Electronic Properties of 4-Substituted Naphthalimides

This paper describes a study of excited-state properties of naphthalimide (NI) and four 4-substituted derivatives: 4-chloronaphthalimide (Cl-NI), 4-methylthionaphthalimide (MeS-NI), 4-nitronaphthalimide (O2N-NI), and 4-(N,N-dimethylaminonaphthalimide (Me2N-NI). Steady-state absorption and fluorescence spectra were collected in solvents of varying polarity to determine the excited-state character of NI derivatives. Furthermore, the excited-state dynamics were studied using femtosecond transient absorption spectroscopy. The experimental findings were compared to calculated data obtained using time-dependent density functional (TD-DFT) methods. We found that light absorption by all NI derivatives leads to the production of the second excited state (S2), which was found to have a n,π* character. Within ∼40 ps, the S2 state undergoes internal conversion to produce the S1 state. The S1 state is relatively long-lived (∼4 ns) and has charge-transfer character in NI derivatives with electron-withdrawing and electron-donating groups (MeS-NI, O2N-NI, and Me2N-NI). In the case of NI and Cl-NI, the S1 state has a π,π* character and undergoes intersystem crossing to produce the T1 state within 400 ps.