James B. Foresman

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.

Application of the CI-Singles Method in Predicting the Energy, Properties, and Reactivity of Molecules in Their Excited States

Configuration interaction with all single excited determinants, CIS, is an easy to use, cost effective method for surveying many excited states of chemical interest. Geometry optimization and vibrational frequency calculations can be carried out using analytical gradients, energy differences can be improved by second order perturbation theory, and large systems can be studied by direct methods. The capabilities and limitations of CIS are illustrated by calculations on the excitation energies of benzene, bicyclobutane and Li2Na2, optimized geometry and properties of n-π* excited state of formaldehyde, vibrational frequencies of n-π* excited acrolein, and excited state potential energy curves for breaking σ and π bonds. These examples are also used to discuss the practical aspects of CIS calculations.