State-specific Electrostatic Potential Descriptors for Estimating Solvatochromic Effects

The Lippert-Mataga equation is used widely to describe the solvatochromic effects of fluorescent molecules through the evaluation of solute–solvent interactions on the basis of the point-dipole model. A large dipole deviation of molecules in the ground-state and the lowest excited-state is a basic requirement for the design of a polarity-sensitive fluorescent probe. Some recently synthesized probes with center-symmetry have near zero dipole deviation while undergoing notably solvatochromic behaviors. Thus, it is necessary to find a new method beyond the Lippert-Mataga model to qualitatively estimate the molecular solvent shifts. To this end, a state-specific descriptor (SSD) based on molecular surface electrostatic potentials (ESP) is proposed to explain the solvatochromic behaviors of the well-studied coumarin C153 and center-symmetric DCB-1d. In contrast to the experimental solvent shifts and state-specific TD-DFT calculations, the SSD successfully explains the solvatochromic effect of C153 and DCB-1d molecules. In addition, the SSD was tested by using eight selected polarity-sensitive fluorescent molecules. The SSD was found to provide a good linear relationship with solvatochromism.