Theoretical Study on the Electronic Structures and Spectral Properties of 1,8-Naphthalimide Derivatives

The molecular geometries, frontier molecular orbital properties, and absorption and emission properties of three 4-phenoxy-1,8-naphthalimide derivatives, namely 4-phenoxy-N-(2 -hydroxyethyl)-1,8-naphthalimide(1),4-(2-tert-butylphenoxy)-N-(2-hydroxyethyl)-1,8-naphthalimide (2), and 4[2,4-di(tert-butyl)]phenoxy-N-(2-hydroxyethyl)-1,8-naphthalimide (3), are investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations in conjunction with polarizable continuum models (PCMs). Four functionals and ten basis sets are employed for 1 to calculate the electron transition energies, which were compared with the experimental observations. Our results reveal that the B3LYP/6-311+G(d,p) method is the best choice to reproduce the experimental spectra. Moreover, the effects of substituents on the molecular geometries, electronic structures, absorption and emission spectra are also studied at the B3LYP/6-311+G(d,p) level. We find that the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases with increasing the number of tert-butyl substituents onto the phenoxy groups, suggesting red-shift of the absorption and emission bands. This is related to the increase of conjugation from 1 to 2 and 3. Our calculations are in good agreement with the experimental results.