Further solvatochromic, thermochromic, and theoretical studies on Nile Red

Experimental steady-state solvatochromic and thermochromic studies of Nile Red absorption and emission in nine dipolar solvents are reported, as well as theoretical modeling results concerning the ground and excited states of Nile Red in solution. Solvatochromic absorption and emission data analyzed according to conventional methods yield ground- and excited-state dipole moments of 8.9 +/- 0.5 and 14.4 +/- 0.5 D. Application of this conventional model to thermochromic shift data gave dipole moments of 8.4 +/- 1.0 and 13.4 +/- 1.0 D. The thermochromic data were also analyzed using a novel solute monopole-solvent dipole model; this model did not appear to reproduce trends in the thermochromic shift data as well as the conventional model. Results of semiempirical ZINDO/S calculations on Nile Red combined with an Onsager model for solvation were also used to examine the energetics of the excited states of Nile Red in solution. These calculations suggest the presence of a TICT state in Nile Red comparable in energy to an emitting excited state at high solvent polarity. Conventional models for solvatochromic and thermochromic response, however, appear to explain the experimental results independently of any emission from this TICT state in the present solvents.