Organometallic Complexes for Nonlinear Optics. 33. Electrochemical Switching of the Third-Order Nonlinearity Observed by Simultaneous Femtosecond Degenerate Four-Wave Mixing and Pump-Probe Measurements

The molecular inorganic compound 1,3,5-{trans-[RuCl(dppe) ₂]C≡C-4-C₆H₄C≡C}₃C ₆H₃ (1) is reversibly oxidized in solution using an optically transparent thin-layer electrochemical (OTTLE) cell, the oxidation to 13⁺ resulting in the appearance of a strong absorption band at 11 200 cm^-1 in contrast to 1, which is optically transparent at frequencies below 20 000 cm^-1. This linear electrochromic behavior is accompanied by third-order nonlinear optical electrochromic behavior, which has been probed by a combination of femtosecond degenerate four-wave mixing (DFWM) and pump-probe experiments at 12 500 cm^-1, DFWM studies of 1 and 13⁺ reveal an order of magnitude increase in the hyperpolarizability, |γ|, upon oxidation with |γ| values of 2000 × 10^-36 esu and 20 000 × 10^-36 esu, respectively; the DFWM signal of the latter is dominated by a delayed response (relaxation time ca. 1 ps). The transient absorption (TA) data for 1 reveal efficient two-photon absorption (σ₂ ≈ 10³ GM), whereas the TA data for 13⁺ reveal saturable absorption (decay time ca. 1 ps). The switching of third-order nonlinearity in the 1/1³⁺ pair represents the first demonstration of femtosecond time-scale processes being responsible for nonlinear electrochromism.