A very challenging task in molecular electronics is to assess the nature of the substrate−molecule bond. Research on silicon has evidenced the importance of the establishment of a Si−molecule covalent bond in the reduction of the surface recombination velocity, in the electronic communication between inorganic substrate and organic molecules, in the chemical stabilization in air or in solution of such a functionalized Si surface. In the last two years an extensive characterization on the molecular electroactive hybrid systems resulting from different anchoring reactions of ferrocene derivatives has been conducted in our group by combining electrochemistry, AFM, XPS and DFT calculations. [1] By comparing the measured potentials with the computed redox energies, it was possible to estimate the degree of unsaturation of the C-C arm in the adsorbed monolayer. [2] In this frame, photoemission experiments with synchrotron radiation have been conducted at ELETTRA on three strictly related molecules: ethylferrocene, vinylferrocene and ethynylferrocene. This choice allows to evidence the different role played by a fully saturated CH3−CH2− moiety with respect to the single and double unsaturated group in the overall electronic structure of the iron redox species. The prototypical character of the three C-C substituents exemplifies the full range of interactions possibly presented by the shortest anchoring arm still able to preserve Si−molecule conjugation through a Si−C bond. VB and core gas-phase spectra of the three molecules were acquired at different photon energies and the results compared to state-of-the art calculations. The distinct valence spectral features have been assigned to differently composed molecular orbitals. This allows for the interpretation of the extent of electronic conjugation due to the side-chain and its role in stabilizing the HOMO, a crucial issue in the correct interpretation of the corresponding VB spectra for the hybrid species.
[1] Decker, F.; Cattaruzza, F.; Coluzza, C.; Flamini, A.; Marrani, A.G.; Zanoni, R.; Dalchiele, E.A. J. Phys. Chem. B, 2006, 110, 7374-7379.
[2] Cossi, M.; Iozzi, M. F.; Marrani, A. G.; Lavecchia, T.; Galloni, P.; Zanoni, R.; Decker, F. J. Phys. Chem. B, 2006, 110, 22961-22965.
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