Design of highly efficient deep-blue organic afterglow through guest sensitization and matrices rigidification

Blue luminescence is a significant challenge in organic optoelectronics, essential for solid-state lighting and full-color displays. Organic ultralong room temperature phosphorescence (OURTP), with lifetimes exceeding 0.1 s, offers unique properties and applications. However, most organic afterglow emits in the 500–600 nm range due to low-lying exciton energies and bathochromic shifts. Developing blue OURTP requires populating and stabilizing high-lying triplet excited states simultaneously. Previous strategies like crystallization/H-aggregation and exciplex formation struggle to achieve blue afterglow due to red-shifts. Dispersing emitters in a host matrix is effective in preventing red-shifts and concentration quenching, but requires rigid host matrices to suppress non-radiative decays for high phosphorescent quantum yield (PhQY). However, rigid hosts are often hard to process and have low compatibility with luminescent guests, leading to weak luminance. This research proposes a novel strategy to overcome these limitations by using an active host for triplet excited-state sensitization and water to rigidify the matrix, enhancing both lifetime and PhQY of OURTP.

The literature extensively discusses the challenges in achieving efficient blue luminescence in both fluorescent and phosphorescent materials. Numerous studies have explored strategies to enhance organic afterglow, including crystallization/H-aggregation, exciplex formation, and host-guest systems. However, these methods often lead to red-shifted emissions, hindering the development of blue OURTP. The importance of rigid host matrices in suppressing non-radiative decays and improving PhQY has been established. Nevertheless, finding suitable rigid hosts with good processability and compatibility with blue-emitting guests remains a significant obstacle. The lack of efficient blue/deep-blue organic afterglow with long lifetimes (>1.0 s) and high PhQY (>40%) motivated the current research.

This study employs cyanuric acid (CA) as a universal host due to its ability to suppress luminescence quenching and non-radiative decay processes. Phthalic acid derivatives serve as guest molecules, interacting effectively with CA for single molecular dispersion and suppressing non-radiative vibrations through hydrogen bonding. CA's high singlet (S1) and triplet (T1) excited state energies facilitate efficient energy transfer, acting as an active host to sensitize the guest's triplet excited state. Trimesic acid (TMA) was chosen as the guest molecule. The materials were prepared by ultrasonically mixing aqueous solutions of TMA and CA, followed by solvent removal. The effect of water content (0-70 wt%) and TMA doping concentration (1-40 wt%) on the photophysical properties was investigated. Steady-state and time-resolved photoluminescence spectroscopy, temperature-dependent PL measurements, differential scanning calorimetry (DSC), Raman spectroscopy, solid-state nuclear magnetic resonance (NMR) spectroscopy, and powder X-ray diffraction (XRD) were used to characterize the materials. The performance of other guest molecules (isophthalic acid, terephthalic acid, and phthalic acid) was also evaluated to determine the universality of the strategy. Finally, the water-responsive OURTP materials were used to fabricate a rewritable encryption paper for anti-counterfeiting applications.

The researchers successfully synthesized a series of deep-blue organic afterglow materials using a host-guest approach with water as a matrix rigidifier. The optimized material, CT5-20 (5 wt% TMA, 20 wt% water), exhibited a deep-blue emission peak at 406 nm, a lifetime of 1.67 s, and a PhQY of 46.1%. These values are among the highest reported for organic afterglow. The addition of water significantly enhanced both the lifetime and PhQY, attributed to the formation of hydrogen-bonding networks that rigidify the matrix and suppress non-radiative decays. Temperature-dependent PL measurements confirmed that both low temperatures and water addition suppress non-radiative decays. DSC, Raman, and NMR studies confirmed the formation of hydrogen bonds between CA, TMA, and water, leading to matrix rigidification. The active host (CA) sensitizes the triplet excited state of the guest (TMA) through Dexter energy transfer, contributing to the high efficiency. The strategy proved universal, as other benzoic acid derivatives also showed improved afterglow performance with water addition. A rewritable encryption paper was successfully fabricated using the water-responsive OURTP materials, demonstrating high-resolution, long-lasting (>1 month), and reversible write-erase cycles using water-jet printing and DMSO vapor fuming.

The findings demonstrate a highly effective approach to design efficient deep-blue organic afterglow materials, addressing the long-standing challenge in this field. The combination of triplet sensitization by the active host and matrix rigidification by water offers a facile and versatile strategy, applicable to different guest molecules. The exceptional performance of the developed materials opens new avenues for applications in optoelectronics and anti-counterfeiting. The water-responsiveness of the materials enables the creation of novel rewritable devices with practical implications. This research significantly advances the understanding of organic afterglow and provides a pathway for developing more advanced, high-performance materials.

This study successfully developed a novel strategy for designing highly efficient deep-blue organic afterglow materials by combining guest sensitization and matrix rigidification using water. The resulting materials demonstrated superior performance compared to existing literature, achieving lifetimes up to 1.67 s and PhQYs up to 46.1%. The strategy's universality was confirmed using other guest molecules. The developed materials also found application in creating a rewritable encryption paper for anti-counterfeiting. Future research could explore other host and guest combinations to further optimize the performance and expand the range of applications.

The study primarily focuses on the performance of the materials in powder form and their application in rewritable paper. Further research is needed to investigate the performance of the materials in device applications, such as OLEDs. The long-term stability of the rewritable paper under various environmental conditions also needs to be evaluated. Additionally, a comprehensive exploration of the effect of various solvents on the OURTP characteristics would be beneficial.