| Summary: | A systematic series of η5-monocyclopentadienyliron(II) complexes with substituted oligo-thiophene nitrile ligands of general formula [FeCp(P_P)(NC{SC4H2}nNO2)][PF6] (P_P=dppe, (+)-diop; n=1-3) has been synthesized and characterized. The electrochemical behaviour of the new compounds was explored by cyclic voltammetry. Quadratic hyperpolarizabilities () of the complexes with dppe coligands have been determined by hyper-Rayleigh scattering (HRS) measurements at two fundamental wavelengths of 1.064 and 1.550 m, to uncover the two-photon resonance effect and to estimate static values. The obtained overall results are found to be better than for the related η5-monocyclopentadienyliron(II) complexes with p-benzonitrile derivatives. Although an increase of the resonant at 1.064 m with increasing number of thiophene units in the conjugated ligand was found (up to 910 x 10-30 esu), the static values 0 remain practically unchanged, as shown by the 1.550 m measurements. Combined with the electrochemical and spectroscopic data (IR, NMR, UV-Vis), this remarkable evolution of shows that the increase of conjugation length is balanced by a decrease in charge-transfer efficiency.
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