Azulenylium and guaiazulenylium cations as novel accepting moieties in extended sesquifulvalene type D–π–A NLO chromophores

Abstract

The novel series of monometallic monocations Fc-[n]-Az⁺ and Fc-[n]-Guaz⁺ (where Az⁺ and Guaz⁺ denote azulenylium and guaiazulenylium cations respectively and n = 0–3), and a bimetallic dicationic complex Fc-[0]-Guaz(RuCp)²⁺ have been prepared. Single crystal structures of Fc-[0]-Guaz⁺ and Fc-[0]-Guaz(RuCp)²⁺ have been determined. Analysis of the bond lengths and angles coupled with the electrochemistry data provides evidence for strong ground state charge transfer in the series which diminishes upon π-bridge extension. The electronic absorption spectra reveal that alkylation of the cationic terminus (1) diminishes the electron-accepting ability, (2) results in a larger dipole moment change upon excitation and (3) causes a smaller sequential lowering of the CT transitions with π-bridge extension. The NLO properties which were characterised by hyper-Rayleigh scattering techniques indicate the potential of the azulenylium based donor–acceptor chromophores, but also that the extended complexes exhibit two-photon absorption fluorescence.