The linear and nonlinear optical properties of organometallic chromophores derived from ferrocene, [Fe2(η5-C5H5)2(CO)2(μ-CO)(μ-CCH3)]+[BF4]− and terthienyl spacers. Crystal structure of 2-[(E)-2-ferrocenylethenyl]-5-(2-thienyl)thiophene

Several chromophores based on ferrocene, [Fe2(η5-C5H5)2(CO)2(μ-CO)(μ-CCH3)]+ and terthiophene or dithienylbenzo[c]thiophene have been synthesized. Both thienyls have been shown to act as good donor end groups in their own right but contrary to our expectations the benzo[c]thiophene merocyanine has a lower hyperpolarizablity (344×10−30 esu) than its thienyl congener (650×10−30 esu). The structure of 2-[(E)-2-ferrocenylethenyl]-5-(2-thienyl)thiophene has been determined by single crystal X-ray diffraction. The terminal thienyl (thp) group is disordered over two orientations about the bithienyl C(sp2)C(sp2) bond such that the two sulfur atoms in either thp ring are positioned trans/cis in the ratio 0.628(4):0.372(4). The integrity of the alkenyl CC remains with a bond length of 1.332(2) Å and the CC lengths adjacent are 1.453(4)/1.441(4) Å. Twisting of the rings from planarity occurs along the Fc–CC–(thp)–(thp) axis: the angle which the ethenylthiophene CCC4H2S moiety makes with the C5H4 is 11.2(2)°, and with the major orientation of the disordered terminal thienyl group, 16(1)°.