Room Temperature Phosphorescence with Time-Resolved Color Changes by Tuning the Conformation of Polyheterocyclic Phenothiazine Derivatives through the Electronic Effect

The research on organic luminescent materials has attracted much attention for their wide applications. Generally, the molecular conformation is of great importance but is often overlooked, regardless of its importance for molecular packing in aggregated states and the corresponding performance. In this work, the relationship between the electronic effect of substituents and the thermodynamically stable conformation in phenothiazine derivatives is explored for the first time. When the substituent is phenyl or a 3-position pyridyl group, the quasi-equatorial (QE) conformer is the main thermo-stable conformation. At this time, the lone pair electrons of the 10-position nitrogen are stabilized by the polyheterocyclic phenothiazine core, resulting in the formation of a large p-π-conjugated electron system. When the substituents are a 2-pyridine or 4-pyridine group, the corresponding lone pair electrons are more easily stabilized by the electron-deficient pyridine system, and another thermo-stable p-π conjugated conformation, namely the quasi-axial (QA) conformer, is formed. In addition, the effects of the two conformations on the corresponding photophysical properties are investigated in detail. It was found that the emissive properties in both singlet and triplet states are largely affected by the molecular conformation. Excitedly, based on the adjustable conformation, one molecule containing two conformations is combined with 3D printing and successfully applied to advanced anticounterfeiting with time-resolved phosphorescence color changes.