Organic optoelectronic materials have revolutionized various applications, including OLEDs, OFETs, solar cells, and bio/chemo-probing. Understanding the relationship between material structure and properties is crucial for designing advanced materials. Historically, atomism and later the molecular hypothesis explained some chemical phenomena but fell short in explaining photophysical phenomena observed in aggregated states. The concept of "Molecular Uniting Set Identified Characteristic (MUSIC)" highlights the significant influence of molecular aggregation on material properties. Luminogens, especially those exhibiting room temperature phosphorescence (RTP), are highly sensitive to molecular stacking. Excimers, short-lived dimers, often cause red-shifted emission. The role of molecular dimers in RTP, particularly whether RTP originates from triplet excimers, remains unclear. This study uses phenothiazine-5,5-dioxide derivatives to investigate the relationship between molecular packing and RTP behavior. These compounds, with two phenothiazine-5,5-dioxide groups linked by alkyl chains, serve as an ideal model to distinguish monomer and dimer-based RTP emissions, clarifying the role of dimers in RTP.

The literature extensively explores the influence of molecular packing on the luminescent properties of organic luminogens. Studies have shown that changes in aggregation states can lead to drastically different luminescence properties, exemplified by excimer formation causing red-shifted emissions compared to monomeric states. The role of molecular dimers with various interactions (H-aggregation, hydrogen bonds, halogen bonds, n-π, or π-π interactions) in influencing RTP has also been investigated. However, a comprehensive understanding of the precise role of molecular dimers in generating RTP, particularly the contribution of triplet excimers, remains elusive. Previous work has established the ability of phenothiazine-5,5-dioxide groups to form intermolecular π-π interactions, contributing to persistent RTP.

Eight target compounds (2PtzO-nC, n = 3–10) were synthesized via a two-step process involving C–N coupling and oxidation. Their chemical structures and purity were confirmed using ¹H and ¹³C NMR, HRMS, and HPLC. Thermal stability was evaluated via TGA and DSC. Photophysical properties were studied in solution (dilute dichloromethane) and solid states. UV-Vis absorption, fluorescence, and phosphorescence spectroscopy (at 77 K and room temperature) were employed, along with phosphorescence lifetime measurements. The compounds were also incorporated into PMMA films to study their RTP behavior. Single crystals were grown for structural analysis using X-ray single crystal diffraction, focusing on intermolecular π-π interactions. The strength of these interactions was quantified using displacement angle (θ) and vertical distance (d) between adjacent benzene rings. Time-dependent density functional theory (TD-DFT) calculations, including natural transition orbital (NTO) analysis of the T1 state, were performed to investigate the relationship between π-π stacking and RTP emission.

The eight synthesized compounds exhibited similar UV-Vis absorption and fluorescence in solution, indicating minimal influence of the alkyl chain length on the electronic structure of the chromophore. At 77K, all compounds showed monomer phosphorescence around 400 nm. In the solid state, however, significant differences in RTP behavior were observed. Compounds 2PtzO-3C, 2PtzO-7C, 2PtzO-8C, and 2PtzO-10C showed pure triplet excimer emission around 500 nm. 2PtzO-9C displayed both triplet excimer (500 nm) and monomer (445 nm) RTP emission. 2PtzO-4C, 2PtzO-5C, and 2PtzO-6C exhibited dual RTP emissions from both monomer and triplet excimer. Single crystal analysis revealed that strong π–π stacking in dimers (θ ≤ 20.66°; d ≤ 3.86 Å) led to pure triplet excimer emission, while weaker π–π stacking (27.02° ≤ θ ≤ 40.64°; 3.84 Å ≤ d ≤ 4.41 Å) resulted in dual RTP emissions. TD-DFT calculations confirmed the presence of intermolecular orbital coupling in dimers with strong π–π interactions, supporting the observation of pure triplet excimer emission.

This study directly demonstrates the crucial role of molecular packing, specifically π-π stacking in dimers, in determining the RTP behavior of purely organic luminogens. The results clearly show that strong π–π interactions within dimers favor triplet excimer emission, while weaker interactions lead to a competition between monomer and excimer phosphorescence. This comprehensive investigation, combining experimental and computational approaches, provides a clear understanding of the relationship between molecular packing, electronic structure, and RTP. The findings advance the understanding of RTP mechanisms and offer a valuable platform for designing novel RTP materials with tailored emission properties.

This work provides a direct demonstration of triplet excimer emission in purely organic RTP through rational molecular design. The systematic study of eight phenothiazine 5,5-dioxide derivatives revealed a clear correlation between the strength of intermolecular π–π stacking in dimers and the resulting RTP behavior. The findings offer valuable insights into the mechanisms of RTP and provide a foundation for the design of advanced RTP materials with tunable emission characteristics. Future research could explore the application of this design strategy to other organic luminogens and investigate the influence of other non-covalent interactions on RTP.

The study focuses on a specific class of phenothiazine 5,5-dioxide derivatives. The generalizability of the findings to other molecular systems requires further investigation. The single crystal analysis represents specific packing arrangements and might not fully capture the range of possible packing structures. While TD-DFT calculations support the experimental findings, they provide an idealized representation of the molecular systems.