Compromise between conjugation length and charge-transfer in nonlinear optical η5-monocyclopentadienyliron(II) complexes with substituted oligo-thiophene nitrile ligands: Synthesis, electrochemical studies and first hyperpolarizabilities

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 (β) 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 μm, to uncover the two-photon resonance effect and to estimate static β 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 β at 1.064 μm with increasing number of thiophene units in the conjugated ligand was found (up to 910 × 10−30 esu), the static values β0 remain practically unchanged, as shown by the 1.550 μm measurements. Combined with the electrochemical and spectroscopic data (IR, NMR, UV–vis), this remarkable evolution of β shows that the increase of conjugation length is balanced by a decrease in charge-transfer efficiency.

Graphical abstractNew organometallic complexes [FeCp(P_P)(NC{SC4H2}nNO2)][PF6] (P_P = dppe, (+)-diop; n = 1–3) were prepared and fully characterized. Quadratic hyperpolarizabilities have been determined by hyper-Rayleigh scattering, at 1.064 and 1.550 μm, to reveal the resonant enhancement effects. The quasi-independence of β0 with conjugation length is attributed to the competing effects of charge-transfer efficiency and conjugation length.Figure optionsDownload full-size imageDownload as PowerPoint slide