The effect of different bithienyl spacers on the linear and non-linear optical properties of organometallic chromophores derived from [Fe2(η5-C5H5)2(CO)2(μ-CO)(μ-C–CH3)]+ [BF4]−: Crystal structure of [Fe2(η5-C5H5)2(CO)2(μ-CO)(μ-(E)-C–CHCH-7-(thieno[3,2-e

Two series of organometallic merocyanines incorporating a variety of bithienyl moieties both as spacers and end groups were synthesised. In the first series the bithienyls function as end groups and were appended to an acceptor derived from [Fe2(η5-C5H5)2(CO)2(μ-CO)(μ-C–CH3)]+ [BF4]−. In the second series, ferrocene acting as an organometallic donor was incorporated into the chromophores at the far terminus of the bithienyl spacers. The molecular first hyperpolarisabilities β of the second series were determined by hyper-Rayleigh scattering and the values range from 429×10−30 esu to 867×10−30 esu in dichloromethane. A single crystal X-ray study of [Fe2(η5-C5H5)2(CO)2(μ-CO)(μ-(E)-C–CHCH-7-(thieno[3,2-e][1]benzothiophene))]+ [BF4]−·CH2Cl2 is reported.