Excited-state polarizability and gas phase frequency of the solvatochromic polarity indicator ET(30)

Absorption band maxima of pyridinium-N-phenoxide betaine-30 [C. Reichardt, Chem. Rev. 94 (1994) 2319] are investigated as a function of refractive index and static permittivity of solvents. Spectral shift components due to dispersive forces and solvent polarization were separated, revealing a large increase of polarizability from 70 Å3 in the ground state to ∼200 Å3 in the excited state. Optical band maximum of the nonsolvated chromophore in vacuum is predicted by extrapolation to lie at 40.3 ± 2 kcal/mol, close to the ET(30) value in methyl acetate (38.9 kcal/mol). The contributions of repulsive, dispersive, inductive, and reaction field effects are discussed.