An ab initio and TD-DFT study of solvent effect contributions to the electronic spectrum of Nile Red

A CIS and TD-DFT study using a polarizable continuum solvent model has been conducted to assess non-specific solvent effects on the spectral shifts in Nile Red (NR). The results in this paper show that the electronic band shifts of Nile Red are a consequence of both a field effect on the transition energy and an effect due to a solvent induced geometry change. The S-0 --> S-1 transition experiences a large red-shift with increasing solvent polarity but is relatively insensitive to geometric change. The TD-B3LYP assessment of the S-0 --> S-2 transition yields a blue shift with increasing solvent polarity but a red shift due to the solvent induced geometry change. Ground state dipole moments and polarizabilities are also shown to increase with solvent polarity. CIS optimization of the first excited state of NR is reported, but the solvent effects suggest a locally excited state may have been obtained. Further studies are needed to assess the role of a TICT state.