Acid-based equilibria in the ground and excited states of a solvatochromic merocyanine dye

The solvatochromic merocyanine dye I is found by spectroscopic techniques to have a pK a = 8.57 in the ground state and pK * a = 1.85 in the lowest excited singlet state. The validity of the Förster cycle for compounds of this sort is discussed in terms of molecular distortions occurring upon excitation that are indicated both spectrally and by quantum chemical calculations.