Constantes d'acidité, à l'état excité, des α-cyanostilbènes et des α-cyanodiphénylazométhines paradiméthylaminés

The acidity constants, in the electronic excited state (pK*), of the conjugate acids of eight terms of a series of dimethy-lamino-4, a-cyanostilbenes (X — C6H4 — C(CN) = CH — C6H4 — N(CH3)2) and of eight terms of a series of dimethylamino-4, a-cyano-diphenylazomethines (X — C6H4 — C(CN) = N — C6H4 — N(CH3)2) were calculated using U.V. and visible spectroscopic v values. Plotting these pK* versus substituent constants, better correlations were obtained with a~ values (for electron-withdrawing substi-tuents) and a+ values (for electron-donating substituents) than with the common Hammett values. In the first series p — 6,95 and in the second p = 1,52. These results are briefly commented.