Room-temperature phosphorescence (RTP) with ultralong emission lifetimes is highly desirable for various applications, including displays, anti-counterfeiting, and bioimaging. However, achieving efficient RTP in organic materials remains challenging due to inefficient intersystem crossing (ISC) and significant non-radiative decay. While inorganic materials exhibit RTP, they often suffer from drawbacks like harsh preparation conditions, scarcity of resources, and toxicity. Recent advances have explored strategies such as crystal engineering, host-guest doping, and metal-organic frameworks (MOFs) to enhance organic RTP. However, these methods often compromise processability and flexibility. Polymer-based RTP materials offer advantages in terms of processability and flexibility, but color tunability in a single polymer under ambient conditions remains largely unexplored. Color-tunable luminescent materials are crucial for applications like multicolor displays, biological imaging, and information encryption. Existing strategies for achieving color tunability often involve complex molecular designs or crystal engineering. This research aims to develop a novel approach to achieve color-tunable ultralong organic room temperature phosphorescence (UOP) within a single polymer system under ambient conditions, addressing the limitations of existing methods and opening new possibilities for advanced applications.

The literature extensively documents the pursuit of ultralong organic room-temperature phosphorescence (UOP). Several strategies have been explored to overcome the challenges associated with inefficient intersystem crossing (ISC) and high non-radiative decay rates in organic molecules, including crystal engineering to create rigid environments that restrict molecular motion and suppress non-radiative decay. However, crystal-based approaches often suffer from poor processability and reproducibility. Polymer-based approaches offer improved processability but have yet to achieve widespread success in color-tunable UOP. Significant advances have been made in extending the lifetime of UOP using homopolymerization and binary copolymerization techniques, or by embedding small molecules into rigid polymer matrices. However, achieving color tunability within a single polymer system under ambient conditions has remained a significant challenge. The literature also highlights the importance of color-tunable luminescent materials in diverse fields. Various methods, such as modulating material composition, altering molecular conformations, and controlling crystal packing, have been employed to achieve color-tunable fluorescence. However, extending these strategies to achieve color-tunable phosphorescence, particularly UOP at room temperature, has proven difficult.

The researchers designed a multicomponent copolymer (PDNA) through radical cross-linked copolymerization. This involved the copolymerization of acrylic acid (AA), vinyl-functionalized naphthalene (MND), and benzene (MDP) using 2-azoisobutyronitrile (AIBN) as an initiator. The molar feed ratio of MND:MDP:AA was optimized. The chemical structures of the monomers and the resulting polymer were characterized using nuclear magnetic resonance (NMR) spectroscopy. The number-average molecular weights (Mn) of the polymers were determined. Photophysical properties were investigated using excitation-phosphorescence mapping, time-resolved emission spectroscopy, and Commission Internationale de l'Eclairage (CIE) coordinate analysis to determine the color tunability. The mechanism underlying the color-tunable UOP was explored using phosphorescence excitation spectra, examining the individual monomers' behavior at 77K and room temperature. Wide-angle X-ray scattering (WAXS) was used to determine the amorphous nature of the polymers, ensuring that the color change wasn't due to differing crystalline structures. Theoretical calculations (natural transition orbitals) were performed to understand the electronic transitions and energy levels involved. To establish the generality of the approach, another multicomponent copolymer (PDBA) was synthesized using di(but-3-en-1-yl) (1,1'-biphenyl)-4,4'-dicarboxylate (MBD), MDP, and AA. The photophysical properties of this copolymer were also characterized using similar methods. Finally, the application of the color-tunable UOP was explored by creating patterns using the polymers as inks and demonstrating their use in multilevel information encryption. The effect of humidity on the phosphorescence lifetime was also investigated to understand the role of hydrogen bonding in maintaining UOP.

The study successfully synthesized a multicomponent copolymer (PDNA) exhibiting color-tunable ultralong organic room-temperature phosphorescence (UOP). The emission color shifted from blue to yellow as the excitation wavelength increased from 254 nm to 370 nm. The polymer exhibited a long lifetime of 1.2 seconds and a maximum phosphorescence quantum yield of 37.5% under ambient conditions. The color tunability was attributed to the dynamic ratiometric variation in phosphorescence intensity from the different luminophores (MND and MDP) upon changes in excitation wavelength. The rigid cross-linked polymer network and hydrogen bonding between polymer chains effectively suppressed molecular motion and prevented triplet exciton quenching, enabling long-lived phosphorescence. The amorphous nature of the polymer was confirmed using WAXS, indicating that the color change wasn't caused by variations in crystal structure. Theoretical calculations confirmed the experimental findings. The approach was demonstrated to be generalizable by synthesizing another copolymer (PDBA) that also exhibited color-tunable UOP. Finally, the researchers successfully demonstrated the application of PDNA in multilevel information encryption, highlighting the potential of this material for anti-counterfeiting and secure information storage. The information could be erased using water and later restored by drying.

The findings address the challenge of achieving color-tunable UOP in a single polymer at room temperature. The successful synthesis of PDNA and PDBA demonstrates a versatile strategy for designing and preparing such materials. The mechanism underlying the color-tunable UOP is clearly explained through experimental and theoretical investigations. The application in multilevel information encryption highlights the practical relevance of the developed materials. This work contributes significantly to the field of organic luminescent materials, offering a new route to create advanced materials with diverse functionalities. The results provide valuable insights into the design principles for achieving both long lifetimes and color tunability in organic phosphorescence, paving the way for the development of new applications in areas such as bioimaging, security, and displays.

This study successfully demonstrated a novel strategy for achieving color-tunable ultralong organic room temperature phosphorescence (UOP) in a single polymer via radical multicomponent cross-linked copolymerization. The color tunability, long lifetime (1.2 s), and high quantum yield (37.5%) of the resulting copolymers highlight their potential applications in diverse fields, including information encryption, bio-labeling, and multicolor displays. Future research could focus on exploring a wider range of luminophores to expand the color palette and optimize the material properties for specific applications. Investigating the potential for these materials in biological systems and developing more sophisticated information encryption techniques are also promising avenues for future work.

While the study successfully demonstrates color-tunable UOP, the specific choice of monomers and their ratios might limit the overall range of achievable colors and lifetimes. The impact of long-term environmental factors on the material’s stability and luminescent properties warrants further investigation. The application in information encryption has been demonstrated on a small scale; further work is needed to explore its scalability and robustness in real-world scenarios. The current encryption method is sensitive to humidity; future efforts should focus on developing more robust and environment-resistant materials.