Dynamic room-temperature phosphorescence (DURTP) materials hold significant potential for various applications, such as displays and security features. However, existing DURTP systems often suffer from a high activation threshold, hindering their use in grayscale displays which require a range of intensities. This high threshold, or 'dead time,' in DURTP activation causes a delay in the response of phosphorescence intensity to photoactivation, resulting in a loss of shadow detail in grayscale patterns. This limitation is fundamentally rooted in the insufficient intersystem crossing (ISC) of metal-free dynamic phosphors. Organic phosphors with low ISC rates are highly susceptible to quenching by even low concentrations of triplet oxygen, further limiting the oxygen consumption rate and hence the DURTP efficiency. This study addresses this limitation by exploring halogen doping as a strategy to enhance ISC in CDs-based DURTP composites, aiming to enable high-quality grayscale displays.

Previous research has explored strategies to improve the performance of DURTP systems, including the introduction of functional groups with lone pair electrons, such as nitrogen heterocyclic rings and phosphonates. While these modifications have shown some improvement, they have not yet yielded grayscale display capabilities. Another common approach is heavy atom substitution/doping, particularly with halogen atoms, which enhances spin-orbit coupling and facilitates ISC. However, halogen doping can also affect polarity and hydrogen bonding, potentially hindering compatibility between the phosphor and the polymer matrix. The use of solvothermal-synthesized CDs, with their highly functionalized surface and hydrophilic groups, offers a potential solution to this compatibility issue, as these groups may compensate for changes induced by halogen doping. Previous studies have demonstrated successful halogen doping in CDs, providing a foundation for this research.

The authors synthesized halogen-doped CDs using chlorinated derivatives of *p*-benzoquinone under solvothermal conditions. The chloride content in the resulting CICDs-1 to CICDs-3 gradually increased from 6.0% to 9.3%, as confirmed by XPS and FT-IR analyses. The CDs were then embedded in a solid PVP matrix to create DURTP composite films. The photoactivation and decay of DURTP were characterized by measuring the phosphorescence intensity and lifetime under different conditions, including varying oxygen concentrations and UV irradiation times. Singlet oxygen productivity was measured using electron spin resonance spectroscopy. The grayscale display capability was evaluated by using a transparent mask printed with an ITE grayscale chart to regulate the light dose and create a grayscale step-chart on a flexible film. The relationship between the normalized intensities/lifetimes and the UV light dose was analyzed to determine the grayscale display range.

The study successfully demonstrated that halogen doping significantly enhances the performance of CDs-based DURTP composites. Specifically, increasing the chloride content in CDs led to: 1. A marked increase in the phosphorescent component in the activated steady-state photoluminescence emission (from 17.1% to 54.9%). 2. A significant decrease in the lifetime of the activated DURTP (from 472 ms to 110 ms), consistent with the heavy-atom effect. 3. A significant improvement in singlet oxygen productivity (approximately 4.1 times that of undoped CDs). 4. Faster DURTP activation kinetics, with shorter τ1/2 values (time to reach half-maximal intensity) and a reduced activation threshold. 5. Successful demonstration of a grayscale display with the highest halogen content composite (CICDs-3), showcasing a series of grids with stepwise increasing intensities under controlled UV irradiation doses. A 10-fold enhancement in lifetime (from 11 to 110 ms) and an 84-fold enhancement in intensity were observed during the activation of DURTP in this composite. The results clearly show that the halogen doping strategy effectively improves the response speed and reduces the activation threshold of the DURTP composites, enabling grayscale display capabilities.

The successful achievement of a grayscale display with the halogen-doped CDs/PVP composite demonstrates the effectiveness of the halogen doping strategy in improving the performance of DURTP materials. The enhanced ISC, resulting from the heavy-atom effect, leads to increased tolerance of phosphorescent emission to oxygen and a higher population of triplet excited states. This improvement in photodynamic conversion capacity contributes to faster activation kinetics and a lower activation threshold. The achieved phosphorescence quantum yield of 2.93% for composite 3 is comparable to that of some molecular long afterglow phosphors, showcasing the competitive performance of this material. The successful applications demonstrated in grayscale-based UV photography and steganography highlight the potential of this material for various optical applications.

This work successfully demonstrated that halogen doping is an effective strategy for enhancing ISC in CDs and achieving high-performance DURTP composites with grayscale display capabilities. The results open up new possibilities for applying DURTP materials in advanced optical applications. Future research could explore different halogen dopants and optimization of the synthesis process to further improve the performance and expand the applications of these materials. Investigating other polymer matrices and exploring the use of these materials in flexible displays are also worthwhile future directions.

While this study demonstrates the effectiveness of halogen doping for enhancing DURTP performance, some limitations exist. The study primarily focused on chlorine doping; exploring other halogens could reveal further performance improvements or trade-offs. The long-term stability of the DURTP effect under various environmental conditions needs further investigation. The scalability and cost-effectiveness of the synthesis method should also be evaluated for potential commercial applications.