Molecular design and functional outcomes of RTP and TADF traits in isomers†

This study reports the synthesis and photophysical analysis of three isomeric compounds, namely 3Fmo, 3Fmm, and 3Fmp, which were engineered using carbazole as the electron donor, phthalimide as the electron acceptor, and a benzene ring as the bridging moiety. Among these, 3Fmm was distinguished by its ability to exhibit immediate room-temperature white phosphorescence following the cessation of UV illumination, whereas 3Fmo and 3Fmp demonstrated TADF properties. Crystallographic analysis revealed unique intermolecular π–π stacking interactions within 3Fmm, absent in the other two isomers. Advanced TD-DFT computations indicated that such π–π stacking in 3Fmm not only facilitates intersystem crossing but also effectively reduces the free volume within the crystal, leading to a decrease in non-radiative transitions. These molecular interactions promote the manifestation of room-temperature phosphorescence. Furthermore, leveraging the superior luminescent properties of 3Fmo, the compound was successfully utilized in cellular imaging, where it achieved excellent imaging results, showcasing its potential for biomedical applications.