Modulating the crystal packing to achieve efficient ultralong organic phosphorescence by simple methylation engineering

Molecular stacking plays a critical role in ultralong organic phosphorescence (UOP) generation under ambient conditions. To better understand how the crystal packing affects UOP and achieve efficient UOP, herein, a combined experimental and theoretical study of the synthesized S,S-dioxide phenothiazine derivatives was performed, in which methyl groups with different numbers and substitution sites were used to modulate the solid-state packing. All compounds showed similar photophysical characteristics in solution, but distinct UOP behaviors in the crystalline state. m-DMOPP crystals possessing two methyl moieties and densely ordered stacking can achieve a longer lifetime of 375 ms and higher phosphorescent efficiency. The potential applications in anticounterfeiting and 3D patterning based on m-DMOPP were realized in virtue of its excellent UOP character. Importantly, the changes in displacement between adjacent molecules along the Y-axis had a more significant impact on the intersystem crossing process than that along the Z-axis in such H-aggregation phenothiazine-based materials. Our investigation would provide a profound understanding of the relationship between molecular stacking and UOP properties and a feasible molecule strategy to design ideal UOP materials. Simple methylation engineering strategy used to adjust the packing and UOP behavior is demonstrated. The intrinsic relationship between structure and UOP properties are investigated by combining the experimental and theoretical results.