Nonlinear polarization of solvatochromic betaine 30.

Evidence is presented that solvatochromism of 2,6-diphenyl-4-(2,4,6-triphenylpyridinio)-phenolate (Reichardt's dye or Betaine 30) results not only from a large reduction of dipole moment and hydrogen bonding, but also from the modulation of mesomeric effect. The polarizability change between the ground and the excited state, estimated from the refractive index (n) dependence of absorption energy E(T)(30), increases from 20 to approximately 150 A(3) on going from apolar to highly polar media. The dependence of E(T)(30) on dielectric permittivity (epsilon) could be linearized using an empirical function of susceptibility difference (epsilon-n(2)), but not in terms of conventional expressions written as phi(epsilon)-phi(n(2)). As a result of solvent-induced change in ionicity and bond alternation, the dipole moments and polarizabilities of the chromophore cannot be treated as invariables. Hypsochromic shifts caused by solvent quadrupoles and bond dipoles were quantified.