Theoretical studies of Nile Red by ab initio and semiempirical methods

Ab initio and semiempirical calculations were carried out for the Nile Red (NR) molecule to study the possible occurrence of the twisted intramolecular charge transfer process. The results showed that NR is planar in the ground state (using the CEP-31g basis set) with a high barrier to rotation of the diethylamine group by 90° (0.334 and 0.381 eV with AM1 and CEP-31g, respectively). CIS calculations showed that the charge transfer decreases after the twisting, in contrast to the TICT prediction. The solvatochromic effect was justified through the dipole moments calculated for the first excited state.