Structure-property dependence of the first hyperpolarisabilities of organometallic merocyanines based on the mu-vinylcarbynediiron acceptor and ferrocene donor

In order to investigate the structure-property relationship of nonlinear op tical materials, a series of organometallic chromophores were synthesised u tilising the [Fe-2(B-C5H5)(2)(CO)(2)(mu -CO)(mu -C-)](+) electron-accepting moiety and the ferrocenyl group, Fc, as the electron donor. The pi -linker between these two termini was systematically modified and the mutual elect ronic communication between them was determined using IR, NMR, and electron ic absorption spectroscopy. An X-ray structure determination of [Fe-2(eta - C5H5)(2)(CO)(2)(mu -CO)(muC-CH=CH-CH=C(Cl)-Fc)][BF4] confirmed the strong e lectronic interaction between the donor and the acceptor with reduced pi -b ridge bond-length alternation. The nonlinear optical properties of these co mplexes were examined using the hyper Rayleigh scattering technique. The ex perimental first hyperpolarisabilities are some of the highest obtained for ferrocenyl chromophores and, significantly, no enhancement was found due t o two-photon absorption fluorescence. When polyene linkers -(CH=CH)(n)- are used, the values for beta (0) increase with a ca. n(1.5) dependence with n o sign of saturation up to n = 4. However, the highest values for beta and beta (0) were obtained for linkers which contained an aromatic ring as oppo sed to pure polyenes and in this respect a benzene ring was more effective than a thiophene or furan. Consequently, the higher beta and beta (0) are n ot exhibited by those merocyanines with the highest values for lambda (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 importa nt than a large change in the dipole moment Delta mu (eg). Furthermore, a c hloro substituent on the olefinic double bond proximate to the ferrocenyl g roup has a dramatic effect on the beta and beta (0) values.