Room-temperature phosphorescence (RTP) materials, particularly those based on polymers, hold significant promise for applications in illumination displays, bioimaging, data encryption, and anticounterfeiting. However, the inherent spin-forbidden transition from singlet to triplet excitons and the rapid non-radiative decay of triplet excitons hinder their performance. Traditional approaches often involve doping, which can lead to issues like phase separation and poor compatibility. Maximizing intersystem crossing (ISC) through the introduction of heteroatoms or heavy atoms, and minimizing non-radiative transitions by creating rigid environments through techniques like crystal engineering or host-guest doping, are key strategies to improve RTP. While doping polymers with phosphors has been widely explored, this often results in limitations such as phase separation. Copolymerization of phosphor groups with matrix monomers provides improved stability and weatherability. However, single-step approaches to enhancing phosphorescence often lack the effectiveness of a multi-faceted approach. This work addresses these challenges by introducing a stepwise, three-level confinement strategy for creating intrinsically polymeric RTP materials.

The existing literature extensively covers the development of RTP materials using various strategies. Doping approaches utilize polyvinyl alcohol (PVA) matrices incorporating various dopants like polymer@PVA, small molecule@PVA, and carbon dot@PVA, leveraging hydrogen bonding networks. Copolymerization of phosphor groups with monomers like acrylic acid (AA), acrylamide (AM), styrene sulfonic acid/sodium, and vinyl pyridine (VPy) has also yielded RTP copolymers with enhanced stability. However, these approaches often rely on single-step methods to promote phosphorescence, leading to limitations in lifetime and quantum yield. Quick-click reactions utilizing boric acid and PVA have shown promise but can suffer from increased molecular mobility, potentially increasing non-radiative transitions. The use of covalent crosslinking and hydrogen bonding in concert has been recognized as a promising avenue for improving RTP polymer performance.

This study introduces a novel stepwise three-level confinement strategy for enhancing RTP in intrinsic polymers. The process begins with the copolymerization of phosphors (chromophores) into poly(vinyl acetate) (PVAc) molecular chains, creating the primary confinement. Secondary confinement is achieved through hydrogen bonding networks formed by alcoholysis of PVAc to generate PVA. Finally, tertiary confinement is established through crosslinking with boric acid. Different phosphors, namely 2-vinyl naphthalene (2VN), 1-vinyl naphthalene (1VN), 9-vinyl anthracene (9VA), vinyl imidazole (MZ), and N-vinyl phthalimide (NVP), were incorporated into the PVAc backbone and subjected to this three-step process. The researchers investigated the photophysical properties of these materials using techniques like phosphorescence decay measurements, emission spectroscopy, excitation-dependent emission spectra, time-resolved excitation spectroscopy (TRES), and temperature-dependent phosphorescence and decay measurements. Structural characterization employed powder X-ray diffraction (XRD), thermogravimetric-infrared (TG-IR) spectroscopy, 1H NMR, Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and determination of glass transition temperature (Tg). Theoretical calculations, utilizing an interaction region indicator (IRI) and analysis of electrostatic potential (ESP), were used to understand the role of various interactions in enhancing RTP. The application of the materials in crack detection and information encryption was also demonstrated.

The stepwise confinement strategy significantly improved the RTP properties of the polymers. The phosphorescence lifetime increased dramatically—from 14.3 µs to 256.5 ms—with a substantial reduction in the nonradiative rate constant (k_nr). The maximum phosphorescence quantum yield reached 16.04%. The enhanced performance was attributed to the synergistic effects of weak interactions, hydrogen bonding, and covalent crosslinking. The alcoholysis step created hydrogen bonding networks, leading to secondary confinement and enhanced luminescence. The subsequent crosslinking with boric acid further improved rigidity, reducing k_nr and enhancing lifetime. The findings were consistent across multiple phosphors, leading to the creation of multi-color RTP emitting materials. Theoretical analysis indicated that the crosslinking not only enhanced the rigidity but also increased the vertical excitation energy of the triplet state, further boosting phosphorescence. The water-sensitive nature of PVA was successfully applied to develop a method for detecting microcracks (<2mm) in humid environments. The difference in phosphorescence lifetimes was effectively utilized to implement information encryption using the Morse code.

The results demonstrate the efficacy of the stepwise confinement approach in creating high-performance, intrinsically polymeric RTP materials. The three-level confinement strategy successfully addresses the limitations of traditional methods by simultaneously enhancing ISC and suppressing non-radiative decay pathways. The synergistic interactions among weak interactions, hydrogen bonds, and covalent crosslinks contribute significantly to the observed improvements. The ability to achieve multicolor emission by varying the phosphor and the successful application of the materials for crack detection and information encryption highlight the versatility and potential of this strategy. These findings provide valuable insights for designing and developing advanced RTP materials for various applications.

This study presents a novel stepwise confinement strategy for creating high-performance intrinsically polymeric RTP materials. The approach combines weak interactions, hydrogen bonding, and covalent crosslinking to enhance both ISC and suppress non-radiative decay. The resulting materials exhibit long lifetimes, high quantum yields, and multicolor emission capabilities. The successful application of these materials for crack detection and information encryption showcases their practical utility. Future research could explore the incorporation of different phosphors to expand the color range and explore applications in areas like sensing and bioimaging.

While the study demonstrates the effectiveness of the stepwise confinement strategy, several limitations exist. The simplified model used in theoretical calculations might not fully capture the complexity of the polymeric system. The crack detection method currently relies on the water-sensitive nature of PVA, limiting its applicability in dry environments. Further optimization of the materials and exploration of alternative detection mechanisms could enhance the technique’s versatility. The information encryption method using Morse code might be susceptible to decryption if the lifetime differences are not sufficiently distinct.