Systematic Study of the Structure−Property Relationship of a Series of Ferrocenyl Nonlinear Optical Chromophores

A series of novel nonlinear optical (NLO) chromophores 1−4 incorporating the ferrocenyl (Fc)
group as an electron donor and 2-dicyanomethylene-3-cyano-4-methyl-2,5-dihydrofuran (TCF) derivatives
as electron acceptors are presented. The use of a constant Fc donor and varied acceptors and bridges
makes it possible to systematically determine the contribution of the conjugated bridge and the acceptor
strength to chromophore nonlinear optical activity. The X-ray crystal structures of all four chromophores
allow for the systematic investigation of the structure−property relationship for this class of molecules. For
example, the crystal structures reveal that both cyclopentadienyl groups in the ferrocenyl donor contribute
to the electron donating ability. The first-order hyperpolarizabilities β of these chromophores, measured by
hyper-Rayleigh scattering (HRS) relative to p-nitroaniline are reported. These β values are compared to
those calculated by density functional theory (DFT). The excellent agreement between the theoretical and
experimental β values demonstrates that a linear relation exists between the hyperpolarizability and the
bond length alternation. An electrooptic coefficient, r33, of ∼25 pm/V at 1300 nm, for compound 4,
incorporated into a polymer matrix, is competitive with organic chromophores. Moreover, this r33 is more
than 30 times larger than the previously reported value for an organometallic chromophore in a poled
polymer matrix. This work not only underscores the potential for Fc donor moieties, which have been
underutilized, but also demonstrates that experimental characterization and theoretical simulations are now
congruent, viable methods for assessing potential performance of NLO materials.