Thermally activated delayed fluorescence (TADF) molecules have emerged as a promising class of third-generation organic light-emitting diode (OLED) emitters that can achieve 100% internal quantum efficiency without the use of noble metals. However, the design of high-efficiency red TADF materials has been challenging due to limitations imposed by the energy-gap law. To overcome this challenge, two new TADF emitters, namely, 6-(4-(diphenylamino)phenyl)-2-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (NI-TPA) and 6-(10H-phenothiazin-10-yl)-2-phenyl-1H-benzo[de]-isoquinoline-1,3(2H)-dione (NI-Pz), have been synthesized and characterized. These compounds exhibit strong TADF characteristics with a small energy gap (ΔE_ST) between the lowest excited singlet and triplet states, short delayed fluorescence lifetimes, high thermal stability, and high photoluminescence quantum yields. The OLED devices fabricated using NI-TPA and NI-Pz as emitters show orange and red electroluminescence with emission peaks at 593 nm and 665 nm, respectively, and maximum external quantum efficiencies (EQEs) of 11.3% and 7.6%, respectively. Furthermore, applying NI-TPA to cell imaging yielded excellent imaging results, indicating the potential of red TADF materials in the field of biological imaging.

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.

During heating, NI-mPCz shows a red shift, then a blue shift, and finally another red shift. These shifts result from phase transitions in its crystalline structures at different temperatures, affecting its photophysical properties.