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 S1/S0 conical intersection seam. As a consequence, the excited-state
lifetime of this compound is predicted to be on the order of 100 fs.