Structure−Property Dependence of the First Hyperpolarisabilities of Organometallic Merocyanines Based on the μ‐Vinylcarbynediiron Acceptor and Ferrocene Donor

In order to investigate the structure−property relationship of nonlinear optical materials, a series of organometallic chromophores were synthesised utilising the [Fe 2 (η‐C 5 H 5 ) 2 (CO) 2 (μ‐CO)(μ‐C−)] + electron‐accepting moiety and the ferrocenyl group, Fc, as the electron donor. The π‐linker between these two termini was systematically modified and the mutual electronic communication between them was determined using IR, NMR, and electronic absorption spectroscopy. An X‐ray structure determination of [Fe 2 (η‐C 5 H 5 ) 2 (CO) 2 (μ‐CO)(μ‐C−CH=CH−CH=C(Cl)−Fc)][BF 4 ] confirmed the strong electronic interaction between the donor and the acceptor with reduced π‐bridge bond‐length alternation. The nonlinear optical properties of these complexes were examined using the hyper Rayleigh scattering technique. The experimental first hyperpolarisabilities are some of the highest obtained for ferrocenyl chromophores and, significantly, no enhancement was found due to two‐photon absorption fluorescence. When polyene linkers −(CH=CH) n − are used, the values for β 0 increase with a ca. n 1.5 dependence with no sign of saturation up to n = 4. However, the highest values for β and β o were obtained for linkers which contained an aromatic ring as opposed to pure polyenes and in this respect a benzene ring was more effective than a thiophene or furan. Consequently, the higher β and β 0 are not exhibited by those merocyanines with the highest values for λ max . It is concluded for these compounds that a low excitation energy E eg and a large transition moment M for the electronic excitation are less important than a large change in the dipole moment Δµ eg . Furthermore, a chloro substituent on the olefinic double bond proximate to the ferrocenyl group has a dramatic effect on the β and β 0 values.