Twisting-Based Spectroscopic Measure of Solvent Polarity: The <i>P</i>_<i>T</i> Scale

The <i>P</i>_<i>T</i> scale, which is correlated well with the <i>E</i>_<i>T</i><i>(30)</i>, <i>Y</i>, and <i>Z</i> scales, is the first twisting-based spectroscopic measure of solvent polarity. It is based on two combined mechanisms: (1) the solvent-dependent intramolecular charge-transfer absorption that displays a regular hypsochromic band shift in polar solvents and (2) overcoming the intramolecular hydrogen-bond of <i>o</i>-<b>1</b> by its differentiated solvation in polar solvents, causing a C–C bond to twist that leads to a regular hypsochromic shift of its lowest energy electronic absorption band