Probing Charge-Transfer Processes in a Covalently Linked [Ge₉ ]-Cluster Imine Dyad

C₆₀ donor dyads in which the carbon cage is covalently linked to an electron-donating unit have been discussed as one possibility for an electron-transfer system, and it has been shown that spherical [Ge₉ ] cluster anions show a close relation to fullerenes with respect to their electronic structure. However, the optical properties of these clusters and of functionalized cluster derivatives are almost unknown. We now report on the synthesis of the intensely red [Ge₉ ] cluster linked to an extended π-electron system. [Ge₉ {Si(TMS)₃ }₂ {CH₃ C=N}-DAB(II)^Dipp ]^- (1^- ) is formed upon the reaction of [Ge₉ {Si(TMS)₃ }₂ ]^2- with bromo-diazaborole DAB(II)^Dipp -Br in CH₃ CN (TMS=trimethylsilyl; DAB(II)=1,3,2-diazaborole with an unsaturated backbone; Dipp=2,6-di-iso-propylphenyl). Reversible protonation of the imine entity in 1^- yields the deep green, zwitterionic cluster [Ge₉ {Si(TMS)₃ }₂ {CH₃ C=N(H)}-DAB(II)^Dipp ] (1-H) and vice versa. Optical spectroscopy combined with time-dependent density functional theory suggests a charge-transfer excitation between the cluster and the antibonding π* orbital of the imine moiety as the cause of the intense coloration. An absorption maximum of 1-H in the red region of the electromagnetic spectrum and the corresponding lowest-energy excited state at λ=669 nm make the compound an interesting starting point for further investigations targeting the design of photo-active cluster compounds.