Metal ion-manipulated afterglow on rhodamine 6G derivative-doped room-temperature phosphorescent PVA films

The long-lived room-temperature phosphorescence (RTP) originating from thiophene boronate polyvinyl alcohol (PVA) has enabled the creation of metal-ion-responsive RTP films doped with spirolactam ring-containing rhodamine 6G (1). In this study, RTP-active PVA films, namely, TDB@PVA and ATB@PVA, were prepared through boronate esterification of thiophene-2,5-diboronic acid (TDB) and 5-acetylthiophene-2-boronic acid (ATB) with the diol units of PVA. The delayed emission properties were evaluated, revealing an emission band at 477 nm with a turquoise afterglow for TDB@PVA and at 510 nm with a green afterglow for ATB@PVA after UV light irradiation ceased. The photophysical properties were assessed using TD-DFT and DFT calculations at the B3LYP/cc-pVDZ level. N-(rhodamine-6G)lactam dye with a salicylimine unit (1) was doped into the RTP-based PVA films, producing a multicolored afterglow upon the addition of metal ions. This phenomenon is explained by a triplet-to-singlet Förster-type resonance energy transfer process from the cross-linked thiophene boronate in PVA to the metal-ion-activated colored form of 1. This photophysical feature finds applicability in encryption techniques. Notably, the reversible metal-ligand coordination of 1 in the PVA system enabled a write/erase information process.

1 Introduction

Stimulus-responsive luminescent materials have garnered significant attention as smart materials, with their emission properties tunable by various external stimuli, including chemical species, temperature, water, pH, and mechanical force. (Gu and Ma, 2022; Lei et al., 2023). This responsiveness is closely linked to their sensing function, which has been widely investigated for fluorescence-based chemosensors used to efficiently detect chemically or biologically important analytes. (Wu et al., 2017; Deng et al., 2021; Sasaki et al., 2021; Krämer et al., 2022; Kumar et al., 2023). Recently, the demand for organic materials applicable to various state-of-the-art technologies has driven research into alternative emission modes for luminescent materials. (Zhang et al., 2018). Organic luminogens with room-temperature phosphorescence (RTP) offer the significant advantage of avoiding short-lived autofluorescence and scattering light caused by irradiation, facilitating practical applications (Li J. et al., 2022) such as time-gated bioimaging, (Zhao et al., 2015), information security, (Gmelch et al., 2019; Su et al., 2020), optoelectronics, (Kabe et al., 2016), among others. (Zhao et al., 2020; Shi et al., 2022). Unlike traditional fluorescence, triplet exciton-based emissive relaxation allows for long-lived delay emission, often visually detectable as an “afterglow” that persists after the excitation source is removed. (Yang et al., 2023). Such fascinating optical phenomena provide valid monitoring parameters. However, developing RTP materials with dynamic photophysical properties remains challenging. In principle, efficient RTP materials require both an increase in the triplet exciton population by facilitating intersystem crossing (ISC) between singlet and triplet states, as well as the suppression of nonradiative relaxation channels, including oxygen quenching. (Xu et al., 2023). Several approaches have been employed to create a rigid environment, including host-guest interactions, (Li et al., 2018; Yu et al., 2019; Wang et al., 2020), crystal forms, (Forni et al., 2018; Jia et al., 2020), H-aggregation, (An et al., 2015), intermolecular hydrogen bonding interactions, (Ma H. et al., 2019), and polymer matrices. (Gan et al., 2018). Although stimulus-triggered tuning in the afterglow has been investigated to date, including visual gas sensing, (DeRosa et al., 2017; Liu et al., 2022) temperature sensing, (Jin et al., 2020; Kawaguchi et al., 2024), metal ions detection, (Wei et al., 2020; Guo et al., 2022; Wu et al., 2022; Dai et al., 2023) volatile organic compounds detection, (Mei et al., 2022; Zhang et al., 2022), and NH₃ and HCl detection, (Cheng et al., 2018) the environmental sensitivity of phosphorescence properties often hinders the dynamic manipulation of RTP materials.

Our interest in developing RTP-active materials motivated us to focus on boronic acid derivatives as phosphors. The empty p-orbitals of some boronic acids promote ISC and suppress the phosphorescence rate constant (k_P), which results in outstanding long-lived RTP behaviors in rigid environments such as solid-state, (Chai et al., 2017; Kuno et al., 2017; Shoji et al., 2017; Yuasa and Kuno, 2018) doped films (Li D. et al., 2022) and doped crystals (Zhang et al., 2021; Zhou et al., 2021). Notably, the facile binding of boronic acids with the diol units of polyvinyl alcohol (PVA) is particularly advantageous for preparing RTP-active films. PVA is well-known for its mechanical flexibility and large-area production, making it suitable for a wide range of applications, such as food packaging, (Oun et al., 2022) biodegradable plastics, (Belay, 2023) and biocompatible materials in the medical and pharmaceutical field (Teodorescu et al., 2019). From the standpoint of RTP materials, PVA serves not only as a rigid matrix to protect triplet exciton but also contributes to accelerating intersystem crossing, (Liang et al., 2023) enabling the easy fabrication of RTP-based afterglow materials by suppressing vibrational dissipation and oxygen quenching of the excited triplet state (Al-Attar and Monkman, 2012; Kwon et al., 2014; Ma X. et al., 2019). Zhao et al. achieved ultralong single-molecule phosphorescence in a PVA polymer matrix and investigated the influence of aggregation, conformation, temperature, and moisture on monomer phosphorescence (Wu et al., 2020). Despite the superior properties of PVA matrices, the development of chemically stimulus-controllable RTP systems in PVA remains at the forefront. Previously, we prepared RTP-active thiophene boronate ester-cross-linked PVA (TDB-PVA) (refer to Figure 1), where the rigid environment of the PVA-based matrix stabilized the triplet state of the thiophene linker (Kanakubo et al., 2021). We thus hypothesized that doping N-(rhodamine-6G)lactam dye 1 (Lee et al., 2016) (Figure 1) into PVA would yield metal ion-responsive afterglow films (1-TDB@PVA). The metal ion-induced ring-opening reaction of 1 could induce the appearance of an absorption band with strong emission in the visible region, which would serve as an acceptor for a triplet-singlet Förster-type resonance energy transfer (TS-FRET) process (Sk et al., 2023) from the thiophene boronate in PVA, thus causing metal ion-manipulated multicolored afterglow. Furthermore, the reversible ring-opening/closing reaction of the spirolactam unit motivated us to investigate afterglow manipulation in our systems. With this in mind, pyrophosphate (PPi), which has a high binding affinity for metal ions, can dissociate the metal-ligand coordination. We postulated the dynamic manipulation of the RTP-based afterglow properties through reversible metal-ligand coordination using PPi, which has intriguing applications in the write/erase information process.

Figure 1

Figure 1. Chemical structures of TDB@PVA, ATB@PVA, and 1.

2 Experimental section

2.1 General

Nuclear magnetic resonance (NMR) spectra were measured on a JEOL JNM-ESC400 (¹H: 400 MHz, ¹¹B: 128 MHz). In ¹H and ¹¹B NMR measurements, chemical shifts (δ) are reported downfield from the internal standard Me₄Si and external standard BF₃∙OEt₂, respectively. Mass spectrometry data were taken using a Bruker micrOTOF II-SDT1 spectrometer with atmospheric pressure chemical ionization (APCI) method. The absorption and emission were measured using Shimadzu UV-3600 UV/Vis/NIR spectroscopy and a JASCO FP-8500 spectrofluorometer, respectively. ATR-FTIR spectra were recorded on a JASCO FT/IR-4100 spectrometer with NaCl salt plate. The absolute photoluminescence quantum yields (Φ) for emission up to 650 nm were determined by JASCO FP-8500 spectrofluorometer equipped integral sphere (ϕ = 60 mm). Photographic images were taken with a digital camera (Canon, EOS Kiss X8i).

2.2 Materials

Unless otherwise indicated, reagents used for the synthesis were commercially available and were used as supplied. Thiophene-2,5-diboronic acid TDB and 5-acetylthiophene-2-boronic acid ATB were recrystallized with water. RTP-active PVA matrices TDB@PVA (Kanakubo et al., 2021) and 3′,6′-bis(ethylamino)-2-(2-(2-hydroxy-5-benzylideneamino)ethyl)-2′,7′-dimethylspiro[isoindolin-1,9′-xanthen]-3-one 1 (Lee et al., 2016) were prepared according to the method previously reported. The NMR data of TDB@PVA are as follows; ¹H NMR (400 MHz, DMSO-d₆, ppm) δ 1.23 – 1.52 (-CH(OH)-CH₂-, and -CH(boronate ester)-CH₂-), 3.82 – 3.88 (-CH(boronate ester), CH(OCOCH₃) and -CH(OH)-), 4.24 – 4.25 (-OH), 4.49 (-OH), 4.89 (1H, -OH), 7.45 (thiophene-H); ¹¹B NMR (128 MHz, DMSO-d₆, ppm) δ 21.1.

2.3 Preparation of PVA film by doping 5-acetylthiophene-2-boronic acid (ATB@PVA)

An aqueous EtOH solution of PVA (0.25 unitM) and 5-acetylthiophene-2-boronic acid (0.1–2.0 mol%) was drop-casted on a silicon rubber plate and dried at room temperature overnight, and then dried in vacuo. The assignment was conducted by ATP-FTIR and ¹H and ¹¹B NMR measurements. ATR-FTIR spectra; 659 cm^‒1 (boronate ester bond), ∼1270 cm^‒1 (B‒O stretching), 1654 cm^‒1 (C=O group). ¹H NMR (400 MHz, DMSO-d₆, ppm) δ 1.27 – 1.52 (-CH(OH)-CH₂-, and -CH(boronate ester)-CH₂-), 3.82–3.88 (-CH(boronate ester), CH(OCOCH₃) and -CH(OH)-), 4.21 – 4.23 (-OH), 4.47 (-OH), 4.668 (-OH), 7.46 (thiophene-H), 7.89 (thiophene-H); ¹¹B NMR (128 MHz, DMSO-d₆, ppm) δ 15.6.

2.4 Fabrication of 1-TDB@PVA

A DMSO solution (500 μL) of PVA (number average molecular weight (M_n) of 89,000–98,000; saponification number: ≥99%, 0.4 unitM. The concentration was based on the monomer unit.), thiophene-2,5-diboronic acid (2.0 mM), and 1 (48 μM) was drop-casted on micro slide glass with silicone rubber plate, heated at 60°C for 12 h, and dried in vacuo.

2.5 Evaluation of phosphorescence quantum yield

The delayed emission of RTP-active PVA films was measured by a JASCO FP-8500 spectrofluorometer equipped with an integral sphere (ϕ = 60 mm). The obtained spectra showed separated fluorescence and phosphorescence peaks. Then the integral emission intensities ranging from 400 nm to 650 nm were evaluated as phosphorescence quantum yield.

2.6 DFT/TD-DFT calculations.

Ground state and excited state geometries of thiophene-2,5-di(boronic acid)propane-1,3-diol diester (TDB ester) and 2-(4-acetylphenyl)-1,3,2-dioxaborinane (ATB ester) were optimized by density functional theory (DFT) at ωB97X-D3/def2-TZVP level with Orca 5.0 software, where spin-orbit coupling matrix elements (SOCMEs) and energy gaps (ΔE_ST) between each singlet and triplet pair were computed. Natural transition orbital (NTO) calculation was conducted using B3LYP/cc-pVDZ level in the Gaussian 16 software. (Frisch et al., 2016). All geometries of the compounds at the ground state were fully optimized. Results of TD-DFT calculation are described in Supplementary Tables S1–S6. Optimized structures of ground state and excited states of TDB and ATB esters are indicated in Supplementary Tables S7–S12 as Cartesian coordinates.

3 Results and discussion

3.1 Preparation of RTP-active PVA films using thiophene boronic acids

Aqueous EtOH solutions of thiopheneboronic acids (TDB and ATB) and PVA (number average molecular weight (M_n) of 89,000–98,000; saponification number: ≥99%, 0.4 unitM for TDB and 0.25 unitM for ATB. The concentration was based on the monomer unit.) were drop-casted and dried to produce TDB@PVA and ATB@PVA, respectively. Characterization was conducted using ATP-FTIR, revealing characteristic broad peaks ascribed to the boronate ester bond and B‒O stretching at approximately 650 – 660 cm^‒1 and ∼1300 cm^‒1, respectively. Additionally, a typical stretching band arising from the C=O group was observed at 1655 cm^‒1 for ATB-PVA. Further information came from ¹¹B NMR spectroscopy, where a broad signal at 15.6 ppm suggested that trigonal planar sp² boron was adopted under the conditions. The average degrees of thiophene-based labeling are deduced to be 4.3 and 5.2 for TDB@PVA and ATB@PVA, respectively, based on the ¹H NMR data (Supplementary Figure S1).

The delayed emission of the PVA films was recorded upon irradiation at 254 nm (Figure 2). TDB@PVA showed a significant RTP emission band at 477 nm when 0.2 mol% TDB was doped into PVA, and a turquoise afterglow was observed after ceasing the irradiation light. Based on our previous evaluation, the phosphorescence quantum yield (Φ_P) is 6.3% with a phosphorescent lifetime (τ_P) of 256 ms. (Kanakubo et al., 2021). On the other hand, the delayed emission of ATB@PVA was observed at 510 nm with Φ_P and τ_P values of 6.8% and 97.1 ms, respectively, upon excitation at 254 nm, when 0.5 mol% of ATB was grafted into PVA, as optimized conditions for RTP behavior (Supplementary Figure S2). Consequently, the color of the afterglow was green, and the emission band was red-shifted by 33 nm compared to that of TDB@PVA. Time dependency on the RTP properties was measured (Supplementary Figure S3). The intensity decreased as time passed; the decline ratios in the emission intensity were 7.7% and 16.6% for TDB@PVA and ATB@PVA, respectively, after 10 min passed. On the other hand, deaeration treatment for the films over 60 min enabled it to detect the emission recovery. Those results indicated that the RTP properties of the films are oxygen-sensitive. However, the thiophene-bridged PVA film is relatively stable, enabling it to acquire the photophysical data in this study.

Figure 2

Figure 2. Delayed emission spectra of TDB@PVA (turqoise solid line) and ATB@PVA (green solid line). [TDB] = 0.2 mol%, [ATB] = 0.5 mol%. The afterglow images are also shown.

The impact of the acetyl substituent in ATB@PVA on the delayed emission was evaluated using TD-DFT and DFT calculations at the B3LYP/cc-pVDZ level. Given the limitations of our PC specifications, the current calculations for thiophene boronate PVA systems are based on an optimized single-molecule structure, where the effect of the polymer matrices is not considered. Thus, we calculated TDB ester and ATB ester as model compounds (Figure 3). The global minimized singlet energy level (S₁^GM) of TDB ester is calculated to be 3.77 eV, which is close to the T₂ energy level. A small S₁ ‒ T₂ energy gap (0.47 eV) suggests a plausible ISC transition from S₁ to T₂. According to Kasha’s rule, the RTP property is postulated to be governed by the T₁ → S₀ transition, calculated at 468 nm (λ_calcd), which is close to the experimental value. On the other hand, the S₀ → S₁ transition of ATB ester is a forbidden transition (HOMO‒1 → LUMO), being characterized by (n, π*) configuration, as inferred from NTO analysis. (Martin, 2003). Therefore, light excitation would prompt ATB ester to increase the population of the S₂ state, followed by energy dissipation of S₂ to S₁ through internal conversion. An efficient ISC process to T₁ would be possible based on El-Sayed’s rule, (El-Sayed, 1968), since T₁ → S₀ transition is characterized by (π, π*). This suggests that the carbonyl unit of the acetyl substituent contributes to an increase in the intersystem crossing rate. Although acetyl group of ATB@PVA could participate hydrogen bonding network to assist the rigidity of the system, the smaller lifetime of ATB@PVA (τ_P = 97.1 ms) allows us to consider that (n, π*) configuration of the acetyl group may govern the photophysical properties.

Figure 3

Figure 3. Jablonski diagrams and the related frontier molecular orbitals of TDB ester (A) and ATB ester (B). The transition energies of S₁^FC and S₁^GM were evaluated based on the ground-state optimization and the S₁ optimized structure, respectively. Transition energies of triplet states were assessed based on T₁ optimized structure.

Given that RTP materials inherently exhibit a large Stokes shift in their photophysical properties, we note that TDB@PVA has an afterglow emission of less than 500 nm. (Zheng et al., 2023). In addition, given that a multicolor afterglow can be easily obtained by combining TDB@PVA with complementary emissive materials, we investigated the photophysical properties of metal ion-responsive RTP films using TDB@PVA doped with N-(rhodamine-6G)lactam dye 1.

3.2 Metal ion-responsive rhodamine 6G-doped afterglow films

We screened suitable rhodamine dyes to serve as metal ion-responsive TS-FRET acceptors in PVA. We note that N-(rhodamine-6G)lactam dye with salicylimine unit 1; Supplementary Figure S4 shows the absorption and fluorescence spectra of 1 in MeOH/H₂O (9:1 v/v), which showed a selective response toward metal ions (Supplementary Figure S5). As a preliminary assessment, the detection of Fe³⁺ was more responsive than Al³⁺ and Hg²⁺. Taking the absorption property of Fe³⁺ in the visible region into account (Supplementary Figure S6), our attention was focused on the Al³⁺-induced response to the photophysical properties. Adding Al³⁺ as a putative trivalent metal ion to the solution of 1 led to the appearance of an absorption band ranging from ca. 470–560 nm, whereas a fluorescence band was observed at 555 nm when excited at 530 nm. The association constant was determined by a non-linear curve fitting plot, giving 4.3 ×10³ M^–1 (Supplementary Figure S7). However, it was found that Al³⁺-induced emission enhancement was almost inactive when 1 was doped in PVA, possibly due to an aggregation of 1 with the ring-opened structure in PVA (Supplementary Figure S8). Therefore, we tuned the internal microenvironment of PVA by modifying fabrication conditions. Subsequently, the corresponding RTP-active films prepared from a DMSO solution of TDB and PVA, instead of an EtOH/H₂O solution, showed a much higher response as a factor of 4.2 than that of the film prepared using EtOH/H₂O solution when 0.4 mM of Al³⁺ was added to each film (Supplementary Figure S8). To our delight, chemical stimuli-induced afterglow manipulation in PVA was conducted using TDB@PVA prepared from a DMSO solution of TDB and PVA (see the Experimental Section). The resultant film TDB@PVA has an RTP band at ca. 500 nm, which shows a negligible decrease in the emission intensity (≤21%) upon adding up to 0.6 mM of Al³⁺ (Supplementary Figure S9). It means that afterglow manipulation could be driven from reversible metal-ligand coordination on 1. The photophysical interaction between the thiophene boronate linker and 1 in the PVA was also evaluated (Figure 4). The absorption band from the Al³⁺-triggered colored dye 1 overlapped with the delayed emission band of thiophene boronate. The photophysical properties could cause TS-FRET between them, possibly endowing the system with the dynamic function of a metal-ion-triggered change in the afterglow.

Figure 4

Figure 4. Phosphorescence spectra of TDB@PVA (turquoise solid line), and UV/Vis absorption (red dotted line) and emission spectra (red solid line) of Al³⁺-triggered colored 1 in PVA. The delay time is 50 ms.

With the above in consideration, 1-TDB@PVA was prepared by doping 1 (0.012 mol%) and TDB (0.5 mol%) with PVA. The stepwise addition of Al³⁺ to the film led to a change in the afterglow from turquoise to yellowish-green after ceasing UV irradiation at 254 nm. To understand this behavior, the delayed emission spectra of 1-TDB@PVA were measured (Figure 5A). The emission intensity at 477 nm declined stepwise, whereas the intensity of the band at 565 nm significantly increased, accompanied by an isosbestic point at around 544 nm upon adding Al³⁺. The spirolactam ring-opening reaction of 1 was induced by Al³⁺, leading to an increase in the longer-wavelength emission at 565 nm through TS-FRET from the thiophene-linker unit of PVA to spirolactum ring-opened 1. The presence of the FRET process was supported by a change in the phosphorescence lifetime of the system. When 0.6 mM of Al³⁺ was added into the PVA film, energy transfer efficiency was estimated to be 10.6% based on E = 1 ‒ τ_FRET/τ_D, (Hoshi et al., 2020), where τ_FRET and τ_D are the lifetimes of the donor–acceptor conjugate (1-TDB@PVA with Al³⁺) and donor (1-TDB@PVA), respectively (Supplementary Figure S10). The lifetime (τ) of compound 1 in 1-TDB@PVA with 0.6 mM of Al³⁺ was measured to be 157.5 ms when irradiated at 254 nm. Although the value is slightly lower than that (τ = 179.5 ms) at 477 nm, the lifetime of the longer-wavelength region may be governed by that of the FRET donor. Utilizing the metal ion-dependent ring-opening reaction of 1 doped in the PVA system, delayed multicolored emission was achieved; the addition of Hg²⁺ and Fe³⁺ to the films changed the afterglow to yellowish green. In this context, a lower-intensity phosphorescence band was observed upon the addition of Fe³⁺, which was responsible for extreme quenching by Fe³⁺. This is presumably because Fe³⁺ absorbs the emission from thiophene boronate (Supplementary Figure S6). The relatively low response for Hg²⁺ (Figure 5C) can be interpreted by a low affinity of 1 with Hg²⁺, the association constant being 6.0 × 10² cm^–1 M^–1 in MeOH/H₂O (9:1 v/v) (Supplementary Figure S7). In this way, different color afterglow emissions were fairly observed by adding metal ions (0.6 mM) after ceasing the UV light; green for Al³⁺, orange for Fe³⁺, and yellowish green for Hg²⁺ (Figure 5).

Figure 5

Figure 5. Delayed emission spectra of 1-TDB@PVA upon adding Al³⁺ (A), Fe³⁺ (B), and Hg²⁺ (C). [TDB] = 0.5 mol%, [Mⁿ⁺] = 0–0.6 mM. The images of the corresponding afterglow.

Next, a reversible change in the afterglow was attempted through the dynamic metal-ligand coordination properties of 1. Goswami et al. developed a naked-eye detection method for PPi by dynamic metal-ligand coordination with a rhodamine dye chelated with Al³⁺. (Goswami et al., 2013). In our case, an Al³⁺-induced yellowish-green afterglow film was drop-casted by PPi aqueous solution. After drying, the emission spectrum was superimposed on that of 1-TDB@PVA (Figure 6A). And then the addition of Al³⁺ to the resultant film in the second run led to the appearance of an emission band at 565 nm. This reversible phenomenon is supported by a model experiment using NMR spectroscopy (Figure 6B). The proton resonance of benzylidene amino unit “●” was shifted significantly downfield by 0.172 ppm by adding Al(ClO₄)₃ in CDCl₃, accompanying a slight shift (0.031 ppm) of one set of doublet (7.94 ppm, J = 4.8 and 2.2 Hz) due to rhodamine unit “Δ.” Furthermore, the proton resonance of the ethylene linker unit “#” in the range of 3.31–3.48 ppm was somewhat broadened. Taken together, these results strongly support the Al³⁺-triggered ring-opening reaction and coordination of 1. In contrast, adding PPi to a solution of 1 plus Al³⁺ led to an almost complete recovery of these chemical shifts. The strong binding affinity between Al³⁺ and PPi can release metal ions from 1 to causing a ring-closure reaction. This indicates that PPi, as a chemical stimulus, controls the reversible metal-ligand coordination on 1.

Figure 6

Figure 6. (A) Delayed emission spectra of 1-TDB@PVA plus Al³⁺ (0.6 mM) before and after drop-casting PPi: Adding Al³⁺ (0.6 mM) to 1-TDB@PVA path (a), PPi (0.3 mM) was drop-casted to 1-TDB@PVA with Al³⁺ (path (b)) and adding Al³⁺ (0.6 mM) as the second run (path (c)). (B) ¹H NMR spectra of 1-TDB@PVA (b-1), 1-TDB@PVA plus 4 equiv. of Al³⁺ (b-2), and an excess amount of PPi was added to (b-2) solution, then shaken in CDCl₃ at room temperature (b-3).

A plausible working mechanism is illustrated in Figure 7. A metal ion-triggered spirolactam ring-opening reaction on 1 in PVA caused a TS-FRET process from the crosslinked thiophene boronate to the emission state of 1, causing a change in the afterglow of the film. Reversible metal ion coordination on 1 enabled the manipulation of the afterglow properties through the dissociation of the metal ions by anions with a high affinity for 1. This mechanism is highly applicable in encryption systems (Vide Infra).

Figure 7

Figure 7. Plausible mechanism of chemical stimuli-induced afterglow manipulation with 1-TDB@PVA.

3.3 Applications

We prepared encrypted papers using 1-TDB@PVA to investigate if 1-TDB@PVA would serve as economical safety ink. As shown in Figure 8, the logotype was placed on a filter paper coated with DMSO ink composed of 1-TDB@PVA using a silk screen printing technique. Although this information was not noticeable under ambient light, it was observed under UV irradiation at 254 nm (UV on), and after removal of the excitation (UV off).

Figure 8

Figure 8. Images of the logo printed on filter paper under ambient light, UV irradiation, and after ceasing the UV light.

The dynamic metal-ligand coordination of 1 in PVA motivated us to apply 1-TDB@PVA to an information encryption system. Initially, 1-TDB@PVA was coated on the filter paper, and the letters “TMU” were written on the paper with a DMSO solution of Al(ClO₄)₃ and then dried (Figure 9). Upon ceasing the UV light, the letters appeared as a yellow afterglow. The information was then erased by immersing the paper in an aqueous PPi solution and performing a desiccative process, although the 1-TDB@PVA-based turquoise afterglow remained. This can be explained by the dissociation of Al³⁺ from dye 1 in PVA upon the addition of PPi, as inferred from a model experiment using ¹H NMR (Figure 6B, Vide Supra). However, re-printing by adding Al³⁺ resulted in unclarity, which has made us consider the following reasons: (1) Immersing 1-TDB@PVA-coated film in PPi aqueous solution could cause leaching of 1 doped; (2) Residual PPi on the filter paper may serve as a scavenger for Al³⁺ in the re-printable process. Although some improvement is desired to solve this issue, afterglow systems with reversible metal ion-ligand coordination would provide a potent approach for developing information encryption techniques.

Figure 9

Figure 9. Schematic diagram of encryption systems with 1-TDB@PVA. The information was invisible under ambient light. The letters “TMU” written by Al³⁺ aqueous solution were visible both under UV light irradiation and after ceasing UV light. The written information was erased after immersing the paper in an aqueous solution of PPi. The printing/erasing process was repeated.

4 Conclusion

Our ongoing program for developing chemical stimulus-tunable RTP films has led to the investigation of thiophene boronate-cross-linked PVA. The afterglow properties were influenced by the substituent groups on the thiophene skeleton, as evaluated by TD-DFT and DFT calculations. TDB@PVA exhibits RTP characteristics with a turquoise afterglow. Combined with N-(rhodamine-6G)lactam dye 1 that serves as a metal ion-responsive color element, color-tunable afterglow emissions were obtained through TS-FRET between thiophene boranate with a turquoise emission and the metal-ion-activated colored form of 1 in PVA. More impressively, the reversible metal-ligand coordination on 1 enabled afterglow manipulation in PVA by combining Al³⁺ and PPi as chemical stimuli. This approach was applied to information encryption films, allowing us to propose a write/erase system with 1-TDB@PVA using dynamic coordination behavior.

Data availability statement

The data presented in the study are deposited in the PubCherm repository, accession number 126453.

Author contributions

MM: Data curation, Formal Analysis, Investigation, Writing–original draft. NS: Data curation, Formal Analysis, Writing–original draft. AS: Data curation, Visualization, Writing–original draft. MI: Data curation, Formal Analysis, Writing–original draft. YK: Conceptualization, Funding acquisition, Supervision, Writing–original draft, Writing–review and editing.

Funding

The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. This work was partially supported by JSPS KAKENHI Grant Number 22K19069.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2024.1441452/full#supplementary-material

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