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 Δλ_max60-65 and 100-140nm, respectively. The thionated naphthalimide derivatives do not show observable fluorescence due to intersystem crossing to the triplet nπ*-states localised at the C=S groups. The nπ*-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 nπ*-transitions of N-phenylthioimides have also a small contribution from nπ*-states due to a partial conjugation between C=S group and π-electronic system of the N-phenyl ring. As a result, the nπ* bands of aromatic substituted N-phenylthioimides are red-shifted as compared to those of the aliphatic N-methyl-thioimides.