Theoretical Study of Ground- and Excited-State Charge Transfer in Fulvene-Based Donor–Acceptor Systems

Donor-acceptor systems based on fulvene as the electron-accepting moiety are typified by exotic, strongly polar electronic structures. In this contribution, ab initio calculations have been performed to explore the ground- and excited-state properties of an archetypal compound of this class, which incorporates the exocyclic carbon atom of fulvene into a tetramethylimidazoline-like five-membered ring. In the electronic ground state, the compound under study has a pronounced zwitterionic character and is best described as consisting of a negatively charged cyclopentadienyl ring linked covalently to a positively charged tetramethylimidazolium ring. Both of these rings can be considered as aromatic. The excess negative charge localized on the cyclopentadienyl ring is highly labile in the photochemical sense: the low-lying valence excited states exhibit varying degrees of reverse charge transfer, whereby electron density is transferred from the cyclopentadienyl ring back onto the tetramethylimidazolium ring. The topographies of the excited-state potential energy surfaces favor rapid and efficient internal conversion at an extended, fulvene-like S 1 /S 0 conical intersection seam. As a consequence, the excited-state lifetime of this compound is predicted to be on the order of 100 fs.