Thermosolvatochromism of Phenol Blue in Polar and Nonpolar Solvents

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The solvatochromic properties of phenol blue [N-(4-dimethylaminophenyl)-1,4-benzoquinoneimine] have been investigated in polar and nonpolar solvents using variable-temperature 1H NMR and electronic absorption spectroscopy. In acetonitrile, chloroform, and cyclohexane, protons on the quinoneimine ring are inequivalent at all temperatures, while in methanol an exchange process increasingly broadens the signals from these protons as the temperature is raised. The temperature-dependent absorption spectrum of phenol blue in methanol, acetonitrile, chloroform, and cyclohexane is reported, and the observed increase in peak frequency and bandwidth with temperature is found to exceed the predictions of continuum theory. The large increase in dipole moment on excitation results in a strong inductive contribution to the solvatochromism, and leads to large thermochromic shifts even in nonpolar solvents. The possible contributions of solvent-dependent electronic structure, low-frequency intramolecular vibrations, and conformational flexibility to the thermosolvatochromic properties of phenol blue are discussed. Evidence is presented for solvent dependence of the dipole difference μe − μg between the ground and excited electronic states.