Molecular structure and spectral properties of thionaphthalimides

The molecular structure and absorption spectra of monothio- and dithio-naphthalimides were compared to their naphthalimide analogues using AM1, PM3 and ZINDO/S semiempirical quantum chemical methods. The substitution of the 4R-naphthalimide oxygen atoms by sulphur atoms resulted in a red-shift of the absorption spectra by Δ λ max 60–65 and 100–140 nm, respectively. The thionated naphthalimide derivatives do not show observable fluorescence due to intersystem crossing to the triplet n π ∗ -states localised at the CS groups. The n π ∗ -absorption bands of monothioimides are located at 525–580 nm ( ε =60–80) and those for dithioimides at 535–560 nm ( ε =140–390) and 628–686 nm ( ε =34–68). None of these transitions are solvent sensitive. The n π ∗ -transitions of N -phenylthioimides have also a small contribution from ππ ∗ -states due to a partial conjugation between CS group and π-electronic system of the N -phenyl ring. As a result, the n π ∗ bands of aromatic substituted N -phenylthioimides are red-shifted as compared to those of the aliphatic N -methyl-thioimides.