Phototuning of Multi-Color Emission in PMMA Composite Films for Information Encryption Applications

Abstract

A strategy centered on dynamically tunable excited-state proton transfer (ESPT) processes is proposed for the design and synthesis of luminescent compounds. An emitter based on guanidine-substituted 1,8-naphthalimide (R-1) with ESPT characteristics has been meticulously engineered. Upon incorporation into poly (methyl methacrylate) (PMMA) matrices, the tunable ESPT process, transitioning between blue and yellow-green emission within the composite film, can be precisely controlled through irradiation in different pH environments. Moreover, the luminescence of the R-1/PMMA composite film exhibits variations in response to environmental changes, and demonstrates excellent fatigue resistance. Exploiting this characteristic, information such as “2020” can be encoded, and this encoded information automatically manifests in response to fluctuations in external pH. Specifically, employing a designated method is essential for accurately deciphering the information. The pH-dependent nature of this feature imparts a higher level of security to the material and offers new insights into information encryption.

1. Introduction

The field of information security has garnered significant attention from society, particularly in light of the rapid advancements in information technology. In recent years, substantial efforts have been directed towards the development of information encryption materials that can securely store and transmit data, resulting in noteworthy achievements in safeguarding confidential information [1,2,3,4,5]. Notable examples of these advancements include dynamic codes [6,7,8], anti-counterfeiting materials [9,10,11], fluorescent inks [12,13,14], and self-erasing materials [15,16,17]. In comparison to materials employing multiple encryption steps to enhance information security, materials with simpler compositions have gained interest due to their uncomplicated information encoding process. The dynamic tuning of fluorescent colors in functional materials via external stimuli holds fundamental significance and has piqued great interest in the realm of information encryption. Among the various stimuli-responsive systems explored, fluorescence resonance energy transfer (FRET) systems have demonstrated the ability to finely adjust the fluorescence emission spectra by precisely manipulating the ratio and distance between donor and acceptor components [18,19,20,21,22,23,24]. In contrast to FRET systems, the photoinduced excited-state proton transfer (ESPT) systems, characterized by a single emitter and relying on intermolecular hydrogen bonding, such as interactions with solvent molecules, offer a potent strategy for altering fluorescence color in response to external stimuli [25].

In recent times, organic molecules possessing ESPT properties have drawn significant attention due to their diverse applications in the realms of information storage and security devices [26,27]. Extensive efforts have been directed towards inducing ESPT processes within biological systems and chiral nematic liquid crystal systems [28,29,30]. Simultaneously, researchers have explored the sensitivity of ESPT signals to external stimuli, aiming to fabricate responsive functional materials that exhibit fluorescence color variations. This exploration holds promise for applications in photonics and biomedicine. Nevertheless, the number of reported molecules exhibiting intermolecular ESPT properties remains relatively limited. In a prior study, we successfully engineered a water-soluble chiral emitter with circularly polarized luminescence (CPL) activity, derived from guanidine-substituted 1,8-naphthalimide (R/S-1). The described sequence delineates the process of excited-state proton transfer in R-1 molecules. In an acidic and neutral environment, the predominant species of R-1 molecules exists in the protonated state, denoted as (R-1)⁺. Upon excitation at 350 nm, a subset of the excited molecules undergoes a rapid transformation from the protonated state ((R-1)⁺) to the excited deprotonated form ((R-1)), facilitated by an intermolecular excited-state proton transfer process from the molecules to the solvent. Subsequently, the (R-1)* molecules undergo a transition back to the ground state R-1. Following their return to the ground state, the R-1 molecules undergo a reverse proton transfer, leading them back to the protonated form ((R-1)⁺). This emitter demonstrated the capability to undergo intermolecular ESPT processes across a range of pH values in aqueous solutions, resulting in tunable CPL with varying colors [30].

In this study, we developed a light-responsive emissive composite film system by incorporating R-1 molecules with ESPT properties into poly (methyl methacrylate) (PMMA). As depicted in Figure 1, a homogeneous mixture of R-1/PMMA in a dichloromethane solution was introduced into a glass cell with a 1 mm thickness, featuring a four-leaf clover pattern. Upon exposure to a portable ultraviolet lamp, the composite emitted a sky-blue fluorescence. Subsequent exposure to an alkaline environment induced a transition in fluorescence from blue to yellow-green. This change arises from alterations in the protonation/deprotonation states of the guanidine groups within the R-1 molecule embedded in the PMMA matrix under acid/alkali stimuli. The luminescence of the R-1/PMMA composite film varies with changes in the environment, demonstrating high luminescent quantum efficiency and excellent fatigue resistance. Furthermore, we provided a conceptual demonstration of utilizing these composite films for information encryption through light manipulation. This work introduces novel avenues for applications in photonics utilizing stimuli-responsive luminescent materials.

2. Materials and Methods

2.1. Materials

In this experiment, all reagents and solvents were obtained from commercial sources without further purification for use. The synthesis and characterization of the R(S)-1 molecule have been previously described in our earlier publication and will not be reiterated here. Reagents such as dichloromethane (CH₂Cl₂) were procured from Beijing Chemical Factory in Beijing. Throughout the entire study, distillation was employed for deionized water, with deionized water (18.2 MΩ cm) purified using the Milli-Q Advantage water purification system. Poly (methyl methacrylate) (PMMA) was purchased from Jianglai Biotech Co., Ltd. (Liaoning, China).

2.2. Synthesis of R-1/PMMA Composite Films

Initially, R(S)-1 solid powder was dissolved in a dichloromethane solution (50 µM) containing 10 wt% PMMA. The solvent was allowed to evaporate, resulting in the formation of an R(S)-1/PMMA luminescent polymer film.

2.3. Characterization

UV–vis spectra were acquired using a Hitachi U-3900 spectrophotometer (Tokyo, Japan). Fluorescence spectra of composite films were recorded on an F-4500 fluorescence spectrophotometer (Tokyo, Japan), employing a xenon lamp as the excitation source. The absolute fluorescence quantum yield was determined using an absolute PL quantum yield spectrometer (Edinburgh FLS-980 fluorescence spectrometer) equipped with a calibrated integrating sphere. The thickness of the film used in the fluorescence quantum yield measurements was 1 mm. Fluorescence lifetime measurements were conducted on the same spectrometer using time-correlated single photon counting (TCSPC). The instrument response function (IRF) of our TCSPC setup is below 120 ps. The circularly polarized luminescent spectra were measured on JASCO CPL-300 (Tokyo, Japan). Hydrochloric acid of a certain concentration was placed in a beaker, and then tweezers were used to hold the composite film above the beaker, allowing the R-1/PMMA composite film to react in the acidic atmosphere formed by the volatilization of hydrochloric acid.

3. Results and Discussion

The responsive behavior of R-1 molecules in solution from our previous report suggested that their incorporation into thin films may yield distinct characteristics compared to conventional fluorescent materials [30]. A polymer thin film was fabricated by blending emitters R(S)-1 with PMMA polymers, denoted as R(S)-1/PMMA composite film, that demonstrated responsive fluorescence under acid/base stimuli. Through multiple experimental trials, it has been observed that the R-1/PMMA polymer film remains colorless at a pH of 7.82. With an increase in pH, the film exhibits a progressively yellow hue.

In order to comprehend the photophysical properties of the R-1/PMMA composite film, we systematically examined the absorption and fluorescence spectra of the R-1/PMMA composite film in acidic/alkaline environments. The absorption spectra of R-1/PMMA revealed a singular band at 346 nm, while the emission spectra exhibited maximum emission bands at 425 nm in an acidic atmosphere, as illustrated in Figure 2a. The emission peak at 425 nm corresponds to the intrinsic emissive property of R-1, consistent with the photophysical characteristics observed for R-1 molecules in solution. Additionally, when subjecting the composite film to an alkaline environment, the absorptions at 346 nm and 415 nm can, respectively, be considered as the contributions of the acidic form (R-1⁺) and basic form (R-1) of R-1/PMMA. The absorption spectra of R-1/PMMA in a basic environment is due to the electron-rich neutral guanidine group (deprotonated guanidinium), which causes a significant increase in the push–pull effect of the ICT transition of naphthalimide. The corresponding emission band was located at 506 nm, as shown in Figure 2b. PMMA is an organic polymer with a dielectric constant typically ranging from 2.5 to 3.5. Under standard conditions, PMMA exhibited a lower dielectric constant, indicating a weaker response to an electric field and diminished intermolecular interactions. Consequently, the pKa and pKa* value of R-1 in PMMA film was determined to be 8.73 ± 0.02 and 1.26 ± 0.01, respectively (Figure S1 in Supporting Information). When the R-1/PMMA composite film was excited by ultraviolet light, it is plausible that R-1 molecules within the PMMA matrix came into spatial proximity. This proximity facilitates intermolecular interactions, such as π-π stacking, potentially promoting the transfer of excited-state electrons. Such interactions may result in the charge transfer of excited states between R-1 molecules and PMMA, manifesting transfer properties following illumination.

To gain deeper insights into the luminous efficiency and ESPT process of the R-1/PMMA composite film, time-resolved fluorescence spectroscopy was employed to analyze R-1 within the PMMA matrix (Figure 3 and

Table 1. Summary of time-resolved fluorescence data and fluorescence quantum efficiency (Φ_f) of R-1 in different solvents; λ_ex = 350 nm.

State	τ(R-1)+*/ns	τ(R-1)*/ns	Φf (%)
R-1/PMMA(H+)	τ = 7.33 (425 nm)		52.3
R-1/PMMA(OH−)	τ = 7.34 (425 nm)	τ1 = 5.01, τ2 = 19.55 (506 nm)	54.2

). In the acidic environment of the R-1/PMMA composite film, upon excitation at 350 nm, the emission from excited-state protonated molecules ((R-1)⁺*) exhibited a single-exponential decay at 405 nm, as depicted in Figure 3a. Conversely, when the R-1/PMMA composite film was exposed to an alkaline environment, the emission peak at 405 nm displayed a single-exponential decay, while a double-exponential decay was observed at 506 nm, as illustrated in Figure 3b. This dual decay at 506 nm indicates the presence of two emissive components resulting from the intermolecular ESPT process between the R-1 and PMMA film, where the excited molecules (R-1)⁺* rapidly convert into excited-state deprotonated ((R-1)*). The excited-state deprotonation process of R-1/PMMA was illustrated, as depicted in Figure S2. In an alkaline medium, the majority of R-1 molecules exist in the deprotonated form (R-1). Upon excitation at 350 nm, some molecules in the protonated form (R-1⁺) undergo rapid conversion to the excited deprotonated form (R-1) through intermolecular excited-state proton transfer. Subsequently, R-1* molecules return to the ground state (R-1) through an emission process. The τ₁ represents the rise time of R-1* emission, and τ₂ represents the lifetime of R-1* undergoing radiative transitions. It was noteworthy that the R-1/PMMA composite film exhibited a high quantum yield whether in acidic or alkaline environments. The combination of excellent luminescence stability and distinctive pH-dependent absorption and fluorescence behaviors established the groundwork for subsequent experiments.

Considering that the R-1 molecule is chiral, the chiroptical properties of the R-1/PMMA composite film and its enantiomer S-1/PMMA were further characterized. When R-1/PMMA was exposed to an acidic environment, a pair of blue circularly polarized light signals were observed at 425 nm. Specifically, R-1/PMMA exhibited a right-handed circularly polarized luminescent signal, while conversely, S-1/PMMA exhibited a positive circularly polarized luminescent signal, as shown in Figure 4a. Through calculations, the luminescent dissymmetry factor was found to be on the order of 10⁻⁴. Additionally, when R-1/PMMA was exposed to an alkaline environment, a symmetric set of yellow-green circularly polarized light signals was similarly obtained, as shown in Figure 4b. These results indicated that the film formed by the composite of R-1 and PMMA also possessed chiral luminescent properties.

The stimulus-responsive fluorescence alterations exhibited by the R-1/PMMA film open avenues for its application in information encryption. Figure 5 illustrates an array initially displaying the number ‘8888’, representing the encrypted state of information within the R-1/PMMA film. Upon exposure to an alkaline environment and subsequent irradiation with 365 nm light, the array effortlessly transformed into the number ‘2020’, accompanied by a shift in color from sky blue to yellow-green. This transition signifies the decryption state of the information, with concentrated ammonia water serving as the decryption agent. Furthermore, exposure to an acidic environment, leading to the protonation of guanidine groups in the R-1 molecule, facilitates the return of the mixed film to its original state under ultraviolet light irradiation, utilizing concentrated hydrochloric acid as the densifier. At this point, the entire system has reverted back to its encrypted state. As shown in Figure 6, the pH-induced fluorescence changes of R-1/PMMA was reversible and reproducible for at least ten cycles, demonstrating that the R-1/PMMA composite exhibited elevated fatigue resistance. The stimulus responsiveness and robust fatigue resistance of R-1 molecules allow for multiple cycling of the information security system prepared using the R-1/PMMA film. Consequently, the conceptual demonstration of the R-1/PMMA thin film system with adjustable stimulus-responsive fluorescence showcases promising potential for information encoding and decoding.

4. Conclusions

To fulfill the requirements of practical applications, luminescent systems must possess both high quantum efficiency and stimulus responsiveness. The acid/base stimulus responsiveness of the R-1 molecule within PMMA polymers was measured. This exploration resulted in the attainment of multi-color tunable fluorescence with high fatigue resistance. Notably, the R-1/PMMA film demonstrated the capability of adjusting luminous color, transitioning from sky blue to yellow-green, with remarkable quantum efficiency upon exposure to acid and alkaline environments, respectively. Their application in information encryption is also demonstrated. The findings presented in this work pave the way for the development of intelligent photonic devices featuring adjustable luminescence colors.