Polarity Indicators and Scales

Abstract

Polarity is a crucial property in both chemical and biological systems. It determines the molecular interactions within the system. Therefore, it substantially influences both physical and chemical processes. In solution, change in the solute-solvent interactions due to the local polarity affects the molecular association, stability and energy levels which are evidently reflected by changes in the position and/or intensity of the absorption and emission spectra. There are many compounds, called solvatochromic compounds, whose spectra are sensitive to the local polarity and can shift to longer (bathochromic shift) or shorter (hypsochromic shift) wavelength ranges. They can be used to be the polarity indicators and to develop the empirical polarity scales. Some well-known polarity scales are stated in this review. Likewise, some solvatochromic compounds which have been extensively used to investigate chemical and biological systems are stated as an example. Furthermore, some details and applications of a well-known fluorescent polarity indicator, pyrene, whose spectral characteristic/intensity is sensitive to the local polarity of the system, are reviewed.