On the solvatochromism of the n ↔ π* electronic transitions in ketones.

The solvatochromism of the n ↔ π* electronic transitions of acetone, which is determined in this work by means of absorption and emission spectroscopy, has been studied using the pure solvent scales for polarizability, dipolarity and acidity of the medium. From these analyses, the necessary reduction of the dipole moment and the increase of the polarizability for acetone on electronic excitation are evaluated using Abe's approach (Bull. Chem. Soc. Jpn. 1966, 39, 936). The influence that the increase of the aromatic structure (cf. acetophenone) and the lengthening of the size of the alkyl substituents R- (cf. R-CO-R) cause on the solvatochromism of acetone is discussed. Also, we have shown that the photophysical model proposed by Pimentel (J. Am. Chem. Soc. 1957, 79, 3323), which is widely accepted for explaining the n → π* blue shift phenomenon for hydrogen-bonded complexes, is mistaken due to ignoring the acid-base changes undergone on electronic excitation; accordingly, a new photophysical model has been proposed.