Structural Effect on Absorption and Emission Properties of 1,8-Naphthalimide Derivatives: a DFT Study

Using B3LYP/6-31G(d) model, time depended(TD)-B3LYP/6-31+G(d) method and Conductor-like Polarizable Continuum Model (C-PCM)-TD-B3LYP/6-31+G(d) method, we calculated the structure and the absorption and emission spectra of a series of N-substituted 1,8-naphthalimides in both gas-phase and dichloromethane. The influence of the substituents on the electronic absorption spectra and their emission spectra has been discussed on their calculated frontier molecular orbitals contour and their energy levels. Results show that their rings extension from CN group and the substituents on their naphthalimic ring play an important role in the absorption spectra and the emission spectra properties. Modification of OCNCO group and the substituents in their naphthalimic rings breaks the structural symmetry. The Mulliken atomic charges values of NO2 groups from S0 to S1 in 4 positions are a little greater than the 5-positions, which also mean that the 5 position provide more electrons. For MACs of N(Ph)2 and N(Me)2, the 4 position substituents provide more charges than that of 5 position. They not only lead to bigger dipole moments, but also extend frontier orbital contour. Frontier orbitals also show that the modification of OCNCN and the introduction N(Me)2, N(Ph)2 and NO2 groups extends their π—π* excitation scope and decreases their energy gap accordingly. Besides, those kinds of molecular design enhance intra molecular charge transfer between substituent and naphthalimic ring. Therefore, redshift are shown in their absorption and emission spectra, which is also verified by calculated results. Their absorption and emission spectra in solvent redshift compared with their gas spectra. For the NO2 derivatives, the charge transfer state is in the 5 position substituent compounds. For donor substituents, charge transfer state lies in their 4 position compounds. When the CO group is in the same side with the NO2 group, and the N(Me)2 and the N(Ph)2 are in the different side with the CO group, compounds have better conduction properties. From compound 1 to compound 4, the redshift of the absorption spectra in dichloromethane is about 139 nm. The more intramolecular charge transfer, the bigger absorption maximum those compounds shown. Above result is in good agreement with the 5-position NO2 derivatives and the 4-position N(Me)2, N(Ph)2 derivatives. Above OCNCO structural change and their charge transfer mechanism provide design basis for further 1,8-naphthalmic derivatives.