Effects of the 3- and 4-methoxy and acetamide substituents and solvent environment on the electronic properties of N-substituted 1,8-naphthalimide derivatives.

The photophysical properties of polar molecules in solution with an intramolecular charge-transfer effect in the excited state depend strongly on the polarity and proticity of the solvents. UV-visible spectra of 1,8-naphthalimide and some N-substituted derivatives in acetic acid, acetonitrile, dichloromethane, and p-dioxane were carried out. Several molecular cluster geometries formed with N-substituted 1,8-naphthalimide derivatives and a large set of random positioning of some solvent molecules in their environment were optimized by a semiempirical method. It provided a complete screening of possible solute-solvent configurations and resulted in a multiple minima hypersurface of the supramolecular systems. With such local minima energies, the main thermodynamic association functions were found. They also provided selected cluster geometries for calculations of vertical electronic transitions with a time-dependent density functional theory (TD-DFT), if the lowest energy structures were considered. Calculated vertical electronic transition energies at the TD-DFT level were compared with experimental data. The experimental absorption UV-visible spectra for the six compounds in the four solvents were performed in our laboratory. Moreover, X-ray photoelectron spectroscospy of the 1,8-naphthalimide was carried out in the ICP-CSIC laboratory. Thermodynamic function values show different association energies between each solvent and the molecules, in correlation with the possibility of hydrogen bond formation and the polarity and dielectric constant of the solvents. The 3- and 4-acetamide 1,8-naphthalimide derivatives have the highest conformer number and the most negative Gibbs free association energy values for a determined solvent. This indicates the importance of the entropic factors.