Aminopyrimidine‐Based Donor–Acceptor Chromophores: Push–Pull versus Aromatic Behaviour

Abstract

AbstractNovel 2‐aminopyrimidines substituted with two electron‐donor dialkylamino groups and either one dicyanovinyl (4a–d) or one tricyanovinyl (7a–d) electron‐acceptor group have been synthesized, and the balance between their push–pull character and their aromatic behaviour has been studied by experimental (spectroscopic, electrochemical and X‐ray analysis) and theoretical (DFT/B3LYP/6‐31G**) methods. Calculations reveal that the push–pull character is energetically favoured with respect to the preservation of the aromaticity of the pyrimidine ring. X‐ray analysis of 7a confirms the theoretical predictions and reveals a strong distortion from planarity due to the steric interaction between the tricyanovinyl group and the adjacent dialkylamino moieties. In contrast with what would normally be expected for push–pull compounds, the visible‐region absorption bands of 4a–d and 7a–d are associated with π–π* electronic transitions and not with intramolecular charge‐transfer absorptions. In each compound, both the HOMO and the LUMO are spread over the whole molecule and are not located on the donor and acceptor moieties, respectively, as is the case for typical push–pull chromophores.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)