A Study on the Nature of the Electronic Absorption Bands of Formylpyrroles and Acetylpyrroles

Abstract The electronic spectra of formyl- and acetylpyrroles were studied from both experimental and theoretical viewpoints. On the basis of the solvent effect and pH-dependence of the absorption spectra, the lowest energy transitions of 2- and 3-acetylpyrroles and the corresponding formylpyrroles, as well as the second lowest energy transitions of 3-acylpyrroles, were suggested to be accompanied by the electron-transfer from the pyrryl to carbonyl group. As to 1-acetylpyrrole (or 1-formylpyrrole), the longest wavelength absorption bands disappeared when the conjugation between the pyrryl and carbonyl groups were inhibited by steric hindrance. Solvent effects on the absorption band, however, were so small that the extent of electron-transfer was considered to change little during the transition. The results of VESCF MO calculations also supported the above suggestion based on experimental observations. The behaviors of the second lowest energy transitions of 1- and 2-acylpyrroles were too complicated to be accounted for straightforwardly.