation method. Inspired by nature’s wisdom, we report a phosphor that can utilize monomers in its environment by polymerization to construct a rigid microenvironment under light illumination, leading to a glow-in-the-dark emulsion with a phosphorescence lifetime of 1 s in water. This phosphor can achieve active growth of the aqueous emulsion with the introduction of more monomers. In the presence of trace amounts of oxygen (which has adverse effects on both polymerization and afterglow), this phosphor can still undergo photo-induced polymerization, removing the influence of oxygen and obtaining afterglow emulsion, demonstrating its adaptability to the environment. This phosphor can also catalyze the polymerization of monomers containing yellow fluorophore, obtaining long-lifetime yellow afterglow emulsion through excited state energy transfer. We have also conducted in-depth studies on the photo-catalytic and phosphorescent properties of this phosphor in model systems. This biomimetic intelligent manufacturing provides a new approach for organic phosphorescent materials and is significant for future applications.

Organic afterglow materials show great potential in diverse applications, and their performance particularly depends on their method of preparation. Here, the authors report a biomimetic phosphor that builds a rigid microenvironment to restrain non-radiative decay of triplet excitons, achieving long-lived organic afterglow in water.

Introduction

Organic room-temperature phosphorescent materials have demonstrated potential in oxygen sensing and anti-counterfeiting mapping and information storage, stimuli-responsive systems and high-contrast bioimaging due to their long lifetime characteristics^1–21. However, due to the difficulty in forming large population of triplet states and their easy deactivation characteristic²², the construction of organic room-temperature phosphorescent materials, especially in water, relies heavily on the preparation strategies. Various material manufacturing strategies have been developed, such as molecular aggregation control, two-component systems, chemical reactions, photoinduction, moisture induction, all-aqueous phase processing, self-doping, etc^23–28. Specifically, to enhance the efficiency of organic afterglow, the roles of heavy-atom effects and n-π* transitions have been extensively explored. These factors primarily facilitate intersystem crossing and increase phosphorescence decay rates, thus improving overall afterglow performance^17,29–35. However, the introduction of heavy atoms often leads to a significant reduction in phosphorescence lifetimes due to accelerated decay processes. Similarly, the presence of n-π* transitions within the organic T₁ states also contributes to a rapid decay of phosphorescence, further shortening these lifetimes. This results in a notable trade-off: achieving high afterglow efficiency typically conflicts with the desire for prolonged afterglow lifetimes, a challenge frequently encountered in organic systems^22,23,29,36. Additionally, strategies that incorporate dopant-matrix cooperation—where organic matrices (the second component) modulate the excited state characteristics of luminescent dopants (the first component)—demonstrate considerable potential for developing high-performance organic afterglow materials^37–59. The two-component strategy allows for flexible selection of luminescent dopants and organic matrices, enabling the preparation of high-performance of room-temperature phosphorescence and thermally activated delayed fluorescence afterglow materials, also organic long persistent luminescence afterglow materials that emits light for several hours^2,6,36,60,61. However, the material preparation methods are only crystallization, doping, solution casting, melt casting, grinding and ultrasonic dispersion^{2,6,29,60–63}, where the resulting products are mostly powders, films, suspensions, blocky and fixed shapes.

Nature is a master of synthesis and provides great inspiration for material preparation. The simplest form of life, such viruses, can find a host environment where they synthesize genetic material and proteins, which can form a protective shell that allows for self-replication (Scheme 1A).

Scheme 1.

Schematic of the formation, mechanism and morphology of the afterglow emulsion. A The self-replication process of viruses in host cells. B Preparation of organic afterglow nanoparticles via emulsion polymerization. C The proposed room-temperature phosphorescence mechanism of the spiro-BP-PMMA afterglow emulsions in aqueous medium. D Photographs, morphology, and hydrodynamic diameter of spiro-BP-PMMA afterglow emulsion with a 1:2 MMA feed ratio obtained at 365 nm UV on and off at room temperature.

Inspired by this, if luminescent phosphors can recruit monomers to build a rigid microenvironment, similar to the host environment of a virus, through catalytic polymerization under illumination, this can protect its triplet excited state to exhibit long-lasting phosphorescence in water, while also allowing for active growth and increasing volume with the presence of more monomers (Scheme 1). In the presence of trace amounts of oxygen, this phosphor can effectively clear the oxygen through triplet-state reactions, enabling polymerization of the monomers to form a rigid microenvironment and a glow-in-the-dark emulsion in water. This emitter can also catalyze the polymerization of monomers containing yellow fluorophores, leading to the formation of long-lasting yellow afterglow emulsion through excitation energy transfer. Through in-depth studies using model systems, we discovered that this phosphor exhibits the ability to undergo photo-catalytic radical polymerization. Additionally, its triplet excited state T₁ has localized excitation (LE) properties, which is different from most benzophenone (BP) molecules as it has fewer n-π* components⁶⁴, resulting in a lower phosphorescence rate. In a rigid environment, this phosphor displays organic room temperature phosphorescence with a lifetime of 1 s.

Results and discussion

In this study, we synthesized a class of compounds called spiro-BP as luminescent phosphors (Scheme 2). The UV-Vis spectra and photophysical results of the three compounds in solutions of dichloromethane, tetrahydrofuran, cyclohexane, and acetonitrile are shown in Supplementary Table 1 and Supplementary Fig. 1 respectively. TD-DFT calculations (Supplementary Figs. 2–4) show their S₀-to-S₁ transition of intramolecular charge transfer properties and T₁ states of localized excitation character. Single-component spiro-BP solid does not exhibit room-temperature phosphorescence phenomenon (Supplementary Fig. 5). So, by adding the three spiro-BP molecules into dichloromethane solution and observing at 77 K, an interesting photophysical phenomenon was surprisingly discovered, which exhibited significant phosphorescence (Fig. 1). The phosphorescence lifetimes of spiro-2-BP and spiro-3-BP solutions were found to be longer than 1.0 s, which is longer than that of spiro-4-BP (Fig. 1).

Scheme 2.

Chemical structures of relevant compounds. A Structures of spiro-BP. B Synthetic pathway of D-spiro-3-BP.

Fig. 1. Photophysical properties of spiro-BP in dichloromethane solution.

A 77 K steady-state and delayed emission (1 ms delay) spectra (left) and emission decay curve (right) of spiro-2-BP in dichloromethane solution. B 77 K steady-state and delayed emission (1 ms delay) spectra (left) and emission decay curve (right) of spiro-3-BP in dichloromethane solution. C 77 K steady-state and delayed emission (1 ms delay) spectra (left) and emission decay curve (right) of spiro-4-BP in dichloromethane solution.

Conventionally, in the system of benzophenone, its S₁ state has a typical n-π* characteristic, while its S₁ state (n-π* character) has very close energy levels to its T₂ state (π-π* character). According to the energy gap law and the El-Sayed rule, this energy structure can lead to a high intersystem crossing quantum efficiency (Φ_ISC) in benzophenone system. On the other hand, benzophenone’s T₁ state is of significant n-π* character and thus has a large phosphorescence decay rate (k_P), resulting in a short phosphorescence lifetime (τ_P). In the case of spiro-2-BP, spiro-3-BP, and spiro-4-BP, because the spiro groups have low T₁ energy levels. When the spiro groups are linked to benzophenone, the T₁ state of spiro-BP molecules consist of significant localized excitation character from spiro groups; the contribution of n-π* transition character is largely reduced (Fig. 2 and Supplementary Tables 2–4). Therefore, when the T₁ states of spiro-2-BP, spiro-3-BP, and spiro-4-BP systems are well protected, long phosphorescence lifetimes of hundreds milliseconds and even seconds can be observed. For spiro-4-BP with T₁-to-S₀ spin-orbit coupling matrix elements (SOCME) of 8.38 cm⁻¹, where the spiro group is substituted at benzophenone group’s para position, these two functional groups have well conjugation (Fig. 2). In the case of spiro-3-BP, the meta substitution of spiro group on benzophenone group breaks the conjugation to some extent. This would reduce the contribution of n-π* character to spiro-3-BP’s T₁ state, and consequently spiro-3-BP’s T₁-to-S₀ SOCME value decreases to 5.13 cm⁻¹ (Fig. 2). For spiro-2-BP system with spiro group in the ortho position of benzophenone group, the dihedral angle between the a-phenyl ring and b-phenyl ring has been found to be relatively large (47.56°) due to steric repulsion; the corresponding dihedral angles of spiro-3-BP and spiro-4-BP are 37.55° and 36.65°, respectively (Fig. 2). The steric effect in spiro-2-BP system significantly reduces ³n-π* contribution to its T₁ state, exhibiting SOCME value of T₁-to-S₀ at 2.41 cm⁻¹ (Fig. 2). Therefore, spiro-2-BP and spiro-3-BP systems have longer phosphorescence lifetimes than spiro-4-BP system (Fig. 1).

Fig. 2. TD-DFT calculation of spiro-BP compounds.

Iso-surface maps of electron-hole density difference of spiro-BP’s S₁ and T₁ states, where blue and green iso-surfaces correspond to hole and electron distributions, respectively. The SOCME values of S₁-T₁ or S₀-T₁ were calculated with spin-orbit mean-field (SOMF) methods on ORCA 4.2.1 program with B3LYP functional and def2-TZVP(-f) basis set.

The long-lived excited states imply photocatalytic potential of spiro-BP. We attempted to dissolve a small amount of the spiro-3-BP phosphor in methyl methacrylate (MMA) and add surfactants for ultrasonic emulsification. After polymerization, it showed that spiro-3-BP-PMMA emulsion had a room-temperature phosphorescence duration of 7 s (Fig. 3A), which is worth noting that no conventional initiator was added to this sample. The spiro-3-BP-PMMA emulsion showed maximum fluorescence emission at 441 nm, and delayed phosphorescent emission peaks at 505 nm and 532 nm (Fig. 3B), with the phosphorescent decay following single exponential decay. The τ_P value at 505 nm was 987.0 ms and at 532 nm was 989.9 ms at room temperature (Fig. 3C).

Fig. 3. Photophysical properties of spiro-BP-PMMA afterglow emulsion.

A Photographs of spiro-3-BP-PMMA afterglow emulsions under 365 nm UV and after switching-off the UV lamp at room temperature. B Room-temperature steady-state and delayed emission (1 ms delay) spectra of spiro-3-BP-PMMA emulsions. C Room-temperature emission decay profile of spiro-3-BP-PMMA emulsions.

Furthermore, inspired by the self-reproduction of the virus, we used spiro-3-BP-PMMA as the seed emulsion to achieve active growth. Different core-shell ratios of seed emulsion and MMA solution were selected to explore, and spiro-3-BP-PMMA organic phosphorescent emulsion was obtained by catalytic polymerization under illumination. The emulsion shows 7 s of room-temperature phosphorescence in the 1:2 feed ratio system (Fig. 4A), with maximum fluorescence emission at 441 nm, delayed phosphorescence emission peaks at 508 nm and 530 nm (Fig. 4B). The phosphorescent decay following single exponential decay. The τ_P value was 1000.9 ms at 505 nm and 980.1 ms at 532 nm at room temperature (Fig. 4C). The other ratios data of 1:1.3, 1:2.5, and 1:3 is shown in Table 1 and Supplementary Figs. 6–9. At present, this represents one of the longest τ_P values in the organic emulsion system containing benzophenone groups.

Fig. 4. Photophysical properties of spiro-3-BP-PMMA afterglow emulsion prepared with MMA dosing ratio of 1:2.

A Photographs of spiro-3-BP-PMMA afterglow emulsion with a 1:2 MMA feed ratio at 365 nm UV and with the UV lamp turned off at room temperature. B Room-temperature steady-state and delayed emission (1 ms delay) spectra of spiro-3-BP-PMMA emulsions with a 1:2 MMA feed ratio. C Room-temperature emission decay profile of spiro-3-BP-PMMA emulsions with a 1:2 MMA feed ratio.

Table 1.

Photophysical data of spiro-3-BP-PMMA materials with different monomer feed ratios under environmental conditions

MMA feed ratio	λP (nm)	τP (ms)
1:1	506 527	971.7 970.7
1:1.3	504 528	898.5 887.5
1:2	508 530	1000.9 980.1
1:2.5	507 530	858.6 849.0
1:3	510 531	889.3 860.5

To study the photophysical mechanism in the spiro-3-BP-PMMA system, several mechanisms that can also generate organic phosphorescence should be eliminated. Firstly, the high purity of spiro-3-BP (confirmed by HPLC) can confirm that it is not impurities that cause phosphorescence mechanisms (Supplementary Fig. 10)⁶⁵. Secondly, according to previous research reports, in the RTP mechanism, energy transfer from the matrix to the phosphor can lead to phosphorescence. This is not the case in the spiro-3-BP-PMMA emulsion excited at 370 nm, because PMMA cannot be excited as an energy transfer donor at 370 nm (Supplementary Fig. 11)⁶⁶. Thirdly, PMMA has a lower HOMO and a higher LUMO than spiro-3-BP (Supplementary Table 5), so the intermolecular charge transfer between the phosphor and matrix should be negligible. Room-temperature afterglow related to intermolecular charge transfer can be ruled out². Fourthly, the T₁ level of PMMA is higher than the S₁ and T₁ levels of spiro-3-BP, thus the T₁-mediated RTP mechanism should be absent in spiro-3-BP-PMMA system⁶⁵. Fifthly, the thermal activation delayed fluorescence (TADF) afterglow mechanism can be eliminated by measuring delayed emission at 77 K (Supplementary Fig. 12)^67,68. Accordingly, in summary, the S₁ state of spiro-3-BP is initially formed excited by UV and followed by efficient ISC leading to population of the T₁ state. The PMMA matrix can suppress the nonradiative decay (k_nr) of the T₁ state of spiro-3-BP, protecting the T₁ state from oxygen quenching (k_q). The slow phosphorescent decay (k_P) of spiro-3-BP because of the localized excitation character of T₁ state (Supplementary Fig. 3) in PMMA leads to a long τ_P.

Usually, phenyl benzoate (PhB) is widely used as a rigid matrix for luminescent dopant^60,69,70. To further explore the luminescence mechanism of spiro-BP as a phosphor, we doped spiro-3-BP at different concentrations into PhB (Supplementary Figs. 13-16), where the two-component materials exhibited prominent 7 s room temperature afterglow (Fig. 5A). The spiro-3-BP-PhB sample displays a 450−550 nm major band in their room-temperature steady and delayed spectra, and the phosphorescence lifetime is 1438 ms under ambient conditions (Fig. 5B). Additionally, delayed spectra at 77 K showed a well-resolved phosphorescent band with emission located in a region which is similar to room-temperature phosphorescence (Supplementary Fig. 17). Moreover, when spiro-3-BP doped into different matrices of 4-methoxybenzophenone (MeOBP), benzophenone (BP), and diphenyl carbonate (DPC), all materials exhibit afterglow characteristics (Supplementary Figs. 18-20). We deuterated spiro-3-BP and obtained D-spiro-3-BP-PhB materials, which also showed significant green organic afterglow emission excited under UV lamps, lasting for over 21 s (Fig. 5C) and the phosphorescence lifetime is 2.26 s under ambient conditions (Fig. 5D and Supplementary Figs. 21–24). Given that C-D bond vibration is much weaker than that of C-H bond, intramolecular motion of D-spiro-3-BP’s T₁ states would be reduced and thus nonradiative decay of T₁ states can be suppressed, which can lead to the elongation of phosphorescence lifetime when the T₁ states are well protected in suitable matrix. Therefore, this further confirms the RTP mechanism of the spiro-BP phosphor luminescent system.

Fig. 5. Photophysical property of spiro-3-BP-PhB materials and D-spiro-3-BP-PhB materials.

A Photographs of spiro-3-BP-PhB-0.01% sample obtained under 365 nm UV and after switching-off the UV lamp at room temperature. B Room temperature steady-state and delayed emission (1 ms delay) spectra (left) and room-temperature emission decay curve (right) of spiro-3-BP-PhB-0.01% sample. C Photographs of d-spiro-BP-PhB-0.01% sample obtained under 365 nm UV and after switching-off the UV lamp at room temperature. D Room temperature steady-state and delayed emission (1 ms delay) spectra (left) and room-temperature emission decay curve (right) of d-spiro-BP-PhB-0.01% sample.

In the emulsion system of spiro-BP-PMMA, we also measured the dynamic light scattering properties (DLS). The average hydrodynamic diameter of the seed emulsion was 67.65 nm, and the diameter of spiro-3-BP-PMMA with a core-shell ratio of 1:1 was 79.75 nm (Fig. 6). By testing the situation of latex particles, NMR indeed confirmed the formation of PMMA with high conversion of MMA (Supplementary Fig. 25), and the cubed ratio of the particle size between spiro-3-BP-PMMA and the seed emulsion was 1.64 as characterized by DLS and TEM (Supplementary Fig. 26). Additionally, as PMMA is in the glassy state at room temperature and both luminescent molecules and PMMA are hydrophobic, the luminescent molecules were protected by the glassy microenvironment provided by PMMA latex particles, thus exhibiting room-temperature phosphorescent properties. Furthermore, it is more interesting that with the increase of PMMA ratio, the emulsion size increased by DLS testing. The calculated sizes based on the original average size of latex particles, the amount of PMMA, and the amount of added MMA monomer matched the measured sizes, indicating that seed growth was achieved in this system. MMA swelled the PMMA latex particles, and after in situ photo-polymerization, the swollen latex particles formed larger-sized emulsion droplets.

Fig. 6. Dynamic light scattering measurements.

A–F Hydrodynamic diameter of spiro-3-BP-PMMA afterglow emulsion estimated by dynamic light scattering (DLS) measurements.

In addition, we investigated the phosphorescent emulsion in water formed by photo-polymerization with the presence of a small amount of oxygen (below 0.1 mg/L) and subsequent removal of oxygen by light irradiation. The results showed that even in the presence of oxygen, there was still 4 s of phosphorescence and a 655.8 ms phosphorescent lifetime (Fig. 7A–C), indicating the ability of the phosphorescent system to adapt to the environment and mitigate the influence of oxygen. We further increase the oxygen concentration of the system to perform the photo-polymerization. At oxygen concentration of 1.5–2.0 mg/L, the obtained spiro-3-BP-PMMA emulsion has phosphorescence lifetime of around 450 ms (Supplementary Fig. 27). When the oxygen concentration reaches 4.5–5.5 mg/L, the resultant spiro-3-BP-PMMA emulsion show insignificant afterglow at room temperature (Supplementary Fig. 28). Interestingly, we also perform the photo-polymerization of MMA in the presence of spiro-3-BP and N-methyldiethanolamine (MDEA). It has been found that, by simply mixing the components for polymerization and sealing the reaction system (degassing is not required), the emulsion after photo-polymerization still exhibits room-temperature afterglow with phosphorescence lifetimes of around 300 ms or longer (Supplementary Fig. 29). Therefore, this phosphor system has a strong role in eliminating the influence of oxygen. As an active growing emulsion, it has excellent adaptability to the environment.

Fig. 7. Property of spiro-3-BP-PMMA afterglow emulsion.

A Photograph of spiro-3-BP-PMMA (polymerized in the presence of a small amount of oxygen) afterglow emulsion obtained at 365 nm UV light and with the UV lamp turned off at room temperature. B Room-temperature steady-state and delayed emission (1 ms delay) spectra of spiro-3-BP-PMMA (polymerized in the presence of a small amount of oxygen) emulsions. C Room-temperature emission decay profile of spiro-3-BP-PMMA (polymerized in the presence of a small amount of oxygen) emulsions. D Chemical structure of DCM. E Photographs of the spiro-3-BP-DCM-PMMA emulsion obtained under 365 nm UV light and after switching-off the UV lamp at room temperature. F Room-temperature steady-state and delayed emission (1 ms delay) spectra of spiro-3-BP-DCM-PMMA emulsions. G Room-temperature emission decay profile of spiro-3-BP-DCM-PMMA emulsions.

In order to study the photo-polymerization mechanism in the present system, we perform MALDI-TOF measurement for the PMMA samples after polymerization. Two PMMA samples are made: (1) by the photo-polymerization of MMA in the presence of spiro-3-BP; (2) by the photo-polymerization of MMA in the presence of spiro-3-BP and MDEA. The MALDI-TOF results of sample 2 indicate that the PMMA chains have MDEA end groups (Supplementary Fig. 30A), so the combination of spiro-3-BP and MDEA represent a typical Type II photoinitiator system with the corresponding photo-polymerization mechanism being illustrated in Supplementary Fig. 30B. For sample 1, the MALDI-TOF results show that the PMMA chains didn’t have well-defined end groups (Supplementary Fig. 31), so we propose that the excited-state spiro-3-BP may undergo some nonspecific mechanism to form free radicals and then initiate the polymerization of MMA.

Considering the good spectral overlap between the phosphorescence of spiro-3-BP-PMMA and the UV-vis absorption of 2-(2-(4-(dimethylamino)styryl)-6-methyl-4H-pyran-4-ylidene)malononitrile (DCM) (Fig. 7D and Supplementary Fig. 32), the resulting three-component spiro-3-BP-DCM-PMMA material displayed orange afterglow at 575 nm and an emission lifetime of 498.7 ms (Fig. 7E–G). To study the energy transfer mechanism, we freshly prepare spiro-3-BP-DCM-PMMA afterglow emulsion and monitor the phosphorescence lifetime of spiro-3-BP donor (Supplementary Fig. 33A). The phosphorescence lifetime monitored at 515 nm has been found to be around 0.5 s (Supplementary Fig. 33B), much shorter than that observed in spiro-3-BP-PMMA afterglow emulsion. With reference to the reported studies^71–74 these observations suggest the excited state energy transfer from the spiro-3-BP afterglow donor to the DCM fluorescence acceptor, leading to the emergence of DCM’s afterglow.

Conclusion

In this study, we report a phosphor that forms an afterglow emulsion with a phosphorescence lifetime of 1 s through catalytic polymerization in water. It can also achieve active growth by introducing more monomers. Especially in the presence of trace amounts of oxygen, photo-induced polymerization can still be carried out to eliminate the influence of oxygen and have certain adaptability to the environment. In addition, the phosphor can also catalyze the polymerization of monomer containing yellow fluorophore through excitation energy transfer to obtain a long-life yellow afterglow emulsion. This biomimetic intelligent manufacturing method provides a new strategy for organic phosphorescent materials, which is of great significance for future applications.

Methods

Materials

9,9′-Spirobi[9H-fluorene]-2-boronic acid (98%, Dibai), (4-bromophenyl) phenylmethanone (99.6%, Bide Pharmatech), (2-bromophenyl) phenylmethanone (95%, Meryer), (3-bromophenyl)phenylmethanone (98%, Bide Pharmatech), potassium carbonate (AR, Shanghai Dahe Chemicals Co., Ltd.), palladium(II) acetate (99% Pd 46.0-48.0%, Innochem), N,N-dimethyformamide (AR, Shanghai Experimental Reagent Co., Ltd.), deuterium oxide (D₂O) (99.8%, Admas), Pd/C (Sigma-Aldrich), phenyl benzoate (PhB) (99%, Energy Chemical), methyl methacrylate (MMA) (99%, Adamas), Pluronic F127 (poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide).

Syntheses and structural characterization

The synthetic procedures and details of the compounds mentioned in this report can be found in Supplementary Information. And compound structure characterization is shown in Supplementary Figs. 34–43.

Physical measurements and instrumentation

Nuclear magnetic resonance (NMR) spectra were recorded on a JEOL Fourier-transform NMR spectrometer (400 MHz), including ¹H NMR and ¹³C{¹H} NMR. Mass spectra were performed on Agilent Q-TOF 6520 liquid chromatograph mass spectrometer. FT-IR spectra were recorded on a Nicolet AVATAR-360 FT-IR spectrophotometer with a resolution of 4 cm⁻¹. UV-Vis absorption spectra were recorded on a Techcomp UV1050 UV-vis spectrophotometer. The steady-state and delayed emission spectra were collected by Hitachi FL-4700 fluorescence spectrometer equipped with chopping systems; the delayed emission spectra were obtained with a delay time of approximately 1 ms. The excited state decay profiles in millisecond to second region were collected by Hitachi FL-4700 fluorescence spectrometer equipped with chopping systems. Dynamic light scattering (DLS) experiments were performed on a Malvern Nano-ZS90 Zetasizer with an internal HeNe laser (l = 632.8 nm). Transmission electron microscopy (TEM) experiments were performed on a JEOL JEM-F200. Photographs and videos were captured by Apple iphone 11 cameras. Before imaging, samples were irradiated by a 365 nm UV lamp (5 W) for approximately 5 s at approximately 15 cm.

Theoretical calculations

TD-DFT calculations were performed to study the photophysical properties of molecularly dispersed spiro-BP in PhB matrix. Since the afterglow properties are originated from the excited states of molecularly dispersed spiro-BP in the rigid PhB matrices where intermolecular rotation and vibration are largely restricted, the ground-state geometry of spiro-BP was used for all the TD-DFT calculations. The ground-state geometry of spiro-BP were optimized by a DFT calculation using B3LYP functional and 6–31 G (d, p) basis set. The singlet excited states and triplet excited states were calculated on Gaussian 16 program (Revision A.03) with B3LYP functional and 6-31 G (d, p) functional. Spin-orbit coupling (SOC) matrix elements between the singlet excited states and triplet excited states were calculated with spin-orbit mean-field (SOMF) methods on ORCA 4.2.1 program with B3LYP functional and def2-TZVP(-f) basis set. The obtained electronic structures were analyzed by Multiwfn software. All isosurface maps to show the electron distribution and electronic transitions were rendered by visual molecular dynamics (VMD) software based on the exported files from Multiwfn.

Preparation of afterglow materials by doping spiro-BP into organic matrices

For the preparation of spiro-BP-PhB materials, 500 μL spiro-BP in dichloromethane (1 mg/mL) and 100 mg phenyl benzoate (PhB) were added into a 3 mL sample bottle, and then heated to 80 °C to give a molten mixture. Subsequently, the sample bottle was transferred to a bath of liquid nitrogen to immediately solidify the molten mixture. After standing at room temperature for tens of minutes, melt-cast materials were obtained.

The preparation of spiro-3-BP-PMMA organic afterglow seed emulsion

Methyl methacrylate (750 mL), spiro-3-BP (5 mg), water (12.5 mL), 7.5 mL of surfactant in aqueous solution (25 mg/mL) were added into a 25 mL Schlenk tube to form the liquid precursor. The liquid precursor was treated by 50 min ultrasonication for pre-emulsification. After three cycles of freeze-pump-thaw-degassing procedures, spiro-3-BP-PMMA organic afterglow seed emulsion was prepared under stirring with 365 nm UV light for 1 h.

The preparation of spiro-3-BP-PMMA organic afterglow core-shell emulsion

A part of seed emulsion was used as core and MMA solution as shell for the second step of active growth reaction. The ratio of core to shell is selected as 1:1, 1:1.3, 1:2, 1:2.5 and 1:3. Firstly, added 20 wt% F-127 to a 50 wt% MMA aqueous solution, which the pre-polymerized emulsion was prepared by ultrasonic for 30 min. After three cycles of freezing pump thawing and degassing procedure, the seed emulsion with different ratio of core to shell was added to the pre-polymerized emulsion. Then stirred for 24 h and irradiated with the 365 nm wavelength of UV lamp for 1 h to obtain the spiro-BP-PMMA organic afterglow core-shell emulsion, which it enables the seed emulsion to realize active growth in shell emulsion.

Author contributions

These authors contributed equally to this work. CRediT: Yuming Su data curation, methodology; Guangming Wang software; Boyi Fu visualization; Xixi Piao writing—original draft; Kaka Zhang conceptualization, funding acquisition, supervision, writing—review & editing.

Peer review

Peer review information

Communications Chemistry thanks Guoqing Zhang and the other, anonymous, reviewers for their contribution to the peer review of this work.

Data availability

The data that support the findings of this study are available in the article, supplementary information file, source data file or from the corresponding authors upon request. Source data for the main figures and Supplementary Figs. are provided in Supplementary Data 1 and Supplementary Data 2, respectively.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s42004-024-01347-4.