Twisting-Based Spectroscopic Measure of Solvent Polarity: The PT Scale

The PT scale, which is correlated well with the ET(30), Y, and Z 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 o-1 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.