Enhancing Fluorescence of Naphthalimide Derivatives by Suppressing the Intersystem Crossing

The control over intersystem crossing (ISC) during an excited state of an organic molecule is critical to realize high-efficiency fluorescent materials. We report newly designed naphthalimide (NMI) based emitters (Ph-NMI and TPA-NMI), in which the photoluminescence (PL) efficiency increased by suppressing the intersystem crossing (ISC). The experimental and theoretical analysis revealed that the enlarged energy gap between the lowest singlet state (S-1) and lowest triplet state (T-1) and the mismatched electronic configuration of these two states can effectively suppress the ISC process in Ph-NMI and TPA-NMI as compared to the original H-NMI. Moreover, the electronic configuration of S-1-state changed to the pi-pi* transition in Ph-NMI and TPA-NMI (n-pi* tansition in H-NMI), resulting in large radiation transition rate constant (K-r). Therefore, Ph-NMI and TPA-NMI exhibit significantly improved PL efficiencies when compared with H-NMI. The optimized OLEDs using TPA-NMI as yellow emitting layers in doped device showed high performance with a maximum external quantum efficiency of 5.96%, which is the best device performance ever achieved based on the naphthalimide derivatives.