Phosphorescent materials offer numerous advantages over fluorescent materials, including long emission lifetimes, high signal-to-noise ratios, and large Stokes shifts, making them valuable for various applications such as biological imaging, information encryption, and anti-counterfeiting. Organic room-temperature phosphorescence (RTP) materials are particularly attractive due to their low toxicity, low cost, and high flexibility. However, achieving efficient phosphorescence requires addressing two key challenges: enhancing intersystem crossing (ISC) to populate the triplet excited state and suppressing non-radiative transitions to minimize energy loss. Traditional approaches often involve forming rigid crystal structures or complex supramolecular assemblies, which hinder processability. This limits the practical applications of these materials. Recent research has explored printable and water-soluble phosphorescent polymers, but these often lack complete biodegradability. This research seeks to address these limitations by utilizing the naturally occurring, sustainable and biodegradable polymer cellulose as a starting material to create a new class of processable, environmentally friendly RTP materials with multiple functionalities.

The existing literature highlights the potential of phosphorescent materials in diverse applications, but also points to the challenges of achieving both high performance and processability. Strategies to enhance ISC involve introducing heavy atoms to increase spin-orbit coupling, while suppressing non-radiative transitions relies on creating rigid structures, such as crystals, supramolecular assemblies, or cross-linked networks, to restrict molecular motions and isolate oxygen. While some success has been achieved with printable and water-soluble phosphorescent polymers, the lack of biodegradability remains a concern. The use of cellulose as a base material for RTP has been explored previously but with limited success in terms of long phosphorescence lifetimes and ease of processing. This work aims to overcome these limitations by employing a novel approach to cationize cellulose, leveraging both the inherent properties of cellulose and the introduction of carefully chosen chemical structures.

The researchers synthesized a cationic cellulose derivative, cellulose 1-cyanomethylimidazolium chloride (Cell-ImCNCl), by introducing cyanomethylimidazolium cations (ImCN⁺) and chloride anions (Cl⁻) into cellulose chains using a homogeneous modification process in the ionic liquid 1-allyl-3-methylimidazolium chloride (AmimCl). The synthesis involved two steps: first, the preparation of cellulose 2-chloropropionate (Cell-Cl) by reacting cellulose with 2-chloropropionyl chloride in AmimCl/DMF; second, the conversion of Cell-Cl to Cell-ImCNCl by reaction with 1-cyanomethylimidazole. The degree of substitution (DS) of both Cell-Cl and the imidazolium cation (DS_CN) were controlled by adjusting reaction conditions. The synthesis of control materials, including 1-cyanomethylimidazolium chloride (CNMImCl), cellulose 1-butylimidazolium chloride (Cell-BimCl), and cellulose derivatives with different anions (Cell-ImCNX, where X represents BF₄⁻, PF₆⁻, Tf₂N⁻, C(CN)₃⁻, N(CN)₂⁻, and SCN⁻) was also carried out to investigate the effects of various chemical structures on the RTP performance. Characterization techniques included ¹H-NMR, FTIR, XPS, and fluorescence/phosphorescence spectroscopy to confirm the chemical structure and assess the RTP properties. Different processing methods, including doctor blade coating, dip coating, screen printing, inkjet printing, and mask casting, were employed to fabricate phosphorescent films, fibers, coatings, and patterns on various substrates. Water resistance was improved by introducing glutaraldehyde as a cross-linking agent. Antibacterial properties were evaluated using an inhibition ring assay with *Escherichia coli* and *Staphylococcus aureus*.

The introduction of cyanomethylimidazolium cations and chloride anions into the cellulose structure significantly enhanced its room-temperature phosphorescence properties. The cyano group and nitrogen atoms in the imidazolium cation promoted intersystem crossing (ISC), while the multiple hydrogen bonding interactions (between the cyano group, chloride anions, and hydroxyl groups) and electrostatic attraction interactions effectively suppressed non-radiative transitions. The resulting Cell-ImCNCl exhibited an ultralong phosphorescence lifetime (up to 158 ms) and a high quantum yield (up to 11.81%). The optimal RTP performance was achieved at a specific DS_C (1.24) and DS_CN (0.60). Control experiments with different anions and cations confirmed the crucial role of hydrogen bonding in achieving efficient phosphorescence; the heavy atom effect played a negligible role. The excellent water solubility of Cell-ImCNCl allowed for easy processing into diverse forms, such as films, fibers, coatings, and patterns, using various solution processing methods. The incorporation of glutaraldehyde improved the water resistance and, surprisingly, imparted significant antibacterial properties to the material, likely due to the high concentration of ions in the cross-linked structure disrupting bacterial cell membranes. The dual-mode (fluorescence and phosphorescence) patterns demonstrated the potential of Cell-ImCNCl for advanced anti-counterfeiting applications.

This study successfully developed a novel, easy-to-process, biodegradable, and multi-functional RTP material based on cellulose. The findings address the long-standing challenge of combining high RTP performance with excellent processability and environmental friendliness. The unique combination of strong hydrogen bonding and electrostatic interactions, facilitated by the specific chemical design of the cationized cellulose, proved crucial in achieving the observed ultralong phosphorescence. The facile solution processability of Cell-ImCNCl opens up diverse applications in various fields, including anti-counterfeiting, information encryption, and smart packaging. The unexpected antibacterial property adds another dimension to the material’s potential. The results contribute significantly to the advancement of sustainable and functional materials research.

This research successfully demonstrated the synthesis and characterization of a novel ultralong phosphorescent cellulose derivative (Cell-ImCNCl) with excellent processability, water resistance, and antibacterial properties. The unique combination of the inherent properties of cellulose and the strategically introduced chemical structures resulted in a material with high RTP performance and versatile applications. Future research could focus on further optimizing the chemical structure for even longer lifetimes and higher quantum yields, exploring additional functionalities, and expanding the range of applications.

While the study successfully demonstrates the potential of Cell-ImCNCl, some limitations exist. The long-term stability of the cross-linked material under various environmental conditions warrants further investigation. A more comprehensive study of the antibacterial mechanism, including the exact interaction between the material and bacterial cells, is also needed. The scale-up of the synthesis process and cost-effectiveness also require further examination for practical industrial application.