Donor−Acceptor Interaction in Cationic Archetype Mono‐ and Dinuclear Sesquifulvalene Complexes [(η 5 ‐C 5 H 5 )Fe{µ‐(η 5 ‐C 5 H 4 )(η 7 ‐C 7 H 6 )}M′L′] n + ( n = 1, 2)

In order to gain a deeper insight into structure‐property relationships of the donor (D)−acceptor (A) interactions in cationic sesquifulvalene complexes the mono‐ and dicationic complexes [(η 5 ‐C 5 H 5 )Fe{µ‐(η 5 ‐C 5 H 4 )(η 7 ‐C 7 H 6 )}M′L′] + were synthesized as BF 4 and PF 6 salts: M′L′ = Cr(CO) 3 ( 3 PF 6 ); M′L′ = none ( 4 BF 4 ); M′L′ = Ru(η 5 ‐C 5 H 5 ) + [ 5 (PF 6 ) 2 ]; M′L′ = Ru(η 5 ‐C 5 Me 5 ) + [ 6 (PF 6 ) 2 ]. The ferrocenyl substituent works as the electron donor and the tropylium moiety as the electron acceptor. For the electronic ground state the NMR spectroscopic results demonstrate an increasing charge delocalization in the order M′L′ = Cr(CO) 3 < Ru(η 5 ‐C 5 Me 5 ) + < Ru(η 5 ‐C 5 H 5 ) < none. The same order of the acceptor moieties is observed for the difference between the oxidation and reduction potentials, which are related to an electrochemically reversible and a complete irreversible one‐electron transfer, respectively. A comparison of the structural data of 3 − 6 reveals a small but distinct contribution of the cross‐conjugated mesomeric form to the bis(aromatic) mesomeric form in the ground state of the sesquifulvalene complexes, which increases, when the electron‐withdrawing ability of the acceptor becomes stronger. The UV/Vis spectra are dominated by two intense absorption bands in the visible and near‐UV region, which are assigned to a low‐energy donor(metal)−acceptor charge‐transfer (D−A CT) transition and to a high‐energy donor(ligand)−acceptor charge‐transfer (L−A CT) transition. For these archetype sesquifulvalene complexes the first hyperpolarizability β was determined by means of hyper‐Rayleigh scattering (HRS). Whereas for 3 no second harmonic signal could be detected and 4 shows fluorescence due to two‐photon absorptions, 5 and 6 possess appreciable β values.