Flexible electronics is a rapidly growing field due to its applications in displays, electronic skin, and solar cells. Organic materials, particularly organic crystals, are promising due to their abundance, tunable functions, and processability. However, organic crystals are typically rigid and fragile because of weak intermolecular interactions. Recently, elastic organic crystals (EOCs) have been created using π-π stacking and multi-directional halogen or hydrogen bonds. While EOCs with short-lived luminescence exist, flexible crystals with afterglow (>100 ms) remain unreported. Ultralong organic phosphorescence (UOP) offers long-lived lifetimes and rich excited-state properties, but room-temperature UOP in purely organic materials is rare. Strategies to achieve efficient phosphorescence include introducing carbonyl or halogen atoms to enhance intersystem crossing (ISC) and creating a rigid environment to suppress non-radiative transitions. Crystal engineering is a key method for restricting molecular motions. This research hypothesized that halogen atoms and π-conjugated phosphorescent chromophores would create elastic crystals with high phosphorescence efficiency, with the halogen atoms increasing ISC.

The introduction section extensively reviews the existing literature on flexible electronics, highlighting the use of various materials including metals, inorganic materials, 2D materials, and organics. It emphasizes the potential of organic crystals but notes their inherent brittleness. The review then focuses on the recent development of elastic organic crystals and their luminescent properties, particularly the lack of flexible crystals exhibiting afterglow. Existing research on ultralong organic phosphorescence (UOP) and strategies to achieve it are also discussed, including crystallization, host-guest doping, and self-assembly. The authors cite numerous papers demonstrating various approaches to achieving both flexibility and luminescence in organic materials, setting the stage for their own contribution.

The researchers designed and synthesized three molecules: DCz4Cl, DCz4Br, and DCz4I, incorporating chlorine, bromine, and iodine atoms respectively. These molecules were characterized using NMR spectroscopy and single crystal analysis. Purity was confirmed through HPLC and elemental analysis. Acicular crystals (10-15 mm long) were grown via slow evaporation. Elasticity was assessed by a manual bending test, measuring the elastic strain (ε) using the equation ε = t/2R (where R is the radius of curvature and t is the thickness). Photophysical properties were investigated in both solution and crystal states using UV-Vis absorption and photoluminescence spectroscopy. Phosphorescence lifetimes were measured using time-resolved emission spectroscopy. Phosphorescence quantum yield (Φph) was determined using an absolute PL quantum yield spectrometer. Single-crystal analysis, photophysical parameter calculations (using Gaussian 09), and theoretical simulations were conducted to understand the high phosphorescence efficiency. Attachment energy was calculated using Materials Studio 7.0 to analyze crystal molecular arrangements. Finally, the DCz4I crystal was embedded in a model banknote to demonstrate its anti-counterfeiting potential.

Three elastic organic crystals (EOCs) with ultralong phosphorescence (UOP) were successfully synthesized. The crystals exhibited excellent elasticity, with strains (ε) of 3.99%, 3.42%, and 3.01% for DCz4Cl, DCz4Br, and DCz4I, respectively. The phosphorescence quantum yield (Φph) increased with the atomic mass of the halogen substituent, reaching 19.1% for DCz4I (34 times higher than DCz4Cl). The ultralong phosphorescence lifetimes were 526 ms, 307 ms, and 268 ms for DCz4Cl, DCz4Br, and DCz4I, respectively. Single crystal analysis revealed strong halogen bonds (especially C-I···π interactions in DCz4I) contributing to both elasticity and high phosphorescence efficiency. Calculations showed that the intersystem crossing (kisc) and radiative decay (krp) rates were faster for DCz4I. The strong heavy-atom effect, high kisc, large spin-orbit coupling (SOC) constants, and low reorganization energy in DCz4I explained its high phosphorescence quantum yield. The DCz4I crystal, with its efficient UOP and elasticity, was successfully used in a flexible banknote anti-counterfeiting application, maintaining its luminescence even when bent.

The findings directly address the research question of creating elastic organic crystals with ultralong phosphorescence. The high phosphorescence quantum yield and elasticity of the DCz4I crystal are significant advancements, offering a new material for flexible optoelectronic devices. The successful anti-counterfeiting application demonstrates the practical utility of these materials. The combination of experimental results and theoretical calculations provides a comprehensive understanding of the structure-property relationships, providing valuable insights for future material design. The strong halogen bonds were identified as crucial for both elasticity and phosphorescence efficiency. This work provides a successful strategy for creating flexible materials with long-lived luminescence.

This research successfully synthesized elastic organic crystals (EOCs) with ultralong organic phosphorescence (UOP). The DCz4I crystal, exhibiting both high elasticity (ε = 3.01%) and phosphorescence quantum yield (Φph = 19.1%), was successfully implemented in anti-counterfeiting applications. Future research could explore other halogen atoms or different molecular designs to further enhance both elasticity and phosphorescence. Investigating the long-term stability of these materials under various environmental conditions is also important for practical applications.

The study primarily focuses on the synthesis and characterization of the EOCs and their application in anti-counterfeiting. Further research is needed to explore their full potential in other flexible electronics applications. The long-term stability and durability of the crystals under various conditions (e.g., temperature, humidity, mechanical stress) need to be investigated. The scalability and cost-effectiveness of the synthesis process are also important considerations for practical implementation.