Toward a generalized treatment of the solvent effect based on four empirical scales: dipolarity (SdP, a new scale), polarizability (SP), acidity (SA), and basicity (SB) of the medium.

This paper reports a methodology for analyzing the solvent effect from empirical measurements of solvent acidity (SA), basicity (SB), dipolarity (SdP), and polarizability (SP). The proposed methodology departs from the traditional single-parameter procedures for estimating nonspecific solvent effects by splitting them into a polarizability term and a dipolarity term. In this work, we examined the SA, SB, SP, and SdP values for 160 solvents, the gas phase (the absence of solvent) being the origin of these scales. As shown in this paper, this information allows one not only to accurately describe the solvent effect experienced by any solute whether polar or nonpolar and exhibiting some or no specific interaction with the solvent but also to understand the nature of the well-known solvent parameters E(T)(30), pi, S', and SPP, which are frequently used to describe the overall nonspecific contribution of solvents in terms of a single parameter. The high potential of the proposed empirical methodology is illustrated with its application to the solvatochromic analysis of the spectroscopic behavior of molecular chromophores, thus explaining, for example, the influence of solvent effects in the twist intramolecular charge transfer (TICT), the excited-state intramolecular proton transfer (ESIPT), or LASER (light amplification by stimulated emission of radiation) emissions. Also, this methodology is applied to understanding for the indole chromophore the feasible inversion of the electronic nature for the first electronic excited state due to solvent effects.