1,4-Dihydropyrrolo[3,2-b]pyrrole: The Electronic Structure Elucidated by Photoelectron Spectroscopy

Abstract

Abstract The electronic structures of 1,4-dihydropyrrolo[3,2-b]pyrrole and some of its alkyl derivatives were studied by means of He(I) photoelectron spectroscopy combined with the MNDO calculation and a first-order perturbation theoretical method. The first five bands of the parent pyrrolopyrrole are assigned, from the top, to the π5(au), π4(au), π3(bg), π2(bg), and σ(ag) bands respectively. The MO model, which explains well the spectral data, indicates that the nitrogen lone-pair electrons are mainly delocalized in the π4, π2, and π1 orbitals. The ionization potentials of the HOMO levels of these compounds, which are characterized by the large pz contributions from the α-carbons, are the lowest among the hitherto known π-electron-excessive heteroaromatic compounds. 1,4-Dihydropyrrolo[3,2-b]pyrrole is concluded to be the system with the most efficient π-electron-donating ability among 10π-electron systems.