Theoretical insights into the excited-state properties of room-temperature phosphorescence-emitting N-substituted naphthalimides.

The optical absorption, fluorescence, and phosphorescence properties along with the intersystem crossing (ISC) rate constants of a series of N-substituted naphthalimides (NNIs), which can be used as organic emitters, were calculated using density functional theory (DFT) and time-dependent DFT (TDDFT). The calculated absorption, fluorescence, and phosphorescence energies as well as the fluorescence and ISC rate constants were found to be in good agreement with available experimental data. According to the calculations, the first excited singlet and first excited triplet states are π-π* in nature, and 1π-π* → 3n-π* is the dominant ISC channel for 1,8-naphthalimide (NI) and N-methyl-1,8-naphthalimide (Me-NI). Intermolecular charge-transfer (CT) states are observed in the energy region between the 1π-π* and 3π-π* states for NNIs with an electron-donating group. The presence of these CT states leads to a reduction in the S1-state and T1-state energy splitting in NNIs with an electron-donating group compared to the energy splitting observed in NI and Me-NI. The singlet CT state (1CT) has very weak emission oscillator strength, so it is nonemissive. For NNIs with an electron-donating group, the 1CT → 3π-π* transition was found to be the dominant ISC channel. Our results indicate that for NNIs with an electron-donating group, the nonemissive S1 state (1CT) transits to the T1 state (3π-π*) via ISC. The 1CT → 3π-π* transition increases the population of the T1 state and thus the phosphorescence quantum yield at room temperature, even in the absence of a heavy atom. Graphical Abstract The S1 state (1CT), which is nonemissive, transits to the T1 state (3π-π*) via intersystem crossing and increases the population of the T1 state in N-substituted naphthalimides with an electron-donating group.